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Development of energy storage systems capable of Cu extraction from CuFeS₂ Deen, Kashif Mairaj 2019

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Development of energy storage systems capable of Cu extraction from CuFeS2  by  Kashif Mairaj Deen  B.Sc. (Engineering), University of the Punjab, 2007 M.Sc. (Engineering), University of the Punjab, 2012  A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF  Doctor of Philosophy in THE FACULTY OF GRADUATE AND POSTDOCTORAL STUDIES (Materials Engineering)  THE UNIVERSITY OF BRITISH COLUMBIA (Vancouver)  April 2019 © Kashif Mairaj Deen, 2019 ii  The following individuals certify that they have read, and recommend to the Faculty of Graduate and Postdoctoral Studies for acceptance, the dissertation entitled:  Development of energy storage systems capable of Cu extraction from CuFeS2  submitted by Kashif Mairaj Deen  in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Materials Engineering  Examining Committee: Edouard Asselin, Materials Engineering Supervisor  David Dixon, Materials Engineering Supervisory Committee Member  Wenying Liu, Materials Engineering Supervisory Committee Member David Dreisinger  University Examiner Dan Bizzotto University Examiner   Additional Supervisory Committee Members:  Supervisory Committee Member  Supervisory Committee Member   iii  Abstract Two hybrid energy storage systems, i.e., a fixed bed flow cell (FBFC) and a tri-functional battery (TFB) are introduced, which use either synthetic CuFeS2 or a mineral concentrate (MC) as electrode materials and a source of Cu. In the FBFC, the composite negative electrode is CuFeS2 or MC sandwiched in graphite felt (GF). The FeII/FeIII redox reaction (in the presence of CuII) occurs on a GF positive electrode. Under optimized conditions, the presence of CuFeS2 resulted in a continuous increase in the specific capacity of the FBFC from 9 to 48 mAh g–1 and in the specific energy from 2 to 6.3 Wh kg–1 in 500 galvanostatic charge/discharge (GCD) cycles. However, in the same setup, the MC had an increase in the specific energy from 3.5 to 8.5 Wh kg−1 in 400 GCD cycles. Advantageously, 10.3 and 12.7% Cu is extracted from the synthetic CuFeS2 and MC, respectively. In the TFB, two energy intensive processes, Cu extraction from CuFeS2 and Zn electrowinning, are integrated for energy storage. In this setup, the positive slurry electrode (PSE) composed of CuFeS2 or MC mixed with activated carbon (AC) in H2SO4 was separated by a membrane from the circulating Zn2+ solution in the negative compartment. The Zn deposition/re–dissolution and commencement of reversible reactions in the PSE during GCD cycles are responsible for energy storage akin to a battery. The maximum 388 Wh kg–1 specific energy (1.13 Wh l–1) during the 1st discharge cycle decreased to ≈50 Wh kg–1 over the subsequent 14 GCD cycles. The low coulombic (≈50%) and energy (~40%) efficiencies are offset by ~23% Cu extraction from CuFeS2 in 100 GCD cycles. The cell potential of ~0.95 V and potential efficiency (>70%) imply that the TFB can be used as a hybrid energy storage device. iv  Using MC in the TFB-M, a monotonic increase in energy density from 2.6 to 36.2 mWh l−1 at low energy efficiency (between 14–43%) was obtained for the initial 14 GCD cycles. On the other hand, in 100 GCD cycles, ~16.1% Cu was also extracted from the MC.     v  Lay Summary  Due to rapid industrial growth and high demand for energy, the use of renewable sources has compelled researchers to develop efficient energy storage devices. On the other hand, ores with a high concentration of Cu are being rapidly depleted and those that remain consist largely of CuFeS2 –a mineral that is hard to decompose for Cu extraction. Here we combine both energy storage and extraction processes into hybrid batteries. These batteries serve to extract Cu for its eventual recovery and sale. Two types of hybrid mineral batteries, which use synthetic and naturally sourced CuFeS2, are introduced. For example, a trifunctional battery is developed in which both Cu extraction from CuFeS2 and Zn deposition are possible in addition to energy storage. It is expected that coupling of these energy storage devices with wind turbines or solar cells at remote mining sites might be an attractive option.    vi  Preface The object of this research work was to investigate the possible use of CuFeS2 as an electrode material in a battery-like setup.  This setup would also be capable of extracting Cu. Unlike most battery systems, in which undesirable irreversible reactions are always responsible for capacity fade and deterioration of the electrodes, we are using this deterioration as a beneficial feature in this research. This research work is the first of its kind. The development and performance of two systems, a fixed bed flow cell (FBFC) and a tri–functional battery (TFB) is discussed. Initially, the core idea around the development of a system capable of leaching Cu from CuFeS2 through the use of the FeII/FeIII couple was introduced by Dr. Asselin and myself. Next, the FBFC design, characterization and analysis of the experimental results was conducted by myself as presented in chapters 5 to 7. The integration of two energy-intensive hydrometallurgical processes in one setup was proposed by myself. Briefly, in this setup, Zn electrowinning is coupled with CuFeS2 (or mineral concentrate) oxidation in the negative and positive compartments, respectively. Quantitative analyses of Cu extraction and energy storage capabilities of these systems are discussed in chapter(s) 8 and 9. The journal articles and conference presentations produced from this dissertation are listed as follows. I designed and executed the experiments. Dr. Asselin and I analyzed the data and equally contributed in the writing of these publications.  Journal Articles 1. K. M. Deen, E. Asselin, “Differentiation of the non–faradaic and pseudocapacitive electrochemical response of graphite felt/CuFeS2 composite electrodes”, Electrochimica Acta, 212 (2016), 979–991. 2. K. M. Deen, E. Asselin, “A hybrid mineral battery: Energy storage and dissolution behavior of CuFeS2 in a fixed bed flow cell”, ChemSusChem, 11 (2018), 1533–1548. vii  3. K. M. Deen, E. Asselin, “On the use of a naturally–sourced CuFeS2 mineral concentrate for energy storage”, Electrochimica Acta 297 (2019) 1079–1093. 4. K. M. Deen, E. Asselin, “A trifunctional battery setup that integrates Cu extraction and Zn electrowinning processes”, (Submitted for consideration) Conference Presentations 5. K. M. Deen, E. Asselin, “A hybrid system for energy storage which integrates Zn deposition and Cu leaching processes”, Resources for Future Generations (RFG 2018), June 16–21, 2018, Vancouver, BC, Canada 6. K. M. Deen, E. Asselin, “A Hybrid Energy Storage System Combining Zn Deposition and Cu Leaching in One Setup”, Extraction 2018, August 26–29, 2018, Ottawa, Canada  7. K. M. Deen, E. Asselin, “Cu extraction from naturally-sourced CuFeS2 coupled to Zn electrowinning in a battery-like setup”, COM-2019, (Accepted) The following table indicates the specific publications corresponding to the chapters provided in this dissertation:  Chapter # Publication # (as listed above) 5 1 6 2 7 3 8 4, 5 and 6 9 7         viii  Table of Contents Abstract ................................................................................................................................... iii Lay Summary ...........................................................................................................................v Preface ..................................................................................................................................... vi Table of Contents ................................................................................................................. viii List of Tables ........................................................................................................................ xiii List of Figures .........................................................................................................................xv List of Symbols ................................................................................................................... xxvi List of Abbreviations ......................................................................................................... xxix Glossary .............................................................................................................................. xxxi Acknowledgments ............................................................................................................. xxxii Dedication ......................................................................................................................... xxxiii Chapter 1: Introduction ..........................................................................................................1 1.1 Research goals .......................................................................................................... 3 1.2 Thesis outline ............................................................................................................ 4 Chapter 2: Literature Review .................................................................................................6 2.1 Structure of CuFeS2 .................................................................................................. 7 2.2 Eh-pH diagram of CuFeS2 and discrepancies ........................................................... 9 2.3 Passivation of CuFeS2 ............................................................................................. 11 2.4 Leaching of CuFeS2 ................................................................................................ 14 2.4.1 Leaching in sulfate media ................................................................................... 15 2.4.2 Factors influencing CuFeS2 leaching .................................................................. 20 2.4.3 Reduction of CuFeS2 ........................................................................................... 22 ix  2.5 Zn electrowinning and its use in energy storage ..................................................... 24 2.5.1 Solution purity and its importance ...................................................................... 26 2.5.2 Optimum conditions for Zn deposition ............................................................... 28 2.5.3 Use of Zn as an electrode material in batteries ................................................... 29 2.6 Energy storage and its importance .......................................................................... 30 2.6.1 Electrochemical energy storage .......................................................................... 30 2.6.2 Current state of battery research ......................................................................... 35 2.6.3 Use of CuFeS2 as battery electrode material ....................................................... 38 2.6.4 Electrochemical capacitors or supercapacitors ................................................... 42 Chapter 3: Objectives ............................................................................................................46 3.1 Setup 1: Fixed bed flow cell (FBFC) ...................................................................... 47 3.2 Setup 2: (TFB) ........................................................................................................ 47 Chapter 4: Approach and Methodology ..............................................................................49 4.1 Materials ................................................................................................................. 49 4.2 Experimental methods ............................................................................................ 50 4.2.1 Synthesis of CuFeS2 and pretreatment of mineral concentrate ........................... 50 4.2.2 Schematic of the systems under investigation .................................................... 51 4.2.3 Preparation of the electrodes for electrochemical study ..................................... 52 4.2.4 Construction of fixed bed flow cell (FBFC) ....................................................... 54 4.2.5 Construction and design of the tri-functional battery setup (TFB) ..................... 56 4.3 Physical characterization of electrode materials ..................................................... 57 4.4 Electrochemical Testing .......................................................................................... 60 4.4.1 Characterization of the composite electrode(s) ................................................... 60 x  4.4.2 Electrochemical characterization of electrode(s) for FBFC setup ...................... 62 4.4.3 Comparing the performance of as-synthesized CuFeS2 and MC in FBFC ......... 64 4.4.4 Electrochemical behavior of electrode(s) used in the TFB ................................. 65 4.4.5 Performance evaluation of the TFB .................................................................... 67 4.5 The reaction sequence for Cu extraction and system mass balance ....................... 68 Chapter 5: Electrochemical behavior of CuFeS2/GF composite electrodes .....................70 5.1 Characterization of the synthetic CuFeS2 ............................................................... 71 5.2 Cyclic Voltammetry ................................................................................................ 74 5.3 Galvanostatic cyclic charging and discharging profiles of electrode systems ........ 84 5.4 Mechanistic study of charge distribution on CF and Composite electrode ............ 87 5.5 Summary ................................................................................................................. 98 Chapter 6: The hybrid mineral battery: energy storage and dissolution behavior of CuFeS2 in an FBFC ..............................................................................................................100 6.1 Physical characterization of as-synthesized CuFeS2 ............................................. 100 6.2 Electrochemical behavior of individual electrode systems ................................... 104 6.3 Estimating the charge storage capability by the FBFC system ............................. 120 6.4 Ex–Situ characterization of CuFeS2 ...................................................................... 130 6.5 Proposed reaction sequence during GCD process ................................................ 135 6.6 Summary ............................................................................................................... 140 Chapter 7: Use of a CuFeS2 mineral concentrate in the FBFC .......................................142 7.1 Characterization of synthetic and CuFeS2 mineral concentrate ............................ 142 7.2 Electrode Kinetics ................................................................................................. 146 7.3 Estimating the energy storage capability of CuFeS2 and MS in the FBFC .......... 156 xi  7.4 Ex–situ characterizations of the retrieved CuFeS2 from the C–1 system ............. 168 7.5 Summary ............................................................................................................... 174 Chapter 8: Integrating the Cu extraction and Zn electrowinning processes for energy storage ...................................................................................................................................177 8.1 Physical characterization of the electrode materials ............................................. 177 8.2 Reaction sequence and optimization of the process conditions ............................ 182 8.3 Estimation of energy and Cu extraction capabilities of the TFB .......................... 194 8.4 Cause of capacity fade and mass balance in TFB ................................................. 209 8.4.1 Diagnosing possible reactions in TFB .............................................................. 209 8.4.2 SO42– transport and mass balance ..................................................................... 212 8.5 Polarization behavior of TFB ................................................................................ 218 8.6 Energy storage by TFB equivalent to energy produced by a diesel generator ..... 220 8.7 Summary ............................................................................................................... 221 Chapter 9: Using a CuFeS2 mineral concentrate in the TFB-M .....................................224 9.1 Difference between TFB and TFB-M ................................................................... 224 9.2 Energetics of the TFB-M ...................................................................................... 225 9.3 Energy storage and Cu extraction capabilities ...................................................... 229 9.4 Energy storage equivalent to energy produced by a diesel generator ................... 235 9.5 Summary ............................................................................................................... 237 Chapter 10: Conclusion .......................................................................................................238 10.1 Remarks on the important findings ....................................................................... 238 10.2 The practical implications of the current research work ....................................... 240 10.3 Future perspectives and recommendations ........................................................... 241 xii  References .............................................................................................................................245 Appendices ............................................................................................................................264 Appendix A ....................................................................................................................... 264 xiii  List of Tables Table 2-1 Oxidative acidic sulfate leaching of CuFeS2, the reaction chemistry [48] ............. 19 Table 2-2 Sulfate based Cu extraction processes from ore or mineral concentrate commercialized and/or studied at pilot plant scale [7, 60] ..................................................... 19 Table 2-3 Formation of possible species during electrochemical reduction of CuFeS2 in 1.7 M H2SO4 [32] .......................................................................................................................... 23 Table 2-4 Existing commercialized and developing battery technologies, their performance, and comparison [121, 157] ..................................................................................................... 37 Table 2-5 Performance metrics of EDLCs and recently reported asymmetrical supercapacitors ........................................................................................................................ 45 Table 4-1 The designation and composition of electrode and electrolyte systems used in the FBFC ....................................................................................................................................... 63 Table 4-2 The electrode systems and composition of the electrolytes used in the TFB setups ................................................................................................................................................. 66 Table 5-1 Quantitative measurement of parameters from Multipoint BET surface area analysis .................................................................................................................................... 71 Table 6-1 Impedance parameters evaluated after fitting the spectra by using the EEC model ............................................................................................................................................... 119 Table 7-1 Parameters calculated from the CV scans obtained at various sweep rates at 25 ºC (Note: Geometrical surface area of the GF (14.4 cm2) was used in the calculation) ............ 155 Table 8-1 Quantitative determination of the extent of reaction from GCD and ICP-OES analysis .................................................................................................................................. 212 Table 8-2 Parameters used to determine the mass transport across the AEM ...................... 214 xiv  Table 8-3 Estimation of energy storage by a TFB, which has the capacity to accommodate 1 tonne of CuFeS2 in PSE and comparison with the energy generated by a diesel generator . 221 Table 9-1 Energy storage capability of TFB-M, which can process 1 tonne of MC in one batch and estimation of MC to diesel ratio for the supply of same amount of energy ......... 236  xv  List of Figures Figure 2-1 (a) Unit cell of CuFeS2 in which each Cu atoms are shown in black spheres, Fe and S atoms are represented as small and large grey spheres, respectively [24]. (b) Schematic of CuFeS2 band structure [10] ................................................................................................... 8 Figure 2-2 Potential–pH diagram of Cu–Fe–S–H2O (activity of solute = 0.1 M, except aCu2+= 0.01 M) at 25 ºC [30] .............................................................................................................. 10 Figure 2-3 Polarization trend of CuFeS2 in 0.5 M H2SO4 developed  from the steady-state current values obtained potentiostically [11] .......................................................................... 14 Figure 2-4 (a) Effect of H2SO4 concentration and temperature on the passivation potential of CuFeS2 [56](b) % Cu recovery from the Kristineberg concentrate at 80 ºC. The redox potentials were adjusted by the addition of dilute H2O2 in H2SO4 solution (pH =1.5) containing initial Fe2+ (1 g l–1) [57] ........................................................................................ 17 Figure 2-5 Strategies applied to enhance Cu extraction form CuFeS2. (Note: ORP was controlled by applying DC potential of 0.4–0.45 V vs. Ag/AgCl electrode, bacterial controlled process was carried out at 50 ºC) [60, 61] ............................................................. 18 Figure 2-6 Percentage Cu and Fe extraction at various temperatures, 100 g solids in 1 liter of 1 M H2SO4 solution containing initial 10 g l–1 Cu2+ and 5 g l–1 Fe3+ was used [48] .............. 18 Figure 2-7 General Zn metal production process flow sheet via roasting, leaching, and electrowinning ......................................................................................................................... 25 Figure 2-8 Effect of metallic impurities concentration on the current efficiency of Zn deposition process in industrial acid sulfate solution at 430 A m–2 (a) IV A and VI-A (b) III-A and V-A group metals including Sn [97]. ........................................................................... 27 xvi  Figure 2-9 Types of reactions that may occur on the electrode materials of an electrochemical energy storage system ............................................................................................................. 33 Figure 2-10 Charge storage mechanism in batteries and supercapacitors, formation of double layer (A) at the surface (B) within the bulk of porous carbon. Pseudocapacitive behavior (C) due to surface redox reaction onto a hydrous RuO2 electrode (D) due to intercalation of Li+ ions within the bulk of host material. Cyclic voltammograms of (E) supercapacitor and (F) battery electrode. Pseudocapacitive discharge curve (G) of MnO2 capacitor and (H) LiCoO2 battery electrode materials [130]. ........................................................................................... 33 Figure 2-11 Morphologies of the as-synthesized CuFeS2 reported in the literature (a) pyramidal [165], (b) nanowires [166], (c) plates/sheets [167], and (d) spikelike nanorods [129, 168] ................................................................................................................................ 39 Figure 2-12 Morphology of synthetic CuFeS2 (a) without and (b) in the presence of PVP. Charge/discharge profiles (at various specific currents), cyclic voltammograms and specific capacity of as-synthesized CuFeS2 (c, e & g) without and (d, f & h) in the presence of PVP [162] ........................................................................................................................................ 41 Figure 4-1 Schematic diagram of the electrode systems used in each battery setup. The combination of positive (red lines) and negative (black lines) connect the two electrodes into a cell assembly. ....................................................................................................................... 52 Figure 4-2 Assembly of three electrodes cell in which each electrode i.e., CF electrode, comp electrode, GF-CuFeS2, GF–MC and GF–Fe/Cu was tested in its respective electrolyte. ....... 54 Figure 4-3 Schematic diagram of the FBFC, the original setup demonstrates the overall assembly .................................................................................................................................. 55 xvii  Figure 4-4 (a) Schematic diagram of the TFB (cross-sectional view), (b) assembled cell, the high-density polyethylene end plates in which (c) components of the cell were tightened with screws. (1; GP is the flexible graphite sheet (FGS), 2; pure Al sheet, 3; Si rubber gaskets, 3a; AEM sandwiched in Si rubber gaskets and 4 represents the flow channels in PTFE plates for PSE and anolyte flow through nozzles ................................................................................... 57 Figure 5-1(a) Morphology and (b) particle size distribution curve of as-synthesized CuFeS2. (c) Multipoint BET analysis. (d) XRD pattern of CuFeS2. (e) The XPS survey scan of the CuFeS2 powder sample. (f) Deconvoluted high-resolution spectra of 'S 2p3/2' ...................... 73 Figure 5-2 CV scans (at 5–100 mV s–1) of (a) CF and (b) Composite (CF+CuFeS2) electrode. (c) Trends of current vs. sweep at various potentials. (d) b–values as a function of charge and discharge potential (For comparison, the current is normalized by the mass of GF) ............. 78 Figure 5-3 CV of (a) CF and (b) Composite electrodes at 20 mV s-1, showing the current distribution. (c) Comparison of the total current contributed to the non-faradaic and faradaic processes leading to diffusion controlled processes as revealed from Csp plots of (d) CF and (e) composite electrodes. ('ch' and 'dis' represent the start and end of discharge, respectively). ................................................................................................................................................. 79 Figure 5-4 FTIR spectra of unexposed (as received) and exposed (after GCD in de-aerated 0.2 mol dm–3 H2SO4 at 25ºC) graphite felt (CF) ..................................................................... 82 Figure 5-5 GCD cycling at 0.02 A g–1 (a) of the CF and (b) the composite electrodes. (Note: arrows down and up represent the charging and discharging, respectively, the EOC of composite electrode = 0.470 ± 0.005 VSHE at pH = 0.702 ± 0.005). (c) The discharge-specific capacitance at various currents. (d) Columbic efficiency of the CF and composite electrodes ................................................................................................................................................. 87 xviii  Figure 5-6 The Nyquist plots of (a) CF electrode (from EOC = 0.683 VSHE to Emax –0.216 VSHE (η=–0.9 V) in the cathodic (charge) direction). (b) The impedance behavior at ω → ∞ for CF electrode. (c) The Nyquist plots of the composite electrode at various potentials starting from 0.470 to –0.429 VSHE (η=–0.9 V) (d) high-frequency impedance behavior of composite electrode ................................................................................................................ 90 Figure 5-7 Schematic of (a) CF electrode and morphology of graphite fibers in the GF. The inset shows the EEC. (b) Composite electrode and EEC used to simulate impedance spectra for analysis .............................................................................................................................. 91 Figure 5-8 Variation in Φdl and Φp of CF and composite electrode. (b) The extent of current leakage through faradaic processes (η = 0 corresponds to the EOCP (0.683 VSHE) and mixed potential (0.470 VSHE) for the CF and composite electrodes, respectively) ........................... 93 Figure 5-9 Cathodic polarization trends of (a) CF and composite electrodes starting from their respective OCP to –0.9 V vs. OCP, low field (η→0) polarization trends showing the convergence potential. (b) The proposed mechanism of the overall charge distribution in the composite electrode ................................................................................................................ 97 Figure 6-1 N2 adsorption/desorption isotherm of as-synthesized CuFeS2 particles (77 K). . 102 Figure 6-2 ToF–SIMS analysis of as-synthesized CuFeS2 (a) 3D surface distribution of Fe and Cu (b) positive ion spectrum (c) 3D mapping for sulfur (S) (d) negative ion spectrum 103 Figure 6-3 Potentiodynamic polarization scans of composite electrode (GF–CuFeS2+CB) in 0.2 M H2SO4 solution; the current is normalized by the mass of CuFeS2 ............................ 106 Figure 6-4 CV scans of (a) the composite electrode in 0.2 M H2SO4 (the current is normalized by the mass of composite electrode) and (b) GF electrode in 0.5 M Fe2++ 0.2 M xix  H2SO4 solution. (c) GF in solution as in (b) with 0.1 M Cu2+ (the current is normalized by the wt. of GF) (d) peak current vs. (sweep rate)1/2. ..................................................................... 110 Figure 6-5 Stability analyses of FeIII species in Cu2+ containing solution at a GF electrode. (a) Schematic of sequential protocol applied for the charging (FeII → FeIII) and discharging (FeIII → FeII).  (b) The discharge current profiles of GF after sequential delay (0 to 60 min) in 0.5 M Fe2+ solution and in (c) 0.1 M Cu2+ ions containing electrolyte. (d) Discharge peak current profiles as a function of delay time (Note: mass (187 mg) and geometrical surface area (14.4 cm2) of GF electrode were same for each test). .................................................................... 114 Figure 6-6 Impedance spectra of composite (GF – CuFeS2 + CB), GF–Fe and GF–Fe/Cu electrodes (at 0 V vs. OCP). The EEC is shown in the inset in which Yx is the admittance for the diffusion parameter and x=B, (for finite diffusion) and x=w (for semi-infinite diffusion), similarly σB = Warburg constant for GF–CuFeS2, and σw = Warburg constant for GF–Fe and GF–Fe/Cu electrodes. In addition, the geometrical area of the GF in all electrodes was the same (14.4 cm2). ................................................................................................................... 116 Figure 6-7 (a) Bode plots (b) Phase angle (c) Residual error plots obtained after fitting of impedance spectra with EEC.  (Echem® Analyst 6.25 software; Gamry Instruments Inc.). 117 Figure 6-8 SEM images and EDX analyses of the (a)  as received and (b) treated GF (in 0.5M Fe2+/0.2M H2SO4) (c) GF treated in 0.5M Fe2+/0.1M Cu2+/0.2M H2SO4 ............................. 120 Figure 6-9 CV scans of (a) CFe and (b) CFeCu systems. (c) Trends showing the variation in the specific capacitance as a function of (sweep rate)–0.5 (at 1 V cell potential), (d) comparison of CFe and CFeCu system (voltammograms at 1 mV s–1) ................................ 123 xx  Figure 6-10 GCD cyclic trends of (a) CFe and (b) CFeCu systems (c) the specific capacity behavior of both systems (d) Trends of coulombic (ηc) and energy (ηE) efficiencies for both systems .................................................................................................................................. 126 Figure 6-11 Specific energy trends of CFe and CFeCu system (Note: Charging and discharging was carried out at 200 mA g–1 and 150 mA g–1, respectively) .......................... 129 Figure 6-12 Morphological, compositional and structural changes in the CuFeS2 before and after 500 GCD cycles in CFeCu system, SEM and EDX spectra of (a) as–synthesized, (b) and retrieved CuFeS2 (c) XRD patterns comparison ............................................................ 131 Figure 6-13 Fe and Cu species concentration in the anolyte after 500 GCD cycles (ICP-OES analysis), the % Cu extraction and retrieved anolyte and catholyte from CFeCu cell system after 500 cycles are shown as an inset. ................................................................................. 132 Figure 6-14 Deconvoluted X-ray photoelectron spectra of S 2p depicting the split of peaks associated with mono, di, and polysulfide species on the surface of (a) as–synthesized (b) retrieved CuFeS2 samples (CFeCu cell system) (c) doublet peaks of Cu associated with Cu 2p3/2 and Cu 2p1/2 orbital ................................................................................................... 133 Figure 6-15 Schematic diagram showing the proposed reactions sequence on the negative electrode in the FBFC system during (a) charging and (b) discharging process .................. 136 Figure 6-16 Potentiostatic polarization of CuFeS2+ CB slurry (a) Variation in pH during charge (cell potential = –1.0 V) and discharge cycles (cell potential = 1.0), (b) Change in current profile (Note: only CuFeS2 slurry in 0.2 M H2SO4 was used for pH measurements) ............................................................................................................................................... 140 Figure 7-1 The EDX spectrum of the synthetic CuFeS2 ....................................................... 144 Figure 7-2 Particle size distribution histograms of (a) synthetic CuFeS2 and (b) MC ......... 144 xxi  Figure 7-3 XRD pattern of the mineral concentrate (MC), the inset shows an SEM image of the MC particles .................................................................................................................... 146 Figure 7-4 Cyclic voltammograms of composite (a) GF–CuFeS2, (b) GF–MC electrodes obtained at various sweep rates. (c) log (v) vs. ΔE1, 2 (peak separation) trends for the composite electrodes. (d) CV of GF–Fe/Cu electrode at different sweep rates (For composite electrodes, the current is normalized by the weight of GF + CuFeS2 / MC, whereas for the GF–Fe/Cu electrode, the weight of GF was used to calculate the specific current). ............ 150 Figure 7-5 Peak current (for Pa1 and Pc1 only) variation vs. sweep rate (v) (0.04 ≥ v <0.1 V s–1) ............................................................................................................................................ 151 Figure 7-6 From the CV scans (0.001 ≤ v ≤ 0.02 V s–1) and Equation 6.2, ip vs. v1/2 of (a) GF–CuFeS2 (b) Ep vs. log (v) of GF–CuFeS2 electrode (c) ip vs. v1/2 and (d) Ep vs. log (v) trends for the GF–MC electrode used to calculate charge transfer coefficient, α, values ............... 151 Figure 7-7 The Ep – Epx vs. log (ip) trends for the calculation of kh (Equation 7.4) from the peak shift and peak current variation in the CV scans of (a) GF–CuFeS2 and GF–MC (b) GF–Fe/Cu electrodes, where I represents the intercept on the y-axis after linear fitting of data. 156 Figure 7-8 CV scans obtained for FBFC at various sweep rates when the negative electrode is (a) GF–CuFeS2 (C–1) and (b) GF–MC (C–2), whereas the positive electrode used was GF. 0.5 M Fe2+ + 0.1 M Cu2+ in 0.2 M H2SO4 was circulated from the external thermostatic reservoirs. (Note: The current is normalized by the weight of either synthetic CuFeS2 or MC) (c) The ip vs. v1/2 trends of the C–1 and C–2 cells (d) variation of ipa/ipc and peak separation due to change in sweep rate .................................................................................................. 159 xxii  Figure 7-9 GCD tests showing the (a) 1st, 250th and 500th charge / discharge cycles and (b) discharge curves of C–1 (c) 1st, 250th and 400th charge/discharge cycles of C–2 (d) discharge profiles of C–2. ..................................................................................................... 163 Figure 7-10 (a) Variation in discharge specific capacity and coulombic efficiency of C – 1 and C – 2 cells (b) the maximum specific energy that can be stored by C–1 and C–2 FBFC setups. The inset shows the % Cu extraction calculated based on the weight of CuFeS2 in the C–1 and C–2 cells ................................................................................................................. 166 Figure 7-11 (a) SEM image of retrieved CuFeS2 from C–1 after GCD cycling, the inset shows the lattice structure of CuFeS2 and CuS (b) EDX spectrum shows the elemental composition (c) XRD pattern of CuFeS2 after 500 GCD cycles .......................................... 169 Figure 7-12 XPS core Cu 2p peaks of (a) as–synthesized and (b) retrieved CuFeS2 particles (from the C–1 cell) show the presence of 2p3/2 and 2p1/2 spin orbitals (c) high-resolution S 2p3/2 peak of the as-synthesized and (d) retrieved CuFeS2 ................................................. 171 Figure 7-13 Potentiodynamic cathodic polarization of the GF–CuFeS2 electrode in 0.2 M H2SO4 (anolyte) containing various amounts of Cu2+. The inset shows the cathodic Tafel slope (βc) as a function of Cu2+ concentration. ..................................................................... 174 Figure 8-1 (a) SEM image showing the morphology and (b) Deconvoluted S 2p3/2 high-resolution spectra of synthetic CuFeS2. (c) N2 adsorption/desorption isotherm for surface analysis and (d) multi–pore size/volume analysis curve of CuFeS2 particles ...................... 178 Figure 8-2 (a) XRD pattern (b) XPS survey scan and (c) the deconvoluted high-resolution spectra of Cu 2p1/2 and Cu 2p3/2 bands of as-synthesized CuFeS2 ........................................ 179 xxiii  Figure 8-3 (a) A three-electrode setup for measurement of electrochemical behavior of the PSE containing (80 wt. % CuFeS2 + 20 wt. % AC in 0.2 M H2SO4. (b) SEM image of AC (c) Multipoint BET surface area and pore size distribution curve for AC ................................. 184 Figure 8-4 (a) CV (4th cycle) of positive electrode components; GP, GP–AC and GP–AC–CuFeS2. (Note: exposed area of GP = 6.28 cm2, 20 wt. % total solids in the slurry) (b) CV curves of negative electrode (Al strip ≈ 2 cm2). (c) Chronopotentiometry scans (30 mA cm–2) depicting the effect of CTAB concentration in the anolyte (vs. MSE = 0.62 VSHE) (d) a potential/concentration (PC) diagram developed from the CV and chronopotentiometry results. ................................................................................................................................... 185 Figure 8-5 The CV of the PSE (5mV s-1 sweep rate). (a) The cyclic performance of the positive electrode (5 cycles). (b) The 1st and 5th cycles show the current peaks (as labeled) used to predict the reactions sequence during actual charge/discharge process in the TFB. 188 Figure 8-6 SEM images of Zn deposit on Al substrate obtained in the base solution containing, (a) 0.1 M Na2SO4, (b) a + 1 ppm CTAB, (c) a + 2 ppm CTAB, (d) a + 4 ppm CTAB (e) a + 8 ppm CTAB (f) XRD patterns of Zn deposits formed in a – e .................... 193 Figure 8-7 TFB showing the assembly of a cell connected to a potentiostat ....................... 196 Figure 8-8 Performance evaluation of TFB (a) CV curves of negative and positive electrode, (5 mV s–1). (b) Equilibrium cell potential (EC) and schematic of potential drop within the TFB. (c) CV scans of TFB at various sweep rates and (d) the trends of peak current vs. (sweep rate)0.5 ........................................................................................................................ 197 Figure 8-9 (a) CV curves of TFB as shown in Figure 8-8c indicating peak P2 (b) shift in peak potentials (P1 and P3) vs. sweep rate (c) variation in peak current and potential vs. sweep rate for peak P2 ............................................................................................................................ 200 xxiv  Figure 8-10 The 10 GCD cycles (a) charge and (b) discharge cycles. (c) 20 to 100 GCD cycles; the specific capacity is calculated based on the mass of Zn deposit during the first charge cycle. (d) Repetitive CV scans of TFB obtained at 0.002 V s–1 ................................ 203 Figure 8-11 Quantitative assessment of (a) Specific capacity (discharge), coulombic, energy and Voltage efficiencies of the TFB. (b) Energy storage capability and % Cu extraction as a function of GCD cycles (Note: The % Cu extraction was calculated from the ICP-OES analysis). (c) The filtrate retrieved from the PSE. (d) Instantaneous % Cu extraction from CuFeS2 in the TFB during 100 GCD cycles ......................................................................... 205 Figure 8-12 (a) Discharge profiles of the TFB obtained at various C–rates, (charged at the 1C rate) (b) Ragone plots developed based on the first GCD cycle (c) Cell potential variation of fully charged TFB under an applied load of single LED (d) Illuminated LED when connected to fully charged TFB ............................................................................................................. 208 Figure 8-13 (a) Schematic diagram of ionic species transport across AEM. (b) The molar fraction of ionic species in the anolyte at 25º C (c) The transient electric field and (d) the molar flux of SO42– across AEM .......................................................................................... 216 Figure 8-14 I–V curve of the fully charged TFB (at 1C) showing the polarization trend and variation in specific power at various discharge currents. .................................................... 219 Figure 9-1 (a) OCV of the TFB-M and (b) CV scans of the TFB-M at various sweep rates. ............................................................................................................................................... 226 Figure 9-2 CV curves of TFB-M (at 25 mV s–1) showing the variation in current response during repetitive cycling ....................................................................................................... 229 Figure 9-3 The galvanostatic (a) charge and (b) discharge profiles of TFB-M (initial 3 cycles). .................................................................................................................................. 231 xxv  Figure 9-4 The performance of TFB-M setup (a) energy density vs. no. of cycles, (b) coulombic efficiency, (c) potential efficiency and (d) rate of Cu extraction measured from ICP–OES analysis of the PSE-M filtrate after various GCD cycles. (Note: the specific energy is calculated based on the volume of slurry in the PSE-M). ................................................. 232  xxvi  List of Symbols Symbol Meaning Common Units a & b Adjustable parameters – Am Membrane area m2 av Applied potential V B Time Constant s1/2 C BET constant  – Cas Bulk SO42– concentration in anolyte  mol l–1 Ccs Bulk SO42– concentration in catholyte mol l–1 Cnc Charge capacity mAh g–1 Co* Bulk ionic concentration mol l–1 Csp Specific capacitance F g–1 D Diffusion coefficient cm2 s–1 Ds Diffusion coefficient of SO42–  cm2 s–1 D̅s SO42– diffusion coefficient within membrane cm2 s–1 dt Change in time s dV Change in potential V dV/dt Shift in potential V s–1 E Potential V Eapp Applied potential V ENOCP OCP of negative electrode V (vs. ref) Eº Potential  V (vs. SHE) EPOCP OCP of positive electrode V (vs. ref) Epx Redox potential  V (vs. ref) Epxa Anodic peak potential (x=1, 2, …) V (vs. ref) Epxc Cathodic peak potential (x=1, 2…) V (vs. ref) ∆Ep Peak separation potential V F Faraday’s constant  C mol–1 i Intercept  – I(V) Sweep rate dependant current A ic Charging current mA g–1 id Discharging current  mA g–1 idR Potential drop at the start of discharge V io Exchange current density A cm–2 Ip1 Peak current A ipa Anodic peak current A ipc Cathodic peak current A ipeak Max. peak current  A IR Potential drop V J Instantaneous molar flux mol m–2s–1 k1 & k2 Differential capacitance coefficients  A (V s–1)–1 & A (V s–1)–0.5 kh Heterogeneous rate constant cm s–1 km Membrane constant – m Mass gm M Molecular mass gm m/z Mass to charge ratio –    xxvii  Symbol Meaning Common Units ROMAN   mGF Mass of the graphite felt gm mnc Amount of SO42– transferred to catholyte mol l–1 mZn Mass of deposited Zn gm n No. of electrons – n Number of cycles – ƞ Overpotential V n1 & n2 Phase angle coefficient – NA Avogadro’s number – ȠC Coulombic efficiency % ȠE Energy efficiency % ȠV Potential efficiency % OCP/EOCP Open circuit potential V (vs. ref) P Equilibrium pressure mbar P1 & P2 Potential plateaux – Pax Anodic peak (x=1, 2…) – Pcx Cathodic peak (x=1, 2…) – Pº Saturation potential mbar Qad Constant phase element (for adsorption) mS sn2 Qc Specific capacity mAh g–1 Qcharge Total charge from charging cycle  C Qdischarge Total charge from discharging cycle C Qg Total charge (from gravimetric analysis) C Qgs Total charge (from galvanostatic) C Qirr Irreversible charge  C R Universal gas constant J mole–1 K–1 R Total resistance Ω Rad Resistance (for adsorption)   Ω Rct Charge transfer resistance Ω Rp Parallel resistance kΩ Rs Solution resistance Ω S Slope – s Sweep rate mV s–1 SBET BET surface area m2 g–1 t Time s T Temperature K t1 Initial time s t2 Final time s V Volume of gas adsorbed cm3 V Potential window V v Sweep rate mV s–1 V(t) Decay in cell potential with time V V1 & V2 Initial and final potential V VA Volume of anolyte ml   VC Volume of catholyte ml vp Membrane pore volume  cm3 VS Volume of slurry ml xxviii  Symbol Meaning Common Units ∆V Potential difference V ∆VCF Potential delay (CF electrode) V ∆Vcomp Potential delay (composite electrode) V W Weight of adsorbed N2 gm WCuFeS2 Initial weight of CuFeS2 gm Wm Adsorbed N2 monolayer capacity gm cm–2 x Extent of reaction – xm Membrane thickness m Ys Warburg diffusion coefficient S s z Charge number of SO42– – Zimg Impedance (Imaginary) Ω  ZRe Impedance (Real) Ω    GREEK   αa Charge transfer coefficient (anodic) – αc Charge transfer coefficient (Cathodic) – βa Anodic Tafel slope  mV decade–1 βc Cathodic Tafel slope mV decade–1 ΓT Surface concentration moles cm–2 θ Membrane Tortuosity  – Λ Dimensionless rate constant – π1/2x(bt) Current function  – σB Finite diffusion coefficient Ω s–1/2 σw Infinite diffusion coefficient Ω s–1/2 Φdl Constant phase element (double layer) mS sn1 Φp Constant phase element (pseudocapacitance) S sn2 ω Frequency Hz dΨ Electric field V m–1  xxix  List of Abbreviations AC Activated Carbon AEM Anion exchange membrane ATR Attenuated total reflection BET Brunauer-Emmet-Teller C–2 FBFC: GF-MC (Negative) & GF-Fe/Cu (Positive)  CB Carbon black CD Charge/discharge CF An electrode assembly (GF connected to graphite rod) CFe FBFC: GF-CuFeS2 (Negative) & GF-Fe (Positive)   CFeCu or C–1 FBFC: GF-CuFeS2 (Negative) & GF-Fe/Cu (Positive)  Composite Composite electrode (CuFeS2 encapsulated in GF) CPE Carbon paste electrode CTAB Cetyltrimethylammonium bromide CV Cyclic voltammetry DC Direct current DFT Density functional theory DI Deionized water DMC Dimethyl carbonate  EC Ethylene carbonate EDLC Electrochemical double layer capacitor EDX Energy dispersive x-ray spectroscopy EEC Equivalent electrical circuit EFC Electrochemical flow cell EIS Electrochemical impedance spectroscopy FBFC Fixed bed flow cell FGP/GP Flexible graphite plate/Graphite plate FTIR Fourier-transform infrared  FWHM Full width at half maximum GCD Galvanostatic charging/discharging GF Graphite felt GF-CuFeS2 Composite electrode (GF + synthetic CuFeS2) GF-Fe CF electrode immersed in Fe(II) containing electrolyte GF-Fe/Cu CF electrode immersed in Fe(II)+Cu(II) containing electrode GF-MC Composite electrode (GF + CuFeS2 mineral concentrate) GHG Greenhouse gases GS Galvanostatic polarization HDPE High density polyethylene ICP-OES Inductively coupled plasma-optical emission spectroscopy I–V Current potential curve LED Light emitting diode LiBs Lithium ions batteries LSV Linear scan voltammetry MC Mineral concentrate MRI Magnetic resonance imaging OCP Open circuit potential OCV Open circuit voltage ORP Oxidation/reduction potential xxx  PC Potential-concentration diagram PCP Potentiodynamic cathodic polarization PD Potentiodynamic polarization PEM Proton exchange membrane PSE Positive slurry electrode (synthetic CuFeS2+AC in 0.2M H2SO4) PSE-M Positive slurry electrode (MC + AC in 0.2M H2SO4) PTFE Polytetrafluoroethylene PVDF Polyvinylidene fluoride PVP Polyvinyl pyrrolidone  QXRD Quantitative x-ray diffraction   SCs Supercapacitors SEM Scanning electron microscopy SHE Standard hydrogen electrode SSFC Semisolid flow cell SX/EW Solvent Extraction/Electrowinning  TFB Tri-functional battery  TFB-M Tri-functional battery setup with MC ToF-SIMS Time of flight secondary ion mass spectroscopy UPD Under potential deposition XPS X-ray photoelectron spectroscopy  XRD X-ray diffraction   xxxi  Glossary Capacitive  (non-faradaic) Charge stored in the electrical double layer Capacity Reversible charge storage; mAh Energy density Energy normalized by the volume of PSE; Wh l–1 Internal charge mediator Facilitating the charge transfer within the composite electrode i.e. formation and transportation of intermediate H⁰ from GF to CuFeS2 Non-Capacitive (irreversible) Irreversible faradaic reactions or conversion reactions  Pseudocapacitive Charge storage due to reversible faradaic processes on the surface  Specific capacity Charge normalized by the mass of electrode material; mAh g–1 Specific energy Energy normalized by the mass of electrode material; Wh kg–1 Specific Power Power normalized by the mass of electrode material; W kg–1  xxxii  Acknowledgments I express my deepest appreciation to the administrative and technical staff at the Department of Materials Engineering for their kind support during the course of my Ph.D. studies. My gratitude goes to my supervisory committee, Professor D. Dixon and Dr. W. Liu whose valuable suggestions helped me to improve my work. I also acknowledge the very useful discussions with Dr. Zihe Ren about the synthesis of chalcopyrite. In particular, I am extremely grateful to my supervisor Professor Edouard Asselin, whose consistent moral and technical support and critical reviews on this research work encouraged me to do my best. Thanks to his extended compassion, professionalism, and enchanting personality, which taught me a lesson to covert my deficiencies into perfection and skills into strengths.  The financial support from Natural Science & Engineering Research Council of Canada (NSERC), the Four-Year Fellowship (FYF) from the University of British Columbia and partial funding from the University of the Punjab are highly acknowledged.  I am indebted to all my fellow graduate students and friends, especially Tasawar Javed, Yu Liu and M. Haziq, whose social, moral and competitive support helped me to tune my personality and technical skills. I never felt loneliness even staying thousands of miles away from my hometown in their company.   Special thanks to my daughters and son, who remained desperate for my time and company during my Ph.D. studies. My sincere gratitude to my lovely wife Uzma, whose continuous support, patience and great endurance in every hardship are incomparable. Finally, yet importantly, I do not have qualified words to tribute my father (Mairaj Deen) and mother (Ruqaya Mairaj), who’s unconditional and untiring efforts from my birthday until today are incalculable and inimitable.  xxxiii  Dedication      Dedicated to my teachers, parents, daughters (Ulaika, Tasmia, and Fatima), son (M. Ali Murtza), my wife Uzma and to all my well-wishers1  Chapter 1: Introduction Cu is an essential ingredient of many technologies in our modern society including solar systems, electronics, construction, and other industrial equipment. Due to its excellent electrical and thermal conductivity, Cu is the conductor of choice for most applications. In nature, Cu largely exists in the form of oxide and/or sulfides ores (elemental or “native” Cu is also possible though rare). To produce Cu from oxide minerals via hydrometallurgical processes, the heap leach–solvent extraction-electrowinning (HL–SX–EW) sequence is adopted. On the other hand, depending on the ore grades; the sulfidic ores from mines are processed and concentrated to 25 – 35 % Cu through froth floatation. The mineral concentrate (MC) is then smelted and electro–refined to produce pure 99.99 % Cu metal [1]. At present, globally about 80 % of the total Cu production is via the pyro–metallurgical route by the use of existing smelting processes. To produce 1 tonne of refined Cu, the global GHG emission is equivalent to ~3.3 tonnes of CO2 [2-5]. The energy requirements for Cu production and GHG emission directly relate to the grade of Cu ores. From the previous 100-year history of existing Cu mines, a continuous decline in ore grades has been evident. From 1900 to 2010, on average, global Cu grades have been substantially decreased from 1.5 – 4.0 % to 0.62 %. It is also forecasted that with an annual decline of 1.5 to 3.7 %, Cu grades may further drop to ~0.49 % in 2050 [6].  Chalcopyrite (CuFeS2) is the main economic copper bearing mineral, which is, as discussed above, generally processed through pyro-metallurgical methods to extract the Cu. However, serious environmental issues with smelting, depletion of high-grade ores and a large number of impurities (some toxic) have resulted in continued research aimed at the development of hydrometallurgical technologies to process CuFeS2. There are a number of 2  different proposed routes in the literature to treat CuFeS2 concentrates, but only a few of them have attained commercial acceptance [7]. The literature provides good insight about the possible leaching of CuFeS2 under oxidizing conditions or by the reductive conversion of CuFeS2 into Cu2S, which could in turn be more easily oxidized to Cu2+. The use of FeIII ions in the presence of oxygen to leach Cu2+ from CuFeS2 has been rigorously studied in the past [8-11]. The energy generated during oxidation of CuFeS2 by FeIII ions in the presence of oxygen is lost as heat. In contrast to the direct reduction of FeIII on CuFeS2, we propose that the FeII/FeIII redox reaction could proceed on a separate electrode in a battery like a cell setup. Thus, the amount of useful energy, which may be retrieved and stored for further use, can be quantified.  Zn is another commercially important metal that is mainly produced via acid leaching of roasted sphalerite (ZnS). The pregnant leach solution is rigorously purified to remove all the soluble metal impurities, particularly those that are electrochemically more noble than Zn and present large kinetic activity towards H2 evolution [12-15]. Metal impurities such as Cu, Co, Ni, Cd, Pb, Sb, As, Ge and Fe etc., if they exist in the leach liquor beyond the tolerance limits, can decrease the current efficiency during electrowinning and deteriorate the deposit quality [16]. A complete flow sheet, describing the process steps of Zn metal production from mine site to pure metal is presented in section 2.5 as given elsewhere [17].  Based on the reactions that occur during the electrowinning process, Zn deposition should take place at ~2.0 V and the theoretical energy required for this process would be 1.63 kWh kg–1. But due to large overpotentials at the electrodes (particularly on the anode, ~0.6 V) and a potential drop in the leach liquor, an actual cell potential of ~3.2 V is required for Zn electrowinning, which corresponds to an energy consumption of ~2.8 kWh kg–1 [16-18].  3  With a lens on energy storage and/or sustainable Cu and Zn production, the tri-functional battery (TFB) proposed here could cumulatively save an enormous amount of energy, which is otherwise individually required for both of these hydrometallurgical processes. On the other hand, the aqueous-based chemistry in this setup innately suppresses the fire hazards and other safety concerns that are specifically associated with the scale-up of Li–ions batteries for stationary applications. The cyclic charge/discharge performance and the Cu extraction is another compelling feature of this setup in contrast to the existing Ag–Zn and Zn–air primary batteries [19-21]. 1.1 Research goals To date, the extractive industries have used minerals mainly for producing metals.  However, many minerals possess intrinsic characteristics that could be used for purposes other than metal production e.g. for energy storage. There exists, then, a dichotomy between the current widespread use of these minerals for metal production and their potential application as energy materials. This research work is something radically different: to leverage existing mining infrastructure to rationalize the construction of battery systems and in turn encourage remote locations to build and use renewable energies. The main goals of this work were to develop hybrid battery like setups that can be used as energy storage devices and as units for Cu extraction mainly from CuFeS2. The energy storage and % Cu extraction capabilities of both synthetic CuFeS2 and MC have been measured by using two different setups. In the first, we used as–synthesized CuFeS2/MC as a negative electrode material in a fixed bed flow cell (FBFC). In this hybrid battery like setup, the FeII/FeIII redox couple in the positive compartment promoted the reduction and oxidation reactions in the negative electrode during repetitive charging and discharging cycles, respectively. 4  Secondly, considering the highly energy intensive nature of hydrometallurgical processes, i.e. the oxidative leaching of CuFeS2 and electrowinning of Zn, a radically novel concept of coupling these two processes in one setup is presented. This TFB setup is capable of electrodepositing Zn in synergy with Cu dissolution from CuFeS2 during the charging cycle in the negative and positive compartments, respectively. In addition, this setup can supply back a portion of the energy in the discharge cycle but at the expense of deposited Zn, like a battery. It is speculated that by coupling this setup with a renewable energy generating facility, i.e. wind turbines and/or solar cells, this could be an attractive technology for the remote mining locations. 1.2 Thesis outline Chapter 2 provides a brief literature review on the hydrometallurgical processes mainly focusing on Cu extraction from CuFeS2 in H2SO4 lixiviant. The leaching chemistry and possible Cu extraction mechanisms are explained. In addition, the importance of energy storage, and in particular electrochemical energy storage systems, their functionality and factors influencing their performance are also discussed in this chapter. Chapter 3 states the motivation and key objectives of this research work. The design of the hybrid battery setups, experimental procedures, characterization of electrode materials, and performance evaluation methods are discussed in Chapter 4.  Electrochemical characterization of the composite electrode and its use as the anode (negative electrode) in the final FBFC are summarized in Chapter 5. Also, the possible reactions and charge storage mechanism by the composite electrode is rigorously investigated using electrochemical techniques. Chapter 6 describes the design and performance of the hybrid mineral battery (FBFC) in which synthetic CuFeS2 and the FeII/FeIII couple were used 5  as negative and positive electrodes, respectively. In addition, the electrochemical behavior of the individual electrodes, the cyclic energy storage capability, and % Cu extraction capability of the FBFC is quantified.  The use of a mineral concentrate as a negative electrode and its comparison with synthetic CuFeS2 in the FBFC is explained in Chapter 7. This chapter also describes the electrochemical reactions that occur on the various electrodes and how these relate to the performance of the FBFC during the charge/discharge process.  The construction and analytical description of the TFB, which integrates two hydrometallurgical processes, i.e., Zn electrowinning and Cu extraction from CuFeS2 are discussed in Chapter 8. In this chapter, the operating conditions for each electrode process are optimized. In addition, the energy storage and Cu extraction capabilities of the TFB using synthetic CuFeS2 (as a positive slurry electrode; PSE) are quantified. Chapter 9 explains some preliminary results on the use of a CuFeS2 mineral concentrate (PSE-M) in the TFB-M. Finally, a summary of key findings, practical implications and opportunities for future work are described in Chapter 10.            6  Chapter 2: Literature Review The demand for Cu metal is rapidly growing due to ever increasing population and industrial growth. Mainly, Cu exists either in the form of oxide or sulfide minerals in the earth crust. It is estimated that total Cu reserves in the earth’s crust are 2100 Mt of which approximately 1470 Mt exists in the form of CuFeS2. This sulfide mineral represents about 70–80% of the total world copper resource [10, 22, 23]. Generally, Cu sulfide ores containing more than 0.4 % Cu are mined, processed and concentrated to increase Cu contents to 25 – 35 %. Conventionally, Cu extraction from this mineral concentrate is achieved via smelting and electro–refining processes, which account for about 80 % of the total Cu metal production [1]. The large amount of GHG emissions (3.3 kg of CO2/kg of refined Cu) associated with the mineral processing and pyrometallurgical processes is another big global challenge [2].  The hydrometallurgical treatment of low-grade sulfidic minerals is considered more economical and environmentally friendly compared to pyrometallurgical processes. Currently, approximately 20 % of world Cu production occurs via a hydrometallurgical route but this is largely directed at oxide minerals and secondary sulfide minerals, such as chalcocite and covellite. The hydrometallurgical processes use acidic lixiviants and FeIII to leach Cu from the secondary sulfides but the processing of CuFeS2 is not currently viable using this technology. With the gradual decline in Cu ore grades over the past century, the economic and environmental feasibility of existing mineral processing technologies are threatened. One of the major issues is the high energy demand for such processes, which is mostly met by the burning of fossil fuels. At remote mine sites, renewable energy sources could be an acceptable alternative to fossil fuels. The major drawback of these energy generating units is their intermittent and unpredictable energy supply due to geological conditions, weather, time 7  and/or location etc. However, from such sources, the continuous supply of energy could be ensured by coupling with smart grid systems integrated with efficient energy storage systems i.e. batteries.  Given the above, the focus of this research is the use of CuFeS2 as an electrode material in battery-like setups, in which the possibility of both energy storage and Cu extraction are explored. In this Chapter, our discussion is constrained to the hydrometallurgical treatment of CuFeS2 and the Zn electrowinning process - concepts that are used in these battery setups. Finally, a brief literature review on existing electrochemical energy storage systems e.g. batteries and supercapacitors is provided.    2.1  Structure of CuFeS2 The crystal structure of CuFeS2 is similar to sphalerite (ZnS) in which the tetragonal unit cell contains four Cu, four Fe and eight S atoms representing the I4̅2d space group as shown in Figure 2-1a. The c–length is almost twice the length of ‘a = 0.5289 nm’ in a unit cell and two atoms of each Cu and Fe occupy tetrahedral interstitial sites of S atoms framework at the half of c–axis [24]. Each Fe and Cu atom is coordinated with the S atoms as a tetrahedron. The S atoms in the tetrahedron are slightly shifted from the center towards the Fe–Fe edge resulting in an Fe–S bond length of 0.2257 nm and Cu–S bond length of 0.2302 nm. On the other hand, each S atom in the unit cell is bonded with two atoms of Cu and two atoms of Fe [23].  8   Figure 2-1 (a) Unit cell of CuFeS2 in which each Cu atoms are shown in black spheres, Fe and S atoms are represented as small and large grey spheres, respectively [24]. (b) Schematic of CuFeS2 band structure [10] Neutron diffraction studies have shown that CuFeS2 is antiferromagnetic at room temperature and the bond angles of Fe–S and Cu–S are 109.47º and 108.68–111.06º, respectively [25, 26]. Based on the neutron diffraction results, the zero magnetic moment of Cu, and filled 3d shell corresponds to the oxidation states of +1 and +3 for Cu and Fe, respectively. However, S atom in the chemical formula have the oxidation state of –2, hence suggesting a chemical formula for CuFeS2 of Cu+Fe3+(S2–)2 [24-27]. CuFeS2 is a degenerate semiconductor having a band gap in the range of 0.53–0.6 eV and it behaves like a metal as confirmed from its optical, magnetic and electrical properties [27-29]. As shown in Figure 2-1b, the band gap of CuFeS2 depends on the spin of Fe in the lattice structure and near the Fermi level the individual contribution of Fe(3d) at the bottom of conduction orbitals and Cu(3d)/S(3p) orbitals at the top of valence band decides its value [27]. 9  2.2 Eh-pH diagram of CuFeS2 and discrepancies The stability of CuFeS2 in aqueous solutions and its tendency to form other phases in oxidizing, reducing, acidic and basic conditions can be explained with the help of potential-pH diagram as shown in Figure 2-2. In this diagram, several discrepancies related to the difference between theoretical thermodynamic aspects and experimental evidence have been explained [30]. Thermodynamically, at low pH, the oxidation of CuFeS2 should form bornite (Cu5FeS4), pyrite (FeS2), chalcocite (Cu2S) and covellite (CuS) as a product but all of these reactions would proceed in the presence of H2S as a reactant. None of these products has been reported to form under laboratory conditions even in the presence of H2S. For example, the nucleation and growth of Cu2S and FeS2 would require an extended period and severe geological conditions. On the other hand, the oxidation of CuFeS2 into Cu2+ is possible with the formation of elemental Sº, which occurs under aggressive oxidizing conditions and is one of the main possible reactions. Also, the formation of Cu2+ species should further promote the oxidation of Sº into sulfate (SO42–). But under laboratory conditions, it has been reported that due to the extended stability of sulfide sulfur, experimentally its oxidation into sulfate would require highly aggressive oxidizing conditions contrary to the observations in the potential-pH diagram at ambient conditions shown in Figure 2-2 [30, 31].   10   Figure 2-2 Potential–pH diagram of Cu–Fe–S–H2O (activity of solute = 0.1 M, except aCu2+= 0.01 M) at 25 ºC [30] From Figure 2-2, it may be predicted that CuFeS2 can be reduced to Cu2S and eventually Cu. These reactions have been experimental confirmed by Nava et al. [32], who suggested the reduction of CuFeS2 into intermediate species i.e. talnakhite (Cu9Fe8S16) and Cu5FeS4 prior to conversion into Cu2S at potential < –0.385 V vs. SHE. Under alkaline conditions, the formation of Fe, and Cu oxides as predominant species have been reported in the literature. For instance, Todd et al. [33] examined the nature of surface species formed in air saturated aqueous solutions and suggested the formation of Fe2O3 and Fe–O–OH dominant species at high pH (between 3.28 – 10.67). Yin et al. [34] reported the formation of a Fe2O3 11  and CuS2 surface layer at potential < 0.54 V vs. SHE in an alkaline solution of pH 9.2. With an increase in pH from zero to 13, the sulfur to sulfate ratio decreased to 3.2:1 from 6:1 and very little passivation effect was evident in weakly acid and in strongly alkaline solutions. 2.3 Passivation of CuFeS2 The slow dissolution of CuFeS2 under most hydrometallurgical conditions has been reported to be due to the formation of a passive film on its surface. The extended sulfide sulfur stability and the existence of metal-deficient sulfide sulfur species of descending coordination states i.e. monosulfide (S2–), disulfide (S22–) and polysulfide (Sn2–) due to physical relocation of atoms at the cleaved surface of CuFeS2 have been experimentally confirmed [35]. The surface oxidation of CuFeS2 in air has been reported in various studies. The formation of a thin surface layer (3 – 4 nm) containing CuS, S, Fe2O3, Cu(II)O, Cu(II)S, Fe–O–OH and/or Cu(II)/Fe(III) sulfate species on the surface of CuFeS2 is possible when exposed to air and/or water under ambient conditions [33, 36-38]. An increase in the concentration of Fe as oxide and hydroxide at the surface would lead to the enrichment of Cu+ species bonded with S in the form of CuS2 during the initial stage of oxidation based on the XPS analysis of the S 2p spectrum [39-41].  During acidic oxidative leaching processes, CuFeS2 dissolution is inhibited and formation of the passive film is believed to be one of the main reasons for its slow kinetic response. The nature of this inhibition (and the passive film that may be responsible) is still controversial and a number of studies describe it as elemental S° [42, 43], S22- [34, 44, 45], Sn2- and/or Fe hydroxy-oxide [46, 47]. However, the S2−, S22−, Sn2−, Cu1−xFe1−yS2 (x + y ≈ 1 and y ≫ x), iron deficient sulfide and CuSn (n > 2) species are detected within the surface film and well documented in the literature [48]. Also, another possible reason for slow CuFeS2 12  dissolution could be its n-type semiconductive behavior as reported by Crundwell and coauthors [49]. They proposed that the slow anodic dissolution of CuFeS2 is due to the formation of an electron depleted region at the surface and is exhibited by the decrease in current due to application of potential in the reverse bias. However, recently, Nicol et al. [50] questioned the relevance of the semi-conductive behavior of sulfide minerals i.e. CuFeS2 and FeS2 during anodic dissolution. They proposed that at potentials < 1.0 V, the rate-determining step for the dissolution of CuFeS2 was associated with the solid-state diffusion of Fe or possibly Cu from the passive film. In direct contradiction to the theory proposed by Crundwell et al. [51], the photocurrent generation during oxidation of these minerals in acidic sulfate solutions was shown by Nicol et al. [50, 52]. They proposed that current generation was related to thermal effects in the semiconductive surface film and not to the bulk CuFeS2. Despite these contradictions about the mechanism of anodic dissolution of transition metal sulfide minerals, there exists a large consensus on the formation of a metal-deficient sulfide sulfur enriched layer at the surface, which likely restricts mineral dissolution [52].  The polarization curve of polycrystalline natural CuFeS2 (98.1 wt. %) in 0.5 M H2SO4 is shown in Figure 2-3 [11]. The curve was obtained by polarizing the electrode in both anodic and cathodic regions potentiostatically, the steady state current vs. potential values are plotted. In the anodic polarization curve, the limiting current within the 0.6 – 0.9 V vs. SHE potential region suggests the passivation of CuFeS2 followed by transpassive behavior depicting the breakdown of the passive film. The continuous increase in current in the transpassive region corresponds to the active dissolution of CuFeS2. In the passive region, at low potential (< 0.9 V vs. SHE), the preferential dissolution of Fe from the surface would lead to the formation of 13  a metal deficient polysulfide film (Cu1–xFe1–yS2–z + zSº, where y>x) according to reaction 2.1 as suggested by many authors [11, 48, 53]. CuFeS2 → Cu1–xFe1–yS2–z + xCu2+ + yFe2+ + zSº + 2(x+y)e–   2.1 The formed polysulfide film has semiconductive properties [46] and it has been shown that ferric reduction on the surface of passivated CuFeS2 is faster than the oxidation of chalcopyrite itself [11]. This polysulfide film is further decomposed into another intermediate CuS(n–s) product at about 0.7 V vs. SHE according to reaction 2.2 [48, 53]. Cu1–xFe1–yS2–z → (2–z)CuS(n–s) + (1–y)Cu2+ + (1–y)Fe2+ + 2(1–y)e– 2.2 The formation of these intermediate polysulfide products i.e. bornite type Cu1–xFe1–yS2–z (y>x) and covellite type CuSn during CuFeS2 oxidation were found to be thermodynamically stable products as suggested in the potential-pH diagram [30, 48]. This demonstrates that Fe deficient but Cu enriched polysulfide film provides a barrier to electron and ion transport. Recently, Ghahremaninezhad et al. [11] proposed that during the leaching process, the electron transport through this passive film is controlled by the diffusion of Fe atoms.      14   Figure 2-3 Polarization trend of CuFeS2 in 0.5 M H2SO4 developed  from the steady-state current values obtained potentiostically [11] 2.4 Leaching of CuFeS2 As given in the literature, various leaching approaches have been adopted to increase the Cu dissolution rate under oxidizing conditions, but few of these processes has gained commercial acceptance [7]. The reason for the low Cu extraction rate from CuFeS2 has been widely investigated in the past and there exists a general agreement about the formation of a surface layer, which impedes this process, as discussed above. Nevertheless, the extent of the dissolution reaction and the nature of the surface film remains controversial. Many studies have shown that with an increase in temperature, acid or oxidant concentration, the dissolution of CuFeS2 can be enhanced. Other factors i.e. acidic type of lixiviant, particle size, agitation speed, mechanical activation, redox potential, and pulp density etc. also influence the dissolution kinetics as discussed in the following sections. 15  2.4.1 Leaching in sulfate media Leaching of CuFeS2 in sulfate media is preferred due to the compatibility of this process with the established heap leaching processes for Cu2S and CuS. Also, conventional solvent extraction and electrowinning technologies developed for the SX-EW process can be adopted without significant modification [54]. However, at ambient temperature and pressure, during the leaching process, the formation of a passive film on the surface of CuFeS2 in the sulfate media hinders the dissolution of Cu, which makes this process industrially unfeasible. Sulfate-based (in H2SO4) leaching processes typically use ferric sulfate and oxygen as oxidants. However, to enhance Cu dissolution and to aid in the re-oxidation of ferrous to ferric, leaching is carried out at various temperatures and in the presence of bacteria. The reaction chemistry of CuFeS2 in the acidic sulfate medium and favorable conditions for Cu extraction are summarized in Table 2-1 [48]. Generally, the sulfide sulfur of CuFeS2 oxidizes to either elemental S º or SO42– in the presence of oxidants e.g. Fe3+ and O2. In ferric sulfate solution, the leaching rate of chalcopyrite is slow due to the formation of the secondary reaction products discussed above.  At atmospheric pressure, the conditions for the leaching of Cu from CuFeS2 in sulfuric acid solution depend on the solution potential. Increased Cu dissolution has been reported up to approximately 0.6 V vs. SHE (referred to as the passivation potential) [10, 11, 55]. Beyond this potential, the observed decrease in dissolution is attributed to the passivation of CuFeS2. The effect of H2SO4 concentration and temperature on the passivation potential of CuFeS2 is shown in Figure 2-4a. The passivation potential at low (25 – 40 ºC) and at moderately high temperature (60 – 80 ºC) is found to be independent to the acid concentration. However, the low passivation potential (~0.44 V vs. SCE) at low temperature and relatively high passivation 16  potential (~0.52 V vs. SCE) at high temperature indicate the CuFeS2 transition from active to passive. It can be observed that the active to passive transition at 50 ºC is sensitive to the H2SO4 concentration and the passivation potential decreased from 0.5 V to 0.44 V vs. SCE with an increase in acid concentration from 2 to 70 g l–1 [56]. In agreement with Viramontes et al. [56] and results reported in the following discussion, batch leaching tests conducted at 80 ºC by Khoshkhoo et al. [57] yielded a Cu extraction of ~80 % in 24 h at low redox potential (0.42 V vs. Ag/AgCl), as shown in Figure 2-4b. These results are consistent with the active dissolution of Cu from the mineral concentrate compared to results obtained at high redox potential (0.62 V vs Ag/AgCl), where the formation of the passive film restricted the leaching process.       Increasing the Cu extraction from CuFeS2 is possible either by adjusting the oxidation-reduction potential (ORP) of the solution with the addition of oxidants, bacteria or by applying an external potential. The accelerated Cu leaching under an applied potential of 0.4 – 0.45 V (vs. Ag/AgCl) and in the presence of moderate thermophilic bacteria at 50 ºC, ~90 % Cu extraction was achieved in 20 days as depicted in Figure 2-5.  In comparison, in only the bioleaching process, and under ORP controlled processes by either chemically or via electrochemically, the copper extraction was ~ 65 % and < 30 %, respectively.  In another study, the ORP was controlled at 0.635 V vs. SHE by the addition of KMnO4 in the suspension containing 1 M H2SO4 and FeSO4 at 50 ºC [55]. The leaching reaction was found to be surface controlled and related to the formation of elemental sulfur on the surface of CuFeS2. As reported by Sandstrom et al. [58], the Cu extraction was enhanced in the presence of thermophilic bacteria (Sulfolobus metallicus) and constant redox potential of 0.6 V vs. Ag/AgCl was maintained by sparging CO2 enriched air in the bioreactor. The complete 17  oxidation of sulfide sulfur to sulfate was possible under these conditions but the limited Cu extraction was related to the passivation of CuFeS2 and precipitation of Jarosite.      Figure 2-4 (a) Effect of H2SO4 concentration and temperature on the passivation potential of CuFeS2 [56](b) % Cu recovery from the Kristineberg concentrate at 80 ºC. The redox potentials were adjusted by the addition of dilute H2O2 in H2SO4 solution (pH =1.5) containing initial Fe2+ (1 g l–1) [57] Hackl et al. [48] described the effect of temperature (between 110 – 220ºC) on Cu and Fe extraction from chalcopyrite, as shown in Figure 2-6. The relatively low copper extraction of 45 – 50 % in the range of 130 – 170 ºC was thought to be due to molten sulfur encapsulation of the mineral particles, which is known to restrict metal extraction by blocking solution access to the mineral surface. The rapid increase in the Cu extraction and a sharp decrease in Fe contents beyond this temperature to 220 ºC were attributed to complete Sº oxidation to SO42– and Fe precipitation in the form of Jarosite, respectively.  Various sulfate-based processes have been patented in the recent past, very few of these have been commercialized and only a handful of others have achieved pilot plant or demonstration status [7, 59]. The medium (150°C) and high-temperature (>200°C) processes are out of the 18  scope of this work (Table 2-2). Further details about these processes and operating conditions may be found in the paper by Dreisinger [7] and references cited therein.  Figure 2-5 Strategies applied to enhance Cu extraction form CuFeS2. (Note: ORP was controlled by applying DC potential of 0.4–0.45 V vs. Ag/AgCl electrode, bacterial controlled process was carried out at 50 ºC) [60, 61]  Figure 2-6 Percentage Cu and Fe extraction at various temperatures, 100 g solids in 1 liter of 1 M H2SO4 solution containing initial 10 g l–1 Cu2+ and 5 g l–1 Fe3+ was used [48] 19  Table 2-1 Oxidative acidic sulfate leaching of CuFeS2, the reaction chemistry [48]  Leaching Process Temperature (ºC) Possible Reactions Ref. Bacterial leaching 20 – 40 CuFeS2 + 4O2 → Cu2+ + Fe2+ + 2SO42– 2Fe2+ + 0.5O2 + 2H+ → 2Fe3+ + H2O CuFeS2 + 4Fe3+ → Cu2+ + 5Fe2+ + 2Sº Sº + 1.5O2 + H2O → H2SO4   [62-64] Ferric sulfate leaching 20 – 100 CuFeS2 + 4Fe3+ → Cu2+ + 5Fe2+ + 2Sº CuFeS2 + 16Fe3+ + 8H2O → Cu2+ + 17Fe2+ + 2SO42– +16H+ [65, 66] Pressure oxidation (O2) 100 – 220 CuFeS2 + 4H+ + O2 → Cu2+ + Fe2+ + 2Sº + 2H2O CuFeS2 + 4O2 → Cu2+ + Fe2+ + 2SO42– 2Fe2+ + 0.5O2 + 2H+ → 2Fe3+ + H2O [59, 67]  Table 2-2 Sulfate based Cu extraction processes from ore or mineral concentrate commercialized and/or studied at pilot plant scale [7, 60] Process Temperature (ºC) Pressure (atm) Particle Size, D80 (µm) Special Requirements Ref. BioCOPTM  65 – 80 1 37 Use of thermophilic bacteria  [68] TPOX 200 – 230 30 –40 37 High temperature and pressure is required [69] Mt. Gordon 90 8 100 Pressure oxidation of Cu2S/FeS2 or concentrate in Fe sulfate solution  [70] Activox 90 – 110 10 – 12  5 – 10 Very fine grinding and high O2 overpressure  [71] Albion 85 1 5 – 10 Very fine grinding of concentrate and Fe3+ species [72] Anglo American–UBC 150 10 – 12 10 – 15 Use of surfactant with moderate grinding for CuFeS2 leaching   [73] GalvanoxTM 80 1 53 – 75 Addition of Pyrite (FeS2:CuFeS2 = 2:1) [74] 20  2.4.2 Factors influencing CuFeS2 leaching There are many factors i.e. type and concentration of lixiviant, oxidant, and temperature which significantly affect the leaching kinetics of CuFeS2. In addition, particle size, pulp density, and agitation speed are also found to be important considerations to evaluate the leaching behavior [24]. Both from an economic point of view and from a process optimization perspective, a rigorous analysis of the effect of acid concentration (pH control) is always necessary. The use of H2SO4 is very common for CuFeS2 leaching as explained above and many processes use this lixiviant due to its compatibility with the well–established downstream processes, i.e. solvent extraction (SX) and electrowinning (EW). High concentrations of acid (pH < 0.5) may promote the formation of the Fe deficient surface layer leading to passivation of CuFeS2 and therefore, 0.1 – 1 M concentration is suggested to be optimal for most processes [75]. Among various oxidants, e.g. Cu2+, Cr2O72–, O2, H2O2, ClO3–, O3, MnO4–, HNO3, NaNO3 and S2O82–, Fe3+ is the most inexpensive reagent, which has been widely utilized for the oxidative leaching of CuFeS2 (and other sulfide minerals) and it may be regenerated during the leaching process [24, 60]. The addition of Cu2+ species in H2SO4 solution containing ferrous ions can reduce CuFeS2 to Cu2S at low potential (0.56 V vs SHE) via metathesis, the Cu2S can then be further, and more readily, oxidized by dissolved O2 [76].  H2O2 is another strong oxidant in dilute acidic solution and in the presence of Fe2+: it dissociates into OHº and OH– with the generation of Fe3+ [77]. These species may further oxidize the sulfide sulfur species into more porous elemental Sº and/or SO42– [78]. In other studies, an improvement in the Cu extraction (from 20 to 60 %) was achieved by the addition of ethylene glycol in the H2SO4–H2O2 media at 65 ºC due to the restricted decomposition of H2O2 at such high temperature [79, 80]. However, permanganate ions (MnO4–) are not used as 21  a direct oxidant for the leaching of CuFeS2 but these species have been used as surrogate oxidants, i.e. to oxidize ferrous ions to ferric and therefore maintain the high ORP of the solution [58]. The rate laws, kinetic behavior and the use of other oxidants (e.g. Cr2O72– and ClO3–) are not discussed here but for detailed information it is recommended to consult the reviews by Watling [60] and Li et al. [24] in addition to the references cited therein.   Independent to the impurity contents in the mineral concentrate, pH and temperature, the active dissolution of CuFeS2 takes place below 0.68 V vs. SHE. Careful control of the ORP by the addition of oxidants and/or by the adjustment of temperature have always been important considerations to avoid passivation of CuFeS2 as discussed above.  Many authors reported no positive effect of mixing or agitation on CuFeS2 dissolution even in the presences of various oxidants. For instance, Dutrizac [81] reported that mixing has a very negligible effect on CuFeS2 dissolution in both ferric chloride and ferric sulfate solution. However, in the presence of K2Cr2O7 + H2SO4 [82] and HClO4 (without Fe3+ addition) [35], the highest dissolution rate was achieved with an increase in mixing rate up to 400 and 500 rpm, respectively. At high mixing rate, a significant decrease in dissolution rate was observed and thought to be controlled by bulk diffusion processes which were associated with the depletion of reacting species on the surface of CuFeS2. The benefit of magnetic agitation over mechanical stirring and its positive effect on the increase in abrasion of CuFeS2 particles improved the Cu dissolution rate as highlighted by Nicol et al [83]. In contrast, other work by Sokic et al. [84] and Adebayo et al. [77] showed a decrease in leaching rate with increase in stirring speed in an H2SO4 solution containing NaNO3 and H2O2 oxidants, respectively. These authors found that little particle to oxidant contact and/or H2O2 dissociation at large stirring speed would limit the dissolution tendency of CuFeS2. 22  Recently, mechanical activation of CuFeS2 by grinding in the presence of Fe3+ has been carried out, which resulted in the formation of a very soluble copper sulfate layer on the particles. During successive activation and leaching steps (leaching in H2SO4, pH 1 and at 50 ºC), about 98 % Cu extraction was possible in 1.5 h [85, 86].  The particle size and/or surface area of CuFeS2 particles is very critical in term of dissolution kinetics, as it would directly affect the reactor design and operating cost. During the leaching process, diffusion-controlled processes such as the development of a product layer (passive film) on the CuFeS2 would always require smaller particle sizes to achieve effective dissolution, as discussed in many studies [75, 77, 81-84]. 2.4.3 Reduction of CuFeS2 As learned from the previous discussion, the leaching of Cu from CuFeS2 is usually carried out under oxidative conditions. Despite the significant advances in the oxidative hydrometallurgical processes in the past decades, still there exists the controversial passivation issue, which has forced researchers to investigate alternative strategies for efficient Cu extraction from CuFeS2. This has led to the development of various processes including, high temperature/pressure treatment, use of oxidants, fine grinding etc. and has introduced complex downstream steps for the efficient recovery of Cu and other precious metals.  As depicted in Figure 2-2, thermodynamically, at low pH < 2, CuFeS2 may reduce into Cu2S with the formation of some intermediate products i.e. bornite and elemental Cu. However, it is noticed that the stability lines for these phases are below the H2 evolution line and conversion of CuFeS2 into these species depends, of course, on its kinetic response in a given system. In a detailed study by Warren et al. [87], electrochemical investigation revealed 23  the formation of Cu2S or djurleite (Cu1.96S) during cathodic polarization in H2SO4 containing Cu2+ and Fe2+ species. The reductive decomposition of CuFeS2 is carried out to generate an intermediate Cu2S product, which is more reactive and may be more easily oxidized. Many approaches in the past have been adopted to improve Cu leaching from CuFeS2 via reduction processes, which have included the use of reducing agents i.e. metallic Fe [75], Cu [88], Al [89], and Pb [90] and/or nascent hydrogen (H⁰) in an electro-assisted reduction process [91].  A detailed electrochemical analysis of CuFeS2 reduction (carbon paste electrode) in 1.7 M H2SO4 suggested the formation of intermediate products i.e. talnakhite (Cu9Fe8S16), bornite (Cu5FeS4) before conversion into Cu2S and Cu [32] within various potential ranges as given in Table 2-3. These results support to the electrochemical results discussed by Elsherief [92], who also claimed the formation of Cu2S and metallic Cu at extremely negative potentials. During cathodic polarization, Fe3+ atoms in the crystal lattice of CuFeS2 at the surface accept electrons and preferentially dissolve in the solution by leaving behind Cu and S species. These reorganize themselves to form an Fe deficient structure at the surface as discussed above [93]. Table 2-3 Formation of possible species during electrochemical reduction of CuFeS2 in 1.7 M H2SO4 [32] Potential (E vs. SHE) Possible reactions 0.115 ≥ E > –0.085 9CuFeS2 + 4H+ + 2e− ⇔ Cu9Fe8S16 + Fe2+ + 2H2S –0.085 ≥ E > –0.285 5CuFeS2 + 12H+ + 4e− ⇔ Cu5FeS4 + 6H2S + 4Fe2+ –0.285 ≥ E > –0.385 2CuFeS2 + 6H+ + 2e− ⇔ Cu2S + 2Fe2+ + 3H2S –0.385 ≥ E  Cu2S + 2H+ + 2e− ⇔ 2Cu + H2S CuFeS2 + 4H+ + 2e− ⇔ Cu + Fe2+ + 2H2S  24  2.5 Zn electrowinning and its use in energy storage Zn is another important metal and ~7 million tonne of Zn is produced annually, mainly by acid leaching of roasted sphalerite (ZnS). This is followed by rigorous purification of leach solution to remove all soluble impurities i.e. Cu, Co, Ni, Cd, Pb, Sb, As, Ge and Fe etc. The complete removal of impurities, particularly those that are more electropositive than Zn and have low overpotential for H2 evolution is essential [12-14, 94]. These impurities in the leach liquor may decrease the current efficiency, deteriorate deposit quality and may increase the energy requirements during electrowinning [16]. The flow sheet, describing the process steps of Zn metal production from Zn concentrate to pure metal is shown in Figure 2-7 as reported elsewhere [17, 95]. During roasting, ZnS concentrate is oxidized to ZnO and SO2.  The gas is further processed to produce sulfuric acid which can be reused in the following leach process of the solid residue. Leaching of ZnO is carried out in spent electrolyte generated in the electrowinning step. Initially, leaching is conducted in continuous stirred tanks containing an almost neutral solution of pH ~5 at 60 ºC. The unreacted solids in the concentrate are further classified and leached in H2SO4 (5–10 g l–1) solution before leaching in hot (90 ºC) strong acidic solution (containing 30 –100 g l–1 H2SO4). Iron and other impurity removal is essential prior to the electrowinning. The ferrous species produced during the leaching process are oxidized to ferric which are precipitated as either jarosite, goethite and/or hematite depending on the process conditions applied [96]. 25   Figure 2-7 General Zn metal production process flow sheet via roasting, leaching, and electrowinning Following Fe removal, the Zn leach solution is purified via cementation process prior to electrowinning step. In this process, any soluble metal impurities are precipitated out, particularly those which are more electropositive than Zn. These are removed by the addition of Zn dust, which is oxidized in the leach solution and is recovered in the electrowinning stage. However, the metal impurities, i.e. Ni, Cd, and Co that have a standard potential close to the Zn/Zn2+ redox potential are difficult to remove. A large number of other impurities such as Cu, As, Sb, Fe, and Ge are removed from the Zn leach solution to less than 1-ppm level.  In this study, the Zn/Zn2+ redox reaction, akin to the electrowinning process, is used in the battery setup. Some other issues, which may restrict efficient Zn electrowinning, are also discussed here. The Zn electrowinning process accounts ~30 % of the total energy required to produce Zn metal from as mined ore. On an industrial scale, Zn is electrodeposited (reaction 2.3) on aluminum cathodes while H2O oxidation (reaction 2.4) occurs on Pb–Ag (0.5 – 0.75 %) alloy anodes in an electrolytic cell. The anode material is of significant importance because 26  a relatively large overpotential for oxygen evolution (reaction 2.4) appreciably increases the overall cell potential.    Zn2+ + 2e– → Zn    (Eº = –0.76 V vs. SHE)   2.3 H2O → 2H+ + 0.5O2 + 2e–     (Eº = 1.23 V vs. SHE)  2.4 Based on these reactions, Zn deposition should proceed at ~2.0 V and the theoretical energy required for this process is 1.63 kWh kg–1. But due to a large overpotential on the electrodes (particularly on the anode, ~0.6 V) and a potential drop in the leach liquor, an actual cell potential of ~3.2 V is required for Zn electrowinning, which corresponds to ~2.8 kWh kg–1 of energy consumption [16-18]. 2.5.1 Solution purity and its importance  Owing to the negative reduction potential of Zn, electrolyte purification is very necessary. Otherwise, the preferential deposition of more noble metal impurities, i.e. Cu, Ni, etc. on the cathode would impair the purity of the Zn deposit in addition with accelerated H2 evolution over these catalytic micro–cathodes. These impurities consequently decrease the current efficiency during the electrowinning process. Mackinnon et al. [97] rigorously studied the effect of metal impurities in industrial zinc sulfate solution and found a variable influence on the deposit morphology, orientation and current efficiency. The critical limits of impurity contents were investigated and their effect on the current efficiency is shown in Figure 2-8a and Figure 2-8b.  The tolerable concentration of Cu and Cd in the Zn electrolyte containing 9 mg l–1 Pb is found to be less than 50 mg l–1 in zinc sulfate solution. However, a further increase in concentration beyond this level could significantly influence the deposit morphology and current efficiency [98, 99]. Ni and Co are more deleterious owing to their small overpotential 27  for H2 evolution and low concentrations (> 5 mg l–1) could greatly decrease the current efficiency. The preferential deposition of these more noble impurities results in the catalysis of H2 evolution and the enhanced localized re-dissolution of Zn by galvanic corrosion [12]. Figure 2-8a shows that current efficiency is very sensitive to the presence of Sb, Ge, Te, Se and Sn (< 2 g l–1) in the electrolyte. Surprisingly, the presence of As(III), In and Tl has a negligible effect on the current efficiency as shown in Figure 2-8b [97].   Figure 2-8 Effect of metallic impurities concentration on the current efficiency of Zn deposition process in industrial acid sulfate solution at 430 A m–2 (a) IV A and VI-A (b) III-A and V-A group metals including Sn [97]. In order to decrease the power consumption, to improve the current efficiency and to refine the grain morphology, several organic additives are introduced to the Zn electrolyte in small quantities. These additives induce positive effects on the morphology of deposited crystals and promote the development of a bright and homogenous Zn deposit at high current efficiency by restricting the localized growth of the deposit (dendrite formation). The use of gelatin, glue, gum Arabic, organic surfactants (triethyl-benzyl ammonium chloride [100]), sodium lauryl sulfate [101], quaternary ammonium bromides [102], perfluorocarboxylic acids 28  [103], acid–mist suppressants (Tennafroth 250 (T), saponin, Dowfroth 250 (D) [104]) and flocculants have been found effective to enhance the current efficiency and deposit morphology as reported in the literature [105].  2.5.2 Optimum conditions for Zn deposition Many variables i.e. electrolyte composition, temperature, current density, electrolyte circulation, Zn and acid concentration greatly influence the overall performance of the electrowinning process and may increase the energy requirements. For instance, Frazer et al. [106] investigated the effect of temperature, current density and electrode rotation on the current efficiency of Zn electrowinning in 0.8 M ZnSO4 + 1.07 M H2SO4 in the presence of 1.2 ppm Pb and 0.2 ppm Ni. Using statistical analysis (23 factorial design), a maximum current efficiency of 98.8 % was achieved by adjusting the electrolyte temperature to 50 ºC, fixing the current density as 50 mA cm–2 and by applying the electrode rotation speed of 35 s–1. In another study, a maximum 95 % current efficiency was obtained in synthetic solution containing 160–65 g l–1 Zn2+ in 40–90 g l–1 acid concentration (25 ºC) and by applying 35–60 mA cm–2 current density. The addition of 50 g l–1 gelatin + 10 ppm thiourea significantly improved the grain structure and prevented the nodular growth of the electrodeposit. It is also found that current efficiency decreased to 88 % from 96 % with the increase in Cu concentration from 6.2 to 20 ppm in 120 g l–1 Zn and 50 g l–1 H2SO4 solution [107]. The addition of Mn in the range of 1.5 – 3.0 g l–1 is always recommended to avoid the corrosion of Pb anodes but higher concentration may produce negative effects on the current efficiency and grain size in the Zn deposit [108].  The effect of Zn and acid concentration variation at fixed sulfate concentration in Kidd Creek Zn electrolyte was determined by Alfantazi et al. [16]. They noticed that input energy required for effective Zn deposition decreased from 3.05 to 2.85 kWh kg–1 when the 29  concentration of Zn in the solution was increased from 42 to 50 g l–1 and became constant at high Zn concentrations. In addition, the current efficiency improved significantly to 95 % from 83 %, when the Zn concentration increased from 42 g l–1 Zn to 107.5 g l–1 in 95 g l–1 H2SO4 solution. Similarly, the large deposition efficiency at high Zn concentration (1.4 M ZnSO4) was achieved in 0.1 to 0.7 M H2SO4 solution. But the decrease in current efficiency at much high acid concentrations was attributed to the increase in H2 evolution exchange current density [109].                      2.5.3 Use of Zn as an electrode material in batteries  Zn and its alloys have great potential for use as anode materials in many aqueous-based energy storage systems owing to their low cost and high theoretical specific capacity of 820 mAh g–1 [20]. However, the Zn based batteries exhibit poor cyclic charge/discharge performance and small cyclic life which restrict their use as secondary batteries [21]. Various battery setups i.e. Zn–C, Ni–Zn, Zn–Ag, Zn–MnO2 and Zn–Air have been introduced that utilize alkaline electrolytes. Among these, only Ni–Zn and Zn–air batteries are rechargeable (secondary batteries) but there are several issues related with the electrochemical performance of Zn anodes. The shape change, formation of dendrites, non–uniform dissolution and limited solubility of Zn anode in the alkaline electrolytes are the main limitations to the design of rechargeable batteries [110]. Dendrite formation on Zn anodes during the charge cycle may rupture the membrane and short-circuiting of the electrodes may lead to failure of the battery. Modifications in the Zn anode design, separators, the addition of inhibiting agents in the electrolyte or alloying additions in the electrode and/or pulsed charging strategies have therefore been devised [21, 111-116].  30  Another issue is the simultaneous parasitic H2 evolution during the charge cycle (Zn deposition), which ultimately decreases the coulombic efficiency and cyclic life of the batteries. To avoid this issue, many alloying additions which have large overpotential for H2 evolution are introduced in the Zn anode i.e. Bi, In, and Ca [117, 118]. The major challenges that remain for existing commercialized Zn–Ni rechargeable batteries are related to the Zn based anode materials. For Zn–air batteries, the fade in capacity, self–discharge and low cyclic stability of these systems are the key issues that require further attention [116, 119]. More detail on these issues and remedies are available in the literature [20, 110, 113] and not discussed in this work. 2.6 Energy storage and its importance The ever-increasing energy demand due to rapidly increasing population and industrial growth has spurred interest in the invention of new renewable energy technologies and in the improvement of the performance of existing systems [120, 121]. The energy supply from renewable sources is always intermittent because of their dependence on geological conditions e.g., weather, time of day or year and location. Therefore, efficient energy storage systems are always required for peak shaving during day and night shifts and to ensure continuous supply on demand [122-128]. 2.6.1 Electrochemical energy storage  Among many other energy storage systems, e,g, mechanical and thermal, electrochemical energy storage devices such as batteries and supercapacitors (SCs) are flexible for use in both mobile and stationary applications. Figure 2-9 shows the general type of reactions that may occur on, or in, electrode materials and defines the mechanism of charge storage. In batteries, chemical energy is stored during the charge cycle and retrieved as 31  electrical energy during the discharge cycle. The charge is stored reversibly by the occurrence of faradaic reactions at or within the bulk of the electrode material.  This is referred to as pseudocapacitive behavior. For example, in a lead acid battery, the oxidation reaction at the negative Pb electrode (anode) is supported by the reduction of the positive PbO2 (cathode) to PbSO4 (conversion reaction) and the attendant release of electrical energy during the discharge cycle. However, irreversible reactions, such as electrolyte dissociation and reactions with the electrode material, can deteriorate a battery’s charge storage capability and decrease its cyclic life during repetitive charge/discharge. On the other hand, in supercapacitors (also referred to as electrochemical capacitors), the formation of the electrical double layer (capacitive behavior) or adsorption/desorption processes (pseudocapacitive process) are limited to the surface and/or near surface regions of the electrode materials. Therefore, supercapacitors can endure a large number of charge/discharge cycles.  Thus, both pseudocapacitive faradaic (reversible electron transfer reactions) and pseudocapacitive reactions are desirable in a charge storage device, whereas the occurrence of unwanted parasitic reactions (electrochemically irreversible reactions) deteriorate their cyclic performance.  Electrochemical characterization of battery electrode materials reveals the charge storage mechanism, as shown in Figure 2-10. In batteries, redox reactions (i.e. conversion reactions) and intercalation processes are responsible for charge storage. These processes are diffusion controlled and depend on the transport of ionic species to or from the electrode materials. Thus, batteries require extended time for charging. The slow release of electrical energy during discharge from these mass transfer-controlled redox reactions also appear as current peaks and potential plateaux in the CV and discharge curves, respectively (Figure 32  2-10F and H). On the other hand, SCs can be charged and discharged rapidly due to the storage of charge on the surface or within the near surface regions of porous electrode materials. These surface reactions are not dependent on mass transfer processes and, therefore, SCs present rapid current response upon change in potential during charge/discharge (Figure 2-10E and G) [129-131]. Based on these charge storage characteristics, batteries can store and supply a large amount of energy (high energy density), whereas, SCs can be charged rapidly and are discharged by providing high power density to overcome intermittent and momentary power outages.             The key issues with existing battery technologies are their limited cyclic stability, relatively lower energy density and inferior-than-expected rate capability. These issues are mainly related to the electrochemical properties of the battery electrode materials and their interaction with the electrolytes during charge/discharge operation.  SCs have a long cyclic life, high power density and high charge/discharge rate capabilities [128]. Nevertheless, the low energy density of SCs and limited power density of batteries are the two extremes, which impede their individual performances. 33   Figure 2-9 Types of reactions that may occur on the electrode materials of an electrochemical energy storage system  Figure 2-10 Charge storage mechanism in batteries and supercapacitors, formation of double layer (A) at the surface (B) within the bulk of porous carbon. Pseudocapacitive behavior (C) due to surface redox reaction onto a hydrous RuO2 electrode (D) due to intercalation of Li+ ions within the bulk of host material. Cyclic voltammograms of (E) supercapacitor and (F) battery electrode. Pseudocapacitive discharge curve (G) of MnO2 capacitor and (H) LiCoO2 battery electrode materials [130]. 34  Therefore, the hybridization of batteries and SCs is an attractive way of achieving both higher energy and power densities from a single system with improved cyclic life, good safety, and good efficiency at low cost. In a hybrid system, the contribution of each component -  battery or SC - depends on the configuration [132]. The key goal of hybrid battery/SCs is a rapid electrochemical response of the electrode materials at large applied potential and rapid charging/discharging response without significant polarization and stability loss [133]. These hybrid systems could economically store an enormous amount of energy and could fulfill energy storage and supply requirements for future hybrid vehicles and grid systems.  Research on the development of new hybrid systems has accelerated in recent years. As noted above, high energy and power density are the two characteristics belonging to batteries and SCs, respectively. To achieve both from a single or a pair of materials is still under intense investigation. But currently, these technologies are immature and widespread commercialization is limited because of the various materials’ intrinsic behavior, bulk vs. surface faradaic/non-faradaic and diffusion controlled reactions [132, 134]. The diffusion-controlled processes limit rapid charging/discharging of LiBs due to local overpotential built up across the electrode surface [135]. To enhance ion and electron transport kinetics, various approaches such as modification of the electrode materials [136], application of conductive coatings over electroactive electrode materials [128, 137] and size reduction to nanoscale [122, 131, 138] have been adopted. To achieve ultra-high charging/discharging rates, a decrease in ion diffusion time and a short electron transport path length are always required. The time constant for diffusion is decreased significantly (t~L2/D) by reducing electroactive material dimensions [136, 139]. 35  2.6.2 Current state of battery research Batteries are considered to be the most reliable and attractive technologies for the storage and continuous supply of energy. Many battery systems have been developed and much information about the recent advances in lead-acid batteries [140], Li-ion batteries [141], Li–S batteries [142, 143], Na–ion batteries [144, 145], Na–S [146], vanadium redox flow batteries [147, 148] and Zn–air batteries [149] etc. is readily available in the literature. Table 2-4 summarizes the performance characteristics of some well-established battery technologies for comparison. A detailed discussion about these battery technologies is beyond the scope of this work. However, the types of electrode materials and the use of CuFeS2 as an electrode material in the batteries is discussed here. Among all other battery systems at present, the Li-Ion batteries (LiBs) have been widely researched and account for a major share in the market in almost every application e.g. cell phones, laptops, electric vehicles, and energy storage systems at grid level etc. Nevertheless, the key issue with the LiBs is their limited charging rate due to potential build up (polarization) across the electrode/electrolyte interface. These polarization effects could increase the local temperature during charging/discharging leading to explosion or fire hazards [150]. The selection of electrode materials for LiBs depends on particle size, morphology, electron transport properties, functionalization, crystal structure, electrolyte type, chemical stability, and charge storage mechanism. In the past two decades, the development of highly energetic cathode materials for LiBs has been greatly focused on achieving high energy storage capabilities and electrochemical stability in both organic and aqueous electrolytes [143, 151]. 36  Generally, the cathode materials are categorized into 'intercalation' and 'conversion' types and are selected based on design criteria such as specific capacity and operating potential.  Intercalation is defined as reversible insertion and removal of guest species i.e. Li+, Na+ etc. into the host material having a layer type lattice structure. For example, in LiBs, the intercalating cathode host lattice structure can accept and release guest ions (Li+, Na+) in a reversible manner during charge/discharge cycles. The commonly used cathode (host) materials are transition metal oxides i.e. LiCoO2 [152], LiNiO2 [153], LiNi0.8Co0.15Al0.05O2 [154],  LiMnO2 [155], Li(Ni0.5Mn0.5)O2 , Li(Ni1/3Co1/3Mn1/3)O2 [156], and LiFePO4 [151], which display a wide variety of crystal structures i.e. layered, spinel, olivine, and tavorite.   37  Table 2-4 Existing commercialized and developing battery technologies, their performance, and comparison [121, 157] Battery Type OCV (V) Specific Energy  (Wh kg–1) Temperature (ºC) Cyclic life Energy efficiency (%) Discharge Time (h) Pb acid battery 2.1 25–40 –40 to 60 1000 50 –75 8 Pb–C battery  2.1 25–40 –40 to 60 3000 – 4 Ni–Cd battery 1.3 30–45 –10 to 45 2000 55–70 4 V–redox flow battery 1.4 10–20 10 to 40 5000 65–80 4 to 12 Na–S battery 2.1 150–240 300–350 4000 75–90 4 to 8 Zeolite Battery Research Africa (ZEBRA) 2.6 95–120 300–350 3000 75–90 4 to 8 Li–ion battery with C anode 3 to 4 150 –25 to 40 1000 94–99 4 Li–ion battery with LiFePO4 cathode and Li4Ti5O12 anode 1.7 50–70 –25 to 40 4000 94–99 4  38  On the other hand, Chalcogenides are other promising conversion cathode materials, which are used in LiBs due to their higher thermal stability, electrical conductivity, and good electrochemical properties than metal oxides. The metal sulfides present high specific capacity but Li+ insertion/de-insertion is the diffusion-controlled process in these electrode materials. For instance, low surface area (large particle size) of electrode material could increase the electronic and ionic path length for Li+ ion diffusion and hence may reduce the energy and power densities [158]. The formation of intermediate polysulfide film could also form on the surface of the electrode (via reaction 2.5), which may dissolve in the organic electrolyte and may lead to irreversible capacity fade.  2Li+ + MS1+x + 2e- → Li2Sx + MS  (2 <x < 8)  2.5 The soluble polysulfides are transported to the anode surface during the charging process and decrease the coloumbic efficiency of the cell. This issue is overcome by decreasing the size and/or by modifying the morphology of sulfide-based electrode materials. Also, mixing with other graphitic materials (hybridization) may enhance both the energy and power efficiencies without compromising cyclic performance [142, 159].  2.6.3 Use of CuFeS2 as battery electrode material Recently, CuFeS2 has been studied as a promising material for many applications including solar energy generation [160], energy storage [161, 162], electrochemical sensors [163] and as a thermoelectric material [164]. Many researchers in the past have adopted different procedures to synthesize various morphologies of CuFeS2 such as spherical and pyramidal nanocrystallites [165], nanowires [166], hexagonal 'plate' [167] and 'spike like nanorods' [168] as shown in Figure 2-11a-d. Wu et al. [162] described the solvothermal synthesis of sheet-like nanocrystals of CuFeS2 (Figure 2-12a) and converted them to rod shape 39  particles by adding 'polyvinyl pyrrolidone' (PVP) surfactant during synthesis, as shown in Figure 2-12b. The PVP macromolecules significantly reduced the crystallite size by their ability to interact selectively with crystal facets while promoting anisotropic growth in other directions. The XRD patterns of crystallites synthesized with and without PVP suggested that PVP has no influence on the crystal structure but significantly affected the morphology.   Figure 2-11 Morphologies of the as-synthesized CuFeS2 reported in the literature (a) pyramidal [165], (b) nanowires [166], (c) plates/sheets [167], and (d) spikelike nanorods [129, 168] The charge/discharge cycles (first 3 cycles) of the sheet and rod-like nano-crystallites of CuFeS2 were conducted at 100 mA g–1 within 0.001 – 3.0 V (vs. Li/Li+) potential as shown in Figure 2-12c and Figure 2-12d, respectively. The two potential plateaus at 0.8 and 1.5 V during discharge process (1st cycle) can be clearly observed, corresponding to conversion type reaction (reaction 2.6) and Li+ insertion into the crystal lattice of CuFeS2, respectively [162, 167, 169]. The formation of metallic Cu and Fe was more pronounced than Cu2+ and Fe3+ (few 40  ppm) in the ethylene carbonate (EC)/dimethyl carbonate (DMC) (1:1 by volume) electrolyte containing 1M LiPF6.    LixCuFeS2 + (4–x)Li+ + (4–x)e– → 2Li2S + Cu + Fe  2.6 Similarly, two obvious potential plateaus at ~1.75 and 2.25 V were observed during charging. These were related with the oxidation of Li2S (Li2S → 2Li+ + S + 2e-) and S (S2-/S0 and or S2-/S22-), respectively [162]. The second and third charge/discharge plateaux were shifted slightly to lower capacity values but remained mirror images of the first scan profile. It was observed that the initial discharge capacity of CuFeS2 without PVP was about 780 mA g–1 in contrast to the much higher capacity (1020 mAh g–1) exhibited by PVP–assisted CuFeS2. The cyclic voltammograms of both CuFeS2 also validated the appearance of two reduction peaks at about 1.3 and 0.5 V (vs Li/Li+), which were assigned to Li+ intercalation and conversion reaction (reaction 2.6), respectively as shown in Figure 2-12e and Figure 2-12f. During anodic polarization, the two prominent peaks at 1.9 and 2.3 V were related to the oxidation reactions and deintercalation of Li+ ions in agreement with the charge/discharge behavior. The rate capability of CuFeS2 at various current densities is shown in Figure 2-12g. The CuFeS2 nanorods exhibited higher capacity than sheet-like CuFeS2 nanoparticles even at higher C–rates. With an increase in charge/discharge specific current, the capacity was decreased from 700 mAh g–1 at 100 mA g–1 to 380 mAh g–1 at 1000 mA g–1 but it rebound back again to 620 mAh g–1 when the discharge specific current of 100 mA g–1 was selected. The better rate capability by rod-shaped CuFeS2 nanocrystallite is correlated with the nanofeatures of chalcopyrite obtained through surfactant-assisted growth. The reduction in size and change in morphology promoted the electrolyte/active material interaction and reduced the diffusion path length for Li+ intercalation/de–intercalation reactions [167, 170, 171]. 41  Furthermore, the cyclic stability test at 100 mA g–1 showed a decrease in specific capacity from 700 mA g–1 to 580 mA g–1 after 50 charge-discharge cycles (Figure 2-12h).   Figure 2-12 Morphology of synthetic CuFeS2 (a) without and (b) in the presence of PVP. Charge/discharge profiles (at various specific currents), cyclic voltammograms and specific capacity of as-synthesized CuFeS2 (c, e & g) without and (d, f & h) in the presence of PVP [162] 42  2.6.4 Electrochemical capacitors or supercapacitors The electrochemical capacitors are divided into two types (1) electrochemical double layer capacitor (EDLC) in which charge is stored within a few nanometers of the electrode surface by reversible adsorption/desorption of ions (non-faradaic) at the electrode/electrolyte interface. (2) Pseudo-capacitors involve surface or subsurface redox reactions (faradaic) at the electrode surface by the electrochemical species in the electrolyte [172, 173]. In recent years, the electrode in SC is generally composed of carbon-based materials, transition metal oxides or conductive polymers. The relatively lower power density of pseudocapacitors than EDLC is due to bulk electrochemical reactions dependent on ionic diffusion path length [174, 175]. The use of nano-sized electrode materials provides a large surface area (~1000m2 g–1) and charge separation across a few nanometers. The diffusion time of an ion is proportional to the square of the diffusion length (t~L2/D), thus for high rate capability high power could be realized by decreasing particle size of the active electrode material [131, 139]. The major concerns related to the nano-sizing of electroactive materials are agglomeration, electrical resistivity, cost and electrochemical stability for high rate applications.  Unlike Li–ions batteries, the electrolyte contribution to charge storage and supply is negligible in EDLCs and, therefore, the electrolyte remains stable after a large number of repetitive charging/discharging cycles. Activated carbon and graphitic carbon template and carbide-derived carbon [176], nanotubes [177], onions [178], nano-horns [179] have been investigated for EDLCs. The variety of carbonaceous materials having various shapes, structures, surface functionalities, porosity, and sizes are ideally considered the best electrode material for an EDLC. In terms of energy density, the EDLC cannot replace batteries but these can complement them in many applications for instance in welding, as an initial power source 43  for starting large electric motors, actuators, X-ray, MRI (magnetic resonance imaging) machines etc. Similar electrode materials (both anode and cathode) are usually used in the development of EDLC, termed as symmetrical capacitors. Due to their low potential window, ‘small cell potential’, these devices present low specific energy and power, which is the main hindrance to their commercialization. On the other hand, asymmetrical supercapacitors utilize two different electrodes of large potential difference, which may consequently supply a relatively large amount of specific energy and power compared to EDLC. However, symmetrical capacitors have large cyclic life compared to asymmetrical supercapacitors due to the high stability of the electrode materials. Because of the large cell potential, the asymmetrical supercapacitor devices have relatively smaller specific energy than the battery setups but better than the EDLC. The aqueous electrolyte-based supercapacitors have shown less specific energy than organic based supercapacitor, mostly associated with their stability at large potentials (large potential window). In other words, the small potential window and dissociation of the aqueous electrolyte would limit the specific energy, but these devices could provide relatively high specific power than organic based electrolytes, attributing to the large ionic mobility in H2O. The comparison of EDLC with recently reported asymmetrical supercapacitors is given in Table 2-5, which clearly illustrates the performance metrics of these devices in various electrolytes. However, due to the limited cyclic stability of electrode materials in the asymmetrical supercapacitor setups, and their relatively low specific power compared to EDLC, there is a demand for more research in this area. The development of new efficient electrode materials which can withstand large potential window without degradation during continuous charge/discharge cycles is essential [180].                44  In the past two decades, many flowable electrochemical energy storage systems such as redox flow batteries [181], Na-S molten batteries [182], semisolid 'Li' flow batteries (SSFC) and electrochemical flow capacitors (EFC) [183] have also been introduced. The key feature of these systems is their flexibility to decouple energy and power ratings since energy storage capacity is regulated by the size of electrolyte reservoir and power density could be tuned by controlling the size of the cell stack (size/volume of each cell) and/or by modifying electroactive materials. The EFC systems are highly efficient for rapid charge and discharge for energy storage at grid level due to their excellent cyclic stability. The charge is stored through non-faradaic adsorption phenomena and, therefore, these systems have a high-power rating but suffer from low energy density. Compared to LiBs, the SC can provide 10 times higher power density, 100 times faster charge/discharge rate and exhibit 1000 times longer cyclic life but deliver 20 times lower energy density. On the other hand, the slow charging response (several hours) under applied potential and capacity fade upon repeated cycles are the major issues associated with the LiBs [183, 184]. For hybrid electric vehicles and grid level power storage, researchers are most interested in the development of high surface area carbon-based, metal oxide or hybridized electrode materials which could give high capacitance (~1000 F g–1) in synergy with high energy density at a high rate [123, 183, 185, 186].  45  Table 2-5 Performance metrics of EDLCs and recently reported asymmetrical supercapacitors Electrodes (Positive/Negative) Surface area  (m2 g–1) Cell Voltage (V)  Specific capacitance (F g–1) Specific Energy (Wh kg–1) Specific Power   (kW kg–1) Cyclic stability (%) / # of Cycles Electrolyte Measurement @  Ref. Symmetrical or electrochemical double layer capacitors (EDLC) Carbon nano–spheres  791 1.0 551 20 2.5 97 /1.0k 1.0M HCl 1 A g–1 [187] Carbon nano mesh 1198 3.5 194 56.1 61.25 90.6/100k Ionic liquid 1 A g–1 [188] Activated carbon 2731 1.0 311 8.3 18.75 96.4/5.0k 6.0M KOH 0.5 A g–1 [189] Asymmetrical supercapacitors Activated carbon*/Zn  2000* 1.4 120 24.0 62.8 70.0/0.3k 7.3M KOH 0.005 A cm–2 [190] Ni(OH)2/graphite – 1.2 153 35.7 0.49 97.0/5.0k 1.0M KOH 0.05 V s–1 [191] Carbon nanotubes + NiO / defect induced graphene* 456* 1.6 108 38.1 0.5 93.5/10k 1.0M KOH 0.5 A g–1 [192] Co2AlO4+MnO2/Fe3O4 205.9/137.8 1.6 99.1 35.3 0.8 92.4/5.0k 2.0M KOH 5.0 A g–1 [193] MnO2+nano–porous gold/polypyrrole+ nano–porous gold – 1.8 193 86 25 85.0/2.0k 1.0M LiClO4 0.1 V s–1 [194] MnO2/Fe3O4 – 1.8 20 7.0 0.82 67.0/5.0k 0.1M K2SO4 0.27 A g–1 [195]   46  Chapter 3: Objectives  Based on the literature review, it is determined that hydrometallurgical processing of CuFeS2 is still a challenge owing to its refractory nature and passivation tendency in acidic media. On the other hand, CuFeS2 may be reduced into the less refractory Cu2S species in acidic media, which may in turn be oxidized to dissolve Cu. FeII/FeIII are common species used to control the solution potential and to enhance CuFeS2 oxidation in acidic sulfate media. During this process, the reduction of Fe(III) at the surface and oxidation of CuFeS2 consume energy to produce conversion products. Some fraction of this energy is lost as heat, the recovery of which is possible. However, these two processes could occur separately, in a battery like setup. In the first part of this work, a FBFC is introduced, which utilizes CuFeS2 as a negative electrode. During the charge cycle, CuFeS2 reduces into intermediate products which oxidize to produce Cu in the following discharge cycle. The FeII/FeIII redox couple is used as a supportive electrode reaction in the positive half of this FBFC.  In the second part of this work, two high energy demand hydrometallurgical processes, i.e., CuFeS2 oxidation and Zn electrowinning are coupled in a tri-functional battery setup (TFB) to produce Cu and to store energy simultaneously. The Zn will deposit on a negative electrode material via the oxidation of CuFeS2 in the positive compartment of the battery setup during the charging step.  In this setup, the CuFeS2 slurry is injected intermittently in the positive compartment and zinc solution is continuously circulated in the negative electrode compartment. During the charging cycle, the oxidation of CuFeS2 and deposition of Zn take place simultaneously on the positive and negative electrodes, respectively. The unique feature of this TFB is the simultaneous extraction of Cu and energy storage that can be reused for other purposes but at the expense of deposited Zn.        47  The main objective of this work is to develop hybrid battery like setups in which CuFeS2 can be used as an electrode material for energy storage and as source for Cu extraction. To quantify the energy storage and Cu extraction capabilities of these hybrid systems, the following sub-objectives were set. 3.1 Setup 1: Fixed bed flow cell (FBFC) 1. To establish a battery like hybrid system i.e. FBFC in which CuFeS2 can be reduced in the negative composite electrode (CuFeS2 mixed with carbon black and encapsulated in graphite felt) during the charge cycle with the help of FeII oxidation to FeIII on the positive electrode.  2. To investigate the electrochemical behavior of the negative composite electrode to estimate its cyclic performance and establish the charge storage mechanism in the final FBFC (Chapter 5).  3. To study the effect of CuII addition in the catholyte to evaluate the electrochemical behavior of the positive electrode and to compare the cyclic charge/discharge performance of the FBFC with and without CuII.  Quantify the specific energy, coulombic and energy efficiencies of this system (Chapter 6). 4. Compare the energy storage and Cu extraction capabilities of the FBFC when either synthetic CuFeS2 or a naturally sourced mineral concentrate are used (Chapter 7).  3.2 Setup 2: (TFB) 5. Introduce a hybrid battery-like hydrometallurgical system in which both Cu extraction from CuFeS2 and Zn deposition are simultaneously possible during the charge cycle.   6. Determine the reaction sequence at both electrodes in the TFB (Chapter 8).  48  7. Examine the cyclic charge/discharge performance of the TFB and quantify its energy storage and Cu extraction (Chapter 8). 8. Use a CuFeS2 mineral concentrate as a positive slurry electrode in the TFB and determine its performance (Chapter 9).      49  Chapter 4: Approach and Methodology 4.1 Materials  The lab grade chemicals i.e., ferrous sulfate tetrahydrate (FeCl2.4H2O; 99.95 %) from Fisher–Scientific, thiourea (CS(NH2)2; 99.98 %) from Alfa–Aesar and cupric chloride dehydrate (CuCl2.2H2O; 99.5 %) from Sigma–Aldrich were used as received to synthesize CuFeS2 hydrothermally. Other chemicals i.e. 1 M sulfuric acid (H2SO4) solution (Fisher–Scientific), sodium sulfate (Na2SO4), zinc sulfate (ZnSO4.7H2O; 99.95 %) Sigma–Aldrich were obtained as used without further purification. Graphite felt (GFE-1, CeraMaterials) used in a Fixed Bed Flow Cell (FBFC) was purchased having a thickness of ~0.8 cm. The resistance of the GF was found to be ~1.7 Ω in the uncompressed state as measured by a conductivity meter in the laboratory. For the preparation of carbon paste Acetylene black, poly(vinyl di-fluoride) (PVDF) and 1-methyl-2-pyrrolidone (C5H9NO) as a curing agent were used. The high surface area activated carbon (AC) (EQ-AB-520Y) purchased from MTI Corporation is used as a component to prepare slurry, which was used as positive electrode in the tri-functional flow battery setups i.e. TFB and TFB-M. Pure Al sheets (1.27 mm thick) conforming to ASTM B209 were purchased from onlinemetals.com for use as current collector and substrate for Zn deposition in the negative half of the TFB. The 3.2 mm thick flexible graphite sheet was purchased from equalseal.com and was cut (L100 x W50 mm) to use as a positive current collector plate in the TFB. The 10 mm thick molded polytetrafluoroethylene (PTFE), the UHMW (ultra–high molecular weight) polyethylene sheets (25 mm) and silicone rubber for sealing (Silicone CG White) were purchased from onlinemetals.com and Rubber-Cal, respectively. The AAS grade Fe and Cu standards solutions (TraceCERT) for inductively 50  coupled plasma optical emission spectroscopy (ICP–OES) analysis were obtained from Sigma Aldrich, Canada. 4.2 Experimental methods 4.2.1 Synthesis of CuFeS2 and pretreatment of mineral concentrate The CuFeS2 was synthesized hydrothermally in the laboratory to use as an electrode material in the proposed battery setups. Synthetic CuFeS2 was used to explore its intrinsic electrochemical behavior and to avoid the complexities of associated with the impurities that exist in the natural mineral concentrates. To synthesize CuFeS2, initially, the stoichiometric amounts of CuCl2.2H2O, FeCl2.4H2O, and thiourea were mixed in 150 ml deionized water (DI) for 2 h for homogenization. The mixture was transferred to a Teflon lined stainless steel vessel and the tightly closed vessel was then placed for 12h in a programmable furnace (VF1200D8) with a heating rate of 3.33 °C min–1 and stabilized at 200°C. The reaction vessel was then furnace cooled to room temperature and the resultant product was washed with DI water 3 times followed by stirring in (2% v/v) sulfuric H2SO4 solution for 30 minutes to remove any impurity. The filtered mass was washed with excess of DI water until the filtrate solution pH became equal to the pH of DI water. The filtered particles dried at 50°C for 24h were then stored in an airtight glass bottle for further use. The synthesis process was devised by the authors in the laboratory after rigorous experimentation.     The CuFeS2 mineral concentrate (MC) used in Chapter 7 and 9 was obtained from a mine and concentrator operation in British Columbia. The composition of the MC was determined via QXRD analysis and is reported in section 7.1. Prior to use, this MC was washed in DI water several times, filtered, mixed with dilute sulfuric acid (0.02 M solution) and stirred for 3 h to remove any air formed oxides or other air-related oxidation products. After acid 51  washing, the thoroughly washed MC with DI water (pH 4.7 ± 0.1 at 25 ºC) was dried at room temperature (24 h) prior to storage in a glass bottle for further use. 4.2.2 Schematic of the systems under investigation Two types of battery setups, i.e., FBFC and TFB are introduced in this work. Each positive and negative electrode and their components were rigorously characterized to identify the reaction sequence and to optimize the operating conditions for the operation of these battery setups. Figure 4-1 shows the scheme of the individual electrode system and components used in the battery setups. The combination of each positive and negative electrode for the FBFC (i.e., CFe, CFeCu or C–1, C–2) and TFB (i.e., TFB and TFB–M) were studied individually in their respective electrolytes (the final battery setups are presented as oval shape icons in this figure). In the following chapters, the detailed discussion on these electrode systems and battery setups is given.        52   Figure 4-1 Schematic diagram of the electrode systems used in each battery setup. The combination of positive (red lines) and negative (black lines) connect the two electrodes into a cell assembly. 4.2.3 Preparation of the electrodes for electrochemical study GF cut into 2.0× 2.0 cm square was connected to a graphite rod by carbon paste. Herein, this assembly is referred to as the 'CF electrode'. The carbon paste was prepared by homogeneously mixing 80 wt. % acetylene black, 20 wt. % PVDF and 1-methyl-2-pyrrolidone (C5H9NO) as a curing agent. The composite electrodes (used in Chapter 5) were prepared by encapsulating 0.1 g of as-synthesized CuFeS2 powder in the GF (without adding CB), by cutting the GF in half along its thickness and manually placing a layer of synthetic CuFeS2 between the resultant halves. After incorporating the CuFeS2 in the GF, the edges were sealed with carbon paste and this was then connected to a graphite rod by using the same procedure 53  as used for the CF electrode preparation. These electrodes were left to cure for 12 h at room temperature before use. Similarly, in Chapter 6 the electrochemical kinetics study of both negative and positive electrodes (as used in the FBFC) was carried out separately in a three-electrode cell setup as shown in Figure 4-2. The composite (negative) electrodes were prepared using the same procedures as stated above. Briefly, as–synthesized CuFeS2 or MC powder samples homogeneously mixed with carbon black (4:1) were manually sandwiched between two halves of GF (2 x 2 x 0.8 cm). The edges of the GF were sealed with carbon paste and connected to a graphite rod by carbon paste (Figure 4-2). These composite electrodes are designated as GF–CuFeS2 and GF–MC. Similarly, GF (the positive electrode in the FBFC) connected to a graphite rod with carbon paste are referred to as GF–Fe/Cu electrodes in Chapter 6. 54   Figure 4-2 Assembly of three electrodes cell in which each electrode i.e., CF electrode, comp electrode, GF-CuFeS2, GF–MC and GF–Fe/Cu was tested in its respective electrolyte. 4.2.4 Construction of fixed bed flow cell (FBFC)1 The two-electrode FBFC was used to estimate the charge storage capability of the synthetic CuFeS2 and MC as shown in Figure 4-3 and discussed elsewhere [196]. Briefly, the two compartments of the FBFC were constructed by drilling 10 mm deep circular cavities (Ø = 10 mm) in 30 mm long graphite cylinders. The inlet and outlet channels were made in the side of these cylinders by fitting Teflon nozzles, which opened in the cavities for electrolyte circulation. In one compartment (positive), disk shape GF was inserted whereas, the composite negative electrodes (made by sandwiching the mixture of either as–synthesized CuFeS2 or MC                                                  1 From the published work, K.M. Deen, E. Asselin, ChemSusChem, 11 (2018), 1533–1548.  55  (80 wt. %) and carbon black (20 wt. %) were manually compressed into the other cavity. These two electrodes were compressed (up to ~ 30 % of the original thickness of the GF) against each other and were physically separated by a preconditioned proton exchange membrane (PEM). The anolyte (0.5M Fe2+ + 0.1M Cu2+ dissolved in 0.2M H2SO4) was circulated in the positive compartment via peristaltic pump. In the negative electrode compartment, the 0.2M H2SO4 solution was circulated. As shown in schematic diagram (Figure 4-3), both electrolytes were pumped in a closed loop configuration from the external water jacketed (for temperature control) reservoirs maintained at 25 ºC.   Figure 4-3 Schematic diagram of the FBFC, the original setup demonstrates the overall assembly For electrochemical testing of the FBFC, the GF and composite electrodes (GF–CuFeS2 or GF–MC) in the positive and negative compartments, respectively, were connected to the working/working sense and counter/reference leads of the potentiostat, respectively. Both 56  anolyte and catholyte were circulated separately in the FBFC at a constant flow rate (7.5 ml min–1). 4.2.5 Construction and design of the tri-functional battery setup (TFB)2 The schematic of the two-electrode cell system designated as TFB is shown in Figure 4-4a. This TFB was designed and constructed in the laboratory. Briefly, two PTFE plates having rectangular flow channels (3 cm2) were attached together by inserting the preconditioned anion exchange membrane (AEM) between them. The AEM was sandwiched in silicone gaskets prior to insertion between PTFE plates. On each side of the PTFE plates, the flexible graphite plate (GP) FGS and Al sheet as positive and negative current collectors, respectively were affixed. To avoid any leakage silicone gaskets were inserted between the current collectors and PTFE plates. This whole assembly (as shown schematically in Figure 4-4a) was tightened with screws through the holes made in HDPE end plates as shown in Figure 4-4b. Detailed information for each component of TFB is presented in Figure 4-4c. The catholyte in the form of slurry and composed of 80 wt. % CuFeS2 + 20 wt. % AC in 0.2 M H2SO4 solution was first injected from one end in the positive compartment to fill the flow cavity and to remove air. The other end was then closed with a graphite plug and connected electrically to the positive plate through a conductive copper tape. The initial total solid content in the slurry was (20 wt. %) and this value was retained for all experiments. The amount of slurry injected into the cavity was calculated to ensure that at least 65 % of the volume was filled with solid particles in the form of slurry once these are settled as shown schematically in Figure 4-4a. On the other side of the cell, the anolyte (100 g l–1 Zn2+ dissolved                                                  2 This work is ‘under consideration’ for publication 57  in 0.2 M H2SO4) containing 0.1 M Na2SO4 and 2ppm cetyl-trimethylammonium bromide (CTAB) (optimized concentration) was circulated in a closed loop by a peristaltic pump at 10 ± 0.5 ml min–1 flow rate. The temperature of the anolyte solution was maintained constant at 25 ºC in an external water jacketed cell connected to a circulating thermostatic water bath.  Figure 4-4 (a) Schematic diagram of the TFB (cross-sectional view), (b) assembled cell, the high-density polyethylene end plates in which (c) components of the cell were tightened with screws. (1; GP is the flexible graphite sheet (FGS), 2; pure Al sheet, 3; Si rubber gaskets, 3a; AEM sandwiched in Si rubber gaskets and 4 represents the flow channels in PTFE plates for PSE and anolyte flow through nozzles 4.3 Physical characterization of electrode materials The morphology of the as synthesized CuFeS2 particles and Zn deposit on the Al sheet sample was examined in scanning electron microscope (Hitachi S300N VP–SEM). Energy dispersive X-ray (EDX) analysis was carried out to evaluate any variation in the elemental 58  composition of CuFeS2 particles before and after GCD in the FBFC and/or TFB. A Rigaku MultiFlex X-ray diffractometer was used to determine the crystal structure of the synthetic CuFeS2 as well as the deposited Zn on Al under various conditions, as described below. The Cu Kα1 radiation source (λ = 1.5405 ºA) was accelerated at 40 kV and 20 mA in a vacuum tube used to generate the X-rays. The CuFeS2 powder sample was compressed in the cavity that was produced in a zero-diffraction quartz crystal and installed in the sample holder. A 1º min–1 step size and 0.01º sampling width were selected to obtain diffraction patterns within the range of 2θ = 10 – 90º. The Kα2 signals and background noise was subtracted from the diffraction patterns by using Jade 6 (Material Data Inc.) software.  The particle size of the as-synthesized CuFeS2 and MC powder samples was determined by using a laser diffraction method in Malvern Mastersizer Hydro 2000S. Briefly, 3g of powder sample added in 100 ml of DI water was mixed on a magnetic stirrer to form a uniform dispersion. A representative sample was injected into the sample holder of Mastersizer and the ultrasound mode was turned on for 30 seconds to de-agglomerate and detach any physically interlocked particles.  For surface area measurement and to determine the pore size and distribution in the as-synthesized CuFeS2 and AC, nitrogen (N2) adsorption/desorption isotherms (at 77 K) were recorded with a Quantachrome Autosorb–1 machine. A known mass of synthetic CuFeS2 and/or AC was added in a 6 mm glass bulb and degassed for 92 h at 25 ºC prior to N2 adsorption. The specific surface area and pore size distribution of the synthetic CuFeS2 and AC were calculated by using the Brunauer–Emmet–Teller (BET) relation and density functional theory (DFT) methods, respectively.  The multipoint BET surface area was 59  determined from the isotherms over the 0.025–0.3 relative pressure (P/Pₒ) range by following the procedure explained elsewhere [2].  Time of Flight Secondary Ion Mass Spectroscopy (ToF-SIMS) (TRIFT V nano TOF; Physical Electronics) was used to determine the surface characteristics of as-synthesized CuFeS2 particles. Briefly, the sample powder was mounted on a commercially available (1 cm2) silicon wafer by using double-sided adhesive tape. Both positive and negative mass spectra were collected by pulsing a 30 keV Au* primary ion beam over a 400 µm X 400 µm raster area with a total ion dose < 1012 ions/cm2. In the acquisition of spectral data and images, high mass and spatial resolution were achieved by applying the bunched and un-bunched mode, respectively3.  To investigate the variation in the surface composition of CuFeS2 particles before and after GCD cycling, X-ray photoelectron spectroscopy analyses of as synthesized and retrieved CuFeS2 from FBFC and/or TFB were carried out by using Leybold Max2000 spectrometer. The X–rays generated from the Mg Kα (1253.6 eV) radiation source were incident on the powder sample placed in the high vacuum chamber. The intensity of the emitted photoelectrons at 90º take-off angle was analyzed as a function of their binding energy in a hemispherical photoelectron analyzer. The survey and high-resolution spectra of Cu, Fe, and S were processed in the XPS peak 4.1 software to examine any change in the surface speciation. The deconvolution of the high-resolution spectra was performed by applying the Gaussian (80 %) + Lorentzian (20 %) function after background subtraction (Shirley method). Any peak shifting in the XPS spectra calibrated with respect to C 1s adventitious peak was analyzed.                                                   3 From the published work, K.M. Deen, E. Asselin, ChemSusChem, 11 (2018), 1533–1548. 60  4.4 Electrochemical Testing 4.4.1 Characterization of the composite electrode(s)4  The electrochemical testing of an individual electrode (used in either FBFC or in TFB) was carried out in a water-jacketed three-electrode cell as shown in Figure 4-2. The intrinsic electrochemical behavior of the CF electrode (GF connected to the graphite rod) was initially determined and compared with the composite electrode under the same conditions and in 0.2 M H2SO4. Each experiment was repeated more than three times to verify the reproducibility of the results. It is important to note that no CB was added to CuFeS2 to make the composite electrode analyzed in Chapter 5. However, the composite electrodes prepared for Chapter 6 and 7 contained 20 wt. % CB to enhance the inter-particulate conductivity. A graphite rod and a saturated mercury/mercury (I) sulfate (Hg/Hg2SO4) (0.620VSHE) electrode were installed in the cell as auxiliary and reference electrodes, respectively. A Gamry Reference–600 Potentiostat and Echem Analyst (Software ver. 5.30) were used for all the electrochemical experiments and analyses. All potential values reported in this work were converted to the standard hydrogen electrode (SHE) scale unless otherwise stated. Before every electrochemical test, N2 gas (99.95 %) was sparged for 30 min to eliminate any dissolved oxygen.  A constant temperature of 25 ± 1.0 °C was maintained for all the experiments.  The cyclic voltammograms were obtained at various sweep rates (100, 80, 60, 40, 20, 10 and 5 mV s–1) and potential was kept well within the thermodynamic water stability potential window (~1.23 V). Scans were initiated from the OCP to 0.020 VSHE (segment referred to as 'charging') followed by a reverse scan to a maximum 1.020 VSHE (discharging)                                                  4 As reported in the published work by K.M. Deen, E. Asselin, Electrochimica Acta, 212 (2016), 979–991. 61  and proceeding to OCP to complete the cycle (see Figure 5-2a and b). Four cyclic scans at each sweep rate were obtained for identification of any variation in the behavior. The scans were reproducible, and data presented in this work is from the third cycle. The charge distribution and transport of ionic species was evaluated from the quantitative analysis of these cyclic voltammograms. The galvanostatic charging/discharging cycling tests for the CF and composite electrodes were conducted at various constant specific current densities (based only on the weight of the GF) ranging from 0.01 to 0.1 A g–1. The charging and discharging profiles were obtained within a 1.0 V potential range (–0.6 ≤ EOC ≤ 0.4 V). The potential range was selected based on the measured OCP of the GF in 0.2 M H2SO4 electrolyte. The same potential range for both electrodes was selected (even if they had different OCP) to investigate any difference achieved with the composite electrode. At each current density, 20 charge/discharge cycles were performed to evaluate reversibility and any potential change due to electrochemical reactions. The columbic efficiency ((discharge/charge) x 100) of both electrodes was measured based on the individual charge/discharge specific capacity at each cycle. The procedure to calculate specific capacity is described in section 5.3. In order to fully understand the current distribution and charge transport mechanism during the 'charging' process on both the CF and composite electrodes, the potentiostatic electrochemical impedance spectroscopy (EIS) and potentiodynamic cathodic polarization (PCP) tests were conducted. Within the charging regime, the impedance spectra were taken at constant potentials from the OCP of each electrode to –0.9 V (vs. OCP) with a step of –0.1 V. The constant potential was initially maintained for 3 h before each impedance measurement. At every static potential, a 5 mV alternating potential amplitude was applied through a 62  sequential variation of frequency from 100 kHz to 10mHz. The experimental spectra simulated with equivalent electrical circuits (EEC) were fitted to obtain quantitative information of the physical elements. The proposed model and quantification was validated by potentiodynamic cathodic polarization (PCP) curves obtained at a sweep rate of 5 mV s–1. The final potential of –1.0 V (vs. OCP) below the H+/H2 stability line was intentionally selected to study the adsorption of intermediate species (H°) and any redox reactions on CuFeS2 microspheres. The mechanism of charge transport and possible reactions at the surface of both electrodes is proposed based on these findings. 4.4.2 Electrochemical characterization of electrode(s) for FBFC setup The electrochemical behavior of a given electrode used in the final FBFC was investigated in a three-electrode cell (Figure 4-2). The electrodes designation and composition of each electrolyte is given in Table 4-1. It is also important to note that electrolyte and electrode systems are designated based on the polarity of electrodes in the FBFC during the discharge cycle. The anode (negative) was a composite of GF and as–synthesized CuFeS2 particles (80 wt. %) mixed with carbon black (CB) (20 wt. %). The composite electrode was prepared by sandwiching CuFeS2 particles manually within the two halves of the GF. The edges of the GF were sealed with carbon paste (CB + poly (vinyl di-fluoride); PVDF (4:1)). This composite electrode was then connected to a solid graphite rod with the same carbon paste to complete the working electrode assembly as shown in Figure 4-2. The cathode (positive electrode) was made of only GF and connected similarly with a graphite rod. Potentiodynamic polarization (PD) scans, linear scan voltammetry, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) analyses were carried out to analyze the 63  electrochemical response of the electrode materials and to estimate their performance in the actual FBFC system.  The potentiodynamic scans (PD) were obtained by applying a potential (–1.5 V ≤ Eapp ≤ 1.2 V) vs. OCP with a sweep rate of 2 mV s–1. The geometrical surface area of each (GF and composite) electrode was 14.4 cm2. However, to compare the results, the current was normalized with the weight of the GF. A total of 25 mg of the CuFeS2 + CB (4:1) mixture was used in the composite electrode. Table 4-1 The designation and composition of electrode and electrolyte systems used in the FBFC5 Type Composition of the electrode(s) Electrolyte(s) Anode (Negative) GF / CuFeS2+ CB (synthetic) (Anolyte)  0.2 M H2SO4 Cathode (Positive) GF (Catholyte)  0.5 M Fe2+ in 0.2 M H2SO4 0.5 M Fe2+ + 0.1 M Cu2+ in 0.2 M H2SO4  The impedance spectra were obtained by imposing a 5 mV AC potential amplitude over OCP of each electrode in their respective electrolytes (Table 4-1) and 100 kHz – 0.01 Hz frequency range was selected. Similarly, the cyclic voltammetry scans were obtained at various sweep rates (100, 80, 60, 40, 20, 15, 10, 5, 2 and 1 mV s–1) to evaluate the kinetic response of the individual electrodes in their respective electrolytes – the combination of which we refer to as electrode systems.                                                  5 The electrode and electrolytes are designated based on the electrochemical process during the discharge cycle 64  After optimizing the conditions, the performance of the final FBFC system was evaluated by cyclic voltammetry (CV) and cyclic GCD tests. In order to identify the electrochemical behavior and maximum current response of the FBFC system, the CV scans were obtained at various sweep rates (0.1 V s–1 – 0.001 V s–1). The specific capacity, energy density, coulombic efficiency, energy efficiency and cyclic performance of the FBFC system was calculated from the cyclic GCD profile as detailed in section 6.3.  The reaction sequence during GCD performance was also evaluated by post-experimental analyses of the CuFeS2 retrieved from the composite electrode (negative electrode of the FBFC) after 500 GCD cycles. The surface morphology, phase transformation and any change in the surface composition of the retrieved samples were analyzed and compared via SEM, EDS, XRD and XPS analyses (section 6.4).  4.4.3 Comparing the performance of as-synthesized CuFeS2 and MC in FBFC6  For electrochemical characterization and to compare the energy storage and Cu extraction capabilities in the FBFC system, the GF and composite electrodes (GF–CuFeS2 or GF–MC) in the positive and negative compartments, respectively, were connected to the working/working sense and counter/reference leads of the potentiostat, respectively. Both anolyte and catholyte were circulated separately in the FBFC in a closed-loop and at a constant flow rate (7.5 ml min–1). To investigate the electrochemical response of both the synthetic CuFeS2 and MC in the FBFC, CV scans were obtained at various sweep rates (from 0.1 V s–1 to 0.001 V s–1) within a 1.0 V cell potential. The charge storage capability of the FBFC was evaluated from the cyclic                                                  6 As reported in the published work by K.M. Deen, E. Asselin, Electrochimica Acta 297 (2019) 1079–1093. 65  GCD test. The charging was carried out at 200 mA g–1 and the FBFC was discharged at 150 mA g–1. The specific capacity of the FBFC system (using both synthetic CuFeS2 and MC) was calculated from the continuous GCD cycles. XRD and XPS analyses of the synthetic CuFeS2 after GCD cycling was also carried out to evaluate any structural or compositional changes in the negative electrode. During GCD cycling Cu2+ released from the CuFeS2 in the anolyte and/or migrating from the catholyte could affect the charge storage capacity of FBFC. Therefore, to support the experimental results, potentiodynamic cathodic polarization scans of GF–CuFeS2 electrodes (separately in a three-electrode cell setup) were also obtained in a 0.2 M H2SO4 solution containing various amounts of Cu2+ species. For detail analysis and result please see section 7.3. 4.4.4 Electrochemical behavior of electrode(s) used in the TFB The electrochemical behavior of the positive and negative electrodes in their respective electrolytes was measured individually in a standard three electrode setup. The electrodes were designated as positive (cathode) and negative (anode) in the TFB and TFB-M based on the electrochemical reactions during the discharge cycle as detailed in Table 4-2. For example, CuFeS2 mixed with AC (20 wt. %) and 0.2 M H2SO4 in the form of slurry was used as a positive electrode in the TFB. Thus, the electrochemical response of each positive slurry electrode (PSE) component, i.e. GP (positive current collector in the TFB), AC and CuFeS2 in 0.2 M H2SO4 solution was measured via CV scans obtained at a 5 mV s–1 sweep rate.     66  Table 4-2 The electrode systems and composition of the electrolytes used in the TFB setups7 Type Electrode(s)/Current collector(s) Electrolyte(s) Anode (Negative) Pure Aluminum sheet  (Anolyte) 100 g Zn2++0.1 M Na2SO4+0.2 M H2SO4 Cathode (Positive) Flexible graphite sheet (Catholyte; PSE)  20% solids (CuFeS2+AC) in 0.2 M H2SO4 (Catholyte; PSE-M)  20% solids (MC+AC) in 0.2 M H2SO4  Similarly, at the negative electrode side, it is important to optimize the conditions for efficient Zn2+ deposition on the Al substrate. For this reason, the effect of CTAB addition on the Zn deposition was investigated by galvanostatic polarization (GS), and CV scans. Based on these results, a potential – concentration (PC) diagram was developed to estimate the nucleation/deposition overvoltage as a function of CTAB concentration. The CTAB concentration was varied from 1 ppm to 8 ppm and coulombic efficiency of Zn deposition was determined from gravimetric analysis. By using Equation 4.1 below, the total charge consumed during GS polarization was calculated to evaluate the coulombic efficiency (ηc).  𝜼𝜼𝒄𝒄 = � 𝑸𝑸𝒈𝒈 𝑸𝑸𝒈𝒈𝒈𝒈� � 𝑿𝑿𝑿𝑿𝑿𝑿𝑿𝑿 = �𝒎𝒎𝒎𝒎𝒎𝒎𝑴𝑴 𝒊𝒊𝒄𝒄𝒕𝒕� � 𝑿𝑿𝑿𝑿𝑿𝑿𝑿𝑿   Equation 4-1 Where, Qg and Qgs are the total charges calculated from the gravimetric analysis by using Faraday's law and from GS tests, respectively. m, n, F and M are the mass of Zn deposit, a number of electrons, the Faraday constant (96485 C mole–1) and the atomic mass of Zn,                                                  7 The electrode and electrolytes are designated based on the electrochemical process during the discharge cycle 67  respectively. The ic and t are the charging current and time, respectively, applied in the GS testing. The CV and GS tests were performed to determine the nucleation and steady-state potential values for Zn deposition. The potential/concentration (PC) diagram was developed to evaluate the overvoltage for nucleation and uniform deposition of Zn on Al as a function of CTAB concentration. The morphology of the Zn deposit obtained in the anolyte solution containing various amounts of CTAB after GS testing was examined by SEM. The influence of CTAB concentration on the crystallographic orientations of Zn deposits developed after GS testing was evaluated from XRD patterns. Based on these observations, the solution of 100 g Zn2+ + 0.1 M Na2SO4 dissolved in 0.2 M H2SO4 and containing 2 ppm CTAB was selected as anolyte in the TFB.  4.4.5 Performance evaluation of the TFB The positive and negative current collector plates of the TFB were connected to the working and counter/reference leads of the cell cable attached to a potentiostat (Reference 600, Gamry®). The open circuit voltage (OCV) was determined, and anolyte was set to circulate continuously in a closed loop during the test. CV scans were obtained at various sweep rates (1 – 25 mV s–1) to identify the reaction sequence and to evaluate the system kinetics. Cyclic performance, specific capacity, and specific energy were measured from cyclic galvanostatic charge/discharge (GCD) tests. The charging and discharging were conducted at 1C and 0.5C rates, respectively (where 1C = 4.65 mAh, determined from the mass of Zn deposit during the charging process in a separate experiment). The coulombic (ηC), energy (ηE) and voltage (ηV) efficiencies were also calculated from individual GCD cycles. The rate performance of the TFB was also probed by charging at 1C but discharging at various rates (0.5C, 1C, 1.5C, 2C, and 2.5C). The cell polarization (I–V curve) behavior was estimated from the galvanodynamic 68  polarization curve. The cell was initially charged from OCV to 1.8 V at 1C, and the variation in cell voltage during discharge was observed (up to 0.6 V) by varying the load from 0 to 0.025 A at a 0.1 mA s–1 scan rate.  To demonstrate the use of the TFB as an energy storage unit, the setup was initially charged to 1.8 V and a light emitting diode (LED) mounted on a pre–built microcontroller was connected with the current collector plates. The internal circuit of the microcontroller was designed to visually display the cell operation by turning on the LED at potential ≥ 1.0 V. In this test a constant LED load (~ 0.5 mA) would consume the stored charge, and under an applied load, the change in the cell potential as a function of time was measured. 4.5 The reaction sequence for Cu extraction and system mass balance During repetitive charge/discharge cycles, the occurrence of irreversible faradaic processes i.e. oxidation/reduction of CuFeS2 in the positive slurry electrode could significantly affect the reversible performance of the TFB. Based on GCD cycling, the total irreversible charge (Qirr = Qcharge –Qdiscahrge) was calculated and the % Cu extraction was determined from ICP–OES (Varian 725–ES) analysis of both retrieved catholyte and anolyte after 20, 40, 60, 80 and 100 GCD cycles. For these analyses, standard solutions of various concentrations were prepared as background solution (2 % HNO3 and 0.2 M H2SO4 (1:1)) for dilution. Based on Qirr and Cu extraction (from ICP–OES analysis) as a function of a number of GCD cycles, the reaction sequence has been investigated (see section 8.4). During the charging cycle, the transport of anionic species (specifically SO42– species) from anolyte to catholyte, and vice versa during discharge process, was calculated from the instantaneous electric field across the AEM. During Zn deposition on the negative electrode (charging step), the transport (diffusion + migration) of SO42– toward the catholyte and reverse (only migration) was calculated using 69  the extended Nernst–Planck equation and compared with the Cu + Fe contents (extracted from CuFeS2) for mass balance. 70  Chapter 5: Electrochemical behavior of CuFeS2/GF composite electrodes8 In this Chapter, detailed electrochemical analyses of synthetic CuFeS2 in a composite electrode were carried out. The motivation to conduct these analyses was to separate out the current distribution on the composite electrode and to predict its cyclic performance as a negative electrode in the FBFC. GF possesses relatively large surface area and high electrical conductivity compared to CuFeS2. Therefore, the effect of GF in this composite electrode and its contribution to charge storage was also determined to predict the reaction sequence during reversible charge (reduction) and discharge (oxidation) cycles in the anolyte (0.2 M H2SO4). Synthetic CuFeS2 was used in this study to avoid the complexities associated with the many impurities in natural mineral concentrates, and to avoid the variability in particle size and shape that could mask the actual electrochemical response of CuFeS2.  It is proposed that in an acidic electrolyte the double layer can form quickly on the GF during charging and an intermediate species (Hº) can adsorb by the interaction of H+ ions with surface functional groups without further reaction to H2 evolution. This charge transfer process and formation of the intermediate Hº species is referred to as underpotential deposition (UPD). Based on the literature, and experimental results described in this Chapter, the increase in the charge storage capacity of the composite electrode was attributed to the reversible transformation of the sulfide sulfur species on the surface of CuFeS2, and to the formation and transportation of intermediate species (H°) to the CuFeS2 crystallites.                                                  8 From published work, by K.M. Deen, E. Asselin, Electrochimica Acta, 212 (2016), 979–991. 71  5.1 Characterization of the synthetic CuFeS2 The as-synthesized CuFeS2 particles presented spherical morphology in which a platelet-like open structure was attached to the core similar to pompom dahlia flowers as shown in Figure 5-1a. The average particle size (D80) of these microspheres was 23.48 ± 2.00 µm. A wide range of particles having variable size and morphology were formed during the hydrothermal synthesis process as depicted in the particle size distribution profile (Figure 5-1b). The micro-spherical morphology of these particles was similar to the results described elsewhere [145, 162, 167] in the literature. The surface area of the GF and synthetic CuFeS2 microspheres were 4.201 and 3.511 m2 g–1, respectively, as determined from the N2 adsorption/desorption isotherm and multipoint BET surface area analysis (Figure 5-1c). The relatively large surface area of the CuFeS2 microspheres attributed to the open platelet-like structure. Table 5-1 provides the quantitative information about the multipoint BET surface area analyses. Table 5-1 Quantitative measurement of parameters from Multipoint BET surface area analysis  Parameters GF Chalcopyrite (CuFeS2) BET constant; C 6.71 59.25 Slope; s 704.9 975.2 Intercept; i 123.3 16.74 Surface Area; SBET (m2 g–1) 4.205 3.511  The XRD pattern of the synthetic CuFeS2 perfectly matched to the JCPDS 37-0471, reference pattern for pure CuFeS2 (Figure 5-1d). This validated the formation of pure crystalline phase having a tetragonal crystal structure with lattice parameters (a, b = 5.2893; c = 10.423), belonging to the I-42d (122) space group. The sharp diffraction peaks at 2θ i.e., 72  29.4°, 48.65°, 49.04°, 57.85°, and 79.48° were attributed to single phase CuFeS2 originating from the (112), (220), (204), (312) and (316) lattice planes, respectively.  Figure 5-1e shows the full range XPS survey scan of as synthesized CuFeS2, which confirmed the presence of copper, iron and sulfur core peaks. The 'C1s peak' commonly referred to as 'adventitious peak' originated at (284.87 eV) also confirmed no charging effects on the powder sample. This peak appeared possibly due to the presence of unreacted organic species left in the product or this may also arise due to the contamination of the specimen holder in the vacuum chamber. The origin of the peak for 'O 1s' at 531.7 eV was possible due to air oxidation of the as synthesized CuFeS2 crystallites. The spin-orbital splitting of S 2p3/2 and S 2p1/2 in the XPS spectrum occur as doublets of the 'S 2p' peak. The high-resolution spectra of sulfur species (S 2p3/2) core peaks were fitted by using XPS peak 4.1 software and by applying a Gaussian-Lorentzian function (weighing; 80% Gaussian + 20% Lorentzian). In addition, the Shirley method was used for background subtraction as shown in Figure 5-1f [197]. This function resolved the spectra and clearly fractioned the doublet peaks of S 2p3/2 at 161.42 and 162.45 eV corresponding to the S2– and S22– in the bulk phase, respectively. The 1.03 eV shift in the binding energy of the S 2p3/2 doublet peaks was slightly lower than the reported value (1.1 eV) for fully coordinated sulfur in the CuFeS2 lattice structure [47, 198, 199]. Klauber et al. [45] reported the S22– species S 2p3/2 peak position at 162.48 eV, which agreed well with our experimental value (162.45 eV). The broad collar peak at 163.73 eV (FWHM=2.044 eV) is due to the presence of Sn2– species with (n > 2) at the crystallite surface [48]. In addition, the core level Fe 2p signatures (i.e., Fe 2p3/2, Fe 2p1/2 peaks at 711.45 and 725.45 eV, respectively) were very weak and the dominant doublet peaks of copper (Cu 2p3/2, 2p1/2) at the crystallite surface corresponded to the association of copper with disulfide in the 73  form of CuSn. The very low-intensity satellite peak at 942 eV was observed for the divalent Cu2+ species and dominant Cu 2p3/2, 2p1/2 peaks at 932.48 and 952.53 eV, respectively, were attributed to mono-valent copper (Cu+) species which are characteristic of CuFeS2.   Figure 5-1(a) Morphology and (b) particle size distribution curve of as-synthesized CuFeS2. (c) Multipoint BET analysis. (d) XRD pattern of CuFeS2. (e) The XPS survey scan of the CuFeS2 powder sample. (f) Deconvoluted high-resolution spectra of 'S 2p3/2' 74  5.2 Cyclic Voltammetry  Within 1.0 V potential range (well within the thermodynamic stability of H2O), the total current response was the sum of charge involved in the double layer charging and faradaic UPD of H+. In addition to these components, the faradaic current may also result from possible redox reactions occurring at the surface of the CF or on the CuFeS2 microspheres in the composite electrode. Figure 5-2 shows the cyclic voltammograms of CF and composite electrode (GF+CuFeS2), in 0.2 M H2SO4 (pH = 0.702±0.005). The sulfuric acid is dissociated into H+ and HSO4– ionic species (reaction 5.1) and were the dominant ionic species depending on their thermodynamic stability at 25 °C and the pH of the electrolyte [200]. H2SO4 + H2O ↔ H3O+ + HSO4–     5.1 The polarization effects possibly associated with the potential drop due to high resistivity of CuFeS2 or due to diffusion controlled processed within the porous GF are pointed out in Figure 5-2a, b, where delay in current response upon potential sweep reversal is shown as 'VCF' and 'Vcomp' for the CF and the composite electrode, respectively. During charging (reverse scan), the total negative charge at the electrode surface is the sum of the charge in the electrical double layer and charge involved in the faradaic reactions across the electrode/electrolyte interface. These additional faradaic reactions (pseudocapacitive behavior) could further increase the overall capacitance [183]. In this case, these pseudocapacitive effects may either emerge due to (i) charge transfer through UPD of H3O+ (adsorption) reaction 5.2, or via (ii) ionic transport through the porous structure of GF and within the platelet structure of the micro-spherical CuFeS2 particles. The pseudocapacitive response may also originate due to the occurrence of (iii) reversible redox reactions at the electrode/electrolyte interface [129, 131].  75  Cx + nH3O+ + ne– → (CxHn) ads + nH2O     5.2    Where Cx is the number of available sites on the graphite surface, 'Hn' is the number of adsorbed Ho species at the surface of GF during charging. For the CF electrode, the charge in the electrical double layer is distributed within the porous structure of GF and therefore the polarization effects due to the electrolyte resistance down the pore during charge/discharge could influence charge storage capacity [201]. The internal charge distribution within the pores (adsorption leading to depletion of H+ species within the porous structure) is controlled by diffusion and therefore, the current response in cyclic voltammetry depends on the sweep rate as shown in Figure 5-2a and b. The relatively large potential delay (ΔVcomp > ΔVCF) observed for the composite electrode (Figure 5-2b) was possibly related to the slow kinetic reactions during charging/discharging cycles [202]. The potential dependent but relatively higher specific current density of the composite electrode was attributed to double layer charging complimented with pseudocapacitive reversible redox reactions. The open porous platelet structure of the synthetic CuFeS2 microspheres (surface area; 3.51 m2 g–1) may enhance the overall current response due to the high interaction of cationic species (H3O+) at the interface. The transport of ionic species within the porous structure of the microspheres and reversible surface limited adsorption/desorption reactions may occur in addition to the electrostatic charge built up in the double layer. This is referred to as pseudocapacitive behavior and is considered to be one of the reasons for the higher specific current density registered by the composite electrode compared to the CF electrode (Figure 5-2a and b). The fractional contribution of surface limited current due to charge in the double layer and due to specifically adsorbed species from the CV analysis can be separated from the 76  current associated with the faradaic (diffusion controlled processes) by employing the power law (Equation 5.1) [129, 131].  𝑰𝑰(𝑽𝑽) =  𝒂𝒂𝒈𝒈𝒃𝒃       Equation 5-1 Where 'I(V)' is the current response at specific potential and 'a' and 'b' are adjustable parameters. During charging/discharging scans of CF and composite electrodes, the total current at each potential presented a linear relation with the sweep rate (s) for both electrodes (as shown in Figure 5-2c; the trends for the composite electrode are provided as an example). The 'b–value' is the slope of the linear trends, which depends on the applied potential. b = 1 corresponds to pure double layer charge.  b = 0.5 corresponds to faradaic adsorption and/or semi-infinite diffusion processes [131, 138]. The b–values calculated for the CF electrode ranged between (0.68 ≤ b ≤ 1) (as shown in Figure 5-2d), which can be related to mixed double layer charging and pseudocapacitive behavior i.e. by interaction of ionic species with the surface functional groups on GF (reaction 5.2) [131]. After an initial delay (ΔVCF = ~0.20 V), during the discharge scan, the pseudocapacitive (desorption) reactions were followed by the non-faradaic double layer transformation (b →~1). In the case of the composite electrode, during charging, within the 0.4 to 1.0 V potential range, the occurrence of faradaic reactions was predicted from the current response controlled by diffusion processes (b =~0.5). 'b' values below 0.4 V indicate the progress of irreversible reduction reactions at the surface of CuFeS2 platelets. The b-value < 1.0 observed for the composite electrode can be assigned to surface limited faradaic reactions leading to a diffusion-controlled process. This behavior could also be related to the partial reduction of the ionic and surface species present on the CuFeS2 particles during the charging cycle. The hybridized electrochemical response and relatively large specific current density presented by the composite electrode compared to the CF 77  confirmed the occurrence of such additional pseudocapacitive reactions on the surface of CuFeS2 microspheres.  In order to quantify and separate the current contributions of non-faradaic and faradaic reactions leading to diffusion-controlled processes, many authors [131, 138, 183] have used the approach adopted here. As discussed above the total current output at various sweep rates as a function of potential is composed of non–faradaic capacitive (𝑘𝑘1𝑠𝑠) and diffusion controlled (faradaic) processes (𝑘𝑘2𝑠𝑠1 2⁄ ) according to the current partitioning Equation 5.2. 𝑰𝑰(𝑽𝑽) =  𝒌𝒌𝑿𝑿𝒈𝒈 +  𝒌𝒌𝟐𝟐𝒈𝒈𝑿𝑿 𝟐𝟐�       Equation 5-2 The manipulation of the above equation to determine ′𝑘𝑘1′ and ′𝑘𝑘2′may be used to calculate the fraction of each current contribution Equation 5.3.  𝑰𝑰(𝑽𝑽) 𝒈𝒈𝑿𝑿 𝟐𝟐�⁄ =  𝒌𝒌𝑿𝑿𝒈𝒈𝑿𝑿 𝟐𝟐� +  𝒌𝒌𝟐𝟐     Equation 5-3 The sweep rate dependent 'I (V)' for both electrodes is plotted according to Equation 5.3 at each potential (V) and the linearly fitted curves provided the values of 'k1' and 'k2'. In Figure 5-3a and b, the respective voltammograms of both 'CF' and composite electrodes at 20 mV s–1 are presented in which the total specific current density was partitioned into non-faradaic (shaded area) and faradaic contributions. It is shown that the major current response (~93 %) from the composite electrode was controlled by diffusion controlled faradaic electrochemical reactions (pseudocapacitive current), whereas, the CF electrode provided a far greater amount of electrostatic current density (~52.5 %) attributed to the double layer charge as shown in Figure 5-3c. The differential specific capacitance (Csp) involved during charging and discharging was measured by integrating the total current within the applied potential and normalizing with sweep rates according to the following Equation 5.4 [203]. 78   Figure 5-2 CV scans (at 5–100 mV s–1) of (a) CF and (b) Composite (CF+CuFeS2) electrode. (c) Trends of current vs. sweep at various potentials. (d) b–values as a function of charge and discharge potential (For comparison, the current is normalized by the mass of GF) 𝑪𝑪𝒈𝒈𝒔𝒔 = 𝑰𝑰(𝑽𝑽,𝒕𝒕)𝒈𝒈𝒎𝒎𝑮𝑮𝒎𝒎       Equation 5-4 Where 's' is the sweep rate, 'dV/dt' (V s–1), mGF is the weight of the GF in the CF electrode; V1 and V2 are the potential extremes of 0.02 and 1.02 VSHE, respectively. The increase in differential capacitance with a decrease in sweep rate (from 100 to 5 mV s–1) confirmed the delay in the ionic charge redistribution within the porous structure of the CF electrode (Figure 5-3d) and growth of the diffusion layer at the electrode/electrolyte interface due to the progress of faradaic (pseudocapacitive) processes. Similarly, in the case of the composite electrode, reversible charging/discharging and highly distorted differential capacitance at low sweep rate 79  (compared to GF) also indicate the progress of pseudocapacitive faradaic reactions preferentially on the surface of CuFeS2 crystallites as depicted in Figure 5-3e. One can expect that the substantial amount of pseudocapacitive behavior of the composite electrode was affiliated with the morphology of the CuFeS2 crystallites and/or to the occurrence of reversible reactions at the surface of these particles.  Figure 5-3 CV of (a) CF and (b) Composite electrodes at 20 mV s-1, showing the current distribution. (c) Comparison of the total current contributed to the non-faradaic and faradaic processes leading to diffusion controlled processes as revealed from Csp plots of (d) CF and (e) composite electrodes. ('ch' and 'dis' represent the start and end of discharge, respectively). The large surface area (3.51 m2 g–1) and porous structure of the microspheres also provided a shorter path length for H+ ion diffusion and therefore registered high current density 80  in the cyclic voltammetry scans. Furthermore, a significant amount of pseudocapacitive current contribution presented by the CF electrode was due to the reversible reactions of H+ and quinone/pyrone type functional groups present at the surface of GF. This could be predicted from the broad anodic and cathodic peaks (centered at ~0.6 VSHE) as evident in Figure 5-2a. From the cyclic voltammograms, it was evaluated that the overall faradaic contribution (~47 %) included reversible redox reactions associated with these functional groups and formation of transient intermediate species (H°) (adsorption) at the surface of the graphite fibers during the UPD process [173, 202, 204]. In Figure 5-3a and c, about 47.47% of the total charge associated with the GF in the CF electrode was faradaic in nature.  To further confirm the cause of this significant amount of pseudocapacitive (faradaic) behavior affiliated with the surface functional groups, the FTIR spectra of 'un-exposed' GF and 'exposed' (after charge/discharge) were obtained as shown in Figure 5-4. The IR spectra of both samples were found to be similar to each other and no change in the band vibrations was observed. This behavior also shows the reversible electrochemical character of surface functional groups which remain stable even after exposure to the acidic solution. The spectra showed broad hydroxyl (–OH) stretching bands at 3500–3250 cm–1 and at 2670 cm–1, indicated the strong hydrogen bonding in the structure and signatures of carboxylic groups on the GF. The free water (–OH) bands (3600 – 3200 cm–1) were neglected in the analysis and are not shown in the spectra [205]. The stretching bands within 3000 – 2830 cm–1 and small bending bands at 1380 cm–1 and 875 cm–1 correspond to the alkane (–C–H) group. The sharp background signatures at 2550 – 2000 cm–1 were also prominent in the spectra and may originate due to the air and CO2 present within the porous structure of the GF and from the ATR diamond crystal used during analysis. These bands were consistent with the spectra of 81  the air background taken before experiments (not shown here). The strong stretching vibrations within 1850 – 1600 cm–1 consisted of multiple bands mostly associated with the carbonyl (=C=O) group. These bands reflected the existence of the carbonyl group attached to aldehyde (1740 – 1720 cm–1), ester (1750 – 1735 cm–1) and multi-membered cyclic ketonic structures as reported by Fuente et al. [206]. The small but sharp bending vibrations at 1570 cm–1 and 1460 cm–1 represent the presence of aromatic (–C=C–) groups in the structure of GF [206, 207]. The strong vibration detected in the IR spectra of both samples within 1850 – 1600 cm–1 and bands at 1570 cm–1 and 1460 cm–1 are well documented in the literature and supported to the presence of quinone/pyrone (poly-aromatic and cyclic ketone) type functional groups at the surface of carbon-based materials [206, 208, 209].  The pseudocapacitive behavior of carbon-based materials is also well known due to reversible redox reactions of H+ ions in acidic solutions with the functional groups (i.e. quinone/hydroquinone/pyrone) present at the edges of graphene layers as given in reaction 5.3 below [210, 211].  =C=O + H+ + e– ↔ =C–OH      5.3 82   Figure 5-4 FTIR spectra of unexposed (as received) and exposed (after GCD in de-aerated 0.2 mol dm–3 H2SO4 at 25ºC) graphite felt (CF) In this study, the OCP of the CF electrode in 0.2 M H2SO4 was about 0.683 ± 0.01 VSHE which also strengthens this feature of the quinone/hydroquinone/pyrone (Eº range between 0.5–0.7 V) reversible redox activity in the GF [208, 210]. The current peaks centered at ~0.6 VSHE in the CV scans (Figure 5-2a and b) of CF and composite electrodes validate this reversible behavior. Andreas et al. [211] also conducted a detailed study to investigate the pseudocapacitive behavior of carbon cloth in both acidic and basic electrolytes. They concluded that a significant amount of total capacitance in the acidic electrolyte was associated with the reversible redox reactions of quinone/pyrone type functional groups at the GF surface. These results were in support of the pseudocapacitive behavior of the GF (CF electrode) in our case.  83  XPS analysis of as synthesized CuFeS2 confirmed the existence of sulfur enriched species (S22–, S2–and Sn2–) at the crystallite surface as shown in Figure 5-1f. Conway et al. [173, 212] discovered the reversible pseudocapacitive behavior of disulfide (S22–) species present at the surface of pyrite (FeS2) as follows (reactions 5.4 & 5.5). S22–+ 2e ↔ 2S2–       5.4 S2–+ H+ ↔ SH–       5.5 The S22– may reversibly transform to S2– at the particle/electrolyte interface and, during the reduction of the S2– species, may be protonated to form SH−. The repetitive CV scans of the composite electrode demonstrate the reversibility of the charge/discharge process which is attributed to the overall current response of both GF and CuFeS2. However, under similar conditions, compared to the GF only, the large faradaic current response of the composite electrode also indicates the reversible character of sulfide sulfur species (i.e. S22–, S2–), which are present on the surface of CuFeS2 as confirmed from the XPS analysis (Figure 5-1f). At low sweep rate (5 mV s–1), the small current peaks at ~0.3 VSHE, as shown in Figure 5-2, may be associated with the reversible S22–/S2 redox reaction.    Based on the above, the reversible charging/discharging behavior of the composite electrode is related to the reversible adsorption/desorption of H+ on the surface functional groups of GF and the reversible transformation of S22– into S2– (reaction 5.4) on the polysulfide surface film. The increase in differential specific capacitance during charging (reduction of H+) was due to its the specific adsorption at potentials that are more negative but above the H+/H2 couple potential under applied conditions. The occurrence of irreversible reactions on the composite electrode cannot be neglected during cycling and these were attributed to the reaction of H+ ions with the surface species and to the formation of conversion products on the 84  surface of CuFeS2. The irreversible behavior was found to be more pronounced for the composite electrode where maximum differential capacitance during discharge at all scan rates was lower than the extreme capacitance obtained during charging sweeps (ch > dis) as shown in Figure 5-3d and e. This behavior may also arise due to potential drop across the interface by the accelerated ingress of cations inside the electrode or due to temporary depletion of H+ ions during charge cycle. For the composite electrode (Figure 5-3e), the increase in differential capacitance with decrease in sweep rate is attributed to the effective penetration of the electrochemical signal down the porous GF structure and the occurrence of quasi-reversible reactions (as identified and discussed in Chapter 7) on the polysulfide film that was developed on the surface CuFeS2. In other words, the electrochemical response of CuFeS2 was sluggish at high sweep rate but improved significantly at low sweep rate. This behavior suggests that to achieve maximum specific capacity from this type of electrode material would require a slow charge/discharge rate similar to many existing battery electrode materials [203, 212, 213]. 5.3 Galvanostatic cyclic charging and discharging profiles of electrode systems The galvanostatic charging (GC) and discharging (GD) profiles of both the CF and composite electrodes were obtained to support cyclic voltammetry analysis. The quantitative estimation of the total charge stored and released by these electrodes can be estimated from the GCD profiles according to Equation 5.5 [173]. 𝑪𝑪𝒈𝒈𝒔𝒔 =  ∫ 𝒊𝒊(𝒅𝒅𝑽𝑽).𝒎𝒎𝑮𝑮𝒎𝒎𝒕𝒕𝟐𝟐𝒕𝒕𝑿𝑿 . (𝒅𝒅𝒕𝒕)      Equation 5-5 Where, 'Csp' is the specific capacitance (F g–1), 'dV/dt' (V s–1) is the shift in potential within initial 't1' and maximum time 't2' during charge or discharge under the applied potential range, whereas, 'i' is the constant specific current (A g–1) and 'mGF' has its usual meanings. The potential limits of –0.6 V ≤ EOCP ≤ 0.4 V were selected for both electrodes during charging and 85  discharging at various constant current densities (ranging from 0.01 – 0.10 A g–1). The reason for selecting this potential range was to avoid the occurrence of any side reactions, i.e. H2 evolution and/or water decomposition at the electrode surface. Since the charging was started from the OCP of the respective electrodes, we neglected the first cycle in the analysis. The discrete GCD cycles are shown in Figure 5-5a and b for both CF and composite electrodes, respectively. The CF electrode showed nearly symmetrical charging and discharging profiles corresponding to reversible behavior mostly arising from double layer capacitance and from the pseudocapacitive response of the surface functional groups. The reason for the sharp increase in potential after each charging step depicted the interfacial resistance possibly arising due to the mass transfer-controlled processes down the porous structure. During charging, the depletion of H+ species within the porous structure of the GF could also contribute to the overall potential drop across the electrode/electrolyte interface. This behavior is mostly due to the porous structure and has been well explained and in detail by Pell et al. [201].  The composite electrode showed asymmetrical but reversible charging/discharging behavior. During charging, the steep potential decay (plateau at about –0.365 ± 0.015 V vs. OCP (or +0.105 VSHE) above E°(H+/H2) was observed, which could be related with the quasi-reversible reaction of S22-/S2– and/or to the adsorption (reaction 5.3 and 5.4) of H+ ions at the surface of the CuFeS2 crystallites. The existence of irreversible faradaic reactions at the surface of CuFeS2 cannot be neglected during cycling at low current densities (0.01 and 0.02 A g–1). The limited reactions based on ion transport are always sluggish and at low current densities, the ions have sufficient time to diffuse within the porous structure and to interact with the thin polysulfide film that covers the CuFeS2 crystallites. This behavior was prominent in Figure 5-5c in which we can see wide variation in Csp at low current densities. This monotonic 86  variation in Csp was apparent and attributed to partial oxidation of sulfide-rich species to elemental sulfur (S°) and Cu+ to Cu2+ at the CuFeS2 surface during discharge (pseudocapacitive faradaic reactions) process of the composite electrode. This fact was also apparent in Figure 5-5d, where coulombic efficiency at low current (0.01 A g–1) was much greater than 100 % [214, 215]. The polarization effects beyond thermodynamic stability, E° (H+/H2) also indicated the depletion of ionic species within the porous structure, possibly because of surface limited faradaic reactions i.e. H2 evolution and the formation of a conversion layer over the surface of CuFeS2 particulates [203]. At relatively higher current densities (above 0.04 A g–1), the variation in 'Csp' was much lower and both electrodes provided reversible behavior. However, the overall coulombic efficiency of the composite electrode (~95 ± 1.5 %) was lower than the CF electrode (98 ± 1.4 %). It is interesting to note that the low conductivity of the surface film on CuFeS2 (due to oxidation of sulfide species to elemental sulfur) could limit the reversible adsorption/desorption of H+ during charge/discharge [38]. The 'Csp' of the composite electrode (1.265 F g–1) at a high current density (0.1 A g−1) was almost double than the value obtained for the CF electrode (0.588 F g–1). Here we considered that the relatively large reversible Csp manifested by the composite electrode at all current densities was caused by the reversible faradaic reactions (pseudocapacitive behavior) of the sulfide-rich passive film, which covers the CuFeS2 crystallites. 87   Figure 5-5 GCD cycling at 0.02 A g–1 (a) of the CF and (b) the composite electrodes. (Note: arrows down and up represent the charging and discharging, respectively, the EOC of composite electrode = 0.470 ± 0.005 VSHE at pH = 0.702 ± 0.005). (c) The discharge-specific capacitance at various currents. (d) Columbic efficiency of the CF and composite electrodes 5.4 Mechanistic study of charge distribution on CF and Composite electrode The potential-dependent current response in the CV and origin of potential plateau in the GCD of the composite electrode is believed to be due to pseudocapacitive effects controlled by the surface limited faradaic and non–faradaic reactions. The porous structure (large surface area) and the presence of sulfide species over the surface of CuFeS2 crystallites are possible causes for the observed higher charge storage capability of the composite electrode. The porous 88  GF network and open structure of the CuFeS2 microspheres would definitely induce polarization effects and the faradaic reactions would depend on the availability of H+ ions during charging of the composite electrode.  In order to understand the reaction mechanism, and for quantitative estimation of the charge distribution within the composite electrode, impedance spectra (Nyquist plots) were obtained at constant bias potentials. The character of GF in the composite electrode was estimated separately through EIS analysis. To evaluate the pseudocapacitive effects arising due to current leakage through adsorption and other possible redox reactions, the impedance spectra at very negative bias potential were obtained as shown in Figure 5-6. In this figure, the impedance behavior (Nyquist plots) can be split into two frequency regimes. At high frequency (ω→∞) the impedance spectra represent coupled charge transport and polarization effects, as shown in Figure 5-6b and d. On the other hand, the impedance behavior at low frequency (ω→0) is associated with interfacial charge transfer due to the occurrence of electrochemical faradaic reactions (Figure 5-6a and c). At high frequency, the low applied potential (av) signals (5 mV) were restricted to the surface of the electrode and resulted in the impedance response of a depressed semicircle. These signals can penetrate well into the porous structure of GF and CuFeS2 at low frequency. This behavior is reflected as steep curves (large –Zimg component) in the impedance plots. The impedance response in this regime corresponds to the pseudocapacitive behavior of the electrode materials [216]. It is interesting to note that with an increase in reduction potential (< EOCP) of the GF and composite electrodes, the –Zimg component became smaller, which indicates an increase in current leakage due to the occurrence of non-capacitive irreversible (conversion reactions) faradaic reactions. The 89  frequency response behavior was analyzed and simulated by using an EEC, as shown in Figure 5-7. The overall ohmic drop across the electronic and ionic transport channels also contributes to the overall impedance and is designated as 'Rs'. The high frequency semicircles (Figure 5-6b and d) are represented as constant phase elements Φdl.  The faradaic resistance is represented as RF in the EEC. The under potential H+ adsorption (EOCP > E > E°H+/H2) at the electrode surface is helped by a small (RF) and would permit the charge to either adsorb at the surface (presented as interfacial pseudocapacitive element (Φp)) or may lead to current leakage due to the progress of faradaic reactions at the electrode surface. These charge transfer processes are inversely related to the parallel resistive element 'Rp' as shown in Figure 5-7. The n1 and n2 are the charge relaxation coefficients for the Φdl and Φp, respectively. Physically, after the formation of the double layer at the electrode/electrolyte interface (presented as Φdl in the EEC), the current leakage through RF could be related to the faradaic reactions such as hydrogen evolution and/or reduction reactions at the CuFeS2 crystallites during charging at very low (negative) bias potentials [217]. The progress of reversible faradaic processes (pseudocapacitive response) result in high values of Φp. However, thermodynamically (at pH 0.702) under the applied conditions, potentials below –0.0415 V (E < E°H+/H2) may possibly increase the current leakage through Rp due to progress of irreversible faradaic reactions such as H2 evolution (via either reaction 5.6 or reaction 5.7) on the surface of the electrode materials [218, 219]. In other words, the charge storage (pseudocapacitive behavior) capacity of an electrode material would decrease with decrease in Rp at more negative potentials due to the progress of irreversible reactions. 90   Figure 5-6 The Nyquist plots of (a) CF electrode (from EOC = 0.683 VSHE to Emax –0.216 VSHE (η=–0.9 V) in the cathodic (charge) direction). (b) The impedance behavior at ω → ∞ for CF electrode. (c) The Nyquist plots of the composite electrode at various potentials starting from 0.470 to –0.429 VSHE (η=–0.9 V) (d) high-frequency impedance behavior of composite electrode 91   Figure 5-7 Schematic of (a) CF electrode and morphology of graphite fibers in the GF. The inset shows the EEC. (b) Composite electrode and EEC used to simulate impedance spectra for analysis H3O+ + H° + e– → H2(g) + H2O     or      5.6 H° + H° → H2(g)       5.7  Quantitative information of the calculated parameters of both CF and composite electrodes are tabulated in Table A-1 (Appendix A). Figure 5-8a shows the relatively high Φdl value of the composite electrode vs. the CF electrode (above η > –0.5 V), which suggests a large amount of charge stored within the electrical double layer. On the other hand, the larger Rp and large ΦP within the same potential regime for the CF electrode is attributed to the high adsorption capacity for H+ (or H3O+) (reaction 5.2) and reversible redox reaction (reaction 5.3) at its surface (pseudocapacitive behavior).  For the composite electrode, the relatively small value of Rp (from ⁓3000 to 1.23 kΩ) compared to the CF electrode (from 22770 to 2.3 kΩ), from 0 ≥ ƞ ≥ –0.5 V, is related to the reversible faradaic redox reactions at the surface of CuFeS2 (i.e. reaction 5.4). The appreciable 92  hindrance provided by GF for recombination reactions (reactions 5.6 and 5.7) (large Rp and ΦP) indicates its character as an ‘internal charge mediator’ that promotes the adsorption of H+ as an intermediate (H°) species. The appreciable decrease in Rp of the composite electrode (Figure 5-8b) (from 3000 to 1.23 kΩ) within the same cathodic overpotential regime (0 ≥ ƞ ≥ –0.5 V) is due to the reaction of CuFeS2 with this adsorbed intermediate species (conversion reaction; reaction 5.8) and has been discussed in detail by other researchers [91, 215, 220]. In simple words, the carbon fiber network behaves as a catalyst in the composite electrode to preferentially adsorb H+ ions and supply the formed intermediate (Ho) species to the coupled electroactive CuFeS2 crystallites. 2CuFeS2 + 2Ho + 4H+ → Cu2S + 3H2S + 2Fe2+   5.8 The GF provides almost two times higher Rp (2.13 kΩ) than that (1.231 kΩ) measured for the composite electrode at low potential (η= –0.5 V), which indicates the occurrence of conversion reactions on the composite electrode. η= –0.5 V was well within the thermodynamic stability of the electrolyte at the surface of GF (EOCP = 0.683 VSHE). However, at this large overpotential applied to the composite electrode (EOCP = 0.470 VSHE), there are fair chances of current leakage due to H2 evolution and/or due to the progress of conversion reactions at the surface of CuFeS2. But the high Rp value for the composite electrode at η > –0.6 (Figure 5-8b) highlights the delay in the H2 evolution reaction.  93   Figure 5-8 Variation in Φdl and Φp of CF and composite electrode. (b) The extent of current leakage through faradaic processes (η = 0 corresponds to the EOCP (0.683 VSHE) and mixed potential (0.470 VSHE) for the CF and composite electrodes, respectively) 94  The increase in Φdl of the CF electrode at η <−0.5 V is due to surface charge balancing by H+ (or H3O+) ions. At large negative potentials, the ingress of a large amount of H+ ions toward the electrode surface would decrease once the maximum adsorption capacity (surface coverage) is achieved.  This can be seen by the decrease in Φp. At overpotential below η < −0.5 V, the ΦP of the composite electrode is significantly decreased due to parallel current leakage through Rp and is comparable to the value of the CF electrode as shown in Figure 5-8b. This behavior corresponds to the progress of faradaic (irreversible) reactions on these electrodes at such negative potentials.  During charging, the mechanism of charge distribution and character of GF as an internal charge mediator (which may supply charge and intermediate (Hº) species to the CuFeS2 internally in a composite electrode) was also estimated from cathodic polarization scans as shown in Figure 5-9a. The polarization trends of the CF and composite electrodes are divided into three regions. The polarization trends for both the CF and composite electrodes at their respective 'EOCP' values in 'region 1' were similar. The 'EOCP' of GF was ~0.213 V more positive than the mixed potential of the composite electrode in 0.2 M H2SO4. It was seen that at small-applied potentials, the composite electrode had larger (0.0995 V decade–1) slopes than the CF electrode (0.0584 V decade–1) as shown in the inset of Figure 5-9a.  Almost half the value of the slope for the CF electrode at low applied potential suggested its quick electrochemical response (as indicated by its high iₒ, CF = 30.55 mA cm–2 compared to iₒ, comp = 17.93 mA cm–2) for the progress of reversible charge transfer processes [221, 222]. It is, therefore, considered that the kinetics of hydrogen adsorption at low field prior to the potential 0.457 VSHE (where the two plots intersect) was 1.7 times faster on GF than on CuFeS2 on the 95  composite electrode based on the apparent exchange current density (iₒ, CF) values (based on the geometrical surface area of the CF electrode) [223]. Below this potential, in 'region 2', the higher current provided by the composite electrode was therefore associated with faradaic reactions i.e. adsorption of H+ preferentially on GF and pseudocapacitive processes (reaction 5.4) over the CuFeS2 crystallites. Individually, in this region, the relatively small current response (slope = 0.981 V decade–1) by the CF electrode compared to the composite electrode (0.284 V decade–1) indicated the large polarization of the GF possibly due to preferential adsorption of H+ ions. In other words, it was evaluated that in composite electrode the rapid ingress of H+ ions towards the porous GF at such negative potentials (as in region 2) could produce the transient intermediate species (H°) (reaction 5.2) and supply to the CuFeS2 crystallites [89, 91]. To verify the internal charge mediator character of GF in the composite electrode, electrochemical behavior of a CuFeS2-carbon paste electrode (CPE) (CuFeS2 85 wt% +10 wt % CB and 5 wt% PVDF deposited on a platinum foil) was also determined and compared with the composite electrode under the same experimental conditions, as shown in Figure A- 1 (Appendix A). The 'EOCP' and low potential electrochemical behavior of the CPE was comparable to the composite electrode. At relatively large overpotential the limited current density (2.2 mA g–1) followed by a gradual increase in the current density within the UPD region (before H2 evolution) was observed below 0.420 VSHE. The overall cathodic current density of the composite electrode was approximately 3 times higher than the CPE at E ≥ 0 VSHE, which also indicated the preferential hydrogen adsorption on GF and effective transportation of intermediate species to the CuFeS2. 96  The impedance behavior of the composite electrode in the same potential range (E > E°H+/H2) (region 2 in Figure 5-9) also provided higher Φdl and large Rp (Figure 5-8), which justified the internal transportation of Ho from the GF fibers to the CuFeS2 crystallites. The maximum ΦP of the CF electrode (at –η = ~0.2 V), would be related to the adsorption of H+ at the graphite fibers and the shuttling of the H° to the CuFeS2 crystallites within the composite electrode [224]. The composite electrode also presented relatively lower polarization such that the shift in potential towards more negative values was delayed in comparison to the CF electrode (shaded area in Figure 5-9a). The faradaic response of the composite electrode was enhanced by approximately a factor of 5 within the region–2 as supported by the higher current in the polarization scans (Figure 5-9a). The impedance behavior of the composite electrode at (−η=0.5V) also showed low Rp (1.23 kΩ) compared to CF electrode which further reinforces this argument. The very low Rp provided by CF and composite electrode at much more negative potentials represented the internal current leakage due to H2 evolution and faradaic conversion reactions on the CuFeS2 as evident form the current response in the polarization curve in region 3. The conversion of CuFeS2 into the intermediate reduction products i.e. talnakhite (Cu9Fe8S16), bornite (Cu5FeS4) and chalcocite (Cu2S) is possible as proposed by Nava et al. [32]. 97   Figure 5-9 Cathodic polarization trends of (a) CF and composite electrodes starting from their respective OCP to –0.9 V vs. OCP, low field (η→0) polarization trends showing the convergence potential. (b) The proposed mechanism of the overall charge distribution in the composite electrode 98  Schematically, the electrochemical mechanism (Figure 5-9b) involved in the overall charge distribution within the composite electrode was proposed based on the experimental results as shown in Figure 5-9a. At very low overpotential (η→0), the internal charge distribution within the composite electrode was a function of intrinsic surface potential and charge storage capacity of both the GF and the CuFeS2. The difference in the electronic energy density of each component in the composite electrode i.e. GF and CuFeS2 will ultimately establish a compromise potential in the electrolyte referred to as 'mixed potential'. In 'region 1' when a small potential perturbation (η > –0.1 V) is applied, the H+ ions tend to ingress preferably towards the highly conductive GF, which also contains quinone/pyrone type functional groups. A further decrease in potential (η < –0.1 V) in 'region 2' results in the H+ ions being adsorbed preferentially (UPD) according to reaction 5.2 and the intermediate transient species (H°) is transported to CuFeS2. These species (H+ and H°) have the ability to react with the sulfide species in a reversible manner at the surface of CuFeS2 according to reactions 5.3 and 5.4. The H2 evolution (reaction 5.6 and 5.7) and conversion reactions may also occur when the potential applied is very negative (η <–0.5) in 'region 3'. 5.5 Summary The main intension of this Chapter was to investigate the electrochemical behavior of the composite (GF+CuFeS2) electrode in 0.2 M H2SO4 for its prospective use as negative electrode in the battery setup (as discussed in subsequent Chapters 6 and 7). Based on the impedance analysis and potentiodynamic polarization results, the mechanism of charge distribution within this composite electrode was proposed. Under similar conditions, detailed CV analyses suggest that the large current response by the composite electrode was mostly due to the progress of reversible faradaic processes on its surface. However, the CF electrode had 99  a relatively large non–faradaic current response compared to the composite electrode, as determined quantitatively from the current partitioning analysis. The large specific capacitance of the composite electrode (1.265 F g–1 at 0.1 A g–1) was also found to be almost double that of the GF alone. The reversible charging/discharging character of the composite electrode, and the appearance of an additional potential plateau at about –0.365 VOCP (well within water stability), revealed its higher specific capacitance in comparison to GF. The interfacial charge distribution and transfer processes within the composite electrode were also explored through potentiostatic impedance spectroscopy and potentiodynamic polarization curves. The reversible behavior of the composite electrode is attributed to the preferential adsorption/desorption of H+ (H° species) on the GF which may transport to the surface of CuFeS2 internally within the composite electrode. This 'internal charge mediator' character of the GF in the composite electrode was predicted from the impedance analysis, which demonstrated very high leakage resistance (Rp) and relatively large pseudocapacitance (ΦP) at η > −0.5 V conditions that simulate charging in a battery setup. The decrease in Rp and large current response of the negative composite electrode at very negative applied potentials (η < −0.5 V) indicated the possibility of irreversible faradaic reactions on the CuFeS2 crystallites possibly due to the internal transport of intermediate (H°) species from the GF. 100  Chapter 6: The hybrid mineral battery: energy storage and dissolution behavior of CuFeS2 in an FBFC9 In this chapter, the first steps toward the development of a hybrid mineral battery-like setup are presented. This setup can be used simultaneously as an energy storage device and as a unit for metal extraction. Initially, the synthetic CuFeS2 is used as an active anode material due to its unique ability to reduce or oxidize under different conditions, a characteristic that motivated this work. This hybrid setup consists of two electrodes in a fixed bed flow cell (FBFC) configuration. The composite electrode (synthetic CuFeS2 mixed with carbon black and sandwiched in GF) was used as the negative electrode in which acidic solution (0.2 M H2SO4) was pumped from an external circuit. In the positive half of the cell, the acidic ferrous sulfate solution (anolyte) was circulated through the GF and separated by a PEM from the negative compartment. During charging, the oxidation of FeII is expected to reduce CuFeS2 into Cu2S. In the subsequent discharge cycle, the reduction of FeIII to FeII is anticipated, which facilitates the oxidation of Cu2S to CuS/Cu2+. Both electrodes were also characterized individually, in their respective electrolytes, to elucidate their respective electrochemical performances in the final FBFC. Based on the ex-situ characterization of the retrieved CuFeS2 product from the FBFC after GCD cycling, a possible reaction mechanism is also proposed.  6.1 Physical characterization of as-synthesized CuFeS2   The as-synthesized CuFeS2 powder particles were examined in SEM and the formation of pure CuFeS2 was confirmed by X-ray diffraction. Variable size open-pored and platelet-like spherical particles were formed during the hydrothermal synthesis process as shown in Figure                                                  9 From the published work, K.M. Deen, E. Asselin, ChemSusChem, 11 (2018), 1533–1548. 101  5-1a.  The open pores and thin platelet-like morphology could form during the synthesis process by the thermal decomposition of the reagents under high temperature and pressure conditions. Figure 5-1d shows the diffraction pattern of as synthesized CuFeS2 particles, which matches well with the PDF 37–0471 reference pattern peaks and without presenting any impurity signatures. Laser particle diffraction of the as-synthesized CuFeS2 showed a wide particle size distribution in the range of ~3 – 45 µm. However, from the cumulative distribution curve, it was determined that 80 vol. % of the particles (D80) were below ~23.5 ± 2.0 µm as reported in the previous Chapter (Figure 5-1b) [225]. The BET surface area, pore size distribution and pore volume of as-synthesized CuFeS2 was evaluated from the N2 adsorption/desorption isotherms as shown in Figure 6-1. The magnified region of the isotherm hysteresis (inset) observed at large relative pressure (0.8 < P/Po< 0.97) is associated with the mesoporous and macroporous platelet-like structure of spherical CuFeS2 particles. The BET surface area of the CuFeS2 particles (3.5 m2 g–1) was calculated from the linear portion of the isotherm as shown in Figure 5-1c. The surface area is related to the particle size, morphology, pore size, and distribution. The DFT method was employed to evaluate the pore size distribution in the as-synthesized CuFeS2 particles.  This assumes that the pore-filling takes place by either micro-pore filling or through capillary condensation. The pores of different sizes are considered to be of a regular shape (cylindrical or slit) and the adsorbent surface is assumed to be homogeneous as reported elsewhere [226]. It was found that the variable sized-pores (5 – 25 nm) were distributed within the CuFeS2 particles and the total pore volume was calculated to be 0.011 cm3 g–1. 102   Figure 6-1 N2 adsorption/desorption isotherm of as-synthesized CuFeS2 particles (77 K). To analyze the surface chemistry of as synthesized CuFeS2, the positive (Figure 6-2a and b) and negative (Figure 6-2c and d) ToF–SIMS mass (m/z) spectra were obtained. The positive ion mapping showed Cu enrichment at the surface with relatively small Fe (blue area) concentrations. The mass spectra also provided a Cu/Fe ratio of 1.88, calculated from the corresponding normalized peak intensity. The characteristic peaks for Cu and Fe are shown in Figure 6-2b with some signals related to the hydrocarbon (HC) impurities [227].  The presence of HCs in the positive spectrum may be due to airborne moieties in the ambient environment during sampling and/or, most likely, the products formed because of decomposition of the mineral oil used in the vacuum system. These contaminants could adhere to the surface and may reflect in the spectrum. 103   Figure 6-2 ToF–SIMS analysis of as-synthesized CuFeS2 (a) 3D surface distribution of Fe and Cu (b) positive ion spectrum (c) 3D mapping for sulfur (S) (d) negative ion spectrum The negative ion mapping (Figure 6-2c) clearly identified the presence of sulfur species at the surface. The mass spectrum of CuFeS2 particles also provided prominent signals for O–, S– and HS– species, as shown in Figure 6-2d. The significantly higher intensity of O– in the spectrum could be associated with the oxidation of the surface during exposure to air. However, the larger signals associated with the Cu and S (mostly ionic in nature as elemental sulfur is volatile under ultrahigh vacuum conditions) species at the surface were evident in both positive and negative ion mapping, respectively, which suggest the presence of Cux–Sz enriched species 104  at the surface of the CuFeS2 particles. These species may form during synthesis, washing, drying and/or the handling process. Buckley et al. [41] also explained the possibility for the formation of iron deficient CuS2 and Cu0.8S2 species on the surface of CuFeS2 upon exposure to air and during conditioning in acidic solutions, respectively. During exposure to air, a reconstruction process could occur on the fresh surface of CuFeS2 implying that migration of Fe to the surface and reduction of Fe3+ would lead to the formation of thin layer enriched with oxide and Cu1-xS2-x (0 < x ≤ 1) species as discussed by Li et al. [24]. This proposed mechanism agrees with the ToF–SIMS and XPS results presented in this study, which also revealed the formation of an Fe deficient, CuS2 layer on the as-synthesized CuFeS2 particles. 6.2 Electrochemical behavior of individual electrode systems Both the cathodic and anodic potentiodynamic polarization curves for the composite electrode were obtained at 1 mV s–1 scan rate after achieving stability of the OCP (0.01 mV s–1) at 25 ºC, as shown in Figure 6-3. The composite electrode was prepared by mixing 10 wt. % CB in CuFeS2 and by sandwiching in the GF. The cathodic and anodic polarization responses were measured separately on freshly prepared composite electrodes to avoid any effect the conversion products may have had on the polarization scan. The OCP of the composite electrode in 0.2M H2SO4 solution was measured to be 0.485 ± 0.01 V, which stabilized after three hours of immersion. The anodic and cathodic Tafel slopes (βa and βc) (measured within linear Tafel region but beyond OCP > 50 mV) were 0.202 and 0.303 V/decade corresponding to the charge transfer coefficients 0.29 and 0.19, respectively. Olvera et al. [228] also reported similar charge transfer values (αa = 0.28 and αc = 0.17 – 0.22) for CuFeS2 in 0.5M H2SO4 solution containing ferric and ferrous at 25 ºC. A possible reason for the larger βa and βc (low charge transfer coefficients < 0.5) could be the formation of intermediate species at the surface 105  of CuFeS2 and preferential adsorption of Hº at GF, respectively. However, the open porous structure of GF and the microporosity of CuFeS2 may also distort the polarization curves, which may result in larger Tafel slopes and lower transfer coefficients, as discussed in the literature [229]. Also at large overpotentials (> ± 0.2 V), the effect of an increase in solution resistance within the porous structure of the composite electrode cannot be neglected. Three polarization regimes can be observed for the anodic scan in Figure 6-3. In the low potential range (OCP ≤ E < 0.65 V) (1a), the relatively rapid increase in current is associated with the formation of non-stoichiometric metal deficient polysulfide (Cu1-xFe1-yS2) at the surface of CuFeS2 via reaction 6.1 as proposed by other researchers [47, 48]. This polysulfide film may restrict further dissolution of CuFeS2 as indicated by the very small increase in anodic current within the (0.65 < E < 1.05 V) potential region (2a). CuFeS2 → Cu1-xFe1-yS2 + xCu2+ + yFe2+ + 2(x + y) e–   (y>x)   6.1 A further increase in potential leads to the transformation of this polysulfide film into CuSx due to the preferred dissolution of iron over copper and with the enrichment of sulfide sulfur at the surface of CuFeS2 [48, 230]. The stability of this polysulfide passive film is reduced at potentials above 1.05 V as indicated by the large increase in the current density (in region 3a). Ghahremaninezhad et al. [47] proposed that beyond this potential, the Fe is preferentially released and the dissolution rate of CuFeS2 is further controlled by the copper enriched polysulfide film. Cathodic polarization of a separate electrode also resulted in three potential regions. The relatively linear increase in current with shift in potential (OCP > E > 0.18 V) was followed by relatively large polarization effects within (0.2 > E > –0.5 V). This behavior is possibly related to the preferential underpotential deposition of H+ ions at the GF surface, followed by 106  transport to the CuFeS2 within the composite electrode at more negative potentials (designated as region 1c in Figure 6-3). The electro-assisted reductive conversion of CuFeS2 into less refractory Cu2S by monoatomic hydrogen species has also been studied in an undivided cell by Fuentes-Aceituno et al. [91]. In the composite electrode, the internal mediator character of GF to generate Hº species and possible reduction of CuFeS2 has been experimentally evaluated in our previous work (Chapter 5) [225]. Also, it has been discussed in the literature that atomic hydrogen was mobile in graphite and can diffuse along graphene planes at room temperature [231]. Details on the electrochemical reduction of CuFeS2 and formation of intermediate species i.e. talnakhite (Cu9Fe8S16) and bornite (Cu5FeS4) before conversion into chalcocite (Cu2S) have been given by Nava et al. [32].  Figure 6-3 Potentiodynamic polarization scans of composite electrode (GF–CuFeS2+CB) in 0.2 M H2SO4 solution; the current is normalized by the mass of CuFeS2  107  As exhibited in Figure 6-3, it is expected that in region 2c, the increase in current below 0.18 V is associated with the formation of these Fe deficient reduced intermediate species on the surface of CuFeS2. Further increase in current below –0.4 V (region 3c) may also indicate the reduction of CuFeS2 directly into metallic Cu along with H2 evolution.     In the two-electrode cell setup, during charging, the reduction of CuFeS2 (at the negative electrode; GF – CuFeS2) is facilitated by the oxidation of FeII into FeIII on the positive electrode (GF – Fe). It is therefore important to study the electrochemical behavior of the individual electrode system and to explore the kinetic behavior of any possible faradaic redox reactions that may increase the charge storage capacity in the proposed FBFC system. The influence of 0.1 M Cu2+ in 0.5 M Fe2+ solution (catholyte) has also been investigated and this referred to as the GF – Fe/Cu electrode system. The CV curves of these electrodes were obtained at various sweep rates and are superimposed to verify the kinetic response as shown in Figure 6-4. In order to simulate the actual conditions in the final setup, the CV scan for the GF – CuFeS2 (negative electrode) was initiated in the reverse direction before scanning in the positive (forward) direction. It can be seen that the rapid increase in the current density beyond –0.4 V and 1 V both in the reverse and forward directions, respectively, is associated with the dissociation of the electrolyte.  Independent of the sweep rate, the most dominant cathodic and anodic peaks were centered at ~0.50 V vs SHE for the GF–CuFeS2 composite electrode system. Both cathodic and anodic peaks were composed of two separate peaks indicating the faradaic response of the surface functional groups present on the GF (indicated as Pc1 and Pa1) and possibly to the reversible character of the sulfide surface species (i.e. polysulfide) present on the CuFeS2 (Pc2 and Pa2) as shown in Figure 6-4a. With the increase in sweep rate, a shift in cathodic peak 108  potentials (Epc1and Epc2) and anodic peak potentials (Epa1 and Epa2) in both negative and positive directions, respectively, was observed.  This is most likely associated with the quasi-reversible nature of the redox reactions. However, the solution resistance may also increase within the porous structure during repetitive cathodic and anodic scans due to the development of a depletion region that could influence the peak separation. Due to the overlapping current response (broad peaks) by the faradaic reactions affiliated with the surface functional groups and sulfide species (i.e. S22–, S2–) at similar potentials (from –0.2 V to 0.7 V vs. SHE), it is difficult to distinguish the individual contributions from the GF and CuFeS2 [32, 211, 232-234]. But it is shown in Figure 6-4d that peak currents (Pa1 and Pc1) vary directly with v1/2, corroborating the occurrence of facile reversible electron transfer (pseudocapacitive; faradaic) reactions at the electrode leading to a diffusion-controlled process. During the cathodic scan, the reduction of CuFeS2 into talnakhite (Cu9Fe8S16), bornite (Cu5FeS4) and chalcocite (Cu2S) are also possible. At more negative potentials (< –0.1 V), iron is completely removed and the formation of Cu2S occurs with H2S generation according to reaction 6.2 as evident from region 2c in Figure 6-3 and discussed by Nava et al. [32].  2CuFeS2 + 6H+ + 2e–→ Cu2S + 2Fe2+ +3H2S    6.2 CuFeS2 + 4H+ + 2e–→ Cu + Fe2+ + 2H2S    6.3  For a further decrease in potential beyond –0.4 V the formation of metallic copper (reaction 6.3) in addition to H2 evolution is also expected. This behavior is evident from region 3c in Figure 6-3 as discussed above. The small but relatively sharp peak at –0.15 V observed during the forward scan could either be associated with the oxidation of metallic copper into Cu2+ ions or to the formation of chalcocite as given in reaction 6.4. The relatively minor peaks observed (from –0.2 V to +0.6 V) during the anodic potential sweep were related to the 109  oxidation of the intermediate species (reaction 6.5, 6.6) formed during the reduction reactions [92, 234]. These results corroborate the potentiodynamic polarization behavior as discussed in the previous section. 2Cu + H2S → Cu2S + 2H+ + 2e–     6.4 Cu2S → Cu2–xS + xCu2+ + 2xe–     6.5 H2S → Sº + 2H+ + 2e–      6.6  The anodic and cathodic peaks in Figure 6-4b and c correspond to pseudocapacitive FeII/FeIII and Cu2+/Cu+ads, GF redox reactions taking place on the GF electrode surface. Thermodynamically, in sulfate media, Cu+ in the bulk solution is not stable but it is thought that this intermediate species can form at the surface of GF during reduction of Cu2+. This species may form complexes with the surface functional groups i.e. carbonyl, carboxylic, quinone, hydroquinone etc. present on the GF as evaluated from the IR spectra and reported in section 5.2, Figure 5-4 [225]. These redox reactions were further studied from the anodic and cathodic current peak dependency on the sweep rate in the voltammograms. However, the significant distortion in the voltammograms at higher sweep rates, i.e. the shift in both anodic and cathodic peak potentials toward more positive and negative potential values, respectively, was related to the hindered electron transfer processes at the electrode. This behavior could be associated with an increase in solution resistance within the porous structure of GF during forward and reverse scanning. The additional peak in Figure 6-4c, seen at relatively lower sweep rates (<10 mV s−1), corresponds to the Cu2+/Cu+ads, CF couple. The typical behavior of increase in peak current density with the square root of sweep rate (Figure 6-4d) corresponds to the faradaic but diffusion controlled electrochemical processes at the GF surface. The peak current for the 110  reverse scan was calculated by subtracting the background current [235] from the baseline produced by Echem® Analyst software 6.3 (Gamry Instruments).   Figure 6-4 CV scans of (a) the composite electrode in 0.2 M H2SO4 (the current is normalized by the mass of composite electrode) and (b) GF electrode in 0.5 M Fe2++ 0.2 M H2SO4 solution. (c) GF in solution as in (b) with 0.1 M Cu2+ (the current is normalized by the wt. of GF) (d) peak current vs. (sweep rate)1/2. The pseudocapacitive redox reactions for the FeII/FeII couple with and without the presence of Cu2+ may occur both at the surface and within the porous structure of the GF. This may lead to diffusion controlled kinetic reactions due to the depletion of ionic species within the porous structure of GF and may be validated from the linear dependency of both the anodic and cathodic peak currents on v1/2 as shown in Figure 6-4d. 111  The GF–Fe/Cu electrode exhibited relatively larger slopes (peak current vs. v1/2) (1.76 ± 0.05) (similar for both anodic and cathodic processes) compared to the GF–Fe electrode.  This is attributed to the rapid kinetics of the FeII/FeIII redox reaction at the GF electrode in the presence of Cu2+. The redox potential for the FeII/FeIII couple was calculated to be 0.63 ± 0.03 V and the addition of 0.1 M Cu2+ did not influence this potential significantly. However, the addition of Cu2+ to the electrolyte resulted in an additional peak in the CV scans (Figure 6-4c), associated with the Cu2+/Cu+ads, GF redox reaction. This species also increased the anodic and cathodic peak current density for the FeII/FeIII couple (at slow sweep rates) compared to the GF–Fe electrode. This behavior indicates an increase in the stability of FeIII obtained through the addition of Cu2+ in the solution according to reaction 6.7. However, the effect of increased ionic strength with the addition of Cu2+ to the electrolyte cannot be neglected either and further experiments are discussed below to rule this out. The Cu+ads, GF species would form as an intermediate species before reduction to metallic ‘Cu’ and may adsorb by interacting with the surface functional groups present at the surface of GF. This explains the higher discharge current (reduction of FeIII) for this electrode system.      FeII + Cu2+ ↔ FeIII + Cu+ads, GF     6.7 To validate the catalytic effect of Cu2+ on FeII oxidation, step-wise linear sweep voltammetry (LSV) was applied on the GF electrode at 5 mV s–1 to first simulate the charge cycle (oxidation of FeII into FeIII) the electrolyte on the surface of a GF electrode (positive electrode), as shown in Figure 6-5. N2 was sparged for 30 min prior to each experiment and the temperature was kept constant at 25 ºC. After charging, and before each discharge (reduction of FeIII into FeII) cycle, the potential of the GF electrode was relaxed at 0 V vs. OCP for 0, 1, 2, 4, 10, 20, 40 and 60 min, respectively in a sequential manner as presented in the 112  Figure 6-5a. The electrolytes were freshly prepared before each experiment by mixing FeSO4. 7H2O (0.5 M) into 0.2 M H2SO4. The LSV discharge curves for the GF electrode in both the electrolytes (without and with the addition of Cu2+ species) are shown in Figure 6-5b and c, respectively.  It is evident in Figure 6-5d that the discharge current was high in Cu2+ containing solution even after a prolonged delay at OCP. This increase in current even after an extended delay during the discharging (negative scan) was directly related with the concentration of Fe3+ ions in the solution which was produced during the charging (positive scan) step and catalyzed by Cu2+ ions. This behavior also predicts the increased stability of FeIII ions in the electrolyte with the addition of Cu2+ species under applied conditions through reaction 6.7. The reduction of Cu2+ during discharge/negative scan would produce intermediate ‘Cu+ads, GF’ species at the surface of GF. This species would adsorb at the surface of GF by interacting with the surface functional groups and may increase the discharge current (reduction of FeIII) as confirmed from the experimental sequence. Zhang et al. [236] reported the effect of Cu addition on the oxidation of FeII ions and proposed that Cu2+ could catalyze its oxidation in the presence of oxygen. In another study, Biniak et al. [237] described the mechanism of copper adsorption on activated carbon and the change in its oxidation state after interacting with the acid-base surface functionalities. Many other reports also described the cupric catalyzed oxidation of FeII at high temperature and pressure in sulfate media [238, 239]. Based on the experimental evidence, and with the support of available literature, we have demonstrated the effect of cupric on the catalytic oxidation of ferrous on GF electrodes, which may be beneficial for increasing the discharge current density, hence the specific energy of the final FBFC device.  113  It was also inferred (Figure 6-5d) that the addition of Cu2+ ions increased the cathodic current for FeIII reduction, which is directly related to its high concentration and availability at the surface of GF even after a 60 min delay (between charging and discharging). In comparison, the GF–Fe system registered low peak current and validated the beneficial effect of Cu2+ addition in the electrolyte. As noted above, this was considered to occur due to the extended stability of FeIII ions in the solution and catalytic oxidation of FeII in the presence of Cu2+ ions. However, the addition of 0.5 M Fe2+ in both electrolytes demonstrated that the increase in current for the FeIII reduction peaks cannot be related to the increased ionic strength of the solution. 114   Figure 6-5 Stability analyses of FeIII species in Cu2+ containing solution at a GF electrode. (a) Schematic of sequential protocol applied for the charging (FeII → FeIII) and discharging (FeIII → FeII).  (b) The discharge current profiles of GF after sequential delay (0 to 60 min) in 0.5 M Fe2+ solution and in (c) 0.1 M Cu2+ ions containing electrolyte. (d) Discharge peak current profiles as a function of delay time (Note: mass (187 mg) and geometrical surface area (14.4 cm2) of GF electrode were same for each test). The impedance spectra for the GF–CuFeS2, GF–Fe and GF–Fe/Cu electrode systems were obtained by applying a 5 mV AC potential amplitude over OCP within the frequency range of 10 mHz –100 kHz, as shown in Figure 6-6 (Nyquist plots) and Figure 6-7 (Bode plots). 115  The impedance at high frequency is associated with the solution resistance (Rs) and with the charge accumulated in the double layer, which is represented by the constant phase element (Qdl).  A constant phase element is used to account for the non–uniform charge distribution within the porous structure of the electrodes. The intermediate and low-frequency regimes correspond to the reversible faradaic response of the electrodes, which is inversely related to the charge transfer resistance (Rct). These faradaic reactions (pseudocapacitive behavior) incorporate specific adsorption of ionic species at the electrode surface (Qad). In a physical system, the ionic species in the electrolyte (H+, HSO4–, Fe2+ etc.) may interact with the CuFeS2 (in the composite electrode) and/or with the surface functional groups present on the GF [92, 225]. In the EEC model, the parallel resistor (Rad) is used to model the barrier to charge transfer for the adsorption/desorption process.  The significant difference in the impedance behavior of the composite and GF electrodes at low frequency can be clearly seen in Figure 6-7a and b. For the composite electrode, the continuous increase in –Zim (at low frequency) is associated with the concentration gradient within the porous structure and to the pre-existing sulfide sulfur enriched species present on the CuFeS2 particles (as confirmed from the ToF–SIMS and XPS analyses, Figure 6-2 and Figure 6-14a). This behavior is attributed to the limited mass transport across this film (finite length diffusion) and is modeled by a Warburg coefficient (σB) and a time constant (B) in the EEC. However, both GF–Fe and GF–Fe/Cu electrodes had similar trends in the low-frequency regime with a slight variation in the phase angle (Figure 6-7b). This behavior is most likely associated with the semi-infinite diffusion characteristics (σw) of the open porous structure of GF. 116   Figure 6-6 Impedance spectra of composite (GF – CuFeS2 + CB), GF–Fe and GF–Fe/Cu electrodes (at 0 V vs. OCP). The EEC is shown in the inset in which Yx is the admittance for the diffusion parameter and x=B, (for finite diffusion) and x=w (for semi-infinite diffusion), similarly σB = Warburg constant for GF–CuFeS2, and σw = Warburg constant for GF–Fe and GF–Fe/Cu electrodes. In addition, the geometrical area of the GF in all electrodes was the same (14.4 cm2). 117   Figure 6-7 (a) Bode plots (b) Phase angle (c) Residual error plots obtained after fitting of impedance spectra with EEC.  (Echem® Analyst 6.25 software; Gamry Instruments Inc.). The data obtained after fitting the EEC model to the impedance spectra is provided in Table 6-1.  The fitting was carried out through an iterative process and by adjusting the parameters of elements in the model editor (Echem Analyst 6.25 software; Gamry Instruments Inc.). There were small fitting errors for the EEC, as shown in Figure 6-7c. Compared to GF electrodes, the large ‘Qdl’ (1.12 mS sn1) registered by the composite electrode is most likely associated with the presence of CuFeS2 in the composite electrode. The sulfide sulfur species (see Figure 6-14a) on the surface of CuFeS2 and surface functional groups on GF can reversibly interact with ionic species resulting in an increase in ‘Qdl’. The kinetic activity (faradaic 118  response) of the composite electrode can be estimated from ‘Rct’ (84.74 Ω) and is comparable to the GF–Fe/Cu (93.11 Ω) electrode. The higher ‘Rct’ of GF–Fe (130.2 Ω) compared to GF–Fe/Cu further validated the catalytic behavior of Cu2+ in the catholyte as indicated by the improved current response by this electrode (Figure 6-4c, d and Figure 6-5). The reversible faradaic response of the surface sulfide sulfur species and surface functional groups in the composite electrode is reflected by the low ‘Rad’ (22.34 Ω) value. The small values of ‘Qad’ and power index (n2 = 0.39) are due to a highly distributed faradaic response of the composite electrode in which CuFeS2 particles make a fix bed with additional inter-particulate porosity [32]. However, limited mass transport across the pre-existing surface film could restrict the non–capacitive faradaic (irreversible) reactions under applied conditions (5 mV AC perturbation at OCP). This behavior can be estimated from the ‘σB’ (606.4 Ω s–1/2) and ‘B’ (1.99 s1/2). The relatively large capacitive response ‘Qdl’ by the GF–Fe/Cu (330.5 µS sn1) compared to GF–Fe (93.9 µS sn1) clearly demonstrated the influence of Cu2+ ions in the electrolyte. Due to the porous structure of GF, the non–uniform charge distribution in both electrodes was predicted from n1< 1 and is evident in the phase shift at high frequency (Figure 6-7b). The almost double ‘Qad’ value (54.9 mS sn2) and very small ‘Rad’ (16.92 Ω) for the GF–Fe/Cu compared to the GF–Fe electrode further validated the improved kinetic response of the GF in the presence of Cu2+ in the electrolyte. The n2 value equal to 1 indicates the pure capacitor-like behavior in which charge is homogenously distributed within the porous GF electrode as discussed by Cuenca et al. [240].  From these results, it can be evaluated that the reduction of Cu2+ at the GF/electrolyte interface (reaction 6.7) could significantly improve the charge transport characteristics of the GF [241]. On the other hand, the significantly reduced ‘σw’ (7.6 Ω s–1/2) for the GF–Fe/Cu 119  electrode compared to GF–Fe electrode is also due to the facile charge transfer process (reaction 6.7) leading to mass transfer control (semi-infinite diffusion characteristics). Table 6-1 Impedance parameters evaluated after fitting the spectra by using the EEC model Parameters GF – CuFeS2 (Anode) GF – Fe (Cathode) GF – Fe/Cu (Cathode) Rs (Ω) 7.77 11.13 8.31 Qdl (µS sn1) 1119 93.9 330.5 n1 0.54 0.67 0.68 Rct (Ω) 84.74 130.2 93.11 Qad (mS sn2) 6.05 30.2 54.93 n2 0.39 0.99 1.00 Rad (Ω) 22.34 57.64 16.92 σB (Ω s–1/2) 606.4 – – σw (Ω s–1/2) – 13.8 7.6 B (s1/2) 1.99 – – Chi-square 2.0x10–4 4.5x10–4 4.9x10–4  To verify this behavior, we immersed the as-received GF samples in each electrolyte for 48 h at 25 °C. The samples washed with DI–water several times were left in the air to dry. The SEM images and EDX analyses of these soaked GF samples showed clear morphological and compositional differences as shown in Figure 6-8. Compared to the as-received GF and GF–Fe, the GF–Fe/Cu contained higher Fe concentrations. Similarly, the signals for O and S species on the GF–Fe/Cu electrode were dominant, which we believe is due to the presence of surface functional groups and/or to the specifically adsorbed ionic species i.e. HSO4–, SO42–, Fe2+, Fe3+, and Cu2+ etc.  Pakula et al. [241] also demonstrated the electro-adsorption of Fe3+ ions on activated carbon. They concluded that with an increase in the density of oxygen-containing acidic functional groups on carbon, they could significantly improve the adsorption capacity for water molecules and Fe3+ ions. The addition of Cu2+ to the electrolyte appears to catalyze the oxidation of Fe2+, as proposed in the preceding discussion, which is thought to be the reason 120  for the improved stability of the Fe3+ species according to reaction 6.7 and its adsorption on GF.   Figure 6-8 SEM images and EDX analyses of the (a)  as received and (b) treated GF (in 0.5M Fe2+/0.2M H2SO4) (c) GF treated in 0.5M Fe2+/0.1M Cu2+/0.2M H2SO4 6.3 Estimating the charge storage capability by the FBFC system The charge storage capability of the composite electrode containing CuFeS2 in the negative compartment of the FBFC system (as shown in Figure 4-3) was estimated using the Fe2+/Fe3+ couple with and without the addition of Cu2+ ions as positive electroactive species. Figure 6-9a and b show the CV for the CuFeS2|Fe2+ (CFe) and CuFeS2|Fe2+– Cu2+ (CFeCu) FBFC systems, respectively, at different sweep rates (0.1 – 0.001 V s–1). A higher specific current density was achieved for the CFeCu system vs. the CFe system during charging (forward) and discharging (reverse scan) cycles. The decrease in specific current density at slow sweep rate is typical behavior in the CV analysis and depends on the kinetics of the charge transfer reactions and mass transport of the electroactive species at the electrode surface. The 121  charge transfer reactions at the high surface area GF electrode could also induce polarization effects due to the development of a concentration gradient within the porous structure of the electrode compared to the bulk electrolyte. The continuous flow of electrolyte (7.5 ml/min) within both compartments of the FBFC ensured the continuous supply of ionic species within the porous electrode to avoid diffusion control processes by the depletion of ionic species at the surface of each electrode. Also, the conversion reactions (reduction of CuFeS2 and formation of intermediate species) that occur within the negative (composite) electrode during the charging cycle affect the charging and discharging behavior of the FBFC in successive cycles. For instance, the large potential drop with increase in sweep rate (Figure 6-9a and b) for both the CFe and CFeCu systems during forward (charging) and reverse cycles (discharging) is possibly due to the slow kinetics of the electroactive materials.  In CV scans at a high sweep rate, the relationship between current and cell potential indicates the limited kinetic response of the electrode materials, particularly associated with the charge transfer characteristics of the CuFeS2 in the composite (negative) electrode. The mixing of CB (20 wt %) with the CuFeS2 likely minimized the inter-particulate contact resistance but the formation of a sulfide sulfur enriched film on the surface of CuFeS2 during the charge/discharge process likely restricts the electron transfer to or from the CuFeS2. The sweep rate dependency of the Fe2+/Fe3+ redox reaction on the GF electrode has already been discussed in the preceding section. Figure 6-9c shows a decrease in differential specific capacitance with an increase in sweep rate. This indicates a drop in the charge storage capability of the proposed system possibly either due to the formation of intermediate species on the surface of CuFeS2 particles or by the quasi-reversible nature of the redox reactions in the catholyte (Fe2+/Fe3+ and or Cu2+/Cu+) as explained in section 7.2 (Chapter 7).  122  The rapid fall in the specific capacitance at high sweep rate (Figure 6-9c) also confirmed the slow kinetic response of the electrode materials [242]. This behavior suggests that the current density for charging and discharging of the proposed setup should always be lower than the maximum current density obtained in the CV scan at slow sweep rate (0.001 V s–1). In this way, the maximum faradaic response of the electrode materials can be ensured in the FBFC setup. The average specific capacitance (Csp) at slow sweep rate (0.001 V s–1) during charging/discharging (at 1.0V cell potential) of the CFe and CFeCu systems was calculated from Equation 6.1. For this system, the specific capacitance was found to be 99.1 and 149.3 F g–1, respectively, which decreased monotonically with increasing sweep rate as shown in Figure 6-9c. 𝑪𝑪𝒈𝒈𝒔𝒔 =  𝑿𝑿𝒎𝒎𝒈𝒈𝑽𝑽 ∫ 𝒊𝒊(𝑽𝑽).𝒅𝒅𝑽𝑽𝑽𝑽𝟐𝟐𝑽𝑽𝑿𝑿       Equation 6-1 Where, ‘m’ is the mass (gm) of CuFeS2 used in the negative electrode, ‘s’ corresponds to the sweep rate (V s–1), ‘V’ is the potential window (V), ‘i(V)dV’ is the potential dependent current response (A) , V1 and V2 correspond to initial and final potential, respectively. The asymmetrical cyclic current response of both systems obtained at 0.001 V s–1 is shown in Figure 6-9d. This behavior was associated with the slow kinetic response of the electrode materials. The well–known refractory nature of CuFeS2, its low electrical conductivity and tendency to form a passive film under acidic conditions could be the possible reasons for very low specific capacitance with an increase in sweep rate. 123   Figure 6-9 CV scans of (a) CFe and (b) CFeCu systems. (c) Trends showing the variation in the specific capacitance as a function of (sweep rate)–0.5 (at 1 V cell potential), (d) comparison of CFe and CFeCu system (voltammograms at 1 mV s–1) In the CFe system, during the charging cycle, the oxidation reactions taking place on the positive electrodes are supported by the reduction reactions at the negative composite electrode. Similarly, the oxidation reactions on the negative electrode would occur in the following discharge cycle facilitated by the reduction of Fe3+ and/or Cu2+ on the positive electrode. It can be estimated that for effective charge storage and retrieval, one would require high stability of Fe3+ ions in the catholyte. This could be achieved with the addition of Cu2+ in 124  the solution as proposed in the stability analysis (Figure 6-5). In a typical CV scan of the CFeCu system, higher anodic and cathodic current densities were observed, which were attributed to the increased stability of Fe3+ ions. However, based on the literature, the quasi-reversible minor redox peaks centered at about 0.55V in both the CFe and CFeCu systems represented the pseudocapacitive behavior of the GF due to the presence of surface functional groups [211]. To verify the charge storage behavior of the proposed systems and to estimate the coulombic (ηC) and energy (ηE) efficiencies of the process, GCD cyclic tests were performed from 0 to 1.05 V. The charging and discharging was carried out at 200 mA g−1 and 150 mA g−1, respectively, and potential profiles for 500 cycles are provided for comparison in Figure 6-10. Analogous to the CV scans, the asymmetrical GCD plots reconfirmed the pseudocapacitive behavior of the proposed FBFC systems. A sudden potential drop of ~0.2 V at the onset of each discharge cycle was observed and may arise from the ionic resistance of the PEM, the electrolyte and/or due to the contact resistance between electrical connections. However, as noted above, the inter-particulate contact resistance in the negative composite electrode is expected to be small due to the mixing of 20 wt. % conductive CB with the CuFeS2. Compared to CFe, larger potential plateaus at approximately 0.2 and 0.6 V were observed in the CFeCu system during the 5th GCD cycle (Figure 6-10a and b), which indicated an improvement in the electrochemical response of the FBFC due to Cu2+ addition in the catholyte.  In the CFe system, during GCD cycling, the repetitive potential plateaus verify the occurrence of reversible redox reactions on the surface of both electrodes. In addition, the relatively lower current response of the FeII/FeIII redox reaction (in the absence of Cu2+ as discussed above) resulted in short potential plateaus in the GCD curves as evident in Figure 6-10a. This behavior also validates the beneficial effect of Cu2+ addition in the catholyte 125  enhances the FeIII stability (as discussed in section 6.2) and the results are in agreement with the higher current density given by the CFeCu system as shown in Figure 6-9d and Figure 6-5. The formation of intermediate species at the GF–CuFeS2 negative electrode is also possible. The enrichment of the CuFeS2 surface with copper and sulfide sulfur species by the release of iron in the electrolyte under repetitive charging/discharging cycles changed the electrochemical response of the composite electrode. Sulfide species i.e. S22- and S2– formed on the surface of CuFeS2 during cyclic charging/discharging and this is seen by the potential plateau at 0.2 V. Conway et al.[212], also explained the reversible pseudocapacitive character of these species over the surface of FeS2 (pyrite) (reaction 6.8). S22– + 2e–↔ 2S2–       6.8 During the charging process, reduction of CuFeS2 to talnakhite (Cu9Fe8S16), bornite (Cu5FeS4) and/or chalcocite (Cu2S) occurs at the negative electrode (CuFeS2) [32]. These reactions are supported by the oxidation of Fe2+ to Fe3+ in the other half of the FBFC, at the positive electrode. It is also noted that during charging the potential plateaus were higher than the discharge potential for all cycles, which results from the irreversible faradaic reactions taking place on the surface of CuFeS2. This behavior limits the reversible charge transfer processes during repetitive charge/discharge cycles and hence deteriorates the charge storage capability of the system, as observed in the following 300 cycles. For the CFeCu system, due to improvement in the kinetic response of the Fe2+/Fe3+ redox reaction on the GF electrode (in the positive compartment of the FBFC) in the presence of Cu2+, the span of the plateau at ~ 0.25 V was increased during continuous GCD cycling as seen in Figure 6-10b. 126   Figure 6-10 GCD cyclic trends of (a) CFe and (b) CFeCu systems (c) the specific capacity behavior of both systems (d) Trends of coulombic (ηc) and energy (ηE) efficiencies for both systems These plateaus represent the concurrence of reversible redox reactions on both the electrodes and the increase in span indicates the improvement in the discharge capacity as calculated from Equation 6.2. The second plateau in the GCD plots also shifted to a lower cell potential, which is likely due to specific adsorption of copper species on the GF at the positive electrode. This is also corroborated by the low Rct value obtained for the GF-Fe/Cu compared to GF–Fe electrode in the impedance analyses. A steep discharging profile for the CFeCu cell system is observed at higher cell potential (> 0.3 V) with a small potential plateau at 127  approximately 0.40 V. This shorter sloping plateau could be related with the oxidation of product formed in the negative electrode during the charging process. The appearance of this plateau in the repetitive discharge cycles also indicates the reversible pseudocapacitive character of some of the intermediate species. With the increase in GDC cycles, the gradual decrease in the potential below 0.25 V corresponds to an increase in charge storage capacity.  The similarity in the CV and GCD profile of the CFeCu system and relatively extended discharge cycles were linked with the increased stability of Fe3+ ions when Cu2+ was present. In simple words, the Fe3+ ions formed in the presence of Cu2+ ions during charging, would be readily available during discharge and hence could increase the overall specific capacity of the system. Mai et al. [243] also reported an increase in specific capacitance of functionalized porous carbon with the addition of Cu2+ in the electrolyte. They also proposed that the reversible adsorption/desorption of Cu+ species with the carbonyl functional group over porous carbon could be the reason for their observed ultrahigh pseudocapacitance (4700 F g–1).     The anomalous increase of the discharge specific capacity in the CFeCu system beyond 10 GCD cycles was calculated from Equation 6.2 and can be seen in Figure 6-10c. The specific capacity gradually increased to 25.4 mAh g–1 in 390 GCD cycles, which dropped approximately ~5% before reaching a maximum of 26.2 mAh g–1 in the following (up to 500 DCD) cycles. This behavior is thought to be due to the reversible character of the sulfide sulfur species present on CuFeS2 (reaction 5.8). 𝑺𝑺𝒔𝒔𝑺𝑺𝒄𝒄𝒊𝒊𝑺𝑺𝒊𝒊𝒄𝒄 𝑪𝑪𝒂𝒂𝒔𝒔𝒂𝒂𝒄𝒄𝒊𝒊𝒕𝒕𝑪𝑪 (𝒎𝒎𝒎𝒎𝒎𝒎 𝒈𝒈−𝑿𝑿) =  𝒊𝒊𝒅𝒅𝟑𝟑.𝟔𝟔𝒎𝒎(𝑽𝑽−𝒊𝒊𝒅𝒅𝑹𝑹)∫ 𝑽𝑽(𝒕𝒕).𝒅𝒅𝒕𝒕𝒕𝒕𝟐𝟐𝒕𝒕𝑿𝑿   Equation 6-2  Where ‘id’ corresponds to discharge current (A), ‘V’ and ‘R’ are the overall cell potential and total cell resistance (Ω), respectively. 128  The function ‘∫ 𝑉𝑉(𝑡𝑡).𝑑𝑑𝑡𝑡𝑡𝑡2𝑡𝑡1 ’is the area under galvanostatic discharge profile from the start (t1) to the end of discharge (t2) (s), respectively. The faradaic contribution of the Cu2+/Cu+ redox couple in the catholyte could also facilitate the reversible pseudocapacitive response of the CFeCu system. Compared to CFeCu, the CFe system had a lower discharge capacity (13.9 mAh g–1) in the initial 200 GCD cycles with a similar increasing trend. In the following 300 cycles, ~18% gradual decrease in capacity was seen with this system. This fade in capacity was related to the relatively lower stability of FeIII species (within the positive electrode compartment) and its limited support for the S22–/S2– (on the negative CuFeS2 electrode surface) reversible redox reaction in the cell. The existence of large polarization (relatively steep profile) and contraction in the discharging curves confirmed this behavior. In addition, the refractory nature of CuFeS2, its passivation, and low electrical conductivity could effectively hinder the pseudocapacitive response that may ultimately deteriorate the performance of the designed cell systems.  Figure 6-11 represents the variation in specific discharge energy during 500 GCD cycles. The specific energy of both systems was calculated from Equation 6.3 in which each parameter has its usual meanings. The monotonic increase in the specific energy (3.29 Wh kg−1) of the CFe for the initial 150 cycles was followed by a ~30 % gradual decrease over 500 cycles. The CFeCu system had a continuous increase in specific energy over 200 cycles and plateaued (3.60 ±0.05 Wh kg−1) for the successive 390 cycles. However, the atypical behavior of a sudden decrease (~10 %) and then increase of specific energy in the last 100 cycles could be related to the surface limited irreversible faradaic reactions (reactions 6.1–6.4 and 6.9–6.10 in the following discussion) at the CuFeS2 particles in the negative composite electrode.  129  𝑺𝑺𝒔𝒔𝑺𝑺𝒄𝒄𝒊𝒊𝑺𝑺𝒊𝒊𝒄𝒄 𝑬𝑬𝒎𝒎𝑺𝑺𝑬𝑬𝒈𝒈𝑪𝑪 (𝑾𝑾𝒎𝒎 𝒌𝒌𝒈𝒈−𝑿𝑿) =  𝑿𝑿𝟑𝟑.𝟔𝟔𝒎𝒎 ∫ 𝜟𝜟𝑽𝑽(𝒕𝒕). 𝒊𝒊𝒅𝒅𝒕𝒕𝟐𝟐𝒕𝒕𝑿𝑿 .𝒅𝒅𝒕𝒕  Equation 6-3 The high ηC (~90%) obtained for both systems indicated the maximum utilization of charge in the pseudocapacitive faradaic reactions during charging and discharging of the system. Whereas, the low ηE (~30%) as presented in Figure 6-10d, suggested a large loss in specific energy during the discharge process. This loss in supplied energy was related with the cell internal resistance, IR drop and more importantly to the significant polarization effects seen in Figure 6-10a and b. In other words, the energy supplied to the systems during the charging process was utilized in the irreversible reactions specifically associated with CuFeS2 reduction into talnakhite, bornite and/or chalcocite, which might not be available during the discharge process (for the oxidation of the CuFeS2 and any reaction products). To understand this behavior, the CuFeS2 retrieved from the negative electrode of the CFeCu cell (after 500 GCD cycles) was compared with the as-synthesized CuFeS2.  Figure 6-11 Specific energy trends of CFe and CFeCu system (Note: Charging and discharging was carried out at 200 mA g–1 and 150 mA g–1, respectively) 130  6.4 Ex–Situ characterization of CuFeS2 SEM, EDX, XRD, and XPS analyzed the surface morphology, composition, phase transformation and surface chemistry of the CuFeS2 samples. The surface morphology of the retrieved CuFeS2 samples was changed after 500 GCD cycles. The formation of a reaction product at the surface of the retrieved CuFeS2 microspherical particles was evident as shown in Figure 6-12a and b. The EDX analysis confirmed the enrichment of copper and sulfur species in the final product after GCD cycling due to the dissolution of Fe from the surface of CuFeS2 particles. The formation of copper and sulfur enriched species and depletion of iron during reduction (herein charging) and oxidation (discharging) of CuFeS2 in the acidic solution has been widely discussed in the literature [87, 91, 244]. An increase in the copper content of the retrieved sample (47.8 vs. 26.3 at. %) was also detected. To probe the cause, ICP-OES was used (Varian 725–ES) to analyze both the anolyte and catholyte after cycling.  A comparison of these results with the as-prepared electrolytes is shown in Figure 6-13. It was found that during the initial 200 cycles, the concentration of Fe and Cu in the anolyte was low (0.61 and 0.03 g l–1, respectively), but that it gradually increased in the remaining 300 cycles. Approximately 2.23 and 0.45 g l–1 of Fe and Cu, respectively, were found in the anolyte (0.2 M H2SO4) after 500 GCD cycles. These values include the Fe and Cu that migrated through the PEM from the catholyte and leached from the CuFeS2. In order to quantify the Cu dissolution from CuFeS2, the loss in mass of CuFeS2 was determined after 500 cycles.  It was determined that ~10.75 % Cu extraction was achieved as shown in the inset of Figure 6-13. The dual functionality of energy storage with an additional benefit of Cu extraction by this system makes it a hybrid for both purposes. The Fe and Cu species in the anolyte could also adsorb or deposit on the composite electrode. It is for this reason that the 131  retrieved CuFeS2 + CB mixture was thoroughly washed with DI water, filtered and dried in air prior to EDX and XRD analyses. Still, the presence of these species on the CuFeS2 particles cannot be neglected and this may have increased the Cu contents in the retrieved product as determined by the EDX analysis (Figure 6-12b).   Figure 6-12 Morphological, compositional and structural changes in the CuFeS2 before and after 500 GCD cycles in CFeCu system, SEM and EDX spectra of (a) as–synthesized, (b) and retrieved CuFeS2 (c) XRD patterns comparison 132  The formation of copper and sulfur enriched species were also confirmed from the XRD patterns as shown in Figure 6-12c. The diffraction peaks originating at 2θ values 21.2º, 31.6º, 32.5º, 33.5º, 47.8º, and 52.7º correspond to the formation of CuS and Cu9S8 phases in the retrieved CuFeS2 samples according to the 06–0464 and 36–0379 reference patterns, respectively. However, the diffraction pattern of the as synthesized sample demonstrated the characteristic peaks of pure CuFeS2 that are indexed in Figure 5-1d, in accordance with the standard reference pattern (37–0471).  Figure 6-13 Fe and Cu species concentration in the anolyte after 500 GCD cycles (ICP-OES analysis), the % Cu extraction and retrieved anolyte and catholyte from CFeCu cell system after 500 cycles are shown as an inset. The survey spectra of the as synthesized and retrieved CuFeS2 were obtained on the binding energy scale. The characteristics peaks for sulfur (S) and copper (Cu) are shown in 133  Figure 6-14a, b, and c, respectively. The nature of the ‘S’ species present on the surface of CuFeS2 was evaluated from the high-resolution spectra of the S 2p core peak. The deconvolution of the spectra was carried out by applying Shirley integration for background subtraction and the Gaussian-Lorentzian (80 %:20 %) function was used to split the spin-orbital core S 2p3/2 peak by using Peak 4.1 software. The deconvolution of the core S 2p peak provided the doublet S 2p3/2 peak at 161.41 and 162.44 eV, as shown in Figure 5.15a.  Figure 6-14 Deconvoluted X-ray photoelectron spectra of S 2p depicting the split of peaks associated with mono, di, and polysulfide species on the surface of (a) as–synthesized (b) retrieved CuFeS2 samples (CFeCu cell system) (c) doublet peaks of Cu associated with Cu 2p3/2 and Cu 2p1/2 orbital 134  The peak at the lowest binding energy (161.41 eV) validated the existence of monosulfide (S2–) species at the surface of as-synthesized CuFeS2 particles according to the value (161.4 eV) provided in the literature [47].  The second doublet peak at 162.44 eV was assigned to S22–species. The 1.03 eV difference between the doublet peaks was slightly lower than the 1.1 eV reported elsewhere and corresponds to partially coordinated sulfur species at the surface [45, 225]. It is well known that the S22– can form by the dimerization of S2– species via physical relocation of sulfur atoms at the surface of CuFeS2 bonded directly with the copper and/or iron atoms [245]. The relatively broad satellite peak (FWHM 1.53 eV) observed at high binding energy (163.71 eV) corresponds to polysulfide species (Sn2-) that most likely originate from the S 3p – Fe 3d inter-band excitation [47].    Figure 6-14b shows the S 2p core peak for the retrieved CuFeS2 sample. A clear difference in the peak distribution and change in intensity was observed after 500 GCD cycles. In the retrieved CuFeS2 sample, the binding energy for the S 2p3/2 doublet peaks also shifted to higher energy values compared to the as-synthesized CuFeS2. It would be reasonable to speculate from this behavior that the metal-sulfur bonding energies may change due to the formation of iron deficient species at the surface of CuFeS2 during repetitive charge/discharge cycles as confirmed from the EDX and XRD analyses. Although the peak binding energy 161.48 eV (FWHM=1.63) was lower than the energy of the sulfide (S2–) species in CuS (161.6 ±0.1 eV), it was dominant having 1.57 times higher peak area than shown in Figure 6-14a. However, the peak for the S22– disulfide species at 162.8 eV was consistent with the literature and may be associated with covellite (CuS) [246, 247]. In addition, the broad tail peak at high binding energy 163.78 eV (FWHM=3.27) is also consistent with the existence of polysulfide species (Sn2–). The relatively small peak intensity of this polysulfide species was evident, and 135  it may decrease due to the formation of an iron-deficient surface layer [47]. The presence of elemental sulfur (Sº) cannot be confirmed through XPS analysis due to its volatile nature in ultra-high vacuum. Compared to as–synthesized CuFeS2, the relatively higher binding energies and larger difference (1.33 eV) within the S 2p3/2 doublet peaks also suggest the formation of a metal-deficient structure surface film after GCD cycling at the surface of the CuFeS2 samples [41]. The XPS analysis of as-synthesized CuFeS2 had core peaks at 932.24 eV and 952.14 eV, which are associated with the Cu 2p3/2 and Cu 2p1/2, respectively. However, the small variation in the binding energy after GCD cycling (retrieved sample) and the presence of a small satellite band at around 942.5 eV, is likely due to the presence of divalent copper (Cu2+) species within the surface layer [48] as shown in Figure 6-14c. However, the decrease in binding (0.21 eV) energy of Cu 2p3/2 validated the formation of a CuS phase within the surface layer according to the binding energies (932.0 eV and 951.8) recorded by Nakai et al. [247]. In agreement with the EDX and XRD results, and based on the binding energy shift for both S 2p and Cu 2p species, these data are consistent with the formation of iron deficient CuS and/or Cu9S8 phases at the surface of CuFeS2.  This information is used to predict a reaction sequence that is proposed in the next section. 6.5 Proposed reaction sequence during GCD process The reactions that may proceed during charging and discharge cycles are graphically shown in Figure 6-15. Non–capacitive irreversible faradaic reactions over the CuFeS2 particles in the negative electrode during repetitive cycling significantly decrease the extractable energy and hence adversely affect the energy storage efficiency. One of the possible reasons for the low energy efficiency is that approximately 11 % of the Cu in the CuFeS2 was dissolved.  136  However, this is also a significant benefit to the proposed system as this Cu can be recovered hydrometallurgically for profit. The continuous increase in the specific capacity during repetitive charging and discharging cycles with high coulombic efficiency (~90%) is associated with surface limited pseudocapacitive reversible reactions (reactions 6.7 and 6.8). During initial charging cycles, the CuFeS2 is expected to transform into intermediate species i.e. talnakhite (Cu9Fe8S16) and bornite (Cu5FeS4) before converting into chalcocite Cu2S (reaction 6.2) with the generation of Fe2+ and H2S(aq) species according to reactions 9 and 10, respectively, as pointed out by Biegler et al. [93] and Sauber et al. [248], and as shown schematically in Figure 6-15a. 9CuFeS2 + 4H+ + 2e− → Cu9Fe8S16 + 2H2S + Fe2+   6.9 5CuFeS2 + 12 H+ + 4e-→ Cu5FeS4 + 6H2S + 4Fe2+    6.10  Figure 6-15 Schematic diagram showing the proposed reactions sequence on the negative electrode in the FBFC system during (a) charging and (b) discharging process These non–capacitive irreversible conversion reactions likely consume a large amount of supplied energy during the initial charging cycles, which is consistent with the low ηE. It is 137  proposed that during discharging, any unreacted CuFeS2 is oxidized into metal depleted sulfide Cu1–xFe1–yS2–z (reaction 1) or can convert into CuS. In the repetitive charging and discharging cycles the reduction of CuFeS2 and oxidation of Cu2S into a series of products i.e. djurleite (Cu1.92S), digenite (Cu1.60S) and CuS, is also possible (reaction 5) [232, 246] (Figure 6-15b).                   The possible H2S(aq) generation during the charging process can oxidize into either elemental sulfur (Sº) or to the S22– species by a non–oxidative dissolution mechanism (reaction 6.6 and 6.11, respectively) [24, 246]. These reactions were found to be reversible and enhanced the pseudocapacitive faradaic response as evident from the increased specific capacity and energy of the CFeCu system. Furthermore, the reversible transformation of S22– into S2– (reaction 6.8) during cyclic GCD is also possible as discussed above.   2H2S(aq) = 4H+ + S22– +2e–       6.11 During continuous GCD cycling of the CFeCu cell setup, the generation of FeII and CuII species by the dissolution of CuFeS2 in the negative electrode (or that diffused from the positive half of the cell) have been verified through ICP analysis. These species are beneficial to enhance the overall specific capacity and energy of the system. These species may react reversibly with the surface products formed at the CuFeS2/electrolyte interface, e.g. CuS, Sº and/or H2S(aq) according to reactions 6.6, 6.12, 6.13 and 6.14, respectively [249, 250]. CuS + Cu2+ + 2e– = Cu2S       6.12 Sº + Cu2+ + 2e– = CuS       6.13 H2S + 2Cu2+ + 2e– = Cu2S + 2H+      6.14 The literature also indicates that the addition of Cu2+ ions to the acidic solution could facilitate the reduction of CuFeS2 into Cu5FeS4 and/or Cu2S (reactions 6.15 and 6.16) [92]. 2CuFeS2 + 3Cu2+ + 4e- = Cu5FeS4 + Fe2+    6.15 138  CuFeS2 + 3Cu2+ + 4e- = 2Cu2S + Fe2+    6.16 The formation of iron deficient species at the surface of CuFeS2, and their reversible character (reactions 6.8, 6.12–6.14) has augmented the cyclic performance of the proposed system as evident from the continuous increase in specific capacity and energy during repetitive GCD cycling. However, the large amount of energy supplied during charging was utilized in the conversion reactions e.g. Cu extraction, which adversely affected the energy storage capacity and efficiency of the proposed systems. In other words, the non–capacitive irreversible faradaic reactions e.g. 10.75 % dissolution of Cu2+ from CuFeS2 would consume energy during charge/discharge cycles. Although, the maximum specific energy provided by this system is low compared to the commercially available energy storage systems, the Cu2+ recovery from CuFeS2 provides an additional benefit that may be monetized.  A very low pH is required for reduction reactions (reactions 6.9 and 6.10) to move in the forward direction during the charging process. In order to estimate the in–situ pH change in the CuFeS2 slurry and to support the proposed mechanism, we also performed a few experiments in the two-electrode setup as shown in the inset of Figure 6-16a. The CuFeS2 (80 wt. %) + CB (20 wt. %) slurry was thoroughly mixed and a 0.2 M H2SO4 solution was added to make a homogeneous slurry containing 20 wt. % solids. The measurements were made without stirring and the pH probe and counter electrodes were submerged in the semi-solid bed. The pH probe and the counter electrode were manually adjusted to an approximate 3 mm distance from the working electrode surface. The in-situ pH of the slurry at constant potentials (–1.0 V and +1.0 V) and the transient current were measured at different intervals as shown in Figure 6-16a and b, respectively.  139  The initial pH of the slurry was measured to be 0.72 at OCP. The OCP was 0 V in this case since both the counter and working electrode was graphite.  During the change in cell potential (to –1.0 V), the pH of the slurry suddenly dropped to almost 0 indicating the abrupt migration of H+ toward the negatively charged slurry particles. Under these conditions, it is possible that H+ could reduce the CuFeS2 into intermediate species i.e. talnakhite (Cu9Fe8S16) and bornite (Cu5FeS4) before converting these into chalcocite Cu2S as indicated in reactions 6.2, 6.9 and 6.10. The current was also found to increase (became less negative) due to the growth of a diffusion layer within the slurry, which approached a limiting current (Figure 6-16b). Under these transient conditions (charging process), the pH of the slurry increased to ~0.55 in 900 seconds.  It was interesting to see that the pH of the slurry shifted back toward the initial pH (~ 0.72) when the cell potential was reset to OCP. Following the charging process, the potential was shifted to +1.0 V (discharge) to evaluate the change in pH in this case. The current increased rapidly to a maximum of 35 mA and exponentially decayed before reaching a constant value (3.1 mA). This behavior corresponds to the oxidation of the CuFeS2 and any intermediate species formed during the preceding charging process. The decrease in current and relatively low constant current also suggest that the decrease in dissolution rate is due to the formation of an Fe deficient and sulfide enriched surface film. The pH variation followed a similar trend to the current, and pH approached 1.96 upon sudden discharge attributed to the migration effect of cationic species in the opposite direction. The pH decayed to 0.94 during the initial 300 seconds and approached an almost constant value (0.89) in the next 600 seconds, which is most likely associated with the generation of oxidized species (FeII and Cu2+ etc.) by the partial dissolution of CuFeS2. 140   Figure 6-16 Potentiostatic polarization of CuFeS2+ CB slurry (a) Variation in pH during charge (cell potential = –1.0 V) and discharge cycles (cell potential = 1.0), (b) Change in current profile (Note: only CuFeS2 slurry in 0.2 M H2SO4 was used for pH measurements) 6.6 Summary  The use of synthetic CuFeS2 as the negative composite electrode material in a laboratory designed FBFC system was presented in this chapter. This hybrid system has the dual capability of storing energy and extracting Cu from the CuFeS2. The reduction of CuFeS2 to Cu2S (charge cycle) and oxidation of the reduced species i.e. Cu2S, or any unreacted CuFeS2, to CuS/Cu2+ (during discharge), was supported by the FeII/FeIII redox reaction in this hybrid system. The main conclusions of this study were:   141   The addition of 0.1 M Cu2+ in the catholyte increased the current response of the FeII/FeIII redox reaction, which was shown to be due to the increased stability of the FeIII ionic species and catalytic behavior of Cu2+ towards FeII oxidation over the GF electrode. This increased the specific current density of the CFeCu system during charging and discharging processes as predicted form the CV scans at 0.001 V s–1.  The specific capacity of the CFeCu systems increased continuously to 26.2 mAh g–1 during 500 GCD cycles having a ƞC over ~90%. The CFe system, on the other hand, provided a relatively low capacity of 13.9 mAh g–1 in the initial 150 cycles followed by an 18% gradual decrease in the next 350 cycles.  The energy storage capability of the CFeCu increased gradually to 3.60±0.05 from 1.2 Wh kg–1 in 500 GCD cycles but the energy efficiency remained low at 30 %. However, in the CFe system, the specific energy reached a maximum of 3.29 Wh kg–1 in the initial 150 cycles and decreased 30% in the successive 350 cycles.  The cause of limited energy storage and low energy efficiency in this system (CFeCu) was also identified from the ex-situ analysis of the retrieved product. The energy consumed during charging of the CFeCu system was due to the progress of irreversible conversion reactions on the CuFeS2 surface such as the dissolution of Cu from CuFeS2. While these reactions limit the energy storage of this system, they provide a source of copper that could be readily recovered.   142  Chapter 7: Use of a CuFeS2 mineral concentrate in the FBFC10  In this chapter, we are reporting the use of a naturally sourced CuFeS2 mineral concentrate as a negative electrode material in a hybrid setup, which is capable of storing energy. The additional benefit of this setup is the Cu extraction from CuFeS2 that occurs during repetitive charging/discharging cycles. Thus, electrode degradation, which is normally undesirable in batteries, is an important and desirable feature in the present case.  Indeed, the extraction of Cu in such a battery would result in its beneficial recovery.  The FeII/FeIII couple is used as the supporting reaction at the positive electrode. In addition, the kinetic behavior of the electrochemical reactions taking place on each electrode is also investigated. In this chapter, the energy storage and Cu extraction capabilities of both synthetic CuFeS2 and the mineral concentrate in the FBFC setups are measured and compared. The analysis of the retrieved CuFeS2 (post-testing) is carried out to further understand the reason for the observed increase in specific capacity during repetitive charging/discharging processes. 7.1 Characterization of synthetic and CuFeS2 mineral concentrate The spherically shaped CuFeS2 particles produced in the laboratory via hydrothermal process consisted of platelets, which seemed to be connected at the core of sphere, similar to a pompom morphology. These particles have open pores at the surface which may develop during synthesis (similar to what is shown in Figure 5-1a) when it is believed that thiourea (SC(NH2)2) reduces Cu(II) and the following reaction (reaction 7.1) at high temperature and pressure occurs [251].  CuCl2 + FeCl2 + 2SC(NH2)2 + 2H2O → CuFeS2 + 2OC(NH2)2 + 4HCl  7.1                                                  10 From published work, K.M. Deen, E. Asselin, Electrochimica Acta 297 (2019) 1079–1093. 143  Cu, Fe and S were detected by EDX and their molar ratio (Cu1.08FeS1.83) was close to the stoichiometric composition of chalcopyrite (Figure 7-1). The signals for C and O in the EDX spectrum during analysis possibly arise from the sputtered carbon and from the air oxidation of synthetic CuFeS2, respectively, and can be neglected.  The XRD pattern of the synthetic CuFeS2 powder sample is shown in Figure 5-1d. The pattern confirmed the formation of CuFeS2 that perfectly matched with the PDF # 37–0471 reference pattern. No impurity phases were detected. The diffraction peaks observed at 29.36º, 34.01º, 48.71º, 49.27º, 58.11º, 58.75º and 79.66º are from the (112), (200), (220), (204), (312), (116) and (316) planes, respectively. This is consistent with the typical tetragonal crystal structure of CuFeS2 belonging to the (I – 42d (122)) space group.  The particle size distribution curves of both synthetic CuFeS2 and MC samples were also obtained by the laser diffraction method (Malvern Mastersizer 2000) as shown in Figure 7-2. The D80 particle size (34.9 ± 3.0 µm) was determined from the cumulative particle distribution curve. This signifies that 80 vol. % of the particles have a size less than the 34.9 µm. However, the distribution curve also indicates a wide range of particle sizes (from 5 to 60 µm) in the synthetic product. It was also observed that approximately 5 vol. % of the particles were less than 7 µm. Similarly, the particle size distribution histogram of MC (Figure 7-2b) indicates the presence of particles ranging from < 1 µm to ~100 µm. From the cumulative volume % distribution curve, the D50 and D80 were determined to be 19.5 ± 2.0 and 48.4 ± 3.0 µm, respectively. 144   Figure 7-1 The EDX spectrum of the synthetic CuFeS2  Figure 7-2 Particle size distribution histograms of (a) synthetic CuFeS2 and (b) MC 145  The morphology and XRD pattern of the MC is shown in Figure 7-3. The irregularly shaped particles of the MC can be seen in the SEM image. The variable size and shape particles were generated from the mineral processing operations i.e. crushing, grinding and separation procedures (froth flotation), which were carried out at the mine site to enrich the CuFeS2 mineral content in the ore extracted from mine. Analysis of the XRD pattern indicates the presence of several other phases in the MC. The diffraction peaks at 2θ: 29.3º, 49.3º, and 58.1º are associated with CuFeS2 as confirmed from the standard reference pattern (PDF # 96–901–5235). However, additional peaks indicate the existence of other phases i.e. pyrite (96–901–5843), covellite (96–900–8371) and quartz (96–901–0145) as evident in Figure 7-3. Peaks at 2θ < 20º are also present and they are possibly related to silicate compounds. To further verify the composition of MC, quantitative X-Ray Powder Diffraction (QXRPD) analysis was also carried out. The quantity of all the crystalline phases present in the MC was obtained via Rietveld refinement of the XRD data using Topas 4.2 (Bruker AXS). The MC consists of pyrite (43.3 wt %), covellite (16.6 wt %) and chalcopyrite (26.6 wt %). Other minor impurity phases were quartz (4.0 wt %), illite–muscovite 2M1 (3.3 wt %), brochantite (2.2 wt %), enargite (1.6 wt %), pyrophyllite (1.2 wt %), kaolinite (0.9 wt %) and molybdenite (0.3 wt %).  QXRPD, as practiced here, has a standard error of approximately 2 wt % in phase analysis, meaning that the identification of low concentration impurity phases is subject to significant error and should be considered indicative at best.               146   Figure 7-3 XRD pattern of the mineral concentrate (MC), the inset shows an SEM image of the MC particles 7.2 Electrode Kinetics To simulate the electrochemical behavior of each composite (GF–CuFeS2, GF–MC) and GF–Fe/Cu electrode in the FBFC setup, the kinetic response of each electrode was analyzed through CV scans. In the case of the composite (negative) electrodes, the CV scans were initiated in the cathodic direction from their open circuit potentials (OCP) to –1.5 V (vs. SHE) and reversed to 1.8 V (vs. SHE). In both GF–CuFeS2 and GF–MC electrodes, reproducible cathodic and anodic peaks were observed, indicating the occurrence of redox 147  reactions (pseudocapacitive responses) at the electrode surface. The rapid increase in the current with an increase in potential beyond (< –0.5 V and > 1.2 V (vs. SHE)) is due to the dissociation of the electrolyte. Binary peaks, designated as P1c, P2c, (cathodic) and P1a, P2a, (anodic) are shown in Figure 7-4a and b. The redox potential (𝐸𝐸𝑝𝑝1 =  𝐸𝐸𝑃𝑃1𝑎𝑎+ 𝐸𝐸𝑃𝑃1𝑐𝑐2 ) of the first anodic and cathodic peaks (designated as P1a and P1c) of both composite electrodes was calculated to be 0.64 ± 0.01 V (vs. SHE). These peaks are associated with the interaction of H+ and HSO4– species with surface functional groups present on the GF electrode. Andreas et. al. [211] also reported that in acidic solutions, the redox peaks (centered at ~0.55 V) in CV scans were associated with the adsorption/desorption of H+ species on the surface functional groups i.e. the Quinone functionalities (Q): Q + 2H+ + 2e– ↔ QH2 of the C cloth. At high sweep rates, the current response is only related to surface reactions, whereas the potential dependent current (heterogeneous current response) associated with the redox and conversion reactions is observed at low sweep rates. At high sweep rates (v) ≥ 0.04 V s–1, the surface limited adsorption/desorption processes should obey the Langmuir or Henry isotherms. In this case, the peak current varies linearly with v according to Equation 7.1 as evaluated by Laviron [252]. 𝑰𝑰𝒔𝒔𝑿𝑿 = 𝑿𝑿.𝟐𝟐𝟐𝟐 𝒎𝒎 (𝒎𝒎𝒎𝒎)𝟐𝟐(𝑹𝑹𝑹𝑹)−𝑿𝑿𝜞𝜞𝑹𝑹𝒗𝒗      Equation 7-1 Where, Ip1 is the peak current for either Pa1 or Pc1, A is the surface area of the electrodes, n is the number of electrons involved during the adsorption/desorption process, F is Faraday’s constant, R is the universal gas constant and T is the temperature (298 K). The ΓT in the above relation is the total surface concentration of the electroactive adsorbed species.  At 0.1 V s–1 > v ≥ 0.04 V s–1, the large peak current associated with the P1a and P1c for both GF–CuFeS2 and GF–MC electrodes varied linearly with v, as shown in Figure 7-5. This behavior confirms the occurrence of surface limited reversible adsorption/desorption 148  processes particularly occurring on the GF electrode. It can be seen that the peak potentials were sweep rate dependent and with an increase in sweep rate, the potentials of both cathodic and anodic peaks were shifted to more negative and positive values, respectively. The shift in potential could originate either from quasi–reversible or electrochemically irreversible electrochemical processes on the electrode surface [253, 254]. Also, the potential drop (due to the increase in solution resistance within the porous structure by the development of a concentration gradient across the electrode/electrolyte interface) may influence the peak separation (ΔEp). To circumvent this issue, the electrolyte was continuously stirred and the positive feedback IR compensation mode was applied in the potentiostat during testing. In this case, the ΔEp (ΔE1 = Ep1a – Ep1c or ΔE2 = Ep2a – Ep2c) as a function of v can be used to predict the kinetics of electrochemical reactions according to the Laviron method described elsewhere [252, 255].  At the lowest sweep rate of 0.001 V s–1, the peak separation ΔE1 and ΔE2 for the GF–CuFeS2 electrode was negligible: 0.038 V and 0.044 V, as shown in Figure 7-4c. This is possibly related to the reversibility of H+ adsorption/desorption on the surface of GF and CuFeS2 particles [211]. Similarly, in the case of the GF–MC electrode, ΔE1 and ΔE2 were significantly lower (0.053 and 0.046 V, respectively) than the ΔEp at high v, as shown in Figure 7-4c. Electrochemical reactions that are kinetically facile (Nernstian), as compared to mass transport (dimensionless rate constant (Λ) > 15), are designated as fully reversible. However, based on electron and mass transport characteristics, moderate (15 > Λ > 10–3) and very slow reactions (Λ ≤ 10–3) are considered quasi-reversible and electrochemically irreversible reactions, respectively [256]. Furthermore, from the CV analysis for a Nernstian reaction (completely reversible), the ΔEp should be 2.18�𝑅𝑅𝑅𝑅 𝑛𝑛𝑛𝑛� � = ~57 𝑚𝑚𝑉𝑉 at low sweep rate for a 149  one electron transfer process [257]. These comparable values of peak potential shifts indicate the reversibility of the electrochemical processes (at 0.001 V s–1) progressing on the surface of the composite electrodes. However, the significant increase in the peak separation at higher sweep rates (0.001 < v ≤ 0.02 V s–1) indicates the heterogeneous quasi–reversible and/or irreversible nature of the electrochemical reactions on the electrodes.  To diagnose the nature of the electrochemical processes, further analysis of the peak current (Ip) vs. v1/2 was carried out. This relationship is linear for the GF–CuFeS2 and GF–MC electrodes, as shown in Figure 7-6a and c, respectively. The porous electrodes have an accelerated kinetic response leading to diffusion-controlled processes at (v ≤ 0.02 V s–1). In this particular case, the heterogeneous current response is expressed by the Randle–Sevcik relation (Equation 7.2) [258].  𝒊𝒊 = ± 𝒎𝒎𝒎𝒎𝒎𝒎𝑪𝑪𝒐𝒐∗  (�𝑫𝑫𝒗𝒗𝑫𝑫𝑫𝑫𝒎𝒎𝒎𝒎𝑹𝑹𝑹𝑹  ).𝒙𝒙(𝒃𝒃𝒕𝒕)      Equation 7-2 Where π1/2x(bt) is the current function and it approaches 0.496 at the current peak (i = ip), 𝐶𝐶𝑜𝑜∗ is the concentration of ionic species in the bulk solution, D and α are the diffusion and charge transfer coefficients, respectively. The other parameters in Equation 7.2 have the usual meaning as mentioned above [258].  150   Figure 7-4 Cyclic voltammograms of composite (a) GF–CuFeS2, (b) GF–MC electrodes obtained at various sweep rates. (c) log (v) vs. ΔE1, 2 (peak separation) trends for the composite electrodes. (d) CV of GF–Fe/Cu electrode at different sweep rates (For composite electrodes, the current is normalized by the weight of GF + CuFeS2 / MC, whereas for the GF–Fe/Cu electrode, the weight of GF was used to calculate the specific current). 151   Figure 7-5 Peak current (for Pa1 and Pc1 only) variation vs. sweep rate (v) (0.04 ≥ v <0.1 V s–1)  Figure 7-6 From the CV scans (0.001 ≤ v ≤ 0.02 V s–1) and Equation 6.2, ip vs. v1/2 of (a) GF–CuFeS2 (b) Ep vs. log (v) of GF–CuFeS2 electrode (c) ip vs. v1/2 and (d) Ep vs. log (v) trends for the GF–MC electrode used to calculate charge transfer coefficient, α, values 152  The heterogeneous rate constant (kh) of the redox reactions strongly depends on the D of the ionic species and the symmetry of the energy barrier, which is represented as α. Therefore, to determine D from Equation 7.2, it is important to first calculate the value of α. The Laviron method was used and the trends between log (v) vs. peak potentials are shown in Figure 7-6b and d.  The slopes indicated on the asymptotes were used to calculate the α values according to Equation 7.3 [255, 256, 258, 259].  𝑺𝑺𝑺𝑺𝒐𝒐𝒔𝒔𝑺𝑺 =  − 𝟐𝟐.𝟑𝟑𝑿𝑿𝟑𝟑𝑹𝑹𝑹𝑹𝑫𝑫𝒎𝒎𝒎𝒎 (𝑪𝑪𝒂𝒂𝒕𝒕𝒎𝒎𝒐𝒐𝒅𝒅𝒊𝒊𝒄𝒄) = 𝟐𝟐.𝟑𝟑𝑿𝑿𝟑𝟑𝑹𝑹𝑹𝑹(𝑿𝑿−𝑫𝑫)𝒎𝒎𝒎𝒎 (𝒎𝒎𝒎𝒎𝒐𝒐𝒅𝒅𝒊𝒊𝒄𝒄)  Equation 7-3 It is calculated that for both peaks (P1a, P1c, and P2a, P2c), the α values are comparable for both GF–CuFeS2 and GF–MC electrodes, as given in Table 7-1. Furthermore, α associated with the P1 of both the electrodes is approximately 0.5, which validates the one-electron transfer process. From the slope (ip vs. v–1/2) for peak 1 (P1) as shown in Figure 7-6a and c (Equation 7.2), the D of H+ species within the GF–CuFeS2 and GF–MC electrodes was 1.53 x 10–6 and 1.37 x 10–6 cm2 s–1, respectively. These results also support that P1 is associated with the GF according to reaction 7.2. From the analysis of P2, α = 0.65 and 0.24, for the GF–CuFeS2 and GF–MC, respectively. These charge transfer coefficient values are believed to be due to the complex two electron transfer process occurring within these composite electrodes [258]. The redox potential (Ep2) of the peaks P2a and P2c in both GF–CuFeS2 and GF–MC was 0.45 ± 0.02 V (vs. SHE) (Figure 7-4a and b) and these peaks are due to the pseudocapacitive response of the sulfide sulfur species present on the surface of CuFeS2 or other sulfide minerals in MC (for example FeS2) [212, 260]. In other words, the reproducible P2a and P2c peaks in the CV scan correspond to the redox reaction between S22– and S2– (reaction 7.3) species. Both are expected to be present on the surface of CuFeS2 particles and/or on the surface of other sulfide minerals in the MC.  153  On the surface of GF (P1) xC + H+ + e– = Cx–1 + CHºads      7.2  On the surface of CuFeS2 or other sulfide minerals in the MC (P2) S22– + 2e– = 2S2–       7.3 Similarly, for the positive electrode (GF–Fe/Cu), the redox potentials EpFe (FeII/FeIII) and EpCu (CuII/CuI) were 0.7±0.05 V and 0.15±0.05 V (vs. SHE), respectively, as evident in Figure 7-4d. By using Equation 7.2 and Equation 7.3 for the peaks associated with FeII and CuII, the kinetic parameters of the GF–Fe/Cu electrode calculated are given in Table 7-1. A one order of magnitude higher D (9.45 x 10–5 cm2 s–1) for Cu2+ compared to Fe2+ (3.07 x 10–6 cm2 s–1) was determined, which may effectively improve the kinetic response of the GF–Fe/Cu electrode according to reaction 7.4 as mentioned elsewhere [196]. Comparable values for the FeII/FeIII redox potential (0.68 V (vs. SHE)) and D for FeII (5.6 x 10–6 cm2 s–1) on porous carbon nanotube electrode systems have also been reported by Friedl et al. [261]. Fe(II) + Cu2+ = Fe(III) + Cu+ads     7.4 The heterogeneous rate constant (kh) at 25 ºC was also calculated from the intercept of the linear trends (log ip vs. (Ep–Epx)) as shown in Figure 7-7 according to Equation 7.4 [258]. 𝒊𝒊𝒔𝒔 = 𝑿𝑿.𝟐𝟐𝟐𝟐𝟐𝟐 𝒎𝒎𝒎𝒎𝑪𝑪𝒐𝒐∗ 𝒌𝒌𝒎𝒎𝑺𝑺−(𝑫𝑫𝒎𝒎𝒎𝒎 �𝑬𝑬𝒔𝒔− 𝑬𝑬𝒔𝒔𝒙𝒙� 𝑹𝑹𝑹𝑹� )       Equation 7-4 Where Ep is the peak potential and Epx is the redox potential of the peaks (obtained from the average of the anodic and cathodic peak potentials). The corresponding values of α for each electrode (given in Table 7-1) are calculated from Equation 7.3 (discussed above). kh for reaction 7.2 was sensitive to the composition of the electrode. For instance, for the GF–CuFeS2 electrode, kh (4.1 x 10–4 cm s–1) was almost 1.5 times higher than on GF–MC. This behavior 154  reflects the combined effect of GF, CuFeS2 and/or MC on the kinetics of the H+ adsorption/desorption process.  The relatively larger faradaic response of the composite electrode than that of the GF-only electrode has previously been discussed in detail [225]. It is proposed that the preferential adsorption of H+ on GF and its transport to CuFeS2 in the composite electrode could significantly improve its pseudocapacitive response. For the GF–MC electrode, the large number of impurity phases may influence this process as seen from the relatively small value of kh. On the other hand, the kh of the S22–/S2– reaction (Peak P2) on GF–CuFeS2 (3.6 x 10–4 cm s−1) was almost double the kh value calculated for the GF–MC (1.4 x 10–4 cm s–1) electrode. Thus, other impurity phases in the MC may restrict the electrochemical reversibility of this reaction in the GF–MC electrode.  The kinetics of the FeII/FeIII redox reaction (kh = 4.1 x 10–5 cm s–1) on the GF electrode was two orders of magnitude lower than for the CuII/CuI redox reaction (kh = 1.1 x 10–3 cm s–1), which further validates the well-known catalytic behavior of CuII species in this catholyte [196, 238, 262]. Furthermore, the calculated kh values (> 2 x 10–5 cm s–1) for each reaction were consistent with quasi-reversible reactions [258].  Another criterion proposed by Matsuda et. al. [263] was used to differentiate the nature of the electrochemical reaction according to the following relation (Equation 7.5).  𝜦𝜦 =  𝒌𝒌𝒎𝒎�𝒎𝒎𝑫𝑫𝒗𝒗𝑹𝑹𝑹𝑹         Equation 7-5 Where, Λ is a dimensionless rate constant parameter, and Λ ≥ 15, 15 > Λ > 10–3, and Λ ≤ 10–3 correspond to reversible, quasi–reversible and irreversible electrochemical reactions, respectively. Based on the experimental results and kinetic parameters, the ‘Λ’ values at 0.001 155  V s–1 sweep rate are given in Table 7-1. According to this criterion, the corresponding Λ values of each reaction further confirm the quasi-reversible nature of the electrochemical processes on GF–CuFeS2, GF–MC and GF–Fe/Cu electrodes.   Table 7-1 Parameters calculated from the CV scans obtained at various sweep rates at 25 ºC (Note: Geometrical surface area of the GF (14.4 cm2) was used in the calculation)   Electrode (s)  Species α D (cm2 s–1) kh (cm s–1) Λ* GF – CuFeS2 P1 H+ 0.47 1.53 x 10–6 4.1 x 10–4 1.68 P2 – 0.65 – 3.6 x 10–4 – GF – MC P1 H+ 0.52 1.37 x 10–6 2.6 x 10–4 1.13 P2 – 0.24 – 1.4 x 10–4 – GF – Fe/Cu – Fe2+ 0.24 3.07 x 10–6 4.1 x 10–5 0.12 – Cu2+ 0.24 9.45 x 10–5 1.1 x 10–3 0.57  156   Figure 7-7 The Ep – Epx vs. log (ip) trends for the calculation of kh (Equation 7.4) from the peak shift and peak current variation in the CV scans of (a) GF–CuFeS2 and GF–MC (b) GF–Fe/Cu electrodes, where I represents the intercept on the y-axis after linear fitting of data 7.3 Estimating the energy storage capability of CuFeS2 and MS in the FBFC Figure 7-8 represents the cyclic voltammetry (CV) trends of the GF – CuFeS2/GF and GF – MC/GF FBFC setups, designated as C–1 and C–2, respectively. The CV scans were obtained at various v (from 0.001 to 0.02 V s–1). An increase in the peak current with an 157  increase in v was typically observed, which is normally associated with the rapid migration of electroactive ionic species towards the electrode surface. The anodic peak (pa) during charging indicates the oxidation of FeII in the positive half, which is driven by the reduction reactions (reactions 7.2 and 7.3) in the negative compartment of the FBFC. The reverse of these reactions during the discharge process is also reflected by the peak (pc) in the CV scans, as shown in Figure 7-8a and b. The relatively small anodic and cathodic peaks designated as a and c, respectively, are due to the oxidation of adsorbed Cu+ads species on the positive (GF) electrode (reaction 7.4).  On the negative (composite) electrode, the reduction of surface functional groups on the GF and/or sulfide sulfur species existing on the surface of CuFeS2 and MC particles is also possible. It has been shown (section 7.2) that the peak potentials are v-dependent and a large separation between pa and pc potentials with an increase in v suggests the occurrence of quasi-reversible reactions on both electrodes in the FBFC. The slight decrease in anodic current density beyond pa is either related to scarcity of the electroactive species (i.e. FeII in the electrolyte and/or CuIads) on the GF surface (positive electrode) or to the quasi-reversible reactions on the composite (negative) electrode. This behavior can also be anticipated from the direct relation of both anodic (ipa) and cathodic currents (ipc) with (v1/2), as shown in Figure 7-8c. The linear trends suggest the progress of diffusion-controlled processes within the porous structure of both C–1 and C–2. Since electrolytes were circulated separately at a fixed flow rate (7.5 ml/min) in both compartments in a closed-loop, and since ionic species were readily available on the surface of both electrodes, it is likely that the delay in current response is related to the kinetics of the electrochemical reactions in these cell systems. The larger anodic (50.3 mA V–1/2 s1/2) and cathodic (– 35.7 mA V–1/2 s1/2) slopes (Figure 5c) for C–2 suggest that 158  kinetics of the electrochemical processes on the MC are faster than on synthetic CuFeS2. The presence of a large amount of FeS2 (approx. 43 %) in the MC could effectively improve the overall current response of C–1 due to the progress of reversible reactions on its surface (reaction 7.3, 7.5 and 7.6) as discussed in the literature [212, 264].  FeS2 + H+ + e– = FeS2H       7.5  FeS2 + H2O = FeS2OH2+ + e–      7.6    Figure 7-8d shows the variation in the peak current ratio (ipa/ipc) and peak separation, ΔEp, as a function of v. At high v (> 0.005 V s–1), the ipa/ipc > 1 of the C – 1 setup is attributed to the charge consumed in the partial conversion of CuFeS2 into Cu2S (non–capacitive irreversible faradaic reaction) during the charging process (reaction 7.7). On the other hand, the ipa/ipc < 1 at low v indicates the oxidation of CuFeS2 and/or products formed during the preceding charging step (i.e. the formation of a metal-deficient sulfide film at the surface of CuFeS2 and/or conversion of Cu2S into CuS (reactions 7.8 and 7.9), respectively, with the possible dissolution of CuII). The large ΔEp at high sweep rates demonstrates the quasi-reversible kinetics of the electrochemical processes in these cells. In simple words, the electrochemical response of the electrodes is delayed during repetitive charge/discharge cycles at higher sweep rates. From this behavior, it is clear that to increase the charge storage capacity and to achieve maximum Cu2+ dissolution, these systems should be charged and discharged at low rates i.e. 200 mA g–1 and 150 mA g–1 (as discussed in section 6.3).   2CuFeS2 + 6H+ + 2e– → Cu2S + 3H2S(aq) + 2Fe2+   7.7 CuFeS2 → Cu1-xFe1-yS2-z + xCu2+ + yFe2+ + zSº + 2(x + y)e– 7.8 Cu2S → CuS + Cu2+ + 2e–       7.9 159   Figure 7-8 CV scans obtained for FBFC at various sweep rates when the negative electrode is (a) GF–CuFeS2 (C–1) and (b) GF–MC (C–2), whereas the positive electrode used was GF. 0.5 M Fe2+ + 0.1 M Cu2+ in 0.2 M H2SO4 was circulated from the external thermostatic reservoirs. (Note: The current is normalized by the weight of either synthetic CuFeS2 or MC) (c) The ip vs. v1/2 trends of the C–1 and C–2 cells (d) variation of ipa/ipc and peak separation due to change in sweep rate During the discharge cycle, the availability of FeIII at the surface of GF and its reductions to FeII is facilitated by the CuII species in the catholyte (reaction 7.4). The catalytic behavior of CuII in the oxidation of FeII and the concomitant improvement in the stability of FeIII species have previously been discussed in Chapter 6 (Figure 6-5) [196]. By comparing the current response of the GF electrode in 0.5 M Fe2+ and in 0.5 M Fe2+ + 0.1 M Cu2+ solution, it is concluded that CuII and CuIads species, formed on the surface of GF during reversible cycling, 160  can enhance the current response of the FeII/FeIII redox reaction (reaction 7.4) by approximately 25 %. These reactions on the positive electrode reversibly supported by the S22–/S2– redox reaction at the negative electrode of C–1 are discussed above (reaction 7.3).  These are called pseudocapacitive faradaic reactions due to their reversible charge transport character. The existence of these species on the surface of as-synthesized CuFeS2 particles has been confirmed with XPS analysis, as discussed in the next section of this Chapter.  It has also been reported that H2S(aq) (which may form during the charge cycle on the negative electrode via reaction 7.7) may either oxidize to elemental sulfur (Sº) or may be reversibly transformed into S22– species at the surface of CuFeS2 according to reactions 7.10 and 7.11, respectively [85, 212, 225, 264]. The electrochemical response of the C–2 setup will depend on the composition of MC and presence of other sulfide minerals besides CuFeS2, such as FeS2. In this case, it was predicted that FeS2 in the MC could also contribute to the charge storage capacity by promoting the occurrence of additional reversible adsorption/desorption reactions (reaction 7.5 and 7.6) on the surface.  H2S(aq) ↔ Sº + 2H+ + 2e–       7.10 2H2S ↔ 4H+ + S22– + 2e–       7.11 However, the partial conversion reactions (faradaic but irreversible) in the negative electrode could definitely decrease the discharge capacity and coulombic efficiency. In simple words, the partial amount of charge consumed (during the charging step) in the conversion reactions would not be available during the discharge process. Based on the CV analyses, the decrease in ΔEp and ipa/ipc < 1 at low sweep rates can also be used to estimate these effects in the C–1 and C–2 FBFC cells during GCD cycling.  161  Figure 7-9 presents the GCD plots for C–1 and C–2 obtained within a 1 V cell potential.  During charging/discharging, the potential profiles reflected similar trends to those seen in the CV scans. The CuFeS2 in the negative composite electrodes, either synthetic or in the MC, are reduced to Cu2S (reaction 7.7) during charge cycles. The partial oxidation of the Cu2S or unreacted CuFeS2 in the discharge cycles dissolves CuII in the anolyte (via reaction 7.8 and 7.9) in addition with the release of stored charge/energy.  At the start of each discharge curve, the rapid fall in the potential can be quantified as idR, which includes the total potential drop across the electrode/electrolyte interface, membrane, and electrical contacts. The potential drop within the porous structure of the fixed bed electrode and quasi-reversible nature of the electrode materials may also account for the large polarization effects during initial GCD cycles. This behavior is seen by the steep discharge profiles and potential plateau at approximately 0.38 V and 0.2 V cell potential in the discharge curves, designated as x and y (Figure 7-9b and d). Due to the surface activation (discussed below) and/or formation of the sulfide sulfur species, the discharge period of both setups was also extended, which indicates the improvement in the kinetic response of the electrode materials upon repetitive GCD cycling. In C–1, the steep GCD profiles indicate activation polarization of the electrode materials as shown in Figure 7-9a and b. The sloping potential plateau at 0.5 V and 0.38 V in the charge and discharge curves, respectively, can be observed at the 250th and 500th GCD cycles. These plateaux represent the occurrence of quasi-reversible charge transfer processes on the electrode materials as confirmed from the CV results discussed above (section 7.2). The quasi–reversible nature of the electrochemical processes associated with these potential plateaux (designated as x and y) also replicate the peaks in the CV scans (Figure 7-8).  162  During continuous GCD cycling, the adsorption/desorption of Cu+ads species would enhance the current response of the FeII/FeIII redox reaction (reaction 7.4) on the positive GF electrode. This may also increase the stability of FeIII ions in the catholyte [196]. This faradaic process in the positive compartment is facilitated by the redox reactions (reactions 7.2, 7.3, 7.5, 7.6, 7.10, 7.11) in the negative compartment of C–1 and C–2. However, besides these quasi–reversible reactions (pseudocapacitive, faradaic reactions), the occurrence of conversion reactions (reactions 7.7 – 7.9) cannot be neglected. These reactions are non–capacitive irreversible faradaic reactions. Below ~ 0.2 V cell potential, the descending potential plateau (y) in the discharge curves corresponds to the reversible character of the sulfide sulfur species (reaction 7.3). The reversible faradaic response of the surface species (S2–, S22–, Sn2– etc.) formed on CuFeS2 (in C–1) and/or other sulfide minerals (in the MC, C–2) during GCD cycling is reflected by the potential plateau (y). I It was observed that in C – 1, this potential plateau in the 500th discharge curve was almost 7 times larger than the plateau in the 1st cycle (Figure 7-9b). On the other hand, the ‘y’ potential plateau was also extended (~ 3 times) after 400 GCD cycles in case of C–2, which is evident in Figure 7-9c. In contrast to C – 1, the sharp potential plateaux at 0.48 V and 0.38 V in the charge and discharge curves, respectively, were only observed in the 1s GCD cycle of C–2 (Figure 7-9c). Also, the potential plateaux below 0.2 V were also seen in the GCD plots that represent the overall faradaic (pseudocapacitive) response by the CuFeS2 and/or FeS2 in the MC. Interestingly, after 250 GCD cycles of the C–2 setup, an additional potential plateau at ~ 0.3 V also emerged in the discharge curves as shown in Figure 7-9d. This was most likely associated with the kinetic response of the other sulfide phases in the MC, particularly with the FeS2 phase due to reactions 7.5 and 7.6. In both cells (C–1 and C–2), the continuous expansion 163  in the discharge time was directly related with the increase in specific capacity as calculated from Equation 7.6 [196].  𝑺𝑺𝒔𝒔𝑺𝑺𝒄𝒄𝒊𝒊𝑺𝑺𝒊𝒊𝒄𝒄 𝑪𝑪𝒂𝒂𝒔𝒔𝒂𝒂𝒄𝒄𝒊𝒊𝒕𝒕𝑪𝑪 (𝒎𝒎𝒎𝒎𝒎𝒎–𝒈𝒈−𝑿𝑿) =  𝒊𝒊𝒅𝒅𝟑𝟑.𝟔𝟔𝒎𝒎(𝑽𝑽−𝒊𝒊𝒅𝒅𝑹𝑹)∫ 𝑽𝑽(𝒕𝒕).𝒅𝒅𝒕𝒕𝒕𝒕𝟐𝟐𝒕𝒕𝑿𝑿    Equation 7-6 Where, id is the discharge current, m is the mass of CuFeS2 (in the C–1 cell setup) or mass of the MC (in the C–2 cell), V is the applied cell potential and idR represents the potential drop at the start of each discharging cycle. The operating variable V(t) corresponds to the decay in the cell potential as a function of time during the discharge cycle. The times t1 and t2 represent the start and end of the discharge cycle, respectively.  Figure 7-9 GCD tests showing the (a) 1st, 250th and 500th charge / discharge cycles and (b) discharge curves of C–1 (c) 1st, 250th and 400th charge/discharge cycles of C–2 (d) discharge profiles of C–2.  164  Each potential plateau attributed to the pseudocapacitive behavior of the electrode materials corresponds to the occurrence of faradaic reactions in the FBFC cells. The almost linear increase in the specific capacity of the C–1 cell over 500 GCD cycles (from 9 to 48 mAh g–1) is shown in Figure 7-10a. This behavior confirms the results reported in our previous work (Chapter 6) [196]. Initially (during the first 80 GCD cycles), the discharge specific capacity for C–2 (approx. 20 mAh g–1) was higher than C–1. This relatively high specific capacity for the C–2 setup was likely due to the combined effect of CuFeS2 and FeS2 in the MC. The specific capacity registered by the C–2 cell also remained relatively constant during the first 150 GCD cycles in contrast to the ever-increasing specific capacity of C–1 over 500 GCD cycles. In the following 150 cycles (up to 300 cycles), the specific capacity of C–2 gradually increased to ~ 30 mAh g–1. The additional potential plateau (at 0.3 V) in the GCD plots for C–2 (Figure 7-9d) resulted in the monotonic increase in specific capacity as shown in Figure 7-10a.  From this behavior, it was deduced that the significant faradaic contribution of FeS2 (reaction 7.5, 7.6) and formation of sulfide species (S2–, S22–, Sn2– etc.) on its surface (via reaction 7.3) could enhance the reversible faradaic response of the GF–MC electrode in the C–2 cell setup. The coulombic efficiency of the C–1 setup remained constant (~80 %) over 500 GCD cycles. However, for C–2, the coulombic efficiency reached a maximum (~ 78 %) during the initial 100 cycles and it then decayed gradually to approx. 60 % in the following 300 cycles. The relatively low efficiency of C–2 compared to C–1 corresponds to the significant consumption of supplied charge via non–capacitive irreversible faradaic reactions (reaction 7.7 – 7.9). These faradaic reactions involve Cu dissolution from the MC in addition to energy storage during continuous GCD cycling. 165  The literature generally cites the use of FeS2 in Li-ion batteries, which use aprotic electrolytes [265, 266]. Despite the commercialization of Li/FeS2 batteries, the cyclic performance of this system is limited due to the progress of conversion (irreversible) reactions of Li with S in organic electrolytes [267, 268]. For instance, Choi et al. [269] reported the use of naturally sourced pyrite in Li-ions batteries and identified a decrease in specific capacity from 772 to 313 mAh g–1 in 25 cycles. On the other hand, Wu et al. [162] studied the use of nanorod-shaped synthetic CuFeS2 as an anode material in Li-ion batteries, which exhibited a maximum 865.5 mAh g–1 specific capacity in the first discharge cycle and decreased ~ 60 % in the following 50 cycles.    Figure 7-10b shows the trends for specific energy, which can be stored and retrieved during charge and discharge cycles. Equation 7.7 was used to calculate the specific energy from the discharge curves [196].  𝑺𝑺𝒔𝒔𝑺𝑺𝒄𝒄𝒊𝒊𝑺𝑺𝒊𝒊𝒄𝒄 𝑬𝑬𝒎𝒎𝑺𝑺𝑬𝑬𝒈𝒈𝑪𝑪 (𝑾𝑾𝒎𝒎 𝒌𝒌𝒈𝒈−𝑿𝑿) =  𝑿𝑿𝟑𝟑.𝟔𝟔𝒎𝒎 ∫ 𝜟𝜟𝑽𝑽(𝒕𝒕). 𝒊𝒊𝒅𝒅𝒕𝒕𝟐𝟐𝒕𝒕𝑿𝑿 .𝒅𝒅𝒕𝒕        Equation 7-7 166   Figure 7-10 (a) Variation in discharge specific capacity and coulombic efficiency of C – 1 and C – 2 cells (b) the maximum specific energy that can be stored by C–1 and C–2 FBFC setups. The inset shows the % Cu extraction calculated based on the weight of CuFeS2 in the C–1 and C–2 cells The energy storage capability of C–1 rapidly increased from 2 to 4.5 Wh kg–1 during the initial 100 GCD cycles followed by a gradual rise to 6.3 Wh kg–1 in the succeeding 400 cycles. On the other hand, the specific energy supplied by C–2 remained relatively constant (3.5 ± 0.3 Wh kg–1) during the initial 150 GCD cycles. A monotonic increase in specific energy (up to 8.5 Wh kg–1) was then observed in the following GCD cycles, which corroborates the presence of the additional potential plateau in the discharge curves of C–2, as discussed above. 167  As evident from these results, the formation of sulfide sulfur species during repetitive GCD cycles could significantly enhance the non-capacitive response (faradaic) of the overall cell setup.  The 80 % coulombic efficiency of C–1 and appreciable decrease (~ 20 %) in the coulombic efficiency of C–2 after 400 GCD cycles is due to irreversible faradaic (conversion) reactions. To verify the dissolution of Cu via these conversions (from either the synthetic CuFeS2 or the MC), the anolyte and catholyte concentrations were measured by ICP – OES (Varian 725–ES). The higher copper (~ 300 ppm) and iron (~ 1900 ppm) concentration in the anolyte beyond the theoretical concentration of copper that could originate from CuFeS2 (92 ppm) or from the MC (56 ppm) (used in the negative electrode) confirmed the migration of these ionic species from the catholyte through the PEM during GCD cycling. Based on the original weight of the CuFeS2, the percentage Cu extraction from C–1 and C–2 after GCD cycling was found to be 10.3 and 12.7 %, respectively. Copper (either because of migration from the catholyte and/or via extraction from either CuFeS2 or MC) in the anolyte can interact with the CuFeS2 and may specifically adsorb on its surface. It has been reported in the literature that this adsorbed Cu could also chemically activate the surface of CuFeS2 via metathesis (reaction 7.12), which forms a covellite-type surface layer [85, 270].  CuIFeIIIS2 + CuIISO4 → FeIISO4 + 2CuIIS     7.12                   Such a surface activation could significantly improve the kinetic response of the synthetic CuFeS2, leading to an increase in the specific capacity as shown in Figure 7-10a. To verify this effect, ex–situ characterization of the CuFeS2 retrieved from C–1, after 500 GCD cycles was also carried out as discussed below. 168  7.4 Ex–situ characterizations of the retrieved CuFeS2 from the C–1 system Figure 7-11 shows the morphology and elemental composition of the retrieved synthetic CuFeS2. The difference in the morphology of as-synthesized (Figure 7-1a) and retrieved particles can be clearly seen. The spherical shape of the CuFeS2 was severely damaged during GCD cycling and the clustered platelet structure disintegrated as shown in Figure 8a. EDX analysis (Figure 7-11b) also revealed a difference in the chemical composition of the retrieved and as–synthesized CuFeS2. The depletion of Fe in the former suggests the formation of S and Cu enriched phases in the final product after cycling, which is consistent with the chemical activation of CuFeS2 (reaction 7.12). In comparison with the as-synthesized CuFeS2 (Figure 7-3c), the diffraction pattern revealed the formation of covellite (CuS) in the retrieved CuFeS2. The diffraction at 2θ = 10.8º, 27.6º, 29.2º, 31.7º, 32.8º, 47.9º, and 59.3º correspond to the (002), (101), (012), (013), (006), (110) and (116) planes, respectively, of CuS according to PDF # 96–900–0063 as shown in Figure 8c. However, the existence of CuFeS2 (PDF # 96–901–5637) cannot be neglected due to strong diffraction signals from the (112), (220), (312) and (316) planes. The depletion of ‘Fe’ from CuFeS2 as seen by the EDX analysis and the presence of the covellite phase in the XRD pattern verified the partial conversion of chalcopyrite to CuS during GCD cycling (via reactions 7.7 – 7.9).  169   Figure 7-11 (a) SEM image of retrieved CuFeS2 from C–1 after GCD cycling, the inset shows the lattice structure of CuFeS2 and CuS (b) EDX spectrum shows the elemental composition (c) XRD pattern of CuFeS2 after 500 GCD cycles To detect the nature of the surface species formed on the surface of the CuFeS2 particles after GCD cycling, XPS analysis was carried out and the spectra were with those obtained from the as-synthesized CuFeS2. The background of the spectra was subtracted via Shirley integration and the ionization states of Cu and S species was determined through deconvolution of the Cu 2p core peaks (Cu 2p3/2 and Cu 2p1/2) and S 2p3/2 peak. An 80 % Gaussian – 20 % Lorentzian function was applied to identify the degenerated sub-orbital states within these core peaks using the Peak 4.1 software package. To avoid misinterpretation of the peak position, 170  the experimental spectra were calibrated with the C 1s adventitious peak position (284.8 eV). The variation in the Cu 2p core peaks (compared to Cu 2p peaks of as-synthesized CuFeS2) elucidate the effect of GCD cycling on the surface chemistry of CuFeS2. Figure 7-12 shows the high-resolution spectra of Cu 2p and S 2p3/2 on the binding energy scale. The core level Cu spectra of the as-synthesized CuFeS2 (Figure 7-12a) represents the doublet peaks at 931.85 eV and 951.65 eV which correspond to Cu 2p3/2 (FWHM = 1.61 eV) and Cu 2p1/2 (FWHM = 2.25 eV), respectively for Cu+ species. The shake-up satellite peak associated with the Cu2+ species was not observed in the high-resolution spectra of as-synthesized CuFeS2. The peak area ratio (Cu 2p1/2 : Cu 2p3/2) of 1.8 and the 19.8 eV difference in the binding energy of these peaks further confirms the dominant presence of monovalent Cu species in the as-synthesized CuFeS2 [27, 247]. In the case of the retrieved CuFeS2, after deconvolution and background fitting of the Cu 2p core peaks, an additional doublet peak (FWHM = 2.24) at higher binding energy (934.47 eV) was seen in Figure 7-12b. However, no change in the Cu 2p3/2 peak position (931.87 eV) was observed except for a slight broadening (FWHM = 2.00) in the peak compared to as–synthesized CuFeS2.  On the other hand, the Cu 2p1/2 peak shifted (~1.2 eV) to a higher binding energy for the retrieved CuFeS2 samples. A very strong and broad (FWHM = 3.73) satellite peak appeared at 942.27 eV, which corresponds to divalent Cu species. The broadening of the core peaks and shift of the Cu 2p1/2 peak to a larger binding energy further supports the formation of Cu2+ species on the surface of CuFeS2 after 500 GCD cycles. The evidence of both Cu+ and Cu2+ species on the surface of the retrieved CuFeS2 confirms the formation of a covellite-type (CuS) phase. However, the existence of other species i.e. CuO, Cu(OH)2 and 171  CuSO4 cannot be ruled out due to the splitting of the Cu 2p3/2 peak and the origin of the peak affiliated with Cu2+ species at 934.47 eV [40, 271-273].  Figure 7-12 XPS core Cu 2p peaks of (a) as–synthesized and (b) retrieved CuFeS2 particles (from the C–1 cell) show the presence of 2p3/2 and 2p1/2 spin orbitals (c) high-resolution S 2p3/2 peak of the as-synthesized and (d) retrieved CuFeS2 Figure 7-12c and d show high-resolution S 2p3/2 spectra of the as-synthesized and retrieved CuFeS2 sample, respectively. The S2–, S22– and Sn2– species were evident on the surface of as-synthesized CuFeS2 from the splitting of S 2p3/2 into triplet peaks at 161.4 eV, 162.4 eV, and 163.7 eV. The S bonded with Cu and Fe atoms on the surface of CuFeS2 may 172  relocate and transform from S2– to S22– species through dimerization; whereas due to interband excitation of S 3p and Fe 3d sub-orbitals, the Sn2– species may also form as reported in the literature [47, 245].   For the retrieved CuFeS2 sample (Figure 7-12d), the variation in the peak distribution was clear and the peaks associated with the S2–, S22– and Sn2– species are slightly shifted to higher binding energies compared to the as-synthesized CuFeS2. From this behavior, it can be assessed that depletion of Fe from the surface of CuFeS2 after 500 GCD cycles may change the binding energy of these species. Also, the peak affiliated with the S2– species became more dominant, however, its binding energy (161.5 eV) was lower than the same species on CuS (161.6 eV). On the other hand, the binding energy of S22– (162.8 eV) species and low intensity of the Sn2– broadened peak at 163.8 eV are consistent with the development of a metal-deficient covellite type phase (via reaction 7.9 and/or via reaction 7.12) on the surface of CuFeS2 during GCD cycling [246, 247]. It was shown that a significant amount of CuII can diffuse/migrate from the catholyte to the anolyte during GCD cycling (in addition to the CuII produced from the partial dissolution of CuFeS2). Furthermore, analysis of the retrieved CuFeS2 suggested the formation of an Fe deficient surface film. Therefore, it is important to elucidate the influence of CuII ions on the electrochemical behavior of the negative composite electrode (GF–CuFeS2). Figure 7-13 shows potentiodynamic cathodic polarization data for the composite electrode in 0.2 M H2SO4 with and without CuII in solution. The cathodic polarization was used to simulate the charging cycle in the FBFC. The composite electrode was polarized from its OCP to –1.5 V (vs. OCP) at a scan rate of 1 mV s–1. The OCP of the GF–CuFeS2 shifted gradually from +0.47 V (in the absence of CuII) to +0.55 V (vs. SHE) with 0.07 M Cu2+ in 0.2 M H2SO4 solution. 173  The cathodic Tafel slope (βc) of the composite electrode also decreased rapidly from −0.313 to −0.25 V decade–1 with the addition of 0.02 M Cu2+. At 0.07 M Cu2+, the βc decreased to −0.236 V decade–1, as shown in the inset of Figure 7-13. This trend suggests an improvement in the faradaic response of the composite electrode with the addition of CuII. The increase in cathodic current density beyond the Tafel region and appearance of a current peak in the polarization curves is directly related to the CuII concentration. This increase in current density is consistent with results reported elsewhere [85, 92, 270] and suggests the transformation of CuFeS2 into copper enriched phases which are more electrochemically active. This behavior also corroborates the results obtained from EDX and XPS (Figure 7-11 and Figure 7-12, respectively), which confirmed the formation of CuS in the retrieved CuFeS2 after 500 GCD cycles.  From these results, it can also be concluded that the presence of CuII in the anolyte (either via migration from the catholyte or leached from the CuFeS2) can improve the kinetic response of the GF–CuFeS2 electrode and hence the overall charge storage capacity of the FBFC increases, as discussed above. Warren et al. [87] described the reduction of CuFeS2 to bornite and chalcocite, respectively in the presence of CuII (reactions 7.13 and 7.14). In addition, the reduction of CuII to metallic Cu on CuFeS2 can result in the formation of Cu2S through galvanic coupling (reaction 7.15). This behavior is consistent with the improvement in the reversible faradaic response of the electrode materials in the FBFC due to the formation of Cu enriched phases.  2CuFeS2 + 3Cu2+ + 4e– ↔ Cu5FeS4 + Fe2+     7.13 CuFeS2 + 3Cu2+ + 4e– ↔ 2Cu2S + Fe2+       7.14 CuFeS2 +2H+ + Cu ↔ Cu2S + H2S + Fe2+     7.15 174   Figure 7-13 Potentiodynamic cathodic polarization of the GF–CuFeS2 electrode in 0.2 M H2SO4 (anolyte) containing various amounts of Cu2+. The inset shows the cathodic Tafel slope (βc) as a function of Cu2+ concentration. 7.5 Summary As–synthesized CuFeS2 and MC were sandwiched in GF and used as a negative composite electrode in an FBFC. The FeII/FeIII redox reaction (in the presence of CuII) in the positive compartment was used to support the charge and discharge process in this battery-like setup. The electrochemical kinetic parameters (i.e. α, D and kh) of the GF–CuFeS2, GF–MC and GF–Fe/Cu were also calculated from the CV scans obtained at various sweep rates. This data proved that the redox reactions in the FBFC were quasi-reversible. This was further confirmed from the quantitative values of the dimensionless heterogeneous rate constant (Λ) 175  calculated experimentally from the CV data. The continuous increase in the specific capacity (~ 9 to 48 mAh g–1) and energy (2 to 6.3 Wh kg–1) of the C–1 setup is associated with the reversible faradaic response of the sulfide surface species (most likely S22–/S2–), which were formed over 500 GCD cycles. The coulombic efficiency in this setup was (~80 %). However, in 400 GCD cycles, the monotonic increase in specific capacity (up to 49 mAh g–1) and energy (up to 8.5 Wh kg–1) of the C–2 setup after 300 GCD cycles is attributed to the additional pseudocapacitive faradaic contribution of the FeS2 in the MC. The decrease (~20 %) in the coulombic efficiency over 400 GCD cycles also suggested the occurrence of non–capacitive (irreversible) conversion reactions. Concurrent to the low coulombic efficiency, 10.3 and 12.7 % Cu was also extracted from C–1 and C–2, respectively, which makes these cells attractive units for combined energy storage and Cu extraction. The presence of Cu in the anolyte and ex-situ analyses of the retrieved synthetic CuFeS2 through EDX, XRD and XPS confirmed the formation of an Fe deficient covellite phase on the surface of retrieved CuFeS2 particles.  Also, the presence of CuII species in the anolyte significantly enhanced the current response of the composite electrode, as exhibited by the potentiodynamic cathodic polarization data. For the MC, due to the presence of other impurity phases, the exact sequence of the electrochemical reactions is difficult to deduce. However, based on the experimental observations, the following general reaction sequence in the FBFC is proposed.   Charging step: Fe(II) → Fe(III) + e–       (Positive electrode) 2CuFeS2 + 6H+ + 2e– → Cu2S + 2Fe2+ + 3H2S (Negative electrode; irreversible) S22– + 2e–→ 2S2–     (Negative electrode; pseudocapacitive) xC + H+ + e–→ Cx–1 + CHºads    (Negative electrode; pseudocapacitive) 176  Discharging Step: Fe(III) + e– → Fe(II)     (Positive electrode) Cx–1 + CHºads → xC + H+ + e–   (Negative electrode; pseudocapacitive) Cu2S → CuS + Cu2+ + 2e–    (Negative electrode; non-capacitive) CuFeS2 → Cu1-xFe1-yS2-z + xCu2+ + yFe2+ + zSº + 2(x + y)e–   (Negative electrode; non-capacitive) 2S2–→ S22– + 2e–     (Negative electrode; pseudocapacitive)         177  Chapter 8: Integrating the Cu extraction and Zn electrowinning processes for energy storage11 Considering that the oxidative leaching of CuFeS2 and electrodeposition of Zn are energy intensive, the radically novel concept of coupling these two processes in one setup is described in this chapter. A TFB is introduced which is capable of electrodepositing Zn on the negative electrode and dissolving Cu from CuFeS2 at the positive electrode. In addition, this energy storage unit can release energy during the discharge cycle, but at the expense of deposited Zn. A TFB coupled with renewable energy generating facilities, i.e. wind turbines and/or solar cells, could be an attractive technology for remote mining locations in Canada’s far North or elsewhere. The aqueous-based chemistry in this setup innately suppresses the fire hazards and other safety concerns that are specifically associated with the scale-up of Li–ions batteries for stationary applications. The cyclic charge/discharge performance with sufficient amount of Cu extraction is another compelling feature of this setup in contrast to the existing Ag–Zn and Zn–air primary batteries [19-21]. 8.1 Physical characterization of the electrode materials  The morphology of the as-synthesized CuFeS2 was of spherical-shaped particules having an open porous structure, as shown in Figure 8-1a. Under applied conditions, the reduction of CuII to CuI and oxidation of thiourea into carbamide and FeII into FeIII species would promote the formation of Cu1+Fe3+S22– according to reaction 8.1, as reported in                                                  11 The results in this Chapter are submitted for publication 178  Chapter 7. It is well recognized that Cu and Fe in CuFeS2 retain +1 and +3 oxidation states, respectively [274, 275].   2SC(NH2)2 + CuCl2 + FeCl2 + 2H2O → OC(NH2)2 + 4HCl + Cu1+Fe3+S22– 8.1  Figure 8-1 (a) SEM image showing the morphology and (b) Deconvoluted S 2p3/2 high-resolution spectra of synthetic CuFeS2. (c) N2 adsorption/desorption isotherm for surface analysis and (d) multi–pore size/volume analysis curve of CuFeS2 particles The XRD pattern of the synthetic product confirms the formation of pure CuFeS2 according to the reference pattern (PDF # 37 – 0471) (Figure 8-2a) as discussed in section 5.1 and reported elsewhere [276]. No peak associated with any impurity phases was detected in the diffraction pattern.  179   Figure 8-2 (a) XRD pattern (b) XPS survey scan and (c) the deconvoluted high-resolution spectra of Cu 2p1/2 and Cu 2p3/2 bands of as-synthesized CuFeS2 Figure 8-2b presents the XPS survey spectrum of synthetic CuFeS2 on the binding energy scale. Conforming to the XRD pattern, the only characteristics peaks associated with the Cu, Fe, and S were evident in the survey spectrum. The origin of C 1s (284.94 eV) and O 1s (531.75 eV) peaks was associated with the surface contaminants and to the airborne surface oxidation of CuFeS2. The C 1s adventitious peak typically at 284.8 eV is used as a reference for peak positioning and to evaluate any surface charging effects during XPS measurement. The small deviation of C 1s peak at higher binding energy (0.14 eV) may also be related to the satellite signals arising from the delocalized electrons associated with the surface functional 180  groups, particularly aromatics species [277]. The S 2p3/2 core peak after background subtraction and deconvolution splits into three peaks representing the nature of the sulfide sulfur species present on the surface of CuFeS2 as shown in Figure 8-1b. The doublet peaks at 161.67 eV and 162.57 eV are associated with the monosulfide (S2–) and disulfide (S22–) species. The 1.1 eV difference in the binding energy of these doublet peaks is related to the partially coordinated sulfur species [278]. The relatively larger full width at half maximum (FWHM = 1.293 eV) and intensity of the S22– peak, as compared to S2– (FWHM = 1.072 eV) peak also indicates the dimerization of the later species through physical relocation of S bonded directly with the Cu and Fe atoms in the crystal lattice of CuFeS2 [196, 225, 278]. The excitations of photoelectrons from S 3p – Fe 3d inter–bands may occur as a broad satellite peak observed at 163.87 eV (FWHM = 2.207 eV) and correspond to the existence of polysulfide (Sn2–) species at the surface [45, 278]. The high-resolution core Cu 2p3/2 and Cu 2p1/2 peaks at 932.61 eV (FWHM = 2.043 eV) and 952.42 eV (FWHM = 3.626 eV) is due to Cu+ species (Figure 8-2c). The binding energy of these peaks is slightly higher than the reported values in the literature (932.4 and 952.1 eV) [247, 278]. However, no satellite peak associated with divalent Cu2+ was observed (this usually appear at 942 eV). These results are in agreement with the literature and they confirm the monovalent state of Cu atoms in the CuFeS2 lattice structure [45, 196, 247, 274, 279]. Surface morphology can significantly affect the electrochemical response of synthetic CuFeS2. Therefore, the specific surface area, pore size and distribution were measured from the N2 adsorption/desorption isotherms as shown in Figure 8-1c. For the specific surface area, the physisorption isotherms were transformed into the BET plot shown in the inset of Figure 8-1c. The multipoint BET method was used within the relative pressure (P/Pº) range of 0.025 181  – 0.30 to calculate the sorbed N2 monolayer capacity, (Wm) according to Equation 8.1 [275, 280].  A linear trend between P/Pº and 1 (𝑊𝑊�𝑃𝑃 𝑃𝑃𝑜𝑜� � − 1)⁄  was found within this P/Pº range. The positive slope (s = 623.3) and intercept (i = 7.21) were calculated by linear regression with an acceptable correlation factor of 0.997. 𝑿𝑿𝑾𝑾�𝑷𝑷 𝑷𝑷𝒐𝒐�−𝑿𝑿�=  𝑪𝑪−𝑿𝑿𝑾𝑾𝒎𝒎�𝑷𝑷𝑷𝑷𝒐𝒐� + 𝑿𝑿𝑪𝑪𝑾𝑾𝒎𝒎     Equation 8-1     In this equation, C is the BET constant which exponentially depends on the monolayer adsorption energy, W is the experimentally measured weight of the adsorbed N2, P and Pº are the equilibrium and saturation pressures, respectively. The corresponding values of C, Wm and specific surface area (S) were calculated from Equation 8.2, Equation 8.3, and Equation 8.4.  𝑪𝑪 = 𝑿𝑿 +  𝒈𝒈𝒊𝒊          Equation 8-2 𝑾𝑾𝒎𝒎 =  𝑿𝑿𝒈𝒈+𝒊𝒊                      Equation 8-3 𝑺𝑺 =  𝑵𝑵𝒎𝒎.𝒈𝒈.𝑾𝑾𝒎𝒎𝑽𝑽.𝑾𝑾𝑪𝑪𝑪𝑪𝒎𝒎𝑺𝑺𝑺𝑺𝟐𝟐        Equation 8-4 Where NA is the Avogadro's number, V is the volume of gas adsorbed and WCuFeS2 is the initial mass of CuFeS2 sample after degassing. From these calculations, the specific surface area of the as-synthesized CuFeS2 was found to be 5.52 m2 g–1. It is important to note that the synthetic CuFeS2 used in this study has a relatively larger surface area than the synthetic CuFeS2 used in Chapters 5 and 6.  As shown in Figure 8-1c, the hysteresis loop formed at low relative pressure (0.4 < P/Pº < 0.8) confirms the presence of fine size pores on the surface of CuFeS2. However, at large relative pressure (0.8 < P/Pº < 1), the hysteresis loop indicates porosity in the inter–aggregated particles. This behavior is also evident in the pores distribution curve as shown in Figure 8-1d.  The pore size/volume was calculated from the adsorption/desorption isotherms 182  using DFT which is a reliable method for pore size analysis at the nanoscale [280]. The adsorption/desorption curve was used to calculate an average pore size of ~47.5 Aº, which is consistent with the mesoporous surface structure of as-synthesized CuFeS2 particles. As evident in the SEM image (Figure 8-1a), the hysteresis loop observed at P/Pº < 0.8 (Figure 8-1c), and multimodal distribution of pore sizes in Figure 8-1d confirm the formation of variable dimension pores on the surface of spherical CuFeS2 particles. The specific pore volume is 0.033 cm3 g–1, and this value represents the total specific pore volume that may include the volume of variable size surface and inter–particulate porous network developed during synthesis and/or by the agglomeration of the fine particles after drying.  8.2 Reaction sequence and optimization of the process conditions Initially, the electrochemical behavior of both positive and negative electrodes was evaluated from CV scans obtained separately in a three-electrode cell setup. To simulate the actual charging/discharging scenario in the TFB, the CV curves of GP and other slurry constituents were acquired by sweeping the potential in the positive direction to facilitate oxidation reactions before reversing the scan in the negative direction. The three-electrode cell setup used to study the electrochemical performance of a positive slurry electrode is schematically shown in Figure 8-3a. The minimal current response by GP in 0.2 M H2SO4, as shown in Figure 8-4a, confirms its negligible contribution in the overall charge/discharge process. However, with the addition of AC in 0.2M H2SO4, large anodic and cathodic specific currents were observed. For instance, at 0.4 V (vs. SHE), the specific current of ~20 mA–g–1 registered by the GP–AC electrode was approximately 10 times higher than for the GP electrode. This overall current response was attributed to the sum of the current contributions by the non–faradaic (double layer charging/discharging), and faradaic (charge transfer) 183  processes occurring on the surface of AC. The large specific current response of AC is related to its high surface area (2545 m2 g–1) and fine platelets-like morphology as shown in Figure 8-3b.  Multipoint BET surface area and pore size distribution were also determined from the N2 adsorption/desorption isotherm of the AC sample. The pore size distribution curve (Figure 8-3c) shows the mesoporous structure of AC (pore width < 2.5 nm). The large specific surface area and pore volume (1.24 cm3 g–1) could enhance the charge storage capacity of the positive electrode as manifested by the large anodic and cathodic current response in the CV scan (Figure 8-4a). The broad peaks observed at 0.71 and –0.24 V (vs. SHE) validate the occurrence of faradaic processes (pseudocapacitive behavior) at the surface of AC in sulfuric acid solution. These peaks are most likely affiliated with surface functional groups which may oxidize and reduce in the presence of H+ and/or HSO4– in a reversible manner (pseudocapacitive behavior) as reported in the literature [202, 281, 282].  With the addition of synthetic CuFeS2, the current response of the positive slurry electrode (PSE) composed of GP–AC–CuFeS2 was significantly affected (Figure 8-4a). The cyclic behavior of GP–AC–CuFeS2 was measured form the CV scans obtained at 5 mV s–1. Multiple anodic and cathodic peaks were evident in the forward and reverse scans, respectively, corresponding to the oxidation and reduction reaction on CuFeS2 in synergism with the pseudocapacitive response of AC. During the 1st cycle, the anodic and cathodic currents were found to be slightly higher than observed in the rest 4 cycles, as shown in Figure 8-5a and b. The current peaks originated during forward and reverse CV scans are also labeled. The rapid initial increase in current and origin of peak A at 0.53 VSHE corresponds to the partial oxidation of CuFeS2 into sulfide enriched surface species via reaction 8.2.  184  CuFeS2 → Cu1–xFe1–yS2–z + xCu2+ + yFe2+ + zSº + 2(x+y)e–  8.2  Figure 8-3 (a) A three-electrode setup for measurement of electrochemical behavior of the PSE containing (80 wt. % CuFeS2 + 20 wt. % AC in 0.2 M H2SO4. (b) SEM image of AC (c) Multipoint BET surface area and pore size distribution curve for AC 185   Figure 8-4 (a) CV (4th cycle) of positive electrode components; GP, GP–AC and GP–AC–CuFeS2. (Note: exposed area of GP = 6.28 cm2, 20 wt. % total solids in the slurry) (b) CV curves of negative electrode (Al strip ≈ 2 cm2). (c) Chronopotentiometry scans (30 mA cm–2) depicting the effect of CTAB concentration in the anolyte (vs. MSE = 0.62 VSHE) (d) a potential/concentration (PC) diagram developed from the CV and chronopotentiometry results. With an increase in potential, peak A1 is observed at 0.69 VSHE and is related to the oxidation of surface functional groups present on AC [211]. Following this peak, a limiting current response up to 0.9 VSHE is obtained and it corresponds to the formation of a passive film on the surface of CuFeS2, which is composed of Cu1–xFe1–yS2–z and Sº species [196, 278]. These 186  species can form as a thin layer on CuFeS2, and a limiting current below 0.9 VSHE corresponds to its barrier characteristics. During the reverse scan, the current increased in the cathodic direction and the appearance of current peak C1 (at 0.56 VSHE) corresponds to the reduction of surface functional groups on AC.  As discussed in the literature on the behavior of carbon-based materials in acids, the set of anodic and cathodic peaks centered at ~0.6 VSHE are normally associated with the oxidation-reduction of surface functional groups [211, 283]. The reduction of reaction products formed at peak (A) results in the detection of the current peak (C) at 0.31 VSHE during the reverse scan. The large decrease in reduction current at peak C and a current plateau observed within 0.37 – 0.1 V SHE indicates the partial conversion of CuFeS2 into other intermediate species i.e. talnakhite (Cu9Fe8S16) and bornite (Cu5FeS4) [87].  However, further decrease in cathodic potential resulted in the broad peak (C2) at –0.12 VSHE, which corresponds to the removal of Fe from the lattice and bornite and/or CuFeS2 is converted into chalcocite (Cu2S) (reaction 8.3 and 8.4) [32]. 2Cu5FeS4 + 6H+ + 2e– → 5Cu2S + 2Fe2+ + 3H2S    8.3 2CuFeS2 + 6H+ + 2e– → Cu2S + 2Fe2+ + 3H2S    8.4        At negative potential, below –0.43 VSHE, the rapid increase in current can be attributed to the simultaneous reduction of CuFeS2 into metallic copper Cuº (reaction 8.5) and H2S evolution.  CuFeS2 + 4H+ + 2e– → Cuº + Fe2+ + 2H2S     8.5   Upon positive return of the scan form –0.65 VSHE, a sharp anodic peak A3 observed at –0.27 VSHE is consistent with the oxidation of previously-formed Cuº (at peak C3) by reaction with dissolved H2S and conversion into Cu2S via reaction 8.6. During the forward scan beyond peak A3, the current increases almost linearly with potential shift with the development of multiple 187  peaks (A4 and A5) at 0.23 and 0.37 VSHE which are consistent with the oxidation of H2S and Cu2S into Sº and Cu2–xS species, respectively according to reaction 8.7 and 8.8 [32, 87, 92, 284]. 2Cuº + H2S → Cu2S + 2H+ + 2e–      8.6 H2S → Sº + 2H+ + 2e–       8.7 Cu2S → Cu2–xS + Cu2+ + 2xe–      8.8 The high current response of GP–AC–CuFeS2 was supported by the pseudocapacitive behavior of AC in the slurry, which may possibly enhance the kinetics of redox reactions during cyclic charge/discharge. However, the occurrence of non–capacitive faradaic reactions on the surface of CuFeS2 could decrease the overall current response during repetitive oxidation/reduction processes, as revealed by the variation in the current response of five repetitive CV cycles (Figure 8-5a). It is reported that Cu2S formed during the cathodic scan (a simulation of discharge cycle in a TFB) could transform to various non–stoichiometric intermediate products i.e. Cu2–XS with the generation of Cu2+ in the following forward scan (the charge cycle in a TFB). This repetitive charge/discharge process would thus facilitate Cu extraction from the refractory CuFeS2 aside with energy storage. Mixing CuFeS2 with AC, in the form of a slurry, significantly enhances its charge storage capacity and Cu extraction during cyclic charge/discharge processing.  This behavior was predicted from the large anodic and cathodic current response (large loop) and current peaks in the CV scans. 188   Figure 8-5 The CV of the PSE (5mV s-1 sweep rate). (a) The cyclic performance of the positive electrode (5 cycles). (b) The 1st and 5th cycles show the current peaks (as labeled) used to predict the reactions sequence during actual charge/discharge process in the TFB. Industrially, Zn electrowinning is carried out by depositing Zn on Al from pure acidic zinc sulfate leach solution. Most cationic impurities are removed via purification processes before the electrowinning step to improve the process current efficiency and morphology of the Zn deposit. To increase the ionic conductivity and to avoid an excessive potential drop, Na2SO4 was added here as a supporting electrolyte. To evaluate the influence of the supporting 189  electrolyte (0.1 M Na2SO4) and addition of cationic surfactant (CTAB) in 100 g–l–1 Zn2+ + 0.2 M H2SO4, CV and GS scans were obtained, and they are shown in Figure 8-4b and c, respectively. An apparent increase in the charge (Zn deposition) and discharge current (stripping) was observed with the addition of 0.1 M Na2SO4 in 100 g–l–1 Zn2+ + 0.2 M H2SO4 as pictured in Figure 8-4b. These results show the effectiveness of Na2SO4 addition on the Zn deposition and stripping process. Moreover, the addition of Na2SO4 in the electrolyte also decreases the IR drop in the electrolyte and improves the kinetics of the Zn/Zn2+ couple [285]. CV scans were initiated from point A (Figure 8-4b), which arbitrarily corresponds to the OCP of Al in each electrolyte. Sweeping the potential in the negative current direction, toward point C, resulted in a negligible increase in reduction current. The abrupt increase in current to point D with a slight increase in potential from point C (plating overpotential) indicates the commencement of Zn deposition. Upon reversal, the current approached a maximum (point E), preceded by zero current at point B (crossover potential). The origin of the anodic peak at point E and the following decay in current is attributed to the complete re-dissolution of Zn. At the same potential, the higher current during the positive scan (from D to B) compared to the negative scan (point C to D) also highlights the availability of a large number of dissolving nuclei on the Al substrate during the stripping process. In these scans, the nucleation hysteresis (BCD) and stripping hysteresis (BEA) were greatly affected by the added Na2SO4 and CTAB. For example, the addition of 2 ppm CTAB shifted the nucleation potential (point C) to more negative potential. Similarly, the anodic peak (point E) also moved toward more positive potential in the presence of CTAB which indicates a slight increase in the hindrance of Zn re-dissolution. In the absence of CTAB, the relatively small stripping hysteresis in Na2SO4 is related to the charge consumed by the parasitic H2 evolution reaction 190  during Zn deposition. Also, no shoulder cathodic peak between point B and C was observed in the presence of CTAB, which is consistent with minimal H2 evolution on the electrode surface [286]. Uniform growth of the Zn deposit happened in the diffusion-controlled regime of the CV curve at potential that is more negative than point C.  The effect of CTAB in 0.1 M Na2SO4 was investigated by GS tests as shown in Figure 8-4c. It was evident that in the absence of CATB, the deposition proceeded at a very negative potential (–1.38 VSHE). However, with the addition of CTAB, Zn deposition took place at less negative potential. The least negative deposition potential (–0.98 VSHE) occurred at 2 ppm CTAB. The importance of CTAB can be seen in the potential/concentration (PC) diagram in Figure 8-4d. In this diagram, line–1 represents the nucleation overpotential and line–2 represents the deposition potential. These were obtained separately from the CV and GS test, respectively. Point A is the OCP of the Al substrate in each solution.  Zn nucleation begins beyond the crossover potential (point B); and uniform growth, along with H2 evolution, occurs once the potential becomes more negative than point C, as depicted in Figure 8-4b. In the region below line–1, which is the potential difference between point A and B in the CV scans, there are very minimal chances for Zn nucleation and this is thus designated as the ‘no deposition’ region. Above this line (beyond crossover), there are a fair chance of Zn nuclei formation and this region is referred to as ‘under-potential deposition’.  From the GS tests conducted at 30 mA cm–2 (Figure 8-4c), steady-state potential values were measured after 1 h of polarization in the presence of various CTAB concentrations, and these are shown as a line–2 in Figure 8-4d. At potentials more negative than line–2, one can expect uniform Zn deposition with possible H2 evolution. The relatively large nucleation and deposition overpotentials (> – 0.5 V) (relative to EOCP) below 2 ppm CTAB concentration are 191  directly related to the cell potential and therefore suggested the requirement for a large energy input for Zn deposition. The lowest deposition overpotential (– 0.44 V) at 2 ppm CTAB indicated that nucleation and growth of Zn crystallites on the Al substrate would be most favorable, with less energy consumption, which is always desirable for an efficient energy storage system. An additional increase in CTAB concentration > 2 ppm, the minor effect on the nucleation overpotential was observed (line–1 in Figure 8-4d). However, the deposition overpotential values increased further with the addition of 4 and 8 ppm CTAB (line–2). It is well known that with an increase in CTAB concentration, which is above the critical micelle concentration (CMC) > 1 mM (in water at 25 ºC), micelles may form multilayers on the cathode surface and may increase the overpotential for Zn deposition [287, 288]. Moreover, at low concentration, it is expected that CTAB may dissociate into cetrimonium cations, which may adsorb specifically on the active sites during Zn deposition and effectively suppress dendrite formation and H2 evolution. With the increase in CTAB concentration > 2 ppm, the surface coverage by cetrimonium cations restricted the Zn2+ deposition on the cathode surface as exhibited by < 90 % current efficiency.  Figure 8-6a–e presents the morphology of the Zn deposits formed without and in the presence of CTAB. The localized growth of Zn in the form of flake-type crystal clustered in pyramidal-shape morphology shows the development of dendritic structure in the deposit without CTAB addition (Figure 8-6a). These crystals were branched in multi-directions, indicating the non–uniform current distribution at the surface during electrodeposition. However, in the presence of CTAB, the cetrimonium cation species are expected to adsorb on the active sites and may hinder localized dendritic crystal growth. In this way, the cationic micelles may also tailor the orientation of growing crystals in the Zn deposit and restrict 192  undesirable dendritic growth [114, 289, 290]. At low (1 ppm) CTAB concentration, coarse multi-faceted crystals containing small pores were deposited on the Al substrate. These pores may form due to the generation of H2 bubbles at the local active sites which may disrupt the continuity of the deposit.  The current efficiency of Zn deposition during GS tests was increased to 90.83 % from 83.7 % with the addition of 1 ppm CTAB and 0.1 M Na2SO4. Further increase in the CTAB concentration from 1 to 2 ppm improved the current efficiency to 92.3 %. A smooth and refined polycrystalline structure was developed in the presence of 2 ppm compared to 1 ppm CTAB. The fine platelet-like crystals were formed and oriented in various crystallographic directions as shown in Figure 8-6c. However, at the high CTAB concentration (4 and 8 ppm), a coarse-grained structure and porosity in the deposit were visible (Figure 8-6d and e). The current efficiency at these CTAB concentrations also decreased to 89.1 and 88.8 %, respectively. This behavior is due to parasitic H2 evolution or possibly due to the preferential adsorption of cetrimonium species at the active sites that may restrict Zn deposition if present in large amounts.  The influence of CTAB concentration on the orientation of crystal growth was further evaluated through XRD patterns as shown in Figure 8-6f. In the presence of CTAB, the major diffraction peaks were observed at 36.3º, 39.0º, 43.2º, 54.3º, 70.0º, 70.7º, 82.1º and at 86.6º  corresponds to the (002), (100), (101), (102), (103), (110), (112) and (201) planes, according to standard pattern (JCPDS card no. 00–004–0831) and these are in agreement with the literature [286, 291, 292]. However, variations in the peak intensities represent preferential orientation of the crystals.  193   Figure 8-6 SEM images of Zn deposit on Al substrate obtained in the base solution containing, (a) 0.1 M Na2SO4, (b) a + 1 ppm CTAB, (c) a + 2 ppm CTAB, (d) a + 4 ppm CTAB (e) a + 8 ppm CTAB (f) XRD patterns of Zn deposits formed in a – e The XRD patterns of Zn deposits produced in the presence of 1, 4 and 8 ppm CTAB, registered the major diffraction peak at 43.2º indicating the preferential Zn growth in the (101) 194  planar orientation. In contrast, the 2 ppm CTAB condition results in a dominant peak at 36.3º (the (002) basal plane). The change in growth orientation from the (101) to the (002) crystallographic plane in the presence of optimum (2 ppm) CTAB concentration significantly refined the grain structure (Figure 8-6c) and inhibited the H2 evolution, as predicted from the highest current efficiency achieved (92.3 %). 8.3 Estimation of energy and Cu extraction capabilities of the TFB The original experimental setup used to estimate the energy storage and Cu extraction capability is shown in Figure 8-7. In this setup, the PSE is injected in the positive compartment of TFB and Zn2+ solution (anolyte) is circulated from the water jacketed external reservoir by using peristaltic pump. Figure 8-8a can be used to predict the possible reactions during the charging and discharging process. Initially, during the charge cycle, the partial oxidation of CuFeS2 in the positive electrode compartment is supported by the reduction of Zn2+ and its deposition on the Al surface. In the discharge cycle, the deposited Zn redissolves from the negative electrode and this encourages the reduction of oxidized species formed on the surface of CuFeS2 (charge cycle) in the positive compartment as observed from the cathodic peaks during discharge (Figure 8-8a). From these results, it is also anticipated that intermediate species will form on the surface of CuFeS2 (reaction 8.2) i.e. Cu1–xFe1–yS2–z and Sº, in the positive electrode. Parasitic H2 evolution also occurs on the negative electrode during charge cycles, which may significantly decrease the coulombic and energy efficiencies in the TFB. However, also with the reversible transformation of sulfide sulfur species (S22– ̸ S2–) on the surface of CuFeS2, AC in the positive slurry electrode enhances its pseudocapacitive response. The reversible faradaic reactions on AC are adsorption ̸ desorption of ionic species and their interaction with surface functional groups [209, 211]. The AC also increases the overvoltage 195  for oxygen evolution on the positive electrode because no sharp increase in current during forward scans was observed beyond the water stability region (–0.06 to 1.18 VSHE at pH 0.9 ± 0.06) (Figure 8-8a).  During the reverse scan, the potential for H2 evolution also took place at relatively more negative (–0.43 VSHE) potentials than the thermodynamic value (–0.05 VSHE at pH 0.9). This may correspond to the adsorption of H+ (C + H+ + e– ↔ C–Hads) within the mesoporous structure of the high surface area AC in the positive slurry electrode [281, 293, 294]. Similarly, during Zn deposition in the charging process, the slow kinetics of H2 evolution over Zn is seen by the large overpotential (point B–C in Figure 8-4b). The OCP of each positive slurry (EPOCP) and negative Al current collector (ENOCP) in the anolyte measured separately in three electrodes cell were found to be 0.46 ± 0.02 V and –0.55 ± 0.03 V, respectively. The variation in the potential difference ED (EPOCP – ENOCP) is shown in Figure 8-8b. The experimentally measured cell voltage (EC) of the TFB was 0.95 ± 0.03 V which was ≈ 60 mV lower than ED. This decrease in the EC is due to the IR drop in the positive and negative compartments, as well as across the AEM, as presented schematically in Figure 8-8b. 196   Figure 8-7 TFB showing the assembly of a cell connected to a potentiostat 197   Figure 8-8 Performance evaluation of TFB (a) CV curves of negative and positive electrode, (5 mV s–1). (b) Equilibrium cell potential (EC) and schematic of potential drop within the TFB. (c) CV scans of TFB at various sweep rates and (d) the trends of peak current vs. (sweep rate)0.5 To evaluate the reversibility of the electrochemical reactions in the TFB, the CV scans were obtained at various sweep rates from 1 – 25 mV s–1 as presented in Figure 8-8c. The continuous increase in current from ≈ 1.1 V and the origin of peak P1 at highly positive cell potential (> 1.55 V) during charging process corresponds to the oxidation of CuFeS2 and Zn deposition in the positive and negative electrode compartments, respectively. However, this 198  rapid increase in current includes the electrochemical response of both AC and CuFeS2 in the PSE. In other words, no separate current peak associated with the AC was apparent, owing to the preferential oxidation of CuFeS2 in the positive compartment of the TFB [295]. In the discharge cycle, the peak P2 (Figure 8-9a) was observed at ≈ 1.32 V, which is related to surface adsorption of cationic species within the mesoporous structure of AC (pseudocapacitive behavior) or due to the reduction of species (i.e. FeIII and CuII) formed in the preceding charge cycle. Furthermore, the prominent broad current peak (P3) demonstrates the re-dissolution of Zn in the anolyte. This re-dissolution supports the electro-reduction of unreacted CuFeS2 or the underpotential deposition of various ionic species (i.e. dissolved CuII, FeII, H+ species etc.) in the PSE these species having formed in the preceding charge cycle (forward scan). The overall current associated with the charge (P1) and discharge (P3) peaks could also include the reversible faradaic response of sulfide sulfur species present on CuFeS2 via reaction 8.9 [196]. 2S2– ↔ S22– + 2e–        8.9 The separation between peaks P1 and P3 tended to increase with the rise in sweep rate. In other words, P1 and P3 shifted away from EC in both directions with increasing sweep rate, as shown in Figure 8-9b. This behavior was linked to the sluggish kinetics of reversible electrochemical reactions at the surface of both electrodes in the TFB. On the negative electrode, this may be connected to the presence of CTAB in the anolyte, which has a slightly increased the Zn deposition overvoltage during the charging step as revealed by the extension in the B–C region (Figure 8-4b). From the analysis of P1 and P3, peak currents (ipeak) varied linearly with the square root of the sweep rate (v1/2) as presented in Figure 8-8d. The highest charge and discharge current at a sweep rate of 25 mV s–1 is due to a thin diffusion layer across the electrodes/electrolyte interface in the TFB. With decreasing v, the progress of 199  electrochemical reactions on the electrode surface lead to an increase in the thickness of the diffusion layer and an apparent decrease in the current response follows the Randles–Sevcik relation (Equation 8.5) [296].  𝒊𝒊𝒔𝒔𝑺𝑺𝒂𝒂𝒌𝒌 = 𝑿𝑿.𝟒𝟒𝟒𝟒𝟔𝟔𝒎𝒎𝒎𝒎𝒎𝒎𝑪𝑪(𝒎𝒎𝒎𝒎𝒗𝒗𝑫𝑫𝑿𝑿𝑹𝑹𝑹𝑹 )𝑿𝑿.𝟐𝟐         Equation 8-5   In this equation, n, F, A, C and Do represented the number of electrons involved in the redox reaction, Faraday’s constant (96484 C eq. mol–1), electrode surface area, the concentration of active species in the bulk electrolyte and their diffusion coefficient, respectively.  The continuous flow of anolyte in the TFB restricts the growth of the diffusion layer on the surface of the negative electrode. However, it is expected that during the charging process, at low cell potential (< 1.55 V), charge transport processes are limited by the formation of reaction products on the surface of the CuFeS2 particles (reaction 8.2) [278, 297]. During charging at high cell potential, the stability of the passive film (Cu1–xFe1–yS2–z and Sº species) is decreased and CuFeS2 is forced to directly oxidize into CuII and FeII species which can be justified from the large peak current (P1) in Figure 8-8c.  Due to the significant potential difference (positive cell potential) between positive and negative electrodes, the reduction of oxidized products (i.e. Fe3+, Cu2+ extracted from CuFeS2) and/or reduction of CuFeS2 itself into other species i.e.  Cu9Fe8S16, Cu5FeS4 and/or Cu2S (reactions 8.3, 8.4) is possible in the following discharge cycle, if the TFB is discharged to very low cell potentials. But due to the selection of a large cut–off cell potential (0.8 V), the tendency of these species to form was expected to be low.  However, the reversible (S22– ̸ S2–) behavior of surface species (via reaction 8.9) and the pseudocapacitive behavior of AC in the PSE are matched by the re–dissolution of highly electro–active Zn at the negative electrode during discharge cycles. These reversible processes promote charge storage in this device. 200  Peak P2 in the CV scans (Figure 8-9a) and the linear dependency of its current vs. v (Figure 8-9c), is consistent with adsorption of ionic species in its porous structure. Also, the peak potential (P2) was remained almost independent of v, which is consistent with the adsorption and/or reduction of leached ionic species (from CuFeS2 i.e. Cu(II) and Fe(III)) in the preceding charge cycle [237, 298]. These species may interact with the surface functionalities of AC and may augment the overall charge storage capability (pseudo–capacitance) of the TFB.  Figure 8-9 (a) CV curves of TFB as shown in Figure 8-8c indicating peak P2 (b) shift in peak potentials (P1 and P3) vs. sweep rate (c) variation in peak current and potential vs. sweep rate for peak P2 201  GCD cyclic tests were performed to measure the specific capacity, energy storage capability and % Cu extraction with the TFB.  The charging and discharging were carried out at 1C and 0.5C rates, respectively and the polarization trends are shown in Figure 8-10a and b. It is apparent that the plateaus in the GCD cycles (Figure 8-10a) replicate the current peaks in the CV scans (Figure 8-8c). However, as seen in the charging trends, compared to first charge cycle, the polarization effect in the subsequent cycles is dominant, which is attributed to the hindrance in the progress of pseudocapacitive (reversible) faradaic reactions within the TFB. This may be related to the passivation of CuFeS2 in the PSE during the first charge cycle, or to the parasitic reactions occurring on the negative electrode that affect the reversibility of the Zn/Zn2+ reaction.  The significant decrease in the discharge-specific capacity during 10 repetitive GCD cycles is related to the area under each discharge curve and it was calculated from Equation 8.6.  𝑸𝑸𝒄𝒄 (𝒎𝒎𝒎𝒎𝒎𝒎 − 𝒈𝒈−𝑿𝑿) =  𝒊𝒊𝒅𝒅𝟑𝟑.𝟔𝟔𝒎𝒎𝒁𝒁𝒎𝒎(𝑽𝑽−𝒊𝒊𝒅𝒅𝑹𝑹)  ∫ 𝑽𝑽(𝒕𝒕).𝒅𝒅𝒕𝒕𝒕𝒕𝟐𝟐𝒕𝒕𝑿𝑿     Equation 8-6 In this equation, id, mZn, V and idR represent the discharge current, the mass of Zn deposit, cell potential and potential drop at the start of each discharge cycle, respectively. The general function ∫ 𝑉𝑉(𝑡𝑡).𝑑𝑑𝑡𝑡𝑡𝑡2𝑡𝑡1  presents the area under the discharge potential profile on a time scale. Upon repetitive GCD cycling, particularly in the subsequent discharge cycles, the plateaus (P3) at 1.2 V are becoming shorter as shown in Figure 8-10b (similar to current peak P3 observed in CV scan, Figure 8-8c).  It is also noteworthy that upon repetitive cycling in the following GCD cycles, the plateau (P3) (at ≈ 1.2 V) disappears. The pseudocapacitive response is most likely controlled by either the sulfide sulfur species formed on the surface of CuFeS2 (reaction, 202  reaction 8.9) or by the reversible adsorption/desorption processes on AC (in PSE) as represented by the steep plateaus (P2) in Figure 8-10b and c. It is also interesting that during repetitive CV scans of the TFB, the current response (both charge and discharge) in each successive cycle decrease continuously, as shown in Figure 8-10d. Akin to the GCD plots, this behavior confirms the limited pseudocapacitive response of the TFB, due to the occurrence of irreversible reactions in the PSE (reactions 8.2 and 8.4). Upon repetitive cycling, the shift in peak potentials to higher cell potentials in the CV scan also indicate the commencement of electrochemically irreversible reactions in the TFB, i.e. Cu and Fe dissolution (non–capacitive, irreversible faradaic reactions) from CuFeS2 [53, 296]. At such high cell potential, the pseudocapacitive response of the TFB is fully managed by the reversible (adsorption desorption and S22–/S2–) reactions on the surface of CuFeS2 and AC in the positive electrode. The specific capacity (Equation 8.6), ηC, ηE and ηV efficiencies of the TFB were determined from the repetitive GCD plots as shown in Figure 8-11a. A large specific capacity (343.2 mAh g–1Zn) is measured during the 1st discharge cycle. Based on total volume of slurry (VS) in the PSE compartment, this discharge capacity in the 1st cycle was equivalent to ~ 1.0 Ah l–1. Over the initial 10 GCD cycles, the specific capacity of the TFB decreases abruptly to 69.6 mAh g–1Zn (equivalent to 0.235 Ah l–1). The supplied charge (during charging) is consumed by irreversible conversion reactions in the TFB as verified from the low ηC (44 ± 2 %) and ηE (32 ± 3 %) over the initial 15 GCD cycles. 203   Figure 8-10 The 10 GCD cycles (a) charge and (b) discharge cycles. (c) 20 to 100 GCD cycles; the specific capacity is calculated based on the mass of Zn deposit during the first charge cycle. (d) Repetitive CV scans of TFB obtained at 0.002 V s–1 From these values, it is clear that a large amount of energy is lost in the successive cycles, mostly due to the formation of a polysulfide surface layer (Cu(1–x)Fe(1–y)S(2–z) + Sº). This 204  is also caused by the Cu and Fe dissolution from CuFeS2 in the PSE in parallel with H2 evolution on the negative electrode. After 20 GCD cycles, the TFBS provides almost constant discharge specific capacity (~ 32 mAh g–1Zn or ~ 0.1 Ah l–1) with little improvement in the ηC, ηE, and ηV, which all remain constant at approximately 52 %, 41 % and 80 %, respectively. During the discharge cycles, the occurrence of faradaic processes at large cell potentials (corresponding to P2 and P3) results in high ηV (80 %), which is considered an encouraging feature for any energy storage device. The energy storage capability of the TFB was calculated from the discharge profiles using Equation 8.7 (Figure 8-11b).  𝑺𝑺𝒔𝒔𝑺𝑺𝒄𝒄𝒊𝒊𝑺𝑺𝒊𝒊𝒄𝒄 𝑬𝑬𝒎𝒎𝑺𝑺𝑬𝑬𝒈𝒈𝑪𝑪 (𝑾𝑾𝒎𝒎 𝒌𝒌𝒈𝒈−𝑿𝑿) =  𝑿𝑿𝟑𝟑.𝟔𝟔𝒎𝒎𝒁𝒁𝒎𝒎  ∫ 𝜟𝜟𝑽𝑽(𝒕𝒕). 𝒊𝒊𝒅𝒅.𝒅𝒅𝒕𝒕𝒕𝒕𝟐𝟐𝒕𝒕𝑿𝑿     Equation 8-7 The maximum specific energy of 388.6 Wh kg–1 (energy density = ~1.1 Wh l–1) during the 1st GCD cycle corresponds to the portion of the supplied energy that can be retrieved upon discharge by the re-dissolution of Zn in the anolyte. The specific energy then decreases rapidly to ~ 50 Wh kg–1 (~ 0.17 Wh l–1) in the subsequent 15 cycles. This behavior is due to the non–capacitive irreversible faradaic reactions (Cu and Fe extraction from the CuFeS2), which consume a large amount of input energy within the TFB during each charging cycle, as evident from the 6.7 % Cu dissolution after 20 cycles determined separately from solution concentrations. It is important to note that the % Cu extracted in the 20th, 40 th, 60 th, 80 th and 100 th cycles was determined in separate GCD experiments. 205   Figure 8-11 Quantitative assessment of (a) Specific capacity (discharge), coulombic, energy and Voltage efficiencies of the TFB. (b) Energy storage capability and % Cu extraction as a function of GCD cycles (Note: The % Cu extraction was calculated from the ICP-OES analysis). (c) The filtrate retrieved from the PSE. (d) Instantaneous % Cu extraction from CuFeS2 in the TFB during 100 GCD cycles A maximum 23.4 % Cu extraction was achieved after 100 GCD cycles. Visually, the presence of dissolved Cu was evident from the filtrate color, as shown in Figure 8-11c. The total time accumulated during GCD cycling and the instantaneous % Cu extraction was plotted as shown in Figure 8-11d. The rapid decrease in specific capacity (Figure 8-11a) from 344 mAh g–1 to ≈ 32 mAh g–1 during the initial 20 cycles indicates the progress of irreversible reactions (via reaction 8.2 and 8.10) within the PSE which was reflected from the 6.3 % Cu 206  extraction from CuFeS2. Therefore, the limited charge storage capability of the TFB is related to the continuous increase in Cu dissolution.  CuFeS2 → Cu2+ + Fe3+ + 2Sº + 5e–      8.10 In 100 GCD cycles which were completed in approximately 26 h, a maximum of 23.4 % Cu was extracted. The oxidation of CuFeS2 (irreversible reactions) in the PSE also markedly decrease the ƞC (50 %) and ƞE (40 %).  However, these poor energetic performance numbers are offset by the simultaneous and valuable Cu extraction from an otherwise refractory mineral. Based on these results, it is speculated that this battery-like setup could be utilized as a unit for both energy storage and as a source of Cu production from mineral concentrate on a large scale. In the 100 th GCD cycle the specific energy was found to be 23.5 Wh kg–1 (or 0.07 Wh l–1).  In addition to dissolved Cu, the filtrate also contains dissolved Fe species, which must be removed prior to recovery of valuable Cu metal through an SX/EW (or other appropriate) process. The pH of the PSE filtrate should also be increased to ~2.0 from 0.9 by the addition of lime and to precipitate soluble Fe as oxides or hydroxides. After solid/liquid separation, the catholyte can be further enriched with dissolved Cu through the well-established SX process before circulating it through the electrowinning cell for the recovery of Cu metal.      It is evident in Figure 8-10b that the potential plateau (P3) tends to shrink and the discharge capacity is controlled by the AC and possibly by sulfide sulfur species in the PSE (as demonstrated by the P2 plateau). Also, the reversible transformation of the dissolved species (i. e. FeII and FeIII) from CuFeS2 could also contribute to the formation of this potential plateau. The plateau (P2) at high cell potential, (Figure 8-10c) therefore indicates the transformation of the TFB from battery to capacitor like behavior after repetitive GCD cycles. With this hybrid feature in mind, Figure 8-12a shows the rated capacity by discharging the 207  TFB at various C–rates. The specific capacity decreases significantly from 344 to 92.8 mAh g–1 when the discharge rate is increased from 0.5C to 1C. Furthermore, upon discharging at 1.5, 2.0 and 2.5C, the specific capacity gradually decreases to 44.3, 31.1 and 30.0 mAh g–1, respectively. This behavior is dominated by the reversible reactions occurring on the surface of AC and CuFeS2. As discussed above, the presence of sulfide sulfur species on the surface of CuFeS2 (i.e. S2–, S22– etc.) and adsorption/desorption of cationic species i.e. H+, CuII, FeII, FeIII, on the surface of AC and CuFeS2 likely facilitate this pseudocapacitive response [243, 299]. In other words, the high specific power and low specific energy at large C-rate correspond to surface or near surface reactions for which the charge transfer is limited at the electrode/electrolyte interface. The Ragone plot of the TFB (Figure 8-12b) shows the highest specific power (2.05 kW kg–1) and the lowest specific energy of 34.8 Wh kg–1 when discharged at 2.5C. This typical behavior of high specific power and low specific energy at high C–rates and vice versa is attributed to the kinetic limitation of reversible (pseudocapacitive) faradaic reactions in the TFB as reported in the literature [132, 300, 301].  208   Figure 8-12 (a) Discharge profiles of the TFB obtained at various C–rates, (charged at the 1C rate) (b) Ragone plots developed based on the first GCD cycle (c) Cell potential variation of fully charged TFB under an applied load of single LED (d) Illuminated LED when connected to fully charged TFB To further demonstrate the practical use of the TFB, the cell was initially charged to 1.8 V at a rate of 1C and connected to a light emitting diode (LED), mounted on a single board microcontroller (Arduino). This Arduino was designed to keep the LED illuminated until the cell potential fell to <1.0 V by applying the LED load (discharge). The cell potential of the TFB setup under an applied load of ~ 0.5 mA (single LED) was measured with the external potentiostat as shown in Figure 8-12c. Under applied load, the high initial cell potential (≈ 1.5 V) decreased rapidly to ≈ 1.3 V in 400 s, followed by a gradual potential decay to 1.2 V in 1.4 209  h. The TFB was able to keep the LED lit for ≈ 1.8 h until the 1.0 V cut-off potential was reached (Figure 8-12d), which is an impressive feature of this setup, thus demonstrating its practical applicability as energy storage device. Gradual decay of the  cell potential from ≈ 1.3 V to the cut-off potential under the applied LED load also indicated a negligible corrosion of deposited Zn. However, there is a possibility of solution-level corrosion of deposited Zn once the TFB is fully charged and left at OCV.  This process is called the self-discharge of the TFB. To use the TFB as a hydrometallurgical unit, the solution-level corrosion of the deposited Zn can be prevented by stopping the anolyte flow into the cell and by rapid replacement of the cathode after each charge cycle. 8.4 Cause of capacity fade and mass balance in TFB 8.4.1 Diagnosing possible reactions in TFB During charging, it is expected that CuFeS2 will passivate at low cell potential (< 1.50 V) and polysulfide passive film forms on the surface of CuFeS2 in PSE (regime–1) as discussed above (reaction 8.2). Also, at large cell potentials (regime–2), the stability of this passive film is decreased and CuFeS2 in the PSE can oxidize either via reactions 8.10 and/or 8.11 supported by Zn deposition and H2 evolution on the negative electrode (these potential regimes are shown in Figure 8-10a) during the charge cycle. CuFeS2 + 8H2O → Cu2+ + Fe3+ + 2SO42– + 16H+ + 17e–   8.11  It has been reported that at sufficiently large cell potentials, CuFeS2 may oxidize directly to Cu2+ and FeIII species via these reactions 8.10 and 8.11 [36, 53]. In addition, it is important to note that according to reactions 8.2 and 8.10, the oxidation of CuFeS2 was pH independent. During charging at high cell potential (> 1.7 V), it is expected that H2O may oxidize at the surface of CuFeS2 and some of the sulfide sulfur may oxidized to sulfate. This 210  reaction (reaction 8.11) may decrease the pH of the catholyte due to the generation of a large amount of H+. To determine the extent of these reactions in the TFB, the total charge consumed during each charge/discharge cycle was estimated from the Equation 8.8.   𝑸𝑸𝒄𝒄 =  𝒊𝒊𝒄𝒄(𝑽𝑽−𝒊𝒊𝒄𝒄𝑹𝑹)  ∑ ∫ 𝑽𝑽(𝒕𝒕).𝒅𝒅𝒕𝒕𝒕𝒕𝟐𝟐𝒕𝒕𝑿𝑿𝒔𝒔𝒊𝒊=𝑿𝑿      Equation 8-8 Where, ic is the charging current and V is the maximum applied cell potential (1.8 V) subtracted by the icR drop across the cell during the charge cycle. The number of cycles (p) is the area under each charge curve (i) and is presented as the operator∫ 𝑉𝑉(𝑡𝑡). 𝑑𝑑𝑡𝑡𝑡𝑡2𝑡𝑡1 . Similarly, the total charge during each discharge curve (Qd) is estimated, and the charge consumed for the faradaic (non–capacitive) irreversible reactions (reactions 8.2, 8.10 and 8.11) (Qirr) was calculated as Qc – Qd. Under applied conditions, it was considered that the charge consumed during passive film formation and ionic release of Cu and Fe (reaction 8.2) was small at low cell potentials compared to the total current involved in reactions 8.10 and 8.11 at large cell potentials. It is, therefore, essential to calculate the extent of each reaction (8.10 and 8.11) based on the degree of sulfide sulfur oxidation to sulfate as proposed by Biegler et al [302]. It is assumed that if x is the fraction of CuFeS2 oxidized according to reaction 8.11, then the overall reaction can be written as reaction 8.12. In this regard, the number moles of electrons per mole of Cu2+ (n) were calculated from the total irreversible charge (Qirr = Qc – Qd) from the GCD tests and from the concentration of Cu extracted from CuFeS2 (as determined from ICP–OES analysis), as a function of GCD cycles according to Equation 8.9. 𝒎𝒎 =  ∑ 𝑸𝑸𝒊𝒊𝑬𝑬𝑬𝑬𝒔𝒔𝒊𝒊�𝒎𝒎𝑪𝑪𝑪𝑪 𝑴𝑴� �.𝒎𝒎       Equation 8-9 211  In this equation ∑ 𝑄𝑄𝑖𝑖𝑖𝑖𝑖𝑖𝑝𝑝𝑖𝑖 is the total charge consumed in the non–capacitive faradaic reactions (reactions 8.10 and 8.11). Also, p is equal to 20, 40, 60, 80 and 100 cycles and the subscript i, mCu, M and F represent the cycle number, mass of Cu extracted as determined from the ICP–OES analysis, molecular mass of Cu and Faraday constant (96485 C–mole–1), respectively. By assuming that x is the fraction of CuFeS2, which was oxidized via reaction 8.11, then the cumulative reaction can be written as reaction 8.12.  CuFeS2 + 8xH2O → Cu2+ + Fe3+ + 2xSO42– + 2(1–x)Sº + 16xH+ + (12x+5)e–  8.12 Based on the procedure adopted in other studies [36, 53, 302], the extent of reaction (x) was calculated by equating the number of electrons per mole of Cu2+ produced with the number of electrons per mole of Cu2+ formed via reaction 8.12 that is n = 12x + 5, and the values are given in Table 8-1. It is also important to mention here that the decrease in n (number of electrons transfer per mole of Cu2+ dissolution) with increase in charge/discharge cycles was possibly attributed to the limited dissolution of Cu2+ due to possible thickening of sulfide sulfur layer on the surface of CuFeS2 (via reaction 8.10) during GCD cycling. In other words, the dominant non-capacitive irreversible faradaic process in the PSE was identified as Cu2+ dissolution from CuFeS2 preferably via reaction 8.10. This was validated form the negative x values (Table 8-1) which indicated the restricted oxidation of Sº into SO42– according to reaction 8.11. In addition, these results have been reinforced by the large overpotential for H2O oxidation in the positive slurry electrode as confirmed from the CV scan (Figure 8-8a).   212  Table 8-1 Quantitative determination of the extent of reaction from GCD and ICP-OES analysis  No. of Cycles (p) *∑ 𝑸𝑸𝒊𝒊𝑬𝑬𝑬𝑬𝒔𝒔𝒊𝒊  (C) †mCu (mg) n Extent of reaction (x) 20 58.64 37.28 1.03 –0.33 40 73.17 38.80 1.24 –0.31 60 85.76 54.07 1.04 –0.33 80 96.68 81.85 0.78 –0.35 100 102.6 127.7 0.53 –0.37  8.4.2 SO42– transport and mass balance  Due to Zn deposition in the charge cycle, the excess of SO42– species in the anolyte would transport through the membrane to catholyte and is balanced by CuII and FeIII species generated from the oxidation of CuFeS2 (reaction 8.10). It is therefore suggested that presence of high concentration of total sulfate {(SO42–)T = SO42– + HSO4– + NaSO4–} in the anolyte not only increased its conductivity but also suppressed the acid generation in the catholyte during the charge cycle. It was obvious that anionic diffusion flux from catholyte {low (SO42–)T} to anolyte {high (SO42–)T} was negligibly small compared to migration component during the discharge cycle. Also, due to the high selectivity of AEM (95 %) used in this TFB, it is expected that cations cross over through the membrane would be very small. In support to this, the very low concentration of Fe (1.5 ppm) and Cu (1.7 ppm) in the anolyte after 100 GCD cycles were determined from ICP–OES analysis, which verified the negligible cross-contamination of electrolytes during TFB operation. In the discharge cycle, the cationic species (H+, CuII FeII and FeIII species etc.) can adsorb on the CuFeS2 and AC surface, giving rise to the discharge capacity and by reverse transport of SO42– from catholyte to anolyte to balance the redissolved Zn2+.  213  During charging/discharging, the anionic species in both anolyte and catholyte can transport through the membrane (AEM) via diffusion and/or migration due to the existence of both concentration and electric field gradients in the TFB. Consequently, the mass transport from anolyte to catholyte and vice versa is highly dependent on the membrane characteristics. During operation, as the cell potential is continuously varying during each charge and discharge cycle, the variable electric field (𝒅𝒅𝝍𝝍𝒙𝒙𝒎𝒎) (where, dψ and xm are the cell voltage and membrane thickness, respectively) across the AEM significantly influence the migration of ionic species. To simulate the mass transport during the GCD process, it is therefore essential to consider the membrane characteristics. During charging the anionic species diffuse and migrate from the anolyte to catholyte. However, only migration of anionic species from the catholyte to anolyte was possible during the discharge process due to positive concentration gradient across the membrane. To incorporate the membrane characteristics, the extended Nernst–Planck Equation 8.10 and Equation 8.11 were used [303] in this work.  𝑱𝑱 = 𝒌𝒌𝒎𝒎 �−𝑫𝑫𝑺𝑺���� �𝑪𝑪𝑺𝑺𝑪𝑪−𝑪𝑪𝑺𝑺𝒎𝒎�𝒙𝒙𝒎𝒎 − 𝒛𝒛𝒎𝒎𝑹𝑹𝑹𝑹 𝑫𝑫𝑺𝑺����𝑪𝑪𝑺𝑺𝒎𝒎 𝒅𝒅𝝍𝝍𝒙𝒙𝒎𝒎� + 𝑪𝑪𝑺𝑺𝒗𝒗     Equation 8-10 𝒌𝒌𝒎𝒎 = 𝒗𝒗𝒔𝒔𝑫𝑫�𝑺𝑺𝜽𝜽𝟐𝟐𝑫𝑫𝑺𝑺 . 𝑿𝑿𝒙𝒙𝒎𝒎        Equation 8-11 In Equation 8.10, the km is the membrane constant and can be determined from Equation 8.11. 𝐷𝐷𝑠𝑠, 𝐶𝐶𝑆𝑆𝑐𝑐 ,𝐶𝐶𝑆𝑆𝑎𝑎 , 𝑣𝑣𝑝𝑝,𝐷𝐷𝑠𝑠���,   𝑎𝑎𝑛𝑛𝑑𝑑 𝜃𝜃2 are the diffusion coefficient, bulk concentration of 𝑆𝑆𝑆𝑆42− ions at the membrane/catholyte and membrane/anolyte interface, pore volume fraction in the membrane, the diffusion coefficient of  𝑆𝑆𝑆𝑆42− in the membrane pore solution, and tortuosity of the membrane, respectively.  In this case, a value of km = 0.0074 for the AEM was used, which has been reported by Koter et. al. [304].  214  It is assumed that the convective flux (Csv) of ionic species within the membrane is negligible and by including both diffusion and migration effects on the mass transport of ionic species, the total flux of anionic species during the 1st charge and discharge was calculated. The values of other parameters used in the calculation are given Table 8-2. Due to high selectivity (0.95) of the AEM used in this study and pre–treatment in dilute saline solution, it was assumed that in sulfate based anolyte and catholyte, the transport of bisulfate (𝐻𝐻𝑆𝑆𝑆𝑆4−) and NaSO4– was limited as shown schematically in Figure 6-13c. It has also been reported in the literature that, depending on the water content of the AEM, and its weak base characteristics, only 𝑆𝑆𝑆𝑆42− ions would carry through the membrane as electric charge [305-307]. It is assumed that the diffusion coefficient of 𝑆𝑆𝑆𝑆42−in the pore solution of the membrane is 𝐷𝐷𝑠𝑠��� =  𝐷𝐷𝑠𝑠 10�  is 10 times lower than in the bulk solution [308, 309] as given in the Table 8-2. Table 8-2 Parameters used to determine the mass transport across the AEM  Symbol (Abbreviation) (Units) Value (Reference) km  (Membrane Constant) 0.0074 [304] xm  (Membrane Thickness) (m) 1.3 x 10–4  𝐷𝐷𝑠𝑠��� (SO42–Diffusion coefficient within membrane) (m2/s) 1.06 x 10–10 [280, 310, 311]  Am (Area of the membrane) (m2) 3.0 x 10–4  VA & VC (Volume of anolyte and catholyte) (m3) 3.0 x 10–6  z (Charge number of SO42–) –2 R (Universal gas constant) (J/mole–K) 8.314  T (Absolute Temperature) (K) 298.15  F (Faraday’s constant) (C/mole) 96485   Certainly, the ingress of anionic species towards and from the catholyte during charging and discharging steps, respectively, strongly depends on the AEM characteristics, it is, therefore, important to consider the membrane properties in the calculation. As shown in the 215  schematic diagram (Figure 8-13a), owing to the water content in the pore solution of the membrane, it was considered that SO42– species can preferably transport through AEM which was due to its weakly basic nature and can reject cationic part of HSO4–, NaSO4– species at the electrolyte/membrane interface, as reported elsewhere [305-307]. The excess of SO42– in the anolyte (due to Zn deposition) was partially compensated by these rejected cationic species. In this calculation, the (SO42–)T concentrations in the anolyte and catholyte were used to calculate the total mass transfer during each GCD cycles.  The concentrations of individual species in the anolyte under study were also determined thermodynamically from the equilibrium constant approach by using PHREEQC software package [312, 313]. It was evaluated that free Zn2+ represented approximately 27% of the total ionic concentration (equivalent to ≈ 50 g l–1 free Zn2+ in the anolyte) at pH 1.1 ± 0.1 and 25 ºC (initial experimental conditions). The other dominant species i.e. HSO4– and SO42– in the anolyte accounted approximately 20 % and 17 %, respectively of the total ionic concentration as shown in Figure 8-13b. In addition, a very small fraction of NaSO4– (≈ 2 %) was available to carry the ionic current during the charge/discharge process. From these calculations, it was revealed that large concentration of undissociated ZnSO4(aq) (almost equivalent to Zn2+ concentration) was present in the anolyte which may further increase to (approx. 40 %) if the pH becomes higher than 2. The small amount of H2 evolution during Zn deposition could increase the pH at the surface of the negative electrode.  From anolyte speciation study, it can be predicted that due to a local change in pH change at the electrode surface could result in the decrease of Zn2+ concentration compared to ZnSO4(aq) hence effective Zn deposition may be affected. This effect may also deteriorate the TFB cyclic performance. During charge cycle, in addition to migration, the diffusion of SO42– 216  due to the relatively large concentration gradient of (SO42–)T at the anolyte (1.83 M) | membrane | catholyte (0.2 M) interfaces could promote the SO42– transport towards positive slurry electrode compartment.  Figure 8-13 (a) Schematic diagram of ionic species transport across AEM. (b) The molar fraction of ionic species in the anolyte at 25º C (c) The transient electric field and (d) the molar flux of SO42– across AEM Based on the transient electric field across the AEM (Figure 8-13c), the instantaneous molar flux (J) of the SO42– was determined from the first GCD cycle. It is important to note that only the second term (migration effects) in Equation 8.10 was used to calculate the flux during the discharge cycle. By integrating the instantaneous molar flux curves, shown in Figure 8-13d, the amount of SO42– transferred per unit surface area of the membrane towards and from 217  the catholyte was determined and designated as mnc and mnd, where n is the number of GCD cycles and c and d in the subscripts represent the charge and discharge cycle, respectively.  During 100 GCD cycles, the concentration of overall 𝑆𝑆𝑆𝑆42–transferred to the catholyte (or depleted in the anolyte) was determined by considering the fractional capacity decay during each successive charge/discharge cycle by Equation 8.12.  𝒎𝒎𝒎𝒎𝒄𝒄 =  ∏ [𝒎𝒎𝒎𝒎𝒄𝒄−𝑿𝑿(𝑿𝑿 − (𝑪𝑪𝒎𝒎𝒄𝒄−𝑿𝑿− 𝑪𝑪𝒎𝒎𝒄𝒄𝑪𝑪𝒎𝒎𝒄𝒄−𝑿𝑿𝑿𝑿𝑿𝑿𝑿𝑿𝒎𝒎𝒄𝒄=𝟐𝟐 )     Equation 8-12 Where, mnc is the amount of 𝑆𝑆𝑆𝑆42– transferred to catholyte during n number of charge cycles and Cnc represents the charge capacity. Similarly, the amount of 𝑆𝑆𝑆𝑆42– transferred from the catholyte to anolyte during the discharge cycle (mnd) was calculated. The mn = mnc – mnd was used to quantify the total 𝑆𝑆𝑆𝑆42– transported to the catholyte. The concentration of 𝑆𝑆𝑆𝑆42– species transported to the catholyte during the 100 GCD cycle was calculated to be 0.083 moles l–1. It was considered that the most dominant HSO4– species in the anolyte (Figure 8-13b) and NaSO4– may dissociate to produce H+ and Na+ cations, respectively at the interface of the highly selective AEM. These rejected cationic species may again complex with the 𝑆𝑆𝑆𝑆42– (formed during Zn deposition) to regenerate parent species (HSO4–, NaSO4–) in the anolyte. Under applied conditions, the cumulative SO42– transferred to the catholyte during the 100th GCD cycles (calculated from the Nernst–Planck equation) was found to be approximately equal to the total concentration of Cu + Fe species (0.085 moles–l–1) as measured from ICP–OES analysis after 100 GCD cycles. These results explain that the excess of 𝑆𝑆𝑆𝑆42– transporting from anolyte to catholyte was balanced by the total concentration of Cu and Fe species that were generated in the catholyte during the 100 GCD cycles. However, the local variation of pH on the surface of Al during Zn deposition, continuous anolyte flow, membrane selectivity 218  and generation of Cu and Fe species in the catholyte are considered dominant factors that could directly influence the SO42– transport and overall performance of the battery.   8.5 Polarization behavior of TFB The overall performance and polarization effects in the TFB were evaluated from the I–V curve as shown in Figure 8-14. The cell voltage is found to decrease linearly from 1.3 V to 1.13 V with an increase in discharge current up to 4.2 mA. This polarization trend at low currents is related to the kinetics of electrochemical processes within TFB corresponding to the electrochemical response of the CuFeS2 in the PSE and Zn redissolution in the negative compartment. However, it is expected that kinetically, Zn re-dissolution in the acidic electrolyte is more favorable than CuFeS2 reduction (and/or reduction of surface species formed during the preceding charge cycle) in the PSE during discharge. On the other hand, the reversible adsorption/desorption of ionic species on the AC surface could significantly improve the kinetic response of the slurry system. Maximum specific power delivered by the TFB at 4.2 mA was 0.84 kW kg–1, which is comparable to the value registered by this setup at 1C discharge rate (Figure 8-12b). The narrow potential plateau at 1.1 V exhibited by the TFB could be related with the overall reversible (faradaic) response by sulfide sulfur species (reaction 8.9) and surface adsorption ̸ desorption on AC. The appreciable decay in cell potential at a large discharge current (> 6 mA) points to the dominance of ohmic drop and mass transport-controlled processes within the TFB, most likely associated with the membrane characteristics. Also, the low conductivity of CuFeS2 and its slow kinetic response in addition to IR drop across the electrode/electrolyte/membrane interfaces could cause the steep potential decay at large applied currents. The significant decay in the cell potential at high currents (up to 0.9 V) could also be related to the slow ionic mobility of anionic species across the 219  membrane. The maximum specific power (≈ 2.6 kW kg–1) achieved at high discharge current (15 mA) confirmed the hybrid behavior of the TFB.  This value is comparable to the existing asymmetrical supercapacitors (see Table 2-5 and references cited therein for further details). This behavior is thought to be explicitly controlled by the AC and sulfide sulfur species within the slurry electrode. During the GCD cycling (Figure 8-10b, c and Figure 8-12a), the highly pseudocapacitive response of the TFB was controlled by the potential plateau (P2), which corresponds to the surface limited reversible faradaic reactions on the AC and CuFeS2 (reaction 8.9) as discussed above.  Figure 8-14 I–V curve of the fully charged TFB (at 1C) showing the polarization trend and variation in specific power at various discharge currents. 220  8.6 Energy storage by TFB equivalent to energy produced by a diesel generator Based on the TFB performance, its energy storage capability was compared with the energy produced by a diesel generator. It was assumed that the TFB setup could accommodate one tonne of synthetic CuFeS2 in the PSE.  From the experimental results, this setup might store up to 8.83 kWh t–1 of CuFeS2 at 40 % energy efficiency. The cyclic life of the TFB was 100 GCD cycles, which was completed in 26 h and this setup is capable of extracting 23.5 % Cu from the CuFeS2. According to these parameters, the TFB should be operated for 100 GCD cycles before refreshing the PSE in the positive compartment having capacity of 1.875 m3 to accommodate slurry (20 wt. % solids containing CuFeS2 to AC ratio of 4:1) containing 1 tonne of CuFeS2. Based on these assumptions, the total amount of energy required to operate the TFB for one year was calculated as tabulated in Table 8-3. From these calculations, it was estimated that this TFB setup can store and deliver 2975 kWh year–1. Considering the calorific value of diesel and energy efficiency of the diesel generator (60 %), the same energy can be generated from 0.39 tonnes of diesel per year. Based on these calculations, it is estimated that to supply the energy produced by 1 tonne of diesel (7584 kWh t–1 of diesel) the TFB would require 860 tonnes of synthetic CuFeS2.      221  Table 8-3 Estimation of energy storage by a TFB, which has the capacity to accommodate 1 tonne of CuFeS2 in PSE and comparison with the energy generated by a diesel generator          8.7 Summary In this Chapter, we have presented the coupling of two energy-intensive hydrometallurgical processes in a battery-like setup capable of extracting Cu from CuFeS2 and electrowinning Zn during a charge cycle. In the discharge cycle, part of the supplied energy is retrieved at the expense of deposited Zn. The occurrence of both pseudocapacitive (reversible) Values unitsAssumptionsAmount of energy stored in one cycle 0.0883 kWh/t of CuFeS2Energy storage efficiency 40 %Cyclic life of TFB 100 cyclesTime to complete 100 cycles 26 hoursEfficiency of Cu extraction in 100 cycles 23.4 %Capacity of the TFB 336.92 t of CuFeS2/yearVolume of each compartment in a TFB 1.875 m3/t of CuFeS2Amount of energy stored in 100 cycles 8.83 kWh/t of CuFeS2Amount of energy supplied in 100 cycles 22.08 kWh/t of CuFeS2Cu extraction capacity 27.30 t of Cu/yearTotal amount of energy required to operate TFB 7446.00 kWh/yearTotal anount of energy stored in TFB 2975.03 kWh/yearTotal energy required for TFB including pre-processing24820.00 kWh/yearCalorific value of Diesel 12640 kWh/t of dieselEnergy efficiency of a diesel generator 60 %Amount of energy available from diesel 7584 kWh/t of dieselTotal amount of diesel required to operate TFB 3.27 t of diesel/yearTFB energy storage equivalent to energy supplied by diesel generator0.39 t of diesel/yearCuFeS2 to diesel ratio for energy supply 860 / 1 t/tCalculations based on the performace of TFB and by using synthetic CuFeS2Calculations of energy storage by TFB equivalent to diesel generatorDesign of a TFB, which can accommodate 1.0 t of CuFeS2 in PSE222  and non–capacitive (irreversible) faradaic reactions in the positive slurry electrode containing synthetic CuFeS2 + AC in 0.2 M H2SO4 was identified from the CV scans. On the other hand, the conditions for efficient Zn deposition were optimized with the addition of 0.1 M Na2SO4 and different concentrations of CTAB in 100 g l–1 Zn2+ anolyte prepared in 0.2 M H2SO4.  A potential–concentration (PC) diagram was developed to estimate the overpotential required for efficient Zn deposition as a function of CTAB concentration. The lowest overpotential of –0.44 V and maximum 92.3 % current efficiency for Zn deposition was achieved at 2 ppm CTAB concentration.  The high cell potential (0.96 V) and capability to leach Cu from CuFeS2 during the charging cycle are promising features of this TFB. In this setup, the pseudocapacitive response of AC in synergism with the sulfide sulfur species (S22–/S2–) redox reaction in the positive slurry electrode facilitated the reversible deposition/re–dissolution of Zn in the negative compartment as predicted from the CV data.  Based on the results of GCD cyclic tests and Cu extraction from CuFeS2 (from ICP–OES analysis), it is suggested that no acid is produced during charging up to 1.8 V. The transient molar flux of SO42– from anolyte to catholyte and vice versa through the AEM was determined via mass balance. It was found that during charging, the total sulfate transported from anolyte was compensated by the Cu and Fe species generated in the catholyte.  In the 1st GCD cycle, the highest discharge specific energy of 388 Wh kg–1 was achieved (1.13 Wh l–1), which rapidly decreased to 50 Wh kg–1 (~0.17 Wh l–1) during the initial 15 repetitive GCD cycles followed by gradual fall to 23.5 Wh kg–1 (~0.07 Wh l–1) in 85 cycles. The low specific energy at low coulombic (≈ 50 %) and energy efficiency (≈ 40 %) indicated the progression of irreversible reactions in the TFB.  This was validated by ICP–OES analyses 223  of the catholyte and anolyte. The maximum ≈ 23 % Cu extracted from CuFeS2 is an incentive from this setup in the form of valuable metal. The highest 2.05 kW kg–1 specific power and 34.8 Wh kg–1 specific energy (at 2.5C discharge rate) as well as the polarization response of the system, confirm the hybrid behavior of TFB as battery and supercapacitor. 224  Chapter 9: Using a CuFeS2 mineral concentrate in the TFB-M This chapter reports preliminary results on the use of an industrial MC of CuFeS2 as a positive slurry electrode (PSE-M) in the TFB (designated as TFB-M) setup. Both energy storage and Cu extraction capabilities of the TFB-M setup were measured. In this setup, the electropositive PSE-M separated by a membrane was coupled with the circulating acidic Zn2+ containing anolyte. During charging of the TFB-M, the oxidation of the MC in the PSE-M would promote the deposition of Zn on the negative current collector (Al sheet). Part of the supplied energy could be stored in the form of deposited electroactive Zn metal. However, the large cell potential and re-dissolution of Zn from the negative electrode could release a portion of the stored energy during the discharge cycle. During the charge process, the progress of irreversible reactions in the PSE-M (i.e., Cu and Fe dissolution from CuFeS2) and any H2 evolution on the negative electrode would consume a large amount of energy. Consequently, the overall energy storage capability of the TFB depends on the reversible electrochemical reactions on the MC and on the Zn deposition side of the process.  9.1 Difference between TFB and TFB-M The TFB-M uses the same experimental setup as shown in Figure 4-4 and Figure 8-7. The only difference is the addition of MC + AC mixture to form a positive slurry electrode (designated as PSE-M) in the TFB-M instead of the PSE (as used in the TFB). The same MC was injected (in the form of PSE-M) in this setup as used in the FBFC (C – 2) setup (see section 7.3 for details). Section 4.2.1 and section 7.1, respectively, describe the pre-washing procedure and mineralogy of the MC determined from QXRPD analysis and is reported in our previously published work [276]. The MC contains ~26.6 wt. % CuFeS2 and FeS2 (43.3 wt. %) as a major impurity as evident in the XRD pattern (Figure 7-3). The cumulative distribution curve (Figure 225  7-2b) of MC shows the D80 of 48.4 µm. This distribution curve also indicates the presence of variable size particles in the MC ranging from < 1 µm to ~100 µm. The same anolyte (100 g Zn2+ + 0.1 M Na2SO4 + 2 ppm CTAB in 0.2 M H2SO4) was circulated in the negative compartment of the TFB-M.  9.2 Energetics of the TFB-M The energetics of the battery setup depends on the electrochemical behavior of each electrode and on a sufficiently large potential difference between positive (cathode) and negative (anode) energy storage materials [314]. Similar to the TFB (Figure 8-8b), the approximate 1.0 V OCV of the TFB-M is an attractive feature of this setup, indicating the possible use of MC as an electrode material for energy storage (Figure 9-1a). To confirm this, CV curves of the TFB-M were obtained at various sweep rates (from 100 to 1 mV s–1). The positive current obtained by varying the potential from OCV to 1.9 V cell potential (~0.9 V vs. OCV) in the forward scan exhibits the charging of the TFB-M. On the other hand, by reversing the scan from 1.8 V to low cell potential (~0.5 V) the discharge current profile is measured. During the discharge cycle, the current peaks demonstrated the reversal of electrochemical reactions, which occurred in the preceding charge step. As discussed in Chapter 8, the appreciable increase in the anodic current (during the charge step) as shown in Figure 9-1b, indicates the progress of Zn deposition on negative electrode and oxidation reactions in the PSE-M. The re-dissolution of Zn in the anolyte and occurrence of reversible reaction on the PSE-M during the discharge process were responsible for the current output in the reverse scan. The decrease in both positive and negative current with a decrease in sweep rate attributed to the growth of diffusion layer on the surface of electrodes materials. In other words, with the progress of kinetically controlled electrochemical reactions, the 226  diffusion layer at the electrode/electrolyte interface, which would grow. The decrease in current response at slow sweep rates confirms this behavior. For instance, at 100 mV s–1, the large peak current during the charging/discharging process shows the oxidation/reduction reactions in the PSE-M.  Figure 9-1 (a) OCV of the TFB-M and (b) CV scans of the TFB-M at various sweep rates. 227  In addition, at slow sweep rates, it was interesting to note the disappearance of a discharge current peak (between 1.25 – 1.50 V), which indicated the occurrence of irreversible reactions (non–capacitive faradaic response) in the TFB-M. This behavior corresponds to the oxidation reactions in the PSE-M that consumed a considerable amount of the supplied charge in the charge cycle, possibly due to the formation of reaction product (i.e. metal-deficient polysulfide film via reaction 6.1) on the surface of CuFeS2. Therefore, the relatively small discharge current at low potential (< 1 V) compared to charging current showed irreversibility in the TFB-M. This effect also indicates that impurity phases in the MC would consume a large amount of supplied energy and may induce polarization effects in the TFB-M.  During discharge, the small limiting current response represents diffusion-controlled processes, which are most likely associated with the conversion layer that may form on the surface of MC particles during the charge process and to the large impurity phases in the PSE-M. To further, explore this irreversibility in the TFB-M, CV cycles repeated at 25 mV s–1 sweep rate are shown in Figure 9-2. In cycle–1, the charge current was relatively larger than the charge current response in the second cycle (cycle–2). The increase in current during charge cycle corresponded to the dissolution of Cu and Fe from the PSE-M and Zn deposition in the negative electrode. However, decrease in charge current in the second cycle indicated the restricted oxidation reactions in the PSE-M possibly due to the passivation of CuFeS2 in the PSE-M. During charging, the first peak (P1c) at ~1.5 V (cell potential) was related to the pseudocapacitive response of the AC and active dissolution of CuFeS2 in the PSE-M supported by Zn deposition on the negative electrode. However, an increase in the current beyond 1.5 V and peak P2c at (~1.75 V) corresponds to the formation of a passive film on the surface of CuFeS2. In the reverse scan (discharge cycle), the large IR drop (~0.5 V) (see Figure 9-2) also 228  highlights the restricted kinetic response of the TFB-M, mainly associated with the irreversible charge transfer processes and impurity contents in the PSE-M. A current peak (Pd1) at ~1.35 V similar to the peak P2 observed in case of TFB (see Figure 8-8c) corresponds to the pseudocapacitive response of the AC in the PSE-M.  Further discharging to low cell potential resulted in a slight increase in the current (cycle 1). Owing to the concentration of CuFeS2 in the MC, this limiting discharge current further validated the formation of the surface film and its non-capacitive (irreversible) character in the PSE-M. However, after the first discharge cycle, in the charging scan (cycle–2), a small peak P3c was observed at approximately 1.0 V, which was attributed to the oxidation of the products formed in the PSE-M during the preceding discharge cycle. The small charging current associated with this peak (P3c) compared to the discharge current in cycle–1 confirmed the occurrence of irreversible reactions in the TFB-M. Compare to the discharge current in cycle–1, the relatively small discharge current in cycle–2 indicated that this setup could be used as a primary battery. This is obtained by applying only one charge and discharge cycle on the TFB-M and then replacing the PSE-M with fresh MC feed prior to next GCD cycle.  From the hydrometallurgical perspective, the attractive feature of this TFB-M is its use as a unit for Zn electrowinning. Instead of using the water oxidation reaction in the conventional Zn electrowinning cell, one could use the PSE-M in the TFB-M. The additional benefit of this setup could be the Cu extraction from the MC during repetitive charge cycles and the depleted PSE-M (due to Cu dissolution) could be replaced with the fresh slurry after a certain number of charge cycles (for example after 10 –14 cycles as discussed in the following section). 229   Figure 9-2 CV curves of TFB-M (at 25 mV s–1) showing the variation in current response during repetitive cycling 9.3 Energy storage and Cu extraction capabilities  Based on the electrochemical behavior and small current response of the TFB-M during repetitive charge and discharge (CV scans), it was estimated that this setup should be charged and discharged at low currents. To evaluate the cyclic performance of the TFB-M, the charging and discharging was carried out at 2 mA and 0.5 mA, respectively. In addition, 1.6 and 0.6 V cut-off cell potentials during charge and discharge cycles respectively, were selected for the TFB-M. The mass of the Zn2+ deposited on the negative electrode during the charge cycle is equal to the amount of Zn2+ re-dissolved during the discharge cycle and can be estimated from the discharge capacity. Figure 9-3 shows the three repetitive charge and discharge cycles and the total charge (volumetric capacity) was determined by integrating the charge/discharge 230  profiles according to Equation 8.6. In this relation the charge was normalized by the volume of slurry in the positive electrode compartment of the TFB-M. In the 1st charge cycle, akin to the current peak (P1c) in the CV scan, the extended potential plateau at ~1.53 V represents the effective Zn deposition on the negative electrode and oxidation of the MC in the PSE-M. It is important to note that the total charge consumed by the TFB-M in the 1st charge was approximately 5 times larger than the 1st discharge capacity. This behavior corroborates to the progress of non–capacitive faradaic (irreversible reactions) processes. In confirmation to the discharge peak Pd1 (Figure 9-2), the potential plateau at ~1.4 V during the 1st discharge represents the possible adsorption of CuII and FeII species on the AC in addition to its non-faradaic capacitive response (charge stored in the electrical double layer). During the discharge cycle, these processes in the PSE-M were supported by the Zn re-dissolution in the negative compartment of the TFB-M. CuII and FeII species were generated from the oxidation of CuFeS2 in the PSE-M during the charging step. The appearance of the small potential plateau at ~1.44 V in the 2nd charge cycle is due to the pseudocapacitive response of the AC and oxidation of the reaction products formed in the PSE-M in the preceding discharge. The shift in the potential plateau to 1.57 V in the 2nd charge cycle reflects the hindrance in the charge transfer processes that was associated with the possible irreversible character of the impurity phases in the PSE-M. This was confirmed by the significant decrease in the capacity from 16.5 to 9.4 mAh l–1 in the 2nd discharge cycle.   231   Figure 9-3 The galvanostatic (a) charge and (b) discharge profiles of TFB-M (initial 3 cycles). Large polarization effects in the TFB-M are evident in the 3rd cycle, which has a decrease in the charging potential plateau followed by a limiting non-capacitive faradaic response (rapid increase in the cell potential). This trend indicated the diffusion-controlled processes in the TFB-M, mostly associated with the passivation of CuFeS2 in the MC and possibly to the impurity phases in the PSE-M, which may not support reversible reactions during repetitive GCD cycling. Similarly, in the following discharge (3rd cycle), the rapid potential drops to ~0.8 V and slightly extended decaying potential profile up to 0.6 V (cutoff potential) highlighted the hindrance in the progress of pseudocapacitive faradaic reactions due to kinetic limitations in the PSE-M. In simple words, the discharge capacity of the TFB-M setup is related to the amount of Zn that will deposit during the charge cycle. The cyclic 232  charge/discharge performance of the TFB-M was dependent on both the reversible Zn2+ deposition/dissolution and pseudocapacitive behavior of PSE-M. The discharge capacity of ~4 mAh l–1 and low ƞC (~9 %) was obtained by the TFB-M in the 3rd GCD cycle. This confirmed the progress of irreversible reactions (e.g. conversion reactions and dissolution of Fe and Cu from CuFeS2) in the PSE-M during the initial 3 cycles. The very small ƞC also reflects the significant amount of supplied charge that was consumed in the irreversible reaction (conversion reaction), and that was not available in the following discharge cycle.   Figure 9-4 The performance of TFB-M setup (a) energy density vs. no. of cycles, (b) coulombic efficiency, (c) potential efficiency and (d) rate of Cu extraction measured from ICP–OES analysis of the PSE-M filtrate after various GCD cycles. (Note: the specific energy is calculated based on the volume of slurry in the PSE-M). 233  Repetitive GCD cyclic tests were also conducted to estimate the cyclic discharge energy density, ƞE, ƞC, and ƞV of the TFB-M. As shown in Figure 9-4a, the discharge energy density was calculated from the GCD plots and by using Equation 9-1.  𝑬𝑬𝒎𝒎𝑺𝑺𝑬𝑬𝒈𝒈𝑪𝑪 𝒅𝒅𝑺𝑺𝒎𝒎𝒈𝒈𝒊𝒊𝒕𝒕𝑪𝑪 (𝑾𝑾𝒎𝒎 𝑺𝑺−𝑿𝑿) =  𝑿𝑿𝟑𝟑.𝟔𝟔𝑽𝑽𝑺𝑺  ∫ 𝜟𝜟𝑽𝑽(𝒕𝒕). 𝒊𝒊𝒅𝒅.𝒅𝒅𝒕𝒕𝒕𝒕𝟐𝟐𝒕𝒕𝑿𝑿  Equation 9-1 Where VS is the volume of slurry in the PSE-M and other parameters have their usual meaning as discussed in Chapter 7. The rapid fall in the energy density from 22 to 2.6 mWh l–1 with sharp decrease in ƞE (from 15 to 4 %) in the first three cycles represents the enormous loss in the supplied energy mostly consumed by conversion and parasitic reactions in the TFB-M. Interestingly, in the following 10 cycles, the monotonic increase in the energy density (max. ~36.2 mWh l–1 at 14th cycle) and a considerable increase in energy efficiency to 43% reflects an improvement in the charge storage capability of the TFB-M. As discussed in section 8.3, the enhancement in the pseudocapacitive response is attributed to the reversibility of the S22–/S2– redox reaction on the surface of sulfidic minerals i.e., CuFeS2 and FeS2 in the MC.  In previous chapters (see section 7.2, 8.3 and 8.4.1 and reference cited therein for further details), the formation of a polysulfide film and the existence of sulfide sulfur species on the surface of CuFeS2 are examined and discussed. In addition, the presence of ~43 % FeS2 in the MC and the reversible character of S22–/S2– species on its surface could further augment the pseudocapacitive response of MC as found in the FBFC setup (C–2) (see section 7.3) [212, 264]. Similarly, as shown in Figure 9-4b, the considerable increase in the ƞC (from ~17 to ~ 95 %) between the 4th and 12th GCD cycles further confirms this behavior. The reversible character of sulfide sulfur species formed on the surface of CuFeS2 and FeS2 in the PSE-M was responsible for this increase in the ƞC. On the other hand, the ƞV of the TFB-M setup 234  decreased rapidly from 85.7 to 45.5 % in the first three GCD cycles, which with slight variation, remained almost constant in the successive 97 cycles (Figure 9-4c). As depicted in Figure 9-3b, the disappearance of high potential plateau and origin of the relatively small decaying potential profile at low cell potential was evident and attributed to the reversible character of the sulfide sulfur species as discussed in Chapters 5, 6, and 7. The reason for the rapid decrease in the potential efficiency was associated with the decay in discharge potential and due to kinetic limitation of reversible faradaic processes. From these results, the rapid potential drop (IR drop) during discharge cycles may also be related with the high inter-particulate contact resistance in the PSE-M.  The ICP–OES analysis of the PSE-M filtrate after the 10th cycle (2.4 h) revealed 6.8 % Cu dissolution from the MC. From these results, it can be predicted that during the initial 3 charge/discharge cycles, a large amount of supplied charge is consumed in the formation of a passive film on the surface of CuFeS2 with partial dissolution of Cu and Fe in the PSE-M (irreversible reactions). However, parasitic H2 evolution reaction on the negative electrode during the charging step may also influence the cyclic performance of the TFB-M. Further increase in the ƞC and ƞE in the next 10 cycles were related to the growth of metal-deficient polysulfide surface film that could facilitate the reversible processes in the TFB-M. The Cu extraction increased to approximately 16.1 % in 100 GCD cycles, which completed in almost 12.2 h. Nevertheless, during this GCD cycling, ~ 86 % of the supplied energy was consumed in the progress of irreversible reactions (e.g., dissolution of Cu and Fe from CuFeS2) in the TFB-M as shown in Figure 9-4a. The reversible charge storage capacity (< 1 mAh l–1) of the TFB-M was low in the last 85 GCD cycles.  235  9.4 Energy storage equivalent to energy produced by a diesel generator A design of the TFB-M that could process 1 tonne of MC (containing 26.6 % CuFeS2) is proposed in this section. The PSE-M used in the TFB-M contains 20 wt. % total solids (MC to AC ratio of 4) in 0.2 M H2SO4. The TFB-M can be operated in batches and each batch would consist of 100 GCD cycles, which completes in 12.2 hours. The average energy storage capability of this setup over 100 GCD cycles (each batch) is 1.5 mWh l–1 based on the volume of the PSE-M. This setup could process approximately 718 tonnes of MC per year and could produce 16.1 % of this mass in Cu, which is equivalent to 10.65 tonnes of Cu per year as given in Table 9-1. In addition, this single unit, which accommodates 1 tonne of MC in each batch, could store and supply approximately 673 kWh year–1 of energy at 30 % efficiency. Based on the calorific value of diesel (12.64 kWh kg–1) and considering 60 % efficiency of a commercial diesel generator, 1 tonne of diesel would generate (7584 kWh) of energy. As a result, it is calculated that to store the same amount of energy as produced from 1 tonne of diesel, approximately 8089 tonnes of MC would have to be processed in the TFB-M. Thus to store the energy equivalent to 7584 kWh t–1 of diesel, approximately 11 TFB-M setups each having a capacity to process 718 tonnes of MC per year and energy storage capability of 673 kWh year–1, would be required. Compared to the TFB that uses synthetic CuFeS2 (as given in Table 8-3), the TFB-M would require approximately 9 times more MC over one year to store the same amount of energy (7584 kWh t–1 of diesel). However, this ratio strongly depends on the average amount of energy storage in each cycle during operation of the TFB-M. In addition, the grade of MC and the effect(s) of impurities could significantly influence the energy storage capability of this setup.  236  The requirement for ~8089 tonnes of MC to store the equivalent energy to 1 tonne of diesel at the mine site is not a significant issue in terms of the concentrate mass. Indeed, copper concentrators often produce approximately 35,000 tonnes of mineral concentrate per day.               Table 9-1 Energy storage capability of TFB-M, which can process 1 tonne of MC in one batch and estimation of MC to diesel ratio for the supply of same amount of energy     Values unitsAssumptionsAmount of energy stored in one cycle 0.009 kWh/t of MCEnergy storage efficiency 30 %Cyclic life of TFB-M 100 cyclesTime to complete 100 cycles 12.2 hoursEfficiency of Cu extraction in 100 cycles 16.1 %Capacity of the TFB-M 718.03 t of MC/yearVolume of each compartment in a TFB-M 1.875 m3/t of MCAmount of energy stored in 100 cycles 0.94 kWh/t of MCAmount of energy supplied in 100 cycles 3.13 kWh/t of MCCu extraction capacity 10.65 t of Cu/yearTotal energy required to operate TFB-M 2243.85 kWh/yearTotal energy stored in TFB-M 673.16 kWh/yearTotal energy required for TFB-M including pre-processing7479.51 kWh/yearCalorific value of Diesel 12640 kWh/t of dieselEnergy efficiency of a diesel generator 60 %Amount of energy available from diesel 7584 kWh/t of dieselTotal amount of diesel required to operate TFB 0.99 t of diesel/yearTFB-M energy storage equivalent to energy supplied by diesel generator0.09 t of diesel/yearMC to diesel ratio for energy supply 8089.6 / 1 t/tDesign of a TFB-M, which can accommodate 1.0 t of MC in PSE-MCalculations based on the performace of TFB-M and by using MCCalculations of energy storage by TFB-M equivalent to diesel generator237  9.5 Summary This Chapter focused on the use of the TFB-M as a hybrid unit for both energy storage and Cu extraction. The discharge energy density of the TFB-M increased sharply to 36.2 mWh l–1 during the initial 14 GCD cycles. This is attributed to the formation of a polysulfide passive film and to the reversible character of S22–/S2– species, which exist and may grow on the surface of CuFeS2 and FeS2 particles in acidic media during repetitive charge cycles. The ƞC and ƞE also increased monotonically to 95.6 % and 43 %, respectively, with approximately 6.8 % Cu extraction from the TFB-M during the initial 10 GCD cycles (about 2.4 hours). However, in the following 85 cycles, the low ƞC (~33 %) and ƞE (~14 %) suggested the progress of irreversible reactions in the TFB-M (i.e., the formation of the passive film, dissolution of Cu and Fe in the PSE-M and H2 evolution on the negative electrode). In 100 GCD cycles, a maximum 16.1 % Cu extraction was achieved in 12.2 h.                   238  Chapter 10: Conclusion 10.1 Remarks on the important findings This thesis presents the first use of synthetic CuFeS2 and MC in aqueous-based battery like setups that are capable of extracting Cu and storing energy. In the existing secondary battery setups, the non–capacitive faradaic (irreversible) reactions are always undesired because these reactions deteriorate their cyclic performance and charge storage capability. However, in this research work, irreversible faradaic reactions, i.e., Cu dissolution from CuFeS2 is a desirable feature. Two-hybrid batteries, i.e. FBFC and TFB setups, were introduced. Using synthetic CuFeS2 and MC as electrode materials in these setups, the energy storage and Cu extraction capabilities were determined. Following are the pointwise conclusions from the experimental results discussed in the preceding chapters. Setup–1: (FBFC)  1. The detailed electrochemical analyses of the composite electrode (negative GF–CuFeS2 electrode) and GF (positive) electrode in their respective electrolytes revealed the occurrence of quasi-reversible redox reactions (pseudocapacitive; faradaic reactions) in the FBFC that were responsible for energy storage. 2. The designed FBFC is capable of storing 2 to 6.3 Wh kg–1 energy over 500 GCD cycles. The coulombic and energy efficiencies of this setup were ~80 % and ~30 %, respectively. The cause of limited energy storage and low energy storage efficiency in the FBFC is related to the progress of irreversible conversion reactions on the CuFeS2 surface such as the dissolution of Cu from CuFeS2, which consumed ~70 % of the supplied energy. 239  3. A maximum 10.3 % Cu was also extracted from CuFeS2 during cycling, which is a unique feature of this setup suggesting the simultaneous use of FBFC for both energy storage and as a unit for Cu extraction.   4. The use of naturally sourced MC in this FBFC could supply up to 8.5 Wh kg–1 specific energy in addition with 12.7 % Cu extraction in 400 GCD cycles.  Setup–2: (TFB) 5. We introduced the trifunctional battery setup for the very first time in which both Zn deposition and Cu extraction take place simultaneously during the charge cycle. The deposition of highly electroactive Zn metal on the negative Al electrode was facilitated by the oxidation of CuFeS2 (dissolution of Cu and Fe) in this setup. Both of these processes are very important from a hydrometallurgical perspective. The re-dissolution of deposited Zn supported by the pseudocapacitive reactions in the PSE during the discharge cycle resulted in the release of stored energy which is an important feature of this setup.  6. In the first GCD cycle, the TFB setup provided the highest discharge specific energy of 388 Wh kg–1 (1.13 Wh l–1), which decreased to 50 Wh kg–1 (0.17 Wh l–1) during the initial 15 repetitive GCD cycles and to 23.5 Wh kg–1 (~0.07 Wh l–1) in the following 85 cycles. The low specific energy at low coulombic (≈ 50 %) and energy efficiency (≈ 40 %) indicated the progression of irreversible reactions in the TFB.  7. These irreversible reactions in the TFB resulted in approximately 23 % Cu extraction from the synthetic CuFeS2 in 100 GCD cycles (~26 h). 8. The highest 2.05 kW kg–1 specific power and 34.8 Wh kg–1 specific energy (at 2.5C discharge rate) in the first GCD cycle also highlighted the hybrid character of the TFB. 240  This is another promising feature of this setup and may be used to overcome momentary power outages.   9. By using MC as PSE-M in the TFB-M setup, the energy density increased from 2.6 to ~ 36.2 mWh l–1 monotonically from 3 to 14 GCD cycles and approximately 16.1 % Cu was extracted from the MC in 100 GCD cycles, which were completed in 12.2 h.   10. Based on these results, it is suggested that the TFB-M can be used either as primary battery or as a unit for Cu extraction and Zn electrowinning, as required by the user. For instance, by restricting the process to the charge step, one could use TFB-M as a Zn electrowinning setup, which uses MC in the positive compartment. Also, after various charging steps, Cu extraction from MC is possible as evident from the GCD tests and ICP-OES analyses in Chapter 9.          10.2 The practical implications of the current research work This research work used a widely available natural mineral as an electrode material in battery like setups. The relatively low energy efficiency by these systems compared to existing commercial batteries is offset by the valuable Cu extraction from the mineral and the relatively low cost and inexhaustible supply of the electrode material (MC). To recover Cu, existing hydrometallurgical practices, i.e., solid/liquid separation, solvent extraction, and electrowinning processes can be adopted without modification. These battery setups coupled with renewable energy generating units could be installed at remote mining sites to partially fulfill the energy demands for stationary applications with an additional advantage of Cu extraction. Certainly, to improve both energy efficiency and Cu extraction capabilities, the modification in the electrode(s) system, cell design and change in electrolyte composition is required. In this regard, some key points are highlighted in the following section, which may 241  be considered in the future research work to further enhance the performance of these batteries. By using MC in the TFB-M, the energy storage capacity increased monotonically in the initial 15 cycles. However, the low energy efficiency (14 %) of this setup corresponds to the occurrence of parasitic reactions in the cell, which limit the cyclic life of this setup. However, by injecting fresh PSE every 15 cycles this setup becomes more attractive for a continuous process.  10.3 Future perspectives and recommendations To improve the performance of these setups there are many aspects that need further research. Mainly, testing of various MC concentrates (taken from different sources and of different compositions) in these systems is the key recommendation for future research. However, a few points are highlighted that may be considered in any future research. Setup 1: FBFC Depending on the requirement and use of the FBFC, either as a Cu extraction unit or as a device for energy storage, upscaling and modification in the existing design is proposed based on the following conditions. • In the composite electrode, the surface modification of the GF, the addition of pseudocapacitive activated carbon in the CuFeS2/MC could improve the coulombic and energy efficiencies of FBFC. • Use of highly conductive and kinetically active materials (Pt-doped graphite felt) in the positive electrode may enhance the electrochemical kinetics of FeII/FeIII redox reaction, which may ultimately improve the cyclic charge/discharge performance of the FBFC.  242  • The temperature, electrolyte flow rate and initial acid (in both anolyte and catholyte) concentration effects may be studied to further enhance the Cu extraction and energy storage capability.   • During continuous GCD cycling, the consumption of CuFeS2 (due to Cu dissolution) would require the frequent replacement of the composite electrodes in FBFC. Modification of the existing FBFC design is therefore recommended. For this, the intermittent CuFeS2/MC slurry (instead of fixed bed) flow cell design is suggested. In this setup, slurry could be injected in the negative compartment to make this setup more flexible for a continuous supply of energy and Cu from the mineral concentrate. • The rigorous characterization of the slurry electrode, i.e., the effect of mineral particle size, acid concentration, temperature, impurities and solid contents in the slurry and its effect on the viscosity is recommended due to its direct impact on the pumping requirements and related cost for injection and circulation.  • To simulate the effect of impurities on the electrochemical performance of the mineral concentrates, a varying amount of impurities such as pyrite in the synthetic CuFeS2 may be added to explore the optimum conditions for impurity level in the FBFC in order to achieve maximum Cu extraction and energy efficiency from MC.  Setup 2: TFB In the TFB, for only hydrometallurgical interest, to achieve both Cu extraction and Zn-electrowinning, the operation of this setup must be stopped after the charge cycle. However, both Cu extraction and energy retrieval is possible upon sacrificing the deposited Zn during the discharge cycle. 243  • To enhance the Cu extraction and energy storage capability of the TFB, investigation of the optimum amount of solid contents in the PSE and adjustment of CuFeS2/AC ratio is required. These factors could significantly influence the conductivity of the PSE, pumping requirements and may affect the particle-particle and particle to current collector contact during operation of the TFB. • The amount of deposited Zn on the negative electrode is proportional to the oxidation reactions in the PSE compartment. Thus, the energy storage capability and Cu extraction rate in the TFB can be enhanced by increasing the surface area of the negative current collector. The use of porous metal (Al foam) electrodes could be used in the TFB to increase surface area.  • It is important to understand the local changes on the surface of the negative electrode during Zn deposition in the charge cycle. For instance, hydrogen evolution at the surface will change the local pH, which could affect the Zn deposition efficiency and may deteriorate the cyclic performance of the TFB. Therefore, in-situ pH monitoring at the electrode surface during charge and discharge cycle can provide useful information to adjust the anolyte composition in the external reservoir and to adjust its flow rate.  • To use the TFB as a high-energy storage device, the PSE can be replaced by injecting fresh slurry in the cell after a specific number of cycles depending on the GCD cyclic performance. • To avoid the cross-contamination of the anolyte and catholyte, a novel design of TFB will be studied. 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