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Hybrid PEM fuel cell : redox cathode approach Moraw, Franz Christian 2009

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Hybrid PEM Fuel Cell: Redox Cathode Approach by Franz Christian Moraw B.Sc., Simon Fraser University & University College of the Fraser Valley, 1997 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE in THE FACULTY OF GRADUATE STUDIES (Chemical and Biological Engineering) The University of British Columbia (Vancouver) March 2009 © Franz Christian Moraw, 2009 Abstract The proton exchange membrane fuel cell (PEMFC) is considered to be a promising power device with a broad range of applications. However, there are still a number of challenges especially concerning performance, cost, and reliability of these systems. The redox flow battery utilizes fundamentally simpler chemistry, but has limitations in terms of membranes/materials used in system construction and in terms of redox regeneration requirements. The hybridization of a PEMFC anode with a redox flow battery cathode, replacing the limiting oxygen electrode, leads to both advantages and compromises in performance. Although there are improvements in kinetics, cell and systems design, and cost, there are restrictions imposed by the regeneration method and membrane contamination. In this work, the Fe37Fe2redox fuel cell cathode is characterized over a range of electrolyte concentrations, operating conditions, and electrode materials. A Fe3/F2simulated bio-electrolyte and a simple electrolyte catholyte are studied using CV and ETS to determine kinetic parameters for the electrolyte cathode redox couple, while a prototype single cell fuel cell is used to demonstrate actual fuel cell performance. Electrochemical data shows the effect of ferric ion complexation! polymerization on the operation of both electrolyte systems. The results show that the heterogeneous electron transfer rate constant and diffusion coefficient as well as interface properties all increase with the ratio of total anion species (S042,HS04)to ferric species. Fuel cell testing showed no significant difference in performance between the two systems opening up various possibilities for redox species regeneration. Improvements are also achieved through optimization of cathode materials and operating conditions. This hybrid system, part of a strategic NSERC grant (Novel biofuel cell - methane reforming reactor system for electricity generation, #GHGPJ 269967 — 03) (1), showed promising performance even though components such as the membrane were not optimized. Power densities of greater than 0.25 W/cm2were achieved with no platinum group metals on the cathode. In addition, the liquid redox cathode eliminates the need for external humidification and separate cooling for the fuel cell and provides greater design 11 flexibility. Different aspects of the redox cathode were characterized and showed opportunity for further performance improvement. 111 Table of Contents Abstract.ii Table of Contents iv List of Tables vi List of Figures vii Nomenclature/Abbreviations/Units ix Acknowledgements xiv 1. Introduction 1.1. Worldwide energy/pollution paradox 1.2. Overview of Fuel Cells 1.2.1. Basic Principles 3 1.2.2. Types of FC 3 1.2.3. Thermodynamics and Performance 6 1.3. PEM Fuel Cells in Detail 8 1.3.1. Components of PEMFC 10 1.3.2. Membrane Electrode Assembly (MEA) 10 1.3.3. BipolarPlates 13 1.4. Redox Flow Batteries 14 1.4.1. The Redox Couples and Supporting Electrolytes 17 1.4.2. The Redox System Membrane/Separator 20 1.4.3. The Redox System Electrodes 22 1.4.4. Redox Cell Design 24 1.4.5. Redox Operation 25 1.4.6. Regeneration of Redox Solutions 25 1.4.7. Regeneration Example: Bioregeneration of the Ferric Ferrous couple 27 1.4.8. Types of Redox Systems 30 1.4.9. Current Commercial Applications: Vanadium Redox Batteries 32 1.4.10. Overall Summary of Redox Flow Battery Systems 33 2. Thesis Research in Context of Overall Project Research Objectives 36 3. Thesis Research Approach: Replacement of the Oxygen Reduction Cathode in a Fuel Cell with a Redox Couple (PEM/Redox Fuel Cell Hybrid) 39 4. Experimental Methods 43 4.1. Electrochemical Characterization of Electrolyte and Electrodes 43 4.2. Electrolyte Preparation 47 4.3. Electrode Preparation and Characterization 48 4.4. MEA Fabrication 49 4.4.1. Cutting of GDL/catalyst layer and MEA assembly 51 4.5. Fabrication of Fuel Cell Gaskets 52 4.6. Fuel Cell Testing System Design 53 5. Results and Discussion 57 5.1. Mathematical Analysis of Results (CV/Impedance Results) 57 5.2. Electrochemical and Fuel Cell Results 57 5.2.1. Cyclic Voltammetry Results for Simple Electrolyte 57 5.2.2. Impedance Results for Simple Electrolyte 68 5.2.3. Effect of Electrode Type and Cross-Over on Redox Activity 71 5.2.4. Electrochemical Results for Bio-Electrolyte 74 5.2.5. Fuel Cell Testing Results for “Simple” and “Bio” Electrolytes 77 5.2.6. Redox Fuel Cell Degradation 89 6. Conclusions 95 7. Future work 98 8. Research Contribution, Significance and Ultimate Impact 100 References 101 Appendix 1: Mathematical Techniques and Electrochemical Equations Used 108 Al .1. Additional Electrochemical Data Analysis 112 iv Al .2. Additional Result Available Through Mathematical Analysis 112 Al.3. MATLAB Programming 115 Appendix 2: Integrated System I: The Biological Considerations (Bioreactor System) 124 A2. 1. Overview of Bioreactor Work (University of Western Ontaro) 125 Appendix 3: Integrated System II, The Gas Reformer 127 Appendix 4: Design Schematics of Redox Fuel Cell 128 A4.1. Schematic of 4cm2 Redox Fuel Cell for Parametric Analysis (Exploded View) 128 A4.2. Bladder Plate 128 A4.3. Compression Piston 129 A4.4. End Plate 129 A4.5. Anode Manifold 130 A4.6. Cathode Manifold 130 A4.7. Bus Plates (gold plated) 131 A4.8. Anode (Fuel) Plate (Serpentine) 131 A4.9. Cathode Plate (Serpentine) 132 A4.l0. Cathode Plate (Hollow pocket) 132 A4.ll. Bushings 133 A4.l2. Pin 133 A4.13. Tie Rod 134 Appendix 5: Profile of Organizations Involved 135 A5.l. University of British Columbia Department of Chemical and Biological Engineering 135 AS .2. National Research Council Institute of Fuel Cell Innovation 136 A5.3. Membrane Reactor Technologies (MRT) 137 A5.4. University of Westem Ontario Department of Chemical and Biological Engineering 137 A5.5. National Science and Engineering Research Council (NSERC) 137 Appendix 6: Altemative energy options 138 Appendix 7: Publications and Presentations 143 Appendix 8: Statement of Co-Authorship 145 V List of Tables Table 1: Comparison of Various Fuel Cell Types 3 Table 2: A comparison of potential differences of various redox reactions 18 Table 3: Estimated solubilities for ferrous-ferric couples and ligands measured in 0.5MH2S04(16) 19 Table 4: Typical Physical Properties of Carbon (23) 23 Table 5: Effects of operating conditions on T. ferrooxidans growth (33) 29 Table 6: Compilation of redox fuel cells examined in the last 20 years (26,20,20,13,11,36) 31 Table 7: Performance Aspects of Redox Flow Batteries 34 Table 8: Kinetic parameters obtained on GC using CV and EIS 62 Table 9: Some reported data on the stability constants of iron-sulphate complex species 64 Table 10: Effect on conductivity of various soaking treatments ofNafion® 115 [amembrane exposed overnight in solution at 22°C; b membrane soaked for 1 hour inH2S04 (pH = 1) solution at 22°C after being exposed to the iron solution] 92 Table 11: Differences between oxygen and redox fuel cell cathode 95 vi List of Figures Figure 1: Schematic of a Proton Exchange Membrane Fuel Cell(3) 4 Figure 2: Typical fuel cell polarization curve with labeled regions of loss 9 Figure 3: Exploded view of a standard PEM fuel cell 10 Figure 4: Free radical initiation process polymerization 11 Figure 5: Chemical production of polytetrafluoroethylene (PTFE) 12 Figure 6: Representations of the chemical structure of Nafion 12 Figure 7: A schematic of an electrically rechargeable redox system (7) and a representation of a I OMW/S5MWh redox system (9) 15 Figure 8: Schematic of a chemically regenerative redox fuel cell 16 Figure 9: Effect of concentration on 10 MW, 85MWhr redox system (9) 20 Figure 10: Schematic of overall NSERC proposal with research focus for this thesis (bioreactor insert from Nagpal (37)) 37 Figure 11: Contributions to Polarization of Anode and Cathode in a Phosphoric Acid Fuel Cell (Relative shapes are typical for other types of fuel cells) (38) 39 Figure 12: Schematic Comparison of a PEMFC, Redox Battery, and Hybrid Redox/Hydrogen Fuel Cell (42) 41 Figure 13: Three Electrode Cell Experimental Setup (44,45) 43 Figure 14: Diagram of a cyclic voltammogram illustrating capacitance correction of peak height 44 Figure 15: Example of reversible reaction as measured by cyclic voltammetry (16) 45 Figure 16: Equivalent Circuit Model for single electron transfer 46 Figure 17: Typical Nyquist plot and CV curves ofFe43!F2redox couple inH2S04 46 Figure 18: Results of contact angle tests on Toray carbon paper treated with (a) nitric and (b) sulphuric acid 49 Figure 19: Schematic of GDL cutting tool 52 Figure 20: Schematic of 4cm2 Redox Fuel Cell for Parametric Analysis (Exploded View) 54 Figure 21: Schematic of redox fuel cell system (Testing Setup) 55 Figure 22: Cyclic voltanimograms of the Fe3/F2redox couple ([Fe3]/[2 9) in 0.5 MH2S04at 25°C on Glassy Carbon 58 Figure 23: Peak current vs. the Square Root of the Scan Rate for the anoclic and cathodic reaction of the iron redox couple (0.9M, [Fe3]/[Fe2J=1) on glassy carbon at 25°C and pH 1.5 59 Figure 24: Cyclic voltammograms of the Fe3/F2redox couple ([Fe3]I[F12= 9) in 0.5 MH2S04at 25°C on TGP-H090. Inset: SEM image of carbon TGP-H090 surface 61 Figure 25: Peak current vs. the Square Root of the Scan Rate for the anodic and cathodic reaction of the iron redox couple (0.9M, [Fe3]/[Fe2=1) on TGP-H090 at 25°C and pH 1.5 61 Figure 26: Influence of suiphate/ferric ratio on ferric/ferrous diffusion coefficients at 25°C 63 Figure 27: Comparison of the variation of apparent k0 with suiphate/ferric ratio obtained on GC and TGP H090 at 25°C 64 Figure 28: Variation of apparent k0 with sulphate/ferric ratio obtained with CV in diluted ferric solution (5mM ferric chloride in 1M HC1) at 25°C 67 Figure 29: Impedance spectrum in Nyquist plane ofFe3/F2redox couple on GC at equilibrium potential. Inset: Bode plot 68 Figure 30: Comparison of the variation of apparent k0 with suiphate/ferric ratio obtained with CV and EIS 70 Figure 31: Variation of the double layer capacitance and the parameter 4J) obtained with EIS on GC with suiphate/ferric ratio 70 Figure 32: Cyclic voltammograms (70°C, scan rate 50 mV/s) for hydrogen and the Fe3 / Fe2 redox couple +3 +2([Fe]=0.9M, Fe /Fe =9) mixed on glassy carbon and for the redox couple alone on glassy carbon and platinum 72 Figure 33: Comparison of cyclic voltammograms (25°C, scan rate 100 mV/s) for the Fe3 / Fe2 redox couple on glassy carbon and on Toray TGPH-090 carbon fiber paper in different solutions: a) 5 mM K3Fe(CN)6in 1 M KNO3; b) 0.81 M / 0.09 M Fe(NH4)(S02.12H0/ FeSO4.7H20in 0.5 M H2S04 73 Figure 34: Influence of total iron concentration on k<, and DFe+3 in SBE (CV). [Fe3]/[Fe = 1, pH = 1.5 at 25°C 75 vii Figure 35: Influence of pH on DFe+3 and k0 in SBE ([Fe] = 50 g/L). [Fe3]/[Fe2 1 @25°C (CV) 76 Figure 36: Influence of temperature on k0 and DFe+3 (CV) in SBE ([Fe] = 50 g/L). [Fe3] = 90%[Fe], pH =1.2 77 Figure 37: Fuel Cell Performance using Thick Carbon Felt, Thin Carbon Felt and Carbon Paper with respect to Bladder Pressure (“simple” electrolyte used) 80 Figure 38: Performance Comparison of Bioelectrolyte and “Simple” Electrolyte 81 Figure 39: Direct hydrogen redox fuel cell polarization curves obtained at 40°C for different flow rates of the Fe3 / Fe2 bioelectrolyte. Toray TGPH-090 carbon fiber paper was used for the cathode 82 Figure 40: Comparison of the effect of constant flow versus constant stoichiometries using TGPH-090 electrodes (Simple electrolytes) 83 Figure 41: Effect of different carbon materials on the performance of a redox fuel cell using a SBE 84 Figure 42: Direct hydrogen redox fuel cell polarization and power density curves obtained in the temperature range of 25°C to 70°C for a constant Fe3 I Fe2 solution (SBE) flow rate of 10 mllmin. Toray TGPH-090 carbon fiber paper used for the cathode 85 Figure 43: Polarization Curves of Bio-Redox Fuel Cell (using thick felt, 1 Omllmin catholyte) at various temperatures. Inset: SEM image of carbon felt surface at X5 1 scale 86 Figure 44: Repeatability of results in a “simple” redox fuel cell at a redox solution flow of 10 mI/mm 87 Figure 45: Redox PEM Fuel Cell Performance with Bioelectrolyte at 70°C (thick carbon felt cathode, 10 mi/mm catholyte) 88 Figure 46: Comparison of a 5cm2 PEMFC at 55°C (0.4mgp/cm2catalyst loading) with the 4cm2 Bio-Redox Hybrid Cell (using a thick carbon felt cathode, lOmi/min catholyte) at 55°C 88 Figure 47: Voltage Stability in a redox/PEM fuel cell (bioelectrolyte, thick carbon felt cathode) at 0.62 A/cm2, Sml/min redox flow, I lml/min hydrogen flow, and 55°C 89 Figure 48: XJS data for untreated carbon paper and nitric acid treated carbon paper prior to use the in the hybrid cell and nitric acid treated carbon paper after use in the hybrid cell 90 Figure 49: Comparison of “simple” solution experiments at 40°C at different times throughout the experiment (approximately 4 hours between mns) 91 Figure 50: Membrane recovery in a redox PEMFC at 55°C (thick carbon felt, bioelectrolyte at 5 ml/min).93 Figure 51: Equivalent Circuit Model for Ferric/Ferrous reaction on GC Electrode 108 Figure 52: Peak Current vs. the square root of the scan rate (MATLAB program plots) 109 Figure 53 Extended Nicholson Working Curve (55) 110 Figure 54: N’ vs. the inverse square root of the scan rate (MATLAB program plot) 111 Figure 55: Rate constant vs. Concentration (MATLAB program plot) 113 Figure 56: d(rate constant)/d(Concentration) vs. Concentration using both forward differencing and a differencing relation (MATLAB program plot) 114 viii Nomenclature/Abbreviations/Units peak current density (amps/cm2) exchange current density (amps/cm2) limiting current density (amps/cm2) ri overpotential (Tafel equation) n number of electron involved in the electrode process A electrode area (cm2) D diffusion coefficient (cm2 s’) D0 diffusion coefficients for the oxidized species (cm2 s’) DR diffusion coefficients for the reduced species (cm2 s’) C concentration of the electroactive species (mol cm3) Co concentration of the oxidized form electroactive species (mol cm3) CR concentration of the reduced form electroactive species (mol cm3) CbUlk concentration of the species in the bulk on solution (mol cni3) v scanrate(Vs’) k0 heterogeneous electron transfer rate constant (cm s’) Ic0 rate constant for the oxidation reaction (cm s’) kR rate constant for the reduction reaction (cm s’) dimensionless parameter (used in the Nicholson method) a cathodic charge transfer coefficient (dimensionless, based on symmetry of CV peaks; values range from 0 to 1) v scanrate(Vs’) R solution resistance (2 cm2), fitted parameter of the constant phase element (AC impedance) charge transfer resistance (Q cm2), fitted parameter of the constant phase element (AC impedance) T capacitance parameter (F cm2 fitted parameter of the constant phase element (AC impedance) or temperature (K) degree of homogeneousity of reaction (a parameter related to the time constant distribution), fitted parameter of the constant phase element (AC impedance) ix Cdl double layer capacitance (.tF cm2) fNafion Nafion® loading (fraction Nafion® in Pt+VulcanXC+Nafion®) mNaf So! mass of the Nafion® solution (7Pt catalyst loading (mg per geometric cm2) ,ug Biological specific growth rate (Monod equation) ,Umax Biological maximum specific growth rate (Monod equation) K Biological substrate concentration when pg = O.5flmo R Ideal gas constant (8.314 J K’ mol’) or general resistance (ohm) F Faraday constant (96485 C mor’) E Electrochemical Potential (V) AC Alternative Current ADP Adenosine di-phosphate’ A.f Acidothiobacillus ferrooxidans AFC Alkaline Fuel Cell ATP Adenosine tn-phosphate2 Ca Calcium Ca(N03)2 Calcium Nitrate CCM Catalyst-Coated Membrane CERC Clean Energy Research Centre CHBE Chemical and Biological Engineering (The Department of...) CISTI Canada Institute for Scientific and Technical Information Cl Chlorine CNRC Conseil National de Recherche du Canada CO Carbon Monoxide CO2 Carbon Dioxide DC Direct Current DMFC Direct Methanol Fuel Cell ‘ADP is a glycolysis intermediate and an energy carrier in cellular systems. 2 ATP is the fundamental energy carrier for all biological systems. It is essential in almost all aspects of metabolism. x FC Fuel Cell Fe Iron Fe3 Ferric cation Fe2 Ferrous cation GDL Gas Diffusion Layer H2S04 Sulfuric Acid IFCI Institute for Fuel Cell Innovation K Potassium KC1 Potassium Cloride K2HPO4 Potassium Phosphate KOH Potassium Hydroxide KE Kinetic Energy L.f Leptospirillum ferrooxidans LffV Lower Heating Value MCFC Molten Carbonate Fuel Cell MEA Membrane Electrode Assembly Mg Magnesium MgSO4 Magnesium Sulphate MPL Microporous Layer MRT Membrane Reactor Technologies Inc. MSDS Material Safety Data Sheet NADPH Nicotiamide Adenine Dinucleotide Phosphate3 NASA National Aeronautics and Space Administration (NH4)50 Amnionium Sulphate NRC National Research Council NSERC National Science and Engineering Research Council NTP Normal Temperature and Pressure 02 Oxygen OCV Open Circuit Voltage NADPH is component of the Calvin Cycle (it acts as a reducing agent) and a key component of cellular metabolism. xi ORR Oxygen reduction reaction PAFC Phosphoric Acid Fuel Cell PID Proportional Integral Derivative PEMFC Proton Exchange Membrane Fuel Cell PFD Process Flow Diagram PTFE Polytetrafluoroethylene (Nafion ppb Parts per Billion ppm Parts per Million SOFC Solid Oxide Fuel Cell Soc State of Charge S0j2 Sulfate anion S Sulfur R&D Research and Development SEM Scanning Electron Microscope TPZ Three-phase zone uBc University of British Columbia (IWO University of Western Ontario YSZ Yitria Stabilized Zirconia Micro (unit prefix) k Kilo (unit prefix) c Centi (unit prefix) mol Mole A Ampere °C Degrees Celsius J Joule c Coulomb g Gram L Litres lb Pound xli Acknow1edements While Isaac Newton stood on the shoulders of giants, I have been fortunate enough to stand shoulder to shoulder with amazing people. I wish to thank my supervisors Dr. David Wilkinson of UBC and Dr. Khalid Fatih ofNRC for all of their patience and support, without which this work would not have been possible. Your guidance has allowed me to take my journey as a man of science and engineering to the next level. I would also like to thank my colleagues at both UBC and NRC for their aid in this endeavour, particularly Dr. Francois Girard, Dr. Titichai Navessin, Dr. Elod Gyenge, Robert Chow, Tom Vanderhoek, Jeorg Zimmermann (now of Angstrom Power), Ryan Baker, Alan Illicic, Alfred Lam, Alex Bauer and Alexandre Vigneault. In addition, I wish to thank the National Research Council Institute for Fuel Cell Innovation, the University of British Columbia, and the Natural Science and Engineering Research Council for the opportunity to pursue this post graduate work. I wish to express my eternal gratitude to my parents Franz and Caroline Moraw for both their love and encouragement throughout my life, without which I am certain I would not be where I am today. Finally, words cannot express the love and admiration I have for my wife Jung Sim. Her strength, support and endless patience have allowed me to complete this part of my journey. Now it is time for us to look to the future. xiv 1. Introduction 1.1. Worldwide energy/pollution paradox Access to electrical power is one of the key needs of a developed/developing country. Developed countries use massive amount of energy to support their way of life, their industry, and their economy. Developing countries are also beginning to utilize more and more energy. With the large population represented by these countries energy is rapidly becoming the biggest commodity of the modern world. Globally, the current sources of electrical energy production are varied, but they can be encapsulated under a short list of headings(2): Coal (40%), Oil (10%), Natural Gas (15%), Hydro and Other (19%), and Nuclear (16%). Currently, one of the biggest concerns of society is the effect of global warming and climate change on the world environment. Although some people believe that this change can be accounted for by natural cycles, it is commonly believed that pollution, caused by the burning of fossil fuels to acquire energy, is the major contributor to the overall problem. As can be seen above, fossil fuels account for approximately 65% of the world’s energy production. This has led to the development of a political and social will to investigate alternative sources of “clean” energy. In an effort to decrease pollution output, various technologies are being considered which decrease our reliance on fossil fuels. Some of these alternative energy technologies are hydropower, solar power, wind power, biomass energy utilization, geothermal power, nuclear power, and fuel cells (a brief essay on non-fuel cell alternative energy sources can be found in Appendix 6). 1.2. Overview of Fuel Cells A fuel cell is an energy conversion device, which converts chemical energy (from a fuel and oxidant) to electricity, heat, and chemical by-products. The main difference between a fuel cell and a battery4 is that a fuel cell does not have a stored fuel supply — it is fed a fuel and oxidant (and therefore can run continuously) from an external source. There are various types of fuel cells: proton exchange membrane fuel cells, alkaline In a battery, electrical energy is stored chemically (in ionic conductor solutions, polymers, gels, or ceramic media) and discharged electrochemically via electrons at two electrolytically connected but isolated half-cells (the anode and cathode are kept separate, often separated by a material which is porous to the electrolyte). 1 electrolyte fuel cells, phosphoric acid fuel cells, molten carbonate fuel cells, and solid oxide fuel cells. Each type of fuel cell has different operating conditions, different materials, applications and limitations. The basic principle of a fuel cell was first discovered by Sir Humphry Davy in 1802 using a high temperature carbon cell (C I H20, I{N03102 I C) with a nitric acid electrolyte. The first hydrogen-oxygen fuel cell was built by Sir William Grove (1811- 1896) in 1839 after of an electrolysis experiment accident. After disconnecting the battery from the electrolyser and connecting the two electrodes together he noticed that the current flowed in the opposite direction consuming oxygen and hydrogen. He later connected a series of these “gas batteries” together into a “gas chain”. His plan was to use the electricity produced to power an electrolyser but due to electrode corrosion and material instability problems, he found that the application was not practical. The next major evolution in “fuel cell” technology was the introduction of platinum black catalyst and the use of electrolytes capable of being contained in porous plaster matrices (asbestos) by L. Mond and C. Langer in 1889. This was followed, in 1921, by the high temperature kinetic work of E. Bauer using a carbon anode, an iron oxide cathode, and alkaline carbonate electrolytes. Francis Bacon, beginning in the 1930s, did more significant work at Cambridge University. In 1956, with the help of J.S. Frost, he built a 6kW engine. Harry Karl Ihrig (of Allis-Chalmers) followed this work by building a 15kW fuel cell stack and using it to power a 20 horsepower tractor. Next, a 4-passenger vehicle (using a hydrogen fuel cell/battery hybrid engine) was built by K. Kordesch in 1970. This was followed by a shift in research focus away from alkaline fuel cells, to phosphoric acid systems for stationary applications (1970s), to molten carbonate fuel cells (1980s), and to solid oxide fuel cells and polymer electrolyte membrane fuel cells in the 1990s and onward. So far the most high profile application of fuel cells was their use in the power supply systems of the NASA Gemini, Apollo and Space Shuttle programs. Currently companies such as Ballard Power Systems, Angstrom Power, Ford, Toyota, GM, and Global Thermoelectric are leading the industry in developing and using fuel cell products for micro, portable, stationary, and mobile applications. 2 1.2.1. Basic Principles Simply put, a fuel cell utilizes a fuel, an oxidant, catalysts, and an ion exchange membrane to produce electric power. While liberated electrons (produced from a half- cell reaction at either the anode or the cathode) flow through an electric circuit, the ions of interest (also produced from the half-cell reaction) move across the membrane to complete the overall reaction (the other half-cell reaction). It is critical that only ions and not electrons pass through the membrane so that power is produced. Fuel cell systems differ based on operational temperature, type of membrane, whether the fuel is processed by external or internal reforming, type of fuels and oxidants, system design (e.g., manifolding), etc. 1.2.2. Types of FC While there is a wide range of fuel cell variations, their core technologies can be summarized and grouped based on the short list of parameters detailed in Table I (modified from the 5th edition of the Fuel Cell Handbook, U.S. Department of Energy, 2000). The remainder of this section goes into a more detailed account of the main types of fuel cells. SOFC MCFC AFC PAFC PEMFC/DMFC PEMFC Redox Hybrid Operating 600-1000 650 65-220 205 80 40-80 Temperature (°C) Electrolyte Ceramic Molten Potassium Phosphoric Proton exchange Proton Carbonate Hydroxide Acid membrane exchange membrane or_other Charge O C032 OH H 11 Carrier Base Ceramic Stainless Carbon Graphite Carbon Carbon Component Steel Material Catalyst Perovskites Nickel Platinum Platinum Platinum Platinum (anode) Water Gaseous Gaseous Evaporation Evaporation Evaporation Evaporation Management Product Product Mechanism Heat Internal Internal Electrolyte Cooling Cooling Medium Catholyte Management Reforming Reforming Circulation Medium and Process gas Circulation Mechanism and Process and Process and Process and Process and Process Gas Gas Gas Gas Gas Table 1: Comparison of Various Fuel Cell Types 3 Proton exchange membrane fuel cells (PEMFC) are operated at relatively low temperatures (less than 120°C, but usually around 80°C). They are being considered for a variety of major applications including the replacement of the internal combustion engine in the automotive sector. They use a semi-permeable polymer, ion exchange membrane as the electrolyte (it acts as an excellent proton conductor). Water management plays a critical role in the operation of this type of fuel cell since the membrane electrolyte must remain hydrated for proper proton conduction. As can be seen in Figure 1, PEMFCs use hydrogen gas at the anode and air (or pure oxygen) at the cathode. Hydrogen flows through the flow field of the anode, electrons are catalytically stripped (using a platinum based catalyst5)from hydrogen producing protons and electrons, the produced electrons flow through the anode to the external circuit (providing power) while the hydrogen ions flow through the ion exchange membrane to the cathode, the electrons move through the load to the cathode (where oxygen is present), and the hydrogen ions, oxygen, and electrons combine to form water at the cathode (which is then carried away by the cathode flow). Figure 1: Schematic of a Proton Exchange Membrane Fuel CeIl(3) Since platinum is used as the catalyst it is critical that there be no carbon monoxide in the gas streams, as CO will disrupt the catalytic activity of platinum. PEM FUEL CELL Elecincal Current Electrolyte 4 Direct Methanol Fuel Cells (DMFCs) utilize methanol, instead of hydrogen, directly at the anode catalyst. The catalyst is a mix of ruthenium and platinum (CO is produced as an intermediate during the oxidation reaction and the mixed catalyst resists CO poisoning). The main issues with this type of fuel cell are methanol crossover to the cathode across the membrane (which affects overall system efficiency) and slow anode kinetics for the oxidation of methanol. DMFCs are also classified as low temperature fuel cells. Alkaline Fuel Cells (AFCs) are a moderate temperature fuel cell type (they operate between 65 and 250°C depending on the concentration of the potassium hydroxide electrolyte). The fuel cell has the KOH electrolyte supported in a matrix (e.g., asbestos) and utilizes a variety of different catalysts (noble metals, metal oxides, nickel, gold, etc.). Compatibility of the fuels and system components with the KOH (OW charge carrier) is critical. For example, CO in the fuel will react with the KOH to formK2C03. Phosphoric Acid Fuel Cells (PAFCs) are moderate temperature fuel cells operating between 150°C and 220°C. They use 100% concentrated phosphoric acid as their electrolyte contained in a silicon carbide matrix and platinum as the catalyst. Since phosphoric acid is highly stable, it works well at higher temperatures. This increases the ionic conductivity (Hf) and decreases the effect of CO poisoning on platinum. Another advantage of working at higher temperatures is that water is in the vapour phase, simplifying water management concerns. Molten Carbonate Fuel Cells (MCFCs) are high temperature fuel cells (600°C to 700°C) which usually use alkali carbonates as their electrolytes contained in a LiA1O3 ceramic matrix. At these elevated temperatures the electrolyte forms a highly conductive molten salt (the CO32are involved in the ionic conduction). The anode uses nickel and the cathode uses nickel oxide as catalysts. High temperature fuel cells are often combined with heat exchangers to take advantage of the high temperature exhaust, increasing overall system efficiency. Solid Oxide Fuel Cells (SOFCs) are high temperature fuel cells (600°C to 1000°C) which use an electrolyte composed of a solid, nonporous metal oxide (i.e., Y203 stabilized ZrO2where 02 is the ionic conductor). The anode is usually made of Co-ZrO2 or Ni-Zr02cermet, while the cathode is usually made of Sr-doped LaMnO3).Like molten 5 carbonate fuel cells, these high temperature systems take advantage of combined heat and power to increase system efficiency. 1.2.3. Thermodynamics and Performance When determining the electromotive force of the hydrogen/oxygen fuel cell (H2 + 1/2O2-HO)it is necessary to consider the reaction’s Gibbs free energy of formation (zGf = of the products — Gf of the reactants). In this case, the Gibbs free energy per mole6 can be written as: = ()o — ()H2 ()o2 Eq.1 In the case of a reversible reaction (where there are no losses) all of the Gibbs energy is converted to electrical energy with two moles of electrons used to produce one mole of water. Therefore, the electrical work produced is equal to the Gibbs free energy as shown in the following equation: Agf = —2FE Eq.2 where 2F represents the coulombs produced by two moles of electrons (F being the Faraday constant) and E represents the voltage of the fuel cell. For a typical PEMFC (at 80°C, with liquid water being produced) the voltage can therefore be written as —‘gj 228200 JImolE = =l.18V Eq.3 2F PEMFC@80C 2*96485C This value, of course, represents an idealized case. In reality, fuel cells do not operate at 100% efficiency. To calculate that efficiency one must compare the energy produced to the heat that would be produced by burning the fuels. In the case ofH2, the heat of combustion is —285,840 J/mol. Therefore the overall efficiency can by defined using the heat of formation of water (either as steam or liquid). = Electrical energy generated per mole of fuel E 4q Therefore, for a PEMFC operating at 80°C and producing liquid water (High Heating Value): 6NB Gibbs free energy is temperature dependant and therefore will change under different operating conditions. 6 7max * l00% T1maXPEMFC@SOOC = 228200*100/O = 79.8% Eq.5 It is important to note that if one assumes that all of the heat of formation is converted to electrical power the voltage equation could be rewritten as: -Ah Em = 2F Eq.6 giving a value of 1 .48V (using the HHV) and 1 .25V (using the LHV). These values can then be used to recalculate the observed efficiency of the fuel cell. h1ceii11*h1 Eq.7 AG Where it is assumed that i7, the thermodynamic efficiency (i = ), is equal to 1 andAH that 100% of the fuel is utilized in the reaction to produce electricity7. As previously mentioned, operating parameters such as temperature and pressure play a significant role in fuel cell (in this case PEMFC) performance. At a thermodynamic level, the effect of these parameters on the equilibrium potential can be represented by the Nernst equation. with p° =101.325kPa Eq.8 p0 Temperature plays both a direct and indirect role on the fuel cell performance in terms of its effect on the Gibbs free energy, electrode kinetics, electrolyte ohmic resistance, and mass transfer limitations. In general, it is better to operate a fuel cell at higher temperature but this is limited by the vapour pressure of water in ion exchange membranes since if the temperature is too high the membrane dries out and the ionic conductivity decreases, negatively effecting performance. Another factor can be inserted in to represent fuel efficiency (defmed as the ratio of the amount of fuel used in the desired reaction vs. the amount of fuel supplied). nF PH2O 7 Increased gas pressure has a positive effect on fuel cell voltage. This can be illustrated by the following equation(4) for the effect of increased hydrogen pressure on fuel cell voltage. AVgain = * ln- (p2 > p1) Eq. 9 nF p1 In a non-ideal case the amount of voltage gain varies based on various parameters such as fuel cell type, differing gas compositions/stoichiometries, and flows. The quantifiable effect of these parameters is the subject of much experimentation. It is worth noting that the use of pure oxygen significantly boosts performance (by about 30%) in PEMFCs due to increased reactant concentration, reduced mass transport losses, and an overall increase in oxygen partial pressure. While doing this may be useful in benchmarking, most practical systems run on air. 1.3. PEM Fuel Cells in Detail PEMFCs have various performance limitations and development issues (economic and activity-related optimization of catalyst material, fuel storage/types concerns, water management, minimization of parasitic loads in systems, electrochemical power losses, size/weight minimization, overall system costs, catalyst and membrane poisoning, etc.), which are the current focus of significant research effort around the world. At the basic level of performance, one needs to consider all the power losses and their root causes. As shown in Figure 2, the PEMFC polarization curve losses can be divided into three areas(4), which can have a significant impact in specific regions and/or throughout the polarization curve: • Kinetic effects at the electrode surfaces (activation losses) • Ohmic losses of the overall system (mainly due to membrane resistance) • Mass transport limitations 8 14 1.2 Figure 2: Typical fuel cell polarization curve with labeled regions of loss In addition, fuel directly passing through the electrolyte leads to internal currents and a decrease in fuel utilization (fuel crossover) that affect the open circuit potential (this leads to the difference between the observed OCV and the theoretical, thermodynamically calculated OCV). It is important to note that the losses mentioned, while dominant in certain regions of the polarization curve, are not the only sources of performance loss8. The oxygen reduction reaction at the cathode has a number of fundamental problems: • Low kinetics of the oxygen reduction reaction (even with noble metal catalysts) lead to serious limitations on performance and efficiency. • The cathode-PEM three-phase interface structure complicates the design especially when one uses air. A complex design is needed (hydrophilic hydrophobic boundaries, pore size distribution, PEM conductivity, etc) for water management, reactant distribution (gas diffusion and conditioning), and efficiency/performance optimization. • The need for expensive catalysts with relatively low utilization increases the overall cost of the fuel cell system. 8 There is also the possibility of short circuits occurring at the cell/stack level. 0 1.2 0.8 0.6 0.4 0.2 0 0.8 0.6 0.4 0.2 0 0 0.5 1 1.5 Cent (A) 2 2.5 9 In addition, there are engineering issues to be considered. Proper sealing (via gaskets) and issues of water management are two of the most basic problems, which must be dealt with in order to design a functional PEM fuel cell stack. 1.3.1. Components of PEMFC PEM fuel cells have various components (see Figure 3), some of which are self- explanatory and some of which require more discussion. In brief, a fuel cell consists of a pair of manifold end plates (incorporating some kind of compression mechanism), current collectors (generally gold plated copper or niobium), flow field plates (typically bipolar plates), gaskets, and the membrane electrode assembly (MEA). Figure 3: Exploded view of a standard PEM fuel cell 1.3.2. Membrane Electrode Assembly (MEAl The heart of a PEMFC is the MEA. It consists of an ion exchange membrane (usually Nafion®), catalyst layers and gas diffusion layers. The ion exchange membrane is usually a polymer electrolyte material. Polymer materials are made through a polymerization reaction, usually a free radical initiation a. Maitiflild/Ewl Plate b. Insulating Plate (only one shown) c. Collector Plate d. Flowfiehl Plate e. Gasket f. MEA a. b. Le fI 10 process (see Figure 4), to produce a chemical chain that can be propagated to produce a polymer of varying length and branching9. INITIATION OF REACTION R—O—O—R 2R—O ___ ‘ !,R—0 C=C1 ) R—O—C—(.• II PROPAGATION OF REACTION I i i i R—O—C—C• C=C ) R—O—C—C—C—C• II / liii I iIi iii R—O—C—C—C—C• e=c ) R—O—C—C—C—C—C—C I I I I I I[I 1J1 TERMINATION OF REACTION ri ii i iR_O_C*_C_C_C_C_C. •C—C—C—C—C—C—O—R I i[i iji I I ILl iji I 11 ii r ii r ii R—O—C—C—C—C—C—C—C—C—C—C—C—C—O—R I I LI I] I I I I [I I] I I H it Figure 4: Free radical initiation process polymerization If all the carbon side chains are connected to hydrogen atoms the product is polyethylene and if the carbon side chains are connected to fluorine atoms the product is polytetrafluoroethylene. Since 1967, when it was first introduced, Nafion® has been the basis of polymer electrolyte technology. Nafion® or perfluorosulphonic acid polytetrafluoroethylene (PTFE) copolymer (suiphonated fluoroethylene) is fabricated from tetrafluoroethylene Through use of catalysts like Cr203high density polyethylene can be made (very low branching in the molecules). 11 using a similar mechanism to the one detailed above, but first the hydrogen atoms of the ethylene molecule are substituted with fluorine atoms (perfluorination). The resultant molecules then undergo a free radical initiation reaction to form polytetrafluoroethyelene (see Figure 5). Teti’afluoioethyleue PTFF F F F F F F F F rF F\ / 11111 . —Polvmeiization—— — — — — . — With • — — F F F F F F F F LF F “n” repeated units Figure 5: Chemical production of polytetrafluoroethylene (PTFE) Once PTFE is formed it needs to be modified to allow for proton conduction. A sulphonation reaction adds a suiphonic-acid-ending side chain to the polymer (see Figure 6), creating a molecule with both hydrophilic (water affinity) and hydrophobic (water phobic) regions. FFFFFFFFFFFFF ‘‘‘‘III’’’’’’ —C—c—C—C—C—C—C—C—C—C—C—C—C— ‘‘‘‘IIII’’I’I F F F F FF0 F F F F FF F—C—F F--F 1 / “0HH CF3 0 Figure 6: Representations of the chemical structure of Nafion® Nafion® is a highly studied material with various interesting properties based on its polymer structure. It can absorb large amounts of water, is an excellent proton conductor if well hydrated’°, and has reasonable chemical resistance/mechanical strength. An excellent schematic and a highly informative stylized view can be found in the works 10 The S03 groups can attract protons, allowing for H movement through the membrane via the Grutthus mechanism. 12 of Tjamhage1’(5) and Mauritz (6), respectively, illustrating the complexity of the hydrophobic/hydrophilic (polar/nonpolar microphases) found in hydrated Nafion®. The catalyst layer can be applied to the ion exchange membrane of choice to form a catalyst-coated membrane (CCM). The catalyst layer can also be applied directly to the gas diffusion layer and subsequently be bonded to the membrane. Traditional PEMECs utilize platinum at both the anode and cathode (a large research drive has allowed total platinum loading to be decreased from 28 mg/cm2to about 0.3 mg/cm2). The platinum particles are generally supported/dispersed on larger carbon particles and applied by various technological spraying or printing methods. Finally, the gas diffusion layer serves various purposes including forming an electric connection between the flow field plate and the catalyst layer, creating a method to allow reactants to reach the catalyst at the three phase interface, protecting the catalyst layer, and allowing for reaction products (specifically water) to be removed from the reaction area. The gas diffusion layer is usually made of a carbon-based material such as carbon cloth, carbon fibre paper, or carbon felt. Teflon is usually added to make the material hydrophobic in order to reject liquid water. 1.3.3. Bipolar Plates Bipolar plates serve the purpose of distributing the reactant fuel and oxidant to the respective catalyst sites on the MEA. Input gases are directed via machined flow field channels (of varying design) to the reaction sites and product gases (and unused reactant gases) are carried away by the flow-field. A significant amount of research has been done on the optimization of the flow-field plate design to maximize reactant concentration throughout the reactive area. A discussion of this type is beyond the scope of this work. A few standard flow-field designs are straight flow, serpentine, spiral, and interdigitated. For PEMFCs, graphite plates (produced by injection moulding) are the norm as they are easy to machine, chemically inert and highly conductive. Another function of Illustrates the three different phases present: the hydrophilic inclusions (approximately 30 A in diameter and interconnected by approximately 10 A aqueous channels) where suiphonic acid groups are clustered (and where most of the counter ions and water is found), the Teflon-like hydrophobic polymer backbones, and the partially hydrophilie interfacial domains(5). 13 the bipolar and end plates is cooling. As previously discussed fuel cells do not operate at 100% efficiency and therefore produce a certain amount of waste heat. The waste heat rate is given by the following equation (4) where Pe is the electric power and V is the operating cell voltage. Heating Rate P (l .25V —1) Eq. 10 NB., a voltage ofl.25Vis used fthe product is water vapour but 1.48Vis used f the product is liquid water. Since maintenance of operating temperature is critical to fuel cell operation, this waste heat must be removed. Cooling channels are therefore incorporated into the plate design, which will carry some sort of thermal fluid into and out of the stack. Other types of designs may rely on air-cooling. 1.4. Redox Flow Batteries Walther Nernst (1864-194 1) was the first to point out some of the problems with a direct hydrogen-oxygen fuel cell. He proposed the creation of an indirect hydrogen- oxygen fuel cell (or a redox fuel cell) in which two redox solutions react to produce power and are converted, as a result, to different oxidation states (the solutions can then be regenerated externally by various means). Such a system has the potential to function both as device for direct energy conversion and for energy storage. In the latter half of the 20th century significant work was done investigating the possibilities of redox systems. The economic concerns of the power industry in the mid-1970s led to the serious consideration of large-scale redox systems for reversible energy storage, using intermittent charging methods (such as solar and wind energy), as an alternative to storage batteries(7). Other bulk energy storage alternatives included compressed air reservoirs, hydrogen gas, flywheels, capacitive storage, inductive storage, and other electrochemical methods. Such redox systems as those shown in Figure 7 were considered to have many advantages including comparable cost and size estimates, lower environmental impacts (depending on the initial energy source used and the useful life of the electrolytes involved), higher efficiencies at lower temperatures, faster load levelling (peak shaving) 14 response (illustrated nicely by VRB Power Systems(8)), and less cycle life limitations than other energy storage methods of the time(9). Although redox fuel cells systems were not utilized to the extent initially envisioned, the interest spurred on continued research (specifically in the area of vanadium redox cells) based on it’s great potential as an energy device. Basically, a redox flow cell is an electrochemical device that uses the oxidation states of different redox couples to store energy. The redox reactions of these different couples need to have standard potentials different enough to create the electromotive force necessary to drive the oxidation-reduction reactions, which cause the cell to charge and discharge. Such systems store potential energy in their solutions, which are provided to the cells from an external source and are regenerated externally to the system. This allows for the overall capacity to be controlled by the number and size of electrolytic storage tanks and the overall power to be controlled by the number of cells and the cell area (the size of the redox stacks). This sets them apart from batteries and brings them Chaiged Elecliolyte storage tanks Redox Stacks for power generation Figure 7: A schematic of an electrically rechargeable redox system (7) and a representation of a 1OMW/85MWh redox system (9) 15 closer, in concept, to fuel cells. They have been previously classified as regenerative fuel cells (10). Redox systems have the potential to overcome several of the problems faced in the fuel cell industry. In the redox system, the fuel and oxidant gases used for regeneration (the combination of which are potentially explosive) are kept separate and are used to regenerate their associated redox couple. Hence, the concerns related to the directly catalyzed reactions at the electrodes in a direct fuel cell are avoided. This separation of electrochemical and regeneration reactions has the potential to improve overall performance and safety (1]). Redox cells allow for designs that minimize the use of catalysts at the electrode surface, allow for the use of inexpensive and simple materials, and increase the options in terms of fuels including the use of traditional fuels such as coal (12). In addition, redox systems have fast kinetics with high exchange current densities compared to the fuels and oxidants of traditional direct fuel cells. Therefore, redox systems could potentially produce greater amounts of power/energy. Figure 8 shows a simple schematic of a chemically regenerative redox fuel cell: Figure 8: Schematic of a chemically regenerative redox fuel cell It illustrates some of the basic components of any redox fuel cell system. When designing a redox fuel cell, various key components need to be considered so that maximum performance can be achieved (1]). The major components are as follows: Eleefrodes + Punp Pn11p Meckaitisni Mechaidsm 16 • Redox solutions and supporting electrolytes • Separator/membrane • Electrodes • Overall cell design (flow fields, collector plates, etc.) and operating conditions • System components and pumps (which maintain solution flow) • Regeneration module 1.4.1. The Redox Couples and Supporting Electrolytes The redox couples need to be evaluated experimentally both for their electrochemical properties, handling requirements and potential hazards. The overall efficiency/performance of the system must take into account the conversion of chemical energy to electrical energy and then the reconversion of the products of this reaction back into its reactants12 (see the section on regeneration for more details). One must therefore choose the redox couples and their supporting electrolytes based on the final power outputs (which depend on the cell’s potential-current relationship (13)). In addition, no unwanted potentially hazardous or electrically parasitic/counter-productive side reactions should occur at the electrode interfaces within the cell’3 and the use of expensive catalysts should be minimized to reduce costs14. From an electrochemical point of view, high exchange current densities are desirable since the exchange current density is related to the initial drop in voltage through the Tafel relation (the lower the i, the bigger the voltage drop and the greater the loss of efficiency/power). This initial voltage drop is primarily related to the kinetics of the system and is one of the main advantages for redox systems (reduced activation losses). Redox couple reactions generally have higher kinetics than the more complex direct electrode reactions of fuels and oxidants in more traditional fuel cells, hence 12 Since some redox cells use the redox couples as intermediates instead of direct fuel and oxidants there will be some loss of efficiency. In addition, one still needs to produce/have available the fuels and oxidants for the chemical regeneration reaction (if one uses non-chemical regeneration other parasitic requirements might be necessary). 13 N.B., The various ions involved in the redox couples should be stable in their state of charge otherwise unexpected reactions or performance problems could occur. 14 The use of various catalysts in the catholyte have shown little effect but use at the electrode surface (especially platinum) improves performance(1l) (by counteracting the activation polarization — N.B., the mechanism of catalyst activity in redox systems is not fully understood). 17 creating higher performance systems’5.Redox reactions are fast, due to the fact that the electron transfer reactions involved are based on outer sphere reactions of the redox species. An outer sphere electron transfer reaction occurs when electrons flow from the reductant to the oxidant between the outer or second coordination spheres/shells of the reactants. An inner sphere electron transfer reaction is one in which electrons are transferred via a bridging ligand shared between the reactants. The more direct nature of outer sphere reactions leads to a higher exchange current density than the more complex inner sphere ones that involve more stable electrons. For example, the reaction 2Fe3 + 2e 2Fe has an i of approximately 0.38 A/cm2 (on smooth Pt) while the reaction H2 2H + 2e has an ii., of approximately l0AJcm2(15) under similar conditions. When choosing a redox couple one must consider the standard potential difference of the redox reactions involved and the direction of the electromotive force (see Table 2). The open circuit voltage of the system will vary according to the redox couple and conditions chosen’6. C e4 + e = C e3 E0 = 1.72 V O2+4H+4ec2HO E=1.229 V Br+2e2Br 0=1.065 V VO+2H+eVO E=1.00 V Fe3+eFe 0.771 V Cu+2ecCu E=0.337 V u+eCu =0.153 V Sn + 2eSn E0 = 0.15 V TiO+2H+ecTiO E0= 0.1 V 2H+2ecH E=O.OO0 V Table 2: A comparison of potential differences of various redox reactions In addition, one must consider each reaction with respect to its necessary regeneration reaction chemistry potential. For example, if one were to use oxygen and hydrogen to regenerate the respective redox couples it would be desirable to have redox voltages which would allow for spontaneous regeneration. Ideally, the redox potentials of the two couples involved should be different enough to give as large a terminal cell voltage as 15 The steady-state velocities of a redox reaction can be expressed in terms of the exchange current density (14). 16 The best system investigated showed an OCV of approximately 1 .5V (12) but most OCVs are less than lv. 18 possible. A higher electromotive force leads to a higher overall energy density and power in the system. The redox species needs to be sufficiently soluble in the electrolyte. If a supporting electrolyte is used (includesH2S04,HC1, andH3P04)it must have suitable solubiity, electrolytic conductivity and shouldn’t significantly decrease the solubility of the redox species, etc. As can be seen in Table 3, consideration of the substances used is critical, as both solubility and maximum concentration achievable vary greatly even within a group of related chemicals. Substances Solubility Concentration (gIlOOmI) (M) FeSO47H20 27.8 1.0 Fe(S0)39H 440 0.8 Fe(phen)S04 52.6 0.8 Fe(bpy)3S0 438 0.7 Fe(2,9-dmp)S04 37.2 1.0 Fe(tp)S0 22.4 0.3 Liqands: Phenanthroline 40.0 2.2 Bipyridyl 31.6 2.0 Terpyridine 44 1.8 4-Cyanopryridine 10.5 1.0 Table 3: Estimated solubilities for ferrous-ferric couples and ligands measured in O.5M H2S04(16) The solubility of oxidant/reductant in the supporting electrolyte limits the supply of the species to the cell (mass transport limitations). While a high concentration of the redox couples is desirable, the concentration! composition of the supporting electrolyte must also be considered to give an optimum concentration. The greater the solubility of the electroactive species, the greater the performance and the overall energy/power density will be. Figure 9 shows the effect of redox concentration on overall system size. If the concentration of both the redox couple and/or the supporting electrolyte is too high, one could have issues with chemical complexation producing undesired precipitation leading to performance losses. 19 REACTANT Figure 9: Effect of concentration on 10 MW, 85MWhr redox system (9) 1.4.2. The Redox System Membrane/Separator As part of the development of redox fuel cells, one of the key areas of study has been the use of different types of separators or membranes to keep the anolyte and catholyte apart, and prevent crossover of redox ions. To maximize performance various properties must be considered during the selection process such as chemical compatibility’7,ion transfer effects, and longevity, etc. The membranes/separators need to be chemically resistant to the redox couples and their supporting electrolytes and ideally, resistant to contamination by the redox couples. One interesting strategy used is bi-membrane separators in redox cells (as illustrated in the work of Kummer and Oei (18))which utilize different membrane materials at the anode and the cathode. This type of system could allow for an increased level of flexibility in the choice of electrolytes with respect to chemical compatibility and individual membrane selectivity (decreasing crossover between the cathode and the anode). In addition, membranes should be inexpensive (preferably), resistant to fouling (which causes an increase in the separators electrolytic resistance and therefore large voltages losses (19)), and be selectively conductive to ions. For selective conductivity, positive ions (such as H) need to be able to pass via the membrane from the anode to the 17 only must it be compatible, it must be stable over time. To test for stability, membranes are often immersed in a more concentrated version of their supporting electrolyte for a period of time (at an elevated temperature, e.g., 50°C) and monitored for changes in their electrolytic resistance (17). Fr REACTANT CONC =1 MOLAL f CIIL POWER DENSITY= 30 WIlT2 CELL MODVLES REACTANT CONC =4 MOLAL 20 cathode or negative ions (such as C1 or S042)need to be able to pass via the membrane from the cathode to the anode (7). Studies (e.g. work by Larson and Folkesson(12)) have shown that the ion exchange membrane Nafion® gives the best polarization curve results for a vanadium redox cell. Other materials, which have been tested, include the cheaper silica-filled polyethylene separator (Submicro) (12), polysuiphone membranes (SPS) (12), polymeric ion exchange membranes (produced by a radiation grafting technique) (17), and nylon ion exchange membranes (20). One of the key problems with proton conductive materials, such as Nafion®, is the high level of ohmic loss (due to electrolytic resistance) across the membrane. This is mainly caused by counter productive proton gradients which cause diffusion opposed to the direction of current (from high to low pH) (12) and membrane contamination by the redox species which decreases the overall effective conductivity of the membrane. The pH gradient is often due to the type of supporting electrolytes used with the redox species. Therefore, it is important to make sure that the pH on the anode and cathode side are as close as possible, and are maintained close to their initial values throughout the operation of the cell. In direct fuel cells, increased performance loss in the ohmic region of the polarization curve is mainly the result of problems with the membrane such as drying out, contamination of ionic sites, etc. Diffusion of the ions into the membrane can lead to free radical formation that can cause membrane damage, affecting both membrane stability and ionic conductivity. This was illustrated in work by Poxio et al. (21) where iron contamination (attributed to the end plate material of the fuel cell) led to an increase in fluoride ion concentration in the water produced by the fuel cell. Fluoride ions indicate a break down in the Nafion® structure (while the C-F bond is usually poorly reactive, Fe ions act as a catalyst) caused by hydroxyl radicals produced via a modified Fenton reaction (Nafion®Fe+3+ H20- Nafion®Fe+2+ HO2+ H and Nafion®Fe+2+ H2O -* Nafion®Fe+3+ OH + OW). In addition, research into vanadium redox cell applications has indicated that metal ions could clog/deform pores in membranes (displacing S042 and HSO4 ions which normally act as co-ions aiding in the flow of hydrogen ions across the membrane) decreasing ion exchange capacity and diffusivity (22). 21 Another problem, which is minimized in ion exchange membranes, is metal ion leakage as the redox species migrate across the membrane when the redox cell is in operation (this could lead to unwanted side reactions and depolarization of the cell). The membrane chosen for a redox system should ideally be very selective with respect to what species it allows to cross over. Some systems avoid the concern of cross contamination through careful choice of the redox couple (e.g., vanadium redox batteries). Some types of redox systems under study do not use any type of separator (membrane-less), but they will not be discussed at this time. While there are a number of factors which must be considered, ion exchange membranes result in higher coulombic efficiency, lower electrolytic resistance, higher selectivity, longer life expectancies, and greater resistance to fouling than other types of separators (17). They are an excellent membrane starting point when designing a redox cell. 1.4.3. The Redox System Electrodes The electrode material (solid porous carbon, graphite felt, carbon foam, etc.) and catalyst to be used should have the following properties: • High reaction selectivity with no undesired reactions • High electronic conductivity (the resistivity should at least be less than 40 ilohm*m, e.g., the resistivity of glassy carbon is 30-50 j.tobm*m) • High electrochemical activity • Long service life (high resistance to oxidation) • Minimal catalytic loading (including the use of cheaper non-noble metal catalysts such as Ni and C) Not only is the material important but also the state of the electrode surface due to surface treatment/preparation techniques since the nature of the material affects the rate of the reaction(13). For example, higher porosity leads to higher overall surface area for a reaction. By maximizing the surface/volume ratio of the electrode material, one can minimize mass transfer problems and decrease electrode polarization and power density losses (13). This maximizes volumetric conversion and therefore maximizes current/power density. Additional electrode properties of interest are wet-ability, diffusion coefficients, catalytic load, and oxidation resistance. 22 Based on the type of redox cell under consideration there are many different claims as to which electrode material is the best. When dealing with redox systems metal electrodes are not usually considered due to either chemical compatibility, corrosion and/or cost issues. Carbon based materials have found a market in both PEMFCs and in Redox systems due to their low cost and electrochemical properties (see Table 4). Carbon has many of the properties important to these applications: electronic conductivity, reasonable corrosion resistance and appropriate surface properties. Carbon materials can act as a metallic conductor due to overlap of their valence and conduction bands (23). Melting point (K) —3820 Boiling point (K) 4470 - 5070 Triple point (K) —4020 (—1 10 bar) Electrical resistivity ()m) Glassy carbon 30 — 50 Carbon fiber >600 petroleum coke base 35—46 anthracite base 33—66 lampback base 58 —81 lam pback graphitized 46—66 Graphite fiber --42 petroleum coke base 8—13 Specific gravity Diamond 3.515 Graphite 2.266 umorphous carbon 1.8 — 1.9 C-C bond length (nm) 0.154 Table 4: Typical Physical Properties of Carbon (23) 23 Some research has shown that carbon felt electrodes give excellent performance especially when used in conjunction with solid graphite current collectors (11). At the same time, additional research is occurring into advanced electrode materials such as carbon nanotubes’8and carbonaceous fibers with quasi-graphite crystal structures whose properties can be tailor made to optimize performance (25). These types of materials address the need to maximize the surface area available for electrochemical reaction. 1.4.4. Redox Cell Design Redox cells must be designed with a configuration that will provide optimal performance. Optimization of the cell design to accommodate a liquid fuel and liquid oxidant at the anode and cathode sides, respectively, is necessary. The flow fields of the cells should allow for effective flow of the redox solutions so that maximum reaction area is achieved at the membrane/electrode/solution interface with minimal flow resistance. This requires the careful selection of a porous electrode material, which can have a large effect on flow resistance. It is critical to consider the chemical compatibility of all the cell components based on the properties of redox/supporting electrolyte. For example, the flow field material and collector plates must be selected with respect to their material compatibility with respect to pH and the redox environment (i.e., a copper collector plate should not be used in a copper redox cell). In addition, with respect to overall design and operation of the cell, it is important to minimize all types of cell resistance. There are a number of potential resistance sources: (20) • The use of a separator or an ion exchange membrane can lead to large ohmic losses and fouling issues • Poor cell geometry can cause uneven current distribution at the electrodes • Higher concentration polarization can arise because of no continuous mixing of solution (need good flow conditions) 18 Conductance Quantization: n x (12.9 kf)i where n is an integer; Resistivity: 1 Qcm (approx. I tohm*m); Maximum Current Density: 1013 AIm2 (24) (The resistivity is an order of magnitude lower than glassy carbon!) 24 • Hydrogen and oxygen evolution reactions at the surface of the electrodes can also increase the resistance 1.4.5. Redox Operation The operational parameters of the designed redox system must also be considered. The temperature of operation must be carefully considered, as it will have far reaching effects on the overall system. Many systems are designed to run at ambient temperatures because of the desire to minimize balance of plant concerns and to simplify the choice of construction materials. At the same time, increasing the temperature can lead to significant improvements in performance which can more than make up for the energy losses due to heating the system. Operating the redox cell at elevated temperatures (limited by physical/chemical properties of the electrolytes such as the boiling and decomposition point) has obvious benefits to performance such as the following: • higher solubility of the oxidant and reductant redox couple (making more available to react), • a decrease in internal resistance with increasing temperature, and • increased reaction rate/kinetics with temperature increase. Another key point of operation is the flow rate of the redox solutions. The flow rate needs tc be optimized so that the cell can operate up to its mass transport limit (based on concentration) with less polarization loss. By maximizing the flow one can maximize the cell performance. Work by Pattabiraman et al (13) illustrates this for the charge cycle of a redox system (the same situation would be reflected in the discharge curve) and shows an increase in current density with flow rate for a given polarization voltage. 1.4.6. Regeneration of Redox Solutions The regeneration chemistry of the oxidant and reductant is also very important to the overall efficiency and figures of merit of redox systems. Without proper, relatively rapid, and stable regeneration the concentration of the reactive redox species will decrease causing a decrease in the amount of reaction occurring (to produce electrons within the cell) over time leading to a loss in performance. Open circuit voltage (OCV) changes can be predicted via the Nernst equation with changes in concentration (26). Successful regeneration should lead to approximately constant, or slightly increasing, 25 voltage over time based on the initial species ratio (26). Tn addition, the method used should not become electrochemically parasitic to the overall reaction and preferably should not use an expensive catalyst’9. One of the major disadvantages to redox systems is the need for ancillary regeneration equipment (13). The regeneration of the redox system can be achieved by various means: electrochemical (2Fe3 + 2e <z> 2Fe)0,thermal (catalytic method), chemical [4VO2 +02 +21120 <> 4VO2 + 4H (via N03-/NO)] 21, biological (via bacterial organisms), or photochemical/radiochemical (N02 + hv - NO + 0) (13). It is very common to use hydrogen gas (or some other fuel) to regenerate the anodic couples and oxygen to regenerate the cathodic couples. For a V0/VO2cathode system regeneration is achieved with oxygen and an NO37N0 catalyst, while for aTi’3/T 02anode system regeneration is achieved with hydrogen (26’). Ideally, the redox potentials need to be near to (and lower than) that of the regenerating chemicals in order for the system to work properly. In addition, the exchange current densities for both redox systems should be high (26). The type of regeneration approach will have a critical effect on the regenerative reactor design. One must consider the reaction chemistry including rate determining steps, diffusion of gases, use of catalysts (often Platinum is used), and temperature of operation. The regeneration chamber must be compatible with the conditions contained within it and needs to be operated under conditions that do not lead to unwanted (and potentially dangerous) reactions. A key point that needs to be considered with respect to regeneration is the maintenance of the redox couple ratio of the reactant charged species. This is often optimized through significant experimentation and is critical to the performance of the cell. In reality it is important to realize that it is impossible to completely regenerate a species due to the equilibrium they are in with their counter species, and there will always be some of the other species in solution. It is therefore important to set realistic and affordable expectations on the regeneration reactors to be paired with the energy system, ‘9N.B., Low conversion efficiencies are achieved even with platinum group metal catalysts. 20 Some other examples of electrical regeneration are lithium-chlorine, hydrogen-bromine, hydrogen oxygen, and titanic-titanous couples. 21 Chemical regeneration generally involves a fuel (hydrogen, methanol, coal, etc.) and/or air in separated compartments. 26 i.e., partial regeneration is sufficient. The degree of regeneration is achieved through modifying the residence time in the regeneration reactor, modifying the catalyst, controlling/monitoring the applied current/voltage in the system, use of a rebalance cell (a redox based system used in battery technology), etc. (27). In addition, the use of filters to deal with precipitates and a method to maintain control of pH is essential to the long- term steady state performance of the system (28). 1.4.7. Regeneration Example: Biorepeneration of the Ferric Ferrous couple Although there are various ways to regenerate the iron redox couple, a novel and increasingly attractive method is based on bio-regeneration (29,1). Bio-regeneration is a process within the sphere of bioprocess engineering, a large and relatively new field of science. It utilizes biological species (usually some type of micro-organism) in controlled and often artificial environments to achieve some desired task. Biological organisms are by their nature highly efficient chemical reactors and, depending on their biochemistry, can be used in a vast array of applications. With respect to this work the purpose of the bioreactor would be to convert Fe2 to Fe3. While this could be accomplished by other means, the use of a biological organism (many bacteria are able to oxidize iron sulphate) is both a unique and intriguing choice. Bacteria, like all living organisms, require certain conditions to be able to survive and function properly. The correct combination of pH, temperature, and nutrient media must be considered and compared to the needs of the overall system. In the case of a combined energy system, the needs of the bacteria and of the overall system override the preferences of the energy subsystem since without an efficiently functioning regeneration system (which is dependent on efficiently functioning bacteria) the energy produced would decrease over time. Thermophilic bacteria (which prefer high temperature environments) would seem an idea candidate in terms of potential performance since kinetics improve at high temperature. However, they are unable to survive in high metal concentrated environments because the metals tend to inhibit key metabolic pathways. With respect to energy producing systems, this puts a significant limit on the achievable power density that is unacceptable. 27 The advantage of choosing bacteria such as Acidothiobacillus ferrooxidans (A.f and Leptospirillum ferrooxidans (L.J) is that these organisms are adapted to life in a highly concentrated metal environment. These bacteria are chemolithrophic (acquire energy from inorganic sources), acidophilic (preferred environmental media is at a low pH), autotrophic (carbon needs can be met by C02), aerobic (oxygen is an electron acceptor), and mesophilic (grow and function at moderate temperatures). In fact, their metabolism (30,3 1) allows them to utilize this environment in a manner which has been highly exploited and researched by the mining industry22,in a process known as bioleaching. It uses bacteria such as Acidothiobacillus ferrooxidans (A.J) and Leptospirillum ferrooxidans (U) to extract metals from suiphide ores using the natural biochemistry of the organisms. The two dominant proposed mechanisms for bioleaching of minerals are(32): • the direct mechanism: the micro-organisms interact directly (microbial catalysis) with the mineral, enhancing the rate of dissolution, in addition to what is achieved by chemical leaching; and • the indirect mechanism: the overall process occurs via microbial oxidation of ferric ions and chemical leaching. The overall reaction for oxidation of ferrous iron involving T. ferrooxidans is as follows: 4FeSO + 2HS04+ 02 A.f > 2Fe (SO4)3+ while the biochemistry can be more easily visualized in the work of Nagpal (33)23 As can be seen, the bacterium thrives in low pH environments where it utilizes the pH gradient between the acidic environment and the cell’s neutral interior fluids to drive an ATP generating reaction24via a transmembrane enzyme (ATPase). energy(2H) ADP+P —> ATP 22 These same properties can also be exploited for use in a power system (higher metal concentration equates to higher power density, especially at high temperature). According to Nagpal CO2 is fixated into 3 different cellular products: Xe (enzymes involved in electron transport with Xe being the reduced form and Xe being the oxidized form), Xn (carbon and energy storage), and Xc (Calvin cycle enzymes). 24 ATP, the basic energy unit of the cell, is used in a variety of cellular processes including cellular maintenance, protein/enzyme production, energy storage, etc. 28 At the same time other transmembrane enzymes (Xe) are involved in a coupled reaction (34), which oxidizes ferrous ion to ferric ions outside the cell while producing water from the protons and oxygen that enter the cell (maintaining the pH balance disturbed by ATP formation). T ferrooxidans effectively uses the ferrous iron as the electron donor for the water formation reaction Xe,, 2Fe —2Fe3+2e 3402 +2H +2e—*H0 The electron transport chain (driven by transmembrane electric potential) utilized in the oxidation of ferrous ions is further detailed in work by Nemati et al (34). In this way T. ferrooxidans uses the ferrous ions as electron donors to drive ATP/NADPH production via a coupled reaction. These products coupled with carbon from absorbed CO2 are used in carbon fixation via the Calvin cycle, cellular maintenance, fabrication of extracellular products, and in cell growthldivision (35). Since biological organisms require a complex media and specific environmental conditions to meet their functional needs (see Table 5 for an overview of the impact of various operation conditions on T.ferrooxidans) and purifying/reconstituting/modifying the bio-electrolyte between the energy system and the bioreactor is impractical, the electrolyte must be a compromise between the two parts of the system. Condition Details Comment pH 1-6 Normal strains grow adversely at pH less than 1.5 Temperature 30°C — 40°C This is strain dependent [Fe’] 2 - 20kg/rn3 Needs to be optimized depending on the strain to avoid inhibition and maximize performance [02] Greater than 29mg/L No bacterial growth below 2.OmglL [C02] 5% -5% in air If too high HC03 and H production will be favoured, leading to a decrease in cellular efficiency Applied potential -500 to —l000mV Protein production increased with applied potential Inhibition Temperature, pH and [K’] [Fe3] Less than 2kg/rn3 No bacterial oxidation above 15.6kg/rn3 Table 5: Effects of operating conditions on T. ferrooxidans growth (33) In order for the microbes to survive and function efficiently, nutrients (including oxygen and carbon dioxide) need to be provided to the bioreactor. Other essential nutrients need to be incorporated into the bio-electrolyte medium at levels that meet both the requirements of the bioreactor and the energy system. With respect to T. ferrooxidans, microbiologists have developed the 9K (Silverman-Lundgren) medium, 29 consisting of: (NH4)S0,MgSO47H2O,K2FIPO4, KC1, Ca(N03)2FeSO47H2O, and H2S04(for pH adjustments) in water (34). The concentrations of the various components can be varied based on the specific situation and application but 9K has been shown to be the most relevant for successful cultivation and maintenance ofT ferrooxidans. Due to the complexity of the solution (and the system), optimization of various factors is required with respect to both the energy system and the bioreactor. One must consider the total iron concentration, the ratio of the ferric and ferrous species, the temperature (bacteria are optimally functional within a given range)25,and the pH of the electrolyte. In addition, side reactions such as jarosite precipitation (34) must be avoided26. 1.4.8. Types of Redox Systems Since its initial conception, many types and configurations of redox fuel cells have been investigated with varying results. Table 6 compiles many of the researched redox fuel cells (with performance comments) found in the literature over the past two decades. 25 If the temperature is too low the bacteria’s metabolism will slow and its usefulness as a bioreactor will cease, but if the temperature is too high the bacteria will die. 26pH of operation must consider bacterial needs and chemical issues, such as precipitation. For example: the precipitation of ferric hydroxysulfphates (jarosites): 3Fe + M + + 2HSO + 6H20 -+ MFe 3(SO 4)2 (OH ) ‘1 +8H + where M+ can be K+, Na+, NH4+, H30+. 30 Redox Cell System Comments (anode & cathode) CO/CO2& Cu2/C Unreliable i0 value for the ferrous/ferric couple is highest when palladium is used as a Fe2/F3 & Cr3/Cr’ catalyst e2/Fe3&Ti/TiO*2 1o44 & Mo5 O;&NO *2/5*4 orTi3iTiO2& r2!Bf Bromine is too dangerous Low power density Low cathodic polarization (anadium Oxide at the cathode High voltage Moderate exchange current density Solubility limitation in certain supporting electrolytes (e.g., phosphoric acid) can limi the supply of oxidant VO2/V is difficult to regenerate V3/V2& VONO2 Leakage via the separator does not pose a problem e2/Fe3& VON High exchange current density Iron’s high redox potential can be altered due to complexation, solvation, etc. in various media Low power density Fe2/F ’ ratio is key to performance (i0 & Eo) Fe2/F ’ rate constant is dependant on the supporting electrolyte (HCIO4>HS0HCI) u/Cu2& VO2IV Good results Copper consumption at the anode requires a continuously supply The anode current collector cannot be made of copper (it would be consumed) Sn2/S4& V02WO’ Catalysts are needed to lower the polarization on the tin anode Tin also has a low exchange current density Low power density i3/TiO2with Fe(EDTA) couple & VO2N TheTi3/TiO2reaction is diffusion controlled V2/V3& Ce*4/Ce3 Good results Cerium is relatively available at a low cost Energy density is related to Ce concentration Too high a voltage causes instability Need to optimize concentration of both Cerium and H2S04supporting electrolyte (to minimize precipitation due to complexation) Table 6: Compilation of redox fuel cells examined in the last 20 years (26,20,20,13,11,36) While there are many technical challenges to overcome in the design and operation of redox fuel cells, there are also many significant advantages. For example, some of the performance problems that direct fuel cells (such as PEMFC5) have due to the reaction kinetics at the cathode can be overcome with the use of redox systems. 31 1.4.9. Current Commercial Applications: Vanadium Redox Batteries There are currently at least two groups marketing commercial Vanadium Redox Batteries (VRBs) around the world. Up to 8MWh systems are being supplied by either the University ofNew South Wales, Australia (10) or VRB Power Systems, Inc. in Vancouver, Canada (8). Unlike many systems using two different redox systems, both the UNSW and the VRB Power Systems vanadium redox batteries use vanadium on both sides of the system thus eliminating some of the issues of cross-contamination of the solutions. Cross-contamination can lead to capacity loss and the possibility of unwanted side reactions. The electrochemical reactions of the VRB systems are as follows (10): Cathode: Anode: VQj +2E + e VO÷2 + 1120 V’3 + e 1.OOV E° = -O.26V VRB Power Systems makes the following claims about its system performance: The standard cell potential E° (cell) is 1.26 Volts at concentrations of 1 mole per litre and at 25°C, but under actual cell conditions, the open-circuit cell voltage is 1.4 Volts at 50% state-of-charge (SOC) and 1.6 Volts at 100% SOC. Typically, the electrolyte for the vanadium battery is 2 M vanadium sulphate in 2.5 MH2S04,the vanadium sulphate (initially 1 M V (III) + 1 M V (IV)) being prepared by chemical reduction or electrolytic dissolution ofV205powder.’ (10) It is important to note that the modern redox system is not just a cell and some tanks. It is an integrated system with various components making up the balance of plant27. As mentioned earlier, the extra auxiliary equipment needed in redox systems is a disadvantage in terms of cost and size. Therefore much work has gone into streamlining and minimizing the costs/size of the balance of plant components in redox fuel cell systems and the redox regeneration systems. A typical schematic of the VRB redox system can be found at http://www.vrbpower.comlvrb power.html (8). It is interesting to note that the overall design illustrated is not that much different from the designs 27 Power Conversion Systems (PCS5), Storage Tanks, Cell Stacks, Heat Exchangers (for hot climates), HVAC units (to protect electrical equipment from temperature extremes), PVC piping for redox fluid connections, and Pumps 32 envisioned in the 1 970s (see earlier patent pictures) except that they have yet to reach the scale initially conceived. Systems such as the VRB (8) one have shown many advantages common in refined redox systems: • High efficiencies (70% - 78%) • Low operating temperatures (and less sensitivity to ambient temperature changes) o Can be operated between 0°C and +40°C • Low cost for large installations; projected costs are US$150/kWh from 8 hours of storage capacity and up (10) • Long membrane life (>12 years) • Scalability (simply add more tanks and/or cells/stacks) • Can be easily monitored and controlled28without the need for personnel onsite (this leads to low maintenance costs29) • Low hydrogen evolution during charging • Easy to recharge, refuel, and discharge (with minimal effect on battery life) • Long life of electrolyte (13,000+ discharge cycles) means that the environmental impact is minimal • Low self-discharge means that energy can be stored for useful periods of time The key disadvantage is low energy density when compared to modem battery technology (especially in transportation and portable applications). Research into in-situ chemical regeneration of the vanadium redox couples, improved stabilizing agents, and alternative supporting electrolytes is being conducted in the hope of overcoming this low energy density problem (10). Vanadium redox battery systems are currently being used in North America, Europe, Australia, and Japan in various stationary installations including wind, solar, load-leveling, and remote storage applications. 1.4.10. Overall Summary of Redox Flow Battery Systems Redox Fuel cells have been seriously considered as alternative systems for both energy storage and conversion/production for about the last 30 years (although they were 28 Capacity and state-of-charge of the system are the points of interest for control. 29 VRB, Inc. estimates the maintenance cost at $O.OO1/kWh 33 conceived of far earlier). World energy concerns have driven the research forward. They show great potential and have some significant advantages over other fuel cell systems. While they do suffer from implementation limitations due to geography and environment they have a definite place in the future of distributed energy generation. Their use as energy storage/energy conversion devices has various advantages and disadvantages as outlined in Table 7. Table 7: Performance Aspects of Redox Flow Batteries When designing a redox system, one must optimize all the main components involved. There are many factors to consider. The redox couples and their supporting electrolytes need to have appropriate electrochemical properties to allow for reversible, rapid and efficient reactions. The concentrations of all involved species need to be optimized and long-term use of the solutions must be both stable and safe. The membranes utilized must be stable, ion selective, chemically compatible, and cheap. The electrodes need to be highly electro-active with minimal use of’ expensive catalysts, highly conductive, chemically compatible and stable for prolonged periods of operation. The rest of the cell must be chemically compatible with the often-acidic electrolyte (i.e., high resistance to corrosion), and must be operated at optimal operating conditions for best performance. Finally, the redox solutions need to have simple and efficient regeneration chemistry and should not produce unwanted (and/or potential hazardous) side products at any point in the process. Performance Aspects of Redox Flow Batteries DisadvantagesllssuesAdvantages • High kinetics (outer sphere reaction) • Lower ohmic losses • Mass transport limitations can be controlled by concentration • Lower overpotential at both electrodes • Humidificationlflooding not an issue • Simple temperature control (the heat is easily transported by the solutions) • Flexible electrode structures /simpler flow fields • Easily Scalable • Low energy density • Redox ion species cross over (membrane issue) • Complexation/precipitation (electrolyte issue) • Corrosion concerns • Often have lower gravimetric/ volumetric system energy density 34 While redox systems do have a lower power and energy density than conventional fuel cell systems, they generally have faster kinetics, lower electrode ohmic losses, overall lower overpotential of the electrode reactions, and many other chemical and operational advantages (such as simple operational conditions, good scalability, and flexible component design). This has led some companies around the world to begin to commercially produce redox systems (in particular, Vanadium Redox Batteries) for various applications. In addition, one must consider the over-all system efficiencies of the systems involved. When comparing redox fuel cell systems and fuel cell systems the following equations are generated (1). 7lRedoxFC (SYSTEM) =1lredoxCell1lRe actor iFuel TiOthersl Eq. 11 TiFC (SYSTEM) = 1lCell TiAir sup ply2lFuel TiOthers2 Eq. 12 The significance of these calculations illustrates that due to the various differences in the systems’ components (components with different efficiency parameters) the combined redox fuel cell/reactor could have higher overall system efficiency than the direct fuel cell/compressor system. This is much like the case of DMFCs, where the cell efficiency is less but overall system efficiency can be higher (since no reformer is used in the system, the fuel efficiency is larger and the overall efficiency turns out to be larger than a reformer based system under some conditions). 35 2. Thesis Research in Context of Overall Project Research Objectives This research is part of an overall feasibility assessment of a bioregenerative redox fuel cell system as described by Karamanev (PCT/CA2004/000943) (29,1) in support of a strategic NSERC research grant (GHGPJ 269967-03) entitled “Novel Biofuel Cell - Methane Reforming Reactor System For Electricity Generation” (1). For more information on the other components of this research program see Appendix 2 and 3. The objective of the thesis research was: • To do a comparative investigation of the electrochemical and interfacial properties of the Fe3/F2cathode couple in two supporting electrolytes: one for chemical regeneration (aqueous sulphuric acid) and one for biochemical regeneration (bioelectrolyte).3° • To develop, test, and optimize a hybrid redox/PEM fuel cell using the iron couple and hydrogen fuel. Bio-regeneration was based on the microbial aerobic oxidation of ferrous ions using Acidothiobacillusferroxidans. As can be seen in Figure 10 the initial concept was to pair the bioreactor (to supply regenerated catholyte) with a methane reformer (for hydrogen production) allowing for a novel fuel cell system, which would include partial carbon dioxide sequestration. This system was targeted for stationary applications. 30 Both of these methods can now be considered as possibilities for efficient regeneration of the oxidant (Fe31) since significant progress made in redox research has allowed cheaper and simpler reactors to be developed. 36 REFORMER r Natural Gas Methane Reformer!Bio-fuel cell power generation system Figure 10: Schematic of overall NSERC proposal with research focus for this thesis (bioreactor insert from Nagpal (32)) Use of the above mentioned bio-regeneration method required that the supporting electrolyte be tailor fitted to support the microbes (both environmentally and nutritionally). Strains of Acidothiobacillus ferrooxidans and Leptospirillum ferrooxidans were acquired by a team at the University of Western Ontario from various mining pits and selected for the lowest pH (to avoid jarosite formation and to facilitate bacterial growth while maximizing fuel cell performance). In addition, the stream of electrolyte leaving the bioreactor must have properties appropriate to the running and operation of the redox fuel cell. The characteristics of the fuel cell cathode inlet stream with respect to the cathode redox couple were part of the thesis research project. To achieve maximum performance in the redox fuel cell, a variety of conditions were explored: temperature, pressure, pH, ferric/ferrous ratio, total iron concentration, and the concentration ratio of the ferric/ferrous couple to the concentration of key components in the supporting electrolyte. It was critical to find, characterize, and optimize a bio-electrolyte, which meets the needs of both the bioreactor system and the redox fuel cell31. The electrolyte components were optimized to maximize both efficiency and long-term operation. To do 31 As mentioned earlier, iron plays a critical role in the biochemistry of T.ferrooxidans and this needs to be considered with respect to the limitations on the chemical composition of the total electrolyte. FUEL CELL H , Fe2 M.ASc Research Area BIOREACTOR Air (02, GO2) 412 •4H + ;;:IlII4Ø__I 4Fe+ 2H0 H2 Vent gas Partial 37 this it was necessary to fully characterize the physical and chemical/electrochemical properties of the simple electrolyte and more complex bio-electrolyte32. Once the redox couple and bio-electrolyte had been optimized and characterized, the catholyte was tested in a redox fuel cell setup with hydrogen as the fuel. This research allowed for design optimization of the cell and system components (flow-fields, electrodes, membranes, etc.) and operating conditions (temperature, pressure, etc.) in a cell/stack environment. The performance will be considered based on polarization curves at varying operating conditions and fuel cell material. Various working electrodes materials were compared and discussed with respect to the redox reaction. Electrochemical comparisons of Platinum (Pt), Glassy Carbon (GC), and several types of three-dimensional carbon based electrodes were undertaken. They were then used and analyzed for performance in the fuel cell. This research helped in optimizing the electrode material for the redox fuel cell. 32 example: Uncontrolled precipitation would make it impossible to accurately maintain concentration parameters and could have serious effects on long-term system operation. This situation can be controlled by careful selection of pH (less than 1.5 seems to work) and cell operating conditions. 38 3. Thesis Research Approach: Replacement of the Oxygen Reduction Cathode in a Fuel Cell with a Redox Couple (PEM[Redox Fuel Cell Hybrid) The slow kinetics of the oxygen reduction reaction and the associated high overpotential at the cathode pose a major limitation for PEMFC efficiency and performance. Figure 11 shows the substantial polarization losses on the cathode with air/oxygen compared to the anode with hydrogen. 7O- - 0 20D 4C0 O3 Current density (mAIcm2) Figure 11: Contributions to Polarization of Anode and Cathode in a Phosphoric Acid Fuel Cell (Relative shapes are typical for other types of fuel cells) (38) The following parameters are important to consider for the oxidant reduction reaction and must be considered when looking for a replacement for oxygen (39): • Equilibrium Potential: Ee = (RT/F)ln(kR/ko) + (RT/F)ln(Co/CR) Eq. 13 • Rate of Reaction (Exchange Current Density): i0 =Fke’ Eq. 14 • Overpotential (Tafel equation): ri = (RTktF)ln(i0)— (RT/ctF)ln(i) Eq. 15 • Mass Transfer Limit (limiting current density): 1L = nF(D/a)Cb1k Eq. 16 From these equations, it can be seen that to have high fuel cell reaction efficiency, the oxidant in question requires a high concentration (with a high Cox/Cred ratio), a high 39 temperature, a high reaction rate constant, a high exchange current density, and a high electrode surface area upon which to react. Redox couples with excellent kinetic properties can be used as a substitute for oxygen if they can be used at a relatively high concentration. Low polarization (overpotential) losses for the VO2N andFe2/F3redox couples in redox cells (using Naflon® 415 as the separator) were compared by Oei (11) for low current densities up to lOOmAIcm2.Such redox couples need to have a high standard potential and be able to be “regenerated” at useful rates for application in fuel cells. The approach in this thesis research is to replace oxygen with a redox couple and use a hydrogen anode. Figure 12 illustrates the case of a redox fuel cell with hydrogen anode as a hybrid system between H2/Air PEMFC and a redox flow battery. Yeager et al. (40) in 1962 were first to report the use of H2 with a redox couple (Fe3’7Fe2)based on a radiochemical regeneration fuel cell scheme first proposed in 1957 by J.A. Ghomley at the Parma Research Laboratory at Union Carbide. They were interested in the radioactive regeneration ofFe3 according to the reaction 2Fe + 2H - H2 + 2Fe3 The unit cell was based on a concentric design that used a microporous polyvinyl chloride separator material (PorvicTM), high loaded platinum catalyzed wet-proofed carbon electrodes, and an electrolyte consisting of equimolar FeSO4 and Fe2(S04)3 with 1 M H2S04.Performance results were very poor typically in the voltage range of 0.35 to 0.50V at 20 mA/cm2with very low overall gravimetric and volumetric power density. Surprisingly, the hydrogen redox fuel cell approach appears not to have been revisited in detail since Yeager’ s proceeding article in 1962. The terms redox flow cell, redox battery and redox fuel cell have been used synonymously in the literature to refer to redox couples on both the anode and cathode, and the term redox- hybrid in the case when metallic species are deposited in one half-cell (41). However, direct hydrogen / oxygen fuel cell technology has significantly advanced with respect to materials, design and performance over the last twenty years. In particular, the low-temperature PEMFC is now at a field trial level / early commercialization stage but a number of technology gaps remain. Therefore, it is useful to revisit the redox approach in this thesis research in order 40 to look for opportunities to advance conventional fuel cell technology and to meet the overall project goals of the NSERC strategic grant. PEMFC Hybrid Redox Flow Battery LOAD LOAD LOAD - i—I c N m Regenerative I system It + IN N4 Carbon based Pt/C Catalyst layer electrode Figure 12: Schematic Comparison of a PEMFC, Redox Battery, and Hybrid Redoxfflydrogen Fuel Cell (42) The Fe3/F2 redox couple was chosen as the cathode for the direct hydrogen redox fuel cell with the following electrochemical reactions: Cathode: 2Fe3(aq) + 2e —. 2Fe(a Eo 0.77 V Anode: H2 —* 2H + 2e Eo = 0.00 V Overall reaction 2Fe3a+112 —k 2Fe(a+ 2H Eo = 0.77 V There are a variety of reasons to choose the iron redox couple. Ideally, the redox potentials of the two couples involved (in supporting electrolytes such as H2S04,HC1, and H3PO4) should be different enough to give as large a terminal cell voltage as possible. The higher the electromotive force the higher the overall energy density of the system. Also, if the reversible potential of the selected redox couple is slightly more cathodic/anodic than the potential for the overall reaction in the cathode/anodic compartments, respectively, then the open circuit potentials of the cells will be similar to the case of a fuel cell operating without the redox system(]3). Not only would this be highly efficient but should also allow for spontaneous regeneration of the electro-active 41 species. In the case of the ferric/ferrous33cathode the standard potential is high enough to get useful cell voltage and at the same time low enough for ferric species to be regenerated by oxygen. In addition, the iron redox couple is readily available and inexpensive and has a high solubility (up to 1M, re: Table 3). This redox couple has mainly been extensively studied in the second half of the last century in the Fe-Cr redox flow battery (43), (41). 42 4. Experimental Methods 4.1. Electrochemical Characterization of Electrolyte and Electrodes Various electrochemical techniques were used to study the properties of the redox couples and their supporting electrolytes (20). The electrochemical properties of the simple electrolyte (diluted sulphuric acid and the redox couple) and the bio-electrolyte with the iron redox couple were electrochemically characterized in a classical three- electrode34glass cell in a temperature controlled water bath (see Figure 13) and then studied in a small fuel cell designed for a liquid cathode (using a specially designed testing apparatus). Cyclic voltammetry (CV) with a potentiostat-galvanostat (Solartron 1480, multi-channels) was used to investigate the Fe3!F2redox couple. In addition, electrochemical impedance spectroscopy (EIS) with a frequency response analyzer (Solartron 1255B) was used for determining resistance for IR compensation. P&tJVOt4ith JChcIpWLiMr(for j iic ReeA,er ______ (krCVdicdccb — •Working E2ctrod Counkt Figure 13: Three Electrode Cell Experimental Setup (44,45) Cyclic voltammetry studies the Faradic response to a linear change in applied potential (46). As illustrated in Figure 14, for a typical Fe3/F2scan done in the thesis research, the potential is changed at a constant scanning (or sweeping) rate (V!sec) back and forth between two potential points. By cycling the potential, the oxidation and reduction reactions between the transforming substrate! product can be studied in the Working (Platinwr, glassy carbon, etcj, reference and counter electrode. 43 form of a voltammogram. From the analysis and comparison of various CVs (at different scan rates”) a great deal of information was acquired. 4 2 Figure 14: Diagram of a cyclic voltammogram illustrating capacitance correction of peak height The degree of reversibility of the reaction (the ability of a reaction to reach thermodynamic equilibrium within a given time frame) can be studied via the technique of cyclic voltaminetry. Reversible (or Nemstian) reactions are usually indicated by a characteristic “duck shape” (for simple reactions), as seen in Figures 14 and 15, in which the peak potentials cathodic peak potential; Epa, anodic peak potential) are independent of concentration variations and scan rate (peaks are also usually found at 28.5/n mV after the E° or 56.5/n mV after the half-peak potential, E/2, at 25°C) (47)36• Another sign of reversibility is when current (either peak, L,, or any other point on the CV) is linearly dependent on the square root of the scan rate (this also indicates a diffusion controlled process). In irreversible reactions, E is a function of scan rate (while i, is still dependent on both the square root of the scan rate, v1”2, and reactant The scan rate has a significant influence on the CV it generates. The voltage scan rate will determine the size of the diffusion layer above the electrode. High scan rates will have a more narrow diffusion layer than low scan rates. This will affect the flux to the electrode surface (higher scan rate gives higher flux), which in turn affects the current (it is proportional to the flux). 36 Reversible reactions have very rapid electron transfer kinetics. Low kinetics indicate a lower degree of reversibility. 2 0• EpQ Fe+2+Fe+3+e (oxidzfion) Fe3+ )Fe2 I (idfloii) _____ .600 600 (E-Ej)/ rnV 44 concentrations). The quasi-reversible case is far more complex, with a variety of factors (scan rate, concentration, diffusion coefficients, etc.) affecting the reaction’s E and i values. Each type of system can be mathematically modeled (with varying degrees of complexity) to get specific information such as the diffusion coefficient, rate constant, exchange current density, etc. (46). 1. 20 irwlsec 0* • 2. 0 mv/sec 3. 100 mv/sec 0.4 02 -o -9.2 —0.4 •. Figure 15: Example of reversible reaction as measured by cyclic voltammetry (16) CV analysis can also give further information based on results, for example: • a current which is linearly dependent on scan rate indicates a surface controlled process • rotating disk/ring-disk electrode work can determine the kinetics (exchange current density, etc.) and oxidation/reduction efficiency • reactant concentrations can be optimized via the use ofperformance with respect to concentration experiments37 • charge/discharge testing can model the stability of performance over time (2O). Electrochemical impedance spectroscopy (EIS) was another powerful technique used in the thesis research, which allows one to model a system electrically and gain The generation of a phase diagram for the species involved can also be useful to study complexation/precipitation issues. studying how the voltage drops over time one can determine the point at which charging is necessary to maintain the desired voltage of operation (based on application). 0* .ê 04 0.2 0 E(TwSfF 45 insight into the reaction mechanism. By applying a small sinusoidal (AC) voltage or current signal (of known amplitude and frequency) one is able to observe the AC amplitude and phase response of the cell. Application of Ohms law gives the complex impedance of the systems (including both real and imaginary parts) (48). By applying perturbations of various frequencies one is able to separate many of the processes occurring in the system. Results were plotted in arced Nyquist plots (see Figure 17). These plots were modeled on equivalent circuit models (ECM5, see Figure 16), which can contain ideal resistors, capacitors, constant phase elements and inductors in series and parallel configurations. Figure 16: Equivalent Circuit Model for single electron transfer These circuits represent conductive pathways for ion and electron transfer (48). [rcj = *l.2 OCP@ 25 F, 00% - 40% - 20% - Figure 17: Typical Nyquist plot and CV curves ofFe3IFe2redox couple in H2S04 For a more detailed explanation of AC impedance theory see Bard and Faulkner’s Electrochemical Methods: Fundamentals and Applications (46). Using computer software (CorrWare®, CorrView®, ZPlot®, and ZView®) to fit the Nyquist plots to ECMs and dissect CV data from plots it was possible (with the help of some mathematical treatments) to determine information such as the number of electrons R CT Pt 41 44 41 14 U 44 E1VY. 46 transferred at the interface, double layer charge capacity, solution resistance, charge transfer resistance, diffusion coefficients of the ionic components of the electrolytes, rates of the electrochemical reaction, and even reaction mechanisms. In addition to characterizing chemical reactions, AC impedance was also used to perform conductivity measurements on membrane materials under different conditions (for more information on AC impedance see Appendix 1). 4.2. Electrolyte PreDaration The simulated bio-electrolyte (SBE) was prepared based on a modification of the Silverman-Lundgren medium using double distilled water (Millipore water, 18.2 M2 cm), 0.1 g/L potassium chloride (ACS, Anachemia), 0.5 g/L magnesium sulphate (Baker Analyzed, J.T. Baker), 0.5 g/L potassium phosphate dibasic K2HPO4 (Enzyme Grade, Fisher), and 3.0 g/L ammonium sulphate (OmniPur, EM Science) mixed with ferric ammonium sulphate Fe(NH4)(S0212H (ACS, Fisher), ferrous sulphate FeSO47H2O (ACS, Fisher), and sulphuric acid (ACS, Anachemia) to the desired electrolyte composition and pH. The ratio of ferric to ferrous ions was kept constant at nine to simulate the composition in a real redox fuel cell system with incomplete regeneration of the ferric ions. All chemicals were used as received. Since the state of ionic association of ionic iron changes with pH (49,50,51), this parameter must be carefully controlled. The solutions were allowed to equilibrate for at least 10 hours and the pH was measured using an Accumet AR25 meter39. The simple electrolytic solution was prepared using only DI water, Fe(NH4)(S0212H0,FeSO47H2Oand H2S04as a supporting electrolyte mixed to the desired electrolyte composition. Unless otherwise stated the catholyte used for fuel cell testing was 0.9M total iron with a Fe3/F2ratio of 9 (to mimic a 90% oxidant regeneration process) at a pH of 1 and was preheated to the desired temperature prior to entry in the fuel cell. 39The redox potential of the solutions were measured using an Accumet AR 25 meter (and a combination redox Thermo-Orion electrode). 47 4.3. Electrode Preparation and Characterization Working electrode materials consisted of platinum, Glassy Carbon (GC) and three-dimensional porous carbon fiber paper, TGP-H090 (Toray, E-Tek, thickness 0.28 mm) and thin carbon felt (Electrolytica, thickness 0.3 mm). All electrochemical measurements were conducted in an oxygen-free single compartment three-electrode glass cell with a Pt gauze counter electrode and a Hg/Hg2SO4/K0(saturated) reference electrode. The carbon electrodes were prepared by connecting one edge to a lead wire with silver epoxy (Trancene, mc) and sealed in a glass tube with an epoxy patch (Hysol Cl, Loctite). The GC electrodes were polished with a 1 im diamond suspension (Buehler) then with a 0.05 I.Lm Alumina suspension (Buehler), sonicated in DI water and then stored in concentrated sulphuric acid. The GC electrode was cleaned with DI water prior to use and the porous electrodes were cleaned with isopropanol followed by DI water. Porous electrodes were stored in concentrated sulphuric acid and sonicated in DI water prior to use. The exposed superficial geometric area of the porous electrode was 0.6 cm2 for the TGP-H090 and thin felt. CV measurements were carried out in a static electrolyte solution and were repeated at least three times to ensure reproducibility. With respect to the fuel cell electrodes some contact angle tests40 where first run on carbon paper treated with sulphuric and nitric acid to determine which had the higher hydrophilicity (to allow for optimum accessibility of the solution in the electrode matrix). The results (Figure 18) indicated that the nitric acid treatment was superior. 40 The contact angle is a measure of the interaction across a liquid/vapour/solid interface. When a water droplet meets a solid surface the level of attraction to that surface determines the shape the water droplet will take (and how/if the water wets the solid surface). In the case of a hydrophilic surface (where water is strongly attracted to the surface) the droplet will spread out on the surface giving a contact angle of less than 900 (the lower the number, the greater the hydrophilicity). In the case of a hydrophobic surface (where water is strongly repelled by the surface) the droplet will bead/rest on the surface giving a contact angle of greater than 90° (the higher the number, the greater the hydrophobicity). 48 aTreatment with nthic acid leals to more hydrophuic iiiface piopertie (contact anIe i 1e thaim 900) Treatment with suiftuic acid 1ead to moi’e hydrophobic iuface l)1oPert1e (contact an1e i ‘eater than 900) Figure 18: Results of contact angle tests on Toray carbon paper treated with (a) nitric and (b) sulphuric acid The cathode carbon electrodes were therefore first pre-treated with boiling 1M HNO3 (for 1 hour) and thoroughly rinsed with DI water prior to being cut (with a modification of the tool shown in Figure 19). A variable number of pieces (4 for Toray TGPH-090, 5 for the carbon cloth, 7 for the thin felt, and 1 for the thick felt) were fitted into a square pocket (4cm2 area with a depth of 0.1 cm and 2mm channels on each side) in the carbon cathode current collector plate. In addition, Scanning Electron Microscopy (SEM) and X-ray Photoelectron Spectroscopy (XPS) were used to further characterize the electrodes. A Hitachi S35000N Variable Pressure SEM was used to visualize the electrode materials and gain a better understanding of their three dimensional microstructure. XPS was used to determine changes in the chemical properties of the surface of the electrode as a result of pre treatment and testing. The XPS allows for determination of the elemental composition of the electrode surface and the chemical/electronic state of the elements. 4.4. MEA Fabrication For this research one MEA formulation was primarily utilized: Nafion® 115 with catalyst coated Toray 060 (on one side). The anode GDL/catalyst layer preparation consisted of the following: U 49 • C-paper 10% wet-proof • 0.1 9mg/cm2C-sublayer, 20%PTFE, 80%C4’ • 0.2mg/cm PtJC (dispersed deposit on Vulcan X carbon powder — available through E-tek, Inc.), 30%Nafion® (this Nation® aids in the heat bonding stage and increases the length of the triple phase boundary, 30% is common to avoid delamination) • application of catalyst ink by use of a manual spray gun (Model 07 HVLP, Accuspray Canada Inc.) Once the catalyst loading was decided upon, the mass of the Pt/C powder mixture was calculated using the following formula. “ 1 ‘‘ lg ‘ + VulcanXC (g) = 30 A Geom ___)J t\ J Eq. 200.2 1000 mg where the ap, represents the catalyst loading (mg per geometric cm2; the actual loading can be re-calculated later). It is important to note that to account for application losses (off-edge spraying, air-borne spray), the premixed Pt/C was added with an excess factor of 3 (note the number 3 in the equation, for lower loadings a higher excess factor can be used). The next step involved calculation of the mass of the Nafion® solution (commercially available as a 5% w/w solution) required to mix with the catalyst powder. Nafion 1 mNafsol = Eq. 21 fN00 41 If you wish to add a sublayer (to serve as a hydrophobic layer to aid in water management), choose appropriate Teflon loading (0.2 mg/cm is common) and then calculate the amount of Teflon solution (available as a 60% w/w solution) and carbon needed. I A I g mPTFE gj = ° PTFE Geom i Eq. 171000 mg mPTFE - (g) = mp Eq. 18 mv.c (g) = 4mPTFE Eq. 19 Mix the Vulcan X carbon and the Teflon solution together with first water and then isopropanol and apply via spraying (remember to consider the actual Teflon being applied to get the appropriate loading). Finally sinter the carbon paper in an oven at 350°C for 3Omin so that the Teflon can achieve good hydrophobicity. 50 where fNajìon (fraction Nafion® in Pt+VulcanXC+Nafion®) represents the Nafion® loading and mNaf So! represents the mass of the Nafion® solution required. The catalyst powder was then added to an appropriately sized isopropanol-cleaned beaker. Water was added with vigorous stirring (the powder will naturally agglomerate on the surface, so stirring is required to submerge the powder) to be followed by isopropanol to completely solvate the carbon powder (amounts added will vary). The Nafion® solution (mixed with water) was then added via a micropipette with stirring. The whole solution was then sonicated in the covered beaker for 60 mm until a homogenous dispersion was achieved. The mixture was then applied using an isopropanol-cleaned manual Accuspray spray gun42 (15 psi pressure) to a pre-weighed piece of carbon paper (the dried paper was reweighed during the spraying process so the desired catalyst loading could be achieved) on a hotplate set at approximately 80°C (so the ink can quickly dry). Once the spraying was complete, the paper was dried at 80°C for 30mm to remove the water and isopropanol. The Nafion® membrane was also treated. It was heated at 80°C for 2 hours in 3% (by weight) H20to remove any adsorbed organic impurities. After thorough rinsing with D.I. H20, the membrane was boiled in 15% (by weight) H2S04for 2 hours, followed by rinsing for 30 mm in D.I. water approximately 3 times. The Naflon® was then stored in D.I. H20 until required. 4.4.1. Cutting of GDL/catalyst layer and MEA assembly The prepared catalyst/GDL layer now needs to be applied to the treated Nafion® to construct MEAs. It is very important that the cutting of the material is consistent so a cutting tool was designed and fabricated to the exact dimensions needed. Figure 19 is the schematic of the required tool (fabricated by Peters Rule and Die, Canada). 42 There are various methods to apply catalysts ink. In addition to hand spraying both hand painting and automatic spray techniques are appropriate. 51 Open circle to get sample Out —‘100mm S - S_f - Cutter holder alb-z’ mni Steel Cutter a axa Specified in price quote 100 X 100 mm Cross Side view Top ‘iew Figure 19: Schematic of GDL cutting tool Since the MEA is single sided this presents some novel challenges (to avoid delamination). The MEA was pressed at 145°C (higher than the commonly used 135°C) for 4 minutes (must be below 10 minutes at higher temperature to avoid decomposition) between 90-100 kg/cm2(1280-1422 PSI). The anode side with the GDL/catalyst was placed against a cleaned Niobium sheet while the other side (exposed membrane side) was placed against a thin Teflon sheet (to avoid over adhesion). After pressing the MEA was carefully removed with a scalpel and stored in a Teflon sheet holder. Upon fuel cell assembly the cathode carbon electrode(s) in the cathode flow field pocket were placed in contact with the exposed membrane of the prepared MBA. 4.5. Fabrication of Fuel Cell Gaskets Fuel cell gaskets play a critical role in both sealing and proper electrical contact. If they are the wrong size either leaks will occur or the GDL will have poor contact with the flow field plate. One must consider the how the gaskets material will flow and compress under pressure (this is also important when designing the flow field plate). In addition, material compatibility issues must be considered based on the cathode and anode flows. For this work, 4cm2 silicone seals were prepared for the fuel cell using acetone- cleaned machined molds prepared for the fuel cell. JRTV silicone was mixed with its curing agent in a ratio of 10:1. The curing agent was poured first and then the silicone into a pre-weighed vessel and then mixed. The mix was placed into a vacuum chamber 52 Open circle to get -- - -1U0 mm sample out :•- — - I Cutler holder H1 nun Steel cutter axe Specified in price quote • ----- 100 X 100mm — — Cross Side view Top view Figure 19: Schematic of GDL cutting tool Since the MEA is single sided this presents some novel challenges (to avoid delamination). The MEA was pressed at 145°C (higher than the commonly used 135°C) for 4 minutes (must be below 10 minutes at higher temperature to avoid decomposition) between 90-100 kg/cm2(1280-1422 PSI). The anode side with the GDL/catalyst was placed against a cleaned Niobium sheet while the other side (exposed membrane side) was placed against a thin Teflon sheet (to avoid over adhesion). After pressing the MEA was carefully removed with a scalpel and stored in a Teflon sheet holder. Upon fuel cell assembly the cathode carbon electrode(s) in the cathode flow field pocket were placed in contact with the exposed membrane of the prepared MBA. 4.5. Fabrication of Fuel Cell Gaskets Fuel cell gaskets play a critical role in both sealing and proper electrical contact. If they are the wrong size either leaks will occur or the GDL will have poor contact with the flow field plate. One must consider the how the gaskets material will flow and compress under pressure (this is also important when designing the flow field plate). In addition, material compatibility issues must be considered based on the cathode and anode flows. For this work, 4cm2 silicone seals were prepared for the fuel cell using acetone- cleaned machined molds prepared for the fuel cell. JRTV silicone was mixed with its curing agent in a ratio of 10:1. The curing agent was poured first and then the silicone into a pre-weighed vessel and then mixed. The mix was placed into a vacuum chamber 52 depressurized to around 3OmmHg in order to remove the air bubbles from the silicone43. This was repeated as necessary to prepare enough material for all the seals (approx. 17-20 grams of the mix are needed for the cell seals). The mixture was poured into the molds using a small spatula and/or a syringe. Once the molds were filled with no bubbles present a straight edge was used to remove the excess silicone. The filled mold was then cured for one hour at 100°C. The seals were then carefully removed for use. 4.6. Fuel Cell Testing System Design Fuel cell testing allowed for characterization of alternative fuel cell components and cell operating conditions. The following parameters were investigated in the redox fuel cell: • Fuel cell design (flow fields, etc.) and operational conditions (anolyte/catholyte flow, pressure, and temperature) • Alternative catalysts and electrodes (TGP, PGP, carbon felt, carbon aerogel, etc.) • Variations in reactor outlet composition and operating conditions • Optimization of electrolyte/bio-electrolyte composition for chemical/biological regeneration (these results could be used to make a compromise between fuel cell performance and regeneration reactor performance to optimize overall operational efficiency) o pH, redox species concentration, redox species ratio, and sulphuric acid concentration The hybrid system, as previously mentioned, will require some specific engineering to accommodate and properly utilize the liquid catholyte. Figure 20 shows the 4cm2 cell that was used in the fuel cell polarization testing (detailed engineering diagrams can be found in Appendix 4, including detailed flow field schematics). The baseline cell hardware consisted of a single channel serpentine anode flow field (using Be careful to avoid spillage as the silicone expands to normal size in the weighing vessel. 53 hydrogen as the fuel) and an open space pocket design (can incorporate various porous carbon electrode materials) as the cathode flow field for the redox electrolyte. mIi Figure 20: Schematic of 4cm2 Redox Fuel Cell for Parametric Analysis (Exploded View) It was assembled, optimally compressed with a nitrogen bladder, and electrically connected to a potentiostat I galvanostat (Solartron 1480, multi-channels). The fuel cell was incorporated into an overall testing system (see Figure 21), which allowed for pressure, temperature and flow control of both the hydrogen fuel and the catholyte. The system components can be summarized as follows: • A modified Alloy Products Corp. pressure vessel (rated up to 130 psi between 38°C and —29°C) with N2 input/Electrolyte output ports, a venting mechanism, and a pressure relief valve supplied pressurized catholyte. • A 2L Nalgene bottle served as a storage container for the catholyte in the pressure vessel (this limited the catholyte to 2L batches). • A Haake circulating water bath and custom heat exchanger allowed for the redox solution temperature to be controlled prior to input into the cell. • Teflon flowmeters (available from Omega Engineering, Inc.) with an attached graduated pipette controlled and checked accurate flow of the catholyte. I; // a. Bladdex Plate b Tie Roth (4) C. C ompzesion P1 ton (c-ring) d. Cathode Manifold e. Cathode Seal f. Cathode Bus Plate g. Cathode Plate h. MEASeals(2) i. Anode Platej. Anode Bus Plate k. Anode Seal 1. Anode Manifold in. Bushing(4) ii. Piu(2) o. End Plate 1 54 • Calibrated gas flowmeters available from Porter Instrument Company and mounted hydrogen and inert gas regulators (available Praxair, Inc.) controlled gas flows and pressures. • 4 pressure gauges (available from Praxair, Inc.) allowed for monitoring of pressure and pressure drop of anolyte and catholyte (liquid/gas Teflon isolators, available from Omega, Inc., were used on the catholyte side). • A custom-built temperature control box controlled the fuel cell temperature via thennal strips attached to the backs of the anode and cathode collector plates. • Teflon hosing, stainless steel flexible hose (for the hydrogen feed), and Swagelock components (available from Industrial Pipe and Fittings, Inc.) connected the system together. Figure 21: Schematic of redox fuel cell system (Testing Setup) The catholyte was flowed in a single pass through the cell (no recirculation) to allow for better control of the experimental conditions (and to simulate a regenerated solution). After the fuel cell temperature had stabilized with the flowing redox solution (hydrogen flow was fixed at an outlet flow of 1 lml/min for all current densities to ensure sufficient fuel availability), the open circuit voltage (OCV) was recorded for 5-15 mm followed by a potentiodynamic polarization curve (scan rate of 0.2 mV/s) and a galvanostatic polarization curve (individual current points analyzed until voltage F PT,ItIl(I—(hala%U .1th lreqieocy 55 stabilizes). All fuel cell testing was carried out at least three times to ensure reproducibility. 56 5. Results and Discussion 5.1. Mathematical Analysis of Results (CV/Impedance Results) For inner-sphere charge transfer, as expected withFe3/F2redox couple on carbon-based electrode, the kinetics of the charge transfer strongly depends on the nature of the iron complexes and their electrochemical behaviour. This makes the electrolyte composition and its speciation a relevant aspect of the kinetics study of the Fe3/F2 redox couple. In fact, in a complex medium such as sulphuric acid based solutions, ferrous and ferric species may exist as free ions and/or complex compounds. Therefore, using a redox cathode presents opportunities as well as challenges to tailor the suitable electrolyte composition that provides the desired properties for better kinetics of charge transfer at a given operating condition. Some of the values presented in the following work can be directly determined via CorrView and Zview software packages. Other values need to be calculated from values taken from data analysis. Appendix 1 details the mathematical methods used to determine and calculate the values of the diffusion coefficients, heterogeneous electron transfer rate constants, double layer capacitance, charge transfer resistance, and degree of homogeneousity of surface phenomenon (1). 5.2. Electrochemical and Fuel Cell Results 5.2.1. Cyclic Voltammetry Results for Simple Electrolyte Cyclic voltammetry (CV) responses of the Fe3/F2 redox couple recorded at various scan rates (v) at a GC electrode in O.5M H2S04with the ferric/ferrous ratio of 9 and a total ferric and ferrous concentration of 1 M are shown in Figure 22. For all solutions studied, the cyclic voltanimograms stabilize in the second cycle. The cathodic and anodic peaks exhibit good symmetry. The ratio of the peak currents, ‘pa”po, with the capacitive current being subtracted, is almost unity particularly at scan rates higher than 100 mVs1 while at lower scan rates this ratio is higher than unity. This is because of the existence of both oxidized and reduced forms of the redox couple in the starting solution (Fe3t’F2ratio of 9). Since the bulk of the solution contains both redox species the effect of diffusion to the electrode interface plays a significant role in the observed results. High scan rate experiments represent large Faradaic responses at the electrode surface 57 with the magnitude of the flux to the electrode surface being high. This is behaviour is predicted by the Cottrell equation, i.e. current decreases over time as the thickness of the diffusion layer increases and current increases over time as the thickness of the diffusion layer decreases (52). During high scan rate experiments the effect of species diffusion to the working electrode interface, where the reactions are occurring, is minimal due to the short time frame available, meaning that little change occurs with respect to the concentration of the reacting species and therefore ipJipC tends to unity. Low scan rate experiments represent smaller Faradaic responses at the electrode surface (lower flux). The longer time frame allows diffusion of the redox species to change the reactant concentration values and is manifested by the difference between the corrected ipa and i values. The fact that i,,Ji increases at lower scan rates is indicative of the ferric ion (responsible for the value) having a lower diffusion coefficient than the ferrous ion (responsible for the 1pa value). This observation will be further discussed later. 0.010 0.005 0.000 - -0.005 - -0.010 Figure 22: Cyclic voltammograms of the Fe3/F2redox couple ([Fej/[Fef2=9) in 0.5 M H2S04at 25°C on Glassy Carbon. The peak symmetry also indicates the reversibility of the electrode process. However, the separation between oxidation and reduction peak potentials, AE, corrected for a 250 mV/s 150 20 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 EN vs HgIHg2SO4 58 uncompensated JR drop, shows values around 0.1 85V at 20 mV s1 and increases with increasing scan rate to 0.546V at 400 mV s, suggesting that the Fe3JFe2redox reaction is an electrochemically less reversible process under these conditions. Chen et al. (16) indicate that this effect is due to heterogeneous electron transfer. Similar cyclic voltammetry responses were obtained on GC when varying the total Fe3/F2 concentration and H2S04 concentration. A linear relationship between the peak current and the square root of the scan rate was obtained (an example of which can be seen in Figure 23), which indicates that redox charge transfer is a diffusion-controlled process. 0.005 0004 McReacffoi 0.001 0 I I I -0.001 ( 2 0.25 0.3 0.35 0.4 0.45 0 5 -0.003 y9805 0.004 Cathodic Reaction -0.005 Square Root of Scan Rate Figure 23: Peak current vs. the Suare Root of the Scan Rate for the anodic and cathodic reaction of the iron redox couple (O.9M, IFe ]/IFe21=1) on glassy carbon at 25°C and pH 1.5 As discussed in the section on cyclic voltammetry (14a), different mathematical treatments are used to analyze CV data depending on the reaction’s degree of reversibility. To simplifr the analysis, the reversible case was chosen as a good approximation of the system (even though it is not a truly reversible case). The diffusion coefficients of ferric and ferrous species, DF3+/2+, were therefore estimated using the Randles-Sevcik equation (46). = (2.686 x io )An3/2D112cv112 Eq. 22 where i is the anodic or cathodic peak current (A), n is the number of electron involved in the electrode process, A is the electrode area (cm2), D is the diffusion coefficient of the 59 ferrous or ferric ions (cm2 s’), C is the concentration of the electroactive speciesTM (mol cm3), and v is the scan rate (V s’). The heterogeneous electron transfer rate constant, k0, was estimated from CV measurements using the Crank-Nicholson method (for more information on the mathematical handling of the data see Appendix 1) (54). However, in this case, because of wide anodic and cathodic peak potential separation (over 220 mV for higher scan rates), we have used the extended working curve, nAE versus logu, reported by Mahé et al. (55), (DY kJ ° Eq.23 LDR) J7rD0nFv Where u is a dimensionless parameter, k0 is the heterogeneous electron transfer rate constant (cm &1), D0 and DR are diffusion coefficients for the oxidized and reduced species, respectively (cm2 s’), a is the cathodic charge transfer coefficient, and v is the scan rate (V s’). Other terms have their usual definition. Assuming a = 0.5 (based on symmetry of the CV peaks), the diffusion coefficients and k0 were calculated for different electrolyte compositions. The CV results are summarized in Table 8 for the GC electrode. The obtained values for the diffusion coefficients agree well with values reported in the literature (16), particularly at low ferric and ferrous concentrations. Both DF3+/2+ and Ic show an increase with decreasing total iron species concentration. Figure 24 shows cyclic voltammetry results of the porous carbon fiber-based material, TGP-H090. The concentration of the electroactive species used in Eq. 22 was taken as the total iron in the solution. This is a condition of the Randles-Sevcik equation. The mathematical proof of which (a derivation of the Nernst equation taken at the electrode surface) can be found in Electroanalytical Methods: Guide to Experiments and Applications by Tnzelt et al. (53). In short, after the first cycle in a cyclic voltainmetry experiment, the CVs represent the cycling of all ionic species between their oxidized and reduced forms. Therefore, once the CVs have stabilized the only concentration of relevance is the total of the ionic redox species near the electrode surface (in this case, the total iron concentration). This is why, when conducting experiments in which the ratio of ferric to ferrous ions is changed (with all other factors constant), the values of the diffusion coefficients and the rate constants to not change in any meaningful way. 60 0.150 Square Root of the Scan Rate Figure 25: Peak current vs. the Square Root of the Scan Rate for the anodic and cathodic reaction of the iron redox couple (0.9M, [Fe3]/[Fe2=1) on TGP-H090 at 25°C and pH 1.5 I I b 250 mVI 150 0.100 - 100 60 0.050 - 20 0.000 - -0.050 - -0.100 - -0.150 I I I I -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 EN vs HgIHg2SO4 Figure 24: Cyclic voltammograms of the Fe3/F2redox couple ([Fej/LFe2]=9) in 0.5 M H2S04at 25°C on TGP-H090. Inset: SEM image of carbon TGP-H090 surface. A linear relationship between the peak current and the square root of the scan rate was also obtained on this material, again indicating a diffusion-controlled process (See Figure 25). 0.2 0.15 0.1 0.05 0• O.050 .0.1 -0.15 -0.2 AReacoi I I I I I 2 0.25 0.3 0.35 0.4 0.45 0 y=-0.3636x- 0.008 R2= 0.9978 Cathodic Readion 5 61 However, DF3+/2+ could not be estimated because of the complexity of the diffusion process (non-planar) at a 3D network of carbon fibres for TGP-H090 (inset of Figure 24) and the difficulty to evaluate the electrode surface area with accuracy. For this porous carbon electrode with 3D structure, k0 was estimated using equation 23 from the peak separation corrected for the JR drop, and using DF3+/2+ obtained at GC for each respective composition. It is worth noting that DF3+/2+ should be in theory independent of the electrode material. CV(ct-.’O.5) EIS Electrolyte GC TGP-H090 GC H2S04 Fe3’2 106DF, 1ODF2* 104k0 04k 104k0 Ccj M M cm2 s1 cm2 s1 cm s1 cm s_I cm s pF cm2 0.5 1.0 0.8 0.7 0.7 0.2 3.4 0.64 93 0.5 1.4 1.3 0.9 0.5 3.7 0.71 109 0.1 2.0 2.2 2.9 1.5 7.0 0.82 175 1.0 1.0 0.9 0.8 0.9 0.9 3.3 0.71 99 0.5 1.4 1.4 0.9 1.1 3.1 0.80 118 0.1 1.8 2.1 2.2 2.0 5.3 0.85 168 2.0 1.0 0.8 0.8 1.5 0.7 3.7 0.83 152 0.5 1.3 1.3 1.7 1.2 3.8 0.86 148 0.1 1.9 2.1 2.6 1.5 5.4 0.86 187 Table 8: Kinetic parameters obtained on GC using CV and LIS Table 8 shows that the kinetic behavior of the redox reaction is similar (the k0 values are within the same order of magnitude and follow the same trends) on GC and TGP-H090 for each studied electrolyte composition. The changes which were observed were most likely due to the increase in the concentration of the S042 ions, which could either act to increase surface groups on the GC electrode (via oxidation) or act as bridging ligands in the charge transfer reaction (inner sphere mechanism) (56,57). No experimentally significant variation of DF3+/2+ and k0 was observed when changing sulphuric acid concentration to 1M and 2M H2S04. This suggests that the ionic strength 62 •0 E C.) 0 0 + c.1 4’ U. C Fe2 Fe3* has a negligible effect on the diffusion coefficient and the charge transfer in this range of H2S04concentration. This assumption allows the variation of kinetic parameters versus the ratio of the total suiphates species in the solution (HS04,S042)to ferric species to be determined for all studied solutions as shown in Figure 26 for DF3+/2÷. 2.5 2.0 1.5 - 1.0 - 0.5 0 25 Figure 26: Influence of suiphate/ferric ratio on ferric/ferrous diffusion coefficients at 25°C It can be seen clearly that the diffusion coefficients reached a plateau at higher sulfates/Fe31 ratio (above 6). Interestingly, DF3+ and DF2+ showed similar values at lower su1fates/Fe3ratios up to 7 to 9 but slightly different plateau values with DFe2+ being higher at higher ratios. Figure 27 shows similar behaviour of apparent k0 on both GC and TGP-H090. This indicates that ferric complex coordination also influences the charge transfer properties at the electrode surface. 5 10 15 20 [SO4 63 3.0 E 0 2.0 < 1.0 A careful interpretation of these results needs a fundamental understanding of ferrous, ferric and sulphate speciation in concentrated electrolytes. Table 9 summarizes the stability constants of iron-sulfate complex species. From this data it is clear that ferric sulfate complexes are predominate in the studied electrolytes. One could make the assumption that the diffusion properties (related to charge and size) of ferric and ferrous complexes would be expected to stabilize after the central ion has reached a full coordination by sulfate species (solution viscosity assumed constant). Complex Temp. °C Ionic strength Stability constant Reference FeSO4 25 5 210 (58) FeHSO42 25 2.67 4 (59) FeSO4° 25 1 10 (60) FeHSO4 25 4 1.94 (59) Table 9: Some reported data on the stability constants of iron-sulphate complex species GC 0.0 0 5 10 I I 15 [SO42]I[Fe34 20 25 Figure 27: Comparison of the variation of apparent k0 with suiphate/ferric ratio obtained on GC and TGP-H090 at 25°C 64 Nonetheless, in a recent study on the speciation of iron and sulphate in acid waters (less concentrated solutions), Majzlan and Myneni (61), had demonstrated using FTIR spectroscopy that the interactions of sulphate with ferrous species are negligible. They deduced that sulphate and ferrous either associate in outer-sphere complexes or do not associate at all. As free ions, DFZ+ should mostly be iafluenced by the viscosity of the solution. In contrast, ferric species are known to strongly interact with sulphates (62). However, the speciation and the real nature of the ferric complexes (the oxidant at the fuel cell cathode) at lower pH and in the presence of sulphates are still a subject of debate (62,61). Aqueous ferric (Fe3) species are also known to form hydrolytic complexes according to the following reaction: pFe3 + qH2O —* [Fep(OH)qJ’:31’+ qH Sapieszko et a?. (59) reported that hydrolytic complexes such as Fe0H2;Fe(0H)2;and Fe2(0H)4,which are species derived by deprotonation of the octahedral complex We(0H2)61’3,are present even at relatively low pH (0.7 — 2.2) with a distribution that is pH and temperature dependant. They also reported that the presence of excess suiphates in ferric solutions favours the formation of FeS04 as a predominant complex in the studied pH range of 0.7 — 2.2 and temperatures ranging from 25 to 80°C with no indication whether this is an inner-sphere or outer-sphere complex. Majzlan and Myneni (61) on their side reported that the predominant species in Fe3-SO4 solutions are hydrogen-bonded complexes with a small portion of inner-sphere complexes but in their study the maximum sulphate/ferric ratio was 1.5. Casas et a?. (63) reported by comparing model calculations with experimental results in more concentrated solutions (2.2 M H2S04 and 1.3 M total Fe), that the dominant species are HS04, H, Fe2 and FeH(S04)20. From these literature findings, it seems that at lower sulphate/ferric ratio we are in the presence of strongly hydrogen-bonded ferric complexes or poly-nuclear hydrolytic ferric species, which increase the solution viscosity (decreasing D), while at higher sulphate/ferric ratios, mononuclear ferric species (either as inner-sphere or outer sphere complexes with sulphates) are present, which decrease solution viscosity (increasing D). This in fact explains the diffusion coefficient behaviour for both ferrous and ferric species, taking into account that the structural changes associated with the conversion between Fe3 and Fe2 at the electrode/electrolyte interface is much faster 65 than the diffusion process (this also explains the IJXI/ipC observation noted at the beginning of this section). On the other hand, the difference between D3+ and DFZ+ at higher sulphate/ferric ratio can be related to the difference in size of the diffusing molecular 2+ 3+complex and the solvating process of Fe compared to Fe . In addition to the complexation issues described in the text above, it is also important to note that the Fe2 ion is physically larger than the Fe3 ion , leading to a significant difference in charge concentration on the ions “surfaces”. The simple interaction of these charged surfaces with polar water molecules in the electrolyte will lead to differences in viscosity and therefore ionic diffusion (diffusion is a function of particle size and charge). In the case of the iron redox couple, even though the size of the ferric species is less than the ferrous species, its higher ionic charge concentration plays a role in decreasing its diffusion coefficient due to interaction with the surrounding electrolyte water (which increases solution viscosity). While the diffusion coefficient mostly depends on the electrolyte properties, the case for the apparent k0 is more complex, involving interface and surface properties. It may be possible that working with higher concentrations and electrolytes with different ionic strengths, that the obtained variation of parameters such as the apparent rate constant k0 with sulphate/ferric ratio may be uncertain. From this perspective, CV measurements were conducted on GC in dilute solutions of FeCl3 (5 mM) in 1 M HC1 as supporting electrolyte. The sulphate was added to the solution as a sulphuric acid solution to allow sulphate/ferric ratio values of 0, 1, 3, 6, 12 and 24. The ionic strength of all solutions was kept in the range of 1.0 to 1.2. Typical CV curves characteristic of the redox system were obtained. The AE corrected for uncompensated IR drop, shows values around 0.1 52V at 20 mV/s and increases with increasing scan rate to 0.246V at 400 mV/s, suggesting that the Fe3t’Fe2redox reaction is still an electrochemically less reversible process under these conditions, but more reversible than in concentrated solution. DFe3 and DF2+ were estimated using equation 22 and showed a similar value of —‘4.4x 1 06 cm2/s, which was independent of sulphate content. The apparent rate constant k0 was calculated using the Nicholson method as well as simulated CV with DigiSim 3.03 (CV simulation software from Bioanalytical Systems Inc.). The results presented in Figure 28 showed clearly that k0 is one order of magnitude 66 higher than in concentrated solution but more importantly it varies the same way with suiphate/ferric ratio as was the case in concentrated solutions. 20 E 15 0 x 0 4- 0. 10 5 Figure 28: Variation of apparent k0 with suiphate/ferric ratio obtained with CV in diluted ferric solution (5mM ferric chloride in 1M HCI) at 25°C. While it is clear that increased sulphate species content improves the charge transfer kinetics on carbon materials (GC and TGP-H090), it is still not clear yet whether these species are involved as a bridge in an outer-sphere charge transfer mechanism or they are being chemically adsorbed on the carbon electrode and facilitating the charge transfer. There is a need for more investigation to better understand the effect of sulphate species. Although the Fe3/F2redox couple was taken as a model cathode system, this study indicates in fact, that redox fuel cell performance could be improved by tailoring the catholyte composition and by understanding the kinetic behaviour of the redox oxidant at the cathode. This obviously also can be achieved by improving the fuel cell design and architecture. 0 5 10 15 20 25 [SO2]I[FeJ 67 5.2.2. Impedance Results for Simple Electrolyte Impedance (EIS) measurements were conducted around the equilibrium potential of each solution. The interface properties such as the double layer capacitance and charge transfer resistance were obtained. Figure 29 shows the impedance spectrum at the equilibrium potential of theFe3/F2redox couple (O.77V vs. SHE) in the Nyquist plane. -5 -4 -3 E 0 N -2 0 0 1 2 3 4 5 z I c cm2 Figure 29: Impedance spectrum in Nyquist plane ofFe3/F2redox couple on GC at equilibrium potential. Inset: Bode plot The spectrum consists of two strongly overlapping and depressed semi-circles. The Bode plot (inset of Figure 29) shows the existence of a barely visible shoulder at higher frequencies indicating the presence of a second time constant. The origin of this time constant is not clear. It can be the result of the presence of both ferric and ferrous species in the starting solution, the presence of two or more active species (e.g., different complexes of ferric species) having different charge transfer rates, or due to the heterogeneous electrode surface (different active sites). At lower frequencies the straight line indicates Warburg impedance (diffusion controlled process). In the present work, only the impedance data at high frequencies were fitted assuming one depressed semi- -25 -U 0) -20 CD -15 CD C” E4 C) N 0 -5 10’ 10’ 10’ 10’ -10 10’ 10’ 10’ Freq I Hz •• +4.. 68 circle related to the overall charge transfer of the redox reaction. The equivalent circuit used consists of a solution resistance in series with a parallel constant phase element, CPE, and a charge transfer resistance, (see Figure 16). The apparent k0, was estimated from fitted using the following equation, derived from the low field approximation (46): =RT/n2FAk0CC Eq. 24 Where C0 and CR are the concentrations of the oxidized and reduced forms respectively. The other parameters have their usual definition as mentioned previously in equations 22 and 23. The double layer capacitance, Cdl, in the presence of a faradic process and the parameter, c2>, related to time constant distribution, were estimated from fitted parameters (R, T and P) of the CPE, and the average Cdl was estimated according to the following equation (64): T = c(R;’ + R’r Eq. 25 Where T is a capacitance parameter (F cm2 s’), and R is the solution resistance ( cm2). The results are summarized in Table 8 for the GC electrode. Although the rate constant values obtained by EIS are much higher than those obtained by CV (due to the different mathematical models and techniques45 employed), the magnitude of the variation with the electrolyte composition is similar. These experiments were attempted with carbon fibre paper but due to the inability to determine an accurate overall surface area for the 3D electrode and the tendency for the 3D spaces to fill up with air bubbles, changing the area, reliable results were not possible. It can be seen from Figure 30 that both methods show that the apparent rate constant k0 reaches a limiting value at a higher suiphate/ferric ratio. The same behaviour was observed in Figure 31 with the double layer capacitance, which is related to the electrode/electrolyte interface charge, and with the parameter c1, which is related to the degree of distribution of the time constant related to the charge transfer. k is calculated at OCV when the impedance technique is used (using different equations than for the CV technique). 69 •0) o 2.0 x 0. 0. < 1.0 3.0 0 5 10 15 20 25 8.0 6.0 4.0 00L I I I 2.0 [SO2]I[Fe3 Figure 30: Comparison of the variation of apparent k0 with suiphate/ferric ratio obtained with CV and EIS 200 i 0.90 180 - 0.80 120 - 0.70 100 I— 80 0.60 0 25 [SO42]I[Fe3 Figure 31: Variation of the double layer capacitance and the parameter D obtained with EIS on GC with suiphate/ferric ratio DC dl 5 10 15 20 70 These results mean that at higher sulphate/ferric ratios, the interface charge increases and the charge transfer is more homogeneous as ct —* 1. The increase of the double layer charge can be explained, assuming that double layer thickness is constant over the studied range of concentration, by the increase of the active species charge/size ratio at the electrode/electrolyte interface. The EIS results support the assumption that at a lower suiphate/ferric ratio the ferric complex is hydrogen-bonded or in the form of poly-nuclear hydrolytic ferric species, while at higher suiphate/ferric ratios, mononuclear ferric species either as inner- or outer-sphere complexes with sulphates are present. Also, it is interesting to point out that both solution and kinetic parameters reach a steady state at a sulphate/ferric ratio around 7 to 9. Even if the nature of the active complexes is still not clear and a more fundamental understanding is needed, the plateau values observed with all the obtained reaction parameters likely indicate that the main active ferric complex reached a constant structure with sulphate as an inner or outer-sphere ligand that influences its diffusion and charge transfer properties. 5.2.3. Effect of Electrode Type and Cross-Over on Redox Activity Key to the use of the liquid redox system with hydrogen is a tolerance to cross over and minimal depolarization at the active electrode surface. Figure 32 shows a comparison of cyclic voltammograms for hydrogen and the liquid redox system mixed on glassy carbon, and for the liquid redox system alone on glassy carbon and platinum. These tests were done at 70°C, the maximum temperature used in the study where activity would be greatest. These results show similar activity for the redox system on both glassy carbon and platinum. Also, there is little to no activity of hydrogen on glassy carbon indicating hydrogen permeation to the cathode is not a problem as a result of reaction selectivity. However, since hydrogen and the redox couple are both active on platinum redox species cross-over to the anode is considered to be a possible problem. Although the redox couple showed higher activity in a dilute redox solution on platinum compared to glassy carbon (see Figure 34), this activity was observed to be similar in a concentrated redox solution (42), and highlights the potential for significant platinum group metal reduction in the direct hydrogen redox fuel cell. 71 0.10 0.05 - C. E C., 0.00 - 4- I C.) -0.05 - -0.10 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 Potential (V vs HgIH92SO4) Figure 32: Cyclic voltammograms (70°C, scan rate 50 mV/s) for hydrogen and the Fe3 I Fe2 redox couple ([FeJ=0.9M, Fe3fF2= 9) mixed on glassy carbon and for the redox couple alone on glassy carbon and platinum. Also key to the use of the liquid redox cathode system is the ability to use a three dimensional electrode to extend the reaction zone and increase active surface area. This approach has been used predominantly with liquid fuel feeds such as in the direct methanol fuel cell to increase fuel utilization, and performance, and decrease fuel crossover (65,66). It has generally involved distributing catalyst throughout the electrode structure. In the case of the liquid redox cathode system the carbon diffusion layer itself is active and no further catalyst application is required, thus greatly simplifying the electrode structure and processing. Figure 33 a and 33 b shows a comparison of cyclic voltammograms for the redox system in two different solutions on glassy carbon and on Toray TGP-H090 carbon fiber paper based on the electrode superficial area. Similar potentials are observed for the redox couple in each solution, indicating similar activation on the two different carbons. The real area accessed for the carbon fiber paper (or any other three dimensional electrode) can be estimated from the ratio of peak currents based on the Randles - Sevcik equation (46), i.e. i = (2.686x 10 )An3hf2Dh/12Cvh/2 (see Eq. 22). Fe3IFe2 on GC Fe3*I Fe2 + H2 on GC Fe3IFe’ on Pt I I I I I 72 E U C, 0 1 x ‘a ‘a 1 = C) C. E ‘a C ‘a C I.. 5 C.) Based on this analysis there is an approximate 4 times increase in accessible active area for the three-dimensional carbon fibre paper over that of the glassy carbon electrode. The total potential area of the carbon fibre paper is substantially larger but accessibility is limited by cell design, electrolyte composition and operating conditions. 1.5 0.0 0.2 0.4 0.6 Potential (V vs AgIA9CI) 0.8 1.0 0.5 0.0 -0.5 -1.0 -1.5 -0.2 0.10 — 0.05 0.00 -0.05 -0.10 — -1.2 -0.8 -0.4 0.0 0.4 Potential (V vs HgIHg2SO4) Figure 33: Comparison of cyclic voltammograms (25°C, scan rate 100 mVIs) for the Fe / Fe2 redox couple on glassy carbon and on Toray TGPH-090 carbon fiber paper in different solutions: a) 5 mM K3Fe(CN)6in 1 M KNO3; b) 0.81 M I 0.09 M Fe(NH4)(S0.12H0/ FeSO4.7H20in 0.5 MH2S04. TGP-H090 Glassy Carbon x4 b 0.8 1.2 73 5.2.4. Electrochemical Results for Bio-Electrolyte Efficient redox systems improve power density by maximizing ionic concentration which is limited by solubility of the iron species and the tolerance of T. ferrooxidans to high iron concentrations46.This minimizes concentration gradients and allows for high ionic availability at the electrode during operation. Research by Karamanev et al. (see Appendix 2) has led to the characterization of bacteria which function well at iron solubilities of 50 to 60 gjJL. Figures 34a and 34b show the effect of total iron concentration of the bio catholyte on the apparent rate constant, k0, and the diffusion coefficient, D, respectively. For Pt, GC, and TGP-H090 the k0 and DFe3+tFe2+ decrease as the iron content increases for a ratio of[Fe3J/[Fe2]=1. The behavior was observed with GC to a smaller extent. It can be seen from Figure 34a that the kinetics of the ferric/ferrous couple is material dependent, particularly at lower iron concentrations indicating the inner-sphere nature of the charge transfer. However, from a fuel cell application perspective Pt and TGP-H090 will give the same performance at a working iron concentration of S0gFelLitre (obtained from optimized bioreactor data (29)). The diffusion coefficient, which should be material independent for the Pt and GC, shows significant differences. The change in diffusion constant with total iron concentration is significantly less for GC compared to Pt, suggesting that an irreversible layer is formed on the electrode, probably as a result of an interaction with surface groups on the GC, which would interfere with both chemical kinetics and mass transport at the electrode surface. See section 1.4.7., section 4.2., Table 3 and Table 5 for more details on bioelectrolyte composition and theory. 74 61.6 1.4 4 1.2 11 X3 0.8 C 0.6 0.4 0 0.2 - 0 60 0 10 20 30 40 50 60 [Fe] (gIL) (FeJ (gIL) Figure 34: Influence of total iron concentration on k0 and DFe+3 in SBE (CV). [Fej/[Fe2J= 1, pH = 1.5 at 25°C This trend is also mirrored in double layer charge capacitance on GC (suggesting a possible link between reaction rate, diffusion and double layer charge) obtained with ETS. To understand this behavior, the nature of ferric/ferrous complexation in such a standard bio-electrolyte (SBE) and its effect on the double layer need further characterization. In order to achieve maximum power density for the fuel cell we need to operate in a concentration range characterized by lower reaction kinetics and diffusion. Another important point illustrated by this data is chemical compensation during fuel cell operation. At the beginning of the fuel cell flow-field the amount ofFe3 will be very high, but as the Fe3 is converted to Fe2 it will effectively lower the useable iron concentration and the current density (A/cm2)will drop. However, the kinetics, in terms of both the rate constant and the diffusion constant, will increase. This will help to provide a more even distribution of current in the fuel cell, allowing for more efficient fuel cell design and operation. Maintenance of the pH level is critical to the proper operation of the catholyte in both the fuel cell and bioreactor. In fact, jarosite (ferric hydroxysulfphates: MFe3(S04)2OH)6where M can be K, Na, NH4or H3O) precipitation at high pH (34) must be avoided, while Acidothiobacillusferrooxidans survival will set a lower pH limit for acidity, usually a pH greater than 1 is required. At the same time, the effect of pH on reaction kinetics must be considered. As can be seen from Figures 35a and 35b, low pH b —•-— GC N 10 20 30 40 50 75 enhances both k0 and DFe3+ for the cathode reaction. The double layer charge capacitance also increases with decreasing pH. As a result of these trends, a pH operational parameter of about 1.2 (it can go down as low as 0.5) can be selected to allow for optimal kinetics near the threshold of organism viability. The bacteria strain finally utilized in the bioreactor was selected for functionality at low pH (pH 1). For more details on the biological organisms used see Appendix 2. 0.1 0.7 I I a I b 0.6 E 0.08 o. 006 I, \ 2.5 :: 004 2 02 0.02 1.5 0.1 00.8 1 1.2 1.4 pH 1.6 1.8 2 2 00.8 1 1.2 1.4 H 1.6 1.8 2 2.2 Figure 35: Influence of pH on D+3and k0 in SBE ([Fel =50 gIL). [Fe3j/[Fe2]= 1 @25°C (CV) Temperature plays a similar role to pH. It affects the solubility of the ionic species, the kinetics and diffusion of the reactive species, and the viability of the regenerative organisms. Fuel cells function well at elevated temperatures (‘-8O°C), but from the perspective of the bioreactor there is a limit to how high the temperature can be. Organism functionality can also benefit from higher temperatures up to an optimum temperature, which is usually less than 50°C to avoid denaturation. Figures 36a and 36b illustrate that as temperature increases both the k0 and DFe3+ increase (although not shown, the double layer capacitance also increases with temperature). This change in kinetics and diffusion properties due to temperature, iron content, and pH all point toward the influence of the complex properties of the iron species both in the bulk solution and at the interface. These effects may also have the potential to counteract some of the losses experienced due to the higher total iron concentration discussed earlier. 76 1.60.5 , 0.14 _________ 1.4 0.12 0.45 -12 U I — 10.1 z 0.4- X 1 0 0.08. a 00.8 0.35 0.06 0.6 03 . . 0.4 20 30 40 50 60 70 Figure 36: Influence of temperature on k0 and DFe+3 (CV) in SBE ([Fel = 50 gIL). [Fe31 = 90%[Fe], pH =1.2 This work illustrates the importance of a balanced compromise between electrolyte chemistry, fuel cell performance and bioreactor viability. Both components of the cathode system (fuel cell and bioreactor) must be carefully considered with respect to reaction and solution electrochemical/regenerative parameters in an effort to obtain maximum efficiency and long-term functionality. For example, once the desired concentration of the electrolyte has been decided upon these concentration levels need to be monitored and maintained, so that an optimized fuel cell will be able to function to its potential. A method will need to be incorporated to allow for addition of the supporting media (including water which could diffuse across the membrane to the anode compartment in the fuel cell) to keep the bacteria population constant and functioning. 5.2.5. Fuel Cell Testing Results for “Simple” and “Bio” Electrolytes When oxygen is replaced by a redox oxidant the rate of the cathode reaction even without noble based catalysts is significantly higher than for the ORR. The exchange current density of the Fe3/F2 redox couple is usually several orders of magnitude higher ( 102 A/cm (67)) than for oxygen ( l0° A/cm2 (68)). Also, the rate of mass transfer of the oxidant to the electrode surface is increased by several orders of magnitude compared to oxygen due to the much higher aqueous solubility of the redox oxidant (1 M or higher) compared to oxygen, 2.67x104M at 25°C and l.73x104at 70°C (69). a I —.-LGc b / z .7 -.-GCI 004 20 30 40 50 80 70 Temperature (°C) Temperature (°C) 77 Hence, a redox cathode could provide the following advantages in materials, design and system over the conventional PEMFC47: - The high redox potential and good kinetics of some redox systems on non- noble materials can lead to the elimination of the noble metal catalyst on the cathode, significantly decreasing overall fuel cell costs. Carbon based materials can be used, which represents an 80% reduction of total Pt in a stack compared to H2/Air PEMFC. - The liquid redox cathode greatly simplifies the electrode/electrolyte interface requirements and allows three-dimensional electrodes (high surface area) to be used, which should improve performance. - No need for a complex catalyst layer microstructure (Three Phase Zone, TPZ) as for the ORR to occur - Faster kinetics at the cathode with high exchange current density - No depolarization issues with fuel crossover - Cathode water management is no longer an issue since the oxidant is afready a liquid, therefore the problem of flooding (so common in gas based fuel cells) disappears. - The aqueous oxidant redox couple can provide the humidification necessary to the membrane, thus eliminating the need for a separate or external humidification at the cathode and the anode. - The temperature of the cell can be easily controlled by the liquid cathode redox solution, eliminating the need for separate cooling plates and some of the temperature regulation/control components (heat could be easily removed with the circulating solution). In addition, the redox couple cathode has a relatively high efficiency at lower temperatures. - Alternative (and possibly cheaper) membranes/separators can be used since there is no longer any concerns of an explosive crossover situation as in a PEMFC (which commonly use Nafion®) - More flexible electrode structures and simpler flow-fields can be used on the cathode. of these advantages can result in fuel cell system simplification and cost reduction. 78 Although many benefits are possible with a redox oxidant there are challenges with respect to regeneration (previously mention in the chapter on redox batteries), which need to be considered in an overall system analysis. The overall system must be economically viable and regeneration of the couple adds additional expenses. The iron couple can be regenerated through a variety of means, eliminating this concern. Regeneration can be carried out by a variety of processes including radiochemical methods (40) , chemical methods (70), electrochemical methods (electrically rechargeable redox-flow cell) (41,41), and biological methods (71). Other research groups are working on the regeneration aspect of the system. One of the most intriguing methods being considered is bio-regeneration48.A regeneration level of 90% has been identified as a realistic model (1) that is acceptable to both the bioreactor (the bacteria have biochemical limits) and the fuel cell (a large ferric/ferrous ratio will give superior power density). It is believed that this value will be a good representation of a realistic system using any type of regenerative method. While regeneration is not a major issue for the iron couple there are other disadvantages to the proposed system that must be considered. • As with all redox system low energy density • Anode flooding becomes a possibility due to diffusion from the aqueous cathode across the membrane • Design (particularly at the stack level) becomes a concern with a liquid cathode (to avoid shunt currents, etc.) • Contamination of the membrane by the iron redox couple (which could affect ionic conductivity and membrane stability)49 Finally, it has been reported in the literature that the mechanism of the charge transfer of the aqueous ferric/ferrous complexes in reaction is not an outer-sphere process (as is the case with most redox couples) but rather an inner-sphere process (56). The kinetics of ferric/ferrous couple on glassy carbon was reported to be surface and oxide In this case, the primary concerns are the system operating conditions and the bio-electrolyte composition necessary for efficient and long-term ferric ion bio-regeneration (since these factors influence the redox couple’s chemistry/electrochemistry, which in turn would affect energy production performance). formation can be accelerated by the presence of iron in the membrane and can lead to the breaking of the C-F bonds ofNafion. 79 sensitive (57,72). It is therefore critical to further evaluate the kinetics of this redox couple on a variety of carbon materials (which can be used in redox fuel cell system). Therefore, fuel cell performance for the direct hydrogen redox and direct hydrogen bio redox fuel cell was evaluated for different electrode materials and operating conditions. For all experiments the total iron concentration was kept at 0.9M with the ratio of the ferric / ferrous species kept at 9 to simulate a redox regeneration process with an efficiency of 90%, i.e., a typical feed composition in a redox system. Initial fuel cell testing was done to determine the optimal compression for each type of cathode. A standard cathode pocket depth of 1 mm was used for all cathode tests. Figure 37 shows the effect of cell compression on performance with respect to carbon fibre paper (4 pieces), thick carbon felt (1 piece) and thin carbon felt (7 pieces) three- dimensional cathodes (note the current densities at which each test was run). The carbon cloth (5 pieces) cathode (not shown) had a similar optimal pressure but the high-pressure side tended to plateau outward indicating limited compressibility. 0.28 0.26 ... - / . .‘ // • ã•\ 0.24, I \ / N > 0.22 “ - 0 > 0.2 - I . c.) 0.18’ - 0.16 •‘ TGPH-090 (417 mA/cm2) Thin Felt (250 mA/cm2) I Thick Felt (555 mA/cm2) 0.14 15 20 25 30 35 40 45 Bladder Compression (psi) Figure 37: Fuel Cell Performance using Thick Carbon Felt, Thin Carbon Felt and Carbon Paper with respect to Bladder Pressure (“simple” electrolyte used) 80 -U 0 -‘ U) 3 Figure 38: Performance Comparison of Bioelectrolyte and “Simple” Electrolyte This indicates that even though there are chemical differences (and therefore potentially kinetic differences), at the fuel cell level these differences are not significant. As will be U Performance is optimal at a similar compression pressure of 28-30 psi (1.97-2.11 kg/cm2) for the different cathodes indicating the importance of balancing contact resistance and electrode porosity. If the pressure is too low contact resistance is quite high due to poor contact between the electrode and the flow field. Once the optimal compression pressure is reached additional compression begins to have a negative impact on porosity and decreases performance due to increased mass transport losses. Carbon fibre paper was chosen as the baseline material since it is usually the electrode material of choice in PEMFCs. Both electrolytes (simple and bio) where then optimized for operating conditions within the 4cm2 fuel cell. As can be seen in Figure 38, the standard redox couple in the simple aqueous sulphuric acid electrolyte and the bio-electrolyte show very comparable performance results. For a comparative study, both in terms of temperature and solution flow, with Toray carbon paper, the performance results were effectively identical. 0.16 0.12 0.09 0.04 0 0.6 > 4... 0 ‘I) C, 0.2 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.0 Current density (A cm2) 81 seen, the bulk of the fuel cell performance data was generated using the bio-electrolyte to meet the needs of the NSERC feasibility assessment and to gain a greater comprehension of the challenges involved in operating a biofuel cell. It is important to note that the following bio-electrolyte results are also applicable to the simple electrolyte. As can be seen in Figure 39 the flow (in this case a fixed flow) of the redox solution has a significant impact on overall performance. Using the carbon fibre paper as a base of comparison, one can see that higher flow rates and stoichiometries give better polarization results (40°C was chosen as a baseline temperature for later comparison). Similar trends are noted with different 3D carbon electrode materials. Below a certain flow threshold the fuel cell performance is limited due to design issues. These lower flow rates lead to poor reactant distribution and hence do not allow full utilization of the 3D electrode. At higher flows the performance increases but no mass transfer limiting region is reached indicating severe ohmic control. 0.4 0.2 0.8 Figure 39: Direct hydrogen redox fuel cell polarization curves obtained at 40°C for different flow rates of the Fe3 / Fe2 bioelectrolyte. Toray TGPH-090 carbon fiber paper was used for the cathode. Based on these results (even though 10 ml/min was used as a baseline for comparison), 5ml/min would be sufficient to run a single 4cm2 fuel cell. Increasing flow seems to have 0.8 0.6 0 0 0.1 0.2 0.3 0.4 0. 0.6 0.7 Current Density (AIcm2) 82 a less significant effect once the full electrode geometry is utilized. These observations have far more reaching implications since the fuel cell will be paired with a regeneration reactor. The exit flow rate from the reactor must meet the requirements of the fuel cell in order to get adequate performance. Additional work was done using constant redox stoichiometries of 4 and 6. Figure 40 shows that at 55°C little effect is noted by changing the stoichiometries but both are still inferior to the fixed flow of 10 mi/mm. At 70°C the performance difference is more pronounced at the two stoichiometries, indicating a synergistic link between flow and temperature, but both are also inferior to the fixed flow of 10 mllmin. While these results were not surprising since the greater starting stoichiometries of the fixed flow should give superior performance, it is interesting to note that even with these different flow regimes a mass transport region is not reached (mass transport limitations are only noted at the fixed flows of 1 and 2 mi/mm in Figure 39). The fixed flow method was chosen for the remainder of the work to simplify the experimental method. 0.8 ____________ 0.7 ‘1. _ 0.6 a) 0.4 -J > j 0.2k 0.1 TGPH electrodes 0 - -_____ 0 0.2 0.4 0.6 0.8 1 Current Density (AIcm2) Figure 40: Comparison of the effect of constant flow versus constant stoichiometries using TGPH-090 electrodes (Simple electrolytes) Stoich 4, 55°C —i-— Stoich 6, 55°C Stoich 4, 70°C • Stoich 6, 70°C • lOml/min,55°C • 10 mI/mm, 70°C • 83 Figure 41: Effect of different carbon materials on the performance of a redox fuel cell using a SBE Figure 42 shows the polarization performance of the direct hydrogen redox fuel cell at different temperatures and the associated power density curves. Very stable and reproducible open circuit voltages of around 0.775 V were obtained that were practically independent of the temperature. The calculated Nernst values of the open circuit voltage (OCV) using a total redox species concentration of 0.9M at the cathode with a pH of 1 and a hydrogen pressure of 2 atm (29.4 psi) at the anode are 0.895 V and 0.913 V at 25°C and 70°C, respectively. The observed difference in the OCV compared to the thermodynamic values is likely due to the activity of the redox species at the cathode. 0.8 The above results indicate that the nature of the three dimensional cathode, the cathode flow field design, and the cathode flow regime can have a significant impact on the performance of the redox fuel cell system. Figure 41 illustrates that the carbon electrode material structure and porosity also have a major impact on overall polarization and power density performance. Thick carbon felt and carbon cloth gave superior performance when compared to the other materials. __________ 0.25 • ThinFelt70 C, 10 mtImin • Thick Felt U- Cloth —-A-—— Paper 0.19 0.13 0.7 0.6 • 0.5 0.4 > C) 0, C) 0) C-) 0.3 •0 0 I. 4 a a’ ‘I .1 U U 0.2 0 0.2 0.4 0.6 0.8 Current Density (AIcm2) 0.06 —0 1.2 84 0.9 0.25 -U0 0.15 CD z CD 0.1 3 0.72 0.2 0.54 CD ‘5 0.36 C., 0.18 I 0.05 0 0 0 0.2 0.4 0.6 0.8 Current Density (AIcm2) Figure 42: Direct hydrogen redox fuel cell polarization and power density curves obtained in the temperature range of 25°C to 70°C for a constant Fe3 / Fe2 solution (SBE) flow rate of 10 mllmin. Toray TGPH-090 carbon fiber paper used for the cathode. In fact, the redox potential of the Fe3/F2couple measured at 25°C using a commercial ORP (oxidation reduction potential probe) electrode was around 0.700 V vs NHE. This results in a calculated OCV of 0.768 V, which compares well with the measured fuel cell value of 0.775V. Figure 42 also shows that there is a strong dependence of the cell performance on temperature with an adequate catholyte flow rate. The linearity of the polarization plots at higher current densities and the strong dependence on temperature are indicative of ohmic control. An approximate theoretical calculation assuming all of the electrode surface area is used and a mass transfer coefficient (Km) of about 1 0’ rn/s (common for turbulent liquid flow) reveals that the limiting currents5°should be greater than 1 A/cm2,well beyond what is observed experimentally. Again, this calculation 50 Limiting Current Density (iL) = nFIcCB where CB is the bulk concentration, n is the number of electrons involved in the reaction equation and F is the Faraday constant 85 supports the premise that the polarizations are ohmic controlled and not mass transport controlled. As can be seen in Figure 43, when another cathode material is used (in this case thick carbon felt, pictured in insert at X5 1 scale) a similar trend with temperature is observed. While the performance is enhanced by the increased porosity and available surface area, ohmic control is still the main limiting factor. On the other hand, since a mass transfer limited region has not been reached there is still room for significant improvement. 0.1 1 1.2 Figure 43: Polarization Curves of Bio-Redox Fuel Cell (using thick felt, lOmi/min catholyte) at various temperatures. Inset: SEM image of carbon felt surface at X51 scale. It is important to note at this point that the results achieved and observations noted in these experiments were highly repeatable throughout this work. This can be illustrated in Figure 44 for the simple electrolyte, where temperature experiments from different days with new MEAs were compared and found to be very similar in terms of performance. Good repeatability was also noted for the bioelectrolyte and for the flow experiments. 0.8 0.7 0.6 0.2 0 0 0.2 0.4 0.6 0.8 Current Density (Akin2) 86 0.8 I 55°C 0.7 - • 40°C S1 0.6 - a: i. 05 TGPH electrode 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 Current Density (AIcm2) Figure 44: Repeatability of results in a “simple” redox fuel cell at a redox solution flow of 10 mI/mm To date the best performance achieved was a power density of 0.21 W/cm2 for the thick carbon felt with a 1 Omi/min flow of the bioelectrolyte at 70°C, and is shown in Figure 45. Figure 46 compares the bioredox system at a realistic operating temperature of 55°C with a PEMFC at similar conditions (55°C; 100%RH; hydrogen stoichiometry of 1.5; air stoichiometry of 2, ambient back-pressure). The PEMFC consisted of duel serpentine flow channels, a Nafion® 115 membrane loaded with 0.4 mgpt/cm2 (20% Pt/Vulcan XC-72 from E-Tek and 30% Nafion) at both the anode and the cathode (CCM), and SGL BC-24 gas diffusion layers (carbon fibre paper51). While the performance of the bioredox cell is lower than the PEMFC, if one considers that up to 80% of the Pt catalyst has been removed from the overall fuel cell, the potential for this type of system, as further improvements are made, is evident. 51includes teflon based microporous layers to assist with product water removal 87 0.8 0.25 0.2 0.15 0 I0.1 0.05 0 0.2 0.4 0.6 0.8 1 0.64 0.48 0 > 0.32 0.16 0 0 Current Density (A cm2) Figure 45: Redox PEM Fuel Cell Performance with Bioelectrolyte at 70°C (thick carbon felt cathode, 10 mI/mm catbolyte) 1 - 0.25 0.8 — 0.2 0 — 0.6 0.15 a 0.4 - 0.1 0.2 — 0.05 a 0 . 01 0 0 0.2 0.4 0.6 0.8 1 Current Density (AIcm2) Figure 46: Comparison of a 5cm2 PEMFC at 55°C (0.4mg/cm2catalyst loading) with the 4cm2Bio Redox Hybrid Cell (using a thick carbon felt cathode, lOmi/min catholyte) at 55°C I . . a. a • a 88 5.2.6. Redox Fuel Cell Degradation While the initial results of the redox fuel cell are highly repeatable, longer-term operation of the fuel cell indicates that the performance degrades (see Figure 47). The most probable reasons for this degradation are membrane contamination by the iron species, leading to an overall drop in membrane conductivity, and jarosite formation. In the case of the bio-electrolyte, changing chemical conditions within the electrode microstructure could lead to localized changes in pH and promote jarosite formation that could clog pores, impacting mass transport at the electrode. 0.17 0.16 0.15 > CD 0.14 0.13 0.12 0 50 100 150 200 Time(min) 250 300 350 Figure 47: Voltage Stability in a redoxiPEM fuel cell (bioelectrolyte, thick carbon felt cathode) at 0.62 A/cm, 5mlImin redox flow, llmllmin hydrogen flow, and 55°C. To determine any changes in the chemical properties of the electrode surface after running the redox fuel cell XPS analysis was used. This allowed for not only elemental analysis of the electrodes but also determination of any impurities/contamination. As can be seen in Figure 48, the use of the carbon paper in the hybrid cell has an effect on the surface of the electrode material. An increase in the peaks area at 530 eV and 978 eV 89 indicate a significant increase in oxygen content in the post cell (an indication of oxidation of the carbon surface during operation of the cell). Peaks at 400 and 169 eV (paired with 230 eV) indicate the presence of nitrogen and sulphur groups, respectively, on the surface of the post cell electrode (due to reactions with the electrolyte). Finally the hump at 711 eV indicates the presence of iron (to be expected considering the electrolyte) in the post cell material. This data could be used as an indication of electrode condition when designing a stack and modeling its operating life. It is also interesting that little difference is noted between the untreated and nitric acid treated carbon paper (carbon fibre peak at 284.5 eV). This would tend to indicate that the treatment with nitric acid does not have a direct impact on the level of oxidation of the surface (perhaps oxide groups are converted to hydroxide groups, leading to the increase in hydrophilicity noted in the contact angle work). U, 0 0 In C C Figure 48: XPS data for untreated carbon paper and nitric acid treated carbon paper prior to use the in the hybrid cell and nitric acid treated carbon paper after use in the hybrid cell. 1000 000 600 400 200 0 BE (eV) 90 The order of experimentation plays a key role in accessing the impact of these effects on the results. Experiments were conducted in a sequential order starting with incremental temperature increases to 25°C, 40°C, 55°C, and 70°C (with a redox flow rate of 1 Omi/min) and ending with incremental redox electrolyte flow increases to 1 mllmin, 2 mllmin. 5 ml/min, 10 mllmin, 15 mi/mm, and 2OmlJmin (at 40°C). The time period for these sets of experiments was kept quite consistent due to the limitation of a finite amount of electrolyte solution for each run, i.e., 2 litres. Since any decreases in performance due to the above mentioned effects would be included in the observed results over the experimental series, the polarization curves must be considered to include these negative impacts. It can therefore be concluded that the trends and results achieved can only improve when problems such as jarosite formation (with respect to a bioelectrolyte) and membrane contamination are solved. In addition, as can be seen in Figure 49, little difference was noted with respect to performance when comparing the two experiments run at a flow of 10 ml/min at 40°C (conducted at different points in the experimental run). If anything the performance seems to have improved at higher current density indicating that the sequence of testing had little effect on the performance. 0.8 - ____________________ Experiment 1, Run 1,40°C, 10m/min 0.7 Experiment 1, Run 2, 40°C, lOmi/min -j - Experiment 2, Run 1,40°C, lOmi/min 0.6 — Experiment 2, Run 2, 40°C, lOmi/min - i 0.4 80.3 0.2 - 02 0.3 0.4 06 7 Current Density (AIcm2) Figure 49: Comparison of “simple” solution experiments at 40°C at different times throughout the experiment (approximately 4 hours between runs). 91 Further clarification of the ohmic issue was determined by taking conductivity measurements of a Nafion® 115 membrane before and after exposure to the iron based solution. Table 10 shows the conductivity of Nafion® 115 that had been exposed overnight to DI water, sulphuric acid solution (pH = 1), and a working iron solution (pH 1). It is clear that as a result of the uptake ofFe3 and Fe2 the membrane conductivity is significantly reduced by around 86% from its baseline value. A similar conductivity decrease (‘40%) with Nafion® 117 uptake of Cr3 was reported by Shores and Deluga (73). However, after exposing the contaminated membrane to sulphuric acid solution (pH = 1) for one hour, the conductivity partially recovers to about 40% of the baseline value. Exposure solution DI watera H2S04(pH = Fe3/F2(0.9 M) H2S04(pH = H2S04(pH l) 0.200 ± 0.002 0.028 ± 0.002 0.08 1 ± 0.0 12Conductivity of 0.133 ± 0.0 16 N115 (S/cm) Table 10: Effect on conductivity of various soaking treatments of Nafion® 115 lamembrane exposed overnight in solution at 22°C; b membrane soaked for 1 hour in H2S04 (pH = 1) solution at 22°C after being exposed to the iron solutionj Under other conditions such as higher temperature and more concentrated acid solution it is likely possible to recover more of the baseline membrane conductivity. These results clearly indicate the importance of improving the membrane/separator for the direct hydrogen redox fuel cell in order to reduce contamination and cross-over by the redox species. When a membrane recovery method is introduced to the fuel cell polarization testing, some highly interesting results are observed as shown in Figure 50. 92 0.8 I I 0.7 0.6-- I a 05- 0.4 0.3 02 a 01 Carbon Felt Cathode, 55°C, 5 mllmin •• t 0 -- 0 0.2 0.4 0.6 0.8 1 Current Density (AIcm2) Figure 50: Membrane recovery in a redox PEMFC at 55°C (thick carbon felt, bioelectrolyte at 5 mi/mm) After initial polarization curves were taken at 55°C the fuel cell was exposed to iron solution for approximately 5 hours. A second polarization curve, after this period, showed some negative performance effects as expected. After a 17-hour exposure to a pH 1 (H2S04)aqueous recovery solution, the performance was actually higher than the initial baseline at higher current densities. This observation indicated both a recovery mechanism (diffusion of iron out of the membrane) and a possible membrane conditioning effect. The Day 2 Post Exposure curve shows a decrease in performance, probably due to the same membrane contamination as in Day 1, which for the most part is inferior to the Day 1 Post Exposure. What is interesting is that at high current densities the performance is better than the Day 1 Post Exposure curve, while in the peak power region the performance is lower. This illustrates the complexities of the polarization curve where ohmic forces dominate in the peak power region while both diffusion and ohmic issues play a combined effect at the end of polarization curve. Day 1 Initial -- Day 1 Post Exposure Membrane Recovery Day 2 Initial S Day 2 Post Exposure 93 When the results of Figure 47 are considered, in the light of this membrane recovery/conditioning effect, it leads to the obvious comparison of passive (with no current production) and active (with current production) degradation of the membrane. There seems to be an indication that the degradation mechanism is accelerated in the active case. In addition, as can be seen in the bulk of the polarization curves on Day 2 (where the performance is inferior), there seems to be more than one chemical effect occurring. It is possible that the recovery procedures have the effect of purging jarosites out the pores of the cathode, decreasing the mass transport limits imposed by clogging. This could explain the more linear nature of the Day 2 results (only ohmic problems are present). Further experiments with the “simple” electrolyte, where jarosite formation would not be a factor, would allow for additional understanding of this observation. It is this author’s opinion that further work on this project should include investigation, characterization, and explanation of these phenomena. It may be possible to significantly improve overall performance (both initial and after degradation) via use of a conditioning step prior to performance testing and operation. If this is combined with an iron resistant high performance ion exchange membrane, a highly viable technology based on a redox cathode could be developed for stationary applications. In the case of bio-redox applications it would be desirable to eliminate jarosite formation completely. 94 6. Conclusions Replacing the poorly performing oxygen cathode in a conventional PEMFC by a redox species (ferric/ferrous couple) cathode could be a promising approach to resolve many of the issues with the oxygen reduction reaction (ORR), as illustrated in Table 11. Table 11: Differences between oxygen and redox fuel cell catbode The research presented in this work has led to a variety of conclusions with respect to this alternative cathode approach: Experimentation (both CV and ETS) shows complex association behaviour of ferric ions (which has an affect on reaction kinetics — suggesting inner sphere mechanism) o Complexation of sulphate with ferric ions affects reaction kinetics (reflected in the k0, D, and Cdl) o ko and D values reach a plateau at high sulphate/ferric ratio (on Pt and GC) o High ratio of sulphate/ferric (5-7) increases ferric ions mobility and reaction charge transfer Oxygen Cathode FC Redox Cathode FC •Efficiency Limitation due to low kinetics of the oxygen reduction reaction (even with noble metal catalysts) •Expensive catalysts (with relatively low utilization) are needed •Design complications due to the cathode-PEM three phase interface structure •Complex cathode design (hydrophilic hydrophobic boundaries, pore size distribution, PEM conductivity, etc) for water management, reactant distribution (gas diffusion and conditioning), and efficiency optimization. •Serious concerns with gas humidification, water management, thermal management, and parasitic loads at the stack level. •Fast kinetics (reversible to quasi-reversible reactions) •No noble-metal based catalyst needed •Simpler electrode/electrolyte interface structure/design (no three phase boundaries) •Liquid catholyte leads to 3D electrode structure, alternative membrane options, simple flow fields and a lack of water management/external hum idification/ cooling issues •No Cathode depolarization due to Fuel crossover •Performing PEMFC anode designs can be utilized •High system efficiency possible 95 o Cdl behaviour follows ko and DFe3+/2+ (Suggests ko and DFe3+/2+ are linked to the double layer charge) • The mechanism seems to be an inner-sphere charge transfer • The inner sphere mechanism is supported by dilute concentration kinetics (some material dependence noted) • While the reaction is material independent (particularly at high concentrations), material microstructure (porosity, bydrophilicity, surface area, etc) is critical • Redox fuel cell data showed little difference between the “simple” sulphuric acid based medium and the bio-electrolyte medium • A minimum flow is required to access the full 3D electrode structure • At this point performance is still ohmic controlled; limiting current not yet obtained (though improvement achieved at elevated temperature) • Decreases in membrane conductivity (and therefore fuel cell performance) due to Fe exposure can be reversed via recovery methods (conditioning effect is also possible) • Even with present limitations, redox fuel cell performance is quite good (0.22 W/cm2);with 65-80% reduction in total Pt loading over the state of the art conventional air/hydrogen PEMFC (which can have a maximum power density of 0.7 W/cm2) In summary: • Electrolyte composition can have a significant effect on the electrode kinetics and mass transport limitations • Use of a redox couple to replace the oxygen reduction cathode shows promise for future fuel cell power generation systems • Significant improvements in performance (up to 1.7 W/mgpt) are expected with further optimization of operating conditions and materials (particularly the membrane) • Current performance of this novel system (1.1 W/mgpt) is comparable to PEMFC performance (1.2 W/mgpt) even though ferric/ferrous potential is significantly lower 96 This study of the ferric/ferrous redox couple in a “simple” sulfuric acid medium and a bioelectrolyte medium (practical electrolyte compositions were considered that would be used in a real redox fuel cell with incomplete regeneration of the reduced redox cathode species) has lead to the determination of the best conditions for the combined bioreactor/redox fuel cell operation. In addition, it has opened up a discussion of the application compromises necessary when considering the hybrid bio-redox!reactor system. 97 7. Future work While a significant amount of fundamental work has been completed there is still much to do to fully characterize the redox/PEM fuel cell. It is this author’s recommendation that future work be done primarily on the “simple” redox catholyte to avoid the problems encountered with jarosite formation when using the bio-electrolyte during long-term operation and operation at temperatures over 55°C. This will simplify the work by removing one layer of complexity. In addition, in order to keep costs and lab time to a minimum, an electrochemical regeneration reactor should be constructed to regenerate the ferric species. Some recommendations for future work follow: • Alternative Membranes: Other types of membranes should be considered. This should include both thinner Nafion® and nonNafion® products (preferably ones that are not as sensitive to iron). • Alternative flow field design: While the open pocket flow field filled with thick carbon felt gives excellent performance there is still value in exploring other liquid flow field designs. • Alternative Redox Couples: It would be interesting to look at alternative redox couples to the Fe3/F2couple that may result in less membrane contamination and degradation (Ce3/4could be of interest but would require more chemically compatible cell components, such as a diamond doped carbon electrode which is more resistive to oxidation by the Cerium redox couple). • Redox Electrolyte Modification: Modification of the electrolyte composition and addition of complexing agents could have a significant effect on the electrode kinetics, the level of membrane contamination and the solubility /concentration of the redox species. • Alternative Fuel Sources: There is also great potential value in exploring alternative fuel sources to hydrogen such as methanol, ethanol, and formic acid for use in the redox fuel cell. Fellow student Alan Illicic is currently doing his Ph.D. research in this area. • Degradation Analysis: It is critical to gain further understanding of the effect of the iron redox couple on the membrane and overall cell degradation. In addition, 98 it is important to determine the reversible and the irreversible performance losses in the redox fuel cell. • Conditioning: The interesting results observed toward the end of this work indicate the possibility of a conditioning or recovery method for reversible performance loss in the redox fuel cell. This should be further characterized. • Scale up to stack: Once the above-mentioned tests have been fuiiy explored and if results indicate high overall performance (specifically if the ohmic losses can be overcome) the single cell should be re-engineered to the stack level. This will have a number of challenges associated with it. 99 8. Research Contribution, Significance and Ultimate Impact This project has been an opportunity to utilize multiple advancements in various fields in the development of a novel regenerative redox fuel cell system. It has advanced the areas of fuel cell, redox, and bioreactor research. This project addresses an identified gap between advances in redox technology for wastewater (6) and hydrometallurgical leaching R&D (7) and energy conversion R&D. It has allowed for incorporation of the advantages of PEMFCs and redox batteries into a novel fuel cell and stationary energy supply application, which has the possibility to exceed the performance of current PEMFCs through the utilization of 3-dimensional electrodes, the elimination noble metal catalysts at the cathode, and the replacement of the poorly performing oxygen catholyte with a better performing aqueous redox system. 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Using MATLAB a more streamline and accurate method of data handling and mathematical analysis can be devised. To determine the diffusion coefficients of the ferric and ferrous species the Randles-Sevcik equation(46) can be utilized: = (2.686x 1 O )An2D1I’2cvv2 Eq. 26 i, is the peak current (Amps) n is the number of electron involved in the electrode process A is the electrode area (cm2) D is the diffusion coefficient (cm2 s’) C is the concentration of the electroactive species (mol cm3) v is the scan rate (V 1) First of all an experiment is conducted under fixed conditions in which a series of incrementally increasing scan rate CVs are monitored and recorded. At the same time impedance spectroscopy (and Zplot analysis software using an equivalent circuit model from literature) is used to determine the Rs value (solution resistance) for the experiment. Figure 51: Equivalent Circuit Model for Ferric/Ferrous reaction on GC Electrode R CT 108 The cyclic voltammograms can be analyzed using appropriate software (CorrView) to identifr the peak anodic and cathodic currents and potentials. CV plot analysis is then used to do a capacitance correction (where subtraction of the double layer capacitance effect is done to get accurate peak heights) to get the true peak currents (a basic algebraic operation which is done prior to construction of the data file). By plotting the peak currents vs. the square root of the scan rate (see Figure 52), one can determine the anodic and cathodic diffusion coefficients (which should be approximately equal) from the slope of the line (linear regression) based on the Randles Sevcik equation. io Peak Cuirer1 on. Square Root of Scan Rate 2 io Peak Crereat on. Square Root of Scan Rate -4.4 0.3 0.35 0.2 0,25 0,3 0.35 0,4 0.45 Square Root of Scan Rate Square Root of Scan Rate Figure 52: Peak Current vs. the square root of the scan rate (MATLAB program plots) With the values of the diffusion coefficients available for each species (and taking the Butler-Volmer activation law and the diffusion coefficient equations into account) the heterogeneous electron transfer rate constants can be calculated using the following equation and an extended working curve (nA.E versus logçt) (55) based on the Crank- Nicholson fmite difference method (74,54). (DQY k Eq.27 DR) rD0nFv where: k0 is the heterogeneous electron transfer rate constant (cm s’) ti is a dimensionless parameter D0 and DR are diffusion coefficients for the oxidized and reduced species (cm2 &1) a is the cathodic charge transfer coefficient = 0.5 (based on symmetry of CV peaks) v is the scan rate (V s’) -2.8 -3 -3.2 -3.4 C)-3.8 -3.8 + Cathodic Reaction N -4 109 This mathematical procedure involves the corrected potential peak separation and a new parameter ti. The extended working curve (see Figure 53) was obtained using numerical simulations of cyclic voltammograms (using initial work by Nicholson (54)). The initial curve (a) was valid for peak separations from 61 to 212mV, while the extension to the curve (b, utilizing both simulations and experimental data from iron redox couple experiments) validates the technique over a larger range (see below). 1.6 1.4 1 .2 1 > 0 8 Li: Eended Nicholson data C 0.6 nAE = f(Log 0.4 y = -O.0001x6- O.OO19x - O.OO9x4- O.008x3 + 00675x2 O.1136x + O.0B17 0 - a: Nicholson data.4. R2=O.99 0 I I -8 -6 -4 -2 0 2 Log ‘3’ AEP(corr) = /Ep(obed) — (Ipa + )R Figure 53: Extended Nicholson Working Curve (55) nAE represents the potential peak separation (corrected for using the recorded anodic and cathodic current peaks, Ipa and I, and the solution resistance, R3, obtained from impedance spectroscopy). The value of n depends on the reaction under study, as it represents the number of electron being transferred in the electrochemical reaction (in the case of the ferric/ferrous redox couple n1). With this value as a y-axis input a value for the x-axis 1ogit (and therefore v’) can be determined form the working curve. This can be best achieved through the use of the cubic-spline interpolation method to process the 110 extended Nicholson plot data52 (both manual fitting and use of a complex polynomial model53 give only approximations and are extremely time consuming). By plotting w vs the (scan rate)°5,as shown in Figure 54, one can determine the rate constant (k0; cm/s)54 using the slope of the line (linear regression) based on the Nicholson method equation (diffusion coefficients have already been determined and all other data is known). Pvs. (Inverse Square Root) of Scan 0.03 0.025 0.02 4’ 0015 0.01 0.005 0 I I 2 4.5 Figure 54: w vs. the inverse square root of the scan rate (MATLAB program plot) A program constructed with MATLAB, function Analyze.m (see section 9.3.), can be used to calculate the diffusion coefficients and the electrochemical rate constant for a given set of constant conditions using the equation described above. Analyze.m can then be used in a program that inputs various data for different iron concentrations. In addition, by using the Levenberg-Marquardt non-linear regression method (with Gaussian and Partial Pivot functions to solve the generated matrix) one can fit the calculated data to approximate equations, which can be used to predict relationships between variables. 52 Care must be taken in the programming of this interpolation, specifically in the evaluation of the function (signs and logic errors are common). See polynomial regression program (Polynomial.m) 54Using the rate constant and other data from impedance one can determine Double Layer Capacitance and the Degree of Homogeneousity of reaction -1- 2.5 3 3.5 4 (Inverse Square Root) of Scaii 111 Similar methods can be used to obtain other parameters of interest (such as Charge Transfer Resistance, Degree of Homogeneousity of reaction (CD), and Double Layer Capacitance). A1.1. Additional Electrochemical Data Analysis There are two other equations of note to be detailed. The values of interest can be obtained by algebraic manipulation of impedance data. The heterogeneous electron transfer rate constant can be determined from electrochemical impedance spectroscopy data in addition to the methods describe above. Preliminary data for the sulphuric acid based electrolyte indicates that the reaction is complex in the high frequency range and shows Warburg impedance (diffusion controlled process) in the low frequency range. Using an approximation of the high frequency data and an equivalent circuit (see Figure 51) of a solution resistance in series with a parallel constant phase element, CPE, and a charge transfer resistance, R1, the following equation can be applied (46): =Rl/n2FAk0CC: Eq. 28 where: C0 and CR are the concentrations of the oxidized and reduced forms, respectively. The double layer capacitance, Cdl, can be determined from the impedance data using the following equation (64): 1’ n-i n-i - = -‘dl [A + j Eq. 29 The R, R1, T and are fitted parameters of the Constant Phase Element where is related to the time constant distribution Tis a capacitance parameter (F cm2s1) R is the solution resistance () cm2) A1.2. Additional Result Available Through Mathematical Analysis By altering the total concentration of the iron in the electrolyte (both ferric and ferrous species) one can now illustrate how the rate constant changes with respect to the electrolyte’s iron concentration. 112 -4 k vs. ConcentrationxlO 8 1 0 0.1 In addition, by using the Levenberg-Marquardt non-linear regression method (with Gaussian and Partial Pivot functions to solve the generated matrix) one can fit the calculated data to an approximate exponential equation, which can be used to predict a rate constant for an iron concentration of interest. Electrochemical Rate Constant = O.OOO945e3°67184 (Concentration) Eq. 30 This indicates that rate constant decreases exponentially as iron concentration increases. While this result may seem counter-intuitive, it can be understood if one takes into account the complex nature of the electrolyte. Other chemical groups not directly involved in the electrochemical reaction (such as S042)are involved in complexation with the ferric/ferrous ionic species. As the iron concentration increases the level of complexation (and polymerization of these complexes) increases. This leads a decrease in the diffusion process (reflected in decreasing diffusion coefficients55and a decrease in The diffusion coefficient, which should be material independent for the Pt and GC, shows significant differences. The change in diffusion constant with total iron concentration is significantly less for GC compared to Pt, suggesting that an irreversible layer is formed, probably as a result of an interaction with surface groups on the GC. This is confirmed by a double layer charge capacitance increase (on GC) with 7 6 5 3 2 0.2 0.3 0.4 0.5 0.6 Concentration (mol/l) Figure 55: Rate constant vs. Concentration (MATLAB program plot) 0.7 0.8 0.9 113 the overall availability of the ferric species at the electrode interface. This decreases the frequency of ferric collisions with the electrode surface (limits the number of locations where charge transfer can occur). This “crowding” of the electrode interface leads to the reduction in the rate constant (k) with increasing concentration. It is interesting to note that a lower electrolyte pH decreases the complex polymerization, leading to an increase in rate constant values (partially countering the kinetic losses as the iron concentration increases) (15). It is also interesting to observe how sensitive the change of k is as the concentration of iron increases. Using a forward differencing scheme (or by just differentiating the function) one can observe how much rate constants change with respect to a change in iron concentration at a given iron concentration. 0 C..) Conc Figure 56: d(rate constant)/d(Concentration) vs. Concentration using both forward differencing and a differencing relation (MATLAB program plot) This result indicates that as iron concentration increases the effect of a change in concentration on the rate constant decreases. Assuming that this trend remains constant xl Plot of dkldConc vs. Coiic decreasing iron content, obtained by EIS (not shown here). To understand this behavior, the nature of ferric/ferrous complexation in such SBE and its effect on the double layer need further characterization. 114 there should be a point at which the rate constant becomes independent of iron ion concentration (there is spatial limit at the electrode surface). This also reflects the increasing viscosity of the electrolyte as concentration increases. A1.3. MATLAB Protrammin This program takes electrochemical data from a 3-electrode cell experiment under varying conditions and calculates diffusion coefficients and rate constants for each condition (using various sub-programs utilizing linear regression and Nicholson interpolation). It then plots the data with a non-linear regressed treadline. Finally it uses direct differentiation (Forward Euler’s Scheme and differentiation of a formula) to generate a plot illustrating how the electrochemical rate constant changes with respect to ion concentration at various concentrations. The code listed has been created for this function utilizing new and modified code (from a variety of sources) using standard mathematical techniques. %Proj eat_Main, in clear dc cif %Load the data file N=1; fprintf(\ntor Iron Concentration = 0.1M\n’); DATAO1 = load(’DataOl.txt’); [Diffusion Coeff Fe2, Diffusion Coeff Fe3, Rate Constant]=Analyze (DATAO 1, 0.1); DFe2 (N)=Diffusion_Coeff_Fe2; DFe3(N)=DiffusionCoeffFe3; k (N) =Rate_Constant; Conc (N) =0. 1; N=N+l; fprintf(’\nFor Iron Concentration = 0.2M\n); DATAO2 = load(’DataO2.txt’); [Diffusion Coeff Fe2, Diffusion Coeff Fe3, Rate Constant]=Analyze(DATAO 2, 0.2); DFe2 (N) =DiffusionCoeffFe2; DFe3 (N)=DiffusionCoeffte3; k (N) =Rate Constant; Conc (N) =0 .2; N=N+1; fprintf(’\nFor Iron Concentration = 0.3M\n’); DATAO3 = load(DataO3.txt); [DiffusionCoeffFe2,DiffusionCoeffFe3, Rate Constant) =Analyze (DATAO 3, 0.3); DFe2(N)=DiffusionCoeffFe2; DFe3 (N) =DiffusionCoeffFe3; k(N) =Rate Constant; Conc(N)=0.3; N=N+ 1; 115 fprintf(’\nFor Iron ConcentratLon = 0.4M\n’); DATAO4 = load(DataO4.txt); [Diffusion_Coeff_Fe2, Diffusion Coeff Fe3, Rate_Constant] =Analyze (DATAO 4, 0.4); DFe2 (N) =DiffusionCoeffFe2; DFe3 (N)=DiffusionCoeffFe3; k (N) =Rate Constant; Conc (N) =0 . 4; N=N+l; fprintf(’\nFor Iron Concentratzon = O.5N\n’); DATAO5 = load (‘zta05.txt’); [Diffusion Coeff Fe2, Diffusion Coeff Fe3, Rate Constant]=Analyze (DATAO 5, 0.5); DFe2 (N)=DiffusionCoeffFe2; DFe3 (N) =DiffusionCoeffFe3; k(N) =Rate Constant; Conc (N) =0 . 5; N=N+l; fprintf)’\nFor Iron Concentration = 0.6N\n’); DATAO6 = load(Data06.txt’); [Diffusion Coeff Fe2, Diffusion Coeff Fe3, Rate Constant]=Analyze(DATAO 6, 0.6); DFe2 (N) =DiffusionCoeffFe2; DFe3 (N)=DiffusionCoeffFe3; k (N) =Rate_Constant; Conc (N) =0 . 6; N=N+l; fprintf(’\nFor :ron Concentration = 0.7M\n’); DATAO7 = load(DataO7.txt’); [Diffusion Coeff Fe2, Diffusion Coeff Fe3, Rate Constant)=Analyze (DATAO 7, 0.7); DFe2(N)=DiffusionCoeffF’e2; DFe3 (N)=DiffusionCoeffFe3; k (N) =Rate_Constant; Conc (N) =0 . 7; N=N+l; fprintf(’\nFor Iron Concentration = 0.8N\n’); DATAO8 = load(’DataOS.txt’); [Diffusion Coeff Fe2, Diffusion Coeff Fe3, Rate Constant]=Analyze (DATAO 8, 0.8); DFe2(N)=DiffusionCoeffFe2; DFe3 (N) =DiffusionCoeffFe3; Jc (N) =Rate Constant; Conc (N) =0 . 8; N=N+l; fprintf(’\nFor Iron Concentretinn = 0.9N\n’); DATAO9 = load(’DataOO.txt’); [DiffusionCoeffFe2,DiffusionCoeffFe3,RateConstant]=Analyze(DATAO9,0.9); DFe2 )N)=DiffusionCoeffFe2; DFe3 (N) =DiffusionCoeffFe3; k (N)=Rate Constant; Conc (N) =0.9; N=N+l; fprintf(’\ntor Iron Concentra’ion = l.ON\n); DATA1 = load(Datal.txt’); [Diffusion Coeff Fe2, Diffusion Coeff Fe3, Rate Constant]=Analyze (DATA1, 1); DFe2(N)=DiffusionCoeffFe2; DFe3 (N)=DiffusionCoeffFe3; Ic (N) =Rate Constant; Conc(N)=l; end %Conc fprintf(’\n\n\n\nTherefore the relationship tetween rate constant and Iron Concentration is as follows:\n\n’); [a,b,Coeff,n,Conc,k] = Regression LM(Conc,k); [slope] = Output results3(e,b,n,Conc,k); 116 pause; %Fcrward Differencing (Differentiation) - note for this scheme (N-i with i+i) %one cannot do last point. i will therefore use a backward differentiation %for the last point. N=length(Conc); for i = l:l:(N—l) %Must first deal with round off error (using technique from class) h = Conc(i+l) — Conc(i); temporary = Conc(i) +h; h = temporary — Conc(i); FDderivativek(i) = (k(i+l) — end FDderivativelc(N) = (k(N) — k(N—l))/(Conc(N) — Conc(N—l)); %Differencing (Differentiation) of the analytical expression D(t) achieved %from the non-linear curve fitting. for ± = l:N Analytderivativek(i) = a*(b)*exp(b*Conc(iH; end %Plot results for both techniques plot (Conc, FD derivative k, -k’ , Conc,Analyt derivative k, xlabel ( Conc’); ylabel ( ‘dk/dConc’); t±tle(’Plot of dk/dConc vs. Conc’); legenci( ‘Forward Differencing’, Differencing Relation’); %Anaiyze . m function [Diffusion Coeff Fe2, Diffusion Coeff Fe3, Rate Constant) =Analyze (DATA, CONC); Colunml=2; Column2=5; sqrtv = (DATA(:,Columnl)) /‘O.5; Peakl = DATA(:,Coluxnn2); [alin,blin,n,sqrtv,Peakl] = Regression(sqrtv,Peakl); [slope] Output resultsl(alin,blin,n,sqrtv,PeakI); DiffusionCoeffFe2=(slope/(l8986.2l52*CONC*O.5/lOOO))2; fprintf(’’nDiffusion Coefficient of the Ferrous ±on=%l.lOf\n\n’,DiffusionCoeffFe2); pause; Columnl=2; Column2=9; sqrt_v = (DATA(:,Columnl))/’O.5; Peakl DATA(:,Colunm2); [alin,blin,n,sqrtv,Peakl] Regression(sqrtv,Peakl); [slope] = Output resultsl(alin,blin,n,sqrtv,Peakl); DiffusionCoeffFe3=(slope/(l8986.2152*CQNC*O.5/lOOO))’2; fprintf(’\nDiffusion Coefficient of the Ferric ion=%l.lOf\n’,DiffusionCoeffFe3); pause; DFe3overDFe2=DiffusionCoeffFe3/DjffusjonCoeffFe2; Coluxnn3=7; Colusnn4=ll; n=l; Ea = DATA(:,Column3); Ec = DATA(:,Coluinn4); nDeltaEp = Ea-Ec; [logKye] = Nicholson_interpol (nDeltaEp); Kye=lO. “logKye; FCye=Kye’; negsqrtv = DATA(:,Coluxnnl).(—O.5); [alin,blin, n, neg sqrtv, Kye] = Regression (neg sqrt v, Kye); [slope] = Output results2 (alin,blin,n,negsqrtv,Kye); 117 RateConstant=slope/((DFe3overDFe20.25)*()8.3l44l*298.l5)0.5)/((3.14l592654*Diffusion CoeffFe3*l*96484.56)0.5)) fprintf(\nElectrochemical Rate Constant of the electrode reactiori%1 . lOf\n ,Rate Constant); pause; %Regression . m function [alin,blin,n,xaxisdata,yaxisdata] = Regression (xaxisdata,yaxisdata); %This program does linear regression on inputted hata to get equation icomponents (y-intercept and Slope) % %Linear Regression Sigma = 1; %Assumption n=length (xaxisdata); for i=l:n s(i) = l/(Sigma2); end S=sum(s) IS = length(xaxisdata); %Assumes sigma = 1 (Simplifies taking the sum of l/Siqma2 %for each data point Sx=sum( (xaxisdata) / (Sigma2)); Sy=sum((yaxisdata)/(Sigma2)); Sxx=sum(U(xaxisdata)).”2)/(Sigma’2)); Sxy=suin(((xaxisdata) .*(yaxisdata))/csigma2)); De1ta=S*Sxx_Sx2; Coeff(l) = (((Sy*Sxx)_(Sx*Sxy))/Delta); Coeff(2) ( ( (SSxy) _(Sx*Sy) ) /Delta); alin=Coeff(1) blin=Coeff(2); end %RegressionLM. m function [a,b,Coeff,n,xaxisdata,yaxisdata] = Regression LM(xaxisdata,yaxisdata); %This program utilizes the Levenberg-Marquardt Method (non-linear regression) to %calculate coefficient values for an exponential relationship to fit the supplied data. %I will use two functions modified from other sources: Partial Pivot Function land a Gaussian elimination function. In addition, there will also be an equation function. n=length (xaxisdata); Coeff(l)=1; %Initial guess Coeff(2)=—1; llnitial guess lLevenberg—Marquardt Method (non—linear regression) Lambda = l0—3; dCoeff = [0.0O00l;0.OO0O1; old Chi=10; iteration = 0; tolerance 10—6; %lO-6 tolerance allows for convergence within 100 iterations ISmailer tolerance values lead to convergence error dX = lO”—3; IThis dx (for numerical differentiation) value should %be fine hut depending on the system more consideration %to its value should be given based on experimental %information. dX should be small to avoid truncation 118 lerror but big enough to evoid round off error. whiie max(abs(dcoeff)>toleranre) & iteration < 100 %Criteria to stop loop iteration = iteration + 1; %Celrulate Chi with offset Coeffirients Chi = sum) (yaxisdata-EEquation(xaxisdata,Coeff)) .“2); Chiaadddx = sumUyaxisdata—EEquation(xaxisdata, [Coeff U) + dX;Coeff(2)])) .A2); ChiasubtractdX = sum( (yaxisdata-EEquation(xaxisdata, [Coeff U) — dx; Coeff (2) 1) ) . A) Chibadddx = sumUyaxisdata—EEquation(xaxisdata, [Coeff (1) ;Coeff(2) + dX] )) .‘2); ChibsubtractdX = sum) (yaxisdata—EEquation(xaxisdata, [Coeff(l);Coeff(2) - dx])) .‘2) First derivative Chi(l) = (Chiaadddx - Chi)/dx; First derivative Chi(2) = (ChibadddX — Chi)/dx; Second derivative Chi(1,l) = (Chia add d - 2*Chi + Chiasubtractdx)/(dx”2); Second derivative Chi (1,2) = 2*First derivative Chi (1) * First derivative Chi(2); Second derivative Chi(2,2) = (Chibadddx - 2*Chi + Chibsubtractdx)/(dxA2); Serond derivative Chi(2,1) = 2*First derivative Chi(1) * First derivative Chi(2); A1)l,l) = 0.5*Second derivative Chi(1,1) * (1 + Lambda); A1(2,2) = 0.5*Second derivative Chi(2,2) * (1 + Lambda); A1(1,2) = 0.5*Second derivative Chi(1,2); A1(2,1) = 0.5*SecondderivativeChi(2,l); El = _0.5*First derivative Chi; %Make augmented natrix for gaussian elinination, solve end update regression %pa rame t e r 5 A1(1,3) = 51(1); Al(2,3) = 51(2); dCoeff=Gaussian(A1); Coeff=Coeff + dCoeff; Chi=yaxisdata - EEquation(xaxisdata,Coeff); %Compare Chis and adjust lenba accordingly. if (abs(oldChi—Chi)>0) Lambda=Lambda/10; else Lambda=Lambda* 10; end Old Chi=Chi; end b=Coeff(2) a=Coeff ) 1) *0. 001; %If number of iterations exreeded while trying to solve if iteration >= 100 fprintf(’Convergenre Error — Maximum number of iterations (100) exceeded with no ronvergeore\n’) end %EEquation.m lExponential Linear Eguation (Approzinetion) function y = EEguation(xaxisdata,Coeff) y = 0.00l*Coeff)1)*exp)(xaxisdata)*Coeff(2fl; multiplier increases stability oGaussian. m function dCoeff=Gaussian(Ml); N=size (Ml, 1) 119 for k=l:N—l %Perform operation N—l times Ml=PartialPivotFunction(Ml,k); %Utilize external Partial Pivot Function (PartialPivotFunction . rn) for i=k+l:N %Make upper triangular matrix via gaussian elimination for j=k+l:N+l Ml (i, )=Ml (i,j ) — (Ml (1, k) /Ml (k, k) ) *Ml (k, j) end Ml(i,k)=O; end end dCoeff(N)= Ml(N,N+l)/Ml(N,N); Back substitution to get result for i = N—l:—l:l Summation = 0; for j = j+l:N Summation=Summation + Ml(i,j) * dCoeff(j); end dCoeff(i) = (Ml(i,N+l)— Summation)/Ml(i,i); end %aritaJ.PivotFunction . in t%This function is used for partial pivoting of the matrix Mi function Ml=PartialPivotFunction (Ml, k) N=size(Ml,l); %Deterrnrnes the number of rows in the matrix maxMl=abs(Ml(k,k)); %Deterrnines which row has the largest coefficient Cbiggestcoeff) biggestcoeff=k; for ik+l:N if abs(Ml(i,k))>maxMl maxMl=abs (Ml (i, k)); biggestcoeff=i; end end %Pivot operation (exchanging rows if necessary) if biggestcoeff-=k for j=k:N—l temporary=Ml (biggestcoeff, j); Ml(biggestcoeft,j)=Ml(k,j); Ml(k,j)= temporary; end end %Nicholsoninterpol in iThis function cubic spline interpolation methods to solve for a %value of y(YY) for a given x(XX) within a given data set. % function [logKye) = Nicholson_interpol (nDeltaEp); XX=nDeltaEp; N = lemgth(XX); %Reload the data file and recreate X and Y data DATA = load(’ExtNich.dat.txt); X = DATA(:,l); Y = DATA(:,2); N=length(X); fprintf(\n\nCubic Spline Interpolation for a given value of x\n); %Cubic Spline Interpolation x = X(l:N,l); y = Y(l:N,l); [ddy,YY] = cubic splinemethod(x,y,N,XX,M); logKye=YY; :plot cubic spline results XX = limspace(x(l),x(N),lQO); N = length(XX); [A,B,C,D,YY) = cubic function eval(x,XX,y,ddy,N,M); 120 plot (XX, YY, k’ , y, or title(’Cubic Sczne Interpolation, Y vs X for variable N data points (6, 9, and 12) ) xlabel ( X’); ylabel ( ‘Y ); % cubic_splirie_method. %This function utilizes the cubic splice interpolation to get values fcr ddy %(second derivative) and calculates a value of y (YY) for a given x for a %given N number of data points used. tVaiues for x(x), values for y(y), and number of x values(N) used are inputted. %Corresponding values of YY (for XX value) and second y derivative (ddy) tare outputted. function [ddy,YYJ = cubic spline method(x,y,N,XX,M) for i l:N—1 g(i) = x(i+l) — x(i); deltay(i) = (y(i+1)—y(i))/g(i); end for i = 1:N—2 h(i) = x(i+2) -. x(i); end %Make tridiagonal linear system and solve using tridiagonal solve function for i = 2:N—2 U(i—1) = 1/6*g(i); end L = U; for i = l:N—2 D(i) = l/3*h)i); end for i = l:N—2 3(1) = delta y(i+l) — deltay(i); end ddy = tridiagonalsolve(L,U,D,B); ddy = {O,ddy(1:N—2),O]; %Solve polynomial at XX [A,B,C,D,YY] = cubic function eval(x,XX,y,ddy,N,M); %Output results %fprintf(\nN = %1.Of points used: \n,N); %fprintf(’x = %1.4f\n’,XX); %fprintf(’y=%1.4f\n,YY); % tridiagonal solve. m %This function solves a tridiagonal system. The lower diagonal (L), %upper diagonal (U), diagonal(D), and “right hand side” vector (B) tare inputted and the solution vector (X( is outputted (First it will do %a forward substitution and then a backward substitution) function X = tridiagonalsolve(L,U,D,B) N = length(B); for i = 2:N M = L(i—l)/D(i—1); 8(i) = 8(i) — M*B(i_1); D(i) = D(i) — M*tJ(i_1); end X(N)=B(N) /D(N) for i = (N—1):—1:1 X(i) = (B(i) — U(i)*X(i+1))/D(i); end 121 %cubic function va1 . m %This function evaluates the polynomial of different XX values (given when called) %Values for x(x), values for y(y), desired x values (XX), second y derivative )ddy), %number of x values(N), and number of XX values (N) are inputted. Coefficients A,B,C %and P and corresponding values of YY (for each XX value> are outputted. function [A,B,C,D,YY] cubic function eval(x,XX,y,ddy,N,M) for i = l:M for k l:N—l if ((x(k) >= XX(i)) & (XX(i) >= x(k+1))) %calulates polynomial coefficients A(i) = (XX(i) — x(k+l))/(x(k) — x(k+lfl; B(i) = (XX(i) — x(kH/(x(k+l) — C(i) = 1/6 * (A(j)’3 — A(i))*(x(k+l) — x(k))’2; D(i) = 1/6 * (B(i)3 — B(i))*(x(k+l) — YY(i) = A(i)*y(k) + B(i)*y(k+l) + C(i)*ddy(k) + D(i)*ddy(k+l); end end end %Po].ynomial .m %Alternative program to cubic spline interpolation %This program in an alternative method to determine the iog(Chi) %values(instead of using the cubic spline interpolation of the tof the extended working curve) . This uses the coefficients tdetermined from an EXCEL treadline approximation to solve for %the polynomial representing the curve for given data in poly—y.txt %it then sorts the data and outputs only real (non—imaginary) solutions %that fit the curve data set into poly—x.txt (log(Chi) solutions). s=load( poly-y.txt’); s=s’; N=size(s,2) ; p=[—le—4 —1.9e—3 —9. 6e—3 —8e—3 6.75e—2 —6.36e—2 8. 17e—2] ; c=[O 0 0 0 0 0 11; fid fopen(’poly—x.txt’,’w’); for i =1 :1: N, pl=p— c*s(i); r=roots (p1); for j= 1:1:6 if isreal(r(j)) if and(r(j)>—6,r(j)<l.5) fprintf(fid,%l2.8f\n’,r(j)); end end end end fclose (fid); %Output_resultsl . m function [slope] = Output resultsl (a lin,b lin, n,xaxisdata, yaxisdata); %Print results disp(’Coefficients for Linear Relationship:’); fprintf( ‘Using Linear Regression technique\ny intercept=%1.6f\nslope=%1.6f\n’ ,alin, b_lin); %Plot data and fitting methods for Power Law for i=1:n yaxisdatalin(i) = b lin*xaxisdata(i)+ aim; end plot(xaxisdata,yaxisdata, ‘k±’ ,xaxisdata,yaxisdata lin, ‘g’); xlabel(’Square Root of Scan Rate); ylabel (‘Peak Current); 122 title(’Peak Current vs. Square Root of Scan Rate’); slope = blin; %Output_results2 m function [slope) = Output results2(alin,blin,n,xaxisdata,yaxisdata); %Print results disp(Coefficients for Linear Relationship:’); fprintf( ‘Using Linear Regression technique\ny intercept=%l.6f\nslope=%l.6f\n’,alin, b_lin) %Plot data and fitting methods for Power Law for i=l:n yaxisdatalin(i) = blin*xaxisdata(i)+ alin; end plot (xaxisdata, yaxisdata, ‘k-f ‘ , xaxisdata, yaxisdatalin, ‘g’); xlabel(’ (inverse Square Root) of Scan Rate’); ylabel (‘Rye); title(’Kye vs. (inverse Square Root) of Scan Rate’); slope bun; %Outputresults3 m function [slope) = Output results3(a,b,n,xaxisdata,yaxisdata); %Print results disp( Coefficents for Exponential Relationship (y=a*e (bx) ) : ‘) fprintf(’Using Levenberg—Marquardt Method (non—linear regression) \na=%l.6f\nb=%l.6f\n’,a, b); %Plot data and line fit for i=l:n yaxisdata_lin(i) = a*exp(xaxisdata(i)*b); end plot (xaxisdata, yaxisdata, k-f’ , xaxisdata, yaxisdatalin, g); xlabel(’Concentration (mol/l) ‘); ylabel( k); title(k vs. Concentration’); slope = 1; 123 Appendix 2: Integrated System I: The Bio1oicaI Considerations (Bioreactor System) Biological models are generally based on Henri/Michaelis-Menten and Monod equations, but since biological systems can be quite complex, including temperature, pH, inhibition factors (including substrate, product, and toxin concentrations), etc. a variety of model equations need to be developed to get an accurate overall picture. The following are some of the equations found in literature: Henri/Michaelis-Menten (HMM) equations (35) are used to model single- substrate-enzyme-catalyzed reactions. v= Vm[SI Eq.31 Km + [S] where: v = Enzyme reaction rate; IS] = concentration of substrate; Vm = maximum production rate (k2[E0]); and Km = (Ici+k2)/ki Monod equations (35) are used to model the growth rate of cells. p — Pm[5I Eq.32 K3+[S] where: Ug= specific growth rate; [5] = concentration of substrate; jUm = maximum specific growth rate; and K = substrate concentration where the specific growth rate is half its maximum value. Harvey and Crundwell have produced a model, which includes the effects ofFe2, 02, Fe3 and H.56 p [Fefl 1+ [SJ Eq. 33Kll+ i+[Fe 1±K ,) Ka where Pm 0.16 h’; K= 0.073 kg/rn;K = 0.78 kg/rn3;Ka 21.75 kg/rn3 In addition, Nemati developed a model of the kinetics of the ferrous ion oxidation: d[Fe2+] = Koe_ET[XI[Fe2i Eq 34 dt Km K1) fi) a Nagpal also produced a model, which includes the effects of Fe2, 02, C02,Fe3, As, H and temperature(33). 124 where K0= 6438 kg/m3hr; Km = 0.0672 kg/rn3;K,’ = 2.68E7 cells/mi; E = 68.4 kJ/mol; a= 26.1 kg/m3; j9= 7.8E8 cells/mi A2.1. Overview of Bioreactor Work (University of Western Ontaro) Dr. Kararnanev’s group (1) at the University of Western Ontario investigated a variety of bioreactors (categorized into two groups: free suspended cell and immobilized cell bioreactors57)in terms of the volumetric iron oxidation rate and long-term stability. They found that an immobilized solid bioreactor was the best alternative (sized between 0.5 and 120 litres). Kinetic studies of microbes originally indicated standard Acidithiobacillus ferroxidans as a promising candidate for the regeneration of the ferric ion, but when considered in the light of the biofuel cell it was decided to pursue other options (the low pH, high temperature, and high iron concentrations which have been indicated in this work are not compatible with normal operation of the organism58). The lion Mountain mine (Northern California) drainage waters, which more closely match the conditions preferred by the biofuel cell, provided microbial cultures which efficiently operated at pHs between 0.7 and 1.0, temperatures between 40 and 60°C and iron concentrations between 50 and 60 g/L. Studies where also carried out on the effect of the some of the expected impurities in the hydrogen gas stream, which might affect the bioreactor’s organisms (due to membrane crossover). It was shown that CO2 will act as a carbon source for the organisms and the trace amounts of methane and CO from the reformer’s operation have no significant effect on the bioreactor’s operation. As previously mentioned in this work, ajarosite formation is potentially a serious problem in the operation of this joint system. Jarosites could clog components of the system (piping, pumps, valves, etc.) and could clog the cathode, creating a diffusion Free suspend cell bioreactors: CSTR (Continuous Stirred Tank Bioreactor) and Airlift Bioreactor; Immobilized cell bioreactors: Inverse Fluidized Bed Bioreactor, Fixed Bed Bioreactor and Immobilized Solids Bioreactor. Karanev et al. found that standard T. ferroxidans prefers pH 2.3, iron concentrations less than 20g/L and an environmental temperature around 30°C. 125 barrier for the reactants/products of the fuel cell reaction (effecting overall chemical kinetics). The ferric ion is involved in two competing reactions: ferric ion hydrolysis (Fe3 + 31120 Fe(OH)3+ 3H) and ferric hydroxysuiphate (jarosite) formation (3Fe + W + 2HS04 + 61120 —* MFe3(SO4)2 (OH)6 1- +8H where M can be K, Na, NH4,H3O). It was shown by both Karamanev’s group (Mining Engineering Journal) and in the fuel cell reactions presented in this work that under low pH conditions jarosite formation ceases to become a problem. Another interesting challenge that presented itself to the bioreactor group was the production (excretion) of organic by-products from the microorganisms. To avoid their accumulation, a symbiotic culture of additional microorgasims was studied in the bioreactor (the autotroph Leptospirillum and the mixotroph Ferroplasma). This mixture was successful in controlling the produced organics (Ferroplasma consumed it). Research at the Univeristy of Western Ontario cumulated in the development of an integrated biofuel cell with both catalyst for the hydrogen reaction at the anode and the microrganisms (At.ferroxidans and L.ferriphilum) at the cathode immobilized on fibrous carbon paper. Unfortunately real bioelectrolyte material was unable to be tested by this author at 1JBC (with the fuel cell modifications mentioned in this work) due to limitations of shipping hazardous goods within Canada. Both the work of Karamanev’s and this author’s group support that a biofuel cell with bioregeneration is a promising technology requiring further investigation. Success was obtained at both the single cell and stack level (different architectures) with simulated bioelectrolyte, standard iron electrolyte, and actual bioelectrolyte. 126 Appendix 3: Integrated System II, The Gas Reformer The fluidized bed membrane reactor (FBMR) uses perm-selective membranes containing palladium alloy as part of a novel reactor configuration (a modification of standard steam methane reforming) to selectively extract produced hydrogen during the reforming process. This system drives the reaction equilibrium forward (leading to a decrease in temperature and pressure limitations). The use of fine catalyst particles (as opposed to a conventional fixed bed configuration) leads to increases in catalyst effectiveness and improvements in heat transfer. Since the reaction can now be run at lower temperatures the bulk of the carbon is converted to carbon dioxide that can used to feed the microorganisms in the bioreactor (carbon capture) while the hydrogen produced can be directed to the fuel cell, creating a more holistic and self-contained system. More information on the reformer improvements achieved via MRT during the course of this project can be found in the NSERC Strategic Project Grant Final Report (1). Since these advances do not have any direct effect on the fuel cell (with the exception of the gas separation techniques described above) they will not be further discussed in this work. 127 I a a ES Ca Ca CD a CD X x- 4 * o > x x r - 4 :‘< m m > — > o n r 0 - fl o > N r 4P C o C -c -Z N O fi N fl lf lf l N 3 J N O N 0 a Ca — . a Ct — S a 7 a B a Ct C a C) 7 CD B N_s C) 0 - a C) CD 0 CD CD WI S WI —. C) ii t— 3 00 p c - ” r - z n r A4.3. Compression Piston Sept 20 04 Tom V 0riinaI o-ri,-g mom hond moe Suggest using o 2-028 regular o-rng. The grooe I snug but try t anyway. MATERIAL6 ALUMINUM NRC IFCI T0LERNCE 4 CM’ PEMEC HARDWARE COMPRESSiON PISTON x.x ±o.T JOERG ZTMMERMANN X.XX ±O025 02/23/04 DIMENSIONS IN MILLIMETERS SH1IUWGIWOR A4.4. End Plate MATER I AL ALUM I NUM TOLERANCE X.X yO.I X.xx ±0.026 DIMENSIONS IN MILLIMETERS NRC PCI 4 CM’ PEMEC HARDWARE ENOPLATE IOERO ZIMMERMANN 02/24/04 j- - 129 17.00- 34.00- .6 I0.O°° 1.6 A4.5. Anode Manifold .- 70 500 F0L0 005050 0 00054 4.57 044 HOLE. I000IC*L, A4.6. Cathode Manifold [I ;V 1.0 • 19.00 5.00_i 750 8.0• MATERIAL: <EL F PLAStIC TOLERANCES >( ±0.5 x.x ±0.1 X.XX ±0.025 DIMENSIONS IN MILLIMETEPS SHOID50GIwORX 30.90 I: 44.00 1*4 CR00500 rYPICALI 0 0.50 5 .00 — 15.00 3 .2.50 4 0,50 i. jr:. :r. z±;rr.i r0.00 0 ,50I 0 .00 MATERIAL: KEL F PLASTIC TOLERANCE: ±0.025 DRAWIND iN MILLIMETERS NRC IFCI 4 CM0 PEMFC HARDWARE ANODE MANIFOLD JOERG ZIMMERMANN 02/23/04 REV. A - MAURICID SLANCO 12/14/04 44.00— 5.OO_] MM CHAMFER (TYPICAL) B.50— 4.50— —2 oR 1.8 1.0 3.O0 NRC IFCI 4 CM0 PEMFC HARDWARE CATHODE MANIFOLD JOERO ZIMMERMANN 02/23/04 REV. A - MAURICIO ELANCO 12/14/04 130 A4.7. Bus Plates (gold plated) MATERIAL: COPPER TOLERANCES X = O.5 X.X ±0.1 X.XX ±0.025 DIMENSIONS IN MILLIMETERS NRC IFCI 4 CM PEMFO HARDWARE ANODE EUS PLATE JOERO Z I MMERMANN 02/23/04 MATERIAL: SGL 85P4 NRC IFCI CARBON/PHENOLIC COMPOSITE 4 CM PEMFC HARDWARE FUEL PLATE TOLERANCES: JOERO ZIMMERMANN x ±0.5 02/24/04 XX ±0.1 X.XX ±0.025 DIMENSIONS IN MILLIMETERS SHIDWGIWOR 131 -R S.D 0 3.2- I ti CffiMFER IrrpIcU, j .50 A4.8. Anode (Fuel) Plate (Serpentine) A4.9. Cathode Plate (Serpentine) 1 I\ 3.0’ “—02.0 MATERIAL SGL EIPP4 CAREON-PHENOLIC COMPOSITE TOLERANiCE NRC PCI X.X ±0.05 4 CM PEMFC HARDWARE OXIDANT PLATE X.XX ±0.005 JOENG ZIMMEPLIANN DRAWING IN MILLIMETERS 04/01/04 SH2InWGIW0R A4.1O. Cathode Plate (Hollow pocket) M1SR’.L. . Fep. .F’±( i-r’d I C ‘(NOR T(RAJZ-. ±0.1 OI,4ICx’tS It 1IiTWW p.pc irr4 C) p.r-c ri s,,, Pi. 132 4. A4.11. Bushings Sf41 LWOR MATERIAL: BRASS TOLERANCES x ±0.5 Xx ±0.1 X.XX ±0.025 NRC IFCI 4 Ck4 PEMFC HARDWARE BUSHING J0ERG ZIMMERMANN 02/23/04 MorEpinc: CQO ROLLED CORDON NRC I FC SREDL MIRE (PIANO WIREI 4 CM PEMFC HARDWARE RETA HER PIN INI4ILLIEWOtRO JOERG ZIMMERMANN TOLERAICE: •/- 0.5 /29/04 SHIIDWGIWOR 133 0 4.76 25.0 AAM-1I4’IER AND OUrERCCtAMFER- TYPICAL) A4.12. Pin _____I6j 90 —R 73 I/IS DID. P1040 WIRE A4.13. Tie Rod + 0 476 MATERIAL: 36SS NRC IFCI 4 CM PEMFC RARDWARE TIE ROD TOLERANCES: JOERS ZIMMERMANN 02/24/04 X.X±0.T X.XX ±0.0215 DIMENSIONS IN MILLIMETERS SH1IDWGIWOR I0.0 134 Appendix 5: Profile of Organizations Involved This work, part of a strategic NSERC grant (Novel biofuel cell - methane reforming reactor system for electricity generation, #GHGPJ 269967 — 03) (1), was a collaborative effort encompassing the various components of an overall system. • The fuel cell work was done at the University of British Columbia’s Department of Chemical and Biological Engineering and the National Research Council Institute for Fuel Cell Innovation • The gas reformer work is being done at the University of British Columbia’s Department of Chemical and Biological Engineering in conjunction with Membrane Reactor Technology Inc. (MRT) • The bioreactor work is part of an ongoing program at the University of Western Ontario’s Department of Chemical Engineering Funding for this project was partially supplied by the Natural Science and Engineering Research Council A5.1. University of British Columbia Department of Chemical and Bio1oical • . 59Enrneering UBC’s Department of Chemical Engineering was founded in 1915 (it was the first western Canadian applied science program). The program was extended to Chemical and Biological engineering (at that time called Chemical and Bio-Resource Engineering) in 1996 to service the growing fields of biological/biochemical engineering. The Department offers both undergraduate and various graduate options and has recently relocated to a new facility, which will allow for its excellence in research to continue and expand (particularly with the addition of the Clean Energy Research Centre). The main foci of research include biotechnology, environmental research and process engineering. University of British Columbia, Department of Chemical and Biological Engineering, 2216 Main Mall, Vancouver, B.C., Canada V6T 1Z4 135 With the addition of the CERC building, research in electrochemistry and alternative energy engineering has gained a significant new resource for graduate students in the field to utilize. A5.2. National Research Council Institute of Fuel Cell Innovation60 The National Research Council (NRC) of Canada was established in 1916 as an advisory government body. Laboratories were later developed in Ottawa where military R&D was performed during World War 2. This was followed by significant work in the post-war period in the areas of fundamental and applied science and engineering. The NRC produced a number of significant inventions that have had a profound worldwide impact, these include the Pacemaker (1940s), Canola (1940s), the Crash Position Indictor (1950s), the Caesium Beam Atomic Clock (1960s), Computer Animation Technology (1970s), and the Canadarm (1980s). The current mission of the NRC is to offer cutting-edge research in support of industry and to develop private and public sector partnerships both within Canada and on an international level to stimulate and sustain innovation and wealth creation. Its current areas of focus are Aerospace, Biotechnology, Engineering and Construction, Fundamental Sciences, Industrial Support, Information and Communication Technologies, and Manufacturing. The NRC Institute for Fuel Cell Innovation (IFCI) is located in Vancouver, British Columbia. Its main focus is to aid in the development of a fuel cell technology cluster in Canada and to lead the National Fuel Cell Program. It utilizes a multidisciplinary team of scientists and engineers drawn from the universities, industry and government of both Canada and the global community. This allows for a depth of knowledge and experience to be applied to areas of Low Temperature Fuel Cells, High Temperature Fuel Cells, Demonstration Projects, and Industrial Research Assistance. 60NRC Institute for Fuel Cell Innovation, 3250 East Mall, Vancouver, B.C., Canada V6T 1W5 136 A5.3. Membrane Reactor Technologies (MRT) Membrane Reactor Technologies is a company located in Vancouver. It focuses on developing superior hydrogen generation, purification and recovery technologies using advanced metallic membranes. Their Fluid Bed Membrane Reactor (FBMR) incorporates hydrocarbon reforming, shift conversion and hydrogen purification in a single stage process that can produce pure hydrogen from various raw fuels (coal gas, syn-gas, natural gas, etc.). They also work on catalysts, reactor configuration, design and pilot plants for FBMR research. A5.4. University of Western Ontario Department of Chemical and Biological Engineering61 The Department of Engineering at the University of Western Ontario was founded in 1954. The research foci generally fall into four main areas: Biochemical and Biomaterials, Chemical Reactors, Fluidization, and Particle Technology. A5.5. National Science and Engineering Research Council (NSERC)62 NSERC is a government funding organization founded in 1978 with a budget of $900 million. Its role is to make strategic investments in Canadian science and technology at the university, government and industry level. It achieves this via research grants, fellowships and scholarships. In addition to this, it works with industry to invest in the training of the future scientists and engineers and Canada, while at the same time developing international collaborations with other research groups around the globe. 61 Dept. of Chemical and Biochemical Engineering, Faculty of Engineering, The University of Western Ontario. London, Ontario, CANADA N6A 5B9, Telephone: (519) 661-2131, Fax: (519) 661-3498 62 NSERC, 350 Albert Street, Ottawa, ON, Canada, K1A 1H5 Tel.: 613-995-4273 137 Appendix 6: Alternative energy options While fossil fuels are an immense source of energy, issues such as global warming and long-term availability (assuming the pollution generated can be filtered out by improved technology) make it necessary to consider other alternatives. The basic sequence of events in traditional electrical energy production and usage is fuel to generation to transmission to distribution to services. One must consider various concepts along the pathway (8): fuel availability, chemical hazards, energy utilization and demand (fluctuating demand), infrastructure congestion and security/stability, and power quality (including loss prevention). The norm in developed countries is centrally produced electrical power shipped to consumers over transmission lines. This power is generally created by the proven technologies of fossil fuel/coal fired plants, hydroelectric dams and nuclear reactors. The concept of distributed generation is a relatively new one, but one which is gaining popularity in both the developed and the developing world. It allows for cost savings in terms of the minimal transmission infrastructure needed, decreased security issues, local energy independence, and the possibility to generate income by feeding excess production back into the grid. It involves more local production and distribution of energy usually on a small scale. This situation is ideal for the implementation of some alternative energy solutions (the type chosen would depend on geographic location and local energy requirements and availability). In addition, if combined heat and power systems are used the potential for “green”, low impact, and highly effective energy solutions becomes more plausible. The key issue is whether our energy-needy world can continue on its current course through the utilization of conventional energy technologies. The current mindset is that a combination of old and new methods of energy production coupled with a new global mentality of energy conservation and recycling might be the first step in a long term but challenging earth friendly path. Water makes up approximately 70% of the Earth’s surface. The use of hydraulic turbines (paired with electric generators) allow for the movements of global water to be harnessed to produce electricity. This opens up a variety of options for “green power63” 63 There are environmental issues to be considered such as land flooding/damage, local ecology (both aquatic and terrestrial), impacts on fish migrations, etc. 138 generation. Impounded (and pumped) hydropower utilizes the potential energy of water at different elevations, while diversion hydropower utilizes the energy of water already in motion to produce electricity. Tidal power takes advantage of ocean waves, tides and thermal differences to produce power through a variety of scenarios. The use of various mechanical contraptions to convert the power of waves has been considered but is limited to areas where wave power is both strong and consistent. In addition, there are issues with its vulnerability to storm damage, the effect on tidal ecosystems and local sedimentation, and problems of aesthetics and interference with the maritime economy. More interesting is the use of reservoirs, which after being filled to high levels by tidal water can produce energy when paired with strategically placed turbines. Finally, the ability to harness temperature gradients (due to geological phenomena and sea currents) in the ocean is quite intriguing. Since the cost of such endeavours varies with the inverse of the size of the gradient and the fact that a plant would need to be located at depth (quite far off shore due to continental shelves), make this high potential technology impractical in most cases. The sun offers a great wealth of power if a way can be found to harness it for heating and/or generating electricity. On the technological front there are 3 main methods that have been devised to utilize the sun’s radiation. These methods are increasingly being implemented around the world in areas of high solar exposure, which is improving not only technological expertise but also their economic feasibility. The first method is photovoltaic panels. Through use of semiconductor materials the sun’s rays can be converted to electric power. This is achieved through the photoelectric effect of conductive metals and semiconductors. The material’s electrons can capture the energy of photons striking it. If enough energy is absorbed to exceed the material’s band gap, electrons can be freed and electricity can be produced. First developed by international space programs in the 1 960s their application is primarily limited by cost and efficiency, both of which have been improved upon in the last 40 years. The other two technologies, while less well known to the public, are intrinsically easier to understand and relate to. The sun’s rays produce heat naturally and focusing those rays (with a magnifying glass for example) can produce more intense temperatures than what is normal. Passive solar systems can be incorporated into building design/material concepts 139 to capture both heat and light to minimize the need for energy in these applications. Active solar systems are devices, which use solar collectors filled with heat storing liquids, to capture, store and redirect heat to where it is needed. Solar concentrators take these methods to a more advanced level through the use of large mirrors (in the shape of parabolic troughs, dishes, etc.) and heliostat systems to intercept and divert the energy of the sun (75) to centrally located receivers, which can heat up contained fluids to 1000°C. These heated fluids can then be used to vaporize and superheat steam to produce electricity via Rankine cycle systems. Wind power has a history of over 2000 years. Based on the uneven heating of the earth by the sun the resource of wind power is vast but is mainly concentrated in coastal and some unique interior areas. The key points when choosing an area for development are wind speed, wind consistency, and geographic construction limitations (i.e., cost and safety). The biggest problem of large-scale implementation of wind farms is transporting the power generated to consumers. Presently there is no effective way to store the power generated (with the exception of batteries in small systems) and the use of transmission lines would have large losses over the large distances involved. While the technical problems are substantial (including safety, efficiency of blade design, geographic placement, atmospheric considerations, etc.) to produce a long lasting mechanism, the economic equation must also be considered. This includes negatives such as cost, construction, rental/purchase of land, etc., and positives like the value of the displacement of pollution producing technologies (using systems like carbon credits as proposed in the Kyoto protocols). Overall the technology has a definite place in our energy future and can already be seen developing around the world (especially as the concerns about aesthetics, noise, electrical wave interference, and potential damage to bird life are being progressively addressed). From agricultural and forestry waste to bio-industrial side products to city sewage biomass is a huge potential source of energy with the possibility, if handled correctly, to be CO2 neutral between growth and use. If the CO2 produced in the energy extraction steps is captured! sequestered or recycled, use of biomass-derived energy would aid in the fight against global warming and overall pollution (particularly in farming areas). Fuels produced via biomass (e.g., ethanol) have the potential to help minimize the use of fossil 140 fuels. A truly renewable64and readily available resource, biomass can be utilized in the power sector via a variety of methods: direct combustion, air-blown gasification, oxygen- blown gasification and pyrolysis (methanol synthesis is a possible downstream process), hydrolysis and fermentation to ethanol, and anaerobic digestion/bioconversion(75). Energy can be captured or utilized in the form of process heat, steam, electricity (via Rankine cycle and turbine technology), and eco-fuels. The main limitations to the implementation of biomass energy sources are the cost (both the fossil fuel and nuclear options are cheaper), its low energy density, the high water content issues associated with it, and the relatively new arguments of agricultural land displacement (the growing of energy crops as opposed to food crops)65 and overall balance of plant concerns. The centre of the earth is composed of molten rock. Some of this material intrudes into the earth’s crust and heats both the rock and the fluid in the region leading to such natural phenomena as hot springs and geysers. In 1902 geothermal steam was first used to produce electrical power in Larderello, Italy. Now one of the biggest geothermal projects in the world is underway in Iceland to make it the main source of power, with the aim of displacing fossil fuel use. Key to the evaluation of the viability and effectiveness of an area’s geothermal resource are the factors of accessibility, productivity (as defined by the dimensions of temperature, depth, and permeability! porosity), and overall economics of its development and utilization (75). The actual mechanics of geothermal systems are variable based on the type of resource being harnessed (magma, hydrothermal, hot dry rock and geopressurized) but in general either a coupled transport or purely conductive process is involved. Through the use of geothermal wells, heat exchangers, liquid-vapour separators, condensers, turbines, heat pumps (to a source or sink), and cooling towers the energy of the earth can be utilized to produce power, heat crop soil, heat/cool buildings, dry biomass, or enhance industrial processes. Unfortunately, the development and management of geothermal energy reserves must be handled carefully so as not to deplete the heat reservoir, affect local Renewability can be quantified by use of renewability indices. 65 Also note that, as is the case for many of the alternative energy options presented here, biomass can only be effectively utilized in certain geographic locations. Fortunately, biomass can be converted into liquid fuels which can be transported elsewhere. 141 water tables, cause seismic disturbances, or release large amounts of waste heat and dissolved CO2. Nuclear power holds a place of both incredible promise and terrible fear in the global psyche. In the fission of Uranium 236 (produced via the neutron capture of235U), an exited, unstable compound nucleus is produced via the collision of a neutron with a target nucleus. This product nucleus rapidly decays by splitting apart to more stable components (in the form of ionizing particles). These particles, carrying 192 MeV of kinetic energy (75), interact with the surrounding atoms. The kinetic energy transfer which leads to further ionization and further nuclear reactions, which eventually causes the temperature to the surrounding material to increase (due to an increase in internal energy, i.e., heat). This transferred heat energy can be used in conjunction with heat exchangers and steam turbines to produced significant amounts of electricity and heat, allowing nuclear technology to be both an excellent stopgap measure while other energy technologies are developed/commercialized and a long-term “clean” (no CO2 production, etc.) solution to be used as a foundation stone for our energy future. Unfortunately, nuclear energy suffers from a bad reputation due to incidents like the Three Mile Island and Chernobyl accidents. Even though current safety protocols and systems (for controlling, containing and cooling the reactors) are extremely advanced, the concerns about a nuclear accident cannot be put aside. In addition to this, there are the more “realistic” worries of what to do with the radioactive waste (which has a very long half-life) and the high capital costs associated with building nuclear reactors. 142 Appendix 7: Publications and Presentations The majority of the work described in this paper has been published or is in preparation for publication as follows: • Fatih, Khalid, Wilkinson, David P., Moraw, F., and Girard, Francois “Evaluation of the Fe(III)/Fe(II) Redox Fuel Cell Cathode Couple”. Proceedings ofthe Electrocatalysis Symposiumfor the 207th Meeting ofthe Electrochemical Society (2005). pp. 341-350. (42) • Moraw, F, Fatih, Khalid, Wilkinson, D. P., and Girard, Francois “Evaluation of the Fe(III)/Fe(II) Redox Fuel Cell Cathode Couple in a Bioelectrolytic Solution.”. Advanced Materials Research. Vol.15-17. pp. 315-320. (2007) (15) • Wilkinson, D. P., Fatih, Khalid, Moraw, F, Ilicic, A., and Girard, François “Advancements in the Direct Hydrogen Redox Fuel Cell”. Electrochemical and Solid-State Letters 11(2). pp. Bi 1-B15. (2008). (76) In addition to these works, the following publications were influential in this work (while not being directly part of my research): • Karamanev, D. “Biofuel Cell”. WO 2005/00198 1 A2. (2005). (29) • Karamanev, D. “Strategic Project Grant Final Report: Novel Biofuel Cell - Methane Reforming Reactor System For Electricity Generation (Bioregenerative Redox Fuel Cell Using Thiobacillus Ferrooxidans)”. NSERC (GHGPJ 269967- 03) (12-31-2006). (1) Finally, the following conference presentations of the thesis work were made: • Evaluation of the Fe3 / Fe2 Redox Fuel Cell Cathode Couple. 44th Conference of Metallurgists. Calgary, Alberta. August 21-24, 2005. • Evaluation of the Fe(III)/Fe(II) Redox Fuel Cell Cathode Couple in a Bioelectrolytic Solution. THERIVIEC 2006, 5th International Conference on Processing & Manufacturing of Advanced Materials (Poster Session). Vancouver, British Columbia. July 4-8, 2006. 143 • Evaluation of a Redox Cathode in a Fuel Cell (A New Fuel Cell Approach). Hydrogen and Fuel Cells Conference 2007. Vancouver, British Columbia. April 29-May 2, 2007. • Hybrid PEM Fuel Cell: Redox Cathode Approach. 58th Annual Meeting of the International Society of Electrochemistry. Banff, Alberta. September 9-14, 2007. 144 Appendix 8: Statement of Co-Authorship As mentioned in Appendix 7, some parts of this thesis are the result of coauthored and collaborative work. The details of which are as follows: • The identification of this research work is based on a concept detailed in a Strategic NSERC Grant (GHGPJ269967-03) (1) envisioned by Dr. David P. Wilkinson of the University of British Columbia (UBC), Dr. Karamanev of the University of Western Ontario (UWO), Dr. Khalid Fatih of the National Research Council (NRC) and Dr. Francois Girard of the National Research Council (NRC). • The design of the research program was achieved by working in collaboration with my supervisors at UBC and NRC. • The bulk of my thesis research was performed by me under the supervision of Dr. David P. Wilkinson (UBC) and Dr. Khalid Fatih (NRC). The research resulted in some collaborative published work as detailed in Appendix 7. • Some parts of my thesis include extracts of publications in which I was a coauthor (see Appendix 7) that have since modified and changed for this work. • The research resulting in Figure 32 was done in collaboration with Mr. Alan Ilicic (see Appendix 7). 145


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