Open Collections

UBC Theses and Dissertations

UBC Theses Logo

UBC Theses and Dissertations

Lowering the energy consumption of zinc electrowinning by electrocatalysis of the oxygen evolution reaction… Arfania, Sheida 2020

Your browser doesn't seem to have a PDF viewer, please download the PDF to view this item.

Notice for Google Chrome users:
If you are having trouble viewing or searching the PDF with Google Chrome, please download it here instead.

Item Metadata

Download

Media
24-ubc_2020_november_arfania_sheida.pdf [ 27.05MB ]
Metadata
JSON: 24-1.0394793.json
JSON-LD: 24-1.0394793-ld.json
RDF/XML (Pretty): 24-1.0394793-rdf.xml
RDF/JSON: 24-1.0394793-rdf.json
Turtle: 24-1.0394793-turtle.txt
N-Triples: 24-1.0394793-rdf-ntriples.txt
Original Record: 24-1.0394793-source.json
Full Text
24-1.0394793-fulltext.txt
Citation
24-1.0394793.ris

Full Text

LOWERING THE ENERGY CONSUMPTION OF ZINC ELECTROWINNING BY ELECTROCATALYSIS OF THE OXYGEN EVOLUTION REACTION USING MANGANESE OXIDES by  Sheida Arfania  B.ASc., The University of British Columbia, 2018  A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF  MASTER OF APPLIED SCIENCE in THE FACULTY OF GRADUATE AND POSTDOCTORAL STUDIES (Chemical and Biological Engineering)  THE UNIVERSITY OF BRITISH COLUMBIA (Vancouver)  October 2020  © Sheida Arfania, 2020 ii  The following individuals certify that they have read, and recommend to the Faculty of Graduate and Postdoctoral Studies for acceptance, a thesis/dissertation entitled:  Lowering the Energy Consumption of Zinc Electrowinning by Electrocatalysis of the Oxygen Evolution Reaction Using Manganese Oxides  submitted by Sheida Arfania in partial fulfillment of the requirements for the degree of Master of Applied Science in Chemical and Biological Engineering  Examining Committee: Dr. Elöd Gyenge, Chemical and Biological Engineering Supervisor  Dr. Edouard Asselin, Materials Engineering Supervisory Committee Member  Dr. Fariborz Taghipour, Chemical and Biological Engineering Supervisory Committee Member  iii  Abstract Zinc is employed in a wide range of commercial applications including galvanizing iron and steel and production of various metal alloys. In hydrometallurgical processes, zinc electrowinning from sulfate-based electrolytes is the last step of zinc extraction in which high purity metallic zinc is electrodeposited from a highly acidic solution on an aluminum cathode. The electrowinning stage is very energy-intensive and responsible for approximately 60% of the power requirement of a zinc refinery [1]. Inside the electrowinning cell, oxygen evolution reaction (OER) overpotential on conventional lead-silver (Pb-Ag) anodes contributes to nearly 25% of the overall cell potential and places a heavy financial burden on zinc refining plants [2]. Thus, improving the energy efficiency of electrowinning by lowering the OER overpotential is of primary significance to the zinc refineries. This study aimed to develop and evaluate novel anodes in order to lower the OER overpotential within the zinc electrowinning process. The novel anodes were prepared by electrodeposition of manganese oxides (MnOx) on industrially provided Pb-Ag substrate using various potentiodynamic and galvanostatic polarization techniques. The anodic electrocatalytic activity of the baseline and MnOx electrodeposited Pb-Ag electrodes was investigated in a manganese (II)-containing sulfuric acid electrolyte using linear scan voltammetry and 72-hour galvanostatic electrolysis at 500 A m-2 superficial current density. Additionally, the baseline and novel anodes were studied within the full-cell electrowinning setup by the means of 24-hour galvanostatic electrolysis at 500 A m-2 superficial current density. The electrolyte composition and operating conditions were selected such that to be directly applicable to the industrial zinc electrowinning process. iv  The MnOx electrodeposited Pb-Ag anodes reduced the OER overpotential by a maximum of 155 and 113 mV in the absence and presence of chloride ions. Investigation of the novel electrodes in the full-cell zinc electrowinning operation corroborated the half-cell experiments, revealing a maximum of 133 mV reduction in overall cell potential. The surface morphology and elemental composition of the novel anodes were investigated using SEM/EDX, XRD, and ICP-OES.  This work demonstrated that the MnOx electrodeposited Pb-Ag anodes reveal improved electrocatalytic activity and have great capacity to lower the energy consumption of the conventional zinc electrowinning process.  v  Lay Summary In hydrometallurgical plants, majority of zinc is globally produced through the electrowinning process, which is responsible for nearly 60% of the power consumption of zinc refinery. Within the electrowinning cell, the excess energy required to drive the oxygen evolution reaction on conventional anodes accounts for 25% of the overall cell potential. Thus, designing novel anode electrodes and improving the energy efficiency of the zinc electrowinning stage is of great interest to the zinc refineries. This study aims to utilize electrochemical techniques to develop novel anodes consisting of a dispersed phase of manganese oxides in the matrix of industrially supplied lead-silver electrodes. The present work employs various electrochemical procedures in order to assess the performance of the novel anodes under the industrial zinc electrowinning operating conditions and their potential applicability in the industrial process. The findings in this research provides an effort to support the technological advancement of the mineral processing facilities. vi  Preface This research presented henceforth was conducted under the supervision of Professor Elöd Gyenge in the Fuel Cells and Applied Electrochemistry Lab of the Chemical and Biological Engineering department at the University of British Columbia. I, Sheida Arfania, was responsible for performing the literature review, constructing a hypothesis, outlining the objectives, designing the experimental methodologies, conducting the experiments, analyzing and interpreting the results, and writing the manuscript. I am the primary author of the presented work in which Professor Elöd Gyenge, Professor Edouard Asselin, and Dr. Pooya Benhangi-Hosseini provided continuous insight and Yu Pei assisted intensively with the characterization experiments.  The work has been lectured by Professor Edouard Asselin in the form of an oral presentation at the TMS PbZn 2020 symposium: Sheida Arfania, Pooya Benhangi-Hosseini, Edouard Asselin, Elöd Gyenge, Lowering the Energy Consumption of Zinc Electrowinning by Electrocatalysis of the Oxygen Evolution Reaction Using Manganese Oxides, PbZn 2020: 9th International Symposium on Lead and Zinc Processing, California, USA, Feb. 2020. Furthermore, I have presented this project in the form of the following digital presentation at the ECS PRiME 2020 symposium: Sheida Arfania, Pooya Benhangi-Hosseini, Edouard Asselin, Elöd Gyenge, Lowering the Energy Consumption of Zinc Electrowinning by Electrocatalysis of the Oxygen Evolution Reaction Using Manganese Oxides, PRiME 2020: Advances in Industrial Electrochemistry and Electrochemical Engineering, Oct. 2020.  vii  Table of Contents  Abstract ......................................................................................................................................... iii Lay Summary .................................................................................................................................v Preface ........................................................................................................................................... vi Table of Contents ........................................................................................................................ vii List of Tables ................................................................................................................................ xi List of Figures .............................................................................................................................. xii List of Symbols ........................................................................................................................... xix List of Abbreviations ...................................................................................................................xx Acknowledgements ................................................................................................................... xxii Dedication ................................................................................................................................. xxiii Chapter 1: Introduction ................................................................................................................1 Chapter 2: Literature Review .......................................................................................................4 2.1 Electrolytic Zinc Refining Process ................................................................................. 4 2.2 Zinc Electrowinning........................................................................................................ 6 2.3 Electrolyte Composition and Operating Conditions of Electrowinning Process ............ 6 2.3.1 Chloride Ions and Their Effects on the Electrowinning Process ................................ 8 2.3.2 Manganese Ions and Electrolytic MnO2 Film ............................................................. 8 2.3.2.1 Manganese Ions Effects on the Electrowinning Process .................................... 9 2.3.2.2 Manganese Reactions in the Electrowinning Electrolyte ................................. 11 2.3.2.3 Manganese Dioxide Crystallographic Forms.................................................... 13 2.4 Cell Components and Materials .................................................................................... 13 viii  2.4.1 Oxygen-Evolving Anode Materials .......................................................................... 13 2.4.1.1 Lead-Based Alloy Electrodes ........................................................................... 14 2.4.1.2 Dimensionally Stable Anodes (DSA) ............................................................... 20 2.4.1.3 Composite Anodes ............................................................................................ 20 i. Conventional Pb-Metal Oxide Composite Anodes ............................................... 21 ii. Pb-MnO2 Composite Anodes ................................................................................ 22 2.5 Electrowinning Cell Voltage and Power Consumption ................................................ 24 2.5.1 Electrolyte Conductivity and Ohmic Potential Loss ................................................. 26 2.5.2 Cathodic Zinc Reduction Overpotential ................................................................... 27 2.5.3 Anodic Oxygen Evolution Reaction (OER) Overpotential ...................................... 27 2.6 Research Objectives and Novelty ................................................................................. 28 2.6.1 Previous Work .......................................................................................................... 28 2.6.2 Research Objectives .................................................................................................. 29 Chapter 3: Experimental Materials, Apparatus, and Methodology .......................................31 3.1 OER Anode Material Synthesis .................................................................................... 31 3.1.1 Lead-Silver Anodes .................................................................................................. 31 3.1.2 MnOx Electrodeposited Pb-Ag Anodes .................................................................... 32 3.2 OER Electrochemical Measurements ........................................................................... 34 3.2.1 Anodic Half-Cell Electrochemical Experiments ...................................................... 35 3.2.1.1 Anodic Half-Cell Potentiodynamic Polarization (Linear Voltammetry) .......... 36 3.2.1.2 Anodic Half-Cell 72-Hour Galvanostatic Polarization ..................................... 37 3.2.2 Full-Cell Electrowinning Experiments ..................................................................... 37 3.2.2.1 Full-Cell 24-Hour Galvanostatic Polarization .................................................. 38 ix  3.3 Specific Energy Consumption Calculation in Zinc Electrowinning ............................. 39 3.4 Analytical Measurements.............................................................................................. 39 3.5 Physico-Chemical Characterization of the OER Anode Materials ............................... 41 3.5.1 SEM Morphology Characterization and EDX Elemental Mapping ......................... 41 3.5.2 Large Surface Area SEM Characterization and EDX Elemental Mapping .............. 41 3.5.3 Surface Composition and Phase Analysis Using XRD ............................................ 42 Chapter 4: Results and Discussion .............................................................................................43 4.1 Anodic MnOx Electrodeposition Results ...................................................................... 43 4.2 Anodic Surface Layer Physical Characterization ......................................................... 46 4.2.1 Anodic Morphological and Surface Composition Studies Using SEM/EDX .......... 46 4.2.2 Anodic Composition Determination Using ICP-OES Coupled with Aqua Regia Digestion and XRF ............................................................................................................... 49 4.2.3 Surface Composition Analysis Using XRD .............................................................. 50 4.3 Anodic Performance Evaluation for the OER .............................................................. 54 4.3.1 Anodic Potentiodynamic Studies .............................................................................. 55 4.3.2 OER Overpotential Comparison ............................................................................... 59 4.3.3 Half-Cell Anodic 72-Hour Galvanostatic Polarization ............................................. 60 4.3.3.1 In the Presence of Chloride Ions ....................................................................... 60 4.3.3.2 In the Absence of Chloride Ions ....................................................................... 65 4.4 Full Electrowinning Cell Performance Evaluation ....................................................... 68 4.5 Projected Energy Requirements and Energy Savings of the Electrowinning Process.. 72 Chapter 5: Conclusions and Recommendations for Future Research ....................................75 5.1 Conclusions ................................................................................................................... 75 x  5.2 Recommendations ......................................................................................................... 78 Bibliography .................................................................................................................................80 Appendices ....................................................................................................................................91 Appendix A ............................................................................................................................... 91 A.1 CV Electrodeposition in the Absence and Presence of Mn(II) ................................. 91 A.2 EDX Elemental Mapping Results ............................................................................. 92 A.3 Large Surface Area SEM/EDX Analysis .................................................................. 99 A.4 Anodic Half-cell Potentiodynamic Polarization Experimental Data ...................... 106 A.5 Comparison of the 8th, 9th, and 10th LSV Scan of the Baseline and MnOx Deposited Pb-Ag Anodes ..................................................................................................................... 116 A.6 XRF Mapping of the Manganese Mass Loading .................................................... 119 Appendix B ............................................................................................................................. 120 B.1 Conversion to SHE (Primary Reference) from MSE .............................................. 120 B.2 Current Density Selection for Chronopotentiometric Deposition .......................... 121 B.3 OER Overpotential Computation ............................................................................ 122 B.4 IR-Drop Compensation Calculation ....................................................................... 125 B.5 Annual Energy Consumption and Electricity Costs of Zinc Electrowinning Calculation: ......................................................................................................................... 126  xi  List of Tables Table 2-1: Typical industrial operating conditions of electrolytic zinc electrowinning plants [1], [6]. ................................................................................................................................................... 7 Table 4-1: Area-specific manganese mass loading of the MnOx electrodeposited Pb-Ag anodes determined using ICP-OES coupled with aqua regia digestion. ................................................... 49 Table 4-2: Average final potential of the zinc electrowinning cell after 24 hours of galvanostatic polarization at 500 A m-2; Errors represent the range of replicate measurements. ....................... 70 Table 4-3: Projected specific energy consumption and improvements of an electrowinning tank house as a function of the overall cell potential measured at 500 A m-2; Production capacity=287,000 tons [83]. Electricity cost=0.07$ kWh-1 [84]. Zinc current efficiency=0.91 [6]........................................................................................................................................................ 74 Table B-1: Thermodynamic properties required for calculation of the standard equilibrium potential of OER. ........................................................................................................................ 122  xii  List of Figures  Figure 2-1: Overview of the electrolytic zinc extraction process (RLE) [6], [23]. ........................ 5 Figure 2-2: Pourbaix diagram of lead in the presence of sulfate ions at 25℃. Reproduced from Sharpe TF (1973) Chapter I-5 Lead. In: Bard AJ (ed.) Encyclopedia of Electrochemistry of the Elements, vol. 1, pp. 235–347. New York: Marcel Dekker; Figure 1.1.3, p. 242 (Pergamon Press) [60], [61]. ...................................................................................................................................... 18 Figure 2-3: Schematic diagram of the composition of the corrosion film as a function of potential in sulfuric acid electrolyte [62]. .................................................................................................... 19 Figure 2-4: Anodic OER overpotential of various metal oxide electrodes as a function of the enthalpy of the lower to higher oxide transition. (○) Alkaline and (●) acid solutions are indicated. Overpotentials are measured in the low current density region of less than 1 mA/cm2 [68], [69]........................................................................................................................................................ 23 Figure 2-5: Approximate potential distribution within the electrowinning cell at an average current density of 500 A m-2. ........................................................................................................ 26 Figure 3-1: The CANSCI water-jacketed glass electrochemical cell connected to a water bath with the three-electrode setup utilized for the MnOx electrodeposition procedure. ..................... 33 Figure 3-2: The CANSCI water-jacketed glass electrochemical cell connected to a water bath with the three-electrode setup used for anodic half-cell electrochemical measurements. ............ 36 Figure 3-3: The CANSCI water-jacketed glass electrochemical cell connected to a water bath with the two-electrode setup used for full-cell electrochemical measurements. .......................... 38 Figure 3-4: The grid-lined scanning path defined for the large surface area SEM/EDX analysis........................................................................................................................................................ 42 xiii  Figure 4-1: Cyclic voltammetry potentiodynamic electrodeposition procedure displaying variation of measured current density as a function of anodic potential using the Pb-(0.75 wt.%)Ag substrate as the working electrode; Scan rate: 1 mV s-1; Electrodeposition electrolyte: 0.3 M Mn(II) from Mn(CH3COO)2 and 0.1 M Na2SO4; T=25ºC. ................................................ 44 Figure 4-2: Linear sweep voltammetry potentiodynamic electrodeposition procedure displaying variation of measured current density as a function of anodic potential using the Pb-(0.75 wt.%)Ag substrate as the working electrode; Scan rate: 1 mV s-1; Electrodeposition electrolyte: 0.3 M Mn(II) from Mn(CH3COO)2 and 0.1 M Na2SO4; T=25ºC. ................................................ 45 Figure 4-3: One-hour chronopotentiometric electrodeposition procedure at 6 mA cm-2 displaying variation of anodic potential as a function of time using the Pb-(0.75 wt.%)Ag substrate as the working electrode; Electrodeposition electrolyte: 0.3 M Mn(II) from Mn(CH3COO)2 and 0.1 M Na2SO4; T=25ºC. The polarization was initiated from OCP (-0.15 V vs SHE). .......................... 46 Figure 4-4: Morphology (× 1000) of the fresh Pb-Ag (1) and the MnOx CV-deposited (2), CCD-deposited (3), and LSV-deposited (4) Pb-Ag anodes prepared according to the three described electrodeposition techniques in the Methodology section: (a) SEM, (b) Elemental mapping (Pb for the baseline anode and Mn for the MnOx electrodeposited anodes) using EDX. ................... 48 Figure 4-5: Powder X-ray diffraction pattern of the fresh CV-deposited Pb-Ag anode. .............. 52 Figure 4-6: Powder X-ray diffraction pattern of the fresh CCD-deposited Pb-Ag anode. ........... 53 Figure 4-7: Powder X-ray diffraction pattern of the fresh LSV-deposited Pb-Ag anode. ............ 54 Figure 4-8: OER electrocatalytic activity comparison of the baseline and MnOx electrodeposited Pb-Ag anodes indicated by the comparison of (A) 10th linear scan voltammogram, (B) mass activity in the OER potential range; Scan rate=5 mV s-1; Electrolyte composition: 160 g L-1 sulfuric acid solution with 3 g L-1 Mn2+ and 0.3 g L-1 Cl-; T=37℃. ............................................. 58 xiv  Figure 4-9: OER overpotential comparison of the baseline and MnOx electrodeposited Pb-Ag anodes at 10 mA cm-2 calculated using the 8th, 9th, and 10th linear sweep voltammetry scans in the half-cell setup; Scan rate: 5 mV s-1. Electrolyte composition: 160 g L-1 sulfuric acid solution with 3 g L-1 Mn2+ and 0.3 g L-1 Cl-. T=37℃. The vertical error bars represent the standard deviation of triplicate measurements. ............................................................................................................... 60 Figure 4-10: OER half-cell 72-hour galvanostatic polarization of the baseline and the MnOx electrodeposited Pb-Ag anodes at a current density of 500 A m-2; Electrolyte composition: 160 g L-1 sulfuric acid solution with 3 g L-1 Mn2+ and 0.3 g L-1 Cl-. T=37℃. The measured potentials are iR-drop corrected. The vertical error bars indicate the standard deviation of triplicate measurements. ............................................................................................................................... 62 Figure 4-11: Variation of average anodic potential of the baseline and the MnOx electrodeposited Pb-Ag anodes as a function of time during the initial 4 hours of galvanostatic polarization at a current density of 500 A m-2 using the half-cell setup; Electrolyte composition: 160 g L-1 sulfuric acid solution with 3 g L-1 Mn2+ and 0.3 g L-1 Cl-. T=37℃. The measured potentials are iR-drop corrected. The vertical error bars indicate the standard deviation of triplicate measurements. .... 63 Figure 4-12: Final average anodic potential of the baseline and the MnOx electrodeposited Pb-Ag anodes after 72 hours of galvanostatic polarization at a current density of 500 A m-2 using the half-cell setup; Electrolyte composition: 160 g L-1 sulfuric acid solution with 3 g L-1 Mn2+ and 0.3 g L-1 Cl-. T=37℃. The measured potentials are iR-drop corrected. The vertical error bars indicate the standard deviation of triplicate measurements. ......................................................... 64 Figure 4-13: OER half-cell 72-hour galvanostatic polarization of the baseline, MnOx electrodeposited Pb-Ag anodes, and blank (no Mn) CV-deposited Pb-Ag anode at a current density of 500 A m-2; Electrolyte composition: 160 g L-1 sulfuric acid solution with 3 g L-1 Mn2+ xv  and 0.3 g L-1 Cl-. T=37℃. The measured potentials are iR-drop corrected. The vertical error bars indicate the standard deviation of triplicate measurements. ......................................................... 65 Figure 4-14: OER half-cell 72-hour galvanostatic polarization in the absence of chloride ions at a current density of 500 A m-2; Electrolyte composition: 160 g L-1 sulfuric acid solution with 3 g L-1 Mn2+. T=37℃. The measured potentials are iR-drop corrected. The vertical error bars indicate the standard deviation of triplicate measurements. ....................................................................... 67 Figure 4-15: Final average anodic potential of the baseline and the MnOx electrodeposited Pb-Ag anodes after 72 hours of galvanostatic polarization in a chloride-free electrolyte at a current density of 500 A m-2 using the half-cell setup; Electrolyte composition: 160 g L-1 sulfuric acid solution with 3 g L-1 Mn2+. T=37℃. The measured potentials are iR-drop corrected. The vertical error bars indicate the standard deviation of triplicate measurements. ......................................... 68 Figure 4-16: Variation of the average full electrowinning cell potential using the baseline and the MnOx electrodeposited Pb-Ag anodes during the 24 hours of galvanostatic polarization at a current density of 500 A m-2; Cathode electrode: aluminum. Electrolyte composition: 160 g L-1 sulfuric acid solution with 55 g L-1 Zn2+, 3 g L-1 Mn2+, and 0.3 g L-1 Cl-. T=37℃. Error bars represent the range of replicate measurements. The potentials are not corrected for iR-drop losses. ............................................................................................................................................ 69 Figure 4-17: Appearance of the full-cell zinc electrowinning setup prior and after the 24-hour galvanostatic polarization at 500 A m-2 using either the baseline or novel Pb-Ag anodes; (a) Prior to the electrolysis, (b) After the electrolysis using the baseline Pb-Ag anode, (c) After the electrolysis using the novel MnOx CV-deposited Pb-Ag anode. .................................................. 72 Figure A-1: Comparison of the first cycle of the CV electrodeposition procedure at 1 mV s-1 using the Pb-(0.75 wt.%)Ag substrate as the working electrode inside the electrodeposition xvi  electrolyte composed 0.1 M Na2SO4 with and without 0.3 M Mn(II) from Mn(CH3COO)2 at room temperature. ......................................................................................................................... 91 Figure A-2: EDX elemental mapping of the baseline Pb-Ag anode. ............................................ 93 Figure A-3: EDX elemental mapping of the CV deposited Pb-Ag anode. ................................... 94 Figure A-4: EDX elemental mapping of the LSV deposited Pb-Ag anode. ................................. 96 Figure A-5: EDX elemental mapping of the CCD deposited Pb-Ag anode. ................................ 98 Figure A-6: Potentiodynamic polarization curves (10 LSV scans) of the baseline Pb-Ag anode inside the 160 g L-1  sulfuric acid electrolyte with 3 g L-1 Mn2+ and 0.3 g L-1 Cl- at 37℃ at 1 mV s-1 scan rate. ................................................................................................................................. 106 Figure A-7: Potentiodynamic polarization curves (10 LSV scans) of the baseline Pb-Ag anode inside the 160 g L-1  sulfuric acid electrolyte with 3 g L-1 Mn2+ and 0.3 g L-1 Cl- at 37℃ at 1 mV s-1 scan rate. ................................................................................................................................. 107 Figure A-8: Potentiodynamic polarization curves (10 LSV scans) of the baseline Pb-Ag anode inside the 160 g L-1  sulfuric acid electrolyte with 3 g L-1 Mn2+ and 0.3 g L-1 Cl- at 37℃ at 1 mV s-1 scan rate. ................................................................................................................................. 108 Figure A-9: Potentiodynamic polarization curves (10 LSV scans) of the MnOx CV deposited Pb-Ag inside the 160 g L-1  sulfuric acid electrolyte with 3 g L-1 Mn2+ and 0.3 g L-1 Cl- at 37℃ at 1 mV s-1 scan rate. .......................................................................................................................... 109 Figure A-10: Potentiodynamic polarization curves (10 LSV scans) of the MnOx CV deposited Pb-Ag anode inside the 160 g L-1  sulfuric acid electrolyte with 3 g L-1 Mn2+ and 0.3 g L-1 Cl- at 37℃ at 1 mV s-1 scan rate. .......................................................................................................... 110 xvii  Figure A-11: Potentiodynamic polarization curves (10 LSV scans) of the MnOx CV deposited Pb-Ag anode inside the 160 g L-1  sulfuric acid electrolyte with 3 g L-1 Mn2+ and 0.3 g L-1 Cl- at 37℃ at 1 mV s-1 scan rate. .......................................................................................................... 111 Figure A-12: Potentiodynamic polarization curves (10 LSV scans) of the MnOx CCD deposited Pb-Ag anode inside the 160 g L-1  sulfuric acid electrolyte with 3 g L-1 Mn2+ and 0.3 g L-1 Cl- at 37℃ at 1 mV s-1 scan rate. .......................................................................................................... 112 Figure A-13: Potentiodynamic polarization curves (10 LSV scans) of the MnOx CCD deposited Pb-Ag anode inside the 160 g L-1  sulfuric acid electrolyte with 3 g L-1 Mn2+ and 0.3 g L-1 Cl- at 37℃ at 1 mV s-1 scan rate. .......................................................................................................... 113 Figure A-14: Potentiodynamic polarization curves (10 LSV scans) of the MnOx LSV deposited Pb-Ag anode inside the 160 g L-1  sulfuric acid electrolyte with 3 g L-1 Mn2+ and 0.3 g L-1 Cl- at 37℃ at 1 mV s-1 scan rate. .......................................................................................................... 114 Figure A-15: Potentiodynamic polarization curves (10 LSV scans) of the MnOx LSV deposited Pb-Ag anode inside the 160 g L-1  sulfuric acid electrolyte with 3 g L-1 Mn2+ and 0.3 g L-1 Cl- at 37℃ at 1 mV s-1 scan rate. .......................................................................................................... 115 Figure A-16: OER electrocatalytic activity comparison of the baseline and MnOx electrodeposited Pb-Ag anodes indicated by the comparison of their 8th linear scan voltammogram in the OER potential range; Scan rate=5 mV s-1; Electrolyte composition: 160 g L-1 sulfuric acid solution with 3 g L-1 Mn2+ and 0.3 g L-1 Cl-; T=37℃. ..................................... 116 Figure A-17: OER electrocatalytic activity comparison of the baseline and MnOx electrodeposited Pb-Ag anodes indicated by the comparison of their 9th linear scan voltammogram in the OER potential range; Scan rate=5 mV s-1; Electrolyte composition: 160 g L-1 sulfuric acid solution with 3 g L-1 Mn2+ and 0.3 g L-1 Cl-; T=37℃. ..................................... 117 xviii  Figure A-18: OER electrocatalytic activity comparison of the baseline and MnOx electrodeposited Pb-Ag anodes indicated by the comparison of their 10th linear scan voltammogram in the OER potential range; Scan rate=5 mV s-1; Electrolyte composition: 160 g L-1 sulfuric acid solution with 3 g L-1 Mn2+ and 0.3 g L-1 Cl-; T=37℃. ..................................... 118 Figure A-19: 2-Dimensional (side-view) mapping of the manganese mass loading on the surface of the CV-deposited Pb-Ag anode obtained using the XRF equipment. .................................... 119 Figure A-20: 2-Dimensional (top-view) mapping of the manganese mass loading on the surface of the CV-deposited Pb-Ag anode obtained using the XRF equipment. .................................... 120  xix  List of Symbols Symbol Definition Unit E or V  Equilibrium potential V E  Specific energy consumption (Equation 3-2) kWh kg-1 𝐸𝐸° Standard equilibrium potential V F Faraday’s constant (F = 96,485 C mol-1) C mol-1 I Superficial current density  A m-2 iL Mass transfer limiting current density  A m-2 Km Mass transfer coefficient m s -1 M Molar mass kg kmol-1 m  Mass kg P Specific power consumption kW kg-1 Rs Uncompensated solution resistance Ω cm-2 T Temperature ºC z  Moles of electrons transferred mol t Time s or h η Overpotential V ∆𝜂𝜂  Sum of anodic and cathodic overpotentials  V ∆𝐸𝐸Ω  Potential drop due to ohmic, ionic, and electrical resistance V ∆𝐸𝐸𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐,𝑜𝑜𝑜𝑜𝑜𝑜  Operating overall cell potential  V ∆𝐸𝐸𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐,𝑟𝑟𝑐𝑐𝑟𝑟 Reversible thermodynamic overall cell potential V ∆𝐸𝐸𝑜𝑜  Anodic potential variations due to electrode degradation V xx  List of Abbreviations Abbreviation Definition AMO Amorphous Manganese Oxide CCD Constant Current Density Polarization (Chronopotentiometry)  CE Counter Electrode CER Chlorine Evolution Reaction CV Cyclic Voltammetry DI  Deionized DSA Dimensionally Stable Anode ECE Electrochemical-Chemical-Electrochemical EDX Energy-Dispersive X-ray Spectroscopy  EIS Electrochemical Impedance Spectroscopy EMD Electrolytic Manganese Dioxide EW Electrowinning HER Hydrogen Evolution Reaction ICDD International Centre for Diffraction Data ICP-OES Inductively Coupled Plasma - Optical Emission Spectrometry ICSD Inorganic Crystal Structure Database IR-corrected Ohmic Drop Compensated LSV Linear Sweep/Scan Voltammetry MSE Mercury/Mercury Sulfate Reference Electrode (Hg/Hg2SO4) OCP Open Circuit Potential xxi  OER Oxygen Evolution Reaction REF Reference Electrode RHE Reversible Hydrogen Reference Electrode RLE Roasting-Leaching-Electrowinning SEM Scanning Electron Microscopy SHE Standard Hydrogen Reference Electrode WE Working Electrode XRD X-ray Diffraction Spectroscopy XRF X-ray Fluorescence Spectroscopy xxii  Acknowledgements I humbly express my deepest gratitude towards my supervisor, Dr, Elöd Gyenge, for his exceptional guidance, mentorship expertise, and immense knowledge. It is truly an honor and privilege to be able to shape my professional expertise and career aspirations under his supervision. I would like to express my sincere gratitude to Dr. Edouard Asselin and Dr. Pooya Benhangi-Hosseini for their insightful comments and providing me with the appropriate means to make this research possible. I would also like to thank Dr. Fariborz Taghipour and Dr. Edouard Asselin who have agreed to serve as committee members on my thesis examination defense session. I gratefully acknowledge the University of British Columbia and the Natural Science and Engineering Research Council of Canada (NSERC) for their generous financial support in order to pursue this research project.   I am also very grateful to Dr. Alberto Gonzalez from Teck Resources Limited for providing us with Teck’s applied research and technology expertise and for their in-kind contribution of the raw materials. My sincere thanks are also extended to my classmates and all the members of the Dr. Gyenge and Dr. Wilkinson’s Electrochemical Engineering groups, especially Yu Pei, Dr. Arman Bonakdarpour, and Dr. Lius Daniel and members of the UBC Materials Engineering Department namely, Heli Eunike, Ronny Winarko, and Kresimir Ljubetic for their kind support and extensive assistance throughout my research.  Last, but certainly not least, special thanks are owed to my beloved family: mother, father, brother and my partner, who stood by me throughout all the challenges I have had while seeking my dreams far away from home. I am forever grateful for all their sacrifices, emotional and financial supports that they have provided me throughout this journey of a life. xxiii  Dedication To my beloved parents and brother, for their patience, support, and sacrifices that empowered me through these years.  To my loving partner,  without whose unconditional love, this success would have been impossible.  To all my caring friends, for cheering me up through the most difficult times.  And,  to all the precious lives that are lost in the battle with COVID-19. May we get to see people’s beautiful smiles beyond their masks soon enough.1  Chapter 1: Introduction According to Natural Resources Canada, 13.2 million tons of refined zinc metal is globally produced in 2018, with China, South Korea, India, and Canada ranked among the largest producers of refined zinc [3]. Annually 7 million tons of  metallic zinc is produced through the aqueous electrowinning procedure [1], [4]. Zinc electrowinning (EW) is an electrolytic process in which high purity zinc metal is electrodeposited on cathodic electrodes from an acidic sulfate-based electrolyte. The main anodic and cathodic half-cell reactions in this process are the oxygen evolution reaction (OER) and zinc reduction, respectively [4], [5]. Zinc electrowinning is a very power-intensive process and it accounts for nearly 60% of the energy requirements of the zinc refining plants [1]. This process is typically operated at a constant current density in the range of 450 to 550 A m-2, thus the energy consumption of the electrowinning cell is dictated by the resulting cell potential [6].  An overall potential in the range of 3.2 to 3.7 V is conventionally observed during the operation of the industrial electrowinning cells [4]. An estimation of the energy loss within this process can be obtained by subtracting the reversible thermodynamic potential difference between the cathodic and anodic half-cell reactions from the measured industrial cell potential. The major sources of potential loss include anodic and cathodic overpotentials, ohmic potential drop in the electrolyte, and the potential loss through the electronic hardware. At a given zinc production rate, minimizing the cell potential through reducing the potential losses leads to lowered energy consumption and improved energy efficiency of the electrowinning process [1], [7], [8].   Various literature reports have taken a closer look into the components contributing to the overall cell potential and they have conveyed that the OER overpotential on the conventional lead-silver anodes accounts for nearly 20-25% of the electrowinning cell potential [1], [2]. The OER 2  overpotential is dependent on the type of the anode material that is being implemented in the cell and its electrocatalytic performance in facilitating the OER reaction [9]. Lead-based alloys are widely used as the electrowinning anodes due to their desired properties in the harsh sulfuric acid media and operating conditions of the electrowinning process. These characteristics include low solubility and high stability in acidic electrolytes, high conductivity, and low material cost. High OER overpotential and degradation resistance are the main disadvantages associated with using lead-based alloys as anodic materials [4], [10]–[13]. The most recent industrial electrowinning processes utilize lead-based anodes containing 0.5 to 1.0 wt. % silver in order to improve the corrosion resistance and electrocatalytic behavior of the conventional anodes [4], [12], [14]. Various anode materials have been researched in order to improve the OER electrocatalytic performance and the resulting energy consumption of the electrowinning process [9], [10], [15], [16]. Meanwhile, composite anodes have been developed consisting of various types of metal oxide particles with superior OER electrocatalytic behavior dispersed in a highly conductive and chemically stable lead-based alloy matrix and have been the subject of many studies [17]–[20]. Owing to its several special properties, manganese oxide is one of the most popular metal oxide candidates that is incorporated onto conventional electrowinning anodes. Manganese is an inexpensive, earth abundant, and environmentally friendly element and it exhibits various oxidation states [21]. According to various studies, manganese oxides reveal excellent electrocatalytic activity for the OER in alkaline and acidic media. Thus, extensive research efforts have focused on studying manganese oxides as non-precious metal catalysts with high performance to be utilized in commercial OER applications [18], [21], [22]. As manganese ion is an already-present impurity in the zinc electrowinning electrolyte, its dissolution does not interfere with the electrowinning process. Thus, various researchers integrated manganese oxides into the 3  matrix of conventional lead-based alloys using techniques such as powder pressing, spraying, accumulative roll-bonding and so on and evaluated their durability, electrocatalytic performance, and corrosion resistance behavior under the zinc electrowinning operating conditions [2], [17]–[19].  This work aims to use an anodic electrodeposition technique in order to prepare novel composite anodes made up of dispersed oxides of manganese on industrially provided Pb-Ag electrode substrate. It is hoped that the novel MnOx electrodeposited Pb-Ag anodes improve the OER electrocatalytic behavior of the conventional Pb-Ag anodes under the zinc electrowinning conditions. Therefore, this project employs various electrochemical techniques to evaluate the OER performance and the full electrowinning cell potential when utilizing the novel anodes as compared to the industrially employed Pb-Ag anodes. The results are used in order to determine the improvements in specific energy consumption of the electrowinning cell assuming the stable cell potential is maintained throughout the operation. Finally, various surface characterization techniques are employed to investigate the morphology and elemental composition of the MnOx electrodeposited lead-silver anodes. Additionally, this report provides several recommendations on how to better investigate the applicability of the novel anodes for industrial purposes.         4  Chapter 2: Literature Review 2.1 Electrolytic Zinc Refining Process Zinc has a wide range of commercial applications including but not limited to galvanizing iron and steel and production of various metal alloys such as brass and bronze [23]. To a lesser extent, zinc is utilized as an additive in the manufacturing of fertilizers, tires, and other chemicals. In 2018, approximately 267 thousand tons of mined zinc and 696 thousand tons of refined zinc was produced in Canada, which accounts for respectively 2.2% and 5.3% of the global zinc production. Major Canadian zinc refineries are located in provinces such Manitoba, Quebec, Ontario, and New Brunswick [3].  Zinc ores majorly constitute of sphalerite (zinc sulfide, ZnS) and traces of other metal impurities such as copper, iron, cobalt, and nickel [24].  Zinc recovery from sphalerite ores is primarily carried out through an electrolytic process with three main stages of roasting, leaching, and electrowinning (RLE) [4], [24]. During the roasting step, grinded sphalerite ores are converted into zinc calcine (zinc oxide, ZnO) at temperatures as high as 1000℃. In the leaching process, zinc and other metallic ions are deliberated into a sulfuric acid solution. Metallic impurities such as copper, cobalt, and nickel can lower the purity of the deposited zinc and reduce the current efficiency of zinc electrowinning. Thus, majority of the impurities present in the resulting leached electrolyte are removed within a purification stage. Lastly, in the electrowinning step, relatively pure zinc is electrodeposited on the surface of cathodes from the sulfate-based leaching electrolyte. Within an automated operation, the deposited zinc product is stripped off of the cathodic electrodes periodically (every 48 hours). The resulting zinc is melted in electric furnaces and casted with or without addition of alloying metals [6]. Figure 2-1 schematically illustrates the steps involved in the electrolytic zinc extraction process (RLE). 5     Figure 2-1: Overview of the electrolytic zinc extraction process (RLE) [6], [23].  Impurities (Cd, Co, Cu, Ni, etc.) Zinc Cathodes Zinc Dust Gas Cleaning Sulfuric Acid Production By-product Sulfuric Acid Spent Electrolyte Zinc Oxide Zinc Metal Zinc Sulfide Roasting Grinding and Floatation   Purification Electrowinning Leaching Melting and Casting Fresh Sulfuric Acid  Sulfur Dioxide Zinc Sulfate Air Sulfur Dioxide 6  2.2 Zinc Electrowinning Zinc electrowinning (EW) is electrolytic reduction or deposition of metallic zinc from an acidic sulfate-based electrolyte. This electrolysis process is generally performed inside open tank houses of connected cells using aluminum cathode and lead-based anode busbars. Zinc reduction (Reaction 2-1) is the main cathodic reaction, and water decomposition or namely oxygen evolution reaction (OER) (Reaction 2-2) is the principle anodic reaction. Therefore, the overall reaction inside a sulfate-based medium results in zinc being deposited on the cathode and oxygen deliberated on the anode, as shown in Reaction 2-3. In addition to the mentioned primary redox reactions, other reactions may take place on the surface of these electrodes. These competing reactions inevitably reduce the current efficiency of the electrowinning process. Hydrogen evolution reaction (HER) or proton reduction, as shown in Reaction 2-4, is a competing cathodic reaction with a thermodynamically favorable standard potential. However, on the surface of zinc and at the given electrowinning operating conditions, HER or hydrogen deposition (Reaction 2-4) possesses high overpotential. Thus, this reaction proceeds at significantly slower rates, as compared to the desired zinc reduction (Reaction 2-1) [1], [5], [6], [18], [23], [25].  𝑍𝑍𝑍𝑍2+ + 2𝑒𝑒− → 𝑍𝑍𝑍𝑍 ; 𝐸𝐸298𝐾𝐾,   𝑍𝑍𝑍𝑍2+/𝑍𝑍𝑍𝑍° = −0.76 𝑉𝑉𝑆𝑆𝐻𝐻𝐻𝐻                   (Reaction 2-1) 𝐻𝐻2𝑂𝑂 →12𝑂𝑂2 + 2𝐻𝐻+ + 2𝑒𝑒−; 𝐸𝐸298𝐾𝐾,   𝐻𝐻2𝑂𝑂/𝑂𝑂2° = 1.23 𝑉𝑉𝑆𝑆𝐻𝐻𝐻𝐻                                 (Reaction 2-2) 𝑍𝑍𝑍𝑍𝑍𝑍𝑂𝑂4 + 𝐻𝐻2𝑂𝑂 →   𝑍𝑍𝑍𝑍 + 12 𝑂𝑂2 + 𝐻𝐻2𝑍𝑍𝑂𝑂4;  𝐸𝐸298𝐾𝐾,   𝐻𝐻2𝑂𝑂/𝑂𝑂2° = 1.99 𝑉𝑉𝑆𝑆𝐻𝐻𝐻𝐻                   (Reaction 2-3)  2𝐻𝐻+ + 2𝑒𝑒− → 𝐻𝐻2; 𝐸𝐸298𝐾𝐾,   𝐻𝐻+/𝐻𝐻2° = 0 𝑉𝑉𝑆𝑆𝐻𝐻𝐻𝐻                                (Reaction 2-4) 2.3 Electrolyte Composition and Operating Conditions of Electrowinning Process The composition of the electrolyte of industrial zinc electrowinning process is described in    Table 2-1. Manganese and chloride ions are other impurities that typically exist in the zinc 7  electrowinning electrolyte and significantly affect this process. Manganese ions either originate from the ores or are intentionally added to lower the corrosion rates of conventional electrowinning anodes. Chloride ions are originated from the sphalerite ores or wash water. Oxidation of manganese and chloride ions on the surface of lead-based anodes will be discussed in the later sections of this report [26], [27]. Table 2-1: Typical industrial operating conditions of electrolytic zinc electrowinning plants [1], [6]. Parameter Industrial Range (Unit) Sulfuric Acid Concentration [H2SO4] 160-165 (g L-1) Zinc Concentration [Zn2+] 50-60 (g L-1) Cell Temperature 37-40 (℃) Current Density 450-550 (A m-2) Cell Voltage 3.2-3.5 (V) Current Efficiency 88-93% Energy Use 3000-3300 (kWh t-1) Deposition Time 35-48 (hrs) Number of Cathodes per Cell 40-120  Cathode Materials Aluminum (Al) Anode Materials Lead-Silver (Pb-Ag (0.5-0.75 wt.% ))  As indicated in Table 2-1, zinc electrowinning is a galvanostatic process, which typically operates at a high current density in the range of 450 to 550 A m-2. At this current density, a mean industrial cell voltage of 3.2-3.5 V is measured, which makes this process very power intensive. Furthermore, a current efficiency range of 88 to 93% with respect to zinc deposition is industrially reported for this electrowinning operation [1], [4], [6]. 8  2.3.1 Chloride Ions and Their Effects on the Electrowinning Process Chloride ions are typically present in the industrial electrowinning electrolyte in the range of 1 to 500 mg L-1 [28]. Oxidation of chloride ions (Reaction 2-5), namely chlorine evolution reaction (CER), on the surface of the conventional lead-based anodes generates hazardous chlorine gas and accelerates the anodic corrosion rates [26]–[29]. Moreover, reduction of the resulting dissolved chlorine gas (reverse of Reaction 2-5) on the surface of cathodic electrodes lowers the zinc electrowinning current efficiency [26]–[28]. Higher chlorine content of the electrolyte also leads to an increased contamination of cathodic zinc with dissolved lead [10]. However, it has been shown than addition of manganese ions to the electrolyte significantly lowers the anodic oxidation rate of chloride ions. This can be explained by anodic deposition of the electrolytic manganese dioxide (EMD) layer, which acts as a diffusion barrier to chloride ion transport and occurrence of CER [26]–[28]. 2𝐶𝐶𝐶𝐶− → 𝐶𝐶𝐶𝐶2 + 2𝑒𝑒−; 𝐸𝐸298𝐾𝐾,   𝐶𝐶𝑐𝑐−/𝐶𝐶𝑐𝑐2° =  1.36 𝑉𝑉𝑆𝑆𝐻𝐻𝐻𝐻                       (Reaction 2-5) 2.3.2 Manganese Ions and Electrolytic MnO2 Film Manganese is the second most abundant heavy metal in the earth’s crust and is one of the impurities present in the leach liquor of the RLE process for zinc extraction [23], [30]. In this process, either manganese is originated from the zinc sphalerite ores or it is intentionally added to the leaching liquor. Manganese dioxide (MnO2) or potassium permanganate (KMnO4) are typically introduced to the zinc sulfate electrolyte as the source of manganese in order to reduce the anodic corrosion rates [23], [31]. The concentration of manganese (Mn2+) in the zinc sulfate electrolyte varies substantially and it is adjusted according to the impurities in the leach liquor 9  [32]. Mackinnon and Brannen [32] reported that 1-3 g L-1 of Mn2+ is optimum in minimizing the anodic lead corrosion rates and contamination of the deposited zinc by the dissolved Pb2+. 2.3.2.1 Manganese Ions Effects on the Electrowinning Process  Oxidation of manganese ions on the surface of anodes leads to formation of the protective anodic MnO2 layer or EMD [12], [29], [33], [34]. Manganese electrodeposition seems to take place only when some PbO2 is formed on the anodic surface [34]. Manganese oxidation with a corresponding partial current density of as low as 1 mA.cm-2, can be beneficial to the electrowinning process through various effects [35]. The resulting MnO2 film passivates the anodic surface and is shown to retard the corrosion and dissolution rates of lead-based anodes [12], [23], [25], [29], [33], [34], [36]. McGinnity and Nicol [37] reported than OER occurs on the resulting PbO2/MnO2 layer, which hinders the diffusion of oxygen atoms to the lead surface and further oxidation of lead. Additionally, the amount of lead in the zinc product is minimized in the Mn-containing electrolytes [33]. Moreover, the loss of chloride ions in the electrolyte in the form of chlorine gas is shown to be hindered in the presence of manganese (II) ions [26]. This observation can be explained by the higher activation energy of CER on the anodic MnO2 film than the passivated PbO2 layer [38]. Thus, OER is favored over CER on the surface of lead-based anodes covered with electrolytic MnO2 [26]. Furthermore, the release of dissolved chlorine gas is limited by the scavenging reactions involving manganese (II) ions, as suggested by Reaction 2-6 [26].  𝑀𝑀𝑍𝑍2+ + 𝐶𝐶𝐶𝐶2 + 2𝐻𝐻2𝑂𝑂 ↔ 𝑀𝑀𝑍𝑍𝑂𝑂2 + 4𝐻𝐻+ + 2𝐶𝐶𝐶𝐶−;                                        (Reaction 2-6) Lastly and most importantly for this project, various literature reported high electrocatalytic activity of MnO2 deposited electrodes in catalyzing the OER in alkaline media  [21], [22], [39], [40]. Various authors showed that lead-based anodes in Mn-containing electrolytes exhibit lower OER overpotential and that the resulting MnO2 film is conductive and 10  electrocatalytically active for the OER [25], [35], [41]. This catalytic effect is more prominent with increasing the manganese concentration of the electrolyte, up to a saturation level exceeding 5 g.dm-3 [35]. Cachet at al. [35] concluded that the observed catalytic impact stems from the stimulated rate constant of OER on the generated active sites of the formed MnO2 layer.      In addition to the anodic MnO2 layer, oxidation of manganese ions can lead to formation of suspended MnO2 particles in the electrolyte, commonly known as cell mud or sludge [34]. The sludge might be the result of spalling and detachment of the gradually thickened anodic film containing oxide and sulfate species as oxygen gets evolved on the anode [34]. These MnO2 particles may remain suspended in the electrolyte or they may nucleate and deposit on various components of the tank house. The rate of sludge generation is dictated by the mechanical stability and degree of adherence of the anodic electrodeposited MnO2 film [37]. Analysis of the slime has indicated that amorphous manganese oxide (AMO) makes up nearly 80% of its composition [11]. Increased levels of manganese in the electrolyte results in higher sludge formation, which shortens the cells cleaning cycle [42]. MnO2 slime may benefit the zinc electrowinning process by absorbing electrodepositive ions such as Co2+, Cu2+, Ni2+, and Sb2+ and reducing the detrimental effects of chloride ions [26], [43].  Depending on the amount, presence of manganese ions may adversely have detrimental effects on the electrowinning process and the manganese concentration may need to be adjusted when reaching a specific level [33]. Excessive amounts of Mn2+(>4 g L-1) increases the formation of anodic sludge and the frequency of electrode cleaning [32]. Furthermore, high concentrations of Mn2+ can result in a reduction in zinc deposition current efficiency as some of the higher valence manganese species get reduced on the cathode [32]. Excessive growth of the electrolytic MnO2 11  film on the anode may increase ohmic resistance or cause electrical shorting and damage to the electrodes [23].      2.3.2.2 Manganese Reactions in the Electrowinning Electrolyte In sulfuric acid solution and on the surface of Pb/PbO2 anodes, manganese (II) ions are oxidized to form manganese dioxide and permanganate [26]. When the potential is swept in the positive (anodic) direction, MnO2 electrodeposition takes place according to the overall reaction as shown in Reaction 2-7 [34], [44]: 𝑀𝑀𝑍𝑍2+ + 2𝐻𝐻2𝑂𝑂 → 𝑀𝑀𝑍𝑍𝑂𝑂2 + 4𝐻𝐻+ + 2𝑒𝑒−; 𝐸𝐸298𝐾𝐾,   𝑀𝑀𝑍𝑍2+/𝑀𝑀𝑍𝑍𝑂𝑂2° = 1.22 𝑉𝑉𝑆𝑆𝐻𝐻𝐻𝐻         (Reaction 2-7)     Since Reaction 2-7 involves transfer of two electrons, it is highly unlikely that the reaction takes place in a single step [45], [46]. Thus, various mechanisms have been proposed in order to describe the sequence of steps involved in MnO2 electrodeposition. Gibson et al. [47] reported that the prevailing electrodeposition mechanism is the reason behind the diverse structure and morphology of the deposited MnOx film. At low acid concentrations, the Electrochemical-Chemical-Electrochemical (ECE) mechanism is anticipated to take place, as proposed by the sequence of Reaction 2-8 to Reaction 2-10. This process involves the electrochemical oxidation of Mn2+ (Reaction 2-8) followed by hydrolysis of Mn3+ precursor and formation of the MnOOH insulating intermediate (Reaction 2-8), and finally subsequent nucleation of MnO2 through oxidation of MnOOH, according to the Reaction 2-10 [21], [23], [34], [44], [47], [48]:  12  𝑀𝑀𝑍𝑍2+ → 𝑀𝑀𝑍𝑍3+ + 𝑒𝑒− ; 𝐸𝐸298𝐾𝐾.  𝑀𝑀𝑍𝑍2+/𝑀𝑀𝑍𝑍3+° = 1.51 𝑉𝑉𝑆𝑆𝐻𝐻𝐻𝐻                          (Reaction 2-8) 𝑀𝑀𝑍𝑍3+ + 2𝐻𝐻2𝑂𝑂 → 𝑀𝑀𝑍𝑍𝑂𝑂𝑂𝑂𝐻𝐻 +  3𝐻𝐻+                                      (Reaction 2-9) 𝑀𝑀𝑍𝑍𝑂𝑂𝑂𝑂𝐻𝐻 → 𝑀𝑀𝑍𝑍𝑂𝑂2 + 𝐻𝐻+ + 𝑒𝑒−; 𝐸𝐸298𝐾𝐾,   𝑀𝑀𝑍𝑍𝑂𝑂𝑂𝑂𝐻𝐻/𝑀𝑀𝑍𝑍𝑂𝑂2° = 0.95  𝑉𝑉𝑆𝑆𝐻𝐻𝐻𝐻                              (Reaction 2-10)  Rodrigues et al. [46] claimed that inside a strongly acidic medium, Mn3+ ions are very unstable. Thus, Reaction 2-8 can proceed by disproportionation of Mn3+ ions (Reaction 2-11) to generate Mn4+ and Mn2+ species [46], [49]. Finally, MnO2 deposition occurs as the result of immediate hydrolysis of the resulting unstable Mn4+ ions, according to Reaction 2-12 [21], [45], [47]. 2𝑀𝑀𝑍𝑍3+ → 𝑀𝑀𝑍𝑍4+ + 𝑀𝑀𝑍𝑍2+                         (Reaction 2-11) 𝑀𝑀𝑍𝑍4+ + 2𝐻𝐻2𝑂𝑂 ↔ 𝑀𝑀𝑍𝑍𝑂𝑂2 + 4𝐻𝐻+                            (Reaction 2-12) In an alternative mechanism for MnO2 electrodeposition, Mn2+ ions are oxidized to form initially visible purple permanganate (MnO4-) in the first step of the sequence and at high oxidation potentials (Reaction 2-13) [25], [26], [37], [45]. The resulting permanganate ions may chemically react with the Mn2+ ions to form  wine-red Mn3+ ions, according to Reaction 2-14 [26], [37]. Lastly, Mn3+ ions can hydrolyze and disproportionate to form a black anodic MnO2 film, as shown in  Reaction 2-15    [26], [37], [50]. Upon the formation of a uniform anodic MnO2 layer, generation of MnO4- and Mn3+ are no longer visible [37].   𝑀𝑀𝑍𝑍2+ + 4𝐻𝐻2𝑂𝑂 → 𝑀𝑀𝑍𝑍𝑂𝑂4− + 8𝐻𝐻+ + 5𝑒𝑒−; 𝐸𝐸298𝐾𝐾,   𝑀𝑀𝑍𝑍2+/𝑀𝑀𝑍𝑍𝑂𝑂4−° = 1.512 𝑉𝑉𝑆𝑆𝐻𝐻𝐻𝐻                    (Reaction 2-13) 𝑀𝑀𝑍𝑍𝑂𝑂4− + 4𝑀𝑀𝑍𝑍2+ + 8𝐻𝐻+ → 5𝑀𝑀𝑍𝑍3+ + 4𝐻𝐻2𝑂𝑂                               (Reaction 2-14)  2𝑀𝑀𝑍𝑍3+ + 2𝐻𝐻2𝑂𝑂 ↔ 𝑀𝑀𝑍𝑍2+ + 𝑀𝑀𝑍𝑍𝑂𝑂2 + 4𝐻𝐻+                                  (Reaction 2-15) Furthermore, Tompkins [51] explained that the suspended electrolytic MnO2 particles in the zinc electrowinning electrolyte are formed as a result of the reaction of Mn(II) ions with the permanganate ions, known as the Guyard reaction (Reaction 2-16) [51]: 13  2𝑀𝑀𝑍𝑍𝑂𝑂4− + 3𝑀𝑀𝑍𝑍2+ + 2𝐻𝐻2𝑂𝑂 → 5𝑀𝑀𝑍𝑍𝑂𝑂2 + 4𝐻𝐻+;                (Reaction 2-16) 2.3.2.3 Manganese Dioxide Crystallographic Forms Six crystallographic forms of MnO2 exist, namely R (ramsdellite), β (pyrolusite), γ (electrolytic), δ (birmessite), ε and λ (spinel type). The structure of ramsdellite and pyrolusite forms of MnO2 possess one-directional tunnels consisting of octahedra (MnO6) [47], [52]. Electrolytic MnO2 (EMD; γ – MnO2) constitutes of an intergrowth between ramsdellite and pyrolusite MnO2. The resulting structural dislocations lead to formation of defect sites in the structure of EMD, which accounts for high activity of this MnO2 form [47], [53]. Other forms of MnO2 such as δ and λ reveal 2D layered and 3D spinel structures, respectively [52]. The electrochemical properties of the MnOx film and its ability to catalyze the OER vary greatly with its physiochemical properties especially its crystallographic form [40], [54]. Electrolytic manganese dioxide grown on the surface of anodes in zinc electrowinning conditions is reported to be of  γ − MnO2 type from XRD analysis [26], [42], [48], [55], [56]. Jorgensen [55] reported that a composition gradient in the electrodeposited MnOn layer exists as a function of anodic potential variations. According to this his article, a decrease in n from 1.95 near the substrate/deposit interface to 1.90 at the deposit/electrolyte interface was observed [23], [55]. This phase refers to a solution of manganese oxide species with the general formula of MnOn. (2 −n)H2O, and a varied homogeneity ranging from n = 2 (MnO2) to n = 1.5 (MnO(OH)) [26], [57]. 2.4 Cell Components and Materials  2.4.1 Oxygen-Evolving Anode Materials Electrowinning processes are nominally conducted at high potentials and inside acidic solutions with reported negative pH values. The harsh electrowinning operating conditions necessitates the anode electrodes to possess adequate electrical conductivity, mechanical integrity, 14  and corrosion resistance. In order to lower the anodic overpotential and improve the energy efficiency of this process, it is also critical for the electrowinning anodes to reveal excellent electrocatalytic activity for OER [10], [16]. Lead-based electrodes are the dominant insoluble anode materials employed for electrowinning of copper and zinc [10], [11], [23], [34]. Pure lead electrodes although inexpensive, they lack the physical strength essential for industrial electrowinning. Excessive corrosion rate of pure lead electrodes imposes high financial burden on the zinc refining plants associated with maintenance of anodes. Furthermore, contamination of deposited zinc with lead is resulted from high dissolution levels of pure lead electrodes [10], [12], [16], [23], [42].  Due to the mentioned disadvantages associated with the use of pure lead electrodes, various lead-based anodes and other alternative anode materials have been extensively researched for oxygen evolution in acidic electrolytes [10], [11], [16]. The motivation for this search is to find anode materials that are dimensionally stable and have low OER overpotential. For instance, alloying lead with elements of higher conductivity has revealed to significantly improve the mechanical stability and durability of pure lead electrodes and lower their OER overpotential [4], [7], [16]. Furthermore, various fabrication techniques such as rolling and casting are utilized in order to manufacture industrial lead-based electrowinning anodes. The choice of fabrication technique can impact the mechanical properties, grain structure, and corrosion behavior of the resulting electrodes [12], [16].  2.4.1.1 Lead-Based Alloy Electrodes The use of lead-based alloys as the most common insoluble anodic material in sulfate-based electrolyte dates back to 1909 [4], [29]. Various alloying elements have been researched in an effort to improve the energy and cost efficiency of the electrowinning process through reducing 15  the anodic corrosion rates and OER overpotential [16]. Metals with three types of influence are typically considered for alloying with lead anodes [10]: 1) Metals with electrocatalytic effects: Metals such as Ag, Co, Pt, and Au that are shown to lower lead corrosion rates by the superior electrocatalytic performance of their oxides for OER [10]. 2) Metals with structural effects: Metals such as Tl, In, Sn, Bi, and Sb that are revealed to improve anodic corrosion resistance by altering the structure of the alloy and its protective oxide layer [10].  3) Modifying elements: Metals such as those belonging to alkali and alkali-earth groups, for which their positive alloying impact originates from their effect on the adsorption processes during crystallization [10].  Alloyed lead with metals such as silver (Ag), cobalt (Co), and thallium (Tl) are the most promising insoluble anode candidates. The mentioned elements exhibit higher conductivity as compared to the pure lead metal.  Therefore, these effective alloying elements accept more of the current applied on the anodic surface. Consequently, the potential, current density, and dissolution rate of the lead part of the anodic alloys are reduced. The reported lowered dissolution rate of alloyed lead electrodes can be explained by the formation of a thicker protective oxide layer, which decreases the passage of Pb2+ into the electrolyte. Addition of a third alloying element such as tin (Sn) and Tl has shown to further improve the corrosion resistance of the binary lead-based alloys [10]. Among the lead-based alloys, lead-silver (Pb-Ag) anodes have attracted significant attention owing to their positive effect on catalyzing the OER reaction and reducing the anodic overpotential on the oxide layer [11], [12], [15], [35]. Alloying of lead anodes with 0.5 to 1% silver 16  enhances the physical strength and electrical conductivity of lead-based anodes [4], [12], [14], [23], [41]. Kiryakov et al. [ref. 21 in [10]] reported that addition of 1% Ag to the Pb matrix improves the anodic resistance of pure Pb by four times and reduces its dissolution rate by five times. Similarly, Kozin et al. [ref. 22 in [10]] claimed that by increasing the Ag content of the Pb alloy from 0.1% to 1%, the solubility of the Pb anode decreases by more than 10 times and the slime-formation is noticeably suppressed. Although more expensive, lead-silver anodes also reveal improved corrosion resistance in the chloride-containing electrolytes, due to anodic formation of AgCl insoluble salts [6], [26], [29].  Inside the sulfuric acid electrolyte, lead-based anodes are passivated and oxidized to form various phases when exposed to specific potential ranges [6], [26], [58]. Firstly, the reaction of bare lead anode with the sulfate ions in the electrolyte results in formation of lead sulfate (PbSO4) (Reaction 2-14)  [6], [23], [41]. Since formation of the this passive insulating layer increases the surface pH and enables diffusion of only oxygen molecules, some of the bare electrode molecules gets converted into basic sulfates (PbO.PbSO4) and PbO through Reactions 2-17 and 2-18, respectively [14], [25]. Due to the non-conductive nature of PbSO4, an increased potential and current density is observed on the segments of the anode that are not covered with PbSO4. This increased potential enables the formation of a well-conductive lead dioxide layer (PbO2) through Reactions 2-20, 2-21, and  2-22, which in return reduces the anodic potential and current density [6], [11], [16], [23], [59]. PbO2 is the electrocatalytically active component of the anode, which enables the occurrence of the OER (Reaction 2-2) with high reported overpotentials [10]. The growth of a dense, thick, and adherent oxide layer passivates the surface of lead-based electrodes and lowers it corrosion and dissolution rates. The resulting PbSO4 and PbO2 are semi-permeable and allow for diffusion of specific species such as sulfate ions and oxygen molecules [16]. The 17  Pourbaix pH-potential diagram in Figure 2-2 illustrates the progression of the lead electrode through the aforementioned reactions [60]. 𝑃𝑃𝑃𝑃 + 𝐻𝐻2𝑍𝑍𝑂𝑂4 → 𝑃𝑃𝑃𝑃𝑍𝑍𝑂𝑂4 + 2𝐻𝐻+ + 2𝑒𝑒−; 𝐸𝐸298𝐾𝐾,   𝑃𝑃𝑃𝑃/𝑃𝑃𝑃𝑃𝑆𝑆𝑂𝑂4° = −0.36 𝑉𝑉𝑆𝑆𝐻𝐻𝐻𝐻                                    (Reaction 2-17)  𝑃𝑃𝑃𝑃 + 𝐻𝐻2𝑂𝑂 → 𝑃𝑃𝑃𝑃𝑂𝑂 + 2𝐻𝐻+ + 2𝑒𝑒−; 𝐸𝐸298𝐾𝐾,   𝑃𝑃𝑃𝑃/𝑃𝑃𝑃𝑃𝑂𝑂° = 0.24 𝑉𝑉𝑆𝑆𝐻𝐻𝐻𝐻                                    (Reaction 2-18)  2𝑃𝑃𝑃𝑃 + 𝐻𝐻2𝑂𝑂 + 𝐻𝐻2𝑍𝑍𝑂𝑂4 → 𝑃𝑃𝑃𝑃𝑂𝑂.𝑃𝑃𝑃𝑃𝑍𝑍𝑂𝑂4 + 4𝐻𝐻+ + 4𝑒𝑒−                       (Reaction 2-19) 𝑃𝑃𝑃𝑃𝑂𝑂 + 2𝐻𝐻2𝑂𝑂 → 𝑃𝑃𝑃𝑃𝑂𝑂2(𝑠𝑠) + 2𝐻𝐻+ + 2𝑒𝑒−                       (Reaction 2-20) 𝑃𝑃𝑃𝑃𝑂𝑂 .𝑃𝑃𝑃𝑃𝑍𝑍𝑂𝑂4 + 3𝐻𝐻2𝑂𝑂 → 2𝑃𝑃𝑃𝑃𝑂𝑂2(𝑠𝑠) + 𝐻𝐻𝑍𝑍𝑂𝑂4− + 6𝐻𝐻+ + 4𝑒𝑒−                (Reaction 2-21) 𝑃𝑃𝑃𝑃𝑍𝑍𝑂𝑂4 + 2𝐻𝐻2𝑂𝑂 → 𝑃𝑃𝑃𝑃𝑂𝑂2 + 𝐻𝐻2𝑍𝑍𝑂𝑂4 + 2𝐻𝐻+ + 2𝑒𝑒−; 𝐸𝐸298𝐾𝐾,   𝑃𝑃𝑃𝑃𝑆𝑆𝑂𝑂4/𝑃𝑃𝑃𝑃𝑂𝑂2° = 1.69 𝑉𝑉𝑆𝑆𝐻𝐻𝐻𝐻                   (Reaction 2-22)   18   Figure 2-2: Pourbaix diagram of lead in the presence of sulfate ions at 25℃. Reproduced from Sharpe TF (1973) Chapter I-5 Lead. In: Bard AJ (ed.) Encyclopedia of Electrochemistry of the Elements, vol. 1, pp. 235–347. New York: Marcel Dekker; Figure 1.1.3, p. 242 (Pergamon Press) [60], [61]. Upon continuous exposure of PbSO4 with water, two modifications of PbO2 namely rhombic (𝛼𝛼) and tetragonal (𝛽𝛽) are evolved [11], [14], [41]. Due to pH alterations, various electrochemical mechanisms for formation of different types of Pb4+ complexes have been proposed. Ivanov et al. [11] explained that initially, a thin layer of 𝛽𝛽-PbO2 is formed as the result of the reaction of PbSO4 with the present oxygen in the boundary layer. As the results of further oxidation of Pb surface at increased potentials, more oxygen is trapped in the 𝛽𝛽-PbO2 layer and a 19  ∝-PbO2 film is evolved on the anodic surface, which is responsible for the precedent reduction in anodic potential [10], [16]. As the ∝-PbO2 layer thickens, it becomes depleted of oxygen and it gradually gets converted to tetragonal 𝑃𝑃𝑃𝑃𝑂𝑂 and 𝑃𝑃𝑃𝑃. As the polarization proceeds, a PbOx oxide layer with x in the range of 1.4 to 1.6 is resulted [11]. Once the passivated anodic layer reaches a critical thickness, internal stresses coupled with evolution of the oxygen gas will cause flaking and spallation of the formed oxide layer [11], [16]. Figure 2-3 illustrates the expected composition of the passivated lead alloy electrode as a function of potential in a sulfuric acid electrolyte [62].  Figure 2-3: Schematic diagram of the composition of the corrosion film as a function of potential in sulfuric acid electrolyte [62].  20  2.4.1.2 Dimensionally Stable Anodes (DSA) Dimensionally stable anodes have been widely studied for their application in various electrochemical industries such as metal electrowinning, chlor-alkali industry, electroplating, and electro-organic synthesis [23], [63], [64]. Their popularity as alternative metal electrowinning anodes arises from their superior and selective OER activity and electrochemical stability in acidic chloride-containing electrolytes [23], [64]. Various types of DSAs are made up of a thin binary coating of a conductive component (electrochemically active metal oxides such as RuO2, IrO2, and PtOx) and an inert stabilizing oxide such as TiO2, ZrO2, and Ta2O5 deposited on a base metal such as titanium substrate [23], [63]–[65]. Commercial application of DSA electrodes is economically unfeasible due to the expensive nature of precious metal oxides as the electrochemically active component of the coating. Additionally, challenges exist with long-term stability of the DSA electrodes as OER anode materials [9]. Moreover, the performance of DSA electrodes is hindered in Mn-containing electrolytes due to the problems related to MnO2 deposition on these electrodes. As Mn2+ ions are anodically oxidized, MnO2 particles block the catalytic active sites of the metal oxide coating on Ti substrate [23], [64]. Nijjer reported that after one hour of electrolysis, their synthesized DSA electrodes behaved as MnO2 anode [23]. 2.4.1.3 Composite Anodes Composite anodes constitute of an electroactive metal oxide phase dispersed in an electron-conducting and chemically stable matrix of conventional lead-based alloy substrate [5], [15], [20], [36], [66]. These novel electrodes take advantage of the good electrocatalytic behavior of metal oxides and the high chemical and mechanical stability and good electrical conductivity of the lead-based substrates [36], [66]. The resulting electrodes are anticipated to improve the OER activity and corrosion behavior of the conventional lead-based anodes and act as a low-cost alternative for 21  the industrially used electrowinning lead-based anodes. Matsumoto and Sato [9] explained that the electrocatalytic metal oxide can be selected from one of the following three categories [9], [67]: 1. Rutile-type oxides: this group constitute of a transition metal octahedrally surrounded by oxygen anions. Rutile-type oxides may reveal metallic conductivity (RuO2, IrO2, and RhO2) or semiconducting properties (MnO2, PbO2, and PtO2). 2.  Spinel-type oxides: in this group, the transition metal cation substitutes the two types of octahedral and tetrahedral cation sites surrounded by oxygen anions. Co3O4, Co2NiO4, and Fe3O4 are included in this group. 3. Perovskite-type oxides: species in this group follow the AB3 chemical formula, with a large cation filling in the A site and a transition metal filling in the B site and being octahedrally surrounded by the oxygen anions. LaNiO3 and LaCoO3 belong to this category.  i. Conventional Pb-Metal Oxide Composite Anodes Among the various types of metal oxides that are combined with lead-based substrates, excellent OER electrocatalytic behavior is revealed for the later-mentioned types and in the given sequence: RuO2 > IrO2 > Co and Ni-contained oxides > MnO2 [9], [68]. The selected metal oxide is expected to have the following properties in the zinc electrowinning operating conditions: i. Higher electrochemical activity for the OER than then lead-based substrate ii. Chemical stability in the harsh electrowinning electrolyte iii. No negative effect associated with its dissolution on the electrowinning efficiency iv. Economically feasible  Although rutile-type metal oxides such as RuO2 and IrO2 show the most promising catalytic activity for OER, their application is limited due to costly nature of these precious transition metal 22  oxides [9], [17], [66], [68]. Spinel-type metal oxides such as Co and Ni-containing composite anodes also reveal good electrochemical catalytic activities [9]. However, the use of spinel-type metal oxides in zinc electrowinning is associated with challenges regarding their co-deposition with zinc and lowering of the zinc deposition current efficiency. Meanwhile, MnO2 can be considered as a suitable metal oxide phase to be dispersed in a lead-based substrate and employed in the zinc electrowinning process [2], [5], [34].  ii. Pb-MnO2 Composite Anodes   Manganese oxide is a rutile-type oxide, whose association with conventional lead-based alloys has shown to lower the OER overpotential and anodic corrosion rates in sulfuric acid media [21], [24]. The superior OER electrocatalytic activity of MnO2 particles over PbO2 can be explained by the Sabatier principle of catalysis. According to the volcano plot of OER overpotential versus the enthalpy of a lower to higher oxide transition (Figure 2-4), the most successful catalysts for the OER are those with moderate strength of the reaction intermediate binding. In the case of PbO2, the high enthalpy of transition to a stable and higher oxidation state results in a relatively weak metal-oxygen bond and its lower catalytic activity for the OER as compared to MnO2 [5], [68]. 23   Figure 2-4: Anodic OER overpotential of various metal oxide electrodes as a function of the enthalpy of the lower to higher oxide transition. (○) Alkaline and (●) acid solutions are indicated. Overpotentials are measured in the low current density region of less than 1 mA/cm2 [68], [69].  As manganese ions are already present in substantial amounts in the zinc electrowinning electrolyte, their dissolution does not negatively affect the electrowinning process. Novel composite anodes can be fabricated by integration of manganese oxides into the matrix of conventional lead-based alloy electrodes. For this purpose, various deposition techniques such as cold or hot pressing, spraying, accumulative roll-bonding, and electrodeposition can be employed [2], [17]–[19]. Mohammadi et al. [5] developed Pb-MnO2 composite anodes by single-action cold pressing of different proportions of Pb and MnO2 powder mixtures into a disk-shaped mold. In this article, the electrocatalytic performance of the novel electrodes in lowering the energy consumption of the zinc electrowinning process was investigated. Additionally, the anodic corrosion resistance of the proposed anodes was analyzed in reducing the electrode corrosion rates [5]. Karbasi et al. [19] prepared lead-based composite sheets containing 0.5 wt.% of MnO2 (Pb-0.5 wt.% MnO2) using the accumulated roll-bonding technique. In this paper, improved anodic 24  performance, increased zinc production, and lowered product and electrolyte contamination were reported with the composite anodes as compared to the pure lead anode [19].    Electrochemical deposition or electrodeposition enables deposition of specific metal oxides on a variety of electrically conductive substrates with a satisfying control over the physical properties of the deposited catalyst film [22], [47]. As compared to commonly employed deposition techniques such as spraying and pressing, electrodeposition can result in better associativity of the dispersed phase and the lead matrix and more uniformity of the deposited catalyst [17]. Among the various types of electrodeposition methods, potentiodynamic variations (linear scan and cyclic voltammetry polarizations) may not seem cost-efficient for industrial production of electrowinning electrodes. However, chronopotentiometric (constant current density) electrodeposition technique may seem industrially-applicable, if the achieved energy improvements with the novel anodes outweigh the required electrode manufacturing costs. Various researchers have focused on developing Pb-MnO2 composite anodes using electrodeposition techniques. Li et al. [17] prepared Pb/Pb-MnO2 composite anodes by electrochemical co-deposition of Pb and MnO2 on the surface of casted pure lead substrate. The electrocatalytic activity towards OER and corrosion behavior of the developed anodes were evaluated under the electrowinning operating conditions [17], [36]. 2.5 Electrowinning Cell Voltage and Power Consumption The power consumption of the zinc electrowinning process accounts for nearly 60% of the total electrical energy requirement of the electrolytic zinc refining plants. Thus, a significant economic motive exists in order to improve the power efficiency of this process [1], [6]. Since electrowinning is a constant current density operation, the overall electrowinning cell voltage dictates the power requirement of this stage. The electrowinning operating cell potential 25  (∆𝐸𝐸𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐,𝑜𝑜𝑜𝑜𝑜𝑜) can be expressed as the sum of four main components, as shown in Equation 2-1 [7], [8]. ∆𝐸𝐸𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐,𝑜𝑜𝑜𝑜𝑜𝑜 = ∆𝐸𝐸𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐,𝑟𝑟𝑐𝑐𝑟𝑟 + ∆𝜂𝜂 + ∆𝐸𝐸Ω + ∆𝐸𝐸𝑜𝑜 (Equation 2-1) Where, ∆𝐸𝐸𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐,𝑟𝑟𝑐𝑐𝑟𝑟 represents the reversible thermodynamic potential of overall reaction, ∆𝜂𝜂 denotes the sum of anodic overpotential and absolute value of cathodic overpotential, ∆𝐸𝐸Ω indicates the potential drop or due to ohmic, ionic, and electrical resistance, and ∆𝐸𝐸𝑜𝑜 implies the anodic potential or performance variations due to electrode degradation. Reduction in any of the above-mentioned components can improve the energy efficiency of the electrowinning process [7], [8]. The measured cell potential given the previously-described operating conditions is approximately 3.2-3.5 V [1]. An estimation of the potential distribution within the electrowinning cell operated at 500 A m-2 is displayed in Figure 2-5:          26   Figure 2-5: Approximate potential distribution within the electrowinning cell at an average current density of 500 A m-2. 2.5.1 Electrolyte Conductivity and Ohmic Potential Loss Ohmic potential loss caused by the electrical resistance of the electrowinning electrolyte and of the hardware, largely contributes to the overall cell voltage and is approximated in the order of 500 mV [1]. Thus, the electrowinning cell voltage and its power consumption can be lowered by minimizing the solution resistance. This component can be minimized by improving the electrolyte conductivity through increasing the temperature and acidity of the electrolyte. Furthermore, the potential loss is proportional to the spacing between the electrodes, which can be reduced considering to leave room for the growing zinc deposits and avoid shorting the electrodes. Moreover, gas evolution (oxygen from the OER reaction on the anode and hydrogen from the competing HER reaction on the cathode) further lowers the ionic conductivity of the electrolyte. Thus, electrode designs that can efficiently release the gas without directing it to the electrolyte 0.7 V 1.2 V Ohmic  Potential Drop 0.8 V 0.1 V Anode Overpotential Reversible Anode Potential Reversible Cathode Potential Cathode Overpotential Aluminum Cathode Zn2++ 2e- → Zn Lead-Silver Anode 2H2O → O2+ 4H+ + 4e- Cell Voltage 3.3 V 27  solution are preferred. Despite of the relatively substantial contribution of the ohmic potential drop to the overall electrowinning cell voltage, this component cannot be significantly lowered [11]. 2.5.2 Cathodic Zinc Reduction Overpotential Cathodic overpotential corresponding to deposition of metals (such as zinc, copper, nickel, and cadmium) is typically as small as 100 mV [7]. Specifically, for the zinc reduction reaction (Reaction 2-1), the standard equilibrium potential is 𝐸𝐸298𝐾𝐾,   𝑍𝑍𝑍𝑍2+/𝑍𝑍𝑍𝑍° = −0.76 𝑉𝑉𝑆𝑆𝐻𝐻𝐻𝐻. Given an electrolyte with 150 g L-1 sulfuric acid and 50 g L-1 zinc, the equilibrium potential of this reaction can be calculated to be -0.81 V. At an operating current density of 400 A m-2, the cathodic overpotential corresponding to this half-cell reaction is measured to be in the range of 0.05 to 0.12 V. Therefore, the average cathodic potential can be approximated to be -0.89 V [6]. Similarly, Parada and Asselin [1] reported an average cathodic overpotential of nearly 150 mV for the industrial zinc electrowinning process. Therefore, cathodic overpotential corresponding to the zinc reduction reaction does not substantially contribute to the overall electrowinning cell voltage. 2.5.3 Anodic Oxygen Evolution Reaction (OER) Overpotential OER is the most common anodic reaction for the metal electrowinning processes taking place in a sulfate-based electrolyte, with an associated standard equilibrium potential of  𝐸𝐸298𝐾𝐾,   𝐻𝐻2𝑂𝑂/𝑂𝑂2° = 1.23 𝑉𝑉𝑆𝑆𝐻𝐻𝐻𝐻 (half-cell Reaction 2.2) [7]. Due to its highly irreversible nature and sluggish reaction kinetics, OER overpotential on the commonly-used lead-based anodes is one of the largest known and it accounts for nearly 20% of the overall cell voltage [1], [6], [13]. Depending on the type of anode materials, various OER overpotential values in the range of 0.60 to 1.00 V have been reported for the zinc electrowinning process [1], [7]. Therefore, the electrical energy requirements of the electrowinning process can be substantially reduced by lowering the 28  overpotential associated to the anodic OER. Thus, anodic OER overpotential provides much scope in order to improve the power efficiency of the electrowinning process. 2.6 Research Objectives and Novelty  2.6.1 Previous Work Although conventional Pb-Ag electrodes are most commonly employed as electrowinning anodes, their application imposes issues such as high OER overpotential and high corrosion rates on the electrowinning process. Thus, the continuous research for novel anode materials to improve the energy efficiency of the electrowinning process and extend the anodic electrode lifetime has been the subject of many studies. However, high sensitivity of the electrowinning process to the impurities present in the electrolyte limits the type of species that can be integrated into conventional electrodes. Several literature reviews reported that various metal oxides such as rutile, spinel, and perovskite-type oxides have revealed promising OER electrocatalytic activity in both alkaline and acidic media [9]. Among various types of metal oxides, manganese oxide meets majority of the aforementioned requirements and it can be considered as a suitable candidate to be incorporated into conventional Pb-Ag anodes.  This report summarizes a few of the studies focused on development of Pb/Pb-MnO2 composite anodes. It is also explained that as compared to conventional deposition techniques, electrodeposition method seems most promising as it offers merits such as ease of manufacturing, cost-efficiency, and better control over the properties of deposited film. Thus, novel anodes can be developed by electrodeposition of an electrocatalytic dispersed phase of manganese oxide particles on the industrially provided Pb-Ag substrate. Meanwhile, MnO2 electrodeposition can be achieved either via anodic oxidation of Mn2+ or cathodic reduction of Mn7+ [54]. Various studies claimed that anodic MnO2 electrodeposition enables a better control over the crystallographic phase of the 29  deposited MnOx layer and offers more flexibility for large-scale production [21], [22]. Various electrochemical techniques for anodic MnO2 electrodeposition is possible, namely galvanostatic, potentiodynamic, potentiostatic, and pulse deposition [22], [47], [53], [54], [70]–[72]. Anodic MnO2 electrodeposition is generally performed inside an acetate-containing aqueous solution and in the presence of a supporting electrolyte such as sodium sulfate [21], [22], [70]. In this procedure, anodic manganese oxidation as explained by one of the pathways in Section 2.3.2.2 is coupled by HER taking place on the cathode (Reaction 2-4).  The overall governing reaction corresponding to Mn2+ oxidation is previously described by Reaction 2-7. 2.6.2 Research Objectives The present study aims to prepare novel electrowinning anodes by anodic electrodeposition of manganese oxides (MnOx) on the surface of industrially supplied Pb-Ag substrate and to evaluate their OER activity and stability under conditions relevant for the industrial zinc electrowinning process. The novelty of this work is related to the unique preparation technique using various electrochemical procedures and comparative analysis of their OER electrocatalytic performance. The four specific objectives of this study are as following: 1. To prepare and characterize the surface morphology and chemical composition of Pb-Ag electrodes electrodeposited with MnOx using a wide of range anodic electrodeposition techniques namely, galvanostatic and potentiodynamic polarization methods. 2. To investigate the OER electrocatalytic activity of the prepared MnOx electrodeposited Pb-Ag anodes within a half-cell setup by performing consecutive linear-scan voltammetry (LSV) polarizations in the OER potential range inside the electrowinning electrolyte. 3. To assess the OER long-term activity and durability of the MnOx electrodeposited Pb-Ag anodes within a half-cell setup by conducting three-day galvanostatic polarization at 500 30  A m-2 current density in the zinc electrowinning operating conditions and compare them with the industrially supplied Pb-Ag anodes. 4. To evaluate the overall potential of the full electrowinning cells employing the novel MnOx electrodeposited Pb-Ag anodes and compare them with the conventional electrowinning cell through carrying out one-day galvanostatic polarization at 500 A m-2 current density in the zinc electrowinning operating conditions. 31  Chapter 3: Experimental Materials, Apparatus, and Methodology This work aims to evaluate the performance of MnOx electrodeposited Pb-Ag anodes in comparison to the conventional Pb-Ag anodes under the zinc electrowinning operating conditions. For this purpose, the MnOx electrodeposited Pb-Ag electrodes were prepared using a variety of electrochemical techniques. Microscopic morphology and elemental composition of the fresh MnOx electrodeposited Pb-Ag electrodes were analyzed using scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), X-ray diffraction spectroscopy (XRD), and inductively coupled plasma - optical emission spectrometry (ICP-OES) with two-acid digestion methods. Electrochemical techniques such as galvanostatic polarization and linear scan voltammetry (LSV) tests were employed in order to assess the electrocatalytic activity of the novel anodes. 3.1 OER Anode Material Synthesis 3.1.1 Lead-Silver Anodes Lead-silver (Pb-Ag (0.75 wt.%)) anodes are conventionally used in the industrial zinc electrowinning process. For this study, industrially supplied lead-based alloys (Pb-Ag (0.75 wt.%)) as provided by Teck Resources Limited are employed as the substrate for preparation of the novel OER electrodes. As received from Teck Resources Limited, Pb-Ag metal sheets of 1 cm in thickness were cut into disk-shaped samples of approximately 5 cm2 in surface area. Each piece of electrode metal was connected to a copper wire and casted inside epoxy resin. The performance of the conventional Pb-Ag anodes was evaluated using electrochemical techniques in order to act as a baseline and as an industrial benchmark for comparison with the novel electrodes. The Pb-Ag electrodes were polished using 200, 600, and 800-grit silicon carbide sandpaper and rinsed with deionized (DI) water, prior to electrodeposition and electrochemical experiments.   32  3.1.2 MnOx Electrodeposited Pb-Ag Anodes The aforementioned Pb-Ag electrodes were used as the substrate for MnOx electrodeposition. A conventional three-electrode water-jacketed glass electrochemical cell with the Pb-Ag electrode as the working electrode, Hg/Hg2SO4 (s)/ K2SO4 (sat) (saturated sulfate electrode or MSE) as the reference electrode, and graphite rod as the counter electrode was used for the electrodeposition procedure. Figure 3-1 displays the half-cell electrochemical setup used for the electrodeposition procedure with the respective working, counter, and reference electrodes. Using a reversible hydrogen reference electrode (RHE) from Gaskatel GmbH (HydroFlex), the potential of the MSE reference electrode was measured to be 650 mV vs. RHE in 1 N H2SO4 solution at 298 K. As explained in Appendix B.1, all of the measured potentials in this work are reported against the standard hydrogen reference electrode (SHE) for easier comparison with the available literature. The electrodeposition electrolyte was composed of 0.3 M manganese (II) acetate tetrahydrate (Mn(CH3COO)2.4H2O; Sigma-Aldrich, ≥99%) and 0.1 M sodium sulfate (Na2SO4; Sigma-Aldrich, Reagent Plus, ≥99%) and its pH was measured to be relatively neutral. The temperature of the electrolyte inside the jacketed cell was maintained at 25±0.5℃ using a Cole Parmer Polystat water bath and the agitation rate was controlled at 350 rpm using a Thermo Scientific Cimarec stirring hot plate. These conditions were selected as they have been shown to produce a good quality MnOx film on a conductive substrate [21]. The electrodes were connected to a computer-controlled Princeton Applied Research VersaSTAT 4 potentiostat/galvanostat. 33   Figure 3-1: The CANSCI water-jacketed glass electrochemical cell connected to a water bath with the three-electrode setup utilized for the MnOx electrodeposition procedure. Anodic MnOx electrodeposition in this study was performed using three different electrodeposition techniques namely choronopotentiometric and potentiodynamic polarization methods such as linear sweep voltammetry and cyclic voltammetry. The onset potential for anodic MnO2 electrodeposition overlapped with the OER potential range. Prior to all of the electrodeposition techniques, the substrate was rinsed in DI water and allowed to rest at open circuit potential (OCP) inside the electrodeposition solution for at least 10 minutes. Potentiodynamic electrodeposition using cyclic voltammetry (CV) was carried out by performing five consecutive cycles between 0.05 and 2.25 V vs. SHE at a scan rate of 1 mV s-1. The resulting deposited electrodes were named MnOx CV-deposited Pb-Ag anodes. Similarly, potentiodynamic electrodeposition using linear sweep voltammetry (LSV) was carried out by performing five consecutive linear potential scans in the anodic direction in the potential window WE: Baseline Pb-Ag Anode  REF: MSE CE: Graphite Rod  34  from 0.05 to 2.25 V vs. SHE at a scan rate of 1 mV s-1. The resulting deposited electrodes using this technique were named MnOx LSV-deposited Pb-Ag anodes. The potential window for the potentiodynamic electrodeposition methods were selected to contain the theoretical potential at which Mn2+ oxidation occurs, as shown in Reaction 2-7. Chronopotentiometric electrodeposition was conducted at a constant current density of 6 mA cm-2 for a deposition duration time of one hour. As explained in Appendix B.2, this current density was calculated according to the Faraday’s law, in order to achieve the desired thickness of the MnOx film. The electrodeposited anodes prepared by chronopotentiometric electrodeposition were called MnOx CCD-deposited Pb-Ag anodes, within the context of this report. In order to ensure that the electrodeposited MnOx particles are responsible for the major improvements observed in the following electrochemical measurements, a blank CV-deposited Pb-Ag sample was prepared. This anode was developed by performing the previously-explained electrodeposition technique using cyclic voltammetry in a solution containing of only the supporting electrolyte, (0.1 M sodium sulfate (Na2SO4; Sigma-Aldrich, Reagent Plus, ≥99%)). Following the electrodeposition step, the working electrodes were thoroughly washed with DI water and dried at room temperature. 3.2 OER Electrochemical Measurements The OER electrochemical measurements include half-cell experiments targeted to evaluate the anodic OER performance and full-cell experiments replicating the industrial zinc electrowinning process. Half-cell experiments were carried out inside a sulfuric acid electrolyte solution (H2SO4, Anachemia, ACS grade) with 160 g L-1 concentration. Manganese (II) ions in 3 g L-1 concentration were introduced to the electrolyte through addition of manganese (II) sulfate monohydrate (MnSO4.H2O, Sigma-Aldrich, Reagent Plus, ≥99%). The half-cell electrochemical measurements were performed in the absence and presence of 0.3 g L-1 of chloride ion through the 35  addition of sodium chloride (NaCl, Fisher Chemical, Certified ACS). The electrolyte composition in half-cell experiments resembles that of the industrial zinc-free electrowinning process, in order to solely investigate the anodic OER performance. The full-cell electrochemical measurements were carried out in the above-mentioned electrolyte in the presence of 55 g L-1 zinc (II) and 0.3 g L-1 chloride ions through the addition of zinc sulfate heptahydrate (ZnSO4.8H2O, Sigma-Aldrich, Reagent Plus, ≥99%) and sodium chloride (NaCl, Fisher Chemical, Certified ACS), respectively. The electrolyte composition and operating conditions were selected in such a way to be directly applicable to the industrial zinc electrowinning process.  3.2.1 Anodic Half-Cell Electrochemical Experiments The electrochemical OER half-cell experiments were performed inside a CANSCI water-jacketed glass cell in a three-electrode configuration. In the half-cell experiments, the baseline or MnOx electrodeposited Pb-Ag electrodes served as the working electrode with a Hg/Hg2SO4 (s)/ K2SO4 (sat) (MSE, E=650 mV vs. SHE) as the reference electrode and graphite rod as the counter electrode. Figure 3-2 displays the half-cell electrochemical setup with the three described electrodes.  1 cm2 in surface area of the working electrode was exposed to the electrolyte through the circular opening located on one side of the glass container. This electrochemical setup was connected to a Cole-Parmer Polystat water heater in order to main the electrolyte temperature at 37±0.5℃. A mild mixing of the electrolyte was maintained by setting the agitation rate of the Thermo Scientific Cimarec stirring hot plate to 350 rpm. The three-electrode electrochemical setup was connected to a Princeton Applied Research VersaSTAT 4 potentiostat/galvanostat. The potentials in all the experiments are converted and reported against the standard hydrogen electrode (SHE). All measured currents in this work are normalized by the electrode geometric surface area, thus represent the superficial current density. At least three replicates of each 36  experiment were performed in order to ensure reproducibility and accuracy of the experimental measurements. Electrochemical impedance spectroscopy (EIS) measurements were performed inside the described electrowinning electrolyte in order to determine the uncompensated solution resistance (𝑅𝑅𝑠𝑠) and correct the potentials, accordingly. The measured value for the uncompensated solution resistance for this highly conductive electrolyte was determined to be approximately 1.10 Ω cm-2.  Figure 3-2: The CANSCI water-jacketed glass electrochemical cell connected to a water bath with the three-electrode setup used for anodic half-cell electrochemical measurements. 3.2.1.1 Anodic Half-Cell Potentiodynamic Polarization (Linear Voltammetry) The potentiodynamic experiments were performed using the aforementioned half-cell electrochemical setup and electrolyte (Section 3.2.1) in order determine the OER electrocatalytic activity of the MnOx electrodeposited Pb-Ag anodes as compared to the baseline Pb-Ag. Ten WE: Baseline or MnOx Electrodeposited Pb-Ag Anodes  REF: MSE CE: Graphite Rod 37  consecutive linear sweep voltammograms (LSVs) were performed in the OER potential range, namely 1.2 to 2.0 V vs. SHE with a scanning rate of 5 mV s-1. The voltammograms were obtained after 15 minutes immersion of the working electrode in the described electrolyte at OCP.  For each respective electrode, the measured potential at 10 mA cm-2 current density was utilized in order to calculate and compare the OER overpotential values [73]. Appendix B.3 explains how the OER overpotential calculations were performed. The OER overpotential data were calculated as the average of triplicate of the potentiodynamic polarization measurements to ensure reproducibility of the data.  3.2.1.2 Anodic Half-Cell 72-Hour Galvanostatic Polarization Following the potentiodynamic experiments, the baseline and MnOx electrodeposited Pb-Ag anodes were subjected to 72-hour galvanostatic polarization test (i.e. chronopotentiometry) using the aforementioned half-cell three-electrode setup and electrolyte. As previously explained in Section 2.3, the industrial zinc electrowinning process operates at a constant current density in the range of 450 to 550 A m-2. The galvanostatic polarization experiments were carried out at 500 A m-2 current density in order to compare the electrochemical activity of the novel anodes with the baseline. These half-cell 72-hour galvanostatic polarization tests were conducted in the absence of any zinc (II) cation and in the absence and presence of 0.3 g L-1 chloride ion. As explained in Appendix B.4, the results of these experiments were iR-drop corrected using the determined solution resistance from EIS measurements. 3.2.2 Full-Cell Electrowinning Experiments The full-cell electrowinning experiments were performed inside a CANSCI water-jacketed glass container using a two-electrode setup. In the full-cell experiments, the baseline or MnOx electrodeposited Pb-Ag electrodes served as the anode coupled with an industrially provided 38  aluminum electrode as the cathode. The anodic and cathodic electrodes were placed in a vertical opposition to one another with an inter-electrode gap of 9 cm. 1 cm2 in surface area of both of the electrodes was exposed to the electrolyte through the circular opening located at each side of the glass container. Figure 3-3 displays the two-electrode setup utilized for the full-cell electrowinning experiments. These tests were performed inside the previously-described manganese-containing sulfuric acid electrolyte in the presence of 55 g L-1 zinc (II), 3 g L-1 manganese (II), and 0.3 g L-1 chloride ions. The displayed results indicate the average of at least two replicates of each experiment in order to ensure reproducibility and accuracy of the measurements  Figure 3-3: The CANSCI water-jacketed glass electrochemical cell connected to a water bath with the two-electrode setup used for full-cell electrochemical measurements. 3.2.2.1 Full-Cell 24-Hour Galvanostatic Polarization In order to ensure the validity of results within the electrowinning process, 24-hour galvanostatic polarization tests were performed using the aforementioned cell setup and electrolyte described in Section 3.2.2. Full-cell galvanostatic polarization experiments were conducted following the immersion of the electrodes in the described electrolyte at OCP for 15 minutes. The full-cell experiments are not compensated for iR-drop losses. Baseline or MnOx Electrodeposited Pb-Ag Anodes Aluminum Cathode  39  3.3 Specific Energy Consumption Calculation in Zinc Electrowinning Parada and Asselin explained that combining the Joule’s Law and Faraday’s Law results in the relationship shown in Equation 3-1 [1]. Appendix B.5 explains the details of this calculation. Assuming a 100% current efficiency for zinc production, this equation can be utilized in order to calculate the specific energy consumption of the zinc electrowinning cell given the overall cell potential.  𝑃𝑃𝑜𝑜𝑚𝑚= 𝑉𝑉𝑉𝑉𝑉𝑉𝑀𝑀× 1ℎ3600 𝑠𝑠  (Equation 3-1) 𝐸𝐸 = 𝑃𝑃𝑜𝑜𝑚𝑚  (Equation 3-2) In Equation 3-1, P (kW) is the power consumed, t (h) is the electrowinning duration, m (kg) is the mass of deposited zinc, V (V) is the overall cell potential, F (𝐹𝐹 = 96,485 𝐶𝐶 𝑚𝑚𝑚𝑚𝐶𝐶−1) is the Faraday’s constant, z (𝑧𝑧 = 2 for Zn2+) is the valence of deposited ion, and M (kg kmol-1) is the molar mass of zinc. Therefore, E (kWh kg-1) or the specific energy consumption of the process can be determined according to the term 𝑃𝑃𝑜𝑜𝑚𝑚, as in Equation 3-2. For example, using the equilibrium zinc electrowinning cell potential of 1.99 V, the theoretical electrowinning specific energy consumption can be computed to be 1.63 kWh per kg of zinc produced. As illustrated in Figure 2-5, the actual zinc electrowinning cell potential is greater than 1.99 V, due to various sources of energy loss. Therefore, the real specific energy consumption of this process is significantly higher than its theoretical value, which justifies the necessity for increasing its energy efficiency [1].  3.4 Analytical Measurements The area specific mass loading of the MnOx electrodeposited Pb-Ag anodes were determined through microwave-assisted aqua regia digestion coupled with inductively coupled plasma-optical emission spectrometry (Agilent Technologies 5110 ICP-OES). The 40  electrodeposited film on the known surface area of each of the novel anodes was scraped off and collected. About 200 mg of the solid deposit was digested inside an aqua regia solution made up of 3 mL HNO3 (67% w/v, ACS grade, VWR Chemicals), 6 mL HCl (37% w/v, ACS grade, VWR Chemicals), and 1 mL DI water. One-hour irradiation of the resulting mixture in a microwave heating system ensured a complete digestion of the deposits. A manganese reference electrolyte with a concentration of 1000 ppm (PlasmaCAL, SCP SCIENCE) was utilized in order to prepare the standard solutions required for calibration of the ICP-OES. The ICP-OES equipment was calibrated using a blank sample and 12 standard manganese solutions ranging from 1 to 200 ppm in 2 wt.% HNO3. The final solutions were analyzed for their manganese content by an ICP-OES spectrometer at two wavelengths of 257.610 and 259.372. To confirm the accuracy of the obtained data, the reported results were averaged of the triplicate measurements of manganese content.  In order to assess the accuracy of the manganese mass loading measurements obtained using the ICP-OES technique coupled with aqua regia digestion, one of the novel anodes (CV-deposited Pb-Ag anode) was analyzed using X-ray Fluorescence Spectroscopy (XRF) for its manganese mass loading. For this purpose, a layout with 100 uniformly-spaced blocks was selected on approximately 2 to 3 cm2 surface area of the MnOx electrodeposited Pb-Ag anodes. This grid network was scanned for its manganese mass loading assuming an infinite thickness of the substrate Pb-(0.75 wt.%)Ag. The measurements at each x- and y- coordinate were plotted using 2-dimensional graphs and displayed in the Appendix A.6 section of this report. This analysis was performed by a few members of Dr. Wilkinson’s research team namely Dr. Arman Bonakdarpour, Dr. Lius Daniel in the Chemical and Biological Engineering Department at UBC.   41  3.5 Physico-Chemical Characterization of the OER Anode Materials 3.5.1 SEM Morphology Characterization and EDX Elemental Mapping The surface morphology of the baseline and MnOx electrodeposited Pb-Ag anodes were examined using a FEI Quanta 650 scanning electron microscope (SEM) equipped with an X-Ray detector with an acceleration voltage of 20 eV. Additionally, energy dispersive X-ray spectroscopy (EDX) analysis was conducted on the samples. The elemental surface composition of the MnOx electrodeposited Pb-Ag electrodes were determined through energy dispersive X-ray spectroscopy (EDX). The SEM/EDX analysis were conducted in the Materials Engineering Department of UBC, without any other additional sample preparations. 3.5.2 Large Surface Area SEM Characterization and EDX Elemental Mapping In order to better visualize the surface morphology and elemental mapping of the MnOx electrodeposited Pb-Ag anodes, a rectangular surface with approximately 2 to 3 cm2 area of these electrodes was scanned using a FEI Quanta 650 SEM. The selected surface was divided into uniformly spaced blocks and scanned according to the path shown in Figure 3-4. The elemental surface mapping on these electrodes was then conducted using EDX. 42   Figure 3-4: The grid-lined scanning path defined for the large surface area SEM/EDX analysis. 3.5.3 Surface Composition and Phase Analysis Using XRD In order to perform the XRD analysis, the freshly electrodeposited catalyst on the surface of Pb-Ag anodes was gently scraped off using a spatula and ground up with a mortar and pestle. The resulting powder was placed on a zero-diffraction plate (Si zero plate, P-typed, B-doped) in order to minimize any diffraction from the sample holder. XRD patterns were obtained on a detector using a Bruker AXS D2 Phaser X-ray powder diffractometer with a Cu-Kα radiation as the X-ray source and a Lynxeye detector, operated at 30 kV and 10 mA. The diffractograms were recorded in the 2θ range of 10º to 85º, with a step size of 0.01°. These analytical measurements were performed by a few members of Dr. Wilkinson’s research team namely Dr. Arman Bonakdarpour, Dr. Lius Daniel, and Yu Pei in the Chemical and Biological Engineering Department at UBC.   43  Chapter 4: Results and Discussion 4.1 Anodic MnOx Electrodeposition Results The three variations of electrodeposition procedures were performed as described in the methodology section of this report and the results are illustrated in Figures 4-1, 4-2, and 4-3, respectively. Figure 4-1 reveals the variation of measured current density with anodic potential during the cyclic voltammetry (CV) electrodeposition. In the potential range of less than 1.0 V vs. SHE, the anodic surface was covered by the passivated PbSO4 film and the observed oxidation pick in the potential window of 1.0 to 1.4 V vs. SHE corresponds to the formation and growth of PbO2 from PbSO4 species, according to Reaction 2-22. This observation can also be verified by the Pourbaix diagram in the presence of sulfate ions in the relatively neutral potential range (Figure 2-2). The positive hysteresis observed in the OER potential range was resulted from the higher reactivity of the newly formed electroactive α-PbO2 layer for evolution of oxygen [28]. Comparison of the CV electrodeposition in the presence and absence of manganese (as shown in Figure A-1 in Appendix A) indicates that manganese dioxide formation roughly occurred at the same potentials as the OER, and therefore it was hidden by the large current densities observed in the OER potential range.  Figure 4-2 shows the variation of measured current density with anodic potential during the linear sweep voltammetry (LSV) electrodeposition. Similar to the CV electrodeposition results, the electrode surface was initially covered with the PbSO4 layer. A slight increase in the measured current density was observed in the potential window of 1.0 to 1.4 V vs. SHE, which can be attributed to the oxidation of PbSO4 to PbO2 species. Analogous to the CV electrodeposition results, manganese dioxide formation approximately took place in the similar potentials to the OER. Figure 4-3 displays the variation of potential as a function of time during the one-hour chronopotentiometric electrodeposition procedure. As it can be observed, the 44  measured potential initiated from the OCP and it increased to 2.20 V vs. SHE and it plateaued around 1.86 V vs. SHE. It is conjectured that the MnOx electrodeposition occurred during the first few minutes of this electrodeposition procedure. Further increasing the electrodeposition period did not reveal any improvement in the OER electrocatalytic activity of the CCD-deposited Pb-Ag anodes.  Figure 4-1: Cyclic voltammetry potentiodynamic electrodeposition procedure displaying variation of measured current density as a function of anodic potential using the Pb-(0.75 wt.%)Ag substrate as the working electrode; Scan rate: 1 mV s-1; Electrodeposition electrolyte: 0.3 M Mn(II) from Mn(CH3COO)2 and 0.1 M Na2SO4; T=25ºC.    45   Figure 4-2: Linear sweep voltammetry potentiodynamic electrodeposition procedure displaying variation of measured current density as a function of anodic potential using the Pb-(0.75 wt.%)Ag substrate as the working electrode; Scan rate: 1 mV s-1; Electrodeposition electrolyte: 0.3 M Mn(II) from Mn(CH3COO)2 and 0.1 M Na2SO4; T=25ºC.     46   Figure 4-3: One-hour chronopotentiometric electrodeposition procedure at 6 mA cm-2 displaying variation of anodic potential as a function of time using the Pb-(0.75 wt.%)Ag substrate as the working electrode; Electrodeposition electrolyte: 0.3 M Mn(II) from Mn(CH3COO)2 and 0.1 M Na2SO4; T=25ºC. The polarization was initiated from OCP (-0.15 V vs SHE).   4.2 Anodic Surface Layer Physical Characterization Various surface characterization techniques were utilized in order to study the morphology and surface composition of the MnOx electrodeposited Pb-Ag anodes. The following section presents the results of the conducted characterization analysis. 4.2.1 Anodic Morphological and Surface Composition Studies Using SEM/EDX Figure 4-4 displays the surface morphology and manganese mapping of the fresh baseline and MnOx electrodeposited Pb-Ag anodes using SEM/EDX surface characterization techniques. 47  Figures 4-4.1 A and B display the SEM image and EDX mapping of lead on the surface of the fresh baseline Pb-Ag anode. The complete EDX elemental mappings included in Appendix A.2.i reveal that the surface of the fresh baseline Pb-Ag anode is mainly covered with the passivated oxide and carbonate compounds of lead. The main macroscopic morphology observed with the MnOx electrodeposited Pb-Ag anodes (Figures 4-4.2 to 4-4.4 A and B) is the irregularly-shaped manganese-concentrated islands surrounded by manganese oxide particles dispersed in the bulk matrix of passivated lead oxide and sulfate compounds. Gibson et al. [47] explained that these islands are possibly resulted from the electrodeposition of manganese oxides on previously-formed nuclei, thus forming manganese-concentrated clusters. The vertical growth of these islands is conjectured to increase the real catalytic active surface area of the electrodes above their geometric areas. Appendix A.2 (Figures A-2 to A-5) displays the EDX elemental mapping for all of the elements present on the surface of the baseline and MnOx electrodeposited Pb-Ag anodes including Pb, Mn, Ag, O, C, Na, S, and Si. The carbon present on the surface of these anodes either originated from the passivation of metal alloy with atmospheric CO2 or polishing of the surface with SiC sandpaper.    40 µm 1. A 1. B 48        Figure 4-4: Morphology (× 1000) of the fresh Pb-Ag (1) and the MnOx CV-deposited (2), CCD-deposited (3), and LSV-deposited (4) Pb-Ag anodes prepared according to the three described electrodeposition techniques in the Methodology section: (a) SEM, (b) Elemental mapping (Pb for the baseline anode and Mn for the MnOx electrodeposited anodes) using EDX. The large surface area SEM/EDX scanning analysis was conducted on the MnOx electrodeposited Pb-Ag anodes, and the results are displayed in Appendix A.3. The captured 40 µm 40 µm 40 µm 2. A 2. B 3. A 3. B 4. A 4. B 49  elemental mapping allows for better visualization of the uniformity of the electrodeposited catalyst on the surface of conventional Pb-Ag anodes.  4.2.2 Anodic Composition Determination Using ICP-OES Coupled with Aqua Regia Digestion and XRF The area-specific manganese mass loading of the MnOx electrodeposited Pb-Ag anodes was measured using ICP-OES coupled with microwave-assisted aqua regia digestion and the results are illustrated in Table 4-1. The 2-dimensional XRF analysis performed on the CV-deposited anode reveals the mapping of the manganese mass loading on the anodic surface and it is included in Appendix A.6. Table 4-1: Area-specific manganese mass loading of the MnOx electrodeposited Pb-Ag anodes determined using ICP-OES coupled with aqua regia digestion. Sample Description Estimated Area Specific Manganese Mass Loading (mg cm-2) MnOx CV-Deposited Pb-Ag  0.255 MnOx LSV-Deposited Pb-Ag  0.040 MnOx CCD-Deposited Pb-Ag  0.117  Having measured the area-specific manganese mass loading of the novel anodes, the efficiency of manganese (II) oxidation and electrodeposition using each technique can be determined. In the case of the chronopotentiometric technique, the theoretical area-specific manganese mass loading of the CCD-deposited sample after one hour of constant current density electrodeposition at 6 mA cm2 in the 0.3 M manganese-containing electrolyte and assuming 100% manganese oxidation current efficiency was determined as explained in Appendix B.2. The theoretical area-specific manganese mass loading of CCD-deposited sample was estimated to be 50  6.15 mg cm-2, which results in 1.9% efficiency of this electrodeposition technique on the given substrate. In the case of CV and LSV electrodeposition techniques, the total charge passed through the system must be calculated from the deposition polarization curves (Figure 4-1 and Figure 4-2) and in conjunction with the measured manganese mass loading, the deposition current efficiency can be calculated using Faraday’s law. These calculations were not performed.  4.2.3 Surface Composition Analysis Using XRD The powder X-ray diffraction patterns of the fresh CV, CCD, and LSV-deposited Pb-Ag anodes are displayed in Figures 4-5, 4-6, and 4-7, respectively. The surface composition of each electrode is respectively matched with the available ICSD and ICDD reference database. According to this analysis and as anticipated by literature review, majority of the anodic surface was covered with the lead-based passivated film consisting of PbSO4 and α-PbO2 (orthorhombic) particles. The electrodeposited manganese oxide on the surface of novel anodes was detected to be of the ramsdellite phase (R-MnO2) using the XRD characterization technique. However, it is conjectured that the MnOx present on the surface of the novel anodes is of the EMD or electrolytic (γ-MnO2) phase, which is defined as intergrowth of the pyrolusite (β- MnO2) phase in the matrix of the ramsdellite (R-MnO2) phase. Inability to detect the pyrolusite phase in the structure of EMD MnO2 can be due to limitations of the XRD analysis for characterization of heterogeneous samples. On the surface of novel anodes, different modifications of mixed metals (various oxide compounds of Pb, Ag, and Mn) were present with Pb constituting to more than 90 wt.% of the anodic surface composition. Thus, the peaks generated by the trace amount of β-MnO2 are believed to be diffused and hidden by the sharp peaks resulted from the Pb matrix. Electrolytic crystallographic phase of MnO2 (γ-MnO2) is shown to possess the highest OER electrocatalytic activity as compared to its 51  alternatives such as pyrolusite and ramsdellite MnO2 phases in acidic solutions [74]–[76]. The intergrowth and micro-twinning defect sites in the structure of the γ-MnO2 phase on the surface of novel anodes are believed to be responsible for their increased catalytic active surface area and high OER activity in acidic media [74], [76]. Among the three MnOx electrodeposited samples, the CV-deposited anode has been matched with a higher number of peaks belonging to the ramsdellite MnO2 pattern, followed by the CCD-deposited and LSV-deposited, respectively. Further analysis is required in order to distinguish and explain the structural differences between the three novel anodes.  However, as previously-explained, this characterization technique is most suitable for homogeneous and single-phase materials. Due to peak overlay and high angle reflection, its detection capability is limited for mixed materials such as the catalyst developed in this work.    52   Figure 4-5: Powder X-ray diffraction pattern of the fresh CV-deposited Pb-Ag anode.   53   Figure 4-6: Powder X-ray diffraction pattern of the fresh CCD-deposited Pb-Ag anode. 54   Figure 4-7: Powder X-ray diffraction pattern of the fresh LSV-deposited Pb-Ag anode.  4.3 Anodic Performance Evaluation for the OER As discussed in Chapter 2 of this report and displayed in Reaction 2-4, OER is the primary anodic reaction in the zinc electrowinning process. Therefore, various electrochemical techniques namely, potentiodynamic and galvanostatic polarization tests were utilized in order to assess the OER electrocatalytic activity of the novel MnOx electrodeposited Pb-Ag anodes in comparison with the conventional Pb-Ag anode. 55  4.3.1 Anodic Potentiodynamic Studies Appendix A.4 displays the variation of measured current density as a function of potential for the baseline and the three MnOx electrodeposited Pb-Ag anodes during the ten consecutive linear potential sweeps (LSV) in the zinc-free electrowinning electrolyte. As it can be seen in Appendix A.4, during the few initial anodic scans of both the baseline and MnOx electrodeposited Pb-Ag anodes, no oxidation peak except for the anodic OER branch appeared. By performing further consecutive potential scans, oxidation peaks started to appear and the OER took place at lower onset potentials and led to higher current densities. Figure 4-8 (A) compares the 10th linear potentiodynamic sweep of the baseline and MnOx electrodeposited Pb-Ag anodes in the zinc-free electrowinning electrolyte. The figures displaying the 8th and 9th linear potential scan of these anodes are presented in Appendix A.4. In the zinc-free electrowinning electrolyte while at OCP (-0.34 ≤ E ≤-0.30 V vs. SHE), the surface of lead-based anodes was dissolved and passivated through formation of PbSO4, according to Reaction 2-17 [25], [77]. In accordance to the previous literature, in the potential range of 0 ≤ E ≤ 1.54 V vs. SHE, the slight increase in the current corresponds to the formation and growth of PbO and PbO.PbSO4 species, through Reactions 2-18 and 2-19 [25], [77]. At the potentials more positive than 1.55 V vs. SHE, formation and growth of polymorphs of PbO2 from the previously-formed compounds of Pb(II) namely PbSO4, PbO, and PbO.PbSO4 occurs [77], [78]. Similar oxidation peaks were observed in the anodic sweep of the baseline Pb-Ag in the absence of manganese (II). Thus, in the case of baseline Pb-Ag, this observed oxidation peak corresponds to oxidation of Pb(II) to Pb(IV). However, for the MnOx electrodeposited Pb-Ag anodes, oxidation of the electrodeposited Mn(IV) to its higher valences and oxidation of Pb(II) to Pb(IV) species both contribute to the observed peak. This explains the relatively higher current of this peak associated with the novel anodes as compared to the baseline anode. This eliminated 56  significant contribution of Mn(II) oxidation to the intensity of the peaks observed in this potential range. At higher potentials (E≥1.55 V vs. SHE), the rhombic structure of PbO2 (∝-PbO2) was formed through oxidation of PbO and basic lead sulfates (xPbO.PbSO4), according to 2-20 and 2-21 [77]. At increased potentials (E≥1.85 V vs. SHE), the tetragonal structure of PbO2, namely 𝛽𝛽-PbO2 was resulted from the oxidation of PbSO4, according to Reaction 2-22 [77]. The formation of polymorphs of PbO2 catalyzes the proceeding OER anodic branch [25]. According to the electrochemical literature, manganese (II) oxidation takes place in the in the potential range of 1.00 ≤ E ≤ 2.10 V vs. SHE [25]. Thus, the described anodic peaks corresponding to the formation of various modification of PbO2 and MnO2 might have been hidden by the accelerated OER. Formation of PbO2 proceeded the OER with an approximate onset potential in the range of 1.70 to 1.90 V vs. SHE [5]. The intensity of the resulting current from the oxidation of the lead-based alloy was small as compared to the OER current, however it was amplified by conducting further sweeps and in the presence of higher amounts of PbO2. The increased current density of the oxidation peaks and the improved OER activity of the anodes with sweeping can be attributed to the catalytic and depolarization effects of the gradually forming PbO2 on the respective reactions taking place in the mentioned potential ranges. Multiple reports have emphasized on the catalytic effect of PbO2 on the OER reaction by claiming that formation of PbO2 is essential for OER to take place [25]. As it can be observed from the anodic potentiodynamic polarization of the MnOx electrodeposited Pb-Ag anodes in Figure 4-8 (A) and Appendix A.4, the onset OER for these novel electrodes was shifted to lower potentials. Using the potentiodynamic polarization results, the OER onset potential for the baseline and MnOx electrodeposited Pb-Ag anodes are estimated by the interpolation of the tangent line to the linear ascending section of the OER anodic branch with the 57  zero-current axis. As seen in Figure 4-8 (A), the onset potential associated with the OER of the baseline Pb-Ag anode is observed at 1.778 V vs. SHE. However, the OER onset potential for the MnOx CV, CCD, and LSV-deposited Pb-Ag anodes were shifted to more noble potentials and are observed at approximately 1.685, 1.629, and 1.693 V vs. SHE, respectively. Moreover, similar current densities were achieved at lower potentials with the MnOx electrodeposited Pb-Ag anodes as compared to the baseline Pb-Ag anode, thus higher OER electrocatalytic activities are associated with the novel anodes. Figure 4-8 (B) displays the mass activity of the MnOx electrodeposited Pb-Ag anodes calculated as the ratio of the superficial current density over the manganese area-specific mass loading of each sample at each specific potential. As compared to Figure 4-8 (A), on the mass activity basis, different trends were observed indicating that the highest performance per active catalyst loading was observed with the MnOx LSV-deposited, followed by the CCD-deposited and CV-deposited Pb-Ag anodes, respectively. Considering the inexpensive nature of the MnOx-based catalysts, this consideration may not be as relevant as it is for the design of the electrodes containing precious-metal catalysts.        58    Figure 4-8: OER electrocatalytic activity comparison of the baseline and MnOx electrodeposited Pb-Ag anodes indicated by the comparison of (A) 10th linear scan voltammogram, (B) mass activity in the OER potential range; Scan rate=5 mV s-1; Electrolyte composition: 160 g L-1 sulfuric acid solution with 3 g L-1 Mn2+ and 0.3 g L-1 Cl-; T=37℃.  (A) (B) 59  4.3.2 OER Overpotential Comparison In order to calculate the OER overpotential, the equilibrium potential for this half-reaction was adjusted by applying the Nernst equation for the specific electrolyte solution (160 g L-1 H2SO4) and operating temperature (37°C) used in this study. The difference between this calculated equilibrium potential and the experimentally measured potential at the given current density of 10 mA cm-2 was taken as the OER overpotential and used for performance comparison purposes [73]. This calculation is explained in detail in Appendix B.1. The potentiodynamic polarization results for the 8th, 9th, and 10th scans were compensated for the solution resistance and employed in order to determine the OER overpotential of the conventional and novel anodes (Appendix A.5). Figure 4-9 compares the experimentally determined values of OER overpotential at a given current density of 10 mA cm-2 for the baseline and the MnOx electrodeposited Pb-Ag anodes. Comparing the 10th LSV scan data indicates that an average reduction of 150, 103, and 39 mV in anodic overpotential is observed with the MnOx CCD, CV, and LSV-deposited Pb-Ag anodes, respectively. The observed trends were reproducible between different scans. Lowered OER overpotentials were associated with the novel anodes, which can be explained by their improved OER electrocatalytic activity in the presence of electrodeposited MnOx. It can be observed that MnOx electrodeposited Pb-Ag anodes with greater area-specific manganese mass loadings revealed higher reductions in OER overpotential. This observation can explain the trend observed in the catalytic performance of the novel anodes, as the lowest OER overpotential was seen with the CV-deposited sample, followed by the CCD and LSV-deposited anodes, respectively.    60   Figure 4-9: OER overpotential comparison of the baseline and MnOx electrodeposited Pb-Ag anodes at 10 mA cm-2 calculated using the 8th, 9th, and 10th linear sweep voltammetry scans in the half-cell setup; Scan rate: 5 mV s-1. Electrolyte composition: 160 g L-1 sulfuric acid solution with 3 g L-1 Mn2+ and 0.3 g L-1 Cl-. T=37℃. The vertical error bars represent the standard deviation of triplicate measurements.  4.3.3 Half-Cell Anodic 72-Hour Galvanostatic Polarization  4.3.3.1 In the Presence of Chloride Ions During the anodic half-cell galvanostatic polarization experiments, OER is the main reaction taking place on the anodic surface. The variation of average anodic potential over the 72 hours of galvanostatic polarization of the baseline and the three MnOx electrodeposited Pb-Ag anodes inside the zinc-free electrowinning electrolyte is displayed in Figure 4-10. In order for this series of experiments to be applicable to the industrial zinc electrowinning process, a constant current density of 500 A m-2 was applied throughout this experiment. Figure 4-11 provides a closer view of the initial stages of electrolysis. For both of the baseline and the MnOx electrodeposited 61  Pb-Ag anodes, a significant potential drop occurred at the beginning of polarization. This sudden variation in potential is conjectured to be due to formation of a stable lead dioxide layer on the remaining surface of the fresh anodes [36]. By further proceeding the galvanostatic polarization tests, the anodic potential gradually raised and reached a relatively stable state. During the electrolysis, Mn(II) oxidation to MnO2 proceeded simultaneous to the OER, until a visible thick film of MnO2 fully covered the surface of the previously-formed PbO2 layer [36]. During the prolonged electrolysis, the gradual increase in the average anodic potential of the four anodes could stem from the growth of the electrolytic MnO2 film. Although according to the Sabatier principle, MnO2 particles reveal better electrocatalytic activity than that of the PbO2 species, excessive thickening of the electrolytic MnO2 film decreases the conductivity of the original anodes and can lead to anodic potential rise during galvanostatic polarization [68]. Once the loosely-adhered electrolytic MnO2 film grew sufficiently, it started to break off from the surface as gas formation proceeded on the anode. Moreover, the increase in the average anodic potential of the baseline and MnOx electrodeposited Pb-Ag anodes can be attributed to the thickening of the micro-porous passivated film on lead-based electrodes that increases the travelling distance of the reactants to the active catalytic layer and limits their mass and charge transfer capability. Furthermore, blocking of the micro-porous film with the continuously evolving oxygen gas can be blamed on for the diminished OER activity at high reaction rates [20].   62   Figure 4-10: OER half-cell 72-hour galvanostatic polarization of the baseline and the MnOx electrodeposited Pb-Ag anodes at a current density of 500 A m-2; Electrolyte composition: 160 g L-1 sulfuric acid solution with 3 g L-1 Mn2+ and 0.3 g L-1 Cl-. T=37℃. The measured potentials are iR-drop corrected. The vertical error bars indicate the standard deviation of triplicate measurements.     63   Figure 4-11: Variation of average anodic potential of the baseline and the MnOx electrodeposited Pb-Ag anodes as a function of time during the initial 4 hours of galvanostatic polarization at a current density of 500 A m-2 using the half-cell setup; Electrolyte composition: 160 g L-1 sulfuric acid solution with 3 g L-1 Mn2+ and 0.3 g L-1 Cl-. T=37℃. The measured potentials are iR-drop corrected. The vertical error bars indicate the standard deviation of triplicate measurements.   Figure 4-12 presents and compares the average anodic potential of the baseline and the three MnOx electrodeposited Pb-Ag anodes after 72 hours of electrolysis. The final anodic potential of the MnOx CV, CCD, and LSV-deposited Pb-Ag anodes after 72-hour electrolysis was respectively 113, 52, and 22 mV lower than that of the baseline Pb-Ag anode. The reduced anodic potential attributed to the MnOx electrodeposited electrodes revealed the greater OER electrocatalytic activity of the novel electrodes as compared to the conventional anodes. The highest OER catalytic performance corresponds to the CV-deposited Pb-Ag anode with the highest area-specific mass loading of manganese, followed by the CCD, and LSV-deposited anodes. 64   Figure 4-12: Final average anodic potential of the baseline and the MnOx electrodeposited Pb-Ag anodes after 72 hours of galvanostatic polarization at a current density of 500 A m-2 using the half-cell setup; Electrolyte composition: 160 g L-1 sulfuric acid solution with 3 g L-1 Mn2+ and 0.3 g L-1 Cl-. T=37℃. The measured potentials are iR-drop corrected. The vertical error bars indicate the standard deviation of triplicate measurements.   In order to ensure that the electrodeposited MnOx on the surface of conventional anodes was majorly responsible for the anodic potential improvements, the blank CV-deposited Pb-Ag anodes were prepared and subjected to the 72-hour galvanostatic polarization at 500 A m-2, as shown in Figure 4-13. As described in Section 3.1.2 of this report, the blank CV-deposited Pb-Ag anodes were prepared by performing the CV electrodeposition procedure in a manganese-free electrolyte. As seen in Figure 4-13, the OER electrocatalytic performance of these anodes closely resembled that of the conventional Pb-Ag anodes. The surface of the blank CV-deposited anodes was anticipated to be covered by the passivated film solely composed of sulfate and/or oxide compounds of lead, which can be held accountable for their similar performance to conventional 65  anodes. Therefore, it can be concluded that the OER performance improvements observed in the 72-hour galvanostatic polarization can mostly be attributed to the MnOx species electrodeposited on the surface of novel electrodes.   Figure 4-13: OER half-cell 72-hour galvanostatic polarization of the baseline, MnOx electrodeposited Pb-Ag anodes, and blank (no Mn) CV-deposited Pb-Ag anode at a current density of 500 A m-2; Electrolyte composition: 160 g L-1 sulfuric acid solution with 3 g L-1 Mn2+ and 0.3 g L-1 Cl-. T=37℃. The measured potentials are iR-drop corrected. The vertical error bars indicate the standard deviation of triplicate measurements.   4.3.3.2 In the Absence of Chloride Ions The half-cell galvanostatic polarization experiments were also conducted in the absence of chloride and the results are displayed in Figure 4-14. Except for the MnOx LSV-deposited sample, at least two replicates of each measurement were obtained in the absence of chloride ions. The galvanostatic polarization experiments in the absence and presence of chloride ions revealed similar trends. Thus, it can be concluded that the achieved improvements were independent of the 66  chloride evolution reaction (CER) that takes place in a similar potential range to the OER (refer to  Reaction 2-5). Figure 4-15 compares the average anodic potential of the baseline and the three MnOx electrodeposited Pb-Ag anodes after 72 hours of electrolysis. The final anodic potential of the MnOx CV, CCD, and LSV-deposited Pb-Ag anodes after 72-hour electrolysis was 155, 66, and 27 mV lower than that of the baseline Pb-Ag anode, respectively. Similar to the half-cell galvanostatic polarization experiments in the presence of chloride ions, the highest OER catalytic activity in the absence of chloride ions corresponded to the CV-deposited Pb-Ag anode with the highest area-specific manganese mass loading, followed by the CCD, and LSV-deposited anodes. Comparing the 72-hour galvanostatic polarization results (Figures 4-10 and 4-15) revealed a relatively higher anodic potential observed in the presence of chloride ions, which was expected of electrowinning in the chloride-containing electrolytes [26]. Various literature have reported that in the presence of dissolved manganese in the electrolyte and anodic (electrolytic) deposition of MnO2, chlorine evolution reaction proceeds at significantly lower rates [26], [28], [79]. The resulting electrolytic MnO2 layer acts as a diffusion barrier to mass transport of chloride ions and decreases the release of chloride ions in the form of chlorine gas during the electrowinning process [26], [27]. Similar behavior is presumably observed with the MnO2 electrodeposited Pb-Ag anodes owing to their high electrocatalytic activity and selectivity for OER. In a similar work by Lai et al. [20], the half-cell OER activity of the Pb/Pb-MnO2 co-deposited composite anodes during 72-hour electrolysis at 500 A m-2 in a chloride and manganese-free electrowinning electrolyte was compared with that of the conventional Pb-Ag anodes. In the beginning of the electrolysis, the performance of the Pb/Pb-MnO2 composite anode was superior to that the Pb-Ag anode by more than 100 mV. The stable potential of composite anodes after 72 hours of electrolysis closely resembled that of the conventional Pb-Ag anode. Mohammadi and 67  Alfantazi [5] also investigated the performance of powder pressed Pb-(10 wt.% MnO2) composite anodes with conventional Pb-Ag anodes in chloride-free electrowinning electrolytes. In their work, they reported better OER activity of the composite anodes in the absence of Mn (II) in the electrolyte. However, in the presence of Mn(II) ions and formation of MnO2 film, the OER improvements were shown to be diminished [5]. Contrary to the work presented in this paper where significant reductions in anodic OER activity was achieved with the MnOx electrodeposited Pb-Ag anodes, due to the highly active nature of the electrodeposited MnOx crystallographic phase for OER.    Figure 4-14: OER half-cell 72-hour galvanostatic polarization in the absence of chloride ions at a current density of 500 A m-2; Electrolyte composition: 160 g L-1 sulfuric acid solution with 3 g L-1 Mn2+. T=37℃. The measured potentials are iR-drop corrected. The vertical error bars indicate the standard deviation of triplicate measurements.    68   Figure 4-15: Final average anodic potential of the baseline and the MnOx electrodeposited Pb-Ag anodes after 72 hours of galvanostatic polarization in a chloride-free electrolyte at a current density of 500 A m-2 using the half-cell setup; Electrolyte composition: 160 g L-1 sulfuric acid solution with 3 g L-1 Mn2+. T=37℃. The measured potentials are iR-drop corrected. The vertical error bars indicate the standard deviation of triplicate measurements.  4.4 Full Electrowinning Cell Performance Evaluation  Figure 4-16 displays the variation of the zinc electrowinning cell average potential with time during the 24-hour galvanostatic polarization experiment at 500 A m-2. As it can be observed, investigation of the baseline and novel anodes in the full-cell zinc electrowinning operation corroborated the half-cell anodic experiments. Reduced average cell potential was observed with zinc electrowinning cells that incorporated the MnOx electrodeposited Pb-Ag anodes. Table 4-2 displays the average final cell potential after the 24-hour galvanostatic polarization experiment. An average reduction in average cell potential of 133, 116, and 79 mV was achieved with the CCD, CV, and LSV-deposited Pb-Ag anodes, respectively, as compared to the baseline Pb-Ag anode. It is important to observe that unlike the baseline Pb-Ag anode, MnOx electrodeposited anodes have 69  not yet achieved a stable potential after 24 hours of galvanostatic polarization. The average cell potential associated with the novel anodes seemed to further decrease with time, indicating that potentially greater improvements in overall cell potential and energy consumption can be achieved with the MnOx electrodeposited anodes.   Figure 4-16: Variation of the average full electrowinning cell potential using the baseline and the MnOx electrodeposited Pb-Ag anodes during the 24 hours of galvanostatic polarization at a current density of 500 A m-2; Cathode electrode: aluminum. Electrolyte composition: 160 g L-1 sulfuric acid solution with 55 g L-1 Zn2+, 3 g L-1 Mn2+, and 0.3 g L-1 Cl-. T=37℃. Error bars represent the range of replicate measurements. The potentials are not corrected for iR-drop losses.    70  Table 4-2: Average final potential of the zinc electrowinning cell after 24 hours of galvanostatic polarization at 500 A m-2; Errors represent the range of replicate measurements. Sample Description Final Average Full Electrowinning Cell Potential (V) Baseline Pb-Ag 3.22 ± 0.02 MnOx CCD-Deposited Pb-Ag 3.09 ± 0.01 MnOx CV-Deposited Pb-Ag 3.11 ± 0.02 MnOx LSV-Deposited Pb-Ag 3.14 ± 0.01  The rate of MnO2 formation inside the electrolyte of the conventional Pb-Ag anode visually differed with that of the MnOx electrodeposited Pb-Ag electrodes. As displayed in Figure 4-17 (b), after a few hours of galvanostatic polarization of the baseline Pb-Ag anode, the electrolyte color turned into black, indicating a substantial amount of suspended MnO2 in the electrolyte. As previously-described by various literature, some of the suspended MnO2 particles in the electrolyte formed MnO2 cell mud or sludge and some precipitated over various inert components of the electrochemical cell [80], [81]. In the case of MnOx electrodeposited Pb-Ag anodes, the electrolyte following electrolysis turned color into pink, which is suspected to be the result of Mn(III) ions formation (Figure 4-17 (c), refer to Reaction 2-14). The difference in the electrolyte content is conjectured to arise from altered rate of manganese oxidation on the surface of the baseline and novel anodes. According to Tompkins [51], the suspended MnO2 particles in the electrolyte are formed as a result of the reaction of manganese (II) ions and permanganate, according to Reaction 2-16. This reaction is believed to be catalyzed by the already-existing MnO2 particles in the electrolyte or on the substrate [51], [82]. Various literature reviews have reported that the electrolytic MnO2 layer formed on the surface of lead-based alloys is porous and loosely-adhered 71  to the electrode. Thus, frequent detachment and excessive formation of electrolytic MnO2 particles into the electrolyte was seen with conventional lead-based alloy anodes [51], [80], [81]. While the MnO2 particles electrodeposited on composite Pb-Ag anodes were more strongly adhered to the surface when polarized and they reduced the excessive formation and detachment of electrolytic MnO2 particles that was seen with conventional Pb-Ag anodes [81].   (a) (b) 72   Figure 4-17: Appearance of the full-cell zinc electrowinning setup prior and after the 24-hour galvanostatic polarization at 500 A m-2 using either the baseline or novel Pb-Ag anodes; (a) Prior to the electrolysis, (b) After the electrolysis using the baseline Pb-Ag anode, (c) After the electrolysis using the novel MnOx CV-deposited Pb-Ag anode.  4.5 Projected Energy Requirements and Energy Savings of the Electrowinning Process The measured zinc electrowinning cell potentials displayed in Table 4-2 are utilized in order to estimate the specific energy consumption of the electrowinning process as explained in Section 3.3. It is assumed that this measured potential remains relatively steady for the duration of the electrowinning cell operation. An average industrial zinc current efficiency of 91% was employed for this calculation [6]. The calculated specific energy consumption of an electrowinning tank house using the conventional Pb-Ag (baseline) and the MnOx electrodeposited Pb-Ag anodes are displayed in Tables 4-3. Teck Resources Limited reported a production rate of 287,000 tons of refined zinc in 2019 at its refinery located in Trails, BC [83]. An average industrial electricity cost of 0.07 $ kWh-1 was reported by BC Hydro for large-power customers in 2019 [84]. Using the given production capacity and electricity cost, the impact of this novel anodic technology on the annual electrowinning energy consumption and energy cost was demonstrated, as displayed in Table 4-3. It can be deducted that the specific energy consumption of the electrowinning tank (c) 73  house that employs the CCD, CV, and LSV-deposited Pb-Ag anodes is 4.30%, 3.74%, and 2.51% less than that of the conventional tank hose using the baseline Pb-Ag anode. The projected energy savings can potentially amount to 2.27, 1.98, and 1.35 million dollars reduction in annual electricity costs of the given tank house that utilizes the CCD, CV, and LSV-deposited Pb-Ag anodes, correspondingly. This economic analysis does not take into account the electrode manufacturing costs and assumes equal durability and operating costs for the process employing the baseline and novel anodes. Appendix B.5 explains the details of this calculation. Section 5.2 of this report proposes further studies that would improve the accuracy of this energy and economic estimations.   74   Table 4-3: Projected specific energy consumption and improvements of an electrowinning tank house as a function of the overall cell potential measured at 500 A m-2; Production capacity=287,000 tons [83]. Electricity cost=0.07$ kWh-1 [84]. Zinc current efficiency=0.91 [6].  Sample Description Measured Electrowinning Cell Potential Specific Energy Consumption per kg of Zn Annual Specific Energy Consumption Annual Electricity Cost Annual Economic Savings (Compared to the Baseline Pb-Ag) Improvements in Energy Consumption (V) (kWh kg-1) (kWh year-1) ($ year-1) ($ year-1) (%) Baseline Pb-Ag 3.22 2.91 8.33×108 5.49×107 _ _ MnOx CCD-Deposited Pb-Ag 3.09 2.78 7.99×108 5.26×107 2.27×106 4.31 MnOx CV-Deposited Pb-Ag 3.11 2.80 8.03×108 5.29×107 1.98×106 3.73 MnOx LSV-Deposited Pb-Ag 3.14 2.83 8.13×108 5.36×107 1.35×106 2.51  75  Chapter 5: Conclusions and Recommendations for Future Research 5.1 Conclusions This work developed novel MnOx electrodeposited Pb-Ag anodes using three electrodeposition techniques including galvanostatic, linear scan and cyclic potentiodynamic polarization methods. These anodes were subjected to several electrochemical measurement procedures in order to evaluate the OER electrocatalytic performance and overall cell potential of the zinc electrowinning operation that makes use of the novel anodes as compared to the conventional Pb-Ag anodes. The surface morphology and elemental mapping of the baseline and the MnOx electrodeposited Pb-Ag anodes were captured by the means of SEM/EDX and XRD characterization equipment. Using ICP-OES coupled with microwave-assisted aqua regia digestion, the area-specific manganese mass loading of the MnOx CV, CCD, and LSV-deposited Pb-Ag anodes were estimated to be 0.254, 0.117, and 0.040 mg cm-2, accordingly. Moreover, the powder X-ray diffraction patterns of the MnOx electrodeposited Pb-Ag anodes were obtained and matched with the available reference database in order to analyze their surface composition. The anodic OER potentiodynamic (half-cell) experiments were utilized in order to compare the OER overpotential associated with the novel anodes with the conventional Pb-Ag anodes. It is shown that inside a manganese and chloride-containing sulfuric acid solution, the MnOx CCD, CV, and LSV-deposited Pb-Ag anodes lowered the OER overpotential by an average of 150, 103, and 39 mV, respectively, as compared to the baseline Pb-Ag anode. When the novel anodes were subjected to 72-hour galvanostatic polarization at 500 A m-2 inside the manganese and chloride-containing electrowinning electrolyte, 113, 52, and 22 mV reduction in average anodic OER potential was observed with the MnOx CV, CCD, and LSV-deposited Pb-Ag anodes, correspondingly, as compared to the baseline anode. In the absence of chloride ions, similar trends  76  in galvanostatic polarization revealed an average of 155, 66, and 27 mV reduction in anodic OER potential with the MnOx CV, CCD, and LSV-deposited Pb-Ag anodes, as compared to the baseline at 500 A m-2. In order to ascertain the validity of the half-cell experiment within the industrial electrowinning process, novel anodes were subjected to 24-hour galvanostatic polarization at 500 A m-2 inside the exact industrial electrowinning electrolyte and within a full electrowinning cell. The results of the full-cell experiments corroborated the half-cell anodic OER tests and lowered cell potentials were achieved with the novel anodes. An average of 133, 116, and 79 mV reduction in average cell potential was attained with the CCD, CV, and LSV-deposited Pb-Ag anodes, respectively, as compared to the baseline Pb-Ag. Visual comparison of the electrowinning electrolyte after 24-hour full-cell galvanostatic polarization revealed a significantly higher formation of black-color cell mud (suspended MnO2 particles) in the presence of the conventional Pb-Ag anode as compared to the MnOx electrodeposited anodes. It can be concluded that the electrodeposited MnOx layer on the surface of novel anodes is more strongly adhered to the anodic surface as compared to the electrolytic MnOx (EMD), thus excessive detachment of MnOx and formation of cell mud are minimized when novel anodes are implemented. Several electrochemical and physical explanations can be utilized in order to justify the improved OER electrocatalytical behavior of the MnOx electrodeposited Pb-Ag anodes as compared to the conventional anodes. The main reason being the better performance of the composite anode made up of a dispersed MnO2 phase with excellent OER activity in the matrix of the Pb-Ag substrate. Furthermore, according to the XRD analysis, the MnOx present on the surface of novel anodes is of the ramsdellite crystallographic phase. However, it was explained that the electrodeposited MnOx is conjectured to be of the EMD modification, which majorly constitutes of the structure of the ramsdellite MnOx phase. This phase is shown to possess higher OER  77  catalytic activity as compared to its other alternatives, specifically the pure pyrolusite and ramsdellite MnOx modifications. Moreover, the vertical growth of the deposited catalysts on the surface of novel anodes is expected to increase their catalytic active surface above their geometric area. Finally, the electrodeposition procedure produces a strongly-adhered MnOx film on the surface of the conventional anodes. As opposed to the naturally-deposited MnOx, the electrodeposited MnOx particles on the surface of novel anodes reduce excessive cell mud formation in addition to their catalytic impact on lowering the OER overpotential. The final average cell potentials observed in the full electrowinning cell galvanostatic experiments were utilized in order to predict the specific energy requirements of an electrowinning cell incorporating the novel anodes as compared to the baseline anode. The yearly energy consumption and electricity costs of a zinc electrowinning tank house were predicted given an annual zinc production capacity of 287,000 ton. It is shown than implementation of the novel anodes can improve the annual electricity cost of the conventional zinc electrowinning process by a maximum of $2.3×106. In summary, the CCD, CV, and LSV-deposited Pb-Ag anodes revealed an average of 4.3, 3.7, and 2.5% improvement in specific energy consumption of the electrowinning process as compared to the baseline anode. The energy consumption calculations utilized the cell potentials after 24-hour galvanostatic polarization, where a stable cell potential was not yet achieved with the novel anodes. Thus, operation of these anodes for longer periods of time may result in greater reduction of the cell potential. Moreover, the rising concerns amid global warming are anticipated to drive the electricity costs high in the future, which would render the use of conventional Pb-Ag anodes unfeasible and encourage implementation of novel anode technologies. Additionally, in the case of conventional Pb-Ag anodes, up to six months of operation is anticipated to be required in order to form a stable PbO2/MnO2, which may result in  78  some extents of performance improvement [14], [80]. This technology can take advantage of this waiting time window in order to improve the power consumption of the electrolytic zinc refineries. In conclusion, the results of this study indicated that the MnOx electrodeposited Pb-Ag anodes could lead to considerable energy improvement of the OER during zinc electrowinning. Furthermore, the novel anodes are advantageous as they can be introduced as an extension to the conventional zinc electrowinning technology.  5.2 Recommendations This scope of this research was restricted to evaluating the OER electrocatalytic performance of the novel MnOx electrodeposited Pb-Ag anodes as compared to conventional Pb-Ag anodes. However, in order to further investigate industrial adoption of this novel anode technology, a more comprehensive study is recommended that additionally focuses on evaluation of the long-term stability of these anodes under the industrial electrowinning conditions. This can be performed by measuring the dissolved lead and manganese content of the cell mud and electrolyte solution following the electrowinning tests using ICP-OES and the weight of lead contamination co-deposited with the product zinc on the cathode (similar to the study by Karbasi et al. [19]). Moreover, the full electrowinning cell experiments in the zinc-containing electrolyte solely focused on evaluation and comparison of the overall cell voltage. In this work, measuring the amount of pure deposited zinc on aluminum cathode did not reveal accurate results as precise mechanical separation of the electrodeposited zinc from the aluminum electrode was challenging. Digestion of the whole cathode with the deposited zinc inside an aqua regia solution and measuring the exact amount of zinc using ICP-OES can reveal accurate information regarding the product yield and current density efficiency. This can alternatively be achieved by conducting an ICP-OES  79  test on the electrolyte solution following the electrowinning experiments to measure the amount of remained zinc in the electrolyte, thus the mass of deposited zinc product. In order to better assess the applicability of the novel electrodes in the industrial zinc electrowinning process, it is recommended to perform an economic evaluation that also takes into account the initial anode material costs [13]. This discussion needs to make assumptions regarding the labor and manufacturing costs associated with the electrowinning process. Prior to this analysis, it is essential to optimize the electrodeposition technique (procedure type, solution composition and concentration, and operating conditions) so that it can be employed for economic analysis in longer-term studies. It is also necessary for the novel electrodes to be utilized under the exact electrowinning conditions for extended duration of operation similar to the actual lifetime of industrial electrodes (at least six months). The resulting long-term performance and lifetime per dollar of material costs generated using the optimized electrodeposition technique can be employed in the above-discussed economic evaluation in order to assess the industrial applicability of this novel anode technology and propose further recommendations.  80    Bibliography [1] T. F. Parada and E. Asselin, “Reducing power consumption in zinc electrowinning,” Jom, vol. 61, no. 10. pp. 54–58, 2009, doi: 10.1007/s11837-009-0152-1. [2] S. Toiminen, M. Toiminen, K. Kontturi, O. Forsen, and M. H. Barker, “New oxygen evolution anodes for metal electrowinning : MnO2 composite electrodes,” J. Appl. Electrochem., no. 39, pp. 1835–1848, 2009, doi: 10.1007/s10800-009-9887-1. [3] Natural Resources Canada, “Zinc facts,” 2018. [Online]. Available: https://www.nrcan.gc.ca/our-natural-resources/minerals-mining/minerals-metals-facts/zinc-facts/20534. [4] M. Tunnicliffe, F. Mohammadi, and A. Alfantazi, “Polarization Behavior of Lead-Silver Anodes in Zinc Electrowinning Electrolytes,” J. Electrochem. Soc., vol. 159, no. 4, pp. C170–C180, 2012, doi: 10.1149/2.055204jes. [5] M. Mohammadi and A. Alfantazi, “The performance of Pb-MnO2 and Pb-Ag anodes in  Mn(II)-containing sulphuric acid electrolyte solutions,” Hydrometallurgy, vol. 153, pp. 134–144, 2015, doi: 10.1016/j.hydromet.2015.02.009. [6] R. J. Sinclair, The Extractive Metallurgy of Zinc, First. Carlton Victoria, Australia: The Australian Institute of Minining and Metallurgy, 2005. [7] R. O. Loutfy and R. L. Leroy, “Energy efficiency in metal electrowinning,” J. Appl. Electrochem., vol. 8, no. 6, pp. 549–555, 1978, doi: 10.1007/BF00610801. [8] S. Trasatti, “Electrocatalysis: Understanding the success of DSA®,” Electrochim. Acta, vol. 45, no. 15–16, pp. 2377–2385, 2000, doi: 10.1016/S0013-4686(00)00338-8. [9] E. MATSUMOTO, Y. AND SATO, “Electrocatalytic properties of transition-metal oxides for oxygen evolution reaction,” Mater. Chem. Phys., no. 14, pp. 397–426, 1986.  81  [10] I. Ivanov et al., “Insoluble anodes used in hydrometallurgy Part I. Corrosion resistance of lead and lead alloy anodes,” Hydrometallurgy, vol. 57, pp. 109–124, 2000, doi: 10.1016/S0304-386X(00)00098-0. [11] I. Ivanov et al., “Insoluble anodes used in hydrometallurgy Part II. Anodic behaviour of lead and lead-alloy anodes,” Hydrometallurgy, vol. 57, no. 2, pp. 125–139, 2000, doi: 10.1016/S0304-386X(00)00098-0. [12] W. Zhang and G. Houlachi, “Electrochemical studies of the performance of different Pb-Ag anodes during and after zinc electrowinning,” Hydrometallurgy, vol. 104, no. 2, pp. 129–135, 2010, doi: 10.1016/j.hydromet.2010.05.007. [13] M. S. Moats, “Will lead-based anodes ever be replaced in aqueous electrowinning?,” Jom, vol. 60, no. 10, pp. 46–49, 2008, doi: 10.1007/s11837-008-0135-7. [14] A. Felder and R. D. Prengaman, “Lead alloys for permanent anodes in the nonferrous metals industry,” Jom, vol. 58, no. 10, pp. 28–31, 2006, doi: 10.1007/s11837-006-0197-3. [15] T. Dobrev et al., “Investigations of new anodic materials for zinc electrowinning Investigations of new anodic materials for zinc electrowinning,” vol. 2967, 2013, doi: 10.1179/174591909X438938. [16] M. Clancy, C. J. Bettles, A. Stuart, and N. Birbilis, “The influence of alloying elements on the electrochemistry of lead anodes for electrowinning of metals: A review,” Hydrometallurgy, vol. 131–132, pp. 144–157, 2013, doi: 10.1016/j.hydromet.2012.11.001. [17] Y. Li et al., “Oxygen evolution and corrosion behaviors of co-deposited Pb/Pb-MnO2 composite anode for electrowinning of nonferrous metals,” Hydrometallurgy, vol. 109, no. 3–4, pp. 252–257, 2011, doi: 10.1016/j.hydromet.2011.08.001.  82  [18] M. Mohammadi et al., “Development of Pb-MnO2 composite anodes for electrowinning application: Electrochemical and corrosion evaluations,” no. March. 2016. [19] M. Karbasi, E. Keshavarz, A. Elahe, A. Dehkordi, and F. Tavangarian, “Electrochemical and anodic behaviors of MnO2/Pb nanocomposite in zinc electrowinning,” pp. 379–390, 2018, doi: 10.1007/s10800-018-1163-9. [20] Y. Lai et al., “Electrochemical behaviors of co-deposited Pb/Pb-MnO2 composite anode in sulfuric acid solution - Tafel and EIS investigations,” J. Electroanal. Chem., vol. 671, pp. 16–23, 2012, doi: 10.1016/j.jelechem.2012.02.011. [21] P. Hosseini-Benhangi, C. H. Kung, A. Alfantazi, and E. L. Gyenge, “Controlling the Interfacial Environment in the Electrosynthesis of MnOx Nanostructures for High-Performance Oxygen Reduction/Evolution Electrocatalysis,” ACS Appl. Mater. Interfaces, vol. 9, no. 32, pp. 26771–26785, 2017, doi: 10.1021/acsami.7b05501. [22] J. W. D. Ng, M. Tang, and T. F. Jaramillo, “A carbon-free, precious-metal-free, high-performance O2 electrode for regenerative fuel cells and metal-air batteries,” Energy Environ. Sci., vol. 7, no. 6, pp. 2017–2024, 2014, doi: 10.1039/c3ee44059a. [23] S. Nijjer, “Deposition and reduction of manganese dioxide on alternative anodo materials in zinc electrowinning.” p. 145, 2000. [24] F. Porter, Zinc Handbook: Properties, Processing, and Use in Design. New York: Marcel Dekker, 1991. [25] A. Recéndiz, J. L. Nava, L. Lartundo-Rojas, I. Almaguer, and I. González, “Characterization of the Corrosion Layers Electrochemically Formed on the Lead–Silver/H2SO4+Mn(II) Interface,” J. Electrochem. Soc., vol. 156, no. 8, p. C231, 2009, doi: 10.1149/1.3142365.  83  [26] G. H. Kelsall, E. Guerra, G. Li, and M. Bestetti, “Effects of manganese (II) and chloride ions in zinc electrowinning reactors,” in Proceedings-Electrochemical Society, 2000, vol. 2000, pp. 350–361. [27] J. E. Bennett, “Electrodes for Generation of Hydrogen and Oxygen from Seawater,” Int. J. Hydrogen Energy, vol. 5, no. 4, pp. 401–108, 1980. [28] M. Nicol, C. Akilan, V. Tjandrawan, and J. A. Gonzalez, “The effects of halides in the electrowinning of zinc . I . Oxidation of chloride on lead-silver anodes,” vol. 173, no. August, pp. 125–133, 2017, doi: 10.1016/j.hydromet.2017.08.015. [29] R. H. Newnham, “Corrosion rates of lead based anodes for zinc electrowinning at high current densities,” J. Appl. Electrochem., vol. 22, no. 2, pp. 116–124, 1992, doi: 10.1007/BF01023812. [30] S. Wang et al., “Hydrometallurgy Electrochemical properties of Pb-0.6 wt % Ag powder-pressed alloy in sulfuric acid electrolyte containing Cl−/ Mn2 + ions,” Hydrometallurgy, vol. 177, no. August 2017, pp. 218–226, 2018, doi: 10.1016/j.hydromet.2018.03.018. [31] D. Filippou, “Innovative hydrometallurgical processes for the primary processing of zinc,” Miner. Process. Extr. Metall. Rev., vol. 25, no. 3, pp. 205–252, 2004, doi: 10.1080/08827500490441341. [32] D. J. MacKinnon and J. M. Brannen, “Effect of manganese, magnesium, sodium and potassium sulphates on zinc electrowinning from synthetic acid sulphate electrolytes,” Hydrometallurgy, vol. 27, no. 1, pp. 99–111, 1991, doi: 10.1016/0304-386X(91)90081-V. [33] W. Zhang and C. Y. Cheng, “Manganese metallurgy review. Part III: Manganese control in zinc and copper electrolytes,” Hydrometallurgy, vol. 89, no. 3–4, pp. 178–188, 2007, doi: 10.1016/j.hydromet.2007.08.011.  84  [34] P. Yu and T. J. O’Keefe, “Evaluation of Lead Anode Reactions in Acid Sulfate Electrolytes,” J. Electrochem. Soc., vol. 149, no. 5, p. A558, 2002, doi: 10.1149/1.1464882. [35] C. Cachet, C. Le Pape-Rérolle, and R. Wiart, “Influence of Co2+ and Mn2+ ions on the kinetics of lead anodes for zinc electrowinning,” J. Appl. Electrochem., vol. 29, no. 7, pp. 813–820, 1999, doi: 10.1023/A:1003513325689. [36] Y. Q. Lai, Y. Li, L. X. Jiang, X. J. Lv, J. Li, and Y. X. Liu, “Electrochemical performance of a Pb/Pb-MnO2 composite anode in sulfuric acid solution containing Mn2+,” Hydrometallurgy, vol. 115–116, pp. 64–70, 2012, doi: 10.1016/j.hydromet.2011.12.013. [37] M. N. Justin McGinnity, “The Effect of Periodic Open-Circuit on the Corrosion of Lead Alloy Anodes in Sulfuric Acid Electrolyte Containing Manganese,” Hydrometall. 2008 - Proc. Sixth Int. Symp. - 68.3 Exp. Soc. Mining, Metall. Explor. (SME)., vol. 17–21, pp. 592–599, 2008. [38] R. Mráz, S. Václav, and S. Tichý, “Experimental activation energies for evolution of oxygen and chlorine on oxide electrodes,” Electrochim. Acta, vol. 18, no. 8, pp. 551–554, 1973, doi: 10.1016/0013-4686(73)85017-0. [39] S. E. Ramírez, A.; Hillebrand, P.; Stellmach, D.; May, M. M.; Bogdanoff, P.; Fiechter, “Evaluation of MnOx , Mn2O3 , and Mn3O4 Electrodeposited Films for the Oxygen Evolution Reaction of Water,” J. Phys. Chem. C, vol. 118, pp. 14073–14081, 2014. [40] Y. Meng, W. Song, H. Huang, Z. Ren, S. Y. Chen, and S. L. Suib, “Structure-property relationship of bifunctional MnO2 nanostructures: Highly efficient, ultra-stable electrochemical water oxidation and oxygen reduction reaction catalysts identified in alkaline media,” J. Am. Chem. Soc., vol. 136, no. 32, pp. 11452–11464, 2014, doi:  85  10.1021/ja505186m. [41] C. Cachet, C. Rerolle, and R. Wiart, “Kinetics of Pb and Pb-Ag anodes for zinc electrowinning - II. Oxygen evolution at high polarization,” Electrochim. Acta, vol. 41, no. 1, pp. 83–90, 1996, doi: 10.1016/0013-4686(95)00281-I. [42] J. M. S. Rodrigues and M. J. Dry, “the Production of Particulate Manganese Dioxide During Zinc Electrowinning,” Proc. Int. Symp. Electrometallurigical Plant Pract., pp. 199–220, 1990, doi: 10.1016/b978-0-08-040430-1.50020-1. [43] I. Ivanov, “Increased current efficiency of zinc electrowinning in the presence of metal impurities by addition of organic inhibitors,” Hydrometallurgy, vol. 72, no. 1–2, pp. 73–78, 2004, doi: 10.1016/S0304-386X(03)00129-4. [44] S. Nijjer, J. Thonstad, and G. M. Haarberg, “Oxidation of manganese(II) and reduction of manganese dioxide in sulphuric acid,” Electrochim. Acta, vol. 46, no. 2–3, pp. 395–399, 2000, doi: 10.1016/S0013-4686(00)00597-1. [45] L. Pajunen, J. Aromaa, and O. Forsén, “The effect of dissolved manganese on anode activity in electrowinning,” Proc. TMS Fall Extr. Process. Conf., vol. 2, pp. 1255–1265, 2003, doi: 10.1002/9781118804407.ch14. [46] S. Rodrigues, N. Munichandraiah, and A. K. Shukla, “A cyclic voltammetric study of the kinetics and mechanism of electrodeposition of manganese dioxide,” J. Appl. Electrochem., vol. 28, no. 11, pp. 1235–1241, 1998, doi: 10.1023/A:1003472901760. [47] A. J. Gibson, B. Johannessen, Y. Beyad, J. Allen, and S. W. Donne, “Dynamic Electrodeposition of Manganese Dioxide: Temporal Variation in the Electrodeposition Mechanism,” J. Electrochem. Soc., vol. 163, no. 5, pp. H305–H312, 2016, doi: 10.1149/2.0721605jes.  86  [48] J. P. Petitpierre, C. Comninellis, and E. Plattner, “Oxydation Du MnSO4 en dioxyde de manganese dans H2SO4 30%,” Electrochim. Acta, vol. 35, no. 1, pp. 281–287, 1990, doi: 10.1016/0013-4686(90)85071-T. [49] G. Davies, “Some aspects of the chemistry of manganese(III) in aqueous solution,” Coord. Chem. Rev., vol. 4, no. 2, pp. 199–224, 1969, doi: 10.1016/S0010-8545(00)80086-7. [50] W. H. Kao and V. J. Weibel, “Electrochemical oxidation of manganese(II) at a platinum electrode,” J. Appl. Electrochem., vol. 22, no. 1, pp. 21–27, 1992, doi: 10.1007/BF01093007. [51] F. C. Tompkins, “The kinetics of the reaction between manganous and permanganate ions.,” 1941. [52] S. Devaraj and N. Munichandraiah, “Effect of crystallographic structure of MnO2 on its electrochemical capacitance properties,” J. Phys. Chem. C, vol. 112, no. 11, pp. 4406–4417, 2008, doi: 10.1021/jp7108785. [53] C. J. Clarke, G. J. Browning, and S. W. Donne, “An RDE and RRDE study into the electrodeposition of manganese dioxide,” Electrochim. Acta, vol. 51, no. 26, pp. 5773–5784, 2006, doi: 10.1016/j.electacta.2006.03.013. [54] G. M. Jacob and I. Zhitomirsky, “Microstructure and properties of manganese dioxide films prepared by electrodeposition,” Appl. Surf. Sci., vol. 254, no. 20, pp. 6671–6676, 2008, doi: 10.1016/j.apsusc.2008.04.044. [55] F. R. A. Jorgensen, “Cell Voltage during the Electrolytic Production of Manganese Dioxide,” J. Electrochem. Soc., vol. 117, no. 2, p. 275, 1970, doi: 10.1149/1.2407486. [56] S. Bodoardo, J. Brenet, M. Maja, and P. Spinelli, “Electrochemical behaviour of MnO2 electrodes in sulphuric acid solutions,” Electrochim. Acta, vol. 39, no. 13, pp. 1999–2004,  87  1994, doi: 10.1016/0013-4686(94)85080-1. [57] C. C. Liang, “Manganese in, Encyclopedia of electrochemistry of the elements,” Encyclopedia of electrochemistry of the elements, vol. 1. Marcel Dekker, p. 349, 1973. [58] F. Mohammadi, M. Tunnicliffe, and A. Alfantazi, “Corrosion Assessment of Lead Anodes in Nickel Electrowinning,” J. Electrochem. Soc., vol. 158, no. 12, p. C450, 2011, doi: 10.1149/2.063112jes. [59] J. J. Lander, “Further Studies on the Anodic Corrosion of Lead in H2SO4 Solutions,” J. Electrochem. Soc., vol. 103, no. 1, p. 1, 1956, doi: 10.1149/1.2430227. [60] K. Maksymiuk, J. Stroka, and Z. Galus, “Chemistry of Lead,” Encycl. Electrochem. Power Sources, pp. 762–771, 2009. [61] A. J. Bard and J. A. A. Ketelaar, “Encyclopedia of Electrochemistry of the Elements,” J. Electrochem. Soc., vol. 121, no. 6, p. 212C, 1974, doi: 10.1149/1.2402383. [62] P. Ruetschi and R. T. Angstadt, “Anodic Oxidation of Lead at Constant Potential,” J. Electrochem. Soc., vol. 111, no. 12, p. 1323, 1964, doi: 10.1149/1.2425996. [63] C. Comninellis and G. P. Vercesi, “Characterization of DSA-Type electrodes : choice of a coating,” J. Appl. Electrochem., vol. 21, pp. 335–345, 1991, doi: 10.1016/0040-6031(91)80257-J. [64] G. N. Martelli, R. Ornelas, and G. Faita, “Deactivation mechanisms of oxygen evolving anodes at high current densities,” Electrochim. Acta, vol. 39, no. 11–12, pp. 1551–1558, 1994, doi: 10.1016/0013-4686(94)85134-4. [65] S. Kulandaisamy et al., “Performance of catalytically activated anodes in the electrowinning of metals,” J. Appl. Electrochem., vol. 27, no. 5, pp. 579–583, 1997, doi: 10.1023/A:1018454830073.  88  [66] M. Musiani, F. Furlanetto, and R. Bertoncello, “Electrodeposited PbO2+RuO2: a composite anode for oxygen evolution from sulphuric acid solution,” J. Electroanal. Chem., no. 465, pp. 160–167, 1998, doi: 10.1007/BF01023725. [67] M. Hamdani, N. Singh, and P. Chartier, “Co3O4 and Co-Based Spinel Oxides Bifunctional Oxygen Electrodes,” Int. J. Electrochem. Sci. Int. J. Electrochem. Sci., vol. 5, pp. 556–577, 2010. [68] S. Trasatti, “Electrocatalysis in the anodic evolution of oxygen and chlorine,” Electrochim. Acta, vol. 29, no. 11, pp. 1503–1512, 1984, doi: 10.1016/0013-4686(84)85004-5. [69] S. Trasatti, “Preliminary note ELECTROCATALYSIS BY OXIDES -- ATTEMPT AT A UNIFYING APPROACH,” vol. 111, pp. 125–131, 1980. [70] D. Tench, “Electrodeposition of Conducting Transition Metal Oxide/Hydroxide Films from Aqueous Solution,” J. Electrochem. Soc., vol. 130, no. 4, p. 869, 1983, doi: 10.1149/1.2119838. [71] W. Wei, X. Cui, X. Mao, W. Chen, and D. G. Ivey, “Morphology evolution in anodically electrodeposited manganese oxide nanostructures for electrochemical supercapacitor applications - Effect of supersaturation ratio,” Electrochim. Acta, vol. 56, no. 3, pp. 1619–1628, 2011, doi: 10.1016/j.electacta.2010.10.044. [72] C. W. Lee, K. W. Nam, B. W. Cho, and K. B. Kim, “Electrochemical synthesis of meso-structured lamellar manganese oxide thin film,” Microporous Mesoporous Mater., vol. 130, no. 1–3, pp. 208–214, 2010, doi: 10.1016/j.micromeso.2009.11.008. [73] C. C. L. McCrory, S. Jung, J. C. Peters, and T. F. Jaramillo, “Benchmarking heterogeneous electrocatalysts for the oxygen evolution reaction,” J. Am. Chem. Soc., vol.  89  135, no. 45, pp. 16977–16987, 2013, doi: 10.1021/ja407115p. [74] A. Biswal, B. C. Tripathy, K. Sanjay, T. Subbaiah, and M. Minakshi, “Electrolytic manganese dioxide (EMD): A perspective on worldwide production, reserves and its role in electrochemistry,” RSC Adv., vol. 5, no. 72, pp. 58255–58283, 2015, doi: 10.1039/c5ra05892a. [75] C. Julien, M. Massot, S. Rangan, M. Lemal, and D. Guyomard, “Study of structural defects in γ-MnO2 by Raman spectroscopy,” J. Raman Spectrosc., vol. 33, no. 4, pp. 223–228, 2002, doi: 10.1002/jrs.838. [76] T. Hayashi, N. Bonnet-Mercier, A. Yamaguchi, K. Suetsugu, and R. Nakamura, “Electrochemical characterization of manganese oxides as a water oxidation catalyst in proton exchange membrane electrolysers,” R. Soc. Open Sci., vol. 6, no. 5, 2019, doi: 10.1098/rsos.190122. [77] D. Pavlov, C. N. Poulieff, E. Klaja, and N. Iordanov, “Dependence of the Composition of the Anodic Layer on the Oxidation Potential of Lead in Sulfuric Acid,” J. Electrochem. Soc., vol. 116, no. 3, p. 316, 1969, doi: 10.1149/1.2411836. [78] Y. Yamamoto, K. Fumino, T. Ueda, and M. Nambu, “A potentiodynamic study of the lead electrode in sulphuric acid solution,” Electrochim. Acta, vol. 37, no. 2, pp. 199–203, 1992, doi: 10.1016/0013-4686(92)85003-4. [79] M. Nicol, C. Akilan, V. Tjandrawan, and J. A. Gonzalez, “Hydrometallurgy Effect of halides in the electrowinning of zinc . II . Corrosion of lead-silver anodes,” in Hydrometallurgy, 2017, vol. 173, no. May, pp. 178–191, doi: 10.1016/j.hydromet.2017.08.017. [80] J. Rodrigues, D. Garbers, and E. H. O. Meyer, “Recent developments in the Zincor cell  90  house,” Can. Metall. Q., vol. 40, no. 4, pp. 441–449, 2001, doi: 10.1179/cmq.2001.40.4.441. [81] M. Mohammadi and A. Alfantazi, “Evaluation of manganese dioxide deposition on lead-based electrowinning anodes,” Hydrometallurgy, vol. 159, pp. 28–39, 2016, doi: 10.1016/j.hydromet.2015.10.023. [82] M. J. Polissar, “The kinetics of the reaction between permanganate and manganous ions,” J. Phys. Chem., vol. 39, no. 8, pp. 1057–1066, 1935, doi: 10.1021/j150368a002. [83] Teck Resources Limited, “Teck’s Zinc Facts,” 2019. [Online]. Available: https://www.teck.com/products/zinc/. [84] Hydro-Québec, “Comparison of electricity prices in major North American cities,” Hydro-Québec, pp. 27–28, 2015.   91  Appendices Appendix A   A.1 CV Electrodeposition in the Absence and Presence of Mn(II) Figure A-1 demonstrates that the sole factor responsible for the higher measured current density of the OER in the manganese-containing electrodeposition electrolyte is the manganese oxidation, which takes place at the same potentials as the OER and its peak is hidden by the OER.   Figure A-1: Comparison of the first cycle of the CV electrodeposition procedure at 1 mV s-1 using the Pb-(0.75 wt.%)Ag substrate as the working electrode inside the electrodeposition electrolyte composed 0.1 M Na2SO4 with and without 0.3 M Mn(II) from Mn(CH3COO)2 at room temperature.   92  A.2 EDX Elemental Mapping Results A.2.i Baseline Pb-Ag Anode Elemental Mapping        93  Figure A-2: EDX elemental mapping of the baseline Pb-Ag anode.  A.2.ii CV-Deposited Pb-Ag Anode Elemental Mapping        94    Figure A-3: EDX elemental mapping of the CV deposited Pb-Ag anode.                95  A.2.iii LSV-Deposited Pb-Ag Anode Elemental Mapping        96    Figure A-4: EDX elemental mapping of the LSV deposited Pb-Ag anode.                97  A.2.iv CCD-Deposited Pb-Ag Anode Elemental Mapping         98    Figure A-5: EDX elemental mapping of the CCD deposited Pb-Ag anode.                99  A.3 Large Surface Area SEM/EDX Analysis A.3.i CV-Deposited Pb-Ag Anode   Pb O Mn S  100    Pb Mn  101    O S  102  A.3.ii CCD-Deposited Pb-Ag Anode   Pb O Mn S  103    Pb Mn  104   O S  105  A.3.iii LSV-Deposited Pb-Ag Anode Since the sample was titlted in the case of the LSV-deposited Pb-Ag anode, this analysis was not successful.   106  A.4 Anodic Half-cell Potentiodynamic Polarization Experimental Data A.1.i Baseline Pb-Ag Anodes   Figure A-6: Potentiodynamic polarization curves (10 LSV scans) of the baseline Pb-Ag anode inside the 160 g L-1  sulfuric acid electrolyte with 3 g L-1 Mn2+ and 0.3 g L-1 Cl- at 37℃ at 1 mV s-1 scan rate.      107   Figure A-7: Potentiodynamic polarization curves (10 LSV scans) of the baseline Pb-Ag anode inside the 160 g L-1  sulfuric acid electrolyte with 3 g L-1 Mn2+ and 0.3 g L-1 Cl- at 37℃ at 1 mV s-1 scan rate.   108   Figure A-8: Potentiodynamic polarization curves (10 LSV scans) of the baseline Pb-Ag anode inside the 160 g L-1  sulfuric acid electrolyte with 3 g L-1 Mn2+ and 0.3 g L-1 Cl- at 37℃ at 1 mV s-1 scan rate.          109  A.1.ii MnOx CV-Deposited Pb-Ag Anodes  Figure A-9: Potentiodynamic polarization curves (10 LSV scans) of the MnOx CV deposited Pb-Ag inside the 160 g L-1  sulfuric acid electrolyte with 3 g L-1 Mn2+ and 0.3 g L-1 Cl- at 37℃ at 1 mV s-1 scan rate.    110   Figure A-10: Potentiodynamic polarization curves (10 LSV scans) of the MnOx CV deposited Pb-Ag anode inside the 160 g L-1  sulfuric acid electrolyte with 3 g L-1 Mn2+ and 0.3 g L-1 Cl- at 37℃ at 1 mV s-1 scan rate.      111   Figure A-11: Potentiodynamic polarization curves (10 LSV scans) of the MnOx CV deposited Pb-Ag anode inside the 160 g L-1  sulfuric acid electrolyte with 3 g L-1 Mn2+ and 0.3 g L-1 Cl- at 37℃ at 1 mV s-1 scan rate.         112  A.1.iii MnOx CCD-Deposited Pb-Ag Anodes   Figure A-12: Potentiodynamic polarization curves (10 LSV scans) of the MnOx CCD deposited Pb-Ag anode inside the 160 g L-1  sulfuric acid electrolyte with 3 g L-1 Mn2+ and 0.3 g L-1 Cl- at 37℃ at 1 mV s-1 scan rate.    113   Figure A-13: Potentiodynamic polarization curves (10 LSV scans) of the MnOx CCD deposited Pb-Ag anode inside the 160 g L-1  sulfuric acid electrolyte with 3 g L-1 Mn2+ and 0.3 g L-1 Cl- at 37℃ at 1 mV s-1 scan rate.         114  A.1.iv MnOx LSV-Deposited Pb-Ag Anodes  Figure A-14: Potentiodynamic polarization curves (10 LSV scans) of the MnOx LSV deposited Pb-Ag anode inside the 160 g L-1  sulfuric acid electrolyte with 3 g L-1 Mn2+ and 0.3 g L-1 Cl- at 37℃ at 1 mV s-1 scan rate.   115   Figure A-15: Potentiodynamic polarization curves (10 LSV scans) of the MnOx LSV deposited Pb-Ag anode inside the 160 g L-1  sulfuric acid electrolyte with 3 g L-1 Mn2+ and 0.3 g L-1 Cl- at 37℃ at 1 mV s-1 scan rate.         116  A.5 Comparison of the 8th, 9th, and 10th LSV Scan of the Baseline and MnOx Deposited Pb-Ag Anodes  Figure A-16: OER electrocatalytic activity comparison of the baseline and MnOx electrodeposited Pb-Ag anodes indicated by the comparison of their 8th linear scan voltammogram in the OER potential range; Scan rate=5 mV s-1; Electrolyte composition: 160 g L-1 sulfuric acid solution with 3 g L-1 Mn2+ and 0.3 g L-1 Cl-; T=37℃.    117   Figure A-17: OER electrocatalytic activity comparison of the baseline and MnOx electrodeposited Pb-Ag anodes indicated by the comparison of their 9th linear scan voltammogram in the OER potential range; Scan rate=5 mV s-1; Electrolyte composition: 160 g L-1 sulfuric acid solution with 3 g L-1 Mn2+ and 0.3 g L-1 Cl-; T=37℃.   118    Figure A-18: OER electrocatalytic activity comparison of the baseline and MnOx electrodeposited Pb-Ag anodes indicated by the comparison of their 10th linear scan voltammogram in the OER potential range; Scan rate=5 mV s-1; Electrolyte composition: 160 g L-1 sulfuric acid solution with 3 g L-1 Mn2+ and 0.3 g L-1 Cl-; T=37℃.         119  A.6 XRF Mapping of the Manganese Mass Loading  Figure A-19: 2-Dimensional (side-view) mapping of the manganese mass loading on the surface of the CV-deposited Pb-Ag anode obtained using the XRF equipment.   120   Figure A-20: 2-Dimensional (top-view) mapping of the manganese mass loading on the surface of the CV-deposited Pb-Ag anode obtained using the XRF equipment. Appendix B   B.1 Conversion to SHE (Primary Reference) from MSE For easier comparison purposes, the measured potentials against Hg/Hg2SO4 (s)/ K2SO4 (sat) (saturated sulfate electrode or MSE) reference electrode are converted to a primary reference such as SHE, according to the following equation: 𝐸𝐸(𝑉𝑉 𝑣𝑣𝑠𝑠. 𝑍𝑍𝐻𝐻𝐸𝐸) = 𝐸𝐸(𝑉𝑉 𝑣𝑣𝑠𝑠.𝑀𝑀𝑍𝑍𝐸𝐸) + 𝐸𝐸𝑐𝑐,𝑀𝑀𝑆𝑆𝐻𝐻  In the above-mentioned equation, the term 𝐸𝐸(𝑉𝑉 𝑣𝑣𝑠𝑠. 𝑍𝑍𝐻𝐻𝐸𝐸) indicates the converted electrode potential versus the SHE primary reference electrode, 𝐸𝐸(𝑉𝑉 𝑣𝑣𝑠𝑠.𝑀𝑀𝑍𝑍𝐸𝐸) refers to the measured working electrode potential versus the actual MSE reference electrode, and 𝐸𝐸𝑐𝑐,𝑀𝑀𝑆𝑆𝐻𝐻 is the tabulated MSE reference electrode equilibrium potential versus SHE, which in this case is indicated to be  121  𝐸𝐸𝑐𝑐,𝑀𝑀𝑆𝑆𝐻𝐻 = 0.650 𝑉𝑉 𝑣𝑣𝑠𝑠. 𝑍𝑍𝐻𝐻𝐸𝐸, according to the reference electrode manual (MSE single junction reference electrode by Pine Research Instrumentation Inc.). B.2 Current Density Selection for Chronopotentiometric Deposition The current density to be employed for chronopotentiometric electrodeposition was determined according to the following equation for the mass transfer limiting current density (𝑖𝑖𝐿𝐿): 𝑖𝑖𝐿𝐿 = 𝑧𝑧𝐹𝐹𝑠𝑠𝑗𝑗 𝐾𝐾𝑚𝑚,𝑗𝑗𝐶𝐶𝑗𝑗 In which, 𝑧𝑧 [𝑚𝑚𝑚𝑚𝐶𝐶]  indicates the number of electrons being transferred in the half-cell equation governing the electrodeposition procedure, 𝐹𝐹 [𝐶𝐶 𝑚𝑚𝑚𝑚𝐶𝐶−1] is the Faraday’s constant, 𝑠𝑠𝑗𝑗 is the stoichiometric coefficient of the limiting species, 𝐾𝐾𝑚𝑚,𝑗𝑗 [𝑚𝑚 𝑠𝑠−1] is the mass transfer coefficient, and 𝐶𝐶𝑗𝑗[𝑚𝑚𝑚𝑚𝐶𝐶 𝑚𝑚−3] is the bulk electrolyte concentration. The electrodeposition equation corresponding to manganese (II) oxidation can be written as: 𝑀𝑀𝑍𝑍2+ + 2𝐻𝐻2𝑂𝑂 → 𝑀𝑀𝑍𝑍𝑂𝑂2 + 4𝐻𝐻+ + 2𝑒𝑒−; 𝐸𝐸298𝐾𝐾,   𝑀𝑀𝑍𝑍2+/𝑀𝑀𝑍𝑍𝑂𝑂2° = 1.22 𝑉𝑉𝑆𝑆𝐻𝐻𝐻𝐻     Thus, using an electrolyte composed of 0.3 M manganese (II) acetate solution ( 𝐶𝐶𝑗𝑗 =300 [𝑚𝑚𝑚𝑚𝐶𝐶 𝑚𝑚−3]), the required current density for this electrodeposition procedure was calculated as following: 𝑖𝑖𝐿𝐿 = 2 (𝑚𝑚𝑚𝑚𝐶𝐶)96,485 (𝐶𝐶 𝑚𝑚𝑚𝑚𝐶𝐶−1)1 (10−6𝑚𝑚 𝑠𝑠−1)(300 𝑚𝑚𝑚𝑚𝐶𝐶 𝑚𝑚−3) 𝑖𝑖𝐿𝐿 = 57.9 (𝐴𝐴 𝑚𝑚−2) ≅ 60 (𝐴𝐴 𝑚𝑚−2) 𝑚𝑚𝑜𝑜 6 (𝑚𝑚𝐴𝐴 𝑐𝑐𝑚𝑚−2) Assuming a 100% current efficiency for the manganese oxidation half-cell reaction, the corresponding charge for the chronopotentiometric technique using the above calculated current density of 6 mA cm-2 for a substrate surface area of 1 cm2 (𝐴𝐴 = 1 𝑐𝑐𝑚𝑚2) and one hour of electrodeposition (𝑡𝑡 = 3,600 𝑠𝑠) is calculated:  122  𝑖𝑖𝐿𝐿𝐴𝐴𝑡𝑡 = 6 × 10−3(𝐴𝐴 𝑐𝑐𝑚𝑚−2) 1(𝑐𝑐𝑚𝑚2) 3,600(𝑠𝑠) 𝑖𝑖𝐿𝐿𝐴𝐴𝑡𝑡 = 21.6 𝐶𝐶 The charge associated with the oxidation of one mole of Mn(II) corresponding to 86.9 g of MnO2 can be determined: 𝑍𝑍𝐹𝐹 = 2 (𝑚𝑚𝑚𝑚𝐶𝐶) 96,485(𝐶𝐶 𝑚𝑚𝑚𝑚𝐶𝐶−1) 𝑍𝑍𝐹𝐹 = 192,970 (𝐶𝐶)  Therefore, the mass of the MnO2 film and Mn particles to be electrodeposited on the Pb-Ag substrate using this technique can be calculated to be 9.73 and 6.15 mg cm-2, respectively. The measured area specific manganese mass loading associated with the CCD-deposited sample is 0.117 mg cm-2. The efficiency of the chronopotentiometric deposition technique for MnOx electrodeposition on the given substrate can be computed to be 1.9%.  B.3 OER Overpotential Computation  In this context, overpotential is experimentally determined by subtracting the thermodynamically predicted potential for the OER half-cell reaction from the measured potential at a given current density (10 mA cm-2) [73]. To calculate the standard equilibrium potential of the OER (𝐸𝐸𝑂𝑂𝐻𝐻𝑂𝑂,298𝐾𝐾° ), the following constants are obtained from the Lange’s Handbook: 𝑂𝑂2 + 4𝐻𝐻+ + 4𝑒𝑒− → 2𝐻𝐻2𝑂𝑂 Table B-1: Thermodynamic properties required for calculation of the standard equilibrium potential of OER. Substance (Physical State) ∆𝒇𝒇𝑯𝑯° (kJ.mol-1) ∆𝒇𝒇𝑮𝑮° (kJ.mol-1) 𝑺𝑺°(J.deg-1.mol-1) 𝐇𝐇𝟐𝟐𝐎𝐎 (l) -285.830 -237.14 69.950 𝐎𝐎𝟐𝟐 (g) 0 0 205.152  123  𝐇𝐇+ (aq) 0 0 0  ∆𝐺𝐺° = �∆𝑓𝑓𝐺𝐺°𝑜𝑜𝑟𝑟𝑜𝑜𝑝𝑝𝑝𝑝𝑐𝑐𝑜𝑜𝑠𝑠 −�∆𝑓𝑓𝐺𝐺°𝑟𝑟𝑐𝑐𝑟𝑟𝑐𝑐𝑜𝑜𝑟𝑟𝑍𝑍𝑜𝑜𝑠𝑠𝑖𝑖𝑖𝑖 ∆𝐺𝐺° = 2∆𝑓𝑓𝐺𝐺°𝐻𝐻2𝑂𝑂 − ∆𝑓𝑓𝐺𝐺°𝑂𝑂2 − 4∆𝑓𝑓𝐺𝐺°𝐻𝐻+ ∆𝐺𝐺° = −474.280 (kJ.mol-1) The standard equilibrium potential of a half-cell reaction can be calculated from the change in standard Gibbs free energy as shown: ∆𝐺𝐺°𝑂𝑂𝐻𝐻𝑂𝑂 = −𝑍𝑍𝐹𝐹𝐸𝐸𝑂𝑂𝐻𝐻𝑂𝑂,298𝐾𝐾°  𝐸𝐸𝑂𝑂𝐻𝐻𝑂𝑂,298𝐾𝐾° = −∆𝐺𝐺°𝑂𝑂𝐻𝐻𝑂𝑂𝑍𝑍𝐹𝐹  𝐸𝐸𝑂𝑂𝐻𝐻𝑂𝑂,298𝐾𝐾° = − (−474,280 � 𝐽𝐽𝑚𝑚𝑚𝑚𝐶𝐶�)4 𝑚𝑚𝑚𝑚𝐶𝐶 × 96,485 (𝐶𝐶𝑚𝑚𝐶𝐶𝑚𝑚𝑚𝑚𝐶𝐶) 𝐸𝐸𝑂𝑂𝐻𝐻𝑂𝑂,298𝐾𝐾° = 1.230 𝑉𝑉 The standard potential at a new temperature can be calculated using the following equation: 𝐸𝐸𝑂𝑂𝐻𝐻𝑂𝑂,𝑇𝑇° = 𝐸𝐸𝑂𝑂𝐻𝐻𝑂𝑂,298𝐾𝐾° + ∆𝑍𝑍298𝐾𝐾°4𝐹𝐹 (𝑇𝑇 − 298𝐾𝐾) Where the reaction temperature is 𝑇𝑇 = 37 + 273 = 310 𝐾𝐾 and 𝐸𝐸𝑂𝑂𝐻𝐻𝑂𝑂,298𝐾𝐾° = 1.229 𝑉𝑉. ∆𝑍𝑍298𝐾𝐾° = 2𝑍𝑍298,𝐻𝐻2𝑂𝑂(𝑐𝑐)° − 𝑍𝑍298,𝑂𝑂2° − 4𝑍𝑍298,𝐻𝐻+°  Using the standard entropies obtained from the Lange’s Handbook and shown in the above table: ∆𝑍𝑍298𝐾𝐾° = −65.252 ( 𝐽𝐽𝑚𝑚𝑚𝑚𝐶𝐶.𝐾𝐾)  124  𝐸𝐸𝑂𝑂𝐻𝐻𝑂𝑂,310𝐾𝐾° = 1.230 𝑉𝑉 + −65.252 ( 𝐽𝐽𝑚𝑚𝑚𝑚𝐶𝐶.𝐾𝐾)4 𝑚𝑚𝑚𝑚𝐶𝐶 × �96,485 𝐶𝐶𝑚𝑚𝑚𝑚𝐶𝐶� (310 − 298) 𝐸𝐸𝑂𝑂𝐻𝐻𝑂𝑂,310𝐾𝐾° = 1.228 𝑉𝑉𝑆𝑆𝐻𝐻𝐻𝐻  In order to calculate the non-standard equilibrium potential for the OER (𝐸𝐸𝑂𝑂𝐻𝐻𝑂𝑂,𝑇𝑇) using the experimental conditions, the Nernst equation is applied for the OER as following: 𝐸𝐸𝑂𝑂𝐻𝐻𝑂𝑂,𝑇𝑇 = 𝐸𝐸𝑂𝑂𝐻𝐻𝑂𝑂,𝑇𝑇° − 𝑅𝑅𝑇𝑇𝑍𝑍𝐹𝐹 ln𝐾𝐾 Where, K is the half-cell reaction quotient and can be determined as following: 𝐾𝐾 = �𝑎𝑎𝑗𝑗𝑠𝑠𝑗𝑗𝑗𝑗 𝐾𝐾𝑂𝑂𝐻𝐻𝑂𝑂 = 𝑎𝑎𝑂𝑂2𝑎𝑎𝐻𝐻+4𝑎𝑎𝐻𝐻2𝑂𝑂2  Assuming that liquid water is present in bulk (excess) (𝑎𝑎𝐻𝐻2𝑂𝑂 ≈ 1) and that oxygen is present in excess (𝑎𝑎𝑂𝑂2 ≈ 1), the above quotient can be simplified as following: 𝐾𝐾𝑂𝑂𝐻𝐻𝑂𝑂 = 𝑎𝑎𝐻𝐻+4  Nernst equation with simplifying assumptions: 𝐸𝐸𝑐𝑐,𝑇𝑇 = 𝐸𝐸𝑐𝑐,𝑇𝑇° − 𝑅𝑅𝑇𝑇4𝐹𝐹 𝐶𝐶𝑍𝑍 𝑎𝑎𝐻𝐻2𝑂𝑂2𝑎𝑎𝑂𝑂2𝑎𝑎𝐻𝐻+4  𝐸𝐸𝑐𝑐,𝑇𝑇 = 𝐸𝐸𝑐𝑐,𝑇𝑇° − 𝑅𝑅𝑇𝑇4𝐹𝐹 𝐶𝐶𝑍𝑍 1𝐶𝐶𝐻𝐻+4  𝐶𝐶𝑍𝑍 = 2.303 𝐶𝐶𝑚𝑚𝑙𝑙 𝐸𝐸𝑐𝑐,𝑇𝑇 = 𝐸𝐸𝑐𝑐,𝑇𝑇° − 𝑅𝑅𝑇𝑇4𝐹𝐹 1𝐶𝐶𝐻𝐻+4  𝐸𝐸𝑐𝑐,𝑇𝑇 = 𝐸𝐸𝑐𝑐,𝑇𝑇° + 𝑅𝑅𝑇𝑇𝐹𝐹 𝐶𝐶𝑍𝑍𝐶𝐶𝐻𝐻+  125  𝐸𝐸𝑐𝑐,𝑇𝑇 = 𝐸𝐸𝑐𝑐,𝑇𝑇° + 2.303𝑅𝑅𝑇𝑇𝐹𝐹 𝐶𝐶𝑚𝑚𝑙𝑙𝐶𝐶𝐻𝐻+ 𝑝𝑝𝐻𝐻 = −log (𝐶𝐶𝐻𝐻+) 𝐸𝐸𝑐𝑐,𝑇𝑇 = 𝐸𝐸𝑐𝑐,𝑇𝑇° − 2.303𝑅𝑅𝑇𝑇𝐹𝐹 𝑝𝑝𝐻𝐻 𝑬𝑬𝑶𝑶𝑬𝑬𝑶𝑶,𝟑𝟑𝟑𝟑𝟑𝟑 𝑲𝑲 = 𝟑𝟑.𝟐𝟐𝟐𝟐𝟑𝟑 𝑽𝑽 B.4 IR-Drop Compensation Calculation  Electrochemical impedance spectroscopy (EIS) measurements were performed inside the described electrowinning electrolyte in order to determine the uncompensated solution resistance (𝑅𝑅𝑠𝑠) between the working and reference electrodes and correct the measured potentials, accordingly. The uncompensated solution resistance is extrapolated by the intersection of the extension of the obtained Nyquist plot with the real axis value at high frequency intercept (close to the plot origin). The measured value for the uncompensated solution resistance in this highly conductive electrolyte was determined to be approximately 1.10 Ω cm-2. Thus, using the Ohm’s Law, the potential loss due to the solution resistance as the applied current density of 500 A m-2 was estimated according to the following equation: 𝐸𝐸𝑜𝑜ℎ𝑚𝑚𝑖𝑖𝑐𝑐 𝑐𝑐𝑜𝑜𝑠𝑠𝑠𝑠 = 𝐼𝐼.𝑅𝑅𝑠𝑠 The estimated ohmic potential loss was deducted from all the potentials measured inside the described electrolyte in order to subtract the effect of uncompensated solution resistance and the positioning of the working and reference electrodes. The uncompensated solution resistance was estimated to be: 𝐸𝐸𝑜𝑜ℎ𝑚𝑚𝑖𝑖𝑐𝑐 𝑐𝑐𝑜𝑜𝑠𝑠𝑠𝑠 = 50 (𝑚𝑚𝐴𝐴 𝑐𝑐𝑚𝑚−2). 1.10 ( Ω cm−2)  𝐸𝐸𝑜𝑜ℎ𝑚𝑚𝑖𝑖𝑐𝑐 𝑐𝑐𝑜𝑜𝑠𝑠𝑠𝑠 = 55 (𝑚𝑚𝑉𝑉)  126  B.5 Annual Energy Consumption and Electricity Costs of Zinc Electrowinning Calculation: As explained in Section 3.3, Faraday’s Law indicates: 𝜀𝜀𝐼𝐼 = 𝑍𝑍𝐹𝐹𝑧𝑧𝑡𝑡= 𝑚𝑚𝑀𝑀𝐹𝐹𝑧𝑧𝑡𝑡 Where 𝐼𝐼 (𝐴𝐴) is the applied current of the galvanostatic process, 𝜀𝜀 is the average zinc current efficiency, 𝑍𝑍 (𝑚𝑚𝑚𝑚𝐶𝐶) is the number of moles of the zinc transported, 𝐹𝐹 (𝐶𝐶 𝑚𝑚𝑚𝑚𝐶𝐶−1) is the Faraday’s constant, 𝑧𝑧 is the ionic valence zinc, 𝑡𝑡 (𝑠𝑠) is the deposition time, 𝑚𝑚 (𝑘𝑘𝑙𝑙) is the mass of deposited zinc, 𝑀𝑀 (𝑘𝑘𝑚𝑚𝑚𝑚𝐶𝐶) is the molar mass of zinc. This equation can be rearrange to write: 𝑡𝑡𝑚𝑚= 𝐹𝐹𝑧𝑧𝜀𝜀𝐼𝐼𝑀𝑀 According to Joule’s Law, the electric power 𝑃𝑃 (𝑊𝑊) in terms of overall cell potential 𝐸𝐸 (𝑉𝑉) and applied current 𝐼𝐼 (𝐴𝐴) can be defined as following: 𝑃𝑃 = 𝑉𝑉𝐼𝐼 Therefore, the previously-mentioned equation can be written as following: 𝑡𝑡𝑚𝑚= 𝐹𝐹𝑧𝑧𝑉𝑉𝜀𝜀𝑃𝑃𝑀𝑀 After rearrangement and unit conversion for time: 𝑃𝑃𝑡𝑡𝑚𝑚= 𝐹𝐹𝑧𝑧𝑉𝑉𝜀𝜀𝑀𝑀× 1 ℎ3600 𝑠𝑠 The left-hand side of the above equation carries the unit of (𝑘𝑘𝑊𝑊ℎ 𝑘𝑘𝑙𝑙−1) and it refers to the specific energy consumption (𝐸𝐸) of the zinc electrowinning process:  𝐸𝐸 = 𝑃𝑃𝑡𝑡𝑚𝑚= 𝐹𝐹𝑧𝑧𝑉𝑉𝜀𝜀𝑀𝑀× 1 ℎ3600 𝑠𝑠 The above equation in units of (𝑘𝑘𝑊𝑊ℎ 𝑡𝑡𝑚𝑚𝑍𝑍−1) can be defined as following:  127  𝐸𝐸 = 𝐹𝐹𝑧𝑧𝑉𝑉𝜀𝜀𝑀𝑀(3.6) In this study, using the Pb-Ag anodes, a full electrowinning average cell potential of 3.222 V was measured. Thus, the specific energy consumption of this process using the baseline Pb-Ag anode and assuming an average zinc current efficiency of 0.91 was determined as it follows [6]: 𝐸𝐸𝐵𝐵𝑟𝑟𝑠𝑠𝑐𝑐𝑐𝑐𝑖𝑖𝑍𝑍𝑐𝑐 = 96,485 (𝐶𝐶 𝑚𝑚𝑚𝑚𝐶𝐶−1) 2 × 3.222 (𝑉𝑉)  (0.91)65.38 (𝑘𝑘𝑙𝑙 𝑘𝑘𝑚𝑚𝑚𝑚𝐶𝐶−1𝑍𝑍𝑍𝑍) 3.6  𝐸𝐸𝐵𝐵𝑟𝑟𝑠𝑠𝑐𝑐𝑐𝑐𝑖𝑖𝑍𝑍𝑐𝑐 = 2,903 (𝑘𝑘𝑊𝑊ℎ 𝑡𝑡𝑚𝑚𝑍𝑍−1) = 2.903 (𝑘𝑘𝑊𝑊ℎ 𝑘𝑘𝑙𝑙−1) Assuming an annual refined zinc production capacity of 287,000 (tons), as reported by Teck Resources Limited at BC Trails [83], the annual required energy for the tank house 𝐸𝐸𝐵𝐵𝑟𝑟𝑠𝑠𝑐𝑐𝑐𝑐𝑖𝑖𝑍𝑍𝑐𝑐 employing the baseline Pb-Ag anode can be computed accordingly: 𝐸𝐸𝐵𝐵𝑟𝑟𝑠𝑠𝑐𝑐𝑐𝑐𝑖𝑖𝑍𝑍𝑐𝑐 =  2,903 (𝑘𝑘𝑊𝑊ℎ 𝑡𝑡𝑚𝑚𝑍𝑍−1)287,000 (𝑡𝑡𝑚𝑚𝑍𝑍) 𝐸𝐸𝐵𝐵𝑟𝑟𝑠𝑠𝑐𝑐𝑐𝑐𝑖𝑖𝑍𝑍𝑐𝑐 = 8.33 × 108(𝑘𝑘𝑊𝑊ℎ 𝑦𝑦𝑒𝑒𝑎𝑎𝑜𝑜−1) An average electricity cost 𝑃𝑃($ 𝑘𝑘𝑊𝑊ℎ−1) of 0.07 $ kWh-1 is reported for industrial large-scale companies in BC [84]. Therefore, the annual energy cost for this tank house 𝑃𝑃𝐵𝐵𝑟𝑟𝑠𝑠𝑐𝑐𝑐𝑐𝑖𝑖𝑍𝑍𝑐𝑐 employing the baseline Pb-Ag anode can be estimated as following: 𝑃𝑃𝐵𝐵𝑟𝑟𝑠𝑠𝑐𝑐𝑐𝑐𝑖𝑖𝑍𝑍𝑐𝑐 = 𝐸𝐸𝐵𝐵𝑟𝑟𝑠𝑠𝑐𝑐𝑐𝑐𝑖𝑖𝑍𝑍𝑐𝑐,   𝑜𝑜𝑐𝑐𝑟𝑟 𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐 𝑃𝑃 𝑃𝑃𝐵𝐵𝑟𝑟𝑠𝑠𝑐𝑐𝑐𝑐𝑖𝑖𝑍𝑍𝑐𝑐 = 8.33 × 108(𝑘𝑘𝑊𝑊ℎ 𝑦𝑦𝑒𝑒𝑎𝑎𝑜𝑜−1) 0.07 ($ 𝑘𝑘𝑊𝑊ℎ) 𝑃𝑃𝐵𝐵𝑟𝑟𝑠𝑠𝑐𝑐𝑐𝑐𝑖𝑖𝑍𝑍𝑐𝑐 = 5.49 × 107($ 𝑦𝑦𝑒𝑒𝑎𝑎𝑜𝑜−1) The above-mentioned calculations were repeated for a tank house using the novel anodes and the resulting cost benefits as compared to the baseline were estimated. This economic analysis assumes equal electrode lifetime and manufacturing costs. A more inclusive study is required in  128  order to take optimize the production procedure of the novel anodes and study their performance and durability during longer-time zinc electrowinning operations.   

Cite

Citation Scheme:

        

Citations by CSL (citeproc-js)

Usage Statistics

Share

Embed

Customize your widget with the following options, then copy and paste the code below into the HTML of your page to embed this item in your website.
                        
                            <div id="ubcOpenCollectionsWidgetDisplay">
                            <script id="ubcOpenCollectionsWidget"
                            src="{[{embed.src}]}"
                            data-item="{[{embed.item}]}"
                            data-collection="{[{embed.collection}]}"
                            data-metadata="{[{embed.showMetadata}]}"
                            data-width="{[{embed.width}]}"
                            data-media="{[{embed.selectedMedia}]}"
                            async >
                            </script>
                            </div>
                        
                    
IIIF logo Our image viewer uses the IIIF 2.0 standard. To load this item in other compatible viewers, use this url:
https://iiif.library.ubc.ca/presentation/dsp.24.1-0394793/manifest

Comment

Related Items