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A Study of Molybdenum Redox Couples

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A Study of Molybdenum Redox Couples The University of New South Wales Faculty of Applied Science School of Chemical Engineering and Industrial Chemistry A Study of Molybdenum Redox Couples by PAUL PENNISI A Thesis Submitted as Part of the Requirements for the Degree of Master of Science 1995 U N S W 1 7 JUL 1997 LIBRARY CERTIFICATE OF ORIGINALITY I hereby declare that this submission is my own work and that, to the best of my knowledge and belief, it contains no material previously published or written by another person nor material which to a substantial extent has been accepted for the award of any other degree or diploma of a university or other institute of higher learning, except where due acknowledgment is made in the text. I also declare that the intellectual content of this thesis is the product of my own work, even though I may have received assistance from others on style, presentation and language expression. £maUM Paul Pennisi ACKNOWLEDGMENTS I would like to thank Professor Maria Skyllas-Kazacos for her endless help during this project and the Vanadium Battery Group whose various members have shared their knowledge and expertise, they have been a great help. My gratitude also goes out to the Australian Research Council for their support with this project. The support and knowledge gained from the staff and friendships made, in the School of Chemical Engineering and Industrial Chemistry was greatly appreciated. Much credit goes to my Mother and Father whose support saw me through my undergraduate degree. Most of all I would like to thank my wife for her patience over the last two years. ABSTRACT Evaluation and optimisation of a molybdenum redox cell was the main objective of this work. There is demand for new energy storage systems. Redox flow cells are an attractive alternative because of their long life, simplicity, low long term cost, wide range of applications, high energy efficiency, as well as being electrically rechargeable. Using a single metal ion in different oxidation states for both the positive and negative half cell electrolytes, the cross mixing, low energy density, irreversibility, and cross contamination which most redox systems suffer from can be largely eliminated. Many electrolytes, electrode materials, membranes, and molybdenum salts were screened to optimise the molybdenum redox cell. The methods used to screen the different solutions and electrodes was solubility tests of molybdenum and cyclic voltammetry. The primary criteria required for a good combination is both significant solubility (more than one mole per litre) and significant electroactivity shown by cyclic voltammetry at an electrode surface. Solubility studies of several molybdenum salts in a range of supporting electrolytes have shown that the maximum solubility of a molybdenum compound which also had significant electroactivity at a graphite electrode, was 1.6M, the salt being disodium molybdate dihydrate in 4M sulphuric acid. The kinetics of four molybdenum couples Mo(VI)/Mo(V), Mo(V)/Mo(IV), and two for Mo(IV)/Mo(III) was investigated at a glassy carbon electrode. The heterogeneous rate constants for these couples were 3.7E-4, 3.9E-5, 1.8E-4, and 2.5E-5 respectively. However, the Mo(VI)/Mo(V) reduction reaction could not be observed at the glassy carbon electrode, it was found that Mo(VI) is reduced directly to Mo(IV). The redox couples initially proposed for a Mo - Mo redox flow cell were Mo(VI)/Mo(V) and Mo(III)/Mo(IV) for the positive and negative half cell electrolytes respectively. The molybdenum cell using 1M sodium molybdate in 4M sulphuric acid as the electrolyte, with 25cm2 graphite felt electrodes, and a current density of 20mA/cnr gave an energy efficiency of more than 70% showing that it can operate as a redox cell. The coulombic efficiency was high although further improvements in the total energy efficiency would be achieved by better cell design, and a more electroactive electrode material to catalyse the Mo(VI)/Mo(V) reaction and reduce the voltage losses. Unfortunately, however, the discharge voltage was found to be 0.4V which could limit its applications. Further work is thus required to achieve higher energy efficiencies with the molybdenum redox cell. Overall the molybdenum redox cell has shown some promising features which could make it feasible as a renewable energy source although the low discharge potential does imply that practical applications would be very limited. TABLE OF CONTENTS ABSTRACT Table of Contents CHAPTER 1 INTRODUCTION CHAPTER 2 THEORETICAL BACKGROUND 2.1. Redox Flow Cell 11 2.1.1. Redox Flow Cell Advantages 11 2.1.2. Basic Requirements for a Redox Cell 12 2.2. Why molybdenum? 13 2.3. Safety, Health, and the Environment 14 2.4. Molybdenum Chemistry 15 2.4.1. Different Oxidation States of Molybdenum 17 2.4.2. Molybdenum Half Cell Reaction Standard Reduction Potentials 19 2.4.3. The Effect of pH on Molybdenum Chemistry 22 2.5. Molybdenum Blue 25 2.6. The Effect of Acid Type on Molybdenum Chemistry 29 2.6.1. Hydrochloric Acid 29 2.6.2. Sulphuric Acid 32 2.6.3. Molybdenum Salt Solubilities 34 2.7. Redox Fuel Cell Application of Mo Redox Couples 37 2.8. Battery Definitions 38 2.9. Electrochemical Kinetic Theory 39 2.9.1. Polarisation Losses 40 2.9.2. Cyclic Voltammetry 42 2.9.3. Summary of Heterogeneous Reaction Kinetics 51 2.9.3.1. Irreversible Reactions 51 2.9.3.2. Reversible Reactions 52 2.9.3.3. Quasi-reversible Reactions 53 2.9.4. Instrumentation Used for Cyclic Voltammetry 55 CHAPTER 3 SUMMARY OF LITERATURE REVIEW AND RESTATEMENT OF OBJECTIVES CHAPTER 4 EXPERIMENTAL 4.1. Chemicals and Materials 58 4.2. Instruments 59 4.3. Experimental Procedures 60 4.3.1. Solubility Tests 60 4.3.1.1. Direct Dissolution 60 4.3.1.2. Electrolytic Reduction and Dissolution 61 4.4. Analytical Methods for Mo Concentration Determinations 62 4.4.1. Potentiometric Titrations 63 4.4.2. Atomic Absorption Spectroscopy 64 4.5. Preparation of Solutions of Different Oxidation States 66 4.6. Temperature Stability Tests 67 4.7. Cyclic Voltametric Studies 67 4.7.1. Electrodes Used for Cyclic Voltammetry 68 4.7.1.1. Counter and Reference Electrodes 68 4.7.1.2. Working Electrodes 68 4.8. Molybdenum Redox Flow Cell Performance Tests 70 4.8.1. Flow Cell Components and Setup 70 4.8.2. Cell Electrode Fabrication 72 4.8.3. Cell Resistance Calculation 72 4.8.4. Efficiency Calculation 74 CHAPTER 5 OBSERVATIONS, RESULTS, AND DISCUSSION 5.1. Solubility Studies of Molybdenum Compounds 75 5.1.1. Solubility Studies of Mo(VI) Compounds 75 5.1.2. Solubility of Electrolytically Generated Mo(VI) Solutions 84 5.1.3. Molybdenum Solution Reduction Observations 86 5.1.4. Mo(III) Stability to Atmospheric Oxygen 91 5.2. Thermal Stability Studies of Molybdenum Electrolytes 94 5.3. Cyclic Voltammetry Studies 98 5.3.1. Effect of Acid Concentration 98 5.3.2. Screening the Molybdenum Salts 107 5.3.3. Electrode Selection 109 CHAPTER 6 CYCLIC VOLTAMETRIC STUDY OF THE KINETICS OF MOLYBDENUM REDOX COUPLES 6.1. Mo(III)/Mo(IV) Couple 118 6.2. Mo(V)/Mo(VI) Couple 120 6.3. Mo(IV)/Mo(V) Couple. 121 6.4. Kinetic Parameters of Concentrated Molybdenum in Dilute Sulphuric Acid at a Glassy Carbon Electrode 123 6.5. Electrode Area 129 CHAPTER 7 EFFECT OF COMPLEXING AGENTS, ELECTROLYTE ADDITIVES AND ANION CONCENTRATION ON THE CYCLIC VOLTAMETRIC BEHAVIOUR AND SOLUBILITY OF MOLYBDENUM 7.1. Effect of Sulphate Concentration on Mo Blue Formation 132 7.2. Effect of Complexing Agents 135 CHAPTER 8 MOLYBDENUM REDOX CELL PERFORMANCE TESTING 8.1. Design and Operation 137 8.2. Cell Resistance 138 8.3. Molybdenum Cell Charge-Discharge Testing 139 8.4. Electrode and Membrane Stability to Mo(VI) and Mo(III) 145 8.5. Mo(III) and Mo(VI) Stability Under Paraffin Oil 146 8.6. Molybdenum Source 146 CONCLUSION BIBLIOGRAPHY APPENDICES 1 Mo Standard Solution AAS Data 156 2 Potentiometric Titrations 157 3 ln(ip) vs (E1(-E0’) plots 160 4 ip vs v0-5 plots 168 5 Cyclic Voltammograms at Different v 176 7 CHAPTER 1 INTRODUCTION There is demand for new energy storage systems for a wide range of applications. Redox flow cells which employ two soluble redox couples in the positive and negative half cell electrolytes are an attractive alternative because of their potentially long life, reliability, low maintenance, simplicity, low cost, and wide range of applications. The aim of a redox flow cell system is to have an electrically rechargeable bulk energy storage system with a high overall efficiency, extended cycle life, high reliability, that can operate at ambient temperatures, and at the same time be economical. Earlier work with the Fe-Cr redox couple revealed that an inherent problem with such systems is the cross mixing of the two electrolytes across the membrane (67). Using a single metal ion in different oxidation states for both the positive and the negative half cell electrolytes, the cross mixing, and capacity loss, due to cross contamination which most redox systems suffer from can be largely eliminated. The advantages of the vanadium redox flow cell which employs V(V)/V(IV) anc* V(III)/V(II) have already been demonstrated (15,16,73-78). Like vanadium, molybdenum also exists in a number of oxidation states and therefore has the potential to be used in a redox flow cell. 8 There are primary cells which use molybdenum as the anode (13), and examples of chemically regenerative redox cells can be found (12,14). A molybdenum / vanadium redox cell has been described by Kummer (5).
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