IRON-BASED FLOW BATTERIES: IMPROVING LIFETIME AND PERFORMANCE by STEVEN SELVERSTON Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy Department of Chemical and Biomolecular Engineering CASE WESTERN RESERVE UNIVERSITY August, 2017 CASE WESTERN RESERVE UNIVERSITY SCHOOL OF GRADUATE STUDIES We hereby approve the dissertation of Steven Selverston candidate for the degree of Doctor of Philosophy*. Committee Chair Dr. Robert Savinell Committee Member Dr. Jesse Wainright Committee Member Dr. Rohan Akolkar Committee Member Dr. Gary Wnek Date of Defense May 18, 2017 *We also certify that written approval has been obtained for any proprietary material contained therein. Contents List of Tables iii List of Figures v Acknowledgments ix Abstract x 1 Introduction 1 2 Literature Review 5 2.1 Flow Batteries ............................... 6 2.2 All-Iron Hybrid Flow Batteries ..................... 8 2.3 Electrolytes ................................ 10 2.4 Iron Plating Electrodes .......................... 17 2.5 Rebalancing ................................ 20 2.6 System Modeling ............................. 21 2.7 Zinc-Iron Electroplating ......................... 24 2.8 Accelerated Lifetime Testing ....................... 28 3 Dissertation Research 30 3.1 Electrolyte Rebalancing ......................... 31 3.2 System Modeling ............................. 32 i CONTENTS 3.3 Zinc-Iron Chloride Flow Batteries .................... 32 4 Model for Sealed Flow Batteries 34 4.1 Introduction ................................ 35 4.2 Materials & Methods ........................... 36 4.3 Model Development ............................ 38 4.4 Results & Discussion ........................... 44 4.5 Conclusions ................................ 49 5 In-Tank Recombination 51 5.1 Introduction ................................ 52 5.2 Materials & Methods ........................... 55 5.3 Results & Discussion ........................... 58 5.4 Conclusions ................................ 63 6 Zinc-Iron Chloride Flow Batteries 65 6.1 Introduction ................................ 66 6.2 Materials & Methods ........................... 71 6.3 Results & Discussion ........................... 72 6.4 Conclusions ................................ 83 7 Conclusions and Recommendations for Future Research 84 A Supporting Information 89 A.1 Chemicals Used .............................. 90 A.2 In-Tank Hydrogen-Ferric Ion Recombination .............. 90 A.3 Zinc-Iron Chloride Flow Batteries .................... 110 A.4 System Model ............................... 121 References 142 ii List of Tables 2.1 Commercial status of flow battery startup companies ......... 7 2.2 Complexation reactions in iron chloride electrolytes .......... 10 2.3 Equilibrium constants for acid ferrous chloride ............. 15 2.4 Measured and calculated pH of FeCl2-HCl-H2O solutions ....... 16 2.5 Studies of iron plating in flow batteries ................. 17 2.6 Effects of anions on iron kinetics ..................... 18 2.7 Proposed mechanisms for anomalous codeposition ........... 26 2.8 Compositions and efficiencies of acidic Zn-Fe alloy plating baths ... 27 2.9 Selected flow battery lifetime studies .................. 29 4.1 All-iron hybrid battery reactions ..................... 36 5.1 Electrode reactions in all-iron flow batteries. .............. 52 A.1 Information regarding the chemicals used in the hydrogen-ferric ion recombination, all-iron battery and zinc-iron battery tests. ...... 90 A.2 Conditions for all-iron battery tests. ................... 93 A.3 Sample of raw pressure vs time data for measurement of recombination rate .................................... 98 A.4 Example calculation of hydrogen oxidation rate from pressure data . 99 A.5 Secondary current distribution ...................... 104 A.6 Measured conductivity of zinc-iron chloride electrolytes ........ 112 iii LIST OF TABLES A.7 Example of pressure-based hydrogen generation rate measurements . 120 A.8 Initial concentrations and diffusivities used in the simulation. ..... 122 A.9 Baseline parameter values used ..................... 122 A.10 Measurement of hydrogen generation rate ................ 123 A.11 Example of an iterative simulation setup ................ 135 A.12 Example program output, Part I ..................... 137 A.13 Example program output, Part II .................... 138 A.14 Example program output, Part III .................... 139 A.15 Example program output, Part IV .................... 140 iv List of Figures 1.1 The Duck Chart interpretation of the energy storage challenge in Cal- ifornia. ................................... 3 1.2 Concentrated solar power plant with 5 MWh vanadium redox flow battery developed by Sumitomo Electric Industries .......... 4 2.1 Tree diagram of iron flow batteries ................... 8 2.2 Schematic of the all-iron flow battery .................. 9 2.3 Iron phase diagram and formation of hydroxides ............ 12 2.4 pH of ferrous chloride (FeCl2) ...................... 13 2.5 pH of ferric chloride (FeCl3) ....................... 13 2.6 Comparison between between measured and calculated pH values using different methods ............................. 16 2.7 Iron corrosion rate ............................ 19 2.8 Pourbaix diagram interpretation of the all-iron flow battery ...... 20 2.9 NASA iron-chromium flow battery. ................... 22 2.10 Rebalancing cells for Fe-Cr and All-V batteries. ............ 22 2.11 Relative positions of zinc and iron couples on the hydrogen scale. .. 24 2.12 Illustration of hydroxide suppression mechanism ............ 25 4.1 Illustration of electrode reactions and ion migration .......... 39 v LIST OF FIGURES 4.2 Example of pressure measurements during battery charging and dis- charging .................................. 44 4.3 Effect of pH on hydrogen generation rate ................ 45 4.4 Measured versus simulated gas pressure in the reservoir headspace .. 46 4.5 Variation of headspace pressure with time during continuous charge- discharge battery cycling ......................... 46 4.6 Simulated iron concentrations in positive electrolyte with and without rebalancing ................................ 47 4.7 Simulated hydrogen generation and consumption currents ....... 48 4.8 Effects of separator porosity and thickness on steady-state pressure . 49 5.1 Schematic of a capillary-action galvanic reactor (CGR) ........ 56 5.2 Schematics of pressurized vessels for CGR testing ........... 57 5.3 Schematic of a sealed recombinant flow battery ............ 58 5.4 CGR potential as function of pH2 . .................... 59 5.5 CGR potential versus time during continuous H2 oxidation. ...... 59 5.6 Impedance and polarization measurements of a CGR ......... 60 5.7 Pressure and hydrogen oxidation rate measurements .......... 61 5.8 Pressure measurements during battery cycling ............. 62 6.1 Schematic of a zinc-iron chloride flow battery with mixed electrolytes 68 6.2 Other zinc-iron flow battery designs. .................. 70 6.3 Cyclic voltammogram interpretation of anomalous codeposition in the zinc-iron system .............................. 73 6.4 Effect of electrolyte composition on plating and stripping processes . 74 6.5 Effect of bulk pH on the plating and stripping processes ........ 75 6.6 Effect of rotation rate and negative scan limit on deposition and strip- ping from mixed Zn-Fe electrolytes ................... 77 vi LIST OF FIGURES 6.7 Effect of rotation rate and scan limit on deposition and stripping pro- cesses ................................... 77 6.8 Effect of zinc chloride on the Fe2+=3+ reaction ............. 78 6.9 Battery voltage during a charge-discharge cycle at 25 mA cm−2 ... 79 6.10 Comparison between all-iron and zinc-iron voltage during charging and discharging ................................ 80 6.11 Cell potential and performance during continuous charge-discharge bat- tery testing ................................ 80 6.12 Cell potential during a 30-day battery cycling test ........... 82 A.1 Sealed reservoir schematic and photo .................. 94 A.2 Pressure and recombination rate measurements. ............ 95 A.3 CGR pressure, rate and mixed potential versus time. ......... 95 A.4 Typical 30-cm2 cell hardware ...................... 100 A.5 Photograph of a complete battery system ................ 100 A.6 A reactor array after battery testing .................. 101 A.7 Appearance of electrolytes after 10 days of battery testing ...... 101 A.8 Electrochemical characterization of CGR as function of hydrogen par- tial pressure ................................ 102 A.9 Comparison of battery pressure profiles with and without rebalancing 102 A.10 Daramic separator after battery cycling ................. 103 A.11 Secondary current distribution in the CGR ............... 105 A.12 Simulated potential and current in the CGR as function of height above surface ................................... 105 A.13 Schematic of the H-Cell used for cyclic voltammetry .......... 110 A.14 Effect of total metal ion concentration on voltammograms for mixed zinc-iron chloride electrolytes on a titanium substrate. ......... 111 vii LIST OF FIGURES A.15 Effect of supporting electrolyte concentration on deposition and strip- ping behavior of mixed zinc-iron chloride electrolytes. ......... 111 A.16 Sketch of glass conductivity measurement cell (l=A = 200 cm−1). ... 112 A.17 Zinc-iron chloride conductivity ...................... 113 A.18 Schematic of cell parts when using ribs. ................. 114 A.19 Photograph of an