ELECTROCHEMICAL RECYCLING OF SODIUM BOROHYDRTDE FOR HYDROGEN STORAGE: PHYSICOCHEMICAL PROPERTIES OF SODIUM METABORATE SOLUTIONS. by Caroline R. Cloutier B. A. Sc., The University of Ottawa, 1999 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE m THE FACULTY OF GRADUATE STUDIES (Materials Engineering) THE UNIVERSITY OF BRITISH COLUMBIA July 2006 © Caroline R. Cloutier, 2006 ABSTRACT The large-scale adoption of a "hydrogen economy" is hindered by the lack of a practical storage method and concerns associated with its safe handling. Chemical hydrides have the potential to address these concerns. Sodium borohydride (sodium tetrahydroborate, NaBH4), is the most attractive chemical hydride for H2 generation and storage in automotive fuel cell applications but recycling from sodium metaborate (NaB02) is difficult and costly. An electrochemical regeneration process could represent an economically feasible and environmentally friendly solution. In this thesis, the properties of diluted NaB02 aqueous solutions and concentrated NaB02 alkaline aqueous solutions that are necessary for the development of electrochemical recycling methods have been studied. The conductivity and viscosity of dilute aqueous solutions of NaB02 were measured as a function of concentration at 25°C. Also, the solubility, pH, density, conductivity and viscosity of the filtrate of saturated aqueous NaB02 solutions containing varying weight percentages (1, 2, 3, 5, 7.5 and 10 wt%) of alkali hydroxides (NaOH, KOH and LiOH) were evaluated at 25°C. Selected experiments were repeated at 50 and 75°C to investigate the effect of temperature on the NaB02 alkaline aqueous solution solubility and physicochemical properties. Preliminary experiments to investigate the effect of glycine (C2H5N02), the smallest amino acid, on the solubility and physicochemical properties of NaB02 alkaline aqueous solutions were conducted at 25°C. Furthermore, the precipitates formed in the supersaturated 10 wt% alkaline aqueous NaB02 solutions at 25°C were characterized by X-Ray Diffraction and Scanning Electron Microscopy. The use of KOH as the electrolyte was found to be more advantageous for the H2 storage and generation system based on NaB02 solubility and NaBH4 half-life due to the pH effect. However, the addition of NaOH led to the highest ionic conductivity, and its use seems more ii suitable for the electroreduction of NaB02. Further investigations on the impact of KOH and NaOH on the electroreduction of NaB02 in aqueous media have the potential to enhance the commercial viability of this H2 generation and storage system. iii TABLE OF CONTENTS Abstract ii Table of Contents iv List of Tables ix List of Figures xi List of Symbols xiii List of Acronyms xiv Acknowledgement xvi Dedication xvii CHAPTER I Introduction 1 1.1 Motivation 1 1.2 Objectives 2 1.3 Approach 3 1.4 Overview 4 CHAPTER II Background 5 2.1 NaBH4 Production 5 2.1.1 Natural Resources 5 2.1.2 Current Production Process 5 A) Production Paths 5 B) The Schlesinger and Brown Process 6 C) The Bayer Process 8 2.1.3 Economics 8 A) Statistics 9 B) Borate Usage 9 C) NaBH4Cost •.. 11 2.2 Regeneration 12 2.2.1 Chemical Synthesis 12 A) Modified Schlesinger and Brown Process 13 B) Modified Schlesinger and Brown Process: Amendment 15 C) Three-Step Regeneration 16 D) Using Coke or Methane 17 E) Recycling Through Diborane 18 F) Reduction of NaB02 with H2 19 G) Methods Using Metal Hydrides 19 a) Annealing 19 b) Ball Milling 20 H) Radiolysis Process 21 2.2.2 Electrochemical Synthesis 22 A) Direct Electroreduction 23 B) Kinetic Considerations 25 C) Patent Review 27 D) Reproducibility 31 E) Indirect Electroreduction 32 F) Electroreduction of Borate Ester 33 G) Electroreduction in Molten Hydroxide Media 34 iv H) Hydrogen Assisted Electrolysis 34 a) Molten NaOH Electrolysis 34 b) H2 Assisted Molten Salt Electroreduction of Borate 36 c) Aqueous Alkaline Borate Solutions 38 I) Metathesis 38 J) Simulating Conventional Synthesis 39 2.2.3 Infrastructure 40 2.3 NaBH4 H2 Generation and Storage System Efficiency 41 2.3.1 Gravimetric Capacity 41 2.3.2 Volumetric Capacity 42 2.3.3 Fuel Comparison 43 2.4 Hydrolysis of NaBH4 for H2 Generation and Storage 44 2.4.1 Hydrolysis 45 2.4.2 Effect of pH 46 A) pH Equilibria of Borates 46 B) Effect of pH on the Solution Stability 48 C) Effect of Electrolyte Concentration on the NaBH4 Hydrolysis 49 2.4.3 Effect of Catalysts 50 A) Noble Metal Catalysts 50 a) Supported Platinum 50 b) Supported Ruthenium 51 B) Non-Precious Transition Metal Catalysts 52 a) Cobalt Boride 52 b) Nickel Boride 53 c) Filamentary Nickel Mixed Cobalt 54 d) Metal Hydride 54 e) Metal-Metal Oxide 55 C) Organic Catalysts 55 D) Catalysts Comparison 56 2.4.4 Effect of Temperature 58 2.4.5 Effect of NaBH4 Concentration 59 2.4.6 Steam Hydrolysis of NaBH4 59 2.5 NaBH4 Applications 60 2.5.1 NaBH4 Hydrolysis Reactors 60 A) Kipp Generator 60 B) Remote Fuel-Cell Power Source 62 C) Solid Hydride and Liquid Water or Water Vapour 62 D) Solid Hydride and Steam 63 E) Catalytic Reactors 64 2.4.2 B-PEMFC System 66 A) Hydrogen on Demand™ 66 2.4.3 Vehicle Prototype 67 A) B-PEMFC Vehicle 68 CHAPTER III Physicochemical Properties of NaB02 69 3.1 NaB02 Solution Characteristics 69 3.1.1 Solubility 69 v A) NaB02 Solubility 69 B) NaBH, Solubility 71 C) Effect of Temperature on the Solubility of NaB02 71 3.1.2 Single 1:1 Electrolyte Theory 72 A) Conductivity 73 B) Dynamic Viscosity 76 a) Effect of Temperature on the Viscosity and Ionic Mobility 77 b) Effect of Temperature on the B Coefficients 78 C) Walden's Rule 79 3.1.3 Physicochemical Properties 80 A) Aqueous NaB02 Solutions 80 B) Alkaline Aqueous NaB02 Solutions 82 3.1.4 Organic additives 83 A) Amides 83 a) Urea '. 83 b) Thiourea 84 c) Acetamine 85 B) Ammonia 85 C) Diethylene glycol 86 D) Glycine 86 E) Organic Additive Comparison 87 3.2 Precipitate Characteristics... 88 CHAPTER IV Experimental Methods 90 4.1 Materials 90 4.1.1 Properties 90 4.1.2 Contamination 91 4.2 Experimental Plan 92 4.3 Solution Preparation 93 4.3.1 Dilute Aqueous NaB02 Solutions 93 4.3.2 Concentrated Alkaline Aqueous NaB02 Solutions 93 4.3.3 Organic Additives 94 4.4 Filtration 95 4.4.1 Filtration at 25°C 94 A) Vacuum Filtration 95 B) Syringe Filtration 96 4.4.2 Filtration at 50 and 75°C 97 4.5 Filtrate Characterization 98 4.5.1 Solubility 98 A) Boron Detection Methods 98 B) Induced Coupled Plasma Mass Spectrometry 99 4.5.2 pH 99 A) Glass Electrode :'. „ 100 B) pH Meter 100 4.5.3 Conductivity 100 4.5.4 Viscosity 101 4.5.5 Density 102 vi 4.6 Precipitate Characterization 102 4.6.1 X-Ray Diffraction 102 4.6.2 Scanning Electron Microscopy .. 103 CHAPTER V Results and Discussion. 104 5.1 Aqueous NaB02 Solutions Physicochemical Properties 104 5.1.1 Solubility 104 5.1.2 Transport Properties 105 A) Conductivity 105 B) Viscosity 107 5.2 Aqueous NaB02 Solutions Physicochemical Properties with Alkali Additive 109 5.2.1 Solubility 109 5.2.2 pH Ill A) NaBH4 Solution Half-Life 112 5.2.3 Transport Properties 113 A) Ionic Conductivity 113 a) Effect of Hydroxide Concentration on the Conductivity of Saturated NaB02 Solutions 113 b) Unsaturated NaB02 Alkaline Aqueous Solution 114 B) Molar Conductivity 115 a) Unsaturated NaB02 Alkaline Aqueous Solution 115 b) Effect of Hydroxide Concentration on the Molar Conductivity of Saturated NaB02 Solutions 115 C) Dynamic Viscosity 117 D) Walden Product 119 5.2.4 Specific Gravity 120 5.3 Effect of Temperature 121 5.3.1 Solubility 122 5.3.2 Transport Properties 123 A) Ionic Conductivity 123 a) Effect of Hydroxide Concentration on the Conductivity of Saturated NaB02 Solutions 123 b) Unsaturated NaB02 Alkaline Aqueous Solution 124 B) Molar Conductivity 126 a) Unsaturated NaB02 Alkaline Aqueous Solution 126 5.4 Effect of Glycine Additions 126 5.4.1 Solubility 127 5.4.2 pH 127 A) NaBH4 Solution Half-Life 128 5.4.3 Transport Properties 129 A) Ionic Conductivity 129 B) Molar Conductivity 130 C) Dynamic Viscosity 131 5.4.4 Specific Gravity 131 5.5 Precipitate Characteristics 132 5.5.1 X-Ray Diffraction 132 vii 5.5.2 Scanning Electron Microscopy 134 CHAPTER VI Conclusions 136 6.1 Conclusions 136 6.1.1 System Considerations 137 6.1.2 Future Research and Recommendations 138 6.2 Summary of Contributions 140 References 141 APPENDIX 150 Appendix A: Comparison to DOE Range, Storage Cost, Energy Density and Specific Energy Targets 150 Appendix B: Filtrate Characterization Data 152 Appendix C: Precipitate XRD Data 157 Appendix D: Precipitate SEM Pictures 159 viii LIST OF TABLES Table 1.1: US Department of Energy Freedom Car Targets for On-Board H2 Storage, (Modified from [Department of Energy, (2004)]) 3 Table 2.1: Partial List of Borate Uses (Modified from [Garret, D. E., (1998)]) 10 Table 2.2: List of Possible Chemical Regeneration Synthesis 13 Table 2.3: Chemical Synthesis Patent List 13 Table 2.4: List of Possible Electrochemical Regeneration Synthesis 22 Table 2.5: Electrochemical Borohydride Synthesis Patent List 23 Table 2.6: Exchange Current Densities (io) and Tafel Slopes for the H2 Evolution Reaction at Various Materials in Basic Solutions at Room Temperature., *313K [Gouper, A.M. et al, (1990)]... 26 Table 2.7: H2 Content of Various Chemical Hydrides 41 Table 2.8: Volume Required for the Storage of 1 kg of H2 from Different Borohydrides 43 Table 2.9: Comparison of Some Catalyst Activity (Modified from [Kaufman, C.