Fuel Cell Systems Explained

Since publication of the first edition of Fuel Cell Systems Explained, three compelling drivers have supported the continuing development of fuel cell technology. These are: the need to maintain energy security in an energy-hungry world, the desire to move towards zero-emission vehicles and power plants, and the mitigation of climate change by lowering of CO₂ emissions. New fuel cell materials, enhanced stack performance and increased lifetimes are leading to the emergence of the first truly commercial systems in applications that range from fork-lift trucks to power sources for mobile phone towers. Leading vehicle manufacturers have embraced the use of electric drive-trains and now see hydrogen fuel cells complementing advanced battery technology in zero-emission vehicles. After many decades of laboratory development, a global but fragile fuel cell industry is bringing the first commercial products to market. 

This thoroughly revised edition includes several new sections devoted to, for example, fuel cell characterisation, improved materials for low-temperature hydrogen and liquid-fuelled systems, and real-world technology implementation.  

Assuming no prior knowledge of fuel cell technology, the third edition comprehensively brings together all of the key topics encompassed in this diverse field. Practitioners, researchers and students in electrical, power, chemical and automotive engineering will continue to benefit from this essential guide to the principles, design and implementation of fuel cell systems.

ANDREW L. DICKS, PHD is an Independent Consultant and Adjunct Principal Research Fellow at Griffith University, Brisbane, Australia.

DAVID A. J. RAND, PHD, SCD was a Chief Research Scientist at CSIRO where, among other duties, he served as scientific advisor on hydrogen and renewable energy. In retirement, he is now a CSIRO Honorary Research Fellow, Melbourne, Australia.

Brief Biographies xiii

Preface xv

Acknowledgments xvii

Acronyms and Initialisms xix

Symbols and Units xxv

1 Introducing Fuel Cells 1

1.1 Historical Perspective 1

1.2 Fuel-Cell Basics 7

1.3 Electrode Reaction Rates 9

1.4 Stack Design 11

1.5 Gas Supply and Cooling 14

1.6 Principal Technologies 17

1.7 Mechanically Rechargeable Batteries and Other Fuel Cells 19

1.7.1 Metal–Air Cells 20

1.7.2 Redox Flow Cells 20

1.7.3 Biological Fuel Cells 23

1.8 Balance-of-Plant Components 23

1.9 Fuel-Cell Systems: Key Parameters 24

1.10 Advantages and Applications 25

Further Reading 26

2 Efficiency and Open-Circuit Voltage 27

2.1 Open-Circuit Voltage: Hydrogen Fuel Cell 27

2.2 Open-Circuit Voltage: Other Fuel Cells and Batteries 31

2.3 Efficiency and Its Limits 32

2.4 Efficiency and Voltage 35

2.5 Influence of Pressure and Gas Concentration 36

2.5.1 Nernst Equation 36

2.5.2 Hydrogen Partial Pressure 38

2.5.3 Fuel and Oxidant Utilization 39

2.5.4 System Pressure 39

2.6 Summary 40

Further Reading 41

3 Operational Fuel-Cell Voltages 43

3.1 Fundamental Voltage: Current Behaviour 43

3.2 Terminology 44

3.3 Fuel-Cell Irreversibilities 46

3.4 ActivationLosses 46

3.4.1 The Tafel Equation 46

3.4.2 The Constants in the Tafel Equation 48

3.4.3 Reducing the Activation Overpotential 51

3.5 InternalCurrents and Fuel Crossover 52

3.6 Ohmic Losses 54

3.7 Mass-Transport Losses 55

3.8 Combining the Irreversibilities 57

3.9 The Electrical Double-Layer 58

3.10 Techniques for Distinguishing Irreversibilities 60

3.10.1 Cyclic Voltammetry 60

3.10.2 AC Impedance Spectroscopy 61

3.10.3 Current Interruption 65

Further

Reading 68

4 Proton-Exchange Membrane Fuel Cells 69

4.1 Overview 69

4.2 Polymer Electrolyte: Principles of Operation 72

4.2.1 Perfluorinated Sulfonic Acid Membrane 72

4.2.2 Modified Perfluorinated Sulfonic Acid Membranes 76

4.2.3 Alternative Sulfonated and Non-Sulfonated Membranes 77

4.2.4 Acid–Base Complexes and Ionic Liquids 79

4.2.5 High-Temperature Proton Conductors 80

4.3 Electrodes and Electrode Structure 81

4.3.1 Catalyst Layers: Platinum-Based Catalysts 82

4.3.2 Catalyst Layers: Alternative Catalysts for Oxygen Reduction 85

4.3.2.1 Macrocyclics 86

4.3.2.2 Chalcogenides 87

4.3.2.3 Conductive Polymers 87

4.3.2.4 Nitrides 87

4.3.2.5 Functionalized Carbons 87

4.3.2.6 Heteropolyacids 88

4.3.3 Catalyst Layer: Negative Electrode 88

4.3.4 Catalyst Durability 88

4.3.5 Gas-Diffusion Layer 89

4.4 Water Management 92

4.4.1 Hydration and Water Movement 92

4.4.2 Air Flow and Water Evaporation 94

4.4.3 Air Humidity 96

4.4.4 Self-Humidified Cells 98

4.4.5 External Humidification: Principles 100

4.4.6 External Humidification: Methods 102

4.5 Cooling and Air Supply 104

4.5.1 Cooling with Cathode Air Supply 104

4.5.2 Separate Reactant and Cooling Air 104

4.5.3 Water Cooling 105

4.6 Stack Construction Methods 107

4.6.1 Introduction 107

4.6.2 Carbon Bipolar Plates 107

4.6.3 Metal Bipolar Plates 109

4.6.4 Flow-Field Patterns 110

4.6.5 Other Topologies 112

4.6.6 Mixed Reactant Cells 114

4.7 Operating Pressure 115

4.7.1 Technical Issues 115

4.7.2 Benefits of High Operating Pressures 117

4.7.2.1 Current 117

4.7.3 Other Factors 120

4.8 Fuel Types 120

4.8.1 Reformed Hydrocarbons 120

4.8.2 Alcohols and Other Liquid Fuels 121

4.9 Practical and Commercial Systems 122

4.9.1 Small-Scale Systems 122

4.9.2 Medium-Scale for Stationary Applications 123

4.9.3 Transport System Applications 125

4.10 System Design, Stack Lifetime and Related Issues 129

4.10.1 Membrane Degradation 129

4.10.2 Catalyst Degradation 129

4.10.3 System Control 129

4.11 Unitized Regenerative Fuel Cells 130

Further Reading 132

5 Alkaline Fuel Cells 135

5.1 Principles of Operation 135

5.2 System Designs 137

5.2.1 Circulating Electrolyte Solution 137

5.2.2 Static Electrolyte Solution 140

5.2.3 Dissolved Fuel 142

5.2.4 Anion-Exchange Membrane Fuel Cells 144

5.3 Electrodes 147

5.3.1 Sintered Nickel Powder 147

5.3.2 Raney Metals 147

5.3.3 Rolled Carbon 148

5.3.4 Catalysts 150

5.4 Stack Designs 151

5.4.1 Monopolar and Bipolar 151

5.4.2 Other Stack Designs 152

5.5 Operating Pressure and Temperature 152

5.6 Opportunities and Challenges 155

Further

Reading 156

6 Direct Liquid Fuel Cells 157

6.1 Direct Methanol Fuel Cells 157

6.1.1 Principles of Operation 160

6.1.2 Electrode Reactions with a Proton-Exchange Membrane Electrolyte 160

6.1.3 Electrode Reactions with an Alkaline Electrolyte 162

6.1.4 Anode Catalysts 162

6.1.5 Cathode Catalysts 163

6.1.6 System Designs 164

6.1.7 Fuel Crossover 165

6.1.8 Mitigating Fuel Crossover: Standard Techniques 166

6.1.9 Mitigating Fuel Crossover: Prospective Techniques 167

6.1.10 Methanol Production 168

6.1.11 Methanol Safety and Storage 168

6.2 Direct Ethanol Fuel Cells 169

6.2.1 Principles of Operation 170

6.2.2 Ethanol Oxidation, Catalyst and Reaction Mechanism 170

6.2.3 Low-Temperature Operation: Performance and Challenges 172

6.2.4 High-Temperature Direct Ethanol Fuel Cells 173

6.3 Direct Propanol Fuel Cells 173

6.4 Direct Ethylene Glycol Fuel Cells 174

6.4.1 Principles of Operation 174

6.4.2 Ethylene Glycol: Anodic Oxidation 175

6.4.3 Cell Performance 176

6.5 Formic Acid Fuel Cells 176

6.5.1 Formic Acid: Anodic Oxidation 177

6.5.2 Cell Performance 177

6.6 Borohydride Fuel Cells 178

6.6.1 Anode Catalysts 180

6.6.2 Challenges 180

6.7 Application of Direct Liquid Fuel Cells 182

Further Reading 184

7 Phosphoric Acid Fuel Cells 187

7.1 High- Temperature Fuel-Cell Systems 187

7.2 System Design 188

7.2.1 Fuel Processing 188

7.2.2 Fuel Utilization 189

7.2.3 Heat-Exchangers 192

7.2.3.1 Designs 193

7.2.3.2 Exergy Analysis 193

7.2.3.3 Pinch Analysis 194

7.3 Principles of Operation 196

7.3.1 Electrolyte 196

7.3.2 Electrodes and Catalysts 198

7.3.3 Stack Construction 199

7.3.4 Stack Cooling and Manifolding 200

7.4 Performance 201

7.4.1 Operating Pressure 202

7.4.2 Operating Temperature 202

7.4.3 Effects of Fuel and Oxidant Composition 203

7.4.4 Effects of Carbon Monoxide and Sulfur 204

7.5 Technological

Developments 204

Further Reading 206

8 Molten Carbonate Fuel Cells 207

8.1 Principles of Operation 207

8.2 Cell Components 210

8.2.1 Electrolyte 211

8.2.2 Anode 213

8.2.3 Cathode 214

8.2.4 Non-Porous Components 215

8.3 Stack Configuration and Sealing 215

8.3.1 Manifolding 216

8.3.2 Internal and External Reforming 218

8.4 Performance 220

8.4.1 Influence of Pressure 220

8.4.2 Influence of Temperature 222

8.5 Practical Systems 223

8.5.1 Fuel Cell Energy (USA) 223

8.5.2 Fuel Cell Energy Solutions (Europe) 225

8.5.3 Facilities in Japan 228

8.5.4 Facilities in South Korea 228

8.6 Future Research and Development 229

8.7 Hydrogen Production and Carbon Dioxide Separation 230

8.8 Direct Carbon Fuel Cell 231

Further Reading 234

9 Solid Oxide Fuel Cells 235

9.1 Principles of Operation 235

9.1.1 High-Temperature (HT) Cells 235

9.1.2 Low-Temperature (IT) Cells 237

9.2 Components 238

9.2.1 Zirconia Electrolyte for HT-Cells 238

9.2.2 Electrolytes for IT-Cells 240

9.2.2.1 Ceria 240

9.2.2.2 Perovskites 241

9.2.2.3 Other Materials 243

9.2.3 Anodes 243

9.2.3.1 Nickel-YSZ 243

9.2.3.2 Cathode 245

9.2.3.3 Mixed Ionic–Electronic Conductor Anode 246

9.2.4 Cathode 247

9.2.5 Interconnect Material 247

9.2.6 Sealing Materials 248

9.3 Practical Design and Stacking Arrangements 249

9.3.1 Tubular Design 249

9.3.2 Planar Design 251

9.4 Performance 253

9.5 Developmental and Commercial Systems 254

9.5.1 Tubular SOFCs 255

9.5.2 Planar SOFCs 256

9.6 Combined-Cycle and Other Systems 258

Further Reading 260

10 Fuels for Fuel Cells 263

10.1 Introduction 263

10.2 Fossil Fuels 266

10.2.1 Petroleum 266

10.2.2 Petroleum from Tar Sands, Oil Shales and Gas Hydrates 268

10.2.3 Coal and Coal Gases 268

10.2.4 Natural Gas and Coal-Bed Methane (Coal-Seam Gas) 270

10.3 Biofuels 272

10.4 Basics of Fuel Processing 275

10.4.1 Fuel-Cell Requirements 275

10.4.2 Desulfurization 275

10.4.3 Steam Reforming 277

10.4.4 Carbon Formation and Pre-Reforming 280

10.4.5 Internal Reforming 281

10.4.5.1 Indirect Internal Reforming (IIR) 283

10.4.5.2 Direct Internal Reforming (DIR) 283

10.4.6 Direct Hydrocarbon Oxidation 284

10.4.7 Partial Oxidation and Autothermal Reforming 285

10.4.8 Solar–Thermal Reforming 286

10.4.9 Sorbent-Enhanced Reforming 287

10.4.10 Hydrogen Generation by Pyrolysis or Thermal Cracking of Hydrocarbons 289

10.4.11 Further Fuel Processing: Removal of Carbon Monoxide 290

10.5 Membrane Developments for Gas Separation 293

10.5.1 Non-Porous Metal Membranes 293

10.5.2 Non-Porous Ceramic Membranes 294

10.5.3 Porous Membranes 294

10.5.4 Oxygen Separation 295

10.6 Practical Fuel Processing: Stationary Applications 295

10.6.1 Industrial Steam Reforming 295

10.6.2 Fuel-Cell Plants Operating with Steam Reforming of Natural Gas 296

10.6.3 Reformer and Partial Oxidation Designs 298

10.6.3.1 Conventional Packed-Bed Catalytic Reactors 298

10.6.3.2 Compact Reformers 299

10.6.3.3 Plate Reformers and Microchannel Reformers 300

10.6.3.4 Membrane Reactors 301

10.6.3.5 Non-Catalytic Partial Oxidation Reactors 302

10.6.3.6 Catalytic Partial Oxidation Reactors 303

10.7 Practical Fuel Processing: Mobile Applications 304

10.8 Electrolysers 305

10.8.1 Operation of Electrolysers 305

10.8.2 Applications 307

10.8.3 Electrolyser Efficiency 312

10.8.4 Photoelectrochemical Cells 312

10.9 Thermochemical Hydrogen Production and Chemical Looping 314

10.9.1 Thermochemical Cycles 314

10.9.2 Chemical Looping 317

10.10 Biological Production of Hydrogen 318

10.10.1 Introduction 318

10.10.2 Photosynthesis and Water Splitting 318

10.10.3 Biological Shift Reaction 320

10.10.4 Digestion Processes 320

Further Reading 321

11 Hydrogen Storage 323

11.1 Strategic Considerations 323

11.2 Safety 326

11.3 Compressed Hydrogen 327

11.3.1 Storage Cylinders 327

11.3.2 Storage Efficiency 329

11.3.3 Costs of Stored Hydrogen 330

11.3.4 Safety Aspects 330

11.4 Liquid Hydrogen 331

11.5 Reversible Metal Hydrides 333

11.6 Simple Hydrogen-Bearing Chemicals 338

11.6.1 Organic Chemicals 338

11.6.2 Alkali Metal Hydrides 339

11.6.3 Ammonia, Amines and Ammonia Borane 340

11.7 Complex Chemical Hydrides 341

11.7.1 Alanates 342

11.7.2 Borohydrides 342

11.8 Nanostructured Materials 344

11.9 Evaluation of Hydrogen Storage Methods 347

Further Reading 350

12 The Complete System and Its Future 351

12.1 Mechanical Balance-of-Plant Components 351

12.1.1 Compressors 351

12.1.1.1 Efficiency 354

12.1.1.2 Power 356

12.1.1.3 Performance Charts 356

12.1.1.4 Selection 359

12.1.2 Turbines 361

12.1.3 Ejector Circulators 362

12.1.4 Fans and Blowers 363

12.1.5 Pumps 364

12.2 Power Electronics 365

12.2.1 DC Regulators (Converters) and Electronic Switches 366

12.2.2 Step-Down Regulators 368

12.2.3 Step-Up Regulators 370

12.2.4 Inverters 371

12.2.4.1 Single Phase 372

12.2.4.2 Three Phase 376

12.2.5 Fuel-Cell Interface and Grid Connection Issues 378

12.2.6 Power Factor and Power Factor Correction 378

12.3 Hybrid Fuel-Cell + Battery Systems 380

12.4 Analysis of Fuel-Cell Systems 384

12.4.1 Well-to-Wheels Analysis 385

12.4.2 Power-Train Analysis 387

12.4.3 Life-Cycle Assessment 388

12.4.4 Process Modelling 389

12.4.5 Further Modelling 392

12.5 Commercial Reality 394

12.5.1 Back to Basics 394

12.5.2 Commercial Progress 395

12.6 Future Prospects: The Crystal Ball Remains Cloudy 397

Further Reading 399

Appendix 1 Calculations of the Change in Molar Gibbs Free Energy 401

A1.1 Hydrogen Fuel Cell 401

A1.2 Carbon Monoxide Fuel Cell 403

Appendix 2 Useful Fuel-Cell Equations 405

A2.1 Introduction 405

A2.2 Oxygen and Air Usage 406

A2.3 Exit Air Flow Rate 407

A2.4 Hydrogen Usage 407

A2.5 Rate of Water Production 408

A2.6 Heat Production 409

Appendix 3 Calculation of Power Required by Air Compressor and Power Recoverable by Turbine in Fuel-Cell Exhaust 411

A3.1 Power Required by Air Compressor 411

A3.2 Power Recoverable from Fuel-Cell Exhaust with a Turbine 412

Glossary of Terms 415

Index 437

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