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Integrated Solar Fuel Generators

Integrated Solar Fuel Generators

Ian D Sharp | Harry A Atwater | Hans-Joachim Lewerenz

(2018)

Abstract

With the rapid worldwide increase of interest and excitement about the promise of artificial photosynthesis for renewable fuels, the research community is beginning to focus on the challenges of integrating the various components into complete, unassisted solar fuel generators.

Integrated Solar Fuel Generators discusses the scientific and engineering efforts addressing the challenges of building complete integrated artificial photosystems that will form the basis for developing a solar fuels technology. Building on recent substantial progress towards efficient semiconductor light absorbers and robust, earth abundant heterogeneous catalysts for water oxidation and proton reduction by the community, the integration of these components into efficient durable generators suitable for scale-up moves into focus. To succeed, a broad range of materials, processing, and design issues need to be addressed to meet efficiency, stability and scalability requirements.

This book describes the critical areas of research and development towards viable integrated solar fuels systems, the current state of the art of these efforts and outlines future research needs that will accelerate progress towards a deployable technology.


Ian D Sharp is a Staff Scientist at the Joint Center for Artificial Photosynthesis, Lawrence Berkeley National Laborator, USA.

Harry A Atwater is the Howard Hughes Professor of Applied Physics and Materials Science at the California Institute of Technology, USA.

Hans-Joachim Lewerenz is Department Head for Accelerated Discovery at the Joint Center for Artificial Photosynthesis, Lawrence Berkeley National Laborator, USA.


Table of Contents

Section Title Page Action Price
Cover Cover
Preface vii
Contents xi
Introduction and System Considerations 1
Chapter 1: Concepts of Photoelectrochemical Energy Conversion and Fuel Generation 3
1.1 Introductory Remarks 3
1.2 Semiconductor Junctions and Dark Electrochemical Processes 5
1.2.1 Concept of the Classical Silicon Solar Cell 5
1.2.2 The Semiconductor-redox Electrolyte Contact 7
Electrocatalysis 79
Chapter 3: Understanding the Effects of Composition and Structure on the Oxygen Evolution Reaction (OER) Occurring on NiFeOx Catalysts 81
3.1 Introduction 81
3.2 Thermodynamics of Water Splitting 82
3.3 Catalysts for the OER 83
3.4 The Structure of FeNiOx 87
3.5 Identity of the Active Site in FeNiOx 96
3.6 Factors Affecting the OER Activity of NiFeOOH 101
3.7 Effects of Additives Other Than Fe on the OER Activity of NiMOx 103
3.8 Effects of Additive on the OER Activity of NiFeOx 105
3.9 Conclusions 108
Acknowledgments 108
References 109
Chapter 4: Surface Science, X-ray and Electron Spectroscopy Studies of Electrocatalysis 117
4.1 Introduction 117
4.2 Laboratory Based Methods for Surface Characterization 120
4.2.1 UHV-based Surface Science 120
4.3 Synchrotron-based in situ and operando Spectroscopy 125
4.3.1 Photon-in/photon-out Methods: Experimental Setup for operando Spectroscopy, X-ray Absorption, and High Resolution X-ray Spectroscopy 126
4.3.1.1 Experimental Setup for operando Photon-in/photon-out Spectroscopy 126
4.3.1.2 X-ray Absorption Spectroscopy 128
4.3.1.3 High Resolution X-ray Spectroscopy 130
4.3.1.4 Feasibility of High-energy XAS as operando Surface Analysis Tool 135
4.3.2 Ambient Pressure XPS 137
4.3.2.1 Methods: Tender X-ray APXPS 145
4.4 Summary and Outlook 148
References 149
Chapter 5: Evaluating Electrocatalysts for Solar Water-splitting Reactions 154
5.1 Introduction 154
5.2 Experimental Considerations 155
5.2.1 Cell Design 155
5.2.2 Auxiliary Electrode 156
5.2.3 Reference Electrodes 157
5.2.4 Working Electrode Material 159
5.2.5 Catalyst Deposition and Characterization 160
5.3 Catalyst Performance 161
5.3.1 Elemental Analysis 163
5.3.2 Catalytic Activity 163
5.3.3 Short-term Stability 164
5.3.4 Extended Stability 165
5.3.5 Faradaic Efficiency Measurements 167
5.3.6 Measuring Catalyst Surface Area 168
5.4 Benchmarking Catalyst Performance 170
5.4.1 Primary Figure of Merit 170
5.4.2 Comparing Electrocatalytic Performance 172
5.5 Conclusions 174
References 175
Semiconductor Light Absorbers 183
Chapter 6: Heterojunction Approaches for Stable and Efficient Photoelectrodes 185
6.1 Introduction 185
6.2 Semiconductor-Electrolyte Interface in the Context of Chemical Conversion 188
6.2.1 Overview 188
6.2.2 Simple Picture of an Unpinned Semiconductor-Liquid Junction (SLJ) 188
6.2.3 Electrically Decoupled Photovoltaic and Catalyst 192
6.2.4 Heterojunction Design for Stability and Efficiency 194
6.3 JCAP Experimental Work 194
6.3.1 Photocathodes 194
6.3.2 Photoanodes 197
6.4 Summary and Outlook 205
Acknowledgments 205
References 205
Chapter 7: Artificial Photosynthesis with Inorganic Particles 214
7.1 Why Particles? 214
7.1.1 Photoreactors 216
7.2 Absorber Configurations 218
7.3 Stability 219
7.4 Ideal Limiting Solar-to-hydrogen (STH) Efficiency 220
7.5 Experimental Efficiencies 223
7.6 Mechanism of Water Splitting Photocatalysis 224
7.7 Free Energy of Photocatalysts 224
7.8 Light Absorption and Exciton Generation 225
7.9 Recombination 227
7.9.1 Auger Recombination 228
7.9.2 Shockley-Read-Hall Recombination 229
7.9.3 Surface Recombination 230
7.9.4 Radiative Recombination 233
7.9.5 Overall Lifetime 234
7.10 Charge Transport 234
7.11 Charge Separation 237
7.11.1 Junctions 237
7.11.2 Electric Dipoles 244
7.11.3 Ohmic Contacts 245
7.12 Charge Transfer Reactions at the Cocatalyst-Liquid Interface 247
7.13 Charge Transfer Reactions at Semiconductor-Liquid Interfaces 248
7.13.1 Controlling the Back Reaction 251
7.13.2 Photocorrosion 252
7.13.3 Electrolyte Effects and pH 253
7.13.4 Theoretical Modeling 255
7.13.5 Promising Absorber Materials 256
7.14 Conclusion 258
Acknowledgments 258
References 259
Chapter 8: Degradation of Semiconductor Electrodes in Photoelectrochemical Devices: Principles and Case Studies 281
8.1 Introduction 281
8.2 Thermodynamic and Kinetic Requirements for Material Stability 282
8.2.1 Thermodynamic Aspects 282
New Materials and Components 305
Chapter 9: High Throughput Experimentation for the Discovery of Water Splitting Materials 307
9.1 Mission-driven Materials Discovery: Introduction and Strategies 307
9.1.1 High Throughput Screening for Specific Device Components and Operating Conditions 307
9.1.2 General Strategies for Constructing Experimental Screening Pipelines 309
9.2 Cross-cutting Capabilities: Materials Synthesis and Data Management 310
9.2.1 Inkjet Printing of Functional Metal Oxides 311
9.2.2 Combinatorial Physical Vapor Deposition 314
9.2.3 Thermal Processing 316
9.2.4 Data Management 317
9.3 Experimental Pipeline for Discovering OER Electrocatalysts 319
9.3.1 The Scanning Droplet Cell and Its Deployment for Electrocatalyst Discovery 319
9.3.2 Parallel Screening via Bubble Imaging 323
9.3.3 Screening Libraries with Unstable Catalysts 324
9.3.4 Materials Characterization for Electrocatalysts 325
9.4 Experimental Pipeline for Discovering Photoanodes 325
9.4.1 High Throughput Spectroscopy for Band Gap Screening 325
9.4.2 Colorimetry as a Parallel Screen 328
9.4.3 Photoelectrochemistry with the Scanning Droplet Cell 329
9.4.4 Material Characterization of Photoanodes: Linking to Theory 330
9.5 Combining Materials and Techniques for Discovery of Integrated Materials 333
9.6 Lessons Learned and Future Prospects 337
Acknowledgments 337
References 338
Chapter 10: Membranes for Solar Fuels Devices 341
10.1 Transport Challenges in Membranes for Solar Fuels Devices 342
10.2 Membrane Materials and Structure 344
10.3 Commercial Membranes 346
10.4 Transport of Solutes in Membranes 350
10.5 Solute Sorption 352
10.6 Solute Diffusion 353
10.7 Water Sorption 356
10.8 Electrical Properties 359
10.9 Multicomponent Transport 363
10.10 Measurement of Transport Parameters in Membranes 365
10.11 Phenomena Affecting Transport: Physical Aging and Degradation 369
10.12 JCAP Membrane Research 371
10.13 Outlook for Membranes in CO2 Reduction Devices 376
List of Symbols 377
References 379
Devices and Modelling 387
Chapter 11: Prototyping Development of Integrated Solar-driven Water-splitting Cells 389
11.1 Introduction 389
11.2 Materials and Components 391
11.2.1 Selection and Design Consideration of Light Absorber Materials 391
11.2.1.1 Triple-junction Amorphous Silicon 392
11.2.1.2 Monolithic Tandem and Triple-junction Crystalline Silicon 394
11.2.1.3 Compound Semiconductor Multi-junction Photovoltaics 396
11.2.2 Selection and Design Consideration of Electrolytes 398
11.2.2.1 Electrolyte Effect on Transport Losses in a Device 399
11.2.2.2 Electrolyte Effect on the Stability of Semiconducting Light Absorbers 401
11.2.2.3 Electrolyte Effect on Catalytic Activity, Stability and Optical Transmittance 402
11.2.2.3.1 Effect of Unintentional Cation and Anion in Electrolyte on the Catalytic Activity 402
11.2.2.3.2 Electrolyte Effect on Activity and Stability 403
11.2.2.3.3 Electrolyte Effect on Light Absorption 404
11.2.2.3.4 Electrolyte Effect on Electrochromism of Electrocatalysts 406
11.2.3 Incorporation of Membrane Separators 407
11.2.3.1 Mechanical Compression 408
11.2.3.2 Adhesion 409
11.2.3.3 Others 411
11.2.4 Chassis and Auxiliary Components 413
11.2.5 Integration of Protective Layers with Catalysts and Light Absorbers 415
11.3 Full Device Characterization and Evaluation 427
11.3.1 Different Sources of Lights and Calibration Methods 427
11.3.2 Product Collection 429
11.3.2.1 Experimental Setup 429
11.3.2.2 Measurements 430
11.3.2.3 Analysis 431
11.3.3 Outdoor Testing 431
11.4 System Engineering Approaches 434
11.4.1 Introduction to System Engineering 434
11.4.2 Development of Hierarchical Requirements 435
11.4.3 Testing Plan 436
11.4.4 Reviews 440
11.5 Conclusion and Outlook 442
References 443
Chapter 12: High-efficiency Water Splitting Systems 454
12.1 The Need for High Efficiency in Solar Fuel Generation 454
12.2 Efficiency Limitations and Prospects for Photoelectrochemical Energy Conversion 456
12.2.1 Fundamental Limitations: Detailed Balance Limit and Catalysis 456
12.2.2 Further Relevant Loss Mechanisms and Mitigation Strategies 460
12.2.2.1 Absorption by Electrolyte and Catalyst 460
12.2.2.2 Ohmic Resistivity 462
12.2.2.3 Non-radiative Recombination 463
12.2.2.4 Energetics 463
12.2.2.5 Light Management 464
12.2.2.6 Gas Bubble Management 465
12.2.2.7 Multi-terminal Approaches 466
12.2.2.8 Concentration 466
12.3 III-V Semiconductor Tandem Structures: A Testbed for High Efficiency 467
12.3.1 History of the III-V Compound Semiconductors and High-efficiency Solar Cells 467
12.3.2 Highly Efficient III-V Tandem Structures in Solar Water Splitting 469
12.3.2.1 Classical III-V Tandem Photoelectrochemistry 469
12.3.2.2 Metamorphic Device Concepts 469
12.3.2.3 Inverted Metamorphic Device Concepts 473
12.4 Efficiency Measurement and Characterization Strategies 480
12.4.1 Standard Solar Irradiance vs. Laboratory Light Sources 480
12.4.2 Tandem Device Characterization: A Case Study on Common Practice vs. Result Validation 483
12.4.3 Utilization of Natural Sunlight and Secondary Illumination Errors 486
12.4.4 Differential Spectral Responsivity 492
12.4.5 Solar-to-hydrogen Conversion Reference Laboratories 493
12.5 Summary and Outlook 494
Acknowledgments 495
References 495
Chapter 13: Continuum-scale Modeling of Solar Water-splitting Devices 500
13.1 Introduction 500
13.1.1 Definitions 503
13.2 Modeling Methodology and Approach 505
13.2.1 Model Dimensionality 508
13.2.2 History of Modeling within JCAP 510
13.3 Governing Equations 511
13.3.1 Transport in Electrolytes 513
13.3.1.1 Mass Conservation 513
13.3.1.2 Acid-Base Equilibria 513
13.3.1.3 Mass-species Fluxes 514
13.3.1.3.1 Infinitely Dilute Electrolyte 514
13.3.1.3.2 Moderately Dilute Electrolyte 515
13.3.1.3.3 Concentrated Electrolyte 515
13.3.1.4 Membrane Transport 516
13.3.1.5 Charge Transport and Conservation 517
13.3.1.6 Electron Transport 518
13.3.1.7 Product Gases 518
13.3.1.8 Momentum Conservation 519
13.3.2 Transport in Photoabsorbers 520
13.3.2.1 Light Capture 520
13.3.2.2 Hole and Electron Transport 522
13.3.2.3 Diode Equation 522
13.3.3 Kinetics 523
13.3.4 Heat Transfer 524
13.4 Sample Cases 526
13.4.1 Simplifications to the Flux-expressions 526
13.4.1.1 Shockley-Queisser 527
13.4.1.2 Butler-Volmer 527
13.4.1.3 Strongly Acidic or Alkaline Electrolytes 527
13.4.1.4 Supporting Electrolyte 527
13.4.1.5 Buffered Electrolyte 527
13.4.1.6 1-D Approximation to 2-D and 3-D Solar-fuels Generator 528
13.4.2 Case Study 1: Design Guidelines and Cell Scaling from 2D PEC Device Models 528
13.4.3 Case Study 2: Multiphysics Modeling in Concentrated PEC 531
13.5 Summary 534
Acknowledgments 535
References 535
Subject Index 537