Electrocatalysis for Membrane Fuel Cells: Methods, Modeling, and Applications 9783527348374

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Electrocatalysis for Membrane Fuel Cells

Electrocatalysis for Membrane Fuel Cells Methods, Modeling, and Applications

Edited by Nicolas Alonso-Vante and Vito Di Noto

Editors

University of Poitiers IC2MP-UMR-CNRS 7285 4 rue Michel Brunet F-86073 Poitiers Cedex 9 France

All books published by WILEY-VCH are carefully produced. Nevertheless, authors, editors, and publisher do not warrant the information contained in these books, including this book, to be free of errors. Readers are advised to keep in mind that statements, data, illustrations, procedural details or other items may inadvertently be inaccurate.

Prof. Vito Di Noto

Library of Congress Card No.: applied for

Prof. Nicolas Alonso-Vante

University of Padova Department of Industrial Engineering Via Marzolo 9 I-35131 Padova Italy Courtesy of Vito Di Noto, Nicolas Alonso Vante and Keti Vezzù Cover Image:

British Library Cataloguing-in-Publication Data

A catalogue record for this book is available from the British Library. Bibliographic information published by the Deutsche Nationalbibliothek

The Deutsche Nationalbibliothek lists this publication in the Deutsche Nationalbibliografie; detailed bibliographic data are available on the Internet at . © 2024 WILEY-VCH GmbH, Boschstraße 12, 69469 Weinheim, Germany All rights reserved (including those of translation into other languages). No part of this book may be reproduced in any form – by photoprinting, microfilm, or any other means – nor transmitted or translated into a machine language without written permission from the publishers. Registered names, trademarks, etc. used in this book, even when not specifically marked as such, are not to be considered unprotected by law. Print ISBN: 978-3-527-34837-4 ePDF ISBN: 978-3-527-83055-8 ePub ISBN: 978-3-527-83056-5 oBook ISBN: 978-3-527-83057-2 Typesetting:

Straive, Chennai, India

v

Contents Preface

Part I 1

1.1 1.1.1 1.1.2 1.1.3 1.2 1.2.1 1.2.2 1.2.3 1.2.4 1.3 1.4

2 2.1 2.2 2.3 2.4 2.5 2.6

xv

Overview of Systems 1

System-level Constraints on Fuel Cell Materials and Electrocatalysts 3 Elliot Padgett and Dimitrios Papageorgopoulos Overview of Fuel Cell Applications and System Designs 3 System-level Fuel Cell Metrics 3 Fuel Cell Subsystems and Balance of Plant (BOP) Components 5 Comparison of Fuel Cell Systems for Different Applications 9 Application-derived Requirements and Constraints 10 Fuel Cell Performance and the Heat Rejection Constraint 10 Startup, Flexibility, and Robustness 13 Fuel Cell Durability 14 Cost 16 Material Pathways to Improved Fuel Cells 18 Note 19 Acronyms 20 Symbols 20 References 20 PEM Fuel Cell Design from the Atom to the Automobile 23 Andrew Haug and Michael Yandrasits Introduction 23 The PEMFC Catalyst 27 The Electrode 32 Membrane 38 The GDL 42 CCM and MEA 46

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Contents

2.7 2.8

Flowfield and Single Fuel Cell 50 Stack and System 55 Acronyms 57 References 58 Part II

3 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 3.10 3.11 3.11.1 3.11.2 3.12 3.12.1 3.12.2 3.12.3 3.13

4

4.1 4.2 4.2.1 4.2.1.1 4.2.1.2 4.2.2

Basics – Fundamentals

69

Electrochemical Fundamentals 71 Vito Di Noto, Gioele Pagot, Keti Vezzù, Enrico Negro, and Paolo Sgarbossa Principles of Electrochemistry 71 The Role of the First Faraday Law 71 Electric Double Layer and the Formation of a Potential Difference at the Interface 73 The Cell 74 The Spontaneous Processes and the Nernst Equation 75 Representation of an Electrochemical Cell and the Nernst Equation 77 The Electrochemical Series 79 Dependence of the Ecell on Temperature and Pressure 82 Thermodynamic Efficiencies 83 Case Study – The Impact of Thermodynamics on the Corrosion of Low-T FC Electrodes 85 Reaction Kinetics and Fuel Cells 88 Correlation Between Current and Reaction Kinetics 88 The Concept of Exchange Current 89 Charge Transfer Theory Based on Distribution of Energy States 89 The Butler–Volmer Equation 96 The Tafel Equation 100 Interplay Between Exchange Current and Electrocatalyst Activity 101 Conclusions 103 Acronyms 104 Symbols 104 References 107 Quantifying the Kinetic Parameters of Fuel Cell Reactions 111 Viktoriia A. Saveleva, Juan Herranz, and Thomas J. Schmidt Introduction 111 Electrochemical Active Surface Area (ECSA) Determination 114 ECSA Determination Using Underpotential Deposition 115 Hydrogen Underpotential Deposition (HUPD ) 116 Copper Underpotential Deposition (CuUPD ) 117 ECSA Quantification Based on the Adsorption of Probe Molecules 118

Contents

4.2.2.1 4.2.2.2 4.2.3 4.2.4 4.3 4.3.1 4.3.2 4.3.3 4.3.4 4.3.5 4.3.6 4.4 4.4.1 4.4.2 4.4.3 4.4.3.1 4.4.3.2 4.5

5

5.1 5.2 5.3 5.4 5.4.1 5.4.2 5.5 5.5.1 5.5.2 5.6

CO Stripping 118 NO−2 ∕NO Sorption 119 Double-layer Capacitance Measurements and Other Methods 120 ECSA Measurements in a PEFC: Which Method to Choose? 120 H2 -Oxidation and Electrochemical Setups for the Quantification of Kinetic Parameters 121 Rotating Disc Electrodes (RDEs) 122 Hydrogen Pump (PEFC) Approach 124 Ultramicroelectrode Approach 125 Scanning Electrochemical Microscopy (SECM) Approach 125 Floating Electrode Method 127 Methods Summary 128 ORR Kinetics 129 ORR Mechanism Studies with RRDE Setups 129 ORR Pathway on Me/N/C ORR Catalysts 130 ORR Kinetics: Methods 132 Pt-based Electrodes 132 Pt-free Catalysts: RDE vs. PEFC Kinetic Studies 133 Concluding Remarks 133 Acronyms 134 Symbols 134 References 135 Adverse and Beneficial Functions of Surface Layers Formed on Fuel Cell Electrocatalysts 149 Shimshon Gottesfeld Introduction 149 Catalyst Capping in Heterogeneous Catalysis and in Electrocatalysis 151 Passivation of PGM/TM and Non-PGM HOR Catalysts and Its Possible Prevention 156 Literature Reports on Fuel Cell Catalyst Protection by Capping 161 Protection of ORR Pt catalysts Against Agglomeration by an Ultrathin Overlayer of Mesoporous SiO2 or Me–SiO2 161 Protection by Carbon Caps Against Catalyst Detachment and Catalyst Passivation Under Ambient Conditions 162 Other Means for Improving the Performance Stability of Supported Electrocatalysts 166 Replacement of Carbon Supports by Ceramic Supports 166 Protection of Pt Catalysts by Enclosure in Mesopores 167 Conclusions 170 Abbreviations 171 References 171

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Contents

Part III State of the Art 175 6

6.1 6.2 6.2.1 6.2.2 6.2.2.1 6.2.3 6.2.4 6.3 6.3.1 6.3.2 6.4 6.5

7

7.1 7.2 7.3 7.3.1 7.3.2 7.4

8

8.1 8.2 8.2.1 8.2.2 8.2.3 8.2.4 8.2.5 8.2.6 8.2.7

Design of PGM-free ORR Catalysts: From Molecular to the State of the Art 177 Naomi Levy and Lior Elbaz Introduction 177 The Influence of Molecular Changes Within the Complex 179 The Role of the Metal Center 179 Addition of Substituents to MCs 183 Beta-substituents 184 Meso-substituents 186 Axial Ligands 187 Cooperative Effects Between Neighboring MCs 190 Bimetallic Cofacial Complexes – “Packman” Complexes 191 MC Polymers 191 The Physical and/or Chemical Interactions Between the Catalyst and Its Support Material 193 Effect of Pyrolysis 194 Acronyms 196 References 196 Recent Advances in Electrocatalysts for Hydrogen Oxidation Reaction in Alkaline Electrolytes 205 Indra N. Pulidindi and Meital Shviro Introduction 205 Mechanism of the HOR in Alkaline Media 206 Electrocatalysts for Alkaline HOR 212 Platinum Group Metal HOR Electrocatalysts 212 Non-platinum Group Metal-based HOR Electrocatalysts 214 Conclusions 220 Acronyms 221 References 221 Membranes for Fuel Cells 227 Paolo Sgarbossa, Giovanni Crivellaro, Francesco Lanero, Gioele Pagot, Afaaf R. Alvi, Enrico Negro, Keti Vezzù, and Vito Di Noto Introduction 227 Properties of the PE separators 228 Benchmarking of IEMs 229 Ion-exchange Capacity (IEC) 229 Water Uptake (WU), Swelling Ratio (SR), and Water Transport 231 Ionic Conductivity (𝜎) 233 Gas Permeability 234 Chemical Stability 235 Thermal and Mechanical Stability 237

Contents

8.2.8 8.3 8.3.1 8.3.1.1 8.3.1.2 8.3.2 8.3.2.1 8.3.2.2 8.3.2.3 8.3.3 8.3.3.1 8.3.3.2 8.3.3.3 8.3.4 8.3.4.1 8.3.4.2 8.3.4.3 8.4 8.5

9

9.1 9.2 9.3 9.4 9.5 9.6 9.7 9.8 9.9 9.10 9.11 9.12 9.13

Cost of the IEMs 239 Classification of Ion-exchange Membranes 240 Cation-exchange Membranes (CEMs) 240 Perfluorinated Membranes 240 Nonperfluorinated Membranes 245 Anion-exchange Membranes (AEMs) 246 Functionalized Polyketones 247 Poly(Vinyl Benzyl Trimethyl Ammonium) (PVBTMA) Polymers 248 Poly(sulfones) (PS) 249 Hybrid Ion-exchange Membranes 249 Hybrid Membranes with Single Ceramic Oxoclusters [P/(Mx Oy )n ] 250 Hybrid Membranes Comprising Surface-functionalized Nanofillers 254 Hybrid Membranes Doped with hierarchical “Core–Shell” Nanofillers 254 Porous Membranes 257 Porous Membranes as Host Material 257 Porous Membranes as Support Layer 258 Porous Membranes as Unconventional Separators 259 Mechanism of Ion Conduction 259 Summary and Perspectives 268 Acronyms 271 Symbols 272 References 272

Supports for Oxygen Reduction Catalysts: Understanding and Improving Structure, Stability, and Activity 287 Iwona A. Rutkowska, Sylwia Zoladek, and Pawel J. Kulesza Introduction 287 Carbon Black Supports 288 Decoration and Modification with Metal Oxide Nanostructures 289 Carbon Nanotube as Carriers 291 Doping, Modification, and Other Carbon Supports 293 Graphene as Catalytic Component 293 Metal Oxide-containing ORR Catalysts 296 Photodeposition of Pt on Various Oxide–Carbon Composites 299 Other Supports 301 Alkaline Medium 302 Toward More Complex Hybrid Systems 303 Stabilization Approaches 306 Conclusions and Perspectives 307 Acknowledgment 308 Acronyms 308 References 308

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Contents

Part IV Physical–Chemical Characterization 319 10

10.1 10.2 10.3 10.3.1 10.3.2 10.4 10.5 10.6 10.6.1 10.6.2 10.6.3 10.7

Understanding the Electrocatalytic Reaction in the Fuel Cell by Tracking the Dynamics of the Catalyst by X-ray Absorption Spectroscopy 321 Ditty Dixon, Aiswarya Bhaskar, and Aswathi Thottungal Introduction 321 A Short Introduction to XAS 323 Application of XAS in Electrocatalysis 325 Ex Situ Characterization of Electrocatalyst 325 Operando XAS Studies 330 Δ𝜇 XANES Analysis to Track Adsorbate 334 Time-resolved Operando XAS Measurements in Fuel Cells 338 Fourth-generation Synchrotron Facilities and Advanced Characterization Techniques 340 Total-reflection Fluorescence X-ray Absorption Spectroscopy 341 Resonant X-ray Emission Spectroscopy (RXES) 341 Combined XRD and XAS 342 Conclusions 342 Acronyms 343 References 344

Part V 11

11.1 11.2 11.3 11.4 11.5 11.5.1 11.5.2 11.5.3 11.5.4 11.5.5 11.6 11.7

Modeling 349

Unraveling Local Electrocatalytic Conditions with Theory and Computation 351 Jun Huang, Mohammad J. Eslamibidgoli, and Michael H. Eikerling Local Reaction Conditions: Why Bother? 351 From Electrochemical Cells to Interfaces: Basic Concepts 352 Characteristics of Electrocatalytic Interfaces 355 Multifaceted Effects of Surface Charging on the Local Reaction Conditions 356 The Challenges in Modeling Electrified Interfaces using First-principles Methods 358 Computational Hydrogen Electrode 359 Unit-cell Extrapolation, Explicit Solvated Protons, and Excess Electrons 360 Counter Charge and Reference Electrode 361 Effective Screening Medium and mPB Theory 361 Grand-canonical DFT 362 A Concerted Theoretical–Computational Framework 362 Case Study: Oxygen Reduction at Pt(111) 364

Contents

11.8

Outlook 367 Acronyms 367 Symbols 368 References 368

Part VI Protocols 375 12 12.1 12.2 12.2.1 12.2.2 12.2.3 12.3 12.3.1 12.3.2 12.3.2.1 12.3.2.2 12.3.2.3 12.3.3 12.3.4 12.3.5 12.4 12.4.1 12.4.2 12.4.2.1 12.4.2.2 12.4.2.3 12.4.2.4 12.4.3 12.4.4 12.4.5 12.5 12.6 12.6.1 12.7

Quantifying the Activity of Electrocatalysts 377 Karla Vega-Granados and Nicolas Alonso-Vante Introduction: Toward a Systematic Protocol for Activity Measurements 377 Materials Consideration 378 PGM Group 378 Low PGM and PGM-free Approaches 379 Impact of Support Effects on Catalytic Sites 381 Electrochemical Cell Considerations 382 Cell Configuration and Material 382 Electrolyte 385 Purity 385 Protons vs. Hydroxide Ions 386 Influence of Counterions 388 Electrode Potential Measurements 388 Preparation of Electrodes 391 Well-defined and Nanoparticulated Objects 395 Parameters Diagnostic of Electrochemical Performance 396 Surface Area 396 Hydrogen Underpotential Deposition Integration 397 Surface Oxide Reduction 398 CO Monolayer Oxidation (CO Stripping) 400 Underpotential Deposition of Metals 401 Double-layer Capacitance 402 Electrocatalysts Site Density 402 Data Evaluation (Half-Cell Reactions) 404 The E1/2 and E (jPt (5%)) Parameters 405 Stability Tests 407 Data Evaluation (Auxiliary Techniques) 409 Surface Atoms vs. Bulk 410 Conclusions 411 Acknowledgments 412 Acronyms 412 Symbols 413 References 414

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13

13.1 13.2 13.3 13.3.1 13.3.2 13.3.3 13.3.4 13.4 13.4.1 13.4.2 13.4.3 13.4.4 13.4.5 13.5 13.5.1 13.5.2 13.5.3 13.5.4 13.5.5 13.6

Durability of Fuel Cell Electrocatalysts and Methods for Performance Assessment 429 Bianca M. Ceballos and Piotr Zelenay Introduction 429 Fuel Cell PGM-free Electrocatalysts for Low-temperature Applications 431 PGM-free Electrocatalyst Degradation Pathways 432 Demetallation 432 Carbon Oxidation 436 Micropore Flooding 439 Nitrogen Protonation and Anionic Adsorption 439 PGM-free Electrocatalyst Durability and Metrics 440 Performance and Durability Evaluation in Air-supplied Fuel Cell Cathode 440 Assessment of Carbon Corrosion in Nitrogen-purged Cathode 443 Determination of Performance Loss upon Cycling Cathode Catalyst in Nitrogen 443 Recommendations for ORR Electrocatalyst Evaluation in RRDE in O2 and in an Inert Gas 446 Electrocatalyst Corrosion 447 Low-PGM Catalyst Degradation 447 Pt Dissolution 449 Carbon Support Corrosion 452 Pt Catalyst MEA Activity Assessment and Durability 454 PGM Electrocatalyst MEA Conditioning in H2 /Air 454 Accelerated Stress Test of PGM Electrocatalyst Durability 456 Conclusion 457 Acronyms 459 References 460

Part VII Systems 471 14

14.1 14.2 14.2.1 14.2.2 14.2.3 14.2.4 14.2.5 14.2.6 14.2.7

Modeling of Polymer Electrolyte Membrane Fuel Cells 473 Andrea Baricci, Andrea Casalegno, Dario Maggiolo, Federico Moro, Matteo Zago, and Massimo Guarnieri Introduction 473 General Equations for PEMFC Models 474 Analytical and Numerical Modeling 474 Reversible Electromotive Force 476 Fuel Cell Voltage 477 Activation Overpotential 478 Ohmic Overpotential – PEM Model 479 Concentration Overpotential 480 Examples of Fuel Cell Modeling 482

Contents

14.3 14.4 14.4.1 14.4.2 14.4.3 14.4.4 14.5 14.5.1 14.5.2 14.5.2.1 14.5.2.2 14.5.3 14.5.4 14.5.5 14.6

15

15.1 15.1.1 15.1.2 15.1.3 15.1.4 15.2 15.2.1 15.2.2 15.2.3 15.2.4 15.3 15.3.1 15.3.2 15.3.3 15.3.3.1 15.3.3.2 15.3.3.3 15.3.3.4

Multiphase Water Transport Model for PEMFCs 483 Fluid Mechanics in PEMFC Porous Media: From 3D Simulations to 1D Models 488 From Micro- to Macroscopic Models 490 Porous Medium Anisotropy 491 Fluid–Fluid Viscous Drag 492 Surface Tension and Capillary Pressure 492 Physical-based Modeling for Electrochemical Impedance Spectroscopy 496 Experimental Measurement and Modeling Approaches 496 Physical-based Modeling 497 Current Relaxation 497 Laplace Transform 498 Typical Impedance Features of PEMFC 498 Application of EIS Modeling to PEMFC Diagnostic 500 Approximations of 1D Approach 501 Conclusions and Perspectives 502 Acronyms 503 Symbols 504 References 507 Physics-based Modeling of Polymer Electrolyte Membrane Fuel Cells: From Cell to Automotive Systems 511 Andrea Baricci, Matteo Zago, Simone Buso, Marco Sorrentino, and Andrea Casalegno Polymer Fuel Cell Model for Stack Simulation 511 General Characteristics of a Fuel Cell System for Automotive Applications 511 Analysis of the Channel Geometry for Stack Performance Modeling 514 Analysis of the Air and Hydrogen Utilization for Stack Performance Modeling 516 Introduction to Transient Stack Models 518 Auxiliary Subsystems Modeling 519 Air Management Subsystem 519 Hydrogen Management Subsystem 521 Thermal Management Subsystem 522 PEMFC System Simulation 522 Electronic Power Converters for Fuel Cell-powered Vehicles 525 Power Converter Architecture 527 Load Adaptability 527 Power Electronic System Components 528 Port Interface Converters 530 The PEMFC Interface Converter 530 The Motor Interface Converter 530 The Energy Storage Interface 531

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15.3.3.5 15.4 15.4.1 15.4.2 .

15.4.2.1

Supervisory Control 531 Fuel Cell Powertrains for Mobility Use 532 Transport Application Scenarios 532 Tools for the Codesign of Transport Fuel Cell Systems and Energy Management Strategies 534 Automotive Case Study: Optimal Codesign of an LDV FCHV Powertrain 535 Acronyms 540 Symbols 541 References 541 Index 545

xv

Preface In electrochemical energy converters such as low-temperature membrane fuel cells, the slow kinetics of oxygen reduction (ORR) represents one of the main reasons for a high overpotential in a fuel cell, e.g. polymer electrolyte membrane (PEM) type. Despite this inherent phenomenon, in these systems, fuel cells constitute (i) a cornerstone in the energy technologies of the present twenty-first century for transportation and stationary applications and (ii) one of the two pillars, together with electrolyzers, of the future Green Hydrogen Economy of the world. These scenarios are comforted by the rapid advances in the development of materials based on noble and non-noble electrocatalytic materials that encompass a bunch of applications operating in a wide pH range (acidic and alkaline). In this context, in order to find a utility, to the knowledge obtained to date, for current and future researchers in this field of activity, the repository of such an avalanche of information is thus a central resource to be transmitted with a global perspective. It is for this reason that the present book aims to consolidate and transmit this knowledge while providing the necessary forum to complement what is published daily in specialized journals. Thus, the contributions of experts working in both academic and industrial research and development will serve as a reference source for the fundamentals and applications of fuel cells, establishing the state-of-the-art and disseminating research advances within a scope corresponding to textbooks for undergraduate and graduate students. This book, devoted to fuel cell electrocatalysis, will, we hope, further the development and application of this exciting technology on the road to the successful establishment of a clean and sustainable energy economy in the twenty-first century. For the reader’s convenience, this book, with a total of 15 chapters, is organized in seven sections, namely Overview, Fundamentals, State of the Art, Physical–Chemical Characterization, Modeling, Protocols, and Systems. The first chapter discusses how application requirements and system-level considerations create constraints on fuel cell materials and electrocatalysts, with the goal of informing more strategic and impactful research and development efforts. In the second chapter, the discussion is centered on how an atomically designed catalyst surface efficiently produces protons and electrons from hydrogen on the anode and water from oxygen, protons, and electrons on the cathode. In the third chapter, insights are provided on how fundamental electrochemistry can be exploited to guide fuel cell research, whereas the fourth chapter discloses the quantification of the kinetic descriptors that determine the activity and stability of the anode and cathode electrocatalysts, providing analytical methods and electrochemical set-ups

xvi

Preface

as supports. Moreover, in Chapter 5, the author discusses some means for protecting catalytic sites in order to maintain high performance in the light of recent data from the literature. Chapter 6, furthermore, puts into relevance the state-of-the-art of platinum group metal (PGM)-free ORR catalysts. Herein, the authors provide an overview of important parameters that influence the catalysis of ORR with well-defined ORR catalysts. In Chapter 7, recent development in electrocatalysts for the hydrogen oxidation reaction (HOR) is put on the floor, emphasizing the state-of-the-art PGM- and non-PGM-based electrocatalysts for the HOR in alkaline conditions. An important ingredient in the proton exchange membrane fuel cell (PEMFC) system is the polymeric electrolyte. In this context, Chapter 8 describes the features that a membrane must exhibit to be implemented in a fuel cell. This chapter ends with a comprehensive overview of the mechanisms of ion conduction proposed for fuel cell membranes, followed by a brief summary outlining the perspectives of the research in this field. The characteristics of ORR electrocatalyst support (carbon-based and oxide-based) have been analyzed in Chapter 9. Of importance, in all interface research, is the in operando technique, and/or probing under real fuel cell operating conditions is offered in Chapter 10 with the use of X-ray absorption spectroscopy (XAS). Theoretical modeling and computation to unravel the local reaction environment are given in Chapter 11. This chapter addresses this complex issue by introducing some basic concepts of electrochemical interfaces, especially the surface charging relation. The authors highlight the electrocatalytic interfaces pertaining to the role of chemisorption-induced surface dipoles that could cause nonmonotonicity in the surface charging behavior. The electrocatalytic materials research protocols for investigating fuel cell reactions are deployed in Chapters 12 and 13. In sum, the correct evaluation of fuel cell reactions, selection of reference electrodes, durability tests of PGM-free materials, and fuel cell testing procedures are put forward in the light of the most advanced literature data research. The last section of the book presents Chapters 14 and 15. These chapters analyze the fundamentals of fuel cell simulation by means of a mono-dimensional analytical model considering multiphase water transport affecting the electrical conductivity properties of the cell membrane, whereas Chapter 15 analyzes the optimization of the operative conditions and the prediction of the system durability that back the design of the PEMFC stack and components of the balance of the plant. The editors appreciate the contributing authors of this book, who maintained high scientific standards. N. Alonso-Vante acknowledges financial support from the European Union (ERDF) and “Région Nouvelle Aquitaine.” V. Di Noto thanks the financial support of EIT Raw Materials, project Alpe, and Graphene Flagship, Core 3, of the European Union. Nicolas Alonso-Vante University of Poitiers, IC2MP-UMR CNRS 7285 Poitiers, France Vito Di Noto University of Padova, Department of Industrial Engineering Padova, Italy

1

Part I Overview of Systems

3

1 System-level Constraints on Fuel Cell Materials and Electrocatalysts Elliot Padgett and Dimitrios Papageorgopoulos Hydrogen and Fuel Cell Technologies Office, Office of Energy Efficiency and Renewable Energy, U.S. Department of Energy, 1000 Independence Ave., SW, Washington, D.C., 20585, USA

1.1 Overview of Fuel Cell Applications and System Designs Fuel cells are anticipated to play an important role in the future clean energy economy as versatile energy conversion devices across many applications and sectors. Fuel cells have important current and potential applications in three broad areas: (i) transportation powertrains, in vehicles such as cars, buses, trucks, rail locomotives, ships, and aircraft; (ii) stationary power systems, such as distributed power generation, backup power, and combined heat and power (CHP) systems; and (iii) specialty applications such as material handling equipment as well as portable systems for auxiliary power or devices such as personal electronics or mobile communications equipment. While fuel cells for these diverse applications have some common foundations, the systems for each application have different requirements and priorities, which call for different system designs and technologies to meet them. The development of advanced, application-relevant materials and electrocatalysts is essential to overcoming the technical challenges that remain to bring fuel cells into widespread adoption and realization of their potential. This chapter discusses how application requirements and system-level considerations create constraints on fuel cell materials and electrocatalysts, with the goal of informing more strategic and impactful research and development efforts. The primary focus will be on transportation applications and polymer electrolyte membrane (PEM) fuel cells, but other applications and fuel cell types will also be included for context and comparison.

1.1.1

System-level Fuel Cell Metrics

It is useful to begin by covering the typical high-level metrics for fuel cell systems, which provide a basis for comparing different fuel cell types, application requirements, and alternative technologies as well as for benchmarking technological Electrocatalysis for Membrane Fuel Cells: Methods, Modeling, and Applications, First Edition. Edited by Nicolas Alonso-Vante and Vito Di Noto. © 2024 WILEY-VCH GmbH. Published 2024 by WILEY-VCH GmbH.

4

1 System-level Constraints on Fuel Cell Materials and Electrocatalysts

Fuel cell system

Fuel cell stack

System specific power 0.86 kW kg–1 0.9 kW kg–1

Stack specific power 2.0 kW kg–1 2.7 kW kg–1

Cold start from –20 °C 80% of its cobalt during its operating lifetime [27]. Alternately, combining Pt with iridium (typically as IrOx , where x ∼ 2.0) has been shown to increase catalyst and electrode stability [63]. IrOx is thermodynamically much more stable, even at potentials as high as 1.7 V [44]. As the HOR and ORR are surface reactions, significant work has been done to replace the less-utilized core of the catalyst particle with cheaper metals [64–66]. These types of catalysts are known as core–shell catalysts. Core–shell catalysts have been made in which between one and five surface layers of the particle is Pt and

31

32

2 PEM Fuel Cell Design from the Atom to the Automobile

Figure 2.9 NSTF pyrelene red whiskers on Kapton substrate (left), NSTF coated with catalyst (middle and middle right). An example of layered Pt over Ir catalyst film (right).

inner layers are Pt-alloy, Cu, Fe, Pd, or other metals. Exceptionally high in-cell values >100 m2 gPGM −1 have been reached [66]. Additionally, certain core materials can contract the surface Pt–Pt distance to better optimize surface reactions. Indeed, core–shells such as Cu/Pt{111} place very high on the Volcano plot [53]. However, catalyst cores such as Cu, Co, and others are not inherently stable over long periods in a PEMFC, and may dissolve out, collapsing the particle, and resulting in significant loss of surface area and performance. The more complex catalytic materials are increasingly difficult to scale to mass production. Ultimately, hundreds of kilograms per month of these nanoparticle metals on nanoparticle carbons must be made at low cost and tight specifications. Ensuring that a Pt/C catalyst is within a product specification likely requires meeting a particle size distribution (measured by XRD), a surface area measurement (m2 g−1 measured by CO adsorption), and a carbon surface area measurement (m2 g−1 measured by N2 BET). A PtCo/C nanoparticle additionally requires specifying Pt:Co atomic ratio and alloying extent. A Pt core–shell material further requires ensuring shell uniformity over the multiple billions of trillions of nanoparticles contained in 10,000 FC vehicles. Such extra specifications add to manufacturing complexity and product cost. Many alternate catalyst structures have been developed. Pt and Pt-alloys have been deposited on stable supports including conductive ceramics such as titanium oxide, niobium-doped titanium oxide, titanium nitride, and others [29, 67, 68]. Nanostructured thin film (NSTF) catalysts deposited on organic perylene red (PR) have shown unique promise as an alternate system. Here, PR is grown into nanowhiskers, and then catalyst is deposited upon them using physical vapor deposition (PVD) [31]. Pt, Pt-alloys such as PtNi, layered materials such as Pt over Ir, Figure 2.9 (right), have been made with high precision [69]. With this catalyst system, up to 30 m2 gPGM −1 in-cell, electrochemical area has been achieved with high activity and exceptional durability.

2.3 The Electrode Electrodes are typically 2–15 μm thick porous layers. They contain a sufficient catalyst loading to achieve efficiency, power, and durability targets. This catalyst

2.3 The Electrode

is intimately mixed with sufficient ionomer to transport protons to and from all catalyst sites. Electrode porosity must be sufficient to further transport gas to and water from all reactant sites. The goal of PEMFC electrode design is to optimally combine these core materials and properties to maximally utilize and maintain catalyst surface area over a wide range of operating conditions. In this way, electrodes can minimize precious metal loadings and reduce system cost. Pt/C and Pt-alloy/C electrodes originated in the 1960s and 1970s for PAFC systems. Since PAFCs utilize liquid electrolyte, their electrodes require a specific combination of metal on carbon and polytetrafluoroethylene (PTFE). Sintering these materials at precise conditions creates an appropriate balance of hydrophilicity and -phobicity required to fill roughly 15–35% of pores with phosphoric acid – enabling ionic transport without compromising gas transport. In early PEMFC electrodes, ionomer was frequently coated on top of a similar version of this PAFC electrode, creating poor ionomer distribution and poor catalyst utilization. Thus, to achieve sufficient performance, 4–10 mgPt cm−2 [16, 41, 70–72] was used. Then Wilson and Gottesfeld [73] mixed ionomer and catalyst with solvent as an ink. They then coated and dried the ink to form an electrode with drastically improved material distribution and catalyst utilization. This enabled lowering overall MEA loadings by 10× to ∼0.8 mgPt cm−2 . By the early 2000s, Gasteiger demonstrated that anode loadings of 0.05 mgPt cm−2 were sufficient when using H2 fuel. Improved catalysts, ionomers, and ionomer distribution enabled sufficient performance and durability to be achieved with less than 0.2 mgPt cm−2 [61]. PEMFC electrodes transport four distinct material phases (electrons, ions, gases, and liquid water) to and from the catalyst particle surface. Furthermore, this transport must be maintained for hundreds to ten-thousands of hours at temperatures ranging from −40 to 100 ∘ C. They are relatively thin to minimize transport resistance and contain a combination of micro- and nanopores. Connected micropores are necessary for transporting reactant gas into and product water out of each electrode. Connected electrically conductive material is required to transport electrons between the GDL and the catalyst reaction sites. Finally, connected ionomer film enables proton transport between the membrane and electrode catalyst sites. Porous electrodes containing these phases are shown in Figures 2.4d–f and 2.10. Electrodes are typically manufactured by first combining catalyst, ionomer, and solvent to form an ink. This ink is then coated, or sometimes sprayed, atop a substrate

Catalyst

O2

Ionomer Water Ionomerfree pore

O2

Membrane (a)

H+

Figure 2.10 PEMFC electrode (a) and composite M/C representation of possible catalyst–ionomer interactions (b).

(b)

O2 H+

O2

H+ O2

O2

33

34

2 PEM Fuel Cell Design from the Atom to the Automobile

(a)

(b)

Figure 2.11 Optical images of electrodes coated on a GDL substrate showing severe “mudcracking” (a) and no cracks (b).

such as a PEM, GDL, or release liner such as a Teflon film. For these electrodes, the amount of catalyst content, known as loading (in units of milligrams per geometric cm2 or mgPt cm−2 ), may be varied simply by adjusting the thickness of the electrode. Optimal electrode manufacture balances many variables including Pt or Pt-alloy type, metal particle size, wt% metal on carbon, carbon type, catalyst loading, ionomer type, ionomer-to-carbon weight ratio (I/C), solvent system, ink mixing method and duration, ink solids content and viscosity, coating method, and coating substrate. Examples of these variables and their impact will be discussed. Examples of poor- and high-quality electrode coatings on a GDL are shown in Figure 2.11. Improper choices in solvent, substrate, materials, and processing can lead to a range of electrode problems from “mudcracking” (accelerating decay as discussed in Section 2.6) to yield loss, poor performance, and reduced durability. Design choices can significantly impact cathode catalytic activity. Overall PEMFC activity and efficiency improvements may be achieved by increasing cathode Pt loading, utilizing increased surface area catalysts, and incorporating more active Pt-alloys. Further, Pt particles may exist largely in nanopores [35] of HSC supports, compared to the carbon surface for LSCs. Surface Pt is likely in contact with ionomer as shown in Figure 2.10b. Interactions between the ionomer and catalyst are well known to reduce Pt activity by 70% to over 90% [38, 74–77], reducing PEMFC performance as represented by Figure 2.1c. The protogenic end groups [78], and to some extent the side chain ether groups [79], of some ionomers coordinate with the Pt surface and reduce ORR activity. As shown in Figure 2.7, Pt on HSC is typically 2–4× more active because the Pt resides largely within the carbon support pores, limiting contact with ionomer [80]. This applies to Pt-alloys as well. As an example, by doubling catalyst loading, converting from LSC to HSC supports, and replacing Pt with intermetallic PtCo, activity gains of 10× (65 mV or ∼5% efficiency) are possible. Additionally, incorporating ionic liquids into catalyst pores has been shown to limit ionomer poisoning [81, 82]. Further, some imide-based ionomers have shown gains in the H2 –air activity region (low current densities) [52]. Finally, reducing the ratio of ionomer to carbon (I/C ratio) in the electrode has been shown to increase ORR activity for LSC catalysts [76].

2.3 The Electrode

Without proper design, cathode electrodes incur increased transport resistances and eventual limitations as shown in Figure 2.1. Its pores, ionomer film, and carbon are tortuously connected, reducing transport rates vs. intrinsic material values. Commonly a Bruggeman correction [(1 − 𝜀)/𝜏)]x is used [83, 84] in electrodes to correlate transport values with intrinsic material properties, where 𝜀 is porosity, 𝜏 is tortuosity, and x = 1–1.5. More rigorous studies further quantify this value adding limitations such as the percolation threshold [85], a minimum quantity required for transport. Further, gas transport through cathode pores is multicomponent [83] as O2 diffuses through N2 and product H2 O vapor. There are many types of electrode transport losses that can become significant without proper design. The electrodes such as those represented by Figure 2.10a may contain significant amounts of ionomer-free pores (catalyst agglomerates). This is more common in HSC carbons where significant nonpolar carbon area and van der Waals forces lead to such agglomeration [86]. Poor proton transport through large ionomer-free agglomerates may limit catalyst utilization and activity [83, 87, 88]. Insufficient ionomer humidification may further reduce overall proton conductivity through the electrode, increasing transport resistance losses. Further, pores containing sufficient hydrophilic surface area (from metal particles, low EW ionomer, or oxidized carbon) can flood, also represented in Figure 2.10a. This can limit gas transport to reactant sites. Finally, oxygen and proton transport may be limited close to the catalyst particle. Figure 2.10b shows that the metal particle may be covered with ionomer, requiring oxygen to “locally” dissolve and diffuse through to the catalyst surface. This local transport resistance can be measured [89, 90]. As electrode surface area decreases and current densities grow, these local transport resistances become increasingly significant [66]. Catalyst particles within carbon pores are likely to suffer further limited protonic conductivity due to lack of ionomer present. Oxygen diffusion into the pore may compete with product water leaving the pore, increasing transport resistance. Finally, electrodes that are too thin may create excessive capillary pressure as high quantities of liquid product water are created in small volumes [83, 91] while electrodes overly thick (>15 μm) may incur additional losses from longer material transport distances [92]. At every level, transport within the electrode must be optimized. A sufficient I/C ratio is chosen to provide bulk proton transport while minimizing gas transport losses. Ionomer EW is chosen to prevent electrode flooding even at high current densities where pore capillary pressures may be high. Improved materials and processing may allow for decreased ionomer content, increasing gas transport through the electrode and near catalyst surfaces [93]. It has been proposed and shown from GISAXS [93] that at thicknesses approaching electrode ionomer films (