PEM Fuel Cell Electrocatalysts and Catalyst Layers

Jiujun Zhang Editor

PEM Fuel Cell Electrocatalysts and Catalyst Layers

Fundamentals and Applications

123

Jiujun Zhang, PhD Institute for Fuel Cell Innovation (IFCI) National Research Council Canada (NRC) 4250 Wesbrook Mall Vancouver, BC, V6T 1W5 Canada

ISBN 978-1-84800-935-6 e-ISBN 978-1-84800-936-3

DOI 10.1007/978-1-84800-936-3

British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library

Library of Congress Control Number: 2008934308

© 2008 Springer-Verlag London Limited, except Chapter 9

Apart from any fair dealing for the purposes of research or private study, or criticism or review, as permittedunder the Copyright, Designs and Patents Act 1988, this publication may only be reproduced, stored ortransmitted, in any form or by any means, with the prior permission in writing of the publishers, or in the caseof reprographic reproduction in accordance with the terms of licences issued by the Copyright Licensing Agency. Enquiries concerning reproduction outside those terms should be sent to the publishers.

The use of registered names, trademarks, etc. in this publication does not imply, even in the absence of aspecific statement, that such names are exempt from the relevant laws and regulations and therefore free forgeneral use.

The publisher makes no representation, express or implied, with regard to the accuracy of the informationcontained in this book and cannot accept any legal responsibility or liability for any errors or omissions thatmay be made.

Cover design: eStudio Calamar S.L., Girona, Spain

Printed on acid-free paper

9 8 7 6 5 4 3 2 1

springer.com

Preface

In today’s world, the demand for clean and sustainable energy sources has become a strong driving force in continuing economic development, and thus as well in the improvement of human living conditions. Proton exchange membrane (PEM) fuel cells, as clean energy-converting devices, have drawn a great deal of attention in recent years due to their high efficiency, high energy density, and low or zero emissions. PEM fuel cells have several important application areas, including transportation, stationary and portable power, and micro-power. From the 1960s to the present, great progress has been made in the research and development of PEM fuel cells, in terms of stack power density increases and cost reduction. Nonetheless, two major technical gaps hindering commercialization have been identified: high cost and low reliability/durability. Fuel cell catalysts, such as platinum (Pt)-based catalysts and their associated catalyst layers, are the major factors in these challenges. Although a great deal of effort has been put into the exploration of cost-effective, active, and stable fuel cell catalysts, we have not yet had any real breakthroughs. Therefore, exploring new catalysts, improving catalyst activity and stability/durability, and reducing catalyst cost are currently the major tasks in fuel cell technology and commercialization.

In a PEM fuel cell, both the anodic hydrogen (or liquid fuel) oxidation reaction (HOR) and the cathodic oxygen reduction reaction (ORR) take place within the respective catalyst layers. Electrocatalysts and their corresponding catalyst layers thus play critical roles in fuel cell performance. In our present state of technology, the most practical catalysts in PEM fuel cells are highly dispersed Pt-based nanoparticles. However, Pt-based catalysts have several drawbacks, such as high cost, sensitivity to contaminants, no tolerance for methanol oxidation (in a direct methanol fuel cell, DMFC, application), fewer completed four-electron reduction reactions, and Pt dissolution. In the search for alternative low-cost non-Pt catalysts, researchers have looked at several others, including supported platinum group metal (PGM) types such as Pd-, Ru-, and Ir-based catalysts, bimetallic alloy catalysts, transition metal macrocycles, and transition metal chalcogenides. However, these approaches are as yet in the research stage, as the catalyst activities and stabilities are still too low to be practical in comparison with Pt-based catalysts. Another approach is to reduce Pt loading in a catalyst or catalyst layer

vi Preface

using alloying and carbon supports. However, due to rapid increases in the cost of platinum, all efforts to reduce Pt loading have thus far been offset by rising prices. Non-noble metal catalysts would therefore appear to be a possible solution for the sustainable commercialization of PEM fuel cells.

Another significant challenge is gaining a fundamental understanding of fuel cell catalyst structures and their corresponding catalytic reaction mechanisms. Current approaches rely largely on trial and error. To design new, breakthrough catalysts, we need a well-defined theoretical approach. Theoretical studies will provide a platform not only for understanding catalyst performance but also for exploring the structure-activity relationship at the electron/molecular level, and ultimately for rationally designing new catalysts.

To accelerate breakthroughs in the research and development of PEM fuel cells and their sustainable commercialization, a comprehensive and in-depth book that focuses on both fundamental and application aspects of PEM fuel cell electrocatalysts and catalyst layers is definitely needed, to build upon the several important books that have previously been published in the area of electrocatalysts.

This book contains comprehensive information on PEM fuel cell eletrocatalysts and catalyst layers, with a particular focus on: (1) the fundamentals of electrochemical catalysis within PEM fuel cells, including both H2/O2 (air) and liquid-fuels/O2 (air) fuel cells; (2) electrocatalyst/catalyst layer synthesis, characterization, and activity validation and modelling; and (3) the integration of electrocatalysts/catalyst layers into fuel cells, and their performance validation, including catalyst layer structure functioning and optimization, catalyst degradation and diagnosis, and strategies to mitigate failure modes.

The contributors to the volume are fuel cell scientists and engineers with excellent academic records as well as strong industrial fuel cell expertise. This book contains the latest research and development in PEM fuel cell catalysis, and indicates important new directions for fuel cell commercialization. Readers will find numerous figures, photographs, and data tables, as well as comprehensive reference materials for each chapter. We hope that this book will be used by industry researchers, by scientists and engineers working in the areas of energy, electrochemistry science/technology, fuel cells, and electrocatalysis, and by post-secondary students. We have endeavoured to make easily accessible the latest information on fuel cell catalysis fundamentals and applications.

Each chapter is relatively independent of the others, a structure which we hope will help readers quickly find topics of interest without necessarily having to read through the whole book. Unavoidably, however, there is some overlap, reflecting the interconnectedness of the research and development in this dynamic field.

I would like to acknowledge with deep appreciation all my colleagues at the Institute for Fuel Cell Innovation, National Research Council of Canada (NRC-IFCI). Financial support for this book’s editing and indexing from NRC-IFCI is also gratefully acknowledged. Special thanks go to Dr. Hui Li, Ms. Lei Zhang, Dr. Chaojie Song, Dr. Zheng Shi, Dr. Hansan Liu, Dr. Kunchan Lee, Dr. Jianlu Zhang, Mr. Ryan Baker, Dr. Xiao-Zi Yuan, Dr. Rob Hui, Dr. Dave Ghosh, Ms. Maja Veljkovic, Ms. Eva Sharpe, Dr. David P. Wilkinson, Dr. Simon (Zhong-Sheng) Liu, Dr. Haijiang Wang, Dr. Jun Shen, Dr. Radenka Maric, and Dr. Steve Holdcroft at NRC-IFCI for their strong support, encouragement, and suggestions. I

Preface vii

also wish to thank Dr. Dania Sheldon for her effective editing and indexing services. Finally, my special appreciation goes to my wife, my son, and my daughter for their understanding and support of my work.

If technical errors exist in this book, I and all of the authors would deeply appreciate the readers’ constructive comments for correction and further improvement.

Jiujun Zhang Institute for Fuel Cell Innovation, National Research Council of Canada Vancouver, BC, Canada May 2008

Contents

1 PEM Fuel Cell Fundamentals ........................................................................... 1 Xiao-Zi Yuan and Haijiang Wang

1.1 Overview.....................................................................................................1 1.1.1 Introduction...................................................................................... 1 1.1.2 Main Cell Components and Materials........................................... 11 1.1.3 PEM Fuel Cell Operation .............................................................. 17 1.1.4 PEM Fuel Cell Applications.......................................................... 25 1.2 Thermodynamics ....................................................................................... 31 1.2.1 Basic Reactions .............................................................................. 31 1.2.2 Heat of Reaction ............................................................................ 41 1.2.3 Effect of Operation Conditions on Reversible Fuel Cell Potential ......................................................................................... 42 1.2.4 Open Circuit Voltage ..................................................................... 44 1.2.5 Fuel Cell Efficiency ....................................................................... 48 1.2.6 Summary ........................................................................................ 50 1.3 Reaction Kinetics....................................................................................... 53 1.3.1 Electrode Reactions ....................................................................... 53 1.3.2 Reaction Rate ................................................................................. 53 1.3.3 Mass Transfer................................................................................. 60 1.3.4 Multiple Kinetics ........................................................................... 65 1.3.5 Polarization Curve and Voltage Losses......................................... 67 1.3.6 Measures to Improve Cell Performance........................................ 78 References .......................................................................................................... 79

2 Electrocatalytic Oxygen Reduction Reaction ................................................ 89 Chaojie Song and Jiujun Zhang 2.1 Introduction....... ........................................................................................ 89 2.1.1 Electrochemical O2 Reduction Reactions...................................... 89 2.1.2 Kinetics of the O2 Reduction Reaction.......................................... 90 2.1.3 Techniques Used in Electrocatalytic O2 Reduction Reactions ..... 93 2.2 Oxygen Reduction on Graphite and Carbon........................................... 101 2.2.1 Oxygen Reduction Reaction Mechanisms .................................. 102

x Contents

2.2.2 Kinetics of the ORR on Carbon Materials .................................. 107 2.2.3 Catalytic Sites on Carbon Materials ............................................ 108 2.3 Oxygen Reduction Catalyzed by Quinone and Derivatives ................... 109 2.3.1 AO Process for O2 Reduction to Produce H2O2 .......................... 109 2.3.2 ORR Mechanism Electrochemically Catalyzed by Quinone...... 110 2.4 Oxygen Reduction on Metal Catalysts ................................................... 110 2.4.1 ORR Mechanism on Pt ................................................................ 110 2.4.2 Mixed Pt Surface and Rest Potential on Pt ................................. 112 2.4.3 ORR Kinetics on Pt...................................................................... 113 2.4.4 ORR on Pt Alloys ........................................................................ 114 2.4.5 Catalytic ORR on Other Metals................................................... 116 2.5 ORR on Macrocyclic Transition Metal Complexes ............................... 117 2.5.1 ORR Mechanisms Catalyzed by Transition Metal Macrocyclic Complexes.................................................................................... 117 2.5.2 Transition Metal Macrocycles as ORR Catalysts ....................... 117 2.5.3 ORR Kinetics Catalyzed by Transition Metal Macrocyclic Complexes.................................................................................... 121 2.6 ORR Catalyzed by Other Catalysts......................................................... 122 2.6.1 ORR Catalyzed by Transition Metal Chalcogenides .................. 122 2.6.2 ORR Catalyzed by Transition Metal Carbide ............................. 124 2.7 Superoxide Ion......................................................................................... 125 2.7.1 Production of Superoxide Ion by Other Methods ....................... 125 2.7.2 Properties of Superoxide Ion ....................................................... 126 2.7.3 Stability of Superoxide Ion.......................................................... 127 2.7.4 Superoxide Production by Electrocatalysis................................. 127 2.8 Conclusions.............................................................................................129 References ........................................................................................................ 129

3 Electrocatalytic H2 Oxidation Reaction ....................................................... 135 Hui Li, Kunchan Lee and Jiujun Zhang

3.1 Introduction.............................................................................................135 3.2 Electrooxidation of Hydrogen................................................................. 136 3.2.1 Mechanism of the Hydrogen Oxidation Reaction....................... 136 3.2.2 Thermodynamic Considerations for the Hydrogen Electrode Reaction........................................................................................ 138 3.2.3 Kinetics of the Hydrogen Oxidation Reaction.............................138 3.2.4 Hydrogen Adsorption Behavior...................................................143 3.2.5 Kinetic Parameters of the Hydrogen Oxidation Reaction........... 147 3.3 Electrocatalysis of Hydrogen Oxidation................................................. 149 3.3.1 Platinum and Platinum Group Metals (Pt, Ru, Pd, Ir, Os, and Rh) ........................................................................................ 149 3.3.2 Carbides........................................................................................ 156 3.3.3 Raney Nickel................................................................................ 156 3.3.4 Typical Example Analysis – PtRu Alloy as a CO-tolerant Catalyst for the HOR ................................................................... 157 3.4 Conclusions.............................................................................................159 References ........................................................................................................ 159

Contents xi

4 Electrocatalytic Oxidation of Methanol, Ethanol and Formic Acid......... 165 Elod Gyenge

4.1 Introduction.......... ...................................................................................... 165 4.1.1 Historical Overview: 1960–1990 ................................................ 165 4.1.2 Objectives..................................................................................... 171 4.2 Reaction Pathways, Catalyst Selection and Performance: Example Analysis.................................................................................... 172 4.2.1 Methanol Electrooxidation........ .................................................. 172 4.2.2 Formic Acid Electrooxidation...... ............................................... 201 4.2.3 Ethanol Electrooxidation...... ....................................................... 219 4.2.4 Non-precious Metal Catalysts for Methanol, Formic Acid, and Ethanol Oxidation...... ........................................................... 224 4.3 Advances in Anode Catalyst Layer Engineering: Example Analysis...................................................................................................230 4.3.1 Engineering of the Catalyst Surface and Morphology.................230 4.3.2 The Catalytic Interface: Catalyst/Support/Ionomer Interaction....................................................................................236 4.4 Conclusions.............................................................................................269 References ........................................................................................................ 270

5 Application of First Principles Methods in the Study of Fuel Cell Air-Cathode Electrocatalysis ........................................................................ 289

Zheng Shi 5.1 Introduction.............................................................................................289 5.2 Background.............................................................................................. 290 5.2.1 Theoretical Methods....... ............................................................. 290 5.2.2 Oxygen Reduction Reaction........................................................ 291 5.3 Surface Adsorption.................................................................................. 293 5.3.1 Computational Methods............................................................... 294 5.3.2 Adsorption on Transition Metals................................................. 295 5.3.3 Adsorption on Bimetallic Alloys................................................. 299 5.4 Activation Energy.................................................................................... 306 5.4.1 Computational Method ................................................................ 306 5.4.2 Example Calculations .................................................................. 307 5.5 Thermodynamic Properties: Reversible Potential and Reaction Energy ...................................................................................... 311 5.5.1 Reversible Potential ..................................................................... 311 5.5.2 Reaction Thermodynamics .......................................................... 313 5.6 Study of Non-noble Catalysts ................................................................. 316 5.7 Summary............................................................................................. ... 324 References ........................................................................................................ 324

6 Catalyst Contamination in PEM Fuel Cells ................................................ 331 Hui Li, Chaojie Song, Jianlu Zhang and Jiujun Zhang 6.1 Introduction.............................................................................................331 6.2 Anode Catalyst Layer Contamination..................................................... 331 6.2.1 Impacts of Carbon Dioxide.......................................................... 332

xii Contents

6.2.2 Impacts of Hydrogen Sulfide (H2S) ........................................... 334 6.2.3 Impacts of Ammonium (NH3) .................................................... 337 6.2.4 Modeling of the Contamination of the PEMFC Anode Catalyst............................................................................. 337 6.2.5 Mitigation of Anode Contamination ........................................... 339 6.3 Cathode Catalyst Layer Contamination .................................................. 339 6.3.1 SOx Contamination ...................................................................... 340 6.3.2 NOx Contamination...................................................................... 343 6.3.3 NH3 and H2S Contamination ....................................................... 346 6.3.4 Volatile Organic Compounds (VOCs) Contamination ............... 347 6.3.5 Ozone Contamination .................................................................. 348 6.3.6 The Contamination Effects of Multi-contaminants ................... 348 6.3.7 Modeling of PEMFC Cathode Catalyst Contamination ............. 349 6.4 Additive Effects of Anode and Cathode Contamination........................ 349 6.5 Summary............................................................................................. ... 350 References ........................................................................................................ 351

7 PEM Fuel Cell Catalyst Layers and MEAs ................................................. 355 Pei Kang Shen

7.1 Fundamentals of Catalyst Layers............................................................ 355 7.1.1 Components and Structure........................................................... 356 7.1.2 Functions and Reactions .............................................................. 356 7.1.3 Factors Affecting the Performance of CLs ................................. 359 7.1.4 Catalyst Layers for Liquid Fuel Cells ......................................... 366 7.1.5 Catalyst Layers for Anion Exchange Membrane Fuel Cells ...... 367 7.2 Principles of Membrane Electrode Assembly (MEA) ........................... 369 7.2.1 Classification of MEA Materials................................................. 370 7.2.2 Methods for MEA Fabrication .................................................... 371 7.2.3 Technical Consideration .............................................................. 372 7.2.4 MEA for Anion Exchange Membrane Fuel Cells....................... 373 7.3 Conclusions.............................................................................................374 References ........................................................................................................ 374

8 Catalyst Layer Modeling: Structure, Properties and Performance ......... 381 Michael H. Eikerling, Kourosh Malek and Qianpu Wang 8.1 Introduction.............................................................................................381 8.2 Understanding Structure and Operation of Catalyst Layers................... 383 8.2.1 Challenges for the Structural Design........................................... 383 8.2.2 Porous Electrode Theory: Historical Perspective ....................... 384 8.2.3 Misapprehensions and Controversial Issues ............................... 387 8.2.4 Effectiveness of Catalyst Utilization........................................... 388 8.2.5 Evaluating the Performance of CLs ............................................ 391 8.3 State of the Art in Theory and Modeling: Multiple Scales .................... 395 8.4 Structural Formation of Catalyst Layers and Effective Properties ........ 398 8.4.1 Molecular Dynamics Simulations ............................................... 398 8.4.2 Atomistic MD Simulations of CLs.............................................. 400 8.4.3 Meso-scale Model of CL Microstructure Formation .................. 403

Contents xiii

8.4.4 Structure-related Effective Properties of CLs ............................. 407 8.5 Performance Modeling and Optimization Studies.................................. 412 8.5.1 General Framework of Performance Modeling .......................... 412 8.5.2 Transport and Reaction in Catalyst Layers ................................. 415 8.5.3 Spherical Agglomerates............................................................... 418 8.5.4 Main Results of the Macrohomogeneous Approach................... 425 8.5.5 Water Management in CCLs ....................................................... 428 8.6 Comparison and Evaluation of Catalyst Layer Designs......................... 433 8.6.1 Conventional Catalyst Layers...................................................... 434 8.6.2 Ultra-thin Two-phase Catalyst Layers ........................................ 434 8.7 Summary and Outlook............................................................................. 438 References ........................................................................................................ 439

9 Catalyst Synthesis Techniques ...................................................................... 447 Christina Bock, Helga Halvorsen and Barry MacDougall 9.1 Introduction.............................................................................................447 9.2 Catalysis Synthesis Methods................................................................... 447 9.2.1 Low-temperature Chemical Precipitation ................................... 448 9.2.2 Colloidal ....................................................................................... 448 9.2.3 Sol-gel........................................................................................ 449 9.2.4 Impregnation ................................................................................ 450 9.2.5 Microemulsions............................................................................ 451 9.2.6 Electrochemical............................................................................ 453 9.2.7 Spray Pyrolysis ............................................................................ 454 9.2.8 Vapor Deposition ......................................................................... 455 9.2.9 High-energy Ball Milling ............................................................ 457 9.3 Particle Size and Shape Control.............................................................. 458 9.3.1 Mechanism for Size Control Using Colloidal Synthesis Methods ....................................................................... 460 9.3.2 Size Control Using Electrochemical Methods ............................ 463 9.3.3 Assistance of Templates and Template Preparation ................... 463 9.3.4 Shape Control............................................................................... 467 9.4 Bi-metallic Catalysts ............................................................................... 468 9.4.1 Synthesis of Alloy versus Two-phase Catalysts ......................... 468 9.4.2 Sub-monolayer Deposition of Ad-metals.................................... 472 9.5 Non-noble Metal Catalyst Synthesis....................................................... 474 9.5.1 Macrocyclic Complexes .............................................................. 474 9.5.2 Methanol Tolerance and the Economics of these Catalysts........ 476 9.5.3 Transition Metal Chalcogenides.................................................. 477 9.5.4 Conclusions.................................................................................. 478 References ........................................................................................................ 479

10 Physical Characterization of Electrocatalysts............................................. 487 Shijun Liao, Baitao Li and Yingwei Li

10.1 Introduction.............................................................................................487 10.2 Analysis of Composition and Phase of Catalyst..................................... 488 10.2.1 X-ray Diffraction (XRD) and Electron Diffraction (ED) ........... 488

xiv Contents

10.2.2 X-ray Fluorescence (XRF), X-ray Emission (XRE), and Proton-induced X-ray Emission (PIXE)...................................... 497 10.3 Measurement of Physical Surface Area and Electrochemical Active Surface Area............................................................................................498 10.3.1 BET Method and Physical Surface Area..................................... 498 10.3.2 Electrochemical Hydrogen Adsorption/Desorption.................... 499 10.3.3 Typical Examples Analysis ......................................................... 501 10.4 Morphology of Catalysts and Their Active Components....................... 505 10.4.1 Scanning Electron Microscopy (SEM)........................................ 505 10.4.2 Transmission Electron Microscopy............................................. 506 10.4.3 Typical Examples......................................................................... 507 10.5 The Structure and Crystallography of Surface and Small Active Component Particles................................................................................ 512 10.5.1 Principles of Electron Spectroscopy for Chemical Analysis

(ESCA)........................................................................................ 512 10.5.2 X-ray Photoelectron Spectroscopy (XPS)................................... 513 10.5.3 UV-induced Photoelectron Spectroscopy (UVPS) ..................... 519 10.5.4 Energy Dispersive Spectroscopy (EDS) and its Application...... 522 10.6 Analysis of the Stability of Catalysts by the Thermal Analysis Method............................................................................................. ...... 525 10.6.1 Principles...................................................................................... 525 10.6.2 Application................................................................................... 526 10.6.3 Typical Examples of Analysis..................................................... 527 10.7 Other Structural Techniques for Characterizing the Bulk and Surface of Electrocatalysts ................................................................................... 532 10.7.1 FTIR and UV-VIS........................................................................ 532 10.7.2 TPD/TPR...................................................................................... 534 10.8 Conclusion.............................................................................................. 536 References ........................................................................................................ 536

11 Electrochemical Methods for Catalyst Activity Evaluation ...................... 547 Zhigang Qi 11.1 Electrochemical Cells.............................................................................. 547 11.1.1 Introduction ................................................................................. 547 11.1.2 Conventional 3-Electrode Cells................................................... 548 11.1.3 Half-cells ...................................................................................... 551 11.1.4 Single Cells .................................................................................. 553 11.2 Brief Principles of Electrochemical Instrumentation ............................. 556 11.3 Cyclic Voltammetry ................................................................................ 556 11.3.1 Basic Principles............................................................................ 556 11.3.2 Potential Step Experiment ........................................................... 558 11.3.3 Instrumentation: Potentiostat ....................................................... 559 11.3.4 Applications ................................................................................. 560 11.4 Rotating Disk and Rotating Ring-disk Electrode Techniques................ 567 11.4.1 Theories and Principles ................................................................ 567 11.4.2 Instrumentation............................................................................. 570 11.4.3 Fuel Cell-related Applications ..................................................... 570

Contents xv

11.5 Electrochemical Impedance Spectroscopy ............................................. 573 11.5.1 Theories and Principles ................................................................ 573 11.5.2 Instrumentation............................................................................. 578 11.5.3 Application in Fuel Cells ............................................................. 578 11.6 Current Interruption and Current Pulse Techniques................................ 585 11.6.1 Principles and Instrumentation .................................................... 585 11.6.2 Application in Fuel Cells ............................................................. 587 11.7 Steady-state I-V Polarization ................................................................... 588 11.7.1 Principles and Instrumentation .................................................... 588 11.7.2 Fuel Cell Hardware ...................................................................... 589 11.7.3 Fuel Cell Performance.................................................................. 590 11.8 Durability Evaluation ............................................................................... 592 11.8.1 Introduction .................................................................................. 592 11.8.2 Techniques.................................................................................... 593 11.9 Summary................................................................................................... 602 List of Symbols................................................................................................. 602 References ........................................................................................................ 604

12 Combinatorial Methods for PEM Fuel Cell Electrocatalysts.................... 609 Hansan Liu and Jiujun Zhang 12.1 Introduction.............................................................................................609 12.1.1 Combinatory Material Chemistry................................................ 609 12.1.2 Electrocatalysis in PEM Fuel Cells ............................................. 611 12.2 Combinatorial Methods for Fuel Cell Electrocatalysis .......................... 612 12.2.1 Catalyst Library Preparation........................................................ 612 12.2.2 Catalyst Activity Down-selection................................................ 617 12.3 Combinatorial Discoveries of Fuel Cell Electrocatalysts........................ 622 12.3.1 Low/Non-platinum Content Catalysts for PEM Fuel Cell Cathodes....................................................................................... 623 12.3.2 CO-tolerant Catalysts for PEM Fuel Cell Anodes ...................... 625 12.3.3 Platinum Alloy Catalysts for Direct Methanol Fuel Cell Anodes....................................................................................... . 625 12.3.4 Methanol-tolerant Catalysts for Direct Methanol Fuel Cell Cathodes....................................................................................... 627 12.4 Conclusions........ ...................................................................................... 628 References ........................................................................................................ 629

13 Platinum-based Alloy Catalysts for PEM Fuel Cells.................................. 631 Hansan Liu, Dingguo Xia and Jiujun Zhang 13.1 Introduction.............................................................................................631 13.2 Pt-based Alloy Catalysts for PEM Fuel Cell Cathodes .......................... 632 13.2.1 The Alloying Effect on Cathode Catalyst Activity..................... 632 13.2.2 Mechanism of the Alloying Effect on Cathode Catalysts........... 635 13.2.3 Stability of Pt-based Alloy Cathode Catalysts ............................ 640 13.3 Pt-based Alloy Catalysts for DMFC Anodes.......................................... 643 13.3.1 The Alloying Effect on Anode Catalyst Activity ........................ 643 13.3.2 Mechanism of the Alloying Effect on Anode Catalysts.............. 646

xvi Contents

13.3.3 The Stability of Pt-based Alloy Anode Catalysts........................ 649 13.4 Concluding Remarks ............................................................................... 650 References ........................................................................................................ 651

14 Nanotubes, Nanofibers and Nanowires as Supports for Catalysts ........... 655 Xueliang Sun and Madhu Sudan Saha 14.1 Introduction.............................................................................................655 14.1.1 The Importance of Combining Nanotechnology and Clean Energy..........................................................................................655 14.1.2 One-dimensional Nanomaterials Based New Catalyst Supports.......................................................................................656 14.2 Synthesis and Characterization of Carbon Nanotubes, Nanofibers, and Nanowires........................................................................................ 657 14.2.1 Structure and Synthesis Methods for Carbon Nanotubes............ 657 14.2.2 Structure and Synthesis Methods for Carbon Nanofibers ........... 661 14.2.3 Structure and Synthesis Methods for Nanowires ........................ 661 14.3 Synthesis and Characterization of Pt Catalysts Supported on Carbon Nanotubes, Carbon Nanofibers and Metal Oxide Nanowires ... 665 14.3.1 Introduction .................................................................................. 665 14.3.2 Methods for Depositing Pt Catalysts on Carbon Nanotubes (Pt/CNTs) ..................................................................................... 666 14.3.3 Methods for Depositing Pt Catalysts on Carbon Nanofibers (Pt/CNFs) ..................................................................................... 682 14.3.4 Methods for Depositing Pt Catalysts on Metal Oxide Nanowires (Pt/NWs).................................................................... 684 14.3.5 Methods of Functionalizing of Carbon Nanotubes and Nanofibers-based Fuel Cell Electrodes ....................................... 687 14.4 Activity Validation of the Synthesized Catalysts in a Fuel Cell Operation.................................................................................................693 14.4.1 Fabrication of Membrane Electrode Assembly for Carbon Nanotubes and Nanofibers-based Catalysts ................................ 693 14.4.2 Performance of Carbon Nanotubes and Nanofibers Membrane Electrode Assembly ..................................................................... 697 14.5 Stability of Carbon Nanotubes and Nanofibers-based Fuel Cell Electrodes................................................................................................700 14.6 Conclusions and Future Perspective ........................................................ 702 References ........................................................................................................ 704

15 Non-noble Electrocatalysts for the PEM Fuel Cell Oxygen Reduction Reaction......................................................................................... 715 Kunchan Lee, Lei Zhang and Jiujun Zhang 15.1 Introduction..............................................................................................715 15.2. Transition Metal Macrocycles for the Oxygen Reduction Reaction...... 716 15.2.1. The Central Transition Metal Effect ........................................... 717 15.2.2. The Ligand Effect........................................................................ 719 15.2.3. The Heat-treatment Effect........................................................... 720 15.2.4. The Effect of the Synthesis Method ........................................... 721

Contents xvii

15.3 Non-noble Transition Metal Carbides and Nitrides for the ORR .......... 725 15.3.1 Carbides........................................................................................ 725 15.3.2 Nitrides......................................................................................... 728 15.3.3 Oxynitrides ................................................................................... 730 15.3.4 Carbonitrides ................................................................................ 733 15.4 Transition Metal Chalcogenides for the ORR ........................................ 734 15.5 Metal Oxides for the ORR ...................................................................... 742 15.6 Conclusions.............................................................................................748 References ........................................................................................................ 748

16 CO-tolerant Catalysts .................................................................................... 759 Siyu Ye

16.1 Introduction.............................................................................................759 16.2 Mechanisms of CO Tolerance................................................................. 764 16.2.1 Electrochemistry of Carbon Monoxide and Hydrogen ............... 766 16.2.2 Characteristics of PEMFC CO Poisoning ................................... 770 16.2.3 Bifunctional Mechanism of CO Tolerance.................................. 771 16.2.4 Direct Mechanism of CO Tolerance (Ligand or Electronic Effect) .........................................................................................773 16.2.5 Surface Science Study and Modeling of CO-tolerance Mechanism................................................................................... 774 16.3 Development of CO-tolerant Catalysts................................................... 781 16.3.1 PtRu Binary System..................................................................... 783 16.3.2 PtMo Binary System.................................................................... 787 16.3.3 PtSn Binary System...................................................................... 790 16.3.4 PtM (M = Fe, Co, Ni, Ta, Rh, Pd) Binary Systems..................... 791 16.3.5 PtRuM (M = Mo, Sn, W, Cr, Zr, Nb, Ag, Au, Rh, Os, and Ta) Ternary Systems........................................................................... 794 16.3.6 The Pt, PtRu-MOx (M = Mo, W, and V) System ........................ 796 16.3.7 Ru-modified Pt Catalysts and Pt-modified Ru Catalysts ............ 799 16.3.8 PtRu on Functionalized Carbon and Carbon Nanotube Systems ........................................................................................ 802 16.3.9 PtAu Binary System..................................................................... 804 16.3.10 Pt-free Systems........................................................................... 804 16.4 Preparation of CO-tolerant Catalysts ...................................................... 805 16.5 Conclusions............................................................................................. 809 References ........................................................................................................ 811

17 Reversal-tolerant Catalyst Layers ................................................................ 835 Siyu Ye

17.1 Introduction.............................................................................................835 17.2 Cell Voltage Reversal.............................................................................. 838 17.2.1 Air Starvation............................................................................... 838 17.2.2 Fuel Starvation ............................................................................. 839 17.2.3 Electrocatalyst Degradation in PEM Fuel Cells Caused by Cell Voltage Reversal During Fuel Starvation ................................... 842 17.3 Development of Reversal-tolerant Catalyst Layers................................ 845

xviii Contents

17.3.1 Reversal Tolerance Cathode Catalyst Layer ............................... 846 17.3.2 Reversal Tolerance Anode Catalyst Layer .................................. 847 17.4 Conclusions............................................................................................. 856 References ........................................................................................................ 856

18 High-temperature PEM Fuel Cell Catalysts and Catalyst Layers ........... 861 Chaojie Song, Rob Hui and Jiujun Zhang 18.1 Opportunities and Challenges for High-temperature PEM Fuel Cells... 861 18.1.1 Advantages of High-temperature PEM Fuel Cells ..................... 861 18.1.2 Routes to Increase the Operating Temperature ........................... 867 18.1.3 Challenges of Catalysts/Catalyst Layers ..................................... 867 18.2 Catalysts for High-temperature PEM Fuel Cells .................................... 868 18.2.1 Current Research Activities......................................................... 868 18.2.2 Degradation of Catalysts at High Temperatures ......................... 869 18.2.3 Catalyst Support Strategy to Improve High-temperature Catalysts/Catalyst Layers............................................................. 876 18.2.4 High-temperature Catalyst Layers – Components and Structure ....................................................................................... 877 18.2.5 Strategies for HT Catalyst/Catalyst Layer Performance Improvement and Mitigation ....................................................... 878 18.2.6 Suggestions for Future Work ....................................................... 878 18.2.7 Typical Example Analysis ........................................................... 878 18.3 Summary................................................................................................... 884 References ........................................................................................................ 884

19 Conventional Catalyst Ink, Catalyst Layer and MEA Preparation ......... 889 Huamin Zhang, Xiaoli Wang, Jianlu Zhang and Jiujun Zhang 19.1 Introduction.............................................................................................889 19.2 Principles of Gas Diffusion Electrodes and MEA Structure.................. 889 19.3 Catalyst Layer.......................................................................................... 893 19.3.1 Preparation of Catalyst Ink .......................................................... 893 19.3.2 Preparation of the Catalyst Layer ................................................ 895 19.4 Preparation of the MEA ........................................................................... 911 19.5 Summary and Outlook.............................................................................. 911 References ........................................................................................................ 912

20 Spray-based and CVD Processes for Synthesis of Fuel Cell Catalysts and Thin Catalyst Layers .............................................................................. 917 Radenka Maric 20.1 Introduction.............................................................................................917 20.2 Spray Pyrolysis Approach....................................................................... 919 20.2.1 Current Research Activities ......................................................... 919 20.2.2 Spray Conversion and Aerosol Routes for Powder Manufacturing.............................................................................. 919 20.2.3 Pt Nanoparticle Preparation via Spray Route .............................. 921 20.2.4 Morphology of Catalyst Deposited by Spray Pyrolysis .............. 922 20.2.5 Electrochemical Performance ...................................................... 925

Contents xix

20.2.6 Electrocatalytic Activity and Stability of Pt-based Catalysts...... 926 20.2.7 Typical Example Analysis ........................................................... 928 20.3 Deposition of Catalyst Layer by CVD..................................................... 929 20.3.1 Current Research Activities ......................................................... 930 20.3.2 Film Formation from Vapor Phase by CVD ............................... 931 20.3.3 Morphological and Microstructural Stability .............................. 933 20.3.4 Electrochemical Performance and Catalytic Activity ................. 935 20.3.5 Typical Examples Analysis.......................................................... 939 20.4 Flame-based Processing ........................................................................... 941 20.4.1 Current Research Activities ......................................................... 942 20.4.2 Atomization Process..................................................................... 943 20.4.3 Particle Formation in the Flame................................................... 944 20.4.4 Particle Size Control .................................................................... 946 20.4.5 Electrochemical Performance and Catalytic Activity of the Flame Deposited Catalyst ............................................................ 950 20.4.6 Typical Examples Analysis.......................................................... 954 20.5 Summary................................................................................................... 958 References ........................................................................................................ 958

21 Catalyst Layer/MEA Performance Evaluation........................................... 965 Jianlu Zhang and Jiujun Zhang 21.1 Introduction.............................................................................................965 21.2 Theoretical Analysis................................................................................ 966 21.2.1 Open Circuit Voltage (OCV) of the PEMFC .............................. 966 21.2.2 Exchange Current Density, i0....................................................... 968 21.2.3 Tafel Slope, b ............................................................................... 968 21.2.4 Polarization Curve Analysis......................................................... 971 21.3 Physical Chemistry Evaluation of Catalyst Layer .................................. 973 21.3.1 Pore Structure Analysis of Catalyst Layer .................................. 973 21.3.2 Protonic and Electronic Conductivity in the Catalyst Layer....... 974 21.3.3 Wettability of the Catalyst Layer................................................. 975 21.4 Catalyst Layer Evaluation in a Half-cell................................................. 978 21.4.1 Rotating Disk Electrode (RDE) Test ........................................... 978 21.4.2 Cyclic Voltammetry (CV) Test.................................................... 981 21.4.3 Polarization Curves in a Half-cell................................................ 984 21.5 MEA Evaluation by the Single-cell Test ................................................ 986 21.5.1 Test Station................................................................................... 986 21.5.2 Polarization Curve........................................................................ 988 21.5.3 Resistance Test – AC Impedance Test ........................................ 988 21.5.4 Permeability/Crossover Test ........................................................ 992 21.6 Lifetime/Durability Testing of the MEA ................................................ 994 21.6.1 Mechanisms of MEA Degradation .............................................. 994 21.6.2 Durability Testing ........................................................................ 996 21.7 Conclusions.............................................................................................997 References ........................................................................................................ 997

xx Contents

22 Catalyst Layer Composition Optimization................................................ 1003 Wei Xing 22.1 Catalyst Layer Materials Selection and Evaluation.............................. 1003 22.1.1 Catalyst selection........................................................................ 1003 22.1.2 Gas Diffusion Layer (GDL) and Microporous Layer (MPL) Materials Selection..................................................................... 1011 22.2 Fabrication Optimization Processes for the Catalyst Layer of MEAs................................................................................................1016 22.2.1 GDL Substrate Preparation ........................................................ 1016 22.2.2 Microporous Layer (MPL) Preparation and Optimization........ 1017 22.2.3 Catalyst Ink Composition and Preparation................................ 1019 22.2.4 Carbon-supported Catalyst Layer Fabrication........................... 1023 22.2.5 Pt Catalyst Layer Fabrication..................................................... 1027 22.2.6 MEA Fabrication and Optimization .......................................... 1029 22.3 MEA Performance Verification with its Catalyst Layer Fabrication Optimization Process............................................................................. 1031 22.3.1 MEA Performance Characterization.......................................... 1031 22.3.2 MEA Water Management Characterization .............................. 1032 22.3.3 MEA CO and Other Contamination Tolerance ......................... 1032 22.3.4 MEA Lifetime Enhancement via MEA Fabrication Process Improvement .............................................................................. 1033 References ...................................................................................................... 1033

23 Catalyst Layer Degradation, Diagnosis and Failure Mitigation ............. 1041 Jing Li

23.1 Introduction....... .................................................................................... 1041 23.2 Diagnosis of Catalyst Layer Degradation: Fuel Cell Failure Analysis.................................................................................................1044 23.2.1 Diagnostic Tools to Identify Catalyst Degradation During Fuel Cell Operation: Electrochemical Methods........................ 1045 23.2.2 Ex situ Tools for Characterization of Catalyst Degradation During Fuel Cell Operation ....................................................... 1049 23.2.3 Durability and Accelerated Stress Testing ................................ 1054 23.3 Anode Catalyst Layer Degradation....................................................... 1056 23.3.1 Anode Catalyst Layer Degradation Caused by Contamination............................................................................ 1056 23.3.2 Anode Catalyst Layer Degradation–Voltage Reversal ............. 1061 23.3.3 Ru Leaching and Crossover ....................................................... 1064 23.4 Cathode Catalyst Layer Degradation .................................................... 1066 23.4.1 Platinum Dissolution During Fuel Cell Operation .................... 1066 23.4.2 Pt Accumulation and Distribution in the Membrane after Fuel Cell Operation.................................................................... 1073 23.4.3 Loss of Platinum Surface Area Due to Agglomeration............. 1075 23.4.4 Carbon Corrosion of Catalyst Layer.......................................... 1080 23.5 Summary................................................................................................. 1087 References ...................................................................................................... 1089

Contents xxi

Acronyms and Abbreviations ........................................................................... 1095

Contributor Biographies ................................................................................... 1103

Author Index....................................................................................................... 1117

Subject Index......................................................................................................1119

1

PEM Fuel Cell Fundamentals

Xiao-Zi Yuan and Haijiang Wang

1.1 Overview

1.1.1 Introduction

A fuel cell is an electrochemical device that continuously and directly converts the chemical energy of externally supplied fuel and oxidant to electrical energy. Fuel cells are customarily classified according to the electrolyte employed. The five most common technologies are polymer electrolyte membrane fuel cells (PEM fuel cells or PEMFCs), alkaline fuel cells (AFCs), phosphoric acid fuel cells (PAFCs), molten carbonate fuel cells (MCFCs) and solid oxide fuel cells (SOFCs). However, the popularity of PEMFCs, a relatively new type of fuel cell, is rapidly outpacing that of the others.

Unlike most other types of fuel cells, PEMFCs use a quasi-solid electrolyte, which is based on a polymer backbone with side-chains possessing acid-based groups. The numerous advantages of this family of electrolytes make the PEM fuel cell particularly attractive for smaller-scale terrestrial applications such as transportation, home-based distributed power, and portable power applications. The distinguishing features of PEMFCs include relatively low-temperature (under 90 °C) operation, high power density, a compact system, and ease in handling liquid fuel.

1.1.1.1 A Brief History of PEM Fuel Cells

Invention of the Fuel Cell—1839 The idea of the gaseous fuel cell can be traced back to Sir William Grove, a Welsh judge, inventor, and physicist, who is recognized as “the father of the fuel cell.” A reproduction of his drawing of a fuel cell, from 1838, can be seen in Figure 1.1. In 1839 Grove found that electrolysis (using electricity to split water into hydrogen and oxygen) could be performed in reverse with the right catalyst, producing electricity. In 1842, Grove developed a stack of 50 fuel cells, which he called a “gaseous voltaic battery”. However, for almost a century after Grove’s discovery

2 X-Z. Yuan and H. Wang

the fuel cell did not make any practical progress, remaining only a scientific curiosity.

Figure 1.1. The first fuel cell. (Originally printed in Grove, W. R. (1838). On a new voltaic combination. Philosophical Magazine and Journal of Science 13, 430.)

In 1937, Francis T. Bacon, an Englishman, started to work on practical fuel cells. By the end of the 1950s [1] he had developed a 40-cell stack capable of 5 kW. The stack was able to power a welding machine, circular saw, and forklift.

PEM Fuel Cell Development—1960s The PEM fuel cell was invented at General Electric (GE) in the early 1960s, through the work of Thomas Grubb and Leonard Niedrach. Initially, sulfonated polystyrene membranes were used as the solid electrolytes, but these were soon replaced by Nafion® membranes in 1966. The Nafion membrane has proved to be superior in performance and durability, and it is still the most popular membrane in use today.

Gemini Space Program—1950–1970 PEM fuel cell technology served as part of NASA’s Gemini Program (Figure 1.2), the main objective of which was to test equipment and procedures for Apollo. GE’s PEM fuel cells were selected, but the earliest model PB2 cell repeatedly encountered technical difficulties, including internal cell contamination and leakage of oxygen through the membrane. Gemini I through IV flew with batteries instead. Due to PB2’s malfunctions and poor performance, a new model, P3, was designed. The first mission to utilize PEMFCs was Gemini V. However, they were replaced by alkaline fuel cells in the Apollo program and in the space shuttle. This delayed the development of PEM fuel cells for a decade [2, 3].

PEM Fuel Cell Fundamentals 3

Figure 1.2. Gemini space mission used fuel cells. (Image courtesy of NASA.)

Ballard Breakthrough—1980–2000 Due to their high cost, fuel cell systems were limited to space missions and other special applications. It was not until the late 1980s and early 1990s, when research by Ballard Power Systems (founded in 1979) resulted in a resurgence in interest in PEMFCs and the development of fuel cells, that fuel cells became a real option for wider applications. In 1983, Ballard began developing PEM fuel cells. Proof-of-concept fuel cells followed, and sub-scale and full-scale prototype systems (Figure 1.3) were developed to demonstrate the technology. Their milestones in the 1990s are as follows:

Mk900 (2000) Mk800 (1997) Mk700 (1995) Mk500 (1993) Mk300 (1991) 80 kW 50 kW 25 kW 10 kW 5 kW

(a) (b) (c)

Figure 1.3. Fuel cells produced by Ballard Power Systems. (a) Mark1020 ACS™, (b) Mark1030™ V3, (c) Heavy-Duty Fuel Cell Module (HD6). (Images courtesy of Ballard Power Systems.)

4 X-Z. Yuan and H. Wang

Figure 1.4. A map of the Hydrogen Highway in British Columbia, Canada. (Image courtesy of BC Hydrogen Highway™.)

PEM Fuel Cells Today Since interest in PEM fuel cell research and development has intensified, more and more universities and institutes all over the world are becoming involved. So far several key innovations, such as low platinum catalyst loading, novel membranes, and new bipolar plates, make the application of PEMFC systems more or less realistic. Demonstration activities in every corner of the world are overwhelming. Figure 1.4 shows an example of a hydrogen and fuel cell demonstration project, the Hydrogen Highway in British Columbia, Canada. British Columbia’s Hydrogen Highway is actually both a demonstration program and a market development program; it features an evolving network of hydrogen and fuel cell technologies, including vehicle fuelling infrastructure. The project leaders and participants come from British Columbia’s hydrogen and fuel cell industries, government, industry associations, and academic institutions. Fully implemented in time for the 2010 Olympic and Paralympic Winter Games, the program will operate a fleet of 20 buses in Victoria and Whistler.

After the Canadian government announced funding for the world’s first hydrogen highway, California’s Governor Schwarzenegger committed to the building of a California Hydrogen Highway Network on a similar timescale, by signing an executive order creating a public/private partnership.

PEM Fuel Cell Fundamentals 5

1.1.1.2 Principles of PEM Fuel Cells The conversion of chemical energy to electrical energy in a PEM fuel cell occurs through a direct electrochemical reaction. It takes place silently without combustion. The key part of a PEM fuel cell, which is known as a membrane electrode assembly (MEA), consists of a polymer electrolyte in contact with an anode and a cathode on either side. To function, the membrane must conduct hydrogen ions (protons) and separate either gas to pass to the other side of the cell. A schematic representation of a PEM fuel cell is shown in Figure 1.5.

Figure 1.5. Diagram of PEM fuel cell principle

Unlike in a conventional battery, the fuel and oxidant are supplied to the device from external sources. The device can thus be operated until the fuel (or oxidant) supply is exhausted. As seen in Figure 1.5, on one side of the cell, hydrogen is delivered through the flow field channel of the anode plate to the anode. On the other side of the cell, oxygen from the air is delivered through the channeled plate to the cathode. At the anode, hydrogen is decomposed into positively charged protons and negatively charged electrons. Positively charged protons pass through the polymer electrolyte membrane (PEM) to the cathode, whereas the negatively charged electrons travel along an external circuit to the cathode, creating an electrical current. At the cathode, the electrons recombine with the protons, and together with the oxygen molecules, form pure water as the only reaction by-product, which flows out of the cell.

The splitting of the hydrogen molecule is relatively easy using a platinum catalyst. However, the splitting of the stronger oxygen molecule is more difficult, which causes significant activation loss. So far platinum is still the best option for the oxygen reduction reaction (ORR). Another significant source of performance loss is the resistance of the membrane to proton flow, which is minimized by making it as thin as possible (around 50 μm). Nevertheless, the PEM fuel cell is a

6 X-Z. Yuan and H. Wang

system whose successful operation with a high power output depends on all the sub-systems; its performance depends on components such as flow field design, catalyst, and membrane, and also on parameters such as temperature and humidity.

1.1.1.3 Single Cell, Stack, and System

Single Cell A single PEM fuel cell includes only one anode and one cathode; therefore, the operating voltage of a single cell is less than 1 V. When it is operated under a given current, the voltage is even less. An acceptable performance for a state-of-the-art single cell is at least 1 A/cm2 at a voltage of 0.6 V. Here are two examples of single cell designs.

An in-house fabricated single cell designed by the National Research Council of Canada Institute for Fuel Cell Innovation (NRC-IFCI) is shown in Figure 1.6. The single cell components include the bladder plate, piston, plastic plate, anode current collector, anode plate, gaskets, cathode plate, cathode current collector, and another plastic plate and bladder plate. The graphite plate/current collector are isolated from the aluminum end plate by a plastic plate, which also serves as a gas manifold. Gaskets are used to seal the MEA. A variable pressure bladder, controlled with nitrogen gas, is used to compress and seal the cell assembly. Cell heating is accomplished by two pairs of heat tape pieces. The inner 60 W heat tape, which is glued onto the copper current collector plates, allows the cell to operate at temperatures up to 120 °C.

Figure 1.6. A single PEM fuel cell with an active area of 4.4 cm2 designed by NRC-IFCI. (Reproduced by permission of ECS—The Electrochemical Society, from Tang Y, Zhang J, Song C, Liu H, Zhang J, Wang H, et al. Temperature-dependent performance and in situ ACimpedance of high-temperature PEM fuel cells using the Nafion-112 membrane.)

Figure 1.7 shows the assembly diagram of a diagnostic modeling PEM fuel cell also designed by NRC-IFCI. The active area of this single cell is 192 cm2. A

PEM Fuel Cell Fundamentals 7

straight channel configuration is chosen for the cathode plate to simplify the modeling process, ease the integration of sensors, and result in a relatively small pressure drop. Two-path serpentine channels are used for the anode plate, with the coolant channels in the back covering the entire active area. Two additional plates are used in the assembly of the single cell. One is the end plate, which seals the coolant channels on the anode plate. It is a blank plate, void of any distribution channels or seal grooves, marked only by holes for the reactant gases, coolant, and alignment pin. The other is the coolant plate, which cools the cathode plate and consists of a hole for the alignment pin, a seal groove, and coolant channels. At one end of the distribution plates, the manifold has ports for the hose fittings to connect to the distribution channels in the various plates. The relatively soft insulating material is able to electrically isolate the fuel cell. Between the plates and the insulators are the current collectors, which have a high electrical conductivity. The main components of the compression hardware are the alignment plate, tie rods, tie rod screws, bushings, and compression plate and bladder. The compression plate and bladder provide the compression force. It is a pneumatic system allowing accurate control of the compressive force, which ensures proper sealing and guarantees that the MEA is in contact with the reacting gases. A picture of the assembled diagnostic single cell is shown in Figure 1.8.

Figure 1.7. Assembly diagram of a diagnostic modeling PEM fuel cell with an active area of 192 cm2, designed by NRC-IFCI [6]

Compression Plate

Compression Bladder

Bushings

InsulationPlate

Cathode Current Collector

Coolant Plate

Cathode

Anode End Plate

Anode Current Collector

Manifold

Tie Rods

Alignment Plate Tie Rod Screws

8 X-Z. Yuan and H. Wang

Figure 1.8. Assembled diagnostic modeling cell [6]

Stack Single cells produce less than 1 V of electricity, which is far from enough to power a vehicle. In order to produce a useful voltage, multiple cells must be assembled into a fuel cell stack. This can be achieved in a parallel and/or a series mode to supply feed gas to the stacks. In a parallel gas supply fuel cell stack, all cells are fed in parallel from a common hydrogen/air inlet. In the serial configuration the gas from the outlet of the first cell is fed to the inlet of the second cell and so on until the last cell, which helps prevent non-uniform gas distribution. To avoid a large pressure drop this arrangement can be used only for stacks with a small number of fuel cells [7].

A discussion of the distribution of the reactant gases must address the flow field design. The essential requirement for flow field design is uniform distribution of the reactant gases over the respective active electrode surface. A non-uniform distribution will result in a non-uniform current density distribution leading to lower catalyst utilization, lower energy efficiency, and, last but not least, a reduced cell life. Usually, flow fields are designed around their maximum power operating point with the goals of maximizing performance and minimizing pressure drop between the inlet and outlet of the flow field. Nevertheless, when designing a flow field, several considerations must be taken into account, including flow rate, water management, thermal management, and pressure drop.

For a typical planar PEMFC design, feeding channels are often designed with one of three basic flow field structures: serpentine, parallel, or interdigitated [8]. Due to the length of the channels, serpentine flow fields have large pressure losses between the inlet and the outlet. The straight parallel design exhibits lower pressure differences; however, inhomogeneous reactant gas distribution can easily

PEM Fuel Cell Fundamentals 9

occur. The interdigitated flow field, which consists of dead-ended inlet and outlet channels, forces the reactant gas to flow through the electrode in order to exit, and helps solving the cathode-flooding problem.

In most commercial fuel cell stacks, the separator plates are designed to be bipolar, with one side of the plate being the anode of one cell and the other side being the cathode of the adjacent cell. The series electrical connection between cells is made through the electrically conducting separator plate. The potential power generated by a series-connected fuel cell stack depends on the number and size of the individual fuel cells that comprise the stack and the surface area of the PEM.

A typical PEM fuel cell stack is shown in Figure 1.9. Individual cells are electrically connected with interconnects. The interconnect becomes a separator plate, which provides an electrical series connection between adjacent cells and a gas barrier that separates the fuel and oxidant of adjacent cells.

Figure 1.9. PEM fuel cell stack. (Reprinted and modified from [9]. Journal of Power Sources, 114(1), Mehta Viral and Cooper Joyce Smith, Review and analysis of PEM fuel cell design and manufacturing, 70–79, ©2003, with permission from Elsevier.)

System In addition to the stack, practical fuel cells such as those in fuel cell vehicles (FCVs) require several other sub-systems and components to work as a system. Generally speaking, most fuel cell systems contain the following:

Hydrogen reformer or hydrogen purification: When a fuel other than hydrogen are used, it must be reformed to form a hydrogen-rich anode feed mixture. Even if hydrogen gas is used, it can contain impurities, which can cause deactivation of the catalysts. The impurities are removed through a process of purification. Air supply: This includes air compressors or blowers as well as air filters. Water management: Inlet gases normally must be humidified, and water is a reaction product. Thermal management: All fuel cell systems require careful management of the fuel cell stack temperature [10].

10 X-Z. Yuan and H. Wang

The specific arrangement of the fuel cell systems varies, depending on the fuel cell type, the fuel choice, and the application. An example of a fuel cell system with direct hydrogen gas feeding is depicted in Figure 1.10(a).

Methanol and some other liquid fuels can be fed to a PEM fuel cell directly without being reformed, thus forming a direct methanol fuel cell (DMFC), direct ethanol fuel cell (DEFC), direct formic acid fuel cell (DFAFC), and so on.

Figure 1.10(a). Block diagram of a PEM fuel cell system [11]. (Pukrushpan JT, Stefanopoulou AG, Peng H. IEEE Control Systems Magazine, Control of fuel cell breathing. ©2004 IEEE. Reprinted with permission.)

Figure 1.10(b). A complete fuel cell system integrated with a fuel processor [1]. (Reprinted from Barbir F. PEM fuel cells: theory and practice. New York: Elsevier Academic Press, ©2005, with permission from Elsevier.)

PEM Fuel Cell Fundamentals 11

Typical systems utilize on-board reformation, processing the gasoline, methanol, or other carbon-based fuel into hydrogen-rich gas. A complete fuel cell system integrated with a fuel processor is shown in Figure 1.10(b). The system contains the four sub-systems explained above. Currently, complete systems such as this are available in the market. The most well-known manufacturers of PEM systems include Ballard Power Systems, UTC Power (also known as UTC Fuel Cells), PEMEAS USA, E-TEK Inc., DuPont, 3M, Johnson Matthey, WL Gore, Hydrogenics, and Plug Power [3].

1.1.2. Main Cell Components and Materials

1.1.2.1 Membrane The main function of the membrane in PEM fuel cells is to transport protons from the anode to the cathode; membrane polymers have sulfonic groups, which facilitate the transport of protons. The other functions include keeping the fuel and oxidant separated, which prevents mixing of the two gases and withstanding harsh conditions, including active catalysts, high temperatures or temperature fluctuations, strong oxidants, and reactive radicals. Thus, the ideal polymer must have excellent proton conductivity, chemical and thermal stability, strength, flexibility, low gas permeability, low water drag, low cost, and good availability [12].

Different types of membranes have been tested for use in PEM fuel cells. The membranes are usually polymers modified to include ions, such as sulfonic groups. These hydrophilic ionic moieties are the key for allowing proton transport across the membrane. The favored polymer structure has changed to improve membrane lifetime and slow down membrane degradation [13].

The very early membranes, fabricated by Grubb and Niedrach of GE, were phenol-formaldehyde sulfonic acids produced by the condensation of phenolsulfonic acid and formaldehyde. Unfortunately, they hydrolyzed easily and were extremely weak. These were followed by membranes with a partially sulfonated polystyrene backbone. Their performance was also unsatisfactory, achieving a lifetime of only 200 hours at 60 °C. The first membranes to have sufficient physical strength were “D” membranes, manufactured by American Machine Foundry. They were fabricated by grafting styrene-divinylbenzene into a fluorocarbon matrix, followed by sulfonation. “D” membranes achieved life spans of 500 hours at 60 °C and were utilized in the fuel cells as auxiliary power sources for seven Gemini space missions [14].

Degradation of the “D” membranes was linked to the reactivity of the alpha C-H bond in the polymer. A series of membranes that did not contain this bond were then synthesized. trifluorostyrene sulfonic acid was determined to have chemical and thermal stability, but poor physical properties (Figure 1.11(a) [15]). This was somewhat improved by using a triethyl phosphate plasticizer to combine polyvinylidine fluoride with the trifluorostyrene sulfonic acid polymer, reaching a lifetime of up to 5000 hours at 80 °C. This lifetime was doubled by a fluorocarbon matrix grafted with trifluorostyrene, and then further improved by a membrane composed of trifluorostyrene and substituted trifluorostyrene copolymers [16, 17]. The latter membrane (Figure 1.11(b)), developed by Ballard Power Systems,