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Fuel Cell Engines Matthew M. Mench Copyright © 2008 by John Wiley & Sons, Inc. Fuel Cell Engines Fuel Cell Engines Matthew M. Mench JOHN WILEY & SONS, INC. This book is printed on acid-free paper.  ∞ Copyright  C 2008 by John Wiley & Sons, Inc. All rights reserved Published by John Wi...

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Fuel Cell Engines Matthew M. Mench Copyright © 2008 by John Wiley & Sons, Inc.

Fuel Cell Engines

Fuel Cell Engines Matthew M. Mench

JOHN WILEY & SONS, INC.

∞ This book is printed on acid-free paper. 

C 2008 by John Wiley & Sons, Inc. All rights reserved Copyright 

Published by John Wiley & Sons, Inc., Hoboken, New Jersey. Published simultaneously in Canada. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording, scanning, or otherwise, except as permitted under Section 107 or 108 of the 1976 United States Copyright Act, without either the prior written permission of the Publisher, or authorization through payment of the appropriate per-copy fee to the Copyright Clearance Center, 222 Rosewood Drive, Danvers, MA 01923, (978) 750-8400, fax (978) 646-8600, or on the Web at www.copyright.com. Requests to the Publisher for permission should be addressed to the Permissions Department, John Wiley & Sons, Inc., 111 River Street, Hoboken, NJ 07030, (201) 748-6011, fax (201) 748-6008, or online at http://www.wiley.com/go/permissions. Limit of Liability/Disclaimer of Warranty: While the publisher and author have used their best efforts in preparing this book, they make no representations or warranties with respect to the accuracy or completeness of the contents of this book and specifically disclaim any implied warranties of merchantability or fitness for a particular purpose. No warranty may be created or extended by sales representatives or written sales materials. The advice and strategies contained herein may not be suitable for your situation. You should consult with a professional where appropriate. Neither the publisher nor author shall be liable for any loss of profit or any other commercial damages, including but not limited to special, incidental, consequential, or other damages. For general information about our other products and services, please contact our Customer Care Department within the United States at (800) 762-2974, outside the United States at (317) 572-3993 or fax (317) 572-4002. Wiley also publishes its books in a variety of electronic formats. Some content that appears in print may not be available in electronic formats. For more information about Wiley products, visit our Web site at www.wiley.com. Library of Congress Cataloging-in-Publication Data: Mench, Matthew. Fuel cell engines / by Matthew Mench. p. cm. Includes index. ISBN 978-0-471-68958-4 (cloth) 1. Fuel cells. I. Title. TK2931.M46 2008 621.31′ 2429–dc22

2007046855

Printed in the United States of America 10 9 8 7 6 5 4 3 2 1

Contents

Preface vii Acknowledgments

3.3 xi 3.4

1 1.1 1.2 1.3 1.4 1.5 1.6 1.7

Introduction to Fuel Cells

Preliminary Remarks 1 Fuel Cells as Electrochemical Engines 3 Generic Fuel Cell and Stack 6 Classification of Fuel Cells 9 Potential Fuel Cell Applications and Markets 17 History of Fuel Cell Development 23 Summary 24

Application Study Problems 25 References 26

2 2.1 2.2 2.3 2.4 2.5 2.6 2.7

3.1 3.2

3.5 3.6 3.7 3.8

Application Study Problems 116 References 119

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116

24 4

Basic Electrochemical Principles

4.1 4.2 4.3 4.4 4.5 4.6 4.7

29

Electrochemical versus Chemical Reactions 29 Electrochemical Reaction 31 Scientific Units, Constants, and Basic Laws 35 Faraday’s Laws: Consumption and Production of Species 43 Measures of Reactant Utilization Efficiency 48 The Generic Fuel Cell 50 Summary 56

Application Study Problems 58 References 61

3

1

Determination of Change in Enthalpy for Nonreacting Species and Mixtures 78 Determination of Change in Enthalpy for Reacting Species and Mixtures 83 Psychrometrics: Thermodynamics of Moist Air Mixtures 91 Thermodynamic Efficiency of a Fuel Cell Maximum Expected Open-Circuit Voltage: Nernst Voltage 106 Summary 114

Physical Nature of Thermodynamic Variables 62 Heat of Formation, Sensible Enthalpy, and Latent Heat 74

Polarization Curve 121 Region I: Activation Polarization 126 Region II: Ohmic Polarization 157 Region III: Concentration Polarization 168 Region IV: Other Polarization Losses 175 Polarization Curve Model Summary 181 Summary 183

Application Study Problems 186 References 189 5 5.1 5.2 5.3 5.4 5.5

58

Thermodynamics of Fuel Cell Systems

Performance Characterization of Fuel Cell Systems 121

62

5.6 5.7

185

Transport in Fuel Cell Systems

191

Ion Transport in an Electrolyte 191 Electron Transport 209 Gas-Phase Mass Transport 210 Single-Phase Flow in Channels 233 Multiphase Mass Transport in Channels and Porous Media 239 Heat Generation and Transport 263 Summary 276

Application Study Problems 279 References 281

278

v

vi

Contents

6

Polymer Electrolyte Fuel Cells

6.1 6.2 6.3 6.4 6.5 6.6 6.7

285

Hydrogen PEFC 285 Water Balance in PEFC 298 PEFC Flow Field Configurations and Stack Design 325 Direct Alcohol Polymer Electrolyte Cells PEFC Degradation 356 Multidimensional Effects 362 Summary 369

Application Study Problems 371 References 374

371

8

339

Hydrogen Storage, Generation, and Delivery 426

8.1 8.2 8.3 8.4

Modes of Storage 426 Modes of Generation 438 Hydrogen Delivery 443 Overall Hydrogen Infrastructure Development 446 Summary 448

8.5

Application Study Problems 449 References 450 9

Experimental Diagnostics and Diagnosis

9.1 7 7.1 7.2 7.3 7.4 7.5 7.6

Other Fuel Cells

380

Solid Oxide Fuel Cells 381 Molten Carbonate Fuel Cells 392 Phosphoric Acid Fuel Cells 398 Alkaline Fuel Cells 410 Biological and Other Fuel Cells 418 Summary 418

Application Study Problems 420 References 421

419

449

9.2 9.3 9.4

Electrochemical Methods to Understand Polarization Curve Losses 454 Physical Probes and Visualization 469 Degradation Measurements 478 Summary 478

Application Study Problems 480 References 481 Appendix Index

485 503

479

453

Preface The field of fuel cell science and technology is undergoing a rapid expansion in both applied and fundamental studies. While the explosive growth of prototype systems for portable, stationary, and transportation applications garner most of the public attention, any textbook that tries to capture this aspect would be hopelessly outdated by the time of publication. One only has to read the bevy of press releases hailing the introduction of a real fuel cell product “in about a year” to realize that the landscape of product development is continually evolving and, in some cases, circling back on itself. Some still question if fuel cells will ever have a real impact on power generation at all. While I agree that fuel cells are not the panacea for every power need, I do believe that development has reached a stage of critical mass, where ubiquitous implementation and real product development in at least portable and stationary applications will eventually occur. This book is based on the need for a single textbook that combines the essential elements of the myriad disciplines required to understand fuel cells at a fundamental level. The purpose of this textbook is to prepare the engineering student with a timeless understanding of the fundamentals of fuel cell operation, so that as the specific applications change, the fundamental understanding can be applied. To that end, the book has been structured to be as fundamental as possible, to prepare the student without engineering bias. I have taught my courses and written this book not as an advocate of fuel cells but rather as an engineer and scientist that studies them with an open mind to the alternatives. Too often, the clouds of hope, fear, or funding obscure the light of good science. The subject matter in each chapter could easily be expanded to cover an entire separate textbook. For the sake of brevity, and based on the material I can reasonably cover in a semester, I have limited the material discussed to the undergraduate and some graduate lectures I have developed at Penn State over the course of the past seven years. A majority of the material in the text is based on a senior-level undergraduate technical elective class I have developed. I find that the course material in Chapters 1–4, 6, and 7 can be covered in a normal 45-lecture undergraduate course. The material in Chapters 5 and 9 are mostly from a graduate-level course I teach, although the undergraduate course definitely includes the more basic aspects of the transport theory described in Chapter 5. The textbook has been written for a senior-level undergraduate student but should also serve as a good introductory text and reference for graduate study. Where useful, I have included typical values for many of the parameters introduced but have intentionally tried to avoid topics that may shift in time where possible, so that the book will remain useful into the future as a fundamental principles reference. Chapter 1 presents a global perspective of the field of fuel cells so that the reader can grasp the practical significance and potential applications of the fuel cells they are about to study. The chapter presents a brief history of various types of fuel cell development (many people are unaware that actual fuel cell products have been developed and in use for years now), the basic functions of a fuel cell, and attempts to place the field in proper context

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Preface

as a multidisciplinary collection of engineering disciplines. The field of fuel cells truly is an exciting multidisciplinary arena, where electrical, mechanical, material, chemical, and industrial engineering merge. A new class of engineer educated in these areas is needed to further fuel cell development. Chapter 2 introduces the basic electrochemical principles and terminology needed to understand electrochemical cells, reactant consumption, and product generation. The chapter concludes with a discussion of the generic fuel cell, including the function and desirable qualities in each fuel cell component. I find that early in the semester, when the generic fuel cell concept is presented with a hands-on in-class demonstration of a fuel cell assembly and disassembly, the students begin with a strong understanding of the various internal processes and engineering trade-offs that occur in any fuel cell, which really helps later when the analytical descriptions are derived and the student needs to be able to visualize the various physical phenomena. This element is key to the future understanding they can achieve, and I suggest the professor accompany discussion of the generic fuel cell with a physical example of a fuel cell in class to help this process along. Notes cannot convey the understanding achieved from simply taking a small fuel cell apart. Chapter 3 is an especially detailed description of the fundamental thermodynamics involved in fuel cell science. Some will find this is overwritten, especially for a graduatelevel class. However, in many schools and between different departments, the curricula in thermodynamics have been thinned out so much that many of my undergraduate students were losing touch when the concept of a Nernst voltage or even relative humidity was presented. To address this issue and provide enough material to get all students on the same foundation, this chapter includes a fundamental description of the thermodynamic parameters involved and the thermodynamic concepts needed for fuel cell study. Not all of this material should be covered in class, but it serves as a reference for students who are struggling to follow the concepts presented and helps them keep up with the other students. Since I find many students lose their joy of engineering when it enters the microscale, where possible, I have tried to impart a physical meaning to the parameters that can help link the micro- and macroscales. Chapter 4 is the largest and most important chapter in the text and could easily be separated into several separate chapters. In this chapter, the entire polarization curve is presented and dissected. Starting with the maximum thermal voltage, each departure from this voltage is analyzed in detail. The culmination of the chapter is the development of a zerodimensional fuel cell performance model that includes detailed expressions for losses from kinetic, thermodynamic, ohmic, concentration, crossover, or short-circuit polarizations. I find that assigning a computer project that asks the student to integrate this fuel cell model into a spreadsheet is an extremely valuable way to help cement the physical parameters and concepts in the students’ minds. Although a zero-dimensional model cannot account for many of the more complex effects involved, it is extremely valuable as a qualitative teaching tool. The professor can extend this model to make it as complex as desired. For a graduatelevel class, including more advanced flooding concepts, an extension to an along-thechannel-l-D model can make a good term project. I find that through this modeling project approach, the students realize the limitations of the model and the trade-offs with design parameters such as electrolyte thickness or humidity and achieve a global understanding of the relative importance of the controlling parameters. Also included in Chapter 4 is a semiempirical modeling approach commonly used. Although less fundamental, it can be

Preface

ix

useful to delineate the relative importance of the various losses and as a comparative tool and so is included here. Chapter 5 covers the especially broad area of mass and heat transport in fuel cells. This is another chapter that could easily be expanded to cover an entire book. Some of the basic transport processes discussed can be taught along with the concentration polarization discussion of Chapter 4. This is what I do in my undergraduate class. In fact, the professor may wish to reorder the presentation of this material to cover Chapter 5 first, to set up a more complete understanding for the presentation of the polarization curve. I have tried this in class but have found that the introduction of the polarization curve comes too late in the semester for a significant final project to be accomplished. At the graduate level, the entire chapter can be taught, including the extended discussion of multiphase flow and flooding in the porous media of polymer electrolyte fuel cells (PEFCs). To truly understand flooding in polymer electrolyte fuel cells, a deep understanding of multiphase flow in mixed wettability porous media, such as diffusion media, is necessary and is presented. This topic is far from complete science, and will certainly evolve in the future. Chapter 6 is devoted entirely to PEFC systems, including hydrogen- and direct alcoholbased applications, issues, and degradation concerns. The specific devotion to PEFCs is based on my personal expertise and the fact the PEFC is the most broadly studied system and most likely to have future ubiquitous application in various applications. From a student perspective, the automotive application tends to draw students into the class, so that the PEFC tends to be the system of greatest student interest. Additionally, multiphase management for PEFCs is especially complex compared to other systems where only single phase flow is present in the reactant and product mixture. Due to its importance in stability, performance, and durability, special attention is taken to detail the water balance and flooding in PEFCs. Chapter 7 is a summary chapter of other fuel cell systems, including solid oxide, phosphoric acid, alkaline, and molten carbonate systems. By this chapter, the reader should possess the background information required to integrate the material presented and understand the various design constraints and engineering trade-offs inherent in each system. An understanding of the particular material and degradation issues is also presented. Other types of fuel cells such as biological and microbial fuel cells are given cursory treatment here, but they certainly enjoy the potential for strong future development. Chapter 8 is included for completeness and to present the reader with a summary of perhaps the biggest challenges in achieving real fuel cell applications: the storage, generation, and delivery of hydrogen. While this chapter is less technical in detail than others, it has been written to present the reader with the options available and the implicit engineering trade-offs accompanying the various choices. As with the rest of the textbook, the information is presented, where possible, without deference to the myriad political and economic factors involved. However, the subject of this chapter is one area where debate certainly rages. As an important capstone, Chapter 9 is given to introduce the reader to some of the more common options available to actually measure the parameters of interest used in fuel cell modeling and delineate the different polarization losses from one another. It is by no means complete and again is an example where an entire book could easily be written. The reader is expected to use this chapter as a starting point and reference for available techniques. Successful laboratory implementation will require additional reading, however, from other focused resources. This chapter can be used to prepare students for a laboratory

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Preface

project and is divided into two main sections. The first includes a description of the basic experimental techniques needed to obtain the parameters needed to describe the polarization curve model of Chapter 4. The second includes an overview of some of the laboratorybased diagnostics and visualization techniques available to discern current, species, and temperature distributions that can be used for transport parameter determination or for closing the energy, current, and mass balance equations for detailed model validation. In my class, I typically follow Chapter 4 with a laboratory project that covers some of the basic concepts presented. Finally, there are many topics which are not included in this textbook for various reasons. These include economic and political issues, hydrogen safety and regulation, system components, dynamic operation and instability, and system control issues. This information was excluded for brevity because it is well covered in other texts or to preserve the fundamental nature of the material. Some of these issues may be incorporated into future editions of this text, as my publisher permits and readers request. I sincerely hope the educational goals of this book are achieved and welcome feedback from my colleagues in the field to help me present a more complete and precise picture in future editions. Matthew M. Mench University Park, PA June, 2007.

Acknowledgments First and foremost, I owe whatever accomplishments I have in my life to my God. I also want to thank my wife, Laurel, and my children, Elizabeth Adeline and Michael, for their willingness to let me vaporize from existence ...