Mench M.M. Fuel Cell Engines
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Fuel Cell Engines
Copyright © 2008 by John Wiley & Sons, Inc.
Matthew M. Mench
Fuel Cell Engines
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Fuel Cell Engines
Matthew M. Mench
JOHN WILEY & SONS, INC.
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This book is printed on acid-free paper.
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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
10987654321
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Contents
Preface vii
Acknowledgments xi
1 Introduction to Fuel Cells 1
1.1 Preliminary R emarks 1
1.2 Fuel Cells as Electrochemical Engines 3
1.3 Generic Fuel Cell and Stack 6
1.4 Classification of Fuel C ells 9
1.5 Potential Fuel Cell Applications and
Markets 17
1.6 History of Fuel Cell Development 23
1.7 Summary 24
Application Study 24
Problems 25
References 26
2 Basic Electrochemical Principles 29
2.1 Electrochemical versus Chemical
Reactions 29
2.2 Electrochemical Reaction 31
2.3 Scientific Units, Constants, and Basic
Laws 35
2.4 Faraday’s Laws: Consumption and
Production of Species 43
2.5 Measures of Reactant Utilization
Efficiency 48
2.6 The Generic Fuel Cell 50
2.7 Summary 56
Application Study 58
Problems 58
References 61
3 Thermodynamics of Fuel Cell Systems 62
3.1 Physical Nature of Thermodynamic
Variables 62
3.2 Heat of Formation, Sensible Enthalpy, and
Latent Heat 74
3.3 Determination of Change in Enthalpy for
Nonreacting Species and Mixtures 78
3.4 Determination of Change in Enthalpy for
Reacting Species and Mixtures 83
3.5 Psychrometrics: Thermodynamics of Moist
Air Mixtures 91
3.6 Thermodynamic Efficiency of a Fuel Cell 96
3.7 Maximum Expected Open-Circuit Voltage:
Nernst Voltage 106
3.8 Summary 114
Application Study 116
Problems 116
References 119
4 Performance Characterization of Fuel Cell
Systems 121
4.1 Polarization C urve 121
4.2 Region I: Activation Polarization 126
4.3 Region II: Ohmic Polarization 157
4.4 Region III: Concentration Polarization 168
4.5 Region IV: Other Polarization Losses 175
4.6 Polarization C urve Model Summary 181
4.7 Summary 183
Application Study 185
Problems 186
References 189
5 Transport in Fuel Cell Systems 191
5.1 Ion Transport in an Electrolyte 191
5.2 Electron Transport 209
5.3 Gas-Phase Mass Transport 210
5.4 Single-Phase Flow in Channels 233
5.5 Multiphase Mass Transport in Channels
and Porous Media 239
5.6 Heat Generation and Transport 263
5.7 Summary 276
Application Study 278
Problems 279
References 281
v
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vi Contents
6 Polymer Electrolyte Fuel Cells 285
6.1 Hydrogen PEFC 285
6.2 Water Balance in PEFC 298
6.3 PEFC Flow Field Configurations and Stack
Design 325
6.4 Direct Alcohol Polymer Electrolyte Cells 339
6.5 PEFC Degradation 356
6.6 Multidimensional Effects 362
6.7 Summary 369
Application Study 371
Problems 371
References 374
7 Other Fuel Cells 380
7.1 Solid Oxide Fuel Cells 381
7.2 Molten C arbonate Fuel Cells 392
7.3 Phosphoric Acid Fuel Cells 398
7.4 Alkaline Fuel Cells 410
7.5 Biological and Other Fuel Cells 418
7.6 Summary 418
Application Study 419
Problems 420
References 421
8 Hydrogen Storage, Generation, and
Delivery 426
8.1 Modes of Storage 426
8.2 Modes of Generation 438
8.3 Hydrogen Delivery 443
8.4 Overall Hydrogen Infrastructure
Development 446
8.5 Summary 448
Application Study 449
Problems 449
References 450
9 Experimental Diagnostics and Diagnosis 453
9.1 Electrochemical Methods to Understand
Polarization Curve Losses 454
9.2 Physical Probes and Visualization 469
9.3 Degradation Measurements 478
9.4 Summary 478
Application Study 479
Problems 480
References 481
Appendix 485
Index 503
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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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viii 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 s tudent 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 graduate-
level 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 zero-
dimensional 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 graduate-
level class, including more advanced flooding concepts, an extension to an along-the-
channel-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
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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 alcohol-
based applications, issues, and degradation concerns. The specific devotion to PEFCs is
based on my personal expertise and the fact the PEFC is t he 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 un-
derstand 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 i s 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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x 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 laboratory-
based diagnostics and visualization techniques available to discern current, species, and
temperature distributions that can be used for t ransport 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 f undamental 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.