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THE FUNDAMENTALS OF THE OPERATION OF
POLYMER ELECTROLYTE MEMBRANE FUEL CELLS
UNDER DRY AND FLOODED CONDITIONS WITH
AN EFFICIENT APPROACH TO THE MANAGEMENT OF LIQUID WATER
Erin E. Kimball
A DISSERTATION
PRESENTED TO THE FACULTY
OF PRINCETON UNIVERSITY
IN CANDIDACY FOR THE DEGREE
OF DOCTOR OF PHILOSOPHY
RECOMMENDED FOR ACCEPTANCE
BY THE DEPARTMENT OF
CHEMICAL ENGINEERING
Advisors: Jay Benziger and Yannis Kevrekidis
March 2010
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Copyright by Erin Kimball, 2010.All rights reserved.
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Abstract
Polymer electrolyte membrane (PEM) fuel cells are power conversion devices
that have the potential to become an important part of a distributed energy portfolio
independent of fossil fuels. In order to improve their performance and make their
mainstream use possible, the fundamental physics of PEM fuel cell operation must be
thoroughly understood. Included in this is the effect of the water balance inside the fuel
cell. Either too little water or an excess leading to liquid water accumulation can cause
fuel cell failure. Several factors affect the water balance, some inherent in fuel cells and
others determined by the system design.
The tool used to study fuel cell operation was the Segmented Anode Parallel
Channel (SAPC) fuel cell. Transparent flow channels and a segmented anode allowed for
direct visualization of liquid water and measurement of the current distribution along the
flow channel. Under dry conditions, hydration fronts were observed to propagate along
the flow channel as the fuel cell ignited or extinguished. Under wet conditions, liquid
water accumulation observed in the flow channel created areas of local reactant
starvation. Each segment of the SAPC fuel cell was modeled as a differential element,
coupled together electrically and through the flux of the reaction components.
The design of the SAPC fuel cell was very simple. The fuel cell was allowed to
operate autonomously without potentiostatic or galvanostatic controllers that hide much
of the physics of fuel cell operation. The setup and experimental results elucidated the
importance of flow pattern, temperature, load resistance, flow channel orientation with
respect to gravity, gas diffusion layer material, flow rates, and flow field construction.
Each of these factors changed how much water was removed as a vapor or how severely
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liquid water hindered the fuel cell operation. It was shown that the design of the fuel cell
can be tailored to maintain the fuel cell hydration while also effectively removing excess
liquid water. The flow rates could be kept low, allowing for high fuel utilization and dry
feeds. The need for extra peripheral equipment, such as humidifiers or feed recycling,
was alleviated.
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Acknowledgements
I would first like to thank my advisors, Prof. Yannis Kevrekidis and, in particular,Prof. Jay Benziger, for their guidance and support over the years. Graduate school was a
struggle for me in the beginning, but I am grateful to have been given the chance to learn
and succeed.
I also must thank the Benziger group members, both past and present, for their
invaluable discussions and support. I am particularly grateful for the conversations with
Barclay Satterfield and May Jean Cheah, which brightened many of the days in the lab.
During my years at Princeton, my peers have motivated me, inspired me, and
introduced me to so many things. It is only through being surrounded by the caliber of
people at Princeton that I have had such rich experiences.
The importance of the unwavering support of my family and friends is impossible
to put into words. Thank you to my brothers, sisters, aunts and uncles, grandparents,
closest friends, and Step-mom, Sherry. It is their encouragement that has always pulled
me through the times of doubt and the long nights that often stretched into the next
morning.
Finally, thank you to my parents, Cyndi Erickson and W. Scott Kimball.
Throughout my childhood, college, and graduate school, they have guided me without
lecturing, allowed me to learn and grow without punishment, and love me without
conditions. My Mom, with her grace, strength, and amazing energy has been my greatest
inspiration. My Dad, with his devotion, intelligence, ideals, and great sense of humor, is
someone who I completely admire. Knowing that he is proud of me has been
tremendously fulfilling. I love you both!
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Table of Contents
ABSTRACT ..................................................................................................................................... IIIACKNOWLEDGEMENTS .................................................................................................................. VLIST OF PUBLICATIONS ................................................................................................................ IX
LIST OF FIGURES ............................................................................................................................ XLIST OF TABLES ......................................................................................................................... XXII
1 INTRODUCTION....................................................................................................................... 1
1.1THE CASE FOR FUEL CELLS ...................................................................................................... 11.2CHALLENGES TO COMMERCIALIZATION ................................................................................ 21.3IMPORTANCE OF WATER MANAGEMENT................................................................................. 31.4BASICS OF PEM FUEL CELLS ................................................................................................... 51.4.1NAFION MEMBRANE ................................................................................................................ 51.4.2CATALYST LAYER................................................................................................................... 7
1.4.3GAS DIFFUSION LAYER............................................................................................................ 81.4.4FLOW FIELD PLATES ................................... ........................................................................... 101.5THE STIRRED-TANK-REACTOR FUEL CELL ........................................................................... 111.5.1RATIONALE FOR DESIGN........................................................................................................ 111.5.2EXPERIMENTAL SETUP .......................................................................................................... 141.5.3INSIGHTS FROM EXPERIMENTAL RESULTS............................................................................. 161.6THE SEGMENTED ANODE PARALLEL CHANNEL FUEL CELL................................................. 171.6.1RATIONALE FOR DESIGN........................................................................................................ 171.6.2EXPERIMENTAL SETUP .......................................................................................................... 181.6.3ADVANTAGES OF THE DESIGN ............................................................................................... 211.7OVERVIEW OF DISSERTATION................................................................................................ 211.8REFERENCES ........................................................................................................................... 23
2 HYDRATION FRONTS .......................................................................................................... 25
2.1INTRODUCTION ....................................................................................................................... 252.2IGNITION.................................................................................................................................. 292.2.1CO-CURRENT VS. COUNTER-CURRENT FLOW PATTERN......................................................... 292.2.2DEPENDENCE ON INITIAL CONDITIONS.................................................................................. 332.2.2.1 Hydrated startup ................................................................................................................. 332.2.2.2 Dry startup ......................................................................................................................... 362.2.3TEMPERATURE DEPENDENCE ................................................................................................ 432.3EXTINCTION ............................................................................................................................ 472.3.1CO-CURRENT EXTINCTION .................................................................................................... 472.3.2COUNTER-CURRENT EXTINCTION.......................................................................................... 502.3.3DEPENDENCE ON LOAD RESISTANCE..................................................................................... 522.4PREDICTING IGNITION AND EXTINCTION .............................................................................. 542.4.1ANALYTIC PREDICTION .....
