Kimball Thesis

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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

    iii

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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.

    iv

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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!

    v

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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 .....