MODERN ASPECTS OFELECTROCHEMISTRY

No. 40

Modem Aspects of Electrochemistry

Modern Aspects ofElectrochemistry, No. 39:

• Approaches to Solute-Solvent Interactions including twoapproaches to computational determination of solution properties,and several other procedures that establish correlations betweenproperties of interest and certain features of the solute and/orsolvent molecules

• Porous Silicon, including its morphology and formationmechanisms, as well as anodic reaction kinetics

• Modeling Electrochemical Phenomena via Markov Chains andProcesses gives an introduction to Markov Theory, then discussesapplications to electrochemistry, including modeling electrodesurface processes, electrolyzers, the repair of failed cells, analysisof switching-circuit operations, and other electrochemical systems

• Fractal Approach to Rough Surfaces and Interfaces inElectrochemistry, from a review of Fractal Geometry to theapplication of Fractal Geometry to the classification of surfaces andElectrochemistry

• Phenomenology and Mechanisms of Electrochemical Treatment(ECT) of Tumors, starting from fundamentals and proceeding toelectrochemical treatment of tumors in animals and then in humans

Modern Aspects ofElectrochemistry, No. 38:

• Solid State Electrochemistry encompassing modem equilibriaconcepts, thermodymanics and kinetics of charge carriers in solids

• Electron transfer processes, with special sections devoted tohydration of the proton and its heterogeneous transfer

• Electrosorption at electrodes and its relevance to electrocatalysis andelectrodeposition of metals

• The behavior of Pt and other alloy electrocatalyst crystallites used asthe electrode materials for phosphoric acid electrolyte fuel-cells

• Applications of reflexology and electron microscopy to the materialsscience aspect of metal electrodes

• Electroplating of metal matrix composites by codeposition ofsuspended particles, a process that has improved physical andelectrochemical properties

LIST OF CONTRIBUTORSDAVID BLOOMFIELDAnalytical Power, LLC2-X Gill StreetWobum, Massachusetts 01801

VALERIE BLOOMFIELDAnalytical Power, LLC2-X Gill StreetWoburn, Massachusetts 01801

JOHN O'M. BOCKRISHaile Plantation10515 S. W. 55th PlaceGainesville, FL 32608

THOMAS Z. FAHIDYDepartment of ChemicalEngineeringUniversity of WaterlooWaterloo, OntarioCanada N2L 3G I

BRENDA L. GARCIACollege of Engineering andInformation Technology,Swearingen Engineering CenterDepartment of ChemicalEngineeringUniversity of South Carolina301 South Main Street,Columbia, South Carolina, 29208

CHARLES R. MARTINDepartment of ChemistryUniversity of FloridaGainesville, Florida 32611crmartin@chem.ufl.eduFax: 352-392-8206

PARTHA P. MUKHERJEEElectrochemical Engine Center(ECEC), and Department ofMechanical and NuclearEngineeringThe Pennsylvania State UniversityUniversity ParkPennsylvania 16802

KEITH SCOTTSchool of Chemical Engineering andAdvanced MaterialsUniversity ofNewcastle upon Tyne,United Kingdom

ASHOK K. SHUKLASolid State and Structural ChemistryUnitIndian Institute of ScienceBangalore, India

CHARLES R. SIDESDepartment of ChemistryUniversity of FloridaGainesville, Florida 32611

CHAO-YANG WANGElectrochemical Engine Center(ECEC), and Department ofMechanical and Nuclear EngineeringThe Pennsylvania State UniversityUniversity ParkPennsylvania 16802e-mail: cxw31@psu.edu,Ph: (814) 863-4762Fax: (814) 863-4848

GUOQING WANGPlugpower Inc.968 Albany-Shaker RoadLatham, New York 12110

JOHN W. WEIDNERCollege of Engineering andInformation Technology,Swearingen Engineering CenterDepartment of ChemicalEngineering,University of South Carolina301 South Main Street,Columbia, South Carolina, 29208

MODERN ASPECTS OFELECTROCHEMISTRY

No. 40

Edited by

RALPH E. WHITEUniversity ofSouth Carolina

Columbia, South Carolina

c. G. VAYENASUniversity ofPatras

Patras, Greece

and

MARIA E. GAMBOA-ALDECOManaging Editor

Superior, Colorado

~ Springer

Ralph E. WhiteDepartment of Chemical EngineeringUniversity of South CarolinaColumbia, South CarolinaU.S.A.white@engr.sc.edu

Library of Congress Control Number: 2005939188

ISBN-I0: 0-387-46099-3ISBN-13: 978-0-387-46099-4

Printed on acid-free paper.

e-ISBN-IO: 0-387-46106-Xe-ISBN-13: 978-0-387-46106-9

© 2007 Springer Science-Business Media, LLCAll rights reserved. This work may not be translated or copied in whole or in partwithout the written permission of the publisher (Springer Science+BusinessMedia, Inc., 233 Spring Street, New York, NY 10013, USA), except for briefexcerpts in connection with reviews or scholarly analysis. Use in connection withany form of information storage and retrieval, electronic adaptation, computersoftware, or by similar or dissimilar methodology now known or hereafterdeveloped is forbidden.The use in this publication of trade names, trademarks, service marks, and similarterms, even if they are not identified as such, is not to be taken as an expression ofopinion as to whether or not they are subject to proprietary rights.

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Preface

This volume begins with a tribute to Dr. Brian E. Conway by Dr.John O'M. Bockris, which is followed by six chapters. The topicscovered are state of the art Polymer Electrolyte Membrane (PEM)fuel cell bipolar plates; use of graphs in electrochemical reactionnetworks; nano-materials in lithium ion batteries; direct methanolfuel cells (two chapters); and the last chapter presents simulation ofpolymer electrolyte fuel cell catalyst layers.

David and Valerie Bloomfield begin the first chapter with adiscussion of the difficulties encountered when confronting bipolarplate development and state that the problems stem from the highcorrosive nature of phosphoric acid. The water problems aremitigated but the oxidation problems increase. Bipolar plates arestill not cheap, reliable or durable.

In Chapter 2, Thomas Z. Fahidy reviews analysis of variance(ANOVA) and includes one-way, two-way, three-wayclassification, and Latin squares observation methods. He moveson to a discussion of the applications of the analysis of covariance(ANCOVA) and goes over certain variables such as velocity,velocity and pressure drop, and product yields in a batch and flowelectrolyzer. His conclusion is that proper statistical techniques aretime savers which can save the experimenter and the processanalyst considerable time and effort in trying to optimize the sizeof statistically meaningful experiments.

In Chapter 3 Charles R. Sides and Charles R. Martin presentthe effect of nanomaterials in lithium-ion battery electrode designand they describe the template-synthesis of lithium-ion batteryelectrode materials using polymeric templates. They present aplasma-assisted method to create a carbon replica of an aluminatemplate membrane, which serves as a nanostructured anode forlithium-ion battery design.

vii

viii Preface

Keith Scott and Ashok K. Shukla begin their discussion inChapter 4 with a review of various types of fuel cells: phosphoricacid fuel cells, alkaline fuel cells, polymer electrolyte fuel cells,molten electrolyte fuel cells, solid oxide fuel cells, and directmethanol fuel cells. They follow this by reviewing the advancesmade in the performance of DMFCs since their inception. Thesefuel cells need to be able to stand higher temperatures than 200 °Cwhich require more temperature stable membranes. These fuelcells have a great potential to be used in mixed reactant systemsdue to the simplicity of the fuel cell stack design which may resultin reduced costs.

In Chapter 5, Brenda L. Garcia and John W. Weidner reviewdirect methanol fuel cells (DMFCs). High temperature operation ofDMFCs has not been achievable without pressure on the cathodeside and researchers have not been able to obtain goodperformances at temperatures above the boiling point of water.Novel polymers that operate well with temperatures above theboiling point have not been developed yet. One solution proposedin the literature is to laminate a layer of methanol impermeablepolymer between two Nafion'" membranes. This solution stillrequires hydration at high potentials and adsorption of methanollimits the reaction rate at low potential. Two-dimensional modelshave been developed to explain the variation of the current alongthe flow path. Transient models have been proposed but not muchwork has been accomplished in this area.

Partha P. Mukherjee, Chao-Yang Wang, and Guoqin Wang inChapter 6 discuss the direct numerical simulation of polymerelectrolyte fuel cell catalyst layers and they present a systematicdevelopment of the direct numerical simulation model which wasdeveloped at Penn State Electrochemical Engine Center. Thismethod is used to describe the oxygen, water, and charge transportat the pore level within 2-D and 3-D computer-generated catalystmicrostructures and indicates the importance of developing high-performance catalyst layers.

R. E. WhiteUniversity ofSouth CarolinaColumbia, South Carolina

C. VayenasUniversity ofPatrasPatras, Greece

Memories of Brian Evans ConwayEditor 1955-2005

I remember meeting Brian Conway on the road, Queens Gate,which leads from Imperial College, London University, to theTube Station in South Kensington, London. It was a Saturdayafternoon in 1946. I was one year into a lectureship (assistantprofessorship in U.S. terms) at Imperial College, thetechnologically oriented part of London University. Brian stoppedme to say that he wanted to do his graduate research with me. Hewas 20 and I, 23.

Brian Conway entered my research group at Imperial Collegein its second year. There are some who have called the yearsbetween 1946 and 1950 as the Golden Years of Electrochemistrybecause the development of the subject as a part of physicalchemistry was pioneered in those years, primarily by theremarkable set of people who were in the group at that time andwho spread out with their own students and ideas after they leftme.

First among those in respect to the honors achieved was RogerParsons. He became an FRS and President of the Faraday Divisionof the Chemical Society. Conway is No.2 in this formal rankingalthough he published more extensively and with more originalideas than did Parsons. Then there was John Tomlinson whobecame a professor at the University of Wellington in NewZealand (and Vice Chancellor of the University). E. C. Potter toowas in the group at that time and became in the 1980's thePresident of the Royal Society of New South Wales, apart fromhaving an active career in the Council of Science and Industry ofthe Australian government. Harold Eagan was there too and helater achieved a grand title: The Government Chemist, the man inthe U.K. government who is in charge of testing the purity ofvarious elements of the country's supply.

The only woman in the group, Hanna Rosenburg, should bementioned here because she had an extended influence on BrianConway, based partly on her remarkable ability to discuss widely.

ix

x Memories of Brian Evans Conway

Martin Fleischmann, too, is relevant. He got his Ph.D. in asmall group near to my group in which Conway worked.However, he joined us in various activities, particularly thediscussions. He became well known internationally not onlybecause of his contributions to physical electrochemistry, but alsobecause in 1989 he resuscitated an idea-which had beenintroduced by the French and Japanese in the 1960's-that nuclearreactions could be carried out in solutions in the cold.

Conway's thesis (1949) was very original in content andcontained, e.g., 3-D representations ofpotential energy surfaces.

After Brian Conway got his Ph.D. with me, he went to workwith J. A. V. Butler at the Chester-Beatty Cancer ResearchInstitute, a short walk from Imperial College. Butler's habit ofporing over manuscripts, murmuring, and his sensitivity to noisewhilst thinking, was reflected by some of Conway's habits. Butlerwas known to be absent minded in a way that colored the greatrespect with which he was regarded. Thus, according to Conway,in Butler's year at Uppsala, he was observed by a British colleagueto pass two lumps of sugar to the cashier whilst thoughtfullyadding the ore to his coffee.

Contact with Conway continued from 1949 to 1953 inLondon. We used to meet on Saturday afternoons in a Kensingtoncoffee shop and it was there that some of our electrochemicalproblems were worked out.

After 1951 I decided to look towards the United States forcontinuation of my career. We thought of Brian as a possible postdoc to take with us and wanted to get him married before thetransition. Unfortunately, Brian evaluated girls principally by theirintellectual abilities and the Viennese Ms. Rosenburg had givenhim the model. He found it difficult to meet her equal and we hadeventually to point out to him that he couldn't get a visa to come toAmerica with a woman unless he were married to her. Shortly,after this, he did invite me to Daguize, our Saturday cafe, to meet aformer Latvian pharmacist, Nina; later I served to take Nina up theaisle at the wedding ofNina and Brian (1954).

It had seemed to me that, if we were going to bringElectrochemistry from its moribund state of the 1940s to modernityin physical chemistry, a yearly monograph would help. I thereforeapproached Butterworth's in London with a proposition and the

Memories of Brian Evans Conway xi

first volume was published in 1954.1 I brought Brian into ModemAspects in Volume I as an assistant and as my co-author in achapter on solvation. It soon seemed unfair to continue to receiveBrian's help in editing unless he was made an editor too and,therefore, from the second volume on, we were both named editorsof the series. We kept it that way until in 1987 we invited RalphWhite to join us as the Electrochemical Engineer, thus increasingthe breadth of the topics received.2

I moved to the University of Pennsylvania in Philadelphia in1953 whilst Brian remained at Chester-Beatty, but I went back toLondon for the summer of 1954 and persuaded Brian to come overto Philadelphia at first as a post doctoral fellow.

Brian Conway's move to the U.S.A. facilitated our jointeditorship in running the series. We had had many discussions inLondon and this encouraged us to make the composition of eachvolume an occasion for a meeting, lasting two to three days in aresort. This meant one to two day's hard discussion, deciding thetopics we wanted as well as the authors who might write thearticles, together with a reserve author in case the first one refused.The third day we spent as free time for our own discussions.Colorado Springs was a place in which we had an interesting timeand then there was the Grand Hotel in White Sulfur Springs andseveral resorts in Canada. But there is no doubt that Bermuda wasmore often the locale of the Modem Aspects invitation meetingsthan any other. After Ralph White threw his expertise onto choiceof authors, we continued to meet yearly, e.g., in Quebec City andin San Francisco, though in recent years the meetings drifted backto their original form.

Coming now to Brian's two years at the University ofPennsylvania (1954-1955), we got out of this time three papersthat I think of as being foundation papers of what we now know asphysical electrochemistry. The first concerned the mechanism ofthe high mobility of the proton in water. The second arose out ofthe interpretation of plots of log io against the strength of the metal-hydrogen bond and found two groups of metals, one involving

1Plenum Press purchased Butterworth's in 1955 after the second volume forModem Aspects of Electrochemistry had been published.2 Our policy throughout-as I believe it is with the present editors-was to make inModem Aspects of Electrochemistry a truly broad presentation of the field althoughwe always avoided analytical topics for which there was already so muchpresentation.

xii Memories of Brian Evans Conway

proton discharge onto planar sites and the other proton discharge(rate determining) onto adsorbed H. The last of the three concernedthe early stages of metal deposition, a subject that raises manyfundamental questions in electrochemical theory (e.g., partialcharge transfer). The first two of these works were published inthe Journal of Chemical Physics and the last one in the Proceedingsof the Royal Society. To have papers accepted in these prestigiousjournals is unusual, particularly for electrochemical topics so oftenmired in lowly technological issues.

Conway transferred to the University of Ottawa in 1955 andremained there for the rest of his career. He published near to 400papers whilst in Ottawa and it is interesting to mention a few ofthem.

In Ottawa I would pick out the work with Currie in 1978. Thetopic was on pressure dependence of electrode reactions and gave anew mechanism criterion.

Agar had suggested in 1947 that there might be a temperaturedependence of the electrode kinetic parameter. Conway took thisup in 1982 and showed experimentally that in certain reactions thiswas the case.

The most well known work that Conway and his colleaguescompleted in Ottawa was on the analysis of potential sweep curves.I had been critical of the application of potential sweep theory toreactions that involved intermediates on the electrode surface.Working with Gilaedi and then with Halina Kozlowska, and tosome extent with Paul Stonehart, Conway developed an analysis ofthe effect of intermediate radicals on the shape and properties ofpotential sweep showing how interesting electrode kineticparameters could be thereby obtained.

This is the point to stress the part played by HalinaAngerstein-Kozlowska in Brian's work. She played an importantrole in administration of the co-workers and supervision of thework, particularly in Brian's absences at meetings, many inEurope. Her position was similar to that of the late Srinivasan inrespect to John Appleby electrochemical research group at TexasA&M. It is more creative and directional than her formal positionas a senior researcher indicates.

I continued an active discussion relationship with BrianConway for more than fifty years. This was made possible notonly by the yearly meetings related to Modem Aspects but byletters, most of which concerned discussions of topics apart from

Memories of Brian Evans Conway xiii

electrochemistry. I remember those on the inheritance of acquiredcharacteristics with Hanna Rosenburg in London in the fifties, thediscussions of the position and number of the galaxies, and then,later, consideration of methods by aggressive groups could berestrained from war. In more recent years we have been concernedwith the speed of electrons in telephone lines. Many people thinkthat electrons there must travel very quickly but in fact theirmovement during the passage of messages is measured in a fewcentimeters per second.

Although the correspondence with me concerned topicsdiscussed in terms of the science of the day, Conway's interestswere broad and he had interests in the Arts, stimulated by his wifein this direction (his house was decorated with her paintings). Inthis respect, Conway exemplifies the Scientist as a Great Man, aman of Knowledge in all and any direction. One is reminded ofFrumkin in Moscow who kept abreast of modem literature fromAnglo-America, France and Germany and Hinshelwood at Oxford,a physical chemist whose alternative interests were in JapanesePoetry.

The long lasting scientific correspondence with Conwaystarted to ebb in the mid nineties and I assumed that this was due tonormal processes of aging but it turned out that it was thebeginning of the end-he was frequently in the hospital.

One day in this Summer of 2005, Nina called to tell me thatthe doctors declared the prostate cancer that he had sufferedmetastasized and that they could no longer control it. I placedmyself at Brian's disposal in respect to anything that he wanted-Iworked here through the well-known Barry Macdougall, an ex-Conway student. Through him I passed to Brian photographs fromthe London days, and these contained, among others, the one hemost wanted-that of the student he had known 55 years earlier,Hanna Rosenburg.

Brian leaves behind him Nina (later a professional artist andmusician) who had contributed so much to his career by her helpand fortitude, and a son, Adrian, who is a competent theorist intelephone networks.

Brian chose to have his body cremated.

John O'M. BockrisEditor 1953-2003

Contents

Chapter 1

PEM FUEL CELL BIPOLAR PLATES

D. Bloomfield and V. Bloomfield

I. Introduction................................ 11. NAFION MEA Based Bipolar Plate Problems 22. Polybenzimidazole/Hjl'O, 3

II. Definition.................................................................... 31. Separator Plate 42. Flow Field 53. Port and Port Bridges 64. Seals 75. Frame 8

III. Bipolar Plate Features 81. Tolerances 92. Thermal Management.......................................... 93. Electrical Conduction 104. Water Management 115. Low Cost 136. Stable, Free from Corrosion Products 13

(i) Galvanic Corrosion.................................... 13IV. Materials and Processes 15

1. Comparison of Carbon and Metal 15(i) Operational 16

(ii) Forming Cost 162. Carbon.................................................................. 18

(i) Molded Graphite 18(ii) Paper 19

(iii) Stamped Exfoliated Graphite (Grafoil,Graflex) 19

xv

xvi Contents

3. Metal 19(i) Forming Metal Bipolar Plates 20

(ii) Intrinsically Corrosion Resistant Metals 21(iii) Direct Coatings 21(iv) Conductive Polymer Grafting 24

References 33

Chapter 2

BASIC APPLICATIONS OF THE ANALYSIS OFVARIANCE AND COVARIANCE IN ELECTROCHEMICAL

SCIENCE AND ENGINEERING

Thomas Z. Fahidy

I. Introduction 37II. Basic Principles and Notions 38

III. ANOVA: One-Way Classification 40I. Completely Randomized Experiment (CRE) 422. Randomized Block Experiment (RBE) 42

(i) Example 1: A Historical Perspective ofCaustic Soda Production 43

(ii) Example 2: Metallic Corrosion 45IV. ANOVA: Two-Way Classification 46

1. Null and Alternative Hypotheses 462. Illustration of Two-Way Classification: Specific

Energy Requirement for an Electrolytic Process 47V. ANOVA: Three-Way Classifications 49

VI. ANOVA: Latin Squares (LS) 51VII. Applications of the Analysis of Covariance

(ANCOVA) 531. ANCOVA with Velocity as Single Concomitant

Variable 53(i) Pattern A(CRE) 53

(ii) Pattern B(RBE) 562. ANCOVA with Velocity and Pressure Drop

Acting as Two Concomitant Variables 583. Two Covariate-Based ANCOVA ofProduct

Yields in a Batch and in a Flow Electrolyzer 584. Covariance Analysis for a Two-Factor, Single

Cofactor CRE 60

Contents xvii

VIII. Miscellaneous Topics .. 621. Estimation of the Type II Error in ANOVA........ 622. Hierarchical Classification................................... 643. ANOVA-Related Random Effects 664. Introductory Concepts of Contrasts Analysis. 69

IX. Final Remarks 71Acknowledgments 72List of Principal Symbols 72References 73

Chapter 3

NANOMATERIALS IN LI-ION BATTERY ELECTRODEDESIGN

Charles R. Sides and Charles R. Martin

I. Introduction................................................................. 75II. Templates Used 78

1. Track-Etch Membranes 782. Alumina Membranes 803. Other Templates 81

II. Nanostructured Cathodic Electrode Materials 831. Electrode Fabrication 84

(i) Nanostructured Electrode 84(ii) Control Electrodes 85

2. Structural Investigations...................................... 863. Electrochemical Characterization........................ 87

(i) Cyclic Voltammetry 87(ii) Rate Capabilities......................................... 89

III. Nanostructured Anodic Electrodes 911. Electrode Fabrication........................................... 92

(i) Nanostructured Electrodes 92(ii) Control Electrodes 92

2. Structural Investigations 933. Electrochemical Investigations........................... 95

V. Nanoelectrode Applications 971. Low-Temperature Performance 97

(i) Electrode Fabrication 97(ii) Strategy......... 98(iii) Electrochemical Results. 99

xviii Contents

(iv) Electronic Conductivity 101(v) Cycle Life 102

2. Variations on a Synthetic Theme 102(i) Nanocomposite of LiFePOJCarbon 102

(ii) Improving Volumetric Capacity 109VI. Carbon Honeycomb.................................................... 117

1. Preparation ofHoneycomb Carbon..................... 1182. Electrochemical Characterization. 121

VII. Conclusions 123Acknowledgements 123References 124

Chapter 4

DIRECT METHANOL FUEL CELLS: FUNDAMENTALS,PROBLEMS AND PERSPECTIVES

Keith Scott and Ashok K. Shukla

I. Introduction 127II. Operating Principle of the SPE-DMFC 128

III. Electrode Reaction Mechanisms in SPE-DMFCs 1321. Anodic Oxidation of Methanol............................ 1322. Cathodic Reduction of Oxygen 139

IV. Materials for SPE-DMFCS 1401. Catalyst Materials 140

(i) Anode Catalysts 140(ii) Oxygen Reduction Catalysts 149

(iii) Membrane Materials 156V. Direct Methanol Fuel Cell Performance 163

1. DMFC Stack Performance 1752. Alternative Catalysts and Membranes in the

DMFC 1783. Alkaline Conducting Membrane and Alternative

Oxidants 183VI. Conventional vs. Mixed-Reactant SPE-DMFCs 185

VII. Mathematical Modelling of the DMFC 1921. Methanol Oxidation 1952. Empirical Models for Cell Voltage Behaviour 1983. Membrane Transport 202

Contents xix

4. Effect of Methanol Crossover on Fuel CellPerformance 204

5. Mass Transport and Gas Evolution 2056. DMFC Electrode Modelling................................. 2097. Cell Models 2108. Single Phase Flow 2129. Two- and Three-Dimensional Modelling 213

10. Dynamics and Modelling 21511. Stack Hydraulic and Thermal Models 215

VIII. Conclusions.................................................................. 216List of Symbols 217References 218

Chapter 5

REVIEW OF DIRECT METHANOL FUEL CELLS

Brenda L. Garcia and John W. Weidner

I. Introduction.................................................................. 229II. Anode Kinetics 232

1. Reaction Mechanism 2322. Methanol Oxidation Catalysts 233

(i) Platinum and Platinum Catalyst Structure 233(ii) Platinum and Platinum Alloy Catalyst

Performance 240III. Oxygen Reduction Reaction Catalysts 247IV. High Temperature Membranes 248V. Methanol Crossover 253

1. Magnitude of Crossover 2532. Effect of CO2 Crossover 2583. Mixed-Potential Effects 2604. Novel Membranes to Reduce Methanol

Crossover 261VI. DMFC Modeling Review. 264

1. One-Dimensional Models 2652. Two-Dimensional and Three-Dimensional

Models 273VII. Summary.. 278

References 280

xx

Chapter 6

Contents

DIRECT NUMERICAL SIMULATION OFPOLYMER ELECTROLYTE FUEL CELL CATALYST

LAYERS

Partha P. Mukherjee, Guoqing Wang, andChao-Yang Wang

I. Introduction.................................................................. 285II. Direct Numerical Simulation (DNS) Approach 288

1. Advantages and Objectives of the DNS Approach 2892. DNS Model - Idealized 2-D Microstructure.......... 2903. Three-Dimensional Regular Microstructure 2934. Results and Discussion 299

(i) 2-D Model: Kinetics- vs. Transport-Limited

Regimes 299(ii) Comparison of the Polarization Curves

between 2-D and 3-D Simulations 304III. Three-Dimensional Random Microstructure 305

1. Random Structure 3062. Structural Analysis and Identification.................. 3073. Governing Equations 3114. Boundary Conditions 3145. Results and Discussion......................................... 316

IV. DNS Model- Water Transport 3201. Water Transport Mechanism 3212. Mathematical Description 3233. Results and Discussion 327

(i) Inlet-Air Humidity Effect 327(ii) Water Crossover Effect 330

(iii) Optimization of Catalyst LayerCompositions 331

V. 3-D Correlated Microstructure 3331. Stochastic Generation Method 3332. Governing Equations, Boundary Conditions and

Numerical Procedure 3343. Results and Discussion 337

VI. Conclusions.................................................................. 340Acknowledgements 340References 341

INDEX 343

1

PEM Fuel Cell Bipolar Plates

D. Bloomfield and V. Bloomfield

Analytical Power, LLC, Woburn, MA

I. INTRODUCTION

The objective of this paper is to describe the state-of-the-art ofPEM fuel cell bipolar plate technology. We will present theproblems confronting bipolar plate development along with severalpotential paths to solution.

The paper first describes the functions, properties, andattributes of bipolar plates. The balance of the paper is devoted tothe materials and fabrication processes that have been explored andare currently under investigation. One of the materials is carbon, ina variety of forms from molded graphite, to paper, to exfoliatedstamped graphite. The other set of materials are metals which canbe intrinsically corrosion resistant, or coated with corrosionprotection materials. Metal bipolar plates are generally compositestructures with different materials performing different functions.Sometimes the only metal part in the metal bipolar plate is the flatsheet separator. The recent problems with PEM MEA: durability,cathode catalyst and carbon loss, have place more emphasis on thedevelopment of bipolar plates with high in plane conductivity,namely metal. The high in plane conductivity enables a potentialsystems solution to this problem. A patent is pending by HowardBaker"

The properties of bipolar plates are more or less defined by themembrane electrode assemblies (MEAs). The set of problems

1

2 D. Bloomfield and V. Bloomfield

associated with low temperature membrane MEAs such as Nafionare due to water and, more specifically, the liquid product waterthat leaches out of the membrane and finds its way into the bipolarplate flow fields. The other major problem is that of carbon. Whilecarbon is a base material, its oxidation kinetics are so slow and itshydrogen evolution overpotential so low that it often behaves as ifit was more noble than platinum, see Sedricks."

Recently there has been a push to develop higher temperatureMEAs such as polybenzimidazole gels with phosphoric acid. Theproblems for bipolar plates stem from the high temperaturecorrosive nature of phosphoric acid.

"Things go wrong, and things go wrong faster at hightemperature."

Murphy

While the water problem is mitigated, in the high temperatureMEAs, the oxidation problems are exacerbated. These problemswere extensively studied in the 1960s and 1970s when theconventional PAFC was developed. The PAFC led to the presentday o-molded graphite bipolar plate, as well as carbon supportedplatinum catalyst and carbon paper GDLs.

1. NAFION MEA Based Bipolar Plate Problems

The problem with PEM Fuel Cells is that after 40 years of R&D,they are not cheap, reliable, or durable. (Gemini 3 flew on March23, 1965.) Most of the problems lie in the MEA, but the source ofmany problems can be found in the MEA/bipolar plate interaction;in particular, the initial degradation of the polymer membrane. Thisis due to the presence of short chain sulfonic acid groups in Nationmembranes. These short chain groups have a relatively highsolubility in water and leach out of the fuel cell when it is initiallystarted." The process is simply demonstrated by measuring the pHof water extracted from the reactant cavities of a fuel cell that hasrun for about 100 hours. Water is always found in the reactantcavities of PEM cells. As the cell ages, imperfectly terminatedpolymers will also release HF and increased amounts of sulfonicacid. Peroxides are formed at the cell anode. The membrane attackby peroxides is well known20

,2I and has been well know for a longtime. Peroxides are stable at anode potentials, and they attack the

PEM Fuel Cell Bipolar Plates 3

thin ionomer films surrounding the anode catalyst. Polyvalentcations, especially Fe, Co, and Ni catalyze peroxide formation.

The low pH of the cell water and the use of carbon paper gasdiffusion layers (GDLs) make metal bipolar plates problematic.Carbon has a low hydrogen evolution potential, lower than all butthe precious metals, and it is the most widely used GDL material.Most metal bipolar plates in electrical contact with carbon GDLsbecome anodic with respect to the GDL and corrode. Carbon in thecatalyst support and in bipolar plates is often a source ofpolyvalentcations which form in the low pH water surrounding the bipolarplates diffuse back to the MEA and catalyze the formation ofperoxide.

2. Polybenzimldazole/Hd'O,

A great deal of relevant work was done by UTC, Westinghouseand Fuel Cell Energy in the 1960's and 1970's on phosphoric acidfuel cells. None of it was encouraging for metal bipolar plates. Theelevated operating temperature, 190C, of the PBI membranescurrently limits bipolar plate material choices to graphite. No liquidwater is found in operating cells but condensate can be found onthe surfaces of plates in shutdown cell stacks. The condensate has alow pH from the phosphoric acid it dissolves.

II. DEFINITION

Bipolar plates are used in PEM fuel cell stacks because no one hassuccessfully engineered an alternative to the planar, gas diffusionelectrode. The function of the plates is to ensure that gaseousreactants are uniformly supplied to the cells in a stack anduniformly distributed over the surfaces of the cell or cooler. Thebipolar plate separates the gases and coolants and provides sealingbetween the cell reactant chambers and the environment. Thebipolar plate provides for the continuous conduction of electronsfrom one cell anode to the next fuel cell cathode. In the case ofinternally manifolded stacks, the manifolds are formed by thestacking of plates. In the case of internal manifolds, port design isof great importance. In some designs, the bipolar plate managescondensed product water to allow unimpeded flow of fuel and air.Condensate is undesirable in the cell stack because it facilitatescorrosion, blocks reactant flow, and increases the heat load on thecooling system.

4 D. Bloomfield and V. Bloomfield

Bipolar plates constitute everything in the fuel cell stack that isnot membrane electrode assembly (MEA) or follow up system (tierods and end plates). We generally consider MEAs everythingbetween the gas diffusion layers (GDLs), including the membraneand catalyst layers. Bipolar plates are comprised of severalfunctionally distinct parts including:

• separator• flow field• port and port bridges• seals• frame

1. Separator Plate

The separator plate separates the anode and cathode sides of thebipolar plate so that gases don't mix. The two sides of the plate areelectrically connected so that they are at the same potential. Theseparator can be a separate part, such as a metal sheet, or integralwith the material, such as the web in a molded graphite plate.Graphite plates functionally separate the gases but do not usuallyhave a discrete separator plate part. In the case of a metal separatorplate, only one side of the plate is exposed to low pH water. Theother side is exposed to the coolant cavity. This is an advantage insome cases such as using a clad metal separator where cladding isnot needed on both sides. Type of coolant used influences thematerials of construction. Aluminum, which has a cost advantage,cannot be used with a water coolant but can be used with air,silicone fluid, or hydrocarbon-ether based coolants. In somedesigns the separator is merely aplastic sheet or membrane

Figure I. TiN-coated stainless-steel separator plate showing manifold holes andalignment holes.

PEM Fuel Cell Bipolar Plates 5

that seals gases between the two flow fields but allows electricalconnections through the sheet by hot pressing or other means. Anexample of a separator plate with manifold and alignment holes isshown in Figure 1.

2. Flow Field

The purpose of the flow field is to distribute the reactants acrossthe active region (catalyzed area) of the cell (Figure 2). The flowfield can be a separate part from the plate or integral with the plate.Combining parts reduces assembly time. The gas distributionchannels are optimized to limit partial pressure gradients andtemperature gradients that can lead to performance loss. A longchannel has a higher frictional pressure drop than a short channel.A smaller cross sectional area will result in a higher reactant gaspressure loss. Deep channels are disadvantageous since the extraflow field thickness to accommodate the depth results in longerstacks requiring more material and bigger manifolds . Shallowchannels are preferred but must be wider to maintain sufficientcross sectional area.

The reactant partial pressure gradients exist across anyone cellfrom inlet to exit as well as from the bottom of the channel to thesurface of the membrane . The gas must diffuse perpendicular to theflow direction, through the gas diffusion layer, in order to get at thecatalyst. If the channels are spaced far apart, the gas must diffusein the plane of the GDL as well as through the plane of the

Figure 2. Metal bipolar plate assembly with lanced and offset roll fin flow field .

6 D. Bloomfieldand V. Bloomfield

GDL to access the full catalyst area. Since the in-plane diffusion isa longer path length the catalyst above the lands of the flow field iscalled shadowed and is not as easily accessible to reactants. Therule of thumb for channel spacing is many shallow channels veryclosely spaced. The spacing of channel area to land area must bebalanced with electrical conduction requirements through the stack.

Losses can also exist across multiple cells in the stackdepending on manifold length and size. The rule of thumb here isthat the cell should have an order of magnitude higher pressuredrop than the manifold. The cells act like a shower head to themanifold ensuring uniform distribution to all cells. The bulk of thecell pressure drop takes place in the ports of an internallymanifolded cell.

The flow field may include water management structures suchas wicks. Flow channel cross sectional area changes are also usedto optimize velocity gradients and entrain water droplets. Theinclusion of extra components to manage water generally leads tohigher assembly costs, and pressure drop (read parasite power).These considerations constitute the water management trade off.

3. Port and Port Bridges

The port is the transition from manifold to flow field. It is usuallycomprised of small flow channels that distribute the gas from themanifold to each flow channel or set of flow channels. The port isgenerally designed to be the highest pressure drop in themanifold/port/cell flow path. This limits the pressure variationacross the active area.

However, this is not the case with long serpentine channelswhere the pressure loss is significant from inlet to exit across theface of the active area. A separate port bridge is sometimesnecessary to support the thin cell membrane. The term bridgerefers to the flat top of the port bridge with a grooved underside(Figure 3). There are at least as many port designs as there arePEM fuel cell companies. Molded plates with grooves on bothsides often use a bridge structure with holes drilled in the plate(Figure 4). On one side of the plate the holes communicate with themanifold. On the opposite side, the holes communicate with theflow field grooves. The flat bridge support is needed not just toprevent the membrane from expanding into the grooves but as asealing surface as well. It is usually not a good idea to use the GDL

PEM Fuel Cell Bipolar Plates 7

Figure 3. Plastic frame for metal bipolar plate showing removable ports andovermolded seals.

as the bridge material. It limits the seal compression since highsealing forces will crush the GDL.

4. Seals

Nothing leaks like hydrogen (except, perhaps, aqueous KOH).Seals can be flat, punched gaskets, or O-rings integral to thebipolar plate, or as separate pieces. Seal materials can be over-molded onto a grooved cell frame or molded plate. This savesinsertion labor and prevents seal misalignment or movement duringassembly. Seals prevent the gases and cooling fluid from mixingand keep the reactants from leaking to the surroundings. Seals canbe made part of the bipolar plate or part of the MEA. In somedesigns, the seal is injected into a sealing manifold after the entirestack is built up. As we will show, this feature greatly improvesstack assembly. The sealing load will compress rubber seals about30%. Part tolerances can affect the compression causing crushingor gaps.

Figure 4. Molded graphite bipolar plate with through hole ports .

8 D. Bloomfield and V. Bloomfield

The requirement to compress seals incurs assembly problems.The uncompressed stack is 30% longer and must be uniformlycompressed in order to get a low contact resistance. Misalignmentof parts and seals during compression is a common problem. Theproblem is not trivial because it causes variations in cell to cellpressure drop resulting in cell reactant flow maldistribution.

5. Frame

A frame is required if the flow field is a separately formed part. Aconductive molded material such as resin filled graphite can formthe entire bipolar plate as a unit part except for the seal. Itsadvantage is the reduced number of parts required to assemble astack. The disadvantage is in cost, .weight and volume. Resinmolded graphite requires a thick web on the order of 0.05"- 0.06".This is the minimum thickness of the plate at the depth of thegrooves on both sides. This web thickness is about an order ofmagnitude larger than metal designs. Recent advances in moldingmaterials have reduced this thickness. Thin stamped or formedmetal flow fields often require a separate frame and!or seal. Theframes are typically injection molded. A separate part requiresadditional seals not only to join the MEA to the plate but to join theplate to the frame.

The ratio of frame area to active area should be minimized toreduce the cost of the membrane electrode assembly (MEA). If theframe includes the manifold (called internal manifolding) then themanifold cross section area and frame is constant for all plates inthe stack. The cost of the added material, seal, and additionalmembrane area is multiplied by the number of bipolar plates in thestack. This added cost can be significant. In some cases, externalmanifold are preferred. External manifolds are generally boxesfitted with an inlet or exit pipe. The boxes are simply held bycompression against the sides of the stack where they form a seal.The shape of the external manifold can vary over the stack lengthto balance pressure gradients. Externally manifolded stack cost istypically much lower as well.

III. BIPOLAR PLATE FEATURES

To utilize all of the catalytic area of the cell the MEA must be wellhydrated, have free access to reactants and a very good electricalcontact with the bipolar plate. The heat of reaction must be

PEM Fuel Cell Bipolar Plates 9

removed to prevent temperature gradients and loss of water fromthe membrane hot spots.

1. Tolerances

Tolerances are critical in a fuel cell stack. The higher the stackvoltage (number of cells) the more important tolerances are. Atolerance of +/- 0.002 inch on all bipolar plate components can addup to 0.408 inches in a 48 volt (nominal) stack if all parts are at themaximum plus tolerance (assuming three parts to each bipolarplate). Part flatness is just as important as overall thickness. If oneportion of the frame is consistently thicker than the rest, the sealmust make up the difference. In an injection over-molded seal, 2mils (50 microns) can be 15% of the seal thickness. The result isunder-compression in the thin parts of the frame. Applying highercompression loads to improve seal only results in crushing of theframes and often the MEA and GDL as well. In some cases, thethin frames will crack. Separate flow field and frame assembliesmake this problem even more complex. An advantage of unitizedplates is that the plate can be surface ground to a uniformthickness. Good plate tolerances will not only ensure sealing butwill improve conductivity.

2. Thermal Management

The operating temperature of the cell is set by choice of MEA. Inthe case of Nafion cells, the MEA conductivity is intimatelyassociated with the water content of the membrane. Since there isno vapor pressure suppression of membrane water by theelectrolyte (which is covalently bonded to the membrane solidphase), the amount of water in the membrane is a function of thecell temperature, pressure, reactant water concentrations andvelocity, and the current density. The relationship is difficult toanalyze and even more difficult to control. To remove the cellwaste heat, a single-phase coolant must absorb the waste heat assensible heat. Two-phase coolants can absorb the waste heat aslatent heat. Of course as the reactant gas streams gather moisturetraversing the cell, a temperature gradient is helpful for removingwaste heat and moisture. This can be closely coupled to the bipolarplate material. If the material has a high thermal conductivity thenthe inlet to exit plate temperature is minimized by axialconduction. One can readily see how complex thermalmanagement becomes when one realizes that the flow through the

10 D. Bloomfield and V. Bloomfield

cells in a stack is only approximately equal and that the transientsin power and the performance characteristics of the fluid moversare coupled into the problem.

The in plane thermal conductivity of metals allows a smalltemperature difference between the inlet and the outlet of a(rectangular cell). This axial conduction can be usedadvantageously.

3. Electrical Conduction

In addition to the functions described above, the bipolar platetransmits the electrons from the cell anodes to the adjacent cellcathodes perpendicular to the plane of the plate, or through plane.Monopolar cells transmit electrons in the plane of the cell requiringa minimum bipolar plate thickness and zero conductivity. This typeof cell is rarely used because the current that can be collected froma monopolar cell is only about half that of a conventional bipolarcell for a given cell voltage. The intrinsic conductivity in a bipolarcell is generally not a concern. The interfacial conductivitybetween cell components is a function of the stack contact pressureand contact area and is very important. High thickness and flatnesstolerances are needed to maximize contact area since the parts arenot soft and deflection of carbon can lead to crushing and increaseddiffusion losses. Intrinsic conductivity in bipolar arrangements isonly important when implementing performance enhancementtechniques that require in-plane conduction to promote cathodecatalyst reduction.

Electronic conduction in the GDL and the flow fieldinfluences flow field design. Just as the reactant gas must diffuse inthe plane of the GDL to get to the shadowed catalyst, the electronsmust move in the plane of the GDL from the lands of the bipolarplate to the region over the grooves in order to supply the catalystwith electrons. Several design features can enhance the in-planeconduction. Narrow channels and lands reduce the conduction pathlength. Increased surface area contact between the land and the gasdiffusion layer can be done by micro-texturing the material incontact with the GDL. Improving the in-plane conductivity of theGDL using micro-layers, or conductive fillers is now common. Themicro-layers are either a separate thin layer placed between GDLand MEA or carbon/Teflon filler material that is printed onto thecarbon paper GDL and sintered.

PEM Fuel Cell Bipolar Plates

Bipolar Plate Resistivity

11

NEuEJ:o~0.10~~§+l.~III

ti

0.01 --'-c--'----r--L-+---'----T----'----i--- -,-------i-------,-- i----i

4 10 12 14 16 18 20

1ODD/contact pressure

--+-- baseline --ft- C paper -ee-- BMC 1 Sand -+- BMC 2 Sand-s- Torian CP -s- Roll Fin & CP ...--Rogers w CP

Figure 5. Comparison of bipolar plate resistivity.

Figure 5 shows the resistivity of several bipolar platematerials. The lowest resistivity on the chart is the baseline whichrepresents the test hardware. All other curves include the baselinehardware resistance. The BMC compounds are moldable graphiteand show moderate resistance. The highest resistivity was shownby Torlon, a composite TiB filler embedded in a plastic resin. Rollfin is a lanced offset metal flow field and CP means carbon paper.Samples are often tested as a sandwich with carbon paper on eachside to representing the bipolar plate arrangement.

4. Water Management

The bipolar plate design is an integral part of the PEM cell watermanagement. Product water vapor that is not carried out with thecathode exhaust will condense, forming droplets in the gaschannels and adversely affecting reactant distribution . The dropletsare often not swept out because the Reynolds number in the flow

12 D. Bloomfield and V. Bloomfield

field is '"16. In the case of condensation, flow distribution isdynamically unstable and CFD analysis on flow distributionbecomes moot. When a reactant gas channel becomes clogged witha water droplet, the gas flow preferentially bypasses the cloggedchannel and diverts into unclogged channels. However, the sameamount of current is passing through the clogged and uncloggedregions of the cell. The reactant gas supply in the clogged region isreduced meaning that the clogged region of the cell operates at ahigher utilization and the polarization increases. This causes theclogged region of the cell to run hotter which may clear thechannel. Hot, dry inlet gases which could carry away additionalproduct water will only dry out the inlet region of the membraneleading to gas crossover and failure. Liquid product water at theexit is undesirable yet dry gases at the inlet are also undesirable.All the water that is needed is produced by the cell yet it is in thewrong location. Some designs recycle this water internally bypassively absorbing the liquid at the exit and providinghumidification at the inlet. A porous bipolar plate which is in liquidcommunication with cooling water fluid can accomplish this.26

,27

Care must be taken to balance the pressure between the twocavities to maintain a capillary seal. This is one of the most reliablemethods since the liquid will be absorbed wherever it maycondense. Other designs recycle the water externally but the watermust first exit the bipolar plate by other means. This typicallyrequires additional energy and cost. For example, the enthalpywheel" recycles the moisture from the cathode exit back to thecathode inlet without recycling the spent gases. It is a regenerativeheat and mass transport device. Moisture transfer membranes arealso used for this purpose.

There are six basic methods for water removal. They areconcentration gradient, capillary forces, electro-osmosis, purging(causing high gas velocity for droplet entrainment), thermalgradient (evaporation) and surface tension gradient. These methodsfall into two categories: active and passive. An actively manageddesign removes water from the cathode gas space using controlssuch as increased air flow, gas purging, decreased humidity (a formof concentration gradient), reduced coolant flow (thermalgradient), etc. While most concentration gradient methods focus onhumidity and temperature, a concentration gradient can be set upbetween the cooling fluid and the cell reactants. A membraneseparator is required to limit gas exchange but allow liquid