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Solid State Proton Conductors


Solid

State Proto

n C

on

du

ctors

Properties

and

Applications

in Fuel Cells

Properties and Applications in Fuel Cells

Prop

erties and

Ap

plicatio

ns in

Fuel C

ells

E D I T O R S

Philippe Knauth Maria Luisa Di Vona

E D I T O R S

Philippe KnauthAix-Marseille University - CNRS,Marseille, France

Maria Luisa Di VonaUniversity of Rome Tor Vergata, Rome, Italy

E D I T O R S

Knauth

Di Vona

Proton conduction can be found in many different solid materials, from organic polymers at room temperature to inorganic oxides at high temperature. Solid state proton conductors are of central interest for many technological innovations, including hydrogen and humidity sensors, membranes for water electrolyzers and, most importantly, for high-efficiency electrochemical energy conversion in fuel cells.

Focusing on fundamentals and physico-chemical properties of solid state proton conductors, topics covered include:

• Morphology and Structure of Solid Acids • Diffusion in Solid Proton Conductors by Nuclear Magnetic Resonance Spectroscopy • Structure and Diffusivity by Quasielastic Neutron Scattering • Broadband Dielectric Spectroscopy • Mechanical and Dynamic Mechanical Analysis of Proton-Conducting Polymers • Ab initio Modeling of Transport and Structure • Perfluorinated Sulfonic Acids

• Proton-Conducting Aromatic Polymers

• Inorganic Solid Proton Conductors

Uniquely combining both organic (polymeric) and inorganic proton conductors, Solid State Proton Conductors: Properties and Applications in Fuel Cells provides a complete treatment of research on proton-conducting materials.

GREEN BOX RULES ARE FOR PROOF STAGE ONLY. DELETE BEFORE FINAL PRINTING.

Solid State ProtonConductors

Properties and Applications in Fuel Cells

Edited by

PHILIPPE KNAUTH

Laboratoire Chimie Provence, Aix-Marseille University - CNRS,Marseille, France

and

MARIA LUISA DI VONA

Department of Chemical Science and Technology,University of Rome Tor Vergata, Rome, Italy

This edition first published 2012

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Library of Congress Cataloging-in-Publication Data

Di Vona, Maria Luisa.

Solid state proton conductors : properties and applications in fuel cells /Maria Luisa Di Vona and Philippe Knauth.

p. cm.

Includes bibliographical references and index.

ISBN 978-0-470-66937-2 (cloth)

1. Solid state proton conductors. 2. Solid state chemistry. 3. Fuel cells. I.

Knauth, Philippe. II. Title.

QC176.8.E4D56 2012

621.31’2429–dc23 2011037228

A catalogue record for this book is available from the British Library.

HB ISBN: 9780470669372

Set in 10/12pt Times-Roman by Thomson Digital, India

First Impression 2012

http://www.wiley.com

Contents

Preface xi

About the Editors xiii

Contributing Authors xv

1 Introduction and Overview: Protons, the Nonconformist Ions 1Maria Luisa Di Vona and Philippe Knauth

1.1 Brief History of the Field 2

1.2 Structure of This Book 2

References 4

2 Morphology and Structure of Solid Acids 5

Habib Ghobarkar, Philippe Knauth and Oliver Sch€af

2.1 Introduction 5

2.1.1 Preparation Technique of Solid Acids 5

2.1.2 Imaging Technique with the Scanning Electron Microscope 6

2.2 Crystal Morphology and Structure of Solid Acids 8

2.2.1 Hydrohalic Acids 8

2.2.2 Main Group Element Oxoacids 10

2.2.3 Transition Metal Oxoacids 20

2.2.4 Carboxylic Acids 22

References 24

3 Diffusion in Solid Proton Conductors: Theoretical Aspects andNuclear Magnetic Resonance Analysis 25

Maria Luisa Di Vona, Emanuela Sgreccia and Sebastiano Tosto

3.1 Fundamentals of Diffusion 25

3.1.1 Phenomenology of Diffusion 26

3.1.2 Solutions of the Diffusion Equation 35

3.1.3 Diffusion Coefficients and Proton Conduction 37

3.1.4 Measurement of the Diffusion Coefficient 38

3.2 Basic Principles of NMR 40

3.2.1 Description of the Main NMR Techniques Used in Measuring

Diffusion Coefficients 42

3.3 Application of NMR Techniques 47

3.3.1 Polymeric Proton Conductors 47

3.3.2 Inorganic Proton Conductors 58

3.4 Liquid Water Visualization in Proton-Conducting Membranes

by Nuclear Magnetic Resonance Imaging 62

3.5 Conclusions 66

References 67

4 Structure and Diffusivity in Proton-Conducting Membranes Studied by

Quasielastic Neutron Scattering 71Rolf Hempelmann

4.1 Survey 71

4.2 Diffusion in Solids and Liquids 73

4.3 Quasielastic Neutron Scattering: A Brief Introduction 76

4.4 Proton Diffusion in Membranes 82

4.4.1 Microstructure by Means of SAXS and SANS 82

4.4.2 Proton Conductivity and Water Diffusion 89

4.4.3 QENS Studies 90

4.5 Solid State Proton Conductors 95

4.5.1 Aliovalently Doped Perovskites 96

4.5.2 Hydrogen-Bonded Systems 101

4.6 Concluding Remarks 104

References 104

5 Broadband Dielectric Spectroscopy: A Powerful Tool for the

Determination of Charge Transfer Mechanisms in Ion Conductors 109

Vito Di Noto, Guinevere A. Giffin, Keti Vezzu, Matteo Piga and Sandra Lavina

5.1 Basic Principles 110

5.1.1 The Interaction of Matter with Electromagnetic Fields: The

Maxwell Equations 110

5.1.2 Electric Response in Terms of e*mðoÞ, s*mðoÞ, and Z*mðoÞ 111

5.2 Phenomenological Background of Electric Properties in a

Time-Dependent Field 114

5.2.1 Polarization Events 114

5.3 Theory of Dielectric Relaxation 127

5.3.1 Dielectric Relaxation Modes of Macromolecular Systems 129

5.3.2 A General Equation for the Analysis in the Frequency Domain

of s�(o) and e�(o) 132

5.4 Analysis of Electric Spectra 132

5.5 Broadband Dielectric Spectroscopy Measurement Techniques 141

5.5.1 Measurement Systems 142

5.5.2 Contacts 158

5.5.3 Calibration 165

5.5.4 Calibration in Parallel Plate Methods 165

5.5.5 Measurement Accuracy 172


References 180

vi Contents

6 Mechanical and Dynamic Mechanical Analysis of Proton-Conducting

Polymers 185

Jean-Francois Chailan, Mustapha Khadhraoui and Philippe Knauth

6.1 Introduction 185

6.1.1 Molecular Configurations: The Morphology and

Microstructure of Polymers 185

6.1.2 Molecular Motions 187

6.1.3 Glass Transition and Other Molecular Relaxations 188

6.2 Methodology of Uniaxial Tensile Tests 191

6.2.1 Elasticity and Young’s Modulus E 192

6.2.2 Elasticity and Shear Modulus G 195

6.2.3 Elasticity and Cohesion Energy 196

6.3 Relaxation and Creep of Polymers 197

6.3.1 Stress Relaxation of Polymers 198

6.3.2 Creep of Polymers 199

6.4 Engineering Stress–Strain Curves of Polymers 201

6.4.1 True Stress–Strain Curve for Plastic Flow and Toughness

of Polymers 203

6.4.2 Behavior of Composite Membranes 204

6.4.3 Behavior in the Glassy Regime 205

6.4.4 Influence of the Rate of Deformation 206

6.4.5 Effect of Temperature on Mechanical Properties 209

6.4.6 Thermal Strain 210

6.5 Stress–Strain Tensile Tests of Proton-Conducting Ionomers 211

6.5.1 Influence of Heat Treatment and Cross-Linking 212

6.5.2 Behavior of Composites 214

6.5.3 Conclusions 215

6.6 Dynamic Mechanical Analysis (DMA) of Polymers 217

6.6.1 Principle of Measurement 217

6.6.2 Molecular Motions and Dynamic Mechanical Properties 218

6.6.3 Experimental Considerations: How Does the Instrument Work? 219

6.6.4 Parameters of Dynamic Mechanical Analysis 220

6.7 The DMA of Proton-Conducting Ionomers 222

6.7.1 Perfluorosulfonic Acid Ionomer Membranes 222

6.7.2 Nonfluorinated Membranes 225

6.7.3 Organic–Inorganic Composite (or Hybrid) Membranes 230

Glossary 235

References 236

7 Ab Initio Modeling of Transport and Structure of Solid State

Proton Conductors 241

Jeffrey K. Clark II and Stephen J. Paddison


7.2 Theoretical Methods 244

7.2.1 Ab Initio Electronic Structure 244

Contents vii

7.2.2 Ab Initio Molecular Dynamics (AIMD) 248

7.2.3 Empirical Valence Bond (EVB) Models 249

7.3 Polymer Electrolyte Membranes 251

7.3.1 Local Microstructure 251

7.3.2 Proton Dissociation, Transfer, and Separation 258

7.4 Crystalline Proton Conductors and Oxides 279

7.4.1 Crystalline Proton Conductors 279

7.4.2 Oxides 284


References 290

8 Perfluorinated Sulfonic Acids as Proton Conductor Membranes 295

Giulio Alberti, Riccardo Narducci and Maria Luisa Di Vona

8.1 Introduction on Polymer Electrolyte Membranes for Fuel Cells 295

8.2 General Properties of Polymer Electrolyte Membranes 296

8.2.1 Ion Exchange of Polymers Electrolytes in Hþ Form 297

8.3 Perfluorinated Membranes Containing Superacid –SO3H Groups 303

8.3.1 Nafion Preparation 304

8.3.2 Nafion Morphology 304

8.3.3 Nafion Water Uptake in Liquid Water at Different Temperatures 306

8.3.4 Water-Vapor Sorption Isotherms of Nafion 307

8.3.5 Curves T/nc for Nafion 117 Membranes in Hþ Form 308

8.3.6 Water Uptake and Tensile Modulus of Nafion 311

8.3.7 Colligative Properties of Inner Proton Solutions in Nafion 313

8.3.8 Thermal Annealing of Nafion 315

8.3.9 MCPI Method 315

8.3.10 Proton Conductivity of Nafion 319

8.4 Some Information on Dow and on Recent Aquivion� Ionomers 321

8.5 Instability of Proton Conductivity of Highly Hydrated

PFSA Membranes 321

8.6 Composite Nafion Membranes 323

8.6.1 Silica-Filled Ionomer Membranes 323

8.6.2 Metal Oxide-Filled Nafion Membranes 324

8.6.3 Layered Zirconium Phosphate- and Zirconium

Phosphonate-Filled Ionomer Membranes 324

8.6.4 Heteropolyacid-Filled Membranes 325

8.7 Some Final Remarks and Conclusions 326

References 327

9 Proton Conductivity of Aromatic Polymers 331Baijun Liu and Michael D. Guiver


9.2 Synthetic Strategies of the Various Acid-Functionalized Aromatic

Polymers with Proton Transport Ability 332

9.2.1 Sulfonated Poly(arylene ether)s 332

viii Contents

9.2.2 Sulfonated Polyimides 341

9.2.3 Other Aromatic Polymers as PEMs 344

9.3 Approaches to Enhance Proton Conductivity 349

9.3.1 Nanophase-Separated Microstructures Containing

Proton-Conducting Channels 349

9.3.2 Replacement of –Ph-SO3H by –CF2 –SO3H 353

9.3.3 Synthesis of High-IEC PEMs 355

9.3.4 Composite Membranes 356

9.4 Balancing Proton Conductivity, Dimensional

Stability, and Other Properties 358

9.5 Electrochemical Performance of Aromatic Polymers 361

9.5.1 PEMFC Performance 362

9.5.2 DMFC Performance 363

9.6 Summary 363

References 365

10 Inorganic Solid Proton Conductors 371

Philippe Knauth and Maria Luisa Di Vona

10.1 Fundamentals of Ionic Conduction in Inorganic Solids 371

10.1.1 Defect Concentrations 372

10.1.2 Defect Mobilities 373

10.1.3 Kr€oger–Vink Nomenclature 373

10.1.4 Ionic Conduction in the Bulk: Hopping Model 376

10.2 General Considerations on Inorganic Solid Proton Conductors 378

10.2.1 Classification of Solid Proton Conductors 379

10.3 Low-Dimensional Solid Proton Conductors:

Layered and Porous Structures 381

10.3.1 b- and b00-Alumina-Type 381

10.3.2 Layered Metal Hydrogen Phosphates 382

10.3.3 Micro- and Mesoporous Structures 384

10.4 Three-Dimensional Solid Proton Conductors:

“Quasi-Liquid” Structures 385

10.4.1 Solid Acids 385

10.4.2 Acid Salts 385

10.4.3 Amorphous and Gelled Oxides and Hydroxides 387

10.5 Three-Dimensional Solid Proton Conductors: Defect Mechanisms

in Oxides 387

10.5.1 Perovskite-Type Oxides 388

10.5.2 Other Structure Types 393

10.6 Conclusion 394

References 395

Index 399

Contents ix

Preface

Solid state proton conductors are of central interest for many technological innovations and,

most importantly, for high-efficiency electrochemical energy conversion in fuel cells

working at low or intermediate temperature.

The most recent textbook on all aspects of solid state proton conductors was published in

1992. Although some excellent review papers have been published since then, an updated

textbook summarizing the current knowledge on solid state proton conductors seemed

worthwhile.

This book presents review chapters on selected characterization techniques, modelling

and properties of solid state proton conductors written by us and some of the leading experts

in the field. It focuses on fundamentals and physico-chemical properties; synthesis

procedures are only marginally addressed. Most chapters discuss first and foremost the

basics that require a decent level of abstraction, before presenting detailed descriptions of

solid state proton conductors.

We are confident that this book will close a gap in recent textbook literature.

Writing and editing a book are difficult and time-consuming tasks, but they also comprise

a rewarding adventure and we hope the readers will consider their “journey” through the

pages of this book a gratifying experience as well.

We want to thank all authors and friends, who contributed their knowledge in a timely

manner.Without their commitment and hard work, this book would not have been possible.

We also gratefully acknowledge the financial support by many institutions which

helped to finance our research in the field of solid state proton conductors over the years,

including the European Hydrogen and Fuel Cell Technology Platform (FP7 JTI-FCH), the

Italian Ministry of Education, Universities and Research (MIUR) and the Franco-Italian

University.

Philippe Knauth and M. Luisa Di Vona

Marseille and Roma, June 2011

About the Editors

Philippe Knauth was the recipient of a doctorate in sciences

(Doctor Rerum Naturalium) in 1987 and the Habilitation �a dirigerdes recherches in 1996. He has been a professor of materials

chemistry at Aix-Marseille University since 1999. Awarded the

CNRS Bronze Medal in 1994, he was an Invited Scientist at

the Massachusetts Instuitute of Technology, United States from

1997–1998 and an Invited Professor at the National Institute of

Materials Science (NIMS), Tsukuba, Japan in both 2007 and 2010.

He is currently director of the Laboratoire Chimie Provence (UMR 6264), which includes

130 academic staff working in all fields of chemistry. He has been an elected member of

France’s Conseil National des Universit�es for materials chemistry since 2003 and president

of the Provence-Alpes-Cote d’Azur regional section of the Soci�et�e Chimique de France

since 2010. His principal research topics are ionic conduction at interfaces, electrochemis-

try at the nanoscale and materials for energy and the environment. He is currently mainly

working on solid state proton conductors for fuel cells and micro-electrodes for lithium-ion

batteries, and he is a member of the editorial board of the Journal of Electroceramics.

Maria Luisa Di Vona obtained a doctorate in chemistry cum laude

in 1984. In 1987 she became a researcher in organic chemistry at the

Faculty of Science of the University of Rome Tor Vergata. She was

visiting professor at the Laboratoire Chimie Provence, Universit�ede Provence, Marseille, France, in 2007 and 2009, and at the

National Institute for Materials Science (NIMS), Tsukuba, Japan

in 2010. She is the author of about 100 papers in international

journals on materials synthesis and characterization, multifunc-

tional ‘inorganic and organic–inorganic materials, the formation

and study of nanocomposite materials and characterization by means of multinuclear NMR

(nuclear magnetic resonance) spectroscopy. Her current research interest is in the field of

proton exchange membranes. She is a project leader and recipient of research grants from

the ASI, Italian Ministry, Franco-Italian University (Vinci program) and European Union

(the European Hydrogen and Fuel Cell Technology Platform, or FP7 JTI-FCH). She is a

member of the organizing and scientific committees of several conferences and was the

principal organizer of the 2009 EuropeanMaterials Research Society (E-MRS) symposium

“Materials for Polymer Electrolyte Membrane Fuel Cells” as well as the 2011 Materials

Research Society (MRS) symposium “Advanced Materials for Fuel Cells”.

Contributing Authors

Giulio Alberti, Department of Chemistry, University of Perugia, Via Elce di Sotto 8,

I-06123 Perugia, Italy

Jean-Francois Chailan, Laboratoire MAPIEM, Universit�e du Sud Toulon-Var, F-83957

La Garde, France

Jeffrey K. Clark II, Department of Chemical and Biomolecular Engineering, University

of Tennessee, Knoxville, TN 37996, USA

Vito Di Noto, Department of Chemical Sciences, University of Padua, Via F. Marzolo 1,

I-35131 Padova, Italy

Maria Luisa Di Vona, Dipartimento di Scienze e Tecnologie Chimiche, University of

Rome Tor Vergata, Via della Ricerca Scientifica, I-00133 Roma, Italy

Guinevere A. Giffin, Department of Chemical Sciences, University of Padua,

Via F. Marzolo 1, I-35131 Padova, Italy

Michael D. Guiver, National Research Council Canada, Institute for Chemical Process

and Environmental Technology Ottawa, ON, K1A 0R6, Canada and WCU, Department of

Energy Engineering, Hanyang University, Seoul 133–791, Republic of Korea

Rolf Hempelmann, Physical Chemistry, Saarland University, D-66123 Saarbr€ucken,Germany

MustaphaKhadhraoui, LaboratoireChimieProvence-Madirel,Aix-MarseilleUniversity -

CNRS, Centre St J�erome, F-13397 Marseille, France

Philippe Knauth, Laboratoire Chimie Provence-Madirel, Aix-Marseille University -

CNRS, F-13397 Marseille, France

SandraLavina, Department of Chemical Sciences, University of Padua, Via F.Marzolo 1,


BaijunLiu, Alan G.MacDiarmid Institute, Jilin University, Changchun 130012, P.R. China

Riccardo Narducci, Department of Chemistry, University of Perugia, Via Elce di Sotto 8,

I-06123 Perugia, Italy

Stephen J. Paddison, Department of Chemical and Biomolecular Engineering, University

of Tennessee, Knoxville, TN 37996, USA

Matteo Piga, Department of Chemical Sciences, University of Padua, Via F. Marzolo 1,


Oliver Sch€af, Laboratoire Chimie Provence-Madirel, Aix-Marseille University, Centre

St J�erome, F-13397 Marseille, France

Emanuela Sgreccia, Dipartimento di Scienze e Tecnologie Chimiche, University of Rome

Tor Vergata, Via della Ricerca Scientifica, I-00133 Roma, Italy

SebastianoTosto, ENEACentroRicerche Casaccia, ViaAnguillarese 301, I-00123Roma,

Italy

KetiVezzu, Department ofChemistry,University ofVenice, ViaDorsoduro, 2137, I-30123

Venice, Italy

xvi Contributing Authors

1

Introduction and Overview:Protons, the Nonconformist Ions

Maria Luisa Di Vona and Philippe Knauth

“The Nonconformist Ion” is the title of a review article on proton-conducting solids by

Ernsberger in 1983 [1]. Indeed, many proton properties are peculiar. First of all, the very

particular electronic structure is unique: its only valence electron lost, the proton is

exceptionally small and light and polarizes its surroundings very strongly. In condensed

matter, this will lead to strong interactions with the immediate environment and very strong

solvation in solution.

Second, two very particular proton migration mechanisms are well established. In

“vehicular” motion, a protonated solvent molecule is used as a vehicle. This mechanism

is typically characterized by higher activation energy and lower proton mobility. In

structuralmotion, the so-calledGrotthussmechanism involves site-to-site hopping between

proton donor and proton acceptor sites with local reconstruction of the environment around

the moving proton. This mechanism is related to lower values of activation energy and

higher proton mobility.

Proton conduction can be found in many very different solid materials, from soft organic

polymers at room temperature to hard inorganic oxides at high temperature. The importance

of atmospheric humidity for the existence and stability of proton conduction is another

common point, which goeswith experimental difficulties formeasuring proton conductivity

in solids.

Proton-conducting solids are the core of many technological innovations, including

hydrogen and humidity sensors, hydrogen permeation membranes, membranes for water

electrolyzers, and most importantly high-efficiency electrochemical energy conversion in

fuel cells working at low temperature (polymer electrolyte membrane or proton exchange

Solid State Proton Conductors: Properties and Applications in Fuel Cells, First Edition.Edited by Philippe Knauth and Maria Luisa Di Vona.� 2012 John Wiley & Sons, Ltd. Published 2012 by John Wiley & Sons, Ltd.

membrane fuel cells, PEMFC) or intermediate temperature (proton-conducting ceramic

fuel cells, PCFC).

1.1 Brief History of the Field

Proton mobility is a special case in the field of ion transport. In early textbooks on

the electrochemistry of solids, proton-conducting solids are not even mentioned [2],

except ice [3].

Historically, the existence of protons in aqueous solutions had already been

conjectured by de Grotthuss in 1806 [4]. The study of proton-conducting solids

started at the end of the nineteenth century, when it was noticed that ice conducts

electricity, with the investigation of the electrical conductivity of ice single crystals [5].

A first mention of “vagabond” ions in an inorganic compound, hydrogen uranyl

phosphate (HUP), was due to Beintema in 1938 [6]. However, it was not until the

1950s that the study of solid proton conductors started in earnest: Bjerrum’s funda-

mental study on ice conductivity led the way in 1952 [7], and Eigen and coworkers

discussed the proton conductivity of ice crystals in 1964 [8]. Nevertheless, these

investigations were fundamental studies and the materials could still be considered

only laboratory curiosities.

The first proton-conducting material applied in practice was a perfluorinated sulfonated

polymer, Nafion, adapted by DuPont in the 1960s as a proton-conducting membrane for

PEMFC, used in theGemini andApollo space programs. This gave important momentum to

the domain of solid proton conductors. Several inorganic solid proton conductors were then

reported in the 1970s and 1980s. The rediscovery of HUPwas followed by the discovery by

Russian groups of several acid sulfates showing structural phase transitions, such as

CsHSO4 [9] and zirconium hydrogenphosphate (ZrP), by Alberti and coworkers [10].

Furthermore, oxide gels containing water show nearly always some proton conductivi-

ty [11]. However, with the exception of ZrP, the proton conductivity of these materials is

limited to about 200 �C.An important discovery was, therefore, the report by Iwahara and coworkers in the 1980s

of “high-temperature” proton conduction in perovskite-type oxides in humidity- or

hydrogen-containing atmosphere [12], where the maximum of proton conductivity is

typically observed at temperatures above 400 �C.Nowadays themain fields of development are proton-conducting polymermembranes for

low-temperature applications and proton-conducting oxide ceramics for intermediate- and

high-temperature devices. Given the current interest for the possible future hydrogen

economy, the fuel cell field is mentioned in most articles of this book.

1.2 Structure of This Book

The most recent textbook on all aspects of solid state proton conductors was published in

1992 [13]. Excellent review papers have been published afterward, for example by Norby

in 1999; [14] Alberti and Casciola in 2001 [15]; and Kreuer, Paddison, Spohr, and Schuster

2 Solid State Proton Conductors

in 2004 [16], but an updated textbook summarizing the current knowledge on solid state

proton conductors seemed worthwhile.

In the following chapters, some of the leading experts in the field have written

authoritative review chapters on the characterization techniques, modeling, and properties

of solid state proton conductors.

The chapter “Morphology and Structure of Solid Acids” shows an overview of structural

analysis of some important solid acids by scanning electron microscopy. This beautifully

illustrated chapter is an aesthetic pleasure, and the micrographs are complemented by

polyhedral representations and a short introduction on the technique.

The chapter “Diffusion in Solid Proton Conductors: Theoretical Aspects and Nuclear

Magnetic Resonance Analysis” starts with an overview on fundamentals of diffusion. Then,

principles of nuclear magnetic resonance (NMR) spectroscopy are introduced. Nuclear

magnetic resonance is a very powerful technique for investigation of structure and diffusion

in solid proton conductors;NMR imaging is a newer development, and is also addressed on a

basic level in this chapter.

The chapter “Structure and Diffusivity in Proton-Conducting Membranes Studied by

Quasi-elastic Neutron Scattering” introduces the basics of neutron scattering, which is

obviously of particular importance for the field. Analysis of diffusional processes in

inorganic as well as organic solid proton conductors is presented and discussed.

The chapter “Broadband Dielectric Spectroscopy: A Powerful Tool for the Determi-

nation of Charge Transfer Mechanisms in Ion Conductors” is devoted to the electrical

properties of ion-conducting solids, especially macromolecular systems. This chapter

describes fundamentals and examples of dielectric measurements in a broad frequency

domain, which can be used for a wide range of materials from insulators to

“super-protonic” conductors.

The chapter “Mechanical and Dynamic Mechanical Analysis of Proton-Conducting

Polymers” introduces first some basic principles of the mechanics of materials: elastic and

plastic deformation, creep and relaxation, and dynamic mechanical analysis. Then, the

mechanical properties of proton-conducting polymers and their durability are discussed.

The chapter “Ab InitioModeling of Transport and Structure of Solid Proton Conductors”

presents a rapid introduction on the theoretical methods of choice. Significant examples of

solid proton conductors are discussed, including proton-conducting polymers; solid acids,

such as CsHSO4; and proton-conducting perovskite oxides.

Two chapters are devoted to polymeric proton conductors. The chapter “Perfluorinated

Sulfonic Acids as Proton Conductor Membranes” introduces the field and presents recent

progress for the improvement of the oldest but still leading ionomer, Nafion. This chapter

reviews a physicochemical approach and strategies for future enhancement of the durability

of Nafion membranes.

The chapter “Proton Conductivity of Aromatic Polymers” discusses a main family of

alternative ionomers based on fully aromatic polymers. Their synthesis and electrical

properties and further possibilities for improvement, such as hybrid organic–inorganic

ionomers and cross-linked systems, are discussed.

The last chapter reviews “Inorganic Solid Proton Conductors.” The chapter recalls

fundamentals of ionic conduction in inorganic solids and presents the main classes of

proton-conductingmaterials, including layered and porous solids, “quasi-liquid” structures,

and defect solids, especially perovskite oxides.

Introduction and Overview: Protons, the Nonconformist Ions 3

References

1. Ernsberger, F.M. (1983)Thenonconformist ion. Journal of theAmericanCeramic Society, 66, 747.

2. Rickert, H. (1982) Electrochemistry of Solids, Springer, Berlin.

3. Kr€oger, F.A. (1974) The Chemistry of Imperfect Crystals, North-Holland, Amsterdam.

4. Grotthuss, C.J.T.d. (1806) M�emoire sur la d�ecomposition de l’eau et des corps qu’elle tient en

dissolution �a l’aide de l’�electricit�e galvanique. Annales de Chimie, LVII, 54.

5. Ayrton,W.E. and Perry, J. (1877) Ice as an electrolyte.Proceedings of the Physical Society, 2, 171.

6. Beintema, J. (1938) On the composition and the crystallography of autunite and the meta-

autunites. Recueil des Travaux Chimiques des Pays-Bas, 57, 155.

7. Bjerrum, N. (1952) Structure and properties of ice. Science, 115, 385.

8. Eigen, M., Maeyer, L.D. and Spatz, H.C. (1964) Kinetic behavior of protons and deuterons in ice

crystals. Berichte der Bunsengesellschaft f€ur Physikalische Chemie, 68, 19.9. Baranov, A.I., Shuvalov, L.A. and Shchagina, N.M. (1982) Superion conductivity and phase-

transitions in CsHSO4 and CsHSeO4 crystals. Jetp Letters, 36, 459.

10. Alberti, G. and Torracca, E. (1968) Electrical conductance of amorphous zirconium phosphate in

various salt forms. Journal of Inorganic and Nuclear Chemistry, 30, 1093.

11. Livage, J. (1992) Sol-gel ionics. Solid State Ionics, 50, 307.

12. Takahashi, T. and Iwahara, H. (1980) Protonic conduction in perovskite type oxide solid solutions.

Revue Chimie Min�erale, 17, 243.13. Colomban, P. (1992) Proton Conductors: Solids, Membranes and Gels - Materials and Devices,

Cambridge University Press, Cambridge.

14. Norby, T. (1999) Solid-state protonic conductors: principles, properties, progress and prospects.

Solid State Ionics, 125, 1.

15. Alberti, G. andCasciola,M. (2001) Solid state protonic conductors, presentmain applications and

future prospects. Solid State Ionics, 145, 3.

16. Kreuer, K., Paddison, S., Spohr, E. and Schuster, M. (2004) Transport in proton conductors for

fuel-cell applications: simulations, elementary reactions, and phenomenology. Chemical

Reviews, 104, 4637.


2

Morphology and Structureof Solid Acids1

Habib Ghobarkar, Philippe Knauth and Oliver Sch€af

2.1 Introduction

The objective of this chapter is to introduce some important solid acids from a structural, and

also morphological, point of view. The micrographs were obtained by scanning electron

microscopy (SEM) on samples prepared in situ, according to the techniques described in the

following section.

2.1.1 Preparation Technique of Solid Acids

Almost all solid acids were prepared by rapid evaporation of highly concentrated aqueous

solutions fromopen stainless-steel containers heated either by a gas flame or by an induction

furnace. Different evaporation speeds could be obtained in this way, but over-heating had to

be strictly avoided. During the cooling process, the samples were placed in the sputtering

unit (low-pressure Ar-plasma atmosphere) in order to cover them with a protective gold

layer (necessary for subsequent SEM observations) before rehydration occurred.

High-pressure hydrothermal processing at temperatures below 200 �C and at 100MPa

pressure as described in detail in reference [1] could be used only for the synthesis of the

complex transition – metal phosphoric acids, presented in Sections 2.2.3.1 and 2.2.3.3.

Samples from both synthesis pathwayswere immediately transferred to the SEM in order

to avoid any further degradation.

1 This chapter is dedicated to the memory of Dr. Habib Ghobarkar († 2010).

Solid State Proton Conductors: Properties and Applications in Fuel Cells, First Edition.Edited by Philippe Knauth and Maria Luisa Di Vona.� 2012 John Wiley & Sons, Ltd. Published 2012 by John Wiley & Sons, Ltd.

2.1.2 Imaging Technique with the Scanning Electron Microscope

X-ray diffraction is the first and standard method commonly used for the identification of

crystalline phases. Ghobarkar [2, 3] developed a new method for the identification of

microcrystals that allows the optical identification of crystals observed by the SEM.

In contrast to the optical reflection goniometer, this method allows the measurement of

crystal faces even in the micrometre range applying the crystallographic principle that the

face normal angles of crystals keep constant independent from size. The face normal angles

of an idiomorphous crystal phase, however, are characteristic for each crystallographic

system while the axis ratios are determined. Furthermore, the calculated axis ratios can be

compared to X-ray diffraction data.

The differences in depth created by object points appearing in different spherical

distances with respect to the eyes are called parallaxes. Ghobarkar could show that these

parallaxes can be used to quantify the relative position of a plane of a microcrystal’s face

relative to the next. This is done in order to obtain all angles between the appearing faces

(represented by their face normal angles).

By using SEM, crystals can be indexed and their crystallographic grouping determined.

Furthermore, the energy-dispersive X-ray (EDS) method allows the measurement of the

chemical composition in a semiquantitative way. The two different results are based on

standard measurements in chemical composition and face angles.

The stereo comparator method can be subdivided into different parts. In the electron-

microscopic part, the crystalline phase under investigation is analysed by stereo imaging.

The specimen containing themicrocrystals is installed on the goniometer specimen stage of

the SEM. In a first approximation, the SEM delivers parallel projection images of the

observed objects.

Different perspectives for stereo-comparator processing are created by taking two

different images, the first at a position of 0� and the second after an inclination of 12�

(Figure 2.1). To get useful results, the inclination has to be done precisely in the same

crystallographic zone. Two different image pairs are taken in order to reduce systematic

errors introduced by mechanical movement of the specimen stage. It is important that the

images are taken at the same value of magnification. Generally, the method is useful for

crystals which need magnification higher than 500 times as crystals bigger in size can be

analysed by other methods. The smaller the crystals are, the higher the precision of the final

phase angle measurements.

2.1.2.1 The Calculation of x,y,z from Measured x,y, Px and Py

The calculation of the face angles is done by the determination of x,y as well as the

parallaxes Px and Py for a respective point on a crystal face. Four points (three points

to define a plane, plus one control point) are measured per crystal face. The co-ordinates x

and y can be directly taken, while Py has to be kept constant carefully during the

measurement in order to guarantee accuracy. The z value for the respective point is

calculated by:

z ¼ Px=sin 2 sin1=2 ð2:1Þ

given that Px for both directions of inclination (�12�, 0�, 12�) gives the same value (control

of accuracy).


By doing this for three points (one supplementary point for control), a plane is clearly

defined; the common form of the equation of a plane is:

AxþByþCzþD ¼ 0 ð2:2ÞThe angle between planes 1 and 2 (crystal faces) is then given by:

cos a ¼ A1A2 þB1B2 þC1C2=ffiffið

pA2

1 þB21 þC2

1Þ ðA22 þB2

2 þC22Þ ð2:3Þ

The calculation is simplified by using the vector form of the plane equation. This has the big

advantage that the angle between two crystal faces is identical to the angle between their

normal vectors. The determination of the angle between two faces, therefore, covers two

steps.

The first step is the determination of the normal vectors of both planes: the determined

three points of a plane permit one to calculate two vectors which pass within the plane. The

normal vector of these planes is placed perpendicular to the plane and is the complementary

angle to 180�.Second is the determination of the angles between the normal vectors: these are the

angles between the crystal faces (Figure 2.2) obtained by the cross product of the two

vectors.

2.1.2.2 Crystal Indexing

In order to confirm the results on the face normal angles obtained by the stereo-comparator

with respect to the crystal habit (crystal morphology), the values are written in the

stereographic projection. At last, the stereographic projection has to be turned in such a

way that a standard set-up is achieved. The final indexing has to be accomplished by trial and

Figure 2.1 Position of crystal images after inclination: L:�12� inclination,M: 0�, R: þ12� (twopairs for control and accuracy purposes) [1]. Reprintedwith permission fromTheReconstructionof Natural Zeolites by H. Ghobarkar, O. Schaf, Y. Massiani, P. Knauth, Copyright (2003) KluwerAcademic.

Morphology and Structure of Solid Acids 7

error, while theoretical values can be taken into account once the crystal axis ratios and the

crystal axis angles have been determined. More details on this SEM observation technique

of microcrystals can be found in references [4, 5].

2.2 Crystal Morphology and Structure of Solid Acids

This chapter presents acid morphologies in the crystalline state, while the respective crystal

structures are directly correlated to these morphologies.

The reader may use corresponding crystal visualization software to obtain complemen-

tary three-dimensional orientations of the respective crystal lattices. Crystal structure

references are indicated to facilitate this approach.

2.2.1 Hydrohalic Acids

2.2.1.1 Hydrofluoric Acid

Chemical formula: HF

Crystal morphology (Figure 2.3)

Crystal structure (Figure 2.4)

2.2.1.2 Hydrochloric Acid

Chemical formula: HCl



2.2.1.3 Hydrobromic Acid

Chemical formula: HBr



Figure 2.2 Angles between crystal faces are obtained by determining the face normal anglesfrom the respective plane vectors for each face [1]. Reprinted with permission from TheReconstruction of Natural Zeolites byH.Ghobarkar, O. Schaf, Y.Massiani, P. Knauth, Copyright(2003) Kluwer Academic.


Figure 2.3 Orthorhombic (class: mmm) hydrofluoric acid (SEM, magnification: 2000�).

Figure 2.4 Polyhedral representation of orthorhombic hydrofluoric acid (space group: Bmmb).Data from Reference [6].

Figure 2.5 Orthorhombic (class: mmm) hydrochloric acid (SEM, magnification: 1290�).


2.2.2 Main Group Element Oxoacids

2.2.2.1 Boric Acid

Chemical formula: H3BO3



Figure 2.6 Polyhedral representation of orthorhombic hydrochloric acid (space group:Fmmm) [7].

Figure 2.7 Orthorhombic (class: mmm) hydrobromic acid (SEM, magnification: 5000�).

Figure 2.8 Polyhedral representation of orthorhombic hydrobromic acid (space group:Fmmm). Data from Reference [7].


2.2.2.2 Isocyanic Acid

Chemical formula: HNCO



Figure 2.9 Triclinic (class: P�1) boric acid (SEM, magnification: 1290�).

Figure 2.10 Polyhedral representation of triclinic boric acid (space group: P�1). Data fromReference [8].

Figure 2.11 Orthorhombic (class: mmm) isocyanic acid (SEM, magnification: 2000�).


2.2.2.3 Nitric Acid

Chemical formula: HNO3



Figure 2.12 Polyhedral representation of orthorhombic isocyanic acid (space group: Pnma).Data from Reference [9].

Figure 2.13 Monoclinic (class: 2/m) nitric acid (SEM, magnification: 1590�).

Figure 2.14 Polyhedral representation of monoclinic nitric acid (space group: P121/a1). Datafrom Reference [10].


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