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Solid State Proton Conductors
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 Proton Conductors
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
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
5.6 Concluding Remarks 180
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
7.1 Introduction 241
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
7.5 Concluding Remarks 290
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
9.1 Introduction 331
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,
I-35131 Padova, Italy
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,
I-35131 Padova, Italy
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.
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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-
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4 Solid State Proton Conductors
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).
6 Solid State Proton Conductors
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
Crystal morphology (Figure 2.5)
Crystal structure (Figure 2.6)
2.2.1.3 Hydrobromic Acid
Chemical formula: HBr
Crystal morphology (Figure 2.7)
Crystal structure (Figure 2.8)
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.
8 Solid State Proton Conductors
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�).
Morphology and Structure of Solid Acids 9
2.2.2 Main Group Element Oxoacids
2.2.2.1 Boric Acid
Chemical formula: H3BO3
Crystal morphology (Figure 2.9)
Crystal structure (Figure 2.10)
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].
10 Solid State Proton Conductors
2.2.2.2 Isocyanic Acid
Chemical formula: HNCO
Crystal morphology (Figure 2.11)
Crystal structure (Figure 2.12)
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�).
Morphology and Structure of Solid Acids 11
2.2.2.3 Nitric Acid
Chemical formula: HNO3
Crystal morphology (Figure 2.13)
Crystal structure (Figure 2.14)
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].
12 Solid State Proton Conductors