www.wiley-vch.de

Kharton (Ed.)

Solid State Electrochemistry II

The one-stop reference source for fundamentals, advances and intrigu-ing problems of solid-state electrochemistry. This important and rap-idly developing scientifi c fi eld integrates many aspects of the classical electrochemical science and engineering, materials science, solid-state chemistry and physics, heterogeneous catalysis and other areas of physi-cal chemistry. The range of practical applications includes many types of batteries, fuel cells, electrochemical pumps and compressors, solid-state electrolyzers and electrocatalytic reactors, synthesis of new materi-als with improved properties and corrosion protection, supercapacitors, accumulators, sensors, electrochromic and memory devices. The second volume contains brief reviews dealing with the ionic memory and fuel cell technologies, ceramic membranes and com-posites, nanostructured ionic and mixed conductors, novel electrode materials for a variety of solid-state electrochemical cells, selected theo-retical aspects, and numerous factors related to interfacial and surface processes, stability and reliability of the electrochemical appliances. As for the previous volume “Fundamentals, Materials and their Applications”, particular emphasis is centered on the general methodological aspects, reference information and recent advances. Due to the highly interdis-ciplinary nature of the topic, this handbook is of great interest to indus-trial and academic researchers, engineers and postgraduate students specializing in all related areas of science and technology.

Vladislav Kharton is a principal investigator at the Depart-ment of Ceramics and Glass Engineering, University of Aveiro (Portugal). Having received his doctoral degree in physical chemistry from the Belarus State University in 1993, he has published over 280 scientifi c papers in international SCI journals, including 10 reviews, and coauthored over 40 papers in other refereed journals and volumes, 3 books and 2 patents. He is a topical editor of the Journal of Solid State Electrochemistry, regional editor of Recent Patents on Mate-rial Science, and member of the editorial boards of Materials Letters, The Open Electrochemistry Journal, SRX Chemistry, SRX Materials Science, The Open Condensed Matter Physics Journal, and Processing and Application of Ceramics. In 2004, he received the Portuguese Science Foundation prize for Scientifi c Excellence.

Edited by Vladislav V. Kharton

Solid State Electrochemistry IIElectrodes, Interfaces and Ceramic Membranes

57268File AttachmentCover.jpg

Edited by

Vladislav V. Kharton

Solid State

Electrochemistry II

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Edited byVladislav V. Kharton

Solid State Electrochemistry II

Electrodes, Interfaces and Ceramic Membranes

The Editor

Vladislav V. KhartonUniversity of AveiroCICECO, Dept. of Ceramics and Glass Engin.3810-193 AveiroPortugal

All books published by Wiley-VCH are carefullyproduced. Nevertheless, authors, editors, andpublisher do not warrant the information containedin these books, including this book, to be free oferrors. Readers are advised to keep in mind thatstatements, data, illustrations, procedural details orother items may inadvertently be inaccurate.

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All rights reserved (including those of translation intoother languages). No part of this book may bereproduced in any form – by photoprinting,microfilm, or any other means – nor transmitted ortranslated into a machine language without writtenpermission from the publishers. Registered names,trademarks, etc. used in this book, even when notspecifically marked as such, are not to be consideredunprotected by law.

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Printed in the Federal Republic of GermanyPrinted on acid-free paper

ISBN Print: 978-3-527-32638-9ISBN oBook: 978-3-527-63556-6ISBN ePDF: 978-3-527-63558-0ISBN ePub: 978-3-527-63557-3ISBN Mobi: 978-3-527-63559-7

Contents

Preface XVList of Contributors XIX

1 Ionic Memory Technology 1An Chen

1.1 Introduction 11.2 Ionic Memory Switching Mechanisms 41.2.1 Cation-Based Resistive Switching Mechanism 41.2.2 Anion-Based Resistive Switching Mechanism 61.3 Materials for Ionic Memories 81.3.1 Metal Sulfide Solid Electrolytes 91.3.2 Ge-Based Chalcogenide Solid Electrolytes 91.3.3 Oxide Solid Electrolytes 101.3.4 Miscellaneous Other Solid Electrolytes 111.4 Electrical Characteristics of Ionic Memories 121.4.1 Ionic Memory Device Characteristics 121.4.2 Ionic Memory Array Characteristics 181.4.3 Comparing Ionic Memory with Other Memories 191.5 Architectures for Ionic Memories 201.5.1 CMOS-Integrated Architecture 201.5.2 Crossbar Array Architecture 211.5.3 CMOS/Hybrid Architecture 211.6 Challenges of Ionic Memories 221.6.1 Overprogramming and Overerasing 221.6.2 Random Diffusion of Metal Ions and Atoms 221.6.3 Thermal Stability 231.6.4 Switching Speed 231.6.5 Degradation of Inert Electrodes 241.7 Applications of Ionic Memories 241.7.1 Stand-Alone Memory 251.7.2 Embedded Memory 251.7.3 Storage Class Memory 25

V

1.8 Summary 26References 26

2 Composite Solid Electrolytes 31Nikolai F. Uvarov

2.1 Introduction 312.2 Interface Interactions and Defect Equilibria in

Composite Electrolytes 322.2.1 Defect Thermodynamics: Free Surface of an

Ionic Crystal 332.2.2 Defect Thermodynamics in Composites: Interfaces 372.2.3 Thermodynamic Stability Criteria and Surface Spreading 392.3 Nanocomposite Solid Electrolytes: Grain Size Effects 422.4 Ionic Transport 502.5 Other Properties 552.6 Computer Simulations 562.7 Design of the Composite Solid Electrolytes: General

Approaches and Perspectives 582.7.1 Varying Chemical Nature of the Ionic Salt–Oxide Pair 582.7.2 Changing the Physical State of Ionic Salt 602.7.3 Changing the Physical State of the ‘‘Inert’’ Component 612.7.4 Alteration of the Inert Component Morphology:

The Geometric Aspects 612.7.5 Metacomposite Materials and Nanostructured Systems 622.7.6 Achieving the Nanoscale Level 622.8 Composite Materials Operating at Elevated Temperatures 642.9 Conclusions 65

References 66

3 Advances in the Theoretical Description of Solid–ElectrolyteSolution Interfaces 73Orest Pizio and Stefan Soko»owski

3.1 Introduction 733.2 Theoretical Approaches 743.2.1 Background 743.2.2 Inhomogeneous Electrolyte Solutions: Generalities 773.2.3 Integral Equations for Solid–Electrolyte Solution Problems 803.2.4 Density Functional Theories 883.3 Computer Simulations 1003.3.1 Summation of Long-Range Forces 1013.3.2 Simulations of Simple Models for Electric Double Layer 1043.3.3 Solvent Effects on Electric Double Layer: Simple Models 1093.3.4 Solvent Effects on Electric Double Layer: Models of Water 1123.3.5 Summary 116

References 118

VI Contents

4 Dynamical Instabilities in Electrochemical Processes 125István Z. Kiss, Timea Nagy, and Vilmos Gáspár

4.1 Introduction 1254.2 Origin and Classification of Dynamical Instabilities in

Electrochemical Systems 1264.2.1 Classification Based on Essential Species 1274.2.1.1 ‘‘Truly’’ or ‘‘Strictly’’ Potentiostatic Systems 1284.2.1.2 Negative Differential Resistance Systems 1284.2.2 Classification Based on Nonlinear Dynamics 1314.3 Methodology 1364.3.1 Experimental Techniques 1364.3.1.1 Techniques for Temporal Dynamics 1364.3.1.2 Techniques for Spatial Dynamics 1364.3.2 Data Processing 1384.3.2.1 Digital Signal Processing 1394.3.2.2 Analysis of Time Series Data with Tools of Nonlinear

Dynamics 1404.4 Dynamics 1424.4.1 Bistability 1434.4.2 Oscillations 1444.4.3 Chaos 1504.4.4 Bursting 1514.4.5 Dynamics of Coupled Electrodes 1534.4.6 Dynamics of Pattern Formation 1584.5 Control of Dynamics 1594.5.1 IR Compensation 1604.5.2 Periodic Forcing 1604.5.3 Chaos Control 1624.5.4 Delayed Feedback and Tracking 1634.5.5 Adaptive Control 1634.5.6 Synchronization Engineering 1654.5.7 Effect of Noise 1674.6 Toward Applications 1674.7 Summary and Outlook 1714.7.1 Electrochemistry of Small Systems 1724.7.2 Description of Large, Complex Systems 1724.7.3 Engineering Structures 1724.7.4 Integration of Electrochemical and Biological Systems 173

References 173

5 Fuel Cells: Advances and Challenges 179San Ping Jiang and Xin Wang

5.1 Introduction 1795.1.1 Principle and Classification of Fuel Cells 1805.1.2 Fuels for Electrochemical Cells 183

Contents VII

5.1.2.1 Hydrogen, Methanol, Formic Acid, and Liquid Fuels 1835.1.2.2 Methane, Hydrocarbons, and Biomass-Derived Renewable

Fuels 1855.2 Alkaline and Alkaline Membrane Fuel Cells 1875.2.1 Alkaline Fuel Cells 1875.2.1.1 Historic Development and Prospects 1885.2.1.2 Gas Diffusion Cathode, Electrolyte Carbonation,

and Corrosion 1885.2.2 Alkaline Membrane Fuel Cells 1905.2.2.1 Alkaline Anion Exchange Membranes 1905.2.2.2 Electrocatalysts in Alkaline Medium 1925.3 Polymer Electrolyte Membrane Fuel Cells 1945.3.1 Technology Development of PEMFCs 1945.3.1.1 Nafion and Other Perfluorinated Membranes 1955.3.1.2 Pt, PtRu, and Nonprecious Metal Composite Electrocatalysts 1965.3.1.3 Self-Humidification and Electrode Structure Optimization 1995.3.1.4 Bipolar Plates and Stack Design 2005.3.1.5 Heat, Water Management, and Modeling 2025.3.1.6 Durability and Degradation of Electrocatalysts,

Membrane and Cell Performance 2035.3.2 PEMFCs with Liquid Fuels 2045.3.2.1 Direct Methanol Fuel Cells 2055.3.2.2 Direct Formic Acid Fuel Cells 2085.3.2.3 Direct Ethanol Fuel Cells 2105.4 Phosphoric Acid Fuel Cells and Molten Carbonate Fuel Cells 2115.4.1 Phosphoric Acid Fuel Cells 2115.4.1.1 Electrocatalysts, Electrolyte, and Gas Diffusion Electrode 2125.4.1.2 Sustainability and Challenges 2135.4.2 Molten Carbonate Fuel Cells 2135.4.2.1 NiO Cathode 2145.4.2.2 Anode, Stability and Retention of Electrolyte, Corrosion

of Separate Plate, and Component Stability 2155.4.2.3 Direct Internal Reforming MCFCs 2165.5 Solid Oxide Fuel Cells 2175.5.1 Development of Key Engineering Materials 2185.5.1.1 Anode 2185.5.1.2 Cathode 2225.5.1.3 Electrolytes 2255.5.1.4 Interconnect, Sealing, and Balance of Plant 2285.5.2 SOFC Structures and Configurations 2305.5.2.1 Cell Structures 2305.5.2.2 Stack Design and Configurations 2315.6 Emerging Fuel Cells 2325.6.1 Protic Ionic Liquid Electrolyte Fuel Cells 2325.6.2 Microbial Fuel Cells 232

VIII Contents

5.6.3 Biofuel Cells 2355.6.4 Microfluidic Fuel Cells 2375.6.5 High-Temperature Proton Exchange Membrane Fuel Cells 2385.6.5.1 HT-PEMFCs Based on Phosphoric Acid-Doped PBI PEM 2385.6.5.2 HT-PEMFCs Based on Inorganic and Ceramic PEM 2395.6.6 Single-Chamber Solid Oxide Fuel Cells 2415.6.7 Microsolid Oxide Fuel Cells 2425.6.8 Direct Carbon Fuel Cells 2465.7 Applications of Fuel Cells 2475.8 Final Remarks 249

References 252

6 Electrodes for High-Temperature Electrochemical Cells:Novel Materials and Recent Trends 265Ekaterina V. Tsipis and Vladislav V. Kharton

6.1 Introduction 2656.2 General Comments 2666.3 Novel Cathode Materials for Solid Oxide Fuel Cells:

Selected Trends and Compositions 2676.4 Oxide and Cermet SOFC Anodes: Relevant Trends 2866.5 Other Fuel Cell Concepts: Single-Chamber, Micro-,

and Symmetrical SOFCs 3016.6 Alternative Fuels: Direct Hydrocarbon and Direct

Carbon SOFCs 3096.7 Electrode Materials for High-Temperature Fuel Cells with

Proton-Conducting Electrolytes 3126.8 Electrolyzers, Reactors, and Other Applications Based on

Oxygen Ion- and Proton-Conducting Solid Electrolytes 3176.9 Concluding Remarks 321

References 322

7 Advances in Fabrication, Characterization, Testing, and Diagnosisof High-Performance Electrodes for PEM Fuel Cells 331Jinfeng Wu, Wei Dai, Hui Li, and Haijiang Wang

7.1 Introduction 3317.2 Advanced Fabrication Methods for High-Performance Electrodes 3337.2.1 Conventional Hydrophobic PTFE-Bonded Gas Diffusion

Electrode 3347.2.1.1 Catalyst Layer Coating onto Gas Diffusion Layer 3347.2.1.2 Nafion Impregnation 3357.2.1.3 MEA Assembly: Hot Pressing 3367.2.2 Thin-Film Hydrophilic Catalyst Coated Membrane Method 3367.2.2.1 Screen Printing Method 3377.2.2.2 Modified Decal Method 3387.2.2.3 Inkjet Printing Technology 338

Contents IX

7.2.2.4 Direct Spray Coating 3397.2.3 Advanced Fabrication Methods for Low Pt Loading Electrodes 3407.2.3.1 Electrophoretic Deposition 3407.2.3.2 Pulse Electrodeposition 3427.2.3.3 Electrospray 3457.2.3.4 Plasma Sputtering 3457.2.3.5 Sol-Gel Method 3467.2.4 Fabrication of Gas Diffusion Media 3477.2.4.1 Fabrication of Gas Diffusion Layer 3477.2.4.2 Fabrication of Microporous Layer 3477.3 Characterization of PEM Fuel Cell Electrodes 3487.3.1 Surface Morphology Characteristics 3487.3.1.1 Optical Microscopy 3487.3.1.2 Scanning Electron Microscopy 3497.3.1.3 Scanning Probe Microscopy 3527.3.2 Microstructure Analysis 3547.3.3 Physical Characteristics 3567.3.3.1 Porosimetry 3567.3.3.2 Permeability and Gas Diffusivity 3577.3.3.3 Wetting 3587.3.3.4 Conductivity 3607.3.4 Composition Analysis 3617.3.4.1 Energy Dispersive Spectrometry 3617.3.4.2 Thermal Gravimetric Analysis 3627.4 Testing and Diagnosis of PEM Fuel Cell Electrodes 3647.4.1 Electrochemical Techniques 3647.4.1.1 Polarization Curves 3647.4.1.2 Current Interrupt 3657.4.1.3 Electrochemical Impedance Spectroscopy 3667.4.1.4 Other Electrochemical Methods 3697.4.2 Physical and Chemical Methods 3717.4.2.1 Species Distribution Mapping 3717.4.2.2 Temperature Distribution Mapping 3737.4.2.3 Current Distribution Mapping 3747.5 Final Comments 377

References 378

8 Nanostructured Electrodes for Lithium Ion Batteries 383Ricardo Alcántara, Pedro Lavela, Carlos Pérez-Vicente,and José L. Tirado

8.1 Introduction 3838.2 Positive Electrodes: Nanoparticles, Nanoarchitectures,

and Coatings 3848.2.1 Layered Oxides: LiMO2 3848.2.2 Spinel Compounds 385

X Contents

8.2.2.1 Materials for 4 V Electrodes 3858.2.2.2 Materials for Higher Voltages 3868.2.3 Olivine Phosphates 3878.2.4 Silicates 3908.2.5 Transition Metal Fluorides 3928.3 Negative Electrodes 3938.3.1 Intercalation Nanomaterials 3938.3.1.1 Graphene, Fullerene, and Carbon Nanotubes 3938.3.1.2 Titanium Oxides 3968.3.2 Intermetallic Compounds 3978.3.2.1 Tin and Tin Composites 3978.3.2.2 Silicon 4008.3.3 Nanomaterials Obtained In Situ 4018.3.3.1 Binary Oxides for Conversion Electrodes 4018.3.3.2 Multinary Oxides 4048.3.3.3 Transition Metal Oxysalts 4048.4 Concluding Remarks 406

References 407

9 Materials Science Aspects Relevant for High-TemperatureElectrochemistry 415Annika Eriksson, Mari-Ann Einarsrud, and Tor Grande

9.1 Introduction 4159.2 Powder Preparation, Forming Processes, and Sintering

Phenomena 4169.2.1 Powder Processing and Forming Techniques 4169.2.2 Densification, Grain Growth, and Pore Coalescence 4209.2.3 Sintering of Oxide Electrolytes and Ceramic Membrane

Materials 4249.3 Cation Diffusion 4269.3.1 Theoretical Aspects of Cation Diffusion 4269.3.2 Grain Boundary and Bulk Diffusion 4299.3.3 Experimental Methods for Determination of Cation

Diffusion 4309.3.4 Cation Diffusion in Perovskite and Fluorite Oxide

Materials 4339.3.5 Kinetic Demixing and Decomposition 4349.4 Thermomechanical Stability 4379.4.1 Thermal Expansion of Oxide Electrolyte and Mixed

Conducting Ceramics 4379.4.2 Chemical Expansion 4399.4.3 Mechanical Properties 4409.4.4 Degradation Due to Fracture 4429.4.5 Chemical Compatibility of Materials 4439.4.6 High-Temperature Creep 444

Contents XI

9.5 Thermodynamic Stability of Materials 4479.5.1 Phase Decomposition and Solid-State Transformation 4479.5.2 Reactions with Gaseous Species 4509.5.3 Volatilization of Components 453

References 454

10 Oxygen- and Hydrogen-Permeable Dense Ceramic Membranes 467Jay Kniep and Jerry Y.S. Lin

10.1 Introduction 46710.2 Structure of Membrane Materials 46810.2.1 Fluorite Structure 46810.2.2 Perovskite Structure 46910.2.3 Sr4Fe6O13�d and Its Derivatives 47010.2.4 Brownmillerite and Other Perovskite-Related Structures 47010.2.5 Dual-Phase Membranes 47110.3 Synthesis and Permeation Experimental Methods 47110.4 Gas Permeation Models 47310.5 Characteristics of Oxygen-Permeable Membranes 47610.5.1 Electrical and Ionic Transport Properties 47610.5.2 Oxygen Permeation under Air/Inert Gas Gradients 47910.5.3 Oxygen Permeation in Reducing Gases 48210.5.4 Membrane Stability and Mechanical Properties 48310.6 Characteristics of Hydrogen-Permeable Membranes 48410.6.1 Electrical and Ionic Transport Properties 48410.6.2 Hydrogen Permeation under Oxidizing Conditions 48510.6.3 Hydrogen Permeation in Inert Sweep Gases 48710.6.4 Membrane Stability 48910.6.5 Comparison with Oxygen-Permeable Membranes 48910.7 Applications of Membranes 49010.7.1 Gas Separation and Purification 49010.7.2 Membrane Reactors 49110.8 Summary and Conclusions 494

References 495

11 Interfacial Phenomena in Mixed Conducting Membranes:Surface Oxygen Exchange- and Microstructure-Related Factors 501Xuefeng Zhu and Weishen Yang

11.1 Introduction 50111.2 Surface Exchange 50311.2.1 Theoretical Analysis of Surface Effects: One Relevant

Approach 50411.2.2 Modeling Formulas 51011.2.3 Selected Experimental Methods 51411.2.3.1 Isotopic Exchange 51411.2.3.2 Electrical Conductivity Relaxation 516

XII Contents

11.2.4 Reduction and Elimination of Surface ExchangeLimitations 517

11.3 Microstructural Effects in Mixed Conducting Membranes 52011.3.1 Selected Experimental Methods 52011.3.2 Microstructural Phenomena in Perovskite-Type Membranes 52111.3.3 Composite Membranes 52411.3.3.1 Perovskite Membranes with Second Phase or Impurities 52411.3.3.2 Dual-Phase Membranes 52611.3.4 Asymmetric Membranes 52911.4 Thermodynamic and Kinetic Stability 53011.4.1 Surface Limitations 53011.4.2 Microstructures and Kinetic Stability 531

References 532

Index 541

Contents XIII

Preface

Aiming to combine the fundamental information and brief overview on recentadvances in solid-state electrochemistry, this handbook primarily focuses on themostimportantmethodological, theoretical, and technological aspects, novelmaterials forsolid-state electrochemical devices, factors determining their performance andreliability, and their practical applications. Main priority has been given, therefore,to the information that may be of interest to researchers, engineers, and otherspecialists working in this and closely related scientific areas. At the same time,numerous definitions, basic equations and schemes, and reference data are alsoincluded in many chapters to provide necessary introductory information for new-comers to this intriguing field. In general, solid-state electrochemistry is an impor-tant, interdisciplinary, and rapidly developing science that integratesmany aspects ofthe classical electrochemical science and engineering, materials science, solid-statechemistry and physics, heterogeneous catalysis, and other areas of physical chem-istry. This field comprises, but is not limited to, electrochemistry of solid materials,thermodynamics and kinetics of electrochemical reactions involving at least one solidphase, transport of ions and electrons in solids, and interactions between solid,liquid, and/or gaseous phases whenever these processes are essentially determinedby properties of solids and are relevant to the electrochemical reactions. The range ofapplications includes many types of batteries, fuel cells, and sensors, solid-stateelectrolyzers and electrocatalytic reactors, ceramic membranes with ionic or mixedionic–electronic conductivity, accumulators and supercapacitors, electrochromic andmemory devices, processing of new materials with improved properties, corrosionprotection, electrochemical pumps and compressors, and a variety of other appli-ances. Although it has been impossible to cover the rich diversity of solid-stateelectrochemical devices,methods, and processes, the handbook is intended to reflectstate-of-the-art in this scientific area, recent developments, and key research trends.The readers looking for more detailed information on specific aspects and applica-tions may refer to the list of recommended literature [1–23] that includes severalclassical references and recent interdisciplinary and specialized books.

The first volume of the handbook [24], contributed by leading scientists from 11countries, was centered on the general methodology of solid-state electrochemistry,major groups of solid electrolytes and mixed ionic–electronic conductors, andselected applications of the electrochemical cells. Attention was drawn to the general

XV

aspects and perspectives of solid-state electrochemical science and technology(Chapters 1–6), nanostructured solids and electrochemical reactions involving nano-and microparticles in a liquid electrolyte environment (Chapters 4 and 6), insertionelectrodes (Chapter 5), superionics and mixed conductors (Chapters 2 and 7–9),polymer and hybrid materials (Chapters 10 and 11), principles of selected solidelectrolyte devices such as fuel cells and electrochemical pumps (Chapter 12), andsolid-state electrochemical sensors (Chapter 13). The fundamental principles ofmixed conducting membrane operation and bulk transport properties of selectedsingle-phase materials were briefly analyzed in Chapters 3, 9 and 12. This volumeentitled Solid State Electrochemistry II: Electrodes, Interfaces, and Ceramic Membranescontinues in these directions, with a major emphasis on the interface- and surface-related processes, electrode materials and reactions, and selected practical applica-tions of ion-conducting solids.

Opening the second volume, Chapter 1 is dedicated to the ionic memory devicesand related technologies, an emerging area with new horizons for solid-stateelectrochemistry. Chapter 2 presents an overview of composite solid electrolytes,a separate class of ion-conducting materials where the transport properties areessentially governed by interfacial phenomena. Chapters 3 and 4 deal with the keyaspects of theoretical description and analysis of surface and interfacial processes,started in the first volume, again with a special attention on methodology andmodeling. Chapter 5 provides an exhaustive review on the conventional andemerging fuel cell technologies, giving a brief summary on the relevant processes,materials, recent achievements, and future challenges. Continuing this survey,Chapters 6 and 7 are centered on the developments of novel materials and technol-ogies for electrodes of the electrochemical cells with solid oxide electrolytes andpolymer electrolyte membranes, while Chapter 8 briefly reviews the nanostructuredelectrodes for Li-ion batteries. The important aspects of materials science andprocessing technologies, with numerous examples on solid oxide fuel cells andceramicmembranes, are discussed in Chapter 9. Finally, Chapters 10 and 11 presentreviews on themixed conducting ceramicmembranes for gas separation and catalyticreactors, membrane materials, selected models and experimental methods, andinterfacial phenomena governing the membrane performance. All the chapters arewritten by leading international experts from 12 countries, namely, Australia,Canada, China, Hungary, Mexico, Norway, Poland, Portugal, Russia, Singapore,Spain, and the United States. After presenting a brief overview of the handbook, theauthors and the editor trust that readers would find the contents useful, interesting,and stimulating.

References

1 Kr€oger, F.A. (1964) The Chemistryof Imperfect Crystals, North-HollandPublishing Company, Amsterdam.

2 Kofstad, P. (1972) Nonstoichiometry,Diffusion, and Electrical Conductivity of

Binary Metal Oxides, Wiley-Interscience,New York.

3 Geller, S. (ed.) (1977) Solid Electrolytes,Springer, Berlin.

XVI Preface

References XVII

4 Takahashi, T. and Kozawa, A. (eds) (1980)Applications of Solid Electrolytes, JEC Press,Cleveland, OH.

5 Rickert, H. (1982) Electrochemistry ofSolids: An Introduction, Springer, Berlin.

6 Chebotin, V.N. (1989) Chemical Diffusionin Solids, Nauka, Moscow.

7 Bruce, P.G. (ed.) (1995) Solid StateElectrochemistry, Cambridge UniversityPress, Cambridge.

8 Gellings, P.J. and Bouwmeester, H.J.M.(eds) (1997) Handbook of Solid StateElectrochemistry, CRC Press, Boca Raton,FL.

9 Allnatt, A.R. and Lidiard, A.B. (2003)Atomic Transport in Solids, CambridgeUniversity Press, Cambridge.

10 Bard, A.J., Inzelt, G., and Scholz, F. (eds)(2008) Electrochemical Dictionary,Springer, Berlin.

11 West, A.R. (1984)Solid State Chemistry andIts Applications, John Wiley & Sons, Ltd,Chichester.

12 Goto, K.S. (1988) Solid StateElectrochemistry and Its Applications toSensors and Electronic Devices, Elsevier,Amsterdam.

13 Schmalzried, H. (1995) Chemical Kineticsof Solids, Wiley-VCH Verlag GmbH,Weinheim.

14 Munshi, M.Z.A. (ed.) (1995) Handbook ofSolid State Batteries and Capacitors, WorldScientific, Singapore.

15 Vayenas, C.G., Bebelis, S., Pliangos, C.,Brosda, S., and Tsiplakides, D. (2001)Electrochemical Activation of Catalysis:

Promotion, Electrochemical Promotion, andMetal-Support Interaction, Kluwer/Plenum, New York.

16 Alkire, Richard C. and Kolb, Dieter M.(eds) (2002) Advances in ElectrochemicalScience and Engineering, vol. 8, Wiley-VCHVerlag GmbH,Weinheim.

17 Hoogers, G. (ed.) (2003) Fuel CellTechnology Handbook, CRC Press, BocaRaton, FL.

18 Wieckowski, A.,Savinova, E.R., andVayenas, C.G. (eds) (2003)Catalysis andElectrocatalysis at Nanoparticle Surfaces,Marcel Dekker, New York.

19 Balbuena, P.B. and Wang, Y. (eds) (2004)Lithium-Ion Batteries: Solid-ElectrolyteInterphase, Imperial College Press,London.

20 Sammes, N. (ed.) (2006) Fuel CellTechnology: Reaching TowardsCommercialization, Springer, London.

21 Monk, P.M.S., Mortimer, R.J., andRosseinsky, D.R. (2007) Electrochromismand Electrochromic Devices, 2nd edn,Cambridge University Press, Cambridge.

22 Zhuiykov, S. (2007) Electrochemistry ofZirconia Gas Sensors, CRC Press, BocaRaton, FL.

23 Li, K. (2007) Ceramic Membranes forSeparation and Reaction, John Wiley &Sons, Ltd, Chichester.

24 Kharton, V. (ed.) (2009) Solid StateElectrochemistry I: Fundamentals, Materialsand Their Applications,Wiley-VCH VerlagGmbH, Weinheim.

Vladislav V. KhartonUniversity of Aveiro, Portugal

List of Contributors

XIX

Ricardo AlcántaraUniversidad de CórdobaLaboratorio de Química InorgánicaEdificio Marie Curie C3Campus de Rabanales14071 CórdobaSpain

An ChenGlobalFoundriesApt. P6, 260 N. Mathilda AvenueSunnyvale, CA 94086USA

Wei DaiNational Research Council CanadaInstitute for Fuel Cell Innovation4250 Wesbrook MallVancouver, BCCanada V6T 1W5

Mari-Ann EinarsrudNorwegian University of Science andTechnologyDepartment of Materials Science andEngineering7491 TrondheimNorway

Annika ErikssonNorwegian University of Science andTechnologyDepartment of Materials Science andEngineering7491 TrondheimNorway

Vilmos GáspárUniversity of DebrecenInstitute of Physical ChemistryP.O. Box 74010 DebrecenHungary

Tor GrandeNorwegian University of Science andTechnologyDepartment of Materials Science andEngineering7491 TrondheimNorway

San Ping JiangCurtin University of TechnologyDepartment of Chemical EngineeringCurtin Centre for Advanced EnergyScience and Engineering1 Turner AvenuePerth, WA 6845Australia

Vladislav V. KhartonUniversity of AveiroCICECO, Department of Ceramics andGlass EngineeringCampus de Santiago, 3810–193 AveiroPortugal

István Z. KissSaint Louis UniversityDepartment of Chemistry3501 Laclede AvenueSt. Louis, MO 63103USA

Jay KniepArizona State UniversityDepartment of Chemical EngineeringEngineering CenterG Wing 301Tempe, AZ 85287–6006USA

Pedro LavelaUniversidad de CórdobaLaboratorio de Química InorgánicaEdificio Marie Curie C3Campus de Rabanales14071 CórdobaSpain

Hui LiNational Research Council CanadaInstitute for Fuel Cell Innovation4250 Wesbrook MallVancouver, BCCanada V6T 1W5

Jerry Y.S. LinArizona State UniversityDepartment of Chemical EngineeringEngineering CenterG Wing 301Tempe, AZ 85287-6006USA

Timea NagySaint Louis UniversityDepartment of Chemistry3501 Laclede AvenueSt. Louis, MO 63103USA

Carlos Pérez-VicenteUniversidad de CórdobaLaboratorio de Química InorgánicaEdificio Marie Curie C3Campus de Rabanales14071 CórdobaSpain

Orest PizioUniversidad Nacional Autónoma deMéxicoInstituto de QuímicaCoyoacán04510 México, DFMexico

Stefan SokołowskiMaria Curie-Sklodowska UniversityDepartment for Modelling ofPhysicochemical ProcessesGliniana 33, 20031 LublinPoland

José L. TiradoUniversidad de CórdobaLaboratorio de Química InorgánicaEdificio Marie Curie C3Campus de Rabanales14071 CórdobaSpain

Ekaterina V. TsipisInstituto Tecnológico e NuclearEstrada Nacional 102686-953 SacavémPortugal

XX List of Contributors

Nikolai F. UvarovInstitute of Solid State Chemistry andMechanochemistrySiberian Branch of the RussianAcademy of SciencesKutateladze 18630128 NovosibirskRussia

Haijiang WangNational Research Council CanadaInstitute for Fuel Cell Innovation4250 Wesbrook MallVancouver, BCCanada V6T 1W5

Xin WangNanyang Technological UniversitySchool of Chemical and BiomedicalEngineering639798 SingaporeSingapore

Jinfeng WuNational Research Council CanadaInstitute for Fuel Cell Innovation4250 Wesbrook MallVancouver, BCCanada V6T 1W5

Weishen YangChinese Academy of SciencesDalian Institute of Physical ChemistryState Key Laboratory of Catalysis457 Zhongshan Road116023 DalianChina

Xuefeng ZhuChinese Academy of SciencesDalian Institute of Physical ChemistryState Key Laboratory of Catalysis457 Zhongshan Road116023 DalianChina

List of Contributors XXI

1Ionic Memory TechnologyAn Chen

Ionic memory devices based on ion migration and electrochemical reactions haveshown promising characteristics for next-generation memory technology. Bothcations (e.g., Cuþ, Agþ) and anions (e.g., O2�) may contribute to a bipolar resistiveswitching phenomenon that can be utilized to make nonvolatile memory devices.With simple two-terminal structures, these devices can be integrated into CMOS(complementary metal–oxide–semiconductor) architecture or fabricated with novelarchitectures (e.g., crossbar arrays or 3D stackable memory). Large memory arraysmade with standard CMOS process have been demonstrated in industry R&D.Although ionic memory technology has seen significant progress recently, somechallenges still exist in device reliability and controllability. Ionic memories maypresent a promising candidate for stand-alone and storage class memoryapplications.

1.1Introduction

With flash memories quickly approaching their scaling limit, numerous novelmemory technologies have emerged as candidates for next-generation nonvolatilememories. Examples include phase change memory (PCM), magnetic randomaccess memory (MRAM), ferroelectric RAM (FeRAM), resistive switching memory(also known as RRAM or resistive random access memory), polymer-basedmemory,molecular memory, and so on [1–3]. Figure 1.1 shows a classification of variousmemories presented by the International Technology Roadmap of Semiconductor(ITRS) [1]. Static random access memory (SRAM) and dynamic random accessmemory (DRAM) are called volatilememories because information stored in thesememories cannot be retained when power is turned off. On the other hand,nonvolatile memories are able to retain information for a long period of time afterpower is turned off. A typical requirement of data retention is 10 years at roomtemperature. The mainstream nonvolatile memory in the market today is flashmemory, which is divided into two categories, NAND and NOR, based on two

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different memory architectures. NOR flash memories are preferred for code storagebecause of their random access capability, while NAND flash memories are moresuitable for data storage due to their sequential access in a block of data. These threetypes of mature memory technologies – SRAM, DRAM, and flash memory – are allbased on Si complementary metal–oxide–semiconductor (CMOS) technology. Theirdevelopment has followed the so-called Moores law; that is, the transistor density inthe integrated circuits has doubled approximately every 2 years. This is achieved byshrinking the size of the Si CMOS transistors, a trend that has successfully continuedfor several decades. When transistor size is reduced, not only more bits of infor-mation can be stored on the same area but also better device/circuit performance canbe achieved, for example, fast speed and lower power consumption.

However, with the transistor size being reduced to 22 nm and below, Si CMOStechnology today is facing some fundamental challenges. Although power consumedby each transistor has decreased with scaling, the overall power density of the wafershas increased because of growing transistor density. Increasing power densityinducesmore Joule heating and raises wafer temperature, which degrades transistorperformance. The wafer temperature today is reaching the limit of practical coolingtechniques, constraining further scaling of transistor size. Although it is believed thatSi CMOS technology can be scaled down to 22 nm, it is not clear howmuch further itcan go. These mainstream memory technologies based on Si CMOS are facing thesame obstacles. Therefore, it has become increasingly important to explore alterna-tive nonvolatile memory technologies that may potentially replace Si-based mem-ories when they reach their limits.

Memory

Volatile Nonvolatile

SRAM DRAM Mature EmergingPrototypical

Flash

NOR

NAND

Charge trapping

FeRAM

MRAM

PCM

RRAM

Molecular

Nano

mechanical

Polymer

Ionic

Thermal

Electronic

Figure 1.1 Classification of memory technologies based on information in ITRS roadmap. Theabbreviations are explained in the text.

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Among these emergingmemory devices, resistive switchingmemory (i.e., RRAM)is a broad category involving a large variety ofmaterials and switching characteristics.These memory devices are usually made in a two-terminal metal–insulator–metal(MIM) structure. They can be electrically switched between a high-resistance state(HRS) and a low-resistance state (LRS), and both states can be nonvolatile. Com-monly used terminology refers LRS as the on state and HRS as the off state. Binarydigital data can be recorded in these resistance states, for example, LRS for logic 0and HRS for logic 1. The HRS-to-LRS switching is called program (or write,set) and the LRS-to-HRS switching erase (or reset). Promising characteristicshave been reported on these devices. However, inmany reports of resistive switchingmemories the switching mechanisms are not clearly understood. Some hints of theswitching mechanism may be found in switching characteristics, for example,current–voltage (I–V) relationships, the voltage polarity dependence of switching,the presence or absence of forming processes, the effect of the electrodes on theswitching properties, device size dependence, temperature effect, variation intransport properties, cycling stability, and so on. Unfortunately, systematic studyon all these aspects is still lacking for many resistive switching materials, andcontroversial interpretations of the switching mechanism are widely presented inthe literature.

A coarse-grained classification has been proposed to divide resistive switchingmemories into three types based on the nature of the dominating switchingprocesses: electronic effect, thermal effect, and ionic effect. In electronic effect resistiveswitching memories, some electronic processes (e.g., charge trapping, Mott meta-l–insulator transition, or ferroelectric polarization reversal) alter the band structureand transport properties in the bulk or at the interface and trigger resistance changes.Thermal effect resistive switching memories are related to electric power-inducedJoule heating and often involve the formation and rupture of some localizedconduction paths in an insulating material.

The third type, ionic effect resistive switching memories, involves the transportand electrochemical reactions of cations (e.g., Agþ , Cuþ ) or anions (e.g., O2�). Theswitching is usually bipolar; that is, programming and erasing are in opposite voltagepolarities. This is because the switching between LRS andHRS is realized by drivingcharged ions in opposite directions to induce different electrochemical reactions. Theswitching process related to the migration and reaction of cations is well understood,and the switching process can be captured in microscopic observations. However,resistive switching process involving anions is less well understood, with many openquestions regarding the details of the anion transport and electrochemical redoxreactions. The discussion of ionic memories in this chapter will mainly focus onthese memory devices based on cation migration and reactions. The resistiveswitching mechanisms and materials involving anions, mainly oxygen ions orvacancies, will also be briefly discussed.

This chapter is organized in the following sections. Section 1.2 discusses the ionicresistive switching mechanisms, followed by a review of materials used in thesedevices in Section 1.3. Electrical characteristics of ionic memories, includingindividual device properties and memory array statistics, are summarized in

1.1 Introduction j3

Section 1.4. Section 1.5 addresses issues in the architecture design of ionic mem-ories. In Section 1.6, challenges of ionic memories are discussed. Section 1.7provides some information of potential applications of ionic memories. Finally, thechapter ends with a brief summary in Section 1.8.

1.2Ionic Memory Switching Mechanisms

The switching mechanisms of cation-based devices and anion-based memories aredifferent [3]. The resistance change in the cation-based devices is due to theelectrochemical formation and dissolution of metallic filaments, which can beobserved in sufficiently large devices in well-designed experiments [4–10]. Foranion-based memories, the switching is generally believed to be triggered by thetransport of oxygen ions/vacancies and some redox processes; however, the exactprocess is still not clear. In some cases, it is even unclear which ions are involved inthe switching process and whether the device falls into the category of anion-basedionic memories.

1.2.1Cation-Based Resistive Switching Mechanism

In the MIM structure of cation-based ionic memories, one of the two electrodes ismade of electrochemically activematerials and the other electrode is inert (e.g., Au orPt). Inmost reported cation-based ionicmemory devices, the active electrode is eitherAg or Cu. The two electrodes are separated by a solid-state electrolyte I layer, inwhich cations can transport with themobilitymuch higher than that in regular solid-state materials. These solid electrolytes are sometimes called superionic materials(see Chapters 2 and 7 of the first volume). A typical switching process is illustrated inFigure 1.2. The solid-state electrolytes normally have high resistance initially and areconsidered to be insulators (Figure 1.2a). When positive voltage is applied to theactive electrode, as the active electrode (acting as anode in this voltage configu-ration) is made of electrochemically active materials, metal atoms of the anode areoxidized and dissolved into the solid electrolyte. These metal cations migrate towardthe cathode under electrical field and are reduced there. Therefore, under electricalfield, oxidation and reduction reactions take places at the anode and cathode,

respectively: Mþ þ e��! �reductionoxidation M. The reduced metal atoms form metal filamentsthat grow from the cathode toward the anode. When the anode and cathode areconnected by completemetal filament(s), theMIMdevice switches fromHRS to LRS(Figure 1.2b). When voltage polarity on the two electrodes is reversed, metal atomsdissolve at the edge of the metal filament(s) and eventually break the conductivefilament(s) between the anode and the cathode. Current-induced Joule heating mayalso contribute to the rupture of the filament(s). Consequently, the MIM device isswitched back to a high-resistance state (Figure 1.2c). Note that the metal filament(s)

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(a) Initial state

Inert electrode

Active electrode

(b) ON state

Active electrode

Inert electrode

(c) OFF state

Active electrode

Inert electrode

Figure 1.2 Schematic illustration of cation-based ionic resistive switching process.

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need to be brokenonly partially to cause significant increase in resistance, and it is notnecessary to completely annihilate themetalfilament(s) from the I layer. As a result,theHRS off state during repeated switching processesmay not be as insulating as theoriginal state; however, sufficiently high on/off ratio between the LRS and the HRScan still be achieved. When the active electrode is positively biased again, the metalfilament(s) can be repaired by the same cation migration and redox reactions. TheHRS-to-LRS and LRS-to-HRS switching processes can be repeated continuously.Since the electrochemical reactions ideally do not cause significant damage to theMIM structure, the switching process may in principle work for many cycles.

From the switching process described above, it is clear that cation-based ionicmemories have to be bipolar; that is, programming and erasing processes have to bedone with opposite voltage polarities. This bipolar switching is considered one of thesignatures of ionic memories. Another key feature of ionic memories is thelocalized conduction path of metal filament(s), which has been suggested as anevidence of excellent scalability of ionic memories. In principle, ionic memorydevices can be made as small as one atomic chain of metal atoms. It is also easy tounderstand that the formation of these filaments is a self-limiting process. As soonas one conductive filament is formed, resistance of the I layer reduces dramat-ically, which results in significant decrease in electric field and the chance offorming additional filaments.

Different names have been given to the cationmigration-based resistive switchingdevices, such as atomic switch [11], programmable metallization cell [7],nanoBridge [8], solid-state electrolyte memory [12], conductive bridging RAM[13], and so on.

1.2.2Anion-Based Resistive Switching Mechanism

Inmany oxides, especially transitionmetal oxides, oxygen ions or vacancies aremuchmoremobile than cations. Themigration of oxygen ions and vacanciesmay introduceredox reaction at the electrode or doping effects in the metal oxides, which may alterthe transport properties of the structure and cause resistance changes. Numerousmodels have been proposed to describe the details of the switching processesinvolving oxygen ions and vacancies; however, the exact microscopic processes arestill not clear. The following are a few examples of resistive switching phenomena thathave been suggested to be caused by oxygen ions or vacancies.

A single crystal of 0.2 mol% Cr-doped SrTiO3 is found to switch from an initiallyinsulating state to a conductive state after it is exposed to an electrical field of105Vcm�1 for about 30min, a process known as conditioning (or forming).High-temperature hot spots and high concentration of oxygen vacancies are bothfound close to the anode, by infrared thermal microscopy and laterally resolvedmicro-X-ray absorption spectroscopy, respectively. It is suggested that the condition-ing process introduces a path of oxygen vacancies in thememory, which provides freecarriers in the Ti 3d band and leads to metallic conduction. The Cr dopants play the

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