the crc handbook of solid state electrochemistry 1997 - gellings

The CRC Handbook of Solid State Electrochemistry Edited by P.J Gellings and H.J.M Bouwmeester University of Twente Laboratory for Inorganic Materials Science Enschede, The Netherlands CRC Press Boca Raton New York London Tokyo Acquiring Editor: Project Editor: Marketing Manager: Direct Marketing Manager: Cover design: PrePress: Manufacturing: Felicia Shapiro Gail Renard Arline Massey Becky McEldowney Denise Craig Kevin Luong Sheri Schwartz Library of Congress Cataloging-in-Publication Data The CRC handbook of solid state electrochemistry / edited by P.J Gellings and H.J.M Bouwmeester p cm Includes bibliographical references and index ISBN 0-8493-8956-9 Solid state chemistry—Handbooks, manuals, etc I Gellings, P.J II Bouwmeester, H.J.M QD478.C74 1996 541.3′7—dc20 96-31466 CIP This book contains information obtained from authentic and highly regarded sources Reprinted material is quoted with permission, and sources are indicated A wide variety of references are listed Reasonable efforts have been made to publish reliable data and information, but the author and the publisher cannot assume responsibility for the validity of all materials or for the consequences of their use Neither this book nor any part may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, microfilming, and recording, or by any information storage or retrieval system, without prior permission in writing from the publisher All rights reserved Authorization to photocopy items for internal or personal use, or the personal or internal use of specific clients, may be granted by CRC Press, Inc., provided that $.50 per page photocopied is paid directly to Copyright Clearance Center, 27 Congress Street, Salem, MA 01970 USA The fee code for users of the Transactional Reporting Service is ISBN 0-8493-8956-9/97/$0.00+$.50 The fee is subject to change without notice For organizations that have been granted a photocopy license by the CCC, a separate system of payment has been arranged The consent of CRC Press does not extend to copying for general distribution, for promotion, for creating new works, or for resale Specific permission must be obtained in writing from CRC Press for such copying Direct all inquiries to CRC Press, Inc., 2000 Corporate Blvd., N.W., Boca Raton, Florida 33431 © 1997 by CRC Press, Inc No claim to original U.S Government works International Standard Book Number 0-8493-8956-9 Library of Congress Card Number 96-31466 Printed in the United States of America Printed on acid-free paper Copyright © 1997 by CRC Press, Inc ABOUT THE EDITORS Prof Dr P.J Gellings After studying chemistry at the University of Leiden (the Netherlands), Prof Gellings received his degree in physical chemistry in 1952 Subsequently he worked as research scientist in the Laboratory of Materials Research of Werkspoor N.V (Amsterdam, the Netherlands) He obtained his Ph.D degree from the University of Amsterdam in 1963 on the basis of a dissertation titled: “Theoretical considerations on the kinetics of electrode reactions.” In 1964 Prof Gellings was appointed professor of Inorganic Chemistry and Materials Science at the University of Twente His main research interests were coordination chemistry and spectroscopy of transition metal compounds, corrosion and corrosion prevention, and catalysis In 1991 he received the Cavallaro Medal of the European Federation Corrosion for his contributions to corrosion research In 1992 he retired from his post at the University, but has remained active as supervisor of graduate students in the field of high temperature corrosion Dr H.J.M Bouwmeester After studying chemistry at the University of Groningen (the Netherlands), Dr Bouwmeester received his degree in inorganic chemistry in 1982 He received his Ph.D degree at the same university on the basis of a dissertation titled: “Studies in Intercalation Chemistry of Some Transition Metal Dichalcogenides.” For three years he was involved with industrial research in the development of the ion sensitive field effect transistor (ISFET) for medical application at Sentron V.O.F in the Netherlands In 1988 Dr Bouwmeester was appointed assistant professor at the University of Twente, where he heads the research team on Dense Membranes and Defect Chemistry in the Laboratory of Inorganic Materials Science His research interests include defect chemistry, orderdisorder phenomena, solid state thermodynamics and electrochemistry, ceramic surfaces and interfaces, membranes, and catalysis He is involved in several international projects in these fields Copyright © 1997 by CRC Press, Inc CONTRIBUTORS Isaac Abrahams Department of Chemistry Queen Mary and Westfield College University of London London, United Kingdom Symeon I Bebelis Department of Chemical Engineering University of Patras Patras, Greece Henny J.M Bouwmeester Laboratory for Inorganic Materials Science Faculty of Chemical Technology University of Twente Enschede, The Netherlands Peter G Bruce School of Chemistry University of St Andrews St Andrews, Fife, United Kingdom Anthonie J Burggraaf Laboratory for Inorganic Materials Science Faculty of Chemical Technology University of Twente Enschede, The Netherlands Hans de Wit Materials Institute Delft Delft University of Technology Faculty of Chemical Technology and Materials Science Delft, The Netherlands Pierre Fabry Université Joseph Fourier Laboratoire d’Electrochimie et de Physicochimie des Matériaux et Interfaces (LEPMI) Domaine Universitaire Saint Martin d’Hères, France Heinz Gerischer‡ Scientific Member Emeritus of the Fritz Haber Institute Department of Physical Chemistry Fritz-Haber-Institut der Max-PlanckGesellschaft Berlin, Germany Claes G Granqvist Department of Technology Uppsala University Uppsala, Sweden Jacques Guindet Université Joseph Fourier Laboratoire d’Electrochimie et de Physicochimie des Matériaux et Interfaces (LEPMI) Domaine Universitaire Saint Martin d’Hères, France Abdelkader Hammou Université Joseph Fourier Laboratoire d’Electrochimie et de Physicochimie des Matériaux et Interfaces (LEPMI) Domaine Universitaire Saint Martin d’Hères, France Christian Julien Laboratoire de Physique des Solides Université Pierre et Marie Curie Paris, France Tetsuichi Kudo Institute of Industrial Science University of Tokyo Tokyo, Japan Thijs Fransen Laboratory for Inorganic Materials Science University of Twente Enschede, The Netherlands Janusz Nowotny Australian Nuclear Science & Technology Organisation Advanced Materials Program Lucas Heights Research Laboratories Menai, Australia Paul J Gellings Laboratory for Inorganic Materials Science Faculty of Chemical Technology University of Twente Enschede, The Netherlands Ilan Riess Physics Department Technion — Israel Institute of Technology Haifa, Israel ‡ Deceased Copyright © 1997 by CRC Press, Inc Joop Schoonman Laboratory for Applied Inorganic Chemistry Delft University of Technology Faculty of Chemical Technology and Materials Science Delft, The Netherlands Elisabeth Siebert Université Joseph Fourier Laboratoire d’Electrochimie et de Physicochimie des Matériaux et Interfaces (LEPMI) Domaine Universitaire Saint Martin d’Hères, (France) Copyright © 1997 by CRC Press, Inc Constantinos G Vayenas Department of Chemical Engineering University of Patras Patras, Greece Werner Weppner Chair for Sensors and Solid State Ionics Technical Faculty, Christian-Albrechts University Kiel, Germany IN MEMORIAM Heinz Gerischer 1919–1994 On September 14, 1994, Professor Heinz Gerischer died from heart failure With his death, the international community of electrochemistry lost the man who most probably was its most eminent representative Professor Gerischer was one of the founders of modern electrochemistry, having contributed to nearly all modern extensions and improvements of this science He was born in 1919 and studied chemistry at the University of Leipzig from 1937 to 1944, presenting his Ph.D thesis, under the supervision of Professor Bonhoeffer, in 1946 He worked throughout Germany, was professor of physical chemistry at the Technical University–Munich, and director of the Fritz-Haber-Institut der Max-Planck-Gesellschaft in Berlin He made great contributions to the kinetics of electrode reactions and to the electrochemistry at semiconductor surfaces He also initiated the application of a wide range of modern experimental methods to the study of electrochemical reactions, including nonelectrochemical techniques such as optical and electron spin resonance spectroscopy, and advocated the use of synchroton radiation in surface research His scientific work was published in more than 300 publications and was notable for its great originality, clarity of exposition, and high quality We are grateful that we can publish as Chapter of this handbook, what may be Professor Gerischer’s last publication, in which he again shows his ability to give a very clear exposition of the basic principles of modern electrochemistry Copyright © 1997 by CRC Press, Inc PREFACE The idea for this book arose out of the realization that, although excellent surveys and handbooks of electrochemistry and of solid state chemistry are available, there is no single source covering the field of solid state electrochemistry Moreover, as this field gets only limited attention in most general books on electrochemistry and solid state chemistry, there is a clear need for a handbook in which attention is specifically directed toward this rapidly growing field and its many applications This handbook is meant to provide guidance through the multidisciplinary field of solid state electrochemistry for scientists and engineers from universities, research organizations, and industries In order to make it useful for a wide audience, both fundamentals and applications are discussed, together with a state-of-the-art review of selected applications As is true for nearly all fields of modern science and technology, it is impossible to treat all subjects related to solid state electrochemistry in a single textbook, and choices therefore had to be made In the present case, the solids considered are mainly confined to inorganic compounds, giving only limited attention to fields like polymer electrolytes and organic sensors The editors thank all those who cooperated in bringing this project to a successful close In the first place, of course, we thank the authors of the various chapters, but also those who advised us in finding these authors We are also grateful to the staff of CRC Press — in particular associate editor Felicia Shapiro and project editor Gail Renard, who were of great assistance to us with their help and experience in solving all kinds of technical problems It is a great loss for the whole electrochemical community that Professor Heinz Gerischer died suddenly in September 1994 and we remember with gratitude his great services to electrochemistry We consider ourselves fortunate to be able to present as Chapter of this handbook one of his last important contributions to this field P.J Gellings H.J.M Bouwmeester Copyright © 1997 by CRC Press, Inc TABLE OF CONTENTS Chapter Introduction Henny J.M Bouwmeester and Paul J Gellings Chapter Principles of Electrochemistry Heinz Gerischer Chapter Solid State Background Isaac Abrahams and Peter G Bruce Chapter Interface Electrical Phenomena in Ionic Solids Janusz Nowotny Chapter Defect Chemistry in Solid State Electrochemistry Joop Schoonman Chapter Survey of Types of Solid Electrolytes Tetsuichi Kudo Chapter Electrochemistry of Mixed Ionic–Electronic Conductors Ilan Riess Chapter Electrodics Ilan Riess and Joop Schoonman Chapter Principles of Main Experimental Methods Werner Weppner Chapter 10 Electrochemical Sensors Pierre Fabry and Elisabeth Siebert Chapter 11 Solid State Batteries Christian Julien Chapter 12 Solid Oxide Fuel Cells Abdelkader Hammou and Jacques Guindet Copyright © 1997 by CRC Press, Inc Chapter 13 Electrocatalysis and Electrochemical Reactors Constantinos G Vayenas and Symeon I Bebelis Chapter 14 Dense Ceramic Membranes for Oxygen Separation Henny J.M Bouwmeester and Anthonie J Burggraaf Chapter 15 Corrosion Studies Hans de Wit and Thijs Fransen Chapter 16 Electrochromism and Electrochromic Devices Claes G Granqvist Copyright © 1997 by CRC Press, Inc Chapter INTRODUCTION Henny J M Bouwmeester and Paul J Gellings I Introduction II General Scope III Elementary Defect Chemistry A Types of Defects B Defect Notation C Defect Equilibria IV Elementary Considerations of the Kinetics of Electrode Reactions References I INTRODUCTION As in aqueous electrochemistry, research interest in the field of solid state electrochemistry can be split into two main subjects: Ionics: in which the properties of electrolytes have the central attention Electrodics: in which the reactions at electrodes are considered Both fields are treated in this handbook This first chapter gives a brief survey of the scope and contents of the handbook Some elementary ideas about these topics, which are often unfamiliar to those entering this field, are introduced, but only briefly In general, textbooks and general chemical education give only minor attention to elementary issues such as defect chemistry and kinetics of electrode reactions Ionics in solid state electrochemistry is inherently connected with the chemistry of defects in solids, and some elementary considerations about this are given in Section III Electrodics is inherently concerned with the kinetics of electrode reactions, and therefore some elementary considerations about this subject are presented in Section IV In an attempt to lead into more professional discussions as provided in subsequent chapters, some of these considerations are presented in this first chapter II GENERAL SCOPE The distinction made between ionics and electrodics is translated into detailed discussions in various chapters on the following topics: • • electrochemical properties of solids such as oxides, halides, cation conductors, etc., including ionic, electronic, and mixed conductors electrochemical kinetics and mechanisms of reactions occurring on solid electrolytes, including gas-phase electrocatalysis Copyright © 1997 by CRC Press, Inc 8956ch16.fm Page 601 Monday, October 11, 2004 2:39 PM FIGURE 16.14 Inserted charge per unit area and intercalation/deintercalation cycle during long-term testing of three types of electrochromic display-type devices Dots indicate measured data, and lines were drawn for convenience Arrow indicates an initial value of the charge/area (From Granqvist, C., Handbook of Inorganic Electrochromic Materials, Elsevier Science, 1995 With permission) FIGURE 16.15 Spectral transmittance at three different states of coloration for an electrochromic device of the type shown in the inset (From Granqvist, C., Handbook of Inorganic Electrochromic Materials, Elsevier Science, 1995 With permission.) transmittance of the device in fully colored and bleached states and at intermediate coloration At λ = 0.55 µm, for example, the transmittance can be varied between ~75 and 13% The decrease of the transmittance at λ = µm, irrespective of coloration, is caused by reflectance from the ITO layers The transmitting devices normally are much larger than the reflecting display-type devices, and for the former ones the size itself, and the electrical properties of the TCs, become of critical importance for the response dynamics Figure 16.16 shows the change of the luminous transmittance during one c/b cycle in which a voltage step of ±1 V was applied between ITO layers with a resistance/square of about 10 Ω The smallest device with × cm2 area has a response time of 106 cycles was obtained for some devices.56 Devices using layers of H3OUO2PO4· 3H2O (hydrogen uranyl phosphate, HUP) as a solid ion conductor have been discussed by Howe et al.59 A typical arrangement embodied two glass plates with ITO layers, each having a ~1-µm-thick evaporated W oxide film Intervening HUP was produced by gently pressing a precipitating HUP solution so that a ~100-µm-thick pale yellowish to transparent layer was formed Such devices were tested with charge insertion/extraction of to 10 mC/cm2, and switching times down to 0.3 s were noted Durability up to × 105 c/b cycles was reported HUP was employed also by Takahashi et al.,60 who used flexible to mm thick tablets of this material, pressed together with Teflon® powder, between an evaporated W oxide film and an Ag CoE Up to 16 mC/cm2 was inserted Durability was found for as much as × 106 c/b cycles Antimony oxides have been used in electrochromic prototype devices Thus Lagzdons et al.61 employed HSbO3·2H2O interfaced between two W oxide films, or between one film of W oxide and another film of Ni oxide, and Matsudaira et al.62 studied display-type devices incorporating a white mixture of Sb2O5·pH2O and Sb2O3 Apart for an evaporated W oxide film on ITO-coated glass, the entire unit in the latter work was manufactured by screen Copyright © 1997 by CRC Press, Inc 8956ch16.fm Page 604 Monday, October 11, 2004 2:39 PM FIGURE 16.18 Change in optical density as a function of time for an electrochromic device of the type shown in the inset The data are based on measured reflectance R (From Granqvist, C., Handbook of Inorganic Electrochromic Materials, Elsevier Science, 1995 With permission.) printing These devices withstood c/b cycling at ±1.3 V for >106 times Recently, work by Kuwabara et al.63,64 considered devices with an evaporated W oxide film, a proton conductor of Sb2O5·2H2O, and a graphite CoE The Sb oxide was spray deposited or applied by electrophoresis onto both W oxide and graphite prior to device assembly Among the aprotic bulk-type ion conductors, work has been reported on Na+ conducting Na2O·11Al2O3 (Na-β-alumina)65 and Na1+xZr2SixP3–xO12 (NASICON).66 Devices with β-alumina required heating to >70°C in order to operate, and devices with NASICON were found to be unstable Generally speaking, thin-film ion conductors are more promising and versatile than bulktype ion conductors, and electrochromic devices based on the former class of materials are considered next Numerous thin films have been used It is convenient to start with thin dielectric films incorporating some water, since they can be used in devices that are structurally simple The inset of Figure 16.19 illustrates a typical design with a glass substrate coated with four superimposed layers: a TC such as ITO, electrochromic W oxide, watercontaining dielectric such as MgF2·H2O, and a semitransparent top layer of Au Initial work on this kind of device was reported by Deb67 and others;68 they have subsequently become known as “Deb devices” The most critical part of a Deb device is its water-containing layer It must be porous, in which case water adsorption takes place spontaneously upon exposure to a humid ambience Detailed studies have been reported for porous dielectric films of MgF2,69-73 SiOx,74,75 LiF,69,76 Cr2O3,77,78 Ta2O5,79 and of some other materials For the top layer, 0.01- to 0.02-µm-thick Au films have been used almost universally; this limits the peak transmittance to ~50% Figure 16.19 shows spectral transmittance from Svensson and Granqvist73 through a Deb device with 0.15 µm of evaporated W oxide, about 0.1 µm of MgF evaporated in the presence of × 10–4 Torr of air, and ~0.015 µm of Au In fully bleached state, the transmittance has a peak value of ~50% at λ ≈ 0.52 µm, which is in excellent agreement with results from Benson et al.80 for a similar device Optical changes are small at low applied voltages, but increase rapidly when a “critical” voltage of ~1.3 V is exceeded This effect is related to the decomposition of water into H+ + OH– and ensuing proton insertion into the W oxide film There is a simultaneous electrochemical oxidation at the Au electrode At voltages >1.8 V, O2 gas is evolved, and at a reverse bias exceeding 0.9 V, H2 gas is evolved Gas formation can lead to morphological changes as well as to film delamination,71 and should be avoided in practical implementations The importance of a large film porosity is demonstrated in Figure 16.20, which shows data from Deneuville et al.70 on the c/b dynamics of a Deb device with 0.3 µm of W oxide, Copyright © 1997 by CRC Press, Inc 8956ch16.fm Page 605 Monday, October 11, 2004 2:39 PM FIGURE 16.19 Spectral transmittance at two different states of coloration for a Deb-type electrochromic device of the type shown in the inset (From Granqvist, C., Handbook of Inorganic Electrochromic Materials, Elsevier Science, 1995 With permission.) FIGURE 16.20 Change in optical density during coloration and bleaching for a Deb device of the type shown in Figure 16.19 The W oxide and MgF2 layers were evaporated at the pressures shown The electric field was reversed after 32 s (From Granqvist, C., Handbook of Inorganic Electrochromic Materials, Elsevier Science, 1995 With permission.) 0.05 µm of MgF 2, and 0.015 µm of Au The films of W oxide and MgF were evaporated at the shown pressures of ambient air, and the device was operated at (±3 V, 0.015 Hz) It is seen that the coloration increases gradually with a time constant of ~10 s and reaches a limiting value that increases in proportion with the gas pressure Bleaching is faster and is completed during the course of a few seconds The decisive factor for the ultimate coloration is the amount of incorporated water rather than the porosity as such Deb devices incorporating MgF2, SiOx, and LiF have been shown to withstand ~104 c/b cycles.76 The shelf-life is much longer, though, with times >10 years having been mentioned.81 The relative humidity of the ambience plays a large role for devices operated in air, and designs incorporating MgF2, SiOx, and LiF become nonfunctional if the water is desorbed, as under vacuum Such a strong dependence on the ambient conditions is not a necessary limitation for Deb devices, though, but designs with Cr2O3 are able to maintain their incorporated water at least to a pressure 5 × 106 c/b cycles at a reflectance modulation of 50% Copyright © 1997 by CRC Press, Inc 8956ch16.fm Page 606 Monday, October 11, 2004 2:39 PM The reliance of most Deb devices on ambient water is problematic and is presumably the reason why their cycling durability normally is limited to ~104 times A technically superior “charge-balanced” device design incorporates a CoE that operates in concert with the W oxide film and permits reversible movements of protons (and, perhaps, hydroxyl groups) between two thin films Work on reflecting and transmitting devices with W oxide, Ir oxide, and an intervening Ta2O5·H2O film was reported by Watanabe et al.82 and Saito et al.83 Such multilayers were integrated in prototype sunglasses capable of varying the transmittance between 70 and 10% with a c/b response time of a few seconds and a durability of >106 cycles.84 Recent work with a Sb2O5·pH2O paste replacing the Ta oxide appeared to give a cycling durability exceeding 107 times.85 Several charge-balanced devices with moderate to low humidity dependence have been studied primarily for applications on automotive rear view mirrors Recent work by Bange et al.86-88 was centered on the symmetric and asymmetric designs shown in the insets of Figures 16.21(a) and 16.21(b) Similar structures, with an ion conducting SiOx film, were discussed recently by Kleperis et al.75 The symmetric device incorporates two W oxide films, two proton-conducting SiO2-based films, a reflecting Rh film interposed between the SiO2based layers, and a metallic back conductor which also is a Rh film All of the films were made by evaporation The intermediate Rh film is almost completely permeable to protons and plays practically no role for the dynamics of the electrochromic system It is advantageous to use Rh, rather than, for example, Pd, as the reflector, since the former metal does not take up as much hydrogen and hence remains dimensionally stable The asymmetric device in Figure 16.21(b) is somewhat simpler and includes one film of each of electrochromic W oxide, proton-conducting Ta2O5, anodically coloring Ni oxide, and Al back reflector The main parts of Figure 16.21 illustrate spectral reflectance in fully colored and fully bleached states for the two device types The asymmetric design is capable of showing a maximum reflectance of ~80%, whereas the symmetric design has a limiting reflectance of ~72% Cyclic voltammetry for the asymmetric configuration indicated a “bistable” behavior with no current drawn between –0.5 and +0.2 V The reflectance at λ = 0.55 µm also showed a “bistable” performance in this voltage range and was ~75% for the anodic sweep direction and lower, with a magnitude depending on the Ta2O5 thickness, for the cathodic sweep direction.89 Work with Li+ conducting layers has also been reported Thus constructions incorporating films of LiAlF4,90 Li3AlF6,91 MgF2:Li,92 and Li2WO4 93 have been described The latter type of device had c/b response times of the order of 0.1 s Transparent electrochromic devices with the general design indicated in the inset of Figure 16.22 have been discussed in some detail by Goldner et al.94-97 The construction includes a layer of LiNbO3 The W oxide film was sputter deposited onto a substrate at 450°C and is hence crystalline; the upper ITO film was sputtered with the coated substrate at 200°C, which is less than the optimum temperature Figure 16.21 shows that a rather high degree of optical modulation can be achieved; it is caused by a reflectance change in the crystalline W oxide film The dynamics were slow, with typical c/b response times of even for a small device, which probably is caused by a poor conductivity of the top ITO film For several of the devices with Li+ conductors, dehydration is not assured and hence H+ conduction may contribute to the electrochromism Finally, one could mention some early experiments by Green et al.98,99 on thin-film devices with layers of RbAg4I5, whose Ag+ conductivity can be very large These devices were found to be unstable due to moisture attack and electrochemical reactions C POLYMER ELECTROLYTES The rapid advances in polymer electrolytes during recent years are paralleled by an upsurge of interest in electrochromic devices including such materials The discussion below first regards proton conductors, for which extensive work has been carried out with multilayer Copyright © 1997 by CRC Press, Inc 8956ch16.fm Page 607 Monday, October 11, 2004 2:39 PM FIGURE 16.21 Spectral transmittance at two states of coloration for symmetric (part a) and asymmetric (part b) electrochromic devices of the types shown in the insets (From Granqvist, C., Handbook of Inorganic Electrochromic Materials, Elsevier Science, 1995 With permission.) FIGURE 16.22 Spectral transmittance at two states of coloration for an electrochromic device of the type shown in the inset (From Granqvist, C., Handbook of Inorganic Electrochromic Materials, Elsevier Science, 1995 With permission.) structures based on poly-2-acrylamido-2-methylpropanesulfonic acid (poly-AMPS), polyvinylpyrrolidone (PVP), polyethylene imine (PEI), and others Alkali ion conductors then are considered, particularly devices incorporating polyethylene oxide (PEO), poly (propylene glycol, methyl methacrylate) (PPG-PMMA),100,101 etc Copyright © 1997 by CRC Press, Inc 8956ch16.fm Page 608 Monday, October 11, 2004 2:39 PM Work on electrochromic devices with polymer electrolytes was pioneered by Giglia and Haacke,102 Randin,103 and by Randin and Viennet.104 The studies were focused on polysulfonic acids, and it was found that poly-AMPS was the best Detailed information was given for a display-type device with poly-AMPS.102 The base was SnO2-coated glass upon which a film of W oxide was applied by evaporation The electrolyte comprised a 1- to 10-µm-thick layer of poly-(HEM, AMPS), with HEM denoting 2-hydroxyethylmethacrylate, and a ~0.5-mmthick layer of poly-AMPS/TiO2 pigment/PEO mixed to 8/1/1 by weight The poly-(HEM, AMPS) was included to separate the W oxide from the poly-AMPS, which was necessary for obtaining long-term durability The PEO admixture improved the dimensional stability of the polymer The CoE was prepared by following standard paper-making techniques utilizing acrylic fibers loaded with carbon powder and a MnO2 additive The latter component raised the emf to a sufficient level that bleaching of the device could be accomplished by short circuiting A protective metal encasement completed the design Displays of this kind had a c/b switching time of 0.9 s, could be cycled >107 times, showed open-circuit memory for up to days, and had a shelf life exceeding years Another polymer-based design, studied by Dautremont-Smith et al.,105 used opacified Nafion® in a symmetric configuration between anodic or sputter-deposited Ir oxide films backed by SnO2-coated glass The Nafion was boiled first in an aqueous solution of a barium salt and subsequently in H2SO4 so that a white precipitate of BaSO4 was occluded in the polymer The devices had a c/b switching time of the order of a second and an open-circuit memory of a few days The moderately low coloration of the Ir oxide, as well as the cost, limit the practical usefulness of these devices, though the excellent durability is an advantage Transparent electrochromic devices built around poly-AMPS have been studied by Cogan et al.,106,107 Rauh and Cogan,108 and others The insets of Figure 16.23 illustrate three related designs that have been investigated They incorporate films of disordered W oxide, disordered W-Mo oxide, or hexagonal crystalline KxWO3,109 together with Ir oxide films serving for ion storage and for augmenting the coloration, and Ta2O5 films for protecting the W oxide from degradation and for providing extended open-circuit memory All films were made by sputtering The electrolyte was with or without wt% PEO Prior to lamination, the W oxidebased films were protonated in a H2SO4 electrolyte to a value compatible with the maximum safe charge insertion into the Ir oxide The three devices all show rather high transmittance in the bleached state and low transmittance in the colored state For designs with disordered W or W-Mo oxide, Figures 16.23(a) and 16.23(b) show that the transmittance can be ~60% in the luminous and near-infrared spectral ranges The device with crystalline KxWO3, on the other hand, has a transmittance up to ~80%, as apparent from Figure 16.23(c) The configuration with W-Mo oxide is capable of yielding an exceptionally low transmittance in the colored state The device in Figure 16.23(a) has been c/b cycled successfully for × 105 times under the application of –1.2 and +0.2 V for 35 s each Metal grid electrodes can provide the low resistance needed for optical modulation with acceptable dynamics even in large-area devices This approach was studied recently by Ho et al.,110,111 whose design is illustrated in the inset of Figure 16.24 The grid electrode was made of Ni or Cu; the open areas were ~0.76 mm across and covered ~20% of the glass The electrolyte was poly-AMPS containing some water and N,N-dimethylformamide The device was completed by a glass plate covered with SnO2:F and evaporated W oxide Figure 16.24 shows the change of the transmittance at λ = 0.55 µm during galvanostatic cycling with ±0.15 mA/cm2; the voltage did not exceed V The transmittance changes between ~68% and