This page intentionally left blank ELECTROCHROMISM AND ELECTROCHROMIC DEVICES Electrochromism has advanced greatly over the past decade with electrochro- mic substances – organic and/or inorganic materials and polymers – providing widespread applications in light-attenuation, displays and analysis. Using reader-friendly electrochemistry, this book leads from electrochromic scope and history to new and searching presentations of optical quantification and theoretical mechanistic models. Non-electrode electrochromism and photo-electrochromism are summarised, with updated comprehensive reviews of electrochromic oxides (tungsten trioxide particularly), metal coordination complexes and metal cyanometallates, viologens and other organics; and more recent exotics such as fullerenes, hydrides and conjugated electroactive polymers are also covered. The book concludes by examining device construc- tion and durability. Examples of real-world applications are provided, including minimal-power electrochromic building fenestration, an eco-friendly application that could replace air conditioning; moderately sized electrochromic vehicle mirrors; large electrochromic windows for aircraft; and reflective displays such as quasi-electrochromic sensors for analysis, and electrochromic strips for moni- toring of frozen-food refrigeration. With an extensive bibliography, and step-by-step development from simple examples to sophisticated theories, this book is ideal for researchers in mat- erials science, polymer science, electrical engineering, physics, chemistry, bioscience and (applied) optoelectronics. P. M. S. MONK is a Senior Lecturer in physical chemistry at the Manchester Metropolitan University in Manchester, UK. R. J. MORTIMER is a Professor of physical chemistry at Loughborough University, Loughborough, UK. D. R. ROSSEINSKY, erstwhile physical chemist (Reader) at the University of Exeter, UK, is an Hon. University Fellow in physics, and a Research Associate of the Department of Chemistry at Rhodes University in Grahamstown, South Africa. ELECTROCHROMISM AND ELECTROCHROMIC DEVICES P. M. S. MONK, Manchester Metropolitan University R. J. MORTIMER Loughborough University AND D. R. ROSSEINSKY University of Exeter CAMBRIDGE UNIVERSITY PRESS Cambridge, New York, Melbourne, Madrid, Cape Town, Singapore, São Paulo Cambridge University Press The Edinburgh Building, Cambridge CB2 8RU, UK Published in the United States of America by Cambridge University Press, New York www.cambridge.org Information on this title: www.cambridge.org/9780521822695 © P. M. S. Monk, R. J. Mortimer, D. R. Rosseinsky 2007 This publication is in copyright. Subject to statutory exception and to the provision of relevant collective licensing agreements, no reproduction of any part may take place without the written permission of Cambridge University Press. First published in print format 2007 ISBN-13 978-0-511-50806-6 eBook (NetLibrary) ISBN-13 978-0-521-82269-5 hardback Cambridge University Press has no responsibility for the persistence or accuracy of urls for external or third-party internet websites referred to in this publication, and does not guarantee that any content on such websites is, or will remain, accurate or appropriate. Contents Preface page ix Acknowledgements xii List of symbols and units xiv List of abbreviations and acronyms xvii 1 Introduction to electrochromism 1 1.1 Electrode reactions and colour: electrochromism 1 1.2 Non-redox electrochromism 3 1.3 Previous reviews of electrochromism and electrochromic work 6 1.4 Criteria and terminology for ECD operation 7 1.5 Multiple-colour systems: electropolychromism 17 References 18 2 A brief history of electrochromism 25 2.1 Bibliography; and ‘electrochromism’ 25 2.2 Early redox-coloration chemistry 25 2.3 Prussian blue evocation in historic redox-coloration processes 25 2.4 Twentieth century: developments up to 1980 27 References 30 3 Electrochemical background 33 3.1 Introduction 33 3.2 Equilibrium and thermodynamic considerations 34 3.3 Rates of charge and mass transport through a cell 41 3.4 Dynamic electrochemistry 46 References 51 4 Optical effects and quantification of colour 52 4.1 Amount of colour formed: extrinsic colour 52 4.2 The electrochromic memory effect 53 4.3 Intrinsic colour: coloration efficiency 54 4.4 Optical charge transfer (CT) 60 4.5 Colour analysis of electrochromes 62 References 71 v vi Contents 5 Kinetics of electrochromic operation 75 5.1 Kinetic considerations for type-I and type-II electrochromes: transport of electrochrome through liquid solutions 75 5.2 Kinetics and mechanisms of coloration in type-II bipyridiliums 79 5.3 Kinetic considerations for bleaching type-II electrochromes and bleaching and coloration of type-III electrochromes: transport of counter ions through solid electrochromes 79 5.4 Concluding summary 115 References 115 6 Metal oxides 125 6.1 Introduction to metal-oxide electrochromes 125 6.2 Metal oxides: primary electrochromes 139 6.3 Metal oxides: secondary electrochromes 165 6.4 Metal oxides: dual-metal electrochromes 190 References 206 7 Electrochromism within metal coordination complexes 253 7.1 Redox coloration and the underlying electronic transitions 253 7.2 Electrochromism of polypyridyl complexes 254 7.3 Electrochromism in metallophthalocyanines and porphyrins 258 7.4 Near-infrared region electrochromic systems 265 References 274 8 Electrochromism by intervalence charge-transfer coloration: metal hexacyanometallates 282 8.1 Prussian blue systems: history and bulk properties 282 8.2 Preparation of Prussian blue thin films 283 8.3 Electrochemistry, in situ spectroscopy and characterisation of Prussian blue thin films 285 8.4 Prussian blue electrochromic devices 289 8.5 Prussian blue analogues 291 References 296 9 Miscellaneous inorganic electrochromes 303 9.1 Fullerene-based electrochromes 303 9.2 Other carbon-based electrochromes 304 9.3 Reversible electrodeposition of metals 305 9.4 Reflecting metal hydrides 307 9.5 Other miscellaneous inorganic electrochromes 309 References 309 10 Conjugated conducting polymers 312 10.1 Introduction to conjugated conducting polymers 312 10.2 Poly(thiophene)s as electrochromes 318 Contents vii 10.3 Poly(pyrrole)s and dioxypyrroles as electrochromes 327 10.4 Poly(aniline)s as electrochromes 328 10.5 Directed assembly of electrochromic electroactive conducting polymers 331 10.6 Electrochromes based on electroactive conducting polymer composites 332 10.7 ECDs using both electroactive conducting polymers and inorganic electrochromes 333 10.8 Conclusions and outlook 334 References 335 11 The viologens 341 11.1 Introduction 341 11.2 Bipyridilium redox chemistry 342 11.3 Bipyridilium species for inclusion within ECDs 346 11.4 Recent elaborations 360 References 366 12 Miscellaneous organic electrochromes 374 12.1 Monomeric electrochromes 374 12.2 Tethered electrochromic species 387 12.3 Electrochromes immobilised within viscous solvents 391 References 391 13 Applications of electrochromic devices 395 13.1 Introduction 395 13.2 Reflective electrochromic devices: electrochromic car mirrors 395 13.3 Transmissive ECD windows for buildings and aircraft 397 13.4 Electrochromic displays for displaying images and data 401 13.5 ECD light modulators and shutters in message-laser applications 404 13.6 Electrochromic paper 405 13.7 Electrochromes applied in quasi-electrochromic or non- electrochromic processes: sensors and analysis 406 13.8 Miscellaneous electrochromic applications 407 13.9 Combinatorial monitoring of multiples of varied electrode materials 409 References 410 14 Fundamentals of device construction 417 14.1 Fundamentals of ECD construction 417 14.2 Electrolyte layers for ECDs 419 14.3 Electrodes for ECD construction 422 viii Contents 14.4 Device encapsulation 424 References 425 15 Photoelectrochromism 433 15.1 Introduction 433 15.2 Direction of beam 433 15.3 Device types 434 15.4 Photochromic–electrochromic systems 438 References 440 16 Device durability 443 16.1 Introduction 443 16.2 Durability of transparent electrodes 444 16.3 Durability of the electrolyte layers 445 16.4 Enhancing the durability of electrochrome layers 445 16.5 Durability of electrochromic devices after assembly 446 References 449 Index 452 Preface While the topic of electrochromism – the evocation or alteration of colour by passing a current or applying a potential – has a history dating back to the nineteenth century, only in the last quarter of the twentieth century has its study gained a real impetus. So, applications have hitherto been limited, apart from one astonishing success, that of the Gentex Corporation’s self-darkening rear-view mirrors now operating on several million cars. Now they have achieved a telling next step, a contract with Boeing to supply adjustably darkening windows in a new passenger aircraft. The ultimate goal of contem- porary studies is the provision of large-scale electrochromic windows for buildings at modest expenditure which, applied widely in the USA, would save billions of dollars in air-conditioning costs. In tropical and equatorial climes, savings would be proportionally greater: Singapore for example spends one quarter of its GDP (gross domestic product) on air conditioning, a sine qua non for tolerable living conditions there. Another application, to display systems, is a further goal, but universally used liquid crystal displays present formidable rivalry. However, large-scale screens do offer an attractive scope where liquid crystals might struggle, and electrochromics should almost cer- tainly be much more