Introduction

Like all other types of display device, mechanical or electronic, no electrochromic device will continue to function indefinitely. For this reason, cycle lives are reported. The definition of cycle life has not been conclusively settled. Even by the definition in Section 1.4, reported lives vary enormously: some workers suggest their devices will degrade and thereby preclude realistic use after a few cycles while others claim a device surviving several million cycles. Table 16.1 contains a few examples; in each case the cycle life cited represents ‘deep’ cycles, as defined on p. 12. Some of these longer cycle lives were obtained via methods of accelerated testing, as outlined below.

It is important to appreciate that results obtained with a typical three-electrode cell in conjunction with a potentiostat can yield profoundly different results from the same components assembled as a device: most devices operate with only two electrodes.

The results of Biswas et al., who potentiostatically cycled a thin film of WO3 immersed in electrolyte, are typical insofar as the electrochemical reversibility of the cycle remained quite good with little deterioration. Their films retained their physical integrity, but the intensity of the coloration decreased with the number of cycles.

Some devices are intended for once-only use, such as the freezer indicator of Owen and co-workers; other applications envisage at most a few cycles, like the Eveready battery-charge indicator. Clearly, degradation can be allowed to occur after no more than a few cycles with applications like the latter.

Redox coloration and the underlying electronic transitions

Metal coordination complexes show promise as electrochromic materials because of their intense coloration and redox reactivity. Chromophore properties arise from low-energy metal-to-ligand charge-transfer (MLCT), intervalence charge-transfer (IVCT), intra-ligand excitation, and related visible-region electronic transitions. Because these transitions involve valence electrons, chromophoric characteristics are altered or eliminated upon oxidation or reduction of the complex, as touched on in Chapter 1. A familiar example used in titrations is the redox indicator ferroin, [FeII(phen)3]2 + (phen = 1,10-phenanthroline), which has been employed in a solid-state ECD, the deep red colour of which is transformed to pale blue on oxidation to the iron(III) form. Often more markedly than other chemical groups, a coloured metal coordination complex susceptible to a redox change will in general undergo an accompanying colour change, and will therefore be electrochromic to some extent. The redox change – electron loss or gain – can be assigned to either the central coordinating cation or the bound ligand(s); often it is clear which, but not always. If it is the central cation that undergoes redox change, then its initial and final oxidation states are shown in superscript roman numerals, while the less clear convention for ligands is usually to indicate the extra charge lost or gained by a superscripted + or −. As mentioned in Chapter 1, whilst the term ‘coloured’ generally implies absorption in the visible region, metal coordination complexes that switch between a colourless state and a state with strong absorption in the near infra red (NIR) region are now being intensively studied.

Introduction

The next major group of electrochromes are the bipyridilium species formed by the diquaternisation of 4,4′-bipyridyl to form 1,1′-disubstituted-4,4′-bipyridilium salts (Scheme 11.1). The positive charge shown localised on N is better viewed as being delocalised over the rings. The compounds are formally named as 1,1′-di-substituent-4,4′-bipyridilium if the two substituents at nitrogen are the same, and as 1-substitituent-1′-substituent′-4,4′-bipyridilium should they differ. The anion X− in Scheme 11.1 need not be monovalent and can be part of a polymer. The molecules are zwitterionic (i.e. bearing plus and minus charge concentrations at different molecular regions or sites) when a substituent at one nitrogen bears a negative charge.

A convenient abbreviation for any bipyridyl unit regardless of its redox state is ‘bipm’, with its charge indicated. The literature of these compounds contains several trivial names. The most common is ‘viologen’ following Michaelis, who noted the violet colour formed when 1,1′-dimethyl-4,4′-bipyridilium undergoes a one-electron reduction to form a radical cation. 1,1′-Dimethyl-4,4′-bipyridilium is therefore called ‘methyl viologen’ (MV) in this nomenclature. Another extensively used name is ‘paraquat’, PQ, after the ICI brand name for methyl viologen, which they developed for herbicidal use. In this latter style, bipyridilium species other than the dimethyl are called ‘substituent paraquat’.

There are several reviews of this field extant. The most substantial is The Viologens: Physicochemical Properties, Synthesis, and Applications of the Salts of 4,4′-Bipyridine (1998) by Monk.

Introduction

This chapter introduces the basic elements of the electrochemistry encompassing the redox processes that are the main subject of this monograph. Section 3.2 describes the fundamentals, starting with the origin of the cell emf (the electric potential across it), introducing the use of electrode potentials, and their determination in equilibrium conditions within simple electrochemical cells. In the first example (with electroactive species that resemble type-I electrochromes), the reactants are all ions in solution. In the second example, the cell assembly comprises two electrodes, each a metal in contact with a solution of its own ions, somewhat resembling type-II electrochromes. Though electrochromic electrodes are intrinsically more complicated than the two examples cited here, they follow just the principles established. Details of fabrication for electrochromic devices (ECDs) appear in Chapter 14. Section 3.3 exemplifies the kinetic features underlying electrochromic coloration. In it, the rates of mass transport and those of electron transfer, the three rate-limiting (thus current-limiting) processes encountered during the electrochemistry, are described. Diffusion of both electrochrome and counter ions is discussed more fully in Chapter 5, to illustrate the way charge-carrier movement limits the rate of the coloration/bleaching redox processes within ECDs.

Section 3.4 covers electrochemical methods involving dynamic electrochemistry, particularly cyclic voltammetry, which is important in studying electrochromism; three-electrode systems are required here.

More comprehensive treatments of electrochemical theory will be found elsewhere.

Introduction

While the applications of electrochromism are ever growing, all devices utilising electrochromic colour modulation fall within two broad, overlapping categories according to the mode of operation: electrochromic devices (ECDs) operating by transmission (see schematic in Figure 13.1) or by reflection (see the schematic representation in Figure 13.2).

Several thousand patents have been filed to describe various electrochromic species and devices deemed worthy of commercial exploitation, so the field is vast. Much duplication is certain in such patents, but it is clear how large scale are the investments directed toward implementing electrochromism as viable in displays or light modulation. In this field, vital details of compositions are often well hidden, as these comprise the valued intellectual property rights on which substantial financial considerations rest.

The most common applications are electrochromic mirrors and windows, as below. These and other applications are reviewed at length by Lampert (1998), who cites all the principal manufacturers of electrochromic goods worldwide, and also several novel applications.

Reflective electrochromic devices: electrochromic car mirrors

Mirrors, which obviously operate in a reflectance mode, illustrate the first application of electrochromism (cf. Figure 13.2). Self-darkening electrochromic mirrors, for automotive use at night, disallow the lights of following vehicles to dazzle by reflection from the driver's or the door mirror. Here an optically absorbing electrochromic colour is evoked over the reflecting surface, reducing reflection intensity and thereby alleviating driver discomfort. However, total opacity is to be avoided as muted reflection must persist in the darkened state.

Prussian blue systems: history and bulk properties

Prussian blue – PB; ferric ferrocyanide, or iron(III) hexacyanoferrate(II) – first made by Diesbach in Berlin in 1704, is extensively used as a pigment in the formulation of paints, lacquers and printing inks. Since the first report in 1978 of the electrochemistry of PB films, numerous studies concerning the electrochemistry of PB and related analogues have been made, with, in addition to electrochromism, proposed applications in electroanalysis and electrocatalysis. Fundamental studies on basic PB properties (electronic structure, spectra and conductimetry) underlie the elaborations that follow.

Prussian blue is the prototype of numerous polynuclear transition-metal hexacyanometallates, which form an important class of insoluble mixed-valence compounds. They have the general formula M′k[M″(CN)6]l (k, l integral) where M′ and M″ are transition metals with different formal oxidation numbers. These materials can contain ions of other metals and varying amounts of water. In PB the two transition metals in the formula are the two common oxidation states of iron, FeIII and FeII. Prussian blue is readily prepared by mixing aqueous solutions of a hexacyanoferrate(III) salt with iron(II), the preferred industrial-production route (rather than iron(III) with a hexacyanoferrate(II) salt). In the PB chromophore, the distribution of oxidation states is FeIII–FeII respectively; i.e. it contains Fe3+ and [FeII(CN)6]4−, as established by the CN stretching frequency in the IR spectrum and confirmed by Mössbauer spectroscopy. The chromophore alone thus has a negative charge, therefore in the solid a counter cation is to be incorporated.

Fundamentals of ECD construction

All electrochromic devices are electrochemical cells, so each contains a minimum of two electrodes separated by an ion-containing electrolyte. Since the colour and optical-intensity changes occurring within the electrochromic cell define its utility, the compositional changes within the ECD must be readily seen under workplace illumination. In practice, high visibility is usually achieved by fabricating the cell with one or more optically transparent electrodes (OTEs), as below.

Electrochromic operation of the ECD is effected via an external power supply, either by manipulation of current or potential. Applying a constant potential in ‘potentiostatic coloration’ is referred to in Chapter 3, while imposing a constant current is said to be ‘galvanostatic’. Galvanostatic coloration requires only two electrodes, but a true potentiostatic measurement requires three electrodes (Chapter 3), so an approximation to potentiostatic control, with two electrodes, is common.

The electrolyte between the electrodes is normally of high ionic conductivity (although see p. 386). In ECDs of types I and II, the electrolyte viscosity can be minimised to aid a rapid response. For example, a liquid electrolyte (that actually comprises the electrochromes) is employed in the world's best-selling ECD, the Gentex rear-view mirror described in Section 13.2. The electrolyte in a type-III cell is normally solid or at least viscoelastic, e.g. a semi-solid or polymer, as below.

In fact, virtually all the type-III cells in the literature are designed to remain solid during operation, as ‘all-solid-state devices’, or ‘ASSDs’.

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 contemporary 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 certainly be much more economical than plasma screens. Numerous other applications have been contemplated. There is thus at present a huge flurry of activity to hit the jackpot, attested by the thousands of patents on likely winners.