Electrochromism solid electrochromes

Progress in the development of solid electrolytes is also being achieved from advances in several other fields of technology such as fuel and electrolysis cells, thermoelectric converters, electrochromic devices, and sensors for many chemical and physical quantities. 

Electrolytes for Electrochromic Devices Liquids are generally used as electrolytes in electrochemical research, but they are not well suited for practical devices (such as electrochromic displays, fuel cells, etc.) because of problems with evaporation and leakage. For this reason, solid electrolytes with single-ion conductivity are commonly used (e.g., Nafion membranes with proton conductivity. In contrast to fuel cells in electrochromic devices, current densities are much lower, so for the latter application, a high conductivity value is not a necessary requirement for the electrolyte. 

Bohnke, O., Applications of proton condnctors in electrochromic devices, in Proton Conductors Solids, Membranes and Gels—Materials and Devices, P. Colomban, Ed., Cambridge University Press, New York, 1992. 

Porqueras, I. Person, C. Corbella, C. Vives, M. Pinyol, A. Bertran, E. 2003. Characteristics of e-beam deposited electrochromic Ce02 thin films. Solid State Ionics 165 131-137. 

The discussion of Brouwer diagrams in this and the previous chapter make it clear that nonstoichiometric solids have an ionic and electronic component to the defect structure. In many solids one or the other of these dominates conductivity, so that materials can be loosely classified as insulators and ionic conductors or semiconductors with electronic conductivity. However, from a device point of view, especially for applications in fuel cells, batteries, electrochromic devices, and membranes for gas separation or hydrocarbon oxidation, there is considerable interest in materials in which the ionic and electronic contributions to the total conductivity are roughly equal. 

Doped silicon, conductivity in, 23 35 Doped/undoped electrochromic organic films, 6 580-582 Dope-dyeing, 9 197 Dope-making process, in acrylic fiber solution spinning, 11 204 Dope solids, in air gap spinning, 11 209 Doping, 23 838—839 calcium, 23 842-844 conducting polymers, 7 528-529... 

Electrochromism can be defined as a colour change induced in a material by an applied electric field or current. Some ions in solid compounds can be reduced or oxidised (redox) electrochromically with a consequent change in colour. WO3 and M0O3 solid films have been extensively used for this purpose. The electrochromic reaction is expressed by... 

The first demonstration of a PEM with electrochromic properties was disclosed by SchlenofFand coworkers [66], using poly(butanylviologen)/ PSS films. While this film exhibited strong electrochromic response, it still required the use of an outer electrolyte solution. DeLongchamp and Hammond disclosed for the first time a solid-state device comprised of two electrochromic PEM-modified ITO electrodes separated by a 200-p,m thick poly(2-acrylamido-methane-2-propanesulfonic acid), proton-conducting PAMPS membrane (see Eigure 2.30) [196]. Both PEMs used in... 

Figure 2.30 Scheme of a solid electrochromic device. The device can operate with two electrochromic films as shown or with an electrochromic and an ion-storage film. 

There is a large potential for conducting polymers as corrosion-inhibiting coatings. For instance, the corrosion protection ability of polyaniline is pH-dependent. At lower pH polyaniline-coated steel corrodes about 100 times more slowly than noncoated steel. By comparison, at a pH of about 7 the corrosion protection time is only twice for polyaniline-coated steel. Another area of application involves creation of solid state rechargeable batteries and electrochromic cells. Polyheterocycles have been cycled thousands of times with retention of over 50% of the electrochromic activity for some materials after 10,000 cycles. IR polarizers based on polyaniline have been shown to be as good as metal wire polarizers. 

The materials that change colour on passing a charge are called electrochromes, and these can be classified into three groups. In the first type the colouring species remain in solution in the second type the reactants are in solution but the coloured product is a solid the third type are those where all the materials are solids, e.g. in films. The first type is used in car, anti-dazzle, rear-view mirrors, the second type in larger mirrors for commercial vehicles and the third type in smart windows (see section 1.5.4.2). 

All inorganic electrochromes exist in the solid state in both the colourless and coloured states, e.g. Prussian Blue and tungsten trioxide. Conducting polymers such as polyanilines, polypyrroles and polythiophenes also fall into this class of electrochromes (see 1.5.3.5). 

Fast ion conduction has been made use for developing electrochromic displays. WO3 is a pale yellow solid that becomes deep blue when a small amount of Na is incorporated, owing to formation of Na (W03. Since the change occurs reversibly and very fast, the material has been used in displays. 

While with photochromic solids, light is used to cause colour change and detect it, with electrochromic solids, an electric field is used to change colour. Certain hemiquinones whose colour is externally switchable by an electric field are potentially useful as memory elements. 

Catenane 404+ was also incorporated into a solid-state device that could be used for random access memory (RAM) storage.42 In addition, this compound could be employed for the construction of electrochromic systems, because its various redox states are characterized by different colors.41,43... 

Miscellaneous. Iridium dioxide, like Ru02, is useful as an electrode material for dimensionally stable anodes (DSA) (189). Solid-state pH sensors employing Ir02 electrode material are considered promising for measuring pH of geochemical fluids in nuclear waste repository sites (190). Thin films (qv) of Ir02 are stable electrochromic materials (191). 

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