Electrochromic

The term electrochromism was apparently coined to describe absorption line shifts induced in dyes by strong electric fields (1). This definition of electrocbromism does not, however, fit within the modem sense of the word. Electrochromism is a reversible and visible change in transmittance and/or reflectance that is associated with an electrochemicaHy induced oxidation—reduction reaction. This optical change is effected by a small electric current at low d-c potential. The potential is usually on the order of 1 V, and the electrochromic material sometimes exhibits good open-circuit memory. Unlike the well-known electrolytic coloration in alkaU haUde crystals, the electrochromic optical density change is often appreciable at ordinary temperatures. 

Members of the ion-insertion/extraction group, as inorganic or organic thin films, especially the former, have attracted the widest interest most recently. Tungsten trioxide was the eadiest exploited inorganic compound (4), even before the mechanism of its electrochromic response was understood (5). It is stiU the best known of the important ion-insertion/extraction group. 

The added electron is delocalized on the monovalent radical ion to which it is reduced (3). There is no general agreement on the molecular representation of the reduced stmcture. Various other viologen compounds have been mentioned (9,12). Even a polymeric electrochromic device (15) has been made, though the penalty for polymerization is a loss in device speed. Methylviologen dichloride [1910-42-5] was dissolved in hydrated 

The seminal work on these materials began at American Cyanamid Co. in the 1960s (4,23), though these workers did not author the ion-insertion/extraction model that has become widely accepted (5). Numerous patents were granted to American Cyanamid Co. as a result of its display-oriented work. Much of what others have written in the open Hterature either confirms or adds to what these teach. Important papers (16) about 

Kirk-Othmer Encyclopedia of Chemical Technology (4th Edition) 

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Wruck D A and Rubin M 1993 Structure and electronic properties of electrochromic NiO films J. Eiectrochem. See. 140 1097-104... 

Liu Y, Shigehara K and Yamada A 1989 Purification of lutetium diphthalocyanine and electrochromism of its Langmuir-Blodgett films Thin Soiid Fiims 179 303-8... 

CHROMOGENICMATERIALS - ELECTROCHROMIC] (Void) [CHROMOGENICMATERIALS - ELECTROCHROMIC] (Void)... 

The divalent cation methyl viologen is highly colored in reduced form and is used as an electrochromic in solution cells (Eig. 11). 

Tetrathiafulvalene (TTE) has also been used in electrochromic devices. TTE-based polymers spin-coated onto transparent electrode surfaces form stable thin films with reproducible electrochromic properties (100). The slow response of these devices has been attributed to the rate of ion movement through the polymer matrix. 

Miscellaneous. Iridium dioxide, like RUO2, is useful as an electrode material for dimensionally stable anodes (DSA) (189). SoHd-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) ofIr02 ate stable electrochromic materials (191). 

Fast-Ion-Gonducting Glasses. One possible appHcation of fast-ion-conducting glasses is in soHd-state batteries (qv) for automobiles. The glasses might also be used in electrochromic displays or as sensors (qv) (25). 

Despite the considerable progress made in the few years in which anodic insertion/extraction films have been known, neither film compositions, film properties, nor electrochemical reactions are sufficientiy well characterized. There have been disagreements, as indicated for h-IrO and h-NiO in Table 2, as to whether is being extracted or OH inserted during coloration. The general problem is best illustrated by the important example of Pmssian blue. Early work (47—50) resulted in two different sets of equations for electrochromic reduction ... 

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