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Electrochromism is the phenomenon displayed by some materials of reversibly changing colour stimulated by redox reactions. Various types of materials and structures can be used to construct electrochromic devices. Electrochromic displays are based on any material that changes color depending on the applied potential.
As the color change is persistent and energy need only be applied to affect a change, electrochromic materials are used to control the amount of light and heat allowed to pass through windows ("smart windows"), One popular application is in the automobile industry where it is used to automatically tint rear-view mirrors in various lighting conditions.
Among the metal oxides, tungsten oxide (WO3), has been the most extensively studied material.
For organic materials, viologens have been commercialized on small scale. A variety of conducting polymers are also of interest, including polypyrrole, PEDOT, and polyaniline. Viologen is used in conjunction with titanium dioxide (TiO2) in the creation of small digital displays. It is hoped that these displays will replace liquid crystal displays as the viologen, which is typically dark blue, has a high contrast compared to the bright white of the titania, thereby providing the display high visibility. Rdot (rdotdisplays.com) is offering flexible displays based on similar materials.
Used in the production of electrochromic windows or smart glass and more recently electrochromic displays on paper substrate as anti-counterfeiting systems integrated on packaging. NiO materials have been widely studied as counter electrodes for complementary electrochromic devices, in particular, smart windows.
ICE 3 high speed trains use electrochromatic glass panels between the passenger compartment and the driver's cabin. The standard mode is clear, and can be switched by the driver to frosted/translucent, mainly to conceal "unwanted sights" from passengers' view, for example in the case of (human) obstacles. Electrochromic windows are used in the Boeing 787 Dreamliner.
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