Electrochromism is the phenomenon displayed by some materials of
reversibly changing colour by using bursts of
charge to cause electrochemical redox reactions in electrochromic materials. Various types of materials and structures can be used to construct
electrochromic devices, depending on the specific applications. Transition metal oxides are a large family of materials possessing various interesting properties in the field of electrochromism. Among them,
tungsten oxide (WO
3), has been the most extensively studied material, 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. The world leading institutions on NiO efforts include
National Renewable Energy Laboratory and
Uppsala University. Another example of an electrochromic material is
polyaniline which can be formed either by the
electrochemical or chemical
oxidation of
aniline. If an
electrode is immersed in
hydrochloric acid which contains a small concentration of aniline, then a film of polyaniline can be grown on the electrode. Depending on the
oxidation state, polyaniline can either be pale yellow or dark green/black. Other electrochromic materials that have found technological application include the
viologens and polyoxotungstates.