Electrochromism of colloidal tungsten oxide

Acidification of tungstate solutions leads to the formation of tungstic acids that polymerize, through a polycondensation process, giving rise to colloidal tungsten oxide. Reversible display devices can be obtained by electrochemichromism. The colloidal solution is then deposited onto a conductive transparent electrode and placed into an electrochemical cell containing a liquid electrolyte such as LiClO4 in propylene carbonate. With such a device, an optical densitydo = 0.3 is obtained within 0.3s by applying a negative voltage (-4V). The blue coloration remains after the voltage has been removed. Optical spectroscopy and ESR indicate that the blue layer contains reduced W(V) ions.