Electrochromic Properties of Peroxopolytungstic Acid Thin Films

Electrochromic thin films have been deposited from peroxopolytungstic acid aqueous solutions. These solutions are obtained via the dissolution of tungstic acid in hydrogen peroxide. The nature and properties of the films depend on the drying procedure. Amorphous films are formed upon fast drying. They exhibit good electrochromic properties but have to be heat treated in order to become insoluble. Crystalline films are formed upon slow drying. Their electrochromic properties are not very good but better amorphous or crystalline films are then obtained after a thermal treatment above 200°C. In both cases, the reversibility of lithium insertion is not good during the first cycles but improve significantly upon cycling. This should be due to the electrochemical reduction of peroxo groups.