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 tung stic acid in hydrogen peroxide. The nature and properties of the films depe nd on the drying procedure. Amorphous films are formed upon fast drying. Th ey exhibit good electrochromic properties but have to be heat treated in or der to become insoluble. Crystalline films are formed upon slow drying. The ir electrochromic properties are not very good but better amorphous or crys talline films are then obtained after a thermal treatment above 200 degrees C. In both cases, the reversibility of lithium insertion is not good durin g the first cycles but improve significantly upon cycling. This should be d ue to the electrochemical reduction of peroxo groups.