The dependence of the chemical potential of WO3 films on hydrogen insertion

Knowledge of the chemical potential of electrochromic films coloured by hyd rogen is important for matching the elements of an electrochromic device, f or understanding the colouring mechanism and for obtaining information abou t the microscopic structure of the film. The dependence of the chemical pot ential on the hydrogen concentration was measured electrochemically for tun gsten oxide films of different crystallinity and water content. A new metho d for determining the chemical potential by catalytic coloration by hydroge n gas is introduced. It revealed that the increase in electromotive force w ith increasing crystallinity is due only to different binding energies of t he protons. We expect the protons to be located in the centres of hexagons, which are created by WO,, octahedra. According to our model, amorphous spu ttered films show a hexagonal structure which is similar to that of evapora ted films, but the hexagons are connected, leading to more hexagon centre s ites, which increases the electromotive force. (C) 2002 Elsevier Science B. V. All rights reserved.