The role of water in gasochromic WO 3 films

Abstract

Gasochromic films consist of an electrochromic layer such as WO 3 and a very thin coating of a catalyst. Hydrogen gas is dissociated on the catalyst into H atoms, which colour the electrochromic film. Bleaching is achieved with oxygen. In this paper the role of water for proton diffusion and the reaction process is discussed. Structural water is important to obtain a high proton mobility on the pore surface of the WO 3. During coloration, oxygen vacancies and water are produced. For films with a high initial water content, the additional water produced in coloration leads to saturation of the pore surface, blocking further reactions. For films characterised by rapid proton diffusion, the water blocks the pore surface. This determines the first time constant of the coloration process. The relatively slow desorption of the water leads to a second time constant. Films with a low water content show a deep coloration, which can be explained with the framework of this model. The bleaching in Ar, (the reverse reaction of the coloration in H 2), is incomplete and slow. This is a consequence of the water blocking the reactive surface during the coloration process. The mechanism of the reaction is expressed in differential equations. Their solution agrees very well with the experimental data.