The role of water in gasochromic WO3 films

Gasochromic films consist of an electrochromic layer such as WO3 and a very thin coating of a catalyst. Hydrogen gas is dissociated on the catalyst in to H atoms, which colour the electrochromic film. Bleaching is achieved wit h oxygen. In this paper the role of water for proton diffusion and the reac tion process is discussed. Structural water is important to obtain a high p roton mobility on the pore surface of the WO3. During coloration, oxygen Va cancies and water are produced. For films with a high initial water content , the additional water produced in coloration leads to saturation of the po re surface, blocking further reactions. For films characterised by rapid pr oton diffusion, the water blocks the pore surface. This determines the firs t time constant of the coloration process. The relatively slow desorption o f 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 r eactive surface during the coloration process. The mechanism of the reactio n is expressed in differential equations. Their solution agrees very well w ith the experimental data. (C) 2001 Elsevier Science B.V. All rights reserv ed.