Stability of gasochromic WO3 films

Gasochromic films consist of an electrochromic layer such as WO₃ and a very thin catalyst coating, like Pt. Hydrogen is dissociated on the catalyst into H atoms, which color the electrochromic film. A complete bleaching can be achieved in oxygen, whereas flushing with argon or evacuating the sample leads to a comparatively slow and incomplete bleaching. We discuss two kinds of aging processes, i.e. catalysed poisoning by reactants in air and a change in the water content of the WO₃. Poisoning by air increases the time needed for coloring in H₂ and bleaching in O₂ or in Ar. From results with variable WO₃ film thicknesses, we conclude that poisoning results from adsorption of a blocking species on the interior surfaces of the WO₃ pores and not on the catalyst. The adsorption process is accelerated by the catalyst. After drying the device at 100°C in vacuum, there was a severe decrease in the coloring and bleaching rates due to a reduction of the diffusion in the WO₃. Furthermore, the coloration at steady state was more intense. The variation of the water content of the WO₃ was attempted by exposing it to dry or humid atmospheres and was investigated by IR spectroscopy. No changes in water content could be detected, and no significant change in the coloration velocity could be found. To demonstrate the long-term stability of the film, a 1.1 m×0.6 m large window was switched 20,000 times at 20°C over 2 yr without any significant change in performance.