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 in to H atoms, which color the electrochromic him. A complete bleaching can be achieved in oxygen, whereas flushing with argon or evacuating the sample l eads to a comparatively slow and incomplete bleaching. We discuss two kinds of aging processes, i.e, catalysed poisoning by reactants in air and a cha nge in the water content of the WO3. Poisoning by air increases the time ne eded for coloring in H-2 and bleaching in O-2 or in Ar. From results with v ariable WO3 him thicknesses, we conclude that poisoning results from adsorp tion of a blocking species on the interior surfaces of the WO3 pores and no t on the catalyst. The adsorption process is accelerated by the catalyst. A fter drying the device at 100 degrees C in vacuum, there was a severe decre ase 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 cha nges in water content could be detected, and no significant change in the c oloration velocity could be found. To demonstrate the long-term stability o f the film, a 1.1 m x 0.6 m large window was switched 20,000 times at 20 de grees C over 2 yr without any significant change in performance. (C) 2000 E lsevier Science B.V. All rights reserved.