COUNTERELECTRODE FILMS FOR ELECTROCHROMIC WINDOWS

Thin films examined as candidate materials for the counterelectrode in an electrochromic window exhibit ion insertion reactions which are fa r less ideal than those of WO3 and related materials used for the acti ve electrochromic electrode. The ion-insertion kinetics of ITO, Pr6O11 , NiO and a modified form of vanadium oxide have been studied by non-a queous electrochemical and surface analysis techniques. Short term rev ersibility can be obtained in many film compositions, sometimes unexpe ctedly due to structural changes induced by the electrochemical treatm ent. However, in the case of the first three films above, long term cy cling shows capacity loss due to corrosion and conductivity loss. The analysis of the insertion/extraction kinetics was greatly complicated by large overpotentials and non steady-state behaviour under cycling. The modified vanadium oxide film has been shown to have a large revers ible capacity at a range of potential where degradative side reactions were not detected. The kinetics of the cycling process were found to follow simple diffusion theory without complication and diffusion coef ficients were calculated to be in the range of 10(-16) m(2) s(-1).