Counterelectrode films for electrochromic windows

Thin films examined as candidate materials for the counterelectrode in an electrochromic window exhibit ion insertion reactions which are far less ideal than those of WO3 and related materials used for the active electrochromic electrode. The ion-insertion kinetics of ITO, Pr6O11, NiO and a modified form of vanadium oxide have been studied by non-aqueous electrochemical and surface analysis techniques. Short term reversibility can be obtained in many film compositions, sometimes unexpectedly due to structural changes induced by the electrochemical treatment. However, in the case of the first three films above, long term cycling 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 reversible 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 coefficients were calculated to be in the range of 10−16 m2 s−1.