In situ resonance micro-Raman and UV–visible spectroelectrochemical studies of an electrochromic device with an I 3−/I − redox sol–gel electrolyte

Abstract

An all-sol–gel electrochromic (EC) device was constructed of electrochromic WO 3 and transparent Pt films, both deposited on conductive glass electrodes encapsulating a gel electrolyte with an I 3 −/I − redox couple. WO 3 films (200–400 nm) were deposited by the dip-coating technique from peroxopolytungstic acid sols. The films consisted of nanocrystalline WO 3 grains of dimensions 20–30 nm having monoclinic structure. The gel redox I 3 −/I − electrolyte, which was made via the sol–gel route, consisted of an organic phase (poly(propyleneglycol), PPG, M W=4000) covalently bonded via urea bridging groups to a silica phase. Gelation was performed with acetic or valeric acid. The functioning of the device was studied by the micro-resonance Raman spectroscopic technique, focusing the laser excitation line ( λ=647 nm) on the electrolyte encapsulated in the EC device. Variation in the potential from −2 to 2 V (scan rate 5 mV s −1) resulted in changes of the intensity of the symmetric stretching resonance Raman mode of I 3 − ions ( ν s(I 3 −)) and was accompanied by variation in the background Rayleigh scattering. From this, we concluded that the intercalation/deintercalation of K + ions in the WO 3 film was accompanied by an increase/decrease in the I 3 − ion concentrations and reversible dissociation/association of the KI microcrystallites existing in the redox electrolyte.