In situ resonance micro-Raman and UV–visible spectroelectrochemical studies of an electrochromic device with an I3−/I− redox sol–gel electrolyte

An all-sol–gel electrochromic (EC) device was constructed of electrochromic WO3 and transparent Pt films, both deposited on conductive glass electrodes encapsulating a gel electrolyte with an I3−/I− redox couple. WO3 films (200–400 nm) were deposited by the dip-coating technique from peroxopolytungstic acid sols. The films consisted of nanocrystalline WO3 grains of dimensions 20–30 nm having monoclinic structure. The gel redox I3−/I− electrolyte, which was made via the sol–gel route, consisted of an organic phase (poly(propyleneglycol), PPG, MW=4000) covalently bonded via urea bridging groups to a silica phase. Gelation was performed with acetic or valeric acid. nctioning 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 I3− ions (νs(I3−)) and was accompanied by variation in the background Rayleigh scattering. From this, we concluded that the intercalation/deintercalation of K+ ions in the WO3 film was accompanied by an increase/decrease in the I3− ion concentrations and reversible dissociation/association of the KI microcrystallites existing in the redox electrolyte.