Sol-gel synthesis of vanadate-based thin films as counter electrodes in electrochromic devices

V2O5-based thin films were prepared via the sol-gel method in order to obtain semiconductive films suitable as counter electrodes in electrochemical devices (e.g. electrochromic windows). Different metal precursors (nickel 2,4-pentanedionate, NiCl2, BiCl3, SbCl3) were added, after dissolution, into the starting vanadium oxo-triisopropoxide solution in the presence of acetic acid as a chelating agent. Preventing different synthesis conditions due to the ambient moisture, the hydrolysis reaction was carried out in controlled acidic conditions. Stable sols were obtained and thin films were dipped on transparent substrates coated with a conductive layer (ITO, SnO2:F). Different techniques were employed on as-prepared and heat treated (673 K) films, and the corresponding gels in order to characterize the microstructural, surface and electrochemical features of the samples. Better results were obtained for Ni/V oxides thin films: unless their charge capacity was not very high (∼10 mC/cm2), their electrochemical stability was unchanged after 1000 cycles and their transmittance spectra showed high transparency (>80%) in all the visible region during both the intercalation and deintercalation steps. On the other hand, preliminary analysis on Sb/V mixed oxides showed a higher charge capacity but irreversible electrochemical reactions took place during the first working cycles.