Solid-state electrochemistry of silicotungstic acid immobilized by sol-gel chemistry

Voltammetry of silicotungstic acid (STA), H4SiW12O40, that was encapsulated in silica was performed in the absence of a contacting liquid phase. Two one-electron reductions that are separated by 200 mV were observed, which is the same behavior as in aqueous solution. At scan rates, v, below 10 mV s−1 with a 10 μm dia. carbon fiber indicator electrode, plateaus with limiting currents which are independent of v were observed, which is indicative of spherical diffusion from a field that is much larger than the electrode area. At v > 20 V s−1, peaks were observed with currents directly proportional to v½. For a gel aged for 2 days, an effective diffusion coefficient, Deff, of 3 × 10−7 cm2 s−1 was estimated by voltammetry and chronoamperometry; the concentration of the redox sites thereby determined was about 0.5 M. The Deff values that were obtained in this study were larger than expected for a solid electrolyte, which suggests an important role of residual water. In support of this model, gels that were aged in a humidistat at 33% humidity at room temperature for 2 and 5 days lost 16% and 13%, respectively, of their mass when dried at 120°.