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 v ^½. For a gel aged for 2 days, an effective diffusion coefficient, D _eff, 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 D _eff 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°.