SOLID-STATE ELECTROANALYSIS OF SILICOTUNGSTIC ACID SINGLE-CRYSTALS AT AN ULTRAMICRODISK ELECTRODE

An electroanalytical approach for the quantitative and qualitative characterization of certain fast redox-conducting molecular solids is proposed here. The method utilizes ultramicroelectrodes within a three-electrode solid-state electrochemical cell and is based on a set of potential step and cyclic voltammetry experiments. Such measurements permit the determination of the following parameters: the concentration of mixed-valence redox sites (C(o)) that are electrochemically accessible in the system, the effective diffusion coefficient for charge propagation (D(eff)), the standard heterogeneous rate constant for electron transfer between the working ultramicroelectrode and the electroactive sites (k(o)), and the formal potentials (E(o')) for solid-state redox reactions. For a well-defined 12-tungstosilicic acid tetragonal single crystal (H4SiW12O40.31H2O), the obtained values are C(o) = 1.5 +/- 0.1 M, D(eff) = 2.8 +/- 0.3 x 10(-7) cm2 s-1, and k(o) = (7-13) x 10(-2) cm s-1 for the most positive redox transition at E(o') = -0.225 V versus gel-filled Ag/AgCl (3 M NaCl/apr) reference. This approach can be extended to the characterization of other ionically-conducting solids that contain fixed mixed-valence redox sites on the conditions that (1) the overall charge propagation mechanism can be described in terms of bulk diffusion, (2) the electron transfer between the working electrode and the electroactive sites is not rate-limiting in potential step experiments, and (3) the structural counterions are available in large numbers and are sufficiently mobile to minimize ohmic effects and to support fast diffusive mass transport of electrons. In this context, silicotungstic acid, which shows facile redox processes and hosts large amounts of easily accessible mobile protons, is a model system.