Solid-state electrochemical behavior of Keggin-type borotungstic acid single crystal

We demonstrate the electrochemical characteristics of the, poorly described in the literature, mixed-valence proton conducting solid borotungstic acid single crystal, H₅BW₁₂O₄₀ xH₂O, in the absence of liquid electrolyte phase. We performed electrochemical measurements in an all-solid cell with a gold fiber ultramicrodisk (diameter 10, 25, and 40 μm) working electrode, a silver semi-reference disk electrode, and a glassy carbon ring counter electrode. Diagnostic experiments at different scan rates aimed at probing the model of mass transport and potential kinetic limitations. Such bulk parameters as the effective diffusion coefficient of charge propagation and the concentration of mixed-valence redox centers were determined by two methods. The first method is based on the analysis of both Cottrellian and steady-state currents (the mixed-regimes method), and the second method provides the true diffusion coefficient (transport coefficient free of the migration influence) for both the substrate and the product of the electrode reaction. Together, these methods constitute a double potential step chronoamperometry experiment. The data obtained with these electrochemical experiments (effective diffusion coefficients, concentration of mixed-valence redox centers, etc.) can support the results obtained with other techniques (XRD, FTIR, and TGA).