Interfacial electron transfer and bulk charge propagation in solid-state voltammetry of mixed-valence inorganic material

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We demonstrate the usefulness of cyclic voltammetric measurements for characterization of a model, mixed-valence protonically conducting solid, tetragonal phospho-12-tungstic acid single crystal, H 3PW 12O 40·29H 2O, in the absence of liquid electrolyte phase. The measurements were performed in an all-solid cell which utilizes a carbon fiber ultramicrodisk (diameter 12 μm) working electrode, a silver disk semi-reference electrode, and a glassy carbon ring counter electrode. Diagnostic experiments at various scan rates aimed at probing the model of mass transport and potential kinetic limitations. Such bulk parameters as the effective diffusion coefficient for charge propagation (kinetic) and concentration of mixed-valence redox sites (analytical) were determined from the combination of voltammetric experiments. They were carried out in two time regimes related to the application of slow (0.5 mV s −1) and fast (34 V s −1) scan rates. A modified Nicholson method was used to estimate the interfacial parameter, standard heterogeneous rate constant for electron transfer between the ultramicroelectrode and tungsten redox sites in the crystal. The results are consistent with high dynamics of bulk charge propagation (4 × 10 −7cm 2s −1) and fast interfacial electron transfer (10 −1cm s −1). Simulation of cyclic voltammograms was performed using the calculated kinetic and analytical parameters. The reliability of estimations is discussed. Comparative determinations were done on the standard, highly concentrated (0.86 mol dm −3) phospho-12-tungstic acid solution.