INTERFACIAL ELECTRON-TRANSFER AND BULK CHARGE PROPAGATION IN SOLID-STATE VOLTAMMETRY OF MIXED-VALENCE INORGANIC MATERIAL

We demonstrate the usefulness of cyclic voltammetric measurements for characterization of a model, mixed-valence protonically conducting sol id, tetragonal phospho-12-tungstic acid single crystal, H3PW12O40. 29H (2)O, in the absence of liquid electrolyte phase. The measurements wer e performed in an all-solid cell which utilizes a carbon fiber ultrami crodisk (diameter 12 mu m) working electrode, a silver disk semi-refer ence 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 t he effective diffusion coefficient for charge propagation (kinetic) an d concentration of mixed-valence redox sites (analytical) were determi ned from the combination of voltammetric experiments. They were carrie d 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 wa s used to estimate the interfacial parameter, standard heterogeneous r ate 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 x 10(-7) cm(2) s(-1)) and fast interfacial electron transfer (>10(-1) cm s(-1)). Simulation of c yclic voltammograms was performed using the calculated kinetic and ana lytical parameters. The reliability of estimations is discussed. Compa rative determinations were done on the standard, highly concentrated ( 0.80 mol dm(-3)) phospho-12-tungstic acid solution. (C) 1998 Elsevier Science B.V.