Microelectrode-based probing of charge propagation and redox transitions in concentrated polyoxometallate electrolyte of potential utility for redox flow battery

Concentrated solutions of Keggin-type silicotungstic acid, as well as the system's single crystals (H4SiW12O40*31H2O) and their colloidal suspensions have been tested using the microelectrode methodology to determine mass-transport, electron self-exchange and apparent (effective) diffusion-type coefficients for charge propagation and homogeneous (electron self-exchange) rates of electron transfers. Silicotungstic acid facilitates proton conductivity, and undergoes fast, reversible, multi-electron transfers leading to the formation of highly conducting, mixed-valence (tungsten(VI,V) heteropoly blue) compounds. To develop useful electroanalytical diagnostic criteria, electroanalytical approaches utilizing microdisk electrodes have been adapted to characterize redox transitions of the system and to determine kinetic parameters. Combination of microelectrode-based experiments performed in two distinct diffusional regimes: radial (long-term experiment; e.g., slow scan rate voltammetry or long-pulse chronoamperometry) and linear (short-term experiment; e.g., fast scan rate voltammetry or short-pulse chronocoulometry) permits absolute determination of such parameters as effective concentration of redox centers (C0) and apparent transport (diffusion) coefficient (Dapp). The knowledge of these parameters, in particular of [Dapp1/2 C0] seems to be of importance to the evaluation of utility of redox electrolytes for charge storage. For the colloidal suspension of silicotungstic acid (H4SiW12O40) crystals in the saturated solution, the following values have been obtained: Dapp = 1.8*10-6 cm2 s−1 and C0 = 1.1 mol dm−3, as well as the [Dapp1/2 C0] diagnostic parameter has reached the value as high as 6*10-3 mol/dm−3 cm s−1/2, provided that four electrons are involved in the H4SiW12O40 redox transitions. In this respect, the fact that crystals (dispersed solids) are characterized by high electron self-exchange rate (kex = 1.1*108 dm3 mol−1 s−1) and low activation energy (EA = 18.7 kJ mol−1) facilitating electron transfers between immobilized WVI and WV redox sites is also advantageous.