Countercation intercalation and kinetics of charge transport during redox reactions of nickel hexacyanoferrate

The dynamics of charge propagation in nickel hexacyanoferrate, a model metal-substituted analogue of Prussian Blue-type cyanide-bridged systems, was considered in electrolytes containing potassium and other alkali metal cations. The apparent (effective) diffusion coefficients for charge transport were determined using a large-amplitude potential-step chronocoulometry and small-amplitude potential-step chronocoulometric potentiostatic intermittent titration. The dependence of diffusion coefficient on the potential applied is consistent with the intercalation like model of the counter-cation sorption/desorption during redox processes of nickel hexacyanoferrate. Some differences in diffusion coefficients may originate from distinct charge densities (membrane properties) of the oxidized and reduced metal hexacyanoferrate structures. The existence of strong attractive interactions between an alkali metal cation and the cyanometallate matrix is also expected. The overall dynamics of charge propagation seems to be controlled by transport of electrolyte cations within the film rather than by electron self-exchange (hopping) between the mixed-valence hexacyanoferrate(III,II) ionic sites.