MIXED-VALENCE ELECTRON HOPPING, REDOX CONDUCTION AND MIGRATION EFFECTS IN SOLID-STATE ELECTROCHEMISTRY OF TRANSITION-METAL HEXACYANOFERRATES

A model is proposed and supported with experimental data to describe solid-state electrochemical responses of transition metal hexacyanoferrate powders: K2NiIIFeII(CN)6/KNiIIFeIII(CN)6, KInIIIFeII(CN)6/InIIIFeIII(CN)6 and Berlin Green (K1-x{FeIII[FeIII(CN)6]}x.{FeIII[FeII(CN)6]}1-x, where x < 1). The materials are prepared at various degrees of mixed valency, i.e. ratios of hexacyanoferrate(III, II) sites, contain different amounts of structural K+-counterions, and differ in the extents of hydration. The powders are sandwiched between two glassy carbon slide electrodes and investigated in the absence of an external supporting electrolyte. The model is based on the assertion that the hexacyanoferrate(III, II) sites are fixed, and electron self-exchange (hopping) between them is fast. The nature of the current-potential responses is strongly dependent on the system's degree of hydration, the related potassium mobility and the possibility of coupling electron transfer with the K+ counterion transport. When the materials are largely hydrated (e.g. contain 7-10 structural H2O), and comprise potassium at high concentrations (as in nickel hexacyanoferrates), the applied potential differences produce diffusional concentration gradients and lead to redox transitions. A typical redox conductivity pattern is in the form of well-defined voltammetric peaks, symmetrical around 0 V, and proportional to the degree of mixed valency. Transport of K+ seems to be rate limiting. Chronocoulometry yields effective diffusion coefficients for charge propagation in the nickel(II) hexacyanoferrate(III, II) and indium(III) hexacyanoferrate(III) systems in the range 10(-10) to 10(-9) and 10(-11) to 10(-10) cm2 s-1, respectively. Drying of the materials freezes the counterions and, as the systems are mixed-valence semiconductors, makes a sitewise electron hopping, an ohmic mechanism, predominant. The ohmic effects may also appear in the hydrated systems, owing to migration, when the population or mobility of structural K+ ions is decreased.