Electrochemical charging, countercation accommodation, and spectrochemical identity of microcrystalline solid cobalt hexacyanoferrate

Oxidized and reduced cobalt(II) hexacyanoferrates were fabricated and characterized in the presence of alkali metal (Li+, Na+, K+, Cs+) and Co2+ countercations. Formal potentials of hexacyanoferrate(III,II) redox reactions are sensitive to the choice of electrolyte cation, and they correlate well with the sizes of hydrated Li+, Na+, and K+. Electrochemical quartz cyrstal microbalance measurements clearly indicate that countercations, presumably in partially dehydrated form, are incorporated into reduced cobalt(II) hexacyanoferrate(II). The color of the system reflects primarily the oxidation state of iron sites. But the color of the reduced form is also affected by the nature of an intercalated hydrated countercation. This observation is correlated with the reversible continuous thermochromism of K2CoII[FeII(CN)6]*nH 2O that shall be attributed to the release of structural water molecules interacting with CoII during heating in the temperature range 25-85°C. It is apparent from X-ray absorption near-edge structure (XANES) experiments that the chemical environment of cobalt(II) sites is influenced by the presence of hydrated alkali metal countercations. The results are consistent with the accommodation of countercations in the lattice cavities at interstitial positions. The structural environment of iron ions was the same in all systems studied except that a chemical shift was observed due to change of the oxidation state of iron.