Ion transport in prussian blue and its cobalt analogue

Abstract

Ion and water transport phenomena of electrochemically prepared Prussian blue (Iron(Ⅲ) hexacyanoferrate(Ⅱ)) and cobalt(Ⅱ) hexacyanoferrate(Ⅱ) (CoHCF) films on gold electrode in various supporting electrolyte solutions were investigated by the electrochemical quartz crystal microbalance (EQCM) and the impedance techniques. It was shown that electrochemically deposited Prussian blue is highly hydrated (about 60 water molecules per unit cell) and undergoes an irreversible, cation-dependent mass change during initial cathodic scan, which evidently supports the previously proposed structural reorganization of the insoluble form, $Fe_4[Fe(CN)_6]_3ㆍ6H_2O$, into the soluble form, $KFeFe(CN)_6$. It was also shown that, during the structural reorganization, a substantial amount (ca. 37%) of water molecules are excluded from the Prussian blue film. At the steady-state mass transport of Prussian blue and CoHCF film, ion transport is accompanied by water transport and the number of accompanying water depends on the type of cation in electrolyte solution.