Ion and water transports in Prussian blue films investigated with electrochemical quartz crystal microbalance

Water and ion transport in electrochemically prepared Prussian blue (PB, Iron(III) hexacyanoferrate(II)) films has been investigated with the electrochemical quartz crystal microbalance (EQCM) and the electrochemical/electrogravimetric impedance techniques. It is shown that the freshly prepared PB film is highly hydrated and that it undergoes an irreversible mass change during the first cathodic scan. The latter result supports the previously proposed structural reorganization scheme of the PB film from the insoluble form Fe₄[Fe(CN)₆]₃ · 6H₂O to the soluble form MFeFe(CN)₆ (M is a monovalent cation). It is also shown that, during the first cathodic scan, a substantial amount of water is excluded from the PB film. After the structural reorganization, ion transport during the redox reaction of the PB film is cation-dominant with a small fraction of accompanying water transport.