Electrochemistry of ferric ruthenocyanide (Ruthenium Purple), and its electrocatalysis for proton reduction

Electrochemistry of ferric ruthenocyanide (Ruthenium Purple, RP) was studied in acidic solutions containing a sodium salt (NaH₂PO₄ or NaCl). Two kinds of redox peaks for the Fe^III/II couple were clearly seen in a Na⁺ solution, originating from a composite structure of both Fe₄^IIIRu^II(CN)₆]₃ (insoluble) and Fe^IIIRu^II(CN)₆]⁻ (soluble). In repeated CV scans in a NaCl solution, it was found that only the couple of the redox peak assigned to the insoluble form remains unchanged under the steady state. The ratio of the insoluble form was estimated by coulometry as ca. 40% of the initial coated unit cells of the RP. Efficient electrocatalytic H₂ formation was found to take place with the aid of the RP although this electrocatalysis was dependent on the type of sodium salt employed. In order to investigate the dominant factor affecting the overall kinetics in the H⁺ reduction catalysis, the dependences of both the catalytic activity and the electron transfer rate in the RP film were studied in a NaH₂PO₄ solution as a function of the coated amount. It was found that the overall kinetics are not dominated by electron transfer in the RP film.