Precious metal oxides for electrochemical energy converters: Pseudocapacitance and pH dependence of redox processes

The mechanism of charge storage in hydrous platinum metal oxides is considered in more detail with respect to the pH dependence of redox capacitance for applications in double-layer capacitors, fuel cells, and pH sensors. An amorphous RuO2 electrode is able to work like a pH glass electrode during the titration of hydrochloric acid with sodium hydroxide solution. It is shown that the proton exchange mechanism at a glass membrane can be reproduced by platinum metal oxide-hydrates bound in a polymer matrix. Hydrous RuO2, obtained by alkaline precipitation or thermal decomposition from RuCl3, as well as single crystal RuO2, were characterized by TOF-SIMS, NMR spectroscopy, and electrochemical techniques. The proton conductivity of hydrous RuO2 appears to be due to the dissociative adsorption of water and the formation of acidic OH groups in Ru(III,IV) cluster ions. Depending on the pH of the solution, the electrode potential is determined by the formal hydrogen oxidation or oxygen reduction in ruthenium cluster ions. During aging, the active Ru(III) surface sites are partially oxidized to Ru(IV).