Transition metal oxides as DMFC cathodes without platinum

Transition metal oxides (ZrO2, TiO2, SnO2, Nb2O5, and Co3O4) made by sputtering methods were investigated as new electrocatalysts for the cathode of a direct methanol fuel cell (DMFC) without platinum. The catalytic activity for the oxygen reduction of these transition metal oxides was evaluated in sulfuric acid with and without 0.1 M methanol. The sputtered metal oxides had stable states in acid medium at least in the potential range of the oxygen reduction. The metal oxides were not active for the methanol oxidation. The oxygen reduction activity of the metal oxides decreased in the following order: ZrO2-x > Co3O4-x > TiO2-x approximate to SnO2-x > Nb2O5-x in the presence of methanol. Zirconium oxides showed the best activity for the oxygen reduction among these transition metal oxides. Moreover, in comparison with the sputtered Pt, the potential at i(O2)-(N2) = -5 mu A cm(-2) of ZrO2-x was 60 mV larger than that of the sputtered Pt. This is due to the high selectivity of the metal oxides for the oxygen reduction. The mixed potential as found on the Pt electrode was not observed on the metal oxide catalysts. Zirconium oxides could be a good cathode substituting the platinum cathode for DMFCs. (C) 2007 The Electrochemical Society.