Oxygen reduction on Ru_1.92Mo_0.08SeO₄, Ru/carbon, and Pt/carbon in pure and methanol-containing electrolytes

The oxygen reduction reaction (ORR) activity of a Ru_1.92Mo_0.08SeO₄ catalyst, a Vulcan XC72-supported Ru catalyst and, for comparison, a Vulcan XC72-supported Pt catalyst was studied with a rotating ring-disk electrode. The very similar reaction characteristics of the two Ru catalysts in pure and CH₃OH-containing H₂SO₄ electrolyte, which differ markedly from those of the Pt catalyst, indicate that the reactive centers in both Ru catalysts must be identical. They are highly selective (>95%) toward reduction to H₂O (four electron pathway), independent of the presence of methanol. In the latter case, they are 100% selective toward the ORR, i.e., completely methanol tolerant, while the ORR on Pt catalysts is accompanied by significant CH₃OH oxidation. Based on mass specific current densities, however, the Ru catalysts are significantly less active than the standard Pt catalysts. Only at methanol concentrations above 10-30 mM does their methanol tolerance make them more active than Pt/Vulcan. Implications for their use as cathode catalysts in a direct methanol fuel cell are discussed.