The oxygen reduction activity and methanol tolerance of a range of transition metal sulfide electrocatalysts have been evaluated in half-cell experiments and in a liquid-feed solid polymer electrolyte direct methanol fuel cell. These catalysts were prepared in high surface area form by direct synthesis onto various surface-functionalized carbon blacks. Of the materials tested, mixed-metal catalysts based on ReRuS and MoRuS were observed to give the best oxygen reduction activities. In addition, significant increases in performance were observed when employing sulfur-functionalized carbon black, which were attributed to the preferential deposition of active Ru sites in the catalyst-preparation process. Although the intrinsic activity of the best material tested, namely, Mo2Ru5S5 on sulfur-treated XC-72 was lower than Pt (by ca. 155 mV throughout the entire polarization curve), its activity relative to Pt increased significantly in methanol-contaminated electrolytes. This was due to methanol oxidation side reactions reducing the net activity of the Pt, especially at low overpotentials.
