Methanol Tolerant Oxygen Reduction Catalysts Based on Transition Metal Sulfides

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, 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.