Investigation of a carbon-supported quaternary Pt---Ru---Sn---W catalyst for direct methanol fuel cells

An investigation was carried out in the electro-oxidation of methanol on a carbon-supported quaternary Pt---Ru---Sn---W catalyst prepared by a liquid-phase reduction method. As derived by X-ray diffraction and X-ray photoelectron spectroscopy, the catalyst was composed of metallic Pt, microcrystalline RuO2 and SnO2 phases and amorphous WO3/WO2 species. The electrochemical analysis was carried out in half-cell containing sulfuric acid electrolyte as well as in a liquid methanol-fed solid polymer electrolyte single-cell. The activity of catalyst in the half-cell varied as a function of the methanol concentration, it increased with CH3OH molarity in the activation-controlled region and showed a maximum in 2 M CH3OH at high currents. IR-free polarization curves showed that the activity of the quaternary catalyst was superior to Pt metal/C samples having the same Pt amount. The presence of semi-insulating metal oxides such as RuO2, SnO2 and WO3 on the electrode surface exhibited a significant uncompensated resistance. The single-cell performance was lower than that predicted by the half-cell experiments mainly due to the methanol cross-over through the Nafion membrane.