Role of Hydrous Ruthenium Oxide in Pt−Ru Direct Methanol Fuel Cell Anode Electrocatalysts: The Importance of Mixed Electron/Proton Conductivity

Abstract

Pt−Ru is the favored anode catalyst for the oxidation of methanol in direct methanol fuel cells (DMFCs). The nanoscale Pt−Ru blacks are accepted to be bimetallic alloys as based on their X-ray diffraction patterns. Our bulk and surface analyses show that although practical Pt−Ru blacks have diffraction patterns consistent with an alloy assignment, they are primarily a mix of Pt metal and Ru oxides plus some Pt oxides and only small amounts of Ru metal. Thermogravimetric analysis and X-ray photoelectron spectroscopy of as-received Pt−Ru electrocatalysts indicate that DMFC materials contain substantial amounts of hydrous ruthenium oxide (RuO x H y ). A potential misidentification of nanoscale Pt−Ru blacks arises because RuO x H y is amorphous and cannot be discerned by X-ray diffraction. Hydrous ruthenium oxide is a mixed proton and electron conductor and innately expresses Ru−OH speciation. These properties are of key importance in the mechanism of methanol oxidation, in particular, Ru−OH is a critical component of the bifunctional mechanism proposed for direct methanol oxidation in that it is the oxygen-transfer species that oxidatively dissociates −C⋮O fragments from the Pt surface. The catalysts and membrane-electrode assemblies of DMFCs should not be processed at or exposed to temperatures >150 °C, as such conditions deleteriously lower the proton conductivity of hydrous ruthenium oxide and thus affect the ability of the Ru component of the electrocatalyst to dissociate water. With this analytical understanding of the true nature of practical nanoscale Pt−Ru electrocatalysts, we can now recommend that hydrous ruthenium oxide, rather than Ru metal or anhydrous RuO 2, is the preferred Ru speciation in these catalysts.