Oxygen reduction on carbon-supported Pt-Ni and Pt-Co alloy catalysts

We describe a comparative study of the oxygen reduction reaction on two carbon-supported Pt-based alloy catalysts in aqueous acidic electrolyte at low temperature. Both alloys have the bulk compositions of 50 and 75 at. % Pt, with the alloying elements being Ni and Co. Comparison is made to a pure Pt catalyst on the same carbon support, Vulcan XC-72, having the same metal loading (20 wt %) and nominally the same particle size (4 +/- 2 nm). High-resolution electron microscopy was used to determine the size and shape of the particles as well as the particle size distribution on all catalysts. Electrochemical measurements were performed using the thin-film rotating ring-disk electrode method in 0.1 M HClO4 at 20-60 degreesC. Hydrogen adsorption pseudocapacitance was used to determine the number of Pt surface atoms and to estimate the surface composition of the alloy catalysts. Kinetic analysis in comparison to pure Pt revealed a small activity enhancement (per Pt surface atom) of ca. 1.5 for the 25 at. % Ni and Co catalysts, and a more significant enhancement of a factor of 2-3 for the 50 at. % Co. The 50 at. % Ni catalyst was less active than the Pt standard and unstable at oxygen electrode potentials. Ring-current collection measurements for peroxide indicated no significant differences between the Pt-Co catalysts or the 25 at. % Ni catalyst and pure Pt, while the 50 at. % Ni catalyst had a higher peroxide yield. Together with the observed Tafel slopes and activation energies, it was concluded that the kinetic enhancement is contained in the preexponential factor of the conventional transition state theory rate expression. It is, however, not clear why the alloying with Ni or Co produces this change in the preexponential factor.