Tailoring, structure, and activity of carbon-supported nanosized Pt-Cr alloy electrocatalysts for oxygen reduction in pure and methanol-containing electrolytes

The oxygen-reduction kinetics on Vulcan XC-72 carbon-supported nanosized Pt-Cr alloy catalysts were studied using the porous rotating disk electrode technique in pure and methanol-containing electrolytes. The Vulcan XC-72 carbon-supported Pt-Cr alloy catalysts with different Pt/Cr atomic ratios were prepared via a Pt-carbonyl route. X-ray diffraction data showed that the as-prepared nanosized Pt-Cr alloy catalysts mainly have the disordered structures (solid solution) and that the lattice parameter decreases with the increase in Cr content. Energy-dispersive X-ray analysis indicated that the catalyst compositions are nearly the same as the nominal ones. The obtained Pt-Cr alloy nanoparticles are well dispersed on the surface of carbon with a relatively narrow size distribution. For example, the mean particle size of the as-prepared Pt-Cr (1: 1)/C catalyst with 20 wt % metal loading is about 3.1 nm in diameter with a standard deviation of 1.3 nm, and the particle size distribution is relatively narrow. As compared to the Pt/C catalyst, the bimetallic alloy catalysts with the different Pt/Cr atomic ratios showed slightly enhanced mass activity (MA) for the oxygen reduction reaction (ORR); however, the significant enhancement in the specific activity (SA) by a factor of about 1.5-3 for the ORR was found on the Pt-Cr alloy catalysts in pure HClO4 solution. This enhancement in SA of the Pt-based catalysts was correlated to the changes in the lattice parameter and Pt/Cr surface composition. Moreover, the bimetallic Pt-Cr alloy catalysts with the different Pt/Cr atomic ratios exhibited much higher methanol tolerance during the ORR than the Pt/C catalyst. Furthermore, the catalytic activity for methanol oxidation on the Pt-Cr alloy catalysts was much lower than that on the Pt/C catalyst. Thus, the high methanol tolerance of the carbon-supported Pt-Cr alloy catalysts for the ORR can be ascribed to the weak adsorption of methanol induced by the presence of Cr atoms in the alloys.