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Composition-controlled synthesis of carbon-supported Pt-Co alloy nanoparticles and the origin of their ORR activity enhancement.

The ability to precisely tune the chemical compositions and electronic structures of nanoalloy catalysts is essential to achieve the goals of high activity and selectivity for the oxygen reduction reaction (ORR) on the catalysts by design. In this work, we synthesized carbon-supported Pt-Co alloy nanoparticles with controlled bimetallic compositions (Pt/Co atomic ratio = 81 : 19, 76 : 24, 59 : 41, 48 : 52, 40 : 60 and 26 : 74) by regulating solution pH and the amount of Pt and Co precursor salts to elucidate the effect of catalyst composition on ORR activity. The obtained Pt-Co alloy nanoparticles have face-centred cubic (fcc) structures and are well-dispersed on the surface of the carbon support with a narrow particle size distribution (2-4 nm diameters). The electrocatalysis experiments in alkaline solution reveal a strong correlation between ORR activity and the alloy composition of the catalysts. Interestingly, the mass-specific activities of the catalysts manifest a typical double-volcano plot as a function of alloy composition. In this Pt-Co alloy series, the catalyst with a Pt : Co atomic ratio of 76 : 24 exhibits the best ORR performance, which is remarkably higher than that of the commercial Pt/C (E-TEK). X-ray photoelectron spectroscopy (XPS) measurements demonstrate that the electronic structures of these catalysts can be tuned by controlling their alloy compositions, which are highly correlated with the trends in ORR activity. The origin of the enhancement in ORR activity may be strongly related to the unique chemical surface structures of the catalysts.