Abstract
Pt−Ni alloy nanoparticles supported on carbon black (Pt:Ni = 1:1) were prepared by the borohydride reduction method using acetate anions as a stabilizer in anhydrous ethanol solvent. Here, we surveyed the effect of oxide phases in Pt−Ni alloy nanoparticles on the electrocatalytic activity toward oxygen reduction reaction (ORR). As-prepared Pt1Ni1/C, which showed a relatively high degree of alloying, possessed the lower oxygen reduction reaction (ORR) activity as compared to pure Pt. However, following heat treatment in a flow of Ar at 300 °C for 3 h, Pt1Ni1/C showed oxygen reduction activity higher than that of commercial Pt/C (40 wt % Pt/C, Johnson-Matthey). The potential of zero total charge (PZTC) was calculated from cyclic voltammograms and the CO-displacement charge at dosing potentials at which anions are the main adsorbed species. The calculated value then shifted to a more positive potential after heat treatment. This indicates that the surface of the Pt−Ni nanoparticles became less oxophilic mainly due to the clustering of Pt. This anodic shift of the PZTC is consistent with the results of X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and X-ray absorption near-edge structure spectroscopy (XANES). Consequently, the observed catalytic enhancement by heat treatment is due to the increase of metallic Pt and NiO and the phase separation between metallic Pt and Ni oxides.
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