Oxygen reduction catalysis of the Pt3Co alloy in alkaline and acidic media studied by x-ray photoelectron Spectroscopy and electrochemical methods

The surface structure, stability, and electrocatalytic activity of the Pt3Co alloy toward oxygen reduction reaction (ORR) (with reference to Pt black) in both alkaline and acidic media were studied by X-ray photoelectron spectroscopy (XPS), cyclic voltammetry, and by the rotating disk electrode method. The XPS data show that two species of cobalt, metallic cobalt and cobalt oxides, coexist in the as-received Pt3Co sample. In combination with the voltarnmetric results, the XPS data demonstrate that there is dissolution of cobalt upon exposure of the sample to acidic media, whereas the cobalt forms in the catalyst are stable in alkaline media. In addition, the Pt 4f binding energy was found by XPS to be upshifted by 0.2 eV vs that of the pure Pt black reference. In acids, Pt3Co alloy has a slightly but demonstratively higher activity than Pt black toward the oxygen reduction reaction as shown by the positive half-wave potential shift of ca. 10 +/- 2 mV, while in alkaline media a decrease in the ORR activity was found. The latter was indicated by the half-wave potential negative shift of about 30 +/- 4 mV vs the half-wave potential of the Pt reference. However, the activity of the Pt3Co alloy catalyst (vs the Pt black reference) was restored by holding the alloy catalyst at 0.85 V in acid. The observed catalytic enhancement is overall due to Pt skin formation as concluded from the literature study and from the data of this work.