Effect of Alloy Composition and Crystal Face of Pt-Skin/Pt100–xCox (111), (100), and (110) Single Crystal Electrodes on the Oxygen Reduction Reaction Activity

We have examined and compared for the first time the oxygen reduction reaction (ORR) activities at a series of (111), (100), and (110) faces of Pt100–xCox single crystals as a function of Co content x with rotating disk electrodes in air-saturated 0.1 M HClO4 solution. The low energy electron diffraction and low energy ion scattering indicated that the topmost surfaces of Pt–Co(111), (100), and (110) have well-defined (1×1) Pt-skin layers as a result of annealing in pure H2. Among the three faces examined, the effect of Co content was found to be the most dramatic for the (111) face: specifically, the jk values on the Pt-skin/Pt–Co(111) electrodes increased greatly as x increased and reached a maximum value at x = 27 atom %, which is ca. 27 times larger than that for pure Pt(111). On the Pt-skin/Pt100–xCox(110) electrodes, the jk values reached a maximum at x = 18 atom %, which is 4 times larger than that of pure Pt(110). Unlike other faces, the Pt-skin/Pt–Co(100) electrodes exhibited a weak dependence of jk on the Co content x. The overall order of activity was (100) < (110) ≪ (111), so that the enhancement factor for jk on the (111) face was unique, being significantly larger than the others. Our results can provide an important insight for the significant increase in the ORR activity depending not only on the alloy composition but also on the atomic arrangement of the exposed Pt-skin surface.