Activity for the ORR on Pt-Pd-Co ternary alloy electrodes is markedly affected by surface structure and composition

The oxygen reduction reaction (ORR) has been studied on single crystals of the ternary alloys Pt₄₅Pd₄₅Co₁₀ and Pt₇₇Pd₈Co₁₅ in O₂-saturated 0.1 M HClO₄ using a hanging meniscus rotating disk electrode. Specific activity for the ORR depends markedly on the composition and surface structure of the single crystal electrodes. The activity on Pt₄₅Pd₄₅Co₁₀(hkl) increases in the order Pt₄₅Pd₄₅Co₁₀(1 0 0) ≈ Pt₄₅Pd₄₅Co₁₀(1 1 1) < Pt₄₅Pd₄₅Co₁₀(1 1 0), whereas the activity on Pt₇₇Pd₈Co₁₅(hkl) is Pt₇₇Pd₈Co₁₅(1 0 0) < Pt₇₇Pd₈Co₁₅(1 1 0) < Pt₇₇Pd₈Co₁₅(1 1 1). The ORR activity of Pt₄₅Pd₄₅Co₁₀(1 1 0) is 2.6 times higher than that of Pt₃Co(1 1 0), which is known to have high ORR activity. The activity of Pt₇₇Pd₈Co₁₅(1 1 1) is 2.4 times higher than that of Pt₃Co(1 1 1). Low energy ion scattering spectroscopy (LE-ISS) shows that Pt and Co coexist at the topmost layer of Pt₇₇Pd₈Co₁₅(1 1 1). Pt, Pd and Co atoms are dissolved uniformly in the inner layers according to elemental mapping by scanning transmission electron microscopy with energy-dispersive X-ray spectroscopy (STEM-EDS).