Oxygen reduction reaction (ORR) activity and stability of a molecular beam epitaxially (MBE) prepared Pt-enriched Ni
/Pt(111) surface was investigated. Reflection high-energy electron diffraction (RHEED) patterns, and a scanning tunneling microscopic (STM) image indicated that the Pt-enriched surface has long-range-ordered six-fold symmetry with atomic-scale corrugations. ORR activity was estimated by kinetically-controlled current density at 0.9 V vs. a reversible hydrogen electrode, and the as-prepared Pt-enriched surface showed 8-times-higher ORR activity than clean Pt(111). The activity steeply reduced during potential cycling between 0.6 V and 1.0 V. After 1000 potential cycles, the enhancement factor was 2.5 and a cyclic voltammetry (CV) curve exhibited an increase in EC charge for the H
region accompanied by the emergence of a 0.13 V redox feature caused by (110) surface defects. These results suggest that the electrochemical stability of the underlying Ni atoms determines the durability of Pt-Ni alloy catalysts.