LAUSR

DOI: 10.1002/aenm.201803771 proton exchange membrane fuel cells (PEMFCs).[1–4] Pt nanoparticles (NPs) supported on carbon (Pt/C) is the-stateof-the-art ORR catalyst equipped at the cathode of a PEMFC. However, the prohibitive cost and unsatisfactory activity/ durability of the Pt/C catalyst, mainly caused by the Pt NP dissolution/migration in acidic environments, have been the greatest obstacle of the commercialization of PEMFCs. To this end, considerable research efforts have been devoted to developing advanced Pt-based ORR electrocatalysts with low Pt content, enhanced activity, and high stability.[5–12] During the last decade, alloying Pt with early transition metals (M) such as Fe, Co, Ni, and Cu with well-controlled size, shape, and composition represents the leading research activities in the community and dramatic enhancements in ORR activity and reduced usage of Pt have been achieved.[8,13–17] Specifically, theoretical calculations have indicated that the ORR activity improvement on such Pt–M catalysts can be attributed to the downshift of d-band center and the optimized adsorption strength of oxygenated reaction intermediates (OH*, O*, OOH*, etc.) compared to the Pt/C catalyst.[14,18] However, Pt–M alloy NPs usually Engineering the crystal structure of Pt–M (M = transition metal) nanoalloys to chemically ordered ones has drawn increasing attention in oxygen reduction reaction (ORR) electrocatalysis due to their high resistance against M etching in acid. Although Pt–Ni alloy nanoparticles (NPs) have demonstrated respectable initial ORR activity in acid, their stability remains a big challenge due to the fast etching of Ni. In this work, sub-6 nm monodisperse chemically ordered L10-Pt–Ni–Co NPs are synthesized for the first time by employing a bifunctional core/shell Pt/NiCoOx precursor, which could provide abundant O-vacancies for facilitated Pt/Ni/Co atom diffusion and prevent NP sintering during thermal annealing. Further, Co doping is found to remarkably enhance the ferromagnetism (room temperature coercivity reaching 2.1 kOe) and the consequent chemical ordering of L10-Pt–Ni NPs. As a result, the best-performing carbon supported L10-PtNi0.8Co0.2 catalyst reveals a half-wave potential (E1/2) of 0.951 V versus reversible hydrogen electrode in 0.1 m HClO4 with 23-times enhancement in mass activity over the commercial Pt/C catalyst along with much improved stability. Density functional theory (DFT) calculations suggest that the L10-PtNi0.8Co0.2 core could tune the surface strain of the Pt shell toward optimized Pt–O binding energy and facilitated reaction rate, thereby improving the ORR electrocatalysis.