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Exploring a Pt-based catalyst with a low coordination number is crucial to develop efficient oxygen reduction reaction (ORR) catalysts in fuel cells. Herein, an open-structured Pt-rich PtCo nanoflower (NF) catalyst assembled by radial ultrathin nanosheets (∼1.5 nm) with a low coordination number (8.65) is synthesized. The Pt-rich PtCo NFs exhibit excellent ORR performance in aspects of mass and specific activity (2.63 A mgPt –1 and 11.23 mA cm–2, respectively), which are 17.5 and 38.7 times that of commercial Pt/C, respectively, and a current density of 1854 mA cm–2 is achieved at 0.6 V in a single fuel cell. The superior ORR performance is mainly attributed to the low coordination number of Pt, which is induced by the optimized catalytic interface, including defect sites, high-index facets, and twin boundary. Also, theoretical calculations reveal that the ORR performance of a low-coordination site is much higher than that of a high-coordination site, the tendency of which is contrary to the d-band center. This work provides an in-depth understanding of the relationship between coordination number and electrocatalytic performance at the atomic scale, which is conducive to designing more robust and efficient alloy catalysts.

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