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Shape-controlled Pt-Ni alloys usually offer an exceptional electrocatalytic activity toward the oxygen reduction reaction(ORR)of polymer electrolyte mem-brane fuel cells(PEMFCs),whose tricks lie in well-designed structures and surface morphologies.In this paper,a novel synthesis of truncated octahedral PtNi3.5 alloy catalysts that consist of homogeneous Pt-Ni alloy cores enclosed by NiO-Pt double shells through thermally annealing defective heterogeneous PtNi3.5 alloys is reported.By tracking the evolution of both compositions and morphologies,the outward segregation of both PtOx and NiO are first observed in Pt-Ni alloys.It is speculated that the diffusion of low-coordination atoms results in the formation of an energetically favorable truncated octahe-dron while the outward segregation of oxides leads to the formation of NiO-Pt double shells.It is very attractive that after gently removing the NiO outer shell,the dealloyed truncated octahedral core-shell structure demonstrates a greatly enhanced ORR activity.The as-obtained truncated octahedral Pt2.1Ni core-shell alloy presents a 3.4-folds mass-specific activity of that for unannealed sample,and its activity preserves 45.4％after 30000 potential cycles of accelerated degradation test(ADT).The peak power density of the dealloyed truncated octahedral Pt2.1Ni core-shell alloy catalyst based membrane electrolyte assembly(MEA)reaches 679.8 mW/cm2,increased by 138.4 mW/cm2 relative to that based on commercial Pt/C.