Thermal annealing synthesis of double-shell truncated octahedral Pt-Ni alloys for oxygen reduction reaction of polymer electrolyte membrane fuel cells

Shape-controlled Pt-Ni alloys usually offer an exceptional electrocatalytic activity toward the oxygen reduction reaction (ORR) of polymer electrolyte membrane fuel cells (PEMFCs), whose tricks lie in well-designed structures and surface morphologies. In this paper, a novel synthesis of truncated octahedral PtNi 3.5 alloy catalysts that consist of homogeneous Pt-Ni alloy cores enclosed by NiO-Pt double shells through thermally annealing defective heterogeneous PtNi 3.5 alloys is reported. By tracking the evolution of both compositions and morphologies, the outward segregation of both PtO x 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 octahedron 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 Pt 2.1Ni 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 Pt 2.1Ni core-shell alloy catalyst based membrane electrolyte assembly (MEA) reaches 679.8 mW/cm 2, increased by 138.4 mW/cm 2 relative to that based on commercial Pt/C.