Octahedral to Cuboctahedral Shape Transition in 6 nm Pt 3 Ni Nanocrystals for Oxygen Reduction Reaction Electrocatalysis

Pt 3 Ni stands as one of the most active electrocatalysts for the oxygen reduction reaction (ORR). The activity varies with the morphology of the nanocrystals with a high activity observed for the octahedral shape where only the high density {111} crystallographic planes are exposed. Herein, the synthesis of 6 nm Pt 3 Ni octahedral nanocrystals with a Pt enriched shell or cuboctahedral nanocrystals with a Ni enriched shell is described. Interestingly, the cuboctahedral nanocrystals display a six‐pointed star/skeleton of platinum, which features a very uncommon atomic distribution. In the synthesis, a decrease in the oxygen partial pressure induces the transition from octahedral to cuboctahedral morphology. The octahedral and cuboctahedral nanocrystals both demonstrate high ORR activity (1.1 mA cm −2 Pt and 1.2 A mg −1 Pt at 0.9 V vs reversible hydrogen electrode (RHE) are the highest values obtained for PtNi‐20 and PtNi‐15, respectively). After exposure to oxidative conditions in the acidic electrolyte, the cuboctahedral nanoparticles with a pristine Ni‐rich skin show a Pt skin and retain their cuboctahedral morphology.