Facet-controlled Pt–Ir nanocrystals with substantially enhanced activity and durability towards oxygen reduction

Abstract

Nanocrystals made of Pt–Ir alloys are fascinating catalysts towards the oxygen reduction reaction (ORR), but the lack of control over their surface atomic structures hinders further optimization of their catalytic performance. Here we report, for the first time, a class of highly active and durable ORR catalysts based on Pd@Pt–Ir nanocrystals with well-controlled facets. With an average of 1.6 atomic layers of a Pt4Ir alloy on the surface, the nanocrystals can be made in cubic, octahedral, and icosahedral shapes to present Pt–Ir {1 0 0}, {1 1 1}, and {1 1 1} plus twin boundaries, respectively. The Pd@Pt–Ir nanocrystals exhibit not only facet-dependent catalytic properties but also substantially enhanced ORR activity and durability relative to a commercial Pt/C and their Pd@Pt counterparts. Among them, the Pd@Pt–Ir icosahedra deliver the best performance, with a mass activity of 1.88 A·mg−1Pt at 0.9 V, which is almost 15 times that of the commercial Pt/C. Our density functional theory (DFT) calculations attribute the high activity of the Pd@Pt–Ir nanocrystals, and the facet dependence of these activities, to easier protonation of O* and OH* under relevant OH* coverages, relative to the corresponding energetics on clean Pd@Pt surfaces. The DFT calculations also indicate that incorporating Ir atoms into the Pt lattice destabilizes OH–OH interactions on the surface, thereby raising the oxidation potential of Pt and greatly improving the catalytic durability.