Precise Pt-skin manipulation of strain and ligand effects for oxygen reduction

Abstract

Nanostructured platinum (Pt)-skin alloys are promising electrocatalysts for oxygen reduction reaction (ORR) due to their tunable strain and ligand effects which essentially regulate the surface electronic structures. In addition to the chemical nature of alloying elements, the layer numbers of Pt-skin crucially determine the strain and ligand effects. So far, the effects of Pt-skin layer numbers have been generally investigated through vapor deposition on bulk metals and alloys with extended surfaces, while the precise Pt-skin control of nanostructured alloy electrocatalysts in wet chemical synthesis remains fairly challenging. Herein, we develop a Pt-skin engineering strategy to construct a family of dendrite-like porous PtCu@Pt_nL nanospheres (NSs) with precisely controlled Pt-skin layers by adjusting the reducibility of Cu ions. Density functional theory calculations and X-ray photoelectron spectroscopy-based valence band spectra results indicate a concave parabolic trend of d-band center with varying the Pt-skin layer from 0 to 5, jointly resulting from the skin layer-dependent electron transfer numbers and compression strain. The two-layer Pt-skin alloy, PtCu@Pt_2L NS, is identified to have the lowest d-band center and therefore locates at the summit of ORR activity volcano. Accordingly, this carbon supported PtCu@Pt_2L NSs catalyst achieves excellent ORR electrocatalysis in H₂–O₂ proton exchange membrane fuel cells.