Structural and Electronic Stabilization of PtNi Concave Octahedral Nanoparticles by P Doping for Oxygen Reduction Reaction in Alkaline Electrolytes.

The enhancement in the catalytic activity of PtM (transition metals, TMs) alloy nanoparticles (NPs) results from the electronic structure of Pt being modified by the TM. However, the oxidation of the TM would lead to the electronegativity difference between Pt and TM being much lowered, which induces a decrease in the number of electrons transferred from the TM to Pt, resulting in excessive oxygenated species accumulating on the surface of Pt, thus deteriorating their performance. In this work, the oxygen reduction reaction (ORR) performance of PtNi (Pt68Ni32) concave octahedral NPs (CONPs) in alkaline electrolytes is much improved by doping small amounts of phosphorus. The P-doped PtNi CONPs (P-PtNi) show about 2 and 10 times enhancement for ORR compared to PtNi and commercial Pt/C catalysts. The high-angle annular dark-field scanning transmission electron microscopy and energy-dispersive X-ray spectroscopy mapping characterizations reveal that the P dopant uniformly distributes throughout the CONPs, Pt mainly locates at the edges and corners, whereas Ni situates at the center, forming a P-doped Pt-frame@Ni quasi-core-shell CONP. The X-ray photoelectron spectroscopy spectra indicate that the P dopant obviously increases the electron density of Pt compared with that of PtNi NPs, which contributes to the stabilization of the electronic structure of PtNi CONPs, thus restraining the excessive HO2- species produced on the catalysts, which endow them with a high catalytic performance in the ORR. In addition, the P attached to the Ni sites in the PtNi NPs partially prevents the Ni atoms being oxidized by the external O species, which is conducive to the structural and electrochemical stability of the PtNi NPs during the ORR. The present results provide a new insight into the development of ORR catalysts with low utilization of Pt.