Structural insights and electronic state analysis of PtNi nanowire catalysts by operando high-energy resolution fluorescence detection X-ray absorption spectroscopy

Abstract

Low-platinum alloy nanowire (NW) catalysts improve the performance of polymer electrolyte membrane fuel cells, but ultrafine PtNi-NW catalysts, particularly those with high Ni content, often experience significant Ni leaching during electrochemical processes. In this study, a Pt-rich surface layer was successfully formed on PtNi NWs through a simple post-annealing process. Pair distribution function (PDF) analysis revealed a Pt surface layer with a face-centered cubic (fcc) structure, along with internal body-centered tetragonal (bct) PtNi and fcc Ni phases. Operando X-ray absorption spectroscopy (XAS), including conventional and high-energy resolution fluorescence detection (HERFD) XAS, was used to investigate the catalysts' electronic states and structural changes under oxygen reduction reaction (ORR) conditions. The results showed that the Pt-rich surface layer of PtNi-NW/C catalysts, with its short Pt–Pt bond lengths, effectively suppresses Pt oxidation at high polarization potentials and restricts Ni leaching, leading to a significant increase in the ORR-specific activity of 2.07 mA cm_Pt⁻², which is a 2.5- and 7-fold activity enhancement compared to Pt-NW/C and commercial Pt/C catalysts.