Enhanced Oxygen Reduction Reaction Activity and Characterization of Pt–Pd/C Bimetallic Fuel Cell Catalysts with Pt-Enriched Surfaces in Acid Media

Three types of bimetallic Pt–Pd nanoparticles with different core–shell structures besides Pt and Pd nanoparticles were synthesized by coreduction and sequential reduction methods in ethylene glycol. The synthesized nanoparticles were supported on carbon to prepare five different electrocatalysts Pt/C, Pd/C, PdPt alloy/C, Pd­(core)–Pt­(shell)/C, and Pt­(core)–Pd­(shell)/C for oxygen reduction reaction (ORR) in fuel cells. The nanoparticles and supported catalysts were characterized by means of transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FT-IR), X-ray powder diffraction (XRD), extended X-ray absorption fine structure (EXAFS), and cyclic voltammetry (CV). It was proposed by these characterizations that the PdPt alloy/C, Pd­(core)–Pt­(shell)/C, and Pt­(core)–Pd­(shell)/C catalysts constituted Pd4Pt1(core)–Pt­(two-layers shell), Pd (core)–Pd2Pt1(three-layers)–Pt­(three-layers shell), and Pt­(core)–Pt2Pd1(two-layers)–Pd (microcrystal shell), respectively. The Pt surface-enriched catalysts were more stable than the Pd surface-enriched catalysts in long-term CV scanning in acid electrolyte. The Pt/C, PdPt alloy/C, and Pd­(core)–Pt­(shell)/C catalysts with Pt-enriched surfaces showed much higher ORR specific activity than the Pd/C and Pt­(core)–Pd­(shell)/C catalysts with Pd-enriched surfaces. The Pt surface-enriched bimetal catalysts with core–shell structures showed the higher Pt-based mass activity than the Pt monometal catalyst. The PdPt catalysts with Pd/Pt = 2 and 4 in an atomic ratio were also prepared by the coreduction method. The Pt-enriched surfaces formed also with these samples, but the ORR specific activity and (Pd + Pt)-based mass activity decreased with increasing Pd/Pt ratios (1, 2, and 4). The present study provided core–shell catalysts with better ORR activity, which may be useful for understanding key issues to develop next-generation fuel-cell cathode catalysts.