Palladium–Platinum Core–Shell Electrocatalysts for Oxygen Reduction Reaction Prepared with the Assistance of Citric Acid

Core–shell structure is a promising alternative to solid platinum (Pt) nanoparticles as electrocatalyst for oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFCs). A simple method of preparing palladium (Pd)–platinum (Pt) core–shell catalysts (Pd@Pt/C) in a gram-batch was developed with the assistance of citric acid. The Pt shell deposition involves three different pathways: galvanic displacement reaction between Pd atoms and Pt cations, chemical reduction by citric acid, and reduction by negative charges on Pd surfaces. The uniform ultrathin (∼0.4 nm) Pt shell was characterized by in situ X-ray diffraction (XRD) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images combined with electron energy loss spectroscopy (EELS). Compared with state-of-the-art Pt/C, the Pd@Pt/C core–shell catalyst showed 4 times higher Pt mass activity and much better durability upon potential cycling. Furthermore, both the mass activity and durability were comparable to that of Pd@Pt/C synthesized by a Cu-mediated-Pt-displacement method, which is more complicated and difficult for mass production.