Atomic Dispersion and Surface Enrichment of Palladium in Nitrogen-Doped Porous Carbon Cages Lead to High-Performance Electrocatalytic Reduction of Oxygen

Metal-nitrogen-carbon (MNC) nanocomposites have been hailed as promising and efficient electrocatalysts toward oxygen reduction reaction (ORR), due to the formation of MN_x coordination moieties. However, MNC hybrids are mostly prepared by pyrolysis of organic precursors along with select metal salts, where part of the MN_x sites are inevitably buried in the carbon matrix. This limited accessibility compromises the electrocatalytic performance. Herein, we describe a wet-impregnation procedure by facile thermal refluxing, whereby palladium is atomically dispersed and enriched onto the surface of hollow, nitrogen-doped carbon cages (HNC) forming Pd-N coordination bonds. The obtained Pd-HNC nanocomposites exhibit an ORR activity in alkaline media markedly higher than that of metallic Pd nanoparticles, and the best sample even outperforms commercial Pt/C and relevant Pd-based catalysts reported in the literature. The results suggest that atomic dispersion and surface enrichment of palladium in a carbon matrix may serve as an effective strategy in the fabrication of high-performance ORR electrocatalysts.