Brønsted Basic Imidazole Nitrogen Induced by Sb‑N₄ Sites Serves as the Active Site for Efficient Oxygen Reduction

The conventional understanding of metal-nitrogen-carbon (M-N-C) catalysts for the oxygen reduction reaction (ORR) has long been dominated by a metal-centered paradigm, wherein metal sites are regarded as the sole or primary active centers, while nitrogen species within the carbon framework are generally considered to be of secondary significance or even negligible. Here, we developed a potentiometric titration method that allows for the direct identification and quantification of Brønsted basic sites to investigate the origin of catalytic activity in atomically dispersed Sb supported on graphene. We provide convergent evidence that the Sb-N₄ motif induces the formation of Brønsted basic imidazole nitrogen species, and these imidazole-N sites, rather than the Sb centers, serve as the main active sites for ORR. Furthermore, phosphorus doping is shown to enhance the ORR kinetics significantly through N 2p-P 3p orbital coupling, which fine-tunes the electronic structure of the imidazole-N sites and optimizes intermediate adsorption. Our findings demonstrate that Brønsted basic imidazole nitrogen species, which have long been overlooked in M-N-C catalysis, make a substantial and mechanistically significant contribution to ORR performance, and that their formation and precise regulation are equally critical to catalyst activity as the traditionally emphasized metal-site engineering.