Impact of Active Site Density on Oxygen Reduction Reactions Using Monodispersed Fe-N-C Single-Atom Catalysts.

Exploring the impact of active site density on catalytic reactions is crucial for reaching a more comprehensive understanding of how single-atom catalysts work. Utilizing density functional theory calculations, we have systematically investigated the neighboring effects between two adjacent Fe-N-C sites of monodispersed Fe-N-C single-atom catalysts on oxygen reduction reaction (ORR). While the thermodynamic limiting potential (UL) is strongly dependent on the intersite distance and the nature of adjacent active sites in FeN3, it is almost invariable in FeN4 until two FeN4 sites are ∼4 Å apart. Further, under certain conditions, an otherwise unfavorable physisorbed-O2-initiated 2e- pathway becomes feasible due to charge transfer between reactive species and graphene support. Our results cast new insight into the rational design of high-density single-atom catalysts and may create an alternative route to manipulate their catalytic activities.