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 (U_L) 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.