Potential-Dependent Active Moiety of Fe-N-C Catalysts for the Oxygen Reduction Reaction.

The real active moiety of Fe-N-C single-atom catalysts (SACs) during the oxygen reduction reaction (ORR) depends on the applied potential. Here, we examine the ORR activity of various SAC active moieties (Fe-N, Fe-(OH)N, Fe-(O)N, and Fe-(OH)N) over a wide potential window ranging from -0.8 to 1.0 V (vs. SHE) using constant potential density functional theory calculations. We show that the ORR activity of the Fe-N moiety is hindered by the slow *OH protonation, while the Fe-(OH)N (0.4 V ≤ ≤ 1.0 V), *O-assisted Fe-N (-0.6 V ≤ ≤ 0.2 V), and Fe-(OH)N ( = -0.8 V) moieties dominate the ORR activity of the Fe-N-C catalysts at different potential windows. These oxygenated species modified the single-atom Fe sites and can promote *OH protonation by regulating the electron occupancy of the Fe 3d (spin-up) and Fe 3d (spin-down) orbitals. Overall, our findings provide guidance for understanding the active moieties of SACs.