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-N₄, Fe-(OH)N₄, Fe-(O₂)N₄, and Fe-(OH₂)N₄) 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-N₄ moiety is hindered by the slow *OH protonation, while the Fe-(OH₂)N₄ (0.4 V ≤ U ≤ 1.0 V), *O₂-assisted Fe-N₄ (-0.6 V ≤ U ≤ 0.2 V), and Fe-(OH)N₄ (U = -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_z² (spin-up) and Fe 3d_xz (spin-down) orbitals. Overall, our findings provide guidance for understanding the active moieties of SACs.