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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 3dz2 (spin-up) and Fe 3dxz (spin-down) orbitals. Overall, our findings provide guidance for understanding the active moieties of SACs.
Liu et al. (Wed,) studied this question.