Potential-Dependent Active Moiety of Fe–N–C Catalysts for the Oxygen Reduction Reaction
Kang Liu, Junwei Fu, Tao Luo, Ganghai Ni, Hongmei Li, Li Zhu, Ye Wang, Zhang Lin, Yifei Sun, Emiliano Cortés, Min Liu
Abstract
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 4, Fe–(OH)N 4, Fe–(O 2 )N 4, and Fe–(OH 2 )N 4 ) 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 4 moiety is hindered by the slow *OH protonation, while the Fe–(OH 2 )N 4 (0.4 V ≤ U ≤ 1.0 V), *O 2 -assisted Fe–N 4 (−0.6 V ≤ U ≤ 0.2 V), and Fe–(OH)N 4 ( 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 z 2 (spin-up) and Fe 3d xz (spin-down) orbitals. Overall, our findings provide guidance for understanding the active moieties of SACs.