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-4, Fe-(OH)N-4, Fe-(O-2)N-4, and Fe-(OH2)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-N4 moiety is hindered by the slow *OH protonation, while the Fe-(OH2)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) (spindown) orbitals. Overall, our findings provide guidance for understanding the active moieties of SACs.

Automated summary

The active moieties of Fe-N-C single-atom catalysts during the oxygen reduction reaction (ORR) depend on the applied potential. Through constant potential density functional theory calculations, we investigated the ORR activity of various active moieties (Fe-N-4, Fe-(OH)N-4, Fe-(O-2)N-4, and Fe-(OH2)N-4) over a wide potential window. Our findings revealed that the Fe-(OH2)N-4, *O-2-assisted Fe-N-4, and Fe-(OH)N-4 moieties dominate the ORR activity of Fe-N-C catalysts at different potential windows by modifying the single-atom Fe sites and promoting *OH protonation. These results provide guidance for understanding the active moieties of single-atom catalysts.