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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 3dz2 (spin-up) and
Fe 3dxz (spin-down) orbitals. Overall,
our findings provide guidance for understanding the active moieties
of SACs