Substantial Impact of Spin State Evolution in OER/ORR Catalyzed by Fe–N–C

The Fe-embedded N-doped graphene (Fe–N–C) is the most representative single atom catalyst (SAC) that has shown great potentiality in electrocatalysis, such as oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). However, the active moiety of Fe–N–C is still elusive due to contradictory experimental results. Moreover, early simulations mainly focus on the thermodynamic potential of adsorbates, while the effect of spin multiplicity receives little attention. To explore the role of spin multiplicity in electrocatalysis, we employ the constant-potential density functional theory (DFT) to systematically study the structural evolution of the high-spin (HS) and intermediate-spin (IS) FeN4 site (marked by FeN4HS/IS) in OER and ORR processes. With the consideration of spin multiplicity, our simulation shows spontaneous oxidation from Fe(II)N4IS to Fe(III)N4HS at potential U = 0.4 V versus SHE. Further simulation indicates that the FeN4IS site undergoes a sequential adsorption of *OH and *OOH along with U increase, which leads to the spin state transition from IS to HS. According to the constant-potential free energy analysis, the FeN4HS*OOH is confirmed to be the practical active centers of OER, while the FeN4HS*OH and FeN4IS are assigned to the active center of ORR in low and high overpotentials. The predicted ORR activity of FeN4HS*OH agrees with the in situ X-ray absorption near-edge spectroscopy (XANES) and 57Fe Mössbauer spectroscopy measurement by Xiao et al. [Microporous Framework Induced Synthesis of Single-Atom Dispersed Fe-NC Acidic ORR Catalyst and its In Situ Reduced Fe-N4 Active Site Identification Revealed by X-Ray Absorption Spectroscopy. ACS Catal. 2018, 8, 2824–2832]. Based on the geometry and orbital analysis, the bond length of Fe–N and coordination number of Fe center are found to have a significant impact on the d orbital splitting energy and thus induce the turnover of HS/IS stability in the OER/ORR intermediates. Our study brings comprehensive insights into the evolution of coordination and spin state in Fe–N–C, which reveals the significance of spin multiplicity in electrocatalysis and benefits further theoretical design of SACs from the perspective of spin effects.