Deciphering potential-driven dynamics in Fe–N–C catalysts: <i>ab initio</i> insights into Fe–N switching and spin-state transition

Pyrolyzed Fe-N-C materials are cost-effective alternatives to Pt for the acidic oxygen reduction reaction (ORR), yet the atomic and electronic structures of their active centers remain poorly understood. <i>Operando</i> spectroscopic studies have identified potential-induced reversible Fe-N switching in the FeN _<i>x</i> active centers of D1 type, which provides a unique opportunity to decode their atomic structures, but the mechanism driving this behavior has been elusive. Herein, using constant-potential <i>ab initio</i> molecular dynamics (CP-AIMD), we reveal that pyridinic FeN₄ sites transit reversibly between planar OH*-Fe³⁺N₄ and out-of-plane H₂O*-Fe²⁺N₄ configurations at 0.8 V, mirroring the experimental Fe-N switching phenomenon. This shift arises from a spin-state transition: intermediate-spin Fe³⁺ (<i>S</i> = 3/2) converts to high-spin Fe²⁺ (<i>S</i> = 2) as potential decreases, driven by the pseudo Jahn-Teller effect and strong H₂O binding on the high-spin Fe²⁺ center. Additionally, a metastable 2H₂O*-Fe^2.5+N₄ configuration exists, acting as a transitional state during the reversible switching process. Calculated X-ray absorption and Mössbauer spectra based on CP-AIMD align closely with experimental data, bridging the theoretical predictions and experimental observations. Crucially, this dynamic Fe-N switching is unique to pyridinic FeN₄ sites, challenging the long-held assumption that D1 sites are pyrrolic FeN₄. This study clarifies the potential-driven dynamics and active center structures in Fe-N-C catalysts and will help to precisely design Fe-based ORR catalysts.