Abstract
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. Operando spectroscopic studies have identified potential-induced reversible Fe-N switching in the FeN (x) 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 ab initio molecular dynamics (CP-AIMD), we reveal that pyridinic FeN(4) sites transit reversibly between planar OH*-Fe(3+)N(4) and out-of-plane H(2)O*-Fe(2+)N(4) configurations at 0.8 V, mirroring the experimental Fe-N switching phenomenon. This shift arises from a spin-state transition: intermediate-spin Fe(3+) (S = 3/2) converts to high-spin Fe(2+) (S = 2) as potential decreases, driven by the pseudo Jahn-Teller effect and strong H(2)O binding on the high-spin Fe(2+) center. Additionally, a metastable 2H(2)O*-Fe(2.5+)N(4) 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 FeN(4) sites, challenging the long-held assumption that D1 sites are pyrrolic FeN(4). 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.