Potential‐Gated Spin Crossover Triggers Fe─N─C Catalyst Demetallation

ABSTRACT Atomically dispersed Fe─N─C catalysts are leading candidates for the oxygen reduction reaction (ORR), yet their rapid demetallation in acidic media remains a critical bottleneck for commercial fuel cell viability. While nitrogen protonation is known to correlate with demetallation, the fundamental electronic and structural triggers—specifically the role of Fe redox and spin‐state transitions at the ∼0.8 V dissolution onset—remain poorly understood. Here, using constant‐potential ab initio molecular dynamics (CP‐AIMD), we uncover a potential‐gated spin crossover mechanism that triggers O 2 ‐independent demetallation in acidic electrolytes. Near 0.8 V, reduction from Fe(III) to Fe(II) is coupled to an intermediate‐spin‐to‐high‐spin crossover that promotes pseudo‐Jahn–Teller out‐of‐plane distortion and dynamic formation of a hydrated FeN 4– 2H 2 O* precursor. The resulting Fe─N bond elongation increases the thermodynamic susceptibility of adjacent N sites to protonation, leading to rapid Fe─N bond cleavage and Fe detachment in acidic electrolyte. Simulated x‐ray absorption spectra derived from this mechanism closely reproduce experimental spectroscopic signatures of potential‐dependent Fe dissolution. These results identify a possible electronic and structural basis for protonation‐assisted Fe loss and motivate testing spin‐state and ligand‐field control as strategies for improving the stability of pyridinic FeN 4 sites.