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

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 O₂-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_4-2H₂O* 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 FeN₄ sites.