Pseudo-Jahn–Teller Effect-Mediated Medium-Spin (S = 3/2) Fe for Robust Fe–N Bonds in Acidic Oxygen Reduction and Fuel Cells

Iron–nitrogen–carbon (Fe–N–C) materials are promising candidates, but their practical application is hindered by rapid demetalation of Fe sites under acidic operating conditions. Here, we show that introducing a <i>p-</i>block metal (Bi) adjacent to the FeN₄ motif triggers a pseudo-Jahn–Teller (p-J-T) distortion.

This distortion stabilizes a medium-spin Fe(III) configuration (S = 3/2) with an empty <i>d</i>_x²–y² orbital. This empty orbital accepts lone-pair electrons from the nitrogen ligands, strengthening the Fe–N bond and suppressing proton-induced demetalation. The resulting FeBi–N–C catalyst exhibits a dramatically strengthened Fe–N bond, as evidenced by a shortened bond length (from 1.99 Å to 1.96 Å) and a 3-fold higher Fe retention after 20,000 cycles (78.8% vs 26.7%).