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

Proton-exchange membrane fuel cells (PEMFCs) require efficient and durable platinum-group-metal-free (PGM-free) cathode catalysts for the oxygen reduction reaction (ORR). 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 p-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 dx2-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%). This structural stability translates into a peak power density of 720 mW cm-2 in H2-O2 PEMFCs and 90% current retention after 150 h at 0.60 V. Our findings establish the p-J-T effect as a new chemical strategy to stabilize metal-nitrogen bonds against acid-induced demetalation.