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

Abstract

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.