The oxidation state and asymmetric coordination of iron control activity-stability tradeoff of Fe-N-C oxygen reduction catalysts

Abstract

Fe-N-C catalysts are the most promising non-precious alternatives to platinum group metals (PGMs) for the acidic oxygen reduction reaction (ORR) in proton-exchange membrane fuel cells (PEMFCs). However, their practical application is significantly limited by the intrinsic trade-off between activity and stability. Herein, we propose an advanced integrated framework combining AI-based large language models (LLMs) with operational DFT calculations to elucidate the factors controlling the ORR activity-stability trade-off in Fe-N-C catalysts under acidic conditions. Results show that Fe single atoms coordinated with asymmetric N atoms at a moderate oxidation state lead to a moderate-strength Fe-N bond. This configuration significantly reduces Fe dissolution, improves the adsorption energetics of key ORR intermediates, and ultimately breaks the activity-stability trade-off. Leveraging this computational guidance, we experimentally synthesize a FeN4 site coordinated with a mixture of asymmetric pyrrolic and pyridinic N atoms in Fe-N-C catalysts. This catalyst intrinsically maintains a moderate Fe oxidation state that facilitates an optimal Fe-N bond strength, thereby delivering a high acidic ORR cycle stability (<4% activity loss after 30,000 cycles) while maintaining high activity (half-wave potential (E1/2) of 0.825VRHE), significantly breaking the activity-stability trade-off. This work unveils the atomic-scale mechanisms controlling the trade-off between ORR activity and stability in Fe-N-C catalysts under acidic conditions, paving the way for accelerating the discovery of more active and stable Fe-N-C catalysts.