Fe-N-C core–shell catalysts with single low-spin Fe(II)-N₄ species for oxygen reduction reaction and high-performance proton exchange membrane fuel cells

Fe-N-doped carbon materials (Fe-N-C) are promising candidates for oxygen reduction reaction (ORR) relative to Pt-based catalysts in proton exchange membrane fuel cells (PEMFCs). However, the intrinsic contributions of Fe-N₄ moiety with different chemical/spin states (e.g. D1, D2, D3) to ORR are unclear since various states coexist inevitably. In the present work, Fe-N-C core–shell nanocatalyst with single low-spin Fe(II)-N₄ species (D1) is synthesized and identified with ex-situ ultralow temperature Mössbauer spectroscopy (T = 1.6 K) that could essentially differentiate various Fe-N₄ states and invisible Fe-O species. By quantifying with CO-pulse chemisorption, site density and turnover frequency of Fe-N-C catalysts reach 2.4 × 10¹⁹ site g⁻¹ and 23 e site⁻¹ s⁻¹ during the ORR, respectively. Half-wave potential (0.915 V_RHE) of the Fe-N-C catalyst is more positive (approximately 54 mV) than that of Pt/C. Moreover, we observe that the performance of PEMFCs on Fe-N-C almost achieves the 2025 target of the US Department of Energy by demonstrating a current density of 1.037 A cm⁻² combined with the peak power density of 0.685 W cm⁻², suggesting the critical role of Fe(II)-N₄ site (D1). After 500 h of running, PEMFCs still deliver a power density of 1.26 W cm⁻² at 1.0 bar H₂-O₂. An unexpected rate-determining step is figured out by isotopic labelling experiment and theoretical calculation. This work not only offers valuable insights regarding the intrinsic contribution of Fe-N₄ with a single spin state to alkaline/acidic ORR, but also provides great opportunities for developing high-performance stable PEMFCs.