Rational Design of Asymmetric FeN₃S Single-Atom Sites for e_g Orbital Engineering Toward Efficient ORR in PEMFCs

Iron and nitrogen co-doped FeNC catalysts with symmetric Fe-N₄ as the active site are the most promising alternative to noble metal catalysts for proton exchange membrane fuel cells (PEMFCs). However, the symmetric structure in the Fe-N₄ site endows oxygen intermediates with strong binding energy, which hinders the activity optimization of the active center. In this work, an asymmetric configuration (Fe-N₃SC) is constructed to alleviate the electron localization caused by the d-π conjugation of Fe-N₄. The asymmetric configuration can effectively reduce the splitting energy of the Fe-3d orbital, leading to a decrease in the energy barrier for the electrons to fill in the e_g orbitals. Increased electron occupancy in the e_g orbitals reduces the bond order/adsorption of the oxygen-containing intermediates on the active site, which leads to increased oxygen reduction (ORR) activity. The results of this work highlight the possibility of asymmetric configurations in manipulating and optimizing electron transfer and spin regulation of single-atom catalysts.