Altering the spin state of Fe-N-C through ligand field modulation of single-atom sites boosts the oxygen reduction reaction

Atomically dispersed Fe-N-C catalysts are the most promising candidates alternatively to Pt-based catalysts for oxygen reduction reaction (ORR). However, the ORR activity of Fe-Nx electrocatalysts in acid are still far from satisfactory; thus far, although some discussions demonstrate the important role of ligand fields of single atom metal-N-C sites on improving catalytic properties, the behind mechanism is still ambiguous. Herein, based on the ligand field theory, the electron spin-state modulation of Fe active centers in SA Fe-N-C achieved from a low-spin state (LS) for FeN5 (Fe-N5-LS) to a high-spin state (HS) for FeN4 (Fe-N4-HS) and FeN3 (Fe-N3-HS) was realized by converting defect-rich pyrrolic N-coordinated FeNx sites, which tune the electron readily penetrating the antibonding π-orbital of oxygen. The Fe-N4-HS exhibits a 3d-electronic structure of t2g3eg2 and significantly accelerate the ORR reaction kinetics. Taking advantage of activity-boosting high spin state (S5/2) of Fe (III), the designed Fe-N4-HS (with two longitudinal parallel coordinated pyrr-N and pyri-N, respectively) catalyst displays excellent ORR activity, which is comparable to commercial Pt/C catalyst. In addition, Fe-N4-HS presents higher proton exchange membrane fuel cell (PEMFC) and Zn-air battery performances than most non-precious-metal electrocatalysts. Our findings provide fundamental and technological insights into the correlation between the electronic spin states/geometric structure and high-efficiency SA Fe-N-C catalysts for ORR process.