Tuning Structural and Electronic Configuration of FeN₄ via External S for Enhanced Oxygen Reduction Reaction

The Fe–N–C material represents an attractive oxygen reduction reaction electrocatalyst, and the FeN₄ moiety has been identified as a very competitive catalytic active site. Fine tuning of the coordination structure of FeN₄ has an essential impact on the catalytic performance. Herein, we construct a sulfur-modified Fe–N–C catalyst with controllable local coordination environment, where the Fe is coordinated with four in-plane N and an axial external S. The external S atom affects not only the electron distribution but also the spin state of Fe in the FeN₄ active site. The appearance of higher valence states and spin states for Fe demonstrates the increase in unpaired electrons. With the above characteristics, the adsorption and desorption of the reactants at FeN₄ active sites are optimized, thus promoting the oxygen reduction reaction activity. This work explores the key point in electronic configuration and coordination environment tuning of FeN₄ through S doping and provides new insight into the construction of M–N–C-based oxygen reduction reaction catalysts.