Axial modulation of Fe sites realizing high-performance oxygen reduction reaction of FeN4 catalysts

The modulation of the microenvironment of Fe-N-C conventional catalysts is considered a feasible strategy to improve the catalytic performance. Herein, we report the synthesis of Fe single-atom catalysts with S (axially oriented) and N (horizontally oriented) coordination modes in a MOF-derived hierarchical porous carbon (Fe-N4S1). High oxygen reduction reaction (ORR) activity with a half-wave potential of 0.88 V has been realized. In addition, we found using density functional theory calculations (DFT) that the introduction of axial S atoms around Fe single atoms can redistribute the d-band electrons of Fe compared with a conventional FeN4 catalyst. This leads to a more deviation of the d-band center from the Fermi energy level, thus weakening its intermediate adsorption behavior, maintaining the advantages of high catalytic activity of FeN4 and solving the drawbacks of conventional FeN4 catalysts with strong adsorption to intermediates. Our findings provide a new idea for the microenvironmental modulation of single-atom catalysts, which can realize controllable electron distribution of single atoms.