Tuning oxygen coordination distance boosts hydrogen evolution of Fe single-atom catalysts

Abstract

The development of highly active Fe-N-C single-atom catalysts (SACs) for the hydrogen evolution reaction (HER) requires precise tuning of the electronic structure of FeN 4 sites. In this work, using density functional theory calculations, we systematically investigate the effect of O atoms incorporated at different spatial distances from the FeN 4 center on HER performance. The calculated Gibbs free energies of hydrogen adsorption (∆G H ) show that FeN 4 -O S2 and FeN 4 -O S3 exhibit values of 0.113 eV and -0.103 eV, respectively, comparable to that of Pt (ΔG H = -0.09 eV) indicating excellent HER activity. Partial density of states (PDOS) analysis reveals that O introduction shifts the Fe-3d orbitals to lower energies and modulates their density near the Fermi level, thereby tuning the Fe-H orbital hybridization. Specifically, strong Fe-3d and H-1s overlap in pristine FeN₄ is markedly reduced in FeN 4 -O S2 but only slightly diminished in FeN 4 -O S3 , consistent with their ∆G H values. This work provides a theoretical reference for atomically designing high-performance catalysts by exploiting spatial distance effects for electronic structure modulation.