Unravelling ligand topology effect on the binding of oxygen reduction reaction intermediates on Fe-N -C single-atom catalysts

The rational design of Fe-N-C single-atom catalysts (SAC) requires a priori estimation of its activity with descriptors that require no additional calculations. In this study, we conducted a thorough investigation into the effect of the amount and the location of N and the position of edge atoms on intermediate adsorption energies, and then unveiled the critical topological factors. Our density functional theory calculations, energy decomposition analysis and bonding analysis on OH adsorption on finite graphitic Fe-Nx-C (x = 0–4) models discover that nitrogen not only concentrates electrons on the ligand onto donor atoms but also induces a more ionic nature in the Fe–ligand (Fe–L) bonds and partially disrupts the aromaticity of the ligand. The latter effect strongly correlates to the location of N. These effects influence the electrostatic, Pauli repulsive and orbital interactions that are involved in the Fe–O bond formation. Furthermore, our investigation also reveals that the size of SAC plays a role in binding only when the edge atom is situated in the ring containing donor atoms. Our work for the first time provides a comprehensive mechanism of OH adsorption on SAC and the influence of N and the size. It emphasizes the necessity of considering the quantity and the location of nitrogen, as well as the atoms surrounding the donor atoms when developing descriptors.