The distance effect of the doped heteroatoms away from the catalytic centers has rarely been reported. In this work, we conducted density functional theory calculations to thoroughly investigate the influence of heteroatom (N, P, B, and S atoms) doping distance on the oxygen reduction reaction (ORR) activity of graphene-based FeN₄ sites. We uncovered a Sabatier-like relationship between heteroatom doping distance and ORR activity of FeN₄ sites. The nearest doping does not significantly improve and even block the ORR activity of FeN₄ sites. Optimal ORR activity is achieved when the heteroatoms are 4–5 Å (N, P, and S atoms) or 6–7 Å (B atoms) away from the Fe atoms. Analysis of electronic structure indicates that distance effect can modulate the local chemical environment of Fe atoms, thereby account for the changes in ORR activity along with the doping distance and doping atoms. This study provides insights into the influence of heteroatom doping on the chemical environment of reaction active centers, and provides the theoretical guidance for controlling the doping distance of heteroatoms to achieve optimal catalytic activity and selectivity.

Atomically-dispersed iron-based electrocatalysts are promising substitutes for noble metal electrocatalysts because of excellent performance in oxygen reduction reaction (ORR). Rationally modulating the local coordination environment of the Fe site and optimizing the binding energy of oxygen reduction intermediates are effective strategies to optimize ORR activity. Herein, we report a new method in which Ni is introduced to construct NiFe dual single atoms and iron nanoclusters loaded on the nitrogen-doped carbon with a highly porous structure. This design plays a synergistic role of dual single atoms and clusters, optimizes the 3d orbital and Fermi level of Fe, breaks the symmetrical structure of Fe-N₄, and effectively improves the adsorption/desorption behavior of the oxygen-containing intermediates. Electrochemical tests show Fe_NCs/NiFe_SAs-NC has an excellent intrinsic activity. Theoretical calculations show the oxygen-containing species on the Ni active site will move to the middle of NiFe (bridge site connection) after optimization and that the key step is OH desorption, with a reaction energy of 0.27 eV. The electron exchange between NiFe-N₆ and Fe-cluster is very strong, further indicating the introduction of Ni species and Fe clusters has a regulatory effect on the electronic structure of Fe-N₄.

Developing efficient and stable catalysts for the electrocatalytic N₂ reduction reaction (NRR) shows promise in nitrogen fixation. Here, we proposed active and stable single-atom catalysts (SACs) toward NRR, where transition metals are anchored on nitrogenated carbon nanotubes (NCNTs). Among the screened nine common transition metals (Ti, V, Cr, Mn, Fe, Mo, Ru, Rh, and Ag) on (4, 4) NCNTs, we found Mo-NCNT possesses the most excellent NRR catalytic activity and selectivity with a low overpotential of 0.29 V. Then, the NRR performance of Mo-NCNT was further engineered by controlling the nanotube diameter, where the lowest overpotential is 0.18 V at a diameter of 9.6 Å. In addition, we found a linear scaling relation between *NNH and *NH₂ on the studied catalysts with the exception of (2, 2) and (3, 3) Mo-NCNTs, owing to their extremely unstable structures. We attribute the outstanding NRR performance of Mo-NCNT to the moderate adsorption of N₂ due to the slightly low d-band center of Mo, and the charge donating and accepting capacity of NCNTs. This work has provided a deeper insight into designing high-efficiency and stable NRR SACs supported by NCNTs.