Developing highly active iron-nitrogen-carbon catalysts for electrocatalytic oxygen reduction reactions (ORR) is pivotal to future energy technology. The penta-coordinated Fe-N-C with an augmented activity toward the oxygen reduction has been regarded as one of the promising candidates to replace platinum-based ORR catalysts. However, the lack of pertinent fundamental understanding hinders further optimizing the catalytic activity of such catalysts. Herein, through density functional theory (DFT) calculations, we systematically investigated the catalytic activity and ligand/metal coordination effects of 17 penta-coordinated Fe-N-C catalysts (labeled as FeNC-Xs, X denotes axial ligand). Our results not only show the theoretical overpotential of FeNC-Xs is lower than that of conventional tetra-coordinated Fe-N-C catalysts (labeled as FeNC), verifying the preeminent performance of FeNC-Xs, but also further indicate that the axial coordination effect of X ligands can decrease the orbital hybridization of Fe active center with ORR-relevant intermediates, sequentially accelerating the ORR. More importantly, we reveal that the catalytic activity of FeNC-Xs increases with a decreased electronegativity of X ligands, which can be utilized to describe the axial coordination effect for FeNC-Xs. These findings can deeply advance the understanding of penta-coordinated iron-nitrogen-carbon catalysts, which provide timely guidelines for designing optimum Fe-N-C based catalysts.

Designing highly efficient bifunctional electrocatalysts for oxygen reduction and evolution reaction (ORR/OER) is extremely important for developing regenerative fuel cells and metal-air batteries. Single-atom catalysts (SACs) have gained considerable attention in recent years because of their maximum atom utilization efficiency and tunable coordination environments. Herein, through density functional theory (DFT) calculations, we systematically explored the ORR/OER performances of nitrogen-coordinated transition metal carbon materials (TM-N_x-C (TM = Mn, Fe, Co, Ni, Cu, Pd, and Pt; x = 3, 4)) through tailoring the coordination environment. Our results demonstrate that compared to conventional tetra-coordinated (TM-N₄-C) catalysts, the asymmetric tri-coordinated (TM-N₃-C) catalysts exhibit stronger adsorption capacity of catalytic intermediates. Among them, Ni-N₃-C possesses optimal adsorption energy and the lowest overpotential of 0.29 and 0.28 V for ORR and OER, respectively, making it a highly efficient bifunctional catalyst for oxygen catalysis. Furthermore, we find this enhanced effect stems from the additional orbital interaction between newly uncoordinated d-orbitals and p-orbitals of oxygenated species, which is evidently testified via the change of d-band center and integral crystal orbital Hamilton population (ICOHP). This work not only provides a potential bifunctional oxygen catalyst, but also enriches the knowledge of coordination engineering for tailoring the activity of SACs, which may pave the way to design and discover more promising bifunctional electrocatalysts for oxygen catalysis.