Establishing a theoretical insight for penta-coordinated iron-nitrogen-carbon catalysts toward oxygen reaction

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.