Establishing a theoretical insight for penta-coordinated ironnitrogen-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 FeN-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-nitrogencarbon catalysts,which provide timely guidelines for designing optimum Fe-N-C based catalysts.