Edge-Rich Fe-N4 Active Sites in Defective Carbon for Oxygen Reduction Catalysis.

Controllably constructing nitrogen-modified divacancies (ND) in carbon substrates to immobilize atomic Fe species and unveiling the advantageous configuration is still challenging, but indispensable for attaining optimal Fe-N-C catalysts for the oxygen reduction reaction (ORR). Herein, a fundamental investigation of unfolding intrinsically superior edge-ND trapped atomic Fe motifs (e-ND-Fe) relative to an intact center model (c-ND-Fe) in ORR electrocatalysis is reported. Density functional theory calculations reveal that local electronic redistribution and bandgap shrinkage for e-ND-Fe endow it with a lower free-energy barrier toward direct four-electron ORR. Inspired by this, a series of atomic Fe catalysts with adjustable ND-Fe coordination are synthesized, which verify that ORR performance highly depends on the concentration of e-ND-Fe species. Remarkably, the best e-ND-Fe catalyst delivers a favorable kinetic current density and halfwave potential that can be comparable to benchmark Pt-C under acidic conditions. This work will guide to develop highly active atomic metal catalysts through rational defect engineering.