The Fe₃C–N_<i>x</i> Site Assists the Fe–N_<i>x</i> Site to Promote Activity of the Fe–N–C\nElectrocatalyst for Oxygen Reduction Reaction

Abstract

Fe–N–C\nmaterials containing Fe–N_<i>x</i> sites\nhave emerged as promising electrocatalysts\nto substitute precious metal Pt in oxygen reduction reaction (ORR).\nNevertheless, in-depth understanding the origin of iron species in\nFe–N_<i>x</i> sites is essential for the\ndesign of promising performance catalysts. Herein, Fe–N–C\nmaterial composed of Fe and Fe₃C embedded in bamboo-like\nnitrogen-doped carbon nanotubes (Fe–Fe₃C–NCT)\nhave been constructed via a simple pyrolysis strategy and can be used\nas excellent ORR electrocatalysts. DFT calculations uncover that the\ninteraction between Fe and Fe₃C could promote the electron\ndensity of the center metal iron species, creating a stronger O₂ adsorption and faster ORR kinetics. Furthermore, the oxygen\nintermediates would more readily dissociate on the Fe–N_<i>x</i> sites formed by the coordination of metallic\nFe and N. Inspired by these structural characterizations, Fe–Fe₃C–NCT exhibits a positive onset potential of 0.99 V\n(vs RHE) and a high diffusion-limited current density of 8.00 mA cm^–2 toward ORR, accompanied by an outstanding stability\n(only 4 mV negative shift after 10 000 cycles). The primary\nZn–air battery displays a power density of 195 mW cm^–2 and an energy density of 840 mAh g^–1 at 10 mA\ncm^–2, much superior to Pt/C (123 mW cm^–2, 647.7 mAh g^–1).