Hexagonal Fe2N Coupled with N-Doped Carbon: Crystal-Plane-Dependent Electrocatalytic Activity for Oxygen Reduction

Currently, nonprecious metal nitrides have attracted increasing attention due to their affordable cost, high catalytic performance, and good stability for the oxygen reduction reaction (ORR). Herein, a facile strategy has been provided for synthesizing hexagonal Fe2N nanocrystals dispersed on nitrogen-doped carbon (NC), through direct pyrolysis of a mixture of carbon black, ferric chloride (FeCl3), and melamine in an argon atmosphere, followed by ammonia activation at 700 °C. The obtained Fe2N/NC catalyst exhibits both ultrahigh ORR activity and favorable long-term durability in alkaline solutions, which exceeds the state-of-the-art Pt/C even in Zn–air batteries. Acid etching and KSCN poisoning experiments confirm that the remarkable performances are mainly attributed to the hexagonal nitride with special crystal facets that existed in this hybrid. Density functional theory (DFT) calculation results reveal that the oxygen adsorption energy of the exposed Fe2N(1̅1̅1) plane is −1.14 eV, very close to that of Pt(111). More importantly, at 0.85 V vs reversible hydrogen electrode (RHE), the free energy diagram of the ORR on this exposed crystal facet is closer to the ideal path than that of the Fe–N4 bonds that existed in this hybrid, suggesting that the ORR process over Fe2N is easier than that over Fe–N–C catalysts. This work may provide an idea for fabricating nitrides or carbides with controlled crystal facets as efficient non-Pt catalysts for hydrogen–oxygen fuel cells or metal–air batteries.