The role of iron nitrides in the Fe-N-C catalysis system towards the oxygen reduction reaction

Fe-N-C series catalysts are always attractive for their high catalytic activity towards the oxygen reduction reaction (ORR). However, they usually consist of various components such as iron nitrides, metallic iron, iron carbides, N-doped carbon and Fe-N-4 moieties, leading to controversial contributions of these components to the catalysis of the ORR, especially iron nitrides. In this work, to investigate the function of iron nitrides, FexN nanoparticles (NPs) embedded in mesoporous N-doped carbon without Fe-N-4 moieties are designed and constructed by a simple histidine-assisted method. Herein, the use of histidine can increase the N and Fe contents in the product. The obtained catalyst exhibits excellent ORR catalytic activity which is very close to that of the commercial Pt/C catalyst in alkaline electrolytes. Combining the catalytic activity, structural characterization (especially from Mbssbauer spectroscopy), and the results of DFT calculations for adsorption energies of oxygen on the main surfaces of Fe2N including epsilon-Fe2N and zeta-Fe2N, it can be deduced that Fe2N NPs as active species make a contribution to the ORR catalysis, of which epsilon-FexN (x <= 2.1) is more active than zeta-Fe2N. In addition, we find that there exists an obvious synergistic effect between Fe2N NPs and N-doped carbon, leading to the greatly enhanced ORR catalytic activity.