Rational Generation of Fe-Nx Active Sites in Fe-N-C Electrocatalysts Facilitated by Fe-N Coordinated Precursors for the Oxygen Reduction Reaction

Fe-N-C catalysts synthesized by pyrolysis of Fe and N precursors have been intensively studied due to their remarkable activities for the electrochemical oxygen reduction reaction (ORR). Although Fe-N-4 coordinated structures have been suggested as active sites by recent spectroscopic studies, the influence of precursor coordination on the generation of the active sites during high-temperature pyrolysis is not well understood. In this work, phenanthroline isomers were used as systematic model precursors to reveal the correlation between precursor coordination and active site formation in Fe-N-C catalysts. Coordination between Fe and each phenanthroline isomer was effectively controlled by the molecular structure: monodentate (1,7- and 4,7-phenanthroline) and bidentate coordination (1,10-phenanthroline). Through X-ray absorption spectroscopy and X-ray photoelectron spectroscopy study, large difference in atomic distribution of both Fe and N was revealed; the preferential formation of Fe-N-x active sites was featured only in Fe(1,10-phenanthroline)/KB with homogeneously distributed Fe and highly retained pyridinic N. With Fe-N-x active site moieties, Fe(1,10-phenanthroline)/KB exhibited superior ORR activity and stability in alkaline half-cell and full-cell tests. These results highlight the importance of the use of precursors with multiple coordination (i. e. bidentate) for the effective derivation of Fe-N-x active sites for highly active and stable ORR electrocatalysts.