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

Abstract

AbstractFe−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−N4 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‐Nx active sites was featured only in Fe(1,10‐phenanthroline)/KB with homogeneously distributed Fe and highly retained pyridinic N. With Fe‐Nx 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‐Nx active sites for highly active and stable ORR electrocatalysts.