Fourteen-Membered Macrocyclic Fe Complexes for Oxygen Reduction Catalysis

Abstract

For the broad application of polymer electrolyte fuel cells (PEFCs), the development of nonprecious-metal (NPM) catalysts for oxygen reduction is extremely important. To date, many NPM catalysts have been synthesized by pyrolyzing Fe-, N-, and C-containing precursors; however, they suffer from low density and uncertain chemical structure of their active sites. To date, pyrolyzed Fe/N/C catalysts have been regarded as more active than typical macrocyclic MN4 complexes such as Fe porphyrin. The exact reason for this difference in catalytic activity has remained obscure because of the indistinct chemical structure of the active sites on the pyrolyzed catalyst. However, the recent significant progress in the direct microscopic observation of atomically dispersed Fe sites has prompted researchers to embed FeN4 sites in graphene sheets. Interestingly, these Fe centers are considered to be surrounded by a 14-membered ring consisting of C and N atoms, unlike the case of typical macrocyclic MN4 complexes that have 16-membered rings (e.g., porphyrin and phthalocyanine). This has inspired our research group to focus on the ORR catalytic activity of 14-membered macrocyclic Fe complexes. This study reports a novel 14-membered macrocyclic Fe complex. This 14-membered macrocycle possesses strong Fe–N bonds with an average bond distance of 1.90 Å, as evidenced by single-crystal X-ray diffraction (XRD), which are markedly shorter than those in porphyrin (2.0 Å). This complex demonstrates high electrocatalytic activity for oxygen reduction in both acidic and basic media. Figure 1