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. This study reports a novel 14-membered macrocyclic Fe complex, which was inspired by FeN4 centers in pyrolyzed catalysts, unlike typical macrocyclic MN4 complexes that have 16-membered rings. 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.

Nonplatinum metal (NPM) catalysts for the oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFCs) have been developed; however, NPM catalysts still need to be improved in terms of both their catalytic activity and durability. To overcome these problems, an Fe active site contained within a more compact ligand than conventional, porphyrinic, 16-membered ring ligands, or more specifically, a hexaaza macrocyclic ligand with a 14-membered ring (14MR), was developed. In this study, the durability of the Fe-14MR complex was compared to that of Fe phthalocyanine (FePc), which has a 16-membered ring ligand, using in situ X-ray absorption spectroscopy; demetalation of the Fe complexes was directly observed during electrochemical experiments performed under acidic ORR conditions. It was found that Fe-14MR is significantly more resistant to demetalation than FePc during the ORR.

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

For the globalization of polymer electrolyte fuel cells, the development of non-precious-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, but they suffer from the density and uncertain chemical structure of their active sites. This study reports a 14-membered macrocyclic Fe complex, which was inspired by FeN₄ centers in the pyrolyzed catalysts, whereas typical macrocyclic MN₄ complexes have 16-membered rings. This 14-membered macrocycle has strong Fe-N bonding with an average bond distance of 1.90 Å, which has been evidenced by single-crystal X-ray diffraction, and is markedly shorter than that in porphyrin, 2.0 Å. Promising electrocatalytic activities for oxygen reduction have been demonstrated in both of acidic and basic media.