Investigating the Effect of Fe-N5 Configuration in the Oxygen Reduction Reaction Using N-Heterocycle Functionalized Carbon Nanotubes

Atomic iron embedded in N-doped carbon materials (Fe–N–C) are regarded as the most promising alternative to platinum-based catalysts in anion exchange membrane fuel cells, due to their high oxygen reduction reaction (ORR) activity. The molecular catalyst iron phthalocyanine (FePc) is often employed to explore the correlation between a model single Fe structure and ORR activity. Recent advancements have introduced a penta-coordinated Fe-N5 structure achieved by binding FePc onto pyridine-functionalized carbon surfaces, demonstrating outstanding ORR activities. To investigate the effect of the axial pyridine ligand, in this work, four different N-functionalized carbon nanotubes (N-CNTs) were prepared, each adorned with a unique N-heterocycle (pyridine, quinoline, isoquinoline, or acridine). Subsequent binding of FePc (FePc/N-CNTs) afforded penta-coordinated complexes in all cases. The characterization results demonstrated that the penta-coordination Fe-N5 configuration could be effectively constructed by this method. The ORR activities of these FePc/N-CNTs suggest that the local surface structure around the axial ligand on the functionalized CNTs influences catalytic performance.