Exploration of spatial confinement and ligand effects for the oxygen reduction reaction on Fe-N-x embedded hole-graphene

In this work, we constructed theoretical models by embedding Fe-TCPP and Fe-(mIM)(n) (n = 2,3,4) active sites into hole-graphene, and the structural stability was evaluated using molecular dynamics simulations. Based on the theoretical models, we systematically studied the oxygen reduction reaction (ORR) mechanism and the effect of spatial confinement and ligands with DFT calculations. The analysis of the ORR reaction pathway shows that Fe-TCPP and Fe-(mIM)(4) have good catalytic activity. Subsequently, the confinement effect (5-14 angstrom) was introduced to investigate its influence on the catalytic activity. The Fe-TCPP and Fe-(mIM)(4) active sites have the lowest overpotential at an axial space of 8 angstrom and 9 angstrom, respectively. We select four ligands (bpy, pya, CH3, and bIm) to explore their effect on the catalytic activity of the Fe-TCPP active site. With the modification of bpy, pya, and bIm_N (Fe-N-4 sites become Fe-N-5 active sites), the overpotential decreases by 26-31%. In the present work, the best catalytic system is Fe-TCPP_pya, which is on the top of the volcano plot.