A density functional theory study of catalytic sites for oxygen reduction in Fe/N/C catalysts used in H2/O2 fuel cells

Abstract

The oxygen reduction catalytic activity of carbon-supported FeN₄ moieties bridging micropores between two graphene sheets was investigated by density functional theory (DFT). Based on the FeN₂₊₂/C structure proposed earlier by our group, two types of FeN₂₊₂/C structures were considered: one mostly planar and one in which the Fe ion is significantly displaced out of the graphitic plane. A structure in which the FeN₄ moiety is embedded in an extended graphene sheet (FeN^pyri₄/C) was also considered. In addition, we have investigated the influence of an axial pyridine group approaching the Fe centre. The formation energy is lowest for the planar FeN₂₊₂/C structure. The overall downhill behaviour of the relative free energy vs. the reaction step suggests that most structures have catalytic activity near zero potential. This conclusion is further supported by calculations of the binding energies of adsorbed O₂ and H₂O and of the O–O bond lengths of adsorbed O₂ and OOH. The side-on interaction of adsorbed O₂ is preferred over the end-on interaction for the three basic structures without the axial pyridine. The pyridine coordination produces a stronger binding of O₂ for the planar FeN₂₊₂/C and the FeN^pyri₄/C structures as well as a dominant end-on interaction of O₂. The energy levels of the planar FeN₂₊₂/C structure with and without the pyridine ligand are nearly equal for iron spin states S = 1 and S = 2, suggesting that both configurations are formed with similar concentration during the preparation process, as also previously found for two of the iron sites by Mössbauer spectroscopy experiments.