Oxygen reduction reaction of FeN4 center embedded in graphene and carbon nanotube: Density functional calculations

A proton exchange membrane fuel cell is an energy device in which chemical energy is directly converted into electric energy through the oxygen reduction reaction (ORR). In this work, we have performed first-principles density functional theory calculations for the ORR of FeN4 center embeded in graphene (Gr) and carbon nanotube (CNT) to investigate reaction dynamics. At the beginning of reaction, an O2 molecule is adsorbed on the center with the end-on bent geometry and an electron of the Fe atom is transferred to the O2 molecule (Fe(3dz2)−O2(1πga)). The successive adsorption of two hydrogen atoms generates a water molecule which immediately dissociates from the surface. The remaining oxygen atom on the Fe atom also adsorbs hydrogen atoms and generates the second water molecule. We found that the in-plane Fe atom embedded in Gr becomes out-of-plane with the height of 0.344 Å and this height is reduced in the CNT case due to the mechanical surface tension. After the ORR, the FeN4 centers on Gr and CNT recover their initial electronic and geometrical structures, enabling the subsequent ORR. These results demonstrates the feasibility of the ORR of FeN4 center in carbon systems.