Defect engineering of Fe-N-C single-atom catalysts for oxygen reduction reaction

Fe-N-C single-atom catalysts (SACs) have been widely considered as a promising candidate for oxygen reduction reaction (ORR), and its intrinsic activity is closely related to electronic and geometric structure of graphene supports. The carbon defect is widely existed in graphene, of which the intrinsic effect on ORR activity of Fe-N-C is still unclear. Here, we investigate ORR activity of 43 models representing Fe-N-C SACs accompanying with defects, including 555777, 5775 and 585-defects in three shell distances around FeN4 site. Both pre-adsorption of hydroxide radical during ORR and the distance between Fe SAC and defect are demonstrated to affect the orbital hybridizations between Fe SAC and *OH intermediate, including Fe(dxz)-O(px), Fe(dyz)-O(py) and Fe(d22)-O(pz+s) orbitals, which can accordingly regulate ORR activity of defective Fe-N-C materials. Importantly, we establish a geometrical structure descriptor to quantitatively predict the ORR activity of defective Fe-N-C catalysts without any requirements of performing DFT calculations. With the assistance of the structure descriptor, we find that the 585 and 5775-defects of the large ring adjacent to the FeN4 pentagonal in fourth shell significantly boost the ORR performance of Fe-N-C. This work reveals the ORR activity origin of defective Fe-N-C materials, which provides intuitive guidance to boost the ORR performance of Fe-N-C materials by defect engineering, and may be extended to other types of defects and other single-atom catalysts.