Rational Design of Heteroatom-Doped Fe–N–C Single-Atom Catalysts for Oxygen Reduction Reaction via Simple Descriptor

The coordination engineering of Fe–N–C single-atom catalysts (SACs) through introducing heteroatom dopants has attracted widespread attention in the oxygen reduction reaction (ORR). However, the common regularity for tuning the ORR activity by coordinated and environmental heteroatoms has not been sufficiently studied. Herein, we study the ORR activity on 100 Fe–N–C SACs with S, P, and B heteroatoms in diverse coordination shells by density functional theory calculations. Based on the energy level distribution of frontier orbits and molecular orbital theory, it is found that the origin of Fe–N–C ORR activity is the hybridization of molecular orbitals of Fe 3dz2, 3dyz (3dxz), and O2*/OH* intermediates, where hybrid orbitals are adjusted by coordinated and environmental S, P, and B heteroatoms, and then the protonation process of O2* or OH* intermediate is determined. Moreover, we found that the Fe–O bond length, the d-orbital gap of spin states, the d-orbital center, and the valence state of the Fe site can be used as structural descriptors to predict the ORR activity governed by the protonation of O2* or OH* intermediate as potential-determining steps. Our structural descriptors rationalize the superior ORR performance of Fe–N–C with S or B atoms doped in the second coordination shell to those in the first coordination shell, as well as the fact that the P heteroatom is more suitable as a coordinated atom than the environmental atom to enhance the ORR activity of Fe–N–C, in available experimental references. Thanks to structural descriptors, the codoping synergistic effect between P in the first coordination shell and S in the second coordination shell is predicted and confirmed to greatly enhance the ORR activity. This study provides a unified mechanistic understanding on the ORR activity trend among Fe–N–C SACs regulated by coordinated and environmental heteroatoms.