Toward rational understandings of the oxygen reduction reaction on Fe-N-C single-atom catalysts

Rationalization of experiments of oxygen reduction reaction (ORR) on Fe-N-C single-atom catalysts (SACs) is essential to reveal the underlying mechanism. While extensive investigations have been carried out, it is still challenging for theoretical simulations to accurately describe experimental observations, owing to the complexities in both experiments and simulations. In addition, the predictive ability of the activity descriptor is limited by its insufficient connection with experimental measurements. In this work, by using first-principle calculations and microkinetic simulations, we systematically study the ORR on Fe-N-C sites in various chemical environments. A theoretical scheme is first established that simultaneously takes account of the *O2 adsorption, the Hubbard correlation of Fe 3d electrons, the solvent effect, and the kinetic factors to rationalize relevant experiments. Using this scheme, we then investigate the influence of axial ligands and adjacent site on the ORR activity of Fe-N-C. We find that the covered axial ligands result in activity enhancement on pyridinic FeN4C10 and activity reduction on pyrrolic FeN4C12, respectively, clarifying the better durability of pyridinic FeN4C10. In addition, the activity of the Fe-N-C site is degraded by the adjacent one when inter-site distance is too small. Finally, a three-dimensional (3D) diagram is constructed to predict the experimentally measurable half-wave potential as a function of the adsorption free energies of *OH and *O2, which can provide a unified description of the ORR activity trend. The 3D diagram can be extended to other SACs and metal (111) surfaces. This work demonstrates the significance of assessing the microkinetics in deeper insights and provides comprehensive guidance for rational catalyst design.