Revealing insights into the axial coordination effect of M–N4 catalysts on electrocatalytic activity towards the oxygen reduction reaction

Abstract

Axial coordination is a powerful strategy to effectively regulate the activity of M–N₄ catalysts. However, the underlying mechanism is still not clear. Here, we explore the influence of axial coordination on the activity of Fe/Co/Ni-tetraphenylporphyrin covalent organic frameworks (Fe/Co/Ni-TPP COFs) towards the oxygen reduction reaction (ORR) using the DFT method. It is found that the widely used descriptors (e.g., d-band centers) are not applicable to explain the axial coordination effect on metal reactivity. The reason is that the hybridization of the d_z² orbital of metal centers and p_z orbital of axial coordination atoms leads to the formation of new active orbitals, where the effect of the p_z orbital should be considered in the design of the activity descriptor. Based on this, an activity descriptor using the total number of electrons in d_z² and p_z orbitals is proposed. The results show that the reactivity of the metal center is enhanced by axial coordination if N(d_z²) + N(p_z) > 2, while it is reduced if N(d_z²) + N(p_z) ≤ 2. The application of the descriptor is further extended to explain the activity change of Fe/Co/Ni-TPP COFs by different types of axially coordinated atoms. This work provides new insights into the regulation mechanism of the axial coordination effect on metal reactivity.