Theoretical Understanding of the Effect of Coordination Environment on the Activity of Metal Macrocyclic Complexes as Electrocatalysts for Oxygen Reduction Reactions

Abstract

Transition metal macrocyclic complexes are not only appealing catalysts for electrochemical oxygen reduction reaction (ORR), but also can be seen as the functional motifs in single atom catalysts (SACs). Here, we perform first-principles calculations to gain a comprehensive understanding on the structure-property relationship of the metal macrocyclic complex systems. Various modifications of the complexes are considered, including centered metal, axial ligand, coordination atom, substituent and macrocycles. Based on simulations, the binding between Fe/Co and the oxygenated intermediates is too strong, leading to desorption of *OH hindering the ORR process. In contrast, the interaction between Ni and oxygen is too weak, thus the formation of *OOH limits the reaction. Introduction of appropriate apical ligand can improve the performance of all the three metals on ORR catalysis. Replacement of nitrogen with oxygen or carbon as the coordination atoms strengthens the interaction with oxygen, herein enhancing the Ni-centered systems. The macrocyclic structure also affects the activity that the antiaromatic ring stabilizes the *OOH intermediate whereas the macrocycle with reduced electron density inhibits the formation of *OOH. In comparison, substitution effect is not very significant. By regulating the coordination environment, the complexes can possess an overpotential below 0.5 V. This work may assist the rational design of ORR catalyst and is of great significance for the future development of oxygen reduction catalysts based on the transition metal macrocyclic structures.