Decrypting the Influence of Axial Coordination on the Electronic Microenvironment of Co-N5 Site for Enhanced Electrocatalytic Reaction

Metal porphyrins are star molecules that possess welldefined coordination metal centers for versatile catalytic reactions. However, most previous work has focused on the correlations between in-plane symmetric configura-tion of metal-N4 sites and their catalytic performance. Addressing the catalytic contribution of additional axial coordination to such symmetric configuration remains a challenge. Theoretical calculations revealed that axially anchoring an extra pyridine on the tetra-coordinated cobalt porphyrin (Co-N4) to construct penta-coordinated cobalt porphyrin (Co-N5) renders cobalt a higher electron density, thereby favoring the rate-determining O2 adsorption/activation and reducing the oxygen electroreduction barrier. Therefore, a well-defined Co-N5 site is rationally introduced into the azo-linked polymer framework for a fundamental structure-catalytic performance correlation study. As-prepared Co-N5 catalyst exhibits a 26 mV positive shift in half-wave potential compared with the pyridine-free Co-N4 counterpart, discloses a markedly higher power density (141.4 mW cm-2), and possesses better long-term durability (over 160 h cycles) in a Zn-air battery. Moreover, such a Co-N5 catalyst also showcases potential applications for CO2 reduction with high CO2-to-CO conversion faradic efficiency and better selectivity than the Co-N4 counterpart because coordination of the fifth pyridine evokes electronic localization that suppresses a competitive side reaction. This work proves the positive electrocatalytic contribution of axial penta-coordination on well-defined metal-porphyrin-based catalysts and offers atomic understanding of the structure-performance correlation on single atom catalysts for future catalyst design.

Automated summary

Metal porphyrins are star molecules with well-defined coordination metal centers for versatile catalytic reactions. This study introduces additional axial coordination to explore its contribution to catalytic performance on cobalt porphyrins. The results show that axial coordination increases electron density and enhances catalytic reactions, resulting in improved performance in Zn-air batteries and CO2 reduction.