Alkoxy Side Chain Engineering in Metal-Free Covalent Organic Frameworks for Efficient Oxygen Reduction

Abstract

Metal-free covalent organic frameworks (COFs) gain significant attention as catalysts for the oxygen reduction reaction (ORR), a key process in energy conversion technologies like fuel cells and metal-air batteries. While substantial efforts are devoted to unravelling the mechanisms, by which heteroatom-containing building blocks in linkers, vertices, and linkages, enhance catalytic activity and selectivity, the potential of side-chain engineering to modulate pore wall surfaces and optimize the catalytic environment remains largely underexplored. This study investigates the role of alkoxy side chains in modulating the properties of COFs to enhance ORR performance. The synthesized COFs have adjustable pore surfaces, integrating triazine rings and alkoxy groups to enhance channel hydrophilicity by modulating interactions with water molecules. Moreover, the alkoxy side chains act as electron donors through p-pi conjugation, creating active and tuneable electronic sites, further enhancing hydrophilicity and facilitating efficient catalytic cycles. Notably, COFs with longer alkoxy side chains exhibit superior ORR activity, with a half-wave potential of 0.77 V, surpassing previously reported metal-free COFs. Theoretical calculations suggest that this enhancement is because of the stronger binding affinity of water molecules and *OOH intermediates to the carbon atoms adjacent to the alkoxy side chains.