Positional Engineering of Methyl Groups in Covalent Organic Frameworks for Enhanced Oxygen Reduction Reaction

Covalent organic frameworks (COFs) have been developed as metal-free catalysts for oxygen reduction reaction (ORR) due to their well-defined catalytic sites and pores. While heteroatom doping is a common strategy to modulate catalytic properties, the role of low polar, steric substituents such as methyl groups remain largely unexplored due to their minimal direct electronic effect. Herein, we systematically investigate the influence of methyl groups in COFs on ORR catalysis by precisely controlling their location (knot vs. linker) and density. We demonstrate that methyl positioning dictates the interlayer stacking mode, shifting from an eclipsed (AA) to a staggered (ABC) arrangement as groups migrate from linkers to knots. This methyl group engineering optimizes the local electron density at the imine-adjacent carbon active sites. The optimal catalyst with methyl groups located at the knot sites achieves a half-wave potential (E_1/2) of 0.76 V in 0.1 M KOH and a turnover frequency (TOF) value of 1.04 × 10^-3 s^-1. Theoretical calculations reveal that methylation at the knot facilitates the stabilization of key ^*OOH intermediates, contributing to the enhancement of catalytic activity. This work highlights the critical role of steric engineering in COFs and provides a design principle for advanced metal-free electrocatalysts.