Benzothiophene-based covalent organic frameworks for H2O2 electrosynthesis: the critical role of conjugated structure†

Abstract

Covalent organic frameworks (COFs), a class of emerging porous crystalline polymers with high specific surface areas and tunable framework structures, exhibit great potential in the oxygen reduction reaction (ORR). Herein, we synthesized a series of benzothiophene-based COFs with varying benzene ring counts in the linkers and employed these materials to unveil the correlation between the conjugated structure and the selectivity toward H₂O₂ electro-synthesis. The experimental results display that the highest H₂O₂ selectivity (∼90%) is offered by the benzothiophene-based COFs bearing one benzene ring in the linker, exhibiting a negative correlation with the number of benzene rings in the linkers. Theoretical calculations reveal that variations in the number of benzene rings modulate the adsorption strength and sites of key reaction intermediates, thereby altering the 2e⁻ ORR pathway. The decrease in benzene ring counts enables the dominant pathway for 2e⁻ ORR to become the H₂O₂ formation through the nucleophilic attack of the active *O₂⁻ species, which originates from the electron transfer of ³O₂, on the carbon atom near the sulfur atom of the thiophene ring. This work highlights the importance of appropriate linkers and provides valuable insights for designing metal-free COF electrocatalysts.