Substituent engineering of covalent organic frameworks modulates the crystallinity and electrochemical reactivity

Abstract

Covalent organic frameworks (COFs) are emerging as powerful electrochemical energy storage/conversion materials benefiting from the controlled pore and chemical structures, which are usually determined by the regulation of the molecular building blocks. In contrast, the substituents are not considered significant for the electrochemical reactivity as they are usually removed during carbonization, which is necessary for improving the electrical conductivity of an electrode material. Here we show that the substituents play key roles not only in synthesizing COFs but also in controlling the COF structures during carbonization and thus the related electrochemical reactivity. Five characteristic substituents were used when synthesizing a new COF structure and it was found that electron-withdrawing strength of the substituents significantly influences the crystallinity of the COFs by tuning the reactivity of building blocks, or even determines whether the crystalline COF can be constructed. Moreover, the differences in chemical groups, sizes, and thermal stabilities of the substituents result in varied pore-collapse behaviors and the structures of the carbonized COFs, which show diverse effects on the electrochemical performances. An optimal material shows the highest surface area of 2131 m2/g, rich pores around 1 nm, and the highest ratio of sp2 carbon among the samples, corresponding to the largest double-layer specific capacity over 125 F/g in an ionic liquid electrolyte, while another material with the lowest surface area and N-doping level exhibits a high H2O2 production selectivity over 80% through selective oxygen reduction. This study shows guiding significance for the design of building blocks and substituents for COFs and further the carbonized carbons, and also exhibits the great potential of substituent engineering in modulating the electrochemical reactivity.