Ionic liquid-impregnated covalent organic framework/silk nanofibril composite membrane for efficient proton conduction

Abstract

Covalent organic frameworks (COFs) hold great potential for proton-conducting materials due to the intrinsic pore channels and high surface areas. However, their limited functional groups and poor film-forming properties remain significant challenge. Silk nanofibrils (SNFs) have good mechanical property and abundant functional groups, which can be exfoliated from natural bombyx mori silks. Herein, we demonstrate a novel strategy of utilizing the flexibility of SNFs to assemble conductive COFs into a stable membrane form. Ionic liquids (ILs)-impregnated sulfonic acid-based COF (IL-COF-SO3H) is prepared and then is combined with SNF to fabricate IL-impregnated COF/SNF composite membrane. The uniformly distributed ILs inside COF-SO3H increase the number of proton hopping sites. Meanwhile, the –SO3H groups can form strong interactions with imidazole-type ILs and immobilize ILs within the channels, endowing COF-SO3H pores low-barrier proton transfer pathways. Additionally, the hydrogen-bonding interactions between IL-COF-SO3H and SNFs enable interfacial binding and structure stability, as well as facilitate proton transfer. Thus, the resultant IL-COF-SO3H@SNF composite membrane simultaneously obtains enhanced conductivity (224 mS cm−1 at 90 °C and 100% RH) and mechanical performance (tensile strength of 28.6 MPa). This study provides novel insight into fabrication of stable COF-based membrane for high-performance conduction materials.