Silk nanofibril as nanobinder for preparing COF nanosheet-based proton exchange membrane

Two-dimensional covalent organic framework nanosheets (CONs) with ultrathin thickness and porous crystalline nature show substantial potential as novel membrane materials. However, bringing CONs materials into flexible membrane form is a monumental challenge due to the limitation of weak interactions among CONs. Herein, one-dimensional silk nanofibrils (SNFs) from silkworm cocoon are designed as the nanobinder to link sulfonated CON (SCON) into robust SCON-based membrane through vacuum-filtration method. Ultrathin and large lateral-sized SCONs are synthesized via bottom-up interface-confined synthesis approach. Benefiting from high length-diameter ratio of SNF and rich functional groups in both SNF and SCON, two-dimensional (2D) SCONs are effectively connected together by physical entanglement and strong H-bond interactions. The resultant SCON/SNF membrane displays dense structure, high mechanical integrity and good stability. Importantly, the rigid porous nanochannels of SCON, high-concentration –SO3H groups insides the pores and H-bonds at SCON–SNF interfaces impart SCON/SNF membrane high-rate proton transfer pathways. Consequently, a superior proton conductivity of 365 mS cm−1 is achieved at 80 °C and 100% RH by SCON/SNF membrane. This work offers a promising approach for connecting 2D CON materials into flexible membrane as high-performance solid electrolyte for hydrogen fuel cell and may be applied in membrane-related other fields.