Solution-processable ionic covalent organic framework nanosheets for robust mixed matrix membranes with enhanced proton conductivity

Proton exchange membranes (PEMs) with robustness and high proton conductivity remain scarce. Here, solution-processable sulfonated covalent organic framework (COF) nanosheets are used as functional fillers and incorporated into a sulfonated polyethersulfone (SPES) matrix to prepare mixed matrix membranes (MMMs). Remarkably, the mixed casting solution comprising sulfonated COF nanosheets and SPES demonstrates exceptional stability (no sedimentation/agglomeration over 12 months), primarily due to charge-induced electrostatic repulsion. The optimized MMMs, prepared from the above highly stable casting solution, demonstrate uniform filler dispersion and excellent filler-polymer interfacial compatibility, achieving both high mechanical strength (∼36 MPa) and superior proton conductivity (∼453 mS cm⁻¹, 70 °C, 95 % RH). The unprecedented performance of MMMs mainly benefits from the excellent solution processability of the functional fillers, which enables a uniformly mixed solution with superior stability, thereby minimizing adverse effects of filler agglomeration during membrane preparation. Leveraging the unique advantages of the stable casting solution, large-area MMMs (528 cm²) with comparable performance (∼42 MPa, ∼449 mS cm⁻¹, 70 °C, 95 % RH) are readily prepared and tested in fuel cell. The successful preparation of various MMMs validates the generality of this method. This work offers a new roadmap for preparing MMMs, representing a critical step toward their broader application.