Free-Standing Polymer Covalent Organic Framework Membrane with High Proton Conductivity and Structure Stability

The two-dimensional covalent organic framework (COF) with continuous and stable pores as well as tunable functional groups is considered as promising proton exchange membrane (PEM) material. However, due to the strong rigidity and inferior processability of COFs, it still remains challenging to prepare a self-standing and robust membrane. Herein, self-standing and robust thin COF membranes (20 μm in thickness) were fabricated through the in situ involvement of hyperbranched polyethylenimine (HPEI). The HPEI cross-links the adjacent COF particles and enhances the interactions among them. The involvement of HPEI enables polyCOM to have excellent structure ability, mechanical property, and proton conduction ability. As a result, poly0.2COM obtains a high water uptake of 72.3% at 80 °C (vs 49.11% for Nafion) with a negligible swelling ratio of 1.5%. In addition, the tensile strength of poly0.2COM reaches 45.4 MPa, 2.4 times higher than that of the TpPa-SO3H membrane (13.26 MPa). Importantly, polyCOM shows high proton conductivities of 223 and 35 mS cm–1 under saturated humidity and a low relative humidity (RH) of 30%, respectively, far exceeding those of the commercial Nafion membrane (80 and 5.5 mS cm–1). Moreover, poly0.02COM acquires a remarkable power density of 111.1 mW cm–2 and a current density of 664 mA cm–2 at 80 °C and 40% RH.