Chitosan-Linked Dual-Sulfonate COF Nanosheet Proton Exchange Membrane with High Robustness and Conductivity.

2D materials that can provide long-range ordered channels in thin-film form are highly desirable for proton exchange membranes (PEMs). Covalent organic framework nanosheets (CONs) are promising 2D materials possessing intrinsic porosity and high processability. However, the potential of CONs in PEMs is limited by loose sheet stacking and interfacial grain boundary, which lead to unsatisfied mechanical property and discontinuous conduction pathway. Herein, chitosan (CS), a natural polymer with rich NH groups, is designed as the linker of dual-sulfonate CONs (CON-2(SO H)) to obtain CON-2(SO H)-based membrane. Ultrathin CON-2(SO H) with high crystallinity and large lateral size is synthesized at water-octanoic acid interface. The high flexibility of CS chains and their electrostatic interactions with SO H groups of CON-2(SO H) enable effective connection of CON-2(SO H), thus endowing membrane dense structure and exceptional stability. The stacked CON-2(SO H) constructs regular hydrophilic nanochannels containing high-density SO H groups, and the electrostatic interactions between CON-2(SO H) and CS form interfacial acid-base pairs transfer channels. Consequently, CON-2(SO H)@CS membrane simultaneously achieves superior proton conductivity of 353 mS cm (under 80 °C hydrated condition) and tensile strength of 95 MPa. This work highlights the advantages of proton-conducting porous CON-2(SO H) in advanced PEMs and paves a way in fabricating robust CON-based membranes for various applications.