Enhanced interlayer interaction in sulfonated CONs membrane by amino-rich CONs enabling ultrafast proton transport

Covalent organic framework nanosheets (CONs) with porous crystalline features and ultrathin thickness are ideal candidates as membrane building blocks to form well-defined transfer nanochannels. The formidable challenge behind self-supporting CONs membrane lies in weak non-covalent interlayer interactions and thus loose stacking, insufficient strength and structure stabilities. Herein, we propose the fabrication of interlayer force-strengthened freestanding CONs membrane through the electrostatic attraction bridge effect of positively-charged amino-rich CONs (CON-NH₂) to negatively-charged sulfonated CONs (CON-SO₃H). Ultrathin and large lateral sized CON-SO₃H and CON-NH₂ are synthesized, followed by restacking to prepare freestanding CONs membrane with CON-SO₃H as the membrane bulk. Benefiting from effective interlayer interconnection due to strong electrostatic interaction, the obtained CON-SO₃H/CON-NH₂ membrane displays features of ultrahigh integrity, dense stacking, eminent water/acid/base/organic solvents stabilities and mechanical strength (109 MPa). The shortened -SO₃H distance contributes to construct site-continuous transfer pathways, and the deprotonated -SO₃H and protonated -NH₂ form acid-base pairs to decrease interfacial resistance, which impart membrane superior proton conductivity of 486 mS cm^-1 (80 ℃, 100% RH). This interlayer force enhancement strategy offers a promising perspective on achieving densely-stacked CONs membrane with ultrahigh mechanical property and conduction performance for fuel cell application.