Tight Covalent Organic Framework Membranes for Efficient Anion Transport via Molecular Precursor Engineering

Fabricating covalent organic frameworks (COFs) membranes with tight structure, which can fully utilize well‐defined framework structure and thus achieve superior conduction performance, remains a grand challenge. Herein, through molecular precursor engineering of COFs, we reported the fabrication of tight COFs membrane with the ever‐reported highest hydroxide ion conductivity over 200 mS cm−1 at 80 °C, 100 % RH. Six quaternary ammonium‐functionalized COFs were synthesized by assembling functional hydrazides and different aldehyde precursors. In an organic‐aqueous reaction system, the impact of the aldehyde precursors with different size, electrophilicity and hydrophilicity on the reaction‐diffusion process for fabricating COFs membranes was elucidated. Particularly, more hydrophilic aldehydes were prone to push the reaction zone from the interface region to the aqueous phase of the reaction system, the tight membranes were thus fabricated via phase‐transfer polymerization process, conferring around 4–8 times the anion conductivity over the loose membranes via interfacial polymerization process. The molecular precursor engineering was implemented for modular fabrication of covalent organic framework (COF) membranes. The COF membranes fabricated via phase‐transfer polymerization process with more hydrophilic aldehyde precursors exhibited tighter structure than the COF membranes fabricated via interfacial polymerization process with less hydrophilic aldehydes, thus achieving the ever‐reported highest hydroxide ion conductivity.