Ultrahigh Proton Selectivity by Assembled Cationic Covalent Organic Framework Nanosheets

Ionic covalent organic framework (COF) nanosheets are becoming increasingly attractive as promising two‐dimensional (2D) materials for proton transport due to their ionic functionality and tailor‐made pores. However, most synthetic methods for nanosheets rely on surface‐assisted methods or phase transformation often yielding nanosheets with low aspect ratios. In this study, we present a bottom‐up approach utilizing an oil‐oil‐water triphase system to achieve the large‐scale synthesis of ionic COF nanosheets. The intermediate oil layer in this system modulates the diffusion rate of monomers from the top oil phase into the aqueous phase, enabling in‐plane anisotropic secondary growth from the initial discrete fibrous structure into large and crystalline COF nanosheets. The ionic COF nanosheets exhibit excellent proton permeability while simultaneously excluding other cations by casting into crack‐free membranes, demonstrating efficient HCl extraction from acidic water waste. This strategy for larger‐scale COF nanosheet growth will offer an alternative platform for designing multifunctional COF membranes with applications in sophisticated separation technologies.