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Ultrathin Covalent Organic Framework Membranes via a Multi‐Interfacial Engineering Strategy for Gas Separation

Covalent organic frameworks (COFs) are promising membrane materials due to their high porosity, ordered arrangements, and high stability. However, the relatively large pore size and complicated membrane preparation processes of COFs limit their applications in sieving small gas molecules, even at a lab scale. Herein, a multi‐interfacial engineering strategy is proposed, that is, direct layer‐by‐layer interfacial reaction of two COFs (TpPa‐SO3H and TpTGCl) with different pore sizes to form narrowed apertures at the COF–COF interfaces atop a relatively large‐pore COF (COF‐LZU1) film. At 423 K, one fabricated 155 nm‐thick ultrathin COF membrane displays H2 permeance as high as 2163 gas permeation units (GPU) and a H2/CO2 selectivity of 26, transcending the 2008 Robeson upper bound. This strategy not only provides high‐performance membrane candidates for H2 separation, but also enlightens the interfacial engineering and pore engineering manipulation for other COFs, porous polymers, and their membranes.