Confined Growth of 2D Covalent Organic Framework Nanosheets with Controlled Thickness for Osmotic Energy Conversion

Abstract Achieving precise layer number control in scalable synthesis of 2D polymers (2DPs) remains a critical challenge. Here, the confined growth of 2D covalent organic frameworks (2D COFs) is reported within the nano‐adsorption layer at the solid–liquid interface under ambient conditions. The method allows the gram‐scale preparation of 2D COF nanosheets (2D CONs) with high crystallinity, controlled thicknesses ranging from 0.8 nm (bilayer) to 10.1 nm (29 layers), and lateral sizes of ≈2.0 µm within 6 h. The resulting nanosheets can be well dispersed in various solvents at concentrations up to 0.028 g L −1 . They allow fabrication of thin membranes (0.5–3.0 µm) with high anion selectivity and stability for osmotic energy harvesting, achieving an output power density of 78.5 W m −2 under a 50‐fold NaCl concentration gradient—≈15 times higher than the commercial benchmark (5 W m −2 ) and outperforming most previously reported membranes of comparable thickness based on 2D COFs or other 2D nanosheets. Moreover, the method can be extended to reactions involving different monomers or inorganic substrates, facilitating the preparation of diverse 2D polymers and their inorganic hybrids. This adsorption layer‐confined growth of 2D CONs paves the way for efficient preparation and advanced applications of 2D polymeric materials.