Single Solution‐Phase Synthesis of Charged Covalent Organic Framework Nanosheets with High Volume Yield

Covalent organic framework nanosheets (COF-NSs) are emerging building blocks for functional materials, and their scalable fabrication is highly desirable. Current synthetic methods suffer from low volume yields resulting from confined on-surface/at-interface growth space and complex multiple-phase synthesis systems. Herein, we report the synthesis of charged COF-NSs in open space using a single-phase organic solution system, achieving magnitudes higher volume yields of up to 18.7 mg mL^-1 . Charge-induced electrostatic repulsion forces enable in-plane anisotropic secondary growth from initial discrete and disordered polymers into large and crystalline COF-NSs. The charged COF-NS colloidal suspensions are cast into thin and compact proton exchange membranes (PEMs) with lamellar morphology and oriented crystallinity, displaying outstanding proton conductivity, negligible dimensional swelling, and good H₂ /O₂ fuel cell performance.