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

Abstract 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 H 2 /O 2 fuel cell performance.