"Crystallinity Wave"-Driven Synthesis of Hollow Multi-Shell Covalent Organic Frameworks for Enhanced Supercapacitors

Hollow multi-shell covalent organic frameworks (COFs) with abundant modular interfaces, high loading capacity, and various microenvironments are expected to hold great potential for chemical separation, heterogeneous catalysis, and energy storage/conversion. However, the synthetic methodology of COF hollow multi-shell nanoarchitectures has not been established. Herein, we demonstrate an ingenious "crystallinity wave"-induced regional difference ripening strategy to synthesize a series of hollow multi-shell COF particles with controllable shell numbers and shell thickness. The methodology relies on the isolation effect of the local crystalline COF thin layer inserted between the two layers of amorphous covalent organic polymer by the short-time Ostwald ripening, so that different regions of the particles exhibit distinct reaction stages before reaching chemical equilibrium in the subsequent dynamic imine exchange reaction, and then regions that tend to hydrolyze dissolve during the complete ripening process to form a hollow multi-shell structure. Remarkably, this strategy can be extended to prepare other hollow multi-shell COFs by altering monomers. As a proof-of-concept application, the obtained hollow multi-shell COFs are used as the electrode materials for supercapacitor. Benefiting from the short mass transfer path of the hollow multi-shell structure, ordered channels of the COF, and their high surface area, the as-prepared particles exhibit remarkably enhanced specific capacitance.