Covalent Organic Framework Nanofilm Heterojunctions: Lamination Effect and Suppressed Self‐Discharge in Flexible Micro‐Supercapacitors Energy Storage

Covalent organic frameworks (COFs) nanofilms with well-ordered channels and highly active interfaces have great potential in in-plane micro-supercapacitors (MSCs). COF heterojunction nanofilms integrate the benefits of individual COF phases through alternating stacking. Herein, sandwich-type COF heterojunctions are prepared under van der Waals bonding, controlling the larger outer aperture (vs. inner) to create the stereoscopic lamination effect in axial channel structure, which enhances the rapid transport of electrolyte H⁺ and their concentrated accumulation on active interfaces. Simultaneously, the unique heterojunction structure effectively reduces resistance to electron transport, enabling electrons to conduct through in-plane π-electron clouds and facilitating π-π electron transitions across interfaces. In addition, the outer aperture in COF heterojunctions is also adjusted to inhibit H⁺ overflow causing the self-discharge phenomenon. The results show that the optimal MSC-COF_1.0-0.6-1.0 exhibits a high volumetric specific capacitance (C_V) of 598.6 F cm^-3, high energy density of 40.7 mWh cm^-3 at 2095.2 mW cm^-3, good self-discharge property up to 36 h, and excellent cycling and bending-resistant stability. This work about the optimal integration of multiple factors in COF heterojunctions, including lamination effect, interface contribution, and ion overflow, can provide theoretical guidance for the application of COF heterojunctions in miniature or flexible/wearable devices.