Metal ion-bonded two-dimensional framework non-Van der Waals sandwich heterojunctions for fast mass transfer in flexible in-plane micro-supercapacitors

Two-dimensional (2D) reticular framework films featuring highly accessible surface areas, tunable active sites, and well-defined channels are promising candidates for flexible in-plane micro-supercapacitor (MSC) electrodes. However, the interlayer Van der Waals forces in 2D heterojunctions can limit mass/charge transport. Herein, we design a non-Van der Waals force bonded heterojunction of covalent organic frameworks (COFs) and metal–organic frameworks (MOFs) linked by metal-ion coordination. A COF@MOF monolithic nanofilm is constructed by growing MOF (M3(HHTP)2) in situ on the COF (COFTD) surface, using nickel (Ni) as the optimal metal to connect the two layers and form a sandwich electrode. We further explore various transition metals in M3(HHTP)2, from manganese (Mn) to zinc (Zn), to adjust the electronic structure and charge redistribution. The optimal MSC-Ni-COFTD@Co3(HHTP)2 device exhibits an impressive specific capacitance (1645.3 ​F ​cm−3 at 10 ​mV ​s−1), a high energy density (146.3 ​mWh cm−3), as well as superior cycling and bending stability. This work offers an innovative perspective on overcoming the mass transfer and electron migration limitations of 2D reticular frameworks for miniaturization and wearable energy storage electronics.