A novel COF/MXene film electrode with fast redox kinetics for high-performance flexible supercapacitor

Covalent organic frameworks (COFs) are an emerging class of crystalline porous polymer due to their well-defined channels, highly accessible surface areas and tunable active sites, showing great application potential in the field of energy storage. However, their low inherent conductivity and poor accessibility (difficult to dissolve or melt) have largely limited their electrochemical performances and practical applications. Here, a cation-driven self-assembly process was used to prepare flexible carbonyl containing COF/MXene composite film electrode (CMFs). The uniform intercalation of porous COFs in layered MXene was realized by electrostatic attraction between negatively charged 2D MXene nanosheets and the protonated DAAQ-COFs. Porous COFs tightly contact with MXene and effectively alleviate the self-restacking of MXene nanosheets. In addition, under the synergistic effect, the ordered one-dimensional pore structure of COFs and excellent intrinsic conductivity of MXene can greatly enhances the electron transfer and ion migration rates and further improves the reaction kinetics for flexible composite electrodes. In the three-electrode system, the integrated CMFs film electrode delivers excellent electrochemical performances (the capacitance of 390 F g−1 at 0.5 A/g), excellent kinetic energy storage characteristics (capacitive contribution can be up to 96.7 % at 50 mV/s) and superb mechanical strength (repeated bending 100 times). By assembling into all-solid-state flexible supercapacitor, CMFs//CCMP asymmetric supercapacitors (ASC) achieves the maximum energy density of 27.5 Wh kg−1 (at a power density of 350 W kg−1), maximum power density of 7000 W kg−1 (at an energy density of 19.7 Wh kg−1) and the ultrahigh capacitance retention of 88.9 % after 20,000 charging-discharging long cycles, suggesting great potentials for the applications in future flexible and wearable devices.