Surface chemistry regulation of conductive two-dimensional nanosheets with highly pseudocapacitive covalent groups...

Flexible microsupercapacitors are attracting much more attention for use in miniature energy storage devices due to their attractive electrochemical characteristics. However, it is difficult to achieve high energy density and long-term stability at the same time due to a lack of suitable high-specific-energy materials. Herein, an innovative asymmetric microsupercapacitor is obtained by the highly efficient surface chemistry regulation of conductive two-dimensional nanosheets of both graphene and MXene with highly active covalent groups. To optimize the positive graphene material, graphene oxide powder is first employed, undergoing a fast expansion and exfoliation process via water molecule explosive vaporization. Then, the conductive graphene sheet layer with a high specific surface area is oxidized selectively by acid treatment to introduce pseudocapacitive groups at an optimized proportion and density (more -COOH and less -OH), and this is labelled as expanded-and-oxidized graphene (EOG). To optimize the negative MXene material, delaminated MXene nanosheets undergo hydroxyl substitution through alkalization, and the two-dimensional (2D) lamellae become wrinkled using coulombic attraction for fast intercalation pseudocapacitance, and this is labelled as wrinkled-and-hydroxylated MXene (WOM). Through surface chemistry modification and all-pseudocapacitive 2D structure design, flexible EOG and WOM composite electrodes exhibit capacities of 382 F g^-1 and 550 F g^-1, respectively, and have remarkable stability. An assembled proton-type solid-state microsupercapacitor with a voltage window of 1.5 V readily achieves an energy density of 47.25 mW h cm^-3 (23.62 W h kg^-1) at a power density of 1900.55 mW cm^-3 (950.27 W kg^-1), with high capacity retention of 86.8% after 8000 cycles. This work shows a well-designed microdevice with flexible and integrable properties based on 2D microstructure engineering for use in flexible electronics.