Surface Chemistry Regulation of Conductive Two-dimensional Nanosheets with Highly Pseudocapacitive Covalent Groups for A High-performance Flexible Asymmetric Microsupercapacitor

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