Nitrogen and sulfur co-doped carbon quantum dot-decorated Ti3C2Tx-MXenes as electrode materials for high-performance symmetric supercapacitors

Considering the huge consumption of fossil fuels and cumulative energy demands in the high-tech society, energy storage devices, particularly supercapacitors, play a pivotal role in exploring alternative sources of renewable energy. To focus on potential supercapacitor electrode materials, they should possess essential features such as a massive surface area, good conductivity, and a plentiful number of active sites. Therefore, the integration of a large surface area of two-dimensional (2D) materials with high conductivity with carbon quantum dots (CQDs) containing a large number of active sites is an elegant approach for achieving excellent electrode materials for supercapacitor applications. In the present work, we decorate highly conducting 2D Ti3C2Tx MXene sheets with nitrogen (N) and sulfur (S) co-doped CQDs (NS-CQDs) as a source of numerous active sites to explore their potential as electrode materials for supercapacitor applications. We synthesize three samples exhibiting impressive specific capacitances (Csp) of 562.7 F g-1, 725.7 F g-1, and 523.4 F g-1, respectively, at a current density of 1 A g-1 with excellent cycling stabilities of 95.5%, 98.3%, and 94.1%, respectively, at a current density of 10 A g-1 over 10 000 cycles. The origin of this excellent Csp is the electron clouds near the N- and S-doped atoms which act as active sites. Finally, these unique hybrid composite materials with high Csp, high energy density (170.34 W h kg-1 at a power density of 1290.98 W kg-1), and outstanding electrochemical stability show significant promise in the field of storage device applications.