Nitrogen and sulfur co-doped carbon quantum dot-decorated Ti₃C₂T_<i>x</i>-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 Ti₃C₂T_<i>x</i> 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 (<i>C</i>_sp) 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 <i>C</i>_sp is the electron clouds near the N- and S-doped atoms which act as active sites. Finally, these unique hybrid composite materials with high <i>C</i>_sp, 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.