Bio-derived graphitic carbon quantum dot encapsulated S- and N-doped graphene sheets with unusual battery-type behavior for high-performance supercapacitor

Electrochemical supercapacitors (ECs) have increased large attention in recent years due to their faster charging and discharging process, large specific capacity, and high power density. Herein, we designed the preparation of carbon quantum dots (CQDs), and N-/S-doped CQDs, encapsulated within graphene sheets via a lemon-juice-derived sustainable route for high-performance positive electrode material in supercapacitor application. The as-prepared electrodes revealed an unusual battery-type behavior due to the presence of large number of O-/N-/S-containing functional groups within the graphitic network of the CQDs-graphene composite, which enhances the redox-active sites. The specific capacities were obtained for CQDs, N-doped, and S-doped CQDs at 284, 181, and 125 mAh/g at 1 A/g current density. The S-doped CQDs delivered 94.5% capacitance retention over 10,000 cycles with 98% Coulombic efficiency. The real-time symmetric supercapacitor was constructed using as-prepared S-doped CQDs with an operated potential window of 1.4 V. The constructed symmetric supercapacitor shows high specific capacity, energy, and power densities of 21.7 mAh/g (55.7 F/g at 1 A/g), 15.2 Wh/kg, and 700 W/kg, respectively. The outstanding electrochemical results are based on the easy formation of heteroatom doped S and N-CQDs to enhance the better electronic/ionic motion.