Tunable synthesis of carbon quantum dots from the biomass of spent tea leaves as supercapacitor electrode

A simple and cost-effective method has been used for obtaining highly conductive carbon quantum dots (CQDs) by hydrothermal oxidation of spent tea leaves. The obtained CQDs are characterized by various analytical techniques demonstrating a crystalline structure. CQDs proved to be exceptional working electrodes in the three-electrode cell in H₂SO₄ aqueous electrolyte. Based on X-ray diffraction and microscopic analysis, the synthesized CQD is crystalline, with a diameter of 2–5 nm. Electrochemical impedance spectroscopy, cyclic voltammetry and charge-discharge measurements are used to evaluate the electrochemical performance of CQDs in an aqueous electrolyte. The CQD-based supercapacitor demonstrated a superior specific capacitance of 302.0 F g^–1 at a current of 0.5 Ag^–1, cyclability with 144.4 F g^–1 at 20 A g^–1 after 5000 cycles, and rate performance of 186.4 F g^–1 at 20 A g^–1. The CQDs electrode also exhibited an enhanced energy density and power density of 41.9 Wh g^–1 and 250 W g^–1, respectively at a current rate of 0.5 A g^–1. In experiments, it is demonstrated that many thousands of cycles could be run consistently at near 100% efficiency and a stable capacity. This explains the possibility of using spent tea leaves as a bioresource, the ease of synthetically producing these materials, and the favourable redox reaction of CQDs for high-energy devices.