Electrochemical performance and characterization of a supercapacitor device using carbon nanotube/graphene nanoplatelet composites with biomass-derived carbon dots

The commercialization of flexible electronic goods has driven interest in flexible wearable energy storage devices, notably supercapacitors (SCs). The supercapacitive electrodes made from green, renewable materials are essential for facilitating an eco-friendly approach. This work focused on integrating doped carbon dots (CDs) on the surface of the carbon nanotube/graphene nanoplatelet (CNT : GNP) composites, exhibiting excellent structural/electrochemical characteristics. The CNTs were purified by thermal treatment in a CVD furnace, while the CDs were extracted and hydrothermally synthesized from the biowaste of black lentils. The CDs/CNT : GNP hybrid composite electrode and device demonstrated excellent electrochemical behavior in 1 M Na₂SO₄ electrolyte. The combination of CNTs and GNPs was synergistic as CNTs dispersed evenly in the material prevented GNPs from stacking, enhancing the surface area, porosity, and conductivity. Additionally, CNT intercalation created a porous, linked network for electrolyte ion transfer. The resulting electrode material S-doped CDs/CNT : GNP exhibited better electrochemical performance, with capacitance values as high as 888 F g^-1 at a scan rate of 5 mV s^-1 (recorded in the 0 to 0.8 V voltage window, with a low solution resistance of 0.28 Ω). Moreover, the doped CDs/CNT : GNP device showed remarkable cyclic retention, i.e., ∼80% after 10 000 cycles, due to the imparted structural stability, making it a good contender in SCs.