Novel onion flower-derived biochar for high-performance sustainable supercapacitor applications

Onions, scientifically known as Allium cepa L., are commonly used as a vegetable and offer a variety of health benefits. However, this study explored a novel part of the onion plant, such as the flower seed cover often regarded as waste, by converting it into biochar (BC) via pyrolysis. Through careful optimization and activation with potassium hydroxide, the resulting novel biochar exhibited a unique graphitic nanosheet-like structure with an impressive surface area of 2538.31 m²/g. To serve as a control, rice straw, one of the most commonly used biomasses, was also activated using the same protocol. Various techniques, including XPS, HRTEM, SAED, FTIR, BET, XRD, and Raman, were used to comprehensively understand the surface and structural properties of the biochar. When the novel biochar was tested as an energy storage material, it exhibited a specific capacitance of 200.37 F/g at 1 A/g and 143.82 F/g at 5 A/g in a 1 M H₂SO₄ electrolyte with a three-electrode system. In a two-electrode system, it showed 158 F/g at 1 A/g and 102 F/g at 5 A/g, indicating its effectiveness at high rates. The long-term cyclic stability analysis demonstrated 100 % capacity retention over 1000 cycles in the three-electrode system, and 90.77 % and 74.40 % retention after 5000 cycles and 10,000 cycles, respectively, in the two-electrode system. The fabricated symmetric supercapacitor yielded a power density of 500 W/kg and an energy density of 21.94 Wh/kg at a current density of 1 A/g. These findings provide valuable insights into the valorization of biomass in addressing concerns regarding the current energy storage dilemma.