Methylene blue as a redox additive in electrolytes for advanced charcoal-based hybrid supercapacitors

Carbon-based supercapacitor electrodes derived from biomass have recently garnered significant attention due to their low cost, natural abundance, and environmental sustainability. In this study, charcoal was pretreated using a simple ultrasonic method and was employed as the active electrode material in both three-electrode and symmetric supercapacitor configurations. To further enhance electrochemical performance, a sustainable and dual-functional strategy was implemented by introducing methylene blue, a redox-active additive, into an aqueous sodium chloride electrolyte. Structural and morphological characterizations revealed that charcoal possessed a highly porous architecture with preserved plant-based vascular channels, facilitating efficient electrolyte penetration and ion transport. Electrochemical analyses demonstrated that the incorporation of methylene blue significantly enhanced charge storage through a synergistic combination of electric double-layer capacitance and pseudocapacitive behavior. The optimal device, utilizing the MB35 electrolyte composition, delivered a high specific capacitance of 212.23 F g^-1 at 0.5 A g^-1, an energy density of 15.34 Wh kg^-1 at a power density of 350 W kg^-1, and excellent cycling stability, retaining 91.3% of its initial capacitance after 2000 cycles and 84.3% after 5000 cycles at the high loading mass of 2 mg cm^{- 2}. This work presents a cost-effective route for fabricating high-performance biomass-derived supercapacitors while offering a novel approach for the reutilization of dye pollutants in sustainable energy storage applications.