Heteroatom-free activated carbon derived from wasted geranium leaves as high-performance electrode material in symmetric supercapacitors

Converting biomass waste into high-performance supercapacitor carbon electrode materials often relies on additional heteroatom doping or the use of aqueous electrolytes, which have limited voltage windows. On the contrary, this study presents the conversion of Geranium leaves before and after essential oil extraction into heteroatom-free activated carbon for use as an electrode material in symmetric supercapacitors. The performance of the heteroatom-free activated carbon is evaluated in 3 V symmetric supercapacitor coin cells using ionic electrolyte (ACN:[BMIM]BF 4 ). The performance of activated carbon from both pre- and post-oil-extracted Geranium leaves is compared. The design-directed carbonization followed by the KOH activation process produced an exceptionally high specific surface area (as high as ∼3516 m 2 /g) with hierarchical micro-mesoporous structures and a defect-rich carbon framework. The activation process yielded a heteroatom-free surface carbon, ensuring that the electrochemical performance arises solely from the intrinsic pore structure and carbon framework. Additionally, the symmetric supercapacitors in coin cell assembly are fabricated using an environmentally friendly binder and solvent, as well as an ionic liquid electrolyte. In this study, a high specific capacitance of 445 F/g at 1 A/g, an energy density of 69 Wh/kg, and a power density of 6925 W/kg, along with excellent cycling stability over 10,000 cycles, are achieved. Power-law analysis of cyclic voltammetry data reveals a balanced contribution of capacitive and diffusion-controlled processes, providing insights into the structure-ion interaction pathways. Further analysis confirms the robust structural integrity of the electrode material. This study also sheds light on the structure–ion interaction mechanisms in energy storage. AC/DC conductivity analysis reveals that, while the bulk ionic conductivity of the electrolyte remains consistent across all samples, the frequency-dependent ion transport, and consequently the capacitive performance, are primarily influenced by the pore architecture of the carbon electrodes. The coin cells demonstrate a one-year shelf-life. This work positions the carbon derived from waste Geranium leaves as a competitive, eco-friendly electrode material for high-performance supercapacitors. • Heteroatom-free activated carbon (AC) is derived from wasted Geranium leaves. • The prepared AC exhibited an extremely high specific surface area of 3516 m 2 /g. • AC/AC symmetric coin cell exhibited an energy density of ∼69 Wh/kg. • AC/AC symmetric coin cell exhibited a power density of ∼6925 W/kg. • AC/AC symmetric coin cells exhibited excellent cyclability.