Revealing the super capacitive performance of N-doped hierarchical porous activated carbon in aqueous, ionic liquid, and redox additive electrolytes

Supercapacitors are generally high-power devices, yet their energy is low in contrast to batteries. In the present study, the N-doped porous carbon optimized from brinjal bio-mass waste (solanum melongena) using KOH+ Urea activation confirms its ability as a supercapacitor electrode in presence of an aqueous (1 M H₂SO₄), ionic liquid (1-ethyl-3-methylimidazolium-tetrafluoroborate-EMIMBF₄) and an improved redox additive (0.01 M Hydroquinone-HQ) electrolytes. The hierarchical porous nature of the activated brinjal bio-mass waste carbon with desired compositions and structure is analyzed using FE-SEM and HR-TEM analysis. In a two electrodes symmetric configuration, brinjal waste-derived activated carbon (BC-700) delivers a high specific capacitance of 460 F/g at 1 A/g in 1 M H₂SO₄ aqueous electrolyte. In ionic liquid, it delivers 133 F/g high specific capacitance with an energy of 41 Wh/kg. This outstanding electrochemical performance is due to the electrolyte-ion movement of heteroatoms into the carbon matrix resulting in high specific surface area (850 m² g⁻¹) and effective microporosity. The energy density of the supercapacitor device is further enriched using the novel redox additive 0.01 M Hydroquinone in H₂SO₄ electrolyte with a specific capacity of 888 C/g and a maximum energy density of 61 Wh/kg, which is very high compared to batteries. Outstanding cyclic stability of 77 % capacitance retention after 5000 cycles is achieved in HQ-added aqueous electrolyte. Hence, the bio-mass waste-derived activated carbon performs as an excellent low-cost material for various electrochemical applications.