Anion-dependent electrochemical capacitive behavior of biomass-derived activated carbon in imidazolium ionic liquid electrolytes

Abstract

Biomass-derived activated carbons (ACs) and ionic liquids (ILs) are attractive materials for developing high-performance electrochemical capacitors. However, their combined applications are limited by an inadequate understanding of their interactions. In this study, we have investigated the electrochemical capacitive behaviour of three imidazolium-based ILs: [bmim]Br, [bmim]Cl, and [bmim]PF_6, and explored the effect of anions on the performance of a biomass-based AC supercapacitor. A hierarchical porous AC was prepared from banana leaf, which contains abundant micropores and mesopores. The electrochemical capacitive performance was tested using a symmetric two-electrode cell. The results demonstrated strong anion-dependent capacitance behaviour. The [bmim]PF₆ showed the widest stable cell voltage window (0–3.0 V), achieving the highest energy density of 87 Wh kg⁻¹ and the highest power density of 375 W kg⁻¹. The [bmim]Br exhibited the shortest voltage window of 0–1.0 V with the lowest energy density of 6 Wh kg⁻¹ and power density of 125 W kg⁻¹. Whereas the [bmim]Cl provided a competitive energy density of 28 Wh kg⁻¹ and power density of 200 W kg⁻¹ within the stable voltage window of 0–1.6 V. The [bmim]Cl and [bmim]Br showed remarkable cyclic stability with 98% capacitance retention over 10 000 cycles. These results demonstrate that anion size, ion pairing, and IL viscosity critically determine the scaling of energy and power density. The synergy between hierarchical porosity in biomass-derived AC and carefully selected ILs establishes a promising platform for high-performance, sustainable supercapacitors.