Halogen (iodide, I⁻) added aqueous electrolyte facilitates the capacitive behaviour of biomass derived activated carbon based electric double layer capacitors. To produce economically viable electrodes in large scale for supercapacitors (SCs), the activated carbons (ACs) prepared from Eichhornia crassipes (common water hyacinth) by ZnCl₂ activation. The prepared ACs were characterized by XRD, Raman, FT-IR and surface area, pore size and pore volume analysis. The electrochemical properties of the SCs were studied using cyclic voltammetry (CV), galvanostatic charge–discharge (GCD), electrochemical impedance spectroscopy (EIS) and cycling stability. The 3I⁻/I₃⁻, 2I⁻/ I₂, 2I₃⁻/3I₂ and I₂/IO₃⁻ pairs produce redox peaks in CV and a large Faradaic plateau in charge–discharge curves. Similarly, I⁻ ions improves the good ionic conductivity (lower charge transfer resistance) at the electrode/electrolyte interface which was identified through EIS studies. The calculated specific capacitance and energy density was 472 F g⁻¹ and 9.5 W h kg⁻¹ in aqueous solution of 1 M H₂SO₄. Interestingly, nearly two-fold improved specific capacitance and energy density of 912 F g⁻¹ and 19.04 W h kg⁻¹ were achieved when 0.08 M KI was added in 1 M H₂SO₄ electrolyte with excellent cycle stability over 4000 cycles. Subsequently, this improved specific capacitance and energy density was compared with 0.08 M KBr added to 1 M H₂SO₄ (572 F g⁻¹, 11.6 W h kg⁻¹) and 0.08 M KI added to 1 M Na₂SO₄ (604 F g⁻¹, 12.3 W h kg⁻¹) as electrolytes.