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.

Currently, supercapacitors (SCs) are a promising field in the area of energy storage devices. In the last few decades, different types of carbon based materials with suitable surface modifications, metal oxides, metal hydroxides, conducting polymers and their various composites have been used as electrodes to improve the energy performance of SCs. In addition, different technologies like asymmetric and hybrid systems have also been introduced. Interestingly, another alternative approach has been proposed recently by a few research groups, wherein electrolytes (liquid and polymer) can enhance the performance of the SCs via redox reactions at the electrode–electrolyte interface, by introducing redox additives or mediators in the electrolytes. The main advantage of this new technique is its simple and safe preparation method, along with cost effectiveness compared to the preparation of some active electrode materials. Hence it is believed that identification of suitable redox additives or species in electrolytes will be a hotspot in the field of SCs in the coming years.

A hydroquinone mediated PVA–H₂SO₄ gel electrolyte (PHHQ) and activated carbon from bio-waste were prepared for supercapacitor fabrication. PHHQ delivered a higher capacitance (941 F g⁻¹ at 1 mA cm⁻²) and energy density (20 Wh kg⁻¹ at 0.33 W g⁻¹) than the PVA–H₂SO₄ gel electrolyte (425 F g⁻¹ at 1 mA cm⁻², 9 Wh kg⁻¹ at 0.33 W g⁻¹).

Electric double layer capacitors (EDLCs) were fabricated using biomass derived porous activated carbon as electrode material with 1 M H₂SO₄ and VOSO₄ added 1 M H₂SO₄ as electrolytes. Here, VOSO₄ was used as redox additive to improve the overall performance of EDLC. As expected, the VOSO₄ electrolyte showed ∼43% of improved specific capacitance of 630.6 F g⁻¹ at 1 mA cm⁻² compared to pristine 1 M H₂SO₄ (440.6 F g⁻¹) due to the contribution of VO²⁺/VO₂ ⁺ redox reaction at the electrode–electrolyte interface. Possible redox reaction mechanism of VO²⁺/VO₂ ⁺ pair is also briefly illustrated. The good cycling performance of 97.57% capacitance retention was observed even after 4000 cycles. For comparison, the polymer gel electrolyte (PVA/VOSO₄/H₂SO₄) was also prepared and then the performance of the fabricated EDLCs was studied. Overall, these findings could open up a simple and cost effective way to improve the performance of EDLCs significantly.