Unlocking the potential of KI as redox additive in supercapacitor through synergistic enhancement with H2SO4 as a co-electrolyte

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Adding potassium iodide (KI) as a redox additive to any non-faradaic electric double layer forming aqueous electrolyte (EE) has been shown to significantly enhance the charge storage capacity in a supercapacitor. However, whether the choice of EE is immaterial, or if there are advantages with any specific EE when used with KI is an open question. Herein, we demonstrate that the extent to which performance enhancement can be attained is dependent on the EE used in conjugation with KI. EEs as co-electrolyte in KI-EE systems influence (inhibit/promote) the specific redox reactions, their kinetics, solution conductivity as well as the wettability that dictates its pore penetration. While the pH of the electrolyte dictates the participating redox reactions, the kinetics and the solution conductivity are determined by the characteristic ions constituting the EEs. Our investigations reveal that H2SO4 (acidic) as EE when used with KI showed a synergistic enhancement in charge storage capacity, which was otherwise not obtained for other commonly used EEs such as Na2SO4 and KOH. The synergistic enhancement was found emanating from the enhanced reaction kinetics of the iodine-iodide redox reactions and augmented pore penetration through enhanced wetting resulting in the access to an increased electrochemical surface area. Conversely, Na2SO4 (neutral) as EE with KI offers impeding effect on the charge storage through iodine-iodide redox reactions along with poor wetting of the electrode, thereby resulting in a lower specific capacitance. While KOH (basic) as EE with KI, leads to suppression of the iodine-iodide redox reactions offered by KI thereby lowering the charge storage capacity as compared to KI.