Electrochemical properties of KI-modified and H2SO4-protonated polyvinyl alcohol gel polymer electrolyte applied for activated carbon paper electrode.

The redox additive KI-modified and H₂SO₄-protonated polyvinyl alcohol gel polymer electrolyte (KI-H₂SO₄-PVA GPE) is used to match well with an activated carbon paper (ACP) electrode, which is adopted for solid-state pseudocapacitor applications. Enhanced hydrogen bonding interactions are established between oxygen-containing functional groups of ACP and multi-hydroxyl groups of PVA, promoting interfacial charge transfer. An intensified electrostatic adsorption is also promoted between H₂SO₄-protonated PVA and free iodide anions, enhancing the ionic and electrical conductivity of GPE. The electroactive iodide anion with polyvalent states of I⁻, I₃⁻ and IO₃⁻ in KI-H₂SO₄-PVA GPE participates in reversible redox reactions at the ACP electrode interface, which mostly contributes to Faradaic capacitance. The mass ratio of redox additive KI was optimized to be 0.166, which could balance the capacitance and the cyclability of KI-H₂SO₄-PVA GPE. The KI-H₂SO₄-PVA GPE reveals lower ohm resistance, charge transfer resistance and Warburg impedance, leading to its higher electrical and ion conductivity compared to H₂SO₄-PVA GPE. Theoretical simulations proved that the protonation of H₂SO₄-PVA and electrostatic interaction between H₂SO₄-protonated PVA and KI could cause an effective decrease in the interfacial energy of the activated carbon paper electrode. The KI-H₂SO₄-PVA GPE/ACP accordingly presents higher specific capacitance (24.75 mF cm⁻²) and rate capability (45.5%) than H₂SO₄-PVA GPE/ACP (12.93 mF cm⁻², 15.6%). The redox-activated KI-H₂SO₄-PVA GPE exhibits a lowered cycling stability (64.5%) compared to H₂SO₄-PVA GPE (78.3%) in ACP supercapacitor applications due to the gradually decreasing irreversibility of the redox reactions of polyvalent iodide ions. The KI-H₂SO₄-PVA GPE matches well with ACP electrode and accordingly is suitable for solid-state pseudocapacitor applications with high capacitance performance. The KI-modified and H₂SO₄-protonated polyvinyl alcohol gel polymer electrolyte (KI-H₂SO₄-PVA GPE) is used to match well with the hydroxyl and epoxy groups activated carbon paper (ACP) electrode to form the intensified hydrogen bonding interface, contributing to superior Faradaic capacitance for the solid-state supercapacitor application. [ABSTRACT FROM AUTHOR]