Superior hydrogel electrolytes in both ionic conductivity and electrochemical window from the immobilized pair ions for carbon-based supercapacitors

In this study, hydrogel electrolytes (HEs) with the immobilized pair ions (IPIs) were studied for carbon-based supercapacitors. HEs with immobilized cations (ICs), IPIs, and immobilized anions (IAs) were prepared by controlling the mole ratio of [2-(acryloyloxy)ethyl]trimethylammonium chloride and 2-acrylamido-2-methyl-1-propanesulfonic acid sodium via random copolymerization with poly(ethylene glycol) diacrylate. HEs (IC-Na₂SO_4, IPI-Na₂SO_4, IA-Na₂SO_4, and PVA(Polyvinyl alcohol)-Na₂SO₄) was simply prepared by 1 M Na₂SO_4(aq) intake. The ionic conductivities of IC-Na₂SO_4, IPI-Na₂SO_4, IA-Na₂SO_4, 1 M Na₂SO_4(aq), and PVA-Na₂SO₄ were 37.7 mScm⁻¹, 81.6 mScm⁻¹, 62.9 mScm⁻¹, 56.8 mScm⁻¹, and 0.135 mScm⁻¹ at 25 °C via electrochemical impedance spectroscopy. Based on the dielectric analysis, IPI-Na₂SO₄ with showed the largest dielectric constant was chosen for the supercapacitors test. Carbon-based symmetric supercapacitors (CSSs) were assembled with 1 M Na₂SO_4(aq) (CSS-aq), PVA-Na₂SO₄ (CSS-PVA), and IPI-Na₂SO₄ (CSS-IPI). Electrochemical windows (EWs) of CSS-aq, CSS-PVA, and CSS-IPI were respectively 1.2 V, 1.3 V, and 1.9 V via cyclic voltammetry. The specific capacitances of CSS-aq, CSS-PVA, and CSS-IPI were respectively 125 Fg⁻¹, 71 Fg⁻¹, and 246.2 Fg⁻¹ at current density of 0.5 Ag⁻¹. Since IPI-Na₂SO₄ possesses 1.43 times ionic conductivity and 1.58 times EW of the 1 M Na₂SO_4(aq), it also possessed the remarkable energy density of 121.3 Wh Kg⁻¹ and high power density of 38,424.6 W Kg⁻¹ with 5000 charge/discharge cycles.