Hydrogel electrolytes with immobilized pair ions via one-pot copolymerization for flexible supercapacitors

In this study, hydrogel electrolytes with immobilized pair ions are studied for the possibility of the flexible supercapacitors. One-pot copolymerization of 2-hydroxyethyl methacrylate (HEMA), [2-(acryloyloxy) ethyl] trimethylammonium chloride (META), 2-acrylamido-2-methyl-1-propanesulfonic acid sodium (AMPS) and crosslinker is employed to obtain five polymeric samples, increasing number with increasing HEMA. Fourier-transform infrared spectra confirm the reactions among HEMA, META and AMPS. Ionic conductivities of hydrogel electrolytes increase with increasing immobilized pair ions by electrochemical impedance. Raman spectra of ClO₄⁻ stretching can be split into free anions (FA), solvent separated ion pairs and contact ion pairs (CIP). Hydrogel electrolytes with more FA and less CIP possess the higher ionic conductivity. The compressive strength increases with increasing HEMA from dynamic mechanical analysis. HAS1, HAS4 and HAS5 sandwiched with electrodes are selected for electrochemical studies including electrochemical impedance, cyclic voltammetry, and galvanostatic charge/discharge. Among them, HAS4 covers the most requirements; possesses 64.8 mS cm⁻¹ ionic conductivity, 65% compressive strain, and 0.45 MPa compressive strength. Therefore, HAS4 supercapacitor is chosen for cycle and bending tests. HAS4 supercapacitor has 96.3% capacitance retention after 5000 charge/discharge cycles at 2 A g⁻¹. Under bending different angles, there is no significant loss for the capacitance of HAS4 supercapacitor.