Acrylic acid-based zwitterionic hydrogel for improving the flexibility and performance of supercapacitors

Background: Acrylic acid (AA) is cheap and easy for polymerization to anionic polymer (PAA) as hydrogel. However, the PAA hydrogel electrolyte is fragile in compressive strength due to its high intake of water. Methods: AA is neutralized for sodium acrylate (SA), which is polymerized to obtain PAA with crosslinker. PMA is copolymerized from [2-(acryloyloxy)ethyl] trimethylammonium chloride (META) and SA with crosslinker. Both PAA and PMA are confirmed by Fourier-transform infrared spectra and with 1 M Na₂SO₄ for Raman spectra, electrochemical impedance and dynamic mechanical analysis in comparison. Finally, PAA and PMA hydrogel electrolytes are sandwiched with carbon electrodes as supercapacitors which are bended for electrochemistry test. Significant findings: PMA hydrogel possesses more bound water than PAA hydrogel via Raman spectra analysis, and the ionic conductivity of 72.8 mS∙cm⁻¹ in PMA hydrogel is higher than one of 60.3 mS∙cm⁻¹ in PAA hydrogel. The toughness of PMA hydrogel is better than PAA hydrogel from dynamic mechanical analysis. For supercapacitors, the specific capacitance of PMA supercapacitor is higher than PAA supercapacitor. Furthermore, PMA hydrogel is used for supercapacitors under bending tests. Surprisingly, PMA supercapacitor for bending 90° angle still has over 90% capacitance retention in comparison to one without bending.