In this study, a quasi-solid-state supercapacitor with high energy density was successfully developed using a zwitterionic gel electrolyte with a dual-function redox additive. A polyzwitterion charged polymer, poly([2-(methacryloyloxy)ethyl]dimethyl-(3-sulfopropyl)ammonium hydroxide) (PPDE), was used with LiCl as the flexible gel electrolyte. This proposed electrolyte demonstrates promise for use in practical supercapacitors due to its environmental friendliness, transparency, flexibility, and high ionic conductivity. To further improve the electrochemical performance, a dual-function redox additive, ethyl viologen dibromide, was incorporated into the polyzwitterion. With the redox mediator in the polyzwitterionic electrolyte (PPDE-LiCl-EV), the specific capacitance improved by a factor of three compared to the pristine zwitterionic polymer (178 F g−1 for PPDE-LiCl and 677 F g−1 for PPDE-LiCl-EV) due to additional faradaic reactions. In addition, a quasi-solid-state supercapacitor incorporating PPDE-LiCl-EV exhibited a high energy density of 542 Wh kg−1 and a power density of 2.88 kW kg−1 over a wide potential window of 2.4 V. Fourier transform infrared spectroscopy in attenuated total reflectance mode was employed for in situ monitoring of the redox reactions of additives on the electrode surface. To the best of our knowledge, this is the first attempt to analyze the redox reactions of redox additives.