Redox-active hydrogel electrolytes for carbon-based flexible supercapacitors over a wide temperature range

Hydrogel electrolytes are attractive for fabricating flexible supercapacitors (FSCs) due to their relatively high ionic conductivity, adjustable mechanical properties, and easy preparation processes. However, they still face challenges such as a narrow operational temperature range due to the presence of abundant free water molecules, and comparatively less favorable electrochemical properties compared to liquid electrolytes. In this study, a redox-active hydrogel electrolyte was synthesized, consisting of polyvinyl alcohol-H2SO4, TiO2 nanoparticles, carbon nanofibers and a mixture of redox-active species (FeBr3, Fe2(SO4)3 and KBr). The addition of thermally stable TiO2 and carbon nanofibers not only increased the thermal stability but also enhanced the ionic conductivity of the hydrogel. The Faradaic reactions of redox-active species during the charging/discharging processes contributed to improved electrochemical performance. Subsequently, binder-free carbon nanofiber@carbon cloth electrodes were prepared by directly growing carbon nanofibers on carbon cloth. By combining the electrodes with the redox-active hydrogel electrolyte, a redox electrolyte-enhanced FSC was assembled. It exhibited a capacitance of 182.8 mF cm- 2 at 3.5 mA cm- 2, an energy density of 8.3 Wh kg- 1, and a capacitance retention of 84.3 % after 5000 charging/discharging cycles. The device was capable of operating under various temperature conditions (0-60 degrees C) and bending angles (0-180 degrees). Overall, this work provides a novel but facile approach for fabricating FSCs with high electrochemical performance and versatile applications in different environments.