High-Temperature Tolerant Flexible Supercapacitors with Redox-Active Water-in-Salt Hydrogel Electrolytes

High-temperature-tolerant flexible supercapacitors are essential for powering emerging wearable and portable devices operating under harsh environmental conditions. Conventional hydrogel electrolytes exhibit excellent ionic conductivity, flexibility, and biocompatibility; however, their practical application is hindered by water evaporation at elevated temperatures and a narrow electrochemical window limited by the water decomposition voltage (1.23 V). In this work, we present a novel heat-resistant and redox-active hydrogel electrolyte containing a ZnCl2 water-in-salt solution and a redox-active additive of (NH4)2Fe(SO4)2. The strong coordination between Zn2+ ions and water molecules effectively suppresses water activity, thereby enhancing the thermal stability and expanding the electrochemical window. Furthermore, the incorporation of (NH4)2Fe(SO4)2 contributes to both faradaic charge storage through Fe2+/Fe3+ redox reactions and improved thermal stability via enhanced water coordination. When paired with KOH-activated carbon nanofiber electrodes, a redox electrolyte enhanced flexible supercapacitor has been constructed. The device delivers a wide working voltage of 2.0 V, a high energy density of 23.2 Wh kg–1, and stable operation across a broad temperature range from 0 to 100 °C. This work offers a promising strategy for developing high-performance, thermally stable energy storage devices suitable for next-generation flexible electronics.