Ultrathin redox active hydrogel electrolytes for high performance flexible supercapacitors†

Flexible supercapacitors (FSCs) based on hydrogel electrolytes have the advantages of high ionic conductivity, no liquid leakage, flexibility and versatility, making them the most promising power sources for wearable devices. Herein, a flexible and stretchable, ultrathin polyvinyl alcohol/carboxymethyl chitosan incorporated with a redox active ionic liquid (PVA/CMCS-[ViEtIm][Br]) hydrogel electrolyte is prepared by a facile coating and freezing/thawing method, which is used to improve the practical performance of supercapacitors. The PVA/CMCS-[ViEtIm][Br] hydrogel film has good mechanical properties. More importantly, the redox reaction caused by [ViEtIm][Br] in the hydrogel electrolyte provides a crucial pseudocapacitive contribution to supercapacitors. Thus, the flexible supercapacitor assembled with the PVA/CMCS-[ViEtIm][Br] hydrogel at a thickness of 0.1 mm has an areal specific capacitance of 314.4 mF cm⁻² and an energy density of 78.6 μWh cm⁻² at 540 μW cm⁻², with a capacitance retention of 87.5% after 10 000 charge/discharge cycles. Moreover, the flexible supercapacitor can also exhibit stable performance at different bending angles. This work provides a simple and feasible method for realizing ultra-thin flexible capacitors with high energy density.