Flexible Supercapacitor with Wide Electrochemical Stable Window Based on Hydrogel Electrolyte

Hydrogel electrolyte can endow supercapacitors with excellent flexibility, which has developed rapidly in recent years. However, the water-rich structures of hydrogel electrolyte are easy to freeze at subfreezing and dry at high temperatures, which will affect its energy storage characteristics. The low energy density of micro supercapacitors also hinders their development. Herein, a strategy is proposed to reduce the free water activity in the hydrogel to improve the operating voltage and the energy density of the device, which is achieved through the synergistic effect of the hydrogel skeleton, N, N'-dimethylformamide (DMF), NaClO₄ and water. High concentrations of DMF and NaClO₄ are introduced into sodium alginate/polyacrylamide (SA/PAAM) hydrogel through solvent exchange to obtain SA/PAAM/DMF/NaClO₄ hydrogel electrolyte, which exhibited a high ionic conductivity of 82.1 mS cm^-1, a high breaking strength of 563.2 kPa, and a wide voltage stability window of 3.5 V. The supercapacitor devices are assembled by the process of direct adhesion of the hydrogel electrolyte and  laser induced graphene (LIG). The micro-supercapacitor exhibited an operating voltage of 2.0 V, with a specific capacitance of 2.41 mF cm^-2 and a high energy density of 1.34 µWh cm^-2, and it also exhibit a high cycle stability, good flexibility, and integration performance.