All-bio-based, adhesive and low-temperature resistant hydrogel electrolytes for flexible supercapacitors

For the development of supercapacitors, multifunctional hydrogel electrolytes with flexibility, robust adhe-siveness and low-temperature resistance have attracted increasing interest. Lignin is the most abundant renewable aromatic biopolymer in nature, however, it has not gained appropriate attention for hydrogel elec-trolytes due to its insolubility and lack of flexibility. Herein, an all-bio-based hydrogel is constructed by cova-lently cross-linked lignin (Lig) with interpenetrating gelatin in alkaline solutions. The multifunctional hydrogel electrolytes with a high ionic conductivity of 0.06 S cm-1, robust adhesiveness and anti-freezing properties are designed. The optimized flexible supercapacitor constructed by Lig/Gelatin hydrogel electrolyte shows a remarkable specific capacitance of 145.14F/g and a high energy density of 4.86 W h kg-1 at 0.5 A g-1 at room temperature. It can retain a comparable specific capacitance of 110.6F/g at 0.5 A g-1 even at-40 degrees C. The multifunctional lignin-based hydrogel has promising potential for supercapacitors under a broad range of environmental conditions. With this effective strategy, lignin can potentially be used in multifunctional energy storage devices.