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

For the development of supercapacitors, multifunctional hydrogel electrolytes with flexibility, robust adhesiveness 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 electrolytes due to its insolubility and lack of flexibility. Herein, an all-bio-based hydrogel is constructed by covalently 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 °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.