Stretchable and conductive lignin hydrogel electrolyte for flexible supercapacitor

Although lignin is the second most abundant forest biomass polymer, it has been largely neglected in hydrogel electrolytes due to its insolubility and inflexibility. In this study, a double-crosslinked hydrogel was prepared using aspartic acid-modified lignin and sodium alginate, significantly improving the mechanical properties. The hydrogel exhibited an exceptional strain of 3008% and a tensile strength of 0.03 MPa, demonstrating its remarkable mechanical properties. In addition, high ionic conductivity (11.7 mS·cm⁻¹) was obtained due to the abundant presence of hydrophilic groups in the hydrogel. The hydrogel-assembled supercapacitor manifested an impressive specific capacitance of 39.46 F·g⁻¹. Notably, the supercapacitor showed a wide potential window of 0–1.5 V and achieved a maximum energy density of 5.48 Wh·kg⁻¹ at the power density of 499.9 W·kg⁻¹. The capacitance retention remained at 115% after 10000 charge-discharge cycles. Finally, the coulombic efficiency was almost 100% during the cycles. Upon reaching a bending angle of 90°, the specific capacitance retention remained impressively high at 94%. These results suggest that the supercapacitor cans maintain normal electrochemical performance under extremely harsh conditions.