Design of Fe 3+ -Rich, High-Conductivity Lignin Hydrogels for Supercapacitor and Sensor Applications.

Preparation of natural polymer-based highly conductive hydrogels with tunable mechanical properties for applications in flexible electronics is still challenging. Herein, we report a facile method to prepare lignin-based Fe 3+ -rich, high-conductivity hydrogels via the following two-step process: (1) lignin hydrogels are prepared by cross-linking sulfonated lignin with poly(ethylene glycol) diglycidyl ether (PEGDGE) and (2) Fe 3+ ions are impregnated into the lignin hydrogel by simply soaking in FeCl 3 . Benefiting from Fe 3+ ion complexation with catechol groups and other functional groups in lignin, the resultant hydrogels exhibit unique properties, such as high conductivity (as high as 6.69 S·m -1 ) and excellent mechanical and hydrophobic properties. As a strain sensor, the as-prepared lignin hydrogel shows high sensitivity when detecting various human motions. With the flow of moist air, the Fe 3+ -rich lignin hydrogel generates an output voltage of 162.8 mV. The assembled supercapacitor of the hydrogel electrolyte demonstrates a high specific capacitance of 301.8 F·g -1 , with a maximum energy density of 26.73 Wh·kg -1 , a power density of 2.38 kW·kg -1 , and a capacitance retention of 94.1% after 10 000 consecutive charge-discharge cycles. These results support the conclusion that lignin-based Fe 3+ -rich, high-conductivity hydrogels have promising applications in different fields, including sensors and supercapacitors, rendering a new platform for the value-added utilization of lignin.