Lignin-containing hydrogels with anti-freezing, excellent water retention and super-flexibility for sensor and supercapacitor applications

We developed a highly conductive and flexible, anti-freezing sulfonated lignin (SL)-containing polyacrylic acid (PAA) (SL-g-PAA-Ni) hydrogel, with a high concentration of NiCl2. Ni2+ contributes multi-functions to the preparation of the hydrogel and its final properties, such as fast polymerization reaction as a result of the presence of redox pairs of Ni3+/Ni2+ and hydroquinone/quinone, and anti-freezing properties of the hydrogel due to the salt effects of NiCl2 so that at -20 °C the hydrogel shows similar properties to those at the room temperature. Thanks to the effective coordinations of Ni2+ with catecholic groups and carboxylic groups, as well as the rich hydrogen bonding capacity, the resultant hydrogel possesses excellent mechanical properties. High ionic conductivity (6.85 S·m-1) of the hydrogel is obtained due to the supply of high concentration of Ni2+. Moreover, the ionic solvation effect of NiCl2 in the hydrogel imparts excellent water retention ability, with water retention of ~93 % after 21-day storage. The SL-g-PAA-Ni hydrogel can accurately detect various human motions at -20 °C. The supercapacitor assembled from SL-g-PAA-Al hydrogel at -20 °C manifests a high specific capacitance of 252 F·g-1, with maximum energy density of 26.97 Wh·kg-1, power density of 2667 W·kg-1, and capacitance retention of 96.7 % after 3000 consecutive charge-discharge cycles.