Lignin promoted the fast formation of a robust and highly conductive deep eutectic solvent ionic gel at room temperature for a flexible quasi-solid-state supercapacitor and strain sensors

The green synthesis of ionic gels using alternative feedstock enables the preparation of sustainable materials and devices. Due to the excellent solubility of lignin in deep eutectic solvent (DES), lignin was first dissolved into a DES composed of betaine and ethylene glycol (EG) directly, and the dynamic redox reaction between lignin and Fe2(SO4)3 was employed in the catalytic polymerization of acrylic acid (AA) in DES at room temperature within minutes. The as-prepared ionic gel had high strength, extreme temperature stability, and good electrical conductivity. The synergistic effect of the dynamic cross-linking of lignin and Fe2(SO4)3 gives the gel a high strength of about 1.8 MPa and excellent stretchability, over 1000% strain. Due to the electrical conductivity of Fe2(SO4)3 and the DES itself, as well as the stability of the DES, the electrical conductivity of the prepared gel increased from 0.09 mS cm−1 to 4.07 mS cm−1 at −20 °C to 120 °C. Based on these excellent properties, the application of the new ionic gel in a supercapacitor and wearable sensors was demonstrated. This simple gel preparation method has great potential in the future of flexible electronics.