Mechanistic insights and performance of fully lignin-based hydrogels for next-generation ionic thermoelectric supercapacitors

Lignin, an abundant by-product from the pulp and paper industry, offers a sustainable route to advanced energy materials. Here, we present the first fully lignin-derived ionic thermoelectric supercapacitors (i-TE SCs) that seamlessly integrates low-grade thermal energy harvesting and electrochemical energy storage within a single, sustainable device. A chemically crosslinked lignin hydrogel (LH-1.0) functions as the ionic thermoelectric electrolyte, exhibiting a high Seebeck coefficient of 9.4 ± 0.9 mV/K, an ionic conductivity of 93.63 mS/cm, a low thermal conductivity of 0.45 W/mK, and a significant ionic Figure of Merit (ZTi) of 0.55. Upon carbonization, the same lignin precursor produces a porous activated carbon electrode (LC-1.5) with a high specific capacitance of 262.5 F/g at 0.25 A/g, excellent rate capability, and 94.7 % retention over 5000 cycles. These components are integrated into an i-TE SC, delivering an output power density of 6.24 mW/m²under an 8 K temperature gradient. This dual-functionality, derived entirely from lignin, offers a novel pathway toward sustainable and multifunctional energy devices for next-generation wearables, sensors, and low-grade heat utilisation.