Multifunctional lignin-derived ether-based ionic liquid/PVA-GEL double-network composite hydrogel electrolytes via multiple freezing strategy for flexible energy storage and wearable strain sensors

In the quest for sustainable and environmentally friendly materials, lignin hydrogels have emerged as a revolutionary alternative to traditional synthetic polymers due to their eco-friendliness, low-cost, and biocompatibility. However, the inadequate mechanical properties and poor conductivity limit their applicability in flexible energy storage and electronic devices. This work proposes employing a so called multiple freezing strategy to incorporate lignin/ionic liquid into the dual-cross-linked network of polyvinyl alcohol (PVA) and gelatin (GEL), thereby facilitating the preparation of a multifunctional green composite hydrogel. Through experimental characterizations and theoretical simulations, we elucidated the interactions among the various species. The hydrogen bonds and electrostatic interactions within this hydrogel contribute to its remarkable mechanical properties, high conductivity, and enhanced sensitivity. Leveraging these attributes, we fabricated flexible writing sensors and human motion detection sensors capable of recognizing handwriting and monitoring daily movements. The devices demonstrate excellent stability and anti-fatigue properties, with signal stability maintained after 500 cycles. When utilized as the electrolyte in flexible supercapacitor assembly, the supercapacitor exhibited a specific capacitance of 104.8 F g⁻¹ at a current density of 1 A g⁻¹ , with a capacitance retention rate of 70 % after 5000 cycles. After various degrees of bending and folding, the electrochemical performance remained very stable. This research presents a new candidate materials for environmentally friendly multifunctional hydrogels and paves the way for advancements in flexible energy storage, text recognition, and wearable sensor devices.