Lignin‐Derived Ionic Conducting Membranes for Low‐Grade Thermal Energy Harvesting

Abstract Wood‐based ionic conductive membranes have emerged as a new paradigm for low‐grade thermal energy harvesting applications due to their unique andtailorable structures. Herein, a lignin‐derivedionic conducting membrane with hierarchical aligned channels is synthesized viaa double network crosslinking approach. Their excellent thermal stability andsuperior swelling ratio allow their optimization as low‐grade heat recovery technologies. Several vertically aligned nanoscaleconfinements are found in the synthesized membranes, contributing towardenhanced ionic diffusion. Among all the combinations, the membrane comprising69.2 wt.% of lignin and infiltrated with 0.5 m KOH exhibits anexceptional ionic figure of merit (ZTi) of 0.25, relatively higher ionic conductivity(51.5 mS cm^‒1), lower thermal conductivity(0.195 W m^‒1·K), and a remarkable ionic Seebeck coefficientof 5.71 mV K^‒1 under the application of an axialtemperature gradient. A numerical model is also utilized to evaluate theveracity of experimental observations and to gain a better understanding of thefundamental mechanisms involved in attaining such values. These results displaythe potential of lignin‐basedmembranes for future thermal energy harvesting applications and are a new facetin thermoelectric energy conversion which is certain to pave the way forfurther investigations on sustainable ionic conductive membranes.