Epichlorohydrin‐Crosslinked Lignin–Polyvinyl Alcohol Hydrogel for Ionic Thermoelectric Applications

Abstract

ABSTRACT Lignin, a renewable by‐product of the pulp and paper industry, offers functional versatility and sustainability advantages for hydrogel development. This study synthesizes lignin‐based poly(vinyl alcohol) hydrogels for ionic thermoelectric applications by incorporating kraft lignin and lignosulfonate, using epichlorohydrin as a crosslinker and KOH electrolyte as an ionic source. The objective is to investigate the role of lignin structure and functionality in governing ion transport and the Seebeck effect within the hydrogel matrix. The results show that higher lignin concentration decreases swelling capacity due to stronger polymer interactions, but simultaneously improves the Seebeck effect, enhancing thermoelectric performance. Hydroxyl ions from KOH interact with the hydroxyl groups of PVA and lignin, confirmed by FTIR spectra with strong ‐OH stretching at 3260–3186 cm −1 . These interactions support more effective thermo‐diffusion across the hydrogel. Furthermore, sulfonic acid groups in lignosulfonate significantly increase ionic conductivity, yielding the highest Seebeck coefficient of 0.6059 mV/K. KOH infiltration treatment induced the formation of porous structures in the hydrogels, mainly due to ionic interactions and the consequent disruption of hydrogen bonding within the polymer matrix. Overall, this work demonstrates a promising strategy for developing sustainable lignin‐based ionic thermoelectric hydrogels, with lignosulfonate‐PVA/KOH systems showing strong potential for low‐grade heat‐to‐electricity conversion.