The efficient and economical conversion of low-grade waste heat into electricity has promising potential to combat the greenhouse effect and expedite the shift towards sustainable development. This study presents an innovative and appealing approach through the utilization of lignin, an abundant waste product derived from the paper and pulp industry, to develop hydrogels as compelling and sustainable materials for application in ionic thermoelectricity. Various compositions were evaluated to examine the impacts of varying lignin concentrations, types of electrolytes, concentrations of crosslinkers, and electrolyte concentrations on the ionic thermoelectric performance of the hydrogels. The optimized lignin-derived hydrogel, infiltrated with a 6 M KOH electrolyte, demonstrates high ionic conductivity (226.5 mS/cm) and a superior Seebeck coefficient of 13 mV/K. This results in a remarkable power factor (3831 µW/m·K²) that leads to an impressive Figure of merit (ZT_i) (3.75), surpassing most of the existing state-of-the-art materials and making it the most efficient sustainable ionic thermoelectric material reported until now. These findings underscore the exceptional performance of lignin-based hydrogels in the realm of low-grade waste energy harvesting applications. The present study contributes to address the challenges posed by waste heat through effectively harnessing low-grade waste heat through the utilization of sustainable lignin-based hydrogels while reducing the reliance on fossil fuels and minimizing greenhouse gas emissions.

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