Lignin-derived materials for energy harvesting and storage devices

Harnessing waste heat presents a significant opportunity for sustainable energy generation. Conventional thermoelectric materials, despite their potential, are often limited by toxicity and low conversion efficiencies. In contrast, ionic thermoelectric materials have emerged as a promising alternative due to their superior thermoelectric performance, highlighting the need for exploring new, efficient, and environmentally friendly materials. Additionally, integrating ionic thermoelectric materials with supercapacitors offers the dual benefit of energy conversion and storage. This thesis investigates the development of eco-friendly energy harvesting and storage devices, specifically focusing on ionic thermoelectric supercapacitors derived from lignin, a renewable and abundant resource. The research involves synthesizing lignin-derived hydrogels and membranes to enhance ionic diffusion and optimize heat-to-electricity conversion. Lignin-derived hydrogels, synthesized with polyvinyl alcohol (PVA) and lignin, infused with a 6 M KOH electrolyte, achieved a Seebeck coefficient of 13 mV/K. Furthermore, lignin-derived membranes with vertically aligned channels demonstrated an enhanced Seebeck coefficient of 18 mV/K under the same conditions. The study also synthesized lignin-derived porous carbons, designed to exhibit high surface areas to function effectively as supercapacitor electrodes. These materials were thoroughly analysed to identify the optimum sample, which exhibited a specific capacitance of 102 F/g. When combined into an ionic thermoelectric supercapacitor, the device achieved a power density of 17.12 µW/m2 and an energy density of 228 mJ/m2 . This work represents a significant advancement toward sustainable energy solutions, demonstrating the potential of lignin-based materials in the development of efficient thermoelectric devices with applications in wearable electronics and beyond.