Tunable electrical properties of lignin: morphology-dependent ionic conduction in Kraft and organosolv-derived materials

Abstract Lignin, a by-product of the pulp and paper industry and biomass processing, features a chemical structure rich in aromatic rings and functional groups such as ethers, alcohols, phenols and carboxyls, as well as electrical properties that can make it a promising material for various uses in a waste-to-application approach. This study investigates the composition, morphology, and DC electrical behavior of three distinct lignins: two derived from the Kraft extraction process and differing in their post-treatment (L1, L2) and one (L3) extracted from Cynara cardunculus using an ethanolic organosolv process catalyzed by aqueous ammonia. Morphological analyses reveal that L3 exhibits features intermediate between the smooth structure of L1 and the fibrillar nanostructure of L2. When used as the active layer in interdigitated devices, their I–V characteristics on a semilogarithmic plot exhibit butterfly-shaped curves, showing strong dependence on temperature and pressure. L1 and L3 are similar, while L2 differs substantially, reflecting variations in functional group density and morphology. The low electrical conductivity, the dependence on morphology, and the hysteretic electrical behavior suggest that ionic conduction plays a significant role in the overall charge transport, with conductivity scaling as L2 > L3 > L1 and increasing with pressure and temperature. Morphology-dependent adsorption of air molecules primarily enhances ionic conduction, and the good fit to the Arrhenius model suggests that charge transport occurs via carrier hopping across localized energy barriers. This study highlights the diverse electrical properties achievable with lignins with different extraction histories and their tunability through processing methods, enabling tailoring to specific applications and making lignin a versatile and sustainable material for electronic devices.