Lignin for sustainable electronics: Interplay of structure, morphology and chemistry in modelling dielectric properties

The search for promising and sustainable materials for advanced electronic applications has recently drawn attention to lignin. As a major by-product of pulping processes, lignin features a complex aromatic structure rich in aliphatic and aromatic ethers, as well as hydroxyl and carboxyl functional groups, which endow it with unique chemical and electronic properties. In this study, we present a comparative analysis of three distinct lignins. Two of them (L1 and L2) are derived from the Kraft pulping process, while the third (L3) is extracted from Cynara cardunculus using an ethanolic organosolv method. These lignins are investigated as active layers in an interdigitated electronic device. To explore in depth the influence of the compositional, structural, morphological and chemical properties of the three lignins on dielectric relaxation dynamics and charge transport mechanisms, several advanced analytical techniques were adopted, including Electrical Impedance Spectroscopy (EIS), Nyquist Plots (NP), Broadband Dielectric Spectroscopy (BDS), and Complex Power (CP) representations. Our consistent workflow included the same interdigitated electrodes (IDE) platform, identical frequency window, a unified BDS formalism, and a common base EIS circuit design tailored to the observed Nyquist Plot features. Our findings revealed that the extraction process enables tuning of the lignins properties. Whilst L1 exhibits smooth, compact morphology and a higher polymerization degree, limiting charge mobility and resulting in inferior electrical and capacitive performance, L2 features a fibrous structure with higher content in carboxyl groups and ashes, which significantly enhances conductivity and capacitance. L3 displays an intermediate morphology with a high concentration of aliphatic hydroxyl groups, offering a balanced blend of chemical and structural properties. Furthermore, we reveal the potential of lignin as a versatile dielectric material exhibiting super capacitive behavior among other properties.