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

Abstract

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 Electrochemical Impedance Spectroscopy (EIS), Nyquist Plots (NP), Broadband Dielectric Spectroscopy (BDS), and Complex Power (CP) representations. Our consistent workflow included the same interdigitated electrode (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 lignin properties. Whilst L1 exhibited smooth, compact morphology and a higher polymerization degree, limiting charge mobility and resulting in inferior electrical and capacitive performance, L2 featured a fibrous structure with higher content in carboxyl groups and ashes, which significantly enhanced the conductivity and capacitance. L3 displayed an intermediate morphology with a high concentration of aliphatic hydroxyl groups, offering a balanced blend of chemical and structural properties. In this work, we reveal the potential of lignin as a versatile dielectric material exhibiting supercapacitive behavior among other properties.