Valorization of lignin as an active material for electrochemical energy storage

Abstract

Lignin is one of the three major architectural biopolymers of lignocellulosic biomass, and the largest available feedstock of natural aromatic polymer on Earth. Its valorization in second-generation biorefineries remains a challenge. Among possible uses, lignin can be carbonized to create porous carbons, which are useful as supercapacitors, a kind of capacitor that also displays battery-like properties. Recent studies show lignin could also be exploited as a bio-sourced, redox-active material in batteries thanks to its numerous quinone-like moieties. Still, we lack fundamental pieces of knowledge about lignin, such as the impact of pretreatment type on lignin structure, molecular weight fractionation, organic solvent solubility or electrochemical properties. Moreover, to the best of our knowledge, no research has been conducted on the use of soda-extracted lignin in batteries. In this master thesis, we fractionated soda-extracted lignin samples from three sources (softwood, hardwood, herbaceous), characterized them by Fourier transform infrared (FTIR) spectroscopy, high pressure size exclusion chromatography (HPSEC), nuclear magnetic resonance (NMR) and scanning electron microscopy (SEM). We linked their characteristics to their electrochemical capacity performances, measured by cyclic voltammetry (CV). Our results showed that organic solvent fractionation and pH-driven fractionation created a range of molecular weight-separated fractions of smaller polydispersity. All fractions had unique relative abundances of oxygenated functionalities. Soda-extracted lignin exhibited a ~16 mAh/g capacity. This showed that soda-extracted samples are as capacitive as Kraft-extracted samples reported in literature. Furthermore, our softwood samples displayed capacities 4 and 8 times higher than herbaceous and hardwood lignins, respectively.