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LIGNIN-DERIVED CARBON AND NANOCOMPOSITE MATERIALS FOR ENERGY STORAGE APPLICATIONS

Abstract

With a growing demand for electrical energy storage materials, lignin-derived carbon materials have received increasing attention in recent years. As a highly abundant renewable carbon source, lignin can be converted to a variety of advanced carbon materials with tailorable chemical, structural, mechanical and electrochemical properties through thermochemical conversion (e.g. pyrolysis). However, the non-uniformity in lignin structure, composition, inter-unit linkages and reactivity of diverse lignin sources greatly influence lignin fractionation from plant biomass, the pyrolysis chemistry, and property of the resulting carbon materials. To introduce a better use of lignocellulosic biomass to biofuels and co-products, it is necessary to find novel ways to fractionate lignin and cellulose from the feedstock at high efficacy and low cost. Deep eutectic solvent (DES) was used to extract lignin from high lignin-content walnut and peach endocarps. Over 90% sugar yields were achieved during enzymatic hydrolysis of DES pretreated peach and walnut endocarps while lignins were extracted at high yields and purity. The molecular weights of the extracted lignin from DES pretreated endocarp biomass were significantly reduced. The native endocarp lignins were SGH type lignins with dominant G-unit. DES pretreatment decreased the S and H-unit which led to an increase in condensed G-units, which may contribute to a higher thermal stability of the isolated lignin. Lignin slow pyrolysis was investigated using a commercial pyrolysis–GC/MS system for the first time to link pyrolysis chemistry and carbon material properties. The overall product distributions, including volatiles and solid product were tracked at different heating rates (2, 20, 40 ℃/min) and different temperature regions (100-200, 200-300 and 300-600 ℃). Results demonstrate that changes in reaction chemistry as a factor of pyrolysis conditions led to changes in yield and properties of the resulting carbon materials. Physical and chemical properties of the resulting carbon material, such as porosity, chemical composition and surface functional groups were greatly affected by lignin slow pyrolysis temperature and heating rate. Lignin-derived activated carbons (AC) were synthesized from three different lignin sources: poplar, pine derived alkaline lignin and commercial kraft lignin under identical conditions. The poplar lignin-derived ACs exhibited a larger surface area and total mesopore volume than softwood lignin-derived AC, which contribute to a larger electrochemical capacitance over a range of scan rates. The presence of oxygen-containing functional groups in all lignin-derived ACs, which participated in redox reaction and thus contributed to an additional pseudo-capacitance. By delineating the carbonization and activation parameters, results from this study suggest that lignin structure and composition are important factors determining the pore structure and electrochemical properties of the derived carbon materials. A 3-dimensional, interconnected carbon/silicon nanoparticles composite synthesized from kraft lignin (KL) and silicon nanoparticles (Si NPs) is shown to have a high starting specific capacity of 2932 mAh/g and a retaining capacity of 1760 mAh/g after 100 cycles at 0.72 A/g as negative electrode in a half-cell lithium-ion battery (LIB) test. It was found the elemental Si and C of the C/Si NPs were most likely linked via Si-O-C rather than direct Si-C bond, a feature that helps to alleviate the mechanical degradation from Si volume change and assure a sound electronic and ionic conductivity for enhanced electrochemical performance. EGA-MS and HC-GC/MS analyses suggest that the interaction of the Si, O and C can be tailored by controlling pyrolysis conditions. This study systematically investigated the interconnecting aspects among lignin source, pyrolysis chemistry, characteristics of the derived carbon materials and electrochemical performance. Such knowledge on the processing-structure-function relationships serves as a basis for designing lignin-based carbon materials for electrochemical energy storage applications

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This paper was published in University of Kentucky.

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