Designing Renewable Carbon from Lignin with a Controlled Morphology for Supercapacitor and Polymer Composite Applications

Lignin is an abundant, underutilized, and renewable material that can also be used as feedstock for many industrial products. As part of the recent push for renewable materials development, this dissertation explores controlling the morphology of lignin-derived carbonaceous products via simple means for energy storage and polymer composite applications.

Lignin is a complex, multifunctional, and diverse molecule, and its functionalities depend on biomass source and isolation methods. Attempts have been made to produce lignin-derived spherical carbon particles but with limited success. In order to better understand the mechanism for controlling carbonized lignin’s morphology, a good first step is to study a simpler carbohydrate. Through the hydrothermal reaction of sugar and biomass pretreatment byproducts in aqueous medium, I learned the self-assembly mechanism of hydrochar spheres. Laboratory experiments and modeling revealed that when hydrothermal reaction was carried out in an emulsion medium, hollow hydrochar spheres with abundant microporosity can be formed. After carbonization and surface activation, the carbon produced can be used as supercapacitor electrodes.

Before attempting hydrothermal processing of lignin, I aimed to find a simple way to control carbonized lignin’s porosity in solid state via induced crosslinking during melt-mixing with a 10% rubbery macromolecule. After stabilization and one-step carbonization and surface activation, derivative carbon’s surface area and supercapacitor electrode performance was increased. With the lessons learned from solid-state lignin crosslinking and synthesis of carbohydrate-derived spherical hydrochar via solvothermal pathways, I focused on designing a method to produce spherical carbon particles from lignin. By subjecting lignin with solvothermal treatment in DMSO, a good solvent for lignin, spherical stabilized lignin particles were made. In a solvothermal condition, lignin crosslinking caused lignin precipitation, much like carbohydrate-derived spherical hydrochar. After carbonization, uniform spherical carbon was formed without templates, scaffolds, surfactants, or copolymers.

Lastly, I discovered that by using a simple self-assembly of lignin in nanoprecipitation, spherical lignin nanoparticles can be produced and stabilized in air without superheating in solvents. Using hydrothermal treatment, lignin nanoparticles could also be stabilized. However, hydrothermally stabilized particles released less volatiles during carbonization, leading to fewer defects and porosity in the resulting free-flowing carbon useful as polymer reinforcing agent.