Enhancing Multifunctional Properties of Renewable Lignin Carbon Fiber via Defining Structure-Property Relationship Using Different Biomass Feedstock

Lignin has been explored extensively as a renewable precursor for carbon materials, considering its abundance as a major component of plant cell wall and its sustainability as a byproduct of both lignocellulosic biorefinery and paper-making industries. Despite the extensive efforts on defining process-property relationship, it remains largely unknown how lignin biosynthesis and its chemistry would impact on the resultant carbon fiber properties, for both mechanical and electroconductive performances. Such inadequate understanding fundamentally limited the feedstock design and selection to improve carbon fiber properties toward broader commercial application. Using lignin from a broad range of biomass feedstock for carbon fiber manufacturing, we have fundamentally explored the structure-function relationship between lignin chemistry and carbon fiber performance. Specifically, lignin extracted from hardwood (sugar maple), softwood (loblolly pine and red cedar), and herbaceous plant (corn stover and switchgrass) were used for carbon fiber manufacturing, considering the very different lignin structures from these feedstock. Linear regression models were established to define the relationship between carbon fiber mechanical properties with lignin structural characteristics. The results highlighted that the content of β-O-4 linkage correlates significantly with tensile strength and elastic modulus of lignin carbon fiber, indicating that the more linear β-O-4 linkage would promote carbon fiber mechanical performance. Moreover, electroconductive property is essential for broader energy device application of lignin-based carbon fiber, yet the mechanisms controlling its electroconductivity is largely unknown. We hereby demonstrated that a higher β-O-4 content also promotes electroconductivity of lignin carbon fiber. Microstructure analysis further revealed that the crystallite size and content of pre-graphitic turbostratic carbon structure in lignin-based carbon fiber was enhanced as the β-O-4 linkages increased. The content of β-O-4 linkage has shown a strong correlation with crystallite content in linear regression model. This study thus revealed the underlying mechanisms regarding to how lignin structure in planta defines the resultant carbon fiber properties. Moreover, the study also highlighted the correlation between mechanical and electroconductive properties of lignin-based carbon fiber, both of which were defined by lignin structure.