Synthesis of Carbon Nanofibers from Biopolymer Blends and its Applications

Carbon nanofibers (CNFs) are one-dimensional advanced materials with unique characteristics such as high specific surface area, aspect ratio, mechanical, chemical, and thermal stability, and hence highly promising for various energy and environmental applications. Among the various methods available for CNF synthesis, electrospinning is the most scalable and widely adopted due to its simplicity and cost-effectiveness. This process involves stretching a polymer solution into thin fibers under an electric field, followed by a series of heat treatments to convert the polymer into carbon. However, despite their excellent properties, the widespread use of CNFs is limited by their high production cost. Conventional CNF precursors, such as PAN and pitch, are expensive and derived from non-renewable resources, and their processing releases toxic gases, raising environmental concerns. As a more sustainable alternative, lignin, a biopolymer found in plant cell walls, has emerged as the lowest-cost precursor for CNF synthesis. The paper and pulp industries are the major source of lignin, producing BL as a byproduct during cellulose extraction. Traditionally, BL is either burned for energy recovery, chemically processed, or discarded, posing significant environmental risks. However, its high lignin content makes BL a promising resource for CNF production. This PhD project aimed to tackle key challenges in CNF production, such as the high cost of conventional precursors, the underutilization of BL, and the optimization of lignin-based CNF synthesis. The study specifically addressed the difficulties in the electrospinning of lignin due to its heterogeneous structure, low molecular weight, and large polydispersity index (PDI) by blending it with biocompatible polymers to improve processability and promoting sustainability, reducing dependency on non-renewable resources. Systematic optimizations were conducted to develop an efficient method for converting BL into carbon materials and lignin into CNFs. Additionally, the research investigated the potential of CNFs as substrates for advanced materials growth, with applications extending from supercapacitors to environmental remediation. This thesis introduces CNFs, focusing on recent research developments, applications, and synthesis methods. Chapter 1 provides a detailed analysis of the electrospinning process, discussing its history, system design, working mechanism, and key factors influencing fiber morphology and properties. It also explains the theoretical and mathematical foundations, along with the heat treatment process that transforms polymeric nanofibers into CNFs. The chapter concludes by identifying critical gaps in the current literature, outlining the motivations for this research, and defining the research questions and objectives. Chapter 2 covers material synthesis, including lignin extraction and conversion into nanofibers and carbon products, along with composite synthesis using 2D materials. Key characterization techniques (SEM, TEM, XRD, Raman, FTIR, XPS) and applications in supercapacitors and microplastic removal are discussed. Chapter 3 examines lignin extraction from BL and its application in CNF production. Due to challenges in the electrospinning of BL-derived lignin, such as impurities and molecular inconsistencies, the focus shifted toward synthesizing activated carbon (AC) from BL and lignin. The chapter explores methods to optimize surface area and porosity in AC production, demonstrating its potential for energy storage applications, especially in supercapacitors, where it showed improved electrochemical performance. Chapter 4 focused on optimizing the electrospinning of commercially purchased lignin blended with PVA (90:10) to improve fiber morphology. Building on previous challenges, this study incorporated SDS surfactant to enhance fiber morphology, leading to reduced diameter of fibers. It also explored the feasibility of omitting the most lengthy and costly stabilization step. While fibers without stabilization maintained morphology, stabilization increased oxygen functionalities, improving structural integrity and enhancing supercapacitor performance. The findings suggest that including stabilization yields better energy storage performance but omitting it could save time and cost for specific applications. Chapter 5 focused on synthesizing CNFs with a higher lignin content (99%) and lower blending polymer that is PEO (1%), to reduce costs and enhance sustainability. Motivated by the growing issue of micro-nano plastic pollution (MNPs), this study aimed to develop an environmentally friendly CNF composite membrane for filtration. The membrane was functionalized with KOH to introduce oxygen groups and grafted with 2D ZIF-67 and Fe(OH)3 to enhance hydrophilicity and adsorption capacity. The membranes demonstrated excellent removal efficiency of MNPs, dyes, and antibiotics, with high reusability and potential for conversion into porous carbon membranes for energy storage applications. Chapter 6 summarizes the key findings, emphasizing advancements in sustainable CNF production and its applications in energy storage and environmental remediation. It further outlines potential avenues for future research, building on the results achieved in this study.