Advanced carbon nanofiber materials for electrochemical energy storage devices

Abstract

Environmental concerns and rising demand for portable electronics and electric vehicles have stimulated the development of energy storage devices such as batteries and supercapacitors, towards higher energy density and power density, which significantly depend on the advancement of new materials used in these devices. Most studies in the literature utilize noble metals as catalysts, complex fabrication procedures, which are not easily scalable. Moreover, these techniques fabricate powder-based materials, which have to be blended with electrically insulating polymeric binders and coated onto conductive substrates to be utilized in a commercial system. Thus, development of nanostructured advanced energy storage materials with high stability, optimum pore structure/morphology, binder-free characteristic, and high catalytic activity is challenging and essential. This dissertation focuses on synthesis and understanding process-structure-performance correlation of binder-free carbon nanofiber-based advanced electrodes for applications in various energy storage devices. Carbon nanofibers (CNFs) are excellent candidates for application as electrodes in electrochemical energy storage (EES) devices because of their unique properties such as high mechanical strength, high electrical and thermal conductivities, high chemical stability, flexibility, and high specific surface area. Electrospinning is a simple and versatile fiber formation technique using a strong electric field to pull or thin out a polymer solution or melt jet forming ultrathin fibers with diameters in the range of 50-500 nm. This continuous fiber-formation technique inherently forms a free-standing non-woven fiber mat, thus, potentially allowing their direct application without the addition of any binders. Carbon nanofiber electrodes fabricated in this work are free-standing with a continuous interconnected network providing fast ion-diffusion as well as fast transport of electrons within the network, a characteristic essential for efficient energy storage, also allowing it to be directly used in any EES system without any further processing. Advanced carbon nanofibers with controlled pore architectures and enhanced functionalities were fabricated and characterized as cost-effective and performance-effective electrodes for supercapacitors, lithium-sulfur, and lithium-air batteries. Each of these EES systems is at a different stage of development and has different requirements of the materials used in it. Thus, studies were conducted focused on the issues to be addressed in each of these areas and varied carbon nanofiber based electrodes were synthesized and studied for each application. Chapter 1 provides the introduction to electrochemical energy storage systems, their working operation, and challenges associated with them, the motivation of using carbon nanofibers as electrodes in these systems and a summary of the dissertation. Chapter 2 discusses a novel technique towards introducing pseudocapacitive functionalities on carbon nanofibers using a low-cost material, sodium chloride, for application in supercapacitors. Chapter 3 includes the study on free-standing carbon nanofibers with controlled morphologies as an interlayer in lithium-sulfur cells, demonstrating improved discharge capacity and cycle life. Chapter 4 comprises of the investigation of cobalt nanoparticles embedded porous carbon nanofibers as efficient bifunctional catalysts for both oxygen reduction and oxygen evolution reactions (ORR/OER) and as an efficient cathode for lithium-oxygen batteries.