Structurally Enhanced Electrodes for Redox Flow Batteries Produced via Electrospinning

Abstract

The vanadium redox flow battery is one of the most promising secondary batteries for energy storage system due to its design flexibility attributed to the large adjustable capacity of the storage tanks filled with electrolyte solution. However, the vanadium redox flow battery is not yet widely deployed owing to its low power density. This thesis describes the way of constructing the fibrous electrode with novel structure to overcome the flaw. The general electrospun materials of polyacrylonitrile were synthesized with substantially lower porosity than standard materials by applying compression during the stabilization stage. This objective was to create flow battery electrodes with higher volumetric surface area. The flexibility of the electrospinning technique combined with adjustable post-processing steps such as stabilization and carbonization allowed for the creation of layers with very specific structural and transport properties. In-plane permeability was found to remain relatively constant compared to the original uncompressed fibrous structure. On the other hand, the fibers compacted and compressed down to the flat ribbon shape hurt the through-plane permeability, so artificial holes were created using a CO_2 laser to perforate the structure. The loss of specific surface area caused by laser perforation was quite negligible and still showed improvement. Overall, the novel flow-through electrode provided from this study successfully contributed to improving the transport properties as well as the electrochemical reaction rate, leading to the optimal power density of a vanadium redox flow battery. In addition to that, 2-dimensional half-cell model was created with multi-physics simulation to predict the change in performance with respect to the structural properties of fibrous electrode. The performance was evaluated based on polarization behavior, required pumping power to operate the cell, and operating efficiency. Moreover, electrode was constructed to multi-layered structure in profiles of permeability, fiber size, and porosity. The vanadium ion could be distributed uniformly over the entire region of electrode, which enabled more portion of fiber surface to be utilized for reaction to improve power density while maintaining low pumping power for operation. Based on the prediction from the model, the actual experimental work was invested for multi-layered structure built with novel electrospun fibrous layers. Two different flow channel designs were considered: interdigitated and parallel. The convective flow was induced with the interdigitated flow channel design. Thus, the vanadium ions could be distributed effectively to the region of electrode, resulting in the higher power density. The electrode created in multi-layer provided higher net power density even though the increased pumping power requirement compared to the case of single layer. The body of work presented in this thesis has contributed significantly to understanding the mass transport phenomena taking place in electrodes built in novel fibrous structures. It highlights the preparation of this media through electrospinning as well as numerical and experimental methods for characterizing and understanding these processes. All the work presented here promoted the development of flow batteries through better understanding of the flow battery electrode

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Last time updated on 15/05/2024

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