Liquid plasma assisted synthesis of manganese oxide electrode materials for high-performance energy storage application

• Main Author: Xiu, Mingzhen
• Other Authors: Huang Yizhong
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2023
• Subjects: Science::Chemistry::Crystallography; Engineering::Materials
• Online Access: https://hdl.handle.net/10356/172581; https://doi.org/10.1016/j.electacta.2022.141620
• Institution: Nanyang Technological University

The global community has recognized the pressing need to confront the challenges posed by limited energy resources and the environmental pollution resulting from excessive dependence on fossil fuels. Renewable energy has emerged as a crucial solution to combat these challenges by reducing carbon emissions and further displacing fossil fuels. Energy storage systems have emerged as a crucial solution in harnessing and converting energy from renewable energy sources to meet the diverse energy demands across various sectors. Over the centuries, energy storage technologies have undergone continuous development and encompass a range of mature techniques. An ideal energy storage device should prioritize safety, cost-effectiveness, rapid charging and discharging capability, and good durability. Supercapacitors have been considered a promising strategy in the pursuit of clean energy, serving as an advanced alternative that combines the advantages of both capacitors and batteries. These devices store electrical energy through electrochemical processes, and their performance is significantly influenced by the selection of electrode materials and processing methods employed. This project aims to optimize the electrocatalytic efficiency of electrode materials made of transition metal oxide/carbon cloth composites. The address is on developing the next generation supercapacitors that exhibit exceptional energy and power densities, effectively addressing the escalating need for uninterrupted power in various consumer electronics applications. In this thesis, the wet chemistry liquid plasma strategy is employed for the synthesis and phase transformation of transition metal oxide composites. Manganese oxide is the primary focus of this study, although other transition metal oxides including Co, Cu, Fe, Ni, and Zn are also investigated to demonstrate the feasibility and effectiveness of the wet chemistry liquid plasma approach. The novel liquid plasma discharge deposition (LPDD) method successfully synthesizes two distinct morphologies of Mn3O4 nanostructures. The growth process is driven by the establishment of an electrical field during the LPDD process, with the orientation of the field determining the morphology of the Mn3O4 nanostructures. In the horizontal mode, where the electric field is parallel to the substrate, well-dispersed nano-octahedron shape Mn3O4 is synthesized following a traditional Ostwald ripening mechanism. In the vertical mode, where the electric field is perpendicular to the substrate, ultrathin and porous Mn3O4 nano-sheets are formed through a hydrothermal process. The growth mechanisms are revealed by using advanced surface characterization techniques, such as high-angle annular dark-field STEM. Results show that Mn3O4 nano-sheets present better electrochemical performance compared to the nano-octahedron Mn3O4. To improve the properties of the synthesized transition metal oxide, a laser-induced liquid plasma strategy is developed for rapid phase transformation. By utilizing ethylene glycol as a mediating agent, the laser-induced liquid plasma method allows the effective manipulation of the oxidation state of various transition metals, including Mn, Cr, Fe, Cu, Ni, and Co. leading to the creation of superior-performance electrode materials for supercapacitors. Surface characterization techniques provide experimental evidence supporting the successful phase transformation achieved through this method. Computational simulations further contribute to the understanding of the thermodynamic processes and the involvement of ethylene glycol in driving the transformation. This research showcases the potential of wet chemistry liquid plasma technology in not only producing transition metal oxides with diverse morphologies but also manipulating their oxidation states to achieve superior electrochemical performance in supercapacitors. The approach offers a range of advantages, including simplicity, speed, controllability, low cost, and one-step synthesis, and demonstrates the versatility for synthesizing other metallic-based electrocatalysts. It can approach can be considered a viable option for large-scale implementation in the industry.