Morphological Control of Mesoporous Iron Oxide and N-doped Carbon Nanomaterials for their Applications to Lithium Ion Battery and Fuel Cell

Abstract

Recently, various mesoporous nanomaterials are extensively investigated for their promising applications in electrochemical devices such as Li-ion batteries (LIB) and fuel cells. Mesoporous nanomaterials have extremely large surface area with high density of active sites as well as short diffusion distance. These properties facilitate mass transfer of reactants and products and make mesoporous nanomaterials ideal candidates especially for electrode materials in electrochemical devices. However, there are a number of issues that need to be resolved to realize the commercially feasible, high-performance electrode materials. High surface-to-volume ratio of mesoporous materials often causes extensive side reactions in uncontrolled ways. Kinetics of mass transfer in the complex pore structures is not well understood. To gain control of the optimization parameters, we need to establish standard design principle based on structure-function relationship. This dissertation presents noble approaches to optimizing electrochemical performance of mesoporous materials which is guided by newly developed analytic methods. By controlling morphology, pore structure, and composition of mesoporous materials on the basis of rational design principles, I accomplished the development of high performance electrodes for electrochemical devices. To apply mesoporous materials to LIB electrodes, a robust and well-designed morphology is essential: the formation of solid electrolyte interphase (SEI) needs to be minimized and volume expansion during charge/discharge cycles should be tolerated. In this study, I focus on utilizing metal oxide nanoparticles. While having large storage capacity, nanoparticles suffer from the excessive generation of solid-electrolyte interphase (SEI) on the surface, low electrical conductivity, and pulverization. To compensate for these shortcomings, I designed and prepared mesoporous iron oxide nanoparticle clusters (MIONCs) using a bottom-up self-assembly approach. These materials exhibit excellent cyclic stability and rate capability owing to their three-dimensional mesoporous nanostructure. By controlling the geometric configuration, I was able to achieve stable interfaces between the electrolyte and active materials, confining the SEI formation on the outer surface of the MIONCs. Using porous carbon materials for fuel cell electrodes is another important issue in energy material research. Fe-N-C catalysts have been extensively studied to replace expensive Pt/C electrodes, which facilitate the oxygen reduction reaction (ORR) at the cathode. The ORR performance not only depends on the atomic configuration of the active sites but also heavily relies on structural factors, such as active site utilization and mass transfer properties. Thus, a systematic optimization of the Fe-N-C catalysts was conducted to maximize the ORR activity in alkaline media. Through comparison with the control groups by complex capacitance analysis, I conclude that the mesoporous structure is essential for increasing the utilization of active sites, whereas a nano-sized particulate morphology is required to achieve superior rate capability. During accelerated durability tests, the optimized catalysts show negligible activity loss over 10,000 potential cycles, demonstrating their excellent long-term stability. In the first chapter of the Thesis, various synthetic strategies for applications of mesoporous materials to LIBs and fuel cells are reviewed with representative examples from the previous studies. In the second and the third chapters, my works on the development of metal oxide nanoparticles-based mesoporous LIB electrode materials and carbon-based non-Pt electrode materials are discussed. Through my study, I have shown that controlling morphology and pore structure can be a solution to improving electrochemical activity and stability of the mesoporous material electrodes simultaneously.