Date of Award
Fall 2023
Document Type
Open Access Dissertation
Department
Mechanical Engineering
First Advisor
Dongkyu Lee
Abstract
Developing novel energy materials is a key step toward satisfying the needs for next-generation energy conversion and storage devices. Complex oxides are widely used in many energy technologies owing to their fascinating properties, including electrocatalytic activity, and ionic and electronic conductivity. In particular, oxides in thin film form yield many intriguing physicochemical properties that are unattainable in bulk oxides and thus have become critical to improving the performance of many energy devices, such as fuel cells, batteries, solar cells, and thermoelectric devices. This thesis is centered on designing oxide thin films and nanostructures via pulsed laser deposition (PLD) based on understanding the relationship of the structure-property of complex oxides to enhance the performance of intermediate temperature electrochemical energy applications. Three promising strategies—strain engineering, crystallographic orientation engineering, and interface engineering—are proposed to design highly active and stable materials, with a specific emphasis on enhancing the oxygen reduction reaction (ORR) and ionic conductivity. Epitaxial strain is a simple means to control the ORR activity of oxides. Tensile strain enhances the ORR activity of La0.8Sr0.2CoO3-δ (LSC113) thin films due to the increased oxygen vacancies. However, tensile strain exceeding 1% simultaneously leads to surface Sr segregation, which hampers the ORR activity. Therefore, an optimized tensile strain that can mitigate Sr segregation while increasing oxygen vacancies is required to promote the ORR of LSC113. In the case of anisotropic structures, such as La1.85Sr0.15CuO4 (LSC214), controlling the crystallographic orientation has a significant impact on the control of ORR activity. Compared to (001)- and (103)-oriented LSC214 films, (114)-oriented LSC214 films exhibit significantly improved ORR activity, stemming from the orientation-dependent oxygen migration channels on the surface. Forming a heterointerface also manipulates the ORR activity of oxides. Increased oxygen vacancies at the heterointerface between LSC214 and LaNiO3 result in a dramatic improvement in the ORR activity of LSC214 films. Furthermore, an innovative 3D micro-nano structures of LSC113 fabricated by a combination of 3D printing technology and PLD demonstrate highly enhanced ORR activity, primarily attributed to the amplified surface area. Epitaxial strain is also known as a key control parameter for the ionic conductivity of oxides. However, the experimental realization of strain-enhanced ion conduction is only achievable through careful material selection. By employing the multilayer thin films consisting of the ionic conductor Gd-doped CeO2 (GDC) and an insulator RE2O3 (RE = Y and Sm), interfacial strain is successfully fine-tuned, which in turn controls the ionic conductivity of GDC. Furthermore, the incremental addition of interfaces exhibits no impact on ionic conductivity enhancement. This limitation may arise from the constraints imposed by the geometric design and the necessity to maintain a nanoscale thickness for strain effects. To overcome such limitations, a novel design concept known as vertical heteroepitaxial nanocomposites (VHNs) is proposed to maintain interfacial phenomena in thin film heterostructures. By successfully creating VHNs using GDC and RE2O3 (RE = Y and Sm), precise control over ionic conductivity in the out-of-plane direction can be achieved by leveraging vertical strain and engineered interfaces. The transition from compressive to tensile strain in GDC-RE2O3 VHNs systematically leads to a remarkable enhancement in the ionic conductivity of GDC as tensile strain increases the concentration of interfacial oxygen vacancies. This new concept of oxide heterostructures can provide new prospects for practical applications in advanced energy devices.
Rights
© 2024, Gene Yang
Recommended Citation
Yang, G.(2023). Rational Design of Complex Oxides by Understanding Structure-Property Relationships for Energy Applications. (Doctoral dissertation). Retrieved from https://scholarcommons.sc.edu/etd/7614