Mechanism study for the concurrent process of exsolution and phase transition in perovskite oxides

Abstract

This thesis examines the relationships between phase transitions, exsolution, and the resultant electronic structures of perovskite materials, through the Density functional theory (DFT), to advance the field of computational catalysis. Focusing on Pr0.5(Ba/Sr)0.5FeO3−δ (PBSF) and Pd-doped La0.6Sr0.4Co0.15Fe0.85O3−δ (LSCF) systems, the study provides profound insights into the underlying mechanisms governing phase transitions, exsolution, and electronic structure modifications in perovskite oxides. In the first chapter, an introductory overview is given on the phenomenon of exsolution and its various applications, with particular emphasis on energy and environmental applications. It also introduces the computational approach that promotes exsolution, focusing on computational studies and engineering strategies. Moreover, it sheds light on the promotion of exsolution through phase transition in perovskite oxides, for the subsequent chapters. In Chapter 2, I propose the key role in the phase transition to R-P perovskite in PBST systems (T: Mn, Fe, Co, Ni) by DFT calculations. This is achieved through the examination of Gibbs free energy for oxygen vacancy formation in perovskite materials containing these transition metals, which has provided significant insights into their role in facilitating the phase transition. By employing in-situ XRD measurements and DFT, it is found that a complete phase transition to R-P perovskite occurs when Sr ratio is larger than Ba ratio in PBSF system at a reduction temperature of 850 °C. A deep understanding of the role of Sr2+ on phase transition and exsolution is also provided. Experimental validation of these findings, in combination with SEM analysis and electrochemical performance measurements, substantiates these computational predictions, providing a concrete basis for the implementation of these strategies in the design of advanced materials for SOFC applications. Chapter 3 extends the concept of computational catalysis to the examination of concurrent processes of exsolution and phase transition in perovskite oxides driven by Pd doping. A significant contribution in this chapter is the decomposition of all elementary steps involved in the exsolution process. Using DFT, all the energy barriers associated with each step were calculated, providing a comprehensive energy landscape of the exsolution process. This has resulted in the identification of the most energy-demanding step, offering a deeper understanding of the exsolution mechanism and the effect of Pd doping on it. Further analysis unveils the role of Pd dopant in promoting the phase transition and exsolution, hence enhancing the material's performance under operational conditions. The computational findings are verified experimentally using in-situ HR-XRD studies, confirming the predictions made by the simulations. Finally, The impact of these processes on the electrochemical performances in solid oxide fuel cells (SOFCs) was verified. The Pd-doped perovskite oxides show significantly improved performances due to the enhanced phase transition and exsolution, demonstrating the practical potential of computational catalysis in guiding the design of more efficient materials for energy applications. The final chapter, Chapter 4, studies the electronic activation during nanoparticle exsolution for enhanced activity at elevated temperatures. The evolution of electronic structures during the exsolution process is probed through advanced microscopic and spectroscopic techniques, backed by DFT calculations. It also explores how the exsolution of B-site cations influences catalytic activity for SOFC application. In conclusion, this thesis utilizes DFT to investigate phase transitions, exsolution processes, and surface electronic structure evolution in perovskite materials. The findings of this research pave the way for the development of more efficient and durable materials for energy and environmental applications, contributing to the realization of sustainable technologies.