Study on atomic-scale observation and control of surface cation segregation of perovskite oxides for energy conversion devices

Abstract

In materials based on perovskite oxides, the phenomenon of surface cation segregation on the surface is evaluated as a typical chemical instability of the oxide surface. In relation to this, during the fabrication stages, the initial deterioration occurs, making it challenging to produce a uniform and flat surface of perovskite oxide. The mechanism of degradation due to segregation is still not fully elucidated, and the role of second-phase particles is not completely understood. Additionally, there is a lack of reports on strategies to suppress this phenomenon. This doctoral thesis aims to enhance the understanding of cation segregation phenomena on the surface and to develop mitigation strategies. In Chapter 2, atomically flat model thin film system was built. By combining high-resolution observations and electrochemical characterizations, the reactivity of an ideal surface without surface segregation was measured, and the causes of degradation were identified. In Chapter 3, the role of surface alkaline metal oxides was reassessed. It was confirmed that those previously known to simply block surface reaction active sites can, in fact, enhance charge transfer, thereby increasing the surface reactivity of mixed-conducting perovskite oxide electrodes. In Chapter 4, an innovative method for effectively introducing fluoride anions into the oxide lattice at low temperatures and ambient pressure, called topotactic fluorination, is introduced. This process contributes to enhancing the activity and stability of perovskite oxide-based electrochemical catalysts.