Mixed ionic and electronic conducting (MIEC) perovskite oxides (ABO3) have a substantial role in carbon-neutral clean energy conversion and storage technologies. Owing to their favourable catalytic properties, high ionic and electronic conductivity, and chemical and redox stability, MIEC perovskite oxides are promising candidates as electrode materials in multiple applications such as oxygen transport membranes (OTMs), OTM-based reactors, and solid oxide fuel/electrolysis cells (SOFC/SOEC). The operation of these high temperature electrochemical devices is based on oxygen ion transport at temperatures typically greater than 600 °C. One of the major challenges in the development of these devices is the limited understanding of the oxygen transport properties and mechanisms of MIEC electrodes in humid atmospheres, despite water vapour being a fundamental oxygen bearing molecule present in the input gas stream in these electrochemical systems.
In this PhD study, taking (La0.8Sr0.2)0.95Cr0.5Fe0.5O3-δ (LSCrF8255) as a model MIEC perovskite oxide, the oxygen mass transport properties and mechanisms were studied under humid oxidizing wet oxygen (pO2 = 200 mbar, pH218O = 30 mbar) and humid reducing water vapour (pO2 < 1 mbar, pH218O = 30 mbar) environments through oxygen isotope exchange depth profiling (IEDP) coupled with secondary ion mass spectrometry (SIMS) from 600 to 900 °C. The results were also compared to those measured under dry oxygen conditions (p18O2 = 200 mbar). LSCrF8255 exhibited a consistent oxygen bulk diffusivity between the wet and dry oxygen conditions, but a limited (up to 2 orders of magnitude lower) water surface exchange kinetics in the wet oxygen condition due to the competition between water and oxygen molecules for the oxygen vacancy sites on the sample surface, as well as the scrambling behaviour between them observed through in-situ residual gas analysis (RGA) carried out during the isotope exchange anneal. A significant enhancement of up to 4 orders of magnitude in the oxygen transport properties of LSCrF8255 was demonstrated in the water vapour condition. The increased concentration of oxygen vacancies generated in water vapour, probed through thermogravimetric analysis (TGA), facilitates the oxygen bulk diffusivity and surface exchange kinetics. The surface exchange kinetics were also enhanced through the water surface exchange mechanism studied through in-situ RGA.
Further, the surface chemistry and bulk stability of the material were investigated under the harsh environments of reactive gas and elevated temperature. The surface composition evolution was studied through X-ray photoelectron spectroscopy (XPS), angle-resolved XPS (ARXPS), low energy ion scattering (LEIS), scanning electron microscopy (SEM), and scanning transmission electron microscopy (STEM). The mass transport phenomenon of Sr surface segregation was studied under humid conditions and compared to that under dry oxygen conditions. Sr segregation has been demonstrated on all samples annealed in wet oxygen, water vapour, and dry oxygen, particularly at high temperatures (≥ 800 °C). However, the extent of Sr surface segregation and enrichment, as well as the chemical composition of the Sr surface species were different in the three atmospheres. During the study, Sr segregation was also correlated to other mass transport phenomena such as Cr evaporation and redeposition and Si deposition. The bulk stability was studied through X-ray diffraction (XRD) and neutron diffraction techniques. Both results indicated a high level of bulk stability in LSCrF8255 in both oxidizing and reducing atmospheres at elevated temperatures.
This PhD study is expected to provide an advancement in the understanding of, and guidelines for, electrode design, performance, and durability, not only for SOC technology, but also a wider range of MIEC perovskite oxide applications such as electrochemical sensors and electrocatalysts for water splitting.