Surface cation heterogeneity in perovskite oxides connected to catalytic activity and stability [electronic resource]
Keywords: Materials Science and Engineering.

Electrochemical processes involving oxygen underlie many alternative energy conversion and storage technologies but are susceptible to poisoning by environmental contaminants, which decreases device efficiency. For example, solid oxide fuel cells and electrolyzers offer more efficient energy conversion and storage than traditional technologies but suffer from degradation. Because the cathode, which performs oxygen incorporation, is the primary source of degradation, this work focused on understanding perovskite oxide cathode surfaces as they relate to catalytic performance in low-contaminant and ambient air conditions. The surfaces of these electrocatalysts deviate significantly from idealized bulk termination and must be understood through fundamental studies in order to design for high stability. Using model, thin film systems, surface cation heterogeneities were identified and differentiated using a range of surface-sensitive techniques, from ex situ electron microscopy to in situ ambient pressure X-ray photoelectron spectroscopy. Surface features were connected to catalytic behavior in well-controlled, synthetic air environments that probe native activity and in ambient air conditions containing environmental contaminants. In addition to connecting surface properties to functionality, these studies help explain catalytic performance discrepancies that can span orders of magnitude in literature and how they can be avoided. Three primary research focus areas are reported in this thesis. First, bulk chemical stability and resulting surface compositional changes were explored in a La0.6Sr0.4CoxFe1-xO3-δ system. Substitution of iron with cobalt showed higher catalytic activity; however, this was at the expense of significant bulk and surface chemical heterogeneity in the form of Co-rich regions, suggesting inherent material instability. Second, because bulk inhomogeneity was not observed in the absence of cobalt, variants of (La, Sr)FeO3-δ were explored to focus on surface heterogeneity. Changes to bulk cation A/B stoichiometry in (La, Sr)FeO3-δ showed dramatically different surface terminations, from an Fe-rich double cation layer to A-site rich termination composed primarily of Sr. The Fe-rich termination was less catalytically active (contrary to conventional belief) in synthetic air environments, but this surface was less susceptible to trace, poisonous gases found in ambient air than A-site termination. This demonstrated the ability to separate native catalytic performance from catalyst behavior in the presence of environmental contaminants. Further, substituting Sr in (La, AE)FeO3-δ with other alkaline earth (AE) substituents Ca and Ba showed that surface precipitate formation was not a good indicator of poor catalytic performance. In fact, (La, Ba)FeO3-δ had the highest density of surface precipitates and was the most catalytically active and stable in both synthetic and ambient air. Third, applying the knowledge gained about cation migration and surface heterogeneity effects, direct surface modification for improved catalytic performance was investigated. Bilayer surface decoration was explored to suppress bulk cation segregation to the surface and showed that barrier layers based on ceria lowered degradation rates while retaining catalytic activity. A trilayer geometry was introduced to separate the "infinite" reservoir of segregate in the bulk from a thin layer of catalyst at the surface using a praseodymium-substituted cerium oxide buffer layer. Absolute degradation rates in ambient air decreased up to 10x by using a trilayer of (La, Sr)FeO3-δ compared to an unmodified surface. By replacing the terminating layer with (La, Ba)FeO3-δ, degradation rates were improved by over 20x in ambient air. This demonstrated the versatility of the trilayer design, which allows for optimization of the top layer for catalytic activity and stability while the bottom layer can be designed for sufficient ionic and electronic transport in addition to electrolyte compatibility. Taken together, the three investigations reported in this thesis separated the native catalytic activity of perovskite oxides from performance affected by gas-phase contaminants. Fundamental studies of surface structure and composition were connected to catalytic performance in both environments. Finally, a surface modification technique involving a trilayer structure was introduced and demonstrated as a platform for rapid design and optimization of catalytic activity and device stability.