Study on control of surface cation enrichment in perovskite-type oxides for energy conversion devices

Abstract

Among the phenomena related to the surface rearrangement of cations in perovskite-based oxide materials ($ABO_3$), cation enrichment near the surface has been frequently observed. Upon annealing in an oxidizing atmosphere, an A-site cation, Sr or La in particular, is often enriched on the surface as compared to the bulk composition of the material, which eventually forms additional phases or rearranges the crystal structure of the surface. This segregation has been suggested to be the key reason behind the chemical instability of perovskite oxide surfaces and the corresponding performance degradation of solid oxide electrochemical cell (SOC) $O_{2-}$electrodes. In addition to A-site cations, surface enrichment of B-site cations has been reported. Upon high-temperature reduction, some transition metals dissolved in the lattice in an oxidizing atmosphere can be partially reduced and decomposed into nano-sized metallic particles on the oxide surface. This in-situ synthesis process of metal nanoparticles is referred to as the redox ex-solution phenomenon and has widely been studied as a way to fabricate metal nanocatalyst-decorated oxide electrodes for SOCs. However, despite much effort by researchers, the underlying mechanisms related to these phenomena are not completely understood. Accordingly, practical solutions that effectively inhibit A-site cation segregation or accelerate B-site cation ex-solutions have not yet been proposed.

In this Ph.D. dissertation, I controlled the degree of cation enrichment at the surface of perovskite-type oxide thin films, a model SOC electrode, through lattice strain. Epitaxial thin films of oxides, in this case Sr$Ti_{0.5}$$Fe_{0.5}$$O_{3-$\delta$}$, Sr$Zr_{0.5}$$Fe_{0.5}$$O_{3-$\delta$}$, LaMn$O_3$, and $La_{0.6}$$Sr_{0.4}$$Co_{0.2}$$Fe_{0.8}$$O_{3-$\delta$}$, were fabricated via pulsed laser deposition onto single-crystal substrates to control the degree of lattice strain according to the lattice mismatch between the film and the substrate. The misfit strain and surface composition of each film were analyzed via high-resolution X-ray diffraction (HR-XRD) and angle-resolved X-ray photoelectron spectroscopy (AR-XPS), respectively. The surface activity of the films for oxygen exchange and CO oxidation was also analyzed by electrical conductivity relaxation (ECR) and quadrupole mass spectroscopy (QMS) respectively.

I found that strain when applied to perovskite-type thin films changes their surface composition and electrode reactivity considerably. Density functional theory (DFT) calculations revealed that Sr or La atoms are intrinsically unstable despite the fact that the overall perovskite structure is stable and the extent of deviation from the optimal A-O bond strength in the most stable state of the A-site cation can be a driving force behind surface segregation. Based on these findings, I successfully demonstrate that when an isovalent dopant is added, Sr or La-excess can be remarkably alleviated, improving the chemical stability of the resulting perovskite electrodes.

Similarly, it is found that lattice strain can promote the redox ex-solution of epitaxial thin films (i.e., a Co ex-solution in Sr$Ti_{0.75}$$Co_{0.25}$$O_{3-$\delta$}$) by controlling the B-O bond length in the perovskite lattice. In this case as well, isovalent doping was able to control the degree of the ex-solution and thus the surface CO oxidation reactivity.

In conclusion, the insights gained from this study provide a means by which to understand cation enrichment on the surface of perovskite-type oxides, focusing on bond strength (length) between the nearest cation-oxygen in the perovskite lattice, and will suggest a new strategy to control surface cation segregation to achieve highly active and durable perovskite-based electrodes for SOCs.