Effect of Sr Content and Strain on Sr Surface Segregation of La_1–xSr_xCo_0.2Fe_0.8O_3−δ as Cathode Material for Solid Oxide Fuel Cells

Abstract

Strontium-doped lanthanum cobalt ferrite (LSCF) is a widely used cathode material due to its high electronic and ionic conductivity, and reasonable oxygen surface exchange coefficient. However, LSCF can have long-term stability issues such as surface segregation of Sr during solid oxide fuel cell (SOFC) operation, which can adversely affect the electrochemical performance. Thus, understanding the nature of the Sr surface segregation phenomenon and how it is affected by the composition of LSCF and strain are critical. In this research, heteroepitaxial thin films of La SrCoFeO with varying Sr content (x = 0.4, 0.3, 0.2) were deposited by pulsed laser deposition (PLD) on single-crystal NdGaO, SrTiO, and GdScO substrates, leading to different levels of strain in the films. The extent of Sr segregation at the film surface was quantified using synchrotron-based total-reflection X-ray fluorescence (TXRF) and atomic force microscopy (AFM). The electronic structure of the Sr-rich phases formed on the surface was investigated by hard X-ray photoelectron spectroscopy (HAXPES). The extent of Sr segregation was found to be a function of the Sr content in bulk. Lowering the Sr content from 40% to 30% reduced the surface segregation, but further lowering the Sr content to 20% increased the segregation. The strain of LSCF thin films on various substrates was measured using high-resolution X-ray diffraction (HRXRD), and the Sr surface segregation was found to be reduced with compressive strain and enhanced with tensile strain present within the thin films. A model was developed correlating the Sr surface segregation with Sr content and strain effects to explain the experimental results.

The effect of sub-surface strontium depletion on oxygen diffusion in La0.6Sr0.4Co0.2Fe0.8O3−δ

“…As such, the exact mechanisms of segregation in LSCF have not yet been fully elucidated, however, it is clear that strontium segregation and secondary phase growth is prevalent at intermediate temperature SOFC (IT-SOFC) operating temperatures. 20 To summarise the literature, it has been observed that: (1) segregated strontium rst forms a Sr-O monolayer at the surface, which, given sufficient time at elevated temperatures, will eventually cover the whole surface of the material, 15,21 (2) secondary phases grow preferentially on microstructural defects such as grain boundaries and twin boundaries, [22][23][24] (3) Sr-based particles form, comprised of a strontium oxide (SrO) core with a 'capping' layer surrounding the core, 25 the composition of which is dependent upon the gas atmosphere present during the anneal, (5) particle growth rate is observed to vary with gas composition 22 and (6) the onset of particle growth begins within minutes at temperatures around 1000 C. 20,22 It is not yet clear how monolayer growth transitions into particle formation, however, it is assumed that once the concentration of strontium at the surface exceeds its solubility limit then phase separation will occur. Diffusion of strontium across the grain surface has been observed, 26 however, no direct evidence has linked surface diffusion to particle growth.…”

134Ba diffusion in polycrystalline BaMO3 (M = Ti, Zr, Ce)

“…39,40 The high processing temperature and segregation of BaO as BaCO 3 give rise to an interesting defect chemistry as discussed for BaZrO 3 with respect to materials stability and Ba-mobility. 7 Novel behavior with the implication to cation intermixing has been observed at the interface of LaAlO 3 /SrTiO 3 heterostructures such as two dimensional metallic conductivity, magnetic scattering and superconductivity.…”

Revealing Strain Effects on the Chemical Composition of Perovskite Oxide Thin Films Surface, Bulk, and Interfaces

“…In perovskites containing strontium, an insulating phase generally forms on the surface and has been shown to have a majority decomposition product of strontium oxide, SrO, as well as hydroxyl, Sr(OH) 2 and carbonate, SrCO 3 species . Angle‐resolved XPS (see Figure ), at emission angles of 60° and 80° between the sample surface normal and the detector position shows clear differences in the strontium environment confirming a higher segregation of Sr in the form of secondary phases under tensile strain These differences are consistent with reported experimental and modeling results for related perovskite systems, where tensile strain has been linked to a surface enrichment of strontium carbonate . With these results we can conclude that while RBS and LEIS prove the overall Sr content to be similar in the different films, the Sr chemical environment changes as a function of strain proving a higher tendency to form surface Sr‐secondary phases with tensile strain.…”

“…Studies of strontium‐containing perovskites frequently highlight substantial strontium segregation, a primary cathode degradation mechanism in SOFCs due to a reduction of active surface sites and related phase changes, at thin film surfaces . Dulli et al linked strontium segregation in the perovskite LSM ( x = 0.35) to a surface layer restructuring to a Ruddlesden–Popper (RP) phase, after their films were annealed at 900 °C.…”