Abstract
The oxygen reduction reaction (ORR) at the cathode is often the rate-controlling step in the electrochemical reactions occurring in the solid oxide fuel cells (SOFCs). 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 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 La1-x Sr x Co0.2Fe0.8O3-δ with various Sr contents (x = 0.4, 0.3, 0.2) were deposited by Pulsed Laser Deposition (PLD) on single crystal NdGaO3, SrTiO3 and GdScO3 substrates, leading to different strains in the films. The LSCF thin films were annealed at 800 °C for 10 hours in air. The extent of Sr segregation at the post-annealed film surface was quantified using synchrotron-based total reflection X-ray fluorescence (TXRF) technique. The formation of secondary phases were found and their surface area coverage was quantified by Atomic Force Microscope (AFM). Transmission electron microscope (TEM) and related spectroscopy techniques were used for microstructural and quantitative elemental analyses of the secondary phases on surface which were found to be Sr-rich phases. The bonding environment of the Sr-rich phases formed on the surface were investigated by synchrotron-based 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 strains of LSCF thin films on various substrates were 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.
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