Surface segregation in strontium doped lanthanum cobalt ferrite: effect of composition, strain and atmospheric carbon dioxide
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Solid oxide fuel cells (SOFCs) convert chemical energy directly into electrical energy, leading to significantly higher conversion efficiencies. The oxygen reduction reaction (ORR) at the cathode is often the rate-controlling step in the electrochemical reactions occurring in the 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, strain, and the CO2 in the gas phase at the cathode, are critical. In this research, heteroepitaxial thin films of La1-x SrxCo0.2Fe0.8O3- 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 extent of Sr segregation at the film surface was quantified using the synchrotron-based total reflection X-ray fluorescence (TXRF) technique, and by Atomic Force Microscopy (AFM). The microstructure and the electronic structure of the Sr-rich phases formed on the surface were investigated by scanning/transmission electron microscopy (S/TEM) and hard X-ray photoelectron spectroscopy (HAXPES), respectively. These studies revealed that the surface phases consisted of SrO covered with a capping layer of SrCO3. The presence of CO2 in the atmosphere was found to enhance the kinetics of Sr surface segregation in LSCF. 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.