title: Effect of Sr Content and Strain on Sr Surface Segregation of La1-xSrxCo0.2Fe0.8O3-δas 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 La1-xSrxCo0.2Fe0.8O3-δwith varying Sr content (x = 0.4, 0.3, 0.2) were deposited by pulsed laser deposition (PLD) on single-crystal NdGaO3, SrTiO3, and GdScO3substrates, 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.
authors.affiliation: Division of Materials Science and Engineering, Boston University , Brookline, Massachusetts 02446, United States.
authors.forename: Yang
authors.initials: Y
authors.lastname: Yu
authors.affiliation: Division of Materials Science and Engineering, Boston University , Brookline, Massachusetts 02446, United States.|Department of Physics, Boston University , Boston, Massachusetts 02215, United States.
authors.forename: Karl F
authors.initials: KF
authors.lastname: Ludwig
authors.affiliation: Material Measurement Laboratory, National Institute of Standards and Technology , Gaithersburg, Maryland 20899, United States.
authors.forename: Joseph C
authors.initials: JC
authors.lastname: Woicik
authors.affiliation: Division of Materials Science and Engineering, Boston University , Brookline, Massachusetts 02446, United States.|Department of Mechanical Engineering, Boston University , Boston, Massachusetts 02215, United States.
authors.forename: Srikanth
authors.initials: S
authors.lastname: Gopalan
authors.affiliation: Division of Materials Science and Engineering, Boston University , Brookline, Massachusetts 02446, United States.|Department of Mechanical Engineering, Boston University , Boston, Massachusetts 02215, United States.
authors.forename: Uday B
authors.initials: UB
authors.lastname: Pal
authors.affiliation: Physical Sciences Division, Pacific Northwest National Laboratory , Richland, Washington 99354, United States.
authors.forename: Tiffany C
authors.initials: TC
authors.lastname: Kaspar
authors.affiliation: Division of Materials Science and Engineering, Boston University , Brookline, Massachusetts 02446, United States.|Department of Mechanical Engineering, Boston University , Boston, Massachusetts 02215, United States.
authors.forename: Soumendra N
authors.initials: SN
authors.lastname: Basu
keywords: Sr surface segregation; hard X-ray photoelectron spectroscopy; high-resolution X-ray diffraction; solid oxide fuel cell; strontium-doped lanthanum cobalt ferrite; thin film; total-reflection X-ray fluorescence