Surface Phase Emergence and Evolution of Lanthanum Strontium Manganite and Lanthanum Strontium Cobalt Ferrite Thin Films for Solid Oxide Fuel Cell Cathodes
Committee Members: Advisor: Soumendra N. Basu; MSE/ME; Co-Advisor: Karl Ludwig Srikanth Gopalan, MSE/ME; Uday B. Pal, MSE/ME; Appointed Chair: Xi Lin, MSE/Physics
Abstract: Solid Oxide Fuel Cell (SOFC) technology is an effective method of energy conversion due to its high efficiency and fuel flexibility. A limiting factor for SOFC performance is the oxygen reduction reaction (ORR) that occurs at cathode surface. This research involves the study of changes in the surface composition and structure at the surface of the cathode, since these changes can directly influence the ORR. Idealized single crystals of cathode materials La0.8Sr0.2MnO3 (LSM) and La1-xSrxCoyFe1-yO3 (LSCF) were grown as heteroepitaxial thin films on lattice matched single crystal substrates. These thin films have well defined solid gas interfaces and extremely flat surfaces that are useful model systems. The as-deposited films were characterized by x-ray diffraction (XRD), atomic force microscopy (AFM), Rutherford backscattering spectrometry (RBS) and transmission electron microscopy (TEM).
Changes upon heating the films to operating temperature and pressures were characterized using various synchrotron x-ray techniques. Total Reflection X-ray Fluorescence (TXRF) measurements, which probe compositional changes, were made at high temperature in real time. The LSM surfaces were found to develop manganese enrichment when heated. Highly strontium doped LSCF were found to develop strontium-rich surfaces. On lowering the strontium doping concentration of LSCF, the amount of surface strontium content is reduced. Quenching preserved the high temperature compositional nature of these perovskite materials. X-ray Absorption Near Edge Spectroscopy (XANES) confirmed that the changes to film surfaces are irreversible. HArd X-ray PhotoElectron Spectroscopy (HAXPES) was used to investigate the electronic structure of the materials. LSM undergoes a redistribution of manganese 3+, 4+, and 2+ surface states depending on annealing conditions. Highly strontium doped LSCF precipitates a surface strontium phase that contains both oxide and carbonate contributions. Although, lowering the bulk strontium doping of LSCF decreases the strontium surface precipitation, these precipitates order as triangles in-plane with the film orientation.