SURFACE SEGREGATION AND PHASE FORMATION IN THIN FILMS OF SOFC CATHODE MATERIALS

摘要

A limiting factor for Solid Oxide Fuel Cell (SOFC) performance is the oxygen reduction reaction (ORR) at the cathode surface. Changes in the surface composition and structure on annealing at 800℃ has been studied, since these can influence the ORR. Idealized single crystals of cathode materials La_(0.8)Sr_(0.2)MnO_3 (LSM) and La_(1-x)Sr_xCO_yFe_(1-y)O_3 (LSCF) were grown as heteroepitaxial thin films on lattice matched single crystal substrates. 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. HArd X-ray PhotoElectron Spectroscopy (HAXPES) was used to investigate the electronic structure of the materials. Highly strontium doped LSCF precipitates a surface strontium phase that contains both oxide and carbonate contributions.