Sulfur poisoning behavior of La1-xSrxCo1-yFeyO3-δ thin films with different compositions

Abstract

Sulfur in the air stream is one of the primary contaminants affecting the stability of cathodes in solid oxide fuel cells (SOFCs). In this work, sulfur poisoning of La1-xSrxCo1-yFeyO3-δ (LSCF) thin films with different compositions was investigated. The composition of the cathode was systematically controlled by preparing La0.75Sr0.42Co0.15Fe0.68O3-δ (A-site excess LSCF) and La0.60Sr0.39Co0.20Fe0.81O3-δ (stoichiometric LSCF) thin films on Gd-doped ceria (GDC) substrates by pulsed laser deposition. The LSCF films were then heat-treated at 800 °C for 100 h in synthetic air with a trace amount of SO2 (ppb level). As an informative complement to the microstructural and compositional analyses, secondary ion mass spectrometry was also performed to effectively determine the extent of sulfur accumulations from LSCF cathode surface to GDC. The results showed that sulfur poisoning behavior of LSCF cathodes depends strongly on composition. The A-site excess LSCF films exhibited significant SrSO4 formation with severe porosity, whereas, only an isolated SrSO4 formation was observed in the stoichiometric LSCF films; instead, the partial poisoning by sulfur minimizes excessive porosity while enhancing Co-rich oxide precipitation. Electrochemical impedance spectroscopy was carried out to further elucidate the influence of cathode composition on cell stability. Thermodynamic considerations have been made to clarify the plausible chemical reaction mechanism between LSCF of different compositions with SO2.