Effect of Chromium Vapor Pressure on Accelerated Poisoning Tests for (La0.6Sr0.4)(Co0.2Fe0.8)O3 Cathode

One of the most critical issues causing degradation of SOFCs is poisoning of cathodes by a trace amount of impurities transferred with air such as sulfur (S) and chromium (Cr), and therefore, we are continuously investigating the test method on accelerated degradation by these impurities.  In our previous studies on Cr poisoning, chromium metal covered with Cr₂O₃ was directly placed on the current collector of the cathode as a source of Cr vapor.  However, the setting accelerated the performance degradation in surplus.  In order to perform accerelated poisoning test at an appropriate rate, we improved the cell operating equipment to control the Cr vapor pressure during the cell tests.  The Cr vapor pressure was controlled by adjusting the place and temperature of the Cr source. The cell tests were operated at 1073 K under controlled feeding rate of Cr vapor after the cell performance became stable. The degradation behavior was analyzed with AC impedance analysis, field emission scanning electron microscopy (FE-SEM) with energy dispersive X-ray analysis (EDX) / wavelength-dispersive X-ray spectrometer (WDS).  The vapor pressure of Cr species was estimated from the amount of Cr trapped by (Sm,Sr)CoO₃cathode in the same equipment of cell tests.  The degradation rate decreased with decreasing the Cr vapor pressure, and the amount of Cr species deposited in LSCF cathode decreased with decreasing the Cr vapor pressure.