Sulphur poisoning of the SOFC cathode material La sub(0.6)Sr sub(0.4)CoO sub(3- delta )

The oxygen exchange kinetics of La sub(0.6)Sr sub(0.4)CoO sub(3- delta ) was investigated at 700 degree C and p(O sub(2)) = 0.10 bar by in-situ conductivity relaxation measurements. The experiments were performed for 1000 h in an O sub(2)-Ar atmosphere with and without 50 ppm SO sub(2). For post-test analyses, scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDXS) and analytic transmission electron microscopy (TEM) were applied. Elemental depth profiles were obtained by X-ray photoelectron spectroscopy (XPS). The decrease in the chemical oxygen exchange coefficient k sub(chem) by a factor of 10 during the course of 1000 h indicated that an accumulation of traces of sulphur (estimated to the range of ppb) in the form of SrSO sub(4) can lead to a significant degradation even in nominally pure O sub(2)-Ar. The exposure to 50 ppm of SO sub(2) for 1000 h caused a severe degradation in k sub(chem) by a factor of 3000. Post-test analyses showed that the perovskite phase La sub(o.6)Sr sub(0.4)CoO sub(3- delta ) is decomposed into SrSO sub(4) and La sub(2)O sub(2)SO sub(4) within the first 500 nm of the surface, and a nano-crystalline Co-rich phase, which is located in the depth range of 500-1400 nm.