Time-Dependence Of Surface Composition, Transport Properties Degradation, And Thermodynamic Consideration Of La0.6sr0.4co0.2fe0.8o3-Delta Under Chromium Poisoning

In order to investigate the long-term degradation of La0.6Sr0.4Co0.2Fe0.8O3-delta (LSCF) cathodes due to chromium (Cr) poisoning, the transport properties were investigated by two different approaches: one is electrochemical impedance spectroscopy (EIS) on porous LSCF electrodes, the other is the oxygen isotope exchange and SIMS analyses on LSCF pellets. The oxygen surface exchange coefficient (k*) determined from EIS decreased with increasing poisoning time. For LSCF pellets, the k* values decreased linearly with poisoning time (the first stage) and then became nearly constant (the second stage). The first degradation of k* was attributed to the formation of SrCrO4 and CoFe2O4 within the initial 7 similar to 8 h poisoning time, whereas in the second stage, LaCrO3 formation was detected on the surface of LSCF. Those chemical behaviors were compared with thermodynamic calculations where the k* was observed to decrease in the first stage corresponding to the precipitation of SrCrO4 and CoFe2O4, and LaCrO3 is formed in the second stage. Those considerations lead to the conclusions that the k* decrease is dominated by the decrease in available sites such as Sr, Co or oxygen vacancies on the LSCF surface, whereas the constant k* values may be attributed to the remaining/formed LaCrO3 component of the reacted LSCF. (c) 2018 The Electrochemical Society.