Chromium Poisoning Effects on Surface Exchange Kinetics of La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ

The presence of Cr has already been reported in literature to cause severe degradation to La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ (LSCF). However, fundamental understanding of Cr effects on the surface exchange kinetics is still lacking. For the first time, in situ gas phase isotopic oxygen exchange was utilized to quantitatively determine Cr effect on oxygen exchange kinetics of LSCF powder as a function of temperature and water vapor. Our investigations revealed that the formation of secondary phases such as SrCrO₄, Cr₂O₃, Cr-Co-Fe-O, and La-Co-Fe-O can affect both the oxygen dissociation step and overall surface exchange. Specifically, Cr-containing secondary phases on the surface not only decrease the active sites for surface reactions but also alter the nearby stoichiometry of the LSCF matrix, thereby limiting surface oxygen transport. In addition, water molecules actively participate in the surface reactions and can further block the active sites.