Surface Chemistry and Phase Evolution in Dense Sr- or Ca-Doped LaCoO3 Ceramics and Their Correlation to Surface Exchange and Chemical Diffusion Coefficients

Abstract

It is now established in porous (La,Sr)CoO3 (LSC) solidoxide cell electrodes that Sr2+ dopant, employed mainlyto generate oxygen vacancies, tends to form an insulating SrO/SrCO3/Sr(OH)(2) phase at the electrode surface, diminishingits oxygen exchange ability. Replacing Sr2+ with Ca2+ (switching from LSC to LCC) is likely a viable approachto the mitigation of surface segregation due to the closer match ofcation radius of the latter to that of La3+. In order todetermine the effect of dopant replacement on the phase and surfacechemistry evolution alone, the influence of microstructure evolutionon the performance degradation rate must be eliminated. Therefore,here, we compared the surface chemistry and phase evolutions of LSCand LCC on bulk, dense ceramics, which did not undergo notable changesin active surface area. To track electrochemical performance evolution,we determined the changes in the oxygen surface exchange (k (chem)) and oxygen diffusion coefficients (D (chem)) via electrical conductivity relaxation(ECR) measurements performed prior to and after 100 h exposure to700 degrees C. LCC had higher k (chem) thandid LSC, but D (chem) values were similar.Similar performance degradation behaviors, but via different mechanisms,were observed. Obtained information is useful for both solid oxidecell and separation membrane applications.