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) solid oxide cell electrodes that Sr2+ dopant, employed mainly to generate oxygen vacancies, tends to form an insulating SrO/SrCO3/Sr­(OH)2 phase at the electrode surface, diminishing its oxygen exchange ability. Replacing Sr2+ with Ca2+ (switching from LSC to LCC) is likely a viable approach to the mitigation of surface segregation due to the closer match of cation radius of the latter to that of La3+. In order to determine the effect of dopant replacement on the phase and surface chemistry evolution alone, the influence of microstructure evolution on the performance degradation rate must be eliminated. Therefore, here, we compared the surface chemistry and phase evolutions of LSC and LCC on bulk, dense ceramics, which did not undergo notable changes in 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 to 700 °C. LCC had higher k chem than did LSC, but D chem values were similar. Similar performance degradation behaviors, but via different mechanisms, were observed. Obtained information is useful for both solid oxide cell and separation membrane applications.