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Solid oxide fuel cells (SOFC) have the potential for highly efficient, fuel flexible, environmentally friendly electricity generation. La_1-xSr_xMnO₃ (LSMO), typically with x = 0.2, is the most widely used cathode material for SOFC because of its ability to catalyze O₂ reduction, high electrical conductivity, relative stability at high temperatures, and good thermal expansion coefficient match with the widely used yttrium stabilized zirconia (YSZ) electrolyte [1-2].  Essential for the performance of LSMO cathodes are oxygen adsorption, dissociation, and surface transport, all of which are likely sensitive to cathode surface composition [3-5].  Specifically, there are many reports of Sr segregation to the surface [3, 6-11], with an associated reduction of cathode performance [7], and possible formation of surface phases such as SrO, SrCO3, and (La_1-xSr_x)₂MnO₄ [6-9, 11].  The precise cause of Sr segregation is still unclear and appears to be a function of temperature, O₂ partial pressure, and cathodic polarization [3, 11].  Sr segregation may be driven by formation of Sr-containing surface phases [7, 10], charge compensation for surface concentrations of positively charged oxygen vacancies [11], reduction of surface polarity [11], or kinetic demixing [12-14], which is the focus of our work.

LSMO has a perovskite crystal structure, which consists of a (sometimes disorted) cubic unit cell with La³⁺/Sr²⁺ cations on the corner sites, Mn^3+/4+ cations on the body-centered sites, and O^2- anions on the face-centered sites.  During SOFC operation there is a gradient in oxygen chemical potential across the cathode, which results in corresponding cation chemical potential gradients.  When one cation diffuses more rapidly than the other, kinetic demixing can occur since the faster diffusing cation wil accumulate on the high oxygen potential side and the slower diffusing cation will accumulate on the low oxygen potential side [15].  We are not aware at this time of experimental results for Sr diffusivity compared to La diffusivity.  Therefore, to understand the rate at which demixing occurs and its possible contribution to the experimentally observed Sr segregation, we are modeling diffusion on the La/Sr lattice using an ab initio and kinetic Monte Carlo (KMC) approach. 

We have performed ab initio calculations of the formation energy for vacancies on the La/Sr lattice, as well as La-vacancy and Sr-vacancy migration energies, to parameterize the energy barriers for La and Sr vacancy-mediated diffusion.  All the ab initio calculations are performed using density functional theory as implemented in VASP [16]. Our results show that the migration barrier for Sr-vacancy exchange (2.42 eV) is significantly smaller than the La-vacancy exchange barrier (2.92 eV), suggesting much faster Sr diffusion and possibly significant demixing.  Further ab initio energetics will be combined with empirical defect models and Monte Carlo simulation to predict diffusion constants for La and Sr, which will then be used to us to model the rate of demixing as a function of realistic SOFC operating conditions.

References

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