AmericanChemicalSociety.com

The overall focus of this work has been using ab initio methods to model oxygen diffusion properties in fast oxygen conductors used in solid oxide fuel cells (SOFCs).  We have focused on La_1-xSr_xMnO₃ (LSMO).  LSMO, typically with x = 0.2, is currently the most common SOFC cathode material.  In this work, we have considered the effect kinetic demixing [1] may play in Sr surface segregation LSMO.  LSMO has a perovskite crystal structure, which consists of a (sometimes distorted) cubic unit cell with La³⁺/Sr²⁺ cations on the corner A-sites, Mn^2+/3+/4+ cations on the body-centered B-sites, and O^2- anions on the face-centered sites.  In response to an oxygen chemical potential gradient across the cathode, there is a related cation chemical potential gradient in the opposite direction.  Kinetic demixing occurs when the flux of one cation is greater than the other and accumulates on the high oxygen potential side.  Segregation of Sr to the surface has been widely reported [2-4] and implicated in decreased cathode performance [3] through the likely sensitivity of oxygen adsorption, dissociation, and surface transport to surface composition [2]. 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. 

Using the results of ab initio calculations, we have created a Hamiltonian describing the interaction of Sr and A-site vacancies. All the ab initio calculations are performed using density functional theory as implemented in VASP [5].  We found that the ab initio formation energies could be well-described using a screened electrostatic potential with the form H(r) = C₁exp(C₂r)/r.  Effectively, Sr and A-site vacancies repulse each other with a maximum energy increase of 0.32 eV when they are in first nearest neighbor positions to each other.  Previously, we have calculated 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.  However, the significant repulsion between Sr and A-site vacancies will slow the Sr diffusion.

To calculate the diffusion coefficients of species on the A-sites we performed KMC simulations on an idealized simple cubic lattice. We found that the activation energy for the tracer diffusion coefficient for Sr was Q=3.15 eV, which is directly related to the height of the maximum barrier a vacancy must cross to move around an Sr without dissociation in order to cause a Sr diffusional hop.  The activation energy for La tracer diffusion was Q=2.98 eV, simply related to the La-vacancy migration energy (2.92 eV). 

We calculated the tendency to demix by comparing the ratio of the initial fluxes of Sr and La to the composition of the LSMO.  If the ratio of the fluxes is not equal to the ratio of the concentrations, oxide forming on the high oxygen potential side will have a different concentration than in the bulk, resulting in demixing.  To calculate the initial fluxes we used the predicted diffusion coefficients from the KMC calculations, and cation chemical potential gradients based on a LSMO defect model proposed by Poulsen [6].  We find that initially the La³⁺ chemical potential gradient is 1.5 times greater than the Sr chemical potential gradient due to differences in their effect on the Mn charge state.  The combination of the difference in chemical potential gradient and reduced Sr diffusion caused by vacancy repulsion slow Sr diffusion to the point that it is actually favorable to enrich La at the high oxygen potential side. 

From these results we can conclude that Sr surface segregation due to kinetic demixing is not likely to occur for dilute Sr concentrations.  It is still be possible for Sr surface segregation to be caused by kinetic demixing at the higher Sr concentrations typical of LSMO used in SOFC.  Vacancy-vacancy and Sr-Sr interactions must be parameterized and included in the KMC Hamiltonian in order to quantitatively model demixing in LSMO with higher Sr concentrations. 

This generous support from ACS has enabled partial funding for two students over a period of two years.  These students are both now continuing on toward PhD’s in Materials Science and Engineering, with a focus on modeling of complex oxides. The results from this work are presently being written up in a paper.  This ACS funding helped seed the PIs work in this area, and provided initial results that helped the PI obtain a DOE grant on SOFC cathode materials.  The PI now has 3 students and a postdoctoral researcher working in the area of ab initio studies of SOFC materials and it is one of his core areas of research.

References

1 H. Yokokawa, N. Sakai, T. Horita, K. Yamaji, M. E. Brito, H. Kishimoto, Thermodynamics and kinetic considerations on degradations in solid oxide fuel cell cathodes, Journal of Alloys and Compounds 452, 41 (2008).

2 N. Caillol, M. Pijolat, and E. Siebert, Investigation of chemisorbed oxygen, surface segregation and effect of post-treatments on La_0.8Sr_0.2MnO₃ powder and screen-printed layers for solid oxide fuel cell cathodes, Appl. Surf. Sci. 253, 4641 (2007).

3 S. P. Jiang and J. G. Love, Origin of the initial polarization behavior of Sr-doped LaMnO₃ for O₂ reduction in solid oxide fuel cells, Solid State Ionics 138, 183 (2001).

4 T. T. Fister, D. D. Fong, J. A. Eastman, P. M. Baldo, M. J. Highland, P. H. Fuoss, K. R. Balasubramaniam, J. C. Meador, and P. A. Salvador, In situ characterization of strontium segregation in epitaxial La_0.7Sr_0.3MnO₃ thin films as a function of oxygen partial pressure, Appl. Phys. Lett. 93, 151904 (2008).

5 G. Kresse and J. Hafner, Phys. Rev. B 47, RC558 (1993).  G. Kresse, Thesis, Technische Universitat Wien 1993.  G. Kresse and J. Furthmuller, Comput. Mat. Sci. 6, 15-50 (1996).  G. Kresse and J. Furthmuller, Phys. Rev. B 54, 11169 (1996).

6 F. W. Poulsen, Defect chemistry modelling of oxygen-stoichiometry, vacancy concentrations and conductivity of (La_1-xSr_x)_yMnO_3±δ, Solid State Ionics 119, 145 (2000).