La_1-xSr_xMnO₃ (LSM) was one of the first perovskites employed as SOFC cathode material and is still used in the form of porous composites with the respective electrolyte [1]. In our previous study [2], we analyzed oxygen adsorption, dissociation and migration on the (001) MnO₂-terminated LM surface. In pure LaMnO₃, the MnO₂ (001) termination was found to be the energetically most stable, however with an increase of Sr doping, the (La,Sr)O termination is predicted to become even more favorable thermodynamically [3].

In this talk, we discuss and compare the results of first principles calculations on the elementary steps (oxygen vacancy formation, O₂ molecule and O atom adsorption) of the oxygen reduction reaction on (La,Sr)MnO₃ on the two (001) polar terminations – (La,Sr)O and MnO₂. The LSM slab calculations were performed using the plane wave VASP computer code with GGA exchange correlation functionals. Properties of oxygen vacancies in different planes from the surface were analyzed. The vacancy formation energy on the MnO₂ terminated surface is smaller than that in bulk. In contrast, the vacancy energy on the (La,Sr)O termination is larger, thus strongly reducing the surface vacancy concentration.

Our approach allows for a clear separation of the two distinct effects -- different surface terminations and slab cation stoichiometry (ratio of a number of (La,Sr) ions to Mn that affects the average Mn oxidation state). It is found that oxygen vacancy formation energy in the bulk (slab center) is almost independent of the Mn oxidation state in the region +2.6 to +3.25, whereas it is monotonically decreasing on both surfaces. This surface-to-bulk discrepancy will be discussed. The oxygen atom adsorption energy on the (La,Sr)O termination considerably exceeds that on the MnO₂ termination.

As a result, the equilibrium oxygen adsorbate concentration was found to be about two orders of magnitude larger for the (La,Sr)O termination, but on the other hand, the surface oxygen vacancy concentration is smaller by nearly six orders of magnitude. Therefore, the oxygen reduction rate on (La,Sr)O termination is expected to be significantly lower than on the MnO₂ termination.

[1]. M. Kuklja, E.A. Kotomin, R. Merkle, Yu.A. Mastrikov, J. Maier, PCCP 15, 5443 (2013).

[2]. Yu.A. Mastrikov, R. Merkle, E. Heifets, E. A. Kotomin, J. Maier, J. Phys. Chem. C 114, 3017 (2010).

[3]. S. Piskunov, E. Heifets, T. Jacob, E. A. Kotomin, E. Spohr, Phys. Rev.B 78, 121406 (2008).