Keynote
EFFECT OF SURFACE TERMINATION ON THE OXYGEN REDUCTION RATE ON CATHODES FOR OXIDE FUEL CELLS

La_1-xSr_xMnO₃ (LSM) was one of the first perovskites employed as SOFC cathode [1,2]. In undoped LaMnO₃, 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 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 VASP computer code with GGA exchange correlation functionals. The vacancy formation energy on the MnO₂ terminated surface is smaller than that in bulk whereas on the (La,Sr)O termination it 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 (La,Sr) ions to Mn that affects the average Mn oxidation state). 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]. O. Yamamoto, Y. Takeda, R.Kanno, M.Noda, Sol. State Ionics, 22, 241 (1987). [2]. M. Kuklja, E.A. Kotomin, R. Merkle, Yu.A. Mastrikov, J. Maier, PCCP 15, 5443 (2013). [3]. S. Piskunov, E. Heifets, T. Jacob, E. A. Kotomin, E. Spohr, Phys. Rev.B 78, 121406 (2008).