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Ab initio Study
of
Surface Properties of LaMnO3
Yueh-Lin Lee, Dane Morgan, Materials Science
Program
(La,Sr)MnO3
(LSM) is the primary cathode
catalyst used in commercial solid oxide fuel cells (SOFCs).
SOFCs are significantly limited by the oxygen reduction
reaction (ORR) at conventional SOFC operating
temperature. However, it is still unclear that how ORR
is limited
by the processes such as dissociation, diffusion, and
incorporation on the LSM surface.
To understand the relationship between surface structure
and ORR in SOFC catalysts, we adopt an atomistic
modeling approach based on ab initio density
functional theory (DFT) simulations to study LSM
surfaces. The long-term goal is to use ab initio
based energetics to determine the structure of the LSM
surface under operating conditions and the rates of ORR
processes of oxygen splitting, surface transport, and
incorporation vs. oxygen partial pressure and
temperature.
Initial work has focused on
understanding the stable surface and near surface
vacancy concentration in LaMnO3 (LMO). The
vacancy concentration is important since it is likely to
couple strongly to oxygen dissociation, transport, and
incorporation.
The energies for LMO surfaces obtained by simply
truncating the bulk orthorhombic phase structure are
calculated to be 64 meV/Å2 (100), 76 meV/Å2
(110), and 75 meV/Å2 (111). This ordering is
consistent with the greater stability of the (100)
surface compared to the (110) surface found by 54 meV/Å2
and 83 meV/Å2 for surfaces obtained by
truncating the cubic perovskite phase [1].
It is presently unknown to
what extent the bare cations on these surfaces may
become terminate with oxygen under SOFC operating
conditions and how this might impact the surface
stability. An ab initio based thermodynamic
model is being developed to include the effects of
oxygen termination.
Vacancy formation energies vs. distance from (100)
surfaces in LMO.
Based on the surface energy
results, energetics of vacancy formation associated with
the likely most stable (100) surface have been further
investigated. The 8-layer slab consists of alternating
MnO2 and LaO layers, and the MnO2
layers in the slab have two distinct oxygen sites. The
vacancy formation energies as a function of position
relative to the (100) surfaces are shown in the figure
above. They are referenced to the calculated most stable
bulk vacancy formation energy and therefore go to near
zero in the bulk-like center of the slab. However, the
results show that the vacancy energies deviate
dramatically from their bulk value near the surface, and
that the deviation is highly dependent on surface type.
This will lead to orders of magnitude differences in
vacancy concentrations near the surface, potentially
strongly impacting the ORR mechanisms.
Although there is good understanding of the bulk defect
energetics in LMO [2], as far as we are aware this is
the first prediction of significant deviations from bulk
values at the surface.
We gratefully acknowledge
financial support from the National Science Foundation
(NSF) MRSEC program, Division of Materials Research (DMR),
award number 0079983. We gratefully acknowledge
computing support from the National Science Foundation
(NSF) National Center for Supercomputing Applications
(NCSA), award number DMR060007
References
1. R. A. Evarestov, E. A. Kotomin,
Y. A. Mastrikov, D. Gryaznov, E. Heifets, and J. Maier,
Phys. Rev. B 72, 214411 (2005).
2. F. W. Poulsen, , Solid State
Ionics 129, 145 (2000) |
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