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First principles calculations of BSCF material for membrane applications Eugene Kotomin Laboratory of Theoretical Physics and Computer Simulations of materials. - PowerPoint PPT Presentation
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13.04.2010 NASA Meeting
First principles calculations of BSCF
material for membrane applications
Eugene KotominLaboratory of Theoretical Physics and
Computer Simulations of materials
13.04.2010 NASA Meeting
One of main priorities of our laboratory: New/more efficient Energy Sources and New Materials for energy applications1. advanced nuclear fuels for Generation IV reactors2. New construction reactor (radiation resistant) materials 3. solid oxide fuel cells: 80% conversion of chemical energy into electricity
4. Ceramic membranes
13.04.2010 NASA Meeting
Development of new materials
• Large scale computer simulations of materials
in close collaboration with state-of-the art experiments [Max Planck Institute, Stuttgart]:
Try-and-error approach does not work!
Limitations of experiments:
Discrimination of processes (O vacancies migration) in the bulk and on surfaces,
A role of different dopands and impurities
Identification of adsorbates at low coverages
13.04.2010 NASA Meeting
General problem
2( ) 4 2g e 2 cathode reaction: O O
Study and control of possible reaction pathways of oxygen reduction and incorporation reaction
Improvement of SOFC and membrane performance requires-- better understanding of
Exciting and challenging multidisciplinary field:-Electrochemistry and materials chemistry,- surface science of advanced oxides,-, chemical kinetics, -large-scale computer simulations
13.04.2010 NASA Meeting
Materials of interest: magnetic perovskites
• LaMnO3 (LMO) – model material
• La1-xSrxMnO3 (LSM)– real cathode material
• Multi-component BSCF type cathodes
These strongly correlated materials reveal
numerous phenomena due to a combination
of spin, orbit, lattice degrees of freedom
-- 2 areas of applications: low-T (spintronics)
-- high T: solid oxide fuel cells
13.04.2010 NASA Meeting
First experience with theoretical modelling of LSM:
• E.Kotomin et al, PCCP 10, 4644 (2008)• R.Merkle et al, J. ECS Trans. 25, 2753 (2009)• Yu.Mastrikov et al. J Phys Chem. C, 114, 3017
2010.• First BSCF paper submitted to Enenrgy and
Environmental Science, 2010.
Standard DFT (GGA) or DFT-HF hybrids calculations of large, low symmetry systems with defects and surfaces (up to 320 atoms per
supercell)
13.04.2010 NASA Meeting
Method
Density Functional TheoryPlane Wave basis set
Generalised Gradient Approximation
Perdew Wang 91 exchange-correlation functional
Projector Augmented Wave method
Davidson algorithm for electronic optimization
Conjugate Gradient method for structure relaxation
Nudged Elastic Bands for energy barriers estimation
Bader charge analysis (Prof. G. Henkelman and co-workers, Universiy of Texas)
4.6.19 08Dec03, Georg Kresse and Jürgen Furthmüller
Institut für Materialphysik,Universität Wien
13.04.2010 NASA Meeting
Computational detailsVASP: GGA PW calculations
• atoms description:
• kinetic energy cutoff:
400 eV > Ecutmax = 269.887 eV
• Monkhorst-Pack k-points sampling < 0.27 Å-1
Element
Valence electrons
Cutoff energy,
eV
Core radius,
Å
La 5s26s25p65d1 219.271 1.48
Mn 3p63d64s1 269.887 1.22
O 2s22p4 250.000 0.98
13.04.2010 NASA Meeting
Test calculations(PCCP 7, 2346 (2005)
a b
c
La Mn O
• Cohesive energy, Structure, ionic charges practically (<1%) do not depend on the specific magnetic ordering• In a good agreement with experimental data• Non-magnetic state – very unfavourable• High covalency of the Mn-O bonding
Orthorhombic (Pbnm)
Structure optimisation for the FM, A-, C-, G-AF and non-magnetic states
Bulk calculations Surface calculations
(001) (110) (111)
strongly under-coordinated surface atoms
polar
+/-1 e +/-4 e +/-3 e
surface energy, eV/surface cell
1.18 2.54 2.74
7-, 8-plane slabs are sufficiently thickfor surface processes modellingSpin-polarized calculations Charges on the two surface planes are not affected by slab stoichiometry
13.04.2010 NASA Meeting
Preliminary results:Ba(0.5)Sr(0.5)Co(0.75)Fe(0.25)O3-δ
• Ba
Sr
O
Co
Fe
Bulk and defect properties40 atom supercells (12.5%) and 320 atoms (1.5%)
13.04.2010 NASA Meeting
Test: pure ABO3 perovskites
Lattice constants (A, cubic phase)
A B
Co* Fe**
Ba 3.96(--) 3.97(4.04)
Sr 3.84(3.83) 3.85(3.85)
*IS, **HS
Pure BSCF: a_o=3.90-3.92 A (expt 3.98A)
13.04.2010 NASA Meeting
Effective (static) Bader atomic charges,e
Ba, Sr 1.57e close to formal +2e
Co 1.71e
Fe 1.88e
O -1.1 e
Strong covalent contribution to the bonding
13.04.2010 NASA Meeting
Co-Vo-Fe vacancy
13.04.2010 NASA Meeting
Vacancy formation energies
• (Ba,Sr)CoO3 ca. 1 eV (LaCoO3 1.5 eV)• (Ba,Sr)FeO3 ca. 2.4 eV (LaFeO3 1.2 eV)• LMO 4.5-5 eV expt 3 eV• STO 5.5-6 eV expt 5 eV
Charge disproportionation effect: 2Fe(3+)=Fe(2+) + Fe (4+) is neglected in
theory
13.04.2010 NASA Meeting
Charge redistribution around Vo
Red is electronic density deficiency, blue- excess
Charge of a missing O2- ion is spread over nearest Co and Fe ions
13.04.2010 NASA Meeting
Calculated lattice constants
Oxygen deficiency,
Incorporation of vacancies improvesagreement with the experiment
13.04.2010 NASA Meeting
Vacancy migration energyCo-Vo-Fe Co-Vo-Co
Co-Vo-Fe
Co-Vo-Co 0.42 eV
0.46 eV
0.52 eV0.46 eV
For comparison: LMO 0.9 eV
13.04.2010 NASA Meeting
Our ultimate goal:--the mechanism of the oxygenreduction in different materials
[LSCF?] under different conditions,--understanding of the limiting
reaction steps,--increase of O reduction efficiency
13.04.2010 NASA Meeting
Milestones:
Atomistic/mechanistic details hardly detectable experimentally:-- Optimal sites for oxygen adsorption-- the energetics of O2 dissociation,-- O and vacancy migration on the surface -- O penetration to cathode surface: what are the rate-determining reaction stages, O diffusion
13.04.2010 NASA Meeting
Mechanism of oxygen reduction M2 in LSM (Merkle et al, J ECS Trans.2009)
O2
Mn
-1.1eV
-O O-
Mn O Mn+0.6eV
-O O-
Mn O Mn
-1.6eV VI
VII
Mn O Mn
O-
O
II
O Mn
+0.7eV
O-
Mn Mn
Mn O2- Mn
-1.5eV
VIIIVIII
O2 adsorption superoxide O2
-
dissociationwithout VO involved..
VO surface diffusion
..
incorporationof O- into VO
..
13.04.2010 NASA Meeting
3 possible mechanisms of oxygen incorporation
-O O-
O Mn O
O-
Mn O- MnO-
Mn O Mn
Mn O Mn
O2
Mn Mn
O2
O-
Mn Mn Mn O2- Mn
O- O-
Mn O Mn
VO..
VO..
O22-
O-
--The rate-determining step is encounter of adsorbed molecular oxygen (superoxide O2- or peroxide O2 (2-) )with a surface oxygen vacancy
--Both vacancy concentration and mobility are important for a fast oxygenIncorporation
13.04.2010 NASA Meeting
3 possible mechanisms for oxygen reduction on LSM
molecular adsorbate coverage surfa
ce ox
ygen
vaca
ncy
conc
entra
tion10-6
reac
tion
rate
per
uni
t cel
l and
sT = 1000 KpO2 = 1 bar
10-6
10-510-4
10-3
10-210-1
10-5
10-4
10-3
10-2
10-1
100
102
104
106
108
M2': diss. without VO..,
VO.. O- limiting
M1: adsorption directly into VO
..
M3: dissociation after O2
- VO.. encounter
M3: dissociation after O2
- VO.. encounter
M3'diss. after O2
- VO..,
VO.. O- limiting
M2:dissociation without VO
..
M2:dissociation without VO
..
13.04.2010 NASA Meeting
Thermodynamics of the O adsorption at different temperatures and O2 gas
pressures
LaO O (110) MnO2+O
13.04.2010 NASA Meeting
Conclusions
• Standard ab initio computer codes are able to shed some additional light on
cathode/surface reactions where expt tools are of a limited applicability
• We reproduce Vo low migration energies • Lattice structure role of structural Vo
• low Vo formation energies • To be used in the analysis of BSCF
cathode/membrane performance