1. Thermochemie und Mikrokinetik und Mikrokinetik Martin Kilo, Institut für Metallurgie, TU Clausthal 18. 03. 2004 1 Fuel Cell Research Symposium ETH Zürich Switzerland 18/19.03.04 Experimental Investigation and Simulation of Oxygen Transport in SOFC Materials Thermochemistry and microkinetics 1.Motivation and systems: ZrO 2, LaGaO 3, LaMnO 3 2.Experimental: Tracer diffusion in electrolyte 3.Experimental: Tracer diffusion in LSM/YSZ pair 4.Modelling: Static lattice => migration mechanism 5.Modelling: Molecular dynamics => diffusion Martin Kilo, Christos Argirusis, Günter Borchardt, Rob A. Jackson * TU Clausthal, Institut für Metallurgie, Robert-Koch-Str. 42 D-38678 Clausthal-Zellerfeld, Germany * Keele University, School of Physics and Chemistry, Keele, Staffs ST5 5GB / UK

2. Thermochemie und Mikrokinetik und Mikrokinetik Martin Kilo, Institut für Metallurgie, TU Clausthal 18. 03. 2004 2 Fuel Cell Research Symposium ETH Zürich Switzerland 18/19.03.04 Motivation: Oxygen mixed and ionic conductors Most common examples: doped ZrO 2 or doped perovskites, e.g. (La 0.8 Sr 0.2 )(Ga 0.8 Mg 0.2 )O 3-δ, LSGM, La x Sr 1-x MnO 3-δ, LSM Doping with aliovalent cations leads to fast oxygen diffusion, but usually to slow cation diffusion T = A(x) exp(-E a (x) / RT) n = 2: Ca Zr 2 ' n = 1: Y Zr 1 ' n x A: 0 < x < x'B: x > x' n = 1 n = 2 x A EaEa σT x A EaEa x A EaEa x' 0.08 – 0.12

3. Thermochemie und Mikrokinetik und Mikrokinetik Martin Kilo, Institut für Metallurgie, TU Clausthal 18. 03. 2004 3 Fuel Cell Research Symposium ETH Zürich Switzerland 18/19.03.04 Open questions Experimental What is the practical connection between the experimental oxygen diffusion coefficient and conductivity? Oxygen diffusion under applied electrical field Influence of thermal ageing on oxygen diffusion Simulation of oxygen diffusion Static lattice: Mechanism of transport Molecular dynamics: Transport coefficients Finite element modelling: Simulation of real systems

4. Thermochemie und Mikrokinetik und Mikrokinetik Martin Kilo, Institut für Metallurgie, TU Clausthal 18. 03. 2004 4 Fuel Cell Research Symposium ETH Zürich Switzerland 18/19.03.04 Gaseous tracer: 16 O / 18 O gas tracer Nat.: 0.2 % 18 O Tracer: >90% 18 O Surface limited: x( 18 O,x=0) < 90 % 18 O 16 O Furnace: T, p, static Sample c(x,t)-c 0 =(c s -c 0 )·(erf(x/2(D O t) 0.5 )-exp(h·x+h 2 ·D O t)·erf(x/2(D O t) 0.5 +h·(D O t) 0.5 ))

5. Thermochemie und Mikrokinetik und Mikrokinetik Martin Kilo, Institut für Metallurgie, TU Clausthal 18. 03. 2004 5 Fuel Cell Research Symposium ETH Zürich Switzerland 18/19.03.04 Isotopic lateral and depth distribution: SIMS Analysis VG SIMS-lab: Quadrupole detection; 7 kV Ar + Detection of positively/negatively charged ions Charge compensation with flood gun Cameca 3f/5f: Magnetic sector field; >10 kV O +/- Detection of positively charged ions Charge compensation by conducting layer

6. Thermochemie und Mikrokinetik und Mikrokinetik Martin Kilo, Institut für Metallurgie, TU Clausthal 18. 03. 2004 6 Fuel Cell Research Symposium ETH Zürich Switzerland 18/19.03.04 ZrO 2 -systems CSZ, ScSZ, YSZ: working conditions CaO-ZrO 2 (Duran, J.Mat.Sci. 22 1987 4348) Sc 2 O 3 -ZrO 2 (Ruh, J.Am.Ceram.Soc. 60 1977 399) Y 2 O 3 -ZrO 2 (Suzuki, SSI 81 1995 211) All ZrO 2 -systems have a cubic part of the phase diagram with fast oxygen transport and slow cation transport Cation diffusion (red line: T = 1000 °C) Oxygen diffusion Working regions: Mostly single crystals

7. Thermochemie und Mikrokinetik und Mikrokinetik Martin Kilo, Institut für Metallurgie, TU Clausthal 18. 03. 2004 7 Fuel Cell Research Symposium ETH Zürich Switzerland 18/19.03.04 Oxygen transport: Self diffusion Oxygen Diffusion - Maximum in D(x) like σT, MD - ΔH not strongly dependent on Y 2 O 3 content - Haven ratio no simple T-function - Fuel cell: Field, ageing H R := D O /D σT = f O /f σT

8. Thermochemie und Mikrokinetik und Mikrokinetik Martin Kilo, Institut für Metallurgie, TU Clausthal 18. 03. 2004 8 Fuel Cell Research Symposium ETH Zürich Switzerland 18/19.03.04 Methods: ml: mechanical loss DC: dielectrical conductivity D O *: self diffusion dl: dielectric loss Oxygen diffusion: Activation enthalpies and ageing Ageing: Preannealing decreases Oxygen diffusion coefficient for x(Y 2 O 3 ) 8mol% Tdiff=973K, Tpre: 1150 °C 1400 °C 1700 °C

9. Thermochemie und Mikrokinetik und Mikrokinetik Martin Kilo, Institut für Metallurgie, TU Clausthal 18. 03. 2004 9 Fuel Cell Research Symposium ETH Zürich Switzerland 18/19.03.04 Static oxygen diffusion: Summary Different experimental methods reveal different information Self diffusion Activation enthalpy of oxygen diffusion lowest Oxygen diffusion is dependant on thermal history Oxygen diffusion under working condition of SOFC ? Conductivity Conductivity nonlinear => association. What are the contributions of association and migration? Mechanical loss What is the difference between local and diffusive jumps?

10. Thermochemie und Mikrokinetik und Mikrokinetik Martin Kilo, Institut für Metallurgie, TU Clausthal 18. 03. 2004 10 Fuel Cell Research Symposium ETH Zürich Switzerland 18/19.03.04 Oxygen incorporation into SOFC electrolyte 20 µm ZrO 2 LSM 3PB 2PB Surface diffusion O ad,LSM O2O2 O 2- e - 300nm Three possible mechanisms: - 3 phase boundary (3PB) - electrode surface - through electrode + 2PB

11. Thermochemie und Mikrokinetik und Mikrokinetik Martin Kilo, Institut für Metallurgie, TU Clausthal 18. 03. 2004 11 Fuel Cell Research Symposium ETH Zürich Switzerland 18/19.03.04 Model system for SOFC electrode/electrolyte 100 μm LSM surface, dense, unstructured 200 μm LSM stripe(s) YSZ substrate LSM stripes 20 µm wide LSM layer ~ 300 nm thick PLD of LSM on YSZ at 800 °C

12. Thermochemie und Mikrokinetik und Mikrokinetik Martin Kilo, Institut für Metallurgie, TU Clausthal 18. 03. 2004 12 Fuel Cell Research Symposium ETH Zürich Switzerland 18/19.03.04 Experimental setup Pt ink reference electrode Pt contact Pt ink counter electrode LSM structured cathode YSZ

13. Thermochemie und Mikrokinetik und Mikrokinetik Martin Kilo, Institut für Metallurgie, TU Clausthal 18. 03. 2004 13 Fuel Cell Research Symposium ETH Zürich Switzerland 18/19.03.04 Oxygen exchange in cathode / YSZ FEM calculation of oxygen distribution after diffusion from a line source LSM YSZ Assumptions: Line source at the 2PB D YSZ >> D LSM k 1 at 2PB (LSM/YSZ) = k 2 at 2PB ( 18 O/LSM) = k 3 at 2PB ( 18 O/YSZ) = 0 FEMlab

14. Thermochemie und Mikrokinetik und Mikrokinetik Martin Kilo, Institut für Metallurgie, TU Clausthal 18. 03. 2004 14 Fuel Cell Research Symposium ETH Zürich Switzerland 18/19.03.04 Experimental results: LSM/YSZ-10 18 O Zr La 3PB activity 0 mV / 10 min -300 mV / 10 min -100 mV / 10 min

15. Thermochemie und Mikrokinetik und Mikrokinetik Martin Kilo, Institut für Metallurgie, TU Clausthal 18. 03. 2004 15 Fuel Cell Research Symposium ETH Zürich Switzerland 18/19.03.04 Depth profile analysis 100 μm LSM surface, dense, unstructured on the LSM stripe on the YSZ (LSM free area) Crater 200x200 µm Oxygen content under dense LSM, LSM stripe, free YSZ

16. Thermochemie und Mikrokinetik und Mikrokinetik Martin Kilo, Institut für Metallurgie, TU Clausthal 18. 03. 2004 16 Fuel Cell Research Symposium ETH Zürich Switzerland 18/19.03.04 18 O content: Variation of overpotential surface concentration bulk

17. Thermochemie und Mikrokinetik und Mikrokinetik Martin Kilo, Institut für Metallurgie, TU Clausthal 18. 03. 2004 17 Fuel Cell Research Symposium ETH Zürich Switzerland 18/19.03.04 Oxygen diffusion under field: Summary The bulk path seems to be very sensitive regarding the applied cathodic overpotential. Even at low cathodic overpotentials, the bulk path is blocking. The 3PB is more active at low cathodic overpotentials. The higher the cathodic overpotential, the more inactive becomes the 3PB. The solid/solid interface-resistance is clearly visible with SIMS and depends on the applied overpotential.

18. Thermochemie und Mikrokinetik und Mikrokinetik Martin Kilo, Institut für Metallurgie, TU Clausthal 18. 03. 2004 18 Fuel Cell Research Symposium ETH Zürich Switzerland 18/19.03.04 ZrO 2 : Modelling oxygen migration Migration energies, hopping energies, migration pathways… Association energies

19. Thermochemie und Mikrokinetik und Mikrokinetik Martin Kilo, Institut für Metallurgie, TU Clausthal 18. 03. 2004 19 Fuel Cell Research Symposium ETH Zürich Switzerland 18/19.03.04 Migration pathway from static lattice calculations - Single jump between two vacancies in undoped ZrO 2 : ΔE(O 2- ) < 0.2 eV - Equilibrium position of O 2- ion: (0.333,0.25,0.25) Code: GULP (J. Gale, London)

20. Thermochemie und Mikrokinetik und Mikrokinetik Martin Kilo, Institut für Metallurgie, TU Clausthal 18. 03. 2004 20 Fuel Cell Research Symposium ETH Zürich Switzerland 18/19.03.04 Association energy from supercell calculations E assoc. (x) = {E latt (x)-E latt (x=0)-x(E(V O 2 )+2*E(Y Zr ')}/x Supercells of 4×4×4 unit cells, varying Y/Zr content Association energy: difference between supercell lattice energy and perfect lattice energies

21. Thermochemie und Mikrokinetik und Mikrokinetik Martin Kilo, Institut für Metallurgie, TU Clausthal 18. 03. 2004 21 Fuel Cell Research Symposium ETH Zürich Switzerland 18/19.03.04 Summary static lattice calculations Results Low migration energy, high association energy Oxygen vacancies affects local oxygen surroundings Limitation of static lattice calculations Calculation of one single jump Assumption of a perfect or at least well-defined surrounding Temperature effects difficult to describe Molecular dynamics Information as function of temperature and time More realistic description of highly disordered systems Trajectory allows conclusions on jump mechanisms But: Slow diffusion difficult

22. Thermochemie und Mikrokinetik und Mikrokinetik Martin Kilo, Institut für Metallurgie, TU Clausthal 18. 03. 2004 22 Fuel Cell Research Symposium ETH Zürich Switzerland 18/19.03.04 Oxygen diffusion: Molecular dynamics on YSZ Jumps between one or two Y ions are less likely than between two Zr ions Restricted diffusion path for high dopant level Cubic unit cell

23. Thermochemie und Mikrokinetik und Mikrokinetik Martin Kilo, Institut für Metallurgie, TU Clausthal 18. 03. 2004 23 Fuel Cell Research Symposium ETH Zürich Switzerland 18/19.03.04 MD: Oxygen diffusion coefficient in YSZ Maximum in D similar to experimental point, but higher values of D Like experimental observed, ΔH independent of x(Y 2 O 3 ). At high x(Y 2 O 3 ), D independent of x(Y 2 O 3 ) MD exp

24. Thermochemie und Mikrokinetik und Mikrokinetik Martin Kilo, Institut für Metallurgie, TU Clausthal 18. 03. 2004 24 Fuel Cell Research Symposium ETH Zürich Switzerland 18/19.03.04 MD: Oxygen diffusion coefficient in LSGM-8282 Activation enthalpy close to the experimental values Diffusion goes along (110) direction

25. Thermochemie und Mikrokinetik und Mikrokinetik Martin Kilo, Institut für Metallurgie, TU Clausthal 18. 03. 2004 25 Fuel Cell Research Symposium ETH Zürich Switzerland 18/19.03.04 MD: Oxygen diffusion coefficient in ULSM Two activation enthalpies due to local hopping Sketch of migration pathway along (100); T = 1200K, 1250 ps green : oxygen pink, grey: La, Sr red: Mn

26. Thermochemie und Mikrokinetik und Mikrokinetik Martin Kilo, Institut für Metallurgie, TU Clausthal 18. 03. 2004 26 Fuel Cell Research Symposium ETH Zürich Switzerland 18/19.03.04 Summary of computer simulation results Static lattice calculations Migration energies too low Supercell method good estimation of association energies What are the limitations ? Molecular Dynamics calculations Diffusion coefficients similar to the experiment Activation enthalpies of O almost identical Percolation network ?

27. Thermochemie und Mikrokinetik und Mikrokinetik Martin Kilo, Institut für Metallurgie, TU Clausthal 18. 03. 2004 27 Fuel Cell Research Symposium ETH Zürich Switzerland 18/19.03.04 Conclusions I: Experimental results Static oxygen diffusion experiments Activation enthalpy of oxygen diffusion lowest Oxygen diffusion is dependant on thermal history Oxygen diffusion under SOFC conditions Even at low cathodic overpotentials, the bulk path is blocking The 3PB is less active at high cathodic overpotential How are the diffusivities affected ?

28. Thermochemie und Mikrokinetik und Mikrokinetik Martin Kilo, Institut für Metallurgie, TU Clausthal 18. 03. 2004 28 Fuel Cell Research Symposium ETH Zürich Switzerland 18/19.03.04 Conclusions II: Modelling results Static lattice calculations O/ZrO 2 Estimate of association energies using supercells Molecular dynamics on YSZ Diffusion coefficients and activation energies are close to the experimental values Existence of percolation pathways? Molecular dynamics on LSGM, ULSM Oxygen migration only along (110) Localised jumps according to the cation surrounding of A- and B-sublattices

29. Thermochemie und Mikrokinetik und Mikrokinetik Martin Kilo, Institut für Metallurgie, TU Clausthal 18. 03. 2004 29 Fuel Cell Research Symposium ETH Zürich Switzerland 18/19.03.04 Future perspectives Dynamic oxygen diffusion YSZ/ULSM : Variation of time, polarisation, p(O 2 ) Variation of the cathode material Oxygen exchange coefficient at the solid/solid interface? Anode/Electrolyte : Hydrogen Computer simulations Atomistic modelling of oxygen transport across interfaces solid/solid and gas/solid Modelling of oxygen transport under electrical field Other materials: LSCF, Apatites Advanced methods: QM, finite elements …

30. Thermochemie und Mikrokinetik und Mikrokinetik Martin Kilo, Institut für Metallurgie, TU Clausthal 18. 03. 2004 30 Fuel Cell Research Symposium ETH Zürich Switzerland 18/19.03.04 Acknowledgements M. Weller, MPI Stuttgart: Experimental results Prof. P. Schmidt, TU Darmstadt: Use of computer centre Deutsche Forschungsgemeinschaft (DFG): Financial support