Thermochemie und Mikrokinetik Thermochemistry and microkinetics Experimental Investigation

Thermochemie und Mikrokinetik Thermochemistry and microkinetics Experimental Investigation and Simulation of Oxygen Transport in SOFC Materials Fuel Cell Research Symposium ETH Zürich Switzerland 18/19. 03. 04 1. Motivation and systems: Zr. O 2, La. Ga. O 3, La. Mn. O 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 ST 5 5 GB / UK Martin Kilo, Institut für Metallurgie, TU Clausthal 18. 03. 2004 1

Motivation: Oxygen mixed and ionic conductors Thermochemie und Mikrokinetik Ø Most common examples: doped Zr. O 2 or doped perovskites, e. g. (La 0. 8 Sr 0. 2)(Ga 0. 8 Mg 0. 2)O 3 -δ, LSGM, Lax. Sr 1 -x. Mn. O 3 -δ, LSM Ø Doping with aliovalent cations leads to fast oxygen diffusion, but usually to slow cation diffusion n n = 1: YZr 1' n=1 x A: 0 < x' B: x > x' A Ea A σT x n = 2: Ca. Zr 2' Fuel Cell Research Symposium ETH Zürich Switzerland 18/19. 03. 04 n=2 A Ea Ea σT σT x x s. T = A(x) exp(-Ea(x) / RT) Martin Kilo, Institut für Metallurgie, TU Clausthal x' 0. 08 – 0. 12 18. 03. 2004 2

Open questions Thermochemie und Mikrokinetik Experimental Ø What is the practical connection between the experimental oxygen diffusion coefficient and conductivity? Ø Oxygen diffusion under applied electrical field Ø Influence of thermal ageing on oxygen diffusion Simulation of oxygen diffusion Ø Static lattice: Mechanism of transport Ø Molecular dynamics: Transport coefficients Fuel Cell Research Symposium Ø Finite element modelling: Simulation of real systems ETH Zürich Switzerland 18/19. 03. 04 Martin Kilo, Institut für Metallurgie, TU Clausthal 18. 03. 2004 3

Gaseous tracer: 16 O / 18 O Thermochemie und Mikrokinetik 18 O 16 O Sample Furnace: T, p, static gas tracer Nat. : 0. 2 % 18 O Tracer: >90% 18 O Fuel Cell Research Symposium ETH Zürich Switzerland 18/19. 03. 04 Surface limited: x(18 O, x=0) < 90 % c(x, t)-c 0=(cs-c 0)·(erf(x/2(DOt)0. 5)exp(h·x+h 2·DOt)·erf(x/2(DOt)0. 5+h·(DOt)0. 5)) Martin Kilo, Institut für Metallurgie, TU Clausthal 18. 03. 2004 4

Isotopic lateral and depth distribution: SIMS Analysis Thermochemie und Mikrokinetik Fuel Cell Research Symposium ETH Zürich Switzerland 18/19. 03. 04 VG SIMS-lab: Quadrupole detection; 7 k. V Ar+ Detection of positively/negatively charged ions Charge compensation with flood gun Cameca 3 f/5 f: Magnetic sector field; >10 k. V O+/Detection of positively charged ions Charge compensation by conducting layer Martin Kilo, Institut für Metallurgie, TU Clausthal 18. 03. 2004 5

Zr. O 2 -systems CSZ, Sc. SZ, YSZ: working conditions Thermochemie und Mikrokinetik Fuel Cell Research Symposium ETH Zürich Switzerland 18/19. 03. 04 Ca. O-Zr. O 2 Sc 2 O 3 -Zr. O 2 Y 2 O 3 -Zr. O 2 (Duran, J. Mat. Sci. 22 1987 4348) (Ruh, J. Am. Ceram. Soc. 60 1977 399) (Suzuki, SSI 81 1995 211) Working regions: Cation diffusion Oxygen diffusion (red line: T = 1000 °C) Mostly single crystals All Zr. O 2 -systems have a cubic part of the phase diagram with fast oxygen transport and slow cation transport Martin Kilo, Institut für Metallurgie, TU Clausthal 18. 03. 2004 6

Oxygen transport: Self diffusion Thermochemie und Mikrokinetik Fuel Cell Research Symposium ETH Zürich Switzerland 18/19. 03. 04 HR : = DO/DσT = f. O/fσT 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 Martin Kilo, Institut für Metallurgie, TU Clausthal 18. 03. 2004 7

Oxygen diffusion: Activation enthalpies and ageing Thermochemie und Mikrokinetik Tdiff=973 K, Tpre: 1150 °C 1400 °C 1700 °C Fuel Cell Research Symposium ETH Zürich Switzerland 18/19. 03. 04 Methods: ml : mechanical loss DC : dielectrical conductivity DO * : self diffusion dl : dielectric loss Martin Kilo, Institut für Metallurgie, TU Clausthal Ageing: Preannealing decreases Oxygen diffusion coefficient for x(Y 2 O 3) ≠ 8 mol% 18. 03. 2004 8

Static oxygen diffusion: Summary Thermochemie und Mikrokinetik Ø Different experimental methods reveal different information Self diffusion Ø Activation enthalpy of oxygen diffusion lowest Ø Oxygen diffusion is dependant on thermal history Ø Oxygen diffusion under working condition of SOFC ? Conductivity Ø Conductivity nonlinear => association. What are the contributions of association and migration? Fuel Cell Research Symposium ETH Zürich Switzerland 18/19. 03. 04 Mechanical loss Ø What is the difference between local and diffusive jumps? Martin Kilo, Institut für Metallurgie, TU Clausthal 18. 03. 2004 9

Oxygen incorporation into SOFC electrolyte Thermochemie und Mikrokinetik Three possible mechanisms: - 3 phase boundary (3 PB) - electrode surface - through electrode + 2 PB O 2 Oad, LSM O 2 e- Surface diffusion LSM 300 nm 2 PB 3 PB Fuel Cell Research Symposium ETH Zürich Switzerland 18/19. 03. 04 Zr. O 2 20 µm Martin Kilo, Institut für Metallurgie, TU Clausthal 20 µm 18. 03. 2004 10

Model system for SOFC electrode/electrolyte Thermochemie und Mikrokinetik PLD of LSM on YSZ at 800 °C LSM surface, dense, unstructured LSM stripe(s) YSZ substrate 100 μm 200 μm Fuel Cell Research Symposium ETH Zürich Switzerland 18/19. 03. 04 LSM stripes 20 µm wide LSM layer ~ 300 nm thick Martin Kilo, Institut für Metallurgie, TU Clausthal 18. 03. 2004 11

Experimental setup Thermochemie und Mikrokinetik Pt contact LSM structured cathode YSZ Pt ink counter electrode Pt ink reference electrode Fuel Cell Research Symposium ETH Zürich Switzerland 18/19. 03. 04 Martin Kilo, Institut für Metallurgie, TU Clausthal 18. 03. 2004 12

Oxygen exchange in cathode / YSZ Thermochemie und Mikrokinetik FEM calculation of oxygen distribution after diffusion from a line source FEMlab LSM YSZ Fuel Cell Research Symposium ETH Zürich Switzerland 18/19. 03. 04 Assumptions: Line source at the 2 PB DYSZ >> DLSM k 1 at 2 PB (LSM/YSZ) = k 2 at 2 PB (18 O/LSM) = k 3 at 2 PB (18 O/YSZ) = 0 Martin Kilo, Institut für Metallurgie, TU Clausthal 18. 03. 2004 13

Experimental results: LSM/YSZ-10 Thermochemie und Mikrokinetik 0 m. V / 10 min Fuel Cell Research Symposium ETH Zürich Switzerland 18/19. 03. 04 -100 m. V / 10 min 18 O -300 m. V / 10 min 3 PB activity Zr La Martin Kilo, Institut für Metallurgie, TU Clausthal 18. 03. 2004 14

Depth profile analysis Thermochemie und Mikrokinetik Oxygen content under dense LSM, LSM stripe, free YSZ LSM surface, dense, unstructured Crater 200 x 200 µm Fuel Cell Research Symposium ETH Zürich Switzerland 18/19. 03. 04 100 μm on the LSM stripe Martin Kilo, Institut für Metallurgie, TU Clausthal on the YSZ (LSM free area) 18. 03. 2004 15

18 O content: Variation of overpotential Thermochemie und Mikrokinetik bulk Fuel Cell Research Symposium ETH Zürich Switzerland 18/19. 03. 04 surface concentration Martin Kilo, Institut für Metallurgie, TU Clausthal 18. 03. 2004 16

Oxygen diffusion under field: Summary Thermochemie und Mikrokinetik Ø The bulk path seems to be very sensitive regarding the applied cathodic overpotential. Even at low cathodic overpotentials, the bulk path is blocking. Ø The 3 PB is more active at low cathodic overpotentials. The higher the cathodic overpotential, the more inactive becomes the 3 PB. Ø The solid/solid interface-resistance is clearly visible with SIMS and depends on the applied overpotential. Fuel Cell Research Symposium ETH Zürich Switzerland 18/19. 03. 04 Martin Kilo, Institut für Metallurgie, TU Clausthal 18. 03. 2004 17

Zr. O 2: Modelling oxygen migration Thermochemie und Mikrokinetik Ø Migration energies, hopping energies, migration pathways… Ø Association energies Fuel Cell Research Symposium ETH Zürich Switzerland 18/19. 03. 04 Martin Kilo, Institut für Metallurgie, TU Clausthal 18. 03. 2004 18

Migration pathway from static lattice calculations Thermochemie und Mikrokinetik Code: GULP (J. Gale, London) - Single jump between two vacancies in undoped Zr. O 2: Fuel Cell Research Symposium ETH Zürich Switzerland 18/19. 03. 04 ΔE(O 2 -) < 0. 2 e. V - Equilibrium position of O 2 - ion: (0. 333, 0. 25) Martin Kilo, Institut für Metallurgie, TU Clausthal 18. 03. 2004 19

Association energy from supercell calculations Thermochemie und Mikrokinetik Eassoc. (x) = {Elatt(x)-Elatt(x=0)-x • (E(VO 2 • )+2*E(YZr')}/x Fuel Cell Research Symposium ETH Zürich Switzerland 18/19. 03. 04 Supercells of 4× 4× 4 unit cells, varying Y/Zr content Association energy: difference between supercell lattice energy and perfect lattice energies Martin Kilo, Institut für Metallurgie, TU Clausthal 18. 03. 2004 20

Summary static lattice calculations Thermochemie und Mikrokinetik Results Ø Low migration energy, high association energy Ø Oxygen vacancies affects local oxygen surroundings Limitation of static lattice calculations Ø Calculation of one single jump Ø Assumption of a perfect or at least well-defined surrounding Ø Temperature effects difficult to describe Molecular dynamics Fuel Cell Research Symposium ETH Zürich Switzerland 18/19. 03. 04 Ø Information as function of temperature and time Ø More realistic description of highly disordered systems Ø Trajectory allows conclusions on jump mechanisms Ø But: Slow diffusion difficult Martin Kilo, Institut für Metallurgie, TU Clausthal 18. 03. 2004 21

Oxygen diffusion: Molecular dynamics on YSZ Thermochemie und Mikrokinetik Cubic unit cell Fuel Cell Research Symposium ETH Zürich Switzerland 18/19. 03. 04 Jumps between one or two Y ions are less likely than between two Zr ions Restricted diffusion path for high dopant level Martin Kilo, Institut für Metallurgie, TU Clausthal 18. 03. 2004 22

MD: Oxygen diffusion coefficient in YSZ Thermochemie und Mikrokinetik MD exp Ø Maximum in D similar to experimental point, but higher values of D Fuel Cell Research Symposium ETH Zürich Switzerland 18/19. 03. 04 Ø Like experimental observed, ΔH independent of x(Y 2 O 3). Ø At high x(Y 2 O 3), D independent of x(Y 2 O 3) Martin Kilo, Institut für Metallurgie, TU Clausthal 18. 03. 2004 23

MD: Oxygen diffusion coefficient in LSGM-8282 Thermochemie und Mikrokinetik Fuel Cell Research Symposium ETH Zürich Switzerland 18/19. 03. 04 Activation enthalpy close to the experimental values Diffusion goes along (110) direction Martin Kilo, Institut für Metallurgie, TU Clausthal 18. 03. 2004 24

MD: Oxygen diffusion coefficient in ULSM Thermochemie und Mikrokinetik Two activation enthalpies due to local hopping Fuel Cell Research Symposium Sketch of migration pathway along (100); T = 1200 K, 1250 ps green : oxygen pink, grey : La, Sr red : Mn ETH Zürich Switzerland 18/19. 03. 04 Martin Kilo, Institut für Metallurgie, TU Clausthal 18. 03. 2004 25

Summary of computer simulation results Thermochemie und Mikrokinetik Static lattice calculations Ø Migration energies too low Ø Supercell method good estimation of association energies What are the limitations ? Molecular Dynamics calculations Ø Diffusion coefficients similar to the experiment Ø Activation enthalpies of O almost identical Fuel Cell Research Symposium Percolation network ? ETH Zürich Switzerland 18/19. 03. 04 Martin Kilo, Institut für Metallurgie, TU Clausthal 18. 03. 2004 26

Conclusions I: Experimental results Thermochemie und Mikrokinetik Static oxygen diffusion experiments Ø Activation enthalpy of oxygen diffusion lowest Ø Oxygen diffusion is dependant on thermal history Oxygen diffusion under SOFC conditions Ø Even at low cathodic overpotentials, the bulk path is blocking Ø The 3 PB is less active at high cathodic overpotential Fuel Cell Research Symposium How are the diffusivities affected ? ETH Zürich Switzerland 18/19. 03. 04 Martin Kilo, Institut für Metallurgie, TU Clausthal 18. 03. 2004 27

Conclusions II: Modelling results Thermochemie und Mikrokinetik Static lattice calculations O/Zr. O 2 Ø Estimate of association energies using supercells Molecular dynamics on YSZ Ø Diffusion coefficients and activation energies are close to the experimental values Ø Existence of percolation pathways? Molecular dynamics on LSGM, ULSM Ø Oxygen migration only along (110) Fuel Cell Research Symposium ETH Zürich Switzerland 18/19. 03. 04 Ø Localised jumps according to the cation surrounding of A- and B-sublattices Martin Kilo, Institut für Metallurgie, TU Clausthal 18. 03. 2004 28

Future perspectives Thermochemie und Mikrokinetik 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 Fuel Cell Research Symposium ETH Zürich Switzerland 18/19. 03. 04 • 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 … Martin Kilo, Institut für Metallurgie, TU Clausthal 18. 03. 2004 29

Acknowledgements Thermochemie und Mikrokinetik Ø M. Weller, MPI Stuttgart: Experimental results Ø Prof. P. Schmidt, TU Darmstadt: Use of computer centre Ø Deutsche Forschungsgemeinschaft (DFG): Financial support Fuel Cell Research Symposium ETH Zürich Switzerland 18/19. 03. 04 Martin Kilo, Institut für Metallurgie, TU Clausthal 18. 03. 2004 30