1. Nanocrystalline Super-Ionic Conductors for Solid Oxide Fuel Cells Daniel Strickland (Seattle University) University of California – Irvine Material Science and Engineering Mentor: Professor Martha L. Mecartney Graduate Student: Sungrok Bang Collaborator: Jeremy Roth Support from NSF REU program UCI IM-SURE

2. Introduction to SOFC Basic fuel cell operation Cathode Reaction Anode Reactions Taken from fuelcellworks.com Daniel Strickland IM-SURE July 27, 2005

3. Electrolyte Material Challenges Operating Temperature Design Challenges –Current materials require high operating T > 800 ºC –Sacrifice long-term stability and encourage material degradation –Similar thermal expansion coefficients –High chemical compatibility K. Sundmacher, L.K. Rihko-Struckmann and V. Galvita, Solid electrolyte membrane reactors: Status and trends, Catalysis Today, Volume 104, Issues 2-4, 30 June 2005, Pages 185-199.

4. Electrolyte Material Challenges Implementation Challenges –Operational costs are significantly increased –Potential applications are limited

5. Ionic conductance SOFC operating temp can be reduced by increasing ionic conductance Two ways to increase: –Increase ionic conductivity –Decrease ion travel distance

6. Increasing Ionic Conductivity Doped zirconia used as electrolyte material (Scandium and Yttrium used) Zirconia grain structure:

7. Increasing Ionic Conductivity Traditional theory: –High ionic conductivity through grain interior –Low ionic conductivity through grain boundaries Increase grain size to increase overall conductivity

8. Decreasing Ion Travel Distance Ion travel distance reduced by decreasing electrolyte thickness Thin film fabrication techniques employed to create electrolytes of sub-micron thickness

9. How to improve overall conductance? Nanocrystalline grain microstructure required for sub-micron thicknessess 2 : –Prevent pinholes –Must be gas-tight It appears as if ionic conductivity must be sacrificed to decrease ion travel distance 2. B.P. Gorman, V. Petrovsky, H.U. Anderson, and T. Petrovsky (2004), “Optical Characterization of Ceramic Thin Films: Applications in Low-Temperature Solid Oxide Fuel-Cell Materials Research,” Journal of Materials Research, 19, 573-578.

10. A potential solution Possible grain boundary conductivity improvements at nano-scale! Other factors may begin to dominate: –Decreased impurity concentration 3 3. H.L. Tuller (2000), “Ionic Conduction in Nanocrystalline Materials,” Solid State Ionics, 131, 143-157.

11. Goal of Research Fabricate yittria stabilized and scandia stabilized zirconia nanocrystalline thin films Characterize microstructure and ionic conductivity Daniel Strickland IM-SURE July 27, 2005 Atomic Force Microscope image of YSZ thin film C.D. Baertsch et al, Journal of Materials Research, 19, 2604-2615 (2004)

12. Fabrication Process Zirconium propoxide Zr(OC 3 H 7 ) 4 Isopropanol (dilutant) Yttrium isopropoxide Scandium isopropoxide 0.05-0.25 M Solution Add 70% Nitric 30% H 2 O (hydrolysis) Spin-coat (silicon wafer) Dry T = 130º C Pyrolyze T = 420º C Crystallize T = 520ºC SEMX-Ray DiffractionImpedance Spectroscopy DSC/TGA (Optimize Heating Regime) Multiple

13. Finding optimized condition Parameters involved: –Solution viscosity –Spin speed and time –Heating regime

14. Viscosity Three factors influence viscosity: –Reaction rate: Hydrolysis Process where H 2 O breaks organics off of propoxides –Reaction Time –Solution concentration

15. Reaction time and concentration Viscosity was assumed constant for initial 48 hours Viscosity linearly dependant of sol- gel concentration Concentration varied from.05M to.30M to find optimized condition

16. Sol-gel concentration 0.05 M 0.10 M 0.15 M 0.30 M

17. Heating regime Nano-CracksDelamination

18. Heating regime

19. Optimized Fabrication Conditions.05 M solution.9:1 water to propoxide molar ratio Spin coating at 2000 rpm, for 30 sec Heat treatment between each coat: –3 ºC/min to 130 ºC –Hold 30 min –2 ºC/min to 520 ºC –Hold 60 min Coat up to 8 layers

20. Optimized thin Films

22. X-Ray Diffraction Studies Confirm crystalline zirconia thin film Calculate grain size Calculate lattice parameters

23. X-Ray Diffraction Studies How XRD works: –Incident X-Rays in phase –Phase shift function of plane spacing and incident angle: –Phase shift = multiple of wavelength, beams react constructively –Detected X-ray intensity peaks Taken from Callister

24. XRD: Confirm Crystalline Zirconia

25. XRD: Calculate grain size Used integral breadth formula: Some interesting trends: –Dopants influenced grain size –Heating to 700 C did not induce grain growth 500 C700 C 8YSZ17 nm 4YSZ18 nm 4Y-4Sc20 nm 8ScSZ21 nm 4Sc22 nm

26. XRD: Lattice parameters Each peak corresponds to a plane of atoms Crystal structure unit cube length can be calculated: Ǻ4YSZ8YSZ4ScSZ8ScSZ4Y-4Sc Thin Film 5.0915.0965.0555.0545.075 Sol-Gel Powder 5.1135.1285.0865.0815.101

27. Impedance Spectroscopy (IS) IS needs to be performed to quantify ionic conductivity Substrate conditions: –Not an ionic conductor –Not and electronic conductor –Smooth surface –Mechanically strong Need silver paint for electrodes

28. Conclusions We can fabricate high quality, 1 μ thin films –Crack free –Highly dense Correlation found between dopants and grain size Lattice parameter for thin film is smaller than that of powder or bulk material Thin films are ready for impedance spectroscopy

29. Acknowledgements Mentor: Prof. Martha L. Mecartney Graduate Students: Sungrok Bang Tiandan Chen Collaboration: Jeremy Roth IM-SURE Program: Said Shokair University of California – Irvine National Science Foundation Daniel Strickland IM-SURE July 27, 2005

30. Thank You!