Nanocrystalline Super-Ionic Conductors for Solid Oxide Fuel Cells

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 thicknessess2: • 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 concentration3 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 Atomic Force Microscope image of YSZ thin film C.D. Baertsch et al, Journal of Materials Research, 19, 2604-2615 (2004) Daniel Strickland IM-SURE July 27, 2005

12. Zirconium propoxideZr(OC3H7)4 Isopropanol(dilutant) Yttrium isopropoxideScandium isopropoxide 0.05-0.25 M Solution Add 70% Nitric30% H2O (hydrolysis) DryT = 130º C PyrolyzeT = 420º C DSC/TGA(Optimize Heating Regime) Spin-coat(silicon wafer) CrystallizeT = 520ºC SEM X-Ray Diffraction Impedance Spectroscopy Fabrication Process 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 H2O 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 0.10 M 0.05 M 0.15 M 0.30 M

17. Heating regime Nano-Cracks Delamination

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

21. Optimized thin Films

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

23. Taken from Callister 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

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

26. XRD: Lattice parameters • Each peak corresponds to a plane of atoms • Crystal structure unit cube length can be calculated:

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!