nanocrystalline super-ionic conductors for solid oxide fuel cells daniel strickland (seattle...
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Nanocrystalline Super-Ionic Conductors for
Solid Oxide Fuel Cells
Daniel Strickland(Seattle University)
University of California – IrvineMaterial Science and Engineering
Mentor: Professor Martha L. MecartneyGraduate Student: Sungrok Bang
Collaborator: Jeremy Roth
Support from NSF REU program UCI IM-SURE
Introduction to SOFC
• Basic fuel cell operation• Cathode Reaction
• Anode Reactions
Taken from fuelcellworks.com
Daniel Strickland IM-SURE July 27, 2005
22 24 OeO
eHH 222
OHOH 22 224
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.
Electrolyte Material Challenges
• Implementation Challenges
– Operational costs are significantly increased
– Potential applications are limited
Ionic conductance
• SOFC operating temp can be reduced by increasing ionic conductance
• Two ways to increase:– Increase ionic conductivity– Decrease ion travel distance
Increasing Ionic Conductivity
• Doped zirconia used as electrolyte material (Scandium and Yttrium used)
• Zirconia grain structure:
Increasing Ionic Conductivity
• Traditional theory:– High ionic conductivity through grain
interior– Low ionic conductivity through grain
boundaries
• Increase grain size to increase overall conductivity
Decreasing Ion Travel Distance
• Ion travel distance reduced by decreasing electrolyte thickness
• Thin film fabrication techniques employed to create electrolytes of sub-micron thickness
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.
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.
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 filmC.D. Baertsch et al, Journal of Materials Research, 19, 2604-2615 (2004)
Fabrication ProcessZirconium propoxide
Zr(OC3H7)4
Isopropanol(dilutant)
Yttrium isopropoxideScandium isopropoxide
0.05-0.25 M Solution
Add 70% Nitric30% H2O (hydrolysis)
Spin-coat(silicon wafer)
DryT = 130º C
PyrolyzeT = 420º C
CrystallizeT = 520ºC
SEM X-Ray Diffraction Impedance Spectroscopy
DSC/TGA(Optimize Heating Regime)
Multiple
Finding optimized condition
• Parameters involved:
– Solution viscosity
– Spin speed and time
– Heating regime
Viscosity
• Three factors influence viscosity:
– Reaction rate: Hydrolysis• Process where H2O breaks organics off of
propoxides
– Reaction Time
– Solution concentration
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
Sol-gel concentration
0.05 M 0.10 M
0.15 M 0.30 M
Heating regime
Nano-Cracks Delamination
Heating regime4Y-4Sc DSC/TGA
50%
55%
60%
65%
70%
75%
80%
85%
90%
95%
100%
0 100 200 300 400 500 600
Wei
ght (
%)
<--exothermic endotherm
ic-->
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
Optimized thin Films
Optimized thin Films
X-Ray Diffraction Studies
• Confirm crystalline zirconia thin film
• Calculate grain size
• Calculate lattice parameters
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
sin2shiftphase d
XRD: Confirm Crystalline Zirconia8 Y XRD
8 Y Thin Film 74.0756260.12550.725
35.15
30.425
30.2
34.98
50.26
59.76
62.66 73.86
8 Y Powder
73.72562.7
59.675
50.2
34.95
30.125
8 Y Bulk
20 25 30 35 40 45 50 55 60 65 70 75 80
2-Theta
XRD: Calculate grain size
• Used integral breadth formula:
• Some interesting trends:– Dopants
influenced grain size
– Heating to 700 C did not induce grain growth
22
2
16sintan
2
tan
)2(e
L
K
500 C 700 C
8YSZ 17 nm 17 nm
4YSZ 18 nm
4Y-4Sc 20 nm
8ScSZ 21 nm
4Sc 22 nm
XRD: Lattice parameters
• Each peak corresponds to a plane of atoms• Crystal structure unit cube length can be
calculated:
Ǻ 4YSZ 8YSZ 4ScSZ 8ScSZ 4Y-4ScThin Film 5.091 5.096 5.055 5.054 5.075
Sol-Gel Powder
5.113 5.128 5.086 5.081 5.101
222 lkhda hkl
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
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
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
Thank You!
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