nanocrystalline super-ionic conductors for solid oxide fuel cells daniel strickland (seattle...

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!