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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.

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nanocrystalline super ionic conductors for solid oxide fuel cells

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

introduction to sofc
Introduction to SOFC
  • Basic fuel cell operation
  • Cathode Reaction
  • Anode Reactions

Taken from fuelcellworks.com

Daniel Strickland IM-SURE July 27, 2005

electrolyte material challenges
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 challenges4
Electrolyte Material Challenges
  • Implementation Challenges
    • Operational costs are significantly increased
    • Potential applications are limited
ionic conductance
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
Increasing Ionic Conductivity
  • Doped zirconia used as electrolyte material (Scandium and Yttrium used)
  • Zirconia grain structure:
increasing ionic conductivity7
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
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
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
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
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

fabrication process

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

finding optimized condition
Finding optimized condition
  • Parameters involved:
    • Solution viscosity
    • Spin speed and time
    • Heating regime
viscosity
Viscosity
  • Three factors influence viscosity:
    • Reaction rate: Hydrolysis
      • Process where H2O breaks organics off of propoxides
    • Reaction Time
    • Solution concentration
reaction time and 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
Sol-gel concentration

0.10 M

0.05 M

0.15 M

0.30 M

heating regime
Heating regime

Nano-Cracks

Delamination

optimized fabrication conditions
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
x ray diffraction studies
X-Ray Diffraction Studies
  • Confirm crystalline zirconia thin film
  • Calculate grain size
  • Calculate lattice parameters
x ray diffraction studies23

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
xrd calculate grain size
XRD: Calculate grain size
  • Used integral breadth formula:
  • Some interesting trends:
    • Dopants influenced grain size
    • Heating to 700 C did not induce grain growth
xrd lattice parameters
XRD: Lattice parameters
  • Each peak corresponds to a plane of atoms
  • Crystal structure unit cube length can be calculated:
impedance spectroscopy is
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
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
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