Solid Oxide Fuel Cells (SOFCs)

1. Tunability of Atomic Layer Deposition of Yttrium-Stabilized Zirconium Oxide for use in Solid Oxide Fuel Cells Anna WalterUniversity of Delaware, Newark, DelawareRunshenXu, Christos G. Takoudis, and Gregory JursichUniversity of Illinois at Chicago, Chicago, Illinois Advanced Materials Research Laboratory (AMReL)

2. Solid Oxide Fuel Cells (SOFCs) • Fuel cells can replace many of our current methods of obtaining energy[1] • SOFCs are more efficient and stable than other types of fuel cells[2] • High operating temperatures - electrolyte can conduct oxygen ions[2] • Lower temperaturesdesired[1]– must reduce electrolyte resistance • Must use different materials or make a thinner electrolyte[1] [1] Brett, Daniel. Chem. Soc. Rev. Vol. 37, 1568-1578, 2008. [2] Jacobson, Allan. Chem. Mater. Vol. 22, 660-674 , 2009.

3. Atomic Layer Deposition (ALD) • ALD: alternating gas-phase reactants (precursors) to create thin film of solid product (can be a molecule) • One ALD cycle:

4. ALD Advantages • Self-limiting process – surface area is limiting factor in the reaction • Capable of depositing very thin, highly conformal films; good thickness control[3] • Less than one monolayer is deposited per cycle[4] - steric hindrance • Composition of composite films can be controlled by changing the number of cycles used to deposit each component[3] [3] George, Steven. Chem. Rev. Vol. 110, 111-131, 2011 [4] Sherman, Arthur. ALD for Nanotech. Ivoryton Press, Ivoryton CT, 2008

5. Project Goals • We are investigating: • use of new ALD precursors in making yttrium-stabilized zirconium oxide (YSZ) • tunability of this process - how composition of film is a function of cycle ratio • New precursors hopefully leave no impurities – thin films • Tunability important for composition-dependent uses such as SOFCs – 8% yttria is ideal[5] [2] Jacobson, Allan. Chem. Mater. Vol. 22, 660-674 , 2009. [5] Pomfret, Michael, et. al. Anal. Chem. Vol. 77, 1791-1795, 2005.

6. Materials • Oxygen source: ozone • Yttrium source: tris(isopropyl-cyclopentadienyl)yttrium • Zirconium source: tris(dimethylamino) cyclopentadienylzirconium

7. Custom-Built Hot-Wall ALD Reactor • Y pulse: 8 s • Zr pulse: 5 s • purge: 10 s • O3 pulse: 1 s

8. X-ray photoelectron spectroscopy (XPS) Determine composition Each element has a characteristic binding energy (eV) XPS measures the energy of the electrons released from the sample Number of electrons – intensity - related to amount of element present [6] http://www.rrc.uic.edu/instruments/12

9. XPS Data [7] Scherrer, Barbara, et. al. J. of Power Sources. Vol. 196, 7372-7382, 2011

10. ALD Tunability

11. Zirconium Composition ZrO2 Cycle ratio = yttria cycles : zirconia cycles

12. Yttrium Composition YOH Y2O3 Cycle ratio = yttria cycles : zirconia cycles

13. Summary • YSZ was successfully deposited by ALD of these precursors • This ALD process is highly tunable – linear relationship • Decreasing number yttria cycles compared to zirconia cycles (decreasing cycle ratio): • Decreases relative amount of yttria / increases relative amount of zirconia • Decreases amount of surface YOH impurity

14. Questions? Acknowledgements: • Financial support from the National Science Foundation, EEC-NSF Grant # 1062943 • Additional financial support from the National Science Foundation, CBET-NSF Grant # 1067424 • Other AMReL members.