Atomic Layer Deposition of Cerium Oxide for Solid Oxide Fuel Cells

Rachel Essex, Rose-Hulman Institute of Technology Jorge Ivan Rossero Agudelo, Christos G. Takoudis, Gregory Jursich University of Illinois at Chicago 1

Benefits of Solid Oxide Fuel Cells as Alternate Power Source      No NO x , SO x , or hydrocarbon emissions Reduced CO 2 emissions Fuel flexibility Higher power density than batteries High efficiency R.M., Ormerod: Chemical Society Reviews, 2003, 32, 17-28.

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How a Solid Oxide Fuel Cell Works  Solid oxide fuel cells components: ◦ Cathode ◦ Solid inorganic oxide electrolyte ◦ Anode O 2 (air) Cathode O -2 H 2 and CO e CO 2 and H 2 O Electrolyte Anode Fuel (hydrocarbon and steam or oxygen) R.M., Ormerod: Chemical Society Reviews, 2003, 32, 17-28.

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The biggest setback for solid oxide fuel cell use is the high operating temperature    Operating temperature: 800-1000 ºC Long heat up and cool down periods Limited materials M. Cassir and E. Gourba: Annales de Chimie Science des Matériaux, 2001, 26, 49-58.

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Decreasing Operating Temperatures    New materials with lower ion resistivity Decreasing thickness can increase ion permeability Thickness can be decreased using thin films M. Cassir and E. Gourba: Annales de Chimie Science des Matériaux, 2001, 26, 49-58.

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Deposition of thin films  Physical vapor deposition -thin film deposition method by the condensation of a vaporized form a desired material onto surface o Purely physical process o High temperature vacuum evaporation or plasma sputter bombardment 6

Deposition of thin film (con’t)

 Chemical vapor deposition -chemical process used to produce high-purity, high performance solid materials ◦ Metal organic chemical vapor deposition (MOCVD) ◦ Atomic Layer Deposition (ALD) 7

Atomic Layer Deposition

   Each exposure to precursor saturates the surface with a monolayer Purge of inert gas in-between precursor exposures Each cycle creates one monolayer S.M. George: Chem. Rev., 2010, 110, 111-131 8

Atomic Layer Deposition is a cyclic process consisting of four steps Step One: Substrate is exposed to precursor Step Two: Reactor is purged of first precursor substrate substrate 9

Step Three: Substrate is exposed to coreactant Step Four: Reactor is purged of coreactant and byproducts substrate substrate

Process is repeated until the film is at the desired thickness

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Cerium oxide was created using atomic layer deposition      Precursor: tris(i-propylcyclopentadienyl)cerium Coreactant/Oxidizer: water Purge and Carrier Gas: Nitrogen Uses in solid oxide fuel cells: anode and electrolyte Cerium oxide has lower ion resistivity at lower temperatures than yttrium stabilized zirconium 11

Goals of This Project

 ◦ ◦ ◦ Find optimum ALD conditions including: ◦ Precursor Temperature Oxidizer Pulse Length ALD window Saturation Curve ◦ Linear Growth 12

ALD Operating Conditions

T Reactor 160 ºC 170 mTorr Plug: short time pulse of precursor 150 ºC 140 ºC 130 ºC Q. Tao, Ph.D. Thesis, University of Illinois at Chicago, 2011 13

Precursor Temperature of 140 ºC 5,0 4,5 4,0 3,5 3,0 2,5 2,0 1,5 1,0 0,5 0,0 225 230 235 240 245

Reactor Temperature, °C

250 255 14

50 ms Water Pulse

0,9 0,8 0,7 0,6 0,5 0,4 0,3 0,2 0,1 0,0 0 2 4 6

Number of Plugs

8 10 15

ALD Window

2,3 2,0 1,8 1,5 1,3 1,0 0,8 0,5 0,3 0,0 180 200 220 240 260

Temperature, °C

280 300 Conditions: 170 mTorr, 130 °C Precursor Temperature, 140 °C Valve Temperature, 150 °C Leg Temperature, 160 °C Manifold Temperature, 50 Cycles, 55 ms Water Pulse, 6 plugs, Silicon Wafer are cleaned with standard RCA-1 treating, Silicon Oxide Layer is reduced using HF 2% giving a oxide layer of 8-10 Å 16

Saturation Curve

1,4 1,2 1,0 0,8 0,6 0,4 0,2 0,0 0 2 4 6 8 10 12

Number of Plugs

Conditions: 170 mTorr, 130 °C Precursor Temperature, 140 °C Valve Temperature, 150 °C Leg Temperature, 160 °C Manifold Temperature, 250 ºC Reactor Temperature, 50 Cycles, 55 ms Water Pulse, Silicon Wafer standard RCA-1 treating, Silicon Oxide Layer is reduced using HF 2% giving a oxide layer of 8-10 Å 17

Linear Growth

600 500 400 300 200 100 y=1.2x-5.4

R 2 =0.9954

0 0 100 200 300 400 500

Number of Cycles

Conditions: 170 mTorr, 130 °C Precursor Temperature, 140 °C Valve Temperature, 150 °C Leg Temperature, 160 °C Manifold Temperature, 250 ºC reactor temperature, 5 plugs, 55 ms Water Pulse, Silicon Wafer are cleaned with standard RCA-1 treating, Silicon Oxide Layer is reduced using HF 2% giving a oxide layer of 8-10 Å 18

Conclusions

 ◦ ◦ Optimum ALD conditions of cerium oxide were found.

Precursor Temperature: 130 ºC Oxidizer Pulse Length: 55 ms ◦ ALD window: 210-280 ºC- previous work indicated the no ALD window existed when tris(i-propylcyclopentadienyl)cerium was used ◦ Saturation: 4 plugs of precursor pulse and higher ◦ Linear growth: deposition follows a linear trend with 1.2 Å/cycle M. Kouda, K. Ozawa, K. Kakushima, P. Ahmet, H. Iwai, Y. Urabe, and T. Yasuda: Japanese Journal of Applied Physics, 2011, 50, 6-1-6-4.

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Future Work

  Dope CeO 2 films with yttrium and test as electrolyte in solid oxide fuel cells Dope CeO 2 films with nickel and test as anode in solid oxide fuel cells 20

Acknowledgements

   National Science Foundation, EEC Grant # 1062943 National Science Foundation, CBET Grant # 1067424 Air Liquide (provided the precursor) 21