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 2 R.M., Ormerod: Chemical Society Reviews, 2003, 32,

How a Solid Oxide Fuel Cell Works Solid oxide fuel cells components: ◦ Cathode ◦ Solid inorganic oxide electrolyte ◦ Anode 3 e-e- O 2 (air) Fuel (hydrocarbon and steam or oxygen) H 2 and CO CO 2 and H 2 O Cathode O -2 Electrolyte Anode R.M., Ormerod: Chemical Society Reviews, 2003, 32,

The biggest setback for solid oxide fuel cell use is the high operating temperature Operating temperature: ºC Long heat up and cool down periods Limited materials 4 M. Cassir and E. Gourba: Annales de Chimie Science des Matériaux, 2001, 26,

Decreasing Operating Temperatures New materials with lower ion resistivity Decreasing thickness can increase ion permeability Thickness can be decreased using thin films 5 M. Cassir and E. Gourba: Annales de Chimie Science des Matériaux, 2001, 26,

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 8 S.M. George: Chem. Rev., 2010, 110,

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

Step Three: Substrate is exposed to coreactant substrate Step Four: Reactor is purged of coreactant and byproducts substrate Process is repeated until the film is at the desired thickness 10

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

Precursor Temperature of 140 ºC 14

50 ms Water Pulse 15

ALD Window 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 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 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: 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,

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 # National Science Foundation, CBET Grant # Air Liquide (provided the precursor) 21