Russell SparksCarnegie Mellon University

Department of Chemical Engineering

Jorge Rossero*, Gregory Jursich#, Alan Zdunek*, Christos G. Takoudis#,*

University of Illinois at ChicagoDepartments of Bioengineering# and Chemical Engineering*

8/1/2013

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Tunabiliy and Electrical Measurements of Atomic Layer Deposited Yttria Doped Cerium Oxide for Fuel Cell Applications

Applications and Advantages of Solid Oxide Fuel Cells (SOFCs)

Advantages

• SOFCs could become clean replacements for fossil fuel

• SOFCs do not produce NOx, SOx, or hydrocarbon emissions

• Reduced CO2 emissions• Fuel flexibility-SOFCs can

use alternative fuels such as H2

Applications

• Stationary electrical power generation

• Replacement for car batteries

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N. Perdikaris, K. D. Panopoulos, P. Hofmann, S. Spyrakis and E. Kakaras, International Journal of Hydrogen Energy, 2010, 35, 2446-2456.J. Kupecki, J. Milewski and J. Jewulski, Central European Journal of Chemistry, 2013, 11, 5, 664-671.

SOFC Background• Current SOFCs require operating

temperatures >800 °C• Reducing this to 500-600 °C

would greatly increase SOFC utility

• CeO2 increases O2- ion conductivity at lower temperatures by creating oxygen vacancies in the electrolyte

• Several physical and chemical methods exist to deposit CeO2 and Ceria-based materials (YDC)

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Schematics of a planar SOFC

Yttrium doped Cerium (YDC)• Ce4+ in the electrolyte tends to reduce to Ce3+ in the anode,

increasing the electric conductivity and causing the cell to short circuit

• Adding Y to Ce film tends to stabilize Ce4+ ions and allow higher O2- conductivity through vacancies in the lattice structure

• Research indicates optimal Y concentration to be 20-34 %

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Z. Fan, C. Chao, F. Hossein-Babaeiaand, F. B. Prinz, J. Mater. Chem., 2011, 21, 10903.Z. Li, T. Mori, D. Ou, F. Ye, G. J. Auchterlonie, J. Zou and J. Drennan, The Journal of Physical Chemistry, 2012, 116, 5435-5443.

Atomic Layer Deposition (ALD)• Gaseous ligand precursor is

pulsed over Si substrate• Metal ion reacts with –OH on

substrate• Reaction is self-limited by

amount of –OH on substrate surface

• Oxidizing gas is pulsed to react with metal ions

• –OH tails present for next ALD cycle

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ALD Process Schematic*

*J. Päiväsaari, Inorganic Chemistry Publication Series, Helsinki University of Technology, 2006.

ALD

• Tris[isopropyl-cyclopentadienyl]cerium (Ce(iPrCp)3) and Tris[isopropyl-cyclopentadienyl]yttria (Y(iPrCp)3) precursors and water are used to deposit CeO2 and Y2O3 onto Si substrates

• Water reacts selectively with metal-ligand bond in ALD precursors

• Common ligand Ce(thd)4 only reacts with O3

thd = 2,2,6,6-tetramethyl-3,5-hepadionate

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Experimental Setup*Ice bath

Hot wall reactor

* P. Majumder, et al., Journal of The Electrochemical Society, vol. 155, pp. G152-G158, 2008.

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Electrical Resistivity

• Ce+4 has high ionic conductivity, Ce+3 has less ionic conductivity

• Electric resistance inversely proportional to ionic resistance

• Film surface resistivity measured by four-terminal sensing

• Resistivity measured by lab-built sensor

• Sensor consists of 4 Pt electrical leads resting on a nonconductive surface to measure resistance

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Electrical Resistivity (cont.)• LCR instrument was calibrated to determine best

operating conditions• >2 V and 100-200 Hz settings were found to give most

precise resistance readings • Wide range of YDC, Si, and CeO2 samples tested

• Thickness: 6-28 nm• Weight added on top of sample: 0-50 g • Voltage Range: 0.20 V-5 V• Frequency Range: 10 Hz-100,000 Hz• 2 V DC bias added to overcome effects of frequency• 20, 30 %Y content in YDC films• Annealed and non-annealed YDC films

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Sheet Resistance of 20% YDC Films with Added Weight

• Extra weight may cause wire leads to rub through film

• Substrate resistance instead of film resistance measured

• Negative readings caused by resistances too high to result in readings

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Samples were analyzed with 75 Hz and 2V.

0 10 20 30 40 500

1

2

3

4

5

6

7

8

9

11 nm22 nm28 nm

Added Weight (g)

Sh

eet

Res

ista

nce

(M

Ω-S

q)

Effects of Annealing 20 %Y YDC Sample

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0 50 100 150 200 250 300 350 400 450 5000

2

4

6

8

10

1 V Non-Annealed2 V Non-Annealed3 V Non-Annealed4 V Non-Annealed5 V Non-Annealed

Frequency (Hz)

Sh

eet

Res

ista

nce

(M

Ω-S

q)

0 50 100 150 200 250 300 350 400 450 5000

2

4

6

8

10

1 V Annealed2 V Annealed3 V Annealed4 V Annealed5 V Annealed

Frequency (Hz)

Sh

eet

Res

ista

nce

(M

Ω-S

q)

Sheet Resistance vs. Frequency of YDC Films

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0 50 100 150 200 250 300 3500

2

4

6

8

10

1V 20% Y YDC2V 20% Y YDC3V 20% Y YDC4V 20% Y YDC5V 20% Y YDC

Frequency (Hz)

Sh

eet

Res

ista

nce

(M

Ω-S

q)

0 50 100 150 200 250 300 3500

2

4

6

8

10

1V 30% Y YDC2V 30% Y YDC3V 30% Y YDC4V 30% Y YDC5V 30% Y YDC

Frequency (Hz)

Sh

eet

Res

ista

nce

(M

Ω-

Sq

)

Effects of 2 V DC Bias for 20% YDC Films

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0 50 100 150 200 250 300 350 400 4500

2

4

6

8

10

1V Without Bias2V Without Bias3V Without Bias4V Without Bias5V Without Bias

Frequency (Hz)

Sh

eet

Res

ista

nce

(M

Ω-S

q)

0 50 100 150 200 250 300 350 400 4500

2

4

6

8

10

1 V With Bias2V With Bias3V With Bias4V With Bias

Frequency (Hz)

Sh

eet

Res

ista

nce

(M

Ω-

Sq

)

YDC Film Stoichiometric Tunability

0.1 0.2 0.3 0.4 0.5 0.6 0.70

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

f(x) = 1.00382577987051 x + 0.0615862271924662R² = 0.999430745808032

ALD CYCLE RATIO Y/(Y+Ce)

Y-at

om ra

tio Y

/(Y+

Ce)

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XPS Background• XPS (X-Ray photoelectron spectroscopy) measures

kinetic energy change of spectral emissions• Sensitivity analysis calculates the atomic percent

composition of each component element by comparing peak areas

• XPS can determine Ce4+/ Ce3+ and Ce/Y ratios based on their respective peak sizes

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XPS Results for 20 % Y YDC film

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• 23% Ce3+ in this sample.• V0, U0 and V’, U’ indicate

Ce3+, other peaks for Ce4+.

• Peak sizes analyzed using Pfau-Schierbaum method

Binding Energy I.D

880.32 Vo

882.21 V

884.75 V'

888.55 V"

898.2 V"'

899.2 Uo

900.81 U

903.14 U'

907.13 U"

Conclusions• Yttria doped cerium oxide films were successfully

deposited via ALD using Ce(iPrCp)3, Y(iPrCp)3 and water

• XPS analysis shows that by increasing the ALD cycle ratio (Y:Ce) the concentration of Yttrium was linearly increased in the film

• Annealed and non-annealed resistances are close and have same order of magnitude

• Higher Y concentration had little effect on measured resistance

• 3 V and 150 Hz produce most accurate resistance results• DC bias unnecessary

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Recommendations for Future Experiments

• Produce resistance probe station capable of taking resistance measurements up to 800 °C

• Repeat calibration measurements at high temperature• Goal: Determine precise YDC doping for maximum

electrical resistance

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Acknowledgments• I would like to gratefully acknowledge the financial support

provided by:• EEC-NSF Grant #1062943• CBET-NSF Grant #1346282

• I would like to gratefully acknowledge the material support provided by:• Advanced Materials Research Lab, University of Illinois at Chicago

for use of laboratory facilities• Air-Liquide USA for providing precursors

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References• M. Fanciulli and G. Scarel (Eds.): Rare Earth Oxide Thin

Films, Topics Appl. Physics, 106, 15–32 (2007) © Springer-Verlag Berlin Heidelberg 2007.

• M. Coll, J. Gazquez, A. Palau, M. Varela, X. Obradors and T. Puig, Chem. Mater. 2012, 24, 3732−3737.

• W. Kim, M. Kim, W. J. Maeng, J. Gatineau,d, V. Pallem, C. Dussarrat, A. Noori, D. Thompson, S. Chu and H. Kima, Journal of The Electrochemical Society, 158 (8) G169-G172 (2011).

• Z. Fan, C. Chao, F. Hossein-Babaeiaand and F. B. Prinz, J. Mater. Chem., 2011, 21, 10903.

• P. Gao, Z. Wang, W. Fu, Z. Liao, K. Liu, W. Wang,• X. Bai and E. Wang, Micron, 2010, 41, 301-305.

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Atomic Percent Comparison (Pfau-Schierbaum Analysis)

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Atomic Percent Comparison•  

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