Russell Sparks Carnegie Mellon University Department of Chemical Engineering Jorge Rossero *, Gregory Jursich #, Alan Zdunek *, Christos G. Takoudis #,* University of Illinois at Chicago Departments of Bioengineering # and Chemical Engineering * 8/1/ 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 NO x, SO x, or hydrocarbon emissions Reduced CO 2 emissions Fuel flexibility-SOFCs can use alternative fuels such as H 2 Applications Stationary electrical power generation Replacement for car batteries 2 N. Perdikaris, K. D. Panopoulos, P. Hofmann, S. Spyrakis and E. Kakaras, International Journal of Hydrogen Energy, 2010, 35, J. Kupecki, J. Milewski and J. Jewulski, Central European Journal of Chemistry, 2013, 11, 5,

SOFC Background Current SOFCs require operating temperatures >800 °C Reducing this to °C would greatly increase SOFC utility CeO 2 increases O 2- ion conductivity at lower temperatures by creating oxygen vacancies in the electrolyte Several physical and chemical methods exist to deposit CeO 2 and Ceria-based materials (YDC) 3 Schematics of a planar SOFC

Yttrium doped Cerium (YDC) Ce 4+ in the electrolyte tends to reduce to Ce 3+ in the anode, increasing the electric conductivity and causing the cell to short circuit Adding Y to Ce film tends to stabilize Ce 4+ ions and allow higher O 2- conductivity through vacancies in the lattice structure Research indicates optimal Y concentration to be % 4 Z. Fan, C. Chao, F. Hossein-Babaeiaand, F. B. Prinz, J. Mater. Chem., 2011, 21, Z. Li, T. Mori, D. Ou, F. Ye, G. J. Auchterlonie, J. Zou and J. Drennan, The Journal of Physical Chemistry, 2012, 116,

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 5 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 CeO 2 and Y 2 O 3 onto Si substrates Water reacts selectively with metal-ligand bond in ALD precursors Common ligand Ce(thd) 4 only reacts with O 3 thd = 2,2,6,6-tetramethyl-3,5-hepadionate 6

Experimental Setup* Ice bath Hot wall reactor * P. Majumder, et al., Journal of The Electrochemical Society, vol. 155, pp. G152-G158,

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 8

Electrical Resistivity (cont.) LCR instrument was calibrated to determine best operating conditions >2 V and Hz settings were found to give most precise resistance readings Wide range of YDC, Si, and CeO 2 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 9

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 10 Samples were analyzed with 75 Hz and 2V.

Effects of Annealing 20 %Y YDC Sample 11

Sheet Resistance vs. Frequency of YDC Films 12

Effects of 2 V DC Bias for 20% YDC Films 13

YDC Film Stoichiometric Tunability 14

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 Ce 4+ / Ce 3+ and Ce/Y ratios based on their respective peak sizes 15

XPS Results for 20 % Y YDC film 16 23% Ce 3+ in this sample. V 0, U 0 and V’, U’ indicate Ce 3+, other peaks for Ce 4+. Peak sizes analyzed using Pfau-Schierbaum method Binding EnergyI.D Vo V V' V" 898.2V"' 899.2Uo U U' 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 17

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 18

Acknowledgments I would like to gratefully acknowledge the financial support provided by: EEC-NSF Grant # CBET-NSF Grant # 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 19

References M. Fanciulli and G. Scarel (Eds.): Rare Earth Oxide Thin Films, Topics Appl. Physics, 106, 15–32 (2007) © Springer-Verlag Berlin Heidelberg 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, P. Gao, Z. Wang, W. Fu, Z. Liao, K. Liu, W. Wang, X. Bai and E. Wang, Micron, 2010, 41,

Atomic Percent Comparison (Pfau- Schierbaum Analysis) 21

Atomic Percent Comparison 22