IN SITU INFRARED EMISSION SPECTROSCOPY
OF SOLID OXIDE FUEL CELL ANODES:
A CHEMICAL UNDERSTANDING OF DURABILITY
AND SAFETY
Michael B. Pomfreta, Daniel A. Steinhurstb, Jeffrey C. Owrutskya
aChemistry
Division, U.S. Naval Research Laboratory, Washington, DC 20375
bNova
Research, Inc., Alexandria, VA 22308
Fuel Cell Seminar and Exposition, November 8th, 2012, Uncasville, CT
Solid Oxide Fuel Cells
•
Electrode materials
– Ni-YSZ cermet anodes
– Perovskite cathodes (lanthanum
strontium manganite)
•
Fuels used
– CH4 and CH3OH
– Effect of oxygenated fuels
•
Fuel utilization
– Fuel oxidation and other reactions
– Identification of intermediates
– Improved performance and safety
www1.eere.energy.gov
Fuel Cell Seminar and Exposition
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Previous In Situ Anode Studies
Raman
(b)
(c)
0
•
•
Vibrational Raman spectroscopy
– Identification of damaging species
– Material-specific information
Near-IR thermal imaging
– Anode/fuel interactions
– Surface changes (∆T) depend on fuel,
voltage, and operational temperature
– Spatial variations as signs of failure
Arb. Units
(d)
(e)
Pomfret, et al. JPCB 2006, 110, 17305.
1000
1200
1400 -1 1600
Wavenumbers (cm )
1800
NIR imaging
•
No oxidation molecular intermediates have
been identified by in situ optical studies
Fuel Cell Seminar and Exposition
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FTIR Emission in SOFCs
•
Lu, et al., J. Electrochem. Soc. 2002, 149, A1293.
•
Oxygen reduction at the cathode
– O2-: 1236 and 1124 cm-1
– O22-: 930 cm-1
•
The 1236 cm-1 peak is due to “perturbed” O2-
•
First probe of gas/surface interactions in
functioning SOFCs
Fuel Cell Seminar and Exposition
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FTIR Emission of Anode Reactions
•
Characterize the complex chemistry of fuel utilization
•
Complete oxidation products: CO2, H2O
•
Incomplete oxidation products: CO, CH4, CH2O, CH3OH
•
Potential adsorbed intermediates: -CHx, -CO, -CO2, -OH, -CHxO
Fuel Cell Seminar and Exposition
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Approach and Design
mirror
FTIR
FTIR Emission Spectroscopy (FTIRES)
• Commercial IR spectrometer
• Factory source removed
• Typical spectrum: 16 averaged scans,
0.5 cm-1 resolution
Drawing courtesy of R.J. Kee, CSM
Fuel Cell Seminar and Exposition
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Spectroelectrochemical Emission
•
Furnace at 800 ºC
•
Fuel flow conditions: CH4/Ar mix exposure for 40 min
– 75%, 50%, and 25% Imax, OCV; 10 min each
– Study effects of carbon reactions on anode
•
Electrochemically oxidizing conditions: Ar only on anode side for 20 min
– Oxidation of anode and deposits
Fuel Cell Seminar and Exposition
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What to Look for
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Of the possible carbon species, three found in the gas
phase are most likely to be observed
•
CH4 @ 3010 cm-1, CO @ 2145 cm-1, CO2 @ 2350 cm-1
•
Reactant and product species are distinct in spectra
Gas-phase emission spectra taken at 800 ºC
CH4
Fuel Cell Seminar and Exposition
CO
CO2
8
FTIR Emission under High Current
•
Cell operated at 75% Imax, 330 mA
•
CH4 as a fuel, in excess
•
Evidence of oxidation: CO2, CO
•
Evidence of a surface COx species
– CO(ads) expected as a partially
oxidized intermediate
– CO2(ads) also known to exist on Ni
CH4
CO2(ads)/CO2(g)
CO
– Is either at the surface in a steady
state?
Fuel Cell Seminar and Exposition
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CO2(ads) as the Surface Species
•
Gas phase CO2 has clear P and R branches
•
In the presence of Ni, the gas phase is obscured
•
Bouwman, et al., Appl. Surf. Sci. 1980, 4, 21
– Adsorbed CO2 has a broad peak at 2345 cm-1
– Only 5 cm-1 from center of CO2(g) peak
– 2350 and 2290 cm-1 features identifies CO2(ads)
•
No evidence of adsorbed CO (2060 cm-1)
– CO(ads) is more likely near the electrolyte
•
CO2(ads) is more prevalent at the anode surface
– Water-gas shift: H2O + CO → H2 + CO2
– Boudouard reaction: CO + CO → C + CO2
– Kaltschmitt, et al., IE&CR 2012, 51, 7536
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CH3OH is More Oxidized Than CH4
330 mA
220 mA
110 mA
0 mA
CH4
CH3OH
•
Current dependence of the four peaks
– CH4
3010 cm-1
– CO2(g)
2370 cm-1
– CO
2140 cm-1
– CO2(ads)
2290 cm-1
•
•
CH4 → CO → CO2(ads) → CO2(g)
CH3OH → H2 + CO → CO2(ads) → CO2(g)
Fuel Cell Seminar and Exposition
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Electrochemical Oxidation
CH4
•
•
•
•
Electrochemical oxidation
– 250 mA without fuel
Intensity trends
– Deposit oxidation from CH4
– CH3OH leads less extensive carbon
At early times (first 600 s)
– Deposits are abundant with CH4
– Small amounts of CO are present
Later times (second 600 s)
– CO(g) signal falls off
– Spectra are dominated by CO2
– CO(g) is product of reforming reactions,
not electrochemical oxidation
Fuel Cell Seminar and Exposition
CH3OH
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Summary
•
FTIR emission has been demonstrated as a viable technique for anode studies
•
Oxidation of a simple hydrocarbon fuels has been characterized
•
IR signature of CO2(g) grows with increased current
– Demonstrates a dependence on cell polarization
•
CO2(ads) intermediate
– Identified by a features at 2350 and 2290 cm-1
– Present at all currents, decreases at OCV
•
CH3OH leads to less carbon deposits on the anode
– CO(g) signal as evidence of fuel pyrolysis
– CO2(g) and CO2(ads) decrease immediately during electrochemical oxidation
•
Future work includes various conditions, bio-derived and larger fuels
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Acknowledgements
•
Office of Naval Research
•
Collaborators:
– Prof. Robert A. Walker and John D. Kirtley
Montana State University
– Prof. Bryan W. Eichhorn
University of Maryland – College Park
•
Helpful Contributions:
– Profs. Robert J. Kee and Anthony M. Dean
Colorado School of Mines
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