Identification of a Methane Oxidation Intermediate on Solid Oxide Fuel

Apr 2, 2013 - Under the latter condition, a constant current was applied to drive oxide ions through the solid electrolyte to electrochemically oxidiz...
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Identification of a Methane Oxidation Intermediate on Solid Oxide Fuel Cell Anode Surfaces with Fourier Transform Infrared Emission Michael B. Pomfret,*,† Daniel A. Steinhurst,‡ and Jeffrey C. Owrutsky† †

Chemistry Division, U.S. Naval Research Laboratory, 4555 Overlook Avenue Southwest, Washington, District of Columbia 20375, United States ‡ Nova Research, Inc., 1900 Elkin Street, Suite 230, Alexandria, Virginia 22308, United States S Supporting Information *

ABSTRACT: Fuel interactions on solid oxide fuel cell (SOFC) anodes are studied with in situ Fourier transform infrared emission spectroscopy (FTIRES). SOFCs are operated at 800 °C with CH4 as a representative hydrocarbon fuel. IR signatures of gas-phase oxidation products, CO2(g) and CO(g), are observed while cells are under load. A broad feature at 2295 cm−1 is assigned to CO2 adsorbed on Ni as a CH4 oxidation intermediate during cell operation and while carbon deposits are electrochemically oxidized after CH4 operation. Electrochemical control provides confirmation of the assignment of adsorbed CO2. FTIRES has been demonstrated as a viable technique for the identification of fuel oxidation intermediates and products in working SOFCs, allowing for the elucidation of the mechanisms of fuel chemistry.

SECTION: Energy Conversion and Storage; Energy and Charge Transport

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features.13,14 Identifying these species will help simplify reaction schemes, thus informing of models and ultimately aiding device design and operational protocols. The effective use of hydrocarbon fuels in SOFCs requires an understanding of fuel oxidation, as well as the detrimental side reactions, that can only come with the knowledge gleaned from in situ species-specific measurements. We report the first application of FTIRES to operational SOFC anodes where the chemistry of fuel utilization of hydrocarbons is complex. Methane (CH4) is used as a representative hydrocarbon fuel with simple, well-defined molecular signatures that are distinct from the expected products and intermediates. The measurements were performed for SOFC operation with fuel flow as well as for electrochemical oxidation of carbon deposits.15 Results show gas-phase carbon monoxide (CO(g)) and gasphase carbon dioxide (CO2(g) ) to be products of CH4 utilization. Additionally, a surprising and intriguing result is that we observe CO2 adsorbed (CO2(ads)) on the Ni/yttriastabilized zirconia (YSZ) anode. At the time of this writing, this is the first identification of a fuel oxidation intermediate molecular species adsorbed on the anode surface of an operating SOFC. The characterization of all three COx species provides insight into the evolution of molecular species and reactions for the specific system of SOFCs operating on methane and demonstrates a new avenue for monitoring gas

olid oxide fuel cells (SOFCs) are alternative energy devices that offer higher efficiencies than combustion engines and more fuel flexibility than other types of fuel cells (e.g., protonexchange membrane fuel cells).1 The ability to use a variety of fuels, including hydrocarbons, is a result of high (>600 °C) operating temperatures required for the electrolyte conductivity.2 Significant challenges still exist concerning cell durability and safety when using hydrocarbon fuels in SOFCs, including the detrimental formation of carbon deposits on the anode that result from incomplete electrochemical oxidation of carboncontaining fuels. Much of what is known about fuel oxidation in SOFCs has been derived from electrochemical measurements, ex situ analytical measurements, and theoretical models.3−9 While capturing information regarding device performance, none of these measurements is capable of positively identifying chemical species on the anode or in the anode chamber. Simulations of fuel chemistry and membrane electrode assembly (MEA) degradation have not been scrutinized in functioning devices due to the difficulties associated with using analytical probes to detect chemical species at the high SOFC operating temperatures. In the past decade, this challenge has been addressed through the use of optical techniques, including Raman and infrared spectroscopies and thermal imaging.10,11 The first application of optical spectroscopy to SOFCs was the Fourier transform infrared emission spectroscopy (FTIRES) study of the oxygen reduction reaction on functioning cathodes of symmetric MEAs.12 Many of the chemical species predicted to be present on anodes, that is, unreacted hydrocarbon fuel and fuel oxidation products, have strong, distinct IR © 2013 American Chemical Society

Received: February 27, 2013 Accepted: April 2, 2013 Published: April 2, 2013 1310

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10−4 Torr, but high-resolution electron energy loss spectroscopy has been able to identify the frequency of the asymmetric stretch of CO2 adsorbed to metals as 2340−2350 cm−1.22−24 Bartos et al. determined that the frequency of the asymmetric stretch for CO2(ads) on Ni is 2349 cm−1.22 This band has also been observed by high-resolution infrared chemiluminescence spectroscopy and infrared reflection adsorption spectroscopy.21,25 In a functioning SOFC, this band is very broad and is centered at slightly lower frequency. CO2 can also bind to carbon nanostructures, which are known to form on SOFC anodes during hydrocarbon operations.4,6,26−28 Infrared bands of CO2(ads) on carbon surfaces have been observed between 2320 and 2340 cm−1,29,30 which, given that CH4 is known to form carbon deposits on the anode, may contribute to the lower-frequency portion of the feature observed in SOFCs. Bouwman et al. demonstrated that CO2(ads) is present on Ni catalyst surfaces during CO oxidation.20 In that experiment, the CO2(ads) and CO2(g) bands coincide with each other, resulting in distortion of both bands. Portions of the band are isolated from the overall observed spectrum and assigned to CO2(ads) by subtracting the gas-phase CO2 spectrum; by this method, a feature near 2350 cm−1 was determined and ascribed to CO2(ads). Supporting FTIRES measurements were made by introducing CO2 into the anode chamber containing only an unwired Ni/YSZ anode at 800 °C and comparing the results to the SOFC spectra to confirm that CO2 is present on the surface of the functioning anode (Figure 2). In the first measurement, 25

and surface anode processes that mediate SOFC chemical mechanisms. FTIR emission of the SOFC anode at 800 °C during hydrocarbon fuel oxidation at relatively high operating current (330 mA) is shown in Figure 1. This is the first published

Figure 1. The C−H stretching region and C−O regions of the in situ FTIRES spectrum obtained from the Ni/YSZ anode chamber of a SOFC operating with CH4 fuel at 330 mA and 800 °C.

FTIRES spectrum of an operating SOFC anode and the surrounding gases in the anode chamber. The bands of the oxidation products are distinct from the CH4 stretching band in the emission of the operating cell. Under these conditions, gasphase CO2, CO, and unreacted CH4 are observed. All three species have been observed previously in room-temperature FTIR spectra of SOFC anode exhaust.16 Gas-phase CH4 has a stretching band centered at 3005 cm−1 that is observed in SOFC spectra.13 The C−H stretching band in the region of ∼3250 to ∼2710 cm−1 exhibits a strong Q-band with rotationally resolved P and R branches. The fundamental vibrational band of CO, a partially oxidized product, is observed from ∼2275 to ∼2010 cm−1,14 in which the P and R branches are rotationally resolved, indicating gas-phase products. Similarly, the band for gas-phase CO2 is seen between 2400 and 2275 cm−1,13 in which the rotational structure is partially resolved for the P and R branches. A broad shoulder is also observed in this region between 2300 and 2200 cm−1, which we attribute to adsorbed CO2. The strong, broad character of the band centered at 2320 cm−1 suggests that it is a COx species adsorbed to the anode surface. CO(ads) is an obvious CH4 oxidation intermediate and would be considered a strong candidate to be present on the surface. However, CO(ads) does not have a broad IR band near 2300 cm−1. Depending on how CO(ads) is bound to the Ni, bands due to various types of adsorption structures would be present between 1900 and 2200 cm−1,17−19 none of which are present in the SOFC spectrum. The feature at 2320 cm −1 most closely resembles physisorbed CO2 (CO2(ads)) on the anode. As a fully oxidized product, CO2 will desorb from a Ni surface rapidly, especially at 800 °C. This indicates that the CO2(ads) band is a result of a steady-state population of CO2 on the anode surface or in nearsurface pores as it leaves the anode structure. CO2 physisorbed to metal surfaces has been observed in IR spectra of systems at ambient pressure and elevated temperature, though not at temperatures as high as 800 °C.19−21 Theoretical surface studies have modeled CO2−surface interactions at temperatures as high as 1000 °C, though not in SOFC systems.17 Many CO2 surface studies are conducted at pressures less than

Figure 2. The CO region of in situ FTIRES spectra of (a) CO2 gas over Ni/YSZ cermet, (b) the anode chamber of a SOFC operating with CH4 fuel at 330 mA, and (c) CO2 gas without a catalyst. Spectrum (c) is subtracted from spectrum (b) to isolate CO2(ads) features in spectrum (d). All spectra were acquired at 800 °C and are offset for clarity.

sccm of CO2 was flowed over the Ni/YSZ surface. In the resulting spectrum (Figure 2a), the most dominant feature is a strong, broad band centered at 2310 cm−1 that is similar to (though more pronounced than) the band in the SOFC spectrum (Figure 2b) and is assigned to CO2(ads). The rotational bands of the P and R branches of CO2(g) are also present between 2310 and 2370 cm−1. They are relatively weak compared to the CO2(ads) band, indicating that most of the CO2 is adsorbed rather than in the gas phase. The P and R branches of CO(g) are also observed (from ∼2275 to ∼2010 cm−1). The formation of CO is likely the result of CO2 oxidizing Ni. 1311

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In a second supporting study, the CO2(ads) spectrum is isolated by subtracting the FTIRES spectrum of CO2(g) at 800 °C, which was measured by flowing 25 sccm of CO2 into the SOFC manifold with a piece of YSZ in place of the more active Ni/YSZ anode material. The spectrum (Figure 2c) clearly shows the P and R branches with minimal intensity in the center (2350 cm−1) and on the low-frequency side (near 2300 cm−1). The CO2(g) and SOFC spectra are normalized to their intensities at 2360 cm−1, where the intensity of the CO2(g) R branch is strongest. The CO2(g) spectrum is then subtracted from the SOFC spectrum to isolate the IR emission that is unique to CO2(ads) (Figure 2d). The resulting features are located at 2295, 2345, 2355, and 2385 cm−1 and closely match the bands that are assigned to CO2(ads) by Bouwman et al.20 Finally, a spectrum with 0.25 cm−1 resolution was taken of the anode side of the MEA while operating at 330 mA with CH4 fuel (Figure 3). The increased resolution confirms that

Figure 4. In situ FTIRES spectra obtained due to electrochemical oxidation of carbon deposits on a Ni/YSZ anode of a SOFC operating at 250 mA and 800 °C: (a) during SOFC operation with CH4, (b) 10 min after CH4 is shut off, and (c) 20 min after CH4 is shut off.

detected in FTIRES spectra is a result of fuel reforming reactions with other products, that is, CO2 and H2O, that lead to partial oxidation of CH4 in the gas phase and throughout the structure of the anode. It is expected that some CO is produced through incomplete electrochemical oxidation of CH 4 . However, the lack of an adsorbed CO species suggests that little, if any, of the CO formed electrochemically near the anode/electrolyte interface survives to reach the optically accessible surface of the anode structure. Besides a second electrochemical oxidation event to convert CO to CO2, a CO molecule produced at the interface will interact with other molecules as the product gases flow out of the anode. CO molecules participate in the Boudouard reaction to produce CO2 and carbon deposits.31−35 Furthermore, CO can also react with H2O in the flow channels to produce H2 and CO2 through the water−gas shift reaction.36,37 The electrochemically oxidizing conditions provide an opportunity to observe CO that is not formed through reforming of CH4. Without CH4 present, CO would have to form as a result of partial electrochemical oxidation of the carbon deposits left behind by CH4. However, as CO2 production peaks (Figure 4b), there is no CO present in the IR spectrum. Therefore, it is concluded that little, if any, of the CO(g) observed in the FTIRES measurements of 1 mm thick anodes is produced as a direct result of electrochemical oxidation. The data presented serve as the first positive in situ identification of a carbon oxidation molecular intermediate on SOFC anode surfaces. Specifically, electrochemical oxidation of CH4 and carbon deposits has been studied, and CO2(ads) is identified as an oxidation intermediate on the surface of functioning Ni/YSZ anodes. Comparing the SOFC FTIRES spectrum to CO2 spectra in the gas phase and that in the presence of a catalyst under various operating conditions confirms the identity of CO2(ads) on the anode. Furthermore, insight is gained regarding chemical mechanisms in the cell through the behavior of CO in electrochemical oxidation experiments, which indicate a significant role of fuel−product interaction in the gas phase of the anode chamber. These early results emphasize the importance of cell architecture (i.e., anode thickness) and product flow (i.e., CO2 and H2O reforming) and the role that FTIRES can play in determining the chemistry occurring in SOFCs and other high-temperature systems. Narrowing the scope of SOFC chemistry by

Figure 3. The C−O region of an in situ FTIRES spectrum of the anode chamber of a SOFC operating with CH4 fuel at 330 mA. Spectra were acquired at 800 °C with a resolution of 0.25 cm−1.

portions of the broad CO2 feature are not rotationally resolved. The expected CO2(g) P and R branches are observed with enhanced rotational−vibrational features between 2320 and 2400 cm−1, and rotational features at frequencies lower than 2250 cm−1 are due to CO(g). The broad band lacking rotational structure at 2350 cm−1 and between 2320 and 2250 cm−1 confirm the presence of surface CO2.19 These supplemental measurements provide compelling evidence that CO2(ads) is present in a steady-state concentration on functioning SOFC anodes. Furthermore, it is the only carbon oxidation intermediate present on the surface of the 1 mm thick anodes. SOFC operation with CH4 is known to form carbon deposits on the anode surface. While detrimental to proper cell function, the deposits allow for the study of carbon oxidation without a fuel flow. This is accomplished with an electrochemical oxidation experiment wherein emission is monitored after terminating the CH4 flow to the anode chamber but with a current of 250 mA. CO2(ads) is observed throughout the experiment, growing from the observed band during CH4 exposure (Figure 4a) before peaking after ∼10 min. (Figure 4b) and diminishing significantly by the 20th min (Figure 4c). The rotational structures of the CO2(g) P and R branches are also present in all three spectra, but they are weak enough that they simply distort the CO2(ads) band. It is not until the last spectrum, when the CO2(ads) feature is at its weakest, that the CO2(g) band structure becomes readily apparent. These spectra confirm that CO2(ads) formation is affected by fuel and oxide concentrations and that they serve as an indirect observation of carbon deposits as a byproduct of CH4 fuel use. At early times, CO is present, but no CO is observed after 10 min of electrochemical oxidation. This indicates that the CO 1312

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characterizing surface intermediates related to specific processes will provide insight regarding the simplification of complex fuel utilization models. Future studies will focus on studying different fuels at a variety of temperatures as well as quantifying the emission signals of the gases involved to determine fuel utilization through optical measurements.

EXPERIMENTAL SECTION The optically accessible SOFC assembly used herein has been discussed previously in our thermal imaging studies.10 The assembled SOFC manifold was placed inside of a 2 in. diameter tube furnace with the anode side of the MEA facing the FTIR spectrometer. The furnace was fitted with a sapphire window to separate the furnace chamber from the ambient-temperature atmosphere, while allowing IR transmission as low as 1550 cm−1. The optical path was enclosed and purged with N2 to reduce interference from atmospheric H2O and CO2. Once fully assembled, the manifold was heated to 800 °C. Constant flows of 150 sccm of Ar and 85 sccm of air were delivered to the anode and cathode, respectively. A flow of 100 sccm of H2 was added to the anode for standardized electrochemical characterization. FTIRES data were collected while 25 sccm of CH4 was flowed into the anode chamber. H2 flows were turned off while CH4 was used as a fuel. Cell polarization was maintained, and electrochemical measurements were conducted with a potentiostat (Gamry Reference 3000 potentiostat/galvanostat/ZRA). The maximum current during CH4 operation was ∼420 mA. All FTIR emission spectra were taken with a Mattson Instruments, Inc. spectrometer (Model #: RS-10000) configured for emission detection. The internal source was removed, and emission was collected from the SOFC anode in reflection with a flat mirror. Data were collected as 16 averaged scans with a resolution of 0.5 cm−1 over a range of 500−5000 cm−1. Spectra were collected during CH4 exposure at various currents and while the MEA was under an electrochemically oxidizing condition after CH4 exposure. Under the latter condition, a constant current was applied to drive oxide ions through the solid electrolyte to electrochemically oxidize both the anode and any carbon deposits that formed during CH4 exposure. Reference spectra were taken immediately prior to CH4 exposure and used for background subtraction, and baselines were leveled with a linear correction. ASSOCIATED CONTENT

S Supporting Information *

Details regarding the SOFC assembly, electrochemical characterization, and full FTIR spectra are presented. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Support for this work was provided by the Office of Naval Research. The authors acknowledge Bryan W. Eichhorn at the University of Maryland, College Park, and Robert A. Walker and John D. Kirtley at Montana State University. 1313

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