ARTICLE pubs.acs.org/EF
Methanol and Ethanol Fuels in Solid Oxide Fuel Cells: A Thermal Imaging Study of Carbon Deposition Michael B. Pomfret,*,† Daniel A. Steinhurst,‡ and Jeffrey C. Owrutsky† † ‡
Chemistry Division, United States Naval Research Laboratory, 4555 Overlook Avenue SW, Washington, DC 20375, United States Nova Research, Inc., 1900 Elkin Street, Suite 230, Alexandria, Virginia 22308, United States
bS Supporting Information ABSTRACT: Near-infrared (NIR) thermal imaging is used to study anodes of anode-supported solid oxide fuel cells (SOFCs) when operating with alcohol fuels. Relative propensities for carbon formation can be determined from surface cooling under fuel flows and subsequent heating under oxidizing conditions at temperatures between 700 and 800 °C. Ethanol forms considerable amounts of carbon at all temperatures and voltages studied as evidenced by substantial cooling related to carbon reactions and heating under oxidizing conditions. Methanol operation depends greatly on cell temperature and voltage. At 700 °C, temperature changes resemble those with ethanol, suggesting carbon deposition is occurring. At 800 °C, there is less cooling, which indicates that the oxide flux at higher polarizations mitigates the effects of endothermic carbon reactions. Under oxidizing conditions after fuel exposure, the small observed temperature increase demonstrates that little carbon is formed. At 750 °C the cooling depends on voltage, revealing a set of conditions where cooling from endothermic reactions and heating from exothermic reactions are balanced. The results show that while dry ethanol is not a clean fuel under any of our conditions, methanol can be at higher temperatures. NIR thermal imaging proves a valuable stand-off technique for identifying cell deterioration in situ, with potential for process monitoring in operating SOFCs.
’ INTRODUCTION Solid oxide fuel cells (SOFCs) are a leading technology for alternative energy production. As with other types of fuel cells, SOFCs are inherently more efficient than combustion devices. Furthermore, SOFCs have several advantages over other fuel cell systems, including the ability to operate using complex, carboncontaining fuels via internal fuel reforming, increased poison tolerance, and fabrication from relatively inexpensive materials. SOFCs require high operating temperatures, typically above 600 °C, to drive thermally activated oxide diffusion through the solid electrolyte.1,2 This facilitates the use of hydrocarbon and alcohol fuels by effectively reforming the fuels in situ.35 Most previous studies of SOFCs operated with fuels other than hydrogen involve hydrocarbon fuels, the use of which has been shown to cause anode damage.6,7 There is growing interest in oxygenated fuels. Some work has been done with ethers and acids, but most studies have been carried out with alcohols, which present unique materials challenges in terms of device development for viable and durable commercial and military applications.6,8 Gaining a better understanding of the chemical mechanisms that mediate anodefuel interactions will help to address these challenges. Small alcohols present a unique situation in terms of the chemistry associated with oxygen-containing fuels. There have been conflicting results leading to some debate about the extent to which both methanol (CH3OH) and ethanol (C2H5OH) form carbon and affect cell function. There is speculation that dry alcohol fuel feeds may form fewer carbon deposits than nonoxygenated fuels of the same size.913 Essentially, the tendency to form carbon is lower when the fuel species includes oxygen, r 2011 American Chemical Society
reducing the need for carbon alleviation processes—i.e., steam reforming. Numerous authors have observed that CH3OH used directly as a fuel can form carbon deposits on SOFC anodes unless operating temperatures exceed 800 °C or the fuel feed is mixed with steam.1315 Other studies have reported no evidence of carbon penetration into SOFC anodes based on ex situ SEM data from anodes that had operated with a direct CH3OH feed.16 More recently, Mat et al. reported that CH3OH does not form carbon on anodes in the lower range—300600 °C—of SOFC operational temperatures, but that C2H5OH does.17 This work used ceria-based composite materials rather than the Ni/YSZ cermets employed elsewhere. There are several processes that can lead to carbon deposition, such as through pyrolysis in the gas-phase inlet flow or through fuel breakdown on the surface of the anode, and there are various factors that affect these processes. For example, CH3OH reforming varies greatly in the range of typical SOFC operating temperatures. At temperatures above 1000 °C, CH3OH converts readily into CO and H2.15 At lower temperatures (e 800 °C), dry CH3OH will form a mixture of products that show a strong tendency to form carbon deposits on Ni-based anode materials.11,14 This indicates that thermodynamically, oxygen in the fuel can reduce the extent of carbon deposition if the operational temperature is increased above the limit for carbon formation. Many theoretical results are based on equilibrium calculations of surface reforming and pyrolysis products that may Received: March 14, 2011 Revised: April 29, 2011 Published: May 02, 2011 2633
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Energy & Fuels not reflect actual fuel flow compositions in the inlet. Furthermore, while results of models can provide important insights, they are highly dependent on operational conditions and cell architectures that are difficult to include in models. Dry C2H5OH is more prone to form carbon deposits than CH3OH over the full range of SOFC operating temperatures unless fuel utilization is high.14 To date, studies of alcohol fuels in SOFCs have used traditional fuel cell evaluation techniques, including electrochemical methods, kinetic modeling, and extensive ex situ analyses.1829 In situ, real-time optical evaluation of the chemical reactions responsible for cell function has been shown to provide valuable information about cell performance, chemical processes, and material deterioration under alcohol fuels.30 Thermal imaging is an optical technique capable of gathering component-level information about SOFC processes, including fuel reactions and carbon deposition. The technique is noninvasive and capable of continuous stand-off monitoring of active cell components. It also avoids problems associated with using thermocouples in the cell chamber to monitor temperature. Thermal imaging provides wide spatial coverage, nominally over an entire side of a button cell, with a spatial resolution limited only by the optical lenses, camera, and working distance. Thermal imaging of SOFCs was first reported by Brett et al.31 who employed a narrowband filter near 4 μm and a mid-infrared (MIR) InSb camera to monitor emission from SOFC cathodes. Near-IR (NIR) thermal imaging of SOFCs with Si-based CCD cameras can be used to determine the temperature for objects as cold as ∼400 °C.32 Practical advantages to using shorter wavelengths for thermal imaging are that cameras and optical components are less expensive, more convenient, and in many respects, yield better performance. A schematic of the in situ NIR thermal imaging apparatus for SOFCs has been previously reported33 and can be found in the Supporting Information. NIR thermal imaging has been used to monitor anodes of SOFCs operating on hydrogen and hydrocarbon fuels34 and recently combined with Raman spectroscopy to study CH3OH and methane fuels in electrolyte-supported fuel cells.30 At 700 °C, CH3OH formed significantly more carbon on Ni cermet anodes than CH4, degrading device performance and showing that fuel composition is kinetically, not thermodynamically, controlled.30 This study applies NIR thermal imaging to anode-supported cells to investigate carbon deposition on the anode through changes in surface temperature while operating with CH3OH and C2H5OH over a range of temperatures (700800 °C) and cell polarizations. To our knowledge, these are the highest temperatures of any in situ optical study of functioning SOFCs. The results indicate that more carbon is formed with C2H5OH than with CH3OH. Furthermore, at 750 °C there is a strong dependence of the anode cooling on voltage with CH3OH, suggesting that the temperature changes associated with the fuel cracking and oxidation reactions are comparable. Also, positiondependent changes in temperature are observed, which indicate regions of the cell where degradation starts. The temperature gradients and changes are largest with C2H5OH, which highlight distinct advantages of thermal imaging over point or singleelement detection. These experiments offer not only a continuous monitor for initial stages of anode fatigue, but also an ability to perform numerous and repeated measurements with a single operating cell, which reduces ambiguities that arise from cellto-cell variations that can introduce spurious results and undermine the reliability of interpretations.
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Figure 1. Typical linear-sweep voltammetry scans (solid lines) and power density curves (dashed lines) (top) of the SOFCs used in this study with H2 as the fuel operated at 700 (blue), 750 (green), and 800 °C (red). Typical electrochemical impedance data (bottom) are shown for cells operated at 700 (0), 750 (Δ), and 800 °C (O).
’ EXPERIMENTAL SECTION Fuel Cell and Assembly. The membraneelectrode assemblies (MEAs) used in this study are button cell SOFCs obtained from Materials & Systems Research, Inc. (Salt Lake City, UT). The 25-mm-diameter MEAs were supported by a ∼1-mm-thick, large-grain, Ni/YSZ composite anode. Other layers included a small-grain, Ni/YSZ 25-μm-thick anode interlayer; a 20-μm-thick YSZ electrolyte; a 25-μm-thick strontium-doped lanthanum manganate (LSM)/YSZ cathode interlayer; and a 50-μm-thick LSM cathode. The structure and electrochemistry of this type of MEA have been characterized previously.35 A gold current collector was attached to the anode with gold ink (BASF A1644) and a platinum current collector was attached to the cathode using Pt ink (Engelhard 6082). The wired disks were attached anode-side-out to a 25.4-mm-OD alumina tube (Sentrotech) with zirconia paste (Aremco Products, Inc.). A 50.8-mm-OD alumina tube surrounded the alumina-supported MEA and a 50.8-mm-diameter, 3.175-mm-thick sapphire window (Swiss Jewel Co.) was attached to one end of the outer alumina tube to contain the reactant and product gases while providing optical access to the anode. The rear of the assembly was sealed with RTV silicone-based paste (Permatex, UltraCopper). The assembled SOFC manifold was placed inside a tube furnace (Thermo Scientific, model F21135) and heated to the operating temperatures of 700, 750, and 800 °C. A K-type thermocouple (Omega) was positioned on the outer surface of the outer alumina tube to provide a temperature reference. A schematic of the optically accessible SOFC assembly has been published previously34 and is included in the Supporting Information. Gas Flows. All flows were regulated with mass flow controllers (Celerity FC-260 V-4S). Constant flows of ∼150 sccm Ar and ∼85 sccm air were delivered to the anode and cathode, respectively. A flow of 100 sccm H2 was added to the anode for standard operation. To study the effects of CH3OH and C2H5OH fuels individually, the anode-side flow was routed through a glass bubbler containing the alcohol. The resultant flows were either 16.7% CH3OH or 7.8% C2H5OH by volume, 2634
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Energy & Fuels delivering the same number of carbon atoms to the anode per unit time. To gain a better understanding of the composition of our C2H5OH fuel flow when it reaches the anode, FTIR spectra were taken of flows after transit through our tube furnace. C2H5OH fuel backed by 150 sccm Ar was fed through a 0.6-cm-diameter alumina tube in a 40-cm-long, openended furnace with set temperatures of 700, 750, and 800 °C. The residence time at the set temperature is ∼0.4 s. The effluent was collected in a gas-phase IR cell with a 10-cm path length and 16-scan spectra were taken at a resolution of 2 cm1. All FTIR spectra were obtained using a Matteson IR-7020 spectrometer. Data are presented in the Supporting Information. Electrochemical Characterization. Cell polarization was maintained and electrochemical measurements were conducted with a potentiostat (Gamry Reference 3000 Potentiostat/Galvanostat/ZRA). It should be noted that lower than optimum currents35 are achieved in our cells because fuel and air flows are optimized for optical measurements rather than performance. Voltammetry and electrochemical impedance measurements were made before and after each exposure to alcohol to monitor cell function. A typical voltammetry scan, calculated power density plot, and electrochemical impedance spectrum for each temperature are shown in Figure 1. Thermal Imaging Measurements. Thermal images were acquired with a previously described CCD camera (AVT, Stingray F033B ASG) with an 18108-mm focal length macro zoom lens (Navitar Zoom 7000) using collection software written inhouse in LabView v8.5. A long-pass filter (Hoya R72) with a nominal cutoff wavelength of 720 nm was used to block emitted and ambient reflected visible light. NIR intensities were analyzed and displayed in real time while simultaneously recording and storing the data for postprocessing of specific regions of interest. To study the effects of the fuels during operation, cells were either kept at OCV or polarized to a voltage referenced to OCV (ΔVOC) of 0.3 and 0.6 V for at least 30 s prior to the introduction of alcohol fuel. ΔVOC is referenced to the OCV, i.e., a 0.3 V ΔVOC corresponds to a 0.700.80 V cell voltage or, more generally, Vcell = OCV ΔVOC. Then the alcohol fuel was introduced and the hydrogen flow was terminated. To oxidize carbon deposits that resulted from fuel flow, the cells were operated at currents of ∼400 mA (2/3 of the maximum current under H2 conditions) under an Ar-only anode-side flow. Data were processed into temperature values that are spatially resolved and can be averaged temporally after the experiment. Analysis of images can be used to track the average temperature of a specific region of the anode over time or to track temperature variations across the cell surface. The spatial and temperature resolutions are 0.1 mm and 0.1 °C, respectively. Data processing procedures have been previously described.34
’ RESULTS AND DISCUSSION Gas-Phase Pyrolysis of Alcohols in the Fuel Feed Inlet Tube. To understand the chemical changes occurring on the
anode surface, it is important to determine the fuel composition impinging on it. The degree to which fuels pyrolyze in the gasphase reactions that occur prior to reaching the optically accessible part of the assembly can greatly affect the anode chemistry. A brief review of literature provides helpful insight as to the species that reach the anode and participate in the reactions occurring there. A large portion of the data concerning alcohol pyrolysis is from thermodynamic models. As mentioned earlier, these studies have predicted that CH3OH and C2H5OH pyrolyze to form species that can contribute to carbon deposition on the anode surface. The predicted pyrolysis products that are most likely to result in carbon on the anode surface are methane and particulate carbon.13,14 However, optical studies of methane fuel showed
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only a slight drop in temperature and suggest that carbon deposits on the surface without participating in endothermic carbon cracking reactions34,36 under the conditions studied. Thermodynamic models yield equilibrium predictions of the stable products of the decomposition of CH3OH and C2H5OH.14 It is unlikely that the alcohol fuels reach the thermodynamic limit and are completely decomposed in our system where the residence time of our fuel in the furnace is only ∼0.4 s. Cimenti and Hill comment that species such as acetaldehyde, acetic acid, ethylene, and formaldehyde—while not present at equilibrium—are likely to be intermediates in the pyrolysis of these fuels.14 Some of the species that contain CC or CdC bonds tend to form carbon deposits. A few kinetic models have been used to predict the compositions of CH3OH and C2H5OH flows on time scales similar to our residence times and are pertinent to understanding our fuel flows. Gupta, et al.37 have published kinetic models of C2H5OH pyrolysis at temperatures between 700 and 800 °C. They demonstrate that most of the pyrolysis is complete in ∼1 s. The results suggest that olefins, especially ethylene, are present in significant quantities at all temperatures studied and are likely to be the major contributors to carbon formation. Larger species—such as benzene, cyclopentene, and cyclopentadiene—were also determined to be present and contribute to carbon deposition on the anode. Norton and Dryer38 investigated kinetic models of CH3OH pyrolysis and conducted shock tube experiments at 2000 K. They measured propylene—a species known to lead to carbon formation39—at very short times (
