Perspective pubs.acs.org/JPCL
High-Temperature Chemistry in Solid Oxide Fuel Cells: In Situ Optical Studies Michael B. Pomfret,† Robert A. Walker,‡ and Jeffrey C. Owrutsky*,† †
Chemistry Division, Naval Research Laboratory, Washington, DC 20375, United States Department of Chemistry and Biochemistry, Montana State University, Bozeman, Montana 59717, United States
‡
ABSTRACT: Solid oxide fuels cells (SOFCs) are promising devices for versatile and efficient power generation with fuel flexibility, but their viability is contingent upon understanding chemical and material processes to improve their performance and durability. Newly developed in situ optical methods provide new insight into how carbon deposition varies with different hydrocarbon and alcohol fuels and depends on operating conditions. Some findings, such as heavier hydrocarbon fuels forming more carbon than lighter fuels, are expected, but other discoveries are surprising. For example, methanol shows a greater tendency to form carbon deposits than methane at temperatures below 800 °C, and kinetically controlled steam reforming with ethanol at high temperatures (∼800 °C) is less detrimental to SOFC performance than operating the device with dry methanol as the fuel. In situ optical techniques will continue to provide the chemical information and mechanistic insight that is critical for SOFCs to become a viable energy conversion technology. problematic than in low-temperature ( 800 °C, a H2O/C2H5OH ratio of close to 1:1 is expected to be sufficient to avoid deleterious coking.70 The large range of SR H2O/fuel compositions expected for clean operation of SOFC emphasizes lingering questions about the operational conditions that affect SOFC performance. Many published thermodynamic models imply that equilibrium is not readily achieved in SOFC systems,30,66 but the kinetic limitations of pyrolysis and reforming reactions of fuels larger than CH4 present complex challenges to models. Some studies, such as those of Cimenti et al. and Sasaki et al., have combined thermodynamic calculations with ex situ experimental data to quantify how fuels known to form carbon deposits are likely to behave.62,70 Similarly, the kinetic models published by Norinaga et al., Gupta et al., and Randolph and Dean may more accurately predict the kinetically limited fuel compositions, but the heterogeneous chemistry that can then occur on the anode surface is not addressed explicitly.56,61,71 Due to the combined effects of gas-phase and surface reforming and the effects of polarization-induced oxidation reactions, resolving the complicated chemistry that results from these processes requires in situ diagnostics.
(∼0.1−10 s) to avoid autocatalytic carbon deposition from CH4. Two additional studies predict that CH4 persists intact in the presence of the Ni catalysts of the SOFC anodes, despite their ability to promote pyrolysis.59,60 Thermodynamic and kinetic calculations, as well as experimental results from Lin et al., show that CH4 cracking or decomposition is kinetically slow and leads to less carbon deposition than is predicted by thermodynamic models. The extent of carbon deposition can be further reduced in polarized SOFCs as carbon species, either fuel or carbon deposited on the anode, can be oxidized to inhibit carbon accumulation Pyrolysis becomes increasingly complex with higher-MW fuels. Norinaga et al. studied the pyrolysis of C2 and C3 hydrocarbons through carbon vapor deposition and found that over 40 product species formed during residence times of 3:1 ratio suggested elsewhere.76 The small surface temperature variations detected in situ by a NIR camera provide real-time analysis of the early stages of SOFC failure, demonstrating the promise of this convenient and versatile imaging technique for system diagnostics capable of aiding SOFC development and operational protocol in a variety of applications.
Solid oxide fuels cells are promising devices for efficient power generation using legacy fuels, and SOFC development will benefit from understanding fundamental mechanisms responsible for both gas and surface chemistry in a manner that resembles the progress made in the area of combustion.
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Noninvasive, nondestructive, real-time monitoring has been achieved, and recent studies have begun to provide the constructive feedback with theoretical models necessary to achieve quantitative, predictive descriptions of the chemistry occurring in SOFCs. Combining in situ optical methods with electrochemical analysis and post mortem data will serve to identify and direct the mitigation of detrimental processes such as carbon deposition and thermal stress to maintain performance and avoid device failure. A thorough understanding of fuel chemistry in both the MEA anode chamber and a fuel reforming reactor would serve as a full-system monitoring tool capable of ensuring safe and reliable operation. Realizing the full potential of in situ optical techniques will provide both the chemical knowledge required for sustained, reliable operation of SOFCs and diagnostics capable of system evaluation to ensure proper cell function. Such information is essential if SOFCs are to mature as a viable bridging energy conversion technology capable of operating with traditional hydrocarbon fuels as well as solar fuels, biofuels, converted waste, and coalbased fuels that will gain importance as society weans itself off of a petroleum-based economy.
The range and impact of in situ studies of SOFC chemistry can be enhanced by developing and combining new methods and applying these techniques to a wider array of devices and conditions. For example, a trove of chemical information is likely to result from applying FTIR emission spectroscopy, pioneered by Lu et al.,46 to SOFC anodes where the chemistry of fuel oxidation will be more varied and complex than O2 reduction on SOFC cathodes. FTIR emission is a passive technique that should be capable of readily detecting intermediates such as adsorbed CO and CO2 that result from electrochemical oxidation of deposited carbon.77 Such data will help resolve the mechanisms involved in charge transfer and fuel oxidation in these high-temperature environments. In addition to studying individual devices, current in situ technologies can be adapted to larger, working SOFC systems such as stacks of MEAs. Recent thermal imaging data of SOFC anodes exposed to C2H5OH have shown large thermal gradients that depend on the types of chemistry occurring on catalysts even on small, button MEAs. This result further emphasizes the importance of the combined efforts of modeling electrochemical experiments and in situ observations to understand chemical changes in the fuel composition, electrochemical reactions, and mass/charge transport for both model and larger systems closer to practical cells. Smaller architectures can also continue to be used to address important questions in electrochemical redox chemistry and device degradation. For example, in situ XPS has explored the chemistry of specific regions of the anode side of SOFCs.33,44,45 In one recent study, for example, position-dependent potentials measured by XPS were identified across a single-chamber solid oxide electroysis cell (SOEC) with ceria anodes to spatially identify the extent of the electrochemical double layer.33 Future work in this area will focus on performing experiments under conditions closer to those found in practical systems and, possibly, using emerging tabletop X-ray sources78 that would facilitate small laboratory experiments free from the constraints of synchrotrons. Through nanostructured patterned architectures, the most active region of the MEAs, that is, near the triple-phase boundary, can be spatially defined and isolated. In situ optical techniques developed to study chemistry in SOFCs can also be applied to other electrochemical energy systems, including PEM fuel cells and lithium ion batteries. For example, the largest concern for durability and safety in lithium batteries is the deterioration of LiCoO2 and other cathode materials. In situ Raman studies of these materials have been conducted in operating batteries79 and model electrochemical cells,80 but the methods have yet to yield the level of quantitative data required to characterize material properties during a wide range of operating conditions. The focus of in situ studies of PEM fuel cell systems has been on processes associated with liquid diffusion through the membrane; the content and distribution of water,81−84 alcohol,84 and protic ionic liquids85 have all been studied using Raman spectroscopy. At the time of this Perspective, numerous studies of PEM catalysts have been reported,86 but in situ studies of catalysts in operating PEM fuel cells are limited to X-ray-based techniques that often require low-pressure experimental conditions. Many of the problems that plague SOFCs, such as carbon and sulfur contamination, also represent the death knell of PEM devices. The optical techniques that have been developed for hightemperature use in functioning SOFCs can easily be applied to lower-temperature systems where an understanding of failurerelated chemistry is needed.
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AUTHOR INFORMATION
Notes
The authors declare no competing financial interest. Biographies Michael B. Pomfret earned his Ph.D. in Chemistry from the University of Maryland, studying electrochemical oxidation mechanisms and fuel utilization in SOFCs. Following a tenure as an NRL/ NRC postdoctoral fellow, he joined the Molecular Dynamics Section in the Chemistry Division at the Naval Research Laboratory. His research focuses on materials development for fuel cell catalysts and electrode supports and the application of optical measurements to high- and low-temperature fuel cells and other high-temperature systems. Robert A. Walker earned his Ph.D. in Chemistry from the University of WisconsinMadison, where his doctoral research used gas-phase molecular beam spectroscopy to study large-amplitude motion in aromatic molecules. He is a Professor in the Chemistry and Biochemistry Department at Montana State University. Walker directs a research program that uses nonlinear optical spectroscopy to study structure, organization, and reactivity at liquid interfaces, and he led the team that first applied in situ vibrational Raman scattering to examine high-temperature solid oxide fuel cells. Jeffrey C. Owrutsky received his Ph.D. from the University of California, Berkeley, in Physical Chemistry investigating, highresolution infrared spectroscopy of gas-phase ions. He is currently head of the Molecular Dynamics Section in the Chemistry Division at the U.S. Naval Research Laboratory, where his research interests include spectroscopy and dynamics of confined and interfacial media, ionic liquids, plasmonic nanomaterials, and in situ optical studies of solid oxide fuel cells.
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ACKNOWLEDGMENTS Support for this work was provided by the Office of Naval Research. The authors acknowledge Daniel Steinhurst at NRL, John Kirtley at Montana State University, Bryan Eigenbrodt at AFRL Wright-Patterson, Bryan W. Eichhorn and Gregory S. Jackson at the University of Maryland College Park, and Robert J. Kee and Anthony M. Dean at the Colorado School of Mines. 3061
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