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Direct hydrocarbon fuel cellsRelated Patent Categories: Chemistry: Electrical Current Producing Apparatus, Product, And Process, Fuel Cell, Subcombination Thereof Or Methods Of OperatingDirect hydrocarbon fuel cells description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20060177706, Direct hydrocarbon fuel cells. Brief Patent Description - Full Patent Description - Patent Application Claims
FIELD OF THE INVENTION [0002] This invention relates generally to fuel cells and assemblies, more particularly, cells and components thereof as can be configured for use with hydrogen fuel or the direct oxidation of hydrocarbons. BACKGROUND OF INVENTION [0003] Fuel cells are promising electrical power generation technologies, with key advantages including high efficiency and low pollution. Most potential near-term applications of fuel cells require the use of hydrocarbon fuels such as methane, for which a supply infrastructure is currently available. However, fuel cells typically operate only with hydrogen as the fuel. Thus, current demonstration power plants and planned fuel-cell electric vehicles must include a hydrocarbon fuel reformer to convert the hydrocarbon fuel to hydrogen. Fuel cells that could operate directly on hydrocarbon fuels would eliminate the need for a fuel reformer, providing considerable system and economic advantages and presumably improving the viability of the technology. [0004] Prior art fuel cells utilizing hydrocarbon fuels directly have encountered significant problems. For example, direct-methanol polymer electrolyte fuel cells produce relatively low power densities and require prohibitively large Pt loading of the anodes. In addition, methanol can permeate the electrolyte. See, for instance, Ren, X., Wilson, M. S. and Gottesfeld, S. High performance direct methanol polymer electrolyte fuel cells. J. Electrochem. Soc., 143, L12-L14 (1996); and Wang, J., Wasmus. S. and Savinell, R. F. Evaluation of ethanol, 1-propanol, and 2-propanol in a direct oxidation polymer-electrolyte fuel cell a real-time mass spectrometry study. J. Electrochem. Soc., 142, 4218-4224 (1995). Furthermore, only alcohol fuels appear feasible with this approach. [0005] Alternatively, prior art solid oxide fuel cells (SOFCs) can utilize hydrocarbons directly via internal or external reforming. In this approach, a hydrocarbon fuel (e.g., methane) is combined with H.sub.2O and/or CO.sub.2, which are typically obtained by recirculating the fuel cell exhaust, and introduced directly to the SOFC anode. Commonly used Ni-based anodes provide the catalyst for the endothermic reforming reactions, CH.sub.4+H.sub.20=3H.sub.2+CO .DELTA.H.degree..sub.298=206 kJ/mol CH.sub.4 (1) CH.sub.4+CO.sub.2=2H.sub.2+2CO .DELTA.H.degree..sub.298=247 kJ/mol CH.sub.4 (2) However, maintaining appropriate gas composition and temperature gradients across a large area SOFC stack is challenging. See, Janssen, G. J. M., DeJong, J. P., and Huijsmans, J. P. P. Internal reforming in state-of-the-art SOFCs. 2nd European Solid Oxide Fuel Cell Forum, 163-172, Ed. by Thorstense, B. (Oslo/Norway, 1996); and Hendriksen, P, V., Model study of internal steam reforming in SOFC stacks. Proc. 5th Int. Symp. on Solid Oxide Fuel Cells, 1319-1325, Ed. by U. Stimming, S. C. Singhal, H. Tagawa, and W. Lehnert (Electrochem, Soc., Pennington, 1997). [0006] For instance, if the reforming reactions are slow, then insufficient H.sub.2 is supplied to the SOFCs. On the other hand, fast reforming reactions cause cooling localized near the fuel inlet, leading to poor cell performance, and possible cell fracture. Thus, current SOFC stacks of the prior art do not take full advantage of internal reforming; rather, they employ a combination of .apprxeq.75% external and 25% internal reforming of hydrocarbon fuels. See, Ray, E. R. Westinghouse Tubular SOFC Technology, 1992 Fuel Cell Seminar, 415-418 (1992). [0007] SOFCs can in principle operate by direct electrochemical oxidation of a hydrocarbon fuel. This approach would be desirable since it eliminates the problems with internal reforming mentioned above, and the theoretical maximum fuel efficiency is as good or better than that for reforming. However, prior art attempts with SOFCs operating at temperatures T.sub.c=900-1000.degree. C. with methane fuel have been less than satisfactory: either power densities were very low or carbon deposition was observed. See, Putna, E. S., Stubenrauch, J., Vohs, J. M. and Gorte, R. J. Ceria-based anodes for the direct oxidation of methane in solid oxide fuel calls, Langmuir 11, 4832-4837 (1995); and Aida, T., Abudala, A., Ihara, M., Komiyama, H. and Yamada, K. Direct oxidation of methane on anode of solid oxide fuel cell. Proc. 4th Int. Symp. on Solid Oxide Fuel Cells, 801-809, Ed. by Dokiya, M., Yamamoto, O., Tagawa, H. and Singhal, S. C, (Electrochem. Soc. Pennington, 1995). [0008] Recently, SOFCs have been developed to produce high power densities with hydrogen at reduced temperatures, T.sub.c=600-800.degree. C. See, Huebner, W., Anderson, H. U., Reed, D. M., Sehlin, S. R. and Deng, X. Microstructure property relationships of NiZrO.sub.2 anodes. Proc. 4th Int. Symp. on Solid Oxide Fuel Cells, 696-705, Ed. by Dokiya, M., Yamamoto, O., Tagawa, H. and Singhal, S. C. (Electrochem. Soc. Pennington, 1995); daSouza, S., Visco, S J. and DeJonghe, L. C. Thin-film solid oxide fuel cell with high performance at low-temperature. Solid State Ionics 98, 57-61 (1997); Fung, K-Z., Chen, J., Tanner, C. and Virkar, A. V. Low temperature solid oxide fuel cells with dip-coated YSZ electrolytes. Proc. 4th Int. Symp. on Solid Oxide Fuel Cells, 1018-1027, Ed. by Dokiya, M., Yamamoto, O., Tagawa, H. and Singhal, S. C. (Electrochem. Soc. Pennington, 1995); Minh, N. Q. Development of thin-film solid oxide fuel cells for power generation applications. Proc. 4th Int. Symp. on Solid Oxide Fuel Cells, 138-145, Ed. by Dokiya, M., Yamamoto, O., Tagawa, H. and Singhal, S. C. (Electrochem. Soc. Pennington, 1995); Godickemeier, M., Sasaki, K. and Gauckler, L. J. Current-voltage characteristics of fuel cells with ceria-based electrolytes. Proc. 4th Int. Symp. on Solid Oxide Fuel Cells, 1072-1081, Ed. by Dokiya, M., Yamamoto, O., Tagawa, H. and Singhal, S. C. (Electrochem. Soc. Pennington, 1995); Tsai, T. and Barnett, S. A. Increased solid-oxide fuel cell power density using interfacial ceria layers. Solid State Ionics 98, 191-196 (1997); and Tsai, T., Perry, E. and Barnett, S. Low-temperature solid-oxide fuel cells utilizing thin bilayer electrolytes. J. Electrochem. Soc., 144, L130-L132 (1997). However, such systems have not been considered or used for direct-hydrocarbon operation, because carbon deposition reaction rates decrease with decreasing temperature. In fact, there are no known reports SOFC operation on hydrocarbons at T.sub.c<800.degree. C. [0009] SOFCs and related stacking configurations have undergone considerable development over the past decade. Tubular-cell-based technologies appear to be a promising approach for SOFC stacking. Tubular stacks avoid sealing and manifolding problems inherent to planar stacks, but take a large volume for a given cell active area and can show significant ohmic losses related to current transport around the tube circumference through the (La,Sr)MnO3 (LSM) cathode. Another problem is the relatively poor mechanical toughness of LSM. [N. M. Sammes, R. Ratnaraj, and C. E. Hatchwell, Proceedings of the 4th International Symposium on Solid Oxide Fuel Cells, Ed. By Dokiya, O. Yamamota, H. Tagawa, and S. C. Singhal (Electrochemical Society, Pennington, 1995) p. 952. B. Krogh, M. Brustad, M. Dahle, J. L. Eilertsen, and R. Odegard, Proceedings of the 5th International Symposium on Solid Oxide Fuel Cells, Ed. By U. Stimming, S. C. Singhal, H. Tagawa, and W. Lehnert (Electrochemical Society, Pennington, 1997) p. 1234.] This is typical of SOFC ceramic materials, which are optimized for electrical properties rather than mechanical toughness. [0010] Alternatively, planar stacks can provide higher power-to-volume ratios than tubular stacks, but are not as mechanically robust as tubes and require excellent seals. Another problem with many planar stack designs is that they require pressure contacts between separate SOFC and interconnect plates. This places stringent requirements on the flatness of large-area ceramic plates, making manufacturing difficult and expensive. Furthermore, there are often relatively high resistances associated with these contacts, which deleteriously affect stack performance. It is clear that a choice between tubular and planar stacks involves trade-offs. Even so, the disadvantages associated with each respective approach present obstacles for effective use of SOFCs and suggest a new direction is needed to better utilize and benefit from this technology. OBJECTS OF THE INVENTION [0011] There are a considerable number of problems and the deficiencies associated with the use of hydrocarbons with solid oxide fuel cells. There is a demonstrated need for the use of such fuels in an efficient, economical fashion so as to improve the viability of the related technology. [0012] Accordingly, it is an object of the present invention to provide various solid oxide fuel cells and/or components which can be used with hydrocarbon fuels thereby overcoming various deficiencies and shortcomings of the prior art, including those outlined above. It will be understood by those skilled in the art that one or more aspects of this invention can meet certain objectives, while one or more other aspects can meet certain other objectives. Each objective may not apply equally, in all instances, to every aspect of the present invention. As such, the following objects can be used in the alternative with respect to any one aspect of the present invention. [0013] It can be an object of the present invention to increase the rate of hydrocarbon oxidation so as to increase and/or otherwise provide useful power densities. Such densities can be increased and/or provided utilizing various catalytic metals in the fabrication of fuel cell anodes, such anodes as can be used in conjunction with a ceria material. [0014] It can be an object of the present invention to utilize solid oxide fuel cells and/or components thereof for low temperature direct hydrocarbon oxidation. [0015] It can also be an object of the present invention to provide various anodes and related cellular components having small particle size obtainable by sputter deposition processes and/or related fabrication techniques. [0016] It can also be an object of the present invention to provide a method for hydrocarbon oxidation, at a temperature lower than 800.degree. C. and/or at a temperature for a specific hydrocarbon whereby there is no carbon deposition. [0017] It can also be an object of the present invention to improve the viability of solid oxide fuel cells, both those described herein as well as those otherwise known in the art, through use of a unique assembly of such cells having a configuration and/or geometry of the type described herein. In particular, it is an object of this invention to provide a cell geometry/configuration whereby all active fuel cell components and interconnects are deposited as thin layers on an electrically insulating support. [0018] It can also be an object of the present invention, in conjunction with one or more of the preceding objectives, to provide a geometry/configuration for an assembly of solid oxide fuel cells whereby assembly design and choice of support material can enhance mechanical durability and thermal shock resistance. A related objective is to decrease overall material cost by providing all cell-active materials in thin layer/film form. [0019] It can also be an object of the present invention to improve a number of solid oxide fuel cell performance or function parameters through integration of the cell components and interconnects on a common support, such advantages including reduction of electrical resistances and interconnect conductivity requirements. As described more fully below, such integration can be accomplished through use of the thin film/layer configurations and related geometries described herein. [0020] Another object of this invention is to provide a cell assembly configuration suitable for SOFCs of the type described herein, especially those operable at low temperatures for direct oxidation of hydrocarbon fuels, such cells as can be prepared to preferentially incorporate the catalytic metal anodes of this invention. [0021] Other objects, features, benefits and advantages of the present invention will be apparent from the following summary and descriptions, and will be readily apparent to those skilled in the art made aware of this invention and having knowledge of various and solid oxide fuel cells in the use of hydrocarbon fuels. Such objects, features, benefits and advantages will be apparent from the above as taken in conjunction with the accompanied examples, tables, data and all reasonable inferences to be drawn therefrom. SUMMARY OF THE INVENTION Continue reading about Direct hydrocarbon fuel cells... 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