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United States Patent [i9] [n] Patent Number: 5,273,837
Aitken et al.  Date of Patent: Dec. 28, 1993
U.S. Patent Dec. 28, 1993 Sheet 1 of 2 5,273,837
 SOLID ELECTROLYTE FUEL CELLS
 Inventors: Bruce G. Aitken; Margaret K. Faber, both of Corning; Thomas D. Ketcham, Big Flats; Dell J. St. Julien, Watkins Glen, all of N.Y.
 Assignee: Corning Incorporated, Corning, N.Y.
 Appl. No.: 996,140
 Filed: Dec. 23,1992
 Int. C1.5 H01M8/10
 U.S. CI 429/30; 429/32;
 Field of Search 429/30-33,
429/34, 193; 204/410, 421; 501/104, 105, 102
 References Cited
U.S. PATENT DOCUMENTS
4,476,196 10/1984 Poeppel et al 429/32
4,476,198 10/1984 Ackerman et al 429/32
4,510,212 4/1985 Fraioli 429/30
4,812,329 3/1989 Isenberg 429/33 X
4,827,07! 5/1989 Hazbun 585/443
4,883.497 11/1989 Claar et al 429/33 X
4,888,254 12/1989 Reichner 429/31
4,988,582 1/1991 Dyer 429/30
5,049.459 9/1991 Akagi 429/33
5,089,455 2/1992 Ketcham et al 501/104
5,145,754 9/1992 Misawa et al 429/32
FOREIGN PATENT DOCUMENTS WO88/08045 10/1988 World Int. Prop. O. .
"Ceramics in Fuel Cells", K. Kendall, Ceramic Bulletin,
vol. 70, #7, 1991 pp. 1159-1160.
N. Q. Minh, "High-Temperature Fuel Cells Part 1",
Chemtech, Jan., 1991, pp. 32-37.
N. Q. Minh, "High-Temperature Fuel Cells Part 2:The
Solid Oxide Cell", Chemtech, Feb., 1991, pp. 120-126.
"Economic Fuel Cells by Design", K. Kendall, 1992
Fuel Cell Seminar, Nov. 29-Dec. 2,1992, Tucson, Ariz.
"Solid Oxide Fuel Cells—The Next Stage", B. Riley, J.
Power Sources, vol. 29 (1990), pp. 223-237.
"Ceramic tape casting for solid oxide fuel (SOFC) elec-
trolyte production", S. V. Phillips et al., Conf. on Ce-
ramics in Energy Applications, Apr., 1990, Sheffield,
Primary Examiner—Anthony Skapars
Attorney, Agent, or Firm—Kees van der Sterre
SOLID ELECTROLYTE FUEL CELLS
BACKGROUND OF THE INVENTION
The present invention relates to fuel cells of the type known as solid electrolyte or solid oxide fuel cells, and more particularly to improved materials and methods for making them.
The use of solid electrolyte materials for fuel cells and oxygen pumps has been the subject of a considerable amount of research for many years. The typical essential components of a solid oxide fuel cell comprise a dense, oxygen-ion-conducting electrolyte sandwiched between porous, conducting metal, cermet, or ceramic electrodes. Electrical current is generated in such cells by the oxidation, at the anode, of a fuel material such as hydrogen which reacts with oxygen ions conducted through the electrolyte from the cathode.
Practical power generation units will typically comprise multiple fuel cells of such configuration interconnected in series or parallel with electronically conductive ceramic, cermet or metal interconnect materials. At the present time, the materials of choice for such devices include ... stabilized zirconia (Zr02) for the electrolyte, nickel-Zr02 cermet for the anode material, strontium-doped lanthanum manganite (LaMn03) for the cathode, and strontium-doped lanthanum chromite (LaCr02) as an electronically conductive layer serving as a cell interconnect material. At sufficient temperatures (e.g., 800° C. or above), zirconia electrolytes can exhibit good ionic conductivity but low electronic conductivity.
Several different designs for solid oxide fuel cells have been developed, including, for example, a supported tubular design, a segmented cell-in-series design, a monolithic design and a flat plate design. All of these designs are documented in the literature, one recent description having been provided by N. Q. Minh in "High-Temperature Fuel Cells Part 2: The Solid Oxide Cell", CHEMTECH, February 1991, pp. 120-126.
The tubular design comprises a closed-end porous zirconia tube exteriorly coated with electrode and electrolyte layers. The performance of this design is somewhat limited by the need to diffuse the oxidant through the porous tube. The segmented cell-in-series design comprises tube-supported and self-supporting variants, with the tube-supported designs being somewhat limited in performance by the need to diffuse the fuel gas through the support tube. The self-supporting variant of the cell-in-series design comprises composite electrode/electrolyte layers which are stacked in series to provide a self-supporting tubular structure. This approach, however, requires relatively thick layers (typically at least 100 microns) which again can limit fuel cell performance.
Flat plate designs offer somewhat higher power density than tubular cell configurations, this type of design being typified by the use of pre-sintered electrolyte sheets. In one subclass of flat plate designs, the electrolyte sheets are coated on opposite sides with anode and cathode layers, coating being accomplished by conventional means such as vapor deposition, slurry coating, or plasma spraying. The electrode coatings thus provided are generally quite thick, typical plasma sprayed anode thicknesses, for example, being on the order of 500 to 1000 micrometers.
To form the final cell structure, the thickly coated electrode/electrolyte sheets thus provided are stacked
tightly with current conducting bipolar plates, typically composed of strontium doped lanthanum chromite (such as ... or ... where x=0.1 to 0.2). These plates allow for manifolding of the oxi
5 dant gas and the fuel. In addition, they serve as collectors of current from across the electrode surfaces (current collectors), and as current-carrying conduits between anode and cathode layers (cell interconnects). Since these bipolar plates are also quite thick (typically
10 3 to 4 mm in thickness), the resulting flat plate fuel cell structures are essentially rigid and non-compliant.
In a second subclass of flat plate designs, the electrolyte sheet is stacked between porous plates of anode, cathode, and current conducting interconnect material.
15 Each stack is held tightly so as to ensure good electrical contact between the various components. The manifolding is provided not by a bipolar plate but by gas delivery tubes which extend through the structure. As with the other flat plate designs, this design requires the
20 components to be rigid in order to function properly. Monolithic designs, which characteristically have a multi-celled or "honeycomb" type of structure, offer the advantages of high cell density and high oxygen
25 conductivity. The cells are defined by combinations of corrugated sheets and flat sheets incorporating the various electrode, conductive interconnect, and electrolyte layers, with typical cell spacings of 1-2 mm and electrolyte thicknesses of 25-100 microns. A key advantage of
30 monolithic designs, in addition to the large surface-areato-volume ratio, is reduced voltage loss from internal electrical resistance, due to the small cell sizes and thin cell components employed. At the present time, monolithic solid oxide fuel cells
35 are made by tape casting or calendar rolling the various sheet components of the cell, forming the entire cell as a green body, and co-firing the resulting green structure into a unitary or monolithic assembly. This so-called "co-sintering" method of fabrication places significant
40 constraints on both cell design and on the manufacturing process, since the electrolyte, electrode, and interconnect materials must have compatible firing temperatures and similar coefficients of thermal expansion to achieve homogeneous consolidation and to avoid struc
45 tural defects such as cracking during firing.
It is a principal object of the present invention to provide an improved fuel cell construction, applicable to the fabrication of any of the above fuel cell designs but particularly applicable to multi-celled or "honey
50 comb" fuel cell structures similar to those of monolithic fuel cells, which avoids many of the difficulties of fuel cell manufacture while providing a cell of improved physical, thermal and electrical properties.
It is a further object of the invention to provide im
55 proved methods for the fabrication of fuel cells similar in structure to monolithic fuel cells, but which avoid many of the difficulties associated with prior art co-sintering manufacturing methods. Other objects and advantages of the invention will
60 become apparent from the following description thereof.
SUMMARY OF THE INVENTION
The present invention encompasses solid electrolyte 65 fuel cell designs incorporating circuit components and/or structural components of improved durability, most importantly, of significantly improved thermal durability. Hence the fuel cells and fuel cell components of the