Chemistry: electrical current producing apparatus – product – and – Having earth feature
Reexamination Certificate
1998-12-31
2001-06-19
Kalafut, Stephen (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Having earth feature
C429S006000, C429S047000, C429S047000, C429S047000
Reexamination Certificate
active
06248468
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to fuel electrodes for solid oxide fuel cells and their method of manufacture.
2. Background Information
Solid oxide electrolyte fuel cells (“SOFC”) are well known in the art and are described generally in U.S. Pat. No. 4,490,444 (Isenberg). Such an SOFC can be of a flat plate, tubular, or other configuration, and typically operates at a temperature of from 700° C. to 1100° C., with a solid ceramic electrolyte disposed between two solid ceramic electrodes. In operation, one electrode is in contact with oxygen or air (“air electrode”) and the other electrode is in contact with fuel (“fuel electrode”).
Nickel/zirconia cermet (ceramic-metal) fuel electrodes are typically used for SOFC's, as exemplified by the tubular configuration of the Isenberg '444 patent. Seven properties, in particular, are desired to produce a commercially viable fuel electrode of this type. These are: high conductivity; adherence which does not deteriorate due to aging or thermal cycling; a large area of electrochemically active sites at the electrode/electrolyte interface; chemical and physical stability over a wide range of fuel atmospheres; minimal microstructural changes with operating time (since such changes are often accompanied by deterioration of SOFC voltage performance); adequate porosity, that is 25% to 45% porous by volume, to minimize concentration polarization; and, very importantly, lower fabrication costs.
A variety of procedures have been tried to satisfy these requirements, as described in U.S. Pat. Nos. 3,503,808 and 5,227,258 (Agruss et al. and Ito et al., respectively); Satoshi Ohara et al.,
Proc. Fifth International Symposium on Solid Oxide Fuel Cells
, Vol. 97-40, p. 815; A. Tsoga et al., ibid, p. 823; R. Wilkenhoner et al., ibid, p. 851; Tomoo Iwata,
J. Electrochem Soc
., 143, p. 1521 (1966); an Nguyen Q. Minh,
J. Am. Ceram. Soc
., 76 [3], p. 563-588 (1993).
The most successful process to make these fuel electrodes to date has been the use of an electrochemical vapor deposition (“EVD”) method. This EVD method forms a yttria stabilized zirconia “skeleton” within and surrounding a porous matrix of nickel particles, as described in U.S. Pat. Nos. 4,582,766 and 4,597,170 (Isenberg et al. and Isenberg, respectively). In one embodiment of this EVD process, nickel fibers, about 3 mm to 15 mm long and 0.01 mm to 0.05 mm diameter, are deposited within the fuel electrode as described in U.S. Pat. No. 4,847,172 (Maskalick et al.). Alternate sintering processes, not requiring expensive EVD equipment, offer a potential cost saving if they are able to meet all technical objectives. One of the serious problems encountered by simple sintering processes for forming the cermet fuel electrode has been microstructural changes in the cermet which, can cause deterioration of the SOFC voltage over time. Thus, what is needed is an improved electrode and method of electrode manufacture which meets all the previous requirements, but which uses materials that can allow ease of low cost fabrication.
SUMMARY OF THE INVENTION
Therefore, it is one of the main objects of this invention to provide a low cost sinter-fabrication process, providing fuel electrodes with improved performance compared to previous sintered electrodes.
It is another main object of this invention to provide a “green”, that is, pre-sintered, fuel electrode structure with a large number of potential active sites providing potential high conductivity upon sintering.
These and other objects of the invention are accomplished by providing a fuel cell structure comprising a pre-sintered nickel/zirconia fuel cell electrode and an air electrode, with a ceramic electrolyte disposed between the electrodes, where the pre-sintered fuel electrode comprises (1) at least one of nickel oxide, cobalt oxide and cerium dioxide particles and (2) titanium dioxide particles, within a matrix of yttria stabilized zirconia particles and spaced-apart filamentary nickel powder beads, where the beads and filaments of the filamentary nickel powder have contact points throughout the fuel electrode structure. Preferably the beads have a average particle size between about 2.0 micrometers and 2.8 micrometers diameter.
The invention also comprises a method of forming a pre-sintered, exterior fuel electrode on a ceramic electrolyte comprising, applying to a ceramic electrolyte surface a fuel electrode slurry composition comprising the admixture in a liquid media:
i) at least one of nickel oxide, cobalt oxide and cerium dioxide particles,
ii) titanium dioxide particles,
iii) filamentary nickel powder beads, and
iv) yttria stabilized zirconia particles,
to provide a non-sagging coating of the fuel electrode composition having a firm, pre-sintered fuel electrode structure attached to the electrolyte, where the fuel electrode structure contains nickel oxide particles and titanium dioxide particles within a matrix of yttria stabilized zirconia and spaced-apart filamentary nickel powder beads where the beads and filaments of the filamentary nickel powder beads have contact points throughout the fuel electrode structure.
Preferably the process will be a dipping process using a viscous, homogeneous slurry of the composition in a liquid media, and removal of the ceramic electrolyte surface from the slurry prior to drying.
These new fuel electrode structures provide, upon sintering, high conductivity, good adherence to the solid electrolyte, a large area of electrochemically active sites, chemical and physical stability, minimal structural changes at operating temperatures over time, low cost fabrication and good porosity for fuel gas interaction. The use of the term “green” structure or electrode here will be interchangeable with the word “pre-sintered”, that is, a structure or electrode that is firm, formed, but not cured or sintered, and which may contain various organic plasticizers, binders, solvents or other additives that will generally be “burned-off” during intense heating or sintering.
REFERENCES:
patent: 3503808 (1970-03-01), Agruss et al.
patent: 4490444 (1984-12-01), Isenberg
patent: 4582766 (1986-04-01), Isenberg et al.
patent: 4597170 (1986-07-01), Isenberg
patent: 4847172 (1989-07-01), Maskalick et al.
patent: 5035962 (1991-07-01), Jensen
patent: 5227258 (1993-07-01), Ito et al.
patent: 5908713 (1999-06-01), Ruka et al.
PCT International Search Report (3pp.) RDV98-002PCT; international filing date Jan. 12, 1999.
PCT International Application (11pp.) WO 99/16140; International publication date Apr. 1, 1999.
INCO Nickel Powders Properties and Applications, Single Spikey Particles INCO Type 123 Nickel Powder, 20pp. Date Unknown.
J. Am.Ceram Soc. 73{3} Ceramic Fuel Cells, Nguyen Q. Minh et al., 563-88 (1993).
Proceedings of the Fifth International Symposium on Solid Oxide Fuel Cells (SOFC-V), vol. 97-40, p. 815-32 and 851-860 Date Unknown.
J. Electrochem Soc 143, Characterization of Ni-YSZ Anode Degradation for Substrate-Type solid Oxide Fuel Cells, T. Iwata, p. 1521-1525. Date Unknown.
Ruka Roswell J.
Vora Shailesh D.
Eckert Seamans Cherin & Mellott , LLC
Kalafut Stephen
Siemens Westinghouse Power Corporation
Wills M.
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