Current collector for SOFC fuel cells

Details Current collector for SOFC fuel cells Current collector for SOFC fuel cells

2002-05-31

2004-05-18

Ryan, Patrick (Department: 1745)

Chemistry: electrical current producing apparatus, product, and

With pressure equalizing means for liquid immersion operation

C429S006000, C429S047000, C429S006000, C429S010000

Reexamination Certificate

active

06737186

ABSTRACT:

BACKGROUND OF THE INVENTION
Field of the Invention
The invention pertains to a current collector made from ferritic iron alloy for electrically connecting and mechanically supporting a set of individual, planar SOFC high-temperature fuel cells (solid oxide fuel cells). The fuel cells comprise an anode, an electrolyte, and a cathode, operate at temperatures of between 700° C. and 900° C., and are equipped with a solid electrolyte.
In recent years, SOFC high-temperature fuel cells have experienced considerable progress in development and are beginning to become economically viable. The SOFC-type fuel cell is wherein by a plate-like structure and a solid oxide ceramic electrolyte. Different oxide ceramic electrolytes, for example doped zirconium oxide (zirconia) or cerium oxide (ceria), are used depending on the working temperature selected for the cell in the range between 500° and 1000° C. The cell voltage of an individual fuel cell is approximately 1 volt, and therefore it is always necessary for a multiplicity of individual cells with surface dimensions which are as large as possible to be stacked and electrically connected in series in order to achieve electrical voltages and power outputs that are technically useful.
In actual fact, nowadays plate-like fuel cell arrangements with a surface area of up to 1000 cm
2
, wherein the thickness of the electrodes and of the solid electrolyte is regularly much less than 100 &mgr;m, are used. The lowest possible electrolyte thickness, which is important for the efficiency of the cell, is between 5 and 30 &mgr;m. In this context, a distinction is drawn between unsupported and supported electrolytes, e.g. of the ASE (anode supported electrolyte) type. Plate-like individual cells of this type stacked on top of one another are separated from one another by so-called current collectors, also known as connecting elements, interconnectors, or bipolar plates. The cells are mainly supplied with the required fuels and the reactive media are removed, and the cells are at the same time also mechanically stabilized, by means of open distribution passages in the current collectors.
It is therefore quite understandable that the development of suitable current collectors has in recent years been the subject of considerable attention, both with regard to the selection of material and with regard to economic fabrication thereof to form complex components. The complexity of the components is primarily determined by the generally filigree, open passage and line systems used for the gaseous media.
To be satisfactorily useable over the entire fuel cell service life, which has to be sufficiently long from an economic viewpoint, the current collectors have to meet high demands imposed on a wide range of mechanical, physical and chemical material properties and at the same time it must be possible to manufacture the current collectors at relatively low cost. The material costs alone must not make the overall fuel cells system commercially unattractive.
The indispensable high material quality demands relate to:
high mechanical strength, in particular high rigidity of even thin current collector plates over the wide temperature range between room temperature and approx. 1000° C.
optimum matching of the coefficient of thermal expansion to that of the solid electrolyte film: this match must be equally present at any temperature in the entire range between room temperature and working temperature.
high thermal and electrical conductivity, low electrical surface contact resistance, including maintaining these values throughout the entire service life of a fuel cell.
high corrosion resistance of the material with respect to the fuel gas and exhaust gas atmospheres in the cell, which on the anode side are substantially hydrogen and H
2
O vapor, CO and CO
2
, and on the cathode side are substantially oxygen and air.
The development of suitable materials for current collectors was initially concentrated on chromium alloys. In recent years, the development concentration has shifted to ferritic iron alloys with significant levels of chromium.
During the efforts to further refine the proposed ferritic alloys for current collectors in SOFC-type fuel cell units, it has been important to suppress the formation of volatile chromium compounds and the vaporization of these compounds from the current collector surface as far as possible. By way of example, one countermeasure proposed has been the addition of suitable quantities of titanium and manganese.
Even with the ferritic materials, which are known to be resistant to corrosion, it has been impossible to completely avoid superficial growth of oxide. To reduce the oxide growth rate, but at the same time also to increase the mechanical strength, it has been proposed to add small quantities of the elements yttrium, cerium, lanthanum, zirconium and/or hafnium.
With materials developments of this type, the person skilled in the art has been relying on the theoretical and empirical knowledge of the action of individual metallic and nonmetallic components. Known ferritic iron-based materials with a multiplicity of additions which have by now been described, in view of the state which has been reached in the demands for matching a wide range of extremely divergent materials properties, make a prediction about measures aimed at further matching of properties impossible or at least rather dubious.
The validated prior art forms an important platform but not a reliable indicator toward materials developments of this nature.
For example U.S. Pat. No. 6,156,448 (European patent EP 0 880 802 B1) describes a high-temperature fuel cell with stabilized zirconia as solid electrolyte, wherein the current collectors consist of an iron-based alloy comprising 17 to 30% by weight of chromium, such that this material has a coefficient of thermal expansion of between 13 and 14×10
−6
K
−1
.
A material that is characterized in this way for current collectors has no guiding significance in the context of this description with regard to matching of properties. Even with regard to the coefficients of thermal expansion, nowadays more refined criteria apply, for example in connection with the design and material of the solid electrolyte used in each case.
U.S. Pat. No. 5,800,152 (European published patent application EP 0 767 248 A1) describes an oxidation-resistant, metallic material, in particular also for use in current collectors for high-temperature fuel cells, of the following composition: 15 to 40% by weight of chromium, 5 to 15% by weight of tungsten, 0.01% to 1% by weight of one or more elements selected from the group consisting of Y, Hf, Ce, La, Nd and Dy, remainder iron, which material has a coefficient of thermal expansion of more than 12×10
−6
and less than 13×10
−6
K
−1
in the temperature range between room temperature and 1000° C.
As an alternative, this material must additionally contain 0.001 to 0.01% by weight of boron.
The document states that this material is specifically designed for use in combination with zirconium oxide as solid electrolyte at working temperatures of between 900° C. and 1000° C.
An article by the two inventors of the noted patent which was published after the priority date of this description (M. Ueda, H. Taimatsu, Thermal Expansivity and High-Temperature Oxidation Resistance of Fe—Cr—W Alloys Developed for a Metallic Separator of SOFC, 4
th
European SOFC Forum Lucerne, Jul. 10-14, 2000) provides a very critical report on difficulties and drawbacks of the said material as a current collector. Alloys containing more than 18% by weight of chromium are considered to be difficult to process. The report refers to layers which are formed on the material as a result of corrosion and which flake off.
Despite tests using the Cr and W contents over the entire range covered by the scope of protection of the alloy, it was impossible for the coefficient of thermal expansion of the alloy to be satisfactorily matched to the coefficient for yttrium-stabilized ZrO
2
solid electrol

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