Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Patent
1996-09-17
1998-03-31
Maples, John S.
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
429 34, H01M 802
Patent
active
057336822
DESCRIPTION:
BRIEF SUMMARY
CROSS REFERENCE TO RELATED APPLICATIONS
This application is a national stage of PCT/DE 95/00432 filed 25 Mar. 1995 and based, in turn, on German National application P 44 10 711.0 filed 28 Mar. 1994.
FIELD OF THE INVENTION
The invention relates to a metallic bipolar plate for a high-temperature fuel cell.
BACKGROUND OF THE INVENTION
High-temperature fuel cells (solid oxide fuel cells--SOFC) offer the possibility of direct conversion of chemical into electrical energy. The fuel (H.sub.2, CH.sub.4, CO, etc.) is separated from the oxidation medium (O.sub.2, air) by an oxygen-conducting solid electrolyte (Y-stabilized ZrO.sub.2). At the operating temperature of the cell (approximately 950.degree. C.) oxygen ions are transported from the cathode side through the electrolyte and react at the anode side with the fuel. Because of charge balance, an electron current flows in the same direction.
For increasing the rates of the described reactions, the electrolyte must be coated with porous catalytically-active electrode materials. In general, the anode or fuel side is provided with an Ni/ZrO.sub.2 -Cermet while the cathode or oxygen side is provided with a LaMn-Perovskite.
The voltage which can be obtained from a single cell is low (<1V). For utilizing SOFC technology for power generation, a large number of cells must be connected together. For this purpose, a further cell component is required, namely the bipolar plate or interconnector. By contrast with the electrolyte and the electrodes, which are generally around 100 .mu.m thick, the bipolar plate of an SOFC is several millimeters thick and in the common designs is not only the gas feeding, and connecting part between the individual cells, but also the load carrying component of the cell (see for example EP 0 338 823 A1).
At the preferred operating temperatures (T approximately 950.degree. C.), the bipolar plate must therefore have the following properties:
1. Sufficient mechanical strength;
2. Gas tightness;
3. Simple (inexpensive) fabricatability;
4. Electrode material thermal expansion similar to that of the ceramic;
5. Good electrical conductivity;
6. Corrosion resistance as to the fuel (e.g. H.sub.2 O/H.sub.2) and to the oxidizing gas (e.g. air); and
7. Compatibility with the electrode materials.
Presently, two groups of materials are being discussed as bipolar plate material, namely, LaCrO.sub.3 -based systems and metallic high-temperature materials. The latter have been favored more recently because of their better ductility, better electrical conductivity and easier fabrication. Because of the aforementioned hot gas corrosion resistance which is required, only Al.sub.2 O.sub.3 -forming and Cr.sub.2 O.sub.3 -forming high temperature alloys are potentially suitable. NiCr-based or NiCrAl-based alloys are, however, unsuitable because of their high thermal expansion coefficients (.apprxeq.20.multidot.10.sup.-6 k.sup.-1 as compared with .apprxeq.10.multidot.10.sup.-6 k.sup.-1 for electrolyte/electrodes) in accordance with present day understandings.
For the present SOFC concepts, mainly two types of potentially suitable alloys are under consideration:
1. FeCrAl-(ODS-) alloys (typical composition in weight % 20Cr, 5Al, 0.5Y.sub.2 O.sub.3, balance Fe) which form Al.sub.2 O.sub.3 surface layers at the service temperature.
2. Cr-based (ODS)-alloys (typical composition in weight %: 5Fe, 1Y.sub.2 O.sub.3 balance Cr) which upon high temperature use form Cr.sub.2 O.sub.3 surface layers.
The alloy compositions (1) have a superior resistance to corrosion because of the corrosion-limiting characteristics of the Al.sub.2 O.sub.3 surface layers forming during high temperature service. However, in SOFCs this alloy layer causes problems due to an increase of the interfacial contact resistance between the bipolar plate and the electrode.
The alloys (2) have the significant advantage of a low thermal expansion coefficient but tend to show oxide scale spallation due to the relatively rapid formation of thick Cr.sub.2 O.sub.3 layers. The scale spallation detrimentally
REFERENCES:
Patent Abstract of Japan, vol. 11, No. 382 (E-564), 12 Dec. 1987 & JP A 62 147 663 (Matsushita Elec. Ind. Co. Ltd.).
Patent Abstract of Japan, vol. 9, No. 61 (E-303), 19 Mar. 1985 & JP A 59 201371 (Kogyo Gijutsuin et al) 14 Nov. 1984.
Baumanns Ferdinand
Nickel Hubertus
Quadakkers Willem
Dubno Herbert
Forschungszentrum Julich GmbH
Maples John S.
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