Solid oxide fuel cell structures

Details Solid oxide fuel cell structures Solid oxide fuel cell structures

1995-11-24

1998-10-27

Kalafut, Stephen

Chemistry: electrical current producing apparatus, product, and

With pressure equalizing means for liquid immersion operation

429 32, H01M 812

Patent

active

058276203

DESCRIPTION:

BRIEF SUMMARY
This is a National Stage Application of PCT/GB94/00549 filed Mar. 17, 1994.
The present invention relates to solid oxide fuel cell (hereinafter referred to as SOFC) structures.


BACKGROUND OF THE INVENTION

SOFCs of three basic designs are currently being developed. These three basic designs are generally referred to by the term tubular, planar and monolithic. All of these fuel cells are based upon a stabilised zirconia electrolyte which is capable of conducting oxygen ions at elevated temperatures. A typical operating temperature for a SOFC is 1000.degree. C. The known cells provide high electrical efficiencies and can be operated on a variety of fuels including hydrogen, carbon monoxide, coal-derived gases and natural gas. SOFCs also offer high quality exhaust heat for co-generation applications. Of potentially the greatest significance, however, is the fact that SOFCs produce very low emissions as compared with, for example, diesel generators and therefore can be located wherever an electrical generator is required. For example, it would be possible to replace a relatively dirty and noisy diesel generator providing power to a hospital by a SOFC.
The paper "Solid Oxide Fuel Cells--The Next Stage", author Brian Riley, pages 223-238 of "Journal of Power Sources", 29(1990) briefly describes the various known SOFC structures and reference should be made to that document for details of the structure of the known planar, monolithic and tubular geometries. It will be appreciated, however, that all of the known structures incorporate a solid electrolyte one side of which supports an anode to which fuel gas is delivered and the other side of which supports a cathode to which air or oxygen is delivered. When an external load is connected to the anode and cathode, oxygen at the cathode reacts with incoming electrons from the external circuit to form oxygen ions which migrate to the anode through the oxygen-ion conducting electrolyte. At the anode, the fuel is oxidised with these oxygen ions, resulting in the liberation of electrons to the external circuit. Thus the overall reaction is simply the oxidisation of fuel. Typically 50 to 90% of the fuel is utilised in the electrochemical cell reaction, the partially depleated fuel being combusted outside the cell. The exhaust gas from the cell can be used in a co-generation system for producing process steam or in a steam turbine for an all-electric system. As each cell has a theoretical open voltage of about 1 volt, it is necessary to interconnect a number of cells to provide an appropriate output voltage.
If SOFCs are to be widely useable, they must be very reliable over long term use. The known cells are prone to two problems which compromise long term reliability, the first problem being the fact that fuel cell structures are very easily damaged if subjected to thermal shocks, and the second problem being related to the difficulty experienced in sealing the fuel cell structures so that fuel and oxygen are reliably delivered to opposite sides of a relatively thin electrolyte and do not come into contact with each other until the fuel has been at least significantly depleated. It has proved difficult to deal with these problems given the high operating temperatures and the fact that it is fundamental to the operation of fuel cells that thin ceramic structures form the interface between the anode and cathode. Such structures crack easily when exposed to varying temperatures. The conventional approach to reducing the significance of these problems is to very slowly heat up the fuel cell structure to the normal operating temperature of 1000.degree. C. and to maintain that temperature continuously. Unfortunately in the real world continuous operation of a system cannot be guaranteed. Until such time as manufacturers can assure potential users that, for example, a power failure resulting in rapid cooling of a SOFC would not cause any structural damage to such a cell it is going to be very difficult to convince potential customers that SOFCs are a viable alternative to conv

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patent: 4910100 (1990-03-01), Nakanishi et al.
patent: 5034288 (1991-07-01), Bossel
Patent Abstracts of Japan, vol. 016-267, Jun. 1992, (E 1217) for Kokai No. 4-62760 (Feb. 1992).
Chemical Abstracts, vol. 117, No. 73159C.
Derwent Abstract No. 91-277684, for Japanese Kokai No. 3-183658 (Aug. 1991) .

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