Chemistry: electrical current producing apparatus – product – and – With pressure equalizing means for liquid immersion operation
Reexamination Certificate
2000-04-10
2002-12-10
Chaney, Carol (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
With pressure equalizing means for liquid immersion operation
C429S006000, C429S006000, C429S010000, C429S010000
Reexamination Certificate
active
06492053
ABSTRACT:
The present invention relates to a fuel cell assembly comprising a stack of a plurality of planar fuel cells, and is particularly concerned with such a fuel cell assembly in which the compressive load on each fuel cell is independent of its position in the stack. The invention also extends to a single fuel cell assembly.
A fuel cell assembly comprising a stack of a plurality of planar fuel cells requires interconnect means between each pair of adjacent fuel cells to transfer electrical current and heat from the fuel cells, to facilitate the conveyance of oxygen-containing gas and fuel gas to respective sides of each fuel cell, and to keep the oxygen-containing gas and fuel gas apart.
In a single fuel cell planar fuel cell assembly the interconnect means are effectively terminal plates which transfer electrical current and heat from the fuel cell and facilitate the conveyance of oxygen-containing gas and fuel gas to respective sides of the fuel cell. Likewise, the end interconnect means in a stack of planar fuel cells are effectively terminal plates. However, for convenience, all the aforementioned interconnect means, whether between adjacent fuel cells or terminal plates, will hereinafter be referred to as “interconnect members”.
Commonly, the fuel cells and interconnect members have the same cross-sectional area and the electrical contact between the cells and interconnect members and the sealing of respective sides of each fuel cell from each other is maintained by using the force imparted by the weight of the cell(s) and/or interconnect members above any one cell. Thus, the fuel cells are fully load bearing. Examples of such an arrangement are described in international patent applications PCT/AU96/00140 and PCT/AU96/00594. The problem with this approach is that the lower cells in the stack carry greater weight than the upper cells. For a stack with a large number of fuel cells the load on the lower cells can be significant.
The carrying load of a ceramic, such as in a solid oxide electrolyte fuel cell, is much higher in compression than in tension and the fully load-bearing arrangement described above assumes that, in a perfect system, the cells carry only a compressive load. This compression-only model acquires near perfect flatness of all load carrying parts since unevenness will lead to tensile forces in the structure and to the possible breakage of the fuel cells. In practice it is not possible to ensure such a quality of flatness in all of the load carrying parts.
The problem of possible breakage of solid oxide electrolyte fuel cells due to tensile forces applied to them in use has not been very substantial in the past due to the relatively high strength structure of previously proposed fuel cells such as those described in the aforementioned international patent applications. In this type of fuel cell, the solid oxide electrolyte layer is relatively thick compared to the anode and cathode layers applied to respective sides and has substantial strength. However, recent developments have introduced solid oxide electrolyte fuel cells in which the electrolyte layer is considerably thinner and is not a primary load bearing layer. Instead the porous anode layer acts as the primary load bearing layer and uneven or excessive loads applied to these fuel cells can be very destructive.
A further problem with a fuel cell in a stack carrying the full mass of the fuel cells and interconnects above it is that the relatively weak porous electrode layers of the fuel cell may collapse under the load.
European patent application EP 0568991 describes a fuel cell assembly comprising a stack of a plurality of planar fuel cell structures, each comprising a fuel cell and a single interconnect member on one side. Each fuel cell structure is located in a hollow plate and is separated from an adjacent fuel cell structure by a hollow intermediate plate, with the interconnect member of one fuel cell structure being maintained in electrical contact with the anode of an adjacent fuel cell structure by a felt-like nickel metal conductive material disposed in the hollow intermediate plate. Each fuel cell carries the load of the associated interconnect member. Furthermore, each fuel cell structure is compressed between a seal element and inlet defining elements of the adjacent intermediate plates so that increasing compressive loads may still be applied to fuel cells down the stack.
It is an object of the present invention to alleviate the aforementioned disadvantages of the prior art.
According to the present invention there is provided a fuel cell assembly comprising a stack of a plurality of planar fuel cells each comprising an electrolyte layer having an anode layer on one side and a cathode layer on the other side and a plurality of interconnect members, each fuel cell being disposed between and in electrical contact with an adjacent pair of interconnect members with oxygen-containing gas passage means being formed between the cathode layer of each fuel cell and the adjacent interconnect member and fuel gas passage means being formed between the anode layer of each fuel cell and the adjacent interconnect member, and wherein a chamber of greater height than the thickness of the respective fuel cell is defined between the adjacent interconnect members in each pair within which the fuel cell is received, and electrically conductive compressible means also disposed within the chamber in electrical contact with a first side of the fuel cell and the adjacent interconnect member urges the fuel cell towards the adjacent interconnect member on the second side thereof to maintain the fuel cell in electrical contact with both adjacent interconnect members.
By this arrangement, each fuel cell is displaceably received within the respective chamber but for the associated electrically conductive compressible means and the compressive load on each fuel cell is provided by the respective compressible means. Thus, the compressive load on each fuel cell is independent of the position of the fuel cell in the stack. This means that a reduced compressive load may be applied to each fuel cell which is particularly advantageous for the aforementioned solid oxide electrolyte fuel cells in which the electrolyte layer is not a primary load bearing layer. It also means that the load conditions in each chamber can be the same throughout the stack, so that the properties of the materials used in the stack do not need to vary according to the position of the fuel cell in the stack.
The invention is also applicable to a fuel cell assembly comprising a single fuel cell. Accordingly, the invention further provides a fuel cell assembly comprising a planar fuel cell having an electrolyte layer with an anode layer on one side and a cathode layer on the other side, the fuel cell being disposed between and in electrical contact with respective interconnect members, oxygen-containing gas passage means being formed between the cathode layer and the adjacent interconnect member and fuel gas passage means being formed between the anode layer and the adjacent interconnect member, and wherein a chamber of greater height than the thickness of the fuel cell is defined between the interconnect members within which the fuel cell is received and electrically conductive compressible means also disposed within the chamber in electrical contact with a first side of the fuel cell and the adjacent interconnect member urges the fuel cell towards the adjacent interconnect member on the second side thereof to maintain the fuel cell in electrical contact with both interconnect members.
The compressible means may take any of a variety of forms which maintain at least a minimum desired compressive force on the fuel cell even at the operating temperature of the fuel cell assembly. It is desirable that the compressible means maintains electrical contact between the fuel cell and the interconnect member during the full life of the fuel cell in use and therefore that it is not subject to more than minimal creep under the compression load. Any creep should no
Donelson Richard
Hickey Darren Bawden
Ceramic Fuel Cells Limited
Chaney Carol
Dorsey & Whitney LLP
Yuan Dah-Wei D.
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