Solid oxide fuel cells with specific electrode layers

Details Solid oxide fuel cells with specific electrode layers Solid oxide fuel cells with specific electrode layers

1998-01-19

2000-01-11

Bell, Bruce F.

Chemistry: electrical current producing apparatus, product, and

With pressure equalizing means for liquid immersion operation

429 32, 429 41, 429 33, H01M 810

Patent

active

060133864

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

1. Field of the Invention
The present invention relates to fuel cells in particular to solid oxide fuel cells.
2. Present State of the Art
Fuel cells are electrochemical devices that convert chemical energy obtained from the reactants into electrical energy. A number of different families of such devices have been developed in the prior art. These vary according to the type of electrolyte used in the cell and the usual temperature of operation. All of the devices consume fuel at the anode or negative electrode and consume an oxidant at the cathode or positive electrode. Solid oxide fuel cells are commonly known and conventionally comprise an industry standard electrolyte tile having in close electrical contact an anode and a cathode.
In most known SOFCs the electrolyte is contained between an anode and a cathode and the anode and cathode of adjacent cells in a stack are connected by an interconnect or bipolar plate which permits electronic conduction between cells and allows reactant gases to be delivered separately to regions adjacent to the anode and cathode. The reactant gases will generally comprise oxygen usually supplied as air as oxidant and hydrogen or a hydrogen containing compound, e.g. hydrocarbon such as methane, as fuel. The interconnect or bipolar plate or a part thereof needs to be gas impervious to keep the reactant gases separate as well as electrically conducting to permit transport of electrons to and from the electrode surfaces to facilitate the electrochemical processes.
These conventional SOFCs, however, face a number of problems being uneconomical, costly or inefficient in their various forms.
Attempts have been made to overcome these problems and to improve the performance of the SOFCs. The efficiency can be increased but this has involved increasing the operating temperature to around 1000.degree. C. Such high temperatures limit the varieties of materials it is possible to use in the construction of cells and associated support structure. Expensive exotic materials are necessary to withstand the operating conditions. Additionally the working life of the cell may be reduced.
Other alternatives have been to reduce the thickness of the electrolyte because it is thought that this will lower the cell resistance. Thin electrolytes, however, require additional strengthening through a support medium or the cells are too fragile to be practicable. This again increases cost and may require exotic materials to be used to withstand the operating conditions.


OBJECTS AND BRIEF SUMMARY OF THE INVENTION

According to the present invention there is provided a solid oxide fuel cell which comprises a layer of electrolyte material, a layer of a first electrode material on one side of the layer of electrolyte material, a layer of a second electrode material on the other side of the layer of electrolyte material, at least one of the electrode materials being reactive with the electrolyte material, and a separator layer comprising a mixed conductor separating the said reactive electrode material from the electrolyte layer.
Mixed conductors in the context of the present invention are materials which will at least partially conduct both electrons and oxygen ions. Mixed conductor layers may conveniently be provided between each of the electrode materials and the electrolyte layer. In such case, the mixed conductor materials of the two layers may be different, but are preferably the same.
It has been realised that the electrode material from a conventional cell spaced from the electrolyte may principally provide a current collecting function (the current collector) with the mixed conductor material providing an electrode function (the electrode).
Desirably, the said separator/electrode material is a ceramic oxide material which is stable in an oxidising or reducing atmosphere depending on the gas delivered to the surface of the adjacent electrode/current collecting material. Desirably, the separator/electrode material is stable in both an oxidising atmosphere and a reducing atm

REFERENCES:
patent: 4052532 (1977-10-01), Tannenberger et al.
patent: 4142024 (1979-02-01), Van den Berghe et al.
patent: 4354912 (1982-10-01), Friese
patent: 4692274 (1987-09-01), Isenberg et al.
patent: 4702971 (1987-10-01), Isenberg
patent: 5064733 (1991-11-01), Krist et al.
patent: 5445903 (1995-08-01), Cable et al.
Wang, L.S., and Barnett, S.A., Lowering the Air-Electrode Interfacial Resistance in Medium-Temperature Solid Oxide Fuel Cells, Journal of the Electrochemical Society, Oct. 1992, vol. 139, No. 10, Manchester, NH, pp. L89-L91.

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