Solid oxide fuel cells

Details Solid oxide fuel cells Solid oxide fuel cells

1994-06-08

1996-04-16

Kalafut, Stephen

Chemistry: electrical current producing apparatus, product, and

With pressure equalizing means for liquid immersion operation

429 33, 429 34, 429 38, 429 41, H01M 802

Patent

active

055081274

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

1. Field of the Invention
The present invention relates to fuel cells and in particular to solid oxide fuel cells.
2. Discussion of Prior 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 deices burn fuel at the anode or negative electrode and consume an oxidant at the cathode or positive electrode. The present invention is concerned with so-called solid oxide fuel cells, herein called "SOFC s", in which the electrolyte is a solid refractory oxide, and a temperature of the order of 900 to 1000 C. is employed in operation of the cell. Such cells normally form one building block of a stack of cells of an electrochemical energy source.
In more known SOFCs the electrolyte is contained between an anode and a cathode and the anode and cathode of adjacent cells in a stack are contained between an interconnect or bipolar plate which permits electronic conduction and allows reactant gases to be delivered separately to regions adjacent to the anode and cathode. The reactant gases will generally comprise oxygen usually supped as air as oxidant and hydrogen or a hydrogen containing compound, e.g. methane, as fuel. The interconnect or bipolar plate 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. As a result, the designs of such cells in the prior art have involved for the interconnect the fabrication and use of materials such as lanthanum chromite which are very expensive. These interconnects have been manufactured in an expensive sandwich structure form to provide the necessary gas permeability barrier between suitably channeled gas transporting layers.
It is a purpose of the present invention to reduce the fabrication cost of such cells and stacks incorporating them.


SUMMARY OF THE INVENTION

According to the present invention a solid fuel cell for high temperature operation includes a refractory solid electrolyte, an anode and a cathode both in intimate contact with the electrolyte and an electronically conducting interconnect medium having pores or channels therethrough permitting oxidant and fuel to be delivered without mixing repsectively to the cathode and the anode, wherein the anode, cathode and interconnect medium are provided as zones within a common unitary material, the anode and cathode being present as zones adjacent to different surfaces of the material and the interconnect medium being present as a zone intermediate to the cathode and anode zones.
According to the present invention in another aspect there is provided a stack of fuel cells each according to the aspect of the invention described above, the stack comprising alternate layers of solid electrolyte and layers of the said unitary material.
The said unitary material may comprise urania, preferably depleted urania, optionally doped with an oxidic material. The oxidic material may comprise one or more rare earth oxides in a doping concentration of up to 50 percent by weight of the doped urania. The oxidic material may comprise one or more of an oxide of yttrium, gadolinium or cerium or other oxide.
The doping concentration of the said material may vary through the said unitary material and/or from cell to cell in a stack of cells to facilitate the provision of a suitable electrochemical potential gradient from the anode of one cell to the cathode of the next and to allow thermal expansion of adjacent layers to be matched. For example, the average doping concentration may vary from 5 to 10 percent by weight from one cell to the next.
The said unitary material will have pores or channels running therethrough to the cathode and anode zones to permit delivery of reactants to those zones. As

REFERENCES:
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patent: 3554808 (1971-01-01), Fischer et al.
patent: 4812329 (1989-03-01), Isenberg
patent: 4894297 (1990-01-01), Singh et al.
patent: 5034288 (1991-07-01), Bossel
patent: 5130210 (1992-07-01), Iwasaki et al.

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