Chemistry: electrical current producing apparatus – product – and – With pressure equalizing means for liquid immersion operation
Reexamination Certificate
2000-05-30
2002-09-03
Kalafut, Stephen (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
With pressure equalizing means for liquid immersion operation
C429S006000, C429S006000
Reexamination Certificate
active
06444340
ABSTRACT:
The present invention relates to a solid oxide fuel cell assembly comprising one or more planar fuel cells and is particularly concerned with collecting electricity in such an assembly.
The current design of solid oxide planar fuel cell assemblies requires that the electric current generated in the electrolyte of a fuel cell be conducted away by a gas separator between adjacent fuel cells known as an interconnect. The gas separator must also be a good thermal conductor to conduct heat generated in the fuel cell away from the fuel cell. In some designs the interconnect is manufactured from a ceramic material which is electrically conducting at the operating temperatures while in other designs the interconnect is manufactured from a metallic material which is also a current carrier. Whatever the material of construction of the interconnects, it is essential that it be a good current carrier at the operating temperature and that the interfaces between the interconnect and the electrolyte also be conducting, i.e. the interconnect—cathode layer interface and the interconnect—anode layer interface. These requirements have proved to be difficult ones to meet in practical fuel cell designs without some compromises of the other properties.
It has proved difficult to avoid the various materials of the fuel cell assembly and the interfaces between them degrading or changing substantially during the life of the fuel cell, in so far their electrical conductivity is affected, because of the tendency of dissimilar materials to interact at the high temperatures which are required for efficient operation of a solid oxide fuel cell. For example, most metallic interconnects contain substantial quantities of the element chromium which is used to impart oxidation resistance to the metal as well as other properties. It has been found that where chromium is present in more than minute quantities it may combine with oxygen to form highly volatile oxide or oxyhydroxide gases under conditions which are typical of those experienced in operating solid oxide fuel cells. These volatile gases are attracted to the cathode-electrolyte interface where they may react to form compounds which are deleterious to the efficiency of the fuel cell. If these chromium reactions are not eliminated or substantially inhibited, the performance of the fuel cell deteriorates with time to the point where the cell is no longer effective. Eliminating these reactions while at the same time maintaining the current carrying efficiency of the interconnect is a difficulty faced by most or all of the high temperature planar solid oxide fuel cell designs in use today.
One proposal for alleviating this problem is described in our Patent Application WO96/28855 in which a chromium-containing interconnect plate is provided with an oxide surface layer which reacts with the chromium to form a spinel layer between the substrate and the oxide surface layer and thereby tie in the chromium.
A further requirement of current fuel cell designs is the need for a good current flowpath across the cathode side interface of the fuel cell assembly, i.e. good electrical contact. In existing designs this is accomplished to a greater or lesser degree by holding two approximately flat surfaces together under load, thus ensuring a large number of very fine point contacts which act as electrical bridges between the fuel cell cathode layer and the interconnect plate. This requirement for flat or well matched surfaces is difficult to achieve in practical fuel cells without recourse to expensive and tightly controlled machining or surface preparation methods. Holding these surfaces together as the temperature in the fuel cell rises or falls is also difficult since the thermal expansion characteristics are very difficult to match perfectly with the various materials of construction.
The present invention aims to alleviate the above difficulties of existing designs by separating the current collecting requirement of the gas separator at the cathode-gas separator interface from the other functions of the gas separator, and meeting the electrical conduction requirement in a separate component.
It has been proposed in patent publication JP 6223846-A to provide a current collecting board composed of a porous electrically conductive perovskite oxide between the cathode of a fuel cell and a gas separator in order to achieve the joint aims of providing a current flow path while still allowing oxygen access to the cathode. However, such oxides are relatively expensive, not very robust physically and, because of their brittle nature, require a close matching of mating surfaces to provide satisfactory electrical contact.
According to the present invention there is provided a solid oxide fuel cell assembly comprising a planar fuel cell having a solid oxide electrolyte layer with an anode layer on one side and a cathode layer on the other side, the fuel cell being disposed between a first thermally conductive heat resisting metal alloy gas separator member adjacent the cathode layer and a second thermally conductive heat resisting metal alloy gas separator member adjacent the anode layer, oxygen containing gas passages being provided between the cathode layer and the first gas separator member and fuel gas passages being provided between the anode layer and the second gas separator member, wherein a layer of electrically conductive material is provided between the cathode layer and the first gas separator member in electrical contact with the cathode layer to conduct electrical current away from the cathode layer, said electrically conductive layer being adapted to permit the oxygen-containing gas in the oxygen-containing gas passages to contact the cathode layer and comprising silver, and wherein the first gas separator member has a layer of alumina adjacent the layer of electrically conductive material.
By the invention, the electrical current is conducted away from the fuel cell via an electrically conductive layer comprising silver positioned between the alumina layer on the first gas separator member and the cathode layer of the fuel cell. We have surprisingly found that notwithstanding the relatively low melting temperature of silver, the electrically conducting layer can continue to provide efficient current collection over extended periods even at the elevated operating temperatures of high temperature solid oxide fuel cells.
The silver may be alloyed with one or more other noble metals and/or with one or more non-noble metals or may be present as in intermetallic compound or as a composite material with a non-metal in which case the silver is preferably present as a major component of the alloy, compound or material, for example at least about 50 wt % and the other component(s) should not contaminate the fuel cell. However, the electrically conductive layer advantageously comprises silver at least substantially alone as it is relatively inexpensive, freely available in the pure form, easy to melt and fabricate, non-toxic, and excellent conductor of heat and electricity, and malleable.
By providing the conductive layer comprising silver between the cathode layer and the alumina layer on the first gas separator member, the conductive layer may be able to selectively distort or comply under typical loads applied during use and thereby permit the use of fuel cell assembly components which are not highly flat, parallel, smooth or accurate. Such compliance would typically be of the order of microns. This allows the use of cheaper methods of fabrication for several components in the fuel cell assembly. The improved level of electrical contact which can result from such compliance yields improved fuel cell performance because electrical resistance of the cathode side may be much lower than with previous designs.
Either the structure or the geometry of the electrically conductive layer has to be such that oxygen containing gas can flow or diffuse through it to the cathode layer and electrolyte. Thus, the electrically conductive layer may be formed of porous silver material, o
Ceramic Fuel Cells Limited
Dorsey & Whitney LLP
Kalafut Stephen
LandOfFree
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