Metal working – Method of mechanical manufacture – Electrical device making
Reexamination Certificate
2001-07-23
2004-08-10
Valentine, Donald R. (Department: 1742)
Metal working
Method of mechanical manufacture
Electrical device making
C204S242000, C204S252000, C204S267000, C427S446000, C427S453000, C427S454000, C427S596000, C427S255230, C427S421100, C427S427000, C156S060000, C228S179100, C228S262900
Reexamination Certificate
active
06772501
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the design and manufacture of and includes single cell, basic units for planar, thin-film, ceramic electrochemical devices such as solid oxide fuel cells, electrochemical oxygen generators, gas separation membranes, and membrane modules and stacks and the fabrication of multi-cell stacks and modules from the single cell units.
2. Description of the Prior Art
A solid oxide fuel cell (SOFC) is typically classified with respect to geometry and interconnect materials. With respect to the former, a SOFC has typically either a tubular, planar, or monolithic geometry. Interconnect materials for a SOFC are typically metallic or ceramic.
Although more advanced, tubular SOFCs have a large disadvantage when compared to both planar and monolithic, SOFCs. The power density, i.e., the amount of power the device produces relative to its volume, is much lower. This can be a severe problem where relatively small-scale, less than 500 kW, power generation is desired. Both planar and monolithic SOFC designs have much higher power densities and are therefore much more suitable for this situation.
A major difference between monolithic and planar SOFC designs is the interconnect material. In general, the state-of-the-art monolithic designs are applicable only to ceramic interconnects. For example, U.S. Pat. Nos. 5,935,727, 5,922,486, 5,882,809, 5,788,788, 5,589,017, 5,256,499, 4,761,349, and 4,666,798 relate various designs for monolithic SOFCs. In general, all these designs require cosintering of the various layers. This precludes the use of metallic interconnects as metals are in general not compatible with typical ceramic sintering conditions. Therefore, typically only ceramic interconnects can be used. The electrically conducting ceramics required for SOFC interconnects are very expensive since they are based on chromium. This high materials cost therefore limits or effectively eliminates the use of monolithic SOFCs in many applications.
An exception to ceramic interconnects in a monolithic SOFC is illustrated in U.S. Pat. No. 5,356,730. This patent relates to the use of a cermet interconnect. This interconnect is comprised of an electrically conducting ceramic, typically based on oxides of chromium, and a metal. However, because of the sintering conditions required, the metal needs to be a noble metal, for example, palladium or platinum. These metals, as well as the electrically conducting ceramic, are expensive and therefore do not solve the problem of SOFC costs.
Several types of planar designs are known in the prior art. In the most conventional approach, individual membrane/electrolyte assemblies (MEAs) are stacked together with interconnects and held together with pressure and high temperature gaskets or cements as seals. See, for example, U.S. Pat. Nos. 6,106,967 and 5,770,327, and as exemplified in FIG.
1
. Here, the long-term stability of these SOFCs is questionable since the physical mechanism for applying pressure to the seals can be distorted due to exposure to elevated temperatures for long periods of time. Additionally, seal materials can react with the SOFC materials degrading performance and ultimately leading to failure of the device.
The prior art also relates to designs in which pressure is not required, for example, U.S. Pat. No. 4,997,727. This patent relates to the use of a MEA with a sealing edge formed from the electrolyte. A separate metallic interconnect is used and shaped so that a seal is made between the interconnect and the electrolyte. This design requires that the MEA be electrolyte-supported. This means that the electrolyte will need to be relatively thick, greater than 100 microns, to provide mechanical strength to the MEA. This thick electrolyte reduces overall device performance by increasing resistive losses through the electrolyte. These losses are directly proportional to the electrolyte thickness.
A second example of the prior art for an alternate design in which the use of pressure is not required is U.S. Pat. No. 6,165,632. This patent relates to sealing of metallic composite printing circuit boards to each other or through an electrolyte of an electrolyte/electrode unit. Sealing is achieved through several layers of glass solder and glass ceramics. This additional sealing layer prevents intimate electrical contact between the interconnect and the electrodes and between two interconnects of different cells. Because a fuel cell stack can contain several tens of individual MEAs and interconnects, this poor electrical contact will greatly reduce overall cell performance by adding significant resistive losses.
The prior art also has examples of SOFC designs that are meant to overcome the problem of poor contact between interconnect and electrodes. These designs are, in general, very complicated. For example, U.S. Pat. No. 5,942,348 relates to the use of electrically conducting plates with an ion conducting active layer disposed between them. The design requires several regions on the plates, including insulating and sealing regions. The design is very complex, both in terms of geometry and the requirement for several different types of materials beyond the active SOFC materials. This complexity in turn increases the difficulty and costs of manufacturing the SOFC.
An additional example is U.S. Pat. No. 5,460,897. This patent relates to a fuel cell stack comprised of at least three different subassemblies, each subassembly further comprised of various components requiring several seal regions. Again, this very complex design results in a complicated and multi-step manufacturing procedure adding cost to the device.
U.S. Pat. No. 5,480,739 relates to an SOFC design which uses plasma spraying to form an interconnect in order to achieve a tight contact between the interconnect and electrode. Likewise, U.S. Pat. No. 5,185,219 also relates to the direct deposition of an interconnect layer onto an electrode surface. However, in these designs, no provision is made for directly applying a second interconnect to the second electrode surface. This results in poor electrical connections between the individual cells that are within the stack. Additionally, U.S. Pat. No. 5,480,739 does not teach how the feature of direct formation of the interconnect can be applied with respect to sealing and gas manifolds within a planar SOFC design.
SUMMARY OF THE INVENTION
The present invention relates to the design and manufacture of and includes single cell, basic units for planar, thin-film, ceramic electrochemical devices such as solid oxide fuel cells, electrochemical oxygen generators, gas separation membranes, and ceramic membrane modules and stacks and multi-cell stacks and modules of the single cell or planar units. The design of the present invention is exemplified by an embodiment based upon a single cell in which all the desired layers of the device are manufactured into an integral unit producing a monolithic structure. This design produces, for example, a single cell that is gas-tight and can be easily assembled into multi-cell stacks and modules without the need for external seals or sealing mechanisms. This design is also compatible with standard ceramic and metallurgical production techniques. The present invention will enhance overall device performance because, for example, the basic single cell units are inherently sealed for gas tightness and manufactured with reduced interfacial electrical resistances. All of these features of the novel monolithically integrated unit cell design of the present invention will result in lower manufacturing costs for ceramic electrochemical devices.
The present invention, for example, overcomes the problem of expensive ceramic interconnects through a SOFC design which is compatible with the use of less expensive metallic interconnects. The present invention also produces a SOFC that does not require pressure or additional seals for operation therefore contributing to device lifetime. The present invention is able, for example, to utilize an electro
Armstrong Joseph
Barker William G.
Berland Brian S.
Schwartz Michael
Simpson Lin
ITN Energy Systems, Inc.
Lathrop & Gage L.C.
Valentine Donald R.
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