Gasket for a battery of molten carbonate fuel cells

Abstract: A gasket [2] for a molten carbonate fuel cell [1] formed of an elongated porous member able to support electrolyte flow and which includes barrier means [4] for retarding such flow.

Description: Background of the Invention This invention relates to fuel cells and, in particular, to the prevention of electrolyte migration in these cells. Fuel cells are well known in the art. These cells typically comprise porous anode and cathode electrodes which sandwich an electrolyte usually contained in a matrix. The electrode/matrix sandwich is situated between plates which define chambers for bringing fuel and oxidant process gases to the anode and cathode electrodes, respectively. Cells formed in this manner are placed one on top of the other to form a fuel cell stack. The resultant fuel cell stack has end faces formed by the end faces of the individual cell components. These end faces communicate with manifolds which carry supply gases, both fuel and oxidant, to the stack as well as spent gases from the stack. To ensure gas-tight mating of the manifolds with the stack end faces, gaskets are typically interposed between the stack end faces and the manifold. These gaskets are usually porous. In fuel cell stacks of the above type, it has been found that there is an undesirable migration of electrolyte through the stack. Thus, it has been found that the electrolyte from the individual cells migrates from the positive to negative end of the stack. This migration is driven by the stack voltage and causes the cells at the negative end of the stack to be flooded with electrolyte and the cells at the positive end of the stack to become depleted of electrolyte. Such migration of electrolyte has been found to be most severe in fuel cell stacks employing molten carbonate fuel cells. In such stacks the migration can severely reduce stack performance and has been observed in stacks of far fewer cells than the several hundred cells contemplated for viable commercial power plants. Moreover, where the molten carbonate fuel cells employ a mixture of carbonates (e.g., K₂ CO₃ and Li₂CO₃) as the electrolyte, there is a preferred movement of one of the carbonates (i.e., the K₂ CO₃) to the negative end of the stack. This redistribution of the electrolyte further reduces stack performance. It is known that the migration of electrolyte in fuel cell stacks is due to shunt currents which promote ionic electrolyte flow. These shunt currents are dependent upon the stack voltage and resistance to electrolyte flow. The latter resistance, in turn, is determined by the resistance of the cell wet seal and the resistance of the above-discussed manifold sealing gasket. These resistances act in series, with the resistance of the sealing gasket playing the dominant role and being rate controlling. A variety of techniques have been proposed for counteracting the electrolyte migration problem. One proposed technique is disclosed in U.S. patent 4,591,438 and contemplates the use of a higher than usual content of Li₂CO₃ in the Li₂CO₃/K₂CO₃ electrolyte to maintain a uniform electrolyte molar ratio along the length of the stack. A second proposed technique is described in U.S. patent 4,643,954. In this technique, a passageway is provided for returning excess electrolyte from the negative end of the stack, where electrolyte flooding occurs, to the positive end of the stack, where electrolyte is depleted. Wicks are provided in the ends of this passageway to promote communication with the cell components and the end of the passageway where there is excess electrolyte is heated to increase the electrolyte vapor pressure. A further attempt at a solution to the electrolyte migration problem is disclosed in U.S. patent 4,7613,348. In this patent, reservoirs are provided at the negative and positive ends of the stack for receiving excess electrolyte and supplying depleted electrolyte, respectively. These reservoirs are developed by forming the manifold gasket to have end portions of increased volume and cross-section. The mid-portion of the gasket, in turn, is of lesser volume and cross-section and formed from a fiberous ceramic strip which acts to retard migration of electrolyte from the positive to negative end of the stack. US-A-4,548,874 discloses an electrical short protection provided in an electrolytic cell stack by the combination of a thin, nonporous ceramic shield and a noble metal foil disposed on opposite sides of the sealing medium in a gas manifold gasket. The thin ceramic shield is placed between the porous gasket and the cell stack face at the margins of the negative end plate to the most negative cells to impede ion current flow. The noble metal foil is electrically coupled to the negative potential of the stack to collect positive ions at a harmless location away from the stack face. In consequence of that, corrosion products from the stack structure deposit on the foil rather than on the stack face to eliminate electrical shorting of the cells at the negative end of the stack. US-A-4,704,340 discloses a electrolyte creepage barrier in the form of a thin layer which is disposed intermediate an outermost anode end of a fuel cell and facing outermost end of the electrolyte matrix in order to prevent electrolyte creepage at said anode end of the fuel cell. US-A-4,467,018 discloses a dielectric barrier which isolates the fuel cell stack from each of the manifolds. The dielectric barrier includes an impermeable ceramic member disposed between two ceramic gaskets at the interface of the fuel cell stack and each of the manifolds so that the dielectric barrier is disposed parallel to that interface. The above techniques have offered some limited solution to the electrolyte migration problem. However, is not believed that they can sufficiently reduce migration to bring it within tolerable limits. Furthermore, the techniques are cumbersome and not easily implemented. The invention as claimed in claims 1, 20 and 24 solves the problem of how to reduce the electrolyte migration in fuel cells. The gasket of the invention can be easily and readily implemented. Various embodiments of barrier means joined to the adjoining sections of the elongated member are disclosed. Brief Description of the Drawings The above and other features and aspects of the present invention will become more apparent upon reading the following detailed description in conjunction with the accompanying drawings, in which: FIG. 1 shows a portion of a fuel cell stack incorporating a gasket apparatus in accordance with the principles of the present invention; and FIGS. 2-4 illustrate various embodiments of the gasket apparatus of FIG. 1. Detailed Description FIG. 1 shows a fuel cell stack 1 utilizing a gasket 2 in accordance with the principles of the present invention. The stack 1 comprises a plurality of fuel cells 3 each of which is comprised of number of components in sandwich arrangement. More particularly, each fuel cell 3 includes a porous anode electrode 3A and a porous cathode electrode 3B between which are arranged a matrix 3C containing an electrolyte. In the present case, it is assumed that the electrolyte is a molten carbonate electrolyte and, therefore, that each cell 3 is a molten carbonate fuel cell. A top separator plate 3D abuts the cathode electrode 3B and includes passages 3E for conveying oxidant process gas to the cathode electrode. A bottom separator plate 3F, in turn, abuts the anode electrode 3A and includes channels 3G for carrying fuel process gas to the anode electrode. As can be seen, the fuel cells 3 are placed one on top of the other to form the fuel cell stack 1. With this configuration, the stack 1 has end faces formed by the end edges of the components making up the individual cells, two such end faces 1A and 1B being visible. In usual practice, manifolds 6, 7 (partially shown in FIG. 1) faced with a dielectric member are placed against these end faces in order to supply the fuel and oxidant process gases to the separator plate channels. Similar manifolds with dielectric faces are placed against the oppositely disposed end faces of the stack to extract spent gases from these channels. In order to ensure a gas-tight relationship between the manifolds and the adjoining stack end faces, the gaskets 2 are provided. The gaskets 2 are made from a material which is sufficiently porous to enable the gaskets to elastically conform to the uneven surfaces of the respective manifolds and stack end faces, thereby maintaining the desired gas-tight seal. Typical materials for the gaskets 2 might be zirconia oxide, alumina, ceramic felt or a cloth material. As can be appreciated and as is seen in FIG. 1, the gaskets 2 when placed against their respective stack end faces come in contact with the end edges of the fuel cell matrices 3C. As a result, the electrolyte in these matrices is made available to the gaskets 2 and, as above-indicated, during operation of the stack, the gaskets 2 enable electrolyte to flow or migrate through the gasket pores from the positive to negative end of the stack 1 driven by the stack voltage. As also above-indicated, such electrolyte flow is ionic in character and is due to shunt currents through the gasket. If the migration is allowed to occur, it can result in electrolyte flooding of the cells at the negative end of the stack and electrolyte depletion in the cells at the positive end of the stack. These effects, in turn, if sufficiently severe can cause stack failure. In accordance with principles of the present invention, in order to retard this flow, the gaskets 2 are provided in their interiors with one or more barriers 4. The barriers 4 are adapted to inhibit flow of electrolyte ions through the gasket body and thereby provide the desired electrolyte flow retardation. As shown, the barriers 4 are thin, sheet or foil-like members. A thickness of a few mils or less for barriers 4 is acceptable and the barriers can be made of non-metallic or metallic materials. Typical, metallic materials might be gold or high temperature alloys such as Kanthal, Fe-Cr-Al-Y alloy or inconnel, or ceramics such as alumina. If the metallic material used for the barriers 4 is conductive, care has to be taken to prevent each barrier from shorting across cell components of opposite polarity voltage. This can be accomplished by controlling the alignment of the gasket 2 or the actual physical configuration of the barriers, so that each barrier contacts only a single cell component. Alternatively, the metallic material used for the barriers 4 may be made non-conductive by, for example, using an alumina coating. The barrier members 4 may be situated in the gasket 2 at predetermined levels depending upon the degree of retardation desired. Thus, the barriers 4 may be disposed at levels to span a predetermined number of cells 3, if only a partial reduction of electrolyte migration to a satisfactory level is desired. However, if substantially total blockage is desired, the barriers may be disposed so that a barrier is situated between each pair of cells 3. FIGS. 2-5 show various configurations for realizing the gasket 2 with interposed barriers 4. In FIGS. 2-4, gasket sections and interposed barrier members are joined together to achieve a full length gasket. Only two gasket sections 2A, 2B and an interposed barrier 4 are actually shown in each figure. By joining further barriers to the ends of each gasket section 2A, 2B and then joining further gasket sections to the other ends of these further barriers and repeating the process, gaskets of any desired length can be realized. In FIGS. 2-4, each gasket section 2A, 2B comprises a gasket layer shown as layers 2A₁ and 2B₁, respectively. These layers are comprised of a porous material. In FIG. 2, the barrier member 4 is in the form of a thin sheet having a flat central section 4A which extends through the thickness t of the gasket layers and which is at angle or slant with respect to the thickness dimension. The barrier 4 also comprises two end sections 4B and 4C which extend along the respective upper surface of the layer 2A₁ and the lower surface of the layer 2B₁, respectively. In this embodiment, the faces of the barrier sections are joined to the abutting surfaces of the gasket layers by gluing. A suitable glue might be spray adhesive "Photo Mount" (catalog #6094; 3M, St. Paul, MN). The gasket configuration in FIG. 3 is similar to that in FIG. 2 and also utilizes glue to join the faces of the barrier sections and adjoining surfaces of the gasket layers. In this case, however, the central section 4A of the gasket 4 is corrugated so as to form oppositely directed V-shaped sub-sections. One V-shaped sub-section engages or joins, the upper surface of gasket layer 2A₁ and the other V-shaped sub-section the lower surface of gasket layer 2B₁. The presence of the corrugations reduces slipping and thus provides a more effective joining of the gasket layers and barrier sections. In FIG. 4, the end sections 4B, 4C of the barrier member 4A are provided with hooked ends 5 which protrude into the respective adjacent gasket layers 2B₁, 2A₁. These hooked ends thus mechanically lock the barrier member 4 to the gasket layers. If desired, with this configuration, gluing of the barrier sections to the layers may be dispensed with. It should be noted that if a conductive metallic material is used to form the barriers 4, each barrier may be further configured to act as a fuse which opens in the event the barrier contacts opposite polarity cell components. In this way, any short circuit would be automatically corrected for by the barrier itself.

Claims: A gasket (2) for use in a stack (1) of fuel cells having end faces and manifolds (6, 7) placed against said end faces, the gasket (2) being disposable between one of said end faces and one of said manifolds (6, 7) and said one end face being accessible to the cell electrolyte, the gasket (2) comprising an elongated porous member able to support electrolyte flow and one or more barrier means (4) within said elongated porous member for retarding said electrolyte flow, characterized by the one or more barrier means (4) extending through the thickness (t) of the gasket (2) to inhibit electrolyte flow along the length of the gasket (2). A gasket in accordance with claim 1 wherein: said electrolyte flow is comprised of flow of electrolyte ions. A gasket in accordance with claim 1 wherein: said gasket (2) includes a plurality of said barrier means (4) situated at positions spaced along the length of said elongated porous member. A gasket in accordance with claim 3 wherein: said spacing of said barrier means (4) is such that when said gasket (2) is placed adjacent said stack (1) of cells, there is a barrier means (4) positioned every predetermined number of cells. A gasket in accordance with claim 4 wherein: said predetermined number is one. A gasket in accordance with claim 1 wherein: the sections (2A, 2B) of the elongated porous member adjoining the opposite sides of said barrier means (4) are affixed to said barrier means (4). A gasket in accordance with claim 6 further comprising: an adhesive disposed between the sections of the elongated porous member and the sides of the barrier means. A gasket in accordance with claim 6 wherein: said barrier means (4) each includes means for mechanically engaging said sections of said elongated porous member. A gasket in accordance with claim 1 wherein: said barrier means comprises a thin member extending through the thickness of said elongated porous member. A gasket in accordance with claim 9 wherein: said thin member includes: a central section (4A) within the thickness of said elongated porous member and first and second end sections (4B, 4C) which extend from the central section (4A) along opposing outer surfaces of said elongated porous member. A gasket in accordance with claim 10 wherein: said central section (4A) of said thin member is flat and at an angle to the thickness dimension of said elongated porous member. A gasket in accordance with claim 10 wherein: said central section (4A) is configured to engage said elongated porous member. A gasket in accordance with claim 12 wherein: said central section (4A) is corrugated. A gasket in accordance with claim 13 wherein: said corrugated central section (4A) includes V-shaped corrugations extending in opposite directions. A gasket in accordance with claim 10 wherein: each of said first and second end sections (4B, 4C) includes a hooked end (5) for engaging said elongated porous member. A gasket in accordance with claim 1 wherein: said barrier means (4) comprises a non-porous material. A gasket in accordance with claim 16 wherein: said non-porous material is one of gold, a high temperature alloy, Kanthal, Fe-Cr-Al-Y alloy and inconnel. A gasket in accordance with claim 16 wherein: said non-porous material is conductive or non-conductive. A gasket in accordance with claim 16 wherein: said material is electrically non-conductive and is alumina coated metal or ceramic material. Apparatus comprising: one or more fuel cells (3) arranged in a stack (1), each fuel cell comprising porous anode and cathode electrodes (3A, 3B) and an electrolyte (3C) disposed between said electrode (3A, 3B) said one or more fuel cells (3) defining at least one end face for said stack (1); a manifold (6, 7) adjoining said end face; and a gasket (2) situated between said manifold (6, 7) and said end face along at least a portion of said end face, said gasket (2) including: an elongated porous member able to support electrolyte flow; and one or more barrier means (4) within said elongated porous member for retarding said electrolyte flow, characterized in that the barrier means (4) extend through the thickness (t) of the gasket (2). Apparatus in accordance with claim 20 wherein: said gasket (2) includes a plurality of said barrier means (4) situated within said elongated porous member at positions along the length thereof such that there is a barrier means (4) every predetermined number of said cells (3). Apparatus in accordance with claim 21 wherein: said number of cells (3) is one. Apparatus in accordance with claim 22 wherein: said electrolyte is a molten carbonate. A method comprising: providing one or more fuel cells (3), each fuel cell (3) comprising porous anode and cathode electrodes (3A, 3B) and an electrolyte (3C) disposed between said electrodes; arranging said one or more fuel cells in a stack (1), said one or more fuel cells (3) defining at least one end face for said stack (1); placing a manifold (6, 7) in adjoining relationship to said end face; and situating a gasket (2) between said manifold (6, 7) and said end face along at least a portion of said end face, said gasket (2) including: an elongated porous member able to support electrolyte flow; and one or more barrier means (4) within said elongated porous member for retarding said electrolyte flow, characterized in that the barrier means (4) extend through the thickness (t) of the gasket (2). A method in accordance with claim 26 wherein: said gasket (2) includes a plurality of said barrier means (4) situated within said elongated porous member at positions along the length thereof such that there is a barrier means (4) every predetermined number of said cells. A method in accordance with claim 25 wherein: said number of cells (3) is one. A method in accordance with claim 24 wherein: said electrolyte is a molten carbonate.

Inventors: ALLEN JEFF, FAROOQUE MOHAMMAD, ALLEN, JEFF, FAROOQUE, MOHAMMAD
Applicants: ENERGY RES CORP, ENERGY RESEARCH CORPORATION
Application year: 1991
Main classification: H01M8
IPC codes: H01M8, C09K3