Patent Application
20060188649
The present invention relates to methods of sealing solid oxide fuel cells using glass based seal tape to seal between cell and anode/cathode flow fields and particularly relates to methods for sealing the fuel cells to minimize or eliminate seal leakage, debonding of seals from the flow fields and/or cells and cell cracking thereby facilitating maintenance of cell integrity during fuel cell operation.
In a planar solid oxide fuel cell (SOFC) stack, sealing between the cell and the anode and cathode flow fields is critical to prevent fuel and oxidant from leaking and mixing. The criticality of sealing in solid oxide fuel cells is well known since hermetic seals, often between ceramic and metal, at high temperatures and during thermal cycling are difficult to obtain and maintain. Close thermal expansion coefficient match and chemical compatibility between these materials are required.
Two major approaches are typically utilized in solid oxide fuel cell sealing practice, namely glass ceramic-based chemical seals and gasket-based mechanical compressive seals. The mechanical compressive seals require a high degree of surface preparation and finish and high-pressure load capacity. A complete hermetic seal oftentimes cannot be achieved due to the flatness limitation of high temperature sintered ceramic cell. Also, contact stresses can readily cause cell fracture during assembly and thermal cycling in SOFC stack operation. Representative examples of mechanical type seals are described and illustrated in U.S. publication No. 2002/0195778, 2003/0203267 and 2003/0215689. Additional examples are set forth in WO 2003/036745 A2, WO 2003/032420 A2 and WO/0217416 A2.
Glass- and glass ceramic-based seals have very good wetting and bonding properties to both ceramic and metals and are capable of forming hermetic seals. Representative examples of this type of sealing for solid oxide fuel cells include U.S. Pat. Nos. 6,291,092, 6,271,158, 6,541,146 and 6,656,625. Additionally EP Publication No. 1211230 A1 discloses a glass matrix composition.
In practice, a standoff issue between rigid interconnect plates and glass seals during seal forming and thermal cycling, i.e. differential thermal expansion and shrinkage, often causes seal surface de-bonding or cell cracking and becomes a formidable barrier to forming and maintaining the necessary glass-based seal. Accordingly there has developed a need to overcome the foregoing and other problems, particularly issues of standoff and seal debonding, in order to form and maintain hermetic seals and maintain the cell and seal integrity throughout the life of the fuel cell operation.
In a preferred embodiment of the present invention there is provided a method of sealing margins of solid oxide fuel cell modules having anode and cathode flow fields on opposite faces of a cell comprising the steps of: (a) providing a seal material along margins of the cell and between margins of the anode and cathode flow fields; (b) disposing anode and cathode buffer layers of a compliant, porous and conductive material between the cell and the respective anode and cathode flow fields; (c) compressing the fuel cell module to compress the seal material and buffer layers between the cell and the anode and cathode flow fields; (d) heating the seal material while the anode and cathode flow field margins are compressed; and (e) solidifying or hardening the seal material to hermetically seal the margins of the fuel cell.
In a further preferred embodiment of the present invention there is provided a method of sealing margins of solid oxide fuel cell modules having anode and cathode flow fields on opposite faces of a solid electrolyte cell comprising the steps of: (a) providing glass-based seal tapes between margins of the cell and the anode and cathode flow fields; (b) disposing anode and cathode buffer layers of a compliant, porous and conductive material between the cell and the respective anode and cathode flow fields, the buffer layers having a thickness in excess of the respective thicknesses of the glass-based seal tapes; (c) compressing the fuel cell module to engage the margins of the anode and cathode flow fields with the seal tapes; (d) heating the seal tapes to a molten state while the cell and anode and cathode flow field margins are engaged to wet the cell and anode and cathode flow field margins; and (e) solidifying or hardening the seal glass to hermetically seal the margins of the fuel cell.
Referring to the drawings, a solid oxide fuel cell module in final assembly is generally designated 10 and illustrated in
The fuel and oxidant supplied to the anode layer 18 and cathode layer 20, respectively, provide known reactions causing a voltage from which a current may be collected. It will be appreciated that the fuel cells may have any one of a variety of shapes, for example square, rectangular and circular, typically planar, and that these solid oxide fuel cell modules typically need to have seals about their margins to prevent undesired fuel and oxidant from leaking and mixing.
In accordance with a preferred embodiment, the present invention provides a marginal seal which eliminates the standoff issue and risks of seal debonding and cell cracking and achieves a robust hermetic seal for fuel cell operation. Referring to
Referring to
Referring to
In
With respect to the materials of the various constituent elements, the cell 16 may be formed of a yttria-stabilized zirconia (YSZ) electrolyte with La1-xSrxMnO3 (LSM) cathode and YSZ/NiO cement anode sintered at each side. The cathode buffer layer 24, which also serves as conducting and mass transfer agent between the cell cathode 20 and cathode flow field 14, can be ceramic felt made of conductive oxides, such as La1-xSrxMnO3, La1-xSrxCoO3, La1-xSrxFeO3, La1-xSrxCo1-yFeyO3, or metal felt made of oxidation resistant metals and alloys, such as certain stainless steels, silver, gold, platinum, palladium, fecralloy, Ebrite®, Inconnel, etc., depending on the operation temperature. The anode buffer layer 22, which also serves as conducting and mass transfer agent between cell anode 18 and anode flow field 12, can be made from a relatively larger selection of metal foam or felt, such as copper, silver, nickel, stainless steel, alloys and conducting oxides. The seal glass can be any glass or glass ceramic with melting point in the range of 600-1000° C., and with CTE closely matched to the cell and metal interconnect, preferably in the range of 9˜13×10−6/K, such as soda lime glass, boron silicate glass and Macor glass ceramic, etc. The glass seal tape can be made by tape casting or rolling with organic binder added. The fuel cell operation temperature is preferably 50 ˜200° C. below the seal working temperature to ensure the glass seal is solidified or hardened and can withstand certain across-seal pressure difference.
It will be appreciated that the flow channels for the fuel and oxidant are illustrated as parallel to one another. However, the flow channels can be at right angles to one another or any other different orientation as desired.
While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.
