Abstract:
A fuel cell cassette for forming a fuel cell stack along a fuel cell axis includes a cell retainer, a plate positioned axially to the cell retainer and defining a space axially with the cell retainer, and a fuel cell having an anode layer and a cathode layer separated by an electrolyte layer. The outer perimeter of the fuel cell is positioned in the space between the plate and the cell retainer, thereby retaining the fuel cell and defining a cavity between the cell retainer, the fuel cell, and the plate. The fuel cell cassette also includes a seal disposed within the cavity for sealing the edge of the fuel cell. The seal is compliant at operational temperatures of the fuel cell, thereby allowing lateral expansion and contraction of the fuel cell within the cavity while maintaining sealing at the edge of the fuel cell.

Description:
GOVERNMENT-SPONSERED STATEMENT 
     This invention was made with the United States Government support under Contract DE-NT003894 awarded by the U.S. Department of Energy. The Government has certain rights in this invention. 
    
    
     TECHNICAL FIELD OF INVENTION 
     The present invention relates to a fuel cell stack assembly; more particularly to a fuel cell cassette of the fuel cell stack assembly; even more particularly to a seal for the fuel cell cassette which allows lateral movement of a fuel cell within the fuel cell cassette; and still even more particularly to such a fuel cell stack assembly which is a solid oxide fuel cell stack assembly. 
     BACKGROUND OF INVENTION 
     Fuel cells are used to produce electricity when supplied with fuels containing hydrogen and an oxidant such as air. A typical fuel cell includes an ion conductive electrolyte layer sandwiched between an anode layer and a cathode layer. There are several different types of fuel cells known in the art, one of which is known as a solid oxide fuel cell. A fuel cell is assembled into a fuel cell cassette which provides flow paths for the fuel and the oxidant and provides support for the fuel cell. Multiple fuel cell cassettes are then joined together to produce a fuel cell stack which is capable of producing a desired electrical output. 
     Fuel cell cassettes must include a seal which provides separation of the fuel and the oxidant. However, fuel cells commonly operate at temperatures in excess of 500° C. which can make it difficult to provide such a seal which is capable of long term operation and which is able to withstand multiple thermal cycles. One known seal is comprised of a silver/copper braze. However, when the silver/copper braze is exposed to the atmosphere experienced in operation of the fuel cell, the silver/copper braze may tend to form porosity which can cause degradation to the sealing capability of the silver/copper braze. Furthermore the silver/copper braze may retain the fuel cell rigidly which may transmit high stresses to the electrolyte layer of the fuel cell and cause durability issues for the fuel cell. 
     What is needed is a fuel cell cassette which minimizes or eliminates one or more of the shortcomings as set forth above. 
     SUMMARY OF THE INVENTION 
     Briefly described, a fuel cell cassette is provided for forming a fuel cell stack along a fuel cell axis. The fuel cell cassette includes a cell retainer, a plate positioned axially to the cell retainer and defining a space with the cell retainer, and a fuel cell having an anode layer and a cathode layer separated by an electrolyte layer. The outer perimeter of the fuel cell is positioned in the space between the plate and the cell retainer, thereby retaining the fuel cell and defining a cavity between the cell retainer, the fuel cell, and the plate. The fuel cell cassette also includes a seal disposed within the cavity for sealing the edge of the fuel cell. The seal is compliant at operational temperatures of the fuel cell, thereby allowing lateral expansion and contraction of the fuel cell within the cavity while maintaining sealing at the edge of the fuel cell. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       This invention will be further described with reference to the accompanying drawings in which: 
         FIG. 1  is an exploded isometric view of a fuel cell stack in accordance with the invention; 
         FIG. 2  is an isometric view of the fuel cell stack of  FIG. 1  now shown assembled; 
         FIG. 3  is a cross-sectional view of a portion of a fuel cell cassette of the fuel cell stack of  FIG. 1 ; and 
         FIG. 4  is the cross-sectional view of the fuel cell cassette of  FIG. 3  now showing an alternative arrangement. 
     
    
    
     DETAILED DESCRIPTION OF INVENTION 
     Referring to  FIGS. 1 and 2 , a fuel cell stack  10 , which is illustrated as a solid oxide fuel cell stack, includes fuel cell cassettes  12   1 ,  12   2 , 12   n-1 ,  12   n  stacked along a fuel cell axis  13  where n is the number of fuel cell cassettes in fuel cell stack  10  and the number of fuel cell cassettes n in fuel cell stack  10  is selected to provide a desired electrical output. Unless reference is being made to a specific fuel cell cassette, each of the fuel cell cassettes will be referred to generically as fuel cell cassette  12  from this point forward. Fuel cell cassette  12  includes a fuel cell  14  mounted within a cell retainer  16  to form a cell-retainer frame assembly  18 , anode spacers  20 , an anode interconnect  22 , a cathode interconnect  24 , and a separator plate  26 . Fuel cell cassette  12  includes sealing surfaces  28  which are complementary to sealing surfaces  28  of the adjacent fuel cell cassette  12  to which it is joined. During assembly of fuel cell stack  10 , a glass composite seal  30  is disposed between sealing surfaces  28  of adjacent fuel cell cassettes  12 . Glass composite seal  30  forms a bonded joint to provide a gas tight seal to separate and contain reactants and electrically isolate adjacent separator plates  26 . 
     With continued reference to  FIGS. 1 and 2  and now with additional reference to  FIG. 3 , fuel cell  14  includes an electrolyte layer  32  sandwiched between a cathode layer  34  and an anode layer  36 . In use, fuel cell  14  converts chemical energy from a fuel that is passed over anode layer  36  into heat and electricity through a chemical reaction with an oxidant, for example air, that is passed over cathode layer  34  as is known in the art of fuel cells. 
     Cell retainer  16  is defined by an outer frame  38  and an inner frame  40  such that outer frame  38  lies in a first plane that is substantially perpendicular to fuel cell axis  13  and such that inner frame  40  lies in a second plane that is substantially parallel to the first plane. Inner frame  40  is located inward from outer frame  38 , i.e. toward fuel cell axis  13 . An intermediate section  42  defines a transition from outer frame  38  to inner frame  40 . Inner frame  40  defines a central opening  44  therethrough which allows the oxidant to access cathode layer  34 . 
     Anode interconnect  22  is positioned between separator plate  26  and anode layer  36  of fuel cell  14  within fuel cell cassette  12 . Anode interconnect  22  is disposed adjacent to anode layer  36  in order to provide electrical communication between anode layer  36  and separator plate  26  and ultimately to an adjacent fuel cell cassette  12  which is in electrical contact with separator plate  26 . Anode interconnect  22  also contains features which define flow passages between anode interconnect  22  and anode layer  36  in order to provide a path for fuel to pass across anode layer  36 . A typical anode interconnect  22  is formed of a woven wire mesh of uniform thickness and is solid in a multitude of points in the direction parallel to fuel cell axis  13 . Anode interconnect  22  may also be stamped sheet metal with flow features and contacts such as flattened nails and ribs. Further details of anode interconnect  22  may be found in U.S. Pat. No. 7,718,295 to Haltiner, Jr. et al., the disclosure of which is incorporated herein by reference in its entirety. 
     As shown in  FIG. 3 , anode interconnect  22  extends to outer frame  38  of cell retainer  16  where anode interconnect  22  may be secured to outer frame  38 , for example by a continuous weld, a plurality of tack welds, or other suitable joining techniques or devices. As a result, a space  46  is formed axially between inner frame  40  of cell retainer  16  and anode interconnect  22  such that the perimeter of fuel cell  14  is retained within space  46  between inner frame  40  of cell retainer  16  and anode interconnect  22 . The edge of fuel cell  14 , cell retainer  16 , and anode interconnect  22  together define a cavity  48  which will be discussed in greater detail later. As described herein, anode interconnect  22  defines a plate which is secured to outer frame  38  of cell retainer  16  and extends inward therefrom such that space  46  is defined between inner frame  40  and anode interconnect  22 . 
     Cathode interconnect  24  is positioned adjacent to cathode layer  34  of fuel cell  14  and a separator plate  26  of an adjacent fuel cell cassette  12 . Cathode interconnect  24  is disposed adjacent to cathode layer  34  in order to provide electrical communication between cathode layer  34  and an adjacent fuel cell cassette  12  via separator plate  26  of the adjacent fuel cell cassette  12 . Cathode interconnect  24  also contains features which define flow passages between cathode interconnect  24  and cathode layer  34  in order to provide a path for air to pass across cathode layer  34 . A typical cathode interconnect  24  is formed of a woven wire mesh of uniform thickness and is solid in a multitude of points in the direction parallel to fuel cell axis  13 . Cathode interconnect  24  may also be stamped sheet metal with flow features and contacts such as flattened nails and ribs. Further details of cathode interconnect  24  may be found in U.S. Pat. No. 7,718,295 to Haltiner, Jr. et al., the disclosure of which is incorporated herein by reference in its entirety. 
     Fuel cell cassette  12  includes a plurality of anode supply passages  50  (for clarity, anode supply passages  50  have only been labeled on fuel cell cassette  12   1  in  FIG. 1 ). Anode supply passages  50  are formed along one side of fuel cell cassette  12  between fuel cell  14  and the outside edge of fuel cell cassette  12 . When fuel cell cassettes  12   1  through  12   n  are assembled together to form fuel cell stack  10 , anode supply passages  50  of each fuel cell cassette  12  are aligned with anode supply passages  50  of adjacent fuel cell cassettes  12  to form a plurality of anode supply chimneys  52 . Fuel supplied at one end of fuel cell stack  10  to anode supply chimneys  52  is communicated through anode supply chimneys  52 , thereby distributing fuel to each anode layer  36 . Anode supply passages  50  for each fuel cell cassette  12  may be formed at regular intervals along the length of fuel cell cassette  12  to distribute fuel evenly across anode layer  36 . 
     Fuel cell cassette  12  also includes a plurality of anode exhaust passages  54  (for clarity, anode exhaust passages  54  have only been labeled on fuel cell cassette  12   1  in  FIG. 1 ). Anode exhaust passages  54  are formed along the side of fuel cell cassette  12  which is opposite to the side with anode supply passages  50 . Anode exhaust passages  54  are disposed between fuel cell  14  and the outside edge of fuel cell cassette  12 . When fuel cell cassettes  12   1  through  12   n  are assembled together to form fuel cell stack  10 , anode exhaust passages  54  of each fuel cell cassette  12  are aligned with anode exhaust passages  54  of adjacent fuel cell cassettes  12  to form a plurality of anode exhaust chimneys  56 . Anode exhaust chimneys  56  allow anode exhaust from each fuel cell cassette  12  to be communicated to one end of fuel cell stack  10 . Anode exhaust passages  54  for each fuel cell cassette  12  may be formed at regular intervals along the length of fuel cell cassette  12  in the same way as anode supply passages  50 . 
     Fuel cell cassette  12  also includes a plurality of cathode supply passages  58  formed along the same side of fuel cell cassette  12  as anode supply passages  50  (for clarity, cathode supply passages  58  have only been labeled on fuel cell cassette  12   1  in  FIG. 1 ). When fuel cell cassettes  12   1  through  12   n  are assembled together to form fuel cell stack  10 , cathode supply passages  58  of each fuel cell cassette  12  are aligned with cathode supply passages  58  of adjacent fuel cell cassettes  12  to form a plurality of cathode supply chimneys  60 . An oxidant, for example air, supplied at one end of fuel cell stack  10  to cathode supply chimneys  60  is communicated through cathode supply chimneys  60 , thereby distributing air to each cathode layer  34 . Cathode supply passages  58  may be formed at regular intervals along the length of fuel cell cassette  12  to distribute air evenly across cathode layer  34  such that cathode supply passages  58  and anode supply passages  50  are arranged in an alternating pattern along the length of fuel cell cassette  12 . 
     Fuel cell cassette  12  also includes a plurality of cathode exhaust passages  62  formed along the same side of fuel cell cassette  12  as anode exhaust passages  54  (for clarity, cathode exhaust passages  62  have only been labeled on fuel cell cassette  12   1  in  FIG. 1 ). When fuel cell cassettes  12   1  through  12   n  are assembled together to form fuel cell stack  10 , cathode exhaust passages  62  of each fuel cell cassette  12  are aligned with cathode exhaust passages  62  of adjacent fuel cell cassettes  12  to form a plurality of cathode exhaust chimneys  64 . Cathode exhaust chimneys  64  allow cathode exhaust from each fuel cell cassette  12  to be communicated to one end of fuel cell stack  10 . Cathode exhaust passages  62  for each fuel cell cassette  12  may be formed at regular intervals along the length of fuel cell cassette  12  in the same way as cathode supply passages  58  such that such that cathode exhaust passages  62  and anode exhaust passages  54  are arranged in an alternating pattern along the length of fuel cell cassette  12 . 
     In order to maintain a separation of fuel and air at the edge of fuel cell  14 , a seal  66  is disposed within cavity  48 . Seal  66  is a material which is compliant at operational temperatures of fuel cell stack  10 . As used herein, operational temperatures of fuel cell stack  10  are temperatures of 500° C. or higher. In one example, seal  66  may be viscous glass. As defined herein, viscous glass is any glass that remains in a fully or partial amorphous phase in the standard operating temperature of fuel cell stack  10 , even after prolonged periods of exposure, and retains its ability to flow. Examples of viscous glass include B—Ge—Si—O glasses which retain approximately 70% amorphous phase after 1500 hours at 850° C.; barium alkali silicate glass; and SCN-1 glass, commercially available from SEM-COM Company, Inc. Alternatively, seal  66  may be a braze material that is compliant at the operating temperature of fuel cell stack  10 , for example only, braze materials that comprise a silver based alloy such as AgCu or AgPd may be used. While seal  66  hermetically seals the edge of fuel cell  14 , fuel cell  14  is mechanically supported by cell retainer  16  and anode interconnect  22 . Consequently, seal  66  does not support fuel cell  14 . Furthermore, fuel cell  14  is able to expand and contract laterally, i.e. in a direction perpendicular to fuel cell axis  13 , since seal  66  is compliant at operational temperatures of fuel cell stack  10  which prevents tensile and compressive stresses at the interface of seal  66  and fuel cell  14 . In this way, durability and reliability of fuel cell  14  and fuel cell stack  10  may be realized. 
     Reference will now be made to  FIG. 4  which shows fuel cell cassette  12 ′ which is an alternative to fuel cell cassette  12 . Fuel cell cassette  12 ′ is substantially the same as fuel cell cassette  12  except that fuel cell cassette  12 ′ includes anode interconnect  22 ′ which does not extend to outer frame  38  of cell retainer  16 , consequently, anode interconnect  22 ′ does not support fuel cell  14 . In order to provide support for fuel cell  14 , fuel cell cassette  12 ′ includes fuel cell support plate  68  which is substantially parallel to inner frame  40  of cell retainer  16 . Fuel cell support plate  68  is secured to outer frame  38 , for example by a continuous weld, a plurality of tack welds, or other suitable joining techniques or devices and extends inward toward anode interconnect  22 ′ such that fuel cell support plate  68  surrounds anode interconnect  22 ′. As a result, a space  46 ′ is formed axially between inner frame  40  of cell retainer  16  and fuel cell support plate  68  such that the perimeter of fuel cell  14  is retained within space  46 ′ between inner frame  40  of cell retainer  16  and fuel cell support plate  68 . The edge of fuel cell  14 , cell retainer  16 , and fuel cell support plate  68  together define a cavity  48 ′ much like cavity  48  is defined in fuel cell cassette  12 . Seal  66  is disposed within cavity  48 ′ and provides the same benefits to fuel cell cassette  12 ′ as it does to fuel cell cassette  12  as described above. 
     While this invention has been described in terms of preferred embodiments thereof, it is not intended to be so limited, but rather only to the extent set forth in the claims that follow.