Abstract:
A co-flow planar solid oxide fuel cell stack with an integral, internal manifold and a casing/holder to separately seal the cell. This construction improves sealing and gas flow, and provides for easy manifolding of cell stacks. In addition, the stack construction has the potential for an improved durability and operation with an additional increase in cell efficiency. The co-flow arrangement can be effectively utilized in other electrochemical systems requiring gas-proof separation of gases.

Description:
[0001] The United States Government has rights in this invention pursuant to Contract No. W-7405-ENG-48 between the United States Department of Energy and the University of California for the operation of Lawrence Livermore National Laboratory. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    The present invention relates to fuel cells, particularly to solid oxide fuel cell (SOFC) stacks, and more particularly to a co-flow or counter flow planar fuel cell stack with an integral, internal manifold and a cell casing holder to separately seal the cell, thus providing improved sealing and gas flow as well as easy manifolding of cell stacks.  
           [0003]    Solid oxide fuel cells are one of the most promising technologies for power generation. Like all fuel cells, SOFC cells are composed of two electrodes (anode and cathode) and an electrolyte. Since each single cell has a maximum voltage of about IV only, several cells must be stacked together in a stack to yield high voltages for practical applications. The stacking of the cells needs to address the gas flow distribution in the stack as well. The SOFC design closest to commercialization is the tubular design which can be assembled into larger units without the need of a seal. This sealess design is its biggest engineering advantage. However, the tubular geometry of these fuel cells limits the specific power density to low values because the electrical conduction paths are long, leading to high energy losses from internal resistance heating. For these reasons, other fuel cell constructions are being actively pursed at the present time.  
           [0004]    The most common alternative design is a planar arrangement with a cross-flow or radial flow arrangement. These planar fuel cells are constructed from alternating flat single cells, which are trilayer cathode electrolyte anode structures, and bipolar plates, which conduct current from cell-to-cell and provide channels for gas flow. Each individual cell, and the bipolar plate associated with every cell in the stack must be sealed together so that they are gas-tight at each manifold face. In addition the manifolds must be sealed gastight to the stack to prevent fuel and oxidant gas cross-leakage. The crossleakage can compromise cell efficiency and is hazardous due to the possibility of explosion. Sealant materials which have thermal expansion coefficent matching with other components of the stack and with satisfactory durability at operating temperatures are not available at the present time. This presents a serious technological shortcoming for planar solid oxide fuel cells.  
           [0005]    Planar fuel cell stacks may also be constructed using a co-flow or counter-flow configuration with internal manifolds, as exemplified by U.S. Pat. No. 4,761,349 issued Aug. 2, 1988; U.S. Pat. No. 5,227,256 issued Jul. 13, 1993; U.S. Pat. No. 5,480,738 issued Jan. 2, 1996; and U.S. Pat. No. 5,549,983 issued Aug. 27, 1996. However, most of these designs were mainly developed for electrolyte-supported cells (thick electrolyte with thin electrodes). Since the electrolyte membranes are impervious, the sealing and the stack design are not as complex as for electrode-supported cells (one thick electrode serving as support and a thin film electrolyte). However, the electrode-supported cells have significantly higher performance than the electrolyte-supported cells because of lower resistance of the thin film electrolyte.  
           [0006]    In some of the proposed designs such as the conventional cross-flow configuration, the sealing must be done at the edge and corner, which result in higher risk of leakage due to small seal area and less durable stack. Most of the stack designs proposed for electrolyte-supported cells are not applicable to electrode-supported cells because of the leakage through the porous electrode support.  
           [0007]    The present invention provides a solution to the above-mentioned problems of planar fuel cell stacks, by providing a planar stack design with separate cell holders for improved sealing and reduced thermal stress problems. This design is particularly suitable for electrode-supported fuel cells because it promote face seal instead of corner seal; however, the design is applicable to electrolyte-supported cells as well. A key feature of the present invention is the cell holder that is separate from the interconnect itself.  
         SUMMARY OF THE INVENTION  
         [0008]    It is an object of the present invention to provide an improved fuel cell stack.  
           [0009]    A further object of the invention to provide a co-flow planar solid oxide fuel cell stack.  
           [0010]    Another object of the invention is to provide a method of constructing a fuel cell stack which can also be used in electrolysis, gas separation, and other electrochemical systems requiring gas-proof separation of gases.  
           [0011]    Another object of the invention is to provide a co-flow planar stack with integral, internal manifolding and a casing/holder to separately seal a cell using coventional sealing materials such as ceramic, glass, or glass-ceraminic based sealants.  
           [0012]    Another object of the invention is to provide a co-flow planar stack which improves sealing and gas flow, and provides for easy manifolding of cell stacks.  
           [0013]    Another object of the invention is to provide a co-flow stack and cell casing/holder which provides improved durability and operation with an additional increase in cell efficiency.  
           [0014]    Other objects and advantages of the present invention will become apparent from the following description and accompanying drawings. Basically the invention provides a co-flow planar stack arrangement which can be utilized for solid oxide fuel cells and other electrochemical systems requiring separation of incompatible gases, such as used in electrolysis, gas separation, gas sensors, etc. The present invention overcomes the cross-leakage and other problems associated with prior planar fuel cell stacks designs, by providing a co-flow planar stack with integral, internal manifolding and a cell casing/holder to separately seal each cell using sealants such as materials based on ceramic, glass, or glass-ceramic. Such construction improves sealing and gas flow, and enables easy manifolding of cell stacks. The present invention utilizes a casing/holder containing a cell located intermediate a pair of flow channel/interconnects, with each cell having two pairs of openings at opposite ends which provide the separated co-flow of the fuel and oxidant gases. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    The accompanying drawings, which are incorporated into and form a part of the disclosure, illustrate an embodiment of the invention and, together with the description, serve to explain the principles of the invention.  
         [0016]    [0016]FIG. 1 illustrates a multi-cell stack incorporating a two cell casing/holder and co-flow manifolding of the present invention.  
         [0017]    [0017]FIG. 2 illustrates an exploded enlarged view of component similar to those of FIG. 1 for providing a single unit of a multi-cell stack. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0018]    The present invention provides a unique construction for solid oxide fuel cells (SOFC) stacks. This construction improves sealing and manifolding problems of the cross-flow, counter flow, and radial-flow panar construction and, at the same time provides for co-flow of fuel and oxidant gases. A multi-cell (two cell) embodiment is shown in FIG. 1, with FIG. 2 illustrating enlarged exploded view of the components forming a single unit of the multi-cell of FIG. 1.  
         [0019]    As seen in FIG. 2 a single cell unit includes a single cell located in a casing/holder plate is sandwiched between an anode plate and a cathode plate, indentified as flow channel/interconnect in FIG. 2. Gas flow is provided to the cell with internal, longitudinal manifolds that are an integral part of the casing/holder plate and the electrode plates, the flow being shown in FIG. 1. Each of the cell casing/holder plate and the electrode plates (flow channel/interconnects) includes two inlets and two outlets for gases. The peripheral surface of the cells are sealed to the surface of the casing/holder plate. This surface sealing of the cell increases effective seal area and improves durability of the seal, as compared to conventional planar designs typically fabricated by edge and/or corner sealing. The FIG. 1 embodiments illustrate a multi-cell assembly utilizing components similar to those of FIG. 2, and shows the co-flow of both air (oxidant) and fuel through the entire stack using manifolds and flow channels in the cathode and anode plates. Note that, as shown in FIG. 1, the outer or end plates include only one pair of gas inlets/outlets, while the inner plates include a pair at each end.  
         [0020]    Referring now to the drawings, the multi-cell of FIG. 1, indicated generally at  10 , comprises a lower end or outer electrode plate or flow channel/interconnect plate  11 , a casing/holder plate  12  retaining therein a cell  13 , an intermediate plate or flow channels/interconnect plate  14 , a casing/holder plate  15  retaining therein a cell  16 , and an upper end or outer plate or flow channel/interconnect plate  17 . End plates  11  and  17  are provided with a pair of gas inlets or openings  18 - 19  and a pair of gas outlets or openings  20 - 21 , respectively, and to which are mounted connects  22 - 23  and  24 - 25 . The casing/holder plates  12  and  15  are each provided with two pairs of openings  2627  and  28 - 29  at opposite ends, and intermediate flow channel/interconnect plate  14  is provided with two pairs of openings  30 - 31  and  32 - 33  at opposite ends. The plates  11 ,  12 ,  14 ,  15 ,  17  when assembled are positioned such that gas inlets or openings  18 - 19  plate  11  are aligned with openings  26 - 27  of plates  12  and  15  and openings  30 - 31  of plate  14 , while gas outlets or openings  20 - 21  of plate  17  are aligned with openings  28 - 29  of plates  12  and  15 , and openings  32 - 33  of plate  14 . Plates  11  and  12  are constructed such that gaseous fuel  34  enters connect  22  in plate  11  and passes through opening  18  in plate  11  and opening  26  in plate  12  where after the fuel flow is divided and passes over cell  13  and through openings  30  in plate  14  and opening  26  in plate  15  and passes over cell  16 , as indicated by arrows. The fuel passing over cell  13  passes through opening  33  in plate  14 , opening  29  in plate  15  and opening or outlet  21  and connect  25  in plate  17 , as shown by arrows and fuel exhaust arrow  35 . Fuel passing across cell  16  also enters outlet  21  and connect  25  in plate  17 . Air (oxidant)  36  enters connect  23  and inlet  19  in plate  11  and the flow is divided to pass over plate  11  and into opening  27  of plate  12  through opening  31  of plate  14  and across the plate  14  as indicated by arrows. The air passing across plate  11  enters opening  28  in plate  12 , opening  33  in plate  14  and joins the air passing across plate  14  and then passes through opening  28  in plate  15 , opening  20  and connect  24  in plate  17  as indicated by arrows and is discharged from connect  24  as indicated by air exhaust arrow  37 . Top plate  17  and bottom plate  11  has flow channels on one side only, with a flat surface on the outer side as seen in FIG. 1.  
         [0021]    The construction of the flow channel/interconnect plates and the cells casing/holder plate is shown in greater detail in FIG. 2. As shown, lower flow channel/interconnect plate  40  is provided with pairs of openings  41 - 42  and  43 - 44  and a plurality of spaced protruding members  45  forming flow channels or passageways  46  there between through which air passes at indicated plates  11  and  14  in FIG. 1. Members  45  and flow channels  46  are located on both sides of any center plate, such as plate  14  in FIG. 1, but are located on only one side of plates  40  and  60  in FIG. 2. Cell casing/holder plate  50 , similar to plates  12  and  15  of FIG. 1, includes a pair of openings  51 - 52  and  53 - 54  at each end, central opening  55  and a cut-away or counter-sink  56  which defines a rim surface or flange  57  on which a cell  58  is mounted in a peripheral surface sealing arrangement. Cutaway  56  is also provided with two sets of angled slots  59  at each end and extending radically from openings  51  and  54  and provide gas flow distribution. Note that the cutaway  56  is configured to include opening  51  at one end and opposite opening  54  at the other end, which allows only passage of fuel across the cell  58  as seen in plates  12  and  15  in FIG. 1. Flow channel/interconnect plate  60  is constructed similar to plate  40  and includes pavis of openings  61 - 62  and  63 - 64  at opposite ends and a pluriality of spaced members  65  forming passageways  66  there between. As shown, when cell  58  is mounted in plate  50  and plates  40 ,  50  and  60  are assembled, the openings in the plates are in alignment for passage there through of air or fuel, as in FIG. 1.  
         [0022]    Should it be desired to form a single cell, the plates  40  and  60  of FIG. 2 would be modified to omit or plug the openings  43 - 44  in plate  40  and the openings  61 - 62  in plate  60 .  
         [0023]    The arrangement of FIG. 1 allows interchange of flows as follows:  1 ) opening  34 ; Fuel,  2 ) opening  36 ; Air,  3 ) opening  35 ; Fuel, and  4 ) opening  37 ; Air. Also, these opening can be interchanges to Air (openings  34  &amp;  35 ) and fuel (openings  36  and  37 ).  
         [0024]    It has thus been shown that the present invention provides a unique stack arrangement which includes a co-flow planar stack with an integral, internal manifold and a cell casing/holder to separately seal the cell. This construction improves sealing and gas flow, and provides for easy manifolding of cell stacks. While the description of the invention has been primarilary directed to a solid oxide fuel cell stack, it can be also utilized in electrolyzers, gas separation systems, etc. which are used in energy production and energy use. The invention has particular application for advanced fuel cells for stationary and transportation power generation. The co-flow arrangement of the present invention may be effectively utilized for planar single cells as well as for planar multi-cell stacks.  
         [0025]    While particular embodiments of the invention have been illustrated and described to exemplify and teach the principles of the invention, and such are not intended to be limiting. Modifications and changes may be come apparent to those skilled in the are, and it is intended that the invention be limited only b the scope of the appended claims.