Abstract:
The present invention is directed to an intermediate layer between end plates of a fuel cell enables individual fuel cells to be removed form the stack in a simple manner, thus reducing production costs.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    The present application is a continuation of international application PCT/CH03/00014, filed Jan. 14, 2003, which designated the United States, and further claims priority to Swiss patent application 118/02 and 1131/02, filed Jun. 30, 2002, all of which are herein incorporated by reference. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    The present invention relates to a fuel cell stack, a fuel cell, an intermediate layer for a fuel cell or fuel cell stack, and a method.  
           [0003]    A fuel cell has a sandwich-like structure with an electrolyte disposed between end plates. Between the electrolyte and one end plate there is an anode, and between the electrolyte and the other end plate a cathode. Solid and liquid electrolytes are known; accordingly, the electrolyte can be accommodated in a supporting structure or may itself possess the required solidity to enable it to be built into the cell. The operating temperatures are likewise significantly different, varying from ambient to several hundred degrees C. or more.  
           [0004]    A known practice is to combine individual fuel cells to form a fuel cell stack as a means of obtaining the required operating voltage by connecting a suitable number of individual cells in series.  
           [0005]    In addition to the individual fuel cells, a fuel cell stack then contains, on each side of the stack, special end plates which complete the stack, and preceding each of said end plates a connection plate (e.g. gold-coated copper plate) with the connections enabling the stack to be connected to the leads of the supplied load.  
           [0006]    The abovementioned elements are stacked until the fuel cell stack contains the required number of fuel cells and are then clamped together including the connection and end plates. This can be performed by tie rods which run through the entire stack and are bolted to the end plates.  
           [0007]    Assembling a stack of this kind is an intricate process, as all the elements have to be individually placed on top of one another and held in the correct fit until they are bolted together.  
           [0008]    In addition, e.g. the connection plates are expensive to manufacture.  
           [0009]    Assembly can be simplified by using bipolar plates. A bipolar plate according to the prior art is produced e.g. by implementing the end plates of two adjacent fuel cells as a single piece, with the advantage that, when assembling a plurality of fuel cells to form a fuel cell stack, the number of elements to be assembled can be reduced. Cooling channels can additionally be disposed in the bipolar plates, resulting in considerably improved thermal management of the fuel cell stack.  
           [0010]    Current flows in a bipolar plate during operation of a fuel cell stack, as said plate constitutes the electrical connection between the anode of one adjacent fuel cell and the cathode of the other adjacent fuel cell.  
           [0011]    It is likewise possible to bond together adjacent end plates of the individual fuel cells to form a bipolar plate.  
           [0012]    Instead of clamping, individual elements or all the elements of the fuel cell and of the entire fuel cell stack could also be bonded together, which yields an operational solution. This is likewise an intricate process:  
           [0013]    Checking the parts to be bonded for damage such as scratches, etc., evenly applying the glue, drying, drawing off solvent vapor, bonding under pressure and temperature, cleaning off excess adhesive (blockage of gas supply and cooling channels, etc), checking that all the media such as air, hydrogen, water, etc. are properly separated from one another, etc.,—all the foregoing prevent quick and easy assembly of the elements to form a fuel cell stack.  
           [0014]    A fuel cell stack may contain a large number of fuel cells: for example, a stack of fuel cells having a polymer membrane as an electrolyte, producing 7 kW and weighing approx. 20 kg possesses around 100 cells. There also arises the question of testing a newly manufactured fuel cell stack and of repair and maintenance: individual defective cells must be able to be removed from the stack, repaired or replaced and reinserted. Although this is essentially not impossible in the case of a bonded stack, it makes little sense because of the time and effort involved. Even with a clamped stack, the time and effort is considerable: the clamp must be undone, which requires particular care in the handling of the cells which do not need to be replaced. These must not detached from one another and also in particular as a unit, so as not to disturb the original fit of the individual elements.  
           [0015]    A disadvantage further arises when using bipolar plates: the selected cell can only be replaced along with the end plates of the adjacent cells. The adjacent cells therefore have to be dismantled; electrodes as well as electrolyte lose their original fit. The advantage for manufacturing the fuel cell stack becomes a disadvantage in the stack&#39;s later life.  
           [0016]    It has therefore become a known practice to form packages of two end plates and seal them with a kind of O-ring seal, thereby creating a unit that is functionally identical to the bipolar plate. Precise machining of the abutting surfaces of the end plates is critical here, as the contact resistance must remain small (otherwise the fuel cell stack will lose efficiency) and the cooling channels must remain tight. The outline and sealing quality of the ring seal are likewise critical, as the gas supply channels must be sealed off from one another depending of the design of the fuel cell stack (one channel carries e.g. oxygen and the other hydrogen). The ring seal is expensive in itself (because it must be individually manufactured for particular end plates) and mounting it is difficult and therefore costly.  
           [0017]    The abovementioned disadvantages also apply to the manufacture and assembly of the connection and end plates of the stack with the associated seals.  
           [0018]    For the purpose of simplifying the description, such packages comprising two joined end plates and similar to bipolar plates,—although not implemented as a single piece—will hereinafter likewise be referred to as bipolar plates, as these packages have the same function in the fuel cell stack as the single-piece bipolar plates.  
           [0019]    As a result either single-piece bipolar plates can be used, with the disadvantage that when a fuel cell is removed from the fuel cell stack, dismantling of adjacent cells is unavoidable. Alternatively, disassemblable i.e. multi-piece bipolar plates as described above can be used which, however, involve considerable effort and cost in terms of manufacture and joining together.  
           [0020]    There has hitherto been no known design which can provide the proper functionality provided e.g. by the single-piece bipolar plate while at the same time simplifying assembly in terms of effort or handling or which will also allow the manufacturing, assembly and maintenance cost/complexity to be reduced for the ends of the fuel cell stack with their connection and end plates which must be kept fluid-tight.  
         SUMMARY OF THE INVENTION  
         [0021]    Accordingly the object of the present invention is to overcome these disadvantages and provide an improved design.  
           [0022]    The use of a conductive intermediate layer obviates the need for direct current-conducting contact between the end plates, which also allows the intermediate layer to be implemented as a planar element over the dimensions of the end plates, with the consequence that there is provided an easily manipulable element for the assembly of a fuel cell stack or the replacement of fuel cells in a fuel cell stack.  
           [0023]    A deformable intermediate layer further permits flexible abutting on the thereto facing surfaces of the adjacent end plates, with the consequence that less exacting requirements can be placed on the manufacturing tolerances of the end plates. This applies not only to the local surface quality but also to the implementation of the surface over its entire dimension, should the latter not be level but e.g. convex or e.g. inclined in form. A deformable intermediate layer can compensate for such machining defects, so that machining itself can be performed more easily and therefore considerably more cheaply.  
           [0024]    In order to assemble a fuel cell stack, the individual elements are first brought together and then clamped in position. When an intermediate layer according to the invention is used, all the end plates or bipolar plates can align themselves in the stack during clamping as the intermediate layer deforms. This eliminates any overstressing e.g. of the electrolyte or its supporting structure (PEM membrane) in the completely assembled stack.  
           [0025]    In a particular embodiment, assembly complexity can be significantly reduced still further if the intermediate layer is fixed to one end plate e.g. by bonding. Instead of 3 elements to be assembled, the bipolar plate according to the invention then has 2.  
           [0026]    By bonding both end plates to the intermediate layer, it is also basically possible to manufacture a bipolar plate according to the present invention as a unit and incorporate it in the fuel cell stack during assembly, which reduces assembly complexity, although this must then, as described above, be weighed against the disadvantage with regard to the then unavoidable disassembly of the adjacent fuel cells when replacing a cell.  
           [0027]    As the end plates can have a thickness in the millimeter range and length and width dimensions of e.g. 10 cm or more, the cooling channels must be implemented as an open labyrinth on the intermediate layer side, so that the labyrinth is covered by the intermediate layer and the cooling channels are sealed.  
           [0028]    The intermediate layer according to the invention is advantageously impermeable to the ambient air and to the media used, such as hydrogen or oxygen and a coolant. A compact stack design can then be implemented.  
           [0029]    Flexible graphite possesses the abovementioned properties. However, the invention is not limited to this material but encompasses all materials that are conductive and can be used as an intermediate layer.  
           [0030]    If the intermediate layer is provided with a current-conducting structure according to claim  17  or  18 , there is no need to use a hitherto indispensable plate for connecting the electrical leads to the fuel cell stack. As the copper plates hitherto necessary had to be provided with corrosion protection (e.g. gold coating), not only is assembly simplified by the present invention, but the manufacturing costs are also relevantly reduced. Further embodiments of the present invention are set forth in the dependent claims. 
       
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS  
       [0031]    The invention will now be explained in greater detail with reference to the accompanying drawings in which:  
         [0032]    [0032]FIG. 1 schematically illustrates the construction of fuel cells having regard to the individual fuel cells according to the prior art,  
         [0033]    [0033]FIG. 2 schematically illustrates the construction of a stack of fuel cells with the elements necessary at the ends of the stack according to the prior art,  
         [0034]    [0034]FIG. 3 shows a view of an intermediate layer according to the invention,  
         [0035]    [0035]FIG. 4 schematically illustrates the construction of a bipolar plate according to the present invention,  
         [0036]    [0036]FIG. 5 schematically illustrates the construction of an intermediate layer according to the invention which can be connected to an external conductor, and  
         [0037]    [0037]FIG. 6 schematically illustrates the construction of a stack of fuel cells according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0038]    [0038]FIG. 1 illustrates schematically and by way of example a stack  1  of fuel cells comprising the two PEM fuel cells n and n+1. Such an arrangement will be familiar to an average person skilled in the art. Although for simplicity&#39;s sake the stack  1  is shown with only two fuel cells n and n  30  1, it can contain any number of cells. For a common application e.g. a hundred cells will be provided. Each fuel cell n and n+1 possesses end plates  2 , the adjacent end plates of the cells n and n+1 being combined to form the bipolar plate  3 . Between the end plates  2 , 3  there is located a polymer membrane  4 , between the membrane  4  and the end plates  2 , 3  there is an anode  5  and a cathode  6  respectively. Channels  7  are used to supply gas, H 2  on the anode side, O 2  or ambient air on the cathode side (other possible reactants will also be known to the average person skilled in the art). The channels are implemented in such a way that the gases can be supplied as evenly as possible over the entire membrane surface. Gas supply and removal channels  10  running along the lateral faces  20 , 21  of the stack  1  (FIG. 3) and which serve the channels  7  are not shown for the sake of representational clarity. Likewise not shown are the cooling channels in the bipolar plate  3 .  
         [0039]    The layout of the channels  10  or of the cooling channels will be familiar to the average person skilled in the art. The channels  10  basically run along the entire length of the stack  1  and end blind at one end of the stack  1 , whereas at the other end of the stack  1  they pass through its end plate so that they can be connected to an external supply. The stack  1  is supplied with coolant in a similar manner.  
         [0040]    An electrical load  8  is connected to an anode  5  and a cathode  6  via a conductor  9 . The Figure schematically illustrates the direct connection of the conductor  9  to the electrodes. FIG. 2 shows the correct layout of a stack  1  with a special connection plate  42 , 52 . FIG. 1 further illustrates that, when a single-piece bipolar plate  3  is used, replacing a cell n involves dismantling the cells n +1 and n−1, with the disadvantage that the electrodes  5 , 6  and the membrane  4  lose their original fit.  
         [0041]    [0041]FIG. 2 shows a conventional stack  1  having individual fuel cells n−x, n and n+x. The ends  40 , 50  of the stack  1  have end plates  41 , 51  (the end plate  41  blanks off the channels  10 , the end plate  51  possesses openings (not shown here) for connecting the channels  10  to the external supply). Also shown are connection plates  42 , 52  with connection lugs  43 , 53  for connecting to a conductor  9 . In the end  40  there is further provided a cooling plate  44  as a patterned graphite plate, and in the end  50  an unpatterned plate  54  (see description for FIG. 6). The plates  44 , 54  prevent media cross flow, the plate  44  as mentioned being additionally provided with cooling channels  60 .  
         [0042]    The fuel cells n−x, n and n+x have the structure depicted in FIG. 1 but are here provided with cooling channels  60  running crosswise through the bipolar plates  3 .  
         [0043]    Between all the abovementioned elements (end plates  41 , 51 ; connection plates  42 , 52 ; cooling plate  44  and plate  54  and in the bipolar plates  3 ) there is provided a ring seal SD which prevents unwanted contact of the various media circulating in the stack.  
         [0044]    [0044]FIG. 3 is a schematic view of an intermediate layer  30  according to the invention having a body  11  as well as fluid-tight sealing edge sections  12  for gas supply or removal channels  10  running along the outer sides  20 ,  21  of the stack  1  and feeding the channels  7  running crosswise through the stack  1  (FIG. 1).  
         [0045]    The intermediate layer  30  further has fluid-tight sealing edge sections  13  for the edges of the end or bipolar plates  2 , 3 , thereby preventing unwanted contact of the various media. Also shown are cutouts  14  for tie elements which hold the stack  1  together mechanically. The geometry of the intermediate layer  30  therefore corresponds to the geometry of the adjacent elements of the stack  1 , whether it be end plates  2  or bipolar plates  3  of the individual fuel cells or the plates for the power connection or the end plates  41 , 51  of the stack  1 .  
         [0046]    The construction of the stack  1  as shown in FIGS. 1 and 2 will, as mentioned, be familiar to the average person skilled in the art. The geometry of the intermediate layer  30  is not limited to the form depicted in FIG. 2; the plates  2 , 3  can be provided with any outline, whether it be in respect of the body  11  or of the channels  10  or the cutouts  14 . It is also conceivable for adjacent fuel cells  1   a ,  1   b  to be equipped with an intermediate layer  30  implemented as a single piece, i.e. for adjacently disposed stacks  1  which are supplied e.g. by a common gas supply channel  10 . The geometry of the intermediate layer in turn corresponds to the adjacent elements of the stack  1 .  
         [0047]    The intermediate layer  30  is preferably made of flexible graphite, such as that available in Germany e.g. from the company SGL Carbon AG under the designation “Sigraflex”.  
         [0048]    The intermediate layer  30  can be implemented with a thickness of 1 mm or 0.5 mm, in the form of a foil. The thickness is preferably less than 1 mm, i.e. between 0.5 and 0.3 mm.  
         [0049]    [0049]FIG. 4 shows a bipolar plate  3  in an exploded view of FIG. 1 with the adjacent anode  5  or cathode  6 . The end plate  2   a  belongs to the fuel cell n and the end plate  2   b  to the fuel cell n+1 (FIG. 1). Instead of a ring seal, there is a flexible intermediate layer  30  according to the present invention between the end plates  2   a  and  2   b . In the end plate  2   a  are disposed cooling channels  60  which are sealed off by the intermediate layer  30 . This layer separates the fuel cells n and n+1 from one another; it is disposed between the end plates  2   a ,  2   b  of adjacent fuel cells n, n+1.  
         [0050]    [0050]FIG. 5 shows a cross-section of an intermediate layer  70  which is a modified version of the intermediate layer  30 . A copper sheet (good conductor) implemented as a lead section  71  and which feeds out the current produced by the stack  1  to the conductor  9  is inserted for the connection lugs  43 , 53 . The copper sheet  71  possesses an outline such that a form fit is produced in the intermediate layer  70  and has as large an area as possible for low contact resistance between the sheet  71  and the intermediate layer  70 . The intermediate layer  70  shown in the Figure is preferably three-layered: two outer layers of flexible graphite possess the configuration according to the outline of FIG. 5; one middle layer of flexible graphite possesses the outline of the hatched area shown in the Figure, so that the copper sheet  71  can be inserted in the vacant space and is covered by the outer layers.  
         [0051]    In another exemplary embodiment, a conductor grid can be incorporated in the intermediate layer  70  as the lead section  71  and connected to the connection lug  43 , 53 . The conductor grid can also be inserted between two intermediate layers  70 ; the deformability of the flexible graphite or other material having the same properties produces a secure grid fit and faultless current flow. The more tightly meshed the grid, the lower the contact resistance.  
         [0052]    It is likewise possible to implement the conductor  9  as a stranded cable and to insert the strands, as described above, between two intermediate layers  70  or incorporate them in an intermediate layer  70 . The discharging of current from the stack  1  using an intermediate layer  70  can if necessary be modified by the average person skilled in the art, but without departing from the scope of the invention.  
         [0053]    A special gold-coated connection plate is eliminated.  
         [0054]    [0054]FIG. 6 shows a stack  1  of fuel cells n−x and n+x constructed according to the invention.  
         [0055]    The end  40  of the stack  1  is formed by the end plate  41  and an intermediate layer  70  with a structure (not shown in the Figure) of lead sections  71  for the connection lug  43 . A cooling plate  44  connects to the intermediate layer  70  and is implemented as a patterned graphite plate, the pattern consisting of channels  60  in which coolant flows. This is again followed by an intermediate layer  30  and then the fuel cell n−x of the stack  1 . The fuel cell n−x abuts with its end plate  2   b  the intermediate layer  30  and contains no cooling channels (FIG. 4), so that the cooling plate  44  must be provided in the end  40 . The intermediate layer  30  between the plate  44  and the cell n−x terminates the channels  10  which end blind here and seals said channels (as well as the cooling channels  60 ).  
         [0056]    The end  50  of the stack  1  is formed by the end plate  51  and the intermediate layer  70  which is of identical construction to the intermediate layer  70  in the end  40  of the stack  1 . This is followed by the fuel cell n+x; as the latter has an end plate  2   a  with cooling channels  60 , there is no cooling plate like the cooling plate  44  in the end  40 .  
         [0057]    This material of the intermediate layers  30  and the intermediate layers  70  flexibly adapts to the surface contour of the plates  2   a,b  and  3  and permits current transfer across the entire relevant contact area with minimal transfer resistance. This flexible adaptation also means that the machining tolerances for producing the plates  2   a  and  2   b  as well as the plates  41 ,  51  and  44  can be relaxed, which reduces the processing costs themselves and reduces wastage in terms of defectively produced plates, thereby saving costs.  
         [0058]    If e.g. the contact area to the intermediate layer  30  of a plate  2   a , 2   b , 41 , 51  or  44  is not aligned exactly parallel to the opposing surface in contact with the anode or cathode (e.g. wedge shape of one of the plates), this will cause the adjacent plate to be tilted. This tilt results in tensions in the stack  1 , e.g. an impermissible local stressing of the membrane  4 , which reduces the performance of the fuel cell affected and therefore of the stack  1  as a whole. This risk is eliminated by the deformable intermediate layer  30 , 70 , as the compensation of the mutual positions of the plates  2   a , 2   b , 41 , 51  can take place via the deformation of the layer  30 , 70 .  
         [0059]    The passage of current through the bipolar plate  3  depends on the ohmic resistance of the intermediate layer  30  itself and on the contact resistance between the intermediate layer  30  and the adjacent end plates  2   a ,  2   b . Test measurements have shown that the contact resistance from end plate  2   a  to end plate  2   b —as is known e.g. when using a ring seal according to the prior art—is the same size as the sum of the resistances (contact and ohmic resistance) when using an intermediate layer  30  made of flexible graphite having a thickness of 0.5 mm in a bipolar plate  3  according to the invention under the pressure obtaining when a fuel cell stack is clamped in the normal manner.  
         [0060]    This means that using the intermediate layer  30  according to the invention and having a correctly selected thickness provides a relevant advantage in current transfer through the bipolar plate, an advantage which is independent of the other advantages such as cheaper manufacture of the end plates, easier assembly, etc.