Abstract:
The invention relates to a fuel cell stack in which, according to the invention, several elements of the fuel cell stack are advantageously sealed by a seal in order to form a continuous operating material channel. The inventive fuel cell stack can be provided with a very compact design, whereby the layer thickness of an individual fuel cell is, on a regular basis, distinctly less than 1.5 mm and, advantageously, even less than 1.2 mm.

Description:
TECHNICAL FIELD  
         [0001]    The invention relates to a fuel cell stack, especially for low-temperature fuel cells.  
         STATE OF THE ART  
         [0002]    A fuel cell has a cathode, an electrolyte, and an anode. The cathode is supplied with an oxidizing medium, for example, air, and the anode is supplied with a fuel, for example, hydrogen.  
           [0003]    Different fuel cell types are known, for example, the SOFC fuel cell of the publication DE 44 30 958 C1 and the PEM fuel cell of the publication DE 195 31 852 C1.  
           [0004]    A plurality of fuel cells are, as a rule, electrically and mechanically connected together in order to produce greater electrical powers by connecting elements. An example of such a connecting element is the bipolar plate which is known from DE 44 10 711 C1. By means of bipolar plates, fuel cells stacked one above another are connected electrically in series. This arrangement is termed a fuel cell stack.  
           [0005]    By providing common anode and cathode compartments for respective pairs of fuel cells, the space required for a fuel cell stack can be significantly reduced by comparison with the individual fuel cells.  
           [0006]    Advantageously, for this purpose, in a fuel cell stack, two cells will have a common cell frame, i.e. will be inserted pairwise in each cell frame. A plurality of cell frames connected in succession, which can be contacted externally, then form a fuel cell stack.  
           [0007]    Up to now, for the cell frames and also the bipolar plates, expensive structures have actually been required and have been complicated to fabricate and have a certain minimum structural height. Aside from the electrical contacting, one must be concerned with the respective feed and discharge of the operating media.  
           [0008]    From WO 98/13891, a bipolar plate for a fuel cell is described which is comprised substantially of a number of planar components. Disadvantageously, this bipolar plate requires sealing between the individual components. They form a layer system with a thickness of about 3 mm.  
           [0009]    It has also been found to be a drawback with this configuration that both the anode and cathode components and also the passages through which the operating substances must pass, must be sealed. Because of the compact configuration of the fuel cell stack, the seals which are typically used are flat seals which mandate high requirements for the surfaces of the components as to roughness and finishing precision and thus usually give rise to higher fabrication costs.  
         OBJECT AND SOLUTION  
         [0010]    The object of the invention is to provide a fuel cell stack which enables a compact construction and does not have the above mentioned drawbacks.  
           [0011]    The object of the invention is achieved with a fuel cell stack according to the main claim. Advantageous embodiments of the fuel cell stack are given in the claims which are dependent thereon.  
         DESCRIPTION OF THE INVENTION  
         [0012]    The fuel cell stack according to claim 1 encompasses at least three neighboring elements which, to form a throughgoing channel, each have a through passage. Furthermore, the through passages of the elements lying at the outer sides are smaller than the passage of the inwardly lying element or elements. For sealing the channel formed between the outwardly lying element, only one seal is provided.  
           [0013]    As elements in the sense of this invention, especially frame elements with gas distributor structures or for receiving a membrane electrode arrangement, bipolar plates or a flat seal should be understood which through corresponding cutouts can serve as an electrode compartment. These elements are typically arranged in neighboring relationship, i.e. layer-like adjacent one another in a fuel cell stack. To form a throughgoing channel in the elements, especially for supply and/or discharge of an operating medium, these elements have corresponding through passages. According to the invention, the passage of the inwardly lying element or the inwardly lying middle element is greater than those of the two outwardly lying elements. The channel formed by the passages of the elements is sealed by a single seal. This seal, according to the invention, lies only between the outwardly lying elements. The passages of the intermediate elements (inwardly lying) have a diameter which is sufficiently greater than the seal between the outwardly lying elements is located within the passage of the intermediate elements. A sealing between the inwardly lying elements with respect to their two neighboring elements can thus be advantageously omitted.  
           [0014]    This arrangement has the effect, by contrast with the earlier state of the art, that in the regions of the passage of each individual element to its neighboring element which formally required sealing, a seal can be avoided. As a result, the fuel cell stack is advantageously correspondingly constructed in a more compact manner. The requisite layer thickness of a single fuel cell and thus of the total fuel cell stack can be reduced as a rule.  
           [0015]    For more than three elements, there is a saving not only of one, but of a correspondingly larger number of additional seals. The advantageous reduction in the layer thickness for an individual fuel cell grows with each seal which is saved.  
           [0016]    In an advantageous embodiment of the invention, the outwardly lying elements have passages with the same diameter. These passages usually form a part of a channel for the operating media. These can advantageously have a single diameter so that flow changes and pressure changes because of different geometries can be avoided.  
           [0017]    Advantageously, also all of the inwardly lying elements can have identical diameters for their passages. These passages should be selected to be so large that a seal inserted between the outwardly lying elements can be so dimensioned that its inner diameter corresponds with that of the outwardly lying elements. This advantageously results in a substantially frictionless transition of sealing to breakthrough within the passage for the operating media.  
           [0018]    Advantageously, the layer thickness of a fuel cell unit of the fuel cell stack according to the invention has only a layer thickness of less than 2 mm, especially less than 1.5 mm. The layer thickness means, in the framework of the invention for a fuel cell unit, the structural distance from one membrane electrode arrangement (MEA) to the next. In the case of serial fuel cells disposed in cascade, one fuel cell unit is comprised of a membrane, a cathode, the cathode compartment, a bipolar plate, the anode compartment and the anode. In the case of fuel cells in which two identical electrodes respectively are disposed in adjoining relationship, a unit is comprised, for example, of a membrane, a cathode, the cathode space and yet another cathode.  
           [0019]    Advantageously, as the seal in a fuel cell stack according to the invention, a circular seal in the form of an O-ring is used. An O-ring does not have as high requirements with respect to surface properties as a flat seal and is easy to handle. A suitable material for an O-ring seal is, for example, EPDM, which is very resistant. In general, O-ring seals are highly elastic seals. In addition, they are cost effective. An O-ring seal need not be made as flat as a flat seal but because of the cost saving for seals, this drawback is more than compensated. Thus it is for example usually possible with the aid of one O-ring as the seal to realize layer thicknesses for a fuel cell unit of less than 2 mm, especially less than 1.5 mm.  
           [0020]    The layer thickness can be reduced especially effectively when in a fuel cell stack as many of the operating medium passages as possible can be sealed. Advantageously a fuel cell stack should have elements with at least two and advantageously however, four passages for the formation of operating medium passages. They can be arranged diagonally in an element to achieve an optimum distribution of the operating medium in the electrode compartment.  
           [0021]    Additional passages in several elements, especially the flat seals give rise advantageously to a simple electrical contacting between two non-directly neighboring elements. The contact between these elements is effected not externally of the elements but through suitable means directly between them. One such suitable means is a conductive lug which can be swung out of the plane of a first element. Through the corresponding passage in the neighboring second element, there is no contact with this element. Only the further, third element, which has no passage at the corresponding location is contacted by the lug so that in a simple way an electrical connection between the first and third elements but not, however, to the intermediate element, can be made. 
       
    
    
     SPECIAL DESCRIPTION PART  
       [0022]    In the following, the invention is described in greater detail based upon the figures without limiting the subject matter of the invention thereby. They show:  
         [0023]    [0023]FIG. 1:  
         [0024]    (a) a schematic cross section of three neighboring elements separated by seals according to the state of the art which have a common throughgoing passage.  
         [0025]    (b) a schematic cross section of an embodiment of the invention with three neighboring elements separated by a seal which have a common throughgoing passage.  
         [0026]    [0026]FIG. 2: a schematic cross section through a fuel cell stack according to the invention;  
         [0027]    [0027]FIG. 3: 1. Embodiment:  
         [0028]    (a) an exploded view of a membrane electrode unit with neighboring gas distributor structure,  
         [0029]    (b) exploded view of a membrane-electrode unit with neighboring gas distributor structure and means for electrical contact,  
         [0030]    (c) exploded view of a fuel cell stack constructed with units from  3   a  and  3   b.    
         [0031]    [0031]FIG. 4: 2. Embodiment:  
         [0032]    (a) exploded view of a membrane-electrode unit with neighboring gas distributor structure,  
         [0033]    (b) exploded view of a membrane electrode unit with neighboring gas distributor structure and means have electrical contacting,  
         [0034]    (c) exploded view of a fuel cell stack made with the units from  4 ( a ) and  4 ( b ). 
     
    
       [0035]    [0035]FIG. 1 a  shows schematically a segment from a fuel cell stack according to the state of the art which is comprised of three elements E, for example, frame elements or bipolar plates. These elements each have a throughgoing hole with a diameter φ and thus form a continuous passage. Each element is sealed for example by a ring-shaped seal D around the throughgoing bore with respect to the neighboring element. Up to now, it has been customary to provide these seals as so-called flat seals which are advantageously very thin but have increased requirements as to the surfaces of the elements with respect to roughness and fabrication precision. In FIG. 1 b,  also shown schematically, is a segment from a fuel cell stack in accordance with an embodiment according to the invention. The three neighboring elements Ea, Ei, Ea, have  10  throughgoing openings for forming a continuous passage. The diameter φ i  of the throughgoing opening of the inwardly lying element E i  is however greater than the diameter φ a  of the through opening of the outwardly lying elements Ea. Thus advantageously a single seal D can seal the passage between the outwardly lying elements.  
         [0036]    Since in this manner one seal is saved, an expensive flat seal can be eliminated and instead the latter, for example, a simple O-ring can be provided as the seal. While the single seal (D) in FIG. 1 b  itself thicker than a flat seal (D in FIG. 1 a ), in this embodiment of the invention, the layer thickness S 2  of the three elements is usually reduced by comparison with the state of the art S 1 . This provides substantial advantages for the construction of a fuel cell stack according to the invention. The more elements in the fuel cell stack which are connected in the system of the invention by a seal the greater is the saving through the reduction of the layer thickness.  
         [0037]    [0037]FIG. 2 discloses an embodiment of the fuel cell stack according to the invention in which each electrode compartment is bounded by two identical electrodes. An electrode compartment is formed by two electrically conductive elements with gas distributor structures  1 ,  5  and a flat seal  7  disposed between them. Toward the exterior each electrode compartment is bounded by two neighboring membrane-electrode arrangements (MEAs)  2 . These are sealed with respect to the neighboring elements and electrically insulated by one or two corresponding seals  3 .  
         [0038]    The individual elements  1 ,  5 ,  7  each have two through openings for forming a fuel passage and an oxidizing agent passage. The part of the oxidizing medium passage  12  which runs from the through opening of the element  1 K of one cathode unit to the neighboring cathodic element  5 K, is only sealed by one seal  9  to the anodic elements  1 A,  7  and  5  lying therebetween.  
         [0039]    The designation of outwardly lying element and inwardly lying elements should not be seen as absolute but dependent upon the respective passage which they form. With respect to the oxidizing medium element passage, the elements form with the gas distributor structure  5 A and  1 A, the inwardly lying elements of the anode unit whose through openings, according to the invention, are larger than those of the outwardly elements of the neighboring cathode unit. The same elements  5 A,  1 A of the anode unit form the fuel medium passage  13 .  
         [0040]    [0040]FIGS. 3 a  through  3   c  show in a first embodiment the construction of a fuel cell stack according to the invention from individual elements. The membrane-electrode units (MEAs)  2  are premounted in cartridges  4 ,  6 . The electrical insulation of the cartridge plates  1  to  5  are effected over an MEA flat seal  3 . The cartridges  4 ,  6  are so arranged in the stack that if two cartridges are in contact over a common anode compartment or cathode compartment. The anode compartment or the cathode compartment between two cartridges  4 ,  6  or one compartment  4 ,  6  and an end plate  10 ,  11  is formed by a cutout  7   b  in the flat seal  7 . The flow distribution within the medium compartments and the tightening of the MEA can be realized by nonconducting inserts  8 .  
         [0041]    The insert  8  is a plastic fabric. The cartridge plate  1  and  5  connect via bores  1   a,    1   b,    5   c,    5   d  into operating medium ducts. The bores  1   a,    5   c  are smaller than the bores  1   b,    5   d.    
         [0042]    The sealing of the medium ducts form, for example, one anode compartment to the next anode compartment are formed by a round sealing ring (O-ring)  9 . The services in the cartridge plates  1 ,  5  which directly surround the smaller bores  1   a,    5   c  are simultaneously abutment services for the sealing ring  9 . The sealing ring  9  is received in the space which is formed by the bores  1   b,    7   a,    5   d  and  12   a.  The cartridges  5 , for electric current delivery are in contact via contact lugs  5   a  which, through the breakouts  7   c,    5   b,    12   b  in the frame elements  7 , engage the cartridge plates  5  and the inserts  12  of the cartridge plates  4  and the end plates  10 ,  11 . The thickness of the cartridge plates  1 ,  5  amounts to about 0.2 mm, the thicknesses of the frame elements  7  amount to about 0.8 mm and the thicknesses of the inserts  12  are about 0.5 to 4.0 mm.  
         [0043]    [0043]FIGS. 4 a  through  4   c  show in a second embodiment the construction of a fuel cell stack of the invention from individual elements. In contrast to the fuel cell stack shown in FIG. 3, the individual cartridges  4  and  6  have in this embodiment in their individual elements  1  and  5  only  2  each of through openings. The through openings for the oxidizing medium passage are omitted. Further, instead of flat seal  7  with inserts provided in the recesses  7   b  of plastic fabric, partial flat seals  7  ( 8 ) are provided which now are completely comprised of the fabric  8  and have through openings for the fuel passage as well as for the lugs  5   a.    
         [0044]    Instead of a continuous oxidizing agent passage, the oxidizing agent in this embodiment flows directly from the edges of the flat seal  7  ( 8 ) into the corresponding cathode compartment. The oxidizing agent is then conducted via blowers into the fabric  7  ( 8 ). For small powers it suffices to provide a fabric  7  ( 8 ) of a thickness of 2 to 4 mm without a blower. The extent that the system cannot operate with an oxidizing at ambient pressure, the entire fuel cell unit can also be connected to a larger distribution passage or a collecting passage which is under subatmospheric pressure.  
         [0045]    Alternatively, instead of a plastic web for the seal  7  ( 8 ) a plastic sheet which has passages or the like set into it can be used.  
         [0046]    The invention then has the following advantages:  
         [0047]    it is attainable in a very flat construction, less than 1.5 mm per individual cell;  
         [0048]    no external wiring is required in spite of common anode and cathode compartments;  
         [0049]    the round sealing ring (O-ring) bridges directly four sealing transitions or component surfaces and insures a reliable seal;  
         [0050]    it requires significantly fewer and then only planar components.  
       Legends for Figures  
       [0051]    E, E i , E a  Element, inner Element, outer Element  
         [0052]    φ i , φ a  Diameter of the through opening (i=inwardly lying, a=outwardly lying)  
         [0053]    D Seal  
         [0054]    S i , S 2  Layer thickness  
         [0055]    [0055] 1  Cartridge Plate with gas distributor structures  
         [0056]    [0056] 1 A=anode side  1 K=cathode side  
         [0057]    [0057] 1 A=Through opening for air passage  
         [0058]    [0058] 1 B=Through opening for fuel passage  
         [0059]    [0059] 2  Membrane electrode arrangement MEA  
         [0060]    [0060] 3  Flat seal for MEA  
         [0061]    [0061] 4  Cartridge example from 1+2+3+1  
         [0062]    [0062] 5  Cartridge plate with gas distributor structures  
         [0063]    [0063] 5 A=Anode side,  5 K=Cathode side  
         [0064]    [0064] 5   a= Contact lugs  
         [0065]    [0065] 5   b= through opening for lugs  
         [0066]    [0066] 5   c= through opening for oxidizing passage  
         [0067]    [0067] 5   d= through opening for fuel passage  
         [0068]    [0068] 6  Cartridge assembly from 5+2+3+5  
         [0069]    [0069] 7  Flat seal with  
         [0070]    [0070] 7   a= through opening for contact lugs  5   a    
         [0071]    [0071] 7   b= cutout for insert  8   
         [0072]    [0072] 8  insert of nonconducting plastic web  
         [0073]    [0073] 7  ( 8 ) Flat seal completely of nonconducting plastic web with  
         [0074]    [0074] 8   a= through opening for lugs  5   a    
         [0075]    [0075] 8 b=through opening for fuel passage  
         [0076]    [0076] 9  O-ring  
         [0077]    [0077] 10  End plate  
         [0078]    [0078] 11  End plate  
         [0079]    [0079] 12  Oxidizing Agent Passage  
         [0080]    [0080] 13  Fuel Passage