Abstract:
A membrane electrode assembly for a fuel cell that includes a membrane electrode unit with a membrane and two electrodes which make surface contact with both faces of the membrane. The membrane electrode assembly has a seal support that surrounds the periphery of the membrane and that overlaps the latter. The membrane electrode also has a connecting layer which continuously overlaps the membrane and the seal support, an inner edge section of the connecting layer being bonded to the membrane electrode unit and an outer edge section of the connecting layer being bonded to the seal support on the same flat face of the connecting layer. A seal is connected outside the membrane to the seal support. A fuel cell is provided that includes a plurality of membrane electrode assemblies. A motor vehicle includes the fuel cell and a method is provided for producing the membrane electrode assembly.

Description:
[0001]    This nonprovisional application is a continuation of International Application No. PCT/EP2013/071862, which was filed on Oct. 18, 2013, and which claims priority to German Patent Application No. 10 2012 020 975.4, which was filed in Germany on Oct. 25, 2012, and which are both herein incorporated by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a membrane electrode arrangement for a fuel cell, having a membrane electrode assembly and a seal. The invention also relates to a fuel cell having a plurality of membrane electrode arrangements and a motor vehicle which includes a fuel cell of this type. 
         [0004]    2. Description of the Background Art 
         [0005]    Fuel cells use the chemical conversion of a fuel to water with the aid of oxygen to generate electrical energy. For this purpose, fuel cells contain the so-called membrane electrode assembly (MEA for membrane electrode arrangement) as a core component, which is an assembly of an ion-conducting membrane and an electrode disposed on each side of the membrane (anode and cathode). Gas diffusion layers (GDL) may also be disposed on both sides of the membrane electrode assembly, on the sides of the electrode facing away from the membrane. As a rule, the fuel cell is formed by a large number of stacked MEAs, whose electrical outputs add up. During the operation of the fuel cell, the fuel, in particular hydrogen H 2 , or a hydrogen-containing gas mixture, is supplied to the anode, where an electrochemical oxidation of H 2  to H +  takes place by emitting electrons. A (hydrous or anhydrous) transfer of the protons H+ from the anode compartment to the cathode compartment takes place with the aid of the electrolyte or the membrane, which separates the reaction chambers gas-tight from each other and electrically insulates them. The electrons provided to the anode are supplied to the cathode over an electrical line. Oxygen, or an oxygen-containing gas mixture, is supplied to the cathode so that a reduction from O 2  to O 2−  takes place by absorbing the electrons. At the same time these oxygen anions react with the protons transported through the membrane in the cathode compartment, forming water. By directly converting chemical energy into electrical energy, fuel cells achieve an improved efficiency compared to other electricity generators by circumventing the Carnot factor. 
         [0006]    The currently most advanced fuel cell technology is based on polymer electrolyte membranes (PEM), in which the membrane itself comprises a polymer electrolyte. Acid-modified polymers, in particular perfluorinated polymers, are used. The most common representative of this class of polymer electrolytes is a membrane made of a sulfonated polytetrafluoroethylene copolymer (trade name: Nafion; copolymer of tetrafluoroethylene and a sulfonyl fluoride derivative of a perfluoroalkyl vinyl ether). The electrolytic conduction takes place with the aid of hydrated protons, which is why proton conductivity is conditional on the presence of water, and a humidifying of the operating gases is necessary during the operation of the PEM fuel cell. Due to the need for water, the maximum operating temperature of these fuel cells under standard pressure is limited to less than 100° C. To distinguish between these fuel cells and high-temperature polymer electrolyte membrane fuel cells (HT-PEM fuel cells), whose electrolytic conductivity is based on an electrolyte which is bound by electrostatic complex binding to a polymer structure of the polymer electrolyte membrane (for example, phosphoric acid-doped polybenzimidazole (PBI) membranes) and which are operated at temperatures of 160° C., this type of fuel cell is also referred to as a low-temperature polymer electrolyte membrane fuel cell (LT-PEM fuel cell). 
         [0007]    As mentioned at the outset, the fuel cell is formed by a large number of individual cells arranged in a stack, which are referred to as a fuel cell stack. As a rule, so-called bipolar plates are disposed between the membrane electrode assemblies, which ensure that the individual cells are supplied with the operating media, i.e., the reactants and usually also a cooling fluid. The bipolar plates also ensure an electrically conductive contact between the membrane electrode assemblies. 
         [0008]    Seals, which seal the anode and cathode compartments to the outside and prevent the operating media from exiting the fuel cell stack, are disposed between the membrane electrode assemblies and the bipolar plates. The seals may be provided by the membrane electrode assemblies and/or the bipolar plates and, in particular, be connected to these components. 
         [0009]    For this purpose, the seals may be vulcanized onto one or both sides of the bipolar plate. The seal may furthermore be deposited onto the bipolar plate in the form of a sealing bead with the aid of a robot. The seal deposited by the robot may have substantial tolerances, which may result in leaks. Up to now, this problem has been counteracted, e.g., by optimizing the process of depositing the sealing bead with the robot. 
         [0010]    The membrane may be also laminated between two films (edge reinforcing films) coated with an adhesive. Seals may then be overmolded onto the membrane electrode assemblies, or they may be insert-molded around the membrane electrode assemblies. However, the maximum temperature is determined by the membrane electrode assembly and is approximately 120° C. This temperature limitation plays a role in the process times for cross-linking the elastomer of the seal and results in high costs due to long process times as well as a large number of rejects. The reject costs resulting therefrom are attributable to faulty insert-molding of the membrane electrode assembly as well as the handling of this extremely sensitive component in a stamping and injection molding process. 
         [0011]    DE 10 2009 003 947 A1, which corresponds to U.S. Pat. No. 7,935,453, discloses a UEA (unitized electrode assembly), comprising an MEA, which includes an electrolyte membrane which does not extend substantially beyond an active region of the MEA. A barrier film is disposed between a seal of the UEA, which surrounds a chemically active region, and the chemically active region. The barrier film may overlap the MEA outside the active region. In addition, the barrier film may act as a support for the seal and be designed, in particular, to form a single piece therewith. The MEA may be coupled with the barrier film with the aid of a chemical adhesive. Depending on the connection configuration, a connecting film may also cover one end of the electrolyte membrane. 
         [0012]    WO 2010/114139, which corresponds to US 20110305976, describes a manufacturing method for a fuel cell and for fuel cell modules of the fuel cell. The fuel cell comprises electrode units, each of which includes an MEA and a porous layer on the anode side and on the cathode side of the MEA. The MEA furthermore includes an electrolyte membrane and a catalytic anode and cathode layer. The porous layer includes a gas diffusion layer (made of carbon paper) facing the MEA and a gas flow field layer (made of sintered metal foam). Separators (corresponding to bipolar plates), each of which includes three steel plates, are disposed between the electrode units. Frame-shaped seals, which surround the electrode units, are disposed between and make contact with the separators. 
         [0013]    A first unit including a separator and an outer frame of the frame-shaped seal and a second unit including the electrode unit and an inner frame of the frame-shaped seal are first formed during manufacturing. The inner frame, along with the electrode unit, is then inserted into the outer seal. As another variant, the electrode unit may also be inserted directly into a unit including the separator and the frame-shaped seal. 
         [0014]    WO 2010/114140 A1, which corresponds to US 20120009506, discloses a manufacturing method for a cell arrangement of a fuel cell. The structure if the fuel cell is essentially similar to the structure of WO 2010/114139. The electrode unit, the separator and a seal preform, which has the shape of a frame, are first produced during manufacturing. The electrode unit, the separator and the seal preform are disposed in a forming die, which includes a pressing device. The individual parts are connected to each other in the forming die by pressure and heat. 
       SUMMARY OF THE INVENTION 
       [0015]    It is therefore an object of the present invention to provide a membrane electrode arrangement, which is easier to manufacture. 
         [0016]    The membrane electrode arrangement according to an exemplary embodiment of the invention for a fuel cell comprises a membrane electrode assembly, which includes a membrane and two electrodes which make surface contact with both sides of the membrane. The membrane electrode arrangement also comprises a seal support which circumferentially surrounds and overlaps the membrane. The membrane electrode arrangement furthermore comprises a connecting layer, which circumferentially overlaps the membrane and the seal support, an inner edge section of the connecting layer being integrally connected to the membrane electrode assembly, and an outer edge section of the connecting layer being integrally connected to the seal support, on the same flat side of the connecting layer. The membrane electrode arrangement also comprises a seal, which is connected to the seal support outside the membrane. 
         [0017]    The membrane can be a proton-conducting membrane (polymer electrolyte membrane). The electrodes form an anode and a cathode and may be coated on both sides of the membrane. 
         [0018]    The inner edge section of the connecting layer can be integrally connected to the membrane and/or an electrode of the membrane electrode assembly. 
         [0019]    In flat elements, the sides (surfaces) whose extensions are significantly larger compared to other sides of the elements are referred to as “flat” sides. 
         [0020]    Due to the integral connection, the connecting layer can establish sealing surfaces to the seal support and to the membrane electrode assembly which surround a chemically active region of the membrane electrode assembly. As a result, an undesirable transfer of operating media between the two sides of the membrane is prevented. A sufficient stability of the membrane electrode arrangement is furthermore ensured by the integral connection. The chemically active region is the region of the membrane electrode assembly to which reactant is applied during operation. The connecting layer can be a film, in particular a plastic film. 
         [0021]    The seal can be designed to circumferentially surround the membrane. As a result, the chemically active region of the membrane electrode assembly is also circumferentially surrounded by the seal. Due to the fact that the seal surrounds the chemically active region, reactants and reaction products are prevented from exiting a fuel cell which comprises the membrane electrode arrangement. The electrodes of the membrane electrode assembly thus are also disposed within the chemically active region which is circumferentially surrounded by the seal. 
         [0022]    The seal can be integrally connected to the seal support. This may be implemented by overmolding the seal onto the seal support, for example by partially melting the affected materials. 
         [0023]    The seal is usually connected to the seal support outside the connecting layer. 
         [0024]    The seal furthermore usually extends on both sides along the seal support, in particular in two subsections. The two subsections extend on both sides of the seal support, in that the first subsection extends along a first flat side of the seal support and the second subsection extends along a second flat side of the seal support. 
         [0025]    The two subsections have sealing surfaces, the sealing surfaces of the first subsection and the second subsection preferably essentially forming congruent, orthographic projection regions on the seal support. Sealing surfaces can refer to, for example, those surfaces which are designed to abut and seal a counter-surface, e.g., a bipolar plate. The sealing surfaces are particularly preferably designed to be mirror-symmetrical with respect to the seal support (or a plane situated therein). 
         [0026]    The seal can have two sealing lips per subsection, which are formed by a corresponding profiling of the seal. Two independent sealing lines are formed thereby, i.e., two sealing regions which act as double protection against leaks. The two sealing lips can run all around a sealed region. 
         [0027]    The seal support can be a seal support film made of a plastic. In particular, the seal support film can be a PEN film (polyethylene naphthalate), or the seal support film includes PEN. 
         [0028]    The seal support can have at least one opening for the passage of operating media, which can circumferentially surrounded by the seal. Openings for the passage of operating media are used to supply the membrane electrode assembly with operating media. As a result, the fuel cell stack may be supplied with the operating media in a compact and space-saving manner. The operating media include reactants, i.e., fuel (e.g., hydrogen) and oxidizing agents (e.g., oxygen or air) as well as cooling media, in particular cooling fluid. Reaction products (e.g., water) may furthermore be discharged through openings. 
         [0029]    The membrane electrode arrangement may comprise gas diffusion layers (GDL), which are disposed within the chemically active region surrounded by the seal. The electrodes may be connected by the gas diffusion layers to so-called gas diffusion electrodes. 
         [0030]    The connecting layer can be disposed on a flat side of the membrane opposite the seal support. As a result, the membrane (for example the entire membrane electrode assembly) is situated in a protected manner between the connecting layer and the film support. 
         [0031]    The integral connection can be an adhesive bond. Adhesive bonds are easy and economical to manufacture. The adhesive bond may be both a pressure-sensitive adhesive bond (e.g., with the aid of an adhesive cement) as well as thermally activatable adhesive bond and/or an adhesive blond which includes a hot glue. 
         [0032]    According to an embodiment of the invention, the connecting layer can be coated with an adhesive, in particular the connecting layer itself can be a self-adhesive film. Due to this embodiment, a connection of the membrane electrode assembly, and thus a connection of the membrane, to the seal support, may be particularly easily established. The connecting layer, in particular the self-adhesive film, is thus easily integrally connected to the membrane electrode assembly and the seal support, which overlap each other. 
         [0033]    An inner edge of the connecting layer can ends with an offset against an inner edge of the seal support. In particular, the inner edge of the connecting layer projects beyond the inner edge of the seal support (in the direction of the chemically active region, i.e., in the direction of the middle of the membrane). Due to this embodiment, a uniform thickness of the membrane electrode arrangement is achieved, and a shearing action upon the membrane between the seal support and the connecting layer is prevented or at least reduced. 
         [0034]    The seal support can have a perforation, along which the seal extends on both sides of the seal support. A first subsection of the seal is disposed on a first flat side and a second subsection is disposed on a second flat side of the seal support. The two subsections are connected to each other as a single piece through the perforation. The seal is thereby connected to the seal support in a form-locked manner. A large number of form-locked connecting points can be provided by the perforation and the seal passing through the perforation. Orthographic projections of the two subsections onto the seal support preferably include a congruent region, the seal support having the perforation within the congruent region. The perforation includes recesses which pass through the seal support and have an arbitrary shape, e.g., a circular shape, an arbitrary arrangement, i.e., spaced a regular or irregular distance apart, and are of an arbitrary number, however having at least one recess. 
         [0035]    A fuel cell is also provided. The fuel cell can include a plurality of alternately stacked bipolar plates and membrane electrode arrangements according to the invention. The seal, in particular its subsections, can seals spaces between the membrane electrode assemblies and the bipolar plates. 
         [0036]    A motor vehicle comprising the fuel cell according to the invention is furthermore provided. The fuel cell is preferable used to supply the motor vehicle with electrical current. In particular, the fuel cell can be provided to supply power to an electrical drive of the motor vehicle. 
         [0037]    A method for manufacturing a membrane electrode arrangement according to the invention is also provided. The method comprises a step for manufacturing the seal in the region of the seal support and a subsequent step for integrally connecting the inner edge section of the connecting layer to the membrane electrode assembly and integrally connecting the outer edge section of the connecting layer to the seal support. 
         [0038]    The inner edge section of the connecting layer can be integrally connected to the membrane and/or the electrodes of the membrane electrode assembly. 
         [0039]    A joining of the seal support and the membrane, i.e., a joining of the seal support and the entire membrane electrode assembly, can be performed prior to the integral connection. 
         [0040]    The manufacture of the seal can be implemented by overmolding the seal onto the seal support. This takes place by overmolding a (source) material of the seal. A step for cross-linking the (source) material can takes place. Due to this embodiment, a tight connection between the seal and the seal support can be established. 
         [0041]    In contrast to insert-molding a membrane electrode assembly (MEA), the seal can be molded only onto the seal support without the membrane electrode assembly, according to an embodiment of the invention, or the seal can be insert-molded with the seal support. A maximum implementable processing temperature thus depends on the seal support, in particular on a film used for this purpose, and not on the membrane electrode assembly. The cost of rejects is reduced to the cost of the seal support and the seal. In the next step, the relatively sensitive membrane, or the entire membrane electrode assembly, is positioned in an opening region (window) of the seal support, which later essentially represents the chemically active region (i.e., a chemically active surface). The adhesive bonding of the membrane electrode assembly usually takes place by applying the connecting layer (for example a frame), which is coated with an adhesive (adhesive cement). The gas diffusion layers may be either laminated (i.e., connected to the membrane using an adhesive-free hot pressing method) or integrally connected according to known methods. 
         [0042]    Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0043]    The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus, are not limitive of the present invention, and wherein: 
           [0044]      FIG. 1  shows a membrane electrode arrangement according to the invention according to one preferred embodiment of the invention; 
           [0045]      FIG. 2  shows an exploded view of the membrane electrode arrangement; 
           [0046]      FIG. 3  shows the seal support; 
           [0047]      FIG. 4  shows the seal support with the seal; 
           [0048]      FIG. 5  shows the seal support with the seal and the membrane; 
           [0049]      FIG. 6  shows the seal support with the seal, the membrane and the connecting layer; 
           [0050]      FIG. 7  shows the membrane electrode arrangement with the diffusion layers; 
           [0051]      FIG. 8  shows a fuel cell comprising the membrane electrode arrangement; and 
           [0052]      FIG. 9  shows a motor vehicle comprising the fuel cell. 
       
    
    
     DETAILED DESCRIPTION 
       [0053]    According to an exemplary embodiment of the invention, a membrane electrode arrangement  10  is illustrated in  FIG. 1  in a top view, a sectional view (A-A) and a detailed view of the sectional view (A-A). 
         [0054]    Membrane electrode arrangement  10  comprises a membrane electrode assembly  12  (MEA), a seal support  14  and a seal  16 , which is connected to seal support  14 . Seal support  14  may have openings  18  for the passage of operating media. 
         [0055]    Membrane electrode assembly  12  (MEA) includes a membrane  20  and electrodes  22  disposed on both sides of membrane  20 . (The electrodes themselves are not illustrated, only their positions are marked.) Membrane electrode arrangement  10  provides a chemically active region  26 , to which reactants are applied during operation and in which the desired reactions take place. Both flat sides of membrane  20  are usually completely covered by electrodes  22 . However, electrodes  22  may furthermore also be limited to the chemically active region and thus only partially cover the flat sides of membrane  20 . 
         [0056]    As is apparent in sectional view A-A and the corresponding detailed view, membrane  20  is fitted into an opening region of seal support  14 . Membrane  20  and, in the example, entire membrane electrode assembly  12  circumferentially overlap seal support  14 , and it is connected to seal support  14  by a connecting layer  24 . This is accomplished in that connecting layer  24  is integrally connected to membrane electrode assembly  12 , i.e., to its membrane  20  and/or electrodes  22 , and is also integrally connected to seal support  14 . For this purpose connecting layer  24  circumferentially overlaps both membrane  20  and seal support  14 . To implement the integral connection, connecting layer  24  may be designed as a self-adhesive film. As a rule, the latter is coated with an adhesive, which (on the same flat side of connecting layer  24 ) makes contact with membrane electrode assembly  12  in an inner edge section  28  and with seal support in an outer edge section  30 . Since both seal support  14  and connecting layer  24  have a closed circumferential shape, they surround membrane  20  and, in particular, chemically active region  26 . Due to the likewise closed, circumferential integral connection between connecting layer  24  and membrane electrode assembly  12 , as well as between connecting layer  24  and seal support  14 , circumferentially closed sealing regions are created, which prevent reactants from passing from one side of membrane  20  to the other side of membrane  20  during operation. 
         [0057]    As is apparent, membrane electrode assembly  12  may be disposed between seal support  14  and connecting layer  24 , so that membrane  20  is stabilized in its edge regions between seal support  14  and connecting layer  24 . In other words, connecting layer  24  may be disposed on a flat side of membrane  20  opposite seal support  14 . 
         [0058]    In addition, an inner edge  32  of connecting layer  24  may end with an offset against an inner edge  34  of seal support  14 , whereby the mechanical load on relatively sensitive membrane electrode assembly  12  is reduced. In the illustrated case, inner edge  32  of connecting layer  24  projects beyond inner edge  34  of seal support  14 . 
         [0059]    Gas diffusion layers  36  may abut membrane electrode assembly  12  on both sides. 
         [0060]    Seal  16  may have a first subsection  38  and a second subsection  40 , which extend on both sides of seal support  14 . The two subsections  38 ,  40  may each form two sealing lips  42 . The subsections can have sealing surfaces  44  for sealing a bipolar plate. To reduce a mechanical load on seal support  14 , sealing surfaces  44  may be provided with a mirror-symmetrical design with respect to seal support  14 . 
         [0061]      FIG. 2  shows an exploded view of membrane electrode arrangement  10 , which is already known from  FIG. 1  and which was already discussed above. The following  FIGS. 3 through 7  shows the individual steps according to one preferred sequence for manufacturing membrane electrode arrangement  10 . 
         [0062]    The manufacturing method can begins with seal support  14  shown in  FIG. 3  (edge reinforcement or film support). It may have operating medium openings  18  and an opening region  46 . In addition, seal support  14  may have a perforation  48 , along which seal  16  is mounted onto seal support  14  in the next manufacturing step. Operating medium openings  18  as well as opening region  48  and recesses of perforation  48  are recesses which pass through seal support  14 . These through-recesses  18 ,  46 ,  48  and the contour of seal support  14  may usually be produced by stamping seal support  14  out of a film (for example a plastic film) of the seal support. 
         [0063]    One option for mounting seal  16  onto seal support  14  is to overmold seal  16  onto seal support  14 . This takes place within an injection molding die, by injecting a reaction mixture, comprising a polymer to be cross-linked or monomers and possibly a cross-linking agent into the injection molding die. Thanks to perforation  48 , a pressure compensation may take place within the reaction mixture of the two subsections  38 ,  40  during the overmolding process. A deformation of seal support  14 , due to possible, different pressures on both sides of seal support  14 , may be prevented thereby. After overmolding, a cross-linking and/or polymerization process usually occur(s), which take(s) place by heating the reaction mixture over a predefined period, Thanks to perforation  48 , the two subsections  38 ,  40  are connected to each other as a single piece through perforation  48 , whereby seal  16  is connected to seal support  14  in a form-locked manner. 
         [0064]    Seal  16 , which is already connected to seal support  14 , i.e., can be overmolded thereon, is apparent in  FIG. 4 . This unit may also be referred to as a film sealing frame. 
         [0065]    A joining of seal support  14  and membrane electrode assembly  12  can takes place as the next step—see  FIG. 5 . For this purpose, membrane electrode assembly  12  is positioned above seal support  14  in the illustrated example, opening region  46 , which is still visible in  FIG. 4 , being closed by membrane electrode assembly  12 . Membrane  20  overlaps seal support  14  in its edge regions. 
         [0066]      FIG. 6  shows membrane electrode arrangement  10  after another manufacturing step. Connecting layer  24  (adhesive frame), e.g., a self-adhesive film, is integrally connected to membrane electrode assembly  12  and seal support  14 , overlapping therewith, in the illustrated frame mold. 
         [0067]    Alternatively, seal support  14  may also be integrally connected directly to membrane electrode assembly  12  in the edge region overlapping membrane  20  by applying an adhesive (on seal support  14  and/or membrane electrode assembly  12 , e.g., its membrane  20 ). Implementing this variant, however, requires a relatively complex step of applying the adhesive to the particular edge region. A bonding with the aid of connecting layer  24  is therefore preferred. 
         [0068]    In the final step, gas diffusion layers  36  may still be integrally connected or laminated onto both sides of membrane electrode assembly  12 .  FIG. 7  shows membrane electrode arrangement  10  after this step. 
         [0069]    A fastening of membrane  20  thus takes place on seal support  14  (i.e., on a single layer of edge reinforcement), which is already provided with seal  16 , by applying connecting layer  24  (for example an adhesive frame). 
         [0070]    Due to the invention, a connection of seal  16  to seal support  14 , i.e., an overmolding of seal  16  onto seal support  14  (or even an insert-molding of seal support  14  with seal  16 ) takes place before seal support  14  is connected to membrane  20 . At the same time, this connection of seal support  14  to membrane  20  takes place in a conceivably easy manner with the aid of connecting layer  24 . Due to the subsequent mounting of membrane electrode assembly  12 , the reject costs are reduced and the process times for overmolding seal  16  are shortened. 
         [0071]    Due to the one-sided bonding, a stable membrane electrode arrangement  10  is achieved, which is more cost-effective and more reliable to manufacture. 
         [0072]      FIG. 8  shows a schematic representation of a fuel cell  50  comprising multiple membrane electrode arrangements  10  according to the invention. Membrane electrode arrangements  10  are stacked, alternating with bipolar plates  52 , to form a fuel cell stack  54  comprising multiple individual cells  56 . 
         [0073]    Bipolar plates  52  supply membrane electrode assemblies  12  of membrane electrode arrangements  10  with reactants via gas fusion layers  36 , for which purpose suitable channels are usually provided in bipolar plates  52 . In addition, bipolar plates  52  electrically conductively connect two adjacent membrane electrode assemblies  12 , whereby they are connected in series. The two end bipolar plates are also referred to as monopolar plates, since they supply adjacent membrane electrode assembly  12  only on one side and, for this purpose, have corresponding channels only on one of their sides. 
         [0074]    Seals  16  of membrane electrode arrangements ( 10 ) seal the spaces between membrane electrode assemblies  12  and bipolar plates  52  to the outside and thus prevent the operating media from exiting fuel cell stack  54  during the operation of fuel cell  50 . 
         [0075]    To ensure the proper functioning of seals  16  as well as an electrically conductive contact of bipolar plates  52  to membrane electrode assemblies  12 , even during vibrations (e.g., due to a use in a motor vehicle), fuel cell stack  54  can be pressed. This is usually done with the aid of two end plates  58 , which are disposed on both ends of fuel cell stack  54 , in combination with multiple tension elements  60 . Tension elements  60  conduct tensile forces into end plates  58 , so that end plates  58  press fuel cell stack  54  together. 
         [0076]      FIG. 9  shows a motor vehicle  62  comprising fuel cell  50 . Fuel cell  50  provides electrical energy during operation of motor vehicle  62 , for example, for an electrical drive system of motor vehicle  62 . 
         [0077]    The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are to be included within the scope of the following claims.