Abstract:
A fuel cell ( 10 ) including at least one membrane assembly ( 13 ) arranged between at least two polar plates ( 14 ) of the fuel cell ( 10 ) is provided. The fuel cell ( 10 ) has at least one pair ( 25 ) of flow regions situated outside an active region ( 16 ) of the membrane assembly ( 13 ), the pair having overlapping flow regions ( 20 ) on both sides of the membrane assembly ( 13 ). A support ( 18 ) for supporting the membrane assembly ( 13 ) is arranged in one of the overlapping flow regions ( 20 ) on one side of the membrane assembly ( 13 ), between the membrane assembly ( 13 ) and one of the polar plates ( 14 ).

Description:
[0001]    The present invention relates to a fuel cell including at least one membrane system which is situated between at least two polar plates of the fuel cell. Outside an active area of the membrane system, the fuel cell includes at least one flow area pair which includes overlapping flow areas situated on either side of the membrane system. 
       BACKGROUND 
       [0002]    Fuel cells utilize the chemical reaction of a fuel with oxygen yielding water in order to generate electrical energy. For this purpose, fuel cells include the so-called membrane electrode assembly (MEA) as a core component, which is a combination of a proton-conducting membrane and a respective electrode situated on either side of the membrane (anode and cathode). In addition, gas diffusion layers (GDL) may be situated on either side of the membrane electrode assembly on the sides of the electrodes facing away from the membrane. In general, the fuel cell is formed by a plurality of MEAs arranged in a stack, whose electric power outputs are added up. During 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 from H 2  to H +  takes place, giving off electrons. A (waterbound or anhydrous) transport of the protons H +  from the anode space to the cathode space takes place via the electrolyte or the membrane which separates the reaction chambers from each other in a gas-tight manner and electrically isolates them. The electrons provided at the anode are fed to the cathode via an electrical line. The cathode is supplied with oxygen, or an oxygen-containing gas mixture, so that a reduction from O 2  to O 2−  takes place, taking up the electrons. At the same time, these oxygen anions react in the cathode space with the protons transported via the membrane, forming water. Due to the direct conversion of chemical into electrical energy, fuel cells achieve an improved efficiency compared to other electricity generators by circumventing the Carnot factor. 
         [0003]    The most advanced fuel cell technology at present is based on polymer electrolyte membranes (PEM), in which the membrane itself is made up of a polymer electrolyte. Acid-modified polymers, in particular perfluorinated polymers, are frequently used for this purpose. The most widely 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 acid fluoride derivative of a perfluoroalkyl vinyl ether). The electrolytic conduction takes place via hydrated protons, which is why the presence of water is a requirement for the proton conductivity, and it is necessary to moisten the operating gases during operation of the PEM fuel cell. Due to the need for water, the maximum operating temperature of these fuel cells is limited to below 100° C. at normal pressure. Distinguishing it from high-temperature polymer electrolytic membrane fuel cells (HT PEM fuel cells), whose electrolyte conductivity is based on an electrolyte bound to a polymer backbone of the polymer electrolyte membrane with the aid of an electrostatic coordinative bond (for example, phosphoric acid-doped polybenzimidazole (PBI) membrane) and which are operated at temperatures of 160° C., this fuel cell type is also referred to as a low-temperature polymer electrolyte membrane fuel cell (NT PEM fuel cell). 
         [0004]    DE 10 2005 057 045 A1, for example, describes a fuel cell unit including bipolar plates which include a distribution area and a collection area between fluid port areas and a flow field. A membrane electrode assembly is clamped between raised support points of the distribution and collection areas. Outside the flow field (outside an active area of the membrane electrode assembly), the distribution and collection areas enable a preferably even supply and discharge of the reactants. 
         [0005]    Other known distribution and collection areas include structures having, e.g., trough-shaped recesses. A barrier layer (often also referred to as an edge reinforcement) enclosing the membrane of the membrane electrode assembly may have deformations in the area of a distribution or collection structure after operation of the fuel cell. This results in a reduction of flow-through cross sections within the distribution or collection structure, and thus in an uneven distribution of the reactants in the flow field. 
       SUMMARY OF THE INVENTION 
       [0006]    It is an object of the present invention to provide a fuel cell system which is characterized by increased durability. 
         [0007]    According to the present invention, a fuel cell including at least one membrane system, which is situated between at least two polar plates of the fuel cell, is provided. Outside an active area of the membrane system, the fuel cell includes at least one flow area pair which includes overlapping flow areas situated on either side of the membrane system. As a characterizing feature, it is provided that a support, which is designed to support the membrane system, is situated in one of the overlapping flow areas, on one side of the membrane system, between the membrane system and one of the polar plates. 
         [0008]    The support reduces or prevents a deformation of the membrane system. By the support being situated only on one side of the membrane system, a possible flow impairment by the support occurs (if at all) only in one of the two flow areas of the flow area pair. 
         [0009]    As a result of the spatial expansion of the support, it is thus possible for a free cross section of the flow area in which the support is situated to be reduced, at a constant thickness of an individual cell of the fuel cell. This reduction of the free cross section is preferably divided between both opposing flow areas of the flow area pair. This may be achieved by an appropriate dimensioning of the polar plate. The polar plate preferably includes channels whose base drops (forms a step) at a transition from the active area into the flow area including the support. In the opposing flow area in which thus no support is present, the base of the channels rises at the transition from the active area to the flow area. This embodiment divides a reduction of a flow-through cross section due to the support (which may protrude into the channels) among both opposing flow areas, whereby more even flow conditions are achieved in the two flow areas. 
         [0010]    The polar plates are often also referred to as separator plates and may in particular be designed as bipolar plates, which are provided on either side to supply membrane system with reactants. As an alternative, the polar plates may also be designed as monopolar plates, which are provided on one side to supply a membrane system with a reactant. Bipolar plates usually include a center plane which, if the bipolar plate (as is known) is formed of two sheet metals, is a separating plane between these two sheet metals. If a reduction of a flow-through cross section due to the support is divided between both opposing flow areas (see above), this separating plane is not a continuous plane, but a surface area which has an offset at the transition from the active area toward the overlapping flow areas. The offset runs in the direction of that flow area which does not include a support. 
         [0011]    The support is in particular designed in such a way that the membrane system rests on the polar plate via the support. The support in particular has a planar shape, whereby a contact pressure of the polar plate on the membrane system is distributed across a larger surface area than would be the case without the support. 
         [0012]    If the fuel cell according to the present invention includes more than one membrane system, the fuel cell may also be referred to as a fuel cell stack. Typically, individual cells of the fuel cell stack, which each include one of the membrane systems, are electrically connected in series. 
         [0013]    During operation of the fuel cell, the flow areas are in particular supplied with reactants in such a way that a lower pressure prevails in the flow area in which the support is situated than in the opposing flow area of the flow area pair. In this way, the membrane system is pressed against the support. When the support is situated on the side having the higher pressure, it is advantageous to mechanically connect, e.g., to bond, the support to the membrane system in order to prevent the membrane system from detaching from the support. 
         [0014]    The two flow areas of the flow area pair overlap in an in particular rectangular top view onto the membrane system, the two flow areas being imagined projected onto a center plane of the membrane system. 
         [0015]    The (chemically) active area of the membrane system is that area in which the fuel cell reaction takes place during operation of the fuel cell. For this purpose, the membrane system may preferably include electrodes on either side in its active area, which may include a catalytic coating. In this case, a compact unit composed of the membrane and the electrodes is formed, which often is also referred to as a membrane electrode system or a membrane electrode assembly. 
         [0016]    Within the context of the present invention, on either side of the membrane system means on both (opposing) flat sides (main sides) of the membrane system. On one side of the membrane system thus means only on one of the two flat sides of the membrane system. In the particular flow area pair, the support is thus situated only on one of the two flat sides, while no support is present on the other flat side in the particular flow area pair. As a result of the support being present only on one side, the support is only present where it is necessary, whereby an unnecessary increase in the thickness of the fuel cell or of a flow loss in the flow areas is prevented. Such an unnecessary increase in the thickness of the fuel cell or of a flow loss would then exist, e.g., if the support were provided in both flow areas of the particular flow area pair. 
         [0017]    Preferably, it is provided that at least one of the overlapping flow areas of the flow area pair is a collection area for collecting a reactant and/or a reaction product discharged from the active area, and the support is situated in the collection area. These embodiments prevent an uncontrolled reduction of a free flow-through cross section in the collection area due to a deformation of the membrane system. The constant cross sections, which do not shrink during operation, reduce, and in particular prevent, an uneven distribution of reactants from increasing over the operating time. As an alternative, the at least one of the overlapping flow areas of the flow area pair may also be a distribution area for distributing a reactant to the active area, the support then being situated in the distribution area instead of in the collection area. 
         [0018]    According to the above embodiment, it is preferably provided that the flow area of the flow area pair overlapping with the collection area is a distribution area for distributing a reactant to the active area. The distribution area on the one side of the membrane system and the overlapping collection area on the other side of the membrane system are in general accompanied by a significant pressure difference. The support in the collection area counteracts a deformation of the membrane system as a result of this pressure difference. 
         [0019]    According to one preferred embodiment of the present invention, it is provided that the fuel cell includes at least two flow area pairs, the distribution area of a first flow area pair and the collection area of a second flow area pair being situated on the same side of the membrane system. From this results that the distribution area of the second flow area pair and the collection area of the first flow area pair share the other side of the membrane system. A flow through the fuel cell thus occurs according to the counter-flow principle, which means that the two reactants flowing on opposing sides of the membrane system flow in opposite directions. The present invention is advantageous especially in the case of such fuel cells since a collection area, having typically a relatively low pressure, overlaps in each case with a distribution area, having typically a relatively high pressure. An alternative definition would be that each of the two sides of the membrane system includes a distribution area, opposite which a respective collection area is situated on the other side. On both sides, the supports are situated only in the collection area. 
         [0020]    When a possible cross section reduction due to the support is divided among both opposing flow areas, the distribution area of the anode side and the collection area of the anode side and/or the distribution area of the cathode side and the collection area of the cathode side within the fuel cell may have identically sized flow-through cross sections. Furthermore, the two opposing flow areas may also have identically sized flow-through cross sections. 
         [0021]    Preferably, it is provided that at least one of the overlapping flow areas of the flow area pair has at least one passage of the polar plate which is fluidically connected to the active area, and the support is situated on the side of the membrane system which faces away from the at least one passage. Passages may in particular be supply openings, through which reactants flow out of the bipolar plate at a relatively high speed. The support on the side of the membrane system which is situated opposite the passage prevents a deformation of the membrane system by the outflowing reactant. 
         [0022]    Preferably, it is provided that the support covers at least two elevations of the polar plate. The membrane system is thus not only pressed at the at least two elevations of the polar plate, but the load of the pressing process is distributed across a larger surface area with the aid of the support. Between the elevations, the polar plate includes recesses which serve as channels for the reactants during operation of the fuel cell. 
         [0023]    The support preferably includes a material of a gas diffusion layer of the fuel cell. The support may furthermore also be formed of the material of the gas diffusion layer, i.e., be made up of the same. The support being made from a material which is pervious to reactants reduces or prevents a possible increase in a flow resistance by the reactants. Further preferably, the support may alternatively also be designed as a metal sheet, which in particular has a thickness of no more than 0.1 mm, preferably of no more than 0.05 mm. 
         [0024]    Preferably, it is provided that the support is designed in one piece with one of the gas diffusion layers of the fuel cell. In addition to the above-mentioned advantages of the material of the gas diffusion layer as the material for the support, in this way also a particularly cost-effective fuel cell is achieved since the gas diffusion layer only has to be designed in an extended manner in the particular flow area to implement the support. 
         [0025]    Preferably, it is provided that the membrane system includes at least one barrier layer, at least in the area of the overlapping flow areas, which in particular peripherally encloses the active area (of the membrane system). Such a barrier layer is often also referred to as an edge reinforcement, in particular as an edge reinforcement sheet, since it may overlap an edge of a membrane of the membrane system, and encloses a portion of the membrane situated in the active area. 
         [0026]    Furthermore, a motor vehicle including a fuel cell according to the present invention is provided. The fuel cell is typically configured to supply an electric drive of the motor vehicle with energy. The motor vehicle is characterized by increased durability and a constant performance capability. 
         [0027]    As an alternative, the fuel cell according to the present invention may also be part of a stationary device, for example a stationary power generation device, which is also characterized by increased durability and a constant performance capability. 
         [0028]    The various specific embodiments of the present invention described in the present application may advantageously be combined with each other, unless they are designed differently in the individual case. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0029]    The present invention is described hereafter in exemplary embodiments based on the accompanying drawings. 
           [0030]      FIG. 1  shows a fuel cell according to one preferred embodiment of the present invention; and 
           [0031]      FIG. 2  shows a bipolar plate according to one preferred embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0032]      FIG. 1  shows a fuel cell  10  according to one preferred embodiment of the present invention. Only a subarea of fuel cell  10  and of a motor vehicle  100  including fuel cell  10  is shown. The upper portion of  FIG. 1  shows a top view onto an anode side  11  of fuel cell  10 , followed by a top view onto a cathode side  12  of fuel cell  10 . Subsequently, a section A-A through the subarea of fuel cell  10  is shown. From section A-A, details C, B, D and E (beneath each other, not true to scale) are shown in an enlarged manner. 
         [0033]    Fuel cell  10  includes at least one membrane system  13 , which is situated between at least two polar plates  14  of fuel cell  10 . In the illustrated example, polar plates  14  are bipolar plates, the fuel cell including multiple polar plates  14  and multiple membrane systems  13 . Membrane systems  13  are each situated between two polar plates  14 , this being the case only for the middle membrane system  13  in  FIG. 1 . Outside an active area  16  of membrane system  13 , fuel cell  10  includes at least one flow area pair  25 , which includes overlapping flow areas  20  situated on either side of membrane system  13 . A support  18  is situated in one of overlapping flow areas  20  on one side of membrane system  13  between membrane system  13  and one of polar plates  14 . Support  18  is designed to support membrane system  13 . Support  18  supports membrane system  13  on an adjoining polar plate  14 . 
         [0034]    One of overlapping flow areas  20  of a first flow area pair  26  of each membrane system  13  is a distribution area  22  for distributing a reactant (in this case a cathode gas  52 ) to active area  16 . The second of overlapping flow areas  20  is a collection area  24  for collecting a reactant discharged from active area  16  (in this case an anode gas  50 ). Support  18  is situated in collection area  24 . 
         [0035]    In a second flow area pair  28  of each membrane system  13  as well, one of overlapping flow areas  20  is a distribution area  22  for distributing a reactant (in this case an anode gas  50 ) to active area  16 , while the second of overlapping flow areas  20  is a collection area  24  for collecting a reactant (in this case a cathode gas  52 ) or a reaction product (e.g., water) discharged from active area  16 . Support  18  is also situated in collection area  24  in second flow area pair  28 , and compared to first flow area pair  26  is thus situated on the opposite side of membrane system  13 . 
         [0036]    Passages through the surface of polar plates  14  may be situated on the opposite side of membrane systems  13  in the area of supports  18 . Passages  30  are fluidically connected to active area  16 . 
         [0037]    In addition to membrane  32 , which may essentially be limited to active area  16 , membrane system  13  includes a barrier layer  34 , which peripherally encloses membrane  32 . Since barrier layer  34  generally overlaps an edge of membrane  32  and stabilizes membrane  32 , barrier layer  34  is often also referred to as an edge reinforcement or an edge reinforcement sheet. Barrier layer  34  may also overlap membrane  32  on its entire surface area. Typically, barrier layer  34  is a plastic sheet. 
         [0038]    Membrane system  13  furthermore includes operating media openings  35  outside active area  16  and outside flow areas  20 , which conduct operating media (anode gas, cathode gas and coolant) through fuel cell  10  during operation of fuel cell  10 . 
         [0039]    Gas diffusion layers  36  are situated between membrane systems  13  and polar plates  14 . These extend across active area  16 , but according to one preferred embodiment also form supports  18 . In this way, supports  18  may be implemented particularly easily by simply extending gas diffusion layers  36  into those flow areas  20 , which in the example are collection areas  24 , in which supports  18  are to be situated. 
         [0040]      FIG. 2  shows cathode side  12  of one of polar plates  14 . Cathode side  12  of polar plate  14  within fuel cell  10  points toward cathode side  12  of membrane system  13 . As a result of the planar design of support  18 , support  18  covers elevations  38  of polar plate  14 . The position and progression of elevations  38  are schematically illustrated in  FIG. 2  between also schematically illustrated superficial channels  40  of polar plate  14 . Elevations  38  and channels  40  in collection area  24  form a collection structure, while elevations  38  and channels  40  in distribution area  22  form a distribution structure. Channels  40  each have a (channel) base, which at a transition from active area  16  drops into flow area  20  including support  18 . The base of channels  40  thus has a depression at the transition from active area  16  to collection area  24 . 
         [0041]    Gas diffusion layer  36  covers active area  16  and collection area  24 , whereby support  18  is formed by gas diffusion layer  36 . Gas diffusion layer  36  partially protrudes into channels  40 . A cross section reduction of channels  40  by gas diffusion layer  36  is thus divided among both sides of polar plate  14  by the depression of the base. 
         [0042]    Furthermore, passages  30  are apparent on the side of distribution area  22 , in whose area on the opposite side of adjoining membrane system  13  a support  18  may also be situated. For example, support  18  may extend into this area. 
         [0043]    Anode sides  11  and cathode sides  12  shown in the two  FIGS. 1 and 2  may be reversed according to one alternative embodiment. Shown anode side  11  may thus be the cathode side. Cathode  12  shown in the figures is then consequently the anode side. In  FIG. 2 , the view would thus be directed at the anode side, so that anode gas  50  flows through passages  30 . 
         [0044]    The hatching in  FIG. 2  schematically indicates the extension of gas diffusion layer  36 . In practice, supports  18  may be assigned either to membrane system  13  or polar plates  14 , whereby easy to handle modules result when fuel cell  10  is assembled. 
         [0045]    Such a membrane system module  42  thus includes also at least one support  18 , in the exemplary embodiment thus two supports  18  and thus also two gas diffusion layers  36 , in addition to the membrane system. 
         [0046]    As an alternative, a module may also be formed with the aid of polar plates  14 , for example one bipolar plate module  44  including at least one support  18 , in addition to a bipolar plate. In the exemplary embodiment, bipolar plate module  44  thus includes two supports  18  and thus both gas diffusion layers  36 . During operation of fuel cell  10 , an anode gas  50  flows into distribution areas  22  through inlet-side passages  30  on anode sides  11  of membrane systems  13  and polar plates  14 . In each membrane system  13 , anode gas  50  consecutively flows through distribution areas  22 , active areas  16 , and collection areas  24 . Thereafter, the remaining, non-reacted anode gas  50  is discharged from fuel cell  10  through outlet-side passages  30  and typically recirculated. On its path through fuel cell  10 , anode gas  50  flows from an anode inlet side  54  to an anode outlet side  56 . 
         [0047]    On cathode sides  12  of membrane systems  13  and polar plates  14 , a cathode gas  52  flows through inlet-side passages  30  into distribution areas  22 . In each membrane system  13 , cathode gas  52  consecutively flows through distribution areas  22 , active areas  16 , and collection areas  24 . Thereafter, the remaining, non-reacted cathode gas  52  and reaction products formed in active area  16  are discharged from fuel cell  10  through outlet-side passages  30 . Cathode gas  50  flows from a cathode inlet side  58  to a cathode outlet side  60 . 
         [0048]    As is schematically apparent from  FIGS. 1 and 2 , anode gas  52  (e.g., hydrogen) and cathode gas  54  (e.g., air) flow through anode sides  11  and cathode sides  12  in a counter flow. On the path through fuel cell  10 , gases  52 ,  54  experience a pressure loss. This means that higher pressures are present at the particular inlets, i.e., in the particular distribution areas  22 , of the reactants than at the particular outlets, i.e., in collection areas  24 , of the reactants. 
         [0049]    These pressure differences between the particular anode inlet and the particular cathode outlet, and between the particular cathode inlet and the particular anode outlet, combined with increased temperatures, may result in creeping of barrier layer  34  in fuel cells according to the prior art. This would result in a permanent deformation in the direction of the lower pressure. 
         [0050]    Due to the one-sided supports of membrane systems  13  according to the present invention, i.e., in particular barrier layers  34  (the edge reinforcements), the forces resulting from the pressure difference are supported via supports  18 , in the example via the extended gas diffusion layers  36  and the collection structures of polar plates  14 . Since supports  18  are designed as extensions of gas diffusion layers  36 , the production of membrane systems  13 , to the extent these are produced as membrane system modules  42 , are influenced only insignificantly. In this way, e.g., no further component is required. The height of the collection structures is reduced in this case only by one layer of a gas diffusion layer  36 . The reduction of the height by gas diffusion layers  36  may furthermore be divided among channel depths of the anode and cathode collection structures and anode and cathode distribution structures. 
       LIST OF REFERENCE NUMERALS 
       [0000]    
       
           10  fuel cell 
           11  anode side 
           12  cathode side 
           13  membrane system 
           14  polar plate 
           16  active area 
           18  support 
           20  flow area 
           22  distribution area 
           24  collection area 
           25  flow area pair 
           26  first flow area pair 
           28  second flow area pair 
           30  passages 
           32  membrane 
           34  barrier layer 
           35  operating media opening 
           36  gas diffusion layer 
           38  elevation 
           40  channel 
           42  membrane system module 
           44  bipolar plate module 
           50  anode gas 
           52  cathode gas 
           54  anode inlet side 
           56  anode outlet side 
           58  cathode inlet side 
           60  cathode outlet side 
           100  motor vehicle