Patent Publication Number: US-10790520-B2

Title: Fuel cell

Description:
The invention relates to a fuel cell having a stack of at least one bipolar plate and a membrane electrode assembly stabilized by a frame. Said bipolar plate comprises a flow field formed by a profiled section of the bipolar plate. 
     Fuel cells use the chemical conversion of a fuel with oxygen into water in order to generate electrical energy. For this purpose, fuel cells contain as a core component what are known as membrane electrode assemblies (MEA), which are a combination of an ion-conductive, in particular proton-conductive, membrane and an electrode (anode and cathode) respectively arranged on both sides of the membrane. Additionally, gas diffusion layers (GDL) may be arranged on both sides of the membrane electrode assembly on the sides of the electrodes facing away from the membrane. Generally, the fuel cell is constituted by a plurality of MEAs arranged in the stack, the electrical power outputs of which MEAs accumulate. During operation of the fuel cell, the fuel, in particular hydrogen H 2  or a gas mixture containing hydrogen, is guided to the anode where an electrochemical oxidation of H 2  to H +  with loss of electrons takes place. Via the electrolyte or the membrane that separates the reaction chambers gas-tight from one another and electrically insulates them, the protons H +  are transported from the anode chamber into the cathode chamber (in a water-bound or water-free manner). The electrons provided at the anode are guided to the cathode via an electrical line. The cathode receives oxygen or a gas mixture containing oxygen, so that a reduction of O 2  to O 2−  with gain of electrons takes place. At the same time, these oxygen anions react in the cathode chamber with the protons transported via the membrane to form water. As a result of the direct conversion of chemical into electrical energy, fuel cells have improved efficiency compared to other electricity generators because the Carnot factor is avoided. 
     The fuel cell is formed by a plurality of individual cells arranged in the stack, which is why this is also called a fuel cell stack. Arranged between the membrane electrode assemblies are bipolar plates which ensure that the individual cells are supplied the operating media, i.e. the reactants and a coolant. In addition, the bipolar plates ensure an electrically conductive contact to the membrane electrode assemblies. 
     Bipolar plates are mostly formed by a pair of profiled plates which respectively comprise a coolant side and a cell side, and the two plates are arranged and connected such with respect to one another that channels for transporting coolant form between the coolant sides facing one another. In their active region, the plates have a group of grooves or channels which form open flow or current fields on their cell sides for distributing the reactants over the surfaces of the respective anodes and cathodes. Between the plates within the bipolar plate, coolant channels are formed and distribute coolant over the fuel cell stack in order to cool it. 
     In order to isolate the anode or cathode chamber physically from the environment of the fuel cell, the cell stacks comprise sealing elements in an edge region of the bipolar plates. These sealing elements serve to seal and electrically insulate, and are typically arranged on the bipolar plate in the shape of a frame extending around the active and inactive region. 
     In known fuel cell stacks, the sealing element and the flow field are spaced apart from one another in order to absorb manufacturer&#39;s tolerances of the membrane electrode assembly, to make a welded seam, and to avoid the diffusion layer and the membrane overlapping the sealing element. An overlapping of the diffusion layer and the sealing element may result in an undesired leakage of gaseous operating media. The sealing element and the flow field spaced apart from it therefore form a gap, which results in the formation of a cavity in the stack. This cavity causes a quantity of reactant gases to bypass the active region of the bipolar plates, which is also called reactant bypass flow. The reactant bypass flow is uneconomical since the reactant gas is not led to the active region of the fuel cell stack and is thus not available for the electrochemical reaction. Moreover, the reactant bypass flow results in a pressure loss in the active region, which must be counteracted by an increased supply of the respective reactant gas. As a result of the lack of reactant, the reactant bypass flow may furthermore have an undesired effect on the durability, reliability, and efficiency of the fuel cell stack. 
     In order to solve this problem, DE 10 2009 017 906 A1 proposes to arrange a barrier layer on the sealing element, which barrier layer extends inward toward the active region of the bipolar plate and thus reduces the reactant bypass flow. 
     This solution is disadvantageous in that a gap must be provided in order to ensure that the membrane electrode assembly is not destroyed by the inflexible bipolar plate if the cell is compressed. 
     The invention is now based on the task of providing an alternative solution to the problems of the prior art, and in particular of further reducing or preventing the reactant bypass flow. 
     This task is achieved by a fuel cell having the features of the independent claim. The invention thus relates to a fuel cell having a stack having a bipolar plate comprising a flow field formed by a profiled section of the bipolar plate, and an elongated sealing element, which at least partially surrounds the flow field. It is provided according to the invention that a filling agent is arranged inside a cavity formed between the sealing element and the flow field, which filling agent extends in the extension direction of the cavity. 
     The advantage of the fuel cell according to the invention exists in that the free volume inside the cavity is reduced. The volume of the reactant bypass flow is thus also reduced. This leads to the reactant consumption being significantly reduced, and a loss of reactant gas as a result of reactant bypass flows not needing to be counteracted by an increase in pressure or an increased mass transport of reactant gas. Moreover, compared to traditional methods for reducing the reactant bypass flows, the membrane electrode assembly remains undamaged to the largest extent possible. The fuel cell according to the invention makes it possible to reduce the reactant bypass flow independently of any manufacturing tolerances of the membrane electrode assembly that have to be maintained. 
     In this document, “cavity” refers to a gap or hollow space that forms between the bipolar plate and a layer adjoining the bipolar plate in the stack, i.e. the membrane electrode assembly and/or a gas diffusion layer of the same, in a region between the sealing element and the flow field of the bipolar plate. The cavity generally results from the profile structure of the bipolar plate and is defined on the one hand by the height of the profile structure in the region of the flow field, and on the other hand by the height of the sealing element. The cavity extends in parallel along the flow field. On each side of a bipolar plate, i.e. on the anode and the cathode side, two cavities thus respectively result which extend between the sealing element and the flow field. Advantageously, both cavities of in particular both bipolar plate sides are provided with the filling agent. The sealing element is a sealing unit which extends along the outer edge of the bipolar plate and, for example, is designed as a bead gasket and/or polymer seal. 
     A preferred embodiment of the invention provides that the filling agent extends along a length of the flow field. The advantage of this embodiment consists in the reactant bypass flow being displaced extensively, and thus effectively, over the length of the flow field in this arrangement. In this case, the filling agent preferably takes up more than half, in particular more than two thirds, preferably more than three quarters of the volume of the cavity. The filling agent may in this case have a round, ellipsoid, rectangular, square, or polygon cross section. 
     It is in particular preferred that a height h F  of the filling agent in non-compressed form, i.e. prior to assembly of the stack, is greater than the height h P  of the profiled section of the flow field. This embodiment advantageously results in a contact being brought about between the filling agent and the layer adjacent to the bipolar plate, i.e. the membrane electrode assembly and/or the gas diffusion layer, this region thus being sealed in a gas-tight manner, and the reactant bypass flow being prevented. In addition, a compression of the gas diffusion layer and the membrane electrode assembly results, and thus a stabilization of the same in case of gas-tight contact. 
     It is furthermore preferred if the filling agent is a polymer, in particular an elastomer or a thermoplastic elastomer (TPE). These materials, in particular elastomers or TPEs, are characterized in that they can be easily and reversibly deformed by external pressure. They can thus adapt to the outer shape of the cavity particularly well, are easy to handle in doing so and are gas tight with respect to reactants. Particularly advantageously, such elastomer materials are used that already deform at a pressure that does not damage the membrane electrode assembly. It is thus achieved that a region between the membrane electrode assembly and the bipolar plate, or between the frame of the membrane electrode assembly and the bipolar plate, is also sealed off by the filling agent in a gas-tight manner, and that no clearance must be maintained in order to not damage the membrane electrode assembly. 
     In another preferred embodiment of the invention, the filling agent is connected to the bipolar plate. In this case, the filling agent is advantageously fixed on the bipolar plate. This embodiment in particular offers a production advantage. The filling agent is in this case preferably arranged on the bipolar plate before the membrane electrode assembly is arranged on the bipolar plate. The arrangement of the filling agent on the bipolar plate can thus take place independently of the tolerances of the membrane electrode assembly to be maintained, without the latter being damaged. In this embodiment, the filling agent is either glued as cured material on the bipolar plate or applied in non-cured form to the bipolar plate by means of a dispenser. 
     Another embodiment of the invention provides that the filling agent is designed in several pieces (with respect to one layer). This advantageously results in a material savings with an increase in flexibility at the same time, in particular with respect to the shape of the bipolar plate and the layers adjoining it. In addition, the arrangement according to the invention of the filling agent in the cavity may be combined with additional measures for reducing the reactant bypass flow. If the filling agent is designed in several pieces, it is preferred if at least one part of the filling agent is respectively arranged on the borders of the flow field, i.e. on the two ends of the elongated cavity. 
     It is furthermore preferred that the sealing element is a bead gasket, i.e. a seal that, like the flow field, also results from the profiling of the bipolar plate. The advantage consists in particular in the seal, or the bead, being able to be formed during the profiling of the bipolar plate. Alternatively or additionally, the sealing element may be a polymer seal which is subsequently applied to the bipolar plate. Advantageously, the manufacturing step of applying a polymer seal can be combined with the application of the filling agent. 
     Another embodiment of the invention provides that the filling agent is connected to the membrane electrode assembly. By means of the direct contact between the filling agent and the membrane electrode assembly, it is achieved that the distance between the membrane electrode assembly and the bipolar plate is filled completely by the filling agent and a reactant bypass flow in this region is thus completely prevented. Alternatively to the arrangement of the filling agent on the bipolar plate, the filling agent is glued on the subgasket (the frame) of the membrane electrode assembly for this purpose. 
     In the arrangement of the filling agent both on the bipolar plate and on the membrane electrode assembly, it is advantageous if the filling agent overlaps the gas diffusion layer of the membrane electrode assembly. This advantageously results in a supporting of the membrane electrode assembly, in particular of the gas diffusion layer, by the filling agent. 
     Another aspect of the invention relates to a fuel cell system which comprises several fuel cells according to the invention and is used, for example, for the electrical supply of an electric motor for a motor vehicle. 
     Additional preferred embodiments of the invention arise from the other features mentioned in the dependent claims. 
     The various embodiments of the invention mentioned in this application may be combined advantageously with one another unless otherwise stated in individual cases. 
    
    
     
       The invention is explained below in exemplary embodiments on the basis of the respective drawings. They show: 
         FIG. 1  a schematic representation of a fuel cell stack, 
         FIG. 2  a schematic representation of the top view of a bipolar plate, 
         FIG. 3  a schematic cross-section drawing of a fuel cell stack, 
         FIG. 4  a schematic representation of a fuel cell stack in a perspective view in a first preferred embodiment, 
         FIG. 5  a schematic cross-section drawing of a fuel cell stack according to the invention in a second preferred embodiment, and 
         FIG. 6  a schematic cross-section drawing of a fuel cell stack according to the invention in another preferred embodiment. 
         FIG. 1  shows a fuel cell stack  11  in a schematic representation. The fuel cell stack  11  comprises two end plates  12 . Between the end plates  12  is arranged a plurality of stack elements, which are stacked on top of one another and which comprise bipolar plates  20  and membrane electrode assemblies  30 . The bipolar plates  20  are stacked alternately with the membrane electrode assemblies  30 . The membrane electrode assemblies  30  respectively comprise a membrane and, on both sides of the membrane, adjacent electrodes, namely an anode and a cathode (not shown). Adjacent to the membrane, the membrane electrode assemblies  30  may also comprise gas diffusion layers  33 . Between the end plates  12 , the fuel cell stack  11  is pressed by means of tension elements  16  such as drawbars or clamping plates. 
     
    
    
     In  FIG. 1 , only the narrow sides of the bipolar plates  20  and the membrane electrode assemblies  30  are visible. The main sides of the bipolar plates  20  and the membrane electrode assemblies  30  are adjacent to one another. The representation in  FIG. 1  is partially not dimensionally accurate. Typically, the thickness of an individual cell consisting of a bipolar plate  20  and a membrane electrode assembly  30  is a few mm, wherein the membrane electrode assembly  30  is by far the thinner component. In addition, the number of individual cells usually is much greater than shown. 
       FIG. 2  shows a schematic representation of a top view of a bipolar plate  20 . The bipolar plate  20  may be divided into three subregions, namely two distributor regions  26  and one active region  27 . In doing so, the distributor regions  26  are respectively arranged such that they are adjacent to the active region  27  at ends of the bipolar plate  20 . The distributor regions  26  comprise the supply channels for coolants and the reactant gases. The active region  27  comprises a flow field  22  resulting from a profile structure of the bipolar plate  20 . Within the flow field  22 , the profile structure forms channels in which the reactant gases are guided from one distributor region  26  over the active region  27  to the other distributor region  26 . The flow field is delimited by a sealing element  21  laterally, i.e. at least on both sides, namely is adjoined by the sealing element  21  on the longitudinal sides of the flow field  22 . Between the sealing element  21  and the flow field  22 , a cavity results  25  along the longitudinal side of the flow field  22 . Like the channels, the cavity  25  also constitutes a fluidic connection of the two distributor regions  26  within the profile structure of the flow field  22 . 
     If the bipolar plate  20  shown in  FIG. 2  is arranged to form a fuel cell stack on a membrane electrode assembly, this takes place such that the catalytic electrodes are arranged to be adjacent to the active region so that the fuel cell reaction only occurs in the region of the flow field  22 . This means that only the reactant gases that are guided within the flow field  22  from one distributor region  26  to the other distributor region  26  are available to the fuel cell reaction. The portion of the reactant gas that is not guided over the flow field  22  but bypasses the flow field  22  via the cavity  25  is, as reactant bypass flow, not available to the fuel cell reaction. 
       FIG. 3  shows a cross-section drawing of a fuel cell stack  11 . Shown is a stack consisting of bipolar plates  20 , which are alternately arranged with membrane electrode assemblies  30 . The membrane electrode assemblies  30  comprise a membrane  31 , which, for example, is designed in the form of a polymer electrolyte membrane, as well as a frame  32  which stabilizes the membrane  31 . The frame  32  is, for example, designed in the form of supporting foils which are laminated on both sides of the membrane  31  and leave an active region  27  adjacent to the flow field  22  open. In the active region  27 , the membrane  31  is enclosed by two catalytic layers (not shown), which may be laminated on the membrane  31 . The membrane electrode assembly  30  furthermore comprises two gas diffusion layers  33 , which are arranged to be adjacent to the membrane  31  and which overlap the frame  32  in the embodiment shown. 
     The sealing element  21  shown in  FIG. 3  is, by way of example, designed to be a bead gasket of the bipolar plate  20 . Alternatively or additionally, the sealing element  21  may also be designed as a polymer seal, which is, for example, applied to the bipolar plate  20  or to a bead of the bipolar plate  20 .  FIG. 3  shows that the sealing element  21  substantially extends parallelly to the flow field  22 . 
     The flow field  22  results from a profile structure of the bipolar plate  20 . In particular, within the profile structure channels  35  result which transport the reactant gases, in particular oxygen and hydrogen, over the active region of the bipolar plate. In order to form coolant channels  34 , two plates  29  are joined together to form a bipolar plate  20  such that closed coolant channels  34  form between the plates  29  in the region of the reactant gas channels  35  and  36 . In order to seal the coolant channels, the plates  20 , arranged directly adjacent to one another, of a bipolar plate  20  are sealed on the edge of the flow field  22  by a welded seam  28 , which extends parallel to the flow field. 
     If such bipolar plates  20  are arranged alternately with the membrane electrode assemblies  30  to form a fuel cell stack  11 , the cavity  25  arises between the sealing element  21  and the flow field  22  as a result of the profile structure of the bipolar plate  20 .  FIG. 4  shows how a filling agent  24  is respectively arranged inside the cavities  25 . In the embodiment shown, the filling agent  24  is provided as a strip with a substantially rectangular cross section, which strip is designed as one piece over the entire length of the flow field  22 . The filling agent  24  may alternatively also be designed in several pieces, wherein parts of the filling agent  24  are arranged at least in the edge region of the flow field  22 , i.e. in the regions of the cavity that are adjacent to the distributor regions. 
     The filling agent  24  is a polymer material. Elastomers and thermoplastic elastomers are particularly suited since they are deformed by external pressure. Particularly suited materials are, for example, silicones and/or fluororubbers. According to the invention, at least one cavity  25  per fuel cell comprises such a filling agent  24 . It is, however, advantageous if the cavities  25  respectively comprise a filling agent  24  both on both sides of the flow field  22  and on both sides of a bipolar plate  20 . In this case, the filling agents  24  are designed in the same way on both cell sides, i.e. on the cathode and on the anode side, or are varied in their design, in particular in their material. 
     The filling agent  24  may be connected and arranged either on the bipolar plate  20  or on the membrane electrode assembly  30 . 
       FIG. 5  shows the arrangement of the filling agent  24  on the bipolar plate  20 . It becomes clear that, in this embodiment, the width of the cavity i.e. from the sealing element  21  to the beginning of the flow field  22  is essentially filled completely by the filling agent  24 . The resulting cavity  25  which results from the bead shape is thus minimized. It is advantageous in this embodiment either to fix the filling agent  24  as a cured elastomer, such as from a soft rubber roll, self-adhesively on the bipolar plate, or to spray a non-cured elastomer onto the bipolar plate  20  inside the cavity  25  by means of a dispenser and to cure it. 
       FIG. 5  shows that the height h F  of the filling agent  24  in the embodiment shown projects somewhat above the profile structure of the flow field  22  but does not compensate for the height of the gas diffusion layer  33 . The filling agent  24  and the gas diffusion layer  33  are thus compressed in the overlapping region with the gas diffusion layer  33  during assembly of the stack, whereby a good sealing effect is achieved. 
     A gap arising between the filling agent  24  and the frame  32  may be eliminated by the height h F  of the filling agent  24  being further increased so that the filling agent  24  is also in contact with the frame  32  of the membrane electrode assembly  30 . In addition to the prevention of the reactant flow, this also has the advantage of the gas diffusion layer being clamped by the filling agent  24  and thus stabilized. 
       FIG. 6  shows an alternative embodiment of the invention in which the filling body  24  is arranged on the frame  32  of the membrane electrode assembly and overlaps the gas diffusion layer  33 . This may, for example, occur by gluing a suitable polymer or by spraying or applying the polymer by means of a dispenser. In the drawing shown in  FIG. 6 , the height h F  of the filling agent projects above the height h P  of the profile structure and fills the height of the cavity  25 . As a result of the manufacturing tolerances of the membrane electrode assembly  30 , the entire width of the cavity (between the bead and the flow field) is not filled with filling agent  24  in the embodiment shown in  FIG. 6 . Compared to  FIG. 5 , the height of the cavity  25  is optimally filled in  FIG. 6 , while the reactant bypass flow through the cavity  25  is significantly reduced in  FIG. 5  with respect to the width of the cavity  25 . 
     The embodiments of the invention shown in  FIGS. 4 to 6  have the function of reducing a reactant bypass flow through the cavity  25  by arranging a volume-filling filling agent  24  inside the cavity  25 , which reduces the volume of the cavity  25  available for reactant gases and thus facilitates a flow of the reactant gases through the channels of the flow field  22 . 
     LIST OF REFERENCE SYMBOLS 
     
         
           10  Fuel cell 
           11  Fuel cell stack 
           20  Bipolar plate 
           21  Sealing element 
           22  Flow field 
           23  Profile structure 
           24  Filling agent 
           25  Cavity 
           26  Distributor region 
           27  Active region 
           28  Welded seam 
           29  Plate 
           30  Membrane electrode assembly 
           31  Membrane 
           32  Frame 
           33  Gas diffusion layer 
           34  Coolant channel 
           35  First reactant gas channels 
           36  Second reactant gas channels