Abstract:
A method for producing a fuel cell stack having the following steps: a) providing a first duplicating unit ( 10 ) with a first sealing surface ( 10   a ) and at least one second duplicating unit ( 16 ) with a second sealing surface ( 16   a ) and b) forming at least one sealing section ( 42 ) between the first sealing surface ( 10   a ) and the second sealing surface ( 16   a ). The step b) includes the steps of: b1) arranging a template ( 22 ) between the first sealing surface ( 10   a ) and the second sealing surface ( 16   a ), whereby the template ( 22 ) has at least one edge area ( 32 ) which is arranged adjacent to the sealing section to be formed ( 42 ), and b2) introducing a sealing compound (40) into an area which is defined by the first sealing surface ( 10   a ), the second sealing surface (16a) and the edge area ( 32 ) of the template ( 22 ).

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The invention relates to a process for producing a fuel cell or a fuel cell stack with the following steps: a) Providing a first duplicating unit with a first sealing surface, and at least a second duplicating unit with a second sealing surface; and b) forming at least one seal section between the first sealing surface and the second sealing surface. 
         [0003]    2. Description of Related Art 
         [0004]    For example, SOFC fuel cell stacks (SOFC=“solid oxide fuel cell”) comprise a plurality of so-called duplicating units between which there are seals which, in many cases, establish the spacing of the duplicating units, and for example, seal openings which extend in the stack direction of the fuel cell stack. Furthermore, there are generally an upper and a lower end plate and current collector plates. Fuel cell stacks are known in which the individual layers are rigidly cemented (for example, by a glass paste), as are fuel cell stacks which have detachable, compressible seals or compound seals. 
         [0005]    One problem in the production of fuel cells or fuel cell stacks is that the individual seals are extremely complex to produce, and in many case, for example, a large amount of waste being formed in punching. Furthermore, at present, only very time consuming series fabrication is possible. For this purpose, doctoring a glass paste which forms the sealing compound in series on each individual duplicating unit or placing it in beads using a dispenser is known. Series fabrication is, furthermore, susceptible to faults. For example, the failure rate of a fuel cell stack is: 
         [0000]      Failure rate=1−(1−p) n , 
         [0000]    n being the number of seals and p being the failure probability of an individual seal. 
       SUMMARY OF THE INVENTION 
       [0006]    The object of the invention is to develop a generic process such that the fault susceptibility is reduced and parallel fabrication becomes possible. 
         [0007]    This object is achieved by arranging a template between a first sealing surface and a second sealing surface, the template having at least one edge area which is located adjacent to the seal section which is to be formed; and placing a sealing compound in the area which is bordered by the first sealing surface, the second sealing surface and the edge area of the template. This approach enables rapid and controlled assembly in which several seals can be aligned at the same time. Furthermore, the material loss, compared to known processes, is low, as is the fault susceptibility. The dimensions of at least one seal section which is to be formed can be easily influenced by the choice of the dimensions of the template. 
         [0008]    The advantages of the process of the invention become especially apparent when it is provided that there is a plurality of duplicating units on top of one another for stacking a fuel cell stack, at least one template at a time being provided between a respective two adjacent duplicating units. In this case, a plurality of seal sections provided between the individual duplicating units can be produced in parallel. 
         [0009]    Furthermore, it is preferred that the template be formed at least in part from an organic fiber material, a carbon fiber material or a corresponding composite material. The respective material can be, for example, a felt, nonwoven, knit or woven fabric. 
         [0010]    Furthermore, it can be advantageously provided that the template be completely or partially removed during and/or after formation of at least one seal section and/or changed partially or entirely in its material properties. For example, the template can be made of a non-temperature resistant (for example, for temperatures exceeding 800° C.), combustible, flat material. In this case, the template can be completely removed by burning after formation of at least one seal section. Alternatively, the template can be made of a material which, instead of being flammable, has the property of losing its mechanical stability under the action of temperature, i.e., collapsing under the action of a force. Furthermore, the material (or its decomposition products) can be electrically insulating. For this purpose, the material can be formed, for example, by an organic or ceramic fiber composite or foam material in which, under the action of temperature, at least one structure-forming component vaporizes, bums or melts. As already mentioned, a composite material which represents a combination of the aforementioned materials is also possible. 
         [0011]    In preferred embodiments of the process of the invention, it is provided that the sealing compound contains dispersed components for a glass solder. 
         [0012]    It is furthermore preferred that the sealing compound is subjected, at least in part, to a curing and/or gelling process to form at least one seal section. The sealing compound preferably contains a curing agent component for curing or gelling. The curing agent component of the sealing compound can be advantageously added just briefly before injection or placement in the corresponding area. The curing or gelling agent can be activated, for example, by air feed, temperature or a chemical activator which has been applied to the template and/or at least one sealing surface. 
         [0013]    Without being limited hereto, it is preferred that at least one seal section is formed adjacent to the first recess in the first duplicating unit. The recess can be intended especially to form a gas channel which extends through the fuel cell stack. 
         [0014]    It can be similarly provided that at least one seal section is formed adjacent to the first recess in the second duplicating unit. Without being limited hereto, the process as claimed in the invention can be especially advantageously used when a first recess in the first duplicating unit is aligned with the second recess in the second duplicating unit in the stacking direction. 
         [0015]    Especially in this connection, it is furthermore preferred that the template has a first recess with dimensions which are larger than the dimensions of the first recess in the first duplicating unit and/or larger than the dimensions of the first recess in the second duplicating unit. In this case, the excess of the first recess in the template establishes the width of the seal section to be formed while the height of the template establishes the height of the seal section which is to be formed. 
         [0016]    In especially preferred embodiments of the process in accordance with the invention, it is provided that the sealing compound according to is applied at least partially by way of the first recess in the first duplicating unit and/or by way of the first recess in the second duplicating unit and/or by way of the first recess in the template. To apply the sealing compound, preferably, a feed means which can comprise, for example, a hose or a tube is used. Preferably, there is a flange or other coupling means with which the supply means can be connected to one of the first recesses with sealing. 
         [0017]    In the above explained connection it is also considered to be advantageous if it is provided that when the sealing compound is applied, a mandrel extends at least partially through the first recess in the first duplicating unit and/or the first recess in the second duplicating unit and/or the first recess in the template. The mandrel, preferably, has only slightly smaller outside dimensions than the inside dimensions of the first openings. The viscosity of the sealing compound is preferably chosen such that only little or no sealing compound runs into the opening which is then cleared when the mandrel is removed. 
         [0018]    In certain embodiments of the process of the invention, it is provided that the first duplicating unit has a second recess and/or the second duplicating unit has a second recess and/or the template has a second recess. Here, it is preferred that the existing second recesses are aligned with one another in the stack direction. 
         [0019]    In this connection, it is preferred that the first recess of the template is connected to the second recess of the template by way of the first channel. In this case, the second recesses which are aligned with one another can form a filling channel for the sealing compound, the sealing compound traveling by way of the first channel from the fill opening to the first recesses in which preferably the aforementioned mandrel is located. Here, it is considered to be advantageous if the fill channel has a cross sectional area which is relatively large compared to the cross-sectional area of the seal section which is to be formed. In this way, the hydrodynamic pressure loss in the fill channel is much smaller than in the sealing channel when a fluid (or the sealing compound) flows through the two channels. 
         [0020]    It can be furthermore provided that the application of the sealing mass takes place at least in part by way of the second recess in the first duplicating unit and/or by way of the second recess in the second duplicating unit and/or by way of the second recess in the template. 
         [0021]    Furthermore, it can be provided that the sealing compound present in the second recess in the first duplicating unit and/or in the second recess in the second duplicating unit and/or in the second recess in the template is at least partially removed again, especially using a second mandrel. Alternatively, the sealing compound can remain in the channel formed by the second recesses and can cure there in order to increase the stability of the overall structure. 
         [0022]    For all embodiments of the process in accordance with the invention, it is preferred that the first duplicating unit and the second duplicating unit are at least temporarily compressed in the course of placing the sealing compound in the area which is bordered by the first sealing surface, the second sealing surface and the edge area of the template. When the sealing compound is being added, this prevents the duplicating units from being moved apart by the sealing compound. In later process steps, the compression can contribute to causing a collapse of the template(s). 
         [0023]    The invention is also intended to encompass any fuel cell stack which is produced with the process of the invention. This applies especially to a fuel cell stack in which at least two seal sections which are at least essentially aligned with one another in the stack direction of the fuel cell stack are connected by sealing compound. The seal sections which are connected by the sealing compound and which are located on top of one another between several duplicating units constitute a distinct indicator that the process in accordance with the invention has been used. 
         [0024]    The finding that, by using templates located between two duplicating units at a time, it is possible to form a plurality of seals which are to be provided on top of one another at the same time is critical to this invention. 
         [0025]    Preferred embodiments of the invention are explained by way of example below with reference to the accompanying drawings. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]      FIG. 1  are top views and cross-sectional views taken along line I-I of the top views of a first and a second duplicating unit and of a template; 
           [0027]      FIG. 2  shows the template from  FIG. 1  located on the first duplicating unit from  FIG. 1  in a top view and a cross-sectional view taken along line I-I of the top views; 
           [0028]      FIG. 3  shows the second duplicating unit from  FIG. 1  located on the arrangement from  FIG. 2  in a top view and a cross-sectional view taken along line I-I of the top view; 
           [0029]      FIG. 4  is a cross-sectional view taken along line I-I of  FIG. 3  and which illustrates application of the sealing compound; 
           [0030]      FIG. 5  is a sectional view of the arrangement shown in  FIG. 4  taken along line II-II thereof; and 
           [0031]      FIG. 6  shows is a view corresponding to  FIG. 5  showing a completed seal section. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0032]      FIG. 1  shows a first duplicating unit  10 , a second duplicating unit  16  and a template  22 . As shown in  FIG. 1 , the first duplicating unit  10  has a first sealing surface  10   a  and a first opening  12  and a second opening  14 . The second duplicating unit  16  has a second sealing surface  16 a and a first recess  18  and a second recess  20 . 
         [0033]    In this case, the structure of the first duplicating unit  10  and the structure of the second duplicating unit  16  are identical, although the invention is not limited to these embodiments since applications are also possible in which seals arranged differently are formed between different duplicating units. 
         [0034]    A template  22  is shown between the first duplicating unit  10  and the second duplicating unit  16 . The template  22  has a first recess  24  and a second recess  26 . The periphery of the first recess  24  in the template  22 , in this case, defines an edge area  32  which is intended to be located adjacent to the seal section which is to be formed. The first recess  24  and the second recess  26  of the template  22  are connected to one another by a first channel  28 . Furthermore, the outside periphery of the template  22  is connected by way of a second channel  30  to the first recess  24 . 
         [0035]      FIG. 2  shows the template  22  from  FIG. 1  located on the first duplicating unit  10  from  FIG. 1  and from which it can be seen that the dimensions of the first recess  24  in the template  22  are chosen to be somewhat larger that the dimensions of the first recess  12  in the first duplicating unit  10 . The excess of the first recess  24  in the template  22  defines the width of the seal section which is to be formed and which is to be made in this case essentially in the shape of a circular ring around of the first recess  12  of the first duplicating unit  10 . 
         [0036]      FIG. 3  shows the second duplicating unit  16  from  FIG. 1  located on the arrangement from  FIG. 2  with the template  22  located between the first duplicating unit  10  and the second duplicating unit  16  such that the first recesses  12 ,  18 ,  24  and the second recesses  14 ,  20 ,  26  are aligned with one another in the stack direction, at least in essence. 
         [0037]      FIGS. 4 &amp; 5  illustrate the application of the sealing compound  40 . For the sake of clarity,  FIG. 4  shows only two duplicating units  10 ,  16  with a template  22  located in between. But, one skilled in the art easily recognizes that the process of the invention entails advantages especially when a fuel cell stack is stacked with a plurality of duplicating units and templates located between them. 
         [0038]    Recesses aligned with one another in the individual duplicating units can form one or more channels which extend essentially parallel to the stack axis through the fuel cell stack, especially gas supply channels. In this case, the seal gaps which form between the individual duplicating units must be sealed to prevent escape of gas to the outside when the fuel cell is in operation. The seals which are made between the duplicating units in accordance with the invention should generally be made electrically insulating so that the duplicating units are not electrically short-circuited. Furthermore, it is necessary in many cases for the seals to be fluid-tight even at high temperatures, and preferably, also under mechanical vibrations, for which reason especially a glass solder is considered as the seal material. 
         [0039]    Furthermore,  FIG. 4  shows only one lower end plate  34 . However, it will be clear to one skilled in the art that the fuel cell stack generally also has an upper end plate which is not own. In the illustrated case, the end plate  34  does not have an opening which is aligned to the first openings  12 ,  18 ,  24  and is therefore used as a permanent blocking element which also remains a component of the arrangement even after the sealing compound  40  is applied. 
         [0040]    Alternatively, the use of at least one temporary blocking element which is removed after the sealing compound is applied is conceivable. Similarly, both permanent and also temporary clamping devices for mechanical bracing of the fuel cell stack are possible. 
         [0041]    Although, as mentioned, with the process of the invention, preferably a plurality of seal sections are produced at the same time, the production of only one seal section  42  which seals the first recesses  12 ,  18  in the duplicating units  10 ,  16  is explained below. 
         [0042]    To produce the seal section, the first duplicating unit  10 , the template  22  and the second duplicating unit  16  are stacked on an end plate  34  such that the respective recesses  12 ,  18 ,  24  or  14 ,  20 ,  26  are aligned with one another. The second recesses  14 ,  20 ,  26  form a supply or fill opening  40  for the sealing compound. A supply means  38  which is shown only schematically in  FIG. 4  is connected to this fill opening with sealing so that the sealing compound  40  which has been applied to the second recesses  14 ,  20 ,  26  travels by way of the channel  28  of the template  22  into the first recesses  12 ,  18  and  24 . The sealing compound is preferably applied under high pressure. According to the hydrodynamic pressure loss, generally, the fill channel formed by the two recesses  14 ,  20 ,  26  is completely filled first. Afterwards, the sealing compound  40  is distributed in the sealing channels. The displaced air can leave the channels, for example, through the second channel  30  of the templates  22  or through the template  22  if it has a porous structure. 
         [0043]    During application of the sealing compound  40 , the entire arrangement is compressed by an externally applied, preferably controlled force F. The second channel  30  of the template  22  makes it possible for a sealing compound  40  which may have been applied in excess to escape again. In the first recesses  12 ,  18 , and  24  which are aligned with one another there is a mandrel  36  with outside dimensions which are somewhat smaller than the inside dimensions of the first recesses  12 ,  18 . The mandrel  36  is used especially to suitably establish the cross-sectional ratio of the fill opening which has been formed by the second recesses and the seal which is to be formed in order to achieve hydrodynamically favorable properties. 
         [0044]    At the end of the filling process, all of the sealing channels are filled with the sealing compound  40 . Further, pressing-in of the sealing compound  40  leads to the sealing compound  40  penetrating into the second channel  30  or into the pore structure of the template  22 . This rapidly increases the pressure in the seal channel, and thus, in the fill channel. This pressure rise can be advantageously detected in order to end the filling process. 
         [0045]    Basically, it holds that the application of the sealing compound  40  can be supported by a negative pressure (vacuum) which prevails against the outer sides of the template  22  relative to the inner sides. Since the pressure is continuously equalized by the second channel  30  and the porous configuration of the template  22 , the negative pressure must be maintained if necessary by continuous after-pumping in the device. The negative pressure provides for the sealing compound being sucked more rapidly into the recess of the template  22  and for preventing the formation of air inclusions/air bubbles. 
         [0046]    As soon as the sealing compound  40  is fixed in its gap, the mandrel  36  can be removed without a significant amount of the sealing compound  40  traveling into the first recesses  12 ,  18 . But, alternatively, it can likewise be provided that the mandrel  36  is formed, for example, by a tie rod which is left in the fuel cell stack in order to maintain bracing of the finished product. After the supply means  38  has been decoupled, the entire arrangement is placed, for example, in a furnace, optionally while maintaining compression by the force F, in order to cure the sealing compound  40 . 
         [0047]    In this case, due to the temperature rise, the sealing compound  40  first releases its solvents or diluents and possible binders. The vapors can escape through the second channel  30  of the template  22  or through the template  22  if it is made porous. As time progresses, after all the binders and solvents have escaped from the sealing compound, the sealing compound is present, for example, as the dry raw substance of a glass solder, i.e., as a porous body with the shape of the seal section which is to be formed. Such a porous base body constitutes a mechanical resistance when the fuel cell stack is compressed (i.e., when the sealing surfaces  10   a ,  16   a  are compressed). Therefore, the pore body will possibly collapse in a controlled manner as the compression increases, and in doing so, decrease in its height (reduction of pore volume). As time passes, the components of the seal section or of the sealing element begin to sinter and melt according to the composition of the glass solder. In doing so, a transition takes place from the solid to a highly viscous liquid consistency of the sealing element. As the temperature increases further, the glass solder melts completely and wets the surfaces of successive duplicating units  10 ,  16 , which surfaces are to be sealed relative to one another. The high quality non-Newtonian flow behavior of such a glass melt and the capillary action within the sealing gap prevent the glass solder from being pressed completely out of the sealing gap within a defined time interval, even as the sealing surfaces are further compressed. The continued compression, and thus, the reduction of the height of the sealing element are advantageous for equalizing the shrinkage of the seal section which can occur by the release of binders and solvents as well as enclosed air and gas bubbles. 
         [0048]    Further compression of the fuel cell stack in the course of the joining process can take place, for example, according to the two following versions. 
         [0049]    In the case of a combustible template  22 , it bums preferably without residues as the temperature continues to rise. Compression of the fuel cell stack caused by temporary or permanent bracing of the fuel cell stack during the combustion process causes the template  22  to collapse during combustion. The successive duplicating units are prevented from touching each other by the limiting and/or metering of the compression force F. Touching would result in that the sealing compound  40  under certain circumstances would be pressed completely out of the sealing gap and the duplicating units  10 ,  16 , and moreover, would electrically short circuit. By limiting the force F, the sealing compound  40  remains essentially in its original form in the plane which was defined by the sealing gap so that a surface seal (seal section  42 ) is formed around the recesses  12 ,  18  which are to be sealed. The height of the seal section  42  or of the sealing element is (somewhat) smaller than the height of the original air gap (negative form) since the sealing compound  40 , as mentioned, releases binder and solvent during the melting process and thus shrinks. It is therefore advantageous to track the bracing of the fuel cell stack during combustion. 
         [0050]    In another version, the template  22  is not burned (without residue), but selective collapse of the template  22  is caused by the bracing of the fuel cell stack, supported by the loss (thermal decomposition) of at least one structure-forming component. In this case, the electrically insulating action of the template  22  or of the corresponding decomposition products can be advantageously used to reliably prevent short circuits between successive duplicating units  10 ,  16 . 
         [0051]      FIG. 6  shows a top view of a completed seal section corresponding to  FIG. 5 .  FIG. 6  shows that the seal section  42  which has been produced in accordance with the invention extends in the shape of a circular ring around the first recess  12  in the first duplicating unit  10  so that the seal section  42  forms a channel which connects the first recesses  12 ,  18 . The sealing compound present in the second recess  14  was removed as shown in  FIG. 6  before curing of the sealing compound  40  by means of another mandrel (not shown). But, embodiments are likewise conceivable in which the sealing compound  40  remains in the second recesses in order to increase the stability of the overall structure. 
         [0052]    Although the above explained embodiments of the invention can be considered especially advantageous, a plurality of modifications is possible. For example, embodiments are conceivable in which there are no second recesses so that the sealing compound is added directly to the first recesses. The mandrel  36  can be abandoned if necessary. It is likewise conceivable for the mandrel  36  to be inserted only after the sealing compound is applied in order to remove the sealing compound from the first recesses. Alternatively, the sealing compound can remain in the first recesses. 
         [0053]    Furthermore, it can be provided that the sealing compound  40  is filtered by a porous configuration of the template  22 . In this case, the second channel  30  of the template  22  is preferably omitted. The sealing compound  40  is pressed from the sealing gap through the template  22  by the internal pressure. In doing so, solid particles (glass solder) are retained while the diluent of the sealing compound  40  (water) is pressed to the outside as a filtrate and runs off. In this way, in spite of highly diluted sealing compound  40  which can therefore flow better, a very compact blank for the seal section which is to be made can be formed (filter cake). 
         [0054]    The rapid and controlled outflow of the diluent through the porous structure of the template can be improved by a surface modification of the fiber or pore structure by increasing the wetting of the fiber/pore structure by the solvent. In the case of water for example, a hydrophilic surface layer or impregnation with a hydrophilic component can improve the discharge of water to the outside. 
         [0055]    The features of the invention disclosed in the description above, in the drawings and in the claims can be significant to the implementation of the invention both individually and also in any combination.