Abstract:
In order to improve an electrochemical conversion device comprising a plurality of functional elements stacked one upon the other into a stack in a stacking direction and interconnected within the stack, some of which have peripheral areas of sheet material, some of which are arranged in a stacked configuration one upon the other in a stacking direction, forming peripheral stacks, and are interconnected by way of a first element-to-element connection and some others of which are interconnected by way of a second element-to-element connection, in such a manner that the strain placed on the element-to-element connections can be kept as low as possible, it is proposed that one of the functional elements comprise a compensating unit and that the compensating unit comprise at least one deformable element which, by deformation, allows for at least one height compensation in the stacking direction.

Description:
CROSS REFERENCE TO RELATED PATENT APPLICATION 
       [0001]    This patent application claims the benefit of German application number 10 2013 213 399.5 of Jul. 9, 2013, the teachings and disclosure of which are hereby incorporated in their entirety by reference thereto. 
       BACKGROUND OF THE INVENTION 
       [0002]    The invention relates to an electrochemical conversion device, comprising a plurality of functional elements stacked one upon the other into a stack in a stacking direction and interconnected within the stack, some of which have peripheral areas of sheet material, some of which are arranged in a stacked configuration one upon the other in a stacking direction, forming peripheral stacks, and are interconnected by way of a first element-to-element connection and some others of which are interconnected by way of a second element-to-element connection. 
         [0003]    Such electrochemical conversion devices are known in the prior art. 
         [0004]    In these electrochemical conversion devices, the problem exists that they are subject to variations in pressure and temperature and this imposes very high strain on the element-to-element connections. 
         [0005]    In particular, in such electrochemical conversion devices it is necessary to use isolating element-to-element connections, and these present a problem in terms of their mechanical stability. 
         [0006]    Hence, the object underlying the invention is to improve an electrochemical conversion device of the kind described at the outset such that the strain placed on the element-to-element connections can be kept as low as possible. 
       SUMMARY OF THE INVENTION 
       [0007]    In accordance with the invention, this object is accomplished in an electrochemical conversion device of the kind described at the outset by one of the functional elements comprising a compensating unit and by the compensating unit comprising at least one deformable element which, by deformation, allows for at least one height compensation in the stacking direction. 
         [0008]    The advantage of the solution in accordance with the invention is seen in that by the use of such a compensating unit, the mechanical stresses acting on the element-to-element connections are either reduced or compensated so that there is less strain on the element-to-element connections and therefore less damage to the element-to-element connections during operation of the electrochemical conversion device. 
         [0009]    The compensating unit need not necessarily be arranged adjacent to a stress-sensitive element-to-element connection. 
         [0010]    In order for the compensating unit to function as effectively as possible, it is preferably provided for the compensating unit to be connected to the adjacent functional elements on the one hand by way of the first element-to-element connection and on the other hand by way of the second element-to-element connection. 
         [0011]    This makes it possible, independently of which of the element-to-element connections has the higher sensitivity to stress, to reduce or substantially relieve these stresses by way of the compensating unit. 
         [0012]    In conjunction with the previously described solutions, no details have been provided as to how the compensating unit is to be configured. 
         [0013]    One solution that is particularly advantageous provides for the compensating unit to comprise at least one sheet material layer as the deformable element for height compensation. 
         [0014]    More advantageously, the compensating unit comprises at least two sheet material layers that are movable relative to each other in the stacking direction. 
         [0015]    The at least two sheet material layers can have a variety of different configurations. 
         [0016]    For example, one of the sheet material layers or both sheet material layers may be formed into a bead. 
         [0017]    In the simplest case, however, the two sheet material layers are configured such that they each extend in a plane when in the undeformed state. 
         [0018]    In order to provide for height compensation when deformed, it is preferably provided for the at least two sheet material layers to be interconnected in connection areas and to be movable relative to each other in the stacking direction in movement areas located outside the connection areas. 
         [0019]    The connection in the connection areas may be effected for example by one of the sheet material layers transitioning into the other one. 
         [0020]    Another advantageous solution provides for the sheet material layers to be interconnected in the connection areas by way of a substance-to-substance bond. 
         [0021]    In particular, provision is made for the substance-to-substance bond between the sheet material layers to be located on a side of the compensating unit that faces away from the peripheral area. This is advantageous in that it provides as large as possible an area in which the sheet material layers are capable of deformation. 
         [0022]    In particular for creating flat-lying movement areas, it is advantageous for the connection areas of the sheet material layers to be arranged on a side of the compensating unit that faces away from the peripheral area of the respective functional element. 
         [0023]    Furthermore, no details have been given so far as to how the movement areas are configured. 
         [0024]    An advantageous solution provides for the movement areas of the sheet material layers to lie one on top of the other in a first position and to extend at a distance from one another in at least one second position, wherein different second positions with different distances can be implemented, allowing for the compensation of stresses or tensile loads of different magnitudes. 
         [0025]    Within the scope of the solution in accordance with the invention, it is further advantageous for the movement areas to be arranged on a side of the compensating unit that faces towards the peripheral area. 
         [0026]    With respect to the connection between the compensating unit and the remaining functional elements it is for example advantageous for one of the sheet material layers of the compensating unit to be connected to the adjacent functional element in the stacking direction by way of a peripheral area and the first element-to-element connection. 
         [0027]    It is further advantageous for one of the sheet material layers of the compensating unit to be connected to the adjacent functional element in the stacking direction by way of the second element-to-element connection. No details have been provided so far on the element-to-element connections. 
         [0028]    It is preferably provided for one of the element-to-element connections to be an electrically isolating element-to-element connection. 
         [0029]    Further, it is preferably provided for another one of the element-to-element connections to be an electrically conductive element-to-element connection. 
         [0030]    No details have been given so far as to how the second element-to-element connection is configured. 
         [0031]    An advantageous solution provides for the second element-to-element connection to be a substance-to-substance bond. 
         [0032]    In particular, provision is made for the second element-to-element connection to comprise a solder connection. 
         [0033]    The solder connection may comprise for example a glass solder connection layer so that the second element-to-element connection may be configured as an isolating element-to-element connection. 
         [0034]    Another way of configuring the second element-to-element connection is for the second element-to-element connection to comprise a solder layer and an isolation layer, wherein the isolation layer may be a ceramic layer for example. 
         [0035]    In this case as well, the second element-to-element connection is an isolating element-to-element connection. 
         [0036]    Furthermore, no further details have been given so far on the first element-to-element connection. 
         [0037]    For example, provision is made for the first element-to-element connection to be a substance-to-substance bond. 
         [0038]    Preferably, the substance-to-substance bond is a welded connection comprising a connection zone. 
         [0039]    For example, the peripheral areas of the functional elements are configured such that they extend to end faces succeeding one another in the stacking direction and that the end faces of the respective peripheral stacks are arranged relative to one another such that they are within the connection zone. 
         [0040]    Furthermore, provision is preferably made for the connection zone to be configured in surrounding relation with the functional elements of the respective assembly group, i.e. such that it forms a surrounding and tightly sealed connection. 
         [0041]    Moreover, it is preferably provided for the connection zone to be configured such that it interconnects all of the peripheral areas of the respective assembly group in a gas-tight manner. 
         [0042]    Finally, an advantageous solution provides for an end face area in which the connection zone is formed to extend starting from the end faces of the peripheral areas into the peripheral areas by a distance no greater than that corresponding to twice the thickness of one of the peripheral areas. 
         [0043]    Furthermore, the invention relates to a method for manufacturing an electrochemical conversion device from individual functional elements that are interconnected in a stack. 
         [0044]    In this method, in accordance with the invention, a second element-to-element connection between some of the functional elements is made first, said second element-to-element connection is subjected to a functional test and thereafter stacking of the functional elements is performed and subsequently any stacked functional elements not yet connected by the second element-to-element connection are interconnected by way of a first element-to-element connection. 
         [0045]    The advantage of the solution in accordance with the invention is that it affords the possibility for the functional elements that are at first interconnected by the second element-to-element connection to be tested with respect to their functions and only then for the functional elements to be stacked, wherein the stacked functional elements, for example all or only some of the functional elements that are not yet connected by the second element-to-element connection, are interconnected by a first element-to-element connection. 
         [0046]    This solution is advantageous in that it permits selecting for example as the second element-to-element connection the element-to-element connection that is technically difficult to perform and therefore leads to a substantial defect rate in making the connection, meaning that any connections found to be defective can be discarded and precluded from use in building the stacks of functional elements. 
         [0047]    In particular, it is then possible to select as the second element-to-element connection an element-to-element connection which has no capability of being reworked, meaning that where the connection is found to be defective, reworking the connection and therefore eliminating the defect is not feasible. 
         [0048]    The solution in accordance with the invention thus allows for technically difficult element-to-element connections to be integrated in the overall process of manufacturing the electrochemical conversion device in such a way that these, when they are defective, lead to reject costs that are as low as possible. 
         [0049]    On the other hand, the element-to-element connection that is preferably selected as the first element-to-element connection is the one that is technically less difficult and therefore less susceptible to defects so that the then stacked functional elements can be provided with the first element-to-element connection subject to a very low defect rate. 
         [0050]    In particular, the element-to-element connection selected as the first element-to-element connection is also one which does have the capability of being reworked in the event of a defect so that rejects can also be avoided by reworking the first element-to-element connection. 
         [0051]    In particular, the scope of the solution in accordance with the invention provides for the second element-to-element connection to be an electrically isolating element-to-element connection. 
         [0052]    Such an electrically isolating element-to-element connection may be implemented in a variety of ways. 
         [0053]    It is for example conceivable for this element-to-element connection to be provided as a connection between a solder layer and an electrical isolation layer, wherein the solder layer adheres to the isolation layer and wherein for example a metallic layer of the electrical insulation layer may be connected to the solder layer. 
         [0054]    Another preferred solution provides for the element-to-element connection to comprise a glass solder layer which itself has an electrically isolating effect. 
         [0055]    No details have been provided so far concerning the functional test of the second element-to-element connection. 
         [0056]    Preferably, provision is made for the functional test of the second element-to-element connection to comprise a tightness test and/or an electrical isolation test. 
         [0057]    With such a functional test, it is possible on the one hand to test for tightness, which is important in electrochemical conversion devices, and also on the other hand to test for the electrical isolation effect of the second element-to-element connection. 
         [0058]    Furthermore, it is advantageously provided for the first element-to-element connection to be subjected to a functional test. 
         [0059]    For example, such a functional test is likewise a tightness test, which is of substantial importance in the case of an electrochemical conversion device. 
         [0060]    A particularly advantageous embodiment of the method in accordance with the invention provides for the functional elements to be stacked into an assembly group and for the functional elements of a respective assembly group to be interconnected by way of the first element-to-element connection insofar as these have not yet been interconnected by way of the second element-to-element connection. 
         [0061]    Furthermore, provision is made that for each assembly group the first element-to-element connection, once made, be subjected to a functional test, particularly a tightness test, thereby performing yet another functional test, from assembly group to assembly group. 
         [0062]    This can in particular be implemented in that in the manufacture of the electrochemical conversion device an assembly group is created by stacking the functional elements and making the first element-to-element connection between the functional elements and is subjected to a functional test together with any assembly groups that may have already been created. 
         [0063]    In particular, it is only after this functional test has been conducted that the next assembly group is created by stacking and by making the first element-to-element connection between the functional elements and is subjected to a functional test together with all of the assembly groups that have already been created. 
         [0064]    In conjunction with what has been described above for the method in accordance with the invention, no details have been provided yet as to how to proceed in the case of a negative functional test. 
         [0065]    It is preferably provided that in the case of a negative functional test, the defect of the first element-to-element connection be localized and reworked so that a further functional test can be passed. 
         [0066]    The functional test, in particular the tightness test, can be performed in different ways. 
         [0067]    One advantageous solution provides for the functional test of the first element-to-element connection of the assembly group to be performed at a station for making the first element-to-element connection so that any defect may already be detected at the very station where the first element-to-element connection is made. 
         [0068]    In particular, this also allows for the reworking of the first element-to-element connection for passing the functional test to be performed at the station for making the first element-to-element connection, since the assembly group has not left said station yet. 
         [0069]    Alternatively, another solution provides that in the case of a negative functional test the assembly group be removed from the production process and, for example, the rework for passing the functional test and the further functional test be performed at a separate station. 
         [0070]    In this instance, after having its first element-to-element connection reworked at a separate station and in particular after passing the further functional test, the assembly group can be returned to the production process to continue further processing thereof. 
         [0071]    This procedure of creating the assembly groups successively has the advantage that it allows immediate testing, by use of the functional test, each time another assembly group has been created, of whether or not said assembly group and also the assembly groups that have already been created have the required functionality so that in particular in the case of a manufacturing defect associated with creating the last assembly group, this can be localized very quickly and in particular removed by reworking. 
         [0072]    Further features and advantages of the invention are the subject of the following description and drawing of the exemplary embodiments. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0073]      FIG. 1  is a perspective view, partly in section, of a detail of a fuel cell, showing assembly groups stacked one above the other; 
           [0074]      FIG. 2  is a top view of a first exemplary embodiment, seen in the direction of arrow A in  FIG. 1 ; 
           [0075]      FIG. 3  is an enlarged view of the top view of  FIG. 2 , showing a first state of compensation; 
           [0076]      FIG. 4  is an enlarged view of the top view of  FIG. 2 , showing a second state of compensation; 
           [0077]      FIG. 5  is an enlarged view of peripheral stacks of the assembly groups prior to forming the melt zone; 
           [0078]      FIG. 6  is a view similar to  FIG. 5 , showing the formed connection zone; 
           [0079]      FIG. 7  is a view similar to  FIG. 5 , showing the laser radiation for forming the connection zone; 
           [0080]      FIG. 8  is a section taken along line  8 - 8  in  FIG. 7 ; 
           [0081]      FIG. 9  is a top view, similar to  FIG. 6 , of a second exemplary embodiment; 
           [0082]      FIG. 10  is a flow chart showing a method in accordance with the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0083]    A detail  10  of a fuel cell as an example of an electrochemical conversion device is shown in  FIGS. 1 and 2 , depicting a plurality of assembly groups  12   1  to  12   3  stacked one above the other in a stacking direction S, each of said assembly groups  12   1  to  12   3  being constructed from a plurality of functional elements  22 ,  24 ,  26  stacked one above the other in the stacking direction S, wherein at least a plurality of assembly groups  12  of the fuel cell are constructed from identical functional elements. 
         [0084]    For example, the first functional element  22  of each of the assembly groups  12  represents a tray element having an outer peripheral area  32  which surrounds the functional element  22  in a closed manner in the style of a frame, said outer peripheral area  32  terminating in an end face  34  and merging, on an inner side  36  opposite the end face  34 , in a tray wall portion  38  extending transversely relative to the outer peripheral area  32  and itself merging in an outer functional area  42  which extends parallel to the outer peripheral area  32  and, on a side opposite the tray wall portion  38 , is adjoined by an inner functional area  44  which is configured for example in the form of contacting and flow conducting elements  46 ,  48  which succeed one another and extend parallel to one another in a longitudinal direction L and which in the present embodiment are shown as being configured as corrugations, but can have other shapes. 
         [0085]    The second functional element  24  is configured as a carrier element and comprises an outer peripheral area  52  which surrounds the functional element  24  in a closed manner in the style of a frame and, starting from an end face  54  thereof, extends to a cell carrier  56  which extends in a closed manner as a frame around an inner opening  64  and carries a first fuel cell element  58  which in turn is connected to the cell carrier  56  via a solder layer  62 . 
         [0086]    The first fuel cell element  58  covers the inner opening  64  enclosed by the cell carrier  56  as a frame and protrudes with a holding periphery  66  thereof so far beyond the inner opening  64  that the holding periphery  66  can be connected to the cell carrier  56  via the solder layer  62 . 
         [0087]    The first fuel cell element  58  in turn carries, in a functional area  68  thereof extending within the inner opening  64 , on a side thereof facing towards the assembly group  12  following next in the stacking direction S, a second fuel cell element  72  and carries, on an opposite side thereof facing towards the inner functional area  44  of the tray element  22  associated with the same assembly group  12 , a contact element  74 . 
         [0088]    The second fuel cell element  72  is for example configured as a layer applied to the functional area  68  of the first fuel cell element  58 . 
         [0089]    The contact element  74  in turn is for example configured as a coating or sheet and is in contact with the functional area  68  of the first fuel cell element  58 . 
         [0090]    The third functional element  26  of the assembly group  12  also has an outer peripheral area  82  which surrounds the functional element  26  in a closed manner in the style of a frame and extends, starting from an end face  84  thereof, to a compensating frame  86  which is configured in surrounding relation with an inner frame opening  88 . The compensating frame  86  itself is formed from two sheet material layers  92  and  94 , for example from spring metal sheets, wherein the sheet material layer  92  represents a base layer which extends from the inner frame opening  88  to the end face  84 , thereby comprising the peripheral area  82 , and the sheet material layer  94  represents a connection layer which extends from an inner edge  96  thereof to an outer edge  98  thereof which extends for example at a distance from the end face  84 . 
         [0091]    The base layer  92  and the connection layer  94  have connection areas  102  and  104  respectively which are arranged for example adjacent to the frame opening  88  and the inner edge  96  respectively, these connection areas  102 ,  104  being interconnected by way of a welded connection  106  and therefore non-movable relative to each other. 
         [0092]    Furthermore, the base layer  92  and the connection layer  94  have movement areas  112  and  114  which are arranged for example facing towards the end face  84 , outside of the connection areas  102  and  104  respectively, these movement areas  112  and  114  being movable relative to each other, particularly in the stacking direction S, preferably by the movement areas  112  and  114  being capable of either lying one upon another in contact, or extending in spaced-apart relationship with respect to each other so that an interspace  116  is formed therebetween as is shown in  FIG. 3 . 
         [0093]    The compensating frame  86  itself can, with a support side  124  of the base layer  92  thereof, be seated on a support side  122  of the cell carrier  56  opposite the holding periphery  66  of the first fuel cell element  58  or, as shown in  FIG. 4 , with the support side  124  of the connection area  102 , it can also be unseated from the support side  122 . 
         [0094]    A connection side  126  of the compensating frame  86  opposite the support side  124  which is formed by the movement area  114  of the connection layer  94  is connected by way of a solder layer  127  to an electrical isolation layer  128  of the next tray element  22 , in the stacking direction S, of the next assembly group  12   x+1 , said electrical isolation layer  128  being for example made from a ceramic material. 
         [0095]    Thus, the compensating frame  86  allows for thermal and/or mechanical stresses, such as tensile stresses acting in the stacking direction S, to be compensated and relieves the strain on the joint connections between the individual assembly groups  12 , in particular the connections made by the solder layer  127  between the connection side  126  of the compensating frame  86  and the isolation layer  128  of the tray element  22  next to the compensating frame  86 , in the stacking direction S, of the next assembly group  12   x+1 . 
         [0096]    In particular, the inner opening  64  is configured so as to be in registration with the inner opening  88 . 
         [0097]    In a fuel cell fabricated from the assembly groups  12  by stacking the assembly groups  12  in the stacking direction S, each assembly group  12   x  has the respective contact element  74  thereof, which preferably extends within the inner opening  64  of the cell carrier  56 , supported on and electroconductively connected with crests  108  of the contacting and flow conducting elements  48  of the inner functional area  44  of the tray element  22  of the same assembly group  12   x  that face towards the contact element  74 , while the second fuel cell element  72  is in contact with and electroconductively connected to the corrugation crests  106  of the contacting and flow conducting elements  46  of the tray element  22  of the next assembly group  12   x+1  in the stacking direction S so that in each case the second fuel cell element  72  of the one assembly group  12   x  contacts the tray element  22  of the next assembly group  12   x+1  in the stacking direction S which itself in turn contacts the contact element  74  that is connected to the first fuel cell element  58  of said next assembly group  12   x+1 . 
         [0098]    As shown in the enlarged view of  FIG. 5 , the peripheral areas  32 ,  52  and  82  of each of the assembly groups  12  together form a peripheral stack  130  in which the peripheral areas  32 ,  52 ,  82  are in contact with one another with flat sides thereof. 
         [0099]    Thus, by way of example, the peripheral area  32  has a lower flat side  132  and an upper flat side  134 . Supported on said upper flat side  134  of the peripheral area  32  is the peripheral area  52  with a lower flat side  152  thereof, while an upper flat side  154  thereof faces towards the peripheral area  82  so that the peripheral area  82  with a lower flat side  182  thereof is supported on the upper flat side  154  of the peripheral area  52  and with an upper flat side  184  thereof faces towards the next assembly group  12 . 
         [0100]    For interconnecting the peripheral areas  32 ,  52  and  82  forming the respective peripheral stack  130 , a melt zone  160  as shown in  FIG. 5  is formed in an end face area  33 ,  53 ,  83  adjoining the respective end faces  34 ,  54 ,  84  of the peripheral areas  32 ,  52 ,  82 , wherein the end face areas  33 ,  53 ,  83 , starting from the end faces  34 ,  54 ,  84 , extend into the peripheral areas  32 ,  52 ,  82  over a portion thereof, namely for a minimum distance that corresponds to a thickness of the one of the peripheral areas  32 ,  52 ,  82  that has the smallest thickness and for a maximum distance that corresponds to twice the thickness of the one of the peripheral areas  32 ,  52 ,  82  that has the greatest thickness. 
         [0101]    In this melt zone  160 , a melt is formed by heating a base material of the peripheral areas  32 ,  52 ,  82 , said melt comprising the base material of the peripheral areas  32 ,  52 ,  82 . 
         [0102]    Where the base material of the peripheral areas  32 ,  52  and  82  is a metal, such as steel, the melt which results overall in the melt zone  160  is one which represents an alloy of all the constituents present in the peripheral areas  32 ,  52  and  82 . 
         [0103]    Where the peripheral areas  32 ,  52 ,  82  comprise coatings, these coatings are either burned or evaporated if they are not temperature-resistant enough to withstand the temperature in the melt zone  160 , or the materials of the coatings are embedded if they are temperature-stable enough to withstand the temperatures generated in the melt zone  160 . 
         [0104]    In the latter case, these coatings can be embedded in the melt forming in the melt zone  160 . Such coatings are for example metal coatings so that the metals are then integrated in the melt of the melt zone  160 . 
         [0105]    Where the functional elements are provided with ceramic coatings, as is for example the first functional element  22  with the electrical isolation layer  128 , then these are to be arranged such that no ceramic material thereof is arranged in the peripheral areas  32 ,  52 ,  82  and thus that none will be integrated in the melt of the melt zone  160 . 
         [0106]    Once the melt zone  160  is hardened, a connection zone  162  is formed which, as depicted in  FIG. 6 , interconnects all of the peripheral areas  32 ,  52  and  82  of the respective assembly group  12 , thereby also permanently interconnecting all of the functional elements  22 ,  24  and  26  of the assembly group  12 . 
         [0107]    For generating the melt zone  160  in the respective assembly groups  12 , at least the functional elements  22 ,  24 ,  26  of one assembly group  12  are stacked one upon the other in the stacking direction S and have a force applied to them in a direction opposite to the stacking direction S so that all of the peripheral areas  32 ,  52 ,  82  lie, with the respective flat sides  134 ,  152  and  154 ,  182  thereof, one on top of the other under the application of forces. 
         [0108]    Alternatively, however, it is also possible for all of the functional elements  22 ,  24 ,  26  of all of the assembly groups  12  to be placed one on top of the other in the stacking direction S and have a force applied to them in a direction opposite to the stacking direction S so that for all of the assembly groups  12  peripheral stacks  130  are formed in which the peripheral areas  32 ,  52 ,  82  of the respective functional elements  22 ,  24 ,  26  lie, with the flat sides thereof, one on top of the other under the application of forces. 
         [0109]    In this condition of the peripheral stacks  130 , as shown in  FIG. 6 , heat is input via the end faces  34 ,  54 ,  84  of the peripheral areas  32 ,  52 ,  82  by way of a laser beam  170  directed from outside the peripheral stack  130  towards the end faces  34 ,  54 ,  84 , said laser beam  170  applying heat to all of the end faces  34 ,  54  and  84  of the respective peripheral stack  130  at the same time, thereby causing the material of the peripheral areas  32 ,  52 ,  82  to melt. 
         [0110]    The laser beam  170  is oriented such that a beam axis  172  of the laser beam  170  with a plane E parallel to the extension of the peripheral areas  32 ,  52 ,  82  encloses an angle smaller than 60°, preferably smaller than 30°, in order to provide for optimal heat application to all of the end faces  34 ,  54 ,  84  of the respective peripheral stack  130 , thereby causing the respective base material in all of the peripheral areas  32 ,  52  and  82  to melt. 
         [0111]    Furthermore, the laser beam  170  preferably has a focus  174  having an extension which is preferably of the order of the extension of the end faces  34 ,  54 ,  84  transverse to the plane E. 
         [0112]    As shown in  FIG. 8 , this results in the melt zone  160  being formed in an impingement zone  176  of the laser beam  170 . 
         [0113]    However, if the laser beam  170  is moved along the end faces  34 ,  54 ,  84  in a direction R, then this results in impingement zones  176   1  to  176   n  being formed which overlap one another so that once the melt zones  160  formed in the impingement zones  176   1  to  176   n  have cooled, a continuous connection zone  162  is formed which interconnects all of the peripheral areas  32 ,  52 ,  82  in the respective peripheral stack  130  in a fixed and permanent and in particular gas-tight manner. 
         [0114]    If the laser beam  170  is moved along all of the end faces  34 ,  54 ,  84  of the peripheral areas  32 ,  52  and  82  of the respective assembly group  12 , it is possible, by virtue of the overlapping impingement zones  176   1  to  176   n , for a continuous connection zone  162  to be formed which surrounds the end faces  34 ,  54 ,  84  of the whole assembly group  12  in a closed manner, thereby providing in particular a gas-tight connection of all of the peripheral areas  32 ,  52 ,  82  of the respective peripheral stack. 
         [0115]    The connection zone  162  represents a first element-to-element connection  200  for forming an assembly group  12 , whereas the connection of the assembly groups  12  with one another is effected by a second element-to-element connection  202  between the last functional element  26 , in the stacking direction S, of one assembly group  12   x  and the first functional element  22 , in the stacking direction S, of the next assembly group  12   x+1  by way of the solder layer  127  and the isolation layer  128 . 
         [0116]    Thus, the solution in accordance with the invention affords the possibility of interconnecting the functional elements  22 ,  24 ,  26  of the respective assembly group  12  in a permanent and gas-tight manner. 
         [0117]    Thus, this method may be used on all of the assembly groups  12  in order to thus provide for a simple and advantageous connection of the peripheral areas  32 ,  52 ,  82  in the respective peripheral stacks  130 . 
         [0118]    In a second exemplary embodiment of the electrochemical conversion device constructed in accordance with the invention, illustrated in  FIG. 9 , the second element-to-element connection  202 ′ is formed by a glass solder connection layer  204  which on the one hand is electrically isolating itself and on the other hand connects the connection side  126  of the compensating frame  86  directly with a support side  206  of the first functional element  22  that faces towards the connection side  126 . 
         [0119]    Apart from the above, the second exemplary embodiment is identical to the first exemplary embodiment; therefore, the same reference numerals are used in the second exemplary embodiment for parts that are the same as those illustrated in the first embodiment so that reference may be made to what has been described for the case of the first exemplary embodiment. 
         [0120]    The above-described method for making the first element-to-element connection  200  may thus be used on all of the assembly groups  12  in order to thus provide in the respective peripheral stack  130  a simple and advantageous connection of the peripheral areas  32 ,  52 ,  82  that is easy to repair also in the case of welding defects. 
         [0121]    In the manufacture of the fuel cell in accordance with  FIG. 1 , it would in principle be possible first to interconnect, for each of the individual assembly groups  12   1  to  12   n , the functional elements  22 ,  24 ,  26  at the peripheral areas  32 ,  52 ,  82  thereof by way of the first element-to-element connection  200 , followed in each case by connecting the compensating frame  86  of the one functional element  12   x  with the connection side  126  thereof to the next assembly group  12   x+1  in the stacking direction S by way of the second element-to-element connection  202  comprising the solder layer  127  and the isolation layer  128  of the tray element  22 , as described for the first exemplary embodiment, or, as described for the second exemplary embodiment, to provide for a connection using the glass solder connection layer  204  instead of the connection between the solder layer  127  and the isolation layer  128 . 
         [0122]    However, a particularly advantageous embodiment of the method in accordance with the invention as illustrated in the flow chart of  FIG. 10  provides, as a first step  212 , prior to making the first element-to-element connection  200  between the peripheral areas  32 ,  52 ,  82  of the individual functional elements  22 ,  24 ,  26 , for making the second element-to-element connection  202  between the third functional elements  26  of a respective assembly group  12   x  that are to be used in the fuel cell and the corresponding first functional elements  22  of the respective next assembly group  12   x+1 . 
         [0123]    This is followed, as shown in  FIG. 10 , by a functional test  214  in the form of a pressure test of the second element-to-element connection  202  between the third functional elements  26  and the first functional elements  22  and a conductivity test of the second element-to-element connection  202  between the first functional elements  22  and the third functional elements  26 , wherein the pressure test and the conductivity test may be performed in any order, i.e. the conductivity test may be performed first and then the pressure test or, conversely, the pressure test may be performed first and then the conductivity test, or the two tests may be performed at the same time. 
         [0124]    The advantage of this solution is seen in that it allows the second element-to-element connection  202  between the third functional element  26  and the corresponding first functional element  22 , which is technically difficult to perform and which, while it must be pressure-resistant on the one hand, must not be electrically conductive on the other hand, to be made first so that here if the connection is found not to be pressure-resistant or found to be conductive, the interconnected functional elements  26 ,  22  can be considered as reject parts and precluded from use. 
         [0125]    The next step involves stacking  216  the functional elements  22 ,  24 ,  26  of the first assembly group  12   1  or of all of the assembly groups  12   1  to  12   n  simultaneously. 
         [0126]    Next, in a further step  218 , the respective functional elements  22 ,  24 ,  26  of the assembly groups  12  are interconnected by making the first element-to-element connection  200  at the peripheral areas  32 ,  52 ,  82  thereof in the manner described above. 
         [0127]    In making the first element-to-element connection  200  at the peripheral areas  32 ,  52 ,  82  of the respective functional elements  22 ,  24 ,  26 , there are also further possibilities for proceeding. 
         [0128]    For example, after stacking  216  the functional elements  22 ,  24 ,  26  of the first assembly group  12   1 , wherein the compensating frame  86  is already connected to the tray element  22 , the first element-to-element connection  200  at the peripheral areas  32 ,  52 ,  82  of the first assembly group  12   1  is made, this being followed, prior to stacking  216  the further functional elements  24 ,  26  of the second assembly group  12   2 , by a pressure test of the first assembly group  12   1  along with the tray element  22  of the next assembly group  12   2  connected thereto. 
         [0129]    If a leak is detected after making the first element-to-element connection  200  at the peripheral areas  32 ,  52 ,  82 , then, once the leak is localized, the connection zone  162  can be re-worked, for example re-welded, at the leak location before proceeding to the steps of stacking  216  and making the first element-to-element connection  200  of the peripheral areas  32 ,  52 ,  82  between the functional elements  24 ,  26  and the functional element  22  of the second assembly group  12   2 . 
         [0130]    Thus, when the first element-to-element connections  200  of the assembly groups  12   1-n  are made successively, it is possible for each of the first element-to-element connections at the peripheral areas  32 ,  52 ,  82  of each individual assembly group  12   x  to be tested for tightness and, if required, reworked. 
         [0131]    Therefore, the advantage of this solution is on the one hand that there is the possibility of having the technically critical second element-to-element connection  202  between the functional element  26  and the functional element  22  made first, then having it tested extensively for its functions such as tightness and isolation and only after that having the technically simpler first element-to-element connection  200  at the peripheral areas  32 ,  52  and  82  made and, if found to be defective, reworked.