Abstract:
The invention relates to a fuel cell stack composed of at least one fuel cell ( 1 ) and at least two separator structures ( 2, 2 ′). Said separator structures ( 2, 2 ′) are open on at least one side towards the exterior in order to allow passive exchange of air. Also, said separator structures comprise a channel system ( 53, 53 ′) for guiding fuel. The fuel cell can be embodied as a bi-fuel cell ( 1 ) composed of two electric cells. The anodes or cathodes of the two electric cells are arranged opposite each other.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a U.S. National Phase of the International Application No. PCT/EP2008/001205 designating the U.S., filed Feb. 12, 2008 and published in German on Aug. 21, 2008 as WO 2008/098791, which claims priority to German Patent Application No. DE 10 2007 007 704.3, filed Feb. 12, 2007. 
     FIELD OF THE INVENTION 
     The invention relates to a fuel cell stack with a lightweight construction, which has at least one fuel cell and at least two separators. The separator structures are accordingly optimised in order to enable as good as possible passive ventilation of the fuel cell and to be as lightweight as possible, and also at the same time to guide out the ion products recombined on the cathode. In particular, the fuel cell can be configured as a bi-fuel cell. 
     BACKGROUND OF THE INVENTION 
     Fuel cells are suitable for obtaining electrical energy from chemical energy carriers without thereby being subjected to the restrictions of the Carnot cycle. In order to increase the power of fuel cells, these are often disposed in so-called fuel cell stacks in which a plurality of fuel cells are connected together adjacently or stacked and thus a higher total power can be achieved. Normally, the fuel cell thereby comprises two electrodes which are separated from each other by a membrane or an ion conductor. The anode is subjected to a flow of fuel which is oxidised there. The oxidised positively charged ions migrate through an electrolyte membrane towards the cathode side where they are recombined with the reduced oxidant, such as e.g. air, and are discharged from the fuel cell. 
     In the construction of modern fuel cell stacks, care must be taken in particular to reduce the size and the weight of the fuel cell stacks so that these can be used in applications where, above all, miniaturisation and weight are to the fore. These are e.g. portable electronic devices or technical-medical devices which are portable on the body or small flying devices. 
     In U.S. Pat. No. 6,986,961 B1, a fuel cell stack is described, which is constructed from individual fuel cells which follow the standard construction with bipolar plates in which anode and cathode alternate. In this construction, the current flows vertically through the stack so that all the components must be electrically conductive. This is achieved by undulating metal sheets which are embedded in a frame comprising glass fibre-reinforced plastic and are sealed, on the anode side, with a sealing frame. Although the construction with an undulating metal sheet is lighter than production with conventional bipolar plates, a fuel cell stack of this type still has a high weight because of the stainless steel undulating sheets, the high number of components and the high sealing complexity and also the ratio of height to spacing of the undulations which are relatively firmly prescribed by the undulating metal sheet. As a result, the ventilation properties of the separators are restricted. 
     A further development is represented by the so-called bi-fuel cells. Here, two cells which are electrically insulated from each other are combined such that a fuel cell stack constructed from these cells is constructed in the sequence of cathode, membrane, anode, anode, membrane, cathode or anode, membrane, cathode, cathode, membrane, anode. In US 2005/0 026 021 A1, a stack in the bi-fuel cell type of construction is described, which again loses the saving in weight because of the bi-fuel cell type of construction as a result of solid separator structures and the immission in the fuel. Furthermore, no satisfactory passive ventilation and satisfactory transport away of the reactands is made possible because of the type of construction of the separator structure. 
     SUMMARY OF THE INVENTION 
     It is therefore the object underlying the invention to produce a fuel cell stack with a lightweight construction, in which as light a weight as possible is present and in which the separator structures are constructed such that they enable good passive ventilation of the fuel cells and, at the same time, good transport away of the reactands can take place. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     This object is achieved according to the invention by a fuel cell stack according to claim  1 . 
     The fuel cell stack hereby comprises at least one fuel cell and at least two separator structures, respectively one separator structure being disposed on respectively one of the two oppositely situated sides of the fuel cell. The construction of a fuel cell is provided by a sequence of anode current conductor, membrane electrode unit and cathode current conductor. Connected to the side of the cathode current conductor which is orientated away from the membrane electrode assembly is a separator structure which serves for the purpose of guiding an oxidant, such as e.g. air, to the cathode, the separator structure according to the invention being distinguished in that in is open to the environment on at least one side in order thus to enable a passive exchange of air of the fuel cell with its surroundings. Furthermore, the separator structure, in the edge region thereof, has a closed channel system for fuel with the intent that the region of the separator for the passive exchange of air and the channel system are configured separately from each other. Furthermore, the contact pressure for operation of the fuel cell can be transmitted via the separator structure. 
     Advantageous developments of the fuel cell stack are described in the dependent claims. 
     An advantageous development of the invention is the use of bi-fuel cells. A bi-fuel cell hereby has at least one anode or cathode and at least two membrane electrode assemblies and also two cathodes or anodes. The sequence is such that, viewed in a cross-section from left to right, a cathode current conductor, a membrane electron assembly, an anode current conductor, a membrane electrode assembly and a cathode current conductor describe the minimum elements of a bi-fuel cell. Consequently, two electrical cells are formed in one bi-fuel cell, which electrical cells are separated at least in parts by the anode current conductor. When using bi-fuel cells, respectively one separator structure is disposed on one of the two oppositely situated sides of the bi-fuel cell. The advantage of the fuel cell stack described here resides in the fact that there is a saving in weight because of the bi-fuel cell construction and, at the same time, a passive exchange of air with the environment can take place via the separator structure so that a further saving in constructional elements is produced. 
     An advantageous development of the invention is that the fuel cell has a closed anode region. The anode region is thereby sealed relative to the environment with seals. This means that the anode current conductor or the membrane electrode assemblies have seals so that, between the at least one membrane electrode assemblies of an individual fuel cell or the two membrane electrode assemblies of one bi-fuel cell, a closed region is produced. In the case of using bi-fuel cells, the advantage resides in the fact that a fuel cell distribution structure can be used equally for two adjacent anodes and hence the number of terminals is halved and also the pressure drop is reduced and the fuel is restricted to the narrow region between the membrane electrode assemblies. When using normal fuel cells, the just-mentioned advantage is exploited. As a result, a further saving in weight is produced. This is advantageous in addition since the individual fuel cell can be tested thus individually for the functionality thereof, before incorporation in the fuel cell stack. Consequently, the possibility is avoided of checking the functionality of the fuel cell stack only by means of the total arrangement of the fuel cell stack. As a result, a further advantage is produced in the case of any possible fault analysis. 
     A further advantageous development of the invention is that the fuel cell has a non-active edge region. This means that the fuel cell has a region which is not covered by the membrane electrode assemblies. As a result, no energy can be obtained in this region since both electrons and ions are conducted into this region. In conjunction with the advantageous development that the fuel cell has a closed anode region, neither an electron exchange nor an ion exchange between the anode- and cathode side takes place in this non-active region. Furthermore, the edge region which forms the non-active region, in comparison with the active region, has an additional metallisation. The advantage of the additional metallisation in conjunction with the non-active edge region resides in the fact that the ohmic resistance of the current conductors is reduced and, at the same time, cooling of the fuel cells is possible. By means of cooling of the fuel cell of this type, a higher power density and hence a lighter construction of the fuel cell stack can be achieved. 
     It is hereby particularly advantageous if the additional metallisation protrudes beyond the separator structure. As a result, the metallisation takes over in addition cooling of the fuel cell and acts like a cooling rib arrangement. The cooling ribs can protrude for example in the central region of the fuel cell further beyond the separator structure since the fuel cell is warmest in the centre and thus a uniform temperature distribution within the fuel cell is made possible. 
     A further advantageous development of the invention is if the bi-fuel cell has a solid, at least one-layer, fuel distributor structure, at least one layer being configured as anode current conductor. The advantage of a fuel distributor structure resides in the fact that the fuel can be distributed better over the entire anode. 
     A further advantageous development of the invention is if the fuel distributor structure, in the transverse direction, has a microflow field with depressions or raised portions on at least one side for better distribution of the fuel. The advantage resides in the fact that, in addition to a rough distribution of the fuel to the active region, a very much finer distribution of the fuel can be achieved with the help of the microflow field. It is hereby advantageous in particular if the fuel distributor layer has a separate fuel inlet- and fuel outlet system. In the case of a bi-fuel cell, respectively one microflow field for each electrical cell is applied on both sides of the fuel distributor structure. 
     A further advantageous development is if the microflow field has separate fuel inlet- and fuel outlet openings since better circulation of the fuel is thus achieved. 
     A further advantageous development of the invention is if the cathode current conductors have openings with a large opening ratio. As a result, sufficient supply of the cathode with air is ensured with the help of the separator structure. As openings, squares or circles are formed so that the cathode current conductor is configured as a continuous surface with large holes. The configuration as a regular grating is particularly advantageous. It is possible to configure the cathode current conductor as a foil and to apply a plurality of grating-shaped cathode current conductors, layered one above the other, on the membrane electrode assembly. The advantage of this arrangement resides in the fact that a further saving in weight can be achieved due to the grating-like arrangement and the foil construction. Furthermore, it is advantageous if the cathode current conductor grating is produced from a porous material in order thus to achieve a maximum reaction surface for the reduction of the oxidant. 
     A further advantageous development of the invention is that the separator structure has a carrier structure and, in the transverse direction relative to this carrier structure, has further additional elements on at least one side. The channels of the separator structure are hereby formed at least in parts by the intermediate space of the additional elements relative to each other. The advantage resides in the fact that a separator structure is produced with low material complexity, which separator structure, via the arrangement of the additional elements, can have a large opening ratio of the channels for supplying an oxidant and for discharging recombined ion products. As a result of the arrangement of the additional elements, as a function of the fuel and the oxidant, an air supply with natural convection can hereby be achieved. It is also possible that a fan with only a low pressure drop blows air into the channels with a large opening. The fan thereby requires only a small amount of power. 
     In order to make possible an advantageous construction of the fuel cell stack, it is advantageous furthermore that the separator structure has additional elements in both transverse directions of the carrier structure. Consequently, there results a sequence of separator, fuel cell, separator, fuel cell, separator. In the case of bi-fuel cells, two cathodes which belong to different bi-fuel cells and are separated merely by one separator structure can share a single air-supplying separator structure, which results in an additional saving in weight. 
     An additional advantage is produced if the carrier structure of the separator structure is configured at least partially as a grating or as struts with struts protruding therefrom since, as a result, a further saving in weight can be achieved. 
     A further advantageous development of the invention is if the additional elements of the carrier structure of the separator structure are configured as plates or columns or pins. The advantage results in that different channel geometries can be achieved. Thus meandering air channels can be produced for example by plates or a very large air supply region can be produced by columns or pins. This improves the supply of an oxidant by means of passive exchange of air. 
     A further advantageous development of the invention is if the spacings of the additional elements of the separator structure relative to each other are essentially equal to the spacings of the grating points of the cathode. The advantage resides in the fact that the end points of the additional elements during assembly are placed on the intersection points of the cathode current conductor grating such that the contact pressure required for operation of the fuel cell, or of the bi-fuel cell, can consequently be achieved solely on the basis of pressing on the separator structure. Such a separator structure can therefore produce an improved air supply and a suitable contact pressure for operation of the fuel cell in this weight-saving manner It is hereby advantageous if the separator structure is manufactured to be stable relative to pressure so that sufficiently high contact pressures can be achieved. The separator structure thereby has a mechanical minimum rigidity which is provided by the material properties and the construction so that contact pressures of up to 100 N/cm^2, typically between 10 N/cm^2 up to 50 N/cm^2, can be transmitted between individual fuel cells without individual additional elements of the separator structure being damaged or bending, or the carrier structure being destroyed. 
     As suitable advantageous materials for the separator structure, it is advantageous to use plastic materials or resin-saturated plastic knitted fabrics, and also composite materials reinforced with glass fibres or carbon fibres or nanotubes. It can also be advantageous to configure the additional elements by means of specially formed undulating metal sheets on a carrier structure produced from plastic material. 
     A further advantageous development of the invention is that the separator structure has additional electrical contactings in the edge region. Furthermore, it is hereby advantageous if the contactings through the edge region of the separator structure are connected via electrical supply lines. In this way, the electrical energy which is produced in the fuel cell or in the individual fuel cells of the bi-fuel cell can be conducted to the exterior via the edge region of the fuel cell and the edge region of the separator structure where it can be used by the most varied of electrical applications. 
     A further advantageous development of the invention is to configure the separator structure as edge element. It is hereby sensible to configure the carrier structure as a closed plate so that the separator can serve as part of a housing and various points exist in order to transmit the contact pressure via the edge elements and the separator structures to all fuel cells present in the fuel cell stack. 
     A further advantageous development of the invention is if all components of the fuel cell are produced from plastic material or plastic composite materials or plastic materials with thin metallisations. By using lightweight plastic materials, a further saving in weight is produced. 
     An advantageous development of the fuel cell stack is if the individual fuel cells use hydrogen or methanol or ethanol as fuel. 
     Furthermore, it is an advantageous development if the volume of the fuel cell stack is between 1 cm 3  and 500 cm 3 . 
     A further advantageous development of the invention is that the electrical cells of the bi-fuel cells or the individual bi-fuel cells are connected to each other in series or in parallel. In the case where the individual cells are connected in parallel, the construction of the bi-fuel cell can be produced almost completely from electrically conductive material. The two anodes and the two cathodes of the electrical individual cells of the bi-fuel cell are thereby connected via current conductors. The only insulation exists therefore between the anode and the cathode and is produced respectively by the electrically insulating membrane of the membrane electrode assembly. In the case of a connection of the electrical cells in series, the two electrical cells must be extensively insulated from each other. This means that at least one layer of the fuel distributor structure must have an insulating configuration. The advantage is produced therefrom that, because of the different connection modes, the fuel cell stack can be adapted to the requirements of the application connected thereto. In the case of using single fuel cells, the process can take place analogously thereto. 
     By connecting together a plurality of fuel cell stacks, a high total battery voltage can be produced easily or a robust total arrangement can be produced. The application to be operated by the fuel cell hereby determines whether a series connection or a parallel connection or a combination of both connections is most sensible. 
     A fuel cell stack according to the invention can be used in particular wherever a lightweight, reliable current supply is required, as is required for example for flying devices, such as a drone or in model construction. The fuel cell stack arrangement can be used both as drive voltage for operating a motor and as operating voltage for devices or circuits used in the flying device. 
     Also portable devices, such as laptops or mobile phones or medical devices carried on or in the body, can be operated with a fuel cell stack described here. 
     Further advantageous developments of the invention are described in the coordinated claims of the invention. 
    
    
     
       Further features and advantages of the invention become clear in the subsequent description of a fuel cell stack with bi-fuel cells, with reference to the Figures. There are shown: 
         FIGS. 1   a ,  1   b ,  1   c  construction of fuel cell stacks comprising bi-fuel cells and separator structures with edge elements, 
         FIGS. 2   a  and  2   b  construction of a bi-fuel cell, 
         FIGS. 3   a  and  3   b  construction of the fuel distributor layer of the bi-fuel cell, 
         FIGS. 4   a  and  4   b  construction of the microflow fields in the bi-fuel cell, 
         FIGS. 5   a  and  5   b  metallisations of the end regions of the bi-fuel cell, 
         FIG. 6  construction of a cathode current conductor, 
         FIGS. 7   a  to  7   g  construction and embodiments of the separator structure, 
         FIGS. 8   a  to  8   d  connection of the bi-fuel cells in series, 
         FIGS. 9   a  to  9   c  connection of the bi-fuel cells in parallel. 
     
    
    
     In  FIG. 1   a , the basic mode of construction of a fuel cell stack with bi-fuel cells and a separator structure according to the invention is explained. The bi-fuel cells  1 ,  1 ′—each taken per se—have two electrical cells. Between the two bi-fuel cells  1 ,  1 ′ is situated a separator structure  2 ′ which separates the right fuel cell of the bi-fuel cell  1  from the non-visible left fuel cell of the bi-fuel cell  1 ′. In the stack construction, the bi-fuel cells  1 ,  1 ′ are disposed between separators  2 ,  2 ′,  2 ″ and completed by further bi-fuel cells and separator structures on the left and right of the elements  2  and  2 ″. 
     In  FIG. 1   b , a similar stack is represented in the X-Z plane. In the stack there are situated four bi-fuel cells  1 , l′,  1 ″,  1 ′″ which respectively are embedded on the right and left by separator structures. The separator structures configured on the left edge and on the right edge are hereby configured as edge elements  3 ,  3 ′ via which the contact pressure for the bi-fuel cells  1 ,  1 ′,  1 ″  1 ′″ is produced. The contact pressure is thereby conveyed further via the separator structures  2 ,  2 ′,  2 ″ to the respectively nearest bi-fuel cell. The advantage of the stack construction resides in the fact that the contact pressure for a large number of fuel cells can be applied at the same time. At the same time, a further saving in weight is achieved in that the bi-fuel cells  1 ,  1 ′,  1 ″  1 ′″ are used, which further reduce the material consumption. In the intermediate spaces of the separator structures  2 ,  2 ′,  2 ″ or of the edge elements  3 ,  3 ′, no fuel is situated, but rather they serve inter alia to ensure a passive air supply to the fuel cells. This becomes obvious also from  FIG. 1   a  in which it is shown that the separator structures  2 ,  2 ′,  2 ″ are open on both sides in the X-direction, which enables a passive exchange of air with the environment. Further features illustrated here of the separator structure and the bi-fuel cells are dealt with in the further Figures. 
     In  FIG. 1   c , a similar embodiment is represented, no end plates being present for the sake of clarity. In a real embodiment, the end plates are present. The fuel cell stack has a large number of bi-fuel cells  1 ,  1 ′,  1 ″  1 ′″, the bi-fuel cells being at a spacing from each other by respectively one separator structure  200 ′,  200 ″,  200 ′″. The illustrated spacing between the bi-fuel cells and the separator structures is present merely for more clarity: in the operating state, the separator structures are situated on the bi-fuel cells in such a manner that the circumferential frame of the separator structure is situated in the outer, non-active region of the fuel cell in which also the seal of the respective MEA of the bi-fuel cell is situated. In this way, a uniform contact pressure can be exerted on the seals. The separator structures  200 ,  200 ″″ have a smaller cross-section in the z-direction since only one fuel cell need be supplied, whilst the separator structures  200 ′,  200 ″,  200 ′″ respectively must supply a fuel cell in the z-direction at the top and bottom. The separator structure  200 ′ or  200  is dealt with more precisely in  FIG. 7   g.    
     In  FIG. 2   a , the construction of a bi-fuel cell used here is represented. The bi-fuel cell hereby comprises a central fuel distributor layer  10  on which respectively one microflow field  20 ,  20 ′ is applied on both sides in the Z-direction. Connected to the microflow field  20  or  20 ′ there is situated the membrane electrode assembly (MEA)  30  or  30 ′, the MEA  30  and the MEA  30 ′ being connected via seals  300  and  300 ′ to the microflow fields  20  and  20 ′ such that a closed inner space is produced between the two MEAs  30 ,  30 ′. Since one of the layers,  10 ,  20 ,  20 ′ is configured as anode current conductor, the anode space has hence a closed configuration. Connected respectively to the MEA  30  or  30 ′ is a cathode current conductor  40 ,  40 ′. The MEAs  30 ,  30 ′ can, on the electrode layers of anode and cathode, have additional gas diffusion layers which are however not illustrated in  FIG. 2   a . Furthermore, the bi-fuel cell  1  has openings  13 ,  13 ′ both for the fuel inflow and the fuel outflow and holes  17 ,  17 ′ which can be configured with an electrical conductor for the electrical contacting of different cells. From the mode of construction of the bi-fuel cell  1  in  FIG. 2   a , it can be detected clearly that two separate electrical cells are present, the fuel distributor layer  10 , both the electrical cell comprising the elements  20 ,  30 ,  40  and the electrical cell comprising the elements  20 ′,  30 ′,  40 ′ being supplied with fuel. One feature of the bi-fuel cell is that almost all the layers, i.e. the fuel distributor layer, the microflow fields  20 ,  20 ′ and the cathode current conductors  40 ,  40 ′, can be produced from plastic material or metallically coated plastic material or plastic woven fabric. In order to be able to produce the bi-fuel cell in a very thin embodiment, it is sensible to produce the microflow fields  20 ,  20 ′ and also the cathode current conductors  40 ,  40 ′ in a foil construction. Irrespective of the structure of the bi-fuel cell  1  shown here, some of the illustrated features in  FIG. 2   a  are not absolutely necessary for the operation of such a bi-fuel cell. 
     It is possible to dispense with the microflow fields  20 ,  20 ′. When these are dispensed with, parts of the fuel distributor layer  10  must be configured as anode current conductor, it depending here upon the subsequent connection whether the fuel distributor layer can be electrically conductive as a whole or whether only the upper layers of the distributor layer  10  or the lower layers of the distributor layer  10  are metallised and whether the actual fuel distributor layer has an insulating configuration. The first case is tantamount to saying that the two electrical cells of the bi-fuel cell share an anode current conductor. The anode current conductor is hereby produced as a single anode current conductor in the fuel distributor layer  10 . In the second case, the two electrical cells have two anode current conductors which are insulated from each other, the anode current conductors respectively being placed on the upper side of the fuel distributor layer or on the underside of the fuel distributor layer, the fuel distributor layer per se having an insulating configuration. 
     In the case where the two electrical cells of the bi-fuel cell  1  have a common anode current conductor, the two different electrical cells of the bi-fuel cell  1  cannot be connected in series. 
     In  FIG. 2   b , an alternative construction of a fuel cell  90  is represented. The fuel cell  90  is a simple fuel cell but can also describe the construction of a bi-fuel cell by mirroring the individual layers. The fuel cell  90  has an anode flow field  10 ′ which comprises a metallised plastic material. The metallisation is applied as a metal grating. Alternatively, the anode flow field itself can have an electrically conductive configuration. A gas diffusion layer  201  is disposed between the anode flow field  10 ′ and the MEA  30 ″. A further gas diffusion layer  202  is disposed between the MEA  30 ″ and the current conductor grating  40 ″. The MEA  30 ″ is sealed at the edge with a seal or adhesive so that a gas-tight anode space is produced between the MEA  30 ″ and the anode flow field  10 ′. 
     The gas diffusion layers can be used alternatively to form a fine structuring of the anode flow field  10  of  FIG. 2   a . Likewise, the gas diffusion layers are sensible in the case of a fine (or even missing) structuring if the surface of the fuel cell grows in the xy-plane. 
     In  FIGS. 3   a  and  3   b , the fuel distributor layer  10  illustrated here is explained in more detail. The fuel distributor layer  10  has fuel inflow and outflow holes  13 ,  13 ′, a channel structure  14  and schematically illustrated channels  16 . 
     In  FIG. 3   b , the channel structure  14  is characterised in that it is connected to the inflow hole  13  via a main channel  15  with the channel arms  16 . An analogous structure  13 ′,  14 ′,  15 ′ and  16 ′ is responsible for the fuel outflow. The holes  17  are configured for guiding through electrical contactings. The portions  18  which protrude respectively beyond the active region of the bi-fuel cell are of particular importance in order to be able to achieve a better lateral current conduction and cooling of the bi-fuel cell there. 
     In  FIG. 4   a , the fuel distributor layer  10  is connected respectively in the X-direction on both sides to a microflow field  20 . In the microflow fields  20 ,  20 ′, electrical contactings  17  are also present, which overlap the electrical contactings  17  of the fuel distributor layer. The microflow fields  20 ,  20 ′ can hereby be separately produced substrates or foils which are applied on the fuel distributor layer  10 . Equally however, they can also be produced jointly during the production of the fuel distributor layer  10 . In a simple case, the structure shown in  FIG. 3  comprises fuel distributor layer  10  and microflow fields  20 ,  20 ′ merely made up of two substrates which are connected to each other in a gas-tight manner: one which contains the fuel distributor layer internally and the flow fields  20 ,  20 ′ externally and one with a smooth inside and flow field  20  outside. In the case where the substrates are produced from an insulator, then a metallisation is applied on the flow fields over the entire surface for the current conduction. This can be effected by chemical coating, sputtering or sputtering and electroplating. Corrosion-stable layers are for example Ag, or TiW—Au, NiCr—Au. Furthermore, thin steel or aluminium foils are conceivable as current conductors, which are provided with an electrically conductive organic protective layer. 
       FIG. 4   b  is an individual microflow field  20  represented in the X-Y plane. The microflow field is fitted on the fuel distributor layer  10  in such a manner that the holes  21  coincide with the end points of the channels  16  of the fuel distributor structure. This means that the fuel, which enters through the hole  13  into the fuel distributor layer and is transported via the channel system  15  and  16 , emerges at the microflow fields through the holes  21 . The flow field structure is thereby produced by webs  22  which are disposed at a specific angle relative to the fuel distributor structure  10 . However, other structures are also possible for the elements  22 , such as e.g. meanders which connect the upper openings  21  to the lower openings  21 ′. Furthermore, a seal  300  which serves to seal the anode space by the membrane electrode assembly  40  is visible at the edge. 
     In  FIG. 5 , the metallisation of an anode current conductor is represented, as it is produced here by the flow field  20 . The metal surface  100  covers the entire active region of one of the electrical cells of the bi-fuel cell, the active region  25  of the fuel cell in  FIG. 5  being formed by the surface which is covered by the microflow field  20  with the seal  300 . Furthermore, the metallisation is extended into the regions  18  and  18 ′ of the fuel distributor structure. The metal surface  100  represents the negative pole. In addition to the metallisation regions  100 , two further metallisations  110  which are advantageously connected to the cathode to be applied subsequently are applied. Advantageously, the structuring of the metal surfaces  100 ,  110  can be effected most simply subtractively by laser ablation. 
     In  FIG. 5   b , a further reinforced metallisation  120  is applied but only outwith the active anode region  25 . By means of the additional metallisation of the edge region, the ohmic losses by the current conduction to the current collectors on the narrow sides along the Y-direction of the metallisation  120  can be reduced. Furthermore, the heat dissipation of the bi-fuel cell is improved by the additional metallisation  120 . As a result, it is possible to operate the bi-fuel cell with a higher power density, which affects the fuels with which the bi-fuel cell can be operated. Since the additional metallisation  120  is applied in the non-active region of the bi-fuel cell and is separated from the anode space by the seal  300 , the corrosion problems which occur in the active anode space, can be reduced and ordinary metals, such as copper, aluminium or nickel, can be used for the metallisations. The layers can hereby be applied by chemical or electroplating reinforcement or by applying electrically conductive metal foils connected by electrically conductive adhesives. 
     In  FIG. 6 , the cathode current conductor  40  is represented. This comprises a grating-shaped structure. The grating-shaped structure has as large an opening ratio as possible so that oxygen can be transported to the cathode unimpeded. A development of the current conductors  40  or  40 ′ is achieved in that a plurality of grating structures is situated one above the other, which form a mechanically stable and simultaneously finely structured pattern on the cathodes. This development is reinforced in addition in that the grating structure is configured as a porous grating structure. Because of the grating-shaped configuration of the current conductor foil, the contact pressure can be uniformly distributed and the ohmic losses minimised. Advantageously, the metallisation of the cathode comprises the same materials or layers as that of the anode. Corrosion problems are avoided by such a configuration. In principle, it is however also possible to use different materials assuming that the material properties thereof are adequate for the role of cathode. 
     Analogously to the anode side, an additional metallisation  400  is situated on the current conductor  40  of the cathode which is disposed outwith the active region of the bi-fuel cell. The active region of the bi-fuel cell should thereby be seen as the equivalent of the sealed region of the anode side. 
     The reinforced metallisation  400 , just like the additional metallisation  120  on the anode side, has the role of minimising the ohmic loses and consequently of achieving a better energy yield. This is necessary in that, from each individual cell, the current is conducted firstly laterally to the exterior and only there is the connection to the other cells of the fuel cell stack effected. Normally the current flows vertically through the fuel cell stack and, for this purpose, has available the entire cross-section of the fuel cell stack or of the membrane. 
     Furthermore, contacting to the substrate terminals  110  can be produced via the reinforced region  400  by an additional contact surface  410 . In order to produce the additional metallisation, chemical or electroplating reinforcements or also electrically conductive metal foils can be used. 
     The bi-fuel cell represented in  FIGS. 2 ,  3 ,  4 ,  5  and  6  has a closed anode space and an additional metallisation outwith the active region both on the anode- and on the cathode side of the individual electrical cell of the bi-fuel cell which has a very low weight in total because of the materials since most substrates can be configured as foils or lightweight plastic material parts. Furthermore, because of the low thickness of the individual bi-fuel cells, the fuel can be discharged into the anode space finely metered. This can also take place without microflow fields  20 ,  20 ′. 
     An essential component of the arrangements in  FIGS. 1   a  and  1   b  are the separator structures in  2 ,  2 ′,  2 ″ which are open respectively on both sides in the Y-direction. These separator structures are explained in more detail in  FIGS. 7   a  to  f.    
     In  FIG. 7   a , a separator structure  2  is shown in the X-Z plane. This separator structure  2  comprises a carrier structure  50  which extends in the X-direction, by which additional elements  60 ,  60 ′ are attached in the z- or transverse direction, a channel  70  being configured between the additional elements  60 ,  60 ′. By means of the large opening ratio of the channels  70 , an air supply with natural convection can be effected. The edge region  51  of the separator structure  2  has both an electrical contacting or line  52  and a channel system  53  which is configured for the fuel supply to the bi-fuel cells. With the help of the electrical contactings  52 , different bi-fuel cells can be connected to each other electrically. 
     The carrier structure  50  can be configured both by a continuous plate and, at least partially or totally, as a grating. In the illustration shown in  FIG. 1   a , the carrier structure  50  is configured as a grating. Likewise, a carrier structure  50  is possible in which a central strut connects the edge regions  51  and  51 ′ together and transverse struts emanate from this edge strut in the Y-direction, on which transverse struts the additional elements  60  or  60 ′ are fitted in turn in the Z-direction. As a result, it is possible that an individual separator structure subjects the oppositely situated cathodes of two different bi-fuel cells  1  and  1 ′ to a flow of air by natural convection. It is also possible that a fan with only a low pressure drop blows air into the channels with a large opening. 
     In  FIG. 1   a , the size ratios of the bi-fuel cells  1 ,  1 ′ and of the separator structures  2 ,  2 ′,  2 ″ can be detected. Since the separator structure serves for natural ventilation and transporting away of the recombined protons, the separator structure  2  is configured to be very much thicker in the z-direction than the bi-fuel cell  1 . In order to achieve a saving in weight, plastic material or plastic composite materials can be used as materials for the separator structure  2 . A configuration as a plastic woven fabric which is covered by a layer of resin can likewise increase the pressure stability which is required to produce the contact pressure of the bi-fuel cell  1  by the separators  2  and  2 ′. The separator structure  2  can thereby be produced in an injection moulding process. A further saving in weight is produced by configuring the carrier structure  50  as a grating. 
     In  FIG. 7   b , a further embodiment of the separator structure  2  is provided. The additional elements  60  are hereby formed by an undulating metal sheet. The undulations are formed thereby such that a large cross-section towards the cathode is produced and only a narrow unused cross-section is present on the carrier structure side. The undulating metal sheets can hereby be glued, welded or soldered to the carrier structure  50 . 
     A particularly lightweight embodiment of the separator structure  2  is provided in  FIG. 7   c . Here, the additional elements  60  are formed as knobs made of woven fabric or knitted fabric and are subsequently saturated with a synthetic resin. As a result, there is produced a very open-pore structure which is very light but, because of the existing widening of the knobs  60  towards the base of the knobs  60 , has very high mechanical rigidity and hence can transmit very high contact pressures. The knobs which can be configured also as webs, are hereby applied on the carrier structure  50 , the carrier structure  50 , as shown in  FIG. 7   d , being configured as a grating. The carrier structure  50  together with the additional elements  60  can thereby be manufactured in one piece. The additional elements  60  can however also be connected subsequently to the carrier structure  50 , for example with a synthetic resin. 
     In  FIG. 7   d , a plate  55  which seals the grating-shaped structure  50  on one side is visible in addition. Hence, the separator structure  2  illustrated here can be used as edge element  3 . For this purpose, the additional elements  60  are fitted only on one side of the plate  55 . 
     Further spatial representations of a separator structure according to the invention are represented in  FIGS. 7   e  and  7   f . In  FIG. 7   e , the additional elements  60  are configured as lamellae and, between two oppositely situated channels  70  and  70 ′, there is no direct connection. The exchange of gas is effected here passively, i.e. by natural convection and diffusion along the direction of the illustrated arrows. A single fan cannot undertake the ventilation of all the channels  70  and  70 ′ if the carrier structure  50  extends in the X-Y plane, as represented in  FIG. 1 . 
     In  FIG. 7   f , a particularly simple separator structure is illustrated. The carrier structure  50  is hereby formed by two pins, which connect numerous additional elements  60  to each other. There are consequently produced large continuous channels  70  which enable both a passive exchange of gas and ventilation by a fan since the air need be conducted only around the carrier structure. 
     According to the descriptions of various embodiments of the separator structure  2  in  FIG. 7 , finally the construction of the fuel cell stack in  FIG. 1   b  is explained once again. The pressing together of the fuel cell stack, which comprises the bi-fuel cells  1 ,  1 ′,  1 ″,  1 ′″, the bi-fuel cells being separated from each other by the separators  2 ,  2 ′ and  2 ″, is effected via the edge elements  3  and  3 ′. Since the electrical connection is effected via the electrical contactings of the bi-fuel cells  17  or the electrical contactings of the separator structures  52 , the contactings  52  of the separator structure  2  respectively being in connection with the contactings  17  of the bi-fuel cells  1  and  1 ′ and analogously thereto the fuel supply via the openings  13 ,  13 ′ of the bi-fuel cells and the openings  53 ,  53 ′ being effected laterally on the separator structures  2 ,  2 ′,  2 ″ and on the separators configured as edge elements  3  and  3 ′, the edge elements  3 ,  3 ′, apart from producing and transmitting the contact pressure to the bi-fuel cells  1 ,  1 ′,  1 ″,  1 ′″ and the separator structures  2 ,  2 ′,  2 ″, need have no further functionality. Similarly to the separator structures  2 ,  2 ′ and  2 ″, they can therefore be produced from lightweight constructional materials. 
     The separator structure  200 ′ is represented in  FIG. 7   g . The separator structure  200  differs merely as a result of the smaller cross-section as shown in  FIG. 1   c . The separator structure  200 ′ has a circumferential frame  65  which connects together the additional elements  60  configured as lamellae. The lamellae thereby extend in the yz-plane. The circumferential frame  65  is, at the edges  66  thereof situated in the x-direction, for sealing purposes continuous and closed. Between the individual elements  60 , webs  67  extend. 
     The frame  65  is configured in such a manner than the latter comes to be situated in the outer, non-active region of the fuel cell. As a result, for example the seals  300  of the bi-fuel cell which are visible in  FIG. 2   a  are covered so that a uniform contact pressure is exerted on the seals  300  or  300 ′. The separator structure  200 ′ or  200  is produced analogously to the separator structure  2  from lightweight constructional materials. 
     There can be used as materials of a low weight and high rigidity, foamed metals, composite materials, phenol resin-saturated woven fabric structures or plates with an octagonal column structure comprising saturated woven fabrics. Additional metal plates which receive thin tensioning screws or tensioning means, such as wires or cables, can be applied on these. The tensioning elements should hereby extend as closely as possible alongside the active region of the bi-fuel cells. Therefore it is favourable to provide surfaces  18  or  18 ′, which extend for cooling and current conduction beyond the active region of the bi-fuel cells, with holes through which the tensioning elements extend. 
     The edge elements  3 ,  3 ′ press the fuel cells stack together. The edge elements can be produced from sandwich-like plates which are filled with metal foam or a honeycomb structure and which are delimited respectively on one side by thin glass- or graphite fibre composite plates. 
     In a further embodiment of the fuel cell stack, it is possible that the fuel supplies  53  of the separators  2  are not used. In this case, the bi-fuel cells must be connected to extra hoses or tubes which are flexible in sections so that no fuel can emerge from the anode region. 
     It may be mentioned yet again at this point that the exchange of gas which is made possible by the separator structure and the transmitted or produced contact pressure is useful also when using normal fuel cells. Although a part of the saving in weight relative to bi-fuel cells is forfeited when using normal fuel cells, the saving in weight gained because of the separator structure is furthermore an advantage relative to the state of the art. 
     In order to explain the electrical connection of the bi-fuel cells of a fuel cell stack according to the invention to each other and of the individual cells of an individual bi-fuel cell to each other, the electrical connections of the bi-fuel cells in  FIGS. 8 and 9  are represented in series connection and in parallel connection. 
     In  FIG. 8   a , the metallisations  100   o  and  110   o  of the upper electrical cell and the metallisations  100   u  and  110   u  of the lower electrical cell of one bi-fuel cell are shown, the metallisations  100  being connected to an anode and the metallisations  110  to a cathode. 
     In order to connect the upper and the lower electrical cell in series, the anode of the upper electrical cell is connected to the cathode of the lower electrical cell via an electrical contacting  81 , as shown in  FIG. 8   b.    
     In  FIG. 8   b , the bi-fuel cells  1 ,  1 ′,  1 ″ are connected to each other in series. In addition to the contactings  81 , the anode of the lower electrical cell  100   u  of the bi-fuel cell  1  and the cathode of the upper electrical cell  110   o ′ of the bi-fuel cell  1 ′ are connected via an electrical contacting  82 . In  FIG. 8   b , the three bi-fuel cells  1 ,  1 ′,  1 ″, in total therefore 6 electrical individual cells, are connected together. 
     In  FIG. 8   c , it is shown how the connections  81 ,  82  are effected with the help of mechanical and flexible clamp elements  83 . The clamp elements  83 , on their inside, have electrical contactings in order to produce the corresponding connection between two oppositely situated anodes and cathodes of different electrical cells of one bi-fuel cell. In order to produce the connection between two anodes and cathodes of different electrical cells of different bi-fuel cells, the two clampings must comprise insulating material and be in connection conductively only via an additional electrically conductive connection  82 . The clampings can be configured in the form of a small flexible cable. It is achieved by this form of connection that the clamps transmit no forces between the bi-fuel cells. The contact pressure of the fuel cells is produced merely by the tensioning of the tensioning elements and transmitted between the individual bi-fuel cells via the separators. 
     In  FIG. 8   d , the electrical connection between the anodes and cathodes is produced not via clamps but via the electrical contactings  17  of the bi-fuel cell and the electrical contactings  52  in the edge region  53  of the separator structure  2 . These contactings can, as is normal in circuit board technology, be produced by internally metallised borings. The connection of the anode  100   u  to the electrical contacting  52  and the cathode  110   o ′ is thereby effected after assembly of the fuel cell stack by soldering or conductive adhesion. 
     A parallel connection of the bi-fuel cells to each other or of the electrical cells of an individual bi-fuel cell can be produced in a similar manner. This parallel connection can be advantageous in the small power field since the main consumers operate only at small voltages between 1 and 2 volts. Possibly, it is also convenient to connect a plurality of parallel-connected fuel cell stacks in series. As a result of the parallel connection of the bi-fuel cells in the stack, the total function of the fuel cell stack becomes more robust since an individual defective bi-fuel cell in fact reduces the power but the stack voltage does not collapse so quickly and pole reversal of the individual electrical cells does not take place. 
     In order to connect the individual electrical cells of an individual bi-fuel cell together in parallel, there are connected together the negative pole  100   o  to the negative pole  100   u  or the positive pole  110   o  to the positive pole  110   u . In  FIG. 9   b , the connection of different bi-fuel cells  1 ,  1 ′ and  1 ″ is achieved via clamping elements  83 . The connection via the clamping elements  83  can thereby have a detachable configuration or be non-detachable in that the clamps are fixed by gluing or soldering. 
     In  FIG. 9   c , the parallel connection of the individual cells is achieved via the electrical contactings  17  or  17 ′. The contacting  17  hereby connects the negative poles of the upper and of the lower electrical cell of the bi-fuel cell  1  and the contacting  17 ′ connects the positive pole of the upper and the positive pole of the lower cell of the bi-fuel cell  1 . Via the contactings  52  and  52 ′ in the end region of the separators  2 , the negative pole  100   u  of the bi-fuel cell  1  is connected together to the negative pole  100   o ′ of the bi-fuel cell  1 ′ via the connections  90  and, analogously thereto, the positive pole via the contacting  52 ′.