Abstract:
An energy storage and power generation system ( 10 ) with at least one fuel cell ( 40 ), in which an oxidant and a fuel which can be stored can be converted electrochemically and in the process an electrical current can be generated, and with at least one electrolysis cell ( 30 ), which is used for generating the fuel which can be stored for the fuel cell ( 40 ), wherein the fuel cell ( 40 ) and the electrolysis cell ( 30 ) share a common electrode ( 21 ), and the fuel cell ( 40 ) and the electrolysis cell ( 30 ) each use a respective further electrode ( 31, 41 ) only for themselves. 
     For this purpose, the invention provides that the fuel can be stored outside the fuel cell ( 40 ) and outside the electrolysis cell ( 30 ) in a store ( 13 ).

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The invention relates to an energy storage and power generation system with at least one fuel cell, in which an oxidant and a fuel which can be stored can be converted electrochemically and in the process an electrical current can be generated, and with at least one electrolysis cell, which is used for generating the fuel which can be stored for the fuel cell, wherein the fuel cell and the electrolysis cell share a common electrode, and the fuel cell and the electrolysis cell each use a respective further electrode only for themselves. 
         [0002]    As a result of an increasing proportion of electrical energy from wind or solar energy, the need to buffer-store energy is increased. A possible solution to this is hydrogen which can be generated by electrolysis in an electrolysis cell in the case of a supply of inexpensive and/or regenerative electrical energy which exceeds demand and which can be used for power generation in the case of a supply of inexpensive and/or regenerative electrical energy which falls below the demand, wherein the hydrogen, as a fuel, reacts electrochemically with an oxidant in a fuel cell. 
         [0003]    WO2005/008824 has disclosed an electrochemical system with an electrolysis cell and a fuel cell. In this case, in the electrolysis cell, water and a metal are first converted into oxygen and a metal hydride. The metal hydride is converted, by virtue of a change in temperature, into hydrogen and metal and the former is converted back into water by oxygen in the fuel cell. The electrode at which metal is converted into a metal hydride and back again is used jointly by the electrolysis cell and the fuel cell. In this case, the metal hydride acts as the only store for the hydrogen. One disadvantage with this is that only at most as much hydrogen is available for producing electrical power as there is metal available for reaction to form a metal hydride. In order to be able to store a large quantity of hydrogen, a correspondingly large and therefore heavy electrode needs to be provided which increases the weight and costs of the electrochemical system. In addition, the temperature of the fuel cell needs to be selected such that the metal hydride releases hydrogen. In addition, there is the risk of hydrogen embrittlement. 
       SUMMARY OF THE INVENTION 
       [0004]    The object of the invention is to provide an energy storage and power generation system which avoids the abovementioned disadvantages. In particular, it should be possible to store a large quantity of fuel and at the same time for the electrodes to have a light and/or space-saving design. The intention is for it to be possible for the temperature and the pressure in the fuel cell to be selected independently of hydrogen generation. 
         [0005]    The invention provides that the fuel can be stored outside the fuel cell and outside the electrolysis cell in a store. Spatially separating a basic unit of the energy storage and power generation system containing the fuel cell and the electrolysis cell from the store of the energy storage and power generation system increases the degrees of freedom of the energy storage and power generation system according to the invention. Thus, a correspondingly large store can be provided which ensures electrical power production over a long period of time, for example for days or months, without fuel again needing to be generated in the electrolysis. The energy storage and power generation system according to the invention can therefore replace or supplement pumped-storage power plants. It is even possible for the energy storage and power generation system according to the invention to store energy over a period of time which is longer than is conventional at present. 
         [0006]    The sufficiently large store also ensures that fuel is available at any time with a desired partial pressure. The basic unit does not need to have a store and can therefore be configured in a space-saving manner. Owing to the fact that large quantities of metal for the store-free electrodes are not required, the weight of the electrodes can be reduced. In this case, the basic unit can additionally be configured and operated in such a way as to correspond in the best possible way to desired temperature and pressure conditions in the fuel cell or to the transport of electrons, ions and reagents. Thus, the temperature and pressure conditions can be matched, for example, to a sensitive polymer-electrolyte membrane, a sensitive catalyst layer or an electrical power requirement without needing to take care to ensure that sufficient fuel is released. 
         [0007]    For example, the fuel used can be hydrogen which is stored in molecular form, in particular under pressure, in the store. Caverns can be used as the store. The hydrogen can be stored therein, for example at 80 bar. If already existing natural gas caverns are used as the store, said caverns are already available at low cost. 
         [0008]    While the fuel cell is used for electrical power generation and can therefore be considered to be a power generation system, the electrolysis cell together with the store can be considered to be the energy storage system, since the fuel generated in the electrolysis cell by electrical energy can be stored and, if required, electrical energy can be generated again. Therefore, in the electrolysis cell, electrical energy is converted into chemical energy which can be stored and, in the fuel cell, chemical energy is converted into electrical energy. The electrolysis generally always takes place when a supply of regenerative and/or inexpensive electrical energy generation exceeds the demand for electrical energy at the consumers. The fuel cell, on the other hand, is operated when the demand for electrical energy exceeds the energy generation from regenerative and/or inexpensive energy. A reserve in conventional power plants is therefore no longer required. 
         [0009]    DE 298 23 321 U1 has disclosed that the fuel cell and the electrolysis cell use both electrodes jointly. One disadvantage here is the fact that a catalyst of a cathode of the fuel cell at which an oxidant, in particular the oxygen, is reduced is insufficiently stable for being used as the catalyst of an anode of the electrolysis cell and for being able to generate the oxidant for the fuel cell. Such a use would result in considerable ageing of the cathode of the fuel cell. Therefore, the invention provides that in total three electrodes are used. For the abovementioned reasons, it is advantageous to form the electrodes at which oxygen is produced or converted, i.e. the cathode of the fuel cell and the anode of the electrolysis cell, in each case separately, with the result that the two oxygen electrodes can each have a different catalyst. The electrode at which hydrogen is produced or converted, on the other hand, can be used as the common electrode jointly by the fuel cell and the electrolysis cell. The common electrode therefore acts as anode of the fuel cell and as cathode of the electrolysis cell. This is possible since this electrode does not prematurely age as a result of the alternate use as anode or cathode. An embodiment in which the cathode of the fuel cell and the anode of the electrolysis cell are separate also enables dynamic operation of the energy storage and power generation system, in which it is possible to change quickly between electrolysis and fuel cell operation. As a result, an electrical buffer store can be dispensed with. The electrolysis cell and the fuel cell can also be operated simultaneously. 
         [0010]    The basic unit comprising the electrolysis cell and the fuel cell can be constructed from anode of the electrolysis cell, electrolyte layer of the electrolysis cell, common electrode, electrolyte layer of the fuel cell, cathode of the fuel cell, arranged next to one another in a row. The common electrode has a first side which points towards the anode of the electrolysis cell and a second side which points towards the cathode of the fuel cell. The thickness of the electrolyte layers is preferably less than 100 μm. The electrolyte layers can be acidic or alkaline, with the electrochemical reactions taking place at the electrodes of the basic unit as a function of said acidic or alkaline state, as shown in table 1. 
         [0000]    
       
         
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Electrolysis cell 
                 Fuel cell 
               
             
          
           
               
                   
                   
                 Common electrode 
                 Common electrode 
                   
               
               
                 Electrolyte 
                 Anode 
                 (cathode) 
                 (anode) 
                 Cathode 
               
               
                   
               
               
                 Acidic 
                 2H 2 O → O 2  + 4H +  + 4e −   
                 4H +  + 4e −  → 2H 2   
                 2H 2  → 4H +  + 4e −   
                 O 2  + 4H +  + 4e −  → 2H 2 O 
               
               
                 Alkaline 
                 4OH −  → O 2  + 2H 2 O + 4e −   
                 4H 2 O + 4e −  → 2H 2  + 4OH −   
                 2H 2  + 4OH −  → 4H 2 O + 4e −   
                 O 2  + 2H 2 O + 4e −  → 4OH −   
               
               
                   
               
             
          
         
       
     
         [0011]    At the common electrode, hydrogen which needs to be supplied to a catalyst of the common electrode is consumed during the fuel cell operation, and hydrogen which needs to be discharged by the catalyst is produced during the electrolysis. In the case of an alkaline electrolyte, water needs to be supplied to the catalyst for the electrolysis and water needs to be discharged from the common electrode during fuel cell operation. In order to save on materials and manufacturing costs, it is conceivable that supply and discharge paths in the common fuel cell can be used jointly both in the electrolysis and in the fuel cell operation. For this purpose, provision can be made for the common electrode to have a porous material, for example a foam or a mesh. The porous material contains pores through which the hydrogen can be transported. A region of the common electrode which has the porous material is referred to below as a porous region. The hydrogen can pass through the porous material both from the first side and towards the second side. In this case, catalysts can be arranged on the first and second sides. 
         [0012]    Preferably, however, only one catalyst of the common electrode can be used both for the electrolysis and for the fuel cell operation. The catalyst which can be used jointly can be located in the porous regions, in particular distributed over a total length, width and height of the regions. In this case, the porous material, for example a nickel foam, can itself act as catalyst. Alternatively, the catalyst is applied to the porous material. The porous material can be electrically conductive and therefore can perform the function of dissipating the electrical current from the common electrode. For ionic conduction, the catalyst which can be used jointly needs to be in contact with an electrolyte, wherein the electrolyte conducts ions both during the electrolysis and during the fuel cell operation. Therefore, the electrolyte likewise needs to be provided in the porous regions, in particular over the entire length, width and height of the regions. The electrolyte is preferably in the form of a solid or liquid film on the catalyst and will be referred to below as electrolyte film, in order to distinguish it from the electrolyte layers. Therefore, in this preferred embodiment, both jointly usable supply and discharge paths, a jointly usable catalyst and a jointly usable electrolyte film are provided for the fuel cell and the electrolysis cell. In order to be able to completely use the catalyst both in the electrolysis and during fuel cell operation, the thickness of the common electrode is preferably small, in particular less than 50 μm. 
         [0013]    The electrolyte film can be a liquid electrolyte which wets the catalyst layer in the porous common electrode. Alternatively, a solid electrolyte can be used as the electrolyte film. Examples of an acidic, liquid electrolyte film are sulfuric acid or phosphoric acid. An example of an alkaline, liquid electrolyte film is potassium hydroxide. A perfluorinated sulfonic acid membrane or a phosphoric acid-doped polybenzimidazole membrane can be used for an acidic solid electrolyte, and an anion exchanger membrane can be used for an alkaline solid electrolyte. Particularly preferred is the use of an alkaline, liquid electrolyte film. The same electrolyte is preferably used both in the electrolyte film and in the electrolyte layers which adjoin the first and second sides. In the case of a liquid electrolyte, a gas-impermeable, but ion-permeable layer, for example consisting of Nafion or porous PTFE, can be arranged between the common electrode and the electrolyte layers. 
         [0014]    The catalyst layer in the porous material firstly needs to be in contact with the electrolyte, but secondly sufficient hydrogen also needs to pass to the catalyst layer during fuel cell operation. In order to conduct more hydrogen into the porous layer, it may be that the common electrode has channels. The channels can vary in terms of their interface with the porous material. The harder it is for the hydrogen to reach the catalyst in comparison with the ionic conductivity of the electrolyte film, the larger the interface can be. 
         [0015]    The channels can extend over the entire width of the common electrode. In order to avoid hydrogen diffusion into the electrolyte layers which adjoin the first and second sides, the channels can have walls which are gas-tight and electrically conductive with respect to the first and second sides and preferably consist of metal. During the production process, the channels can follow, in terms of their physical dimensions, the form of salt bars, which are dissolved at the end of the production process. Alternatively, the channels can be surrounded by the porous material. Two halves of the common electrode with depressions can be used for producing these channels. In this case, the channels result if the two halves are adjoined to one another and the depressions complement one another so as to form the channels surrounded by the porous material. The channels are permeable to the hydrogen towards the porous material. When using a liquid electrolyte film, a gas-permeable, but liquid-impermeable film, for example consisting of PTFE, can be arranged between the channels and the porous material. 
         [0016]    The cathode of the fuel cell can be produced from an electrically conductive plate, in which channel-like cutouts for supplying the oxygen to the catalyst layer are located, with said catalyst layer being adjoined by a gas diffusion layer, for example consisting of carbon. Then, there is the catalyst layer which adjoins the electrolyte layer of the fuel cell. The boundary between the electrolyte layer and the catalyst layer is formed by hydrophobization of the catalyst layer or by a separate boundary layer. The anode of the electrolysis cell can have a similar design to the cathode of the fuel cell, wherein the materials listed in table 2 are suitable as catalyst material. Here, Pt/C or IrPt/C means that the platinum or the IrPt is located on a carbon substrate. 
         [0000]    
       
         
               
             
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Possible catalyst materials for the electrodes 
               
             
          
           
               
                   
                 Electrolyte 
                 Anode of the 
                 Common 
                 Cathode of 
               
               
                   
                 is . . . 
                 electrolysis cell 
                 electrode 
                 the fuel cell 
               
               
                   
                   
               
               
                   
                 Acidic 
                 IrPt/C 
                 Pt/C 
                 Pt/C 
               
               
                   
                 Alkaline 
                 Ni 
                 Ni 
                 Pt/C or Ag 
               
               
                   
                   
               
             
          
         
       
     
         [0017]    If air is supplied as oxygen supplier to the cathode of the fuel cell, the catalyst layer of the cathode of the fuel cell may be contaminated by the CO 2  content of the air and therefore may no longer be usable. There is the risk of this happening in particular in the case of a catalyst layer which contains nickel. In order to avoid contamination, the oxygen which is produced at the anode of the electrolysis cell during the electrolysis can be stored, with the result that pure oxygen is available for the fuel cell operation. For this purpose, the energy storage and power generation system according to the invention can have a further store. The water produced during fuel cell operation can also be stored and reused. Owing to the use of pure oxygen instead of air, the efficiency of the fuel cell can also be increased. 
         [0018]    In order to increase the electrical voltage or the electrical current during fuel cell operation or hydrogen production during the electrolysis, provision can be made for the energy storage and power generation system according to the invention to have a plurality of fuel cells and/or a plurality of electrolysis cells. In particular, the energy storage and power generation system has the same number of fuel cells and electrolysis cells. The basic units comprising in each case one fuel cell and one electrolysis cell are in this case stacked one above the other. 
         [0019]    It is possible for the fuel cells and the electrolysis cells to be stacked alternately one above the other. In this case, the basic units are separated from one another by an electrically insulating and gas-tight layer. The fuel cells can be connected in series or in parallel with one another. The electrolysis cells can also be connected in series or in parallel with one another. 
         [0020]    However, as an alternative, it may also be the case that the anode of the electrolysis cell and/or the cathode of the fuel cell has porous material, in a manner similar to the design of the common electrode. For example, a metal foam or an electrically conductive mesh, for example a porous stainless steel structure, can be used. A cathode of the fuel cell with such a configuration can act as cathode of two fuel cells. An anode of the electrolysis cell with such a configuration can be used as anode of two electrolysis cells. For this purpose, common electrodes are arranged on a first lateral face and an opposite second lateral face of the cathode of the fuel cell and the anode of the electrolysis cell, respectively. By virtue of the porous material, oxygen can be supplied to the catalyst layers on both lateral faces in the case of the fuel cell and discharged in the case of the electrolysis cell. Alternatively, a catalyst which is in contact with an electrolyte film can be provided in the porous material. Owing to the fact that, in this embodiment, in each case two fuel cells can share a cathode and two electrolysis cells can share an anode, in each case two fuel cells and two electrolysis cells can be stacked alternately one above the other. The plates of the electrodes and the electrically insulating and gas-tight layer can in this case advantageously be dispensed with, with the result that space is saved and the number of components is reduced. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]    Further measures improving the invention are given in the description below relating to the exemplary embodiments of the invention, which is illustrated schematically in the figures. All of the features and/or advantages, including design details, physical arrangement and method steps, which can be gleaned from the claims, the description or the drawing can be essential to the invention both individually and in a wide variety of combinations. In the drawing: 
           [0022]      FIG. 1  shows a schematic illustration of an energy storage and power generation system according to the invention, 
           [0023]      FIGS. 2A-E  show a section through various exemplary embodiments of a common electrode of a basic unit shown in  FIG. 1 , and 
           [0024]      FIGS. 3A-C  show various stacks according to the invention consisting of basic units. 
       
    
    
     DETAILED DESCRIPTION 
       [0025]    Elements with the same function and operation have been provided with the same reference symbols in  FIGS. 1 to 3 . 
         [0026]      FIG. 1  shows a schematic illustration of an energy storage and power generation system  10  according to the invention with an exemplary basic unit  20  comprising an electrolysis cell  30  and a fuel cell  40 . The electrolysis cell  30  has an anode  31 , an electrolyte layer  32  and a cathode, which is in the form of a common electrode  21  and is likewise associated with the fuel cell  40 . The common electrode  21  acts as anode in the fuel cell  40 , said anode being adjoined by an electrolyte layer  42  and a cathode  41  in the fuel cell  40 . In  FIG. 1 , the electrolyte layers are acidic and solid in the form of a perfluorinated sulfonic acid membrane. If at one instant more electrical energy can be produced inexpensively or regeneratively than the consumers draw, an electrical voltage is applied to the electrolysis cell and water from a water tank  11  is supplied to the anode  31  of the electrolysis cell  30 . As indicated in table 1, hydrogen is produced at the common electrode  21  during the electrolysis, and said hydrogen passes into a first store  13  via a hydrogen line  12 . In this case, the hydrogen is compressed by a first compressor  14  to  80  bar. Oxygen is produced at the anode  31  of the electrolysis cell  30 . The oxygen is compressed in a second compressor  16  and stored in a second store  17 . The size ratios are very distorted in  FIG. 1 . Owing to the fact that the common electrode  21  and the store  13  are separated from one another according to the invention, the basic unit  20  can be very small. The electrolyte layers  32 ,  42  can have a thickness of less than 100 μm, and the common electrode  21  can have a thickness of less than 50 μm. On the other hand, the stores  13 ,  17  may be caverns, which can enclose several million cubic meters. 
         [0027]    If the electrical energy demand is above the electrical energy which can be generated regeneratively or inexpensively, the operation of the fuel cell  40  is begun. For this purpose, a first valve  18  is opened and hydrogen is supplied to the common electrode  21  from the first store  13 . Likewise, a second valve  19  is opened and oxygen is supplied to the cathode  41  of the fuel cell from the second store  17 . During fuel cell operation, the fuel cell  40  produces electrical power, which is drawn by a consumer (not illustrated), and water. The water produced at the cathode  41  of the hydrogen cell  40  is stored in the water tank  11 . The hydrogen which is not consumed can be supplied back to the common electrode  21  by a third compressor  47  via a recirculation line  16 . 
         [0028]    The anode  31  of the electrolysis cell  30  has a plate  33 , in which at least one cutout  34  for supplying the water and for discharging oxygen is located. The plate  33  is adjoined by a catalyst layer  35 , which is adjoined by the electrolyte layer  32 . Similarly, a plate  43  of the cathode  41  of the fuel cell  40  has at least one cutout  44 . A catalyst layer  45  adjoins the plate  43 , and the electrolyte layer  42  adjoins said catalyst layer  45 . 
         [0029]    The common electrode  21  has at least one region  23  consisting of porous material consisting of a foamed carbon paste. The carbon paste is coated with platinum as catalyst and with a perfluorinated sulfonic acid membrane as electrolyte film. In order to increase the permeability of the electrolyte film with respect to hydrogen, channels  22  can be provided in the common electrode  21 . Five exemplary embodiments of the common electrode  21  are illustrated in  FIGS. 2A-E . In this case, a section is positioned along the line A-A shown in  FIG. 1 . In  FIG. 2A , no channel  22  is provided. In  FIGS. 2B-E , channels  22  are arranged in the common electrode  21 . In  FIGS. 2B-D , the channels  22  extend over the entire width b of the common electrode  21 . During fuel cell operation, the hydrogen is passed from the first store  13  through the channels  22  as shown by the arrows  24  shown in  FIG. 2B . From said channels, the hydrogen passes through a boundary face  26 , into the porous regions  23  and permeates through the electrolyte film to the catalyst, as illustrated by the arrows  25  shown in  FIG. 2B . In order that the hydrogen remains in the common electrode  21  and does not enter the electrolyte layers  32 ,  42 , the channels  22  have gas-impermeable walls  27  consisting of metal, which delimit the channels  22  with respect to the electrolyte layers  32 ,  42 . The width b of the common electrode  21  and a width b′ of the channels  22  increases from  FIG. 2B  to  FIG. 2D . As a result of the increase in the widths b, b′, the boundary face  26  through which hydrogen can diffuse into the porous regions  23  increases. As a result, the catalyst is supplied with hydrogen with greater efficiency. The common electrode  21  shown in  FIG. 2A  is used in the case in which it is sufficient to supply hydrogen to the catalyst even without channels  22 . The quantity of hydrogen which can permeate through the electrolyte film without channels  22  would be too low in  FIGS. 2B to 2D . From  FIG. 2B  to  FIG. 2D , the quantity of hydrogen which eliminates this deficiency which otherwise increases from  FIG. 2B  to  FIG. 2D  increases. In  FIG. 2D , a particularly small quantity of hydrogen would permeate through the electrolyte without channels  22 . 
         [0030]    In order to produce the common electrode  21  shown in  FIGS. 2B-D , the channels  22  can follow, in terms of their physical dimensions, the form of a salt bar. Then, a C paste is foamed between the salt bars, the resultant porous material is coated with platinum as catalyst and with the solid electrolyte and the salt bars are surrounded on the outside by a metal in the form of walls  27 . Then, the salt is dissolved and the channels  22  are thus produced. 
         [0031]      FIG. 2E  illustrates a section through a further alternative of a common electrode  21 , in which the channels  22  are located only in the interior of the porous region  23 . Metallic conductors  5  are provided so as to improve electrical conductivity. The common electrode  21  shown in  FIG. 2E  can be manufactured by assembling two halves  21 . 1 ,  21 . 2 . The halves  21 . 1 ,  21 . 2  have carbon foam which is coated with a platinum catalyst and the solid electrolyte. Depressions  22 . 1 ,  22 . 2  are provided in the two halves  21 . 1 ,  21 . 2 , and these depressions result in the channels  22  when the two halves are assembled. The boundary area  26  can be matched in terms of its size so as to correspond to the desired hydrogen permeation. 
         [0032]    If a liquid electrolyte is used instead of the solid electrolyte, the boundary areas  26  in  FIGS. 2B-D  have a gas-permeable, but liquid-impermeable film. The boundary  28  between the porous regions  23  and the adjoining electrolyte layers  32 ,  42  is in this case a gas-impermeable, but ion-permeable layer, for example coated using screen printing, in order to prevent diffusion of the hydrogen into the electrolyte layers  32 ,  42 , but to enable ion conduction. There is no coating of the porous material with the solid electrolyte. 
         [0033]      FIGS. 3A to 3C  illustrate various exemplary embodiments of stacks  50  of basic units  20 .  FIGS. 3A and 3B  each illustrate the same stack  50 . The anode  31  of the electrolysis cell  30  and the cathode  41  of the fuel cell  40  shown in  FIG. 1  have in this case been summarized schematically as one unit. The basic units  20  are each stacked one above the other in the same direction and separated by an electrically insulating layer  51  in  FIGS. 3A and 3B . This results in a stack sequence in which in each case one electrolysis cell  30  and one fuel cell  40  alternate. The electrodes  21 ,  31  of the electrolysis cell  30  are connected electrically in parallel with an electrical voltage source  52  in  FIG. 3A , wherein the voltage of the voltage source  52  can be applied to the electrolysis cells  30  by a switch  53  being closed. Likewise, the fuel cells  40  are connected electrically in parallel with one another. By virtue of a switch  54 , the fuel cell  40  can be connected to a consumer  55 .  FIG. 3B  differs from  FIG. 3A  merely in that the electrolysis cells  30  are connected in series with one another and the fuel cells  40  are also connected in series with one another. 
         [0034]    In  FIG. 3C , the anode  31  of the electrolysis cell  30  and the cathode  41  of the fuel cell  40  are formed from a porous carbon foam with a metal catalyst, in which an electrolyte film is also located. As a result, the anode  31  of the electrolysis cell  30  or the cathode  41  of the fuel cell  40  can be formed by two common electrodes  21  which are arranged to the right and left thereof, two electrolysis cells  30  and two fuel cells  40 , respectively. The anode  31  of the electrolysis cell  30  formed in this way therefore belongs to in each case two electrolysis cells  30 . The cathode  41  of the fuel cell  40  thus formed in each case belongs to two fuel cells  40 . A stack  56  with anodes  31  of the electrolysis cells  30  and cathodes  41  of the fuel cells  40  formed in this way has a stack sequence in which alternately two electrolysis cells  30  and two fuel cells  40  are stacked one above the other alternately, apart from the ends of the stack  56 . In such a stack  56 , in each case one basic unit  20  in which the electrolysis cell  30  is arranged to the left of the fuel cell  40  alternates with a basic unit  20  in which the electrolysis cell  30  is arranged to the left of the fuel cell  40 .  FIG. 3C  illustrates the same stack  56  twice, wherein the stack sequence is explained in more detail in the illustration at the top, while the electrical wiring of the stack  56  is illustrated in the illustration at the bottom. In this case, both the electrolysis cells  30  and the fuel cells  40  are each connected electrically in parallel with one another.