Abstract:
An electrolyzer ( 2 ) includes at least one electrolysis cell ( 4 ), arranged between two end plates ( 6.8 ), in a polymer electrolyte membrane construction manner, and a pressing device for producing a pressing force between the end plates ( 6.8 ). The pressing force at least partly is controlled in a manner dependent on the gas pressure produced by the electrolysis cell ( 4 ).

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a U.S. National Phase Application of International Application PCT/EP2012/076048 filed Dec. 18, 2012 and claims the benefit of priority under 35 U.S.C. § 119 of European Patent Application EP 12151641.3 filed Jan. 18, 2012, the entire contents of which are incorporated herein by reference. 
       FIELD OF THE INVENTION 
       [0002]    The invention relates to an electrolyzer with at least one electrolysis cell arranged between two end plates ( 6 ,  8 ), in a polymer electrolyte membrane construction manner, and with means for producing a pressing force between the end plates. 
       BACKGROUND OF THE INVENTION 
       [0003]    In electrolyzers, water is broken dawn into its constituents of hydrogen and oxygen by way of an electrical current. Electrolyzers which use electrolysis cells in a polymer electrolyte membrane construction manner are known for this. These electrolysis cells usually consist essentially of a polymer electrolyte membrane, on whose two outer sides a gas diffusion electrode is arranged in each case. In the simplest case, an electrolysis cell, as a rule however a multitude of electrolysis cells in a stacked construction is arranged between two end plates, in the electrolyzer. The end plates on the one hand serve for the supply of electrical power and on the other hand serve for exerting a pressing force onto the electrolysis cell or onto the stack of electrolysis cells, said force being necessary for an effective operation of the electrolyzer. For this, the end plates are clamped to one another. 
         [0004]    It is known that the pressing force which is exerted onto the electrolysis cell or onto the stack of electrolysis cells by the end plates reduces with an increase of the operating pressure of the electrolyzer, i.e. with an increase of the gas pressure in the inside of the electrolyzer, and this has disadvantageous consequences for its efficiency, and in the most unfavorable case this pressing can be lost completely. Accordingly, in the state of the art, one attempts to counter this effect with the help of very solid and thus very rigid end plates, which however is accomplished only to an unsatisfactory extent and leads to very heavy designs. 
       SUMMARY OF THE INVENTION 
       [0005]    Against this background, it is an object of the present invention to provide an electrolyzer of the type mentioned above. with which an adequately large pressing of the electrolysis cell or electrolysis cells is always ensured, independently of the operating pressure prevailing in the electrolyzer. 
         [0006]    The electrolyzer according to the invention which serves for breaking water down into oxygen and hydrogen and preferably serves for the production of hydrogen, in the usual manner comprises at least one electrolysis cell in a polymer electrolyte membrane construction manner, said at least one cell being arranged between two end plates. The electrolyzer according to the invention, between the two end plates can also comprise several electrolysis cells grouped together into a stack, for producing greater hydrogen quantities, and therefore when hereinafter one speaks of an electrolysis cell, this is also to be understood as an electrolysis cell stack. 
         [0007]    The two end plates and the electrolysis cell arranged therebetween form a common construction unit. For this purpose, the end plates and the electrolysis cell located therebetween are clamped together for example with tie rods. In this manner, although a certain pressing force is exerted by the end plates onto the electrolysis cell, this pressing force however alone is not sufficient, in order to ensure an operation of the electrolyzer which is optimal with regard to the efficiency of the electrolysis cell, at higher operating pressures, i.e. gas pressures produced by the electrolysis cell of e.g. above 2 MPa. For this, the electrolyzer according to the invention comprises further means for producing a pressing force between the end plates. 
         [0008]    In this context, the basic idea of the invention is to control the pressing force exerted onto the electrolysis cell, at least partly in dependence on the gas pressure produced by the electrolysis cell. I.e. a control is provided, with which the size of the pressing force onto the electrolysis cell can be changed in a targeted manner in direct relation to a change of the gas pressure produced by the electrolysis cell. This means that when the gas pressure produced by the electrolysis cell increases, the pressing force on the electrolysis cell can be increased preferably proportionally to the increase of the gas pressure, in a manner such that the previously outlined disadvantageous effect of a gas pressure increase on the pressing force is compensated, and a pressing force on the electrolysis cell and which is constant in the ideal case is always set independently of the gas pressure and permits the operation of the electrolysis cell with an optimal efficiency. 
         [0009]    The gas pressure which is produced by the electrolysis cell can be detected as the case may be by way of a pressure sensor arranged in the region of the electrolyzer which is subjected to gas pressure, for the gas-pressure-dependent control of the pressing force on the electrolysis cell. wherein then an electrical output signal of the pressure sensor is led to the control as in input signal and there is converted into a control variable for preferably one, as the case may be however also several actuators for producing the pressing force. 
         [0010]    Particularly advantageously, the pressing force can however be controlled directly by the gas pressure produced by the electrolysis cell. I.e., that an actuator for producing the pressing force is impinged directly with the gas pressure as a control variable, or is actuated by the gas pressure. For this, a closed overflow path is to be provided from the inside of the electrolyzer to the actuator, wherein this overflow path on the electrolyzer can basically be formed in a region, in which the oxygen arises with the water decomposition, thus at the anode side, as well as in a region in which the hydrogen arises, thus at the cathode side. A design of the electrolyzer according to the invention, with which the pressing force is controlled by the gas pressure of the hydrogen produced by the electrolysis cell, is however preferred according to the invention. For this, an overflow path can be formed from a hydrogen outlet of the electrolyzer to an actuator producing the pressing force, or to a control controlling the actuator. 
         [0011]    Usefully, a pressing force exerted onto the electrolysis cell is greater than the gas pressure produced by the electrolysis cell, in order to ensure that an adequately large pressing force is exerted on the electrolysis cell independently of the gas pressure which is produced by the electrolysis cell. With one design of the electrolyzer according to the invention, with which the gas pressure acts directly on an actuator producing the pressing force, the actuator for this can contain a pressure multiplier (amplifier) as the case may be. 
         [0012]    According to a further advantageous design of the electrolyzer according to the invention, the pressing force controlled by the gas pressure is fluidically produced. Accordingly, the pressing force on the electrolysis cell is produced directly or indirectly via a component suitable for this, by a gas or a liquid, wherein liquids as a rule are to be preferred since they behave in a quasi incompressible manner. However, it is also conceivable for the pressing force on the electrolysis cell to be provided in the simplest case directly by the gas produced by the electrolysis cell. 
         [0013]    In combination with a fluidically produced pressing force, one advantageously envisages a pressure-proof chamber being formed on at least one of the two end plates of the electrolyzer, at a side which faces the electrolysis cell. This chamber serves for receiving the fluid which is pressed directly against the flat side of the electrolysis cell which is arranged adjacent the chamber. 
         [0014]    Even more advantageously, a pressing body which is displaceably guided in the direction of the electrolysis cell can be arranged in the chamber formed on the end plate, and this body is pressed by the fluid located in the chamber, against the electrolysis cell, and thus the pressing force is exerted into this. Usefully, the pressing body is hereby sealed off in a fluid tight manner with respect to the edge of the chamber, in order to prevent a flaw of the fluid out of the chamber. 
         [0015]    The chamber is usefully conduit-connected to a pressure-producer for producing the pressing force. A pressure-producer in this context is to be understood as a pump or likewise, with which the fluid is pressed into the chamber formed on the end plate. The pressure producer is usefully arranged at the side of the end plate which is away from the electrolysis cell, wherein a fluid conduit is led from the pressure producer to the chamber, through the end plate. 
         [0016]    A piston-cylinder arrangement can form the pressure producer, in a manner which is simple with regard to design. Thus in the simplest case, a cylinder can be provided, which is divided by a piston which is movably guided therein, into two cylinder chambers, wherein one of the cylinder chambers is conduit-connected to the chamber formed on the end plate, and the piston of the other chamber is pressure-impinged by the hydrogen produced by the electrolysis cell. Accordingly, preferably a cylinder chamber of the piston-cylinder arrangement is conduit-connected to a hydrogen outlet of the electrolyzer. 
         [0017]    As has already been noted, one strives for a pressing pressure exerted on the electrolysis cell to be greater than the gas pressure produced by the electrolysis cell. In this context, the piston-cylinder arrangement is particularly advantageously designed in a manner such that it forms a pressure converter. Hereby, the end-face of the piston which delimits the cylinder chamber at the piston-cylinder arrangement which is conduit-connected at the chamber formed on the end plate, is smaller than the end-face of the piston which is pressure-impinged by the hydrogen, as is common for pressure converters, wherein the pressure conversion ratio of the pressure converter results from the quotient of the last-mentioned and first-mentioned piston end-face. 
         [0018]    Preferably, the pressure converter has a pressure conversion ratio which lies in a region of 1.1 to 2.5 and preferably between 1.2 and 2.0. It has been found that with such a pressure conversion ratio, an adequately large pressing force is exerted onto the electrolysis cell, without the pressing force being so large that the electrolysis cell is damaged by this or even destroyed. 
         [0019]    The invention is hereinalfer explained in more detail by way of one embodiment represented in the drawing. 
         [0020]    The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which preferred embodiments of the invention are illustrated. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]    In the drawings: 
           [0022]    The only Figure is a schematic sectional exploded view showing, in a greatly simplified manner an electrolyzer according to the invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0023]    Referring to the drawings in particular, the electrolyzer  2  comprises an electrolysis cell  4  which is arranged between two end plates  6  and  8 . An opening  10  forming a hydrogen outlet of the electrolyzer  2  is formed on the end plate  6 , and a conduit  12  indicated only schematically connects to this opening at the outer side of the electrolyzer  2 . The hydrogen produced in the electrolysis cell  4  is led away to a storage tank which is not shown in the drawing, via the opening  10  and the conduit  12  connected thereto. 
         [0024]    A polymer electrolyte membrane  14  is arranged centrally in the electrolysis cell  4 . This polymer electrolyte membrane  14  at its two flat sides is covered in each case by a gas diffusion electrode  16  and  18 . A plate  20  is arranged on the outer side of the gas diffusion electrode  16 , and a plate  22  on the outer side of the gas diffusion electrode  18 . The plates  20  and  22  serve for the electricity connection. Seals  24  seal the intermediate spaces between the polymer electrolyte membrane  14  and the two plates  20  and  22 . The electrolysis cell  4  at its end which is at the top in the drawing is sealed by two seals  26 . 
         [0025]    The two end plates  6  and  8  are clamped to one another via tie rods which are not represented in the drawing. A certain pressing force which is necessary for a high efficiency of the electrolysis taking place in the electrolysis cell  4  is exerted onto the electrolysis cell  4  by way of this. If a high gas pressure which is mainly due to the hydrogen occurring with the electrolysis prevails in the electrolyzer  2 , then the pressing force exerted by the end plates  6  and  8  onto the electrolysis cell  4  is no longer sufficient for an electrolysis which takes place with an adequate efficiency. For this reason, an additional pressing force must be exerted onto the electrolysis cell  4 . The constructional measures which are selected for this are described hereinafter. 
         [0026]    A recess  28  which tapers in the direction away from the plate  20  amid the formation of a shoulder  30  is formed on the flat side of the end plate  6  which faces the plate  20 . The shoulder  30  extends around the complete periphery of the recess  28  and serves for receiving a seal  34  which seals the end plate  6  with respect to the plate  20  in a fluid-tight manner. A bore  36  which runs out in the recess  28  extends through the end plate  6 . The bore  36  thus forms an access to the chamber which is formed by the recess  28  and the pressing body  32 . 
         [0027]    A conduit branching  38  is formed on the conduit  12 . A conduit  40  leads from this conduit branching  38  to a piston-cylinder arrangement  42 . The piston-cylinder arrangement  42  comprises a closed hollow cylinder  44 , in which a piston  46  is axially displaceably guided. The piston  46  divides the interior of the hollow cylinder  44  into a first cylinder chamber  48  and a second cylinder chamber  50 . 
         [0028]    An opening  52  which runs out in the cylinder chamber  48  is formed on the hollow cylinder  44  on the peripheral side. The conduit  40  is connected on this opening  52 . Moreover, a further opening  54  is formed on the hollow cylinder  44  on a faceside delimiting the cylinder chamber  50 . A conduit  56  represented schematically in the torm at an arrow leads from the opening  54  to the bore  36  formed on the end plate  6 . 
         [0029]    A piston rod  58  is formed on fhe pisfon  46 , on the face-side which faces the cylinder chamber  50 . This piston rod  58  extends concentrically to a middle axis A of the piston  46  through an opening  60  which is formed on fhe face-side of the hollow cylinder  44  which delimits the cylinder chamber  50 . The pressure-effective surface area of the piston  46  on the face-side tacing the cylinder chamber  50  is smaller than at the tace-side facing the cylinder chamber  48 , on account of this piston rod  58  led out of the cylinder chamber  50 . The piston-cylinder arrangement  42  forms a pressure converter due to this design. 
         [0030]    The manner of functioning of the represented electrolyzer  2  is as follows: 
         [0031]    The hydrogen produced by the electrolyzer  2  is to be stored at a high pressure in a storage tank which is not represented. Accordingly, a high hydrogen pressure prevails in the electrolyzer  2  and in the conduit  12  leading to the storage tank, and this high pressure, with the electrolyzers which have been known until now, would lead to the fact that the gas diffusion electrodes  16  and  18  would no longer be pressed onto one another to a sufficient extent. 
         [0032]    This is not the case with the represented electrolyzer  2 . If the hydrogen pressure in the electrolyzer  2  increases, this has the result that the pressure in the cylinder chamber  48  of the piston-cylinder arrangement  42  also increases, since the cylinder chamber  48  is conduit-connected or conductively connected to the inside of the electrolyzer  2  via the conduits  12  and  40 . This pressure increase in the cylinder chamber  48  leads to the fact that the piston  46  is pressed in the direction of the cylinder chamber  50 , by which means the pressure in the cylinder chamber  50  and the recess  28  of the end plate  6  which is conduit-connected thereto via the conduit  56  increases. Hereby the pressure increase in the cylinder chamber  50  and in the recess  28  is greater than in the cylinder chamber  48 , since the piston-cylinder arrangement  42  forms a pressure converter due to the design of the piston  46 . 
         [0033]    Due to the pressure increase in the cylinder chamber  48 , a fluid located in the recess  28  presses with a correspondingly increased pressure against the plate  20  so that it is ensured that the two gas diffusion electrodes  16  and  18  are pressed onto one another to an adequate extent. 
         [0034]    While specific embodiments of the invention have been shown and described in detail to illustrate the application of the principles of the invention, it will be understood that the invention may be embodied otherwise without departing from such principles.