PATENT ABSTRACT
The present disclosure relates to membranes for use in electrolysis systems. The teachings thereof may be embodied in a method for checking a membrane of an electrolyzer comprises two volumes separated by the membrane and produces two product gases from a starting liquid. The method may include: detecting an electrolysis current strength during electrolysis, measuring a liquid flow rate of the starting liquid between the two electrolyzer volumes, calculating a ratio of the measured liquid flow rate and the detected electrolysis current strength, and using the calculated ratio as an indication of membrane leaktightness.

PATENT DESCRIPTION
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a U.S. National Stage Application of International Application No. PCT/EP2015/078979 filed Dec. 8, 2015, which designates the United States of America, and claims priority to EP Application No. 15151622.6 filed Jan. 19, 2015, the contents of which are hereby incorporated by reference in their entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    The present disclosure relates to membranes for use in electrolysis systems. The teachings thereof may be embodied in a method and a test device for checking a membrane leaktightness of at least one membrane of an electrolyzer which comprises two electrolyzer volumes separated from one another by the at least one membrane and is configured in order to produce two product gases from a starting liquid by means of electrolysis. 
       BACKGROUND 
       [0003]    During the electrolysis of water, two product gases hydrogen and oxygen are formed simultaneously. These product gases must be separated, and must not be mixed with one another. Membranes of the electrolyzer may develop leaks during operation, and then hermetic separation of the two product gases can no longer be ensured. In this case, mixing of the product gases may occur, so that in the extreme case an unsafe operating state may arise. This eventuality must be prevented by suitable measures. 
         [0004]    Leaks of membranes of an electrolyzer may, for example, be detected by checking whether a product gas penetrates through a membrane. This procedure requires separate monitoring of the product gases and is relatively expensive. A challenge in the case of an electrolyzer for water electrolysis is, in particular, that water is present in the system, so that together with the two product gases up to three components may be present simultaneously. In a dynamic process, which generally entails temperature and pressure changes, the water content sometimes varies greatly. This makes calibration more difficult, particularly in the case of simple analysis methods. Furthermore, undesired condensation may occur. In a typical technical implementation of the monitoring of the product gases to detect membrane leaks, a relatively small gas flow, diverted from a product gas, is analyzed. With the aid of an actively cooled condenser, for example, the diverted gas can be dried. Time-varying operating pressures can be regulated by a pressure reducer. For example, gas chromatographs, thermal conductivity detectors, or catalytic sensors may be envisioned as detectors. In the presence of hydrogen and oxygen, the latter cause a chemical reaction and thereupon register a temperature increase. Such a procedure has the disadvantage that additional components are required, and that relatively elaborate calibrations need to be carried out. 
       SUMMARY 
       [0005]    The teachings of the present disclosure may be embodied in a method and a test device for checking a membrane leaktightness of at least one membrane of an electrolyzer which comprises two electrolyzer volumes separated from one another by the at least one membrane and is configured in order to produce two product gases from a starting liquid by means of electrolysis. In particular, the method may include checking of the membrane leaktightness of an electrolyzer for water electrolysis, in which water is decomposed into the product gases oxygen and hydrogen, the electrolyzer being for example configured as a proton exchange membrane electrolyzer (so-called PEM electrolyzer) having at least one proton-permeable polymer membrane (PEM=polymer electrolyte membrane). PEM electrolyzers have the advantage that they can be operated very dynamically and are therefore suitable for the use of regenerative surplus current for the production of hydrogen. 
         [0006]    An example method for checking a membrane leaktightness of at least one membrane ( 7 ) of an electrolyzer ( 1 ) which comprises two electrolyzer volumes separated from one another by the at least one membrane ( 7 ) and is configured in order to produce two product gases ( 10 ,  30 ) from a starting liquid ( 50 ) by means of electrolysis, may include during electrolysis, an electrolysis current strength is detected and a liquid flow rate of the starting liquid ( 50 ) between the two electrolyzer volumes is determined, and a ratio parameter (Q), which is proportional to the ratio of the liquid flow rate determined and the electrolysis current strength detected, is formed and is used to check the membrane leaktightness. 
         [0007]    In some embodiments, to determine the liquid flow rate, a time variation of a liquid volume of the starting liquid ( 50 ) in at least one of the two electrolyzer volumes is determined. 
         [0008]    In some embodiments, each of the two electrolyzer volumes comprises a container volume of a separator container ( 5 ,  6 ), in which a product gas ( 10 ,  30 ) and starting liquid ( 50 ) are collected, characterized in that the time variation of a liquid volume of the starting liquid ( 50 ) in at least one of the two electrolyzer volumes is determined by repeatedly detecting and evaluating a filling level of starting liquid ( 50 ) in the container volume of the electrolyzer volume. 
         [0009]    In some embodiments, the time variation of a liquid volume of the starting liquid ( 50 ) in at least one container volume of an electrolyzer volume is determined by repeatedly detecting and evaluating a gas pressure in the container volume. 
         [0010]    In some embodiments, each of the two electrolyzer volumes comprises a container volume of a separator container ( 5 ,  6 ), in which a product gas ( 10 ,  30 ) and starting liquid ( 50 ) are collected, characterized in that the liquid flow rate is determined by detecting and evaluating a time variation of a pressure difference between gas pressures in the two container volumes. 
         [0011]    In some embodiments, a first ratio threshold value (Q s1 ) for the ratio parameter is specified, and a leak of at least one membrane ( 7 ) is inferred when the ratio parameter (Q) exceeds the specified first ratio threshold value (Q s1 ). 
         [0012]    In some embodiments, a second ratio threshold value (Q s2 ) for the ratio parameter is specified, and a leak of at least one membrane ( 7 ) is inferred when the ratio parameter (Q) falls below the specified second ratio threshold value (Q s2 ). 
         [0013]    In some embodiments, the electrolysis is interrupted for an interruption time, the electrolyzer volumes are filled with mutually different liquid amounts of the starting liquid ( 50 ), and a time requirement for equalization of the liquid amounts in the two electrolyzer volumes is determined with the aid of a liquid flow rate determined during the interruption time and is used to assess the membrane leaktightness. 
         [0014]    In some embodiments, before the determination of the time requirement for equalization of the two liquid amounts, gas pressures in the two electrolyzer volumes are equalized to one another. 
         [0015]    In some embodiments, before the determination of the time requirement for equalization of the two liquid amounts, gas pressures in the two electrolyzer volumes are equalized to an ambient pressure in an environment of the electrolyzer ( 1 ). 
         [0016]    In some embodiments, the liquid flow rate is determined repeatedly during the interruption time, and the time requirement for equalization of the two liquid amounts is determined with the aid of an extrapolation of the liquid flow rates detected. 
         [0017]    The teachings of the present disclosure may be embodied in a test device ( 3 ) for checking a membrane leaktightness of at least one membrane ( 7 ) of an electrolyzer ( 1 ) which comprises two electrolyzer volumes separated from one another by the at least one membrane ( 7 ) and is configured in order to produce two product gases ( 10 ,  30 ) from a starting liquid ( 50 ) by means of electrolysis. The test device ( 3 ) may include: an ammeter ( 60 ) for detecting an electrolysis current strength of the electrolyzer ( 1 ), a measuring device ( 8 ) for detecting a liquid amount of the starting liquid ( 50 ) in at least one of the two electrolyzer volumes and an evaluation unit for determining a liquid flow rate of the starting liquid ( 50 ) between the two electrolyzer volumes with the aid of the measurement values detected by the measuring device ( 8 ). 
         [0018]    In some embodiments, each of the two electrolyzer volumes comprises a container volume of a separator container ( 5 ,  6 ), in which a product gas ( 10 ,  30 ) and starting liquid ( 50 ) are collected, characterized in that the measuring device ( 8 ) comprises at least one filling level sensor ( 9 ) for detecting a filling level of the starting liquid ( 50 ) in a container volume and/or at least one pressure sensor ( 15 ) for detecting a gas pressure in a container volume. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    The above-described properties, features and advantages of this invention, as well as the way in which they are achieved, will become more clearly and readily comprehensible in conjunction with the following description of the exemplary embodiments, which will be explained in more detail in connection with the drawings, in which: 
           [0020]      FIG. 1  shows a block diagram of an electrolyzer and a device for checking a membrane leaktightness of the electrolyzer, and 
           [0021]      FIG. 2  shows a diagram of a time variation of a ratio parameter. 
       
    
    
     DETAILED DESCRIPTION 
       [0022]    In some example methods for checking a membrane leaktightness of at least one membrane of an electrolyzer which comprises two electrolyzer volumes separated from one another by the at least one membrane and is configured in order to produce two product gases from a starting liquid by means of electrolysis, a liquid flow rate of the starting liquid between the two electrolyzer volumes is determined and is evaluated in order to check the membrane leaktightness. 
         [0023]    An example method may be used for monitoring the membrane leaktightness of an electrolyzer not with the aid of gas analyses of product gases, but instead by analysis of a liquid flow rate of the starting liquid through the at least one membrane of the electrolyzer, which is manifested as a liquid flow rate between the two electrolyzer volumes separated by the at least one membrane. Besides the molecules of a product gas, molecules of the starting liquid that are not involved in the electrolysis also penetrate through the at least one membrane and therefore pass from one electrolyzer volume into the other electrolyzer volume. In the event of a leak of a membrane, more molecules of the starting liquid can penetrate through this membrane, which leads to a change in the liquid flow rate between the two electrolyzer volumes. Determination of this liquid flow rate therefore makes it possible to check the membrane leaktightness. 
         [0024]    The method therefore allows checking of the membrane leaktightness without an elaborate gas analysis and calibration. In particular, in contrast to gas analysis methods, the method can be carried out without diverting a gas flow and without additional detectors or gas analysis of the diverted gas flow, such as gas chromatographs, thermal conductivity detectors or catalytic sensors. To carry out the method, only sensors for determining the liquid flow rate between the two electrolyzer volumes, and in one configuration of the method as described below ammeters for detecting electrolysis current strengths, are required. Such sensors are generally provided anyway as component parts of an electrolyzer, so that no additional sensors are required to carry out the method. Furthermore, the method allows reliable checking of the membrane leaktightness because of the high measurement accuracy of sensors for determining the liquid flow rate and electrolysis current strength. 
         [0025]    In some embodiments, to determine the liquid flow rate, a time variation of a liquid volume of the starting liquid in at least one of the two electrolyzer volumes is determined. A time variation of a liquid volume of the starting liquid in at least one of the two electrolyzer volumes can be determined simply and precisely by measurement technology, for example by means of filling level sensors, and is therefore suitable for determining the liquid flow rate between the electrolyzer volumes. 
         [0026]    In general, each of the two electrolyzer volumes comprises a container volume of a separator container, in which a product gas and starting liquid are collected. The time variation of a liquid volume of the starting liquid in at least one of the two electrolyzer volumes is determined by determining a time variation of a liquid volume of the starting liquid in the container volume of the electrolyzer volume. The time variation of a liquid volume of the starting liquid in the container volume of an electrolyzer volume is, for example, determined by repeatedly detecting and evaluating a filling level of starting liquid in the container volume, and/or by repeatedly detecting and evaluating a gas pressure in the container volume, and/or by detecting and evaluating a time variation of a pressure difference between gas pressures in the two container volumes. 
         [0027]    The aforementioned configurations make use of the fact that a liquid volume of the starting liquid in a separator container can be determined particularly simply and precisely by detecting a filling level of the starting liquid and/or a gas pressure in the separator container and/or a pressure difference between gas pressures in the two separator containers. During electrolysis, an electrolysis current strength is detected and a ratio parameter, which is proportional to the ratio of the liquid flow rate determined and the electrolysis current strength detected, is formed and is used to assess the membrane leaktightness. 
         [0028]    In general, the ratio of the amounts of a product gas and of the starting liquid, which penetrate through a membrane, is to a good approximation constant. In the event of a leak of a membrane an additional transport path is formed for the starting liquid through the membrane, and this ratio changes. This ratio is therefore suitable as a parameter for assessing the membrane leaktightness. In this case, the electrolysis current strength is a simply accessible measurement quantity that is a measure of the amount of a product gas penetrating through the membrane. A ratio parameter which is proportional to the ratio of the liquid flow rate determined and the electrolysis current strength detected is therefore particularly advantageously suitable for assessing the membrane leaktightness. 
         [0029]    In some embodiments, a first ratio threshold value for the ratio parameter is specified, and a leak of at least one membrane is inferred when the ratio parameter exceeds the specified first ratio threshold value, and/or a second ratio threshold value for the ratio parameter is specified, and a leak of at least one membrane is inferred when the ratio parameter falls below the specified second ratio threshold value. These embodiments define easily testable criteria for detecting a leak of at least one membrane, which are furthermore proven to be surprisingly reliable. In particular, a specification of the two ratio threshold values defines a tolerance range for values of the ratio parameter, outside which a leak of a membrane is inferred. In this way, the starting liquid can pass a leak of a membrane both in the same direction as a product gas penetrates through the membrane and in the opposite direction thereto, the direction depending on the relative level of the pressures in the two electrolyzer volumes. 
         [0030]    In some embodiments, the electrolysis is interrupted for an interruption time, the electrolyzer volumes are filled with mutually different liquid amounts of the starting liquid, and a time requirement for equalization of the liquid amounts in the two electrolyzer volumes is determined with the aid of a liquid flow rate determined during the interruption time and is used to assess the membrane leaktightness. 
         [0031]    Some methods may include a test procedure for assessing the membrane leaktightness, carried out during an interruption of the electrolysis. In this case, it is merely necessary to determine and evaluate a time requirement for equalization of initially different liquid amounts of the starting liquid in the electrolyzer volumes. A disadvantage, however, is that the electrolyzer is not available for electrolysis operation during the test procedure. 
         [0032]    In some embodiments, before the determination of the time requirement for equalization of the two liquid amounts, gas pressures in the two electrolyzer volumes are equalized to one another, and for example to an ambient pressure in an environment of the electrolyzer. Equalization of the gas pressures in the two electrolyzer volumes may define uniform conditions for the test procedure and thereby simplify evaluation of the test procedure for assessing the membrane leaktightness. Equalization of the gas pressures in the two electrolyzer volumes to the ambient pressure in an environment of the electrolyzer can be carried out particularly simply, for example by controlled opening of blow-off lines of the electrolyzer. 
         [0033]    In the case of the aforementioned test procedure, for example, the liquid flow rate may be determined repeatedly during the interruption time, and the time requirement for equalization of the two liquid amounts is determined with the aid of an extrapolation of the liquid flow rates detected. This may shorten the test procedure, since the test procedure does not need to be continued until equalization of the two liquid amounts is reached. 
         [0034]    Some embodiments may include a test device for checking a membrane leaktightness of at least one membrane of an electrolyzer which comprises two electrolyzer volumes separated from one another by the at least one membrane and is configured in order to produce two product gases from a starting liquid by means of electrolysis comprises a measuring device for detecting a liquid amount of the starting liquid in at least one of the two electrolyzer volumes and an evaluation unit for determining a liquid flow rate of the starting liquid between the two electrolyzer volumes with the aid of the measurement values detected by the measuring device. One configuration of the test device provides an ammeter for detecting an electrolysis current strength of the electrolyzer. According to further configurations of the test device, the measuring device comprises at least one filling level sensor for detecting a filling level of the starting liquid in a container volume and/or at least one pressure sensor for detecting a gas pressure in a container volume. 
         [0035]      FIG. 1  shows a block diagram of an electrolyzer  1  and of a test device  3  for checking a membrane leaktightness of at least one membrane  7  of the electrolyzer  1 . The electrolyzer  1  may produce two product gases  10 ,  30  from a starting liquid  50  by means of electrolysis. The starting liquid  50  is for example water, in which case oxygen as a first product gas  10  and hydrogen as a second product gas  30  are produced during the electrolysis. 
         [0036]    The electrolyzer  1  comprises a cell block  2  having at least one electrolysis cell  4  and two separator containers  5 ,  6 . Only one electrolysis cell  14  is represented in  FIG. 1 . However, it will be assumed below that the cell block  2  comprises a plurality of electrolysis cells  4 . Each electrolysis cell  4  has a membrane  7 , which divides the electrolysis cell  4  into a first subcell  4 . 1  and a second subcell  4 . 2 . Each first subcell  4 . 1  has an anode for the electrolysis, and each second subcell  4 . 2  has a cathode for the electrolysis. Each membrane  4  separates the product gases  10 ,  30  produced in the respective electrolysis cell  4  during the electrolysis. 
         [0037]    The first subcells  4 . 1  are connected by means of a first feed line  20  and a first return line  25  to a first separator container  5 , in which the first product gas  10  produced in the electrolysis cells  4  during the electrolysis and starting liquid  50  are collected. In the first feed line  20 , there is a first heat exchanger  21  for thermally regulating starting liquid  50  and a first feed pump  22 , by means of which starting liquid  50  is pumped from the first separator container  5  through the first feed line  20  into the first subcells  4 . 1 . The first return line  25  is used to convey the first product gas  10  produced in the electrolysis cells  4  during the electrolysis into the first separator container  5 . 
         [0038]    The first subcells  4 . 1 , a container volume of the first separator container  5 , as well as the first feed line  20  and the first return line  25 , form a first electrolyzer volume of the electrolyzer  1 . Starting liquid  50  can be delivered to the first separator container  5  through a supply line  13 . To this end, the supply line  13  contains a supply pump  11  and a solenoid valve  12 , by means of which the supply line  13  can be opened and closed. First product gas  10  can be removed from the first separator container  5  via a first output line  17 . In the first output line  17 , there is a first pressure regulating valve  16  for regulating a gas pressure of the first product gas  10 . 
         [0039]    The second subcells  4 . 2  are connected by means of a second feed line  40  and a second return line  45  to the second separator container  6 , in which the second product gas  30  produced in the electrolysis cells  4  during the electrolysis and starting liquid  50  are collected. In the second feed line  40 , there is a second heat exchanger  41  for thermally regulating starting liquid  50  and a second feed pump  42 , by means of which starting liquid  50  is pumped from the second separator container  6  through the second feed line  40  into the second subcells  4 . 2 . The second return line  45  is used to convey the second product gas  30  produced in the electrolysis cells  4  during the electrolysis into the second separator container  6 . 
         [0040]    The second subcells  4 . 2 , a container volume of the second separator container  6 , as well as the second feed line  40  and the second return line  45 , form a second electrolyzer volume of the electrolyzer  1 . Starting liquid  50  can be removed from the second separator container  6  through a blow-off line  14 . To this end, the blow-off line  14  contains a blow-off valve  31 , by means of which the blow-off line  14  can be opened and closed. Second product gas  30  can be removed from the second separator container  6  via a second output line  37 . In the second output line  37 , there is a second pressure regulating valve  36  for regulating a gas pressure of the second product gas  30 . 
         [0041]    The embodiment of the test device  3  as represented in  FIG. 1  comprises a measuring device  8  for detecting a liquid amount of the starting liquid  50  in each of the two electrolyzer volumes, as well as an evaluation unit (not represented) for determining the liquid flow rate of the starting liquid  50  between the two electrolyzer volumes with the aid of the measurement values detected by the measuring device  8 . For each separator container  5 ,  6 , the measuring device  8  comprises a filling level sensor  9  for detecting a filling level of the starting liquid  50  in the container volume of the respective separator container  5 ,  6 , and/or a pressure sensor  15  for detecting a gas pressure in the container volume of the respective separator container  5 ,  6 . 
         [0042]    In the exemplary embodiment represented in  FIG. 1 , for each separator container  5 ,  6 , the measuring device  8  comprises both a filling level sensor  9  and a pressure sensor  15 . In simpler exemplary embodiments, the measuring device  8  comprises either a filling level sensor  9  or a pressure sensor  15  for each or for only one of the separator containers  5 ,  6 . 
         [0043]    According to a first exemplary embodiment of a method for checking membrane leaktightness of the electrolyzer  1 , the electrolysis is interrupted for an interruption time, and a test procedure for checking the membrane leaktightness is carried out during the interruption time. For the test procedure, the two electrolyzer volumes are initially filled with mutually different defined liquid amounts of the starting liquid  50 . To this end, one of the two separator containers  5 ,  6  is filled with starting liquid  50  up to a specified first filling level and the other of the two separator containers  5 ,  6  is filled with starting liquid  50  up to a specified second filling level, which is different to the first filling level. 
         [0044]    In some embodiments, the gas pressures in the two separator containers  5 ,  6  are furthermore equalized to one another. To this and, for example, the gas pressures in the two electrolyzer volumes are equalized to an ambient pressure in an environment of the electrolyzer  1 . 
         [0045]    Subsequently, a time requirement for equalization of the liquid amounts in the two electrolyzer volumes is determined with the aid of a liquid flow rate that has been determined between the two electrolyzer volumes. To this end, a difference between the filling levels of the starting liquid  50  and/or between the gas pressures in the two separator containers  5 ,  6  is repeatedly determined and evaluated by means of the measuring device  8 . 
         [0046]    The time requirement for equalization of the liquid amounts in the two electrolyzer volumes is, for example, either directly measured by detecting the time until the liquid flow rate vanishes or until a specified liquid amount difference between the liquid amounts or a specified gas pressure difference between the gas pressures in the separator containers  5 ,  6  is reached, or by determining a time requirement for equalization of the two liquid amounts with the aid of an extrapolation of the detected liquid flow rates. 
         [0047]    In some embodiments, a mathematical model of a time variation of the equalization of the liquid amounts may be used to determine the time requirement. For the case in which the liquid filling levels in the separator containers  5 ,  6  correlate linearly with the liquid amounts, as is the case for common forms of separator containers  5 ,  6 , it is for example assumed that the filling level difference Δh between the liquid filling levels in the separator containers  5 ,  6  decreases exponentially with time t according to Δh(t)=h 0 ·exp(−kt), where k is a constant that is a measure of the time requirement for equalization of the liquid filling levels in the two separator containers  5 ,  6 . Evaluation of the logarithmic values ln(Δh) of the measurement values for the filling level difference Δh as a function of time t allows approximate determination of the constant k from the slope of the straight line plotted through these logarithmic values. 
         [0048]    A leak of at least one membrane  7  is, for example, inferred when the time requirement for equalization of the liquid amounts in the two electrolyzer volumes as determined during the test procedure, is less than a specified time requirement threshold value. 
         [0049]    The described test procedure may also be carried out two times in succession, the roles of the separator containers  5 ,  6  being interchanged so that the first time the test procedure is carried out, for example, the first separator container  5  is filled with a larger liquid amount of the starting liquid  50  than the second separator container  6 , while the second time the test procedure is carried out the second separator container  6  is filled with a larger liquid amount of the starting liquid  50  than the first separator container  5 . In this way, the reliability of the checking of the membrane leaktightness can be increased since systematic disruptive effects can be found. 
         [0050]    As an alternative or in addition, the membrane leaktightness of the electrolyzer  1  is checked during electrolysis. To this end, the test device  3  may comprise an ammeter  60  for detecting an electrolysis current strength of the electrolyzer  1 . During electrolysis, an electrolysis current strength is detected by means of the ammeter  60  and a liquid flow rate of the starting liquid  50  between the two electrolyzer volumes is determined by means of the measuring device  8 . The liquid flow rate is, for example, in this case determined by determining the time variation of a liquid volume of the starting liquid  50  in at least one of the two electrolyzer volumes. To this end, for example, a time variation of a liquid volume of the starting liquid in the container volume of the separator container  5 ,  6  of the respective electrolyzer volume is determined by repeatedly detecting and evaluating a filling level of starting liquid  50  in the container volume. 
         [0051]    From the liquid flow rate determined and the electrolysis current strength detected, a ratio parameter Q is formed which is proportional to the ratio of the liquid flow rate determined and the electrolysis current strength detected. The ratio parameter Q is used to assess the membrane leaktightness. To this end, a first ratio threshold value Q s1  and a second ratio threshold value Q s2  for the ratio parameter Q are specified, and a leak of at least one membrane  7  is inferred when the ratio parameter Q exceeds the specified first ratio threshold value Q s1  or falls below the second ratio threshold value Q s2 . 
         [0052]    This formation and evaluation of the ratio parameter Q is based on the idea that, particularly when using water as the starting liquid  50 , a few molecules of water, which are not involved in the electrolysis reaction, also pass through a membrane  7  with each molecule of hydrogen. In this case, the ratio of these two substance flows is to a good approximation constant. If a leak of a membrane  7  should occur, an additional transport path is formed so that this ratio is perturbed. The water flow rate is quantified with the aid of the time variation of the filling level of the water in the second separator container  6 . The water flow rate is given as 
         [0000]        dn   w   /dt=c   w   ·A·dh/dt.   [1]
 
         [0053]    In Equation [1], n w  stands for the water amount in the second separator container  6 , c w  stands for the molar concentration of water, A stands for the cross-sectional area of the second separator container  6 , and h stands for the filling level of water in the second separator container  6 . For example, 55.5 mol/l may be used as a numerical value for c w , temperature effects and possibly existing gas bubbles being neglected in this case. Surprisingly, it has been found that such relatively rough approximations nevertheless lead to a reliable method. The time variation of the filling level is expediently calculated with the aid of a linear regression of the temporally discrete filling level values. For example, 10 values may respectively be employed, which are detected at a time interval of 5 seconds each. 
         [0054]    The hydrogen flow through the membranes  7  is calculated with the aid of Faraday&#39;s laws. In this case, the number of active electrolysis cells  4  of the cell block  2  and the electrolysis current strength are taken into account. Furthermore, an electrical efficiency of 100% is assumed. The hydrogen flow is given as 
         [0000]        dn   H2   /dt=a·I /(2· F ).  [2]
 
         [0055]    In Equation [2], n H2  stands for the amount of hydrogen generated, a stands for the number of active electrolysis cells  4  of the cell block  2 , I stands for the electrolysis current strength and F stands for the Faraday constant. 
         [0056]    The ratio of the water flow rate according to Equation [1] and the hydrogen flow according to Equation [2] is therefore proportional to the ratio (dh/dt)/I and therefore to the ratio parameter Q. 
         [0057]    In the consideration above, the ratio of the flow rates is calculated by means of Equations [1] and [2]. Because of approximations used in this case, the actual values may differ slightly from the values calculated according to Equations [1] and [2]. In the case of intact membranes  7 , the ratio of the water flow rate to the hydrogen flow typically assumes a single-figure numerical value, so that for example the numerical value  10  may be set as an upper limit beyond which a membrane  7  is considered defective. In principle, however, constant factors, for example the cross-sectional area A of the second separator container  6  or the number a of active electrolysis cells  4 , do not need to be taken into account for the definition of the ratio parameter Q and the ratio threshold values Q s1 , Q s2 , so that the pure numerical value (and the unit) of the ratio threshold values Q s1 , Q s2  may be adapted accordingly. 
         [0058]      FIG. 2  shows a diagram of a profile of such a ratio parameter Q as a function of time t, values determined for the ratio parameter Q being represented as crosses. At an overshoot time t 0 , the ratio parameter Q exceeds the first ratio threshold value Q s1 . It is inferred therefrom that at least one membrane  7  has a leak at the overshoot time t 0 . A leak of at least one membrane  7  is correspondingly inferred when the ratio parameter Q falls below the second ratio threshold value Q s2 . The time fluctuations of the ratio parameter Q are attributable to fluctuations of the electrolysis current strength, the temperature and the system pressure. Although the influences of these fluctuations of the electrolysis current strength, the temperature and the system pressure may be reduced by replacing the ratio parameter Q with a simplified parameter, such a simplification is however generally unnecessary since the effects of a leak of a membrane  7  greatly surpass the influences of fluctuations of the electrolysis current strength, the temperature and the system pressure. 
         [0059]    Although the invention has been illustrated and described in detail by exemplary embodiments, the teachings are not restricted to the examples disclosed and other variants may be derived therefrom by the person skilled in the art without departing from the protective scope of the claims below.