Determining an aging state of at least one electrolytic cell of an electrolysis system

Abstract

The invention relates to a method for determining an aging state of at least one electrolytic cell (12) of an electrolysis system (14), wherein a specified electrical current is applied to the at least one electrolytic cell (12), an electrical cell voltage (22) of the at least one electrolytic cell (12) is measured, and the specified current is switched off at a specified switch-off time. According to the invention, the cell voltage (22) measured in a specified evaluation period is evaluated and the aging state is determined on the basis of the evaluation, a start time of the evaluation period coinciding with the switch-off time.

Description

[0001] 2024PF00515

[0002] 1

[0003] Description

[0004] Determining the aging state of at least one electrolysis cell in an electrolysis plant

[0005] The invention relates to a method for determining an aging state of at least one electrolysis cell of an electrolysis system, wherein the at least one electrolysis cell is supplied with a predetermined electric current, an electric cell voltage of the at least one electrolysis cell is detected, and the predetermined current is switched off at a predetermined switching-off time.Furthermore, the invention relates to a device for determining the aging state of at least one electrolysis cell of an electrolysis system, wherein the device comprises at least one current source, a voltage sensor and a control unit, wherein the current source is configured to supply at least one electrolysis cell with a predetermined electric current, wherein the voltage sensor is configured to detect an electric cell voltage of the at least one electrolysis cell, and the control unit is configured to control the current source in such a way as to switch off the predetermined current at a predetermined switch-off time.

[0006] Electrolysis cells and electrolysis systems with a plurality of electrolysis cells are extensively known in the prior art, so that no separate written proof is required in this regard.

[0007] Electrolysis plants typically consist of a large number of electrolysis cells, which are at least partially connected in series. The electrolysis cells serve, among other things, to produce substances that are preferably usable on an industrial scale, for example, hydrogen in hydrogen electrolysis, carbon monoxide in carbon dioxide electrolysis, or the like. For this purpose, at least two electrodes of each electrolysis cell must be connected to a 2024PF00515 during normal operation.

[0008] Two suitable small DC voltages are applied, which can be in the range of a few volts or possibly even less than 1 V. Depending on the amount of substance to be produced by electrolysis, a DC current is supplied by a power source to the electrolysis cell. In electrolysis cells connected in series, this current flows through all of the cells connected in series. The series connection is electrically coupled to the power source. However, it is also possible, in principle, to connect electrolysis cells not only in series but also, at least partially, in parallel.

[0009] Particularly in aqueous electrolysis processes, such as chlor-alkali electrolysis, PEM electrolysis, or similar processes, a membrane is often used to separate the respective reaction chambers of the respective reaction areas within a given electrolysis cell, in which the respective electrodes are arranged. A catalyst is frequently also placed on such a membrane to enable or accelerate the electrolysis process. Electrolysis is generally achieved by applying the cell current or a suitable direct current voltage, also called cell voltage, to the electrodes during normal operation.

[0010] A transition to or from an operating state other than the intended operating state proves to be at least partially critical for the electrolysis cell. This applies in particular to the start-up and shutdown of the electrolysis cell or electrolysis system. Especially during shutdown after intended operation, residual substances, particularly residual gases, remain in the electrolysis cell, which can potentially lead to a 2024PF00515 issue within the electrolysis cell.

[0011] 3

[0012] Fuel cell functionality can occur. However, this can irreversibly damage the electrolysis cell, which is why fuel cell functionality should be avoided.

[0013] For this purpose, it is known to apply a protective voltage, also called a polarization voltage, to the electrolysis cell outside of its intended operation. The protective voltage is generally selected such that the fuel cell functionality can be largely avoided. For an electrolysis cell used to electrolyze water, the protective voltage can, for example, be in the range of approximately 1.25 V. Once the electrolysis cell has cooled sufficiently and residual gases have been removed, the protective voltage can be deactivated. For example, EP 3 982 501 A1 discloses a corresponding device with which electrolysis cells can be protected.

[0014] Significant parasitic electrical currents can occur, particularly in electrolysis cells that utilize proton exchange membranes (PEMs). These parasitic currents do not contribute to electrolysis. Instead, they lead to losses and negatively impact the overall performance of the electrolysis system. Parasitic currents influence the characteristics of the electrolysis cell, especially its electrical properties, making it difficult to predict cell aging. It has been shown that parasitic currents increase with the age of the electrolysis cell and are an aspect of this aging. Once present, parasitic currents are essentially irreversible.However, accurate monitoring of parasitic currents is difficult both in laboratory settings and especially in commercial applications. 2024PF00515.

[0015] 4

[0016] There is a desire to be better informed about the parasitic current, and in particular the aging state, of the electrolysis cell. The operation of the electrolysis plant can be controlled, among other things, depending on the electrolysis cells, especially their aging states. In particular, it is desirable to know when it is advisable to replace an electrolysis cell with a new one. However, this is only inadequately achievable with the methods available in the prior art.

[0017] The invention is based on the objective of, among other things, better determining the parasitic current, in particular the aging state, of a respective electrolysis cell.

[0018] The invention proposes a method and a device according to the independent claims as a solution.

[0019] Advantageous further training opportunities arise from the characteristics of the dependent requirements.

[0020] With regard to a generic method, the invention particularly proposes that the cell voltage recorded in a given evaluation period is evaluated and the aging state is determined depending on the evaluation, wherein an initial time of the evaluation period coincides with the switching-off time.

[0021] With regard to a generic device, the invention specifically proposes that the control unit is further configured to evaluate the cell voltage recorded during a predetermined evaluation period and to determine the aging state depending on the evaluation, wherein the initial time of the evaluation period coincides with the switch-off time. 2024PF00515

[0022] 5

[0023] The invention is based, among other things, on the finding that there is a correlation between the magnitude of the parasitic current in a given electrolysis cell and the cell voltage curve, which exhibits a decay behavior when a predetermined current is switched off. Within the scope of the invention, it was found that a large parasitic current can accelerate the drop in cell voltage after the predetermined current is switched off. Extensive investigations using the invention revealed that, conversely, an increasing parasitic current causes a temporal acceleration of the cell voltage decrease after the predetermined current is switched off. This finding makes it possible to determine the parasitic current and / or the aging of the electrolysis cell by utilizing its switch-off behavior.

[0024] The invention takes into account that the aging state of the electrolysis cell may depend, among other things, on the parasitic current.

[0025] The specified current is a suitably chosen current that can be determined through experimental series and / or modeling. It has been shown that a suitable value for the specified current is lower than the rated current of the electrolysis cell. The specified current can be chosen, among other things, depending on the active area of ​​at least one electrode of the electrolysis cell. Preferably, the value of the specified current is significantly lower than the rated current.

[0026] The invention avoids using a particularly small, predetermined electrical current or cell current to determine the parasitic current of the electrolysis cell. Because it was found within the scope of the invention that the parasitic current of the electrolysis cell depends on the decay behavior of the cell voltage after the predetermined current is switched off, the parasitic current no longer needs to be determined in a complex manner. Rather, it is sufficient to observe the decay behavior of the cell voltage, preferably under 2024PF00515

[0027] 6

[0028] Further cell-specific parameters are taken into account and evaluated. This allows the parasitic current and thus also the aging of the electrolysis cell to be determined.

[0029] It is important to note that very low cell currents can undesirably result in a so-called crossover, where, for example, hydrogen can flow from a cathode side to an anode side where oxygen is present. This can potentially lead to a critical concentration of hydrogen and oxygen, which should be avoided at all costs. While such a crossover can also occur during normal operation at high currents and high substance flows, in this case, large volumes of substances like hydrogen and oxygen are also generated, so the crossover is usually no longer significant. However, these findings are not limited to electrolysis cells for the electrolysis of water.In principle, these considerations can also be applied to any other electrolysis cells where a different substance is electrolyzed.

[0030] Taking into account the previously explained findings, the invention therefore provides that the cell voltage of the electrolysis cell is recorded and evaluated during the specified evaluation period. Depending on the evaluation, the aging state of the electrolysis cell and / or – as required – the parasitic current can then be determined. It can be taken into account that the aging state can correspond to the parasitic currents associated with that aging state. In the invention, the specified current, i.e., the current that is switched off, can be arbitrary. In particular, the specified current can have a current value that is also used in normal operation. The discharge time or decay time of the electrolysis cell is introduced as a parameter for the parasitic current in the invention. It has been found that the influence of the current value of the specified current on the discharge time 2024PF00515

[0031] 7 or the decay time of the electrolysis cell is secondary, in particular need not be significant.

[0032] The evaluation period begins at the switch-off time. The switch-off time can be determined by a control unit of the device. The control unit can provide a current signal to the power source, which can be used to control the power source. This applies in particular to setting the value of the specified current as well as to switching it off.

[0033] The evaluation period preferably has a predetermined duration, which is selected to match the switch-off behavior of the electrolysis cell. The duration of the evaluation period can also be predetermined by the control unit. The control unit can be configured to control the power source accordingly to switch off the predetermined current. Simultaneously, the control unit can issue a start signal for the beginning of the evaluation period, enabling an evaluation unit of the device, particularly the control unit, to perform the desired evaluation. For this purpose, the evaluation unit can evaluate a sensor signal from the voltage sensor.

[0034] The evaluation period is preferably chosen such that the decay behavior of the cell voltage can be reliably recorded for different aging states. The evaluation period is preferably chosen such that a clear difference between the decay behavior of the cell voltage of a new electrolysis cell and that of a heavily aged electrolysis cell can be readily determined. The evaluation period can therefore be at least approximately one second, preferably at least approximately two seconds. 2024PF00515

[0035] 8

[0036] The invention thus makes it possible to reliably determine aging or parasitic flow.

[0037] Preferably, an end time is specified for the evaluation period. This allows the evaluation to be normalized. In particular, by appropriately selecting the evaluation period, it is possible to limit the evaluation to the necessary timeframe. This also enables the process to be implemented as efficiently as possible. The end time defines the duration of the evaluation period. The end time is preferably chosen such that, at least for one aged electrolysis cell, the cell voltage can have dropped to the reference value within the evaluation period. Specifying the end time can take into account specific parameters or properties of the electrolysis cell. The reference value can be chosen based on properties of the electrolysis cell.Preferably, the reference value can essentially correspond to a value of a polarization voltage for the electrolysis cell.

[0038] According to a further training, it is proposed that the cell voltage during the evaluation period be compared with a reference value, whereby the aging state is determined at least as a function of this comparison. For example, an electrical voltage value could serve as the reference value, which could be used to determine that if the decaying cell voltage of an electrolysis cell does not reach the reference value within the evaluation period, the corresponding aging state of the electrolysis cell does not yet need to be considered critical for its intended operation. The end time can be specified by the control unit. The reference value makes it possible to specify at least one criterion for determining the aging state of the electrolysis cell. Furthermore, the choice of reference value can also determine the 2024PF00515

[0039] The aging state of the electrolysis cell or its construction must be adapted.

[0040] It is further proposed that the reference value be determined as a function of the cell voltage during intended operation of the electrolysis cell. This further development makes it possible to adjust the reference value appropriately and thus further improve the function of the invention. For example, it can be provided that the reference value also increases appropriately with increasing cell voltage. In principle, it is of course also possible to reduce the reference value with increasing cell voltage. This can reduce the transition between an electrolysis cell still suitable for intended operation and one that has aged too much.

[0041] Furthermore, it is proposed that the specified evaluation period extend over less than 20 s, preferably less than 10 s, and particularly preferably less than 7.5 s. Investigations within the scope of the invention have shown that the evaluation period selected in this way is sufficient to distinguish an aged electrolysis cell from a slightly aged or new electrolysis cell. The length of the evaluation period can be selected depending on the desired resolution with respect to aging. This enables a rapid determination of the aging.

[0042] Furthermore, it is proposed that the predetermined current be less than 7%, preferably less than 3%, and particularly preferably less than 1%, of the rated current of the electrolysis cell. It has been shown that the invention can be implemented particularly advantageously with a comparatively small predetermined current. With such a small predetermined current, the effect found with the invention becomes particularly pronounced. 2024PF00515

[0043] 10

[0044] At the same time, the specified current can also be chosen such that the effect with regard to crossover is still acceptable. Overall, the use of the invention can be further improved in this way. It proves advantageous if the evaluation period is also chosen to be short. This allows the undesirable mixture of substances resulting from the crossover to be reliably limited.

[0045] It is further proposed that the specified current be determined as a function of the active area of ​​at least one electrode of the electrolysis cell. This allows for a further improvement because an area current density can be taken into account. The invention can be easily adapted to different electrolysis cell designs.

[0046] It is particularly advantageous if the specified current is determined in such a way that the current density at the active surface of at least one electrode of the electrolysis cell is less than 0.15 A / cm². 2 preferably less than 0.035 A / cm² 2 , especially preferred to be less than 50 pA / cm 2At such surface current densities, the inventive effect has proven to be particularly pronounced. At the same time, the current density is sufficiently high that the crossover effect can still be at least partially tolerated. In this respect, the application of the invention can be further improved.

[0047] Furthermore, it is proposed that the evaluation period ends at a specific time determined by a voltage comparison, where the cell voltage is compared to a predetermined reference voltage. This allows the evaluation period to extend over an adjustable time span. The evaluation period can therefore be specifically adapted to a particular application so that the parasitic current or aging can be reliably determined. On the other hand, 2024PF00515

[0048] 11. The evaluation period can be specified as short as necessary to obtain a result as quickly as possible or to minimize the time required. The reference voltage can have a value, particularly a fixed one. For example, the reference voltage can be selected within a range limited on the one hand by a rated voltage for intended operation and on the other hand by a reversible cell voltage. The rated voltage is preferably a value for the electrical cell voltage applied to the electrolysis cell during intended electrolysis operation. For an electrolysis cell intended for the electrolysis of water, the rated voltage can, for example, be approximately 1.9 V. The reversible cell voltage is preferably an electrical voltage at which the cell current is essentially zero.The reversible cell voltage is generally a substantially fixed value for a given electrolysis cell, which may, for example, depend on at least some design or operating parameters. The reference voltage can be advantageously determined based on a difference between the rated voltage and the reversible cell voltage by adding a predetermined fraction of this difference to the reversible cell voltage. This predetermined fraction can be, for example, approximately 1% to approximately 20%, preferably approximately 1.5% to approximately 10%, and particularly preferably approximately 2.5% to approximately 5%.

[0049] It is further proposed that a parasitic current be determined to ascertain the aging state. The parasitic current can be determined, for example, using a model of the electrolysis cell based on the parameters explained above. The model preferably takes into account the electrical properties of the electrolysis cell. For example, it has been shown that the electrolysis cell exhibits, among other things, capacitive properties, particularly with regard to switching off the specified current. The parasitic current of the electrolysis cell can be described as parasitic 2024PF00515

[0050] 12. Electrical resistance is taken into account in the model by an electrical resistance element connected in parallel to the electrolysis cell. The resistance value of this electrical resistance element decreases with increasing aging of the electrolysis cell. This also explains the increase in parasitic current with increasing age of the electrolysis cell. The model can further consider fluid dynamic properties and properties relating to the electrolysis process. Since there is a correlation between the parasitic current and the aging state, the aging state of the electrolysis cell can be determined with a high degree of reliability.

[0051] Furthermore, it is proposed that the parasitic current be determined using a cell model of the electrolysis cell. This cell model specifically considers the electrical properties of the cell. In addition, properties relating to substance transport, electrochemical properties relevant to electrolysis, and / or the like can also be taken into account.

[0052] The advantages and effects stated for the method according to the invention also apply equally to the device according to the invention, and vice versa. Therefore, method features can also be formulated as device features, and vice versa.

[0053] The features and combinations of features mentioned above in the description, as well as the features and combinations of features mentioned below in the figure description and / or shown in the figures alone, can be used not only in the combination specified in each case, but also in other combinations without leaving the scope of the invention.

[0054] The following examples of implementation are preferred embodiments of the 2024PF00515

[0055] 13

[0056] Invention. The features and combinations of features specified above in the description, as well as those mentioned in the following description of exemplary embodiments and / or shown in the figures, are not only usable in the combinations specified, but also in other combinations. Thus, embodiments of the invention are also included or considered to be disclosed that are not explicitly shown and explained in the figures, but which can be derived and produced from the explained embodiments by separate combinations of features. The features, functions, and / or effects illustrated with reference to the exemplary embodiments can each, in themselves, represent individual features, functions, and / or effects of the invention, which can be considered independently of one another and each further develop the invention independently.Therefore, the exemplary embodiments should also include combinations other than those described in the embodiments already explained. Furthermore, the described embodiments can also be supplemented by further features, functions and / or effects of the invention already described.

[0057] In the figures, the same reference symbols denote the same features or functions.

[0058] They show:

[0059] FIG 1 shows a schematic block diagram of an electrolysis plant with one electrolysis cell.

[0060] FIG 2 a schematic diagram representation of several

[0061] Measurement runs of the electrolysis cell according to FIG 1 ,

[0062] FIG 3 shows a schematic diagram of voltage waveforms on electrolysis cells after switching off a predetermined current in a first experimental setup, where the electrolysis cells were operated for 41 hours, 2024PF00515

[0063] 14

[0064] FIG 4 shows a schematic diagram representation as in FIG 3, where the electrolysis cells were operated for approximately 2500 hours.

[0065] FIG 5 shows a schematic diagram representation as in FIG 3 in a second experimental setup, where the electrolysis cells were operated for 95 hours.

[0066] FIG 6 shows a schematic diagram representation as in FIG 5, where the electrolysis cells were operated for 1147 hours.

[0067] FIG 7 shows a schematic circuit diagram of a model for a new electrolysis cell in which essentially no parasitic current occurs.

[0068] FIG 8 shows a schematic diagram representation of the behavior of the electrolysis cell according to FIG 7.

[0069] FIG 9 shows a schematic circuit diagram as in FIG 7 for an aged electrolysis cell in which a parasitic current occurs.

[0070] FIG 10 shows a schematic diagram representation of voltage waveforms at different parasitic resistances for the electrolysis cell according to FIG 9.

[0071] FIG 11 shows a schematic diagram representation of polarization curves at different parasitic resistances for the electrolysis cell according to FIG 9.

[0072] FIG 12 shows a schematic diagram representation of a simulated influence of a parasitic resistance on an activation overpotential of a Tafel equation, and

[0073] FIG 13 shows a schematic diagram of a decay time depending on a parasitic current, determined by analyzing a polarization behavior. 2024PF00515

[0074] 15

[0075] FIG. 1 shows a schematic block diagram of an electrolysis plant 14, of which a single electrolysis cell 12 is shown, and a device 10 for determining the aging state of the electrolysis cell 12. In addition to the electrolysis cell 12 shown, the electrolysis plant 14 includes further electrolysis cells not shown. The electrolysis plant 14 is used here for the production of hydrogen and oxygen by the electrolysis of water. Also not shown are further peripheral devices, which serve, among other things, to guide and control the substance flows that occur during electrolysis.

[0076] As can be seen from FIG. 1, the electrolysis cell 12 comprises, among other things, a PEM coated with a catalyst material. Each surface of the PEM is assigned an electrode 24, 26, with one of the electrodes 24, 26 forming an anode and the other two forming a cathode. In normal electrolysis operation, water is supplied to the electrolysis cell 12, and the hydrogen and oxygen produced by the electrolysis are removed from the electrolysis cell 12. The function and basic structure of the electrolysis cell 12 are known to those skilled in the art, which is why a detailed description is omitted here. In normal operation, in which hydrogen and oxygen are produced by electrolysis of water, the electrolysis cell 12, in particular its electrodes 24, 26, is electrically connected to a power source that provides the cell current required for electrolysis.A cell voltage 22 is established between electrodes 24, 26.

[0077] FIG. 1 further shows that the device 10 is connected to exactly one electrolysis cell 12. In alternative embodiments, it would be conceivable to connect the device 10 to a series connection of several electrolysis cells 12. 2024PF00515

[0078] 16

[0079] In this embodiment, the device 10 comprises, in addition to other units not shown, a power source 16, a voltage sensor 18, and a control unit 20. The power source 16 provides a preset electric current with which the electrolysis cell 12 can be supplied. This will be explained in more detail below. For this purpose, the power source 16 is electrically connected to the electrodes 24, 26 of the electrolysis cell 12.

[0080] Furthermore, the device 10 includes the voltage sensor 18, which is also electrically connected to the electrodes 24, 26. The voltage sensor 18 can detect the cell voltage 22 of the electrolysis cell 12. Depending on the detected cell voltage 22, the voltage sensor 18 provides a voltage signal 34 as a sensor signal.

[0081] The control unit 20 is connected to the current source 16 and the voltage sensor 18 via a signal connection. Thus, the control unit 20 receives the voltage signal 34 from the voltage sensor 18. Furthermore, the control unit 20 can control the current source 16 with a control signal 36. Specifically, the control unit 20 can use the control signal 36 to adjust the current with which the current source 16 supplies the electrolysis cell 12. In particular, the control unit 20 can set a predefined current. Moreover, the control unit 20 can control the current source 16 to switch off the predefined current at a predefined switch-off time.

[0082] The control unit 20 is further configured to evaluate the cell voltage 22 recorded during a predetermined evaluation period 42 and to determine the aging state depending on the evaluation, wherein the evaluation period 42 begins at the switch-off time to. For this purpose, the control unit 20 has an evaluation unit 30, which in turn has a comparison unit 32. As a result of the evaluation, the control unit 20 provides an evaluation signal 2024PF00515.

[0083] 17

[0084] 38, by means of which data relating to an aging state of the electrolysis cell 12 or also a value of a parasitic current can be provided. These functions will be explained in more detail below.

[0085] FIG. 2 shows a schematic diagram representation of several measurement runs of different electrolysis cells 12 according to FIG. 1, with each measurement run represented by a graph. An abscissa represents the time in seconds and an ordinate the cell voltage 22 in volts. A graph 40 shows the reference value, which in this case is approximately 1.25 V. The electrolysis cell 12 is supplied with a different predetermined electric current in each run. This results in current densities of approximately 33 mA / cm² at the electrodes 24 and 26. 2 up to approximately 33 pA / cm 2At a given time to, the specified current is switched off. This is illustrated by graph 44. At time to, the evaluation period 42 is started, during which the cell voltage 22 is recorded and compared with the reference value 40. As soon as the decaying cell voltage 22 reaches the reference value, the cell current is recorded. This is evident from the further graphs in FIG. 2. The cell current recorded here serves as an indicator for the parasitic current. From FIG. 2, it can be seen that the parasitic current can vary depending on the current density, which was present for approximately one hour of operation. It is evident that the parasitic current varies from about 1 A to about 10 mA. This led to the suggestion for the invention to investigate the switch-off behavior of the electrolysis cells 12 with respect to the respective parasitic current.In particular, the relaxation time required by electrolysis cell 12 until the cell voltage 22 reaches the reference value after the current is switched off should be investigated. A reversible cell voltage is indicated by graph 92. 2024PF00515.

[0086] 18

[0087] Figures 3 and 4, each a schematic diagram, illustrate test series showing the dependence of the parasitic current on the decay behavior of the cell voltage 22 of the respective electrolysis cell 12 when the specified current is switched off. An ordinate represents the cell voltage 22 and an abscissa represents time. In Figure 3, graphs UL to U6 represent the cell voltages 22 of six electrolysis cells. The electrolysis cells 12 are in near-new condition. The current is switched off at time zero. The diagram shows that graphs UL to U6 represent the cell voltages 22 very closely spaced, and only after a considerable time interval, which can be approximately 45 s, is a reference voltage reached, which is represented in Figure 3 by graph 88. In this configuration, the reference voltage is approximately 1.25 V.At this time, a parasitic current of approximately 10 mA was detected.

[0088] FIG. 4 shows a schematic diagram representation similar to FIG. 3, where the electrolysis cells 12 were operated in electrolysis mode for approximately 2500 hours as intended. Graphs U1 to U6 are again assigned to the respective electrolysis cells as in FIG. 3. In contrast to the situation in FIG. 3, two of the electrolysis cells 12 have aged significantly, as can be seen from graphs U2 and U6. A parasitic current of approximately 0.6 A was determined for graph U2, and a parasitic current of approximately 0.3 A for graph U6. A parasitic current of approximately 10 mA was determined for the other graphs. The corresponding electrolysis cells therefore show hardly any aging effect.

[0089] FIGS. 5 and 6 show schematic diagrams similar to FIGS. 3 and 4 for a further series of measurements with twelve electrolysis cells 12. For the illustration in FIG. 5, the electrolysis cells 12 were operated in electrolysis mode for approximately 100 hours as intended. For the illustration in FIG. 6, see 2024PF00515.

[0090] The same electrolysis cells 12 were operated in electrolysis mode for approximately 1100 hours as intended. The diagrams show the respective cell voltages 22 of the twelve electrolysis cells 12, labeled Ul to U12. Parasitic currents were determined according to Table 1 below.

[0091] Table 1

[0092] Here too, taking into account the graphs for Ul to U12, the previously explained dependence between the respective parasitic current or the respective aging and the decay behavior of the respective cell voltage 22 becomes apparent. The larger the value of the parasitic current, the faster the cell voltage 22 decays.

[0093] FIG 7 shows a schematic circuit diagram of a model for a new electrolysis cell 12 in which essentially no parasitic current occurs. The model has an equivalent electrical circuit. The equivalent circuit includes a capacitor Cdi, which represents a cell capacitance 48 of the electrolysis cell 12. The 2024PF00515

[0094] 20

[0095] The cell capacity 48 is determined in particular by the properties of the catalyst-coated membrane 28. A cell loss resistance 50 is connected in parallel to the cell capacity 48, representing electrical losses within the electrolysis cell 12 during normal operation. The cell loss resistance 50 depends, among other things, on the current normal electrolysis operation. A line resistance 46 is connected in series with the previously described parallel circuit. The line resistance 46 accounts for electrical losses from the electrical leads to the electrodes 26, 26. The cell voltage 22 is established at the terminals of the equivalent circuit. The resistance value of the cell loss resistance 50 can be determined using the following equation, which is also called the Tafel equation:

[0096] FIG. 8 shows a schematic diagram representing the behavior of the electrolysis cell 12 based on the equivalent circuit diagram according to FIG. 7. In the diagram according to FIG. 8, the left ordinate represents the cell voltage 22 and the right ordinate represents the cell current. An abscissa represents time. Initially, the electrolysis cell 12 is supplied with the specified current. At time t0, the current is switched off. The cell voltage 22 then exhibits a profile represented by graph 52. Graph 54 represents the current. The typical functionality of the electrolysis cell 12 can be seen from FIG. 8.

[0097] FIG 9 shows a schematic circuit diagram as in FIG 7, where the model according to FIG 7 is extended for an aged electrolysis cell 12 in which a significant parasitic current occurs. Accordingly, the model is extended by a 2024PF00515

[0098] 21

[0099] The parasitic resistance 56 is extended, which is connected in parallel to the cell loss resistance 50 and the cell capacitance 48. This makes it possible to model the effect of the parasitic current.

[0100] The cell voltage 22 can be represented by the following equation:

[0101] Since the focus here is on small current densities and small overpotentials, an activation overpotential can be described by the Butler-Volmer equation given below:

[0102] As can be seen, the parasitic current does not contribute to electrolysis. The parasitic current is diverted around the elements that cause electrolysis, as can be seen from the parallel connection of the parasitic resistance 56 to the cell loss resistance 50 and the cell capacitance 48. As can be seen from FIG. 10, this leads to a faster discharge of the cell capacitance 48.

[0103] FIG. 10 shows a schematic diagram of the cell voltage 22 curves at different parasitic resistances 56 of the electrolysis cell 12 according to FIG. 9. One ordinate is assigned to the cell voltage 22, while one abscissa is assigned to time. Graphs 58 to 66 show the respective values ​​for the parasitic resistance 56. Graph 58 corresponds to the value 10 Ω cm⁻¹. 2 assigned. The graph 60 corresponds to the value 100 Q cm. 2 assigned to graph 62 is the value 500 Q cm. 2 assigned. The value 1000 Q cm is assigned to graph 64. 2 assigned. A very large value is assigned to graph 66. From FIG 10 it can be seen that particularly small resistance values ​​of the parasitic resistor 2024PF00515

[0104] 22

[0105] 56 cause a faster decay behavior of the cell voltage 22, as can be seen from graphs 58 and 60.

[0106] FIG 11 shows a schematic diagram of polarization curves for different parasitic resistances 56 for the electrolysis cell 12 according to FIG 9. An ordinate is assigned to a polarization value, whereas an abscissa is assigned to time. Graphs 68 to 76, which correspond to graphs 58 to 66 with respect to the resistance values, represent the corresponding polarization curves. As can be seen from FIG 11, the parasitic resistance 56 has a negligible effect on the polarization curves.

[0107] It is evident that the influence of the parasitic current is particularly noticeable during the decay behavior of the cell voltage 22 and in the polarization curves at low current densities. This is illustrated in FIG. 12. FIG. 12 shows a schematic diagram of a simulated influence of a parasitic resistance with respect to an activation overpotential in a table region. One ordinate is assigned to the activation potential and one abscissa to the current density at turn-off. Graphs 78 to 86, corresponding to the resistance values ​​of graphs 58 to 66, represent the corresponding activation overpotentials. From FIG. 12, it can be seen that the desired effect occurs particularly at current densities less than approximately 0.1 A / cm². 2 are . The effect is particularly evident at a current density of less than approximately 0.2 A / cm². 2 is .

[0108] FIG 13 shows a schematic diagram of the decay time as a function of the parasitic current, determined by analyzing a polarization behavior. The ordinate represents the decay time and the abscissa the determined parasitic current. Points in the diagram were determined by measurement, whereas a range of 90° was calculated using the previously described model 2024PF00515.

[0109] Figure 23 shows that the accelerated decay of the cell voltage 22 and the blocking of the charge transfer resistance at low current densities during operation are essentially determined by the parasitic current. Therefore, the parasitic resistance can be determined by investigating parameters on small electrolysis cells in the laboratory using fixed values ​​for the remaining parameters. The parameters thus obtained can be applied accordingly to large electrolysis cells and their intended operation, in particular to determine the aging of the electrolysis cell 12. Therefore, no further complex investigations are necessary to determine the parasitic current, especially the aging.

[0110] The examples of implementation serve solely to illustrate the invention and are not intended to limit it.

Claims

2024PF00515 24 Patent claims 1. Method for determining the aging state of at least one electrolysis cell (12) of an electrolysis plant (14), wherein the at least one electrolysis cell (12) is supplied with a predetermined electric current, an electric cell voltage (22) of the at least one electrolysis cell (12) is recorded, the predetermined current is switched off at a predetermined switch-off time, characterized in that the cell voltage (22) recorded in a predetermined evaluation period is evaluated and the aging state is determined depending on the evaluation, wherein an initial time of the evaluation period coincides with the switch-off time.

2. Method according to claim 1, characterized in that the cell voltage (22) is compared with a reference value during the evaluation period, wherein the aging state is determined at least depending on the comparison.

3. Method according to claim 2, characterized in that the reference value is determined depending on the cell voltage (22) during intended operation of the electrolysis cell (12).

4. Method according to claim 2 or 3, characterized in that the specified evaluation period extends over less than 20 s, preferably less than 10 s, particularly preferably less than 7.5 s.

5. Method according to one of the preceding claims, characterized in that the predetermined current is less than 10%, preferably less than 5%, particularly preferably less than 1.5% of a rated current of the electrolysis cell (12). 2024PF00515 25 6. Method according to one of the preceding claims, characterized in that the predetermined current is determined depending on an active area of ​​at least one electrode (24, 26) of the electrolysis cell (12).

7. Method according to one of the preceding claims, characterized in that the evaluation period ends at a final time which is determined depending on a voltage comparison in which the cell voltage (22) is compared with a predetermined reference voltage (88).

8. Method according to one of the preceding claims, characterized in that a parasitic current is determined to ascertain the state of aging.

9. Method according to claim 8, characterized in that the parasitic current is determined by means of a cell model of the electrolysis cell (12).

10. Device (10) for determining the aging state of at least one electrolysis cell (12) of an electrolysis plant (14), wherein the device (10) comprises at least one current source (16), one voltage sensor (18), and one control unit (20), wherein the current source (16) is configured to supply the at least one electrolysis cell (12) with a predetermined electric current, wherein the voltage sensor (18) is configured to detect an electric cell voltage (22) of the at least one electrolysis cell (12), and the control unit (20) is configured to control the current source (16) in such a way as to switch off the predetermined current at a predetermined switch-off time, characterized in that the control unit (20) is further configured to evaluate the cell voltage (22) detected in a predetermined evaluation period and, depending on the evaluation, to to determine the state of aging, whereby the start time of the evaluation period coincides with the shutdown time.

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