Method for operating a multiple electrolyzer stack

Abstract

A method of operating a plurality of electrolyzer stacks, each of the plurality of electrolyzer stacks being configured to be supplied with water and electrical energy to electrochemically produce at least a first reactant gas at a first electrode type of each of the electrolyzer stacks, the first reactant gas produced by each of the plurality of electrolyzer stacks being combined into a first gas stream; The method includes determining a concentration of an impurity in the first gas stream originating from a second reactant gas electrochemically produced at a second electrode type of each of the electrolytic cell stacks, generating a trigger signal when a concentration of the second reactant gas impurity in the combined first reactant gas exceeds a specific second reactant gas level, and when the trigger signal is generated, identifying at least one electrolytic cell stack of the plurality of electrolytic cell stacks that is performing poorly with respect to excessive supply of the second reactant gas impurity to the first gas stream by measuring a current density of the at least one electrolytic cell stack of the plurality of electrolytic cell stacks.

Description

[Background technology]

[0001] Many electrolysis processes have industrial applications. For example, the electrolysis of water provides a source of hydrogen and oxygen. Alternatively, the electrolysis of water containing sodium chloride can provide a source of hydrogen, chlorine, and sodium hydroxide. Although other chemistries utilizing water electrolysis are certainly possible, the water electrolysis process essentially involves breaking down water into its components.

[0002] In a water electrolyser, water is split into its components, namely hydrogen gas and oxygen gas. The electrolyser essentially comprises a cathode electrode, an anode electrode, and an electrolyte. Hydrogen is produced at the cathode via the Hydrogen Evolution Reaction (HER). Oxygen is produced at the anode via the Oxygen Evolution Reaction (OER).

[0003] explanation For operation of multiple electrolyzer stacks, it is useful to control the correct performance of each of the electrolyzer stacks and / or each cell of the electrolyzer stack.The present invention relates to the field of electrolysis technology and mainly to an improved method for operating electrolysis cells / stacks with regard to monitoring fault conditions.

[0004] Considering large-scale, multi-megawatt hydrogen production facilities consisting of dozens of water electrolysers, it is necessary to have an efficient method to ensure hydrogen production with high purity and low safety risks.

[0005] When such an electrolyzer park containing multiple electrolyzer stacks is operating at full capacity, oxygen and hydrogen separation within the electrolysis cells may be optimal. However, when reduced renewable energy availability causes the control system to reduce the operating capacity of one or more electrolyzers, pressure gradients within the electrolysis cells may lead to hydrogen diffusion through the membrane to the oxygen side of the cell, or oxygen diffusion to the hydrogen side of the cell.

[0006] Hydrogen diffusion into the oxygen half-cell can create an explosive atmosphere if the safety limit of approximately 2% hydrogen in oxygen is exceeded, while oxygen diffusion into the hydrogen half-cell can reduce the purity of the hydrogen, increasing energy consumption for purification purposes in subsequent process steps and reducing the overall efficiency of the system.

[0007] The use of physical hydrogen and oxygen sensors to monitor conditions at each electrolyzer stack of multiple electrolyzer stacks in a multi-MW facility could lead to the installation of hundreds of sensors. A process requiring hundreds of sensors could be unreliable and have very high maintenance costs associated with ensuring the accuracy of each sensor.

[0008] The present invention is therefore directed to a method of operating a multiple electrolyser stack, an electrolyser stack operating apparatus and uses of the electrolyser stack operating apparatus.

[0009] Advantageous modifications of the invention are set forth in the dependent claims. All combinations of at least two of the features disclosed in the description, the claims and the drawings are included within the scope of the invention. In order to avoid repetition, features disclosed according to the method shall also apply and be claimable according to the mentioned system.

[0010] Throughout this description of the invention, the order of procedural steps is presented so that the process can be easily understood. However, those skilled in the art will recognize that many of the process steps may be performed in a different order and still yield the same or corresponding results. In this sense, the order of the process steps can be changed accordingly. Some features are provided with count words to improve readability or to make the assignment clearer, but this does not imply the presence of certain features. Summary of the Invention

[0011] To achieve these and other advantages, and in accordance with the purposes of the present invention, as embodied and broadly described herein, there is provided a method of operating a plurality of electrolyzer stacks, each of the plurality of electrolyzer stacks configured to be supplied with water and electrical energy to electrochemically produce at least a first reactant gas at a first electrode type of each of the electrolyzer stacks, the first reactant gas produced by each of the plurality of electrolyzer stacks being combined into a first gas stream, the method comprising the steps of: In one step, the concentration of impurities originating from the second reactant gas electrochemically produced at the second electrode type of each of the electrolytic cell stacks in the first gas stream is determined. In another step, a trigger signal is generated when the concentration of the second reactant gas impurities in the combined first reactant gas exceeds a certain second reactant gas level. In another step, at least one electrolytic cell stack of the plurality of electrolytic cell stacks is identified, which has a poor performance in terms of excessively feeding the second reactant gas impurities into the first gas stream by measuring the current density of the at least one electrolytic cell stack of the plurality of electrolytic cell stacks when the trigger signal is generated.

[0012] The electrolyzer stack may be a stack of electrolyzer cells stacked and electrically coupled to each other with respect to the power supplied to the stack, each electrolyzer cell may include an anode electrode and a cathode electrode, which are of different electrode types and are arranged on either side of a membrane that may act as an electrolyte. The amount of gas electrolytically produced at the anode electrode and / or the cathode electrode may be collected from each electrolyzer cell to establish an anode flow of gas for the anode electrode and a cathode flow of gas at the cathode electrode of each electrolyzer stack.

[0013] The electrolyzer stack may electrochemically produce a first reactant gas at an anode electrode and simultaneously produce a different first reactant gas electrochemically at a cathode electrode of the electrolyzer stack. The first reactant gas may be a reaction product formed by the electrolytic reaction at each type of electrode.

[0014] The trigger signal may be, for example, an alarm or an alarm signal that is generated when the concentration of the second reactant gas, as determined by a gas-sensing sensor, exceeds a certain limit.

[0015] The first reactant gas streams of the multiple individual electrolyzer stacks can be combined by a system of pipes fluidly connected to each electrolyzer stack, the pipes being connected accordingly. The combined first gas stream can be collected using a tank, the tank being connected accordingly to the system of pipes. Must we mention that impurities in some stacks can exceed the threshold as long as all mixtures remain below the threshold?

[0016] According to one embodiment, identifying a poorly performing electrolyzer stack by measuring the current density of the electrolyzer stack may alternatively or additionally be performed by measuring the current density of a subsystem of the electrolyzer stack, e.g. as part of an electrolyzer stack cell and / or an individual electrolyzer stack cell.

[0017] According to one embodiment, the measurement of the current density of the at least one electrolyzer stack may be triggered by a trigger signal and / or the measurement of the current density may be continuously monitored and the measurement value may be analyzed when triggered by the trigger signal.

[0018] In other words, the method provides a solution where instead of monitoring the status of all electrolyzer stacks of a plurality of electrolyzer stacks with a plurality of gas-sensing sensors, at least one sensor can be placed in the first gas stream and / or in a storage tank for the combined and / or collected reactant gases, e.g. hydrogen gas and oxygen gas, produced by several electrolyzer stacks. If the impurity level, i.e. the concentration of the second reactant gas, in the first gas stream and / or in the storage tank, respectively, is within the required safety and quality levels, e.g. less than 2% hydrogen gas in oxygen gas, all electrolyzer stacks feeding the storage tank are considered to be operating correctly. If a threshold value of the impurity of the second reactant gas in the first reaction case is exceeded, an alarm signal is triggered and an algorithm can be used to identify the defective electrolyzer stack.

[0019] When the alarm signal is triggered, the control system as the electrolyzer stack operating device can determine the current density generated by the individual electrolyzer stacks that feed the storage tank. For each of these electrolyzer stacks, a calibration curve can be pre-set that relates the determined current density of the individual electrolyzer stack to the concentration of hydrogen gas in the generated oxygen gas, so that the determined value of the current density can be used as a soft sensor or virtual sensor for determining the impurity concentration.

[0020] Hydrogen gas may be generated by a diffusion process from the cathode electrodes of the cells of the electrolyzer stack to the respective anode electrodes of the oxygen side of the cells of the electrolyzer stack. This means that the anode hydrogen content can be determined by using a calibration curve.

[0021] Using an algorithm to determine the current density of each individual electrolyzer stack by comparing the actual current density to a calibration curve can be used to identify faulty or underperforming electrolyzer stacks and the power provided to the underperforming electrolyzer stack can be bypassed or cut off.

[0022] As a result, poorly performing electrolyzer stacks can be maintained, so that the hydrogen gas concentration in the produced oxygen gas can be kept below safe limits, and for hydrogen gas production, the oxygen gas concentration in the produced and collected hydrogen gas can be maintained below a required quality level.

[0023] Advantageously, based on existing calibration curves of the electrolyzer stacks, the amount of anode hydrogen and cathode oxygen can be determined for each electrolyzer stack in real time without disconnecting the individual electrolyzer stacks. An early detection system can be set up by measuring the impurity concentration of the anode hydrogen and / or cathode oxygen in the combined first gas stream in the pipe and / or tank. As a result, redundancy can be provided for the physical sensors placed in the collection tanks where the hydrogen and oxygen gases are stored, thereby improving the reliability of the system.

[0024] This means that instead of using individual sensors to monitor the impurity concentration of each electrolyzer stack, at least one sensor can be placed in the combined first gas stream and / or in a storage tank, where the first gas streams as hydrogen gas streams and / or oxygen gas streams produced by several electrolyzer stacks are collected. If the impurity level determined by the sensor exceeds a threshold value, an alarm or trigger signal can be generated and an algorithm can be used to identify the faulty electrolyzer stack by measuring the current density of each electrolyzer stack using the determined current density value as a soft sensor.

[0025] According to one embodiment, the first reactant gas electrochemically produced at the first electrode type is oxygen and the second reactant gas electrochemically produced at the second electrode type is hydrogen. Alternatively or additionally, the first reactant gas electrochemically produced at the first electrode type is hydrogen and the second reactant gas electrochemically produced at the second electrode type is oxygen.

[0026] That is, oxygen gas can be produced as a first reactant gas at the anode electrode, and / or hydrogen can be produced as a first reactant gas at the cathode electrode.

[0027] This means that using the methods described above, the hydrogen content in the oxygen stream can be monitored in this manner, as can the oxygen content in the hydrogen stream, and corrective action can be taken if a corresponding impurity level threshold is reached. This can increase the purity level of oxygen gas in the hydrogen gas, and by determining the hydrogen level in the oxygen gas, this soft sensing can reduce safety risks.

[0028] According to one embodiment, identifying at least one low performing electrolyzer stack from the plurality of electrolyzer stacks is performed by comparing a measured current density of the at least one electrolyzer stack with a critical current density. In particular, the at least one low performing electrolyzer stack may be identified if the measured current density of the electrolyzer stack is below the critical current density.

[0029] This is because the typical relationship between current density and impurity levels in an electrolyser stack is inversely proportional, which can be illustrated, inter alia, at least as an approximation, by a polarization curve.

[0030] According to one embodiment, when the first reactant gas is oxygen, the specific second reactant gas level is a hydrogen reactant gas level. When the specific second reactant gas level is an oxygen reactant gas level, the first reactant gas is hydrogen. In particular, the hydrogen reactant gas level and the oxygen reactant gas level may be different.

[0031] According to one aspect, a current density for each of some of the plurality of electrolyzer stacks is measured, and each of the measured current densities is compared to an individual critical current density assigned to each of the number of electrolyzer stacks to identify a single low performing electrolyzer stack having a lowest measured current density below its individual critical current density, and the low performing electrolyzer stack is identified.

[0032] In this regard, the identified poor performing electrolyzer stacks can be considered as the electrolyzer stacks that produce the highest percentage of impurities because their current densities are the lowest below their respective critical current densities.

[0033] In other words, the worst performing electrolyzer stack in terms of impurity production should be identified and this identified electrolyzer stack can be specifically disabled or shut down.

[0034] According to one aspect, the critical current density is updated during operation of the multiple electrolyzer stacks to account for degradation of the electrolyzer stacks.

[0035] During operation, the membranes may age or degrade, eventually resulting in a higher diffusion rate of gas from one electrolytic half-cell to the other of the electrolytic cell of the electrolytic cell stack. This may result in an upward shift in the calibration curve of the electrolytic cell stack and / or a change in the critical current density of the electrolytic cell stack. This may occur over time, e.g., when the 2% impurity threshold is lowered to, e.g., 1.0 A / cm. 2 to 1.2A / cm 2 This means that the change in the current density can result from a higher current density.

[0036] If the current density is determined for each electrolytic cell stack, it allows the calibration curves, respectively the critical current densities, to be updated individually as the electrolytic cells age.

[0037] For example, if the electrolyzer stacks have been in production for two years, the calibration curves and / or critical current densities of all or each of the electrolyzer stacks can be updated. For this update, at least hourly or periodically, the current density and voltage of each of the electrolyzer stacks are measured, in particular to update the calibration curves and / or the critical current densities. The calibration curves can be updated by storing this value in order to operate the electrolyzer stacks at different operating points caused by the amount of electrical energy supplied to the electrolyzer stacks over time. Alternatively or additionally, the production load can be distributed between some of the electrolyzer stacks of the electrolyzer stacks to operate the electrolyzer stacks at different set points, whereby the calibration curves and / or the critical current densities can be determined. In other words, the set points of operation of the electrolyzer stacks can be swept over time.

[0038] According to one embodiment, the individual critical current densities of each of several electrolyzer stacks are updated during operation of these electrolyzer stacks to take into account degradation of these electrolyzer stacks.

[0039] According to one embodiment, the updated critical current density of the electrolyzer stacks and / or the updated individual critical current densities are determined by a linear relationship between the operation time of at least one electrolyzer stack and the critical current density of that electrolyzer stack.

[0040] In other words, based on the aging curve of the respective electrolyzer stack of the calibration curve, the shift of the calibration curve over time can be used to determine the critical current density of a particular individual electrolyzer stack.

[0041] According to one embodiment, the updated critical current densities of the electrolyzer stacks and / or updated individual critical current densities are determined during operation of the plurality of electrolyzer stacks by operating at least some of the electrolyzer stacks of the plurality of electrolyzer stacks at different set points.

[0042] The current density determines how much oxygen and hydrogen are produced at each operating point, which in turn means that by comparing that operating point with a given calibration curve that can be updated during operation, the crossover, respectively diffusion, of one gas to the other side of the membrane of the electrolyzer stack can be determined.

[0043] The production load of individual electrolyzer stacks of the multiple electrolyzer stacks can be shifted by shifting or modifying the operating point of the electrolyzer stack. The set point, respectively operating point, of the electrolyzer stack can be defined by operating conditions, determined primarily by the voltage and current supplied to the electrolyzer stack, at which the electrolyzer stack is operating to produce a specified first reactant gas.

[0044] Each electrolyzer stack can be configured to determine DC and / or AC current and DC and / or AC voltage measurements. In particular, these measurements can be used for electrochemical impedance spectroscopy analysis, which ultimately makes it possible to determine the degradation of the analyzed electrolyzer stack after a period of operation of several hours, based on the results of the electrochemical impedance spectroscopy analysis, and to use as input and / or to update the calibration curve.

[0045] According to one embodiment, the updated critical current density of the electrolyzer stack and / or the updated individual critical current densities are determined by storing values ​​of voltage and current density of the electrolyzer stack during operation of a plurality of electrolyzer stacks at different set points within a dynamic operating time.

[0046] According to one embodiment, the updated critical current density of the electrolyzer stacks and / or the updated individual critical current densities are determined by impedance spectroscopy measurements of at least some of the electrolyzer stacks of the plurality of electrolyzer stacks.

[0047] Advantageously, adapting the set points for operating the electrolyzer stack can be done by the results of impedance spectroscopy measurements, in particular to determine the polarization curves at different frequencies (impedance spectroscopy), or at least at the frequencies excited by the power supply of the electrolyzer stack for operation. Particularly low frequency variations can even be triggered by the entire control system of a plant with multiple electrolyzer stacks via the power supply.

[0048] According to one embodiment, impedance spectroscopy measurements are performed by using harmonics injected by the power supplied to the multiple electrolyzer stack.

[0049] According to one aspect, the identified poorly performing electrolyzer stack is disconnected from its provided electrical energy.

[0050] For this purpose, multiple electrolyzer stacks may be electrically connected in series or, if disconnected, for example by opening a switch, the electrical energy may be bypassed.

[0051] According to one aspect, the above steps of identifying the single electrolyzer stack having the lowest current density and disconnecting it from its provided electrical energy can be repeated until the trigger signal is no longer generated.

[0052] Advantageously, using this algorithm to identify electrolyzer stacks to be disconnected can maintain a high level of production rate of the first reactant gas with multiple electrolyzer stacks while still providing high quality and safety standards.

[0053] According to one aspect, a waiting period can be introduced after an identified electrolyzer stack is disconnected, the waiting period being long enough to allow the electrolyzer stack system to approach a new equilibrium or steady state before a new measurement of the impurity concentration is taken.

[0054] According to one embodiment, the concentration of the impurity in the second reactant gas is determined by a gas-sensing sensor for generating a trigger signal.

[0055] Such a gas sensing sensor may be located in a tank collecting the combined first gas stream and / or the gas sensing sensor may be located in a pipe carrying the combined first gas stream before it enters the tank.

[0056] According to one embodiment, the gas sensing sensor is disposed for sensing in the first gas stream. Alternatively or additionally, the gas sensing sensor is disposed for sensing in a tank configured for the multiple electrolyzer stack for collecting a first reactant gas of the first gas stream.

[0057] An electrolytic cell stack actuator is proposed, a current density measurement input configured to receive a current density measurement value for each of a plurality of electrolyzer stacks; a switch control device configured to be signally coupled to a respective switch of the plurality of electrolyzer stacks to disconnect any one of the plurality of electrolyzer stacks from the supplied electrical energy; a trigger signal interface and / or an interface for a gas-sensitive sensor; a control device, in particular including a computer, signally coupled to the current density measurement input, to the switch control device and to the first signal interface and / or to the interface for the gas sensitive sensor. The control device may be configured to perform the method of operating a multiple electrolyzer stack as described above.

[0058] It is proposed to use an electrolyser stack actuator as described above for controlling a plurality of electrolyser stacks.

[0059] The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this application, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. [Brief description of the drawings]

[0060] [Figure 1] FIG. 1 shows the calibration curve. [Diagram 2] FIG. 2 shows a system for the electrochemical generation of a first reactant gas. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0061] Figure 1 shows the A / cm 2 The calibration curve is sketched generally as a diagram showing the relationship 102 between the current density j measured in kV and the concentration of hydrogen gas in oxygen gas measured in percent, in which a limit curve 104 is drawn for identification of the critical current density j, which may be determined where the curve 102 and the limit curve 104 intersect each other.

[0062] FIG. 2 generally sketches a system for the electrochemical generation of a first reactant gas, which may be oxygen gas or hydrogen gas.

[0063] The system includes a power source 210 and a plurality of electrolyzer stacks 202, 204, 206 configured to be electrically coupled in series to the power source 210. Each of the plurality of electrolyzer stacks 202, 204, and 206 can be disconnected from the power source 210 by switches 203, 205, 207 to bypass the current to the next electrolyzer stack 202, 204, 206. A first reactant gas, which may be electrochemically produced by each of the electrolyzer stacks 202, 204, 206, may be combined into a first gas stream 230, for example, by a system of pipes fluidly coupled to each of the electrolyzer stacks 202, 204, 206, and the first gas stream 230 may be supplied to a tank 220, for example, by a system of pipes.

[0064] The gas sensing sensor may be disposed in the first gas stream 230 and / or in the tank 220. The physical gas sensing sensor may be configured to monitor the amount of oxygen gas in the hydrogen stream when the first gas stream 230 mainly comprises hydrogen gas. Alternatively or additionally, the gas sensing sensor may be configured to monitor the amount of hydrogen gas in the oxygen gas stream when the first gas stream 230 mainly comprises oxygen gas. When hydrogen gas and oxygen gas are also produced by the electrolyzer stack, a second pipe system including a second tank, not shown here, is connected to the electrolyzer stack configured to produce oxygen gas and hydrogen gas.

[0065] If such gas sensitive sensors determine an impurity concentration that exceeds a threshold of, for example, 2% oxygen in hydrogen, the system is configured to generate a trigger or alarm signal, for example by an electrolyzer stack operating device not shown here. Once the trigger signal is generated, the electrolyzer stack operating device can identify an optimal way to reduce the amount of oxygen impurity in the generated hydrogen gas with minimal impact on the hydrogen production rate, for example by bypassing the electrolyzer stack 204 while maintaining the electrolyzer stacks 202 and 206 in operation, as will be explained in more detail by way of examples below.

[0066] The three electrolyzer stacks 202, 204, 206 can be configured to produce hydrogen at different production rates and different oxygen impurity levels. The hydrogen gas thus produced by all three stacks 202, 204, 206 can be directed to be collected into one common storage tank 220. By way of example, stack 202 can be configured to produce 300 ml of hydrogen with 3% oxygen impurity. 3 / h, and the stack 204 is a 200m 3 / h, and the stack 206 is a 800m 3 / h of hydrogen. These hydrogen gas streams, when collected in a storage tank equipped with a physical oxygen sensor, have a total oxygen gas impurity content of 2.07%, which is above the 2% threshold 104.

[0067] When the trigger signal is generated, the electrolyzer stack operating device is configured to compare the real-time values ​​of the current density with the limit values ​​of the calibration curve 102 to determine and / or analyze the current density of each of the individual stacks 202, 204, 206. The electrolyzer stack operating device identifies that the stacks 202 and 204 are producing hydrogen with an oxygen content above the tolerance value of 2%, but the electrolyzer stack operating device also determines that by shutting down the electrolyzer stacks 204, for example by disconnecting the electrolyzer stack 204 with the lowest current density, the overall oxygen level in the storage tank will be lowered from 2.07% to 1.54%, avoiding the need to shut down the electrolyzer stacks 202.

[0068] Thus, the electrolyzer stack actuator can operate switches 203, 205, 207 as shown in FIG. 2 by bypassing electrolyzer stack 204 while keeping electrolyzer stacks 202 and 206 in operation, ensuring quality and safety standards while minimizing the drop in production rate.

[0069] Similarly, the hydrogen content in the oxygen stream may be monitored in this manner so that, when a threshold value is reached, corresponding corrective action may be taken.

Claims

1. A method of operating a plurality of electrolyzer stacks (202, 204, 206), each of the plurality of electrolyzer stacks (202, 204, 206) configured to be supplied with water and electrical energy for electrochemically producing at least a first reactant gas at a first electrode type of each of the plurality of electrolyzer stacks (202, 204, 206), the first reactant gas produced by each of the plurality of electrolyzer stacks (202, 204, 206) being combined into a first gas stream (230); determining a concentration of an impurity in the first gas stream (230) originating from a second reactant gas electrochemically produced at a second electrode type of each of the electrolyzer stacks; generating a trigger signal when the concentration of the impurity of the second reactant gas in the combined first reactant gas (230) exceeds a specified second reactant gas level (104); and identifying at least one electrolytic cell stack of the plurality of electrolytic cell stacks (202, 204, 206) that has poor performance with respect to excessive supply of a second reactant gas impurity to the first gas stream (230) by measuring a current density of at least one electrolytic cell stack of the plurality of electrolytic cell stacks (202, 204, 206) when the trigger signal is generated.

2. 2. The method of claim 1, wherein the first reactant gas electrochemically produced at the first electrode type is oxygen and the second reactant gas electrochemically produced at the second electrode type is hydrogen, or the first reactant gas electrochemically produced at the first electrode type is hydrogen and the second reactant gas electrochemically produced at the second electrode type is oxygen.

3. 3. The method according to claim 1 or 2, wherein identifying at least one low performing electrolyzer stack from the plurality of electrolyzer stacks (202, 204, 206) is performed by comparing the measured current density of the at least one electrolyzer stack with a critical current density (104), in particular identifying the at least one low performing electrolyzer stack when the measured current density is below the critical current density (104).

4. 4. The method according to claim 2 or 3, wherein when the first reactant gas is oxygen, the specific second reactant gas level is a hydrogen reactant gas level, and when the first reactant gas is hydrogen, the specific second reactant gas level is an oxygen reactant gas level, in particular the hydrogen reactant gas level and the oxygen reactant gas level are different.

5. 5. The method of claim 3 or 4, wherein a current density of each of some of the plurality of electrolyzer stacks (202, 204, 206) is measured and each of the measured current densities is compared to an individual critical current density assigned to each of the several electrolyzer stacks to identify a single low performing electrolyzer stack having a lowest measured current density below its individual critical current density, thereby identifying the low performing electrolyzer stack.

6. 6. The method of claim 1, wherein the critical current density is updated during operation of the plurality of electrolyzer stacks (202, 204, 206) to account for degradation of the electrolyzer stacks.

7. 6. The method of claim 5, wherein the individual critical current densities of each of the several electrolyzer stacks are updated during operation of these electrolyzer stacks to take into account degradation of these electrolyzer stacks.

8. 8. The method of claim 6 or 7, wherein the updated critical current density of the electrolyzer stacks and / or the updated individual critical current densities are determined during operation of the plurality of electrolyzer stacks by operating at least some of the electrolyzer stacks of the plurality of electrolyzer stacks at different set points.

9. 9. The method according to any one of claims 6 to 8, wherein the updated critical current density of the electrolyzer stacks and / or the updated individual critical current densities are determined by impedance spectroscopy measurements of at least some of the electrolyzer stacks of the plurality of electrolyzer stacks (202, 204, 206).

10. 10. The method of claim 9, wherein the impedance spectroscopy measurements are performed by using harmonics injected by the power supplied to the multiple electrolyzer stacks.

11. 11. The method of any one of claims 1 to 10, wherein the identified low performing electrolyzer stack is disconnected from its provided electrical energy.

12. 12. The method of claim 11 when dependent on claim 5, wherein the trigger signal is repeated until it is no longer generated.

13. 13. The method of claim 1, wherein a concentration of an impurity in the second reactant gas is determined by a gas-sensing sensor for generating the trigger signal.

14. 10. The method of claim 9, wherein a gas sensing sensor is disposed for sensing in the first gas stream (230) or the gas sensing sensor is disposed for sensing in a tank (220) configured for the plurality of electrolyzer stacks (202, 204, 206) to collect the first reactant gas of the first gas stream (230).

15. 1. An electrolyser stack operating apparatus comprising: a current density measurement input configured to receive a current density measurement of each of the plurality of electrolyzer stacks (202, 204, 206); a switch control device configured to be signally coupled to respective switches (203, 205, 207) of the plurality of electrolyzer stacks (202, 204, 206) for disconnecting any one of the plurality of electrolyzer stacks (202, 204, 206) from the supplied electrical energy; a trigger signal interface and / or an interface for a gas sensitive sensor; 15. An apparatus comprising: a control device, in particular a computer, signal-coupled with the current density measurement input, signal-coupled with the switch control device, signal-coupled with a first signal interface and / or the interface for the gas sensitive sensor, the control device being configured to perform the method according to any one of claims 1 to 14.

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