Fuel cell system and rinking method for inertizing a fuel cell system

Abstract

The invention relates to a fuel cell system (1), comprising: a fuel cell stack (2) having an anode side (3), which has an anode circuit (4), and a cathode side (5), which has a cathode circuit (6); a nitrogen tank (7) for providing nitrogen for inerting the fuel cell system (1); a nitrogen supply line (8) having a first valve device (9) for regulating the supply of nitrogen from the nitrogen tank (7) to the anode side (3); a pressure sensor (10), in particular a nitrogen pressure sensor, for adjusting a target pressure (ptarget) in the anode side (3), a control device (11) of the fuel cell system (1) is provided for controlling the first valve device (9) in such a way that a periodic change can be made between the ambient pressure (pambient) and the maximum target pressure (ptarget); a gas supply device (12) which is designed to actively supply nitrogen from the nitrogen tank (7) via the first valve device (9) to the anode side (3) of the fuel cell stack (2); and a water outlet pipe (13) having a second valve device (14) for discharging the separated water from the first water separator (15) of the gas supply device (12) of the anode circuit (4) into the exhaust gas pipe (16) of the cathode circuit (6).

Description

Technical Field

[0001] The invention relates to a fuel cell system having the features of claim 1. The invention further relates to a flushing method for inerting a fuel cell system with nitrogen having the features of claim 4. Background Art

[0002] Polymer electrolyte membrane (PEM) fuel cell systems convert hydrogen into electrical energy with the aid of oxygen by generating waste heat and water. Here, the conversion of hydrogen is synonymous with the consumption or removal of hydrogen molecules on the anode side.

[0003] Here, a PEM fuel cell consists of an anode supplied with hydrogen, a cathode supplied with air and a polymer electrolyte membrane placed between them. In actual applications, a plurality of such individual fuel cells are stacked in order to increase the voltage generated. Within this fuel cell stack, which is called a fuel cell stack, there are supply channels that supply the individual cells with hydrogen and air or carry away depleted, humid air and depleted anode exhaust gas.

[0004] In order to separate the liquid water from the gaseous part of the anode exhaust gas, a special water separator is used. In addition to the separation function, the separator also has the task of storing the separated water. If the storage is full, the water is discharged by opening a so-called outlet valve.

[0005] The nitrogen enters the anode space by diffusion processes. Another source of nitrogen is fresh fuel, which does not consist of 100% pure hydrogen. Nitrogen is an inert gas for the fuel cell and reduces the cell voltage and thus the stack voltage, which in turn leads to efficiency losses. Therefore, during the operating cycle, gas is also repeatedly discharged from the anode space to reduce the nitrogen content. This discharge occurs via a so-called ventilation valve.

[0006] According to the prior art, the supply with fresh hydrogen is carried out by means of a hydrogen metering valve, which can be designed as a proportional valve. A control strategy provides that the gas pressure within the anode path is regulated to a defined set pressure at a defined position by means of the valve, measured by means of a pressure sensor, depending on the system operating point.

[0007] In order to be able to reuse the unconsumed hydrogen, the anode is designed in a closed circuit according to the prior art. In order to initiate the recirculation of the gas mixture, a ventilator is installed in the anode circuit.

[0008] There are further valves, lines and pressure reducers between the hydrogen tank and the hydrogen metering valve. They have the following tasks: safely separate the hydrogen tank from the anode and adapt the hydrogen pressure to the operating pressure of the hydrogen metering valve. If a system has two water reservoirs (main water separator and gas supply), both act as outlet valves and one additionally takes on the task of a ventilation valve.

[0009] According to this system, a hydrogen sensor is located in the exhaust gas path of the cathode air downstream of the introduction point of the ventilation valve and the discharge valve. If anode gas enters the exhaust gas path of the cathode air, this is detected at the hydrogen sensor. Summary of the invention

[0010] A fuel cell system having the features of independent claim 1 and a flushing method having the features of independent claim 4 are disclosed. Further features and details of the invention are derived from the dependent claims, the description and the drawings. Here, of course, features and details explained in connection with the fuel cell system according to the invention also apply in connection with the flushing method according to the invention and vice versa, so that with respect to the disclosure of the individual inventive aspects, reference is always made to one another or can be made to one another.

[0011] According to the present invention, a fuel cell system is provided, which has: a fuel cell stack with an anode side and a cathode side, the anode side having an anode circuit, and the cathode side having a cathode circuit; a nitrogen tank for providing nitrogen for inerting the fuel cell system; a nitrogen supply pipe, the nitrogen supply pipe having a first valve device for regulating the supply of nitrogen from the nitrogen tank to the anode side; and a pressure sensor, in particular a nitrogen pressure sensor, for regulating a target pressure on the anode side, wherein a control device of the fuel cell system is arranged to control the first valve device so that it can be periodically changed between ambient pressure and a maximum target pressure; a gas delivery device, the gas delivery device being constructed to actively deliver nitrogen from the nitrogen tank to the anode side of the fuel cell stack through the first valve device; and a water outlet pipe, the water outlet pipe having a second valve device for discharging separated water from a first water separator of the gas delivery device of the anode circuit into an exhaust pipe of the cathode circuit.

[0012] In this case, the fuel cell stack has a large number of fuel cells which can be rendered inert by means of the fuel cell system according to the invention.

[0013] Before maintenance, all pipelines and components that come into contact with hydrogen must be flushed and replaced with nitrogen, that is, the fuel cell system must be inerted. This is necessary, for example, to minimize the risk of combustion or explosion when disassembling the fuel cell system so that as little hydrogen as possible comes into contact with the ambient air. In addition to avoiding the risk of combustion and explosion, the potentially dangerous voltage of the fuel cell stack is also reduced by inerting it.

[0014] The inertization thus creates and significantly increases the safety with which maintenance work on the fuel cell system can be carried out.

[0015] The periodic change between the ambient pressure and the maximum target pressure generated by the control device of the fuel cell system on the first valve device generates a pulsation type of gas flow. This increases the efficiency and speed of the nitrogen replacement of hydrogen. Therefore, this increases the efficiency of the inerting of the fuel cell system.

[0016] In normal operation, the gas delivery device delivers hydrogen to the fuel cell system. Advantageously, the nitrogen supply line, the first valve device and the pressure sensor, in particular the nitrogen pressure sensor, are flowed through by hydrogen in normal operation. The nitrogen supply line is therefore a hydrogen supply line and the first valve device is a hydrogen metering valve.

[0017] Within the scope of the invention, it can be advantageous to provide a third valve device for conducting the anode gas from the first water separator of the gas supply system of the anode circuit into the exhaust gas line of the cathode circuit.

[0018] The anode gas can be discharged via a third valve device, where it can be used to discharge hydrogen. It is also conceivable to evaluate the discharged anode gas to determine how much nitrogen is already in the fuel cell system. This indicates the efficiency of the flushing, which in turn increases safety during maintenance.

[0019] It is also conceivable that the third valve device corresponds to a ventilation valve of a fuel cell system through which hydrogen flows. This dual use of the valve device ensures a simple and inexpensive construction of the fuel cell system. In addition, possible leaky locations in the fuel cell system are reduced thereby.

[0020] It is also conceivable to provide a second water separator. This second water separator then first separates the first part of the water and supplies the remaining anode gas to the gas supply device or the first water separator of the gas supply device. In this case, the third valve device is associated with the second water separator. It is also conceivable to have a further valve device that discharges the water into the exhaust gas line of the cathode circuit.

[0021] Within the scope of the present invention, it is conceivable to arrange a hydrogen sensor in the exhaust gas line of the cathode circuit.

[0022] The hydrogen sensor can be used to determine the hydrogen content in the separated anode gas. A low or falling hydrogen content in the anode gas indicates that the fuel cell system is inerted or inerted. This allows the safety of maintenance to be optimally determined. At the same time, the duration of the inerting process can be determined.

[0023] A second aspect of the present invention is a flushing method for inerting the above-described fuel cell system with nitrogen, the method comprising the following steps:

[0024] - Changing the gas source of the anode circuit of the fuel cell stack from hydrogen to nitrogen,

[0025] - putting into operation a gas conveying device, in particular a fluid compressor, in order to convey nitrogen into the anode circuit,

[0026] - put the cathode circuit into operation in order to produce dilution air in the exhaust gas,

[0027] - opening the first valve means in the nitrogen feed pipe of the anode circuit,

[0028] - adjusting the target pressure at the anode side of the fuel cell stack by means of a first valve device of the nitrogen feed line,

[0029] - opening the second valve device of the water discharge line of the anode circuit in order to discharge the separated water of the first water separator of the gas supply device into the exhaust gas line of the cathode circuit,

[0030] - Cyclic variation of the target pressure, wherein the target pressure is varied between a maximum target pressure and the ambient pressure, and / or the gas supply device is cyclically switched on and off.

[0031] In this case, the fuel cell stack has a multiplicity of fuel cells which can be rendered inert by means of the fuel cell system according to the invention.

[0032] Before maintenance, all pipes and components that come into contact with hydrogen must be flushed and replaced with nitrogen, that is, the fuel cell system must be inerted. This is necessary, for example, so that as little hydrogen as possible comes into contact with the ambient air when the fuel cell system is disassembled, in order to reduce the risk of combustion or explosion. In addition to avoiding the risk of combustion and explosion, the potentially dangerous voltage of the fuel cell stack is also reduced by inerting it.

[0033] The inertization thus creates and significantly increases the safety of carrying out maintenance on the fuel cell system.

[0034] The cyclical change between the ambient pressure and the maximum target pressure generated by the control device of the fuel cell system on the first valve device generates a pulsation type of gas flow. This increases the efficiency and speed of the nitrogen replacement of hydrogen. Therefore, it increases the efficiency of the inerting of the fuel cell system.

[0035] During normal operation, the gas delivery device delivers hydrogen to the fuel cell system. Advantageously, during normal operation, the nitrogen feed pipe, the first valve device and the pressure sensor, in particular the nitrogen pressure sensor, are flowed through by hydrogen. Thus, the nitrogen feed pipe is a hydrogen feed pipe and the first valve device is a hydrogen metering valve.

[0036] Here, the first water separator can be regarded as part of the gas delivery device. With the help of this part, water is removed from the anode gas both in normal operation and during flushing, as far as possible. Here, the water is discharged into the exhaust gas line of the cathode circuit via the second valve device.

[0037] Here, the nitrogen can be supplied from a pipe or a compressed gas cylinder. If the nitrogen is supplied via a compressed gas cylinder, it is not necessary to put a gas conveying device into operation to convey the nitrogen in the anode circuit.

[0038] Before the anode gas, which can potentially contain large amounts of hydrogen, is introduced into the cathode exhaust gas, the cathode system must also be started up, in which the corresponding gas conveyor devices are started up, thereby providing dilution air in the exhaust gas.

[0039] In the context of the invention, it can be provided that the periodic change is carried out with a change frequency, wherein the change frequency is 0.5 to 10 Hz, preferably 2 to 8 Hz, further preferably 3 to 7 Hz.

[0040] By changing the frequency, the period of the flushing process can be influenced in a simple manner. Changing the frequency thus has a direct influence on the efficiency and speed of the flushing process or the inerting of the fuel cell system. The higher the frequency, the faster the period change and the more effective the flushing process. Furthermore, the changing frequency can be dynamic, so that changes in the fuel cell system can be adjusted. In this case, for example, the changing frequency can be adjusted depending on the nitrogen content in the fuel cell system.

[0041] Furthermore, it is conceivable to carry out a periodic change between different operating states of the gas conveying device.

[0042] For this purpose, the gas delivery device has different power levels, between which it can be changed. Here, the periodic change of the operating state of the gas delivery device can also be the periodic switching on and off of the gas delivery device.

[0043] By changing the operating state, i.e. the power level of the gas delivery device, the period of the flushing process can be influenced in a simple manner. The frequency of change thus has a direct influence on the efficiency and speed of the flushing process or the inerting of the fuel cell system. The higher the frequency, the faster the period change and the more effective the flushing process. Furthermore, the frequency of change can be dynamic so that changes in the fuel cell system can be adjusted. For example, the frequency of change can be adjusted depending on the nitrogen content in the fuel cell system.

[0044] According to the invention, it is conceivable to adjust the maximum target pressure relative to the ambient pressure, wherein the target pressure is adjusted to 0.3 to 5 bar, preferably 1 to 4 bar, further preferably 1.5 to 3 bar relative to the ambient pressure and / or relative to the cathode pressure, wherein the target pressure (p 目标 ) has a maximum difference of 1 bar from the cathode pressure.

[0045] The relative pressure between the anode and cathode can be a limitation here. The relative pressure is limited to 0.8 to 1.2 bar for a duration of 10 seconds. It is therefore conceivable that during the inertization process, pressure builds up in the cathode subsystem, so that the pressure in the anode can increase absolutely.

[0046] The target pressure can be dependent on the respective fuel cell system, the nitrogen content in the fuel cell system or the frequency of the change. It is therefore conceivable that the target pressure can be adjusted dynamically and can be changed during the flushing process.

[0047] It is also conceivable to open the third valve device of the water discharge line of the anode circuit in order to discharge the anode gas of the first water separator of the gas supply device into the exhaust gas line of the cathode circuit.

[0048] The anode gas that can be discharged via the third valve device can be used here to discharge hydrogen. It is also conceivable to evaluate the discharged anode gas to determine how much nitrogen is already in the fuel cell system or how much hydrogen is still in the fuel cell system. This can indicate the efficiency of the flushing, which in turn increases safety during maintenance.

[0049] It is also conceivable that the third valve device corresponds to a ventilation valve of a fuel cell system through which hydrogen flows. This dual use of the valve device ensures a simple and inexpensive construction of the fuel cell system. In addition, possible leaky locations in the fuel cell system are reduced thereby.

[0050] Within the scope of the invention, it is optionally possible to measure the hydrogen content of the fuel cell system by a hydrogen sensor in the exhaust gas line of the cathode circuit. In this case, the hydrogen content is measured continuously or periodically.

[0051] The hydrogen sensor can be used to determine the hydrogen content in the separated anode gas. A low or reduced hydrogen content in the anode gas indicates that the fuel cell system is inerted or inerted. This allows the safety of maintenance to be optimally determined. At the same time, the duration of the inerting process can be determined.

[0052] Furthermore, within the scope of the invention it can be provided that the flushing process is carried out to a hydrogen content of 0.05 to 0.5% by volume, which is related to the dilution air mass flow in the exhaust gas of the exhaust gas line.

[0053] The correlation of the exhaust gas concentration to the anode gas concentration depends strongly on the cathode dilution air mass flow. The goal is to be able to infer a target value of less than 4% in the anode system from the hydrogen content measured in the exhaust gas.

[0054] This content ensures that the fuel cell system is inert, ie the hydrogen has been replaced by nitrogen. Here, it is applicable that the lower the hydrogen content, the more effective the flushing process. With a hydrogen content of less than 4% by volume in the air, the lower ignition limit is not reached.

[0055] In connection with the invention, it is conceivable to measure the cell voltage of the fuel cell stack, wherein the flushing process is measured until the stack voltage of the fuel cell system is less than 60 V, preferably less than 45 V, more preferably less than 30 V.

[0056] For this purpose, voltage measuring devices are provided in the fuel cell system. It is conceivable that the control unit can access these voltage measuring devices in order to display the cell voltage or also to adjust the change frequency or the target pressure.

[0057] The efficiency of the flushing method can be determined and / or displayed in a simple manner using the cell voltage. This increases the safety of the maintenance of the fuel cell system, since a low voltage indicates that there is nitrogen in the fuel cell system or that the hydrogen content in the fuel cell system is so low that there is no risk of combustion or explosion. BRIEF DESCRIPTION OF THE DRAWINGS

[0058] Further advantages, features and details of the invention are apparent from the following description, in which several exemplary embodiments of the invention are described in detail with reference to the drawings. The features mentioned in the claims and in the description may be essential to the invention individually or in any combination. The invention is illustrated in the following drawings:

[0059] Figure 1 Schematic representation of a fuel cell system with a water separator,

[0060] Figure 2 Schematic representation of a fuel cell system with two water separators,

[0061] Figure 3 Schematic representation of a flushing method for inerting a fuel cell system. DETAILED DESCRIPTION

[0062] Figure 1 A fuel cell system 1 is shown, which has a fuel cell stack 2 with an anode side 3 having an anode circuit 4 and a cathode side 5 having a cathode circuit 6. In addition, a nitrogen tank 7 is shown. The nitrogen tank 7 is used to provide nitrogen for inerting the fuel cell system 1. For this purpose, a nitrogen supply pipe 8 is further provided, which has a first valve device 9 for regulating the supply of nitrogen from the nitrogen tank 7 to the anode side 3; and a pressure sensor 10, in particular a nitrogen pressure sensor, for regulating a target pressure p in the anode side 3. 目标 Furthermore, the fuel cell system 1 has a control device 11 which is configured to control the first valve device 9 so that the ambient pressure p 环境 and the maximum target pressure p 目标 Change periodically.

[0063] In order to actively convey nitrogen from the nitrogen tank 7 via the first valve device 9 to the anode side 3 of the fuel cell stack 2, a gas conveying device 12 is provided. In addition, the anode circuit 4 is provided with a water discharge line 13 having a second valve device 14. The water discharge line is used to discharge the separated water from the first water separator 15 of the gas conveying device 12 of the anode circuit 4 into the exhaust gas line 16 of the cathode circuit 6.

[0064] The first water separator 15 separates water from the gas mixture on the anode side 3. A portion of the gas is fed via a third valve device 18 into the exhaust gas line 16 of the cathode circuit 6 and discharged therefrom.

[0065] according to Figure 2 The embodiment of the present invention has an additional second water separator 17 and an additional fourth valve device 19. The additional second water separator then first separates a first part of the water of the anode circuit 4 after the anode and discharges the remaining anode gas to the gas supply device 12 or the first water separator 15 of the gas supply device 12. In this case, the third valve device 18 and the fourth valve device 19 are assigned to the second water separator 17. Furthermore, the fourth valve device 19 discharges the water into the exhaust gas line 16 of the cathode circuit 6.

[0066] Figure 1 and 2The fuel cell system 1 is provided with a hydrogen sensor 20 in the exhaust gas pipe 16 of the cathode circuit 6 in order to measure the hydrogen content in the exhaust gas. This provides information about the state of inertization. In addition, a voltage measuring instrument 21 is provided, because the cell voltage also shows a progression. Here, the smaller the cell voltage, the safer it is to perform maintenance.

[0067] Figure 3 Shown for making Figure 1 or Figure 2 A method 100 for flushing an inert fuel cell system 1 comprises the following steps:

[0068] - Changing the gas source of the anode circuit 4 of the fuel cell stack 2 from hydrogen to nitrogen,

[0069] - starting up 120 the gas conveying device 12 , in particular a fluid compressor, in order to convey nitrogen into the anode circuit 4 ,

[0070] - putting the cathode circuit 6 into operation 220 in order to produce dilution air in the exhaust gas,

[0071] - opening 130 the first valve device 9 in the nitrogen supply pipe 8 of the anode circuit 4,

[0072] - setting 140 a target pressure at the anode side 3 of the fuel cell stack 2 by means of the first valve device 9 of the nitrogen feed line 8,

[0073] - opening 150 the second valve device 14 of the water discharge line 13 of the anode circuit 4 in order to discharge the separated water of the first water separator of the gas supply device 12 into the exhaust gas line 16 of the cathode circuit 6,

[0074] -Periodically change the target pressure p160 目标 , where the target pressure p 目标 At the maximum target pressure p 目标 and environmental pressure p 环境 Change between

[0075] - Periodically opening and closing 210 the second valve means.

[0076] In this case, the periodic change is performed with a change frequency between 0.5 and 10 Hz. In addition, the operation of the gas delivery device 12 is periodically changed. In this case, the gas delivery device 12 can be changed between different operating states or power levels or even switched off or on.

[0077] Maximum target pressure p 目标 It involves a relative pressure, that is, it is relative to the ambient pressure p 环境 and cathode pressure, where the target pressure is relative to the ambient pressure p环境 is regulated to 0.3 to 5 bar. In addition, the target pressure at the anode differs from the cathode pressure by a maximum of 1 bar.

[0078] At the same time or subsequently, the third valve device 18 of the water outlet pipe 13 of the anode circuit 4 is opened 170 to discharge the anode gas of the first water separator of the gas supply device 12 into the exhaust gas pipe 16 of the cathode circuit 6. In addition, the fourth valve device 19 of the water outlet pipe 13 of the anode circuit 4 is opened 180 to discharge water into the exhaust gas pipe 16 of the cathode circuit 6.

[0079] Furthermore, the hydrogen content of the fuel cell system 1 is measured 190 by means of a hydrogen sensor 20 in the exhaust gas line 16 of the cathode circuit 6. The hydrogen content is measured continuously 190. As soon as a hydrogen content of 0.5% by volume, which is related to the dilution air mass flow in the exhaust gas in the exhaust gas line 16, is measured, the fuel cell system 1 is considered to be inert.

[0080] Furthermore, the cell voltage of the fuel cell stack 2 is measured 200 by means of the voltage measuring device 21. In this case, a flushing process is carried out until a stack voltage of less than 60 V is measured.

Claims

1. A fuel cell system (1), comprising: A fuel cell stack (2) with an anode side (3) having an anode circuit (4) and a cathode side (5) having a cathode circuit (6), a nitrogen tank (7) for providing nitrogen for inerting the fuel cell system (1), a nitrogen supply pipe (8) having a first valve device (9) for regulating the supply of the nitrogen from the nitrogen tank (7) to the anode side (3), and A pressure sensor (10), in particular a nitrogen pressure sensor, is used to regulate a target pressure (p 目标 ),in, The control device (11) of the fuel cell system (1) is configured to control the first valve device (9) so that the first valve device (9) can be periodically adjusted between the ambient pressure (p 环境 ) and the maximum target pressure (p 目标 ), a gas delivery device (12) configured to actively deliver the nitrogen from the nitrogen tank (7) through the first valve device (9) to the anode side (3) of the fuel cell stack (2), A water discharge pipe (13) has a second valve device (14) for discharging separated water from a first water separator (15) of the gas conveying device (12) of the anode circuit (4) into an exhaust gas pipe (16) of the anode circuit (6).

2. The fuel cell system (1) according to claim 1, It is characterized in that A third valve device (18) is provided for conducting anode gas from the first water separator (15) of the anode circuit (4) into the exhaust gas line (16) of the cathode circuit (6).

3. The fuel cell system (1) according to claim 1 or 2, It is characterized in that A hydrogen sensor (20) is arranged in the exhaust pipe (16) of the cathode circuit (6).

4. A flushing method (100) for inerting a fuel cell system (1) according to any one of claims 1 to 3 with nitrogen, the flushing method comprising the following steps: - changing (110) the gas source of the anode circuit (4) of the fuel cell stack (2) from hydrogen to nitrogen, - putting into operation (120) a gas conveying device (12), in particular a fluid compressor, in order to convey nitrogen into the anode circuit (4), - putting the cathode circuit (6) into operation (220) in order to produce dilution air in the exhaust gas, - opening (130) the first valve device (9) in the nitrogen supply pipe (8) of the anode circuit (4), - regulating (140) a target pressure (p) at the anode side (3) of the fuel cell stack (2) by means of the first valve device (9) of the nitrogen supply pipe (8) 目标 ), - opening (150) the second valve device (14) of the water discharge line (13) of the anode circuit (4) in order to discharge the separated water of the first water separator of the gas supply device (12) into the exhaust gas line (16) of the cathode circuit (6), - periodically varying (160) the target pressure, wherein The target pressure (p 目标 ) at the maximum target pressure (p 目标 ) and environmental pressure (p 环境 ) and / or periodically turning the gas delivery device (12) on and off.

5. The flushing method (100) according to claim 4, It is characterized in that The periodic changing (160) is performed at a changing frequency, wherein the changing frequency is 0.5 to 10 Hz, preferably 2 to 8 Hz, and more preferably 3 to 7 Hz.

6. The flushing method (100) according to claim 4 or 5, It is characterized in that A periodic change (220) is performed between different operating states of the gas delivery device.

7. The flushing method (100) according to claims 4 to 6, It is characterized in that The maximum target pressure (p 目标 ) relative to the ambient pressure (p 环境 ) to adjust, wherein the target pressure (p 目标 ) relative to the ambient pressure (p 环境 ) and / or adjusted to 0.3 to 5 bar, preferably 1 to 4 bar, more preferably 1.5 to 3 bar relative to the cathode pressure, wherein the target pressure (p 目标 ) differs from the cathode pressure by a maximum of 1 bar.

8. The flushing method (100) according to any one of claims 4 to 7, It is characterized in that The third valve device (18) of the water discharge pipe (13) of the anode circuit (4) is opened (170) in order to discharge the anode gas of the first water separator of the gas supply device (12) into the exhaust gas pipe (16) of the cathode circuit (6).

9. The flushing method (100) according to any one of claims 4 to 8, It is characterized in that The hydrogen content of the fuel cell system (1) is measured (190) by means of a hydrogen sensor (20) in the exhaust gas line (16) of the cathode circuit (6).

10. The flushing method (100) according to any one of claims 4 to 9, It is characterized in that The flushing process is carried out until a hydrogen content of 0.05 to 0.5% by volume is reached in relation to the dilution air mass flow in the exhaust gas of the exhaust gas line (16).

11. The flushing method (100) according to any one of claims 4 to 10, It is characterized in that The single cell voltage of the fuel cell stack (2) is measured, wherein the flushing process is measured (200) until the cell stack voltage of the fuel cell system (1) is less than 60V, preferably less than 45V, and more preferably less than 30V.

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