Method for lowering lower limit of hydrogen explosion in exhaust system of fuel cell system

Abstract

The invention relates to a method for preventing the autonomous sustained combustion of hydrogen in an exhaust system (150) of a fuel cell assembly, in particular of a fuel cell vehicle, during the start-up of the fuel cell assembly, according to the invention, reactive hydrogen is removed from the gas on the cathode side and / or on the exhaust gas side of the fuel cell assembly until the current level of the hydrogen concentration in the exhaust gas (150) falls below a hydrogen explosion lower limit when the fuel cell assembly is started, the method is preferably carried out only if the initial level of the hydrogen concentration of the gas on the cathode side and / or on the exhaust gas side in the fuel cell assembly exceeds the lower limit of the hydrogen explosion for the exhaust gas (150).

Description

Technical Field

[0001] The invention relates to a method for preventing the autonomous continuous combustion of hydrogen in the exhaust gas system of a fuel cell unit, in particular a fuel cell vehicle. The invention also relates to a fuel cell unit, a fuel cell system or a fuel cell vehicle. Background Art

[0002] In a low-temperature polymer electrolyte fuel cell of a fuel cell unit (stationary or mobile), such as a fuel cell system, for example a fuel cell vehicle, two reactants of two operating media are electrochemically converted into electrical energy and heat. In this case, the fuel cell comprises at least one membrane electrode assembly (MEA). Typically, the fuel cell is constructed with a plurality of membrane electrode units arranged in a stack and bipolar plates arranged between them (fuel cell stack or stack).

[0003] The integrity of the catalyst layer of the membrane electrode unit is crucial for the performance characteristics and service life of the fuel cell stack. Therefore, when the fuel cell unit is shut down, the fuel cell stack is shut down on the cathode side and the anode side in order to delay as much as possible the prerequisites for air / air start of the fuel cell unit that damage the catalyst layer of the cathode of the fuel cell stack. If the conditions for air / air start exist, corresponding countermeasures should be taken before starting the fuel cell unit.

[0004] Before the conditions for air / air start exist, nitrogen diffuses from the cathode of the fuel cell stack into the anode and hydrogen diffuses into the cathode of the fuel cell stack. During the start-up of the fuel cell unit, the hydrogen in the cathode leads to an increase in the hydrogen concentration in the exhaust gas of the fuel cell unit, whereby compliance with the lower explosion limit (UEG) is not always ensured there. The object of the present invention is therefore to keep the hydrogen concentration in the exhaust gas of the fuel cell unit low so that the hydrogen in the exhaust gas cannot continue to burn autonomously there, i.e. after the hydrogen has ignited, a flame independent of the ignition source can no longer exist in the exhaust gas. Summary of the invention

[0005] The object of the invention is achieved by a method for preventing the autonomous continuous combustion of hydrogen in the exhaust gas system of a fuel cell assembly, in particular a fuel cell vehicle, during the start-up of the fuel cell assembly and by means of a fuel cell assembly, a fuel cell system or a fuel cell vehicle. Advantageous embodiments, additional features and / or advantages of the invention are apparent from the dependent claims and the following description.

[0006] In the method according to the invention, reactive hydrogen is removed from the gas on the cathode side and / or exhaust side of the fuel cell unit when at least one start-up condition of the fuel cell unit is present until the current level of the hydrogen concentration in the exhaust system is below the lower hydrogen explosion limit when the fuel cell unit is started, wherein the method is preferably performed only when the initial level of the hydrogen concentration in the gas on the cathode side and / or exhaust side of the fuel cell unit exceeds the lower hydrogen explosion limit for the exhaust system.

[0007] That is, according to the present invention, before the fuel cell stack of the fuel cell unit is actually put into operation, the hydrogen concentration in the gas on the cathode side and / or the exhaust side of the exhaust system for the fuel cell unit is reduced so that it is below the lower hydrogen explosion limit of the gas at least in the exhaust system. This can naturally also apply to the cathode. In the cathode, the problem of hydrogen combustion (explosion) is not so obvious, so that the focus of the present invention is on the exhaust system.

[0008] Reference to the cathode side and / or exhaust side should here be understood to mean hydrogen from the cathode of the fuel cell stack of the fuel cell unit, wherein the hydrogen diffuses from the anode of the fuel cell stack into the cathode. In this case, the hydrogen is either in the cathode supply path, in the cathode chamber of the fuel cell stack, in the cathode exhaust path, in the exhaust gas bypass valve or bypass of the cathode supply, downstream of this bypass, in the anode exhaust path (purge line or the like) of the anode supply and / or in the exhaust. It is important that the hydrogen concentration in the exhaust is below the lower hydrogen explosion limit when the fuel cell stack is actually put into operation.

[0009] Reactive hydrogen can be removed from the gas on the cathode and / or exhaust side by oxygen, preferably by contacting the gas on the cathode and / or exhaust side with air. Here, air can diffuse into the gas on the cathode and / or exhaust side, air can be transported toward the gas on the cathode and / or exhaust side, and / or air can be transported into the gas on the cathode and / or exhaust side. That is, the reactive hydrogen in the gas on the cathode and / or exhaust side reacts with oxygen, especially atmospheric oxygen. The fuel cell unit can then be started and the fuel cell stack can be operated. Hydrogen concentration peaks in the exhaust are effectively prevented.

[0010] The fluid connection of the cathode to the surroundings is blocked in time before at least one of the start conditions exists. Leaks may occur here (one of the reasons is a possible late air / air start of the fuel cell unit). The two cathode shut-off valves can be closed and / or are closed. Leaks may also occur here, especially due to the shut-off valves (i.e. one of the reasons is again a possible late air / air start). In addition, the anode of the fuel cell unit is blocked and / or is blocked.

[0011] At least one start-up condition of the fuel cell assembly can be characterized by a signal or a start of an expected start-up of the fuel cell assembly. Furthermore, the start-up condition can be characterized by a predetermined or previously determined date and / or determined by a model based on KI. Furthermore, the start-up condition can be characterized by an internal signal from the fuel cell system of the fuel cell assembly. Furthermore, the start-up condition can be characterized by an external signal from outside the fuel cell system of the fuel cell assembly. In this case, the fuel cell assembly naturally belongs to the fuel cell system.

[0012] The initial level of the hydrogen concentration in the gas on the cathode side and / or the exhaust side can be estimated, determined and / or measured. Here, for example, the initial level can preferably be estimated based on the downtime of the fuel cell unit. The initial level of the hydrogen concentration can also be determined by a known characteristic curve or a known characteristic curve cluster and / or a KI-based model. In addition, the initial level can be measured or determined by a sensor on the cathode side, in particular a hydrogen sensor. In addition, the initial level can be determined by a sensor on the anode side, in particular a hydrogen sensor, if necessary.

[0013] It can naturally be determined from the initial level of the hydrogen concentration in the gas on the cathode side and / or on the exhaust side whether the hydrogen concentration exceeds the lower hydrogen explosion limit for the exhaust. If the former is the case, the method is continued; if the latter is the case, the method can be terminated and the fuel cell unit can be started up further.

[0014] In order to remove reactive hydrogen from the gas on the cathode side and / or the exhaust device side, the fluid conveying device of the cathode supply device of the fuel cell unit can be started and operated. In this case, the exhaust gas bypass valve of the cathode supply device or the cathode side bypass of the fuel cell stack can be opened and kept open or kept closed. In this case, the cathode inlet stop valve can be opened and the cathode outlet stop valve of the cathode supply device can be kept closed. In this case, in particular, the exhaust gas bypass valve or the cathode side bypass is kept closed. Alternatively, the cathode outlet stop valve can be opened and the cathode inlet stop valve of the cathode supply device can be kept closed. In this case, in particular, the exhaust gas bypass valve or the cathode side bypass is opened.

[0015] For example, the fluid delivery device of the cathode supply device can be started first, and the cathode shut-off valve can be opened later in time. Alternatively, the cathode shut-off valve can be opened first and the fluid delivery device of the cathode supply device can be started later in time. Alternatively, the fluid delivery device of the cathode supply device can be started again and the cathode shut-off valve can be opened substantially in parallel with this. The waste gas bypass valve or the cathode-side bypass must be opened (cathode outlet shut-off valve opened) or closed (cathode inlet shut-off valve opened) accordingly.

[0016] In order to remove reactive hydrogen from the gas on the cathode side and / or the exhaust side, the cathode can be loaded or filled with air at least once on the input side and once on the output side, or alternately, if necessary, even at a higher frequency (greater than 0.5 Hz) on the input side and the output side. For example, for this purpose, the fluid conveying device of the cathode supply device is started and the cathode inlet stop valve is opened, wherein the cathode outlet stop valve on the one hand and the waste gas bypass valve or cathode-side bypass of the cathode supply device on the other hand remain closed. In this case, air is squeezed or conveyed into the cathode on the input side. The cathode inlet stop valve is now closed, wherein the fluid conveying device can continue to convey, operate at a lower power or even be closed if necessary.

[0017] Subsequently in time, on the one hand, the cathode outlet shut-off valve is opened and on the other hand, the waste gas bypass valve or cathode-side bypass of the cathode supply device is opened. The valve that does not have to lead to the cathode on the output side can also be closed so that air can be squeezed or transported to the cathode on the output side. In addition, the fluid conveying device conveys air here in a similar manner to the supply of the cathode on the input side. The air is now conveyed or squeezed into the cathode on the output side. Now, in time, air can be supplied to the cathode on the input side again, etc. It can be experimentally determined which frequency is well suitable or best suitable for removing reactive hydrogen from the gas on the cathode side and / or the exhaust device side.

[0018] When removing reactive hydrogen, the current level of hydrogen concentration in the gas on the cathode side and / or exhaust side can be estimated, determined and / or measured. Thus, the current level of hydrogen concentration can be estimated, for example, based on a previous downtime of the fuel cell unit. In addition, the current level can be determined by a known characteristic curve or a known cluster of characteristic curves and / or a model based on KI. In addition, the current level of hydrogen concentration in the gas on the cathode side and / or exhaust side can be measured or determined by a sensor on the cathode side or exhaust side, in particular a hydrogen sensor.

[0019] If, for example, the estimated, determined and / or measured hydrogen concentration in the gas on the cathode side and / or the exhaust device side is lower than the lower hydrogen explosion limit, the cathode shut-off valve, in particular the cathode outlet shut-off valve or the cathode inlet shut-off valve, which is still closed, is opened, and the start-up of the fuel cell unit can be continued. The entire method can be performed, for example, based on an internal signal from the fuel cell unit. In addition, the entire method can be performed based on an external signal from outside the fuel cell unit. In addition, the entire method can be performed substantially at each start-up of the fuel cell unit.

[0020] The invention can be demonstrated simply by observing the fluid pressure or fluid mass flow on the cathode side during the start-up of the fuel cell unit, in particular during the filling of the cathode with fresh air. The demonstration of the invention can also be observed by observing the actuation sequence and time course of the shutoff valves. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The invention is explained in more detail below with reference to the schematic and not to scale drawings according to the embodiments. In the present invention, features can be configured as positive, i.e., present, or negative, i.e., absent. In this specification, negative features are not explicitly interpreted as features, unless it is important according to the invention that the feature is absent. That is, the invention that is actually made and not constructed by the prior art will omit the feature. The lack of a feature in the embodiment (negative feature) means that the feature is optional. The only exemplary figures in the drawings show:

[0022] Figure 1 : A simplified block diagram of an embodiment of a fuel cell stack of a fuel cell system for a fuel cell vehicle,

[0023] Figure 2 :for Figure 1 A simplified block diagram of an alternative embodiment of a cathode supply device for a fuel cell stack, and

[0024] Figure 3 : A possible flow chart for the method according to the invention for preventing autonomous and sustained combustion of hydrogen in the exhaust system of a fuel cell unit. DETAILED DESCRIPTION

[0025] According to the exhaust device 150 (see Figure 1 and Figure 2 ) below the lower explosion limit of hydrogen 200 (see Figure 3 ), namely according to a method 200 for preventing autonomous continuous combustion of hydrogen in an exhaust gas device 150 of a fuel cell unit 1 of a low-temperature polymer electrolyte fuel cell system of a fuel cell vehicle, i.e. a motor vehicle having a fuel cell or a fuel cell system (see Figure 3 ) describes the present invention in detail.

[0026] In the figures only the sections of the fuel cell system which are necessary for understanding the invention are shown ( Figure 1 : Fuel cell unit 1). Although the present invention has been described and illustrated in detail by means of preferred embodiments, the present invention is not limited to the disclosed embodiments. Other variations may be derived therefrom without departing from the scope of protection of the present invention.

[0027] Figure 1A fuel cell unit 1 according to an embodiment is shown, which has at least one, in particular a plurality of electrochemical individual fuel cells 11, 11, ... (single cells 11, 11, ...) bundled into a fuel cell stack 10, which are accommodated in a preferably fluid-tight stack housing 16. Each single cell 11 includes an electrode chamber 12 configured as an anode chamber 12, which preferably has a gas diffusion layer (including a microporous particle layer if necessary), and an electrode chamber 13 configured as a cathode chamber 13, which preferably has a gas diffusion layer (including a microporous particle layer if necessary). The anode chamber and the cathode chamber are separated from each other spatially and electrically by the diaphragm of the membrane electrode unit 15. Here, the gas diffusion layer preferably belongs to the membrane electrode unit 15.

[0028] The bipolar plate 14 (preferably a separator plate assembly consisting of an anode plate (electrode plate) and a cathode plate (electrode plate)) is arranged between two directly adjacent membrane electrode units 15, 15 containing relevant anode chambers 12 and cathode chambers 13, and the bipolar plate is also used to supply / export operating media 3, 5 to the anode chamber 12 of the first single cell 11 and the cathode chamber 13 of the second single cell 11 directly adjacent thereto, and also to realize an electrically conductive connection between these single cells 11, 11.

[0029] In order to supply the fuel cell stack 10 with its actual operating medium 3 (anode operating medium, actual fuel) and 5 (cathode operating medium, usually air), the fuel cell unit 1 has an anode supply device 20 and a cathode supply device 30. The anode supply device 20 comprises in particular: a fuel reservoir 23 for the anode operating medium 3 (inflow); an anode supply path 21 with a shut-off / metering valve 27 and an injector 24; an anode exhaust gas path 22 for the anode exhaust gas medium 4 (usually outflowing into the surroundings 2); preferably, a fuel recirculation line 25 with a fluid conveying device 26 located therein; a water separator and a water container if necessary.

[0030] The cathode supply device 30 includes, in particular: a cathode supply path 31 for a cathode operating medium 5 (usually flowing in from the surrounding environment 2), preferably having a fluid conveying device 33; a cathode exhaust gas path 32 for a cathode exhaust gas medium 6 (usually flowing out into the surrounding environment 2), preferably having a turbine 34, in particular for the fluid conveying device 33; preferably a humidity transmitter 36; if necessary, an exhaust gas bypass valve 35 between the cathode supply path 31 and the cathode exhaust path 22; if necessary, a water separator and, if necessary, a water container.

[0031] The fuel cell unit 1 also includes a cooling medium supply device 40 of a thermal system, in particular a fuel cell vehicle, through which the fuel cell can preferably be connected to the cooling circuit for temperature control by means of its bipolar plate 100 (cooling medium path 43) in a heat transfer manner. The cooling medium supply device 40 includes a cooling medium inflow path 41 and a cooling medium outflow path 42. The cooling medium 7 (inflow) and 8 (outflow) circulating in the cooling medium supply device 40 are preferably transported by means of at least one cooling medium conveying device 44. In addition to the fuel cell unit 1, the fuel cell system includes peripheral system components, such as a controller, which can be a controller of the fuel cell vehicle itself.

[0032] Figure 2 An alternative cathode supply device 30 of a fuel cell stack 10 is shown. Figure 1 The positive or negative characteristics can be applied to Figure 2 , and vice versa. Figure 1 compared to, Figure 2 The cathode supply device 30 also shows an air filter 110 upstream of the fluid conveying device 33 and a heat exchanger 120 downstream of the fluid conveying device 33, wherein the heat exchanger 120 can be connected to the thermal system in particular. Figure 2 The “cathode 130” of the fuel cell stack 10 is shown, which can be shut off when the fuel cell unit 1 is shut down (see below) by means of a cathode inlet shut-off valve 131 in the cathode supply path 31 and a cathode outlet shut-off valve 132 in the cathode exhaust gas path 32. The cathode exhaust gas path 32 leads downstream, for example, to an exhaust gas line 160 of the fuel cell unit 1.

[0033] The actual cathode 130 of the fuel cell stack 10 is arranged in particular in the cathode chamber 13, 13, ... of the individual cells 11, 11, .... The cathode-side gas is located "in the cathode 130", i.e. actually in the cathode chamber 13, 13 ... of the individual cells 11, 11 ..., in at least one cathode supply channel for the cathode operating medium 5 on and in the fuel cell stack 10 and in at least one cathode purge channel for the cathode exhaust medium 6 on and in the fuel cell stack 10. When the fuel cell unit 1 is in a stationary state, the cathode-side gas of the fuel cell unit 1 is enclosed there between the closed cathode inlet shut-off valve 131 and the closed cathode outlet shut-off valve 132.

[0034] When the fuel cell unit 1 is started, the cathode side gas is mixed with the exhaust device side gas of the fuel cell unit 1 located downstream of the exhaust device 150 of the fuel cell unit 1. Here, the exhaust gas line 160 leads downstream into the exhaust device 150. Cathode side and exhaust device side gas is generated, which may have a hydrogen concentration exceeding the lower explosion limit of hydrogen in the exhaust device 150. In addition, in particular, the pressure regulator 140 of the fuel cell unit 1 is connected to / in the exhaust gas line 160 upstream of the exhaust device 150.

[0035] The shutdown method of the fuel cell unit 1, especially in the field of vehicles, generally includes the following steps: First: Dry the cathode 130 (i.e., each cathode in the cathode chamber 13, 13, ...) of the fuel cell stack 10 without current. Second: Close the shut-off valves 131, 132 of the cathode 130. Third: Reduce the oxygen (exhaust) of the cathode 130 by applying a low load, thereby generating a low oxygen partial pressure on the cathode 130. Fourth: Close the shut-off valve of the anode (each anode in the anode chamber 12, 12, ...). As a result, the fuel cell stack 10 is in a state where there is nitrogen on the cathode and hydrogen on the anode, is correspondingly moistened, and has nitrogen residue on the anode.

[0036] When the fuel cell unit 1 is shut down, the cathode-side gas is shut off at and in the fuel cell stack 10 by means of cathode shut-off valves 131, 132. This serves to prevent air or oxygen from penetrating into the cathode 130 in order to prevent a reduced electrochemical potential during a subsequent restart of the fuel cell unit 1. In particular, so-called air / air starts of the fuel cell unit 1 that damage the cathode 130 are to be prevented thereby.

[0037] Over time, hydrogen diffuses from the anode via the membrane electrode units 15, 15 ... to the cathode 130 and remains enclosed in the cathode 130. When the fuel cell unit 1 is subsequently started, the hydrogen is swept away by the air flow. This leads to an increase in the hydrogen concentration in the exhaust device 150. Compliance with the lower explosion limit is not always ensured here. The following describes an embodiment of the present invention, how to deal with a hydrogen concentration that is too high in the cathode 130 and therefore in the exhaust device 150 that is too high when the fuel cell unit 1 is started. This is exemplified in Figure 3 Of course, the present invention is not limited to this (see above).

[0038] Before carrying out the method, the cathode 130 is shut off, wherein in particular the two cathode shut-off valves 131 , 132 are already closed and are closed. Furthermore, the anode is preferably also shut off by means of a shut-off valve. This is Figure 3190. If at least one start-up condition for the fuel cell stack 1 is now present (e.g., the first step 210 of the method 200), the method 200 can be started to prevent autonomous continued combustion of hydrogen in the exhaust gas 150 of the fuel cell stack 1. It is determined 210 whether at least one start-up condition for the fuel cell stack 1 is present (see above).

[0039] If at least one start condition exists 210, it can be queried (estimated, determined and / or measured) in a chronological order (e.g., second step 220 (optional) of method 200) whether the initial level of the hydrogen concentration of the gas on the cathode side and / or the exhaust device side exceeds 220 the lower explosion limit of hydrogen for the exhaust device 150 and / or in the exhaust device 150 (see above). If this is the case (222: Yes), the method 200 continues; if this is not the case, the method 200 can be interrupted and the fuel cell unit 1 can continue to be started (at Figure 3 not shown).

[0040] Now (e.g., third step 230 of method 200), reactive hydrogen (see above) can be removed 230 from the gas on the cathode and / or exhaust side. Here, step 230 may include at least two sequential (232=>234, 234=>232) or two parallel (232 / 234) sub-steps 232, 234. Here, sub-step 232 consists in starting the fluid transport device 33; or opening the cathode inlet stop valve 131, wherein the cathode outlet stop valve 132 remains closed; or opening the cathode outlet stop valve 132, wherein the cathode inlet stop valve 131 remains closed. Here, sub-step 234 consists in opening the cathode outlet stop valve 132, wherein the cathode inlet stop valve 131 remains closed; or opening the cathode inlet stop valve 131, wherein the cathode outlet stop valve 132 remains closed; or starting the fluid transport device 33.

[0041] Here (e.g. fourth step 240 (optional) of method 200) it can be queried (estimated, ascertained and / or measured) whether the current level of the hydrogen concentration of the exhaust gas 150 is below the lower hydrogen explosion limit 240 or whether the hydrogen on the cathode side has essentially reacted 240 (see above)? If this is the case (242: yes), the method 200 can be brought to an end; if this is not the case, continue with step 240. Alternatively, the removal of reactive hydrogen 230 can simply continue for a predetermined time.

[0042] If the expected or current level of hydrogen concentration in the exhaust device 150 for starting the fuel cell unit 1 is below the lower hydrogen explosion limit (242: Yes), then (e.g., the fifth step 250 of the method 200) the other cathode shut-off valve 132 / 131 that is still closed, i.e., the cathode outlet shut-off valve 132 or the cathode inlet shut-off valve 131, can be opened. The method 200 now ends 260.

[0043] A brief summary. When the fuel cell unit 1 is shut down, the cathode side gas is shut off by means of the cathode shut-off valves 131, 132. This is used to ensure that no oxygen or air intrudes into the cathode 130 to prevent a decrease in the electrochemical potential when the fuel cell unit 1 is subsequently started. Over time, hydrogen diffuses from the anode 12, 12... through the diaphragm to the cathode 130 and remains enclosed therein. When the fuel cell unit 1 is started, this amount of hydrogen is flushed away by the air flow. This leads to an increase in the hydrogen concentration in the exhaust device 150. The lower explosion limit may not be observed there.

[0044] To counteract this problem, fresh oxygen is supplied to the cathode 130 so that the amount of hydrogen contained therein can react and does not reach the exhaust 150. When starting the fuel cell unit 1, for this purpose, only one of the cathode shut-off valves 131 / 132 is first opened until a sufficiently large amount of hydrogen reacts with the oxygen in the air in the cathode 130. This increases safety because there is no hydrogen emission or only very low hydrogen emission at startup. After a certain amount of hydrogen has reacted in the cathode 130, the still closed shut-off valve 132 / 131 of the cathode 130 is opened so that air can flow through the cathode 130. The fuel cell unit 1 can be started and run normally. Hydrogen concentration peaks in the exhaust 150 are reliably prevented.

Claims

1. A method (200) for preventing autonomous and sustained combustion of hydrogen in an exhaust device (150) of a fuel cell unit (1) when the fuel cell unit (1) is started, the fuel cell unit being in particular a fuel cell unit of a fuel cell vehicle, It is characterized in that When at least one start-up condition (210) of the fuel cell unit (1) is present, reactive hydrogen is removed (230) from the gas on the cathode side and / or the exhaust device side of the fuel cell unit (1) until, when the fuel cell unit (1) is started up, the current level of hydrogen concentration in the exhaust device (150) is below (242) the lower explosion limit of hydrogen, wherein: The method (200) is preferably performed only when an initial level of hydrogen concentration in the gas on the cathode side and / or the exhaust side of the fuel cell unit (1) exceeds (220) a lower hydrogen explosion limit for the exhaust (150).

2. The method (200) according to the preceding claim, It is characterized in that Removing (230) reactive hydrogen from the cathode-side and / or exhaust-side gas is accomplished by oxygen, preferably by contacting the cathode-side and / or exhaust-side gas with air, wherein: In particular, air is diffused into the cathode-side and / or exhaust-side gas, in particular air is conveyed toward the cathode-side and / or exhaust-side gas and / or in particular air is conveyed into the cathode-side and / or exhaust-side gas.

3. The method according to any one of the preceding claims, It is characterized in that Temporally before the presence (190) of the at least one enabling condition (210): the cathode (130) is blocked and / or has been blocked from fluid communication with the surrounding environment (2), The two cathode cut-off valves (131, 132) are closed and / or have been closed, and / or The anodes (12, 12, . . . ) of the fuel cell unit (1) are switched off and / or have been switched off.

4. The method (200) according to any one of the preceding claims, It is characterized in that At least one start-up condition (210) of the fuel cell unit (1) is achieved by a signal or indication of the expected start-up of the fuel cell unit (1), characterized by a predetermined or pre-determined date and / or determined by a KI-based model, · by internal signal characterization of the fuel cell system from the fuel cell assembly (1), and / or Characterized by external signals from outside the fuel cell system of the fuel cell assembly (1).

5. The method (200) according to any one of the preceding claims, It is characterized in that An initial level of hydrogen concentration in the gas on the cathode side and / or the exhaust side is estimated, determined and / or measured (220), wherein the initial level of hydrogen concentration is preferably Based on the downtime estimation of the fuel cell unit (1), Derived from a known characteristic curve or a known set of characteristic curves and / or a KI-based model, · measurement or ascertainment by means of a cathode-side sensor, in particular a hydrogen sensor, and / or Determined by a sensor on the anode side, in particular a hydrogen sensor.

6. The method (200) according to any one of the preceding claims, It is characterized in that To remove (230) reactive hydrogen from the cathode-side and / or exhaust-side gas: Starting and operating the fluid delivery device (33) of the cathode supply device (30) of the fuel cell unit (1), opening or keeping closed the waste gas bypass valve (35) of the cathode supply device (30) or the cathode side bypass of the fuel cell stack (10), Open the cathode inlet stop valve (131) and keep the cathode outlet stop valve (132) of the cathode supply device (30) closed, and / or • Open the cathode outlet stop valve (132) and keep the cathode inlet stop valve (131) of the cathode supply device (30) closed.

7. The method (200) according to any one of the preceding claims, It is characterized in that In order to remove (230) reactive hydrogen from the gas on the cathode side and / or on the exhaust side, the cathode (130) is charged or filled with air at least once on the input side and once on the output side, or alternately on the input side and the output side.

8. The method (200) according to any one of the preceding claims, It is characterized in that When removing (230) the reactive hydrogen, a current level of hydrogen concentration in the gas on the cathode side and / or the exhaust side is estimated, determined and / or measured (240), wherein the current level of hydrogen concentration is preferably: Based on previous downtime estimates of the fuel cell unit (1), Derived from a known characteristic curve or a known set of characteristic curves and / or a KI-based model, and / or Measurement or ascertainment by means of a cathode-side or exhaust-side sensor, in particular a hydrogen sensor.

9. The method (200) according to any one of the preceding claims, It is characterized in that If the current level of hydrogen concentration in the gas on the cathode side and / or exhaust side is lower than (242) the lower hydrogen explosion limit, the cathode shut-off valve (132 / 131) which is still closed is opened and the start-up of the fuel cell unit (1) is continued.

10. The method (200) according to any one of the preceding claims, It is characterized in that The entire method (200): · Executed based on an internal signal from the fuel cell unit (1), · Executed based on an external signal from outside the fuel cell unit (1), and / or · It is basically performed every time the fuel cell unit (1) is started.

11. A fuel cell unit (1), a fuel cell system or a fuel cell vehicle, It is characterized in that A method (200) according to any one of the preceding claims is performed and / or can be performed by the fuel cell unit (1), the fuel cell system or the fuel cell vehicle to prevent autonomous continuous combustion of hydrogen in the exhaust device (150) of the fuel cell unit (1).

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