Fuel cell system and method for operating a fuel cell system

Abstract

The invention relates to methods for operating a fuel cell system (2) in a freeze-start mode, wherein the fuel cell system (2) comprises at least one fuel cell (26) with an anode (28) and a cathode (30); and a jet pump (22) configured to be driven by a hydrogen mass flow (ṁ H2 ), which is supplied to the anode (28) of the at least one fuel cell (26), gas which escapes from the anode (28) of the at least one fuel cell (26), together with the hydrogen mass flow (ṁ H2 ) to the anode (28) of the at least one fuel cell (26). The process comprises, in freeze-start operation, the hydrogen mass flow (ṁ) supplied to the jet pump (22). H2) to limit it so that it is below a predetermined recirculation threshold of the jet pump (22) so that during the freeze start operation no relevant recirculation of gases exiting from the anode (28) of the fuel cell (26) into the anode (28) of the fuel cell (26) takes place.

Description

[0001] The invention relates to a fuel cell system and a method for operating a fuel cell system, in particular for operating a fuel cell system in a freeze-start mode.

[0002] The device also relates to a motor vehicle equipped with a fuel cell system according to the invention. State of the art

[0003] To reduce harmful emissions from motor vehicles, electric motors are increasingly being used in vehicles instead of combustion engines. Batteries or hydrogen-powered fuel cell systems can be used to supply these electric motors with electrical energy.

[0004] To increase the efficiency of a fuel cell system, the fuel cells are often operated with a so-called anode recirculation. In this process, gas exiting the anode of the fuel cell is returned to the anode inlet via an anode recirculation circuit.

[0005] However, during a freeze start of a fuel cell system with anode recirculation, water and / or ice present in the anode recirculation circuit can enter the anode of the fuel cell and freeze there, which can impair the functionality of the fuel cell.

[0006] It is therefore an object of the invention to provide a fuel cell system and a method for operating a fuel cell system in freeze-start mode that makes it possible to prevent water and / or ice from being introduced into the anode(s) of the fuel cell(s) of the fuel cell system during freeze-start mode. Disclosure of the invention

[0007] The invention comprises a method for operating a fuel cell system in a freeze-start mode, wherein the fuel cell system comprises at least one fuel cell and a jet pump. The at least one fuel cell has an anode and a cathode. The jet pump is configured to return, driven by a hydrogen mass flow supplied to the anode of the at least one fuel cell, gas exiting the anode of the at least one fuel cell together with the hydrogen mass flow to the anode of the at least one fuel cell.

[0008] A method according to the invention comprises limiting the hydrogen mass flow supplied to the jet pump during freeze-start operation in such a way that it lies below a predetermined recirculation threshold of the jet pump, so that during freeze-start operation no relevant return ("recirculation") of gases exiting the anode of the at least one fuel cell to the anode of the at least one fuel cell takes place.

[0009] The invention also comprises a fuel cell system with at least one fuel cell having an anode and a cathode; a hydrogen supply valve configured to regulate a hydrogen mass flow supplied to the anode of the at least one fuel cell; a jet pump arranged between the hydrogen supply valve and the anode of the at least one fuel cell, configured to return gas discharged from the anode of the at least one fuel cell, together with the hydrogen mass flow, to the at least one fuel cell, driven by a hydrogen mass flow supplied to the anode of the at least one fuel cell; and a purge valve arranged at the outlet of the anode of the at least one fuel cell.The fuel cell system also includes a control device designed to control the hydrogen supply valve and the purge valve in such a way that the fuel cell system performs a method according to the invention for operating a fuel cell system in a freeze-start mode.

[0010] The invention also includes a motor vehicle with at least one electric motor and a fuel cell system according to the invention, which is designed and configured to supply the electric motor with electrical energy.

[0011] A fuel cell system according to the invention and a method for operating a fuel cell system according to the invention have at least the following advantages: Short drying time after shutdown, as water remaining in the anode recirculation circuit does not need to be completely removed. This also results in lower flushing / hydrogen losses during anode drying.

[0012] Increased lifespan of the fuel cell(s) and a reduction in hydrogen consumption.

[0013] Preventing circulation-induced icing of the anode. This prevents cell degradation, which can further increase the lifespan of the fuel cells.

[0014] Exemplary embodiments of the invention enable a robust freeze-start strategy, resulting in reliable start-up behavior and thus higher customer satisfaction.

[0015] Monitoring the cell voltage of the fuel cells (CVM) can be omitted. This reduces the costs of the fuel cell system.

[0016] If a system for monitoring the cell voltage of the fuel cells (CVM) is available, the degree of icing of individual fuel cells can be determined by monitoring the cell voltages during purging operation, thereby further improving the freeze-start behavior.

[0017] In one embodiment, the electrical current drawn from the at least one fuel cell is limited during freeze-start operation to prevent recirculation by the jet pump. The electrical current density can be limited, in particular, to a value in the range of 0.1 A / cm². 2 and 0.3 A / cm 2 be limited.

[0018] In one embodiment, the fuel cell system has a purge valve, and the method comprises purging the at least one fuel cell by temporarily opening the purge valve. The hydrogen mass flow rate supplied to the anode of the at least one fuel cell is kept constant during and after the purge is initiated. In other words, the hydrogen mass flow rate supplied to the anode of the at least one fuel cell is not changed when the purge valve is opened. This prevents the purge from triggering an undesirable anode recirculation during freeze-start operation.

[0019] In one embodiment, the method comprises reducing the electrical current drawn from the at least one fuel cell during the purging process in order to prevent an undesirable anode recirculation from being triggered by the purging during freeze-start operation.

[0020] In one embodiment, the method comprises reducing the electrical current drawn from the at least one fuel cell during the purging process to such an extent that the gas pressure at the anode of the at least one fuel cell is at least 100 mbar, and in particular at least 100 mbar to 200 mbar, greater than the gas pressure at the cathode of the at least one fuel cell. This prevents the mass flow through the purging valve from coming to a standstill.

[0021] In one embodiment, the method comprises reducing the electrical current drawn from the at least one fuel cell before opening the purge valve. The method can particularly include reducing the electrical current drawn from the at least one fuel cell for 100 ms to 1000 ms before opening the purge valve.

[0022] In one embodiment, the method comprises reducing the mass flow through the cathode of the at least one fuel cell during purging, analogous to the reduction of the electric current. This prevents the cell voltage of the at least one fuel cell from rising momentarily, which would reduce heat production and increase the risk of cathode icing.

[0023] Particularly in the case of individual very low cell voltages, it can also be useful as a recovery and regeneration measure to not reduce the mass flow through the cathode of at least one fuel cell during purging, thereby allowing the cell voltage to rise briefly during the purging process. A measured increase in cell voltage can also be used to determine the extent to which individual cells are already iced up on the cathode side.

[0024] In one embodiment, the method additionally comprises determining, in particular calculating, the maximum electrical current that can be drawn from the at least one fuel cell during freeze-start operation without activating recirculation by the jet pump, and limiting the electrical current drawn from the at least one fuel cell during freeze-start operation to this maximum electrical current.

[0025] When determining / calculating the maximum electrical current that can be drawn from the at least one fuel cell during freeze-start operation, in particular purging and the interaction of the purging with the hydrogen mass flow supplied to the at least one fuel cell can be taken into account.

[0026] This allows the freeze-start operation of a fuel cell system according to the invention to be improved even further.

[0027] In one embodiment, the method comprises controlling the purge valve in such a way that the increased amount of hydrogen supplied to the at least one fuel cell, caused by the opening of the purge valve, is limited.

[0028] In one embodiment, the flushing valve is a flushing valve that can be adjusted in steps or continuously, and the method comprises controlling the flushing valve so that it opens fully or partially in order to adjust the mass flow passing through the flushing valve.

[0029] In one embodiment, the flushing valve is a switching valve that can only be switched between a fully open state and a fully closed state, and the method comprises periodically controlling the flushing valve so that it periodically opens and closes in order to vary the average mass flow through the flushing valve.

[0030] An embodiment of the invention is described below with reference to the accompanying figures. Brief description of the characters Fig. Figure 1 shows a schematic view of a motor vehicle with a fuel cell system according to the invention. Fig. Figure 2 shows a schematic view of a fuel cell system according to the invention. Fig. Figure 3 shows in a schematic representation the mass flow rate delivered by the jet pump as a function of the pressure difference at the anode. Fig. Figure 4 shows a flowchart illustrating the process of a method according to the invention. Character description

[0031] Fig. Figure 1 shows a schematic view of a motor vehicle 1 powered by an electric motor 5 which is supplied with electrical energy by a fuel cell system 2.

[0032] The motor vehicle 1 has four wheels 3 and at least one electric motor 5, which is intended to drive at least two of the four wheels 3 of the motor vehicle 1. The electric motor 5 can also be intended to drive all four wheels 3 of the motor vehicle 1.

[0033] In an alternative embodiment, which is not explicitly shown in the figures, an electric motor 5 can be provided on at least one of the wheels 3, in particular on each of the wheels 3, of the motor vehicle 1 for driving the respective wheel 3.

[0034] The electric motor 5 is supplied with electrical energy via a motor control 7, which is provided by the fuel cell system 2.

[0035] A fuel cell system 2 according to the invention can also be used in motor vehicles 1 that have more or fewer than four wheels 3.

[0036] Fig. Figure 2 shows a schematic view of a fuel cell system according to the invention.

[0037] The fuel cell system 2 includes a hydrogen reservoir 12, for example a hydrogen tank 12, which provides hydrogen gas for the operation of the fuel cell system 2.

[0038] Through a shut-off valve 14, the hydrogen gas from the hydrogen reservoir 12 is fed to a heating device 16, which makes it possible to heat the hydrogen gas to a desired temperature.

[0039] From the heating device 16, the hydrogen gas passes through a pressure regulator 18, which makes it possible to set a desired pressure of the hydrogen gas, to a hydrogen supply valve (“HGI valve”) 20, which makes it possible to regulate the supply of the hydrogen gas to at least one fuel cell 26 of the fuel cell system 2.

[0040] The hydrogen gas is supplied in particular to an anode 28 of the at least one fuel cell 26.

[0041] At least one fuel cell 26 also includes a cathode 30, which has a [missing information] in the Fig. 2. Oxygen supply system not explicitly shown. Oxygen is supplied, for example in the form of air from the environment.

[0042] A membrane 24, in particular a polymer electrolyte membrane 24, is arranged between the anode 28 and the cathode 30 of the at least one fuel cell 26.

[0043] For the sake of simplicity, the following is shown in the Fig. Figure 1 shows only a single fuel cell 26. A fuel cell system 2 according to the invention can also have several fuel cells 26, which can in particular be arranged in a fuel cell stack (“fuel cell stack”).

[0044] The hydrogen and oxygen supplied to fuel cell 26 react in fuel cell 26 to form water (H2O). This reaction releases electrical energy, which is used by fuel cell 26 as an electric current I. Stack and voltage U Stack is made available.

[0045] A jet pump 22 is provided between an outlet 20b of the hydrogen supply valve 20 and an inlet 28a of the anode 28 of the fuel cell 26.

[0046] A first inlet 22a of the jet pump 22 is fluidically connected to an outlet 20b of the hydrogen supply valve 20, so that the jet pump 22 is supplied by a hydrogen mass flow ṁ H2 , which flows from the outlet 20b of the hydrogen supply valve 20 through the jet pump 22 to the inlet 28a of the anode 28 of the fuel cell 26, is driven.

[0047] A second inlet (suction inlet) 22b of the jet pump 22 is fluidically connected to an outlet 28b of the anode 28 via an anode gas return line 32, so that during operation of the jet pump 22, gas exiting the anode 28 through the outlet 28b is drawn into the suction inlet 22b of the jet pump 22 via the anode gas return line 32 and is supplied together with the freshly supplied hydrogen gas to the inlet 28a of the anode 28.

[0048] Furthermore, a purge valve 34 is provided at the outlet 28b of the anode 28, which makes it possible to purge the anode 28 by opening the purge valve 34.

[0049] The flushing valve 34 can be a stepwise or steplessly adjustable flushing valve 34, which makes it possible to control the flushing mass flow ṁ through the flushing valve 34. H2,Purge by varying the geometry of the flushing valve 34 in stages or continuously.

[0050] Alternatively, the flushing valve can be a switching valve that can only be switched between a fully open state and a fully closed state. In this case, the average flushing mass flow rate ṁ through the flushing valve 34 over time can be H2,Purge by controlling the flushing valve 34 so that it opens and closes periodically, whereby the opening time of the flushing valve 34 in the duty cycle is adjusted to the desired flow rate through the flushing valve 34.

[0051] The fuel cell system 2 also includes a control device 36, which is configured to control the valves 14, 20, the heating device 16, the pressure regulator 18 and the purge valve 34 in order to operate the fuel cell system 2 in a desired operating mode with predetermined operating parameters.

[0052] In the case of a freeze start of the fuel cell system 2, it is desirable that as little recirculation as possible of gas from the anode 28 of the fuel cell 26 through the anode gas return line 32 and the jet pump 22 into the inlet 28a of the anode 28 takes place, so that no water or ice from the anode circuit can enter the anode 28 of the fuel cell 26.

[0053] This is achieved by appropriately controlling the hydrogen supply valve 20 during freeze-start operation to determine the hydrogen mass flow rate ṁ supplied to the jet pump 22. H2The circulation is limited so that it remains below a predetermined circulation threshold of the jet pump 22. This results in no relevant recirculation of gases exiting the anode 28 of the fuel cell 26 into the anode 28 of the fuel cell 26 during freeze-start operation. In this way, it can be reliably prevented that water or ice from the recirculation circuit enters the anode 28 of the fuel cell 26.

[0054] The circulation threshold can be determined in the development phase of the jet pump 22 or the fuel cell system 2 by calculations, simulations or component tests and, if necessary, verified by measurements in the application phase of the fuel cell system 2 on a test bench equipped with additional measuring technology.

[0055] One obvious measurement for this is the pressure difference Δp. Anbetween input 28a and output 28b of anode 28, which can be determined by existing or additional pressure sensors. Alternatively, this pressure difference Δp can be used. An measured in real-world operation and used to determine the area below the recirculation threshold.

[0056] Theoretical calculations and simulations for determining the circulation threshold can incorporate geometric parameters of the jet pump 22, such as the diameter, angle, and / or length, as well as surface properties and operating conditions, such as the pre-pressure at the hydrogen metering valve 20 and the gas composition in the recirculation circuit (concentrations of H2, N2, water vapor), as well as temperatures in the gas phase of the recirculation circuit.

[0057] Fig. Figure 3 shows in a schematic representation the mass flow rate ṁ delivered by the jet pump 22 through the anode gas return line 32. Ras a function of the pressure difference Δp An between the inlet 28a and the outlet 28b of the anode 28.

[0058] Below a minimum pressure difference Δp specified by the design of the jet pump 22 min The recirculation capacity of the jet pump 22 is so low that it can be disregarded. In this area, there is no risk of water and / or ice being conveyed through the recirculation circuit to the inlet 28a of the anode 28 of the fuel cell 26.

[0059] The absence of recirculation during a freeze start is unproblematic, since the nitrogen level in the anode 28 has not yet risen shortly after the start and / or a sufficiently high total pressure can ensure that a sufficiently high hydrogen partial pressure is always present at the membrane 24 of the fuel cell 26.

[0060] Water produced by the operation of fuel cell 26 initially remains in the cathode 30 at low temperatures, especially below freezing, and freezes there. Only at higher temperatures of fuel cell 26, particularly above 30°C, does water bound in membrane 24 evaporate on the anode side of membrane 24 and can then freeze at still-cold outlet areas, leading to blockages. Therefore, the risk of anode 28 icing due to water produced on the cathode side of fuel cell 26 is low as long as the fuel cell 26 has not yet heated up significantly.

[0061] Fig. Figure 4 shows a flowchart illustrating the process of a method according to the invention for operating a fuel cell 26 in freeze-start mode.

[0062] After the start of the freeze-start operation in step 100, in step 200 it is first checked whether a purging process or a drainage process of the at least one fuel cell 26 is being carried out and whether the purging valve 34 or a drainage valve not shown in the figures is open.

[0063] If this is not the case, step 300 is skipped and the process continues with step 400. In step 400, it is checked whether the hydrogen mass flow rate ṁ H2 below a predetermined recirculation threshold ṁ max lies (ṁ̇ H2 < ṁ max If this is the case, the current draw I Stack from at least one fuel cell 26 and thus also the hydrogen inflow ṁ H2 increased in step 500 into the anode 28 of the fuel cell 26.

[0064] Steps 200, 400 and 500 are repeated cyclically until, in step 400, it is determined that the hydrogen mass flow rate ṁ H2 the specified recirculation threshold ṁ max has reached or exceeded (ṁ H2 ≥ ṁ max ). In this case, the current draw I Stack from at least one fuel cell 26 and thus also the hydrogen mass flow ṁ H2 reduced again in the anode 28 of the fuel cell 26 to achieve that the hydrogen mass flow ṁ H2 the specified recirculation threshold ṁ max does not exceed.

[0065] If, in step 200, it is determined that a purging or drainage process is being carried out on the at least one fuel cell 26, with the purging valve 34 or the drainage valve being open, the current draw I StackThe current draw from at least one fuel cell 26 is immediately reduced, since a constant current draw with the purge valve 34 / drain valve open would result in an increased hydrogen supply from the hydrogen reservoir 12. This could initiate an undesirable recirculation of gas from the outlet 28b of the anode 28 of the fuel cell 26 through the anode gas return line 32 and the jet pump 22 during a freeze start.

[0066] The method for operating the fuel cell system 2 may in particular include the electrical current I extracted from the at least one fuel cell 26 during a rinsing or dewatering process. Stack to reduce too much so that the gas pressure p An at the anode 28 of the fuel cell 26 at least 100 mbar, in particular between 100 mbar and 200 mbar, greater than the gas pressure p KatThe cathode 40 of the fuel cell is set to 26, otherwise there is a risk that the rinsing or dewatering process will come to a standstill.

[0067] It is advantageous if the electrical current I taken from the at least one fuel cell 26 Stack The flow rate is reduced before the purge valve 34 is opened. This can lead to a brief drop in the hydrogen mass flow rate ṁ. H2 from the hydrogen supply valve 20 into the jet pump 22. However, this is not a problem for the operation of the fuel cell system 2 and the recirculation through the anode gas return line 32.

[0068] The electrical current drawn from at least one fuel cell can be reduced, for example, 100 ms to 1000 ms before the purge valve is opened.

[0069] In the case of cathode operation with air depletion to increase heat production, it can also be advantageous if the oxygen mass flow through the cathode 39 is briefly analogous to the electric current I. Stack , which is taken from fuel cell 26, is reduced.

[0070] Otherwise, the cell voltage of the fuel cell 26 would briefly increase, which would reduce the heat production of the fuel cell 26 and thus increase the risk of icing of the cathode 30.

[0071] On the other hand, particularly in the case of individual, very low cell voltages, it can be useful as a recovery and regeneration measure to briefly increase the cell voltages during rinsing.

[0072] Optionally, such a sudden increase in cell voltage can be used to detect the extent to which the cathodes 30 of individual fuel cells 26 are already iced up.

[0073] The freeze-start behavior of fuel cell system 2 can be further improved by increasing the maximum electrical current I Stack,max is determined, which can be taken from the at least one fuel cell 26 without increasing the recirculation to such an extent that there is a risk of unacceptably high water ingress and thus a risk of icing at the anode 28, and the electrical current I taken from the fuel cell 26 or from the fuel cell stack Stack to the maximum electric current I thus determined Stack,max is limited: IStack≤IStack,max.

[0074] The maximum permissible water ingress is determined experimentally and / or by simulations during the development / application of the system and is stored in the control device 36 as a fixed limit or in a table depending on state variables, such as the electrical load.

[0075] For the hydrogen mass flow rate ṁ H2,HGI , which flows through the hydrogen supply valve 20, applies In normal operation, i.e. with the flushing valve 34 closed: m˙H2,HGI=m˙H2,Consumed+m˙H2,Crossover,Leakage and in flushing mode, i.e. with the flushing valve 34 open: m˙H2,HGI=m˙H2,Consumed+m˙H2,Crossover,Leakage+m˙H2,Purge

[0076] Here, ṁ H2,Consumed The mass flow rate of hydrogen consumed in the chemical reaction in at least one fuel cell 26. ṁ H2,Consumed This results from the Faraday equation and is proportional to the electric current I. Stack , which is taken from at least one fuel cell 26 or the fuel cell stack.

[0077] ṁ H2,Crossover,Leakage is the mass flow rate lost through losses, e.g., leaks, in fuel cell system 2. ṁ H2,Crossover,Leakage is not significantly influenced during the freezing start.

[0078] ṁH2,Purge is the mass flow rate of hydrogen that exits through the purge valve 34 during purge operation. ṁ H2,Purge depends on the flushing strategy used and the flushing valve 34.

[0079] For the maximum electric current I Stack,max , which, without recirculation, can be taken from the fuel cell 26 or the fuel cell stack, then applies In normal operation, with the flushing valve 34 closed: IStack,max∝m˙H2,Consumed,max=m˙H2,HGI,max−m˙H2,Crossover,Leakage and during rinsing operation, with the rinsing valve 34 open: IStack,max∝m˙H2,Consumed,max=m˙H2,HGI,max−m˙H2,Crossover,Leakage−m˙H2,Purge

[0080] In practical application, the maximum current I depends Stack,max from the corresponding geometry of the hydrogen supply valve 20, the jet pump 22, and the fuel cell(s) 26, as well as from the materials used, in particular their interaction with water droplets.

[0081] Preferably, the maximum flow rate ṁ is determined by controlling the flushing valve 34. H2,Purge,max significantly limited compared to normal operation in order to limit the hydrogen supply valve 20 and thus also the recirculation capacity.

[0082] To this end, it is advisable to keep the pressure differential between the inlet 20a and the outlet 20b of the hydrogen supply valve 20 as small as possible, especially below the critical pressure ratio of 2, and / or to flush more frequently, but only for short periods.

[0083] As soon as the temperature of at least one fuel cell 26 is safely above a predetermined freezing temperature level, e.g. when a coolant inlet temperature is above 5°C or more, the freeze start operation is terminated, and the fuel cell system 2 is operated in normal operation with active recirculation.

Claims

[1] Method for operating a fuel cell system (2) in a freeze-start operation, wherein the fuel cell system (2) comprises: at least one fuel cell (26) with an anode (28) and with a cathode (30); and a jet pump (22) designed to be driven by a hydrogen mass flow (ṁ H2 ), which is supplied to the anode (28) of the at least one fuel cell (26), gas which escapes from the anode (28) of the at least one fuel cell (26), together with the hydrogen mass flow (ṁ H2 ) to the anode (28) of the at least one fuel cell (26); the procedure includes, During freeze-start operation, the hydrogen mass flow rate (ṁ) supplied to the jet pump (22) H2) so that it is below a predetermined recirculation threshold of the jet pump (22) so that during the freeze start operation no relevant recirculation of gases or liquids escaping from the anode (28) of the fuel cell (26) or other components (22, 32) of the anode circuit into the anode (28) of the fuel cell (26) takes place. [2] Method according to claim 1, wherein the method comprises using the electrical current (I) extracted from the at least one fuel cell (26) during freeze-start operation. Stack ) to limit, the method in particular comprising limiting the electric current density to a value between 0.1 A / cm² 2 and 0.3 A / cm 2 to limit. [3] Method according to claim 1 or 2, wherein the fuel cell system (2) has a purge valve (34) and wherein the method comprises purging the at least one fuel cell (26) by at least temporarily opening the purge valve (34) and the hydrogen mass flow (ṁ H2 ), which is supplied to the anode (28) of the at least one fuel cell (26), to keep constant during and after the initiation of purging. [4] The method of claim 3, wherein the method comprises using the electrical current (I) extracted from the at least one fuel cell (26) during the rinsing process Stack ) to reduce; wherein the method in particular comprises reducing the electrical current (I) withdrawn from the at least one fuel cell (26) during the purging process Stack ) to reduce too much so that the gas pressure (p An) in the anode (28) of the fuel cell (26) is at least 100 mbar greater than the gas pressure at the cathode (30) of the fuel cell (26). [5] The method of claim 4, wherein the method comprises extracting the electrical current (I) from the at least one fuel cell (26). Stack ) before opening the purge valve (34), the method in particular comprising reducing the electrical current (I) drawn from the at least one fuel cell (26). Stack ) to reduce the time before opening the flushing valve (34) from 100 ms to 1000 ms. [6] Method according to one of claims 4 or 5, wherein the method comprises reducing the mass flow through the cathode (30) of the at least one fuel cell (26) during purging, analogous to reducing the electric current (I Stack to reduce. [7] Method according to any one of claims 3 to 6, wherein the method comprises determining the maximum electric current (I Stack,max), which can be taken from the at least one fuel cell (26) during the freeze-start operation without activating the recirculation by the jet pump (22), in particular to calculate, and to determine the electric current (I Stack ), which is taken from the at least one fuel cell (26) during freeze-start operation, to the maximum electrical current (I) thus determined Stack,max to limit it. [8] Method according to claim 7, wherein when determining the maximum electric current (I Stack,max ) in particular the purging and the interaction of the purging with the hydrogen mass flow (ṁ H2 ), which is supplied to at least one fuel cell (26), shall be taken into account. [9] Method according to any one of claims 3 to 8, wherein the method comprises controlling the purge valve (34) in such a way as to limit the increased amount of hydrogen supplied to the at least one fuel cell (26) caused by the purge. [10] Method according to claim 9, wherein the flushing valve (34) is a stepwise or continuously adjustable flushing valve (34) and wherein the method comprises controlling the flushing valve (34) so ​​that it opens completely or partially. [11] Method according to claim 9, wherein the flushing valve (34) is a switching valve and wherein the method comprises periodically controlling the flushing valve (34) so ​​that it periodically opens and closes. [12] Fuel cell system (2) comprising: at least one fuel cell (26) with an anode (28) and a cathode (30); a hydrogen supply valve (20) designed to supply a hydrogen mass flow (ṁ H2), which is supplied to the anode (28) of at least one fuel cell (26); a jet pump (22) which is arranged between the hydrogen supply valve (20) and the anode (28) of the at least one fuel cell (26) and which is designed to be driven by a hydrogen mass flow (ṁ H2 ), which is supplied to the anode (28) of the at least one fuel cell (26), gas which is discharged from the anode (28) of the at least one fuel cell (26), together with the hydrogen mass flow (ṁ H2 ) to at least one fuel cell (26); a purge valve (34) arranged at the outlet (28b) of the anode (28) of the at least one fuel cell (26); and a control device (36) configured to control the hydrogen supply valve (20) and the purge valve (34) such that the fuel cell system (2) performs a method for operating a fuel cell system (2) in a freeze-start operation according to one of the preceding claims. [13] Fuel cell system (2) according to claim 12, wherein the purge valve (34) is a purge valve (34) that can be adjusted in steps or continuously. [14] Fuel cell system according to claim 13, wherein the purge valve (34) is a switching valve which can be switched between a fully closed state and a fully open state. [15] Motor vehicle (1) with at least one electric motor (5) and a fuel cell system (2) according to claims 12 to 14, wherein the fuel cell system (2) is designed and configured to supply the electric motor (5) with electrical energy.

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