Fuel cell system and method for monitoring a fuel cell system

Abstract

A fuel cell system (100), comprising: a fuel cell stack (10) with a plurality of fuel cells (11); an anode supply (20) and a cathode supply (30) for supplying operating materials to the fuel cells (11); a voltage sensor (40) for detecting an electrical voltage of at least one fuel cell (40) or the fuel cell stack (10); and a control unit (50), characterized in that the control unit (50) is configured to: (a) shutting down the fuel cell stack (10) by shutting off the supply of operating fluid; (b) reading in a plurality of voltage values ​​recorded on the switched-off fuel cell stack (10) and determining a voltage profile over time from the read-in voltage values; (c) carry out an extreme value analysis of the stress curve; and (d) to output a first control signal (S1) as a result of a local extremum of the voltage curve.

Description

The invention relates to a fuel cell system and to a method for monitoring a fuel cell system.Fuel cells utilize the chemical reaction of a fuel with oxygen to form water to generate electrical energy. For this purpose, fuel cells contain as core component the so-called membrane electrode assembly (MEA), which is a structure of an ion-conducting (mostly proton-conducting) membrane and a catalytic electrode (anode and cathode) arranged on both sides of the membrane. The latter usually comprise supported noble metals, in particular platinum. In addition, gas diffusion layers (GDL) can be arranged on both sides of the membrane electrode arrangement on the sides of the electrodes facing away from the membrane.As a rule, the fuel cell is formed by a multiplicity of MEAs arranged in the stack (stack), the electrical powers of which are added together. Bipolar plates (also called flow field or separator plates) are generally arranged between the individual membrane electrode arrangements, which bipolar plates ensure a supply of the individual cells with the operating media, i.e. the reactants, and usually also serve for cooling. In addition, the bipolar plates provide for an electrically conductive contact to the membrane electrode arrangements.During operation of the fuel cell, the fuel (anode operating medium), in particular hydrogen, is supplied to the anode via a flow field of the bipolar plate and electrochemically oxidized to protons with emission of electrons (H 2→2H ++ 2 e -). Via the electrolyte or the membrane, which gas-tightly and electrically isolates the reaction spaces from one another, the protons are transported from the anode space into the cathode space. The electrons provided at the anode are supplied to the cathode via an electrical line.During operation of the fuel cell, oxygen or an oxygen-containing gas mixture (for example air) is supplied to the cathode as cathode operating medium, so that a reduction from O 2 to O 2- takes place with absorption of the electrons (1⁄2 O 2+ 2 e - → O 2-). At the same time, in the cathode space, the oxygen anions react with the protons transported via the membrane to form water (O 2-+ 2 H + → H 2 O).The supply of the fuel cell stack with its operating media, i.e. the anode operating gas (for example hydrogen), the cathode operating gas (for example air) and the coolant, takes place via main supply channels which pass through the stack in its entire stacking direction and from which the operating media are supplied to the individual cells via the bipolar plates. For each operating medium, at least two such main supply channels are present, namely one for supplying and one for discharging the respective operating medium.When known fuel cell systems are switched off, a circuit connected to the fuel cell stack is opened, so that no electrical load is present at the fuel cell stack. Due to remaining operating media, in particular hydrogen on the anode and air on the cathode, undesired potentials can form over the membrane of the switched-off stack, which can lead to corrosion and degradation of the catalytic electrodes. It is therefore known to passivated the electrodes of the shut-off fuel cell stack by flooding the anode and cathode supply with an inert gas, such as nitrogen.In particular in mobile applications, however, the carrying along of an additional container for the inert gas is associated with additional restrictions on the installation space and increased weight. It is therefore advantageous to use the already entrained hydrogen for passivating the electrodes of the fuel cell stack. In order to achieve this, methods have become established in which, when the fuel cell stack is switched off, initially only the cathode supply is deactivated. By suitable actuation of the anode supply and gas discharge from the stack, excess air is pumped off and converted to water. As a result of this reaction and additionally of diffusion in the fuel cell stack, an inert gas mixture of hydrogen and nitrogen is finally present in the anode and cathode spaces.As long as this gas mixture fills the shut-off fuel cell stack, degradation and corrosion of the electrodes can be at least minimized. In addition, the risk of so-called air / air starts, in which oxygen is present on both sides of the membrane electrode arrangement, is significantly reduced. However, due to leaks and the high volatility of hydrogen, this inert gas atmosphere of the fuel cell stack is lost more and more with increasing service life.Methods are therefore known from the prior art for maintaining a protective gas atmosphere in a shut-off fuel cell stack over a longer time.In MCFC or SOFC fuel cells, the cell voltage dropped across a membrane electrode arrangement depends on the oxygen partial pressures in the anode and cathode spaces. Thus, an oxygen diffusion to the anode side can be deduced from a falling cell voltage. According to WO 02 / 19446 A2, directed oxygen transport across the membrane and opposite diffusion is to be achieved by reversing the stack voltage. According to WO 2013 / 001166 A1, the amount of inert gas required should be able to be reduced by temporary voltage reversal in at least one part of the fuel cells.In order to maintain a protective gas atmosphere in PEFC fuel cells over a long period of time, it should first be highly sealed. In addition, hydrogen can be supplied continuously or discontinuously to a shut-off fuel cell stack during the standstill or a starting phase. It is economically advantageous to keep the amount of hydrogen supplied as low as possible.DE 102008018941 A1, DE 102012000882 A1 and DE 102013015025 A1 disclose methods according to which an oxygen concentration or an oxygen partial pressure is determined in the stack and / or its feed lines by means of sensors, for example zirconium oxide sensors. On the basis of these values, a stoichiometrically adapted amount of hydrogen for reacting the penetrated oxygen is then to be determined.DE 102007016307 A1 discloses a method for checking the tightness of a fuel cell stack, having the steps: operating the fuel cell stack at defined gas feed rates, changing at least one gas feed rate in a defined manner, detecting at least one cell or cell group voltage and evaluating the time profile of the at least one cell or cell group voltage.However, it has been found that, on the basis of the measurement of oxygen concentration and / or oxygen partial pressure at feeds and discharges of an anode side or cathode side of a fuel cell stack, it is not possible to reliably infer the presence or absence of a protective gas atmosphere. Due to this unreliability, unnecessary feeding of hydrogen or unrecognized air / air starts may occur.Another approach known from the prior art is to measure stack or cell voltages when the fuel cell stack is switched on, in order to infer harmful mixed potentials therefrom. However, air / air starts cannot be prevented in this way, and it is generally too late at the time of the measurement for adapting a starting strategy.A further possibility is to measure the maintenance time of a hydrogen protection atmosphere in the shut-off fuel cell stack during its development or production and to store corresponding values. For example, aging-related or temperature-dependent fluctuations of these values are disadvantageously not taken into account in this case.The object of the invention is to provide a fuel cell system and a method for monitoring a fuel cell system, with the presence or absence of a protective gas atmosphere in a fuel cell stack being reliably determinable and the waste of hydrogen and unrecognized air / air starts being avoidable.This object is achieved by a fuel cell system and by a method for monitoring a fuel cell system according to the independent claims.The object according to the invention is achieved by a fuel cell system, having a fuel cell stack having a multiplicity of fuel cells, an anode supply and a cathode supply for supplying operating media to the fuel cells, (at least) a voltage sensor for detecting an electrical voltage of at least one fuel cell (individual cell of the fuel cell stack), and a control unit. According to the invention, the control unit is configured to (a) shut off the fuel cell stack by shutting off the supply of operating medium, (b) read in a plurality of voltage values recorded at the shut-off fuel cell stack and determine a temporal voltage profile from the read-in voltage values, (c) perform an extreme value analysis on the voltage profile and (d) output a first control signal as a result of a local extreme or a predetermined feature of the voltage profile.Depending on the shutdown procedure of the stack, the average cell voltage of a fuel cell stack drops to a value of approximately 0 V / cell a short time after the shutdown signal or the shutdown of the supply of at least the cathode operating agent. It has surprisingly been found that after a first period of several hours, the average stack voltage first increases or decreases to a first value between + / - 0.5 mV / cell and + / - 10 mV / cell, then falls or increases to an opposite value of the same order of magnitude and finally assumes the value of approximately 0 V / cell again for a second period of time and remains at this value until the next operation of the stack.It could be shown by means of test series that during the first period with constant voltage of about 0 V / cell a protective gas atmosphere, in particular hydrogen atmosphere, is present in the stack and that during the second period with constant voltage of about 0 V / cell predominantly oxygen is present in the stack. Thus, the wave-shaped profile of the mean cell voltage or of the stack voltage or of the voltage of at least one or more individual cells of the fuel cell stack is a characteristic signal for the loss of a protective gas atmosphere, in particular a hydrogen atmosphere, in a fuel cell stack.The control unit of the fuel cell system according to the invention can therefore advantageously determine, on the basis of the voltage profile of at least one fuel cell or an individual cell of the fuel cell stack, whether a protective gas atmosphere, in particular a hydrogen atmosphere, is present in a shut-off fuel cell stack. If it is determined on the basis of the characteristic voltage profile that a protective gas atmosphere escapes from the fuel cell stack or has already leaked, the control unit outputs a first control signal, as a result of which corresponding measures for obtaining or restoring the protective gas atmosphere are initiated.The characteristic, wave-shaped curve of the voltage of at least one fuel cell, preferably of a plurality of fuel cells and particularly preferably of the mean cell voltage is determined by first detecting a plurality of voltage values at the shut-off fuel cell stack by means of at least one voltage sensor and reading them into a control unit. A time profile of the detected voltage is determined from the plurality of voltage values. The control unit of the fuel cell system then carries out an extreme value analysis on the voltage profile. In this case, the control unit examines the voltage profile or its time derivatives for the presence of a local extreme or a predetermined feature, which is interpreted as part of the wave-shaped signal. If a local minimum or maximum of the voltage curve or another predetermined feature, such as one or more predefined increases in the voltage curve, is determined, the control unit outputs a first control signal. The extreme value analysis preferably comprises the analysis of the voltage profile or its time derivatives with respect to a local extreme and / or a zero crossing and / or a predetermined increase.A temporal voltage profile of the read-in voltage values is preferably determined on a first time scale, wherein the first time scale is greater than a second time scale of background noise. In other words, a smoothed voltage profile is determined, wherein statistical fluctuations of detected voltage values are not recognized as a local extreme. This can be done in various ways. In one embodiment, the control unit determines a voltage profile by regression analysis of the read-in voltage values and by determining therefrom a continuous, for example polynomial, regression function. A local extreme of the voltage curve is then determined by determining the first and second time derivatives of the regression function. If the first time derivative of this function has a zero point, but the second time derivative of this function does not, a local extreme of the voltage curve is present. Alternatively, a local extreme of the time derivative of the voltage curve, i.e. an inflection point of the voltage curve, is determined. For this purpose, the third time derivative of the voltage curve is preferably determined.In a further embodiment, the control unit purely numerically determines an at least sectionally continuous stress profile, for example from line sections. The route trains are particularly preferably difference quotients of the voltage values read in. A local extreme of the voltage curve is determined only with a view to the rear, that is to say a last detected value can never be recognized as a local extreme, for example during a rise in the read-in voltage. ThroughDetermining a local extreme of the voltage curve can be inferred particularly easily from the presence of the characteristic wave-shaped voltage signal.Metrology artifacts, for example voltage peaks caused by the measuring device itself, can lead to a misdetection of the voltage signal and incorrectly to the assumption that oxygen has penetrated into the switched-off fuel cell stack. The control unit is therefore particularly preferably further configured to analyze the voltage profile with respect to a local extreme and a zero crossing. Alternatively, the control unit is configured to analyze the voltage characteristic only with respect to a zero crossing. In the characteristic wave-shaped signal, a local extreme and a zero crossing always occur as a result. Thus, the safety of its detection is enhanced. According to this embodiment, a first control signal is output only as a result of a local extreme and a zero crossing, alternatively only as a result of a zero crossing, of the detected voltage profile.The control unit is also preferably configured to analyze the voltage characteristic with respect to a first local extreme of the first sign, followed by a zero crossing and a second local extreme of the second sign.On the basis of this voltage profile, it is possible to draw a conclusion with particularly high certainty as to the characteristic wave-shaped signal. Faulty detection of the loss of a protective gas atmosphere is thus advantageously largely ruled out. According to this embodiment, a first control signal is output only as a result of a first local extreme of the first sign, followed by a zero crossing and a second local extreme of the second sign. If a measure initiated as a result of the first control signal consists, for example, in supplying hydrogen into the stack, an unnecessary consumption of hydrogen is thus advantageously reduced.In a preferred embodiment, the control unit is further configured to determine a local extreme of the voltage curve if the magnitude of the detected or read-in voltage exceeds an average basic voltage of the shut-off fuel cell stack, i.e. a temporal average of the fuel cell stack voltage, by a predetermined limit value of 0.5 mV / cell to 10 mV / cell. Thus, false detections due to metrology artifacts, for example due to errors of the measuring device or external electrical influences, are effectively avoided. As a rule, an average base voltage of the shut-off fuel cell stack is approximately 0 mV / cell.Particularly preferably, the at least one voltage sensor and / or the control unit are configured for noise suppression in addition to averaging the detected and / or read-in voltage values. A voltage value read in can be determined as an arithmetic mean of a plurality of voltage values detected within a short time. Alternatively or additionally, a high-pass filter for noise suppression can be applied to the detected and / or read-in voltage values. Thus, the smallest fluctuations of the detected voltage, which can likewise be attributable to the sensor itself used, are completely excluded from the determination of the voltage profile and / or the extreme value analysis. In particular, in the case of a purely numerical determination of a sectionally continuous voltage profile, noise suppression is advantageous.The control unit is furthermore preferably configured to repeat steps (b) to (d) at regular or irregular intervals after the fuel cell stack has been switched off. The characteristic wave-shaped voltage signal generally only occurs a few hours after the fuel cell stack has been switched off and itself has a duration of a few hours. In particular in mobile applications, after the fuel cell stack has been switched off, frequently only energy is available from an energy store, such as a car battery. Thus, the point detection of the voltage values saves energy compared to a continuous detection. Particularly preferably, the control unit shortens the intervals for detecting, reading in and analyzing the voltage values as soon as it determines a long-term trend of the voltage values, for example a / a voltage increase / voltage drop continuing over a plurality of measurements. Thus, in expectation of a local extreme, the frequency of detection of the voltage is advantageously increased.The control unit preferably repeats steps (b) to (d) at least until at least one local extreme and / or one zero crossing is determined in the extreme value analysis and a first control signal is output. Depending on the measure taken as a result of the first control signal, the control unit can also repeat steps (b) to (d) after the first control signal has been output. If, for example, additional hydrogen is supplied to the fuel cell stack as a result of the first control signal, the stack can thus be monitored again with regard to the loss of the hydrogen atmosphere. In particular with regard to an unknown duration from the switched-off state of the stack, an unnecessarily high use of hydrogen can thus be avoided.The control unit is likewise preferably configured to carry out steps (b) to (d) during repeated intermediate activation of the fuel cell system. Intermediate activation in this context is understood to mean a regularly or irregularly repeated and brief startup of one or more components of the fuel cell system. Such intermediate activations or wake ups of fuel cell systems are known from the prior art and are used, for example, to react to changing ambient temperatures in the case of long service lives. The integration of steps (b) to (d) into such intermediate activations advantageously allows the adaptation of existing fuel cell systems or control units.In a likewise preferred embodiment of the fuel cell system according to the invention, the voltage sensor is configured to detect at least one electrical voltage of at least one fuel cell at intervals of minutes. The characteristic wave-shaped signal generally occurs only a few hours, for example between 1 h and 6 h, after the fuel cell stack has been switched off or after the last feeding of hydrogen, and generally takes place for a few hours, preferably for 1 h to 10 h, particularly preferably for 1 h to 6 h. The detection of electrical voltages of at least one fuel cell, preferably of a plurality of fuel cells and particularly preferably of the average cell voltage of a fuel cell stack at intervals of minutes is therefore sufficient to sufficiently resolve the voltage profile.In a likewise preferred embodiment, the control unit is configured to actuate the voltage sensor at intervals of minutes for detecting at least one electrical voltage of at least one fuel cell. A plurality of voltage values then results cumulatively in the control unit itself or by reading the value just detected by the sensor together with all previously detected voltage values into the control unit. Likewise preferably, the voltage sensor automatically detects electrical voltage values at intervals of minutes and passes these values on batch-by-batch to the control unit, for example during an intermediate activation of the control unit.The voltage sensor of the fuel cell system according to the invention is preferably configured to detect electrical voltages in the order of magnitude of 0.1 mV / cell. The voltage sensor according to the invention is preferably designed as a single-cell voltage sensor with a scale of + / - 50 mV, preferably + / - 20 mV and particularly preferably + / - 10 mV. Alternatively, the voltage sensor according to the invention is designed as a multi-cell voltage sensor for approximately 10 individual cells, preferably approximately 20 individual cells and particularly preferably approximately 50 individual cells with a scale of + / - 500 mV, preferably + / - 200 mV and particularly preferably + / - 100 mV.Likewise preferably, the voltage sensor according to the invention is integrated into a sensor for detecting the stack voltage, wherein the stack voltage sensor is operated with a first scale, for example a scale of the order of + / -100 V, during operation of the fuel cell stack and is switched over to a second scale, for example a scale in the range + / - 50 mV, preferably + / - 20 mV and particularly preferably + / - 10 mV, when the fuel cell stack is switched off. For the functionality of the fuel cell system according to the invention, it is essential that the at least one voltage sensor is configured to detect electrical voltages in the order of + / - 0.1 mV / cell. Such voltage sensors are far more favorable and reliable than the gas sensors used in known fuel cell systems.Likewise preferably, the control unit of the fuel cell system according to the invention is configured to initiate measures for protecting the electrodes, in particular the anodes, of the fuel cell stack as a result of the first control signal. These measures can have, for example, the control of an anode supply and / or a cathode supply for supplying an anode operating medium and / or an inert gas into the fuel cells, in particular their anode or cathode spaces. Particularly preferably, as a result of the first control signal, hydrogen is fed into the stack by means of the anode supply. Likewise preferably, another inert gas, for example nitrogen, is introduced from a corresponding reservoir into the anode and / or cathode spaces. A hydrogen atmosphere is thus maintained at least in the anode spaces, whereby degradation and corrosion of the electrodes and air / air starts can be avoided.Alternatively or additionally, the measures initiated by the control unit can comprise applying an external voltage, in particular a voltage opposite to the operating voltage of the fuel cell, to the membrane of at least one fuel cell. By applying an external voltage opposite to the operating voltage, the transport processes via the membrane electrode arrangement can be reversed. For example, the MEA of a PEFC can be operated as a proton pump in order to convey and distribute freshly supplied hydrogen to the cathode side on the anode side.Likewise preferably, the control unit of the fuel cell system according to the invention is configured to change a storage value relating to a starting behavior of the fuel cell stack as a result of the first control signal. With the fuel cell system according to the invention, a protective gas atmosphere can generally be maintained advantageously for the entire standstill time of a fuel cell stack. In certain cases, however, penetration of oxygen into the stack, for example due to inadequate fuel or detection too later, can no longer be prevented. In particular in these cases, the control unit changes a value stored in the control unit or in another memory in response to the characteristic wave-shaped voltage signal, so that a starting method is carried out at the next start of the stack, which is optimized for an air / air start. Such starting methods are known.The control unit of the fuel cell system according to the invention is furthermore configured to set the value of a second control signal as a function of the sign of a local extreme detected first after the fuel cell stack has been switched off or after hydrogen has been fed back into the stack. Advantageously, it can be concluded on the basis of the sign of this first local extreme whether oxygen has penetrated first into the cathode spaces or first into the anode spaces of the fuel cell stack. If the first local extreme is a local maximum, the oxygen is first introduced on the cathode side or into the cathode spaces. If the first local extreme is a local minimum, the oxygen is first introduced on the anode side or into the anode spaces. This information is advantageously used for purposes of diagnostics or for coordinating the measures for protecting the electrodes. Preferably, it is determined on the basis of the value of the second control signal whether hydrogen or an inert gas is supplied to the anode side and / or the cathode side.The control unit of the fuel cell system according to the invention is furthermore configured to output a third control signal if a time period between the switching off of the fuel cell stack and the output of the first control signal falls below a predetermined time period. The control unit is likewise preferably configured to output the third control signal if a time period between the renewed feeding of hydrogen and the output of the first control signal falls below a predetermined time period. For this purpose, the control unit is furthermore configured to record the time of occurrence of local extremes and zero crossings of the voltage profile. If the predetermined time interval is undershot, an increased leakage of the fuel cell stack can be deduced therefrom. The third control signal can be used primarily for diagnostic purposes. For example, in mobile applications of the fuel cell system according to the invention, a warning signal can be output to the driver on the basis of the third control signal, with the result that the driver is informed in good time about the increased leakage and can visit a workshop.The invention likewise relates to a method for monitoring a fuel cell system, having a fuel cell stack having a multiplicity of fuel cells, an anode supply and a cathode supply for supplying operating means to the fuel cells, (at least) one voltage sensor for detecting an electrical voltage of at least one fuel cell and a control unit, wherein the method has the following method steps: shutting down the fuel cell stack by shutting down the operating means supply, reading in a multiplicity of voltage values detected by the voltage sensor on the shut-down fuel cell stack into the control unit, determining a temporal voltage profile from the read-in voltage values; carrying out an extreme value analysis on the voltage profile and outputting a first control signal as a result of a local extreme of the voltage profile.The invention also relates to a vehicle having a fuel cell system as described above.Further preferred embodiments of the invention result from the other features mentioned in the dependent claims.The various embodiments of the invention mentioned in this application can be combined with one another with advantage unless stated otherwise in the individual case.The invention is explained below in exemplary embodiments with reference to the associated drawings. The following are shown: FIG. 1 shows a block diagram of a fuel cell system according to a preferred embodiment; FIG. 2 shows an illustration of the mean cell voltage U and of a range of the voltages U / cell of the individual fuel cells of the fuel cell stack for a fuel cell system according to the invention; and FIG. 3 shows a block diagram of a method according to the invention.FIG. 1 shows a fuel cell system, generally designated 100, according to a preferred embodiment of the present invention. The fuel cell system 100 is in each case part of a vehicle, not shown in any more detail, in particular an electric vehicle, which has an electric traction motor which is supplied with electrical energy by the fuel cell system 100.The fuel cell system 100 comprises, as a core component, a fuel cell stack 10 having a plurality of unit cells 11 arranged in a stack form, which are formed by alternately stacked membrane electrode arrangements (MEA) 14 and bipolar plates 15 (see detail section). Each individual cell 11 thus comprises in each case an MEA 14, which has an ion-conductive polymer electrolyte membrane not shown in detail here, and catalytic electrodes arranged on both sides thereof, namely an anode and a cathode, which catalyze the respective partial reaction of the fuel cell reaction and can be designed in particular as coatings on the membrane.The anode and cathode electrodes comprise a catalytic material, for example platinum, supported on an electrically conductive, high specific surface area support material, for example a carbon-based material. An anode space 12 is thus formed between one bipolar plate 15 and the anode, and the cathode space 13 between the cathode and the next bipolar plate 15. Optionally, gas diffusion layers may be disposed between the membrane electrode assemblies 14 and the bipolar plates 15.In order to supply the fuel cell stack 10 with the operating media, the fuel cell system 100 has, on the one hand, an anode supply 20 and, on the other hand, a cathode supply 30.The anode supply 20 of the fuel cell system 100 shown in FIG. 1 comprises an anode supply path 21 which serves for supplying an anode operating medium (the fuel), for example hydrogen, into the anode spaces 12 of the fuel cell stack 10. For this purpose, the anode supply path 21 connects a fuel accumulator 23 to an anode inlet of the fuel cell stack 10. the anode supply 20 further comprises an anode exhaust gas path 22 which discharges the anode exhaust gas from the anode spaces 12 via an anode outlet of the fuel cell stack 10.The anode operating pressure on the anode side 12 of the fuel cell stack 10 is adjustable via a first adjusting means 24 in the anode supply path 21. In addition, the anode supply 20 of the fuel cell system shown in FIG. 1 has, as illustrated, a recirculation line 25 which connects the anode off-gas path 22 to the anode supply path 21. The recirculation of fuel is customary in order to recycle and use the fuel, which is usually used in superstoichiometric amounts, to the stack. A recirculation conveying device 27, preferably a recirculation blower, is arranged in the recirculation line. Further, a water separator 28 is installed in the anode off-gas path 22 to condense and discharge liquid water discharged from the fuel cell stack 10.In the anode exhaust gas line 22 of the fuel cell system 100 shown in FIG. 1, a second actuating means 26 is arranged downstream of the recirculation line 25. The fuel cell stack 10 and a recirculation circuit can be jointly insulated from the environment by the second actuating means 26. The first and second adjusting means 24, 26 can be used together to largely prevent gases from flowing out of the anode spaces 12. Furthermore, downstream of the fuel cell stack 10 and upstream of the second actuating means 26, a water separator 28 is arranged in the recirculation circuit of the cathode exhaust gas line 22.The cathode supply 30 of the fuel cell system 100 shown in FIG. 1 comprises a cathode supply path 31 which supplies an oxygen-containing cathode operating medium, in particular air, which is drawn in from the environment, to the cathode spaces 13 of the fuel cell stack 10. The cathode supply 30 further comprises a cathode exhaust gas path 32, which discharges the cathode exhaust gas (in particular the exhaust air) from the cathode spaces 13 of the fuel cell stack 10 and optionally feeds this to an exhaust system, not shown.For conveying and compressing the cathode operating medium, a compressor 33 is arranged in the cathode supply path 31. In the exemplary embodiment shown, the compressor 33 is designed as a compressor 33 which is driven mainly by electric motor and whose drive is effected via an electric motor 34 equipped with corresponding power electronics 35. The compressor 33 can furthermore be driven in a supporting manner by a turbine 36 (optionally with variable turbine geometry) arranged in the cathode exhaust gas path 32 via a common shaft (not shown).The fuel cell system 100 shown in FIG. 1 further includes a humidifier module 39. The humidifier module 39 is arranged in each case in the cathode supply path 31 on the one hand such that the cathode operating gas can flow through it. On the other hand, it is disposed in the cathode off-gas path 32 so as to be flown through by the cathode off-gas. A humidifier 39 typically comprises a plurality of water vapor permeable membranes, which are either planar or in the form of hollow fibers. In this case, one side of the membranes is overflowed by the comparatively dry cathode operating gas (air) and the other side is overflowed by the comparatively moist cathode exhaust gas (exhaust gas). Driven by the higher partial pressure of water vapor in the cathode off-gas, water vapor passes via the membrane into the cathode operating gas, which is humidified in this way.According to the exemplary embodiment shown in FIG. 1, the cathode supply 30 further has a third actuating means 37 and a fourth actuating means 38 for isolating the fuel cell stack 10 from the environment. The third and fourth adjusting means 37, 38 can together largely prevent gases from flowing out of the cathode spaces 12. All the adjusting means 24, 26, 37 and 38 of the fuel cell system 100 can be designed as controllable or non-controllable valves or flaps.The fuel cell system 100 further includes a voltage sensor 40 and a control unit 50 (ECU). The voltage sensor 40 is configured to detect the average cell voltage of the fuel cell stack 10. For this purpose, voltage sensor 40 detects the electrical voltage across a plurality of individual cells 11 and averages it across the specific number of this plurality of individual cells 11. The voltage sensor of the embodiment shown in FIG. 1 is configured to determine the average cell voltage at the active and at the shut-off fuel cell stack and has, for this purpose, in particular two different scales, between which it is possible to switch.The determined average cell voltage Ucell is read in by the control unit 50. The control unit 50 of the fuel cell system according to the invention determines a temporal voltage profile from the voltage values read in and then carries out an extreme value analysis. As a result of the extreme value analysis, in particular as a result of a local extreme of the course of the mean cell voltage U cell, the control unit 50 outputs a first control signal S 1. This first control signal S 1 is output by the control unit 50, in particular to the fuel tank 23 and to the first actuating means 24, which thereupon feed hydrogen into the anode spaces 12 of the shut-off fuel cell stack 10.FIG. 2 is a representation of the time profile of the mean cell voltage U and of a shaded region in which the time profiles of the voltage of the individual fuel cells 11 of the fuel cell stack 10 run, in each case for a fuel cell system 100 according to the invention.The start of the illustration coincides with the shutdown of the fuel cell stack 10 by shutting down the cathode supply by shutting down the compressor 33. As a result, the oxygen concentration at the cathode inlet and thus the voltages of the individual cells 11 and of the stack 10 drop significantly. At the same time, the turbine 36 is operated as a fan, whereby remaining oxygen is discharged from the fuel cell stack 10. The decomposition of residual oxygen after the fuel cell stack 10 has been switched off is additionally assisted by a continuous feeding in of hydrogen which reacts with the remaining oxygen to form water for a short time. It can be seen from FIG. 2 that just after the fuel cell stack 10 has been switched off, the average cell voltage has dropped to a value of approximately 0 mV / cell and initially remains at this value for approximately 6 h.The average cell voltage shows a characteristic, wave-shaped profile about 6 h after the fuel cell stack 10 has been switched off. In the course of this, the average cell voltage initially rises to a locally maximum value of approximately 1.2 mV / cell, subsequently has a zero crossing approximately 7.5 h after the fuel cell stack 10 has been switched off and subsequently has a local minimum at a value of approximately -1.8 mV / cell approximately 11 h after the fuel cell stack 10 has been switched off. It is further shown in FIG. 2 that the average cell voltage rises again after the local minimum. This increase ends with the average cell voltage again assuming a value of approximately 0 mV / cell and remaining at this.It could be shown by means of test series that during the first section of about 6 h, in which the average cell voltage has a value of about 0 mV / cell, a hydrogen atmosphere was present in the anode spaces 12 of the fuel cell stack 10. It could further be shown that in the second section, which begins with the local minimum of the mean cell voltage about 11 h after switching off the fuel cell stack, an oxygen atmosphere was present in the anode spaces of the fuel cell stack 10. The characteristic wave-shaped profile of the mean cell voltage is thus a measurement signal, from which it is possible to draw conclusions with high certainty about the loss of a hydrogen atmosphere into or the penetration of oxygen into the anode spaces / s 12.A block diagram of a method according to the invention for monitoring a fuel cell system 100 based on this finding is shown in FIG. 3.The method according to the invention begins with the shutdown of the fuel cell stack 10 by stopping the supply of operating medium (OFF). As described above, this can be done by first adjusting the air supply to the cathode spaces 13 and, shortly later, the hydrogen supply to the anode spaces 12. Shutdown procedures for transferring the fuel cell stack 10 into a defined shut-down state are the subject of current developments.After a certain time Δt, which is in the order of minutes and is, for example, 10 minutes, has elapsed, at least one value of an average cell voltage Ucell detected by means of the voltage sensor 40 is read into the control unit 50. The average cell voltage is averaged over all individual cells 11 of the fuel cell stack 10. In addition, each value of the average cell voltage U cell for noise suppression may be averaged over a short period of time as compared to Δt.The control unit 50 then performs a regression analysis by fitting a polynomial function to the plurality of detected values of the mean cell voltage U celland determining the first and second time derivatives of the thus determined course of the mean cell voltage Ucell. The control unit 50 further determines whether the first time derivative of the regression function has a zero point at which the second time derivative of the regression function does not have a zero point. If such a zero point of the regression function is not determined, the control unit 50 reads in again at least one value of the mean cell voltage Ucell after the specific time Δt has elapsed again.However, if the control unit 50 determines a zero point of the first time derivative of the regression function at which the second time derivative of the regression function does not have a zero point, the control unit 50 outputs the first control signal S 1. As a result of the first control signal S 1, the control unit 50 furthermore determines whether the average cell voltage Ucell has a value of greater than or less than zero at this zero point.If the local extreme is a local minimum, i.e. if the mean cell voltage U cell at this point is less than zero, then the control unit 50 sets the value of the second control signal S 2 equal to 0. From this, it can be concluded that oxygen has first penetrated into the anode spaces 12 of the fuel cell stack 10. If the local extreme is a local maximum, that is to say the average cell voltage Ucell is greater than zero at this point, the control unit 50 sets the value of the second control signal S 2 equal to 1.The control unit 50 furthermore checks whether the time period between the shutdown of the fuel cell stack 10 or the last feeding of hydrogen into the anode spaces 12 and the time tiof the occurrence of the first local extreme falls below a predetermined time period tmin. If this is the case, the control unit 50 outputs a third control signal which indicates an unusually high leak in the fuel cell stack 10. In particular in combination with the value of the signal S 2, it is possible to react at an early stage to damage to the seal of the fuel cell stack 10 on the basis of the signal S 3.List of reference characters100 Fuel cell system 10 Fuel cell stack 11 Individual cell 12 Anode chamber 13 Cathode chamber 14 Membrane electrode arrangement (MEA) 15 Bipolar plate (separator plate, flow field plate) 20 Anode supply 21 Anode supply line 22 Anode exhaust gas line 23 Fuel tank 24 First actuating means 25 Recirculation line 26 Second actuating means 27 Recirculation conveying device 28 Water separator 30 Cathode supply 31 Cathode supply line 32 Cathode exhaust gas line 33 Compressor 34 Electric motor 35 Power electronics 36 Turbine 37 Third actuating means 38 Fourth actuating means 39 Humidifier module 40 Voltage sensor 50 Control unit S 1 First control signal S 2 Second control signal S 3 Third control signal

Claims

A fuel cell system (100) comprising: a fuel cell stack (10) having a plurality of fuel cells (11); an anode supply (20) and a cathode supply (30) for supplying operating means to the fuel cells (11); a voltage sensor (40) for detecting an electrical voltage of at least one fuel cell (40) or the fuel cell stack (10); and a control unit (50), characterized in that the control unit (50) is configured to: (a) shut off the fuel cell stack (10) by shutting off the operating means supply; (b) read in a plurality of voltage values detected at the shut-off fuel cell stack (10) and determine a temporal voltage profile from the read-in voltage values; (c) perform an extreme value analysis of the voltage profile; and (d) output a first control signal (S1) as a result of a local extreme of the voltage profile.Fuel cell system (100) according to Claim 1, characterized in that the control unit (50) is furthermore configured to: (c1) analyze the voltage profile or its time derivative with respect to a local extreme and / or a zero crossing and / or a predetermined slope; and (d1) output a first control signal (S1) as a result of a local extreme and / or a zero crossing and / or a predetermined slope of the voltage profile.Fuel cell system (100) according to Claim 1 or 2, characterized in that the control unit (50) is furthermore configured to: (c2) analyze the voltage profile with respect to a first local extreme of the first sign, followed by a zero crossing and a second local extreme of the second sign; and (d2) output a first control signal (S1) as a result of a first local extreme of the first sign of the voltage profile, followed by a zero crossing and a second local extreme of the second sign.Fuel cell system (100) according to one of the preceding claims, characterized in that the control unit (50) is further configured to determine a local extreme of the voltage profile if the magnitude of the detected voltage exceeds a mean basic voltage of the shut-off fuel cell stack (10) by a predetermined limit value of 0.5 mV / cell to 10 mV / cell.Fuel cell system (100) according to one of the preceding claims, characterized in that the control unit (50) is further configured to repeat steps (b) to (d) at regular or irregular intervals after the fuel cell stack (10) has been switched off and / or after the first control signal (S1) has been output and / or is configured to carry out steps (b) to (d) during an intermediate activation of the fuel cell system (100) which takes place repeatedly.Fuel cell system (100) according to one of the preceding claims, characterized in that the voltage sensor (40) is configured to detect at least one electrical voltage of at least one fuel cell (11) or of the fuel cell stack (10) at intervals of minutes.Fuel cell system (100) according to one of the preceding claims, characterized in that the voltage sensor (40) is configured to detect electrical voltages of the order of magnitude of 0.1 mV / cell.Fuel cell system (100) according to one of the preceding claims, characterized in that the control unit (50) is furthermore configured to initiate measures for anode protection and / or cathode protection as a result of the first control signal (S1) and / or to change a storage value relating to a starting behavior of the fuel cell stack (100).Fuel cell system (100) according to one of the preceding claims, characterized in that the control unit (50) is further configured to - set the value of a second control signal (S2) as a function of the sign of a local extreme detected first after the fuel cell stack (10) has been switched off; and / or - output a third control signal (S3) if a time period between the switching off of the fuel cell stack (10) and the output of the first control signal (S1) falls below a predetermined time period.Method for monitoring a fuel cell system (100), comprising a fuel cell stack (10) having a plurality of fuel cells (11); an anode supply (20) and a cathode supply (30) for supplying operating means to the fuel cells (11); a voltage sensor (40) for detecting an electrical voltage of at least one fuel cell (11); and a control unit (50), wherein the method comprises the following method steps: switching off the fuel cell stack (10) by switching off the operating means supply; reading a plurality of voltage values detected by the voltage sensor (40) on the switched-off fuel cell stack (10) into the control unit (50) and determining a temporal voltage profile from the read-in voltage values; carrying out an extreme value analysis on the voltage profile; and outputting a first control signal (S1) as a result of a local extreme of the voltage profile.

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