Determination of the humidity at an inlet of an anode-side fuel path and / or at an outlet of a cathode-side oxidant path of a fuel cell device

Abstract

The invention proposes a method for determining the humidity at an anode inlet (13) and / or cathode outlet (18) of a fuel cell device (10) which comprises a fuel cell stack (11) of fuel cells (60) and an anode path (12) and a cathode path (16) with the respective inlets (13, 17) and outlets (14, 18).According to the invention, the method comprises the following steps: detecting (S1) at least one operating parameter relating to the current operating state of the fuel cell stack (11); detecting (S2) a flow rate and / or a pressure along the anode path (12) and / or a flow rate and / or a pressure along the cathode path (16); performing (S3) electrochemical impedance spectroscopy on the fuel cell stack (11) in order to detect a number of n complex impedance values ​​for a number of n different excitation frequencies, where n is in the range from 2 to 10; evaluating (S4) the detected variables (d1, d2, ... dN) using a previously determined regression model (M) in order to determine an estimated value (Ha) for the humidity at the anode inlet (13) and / or an estimated value (Hk) for the humidity at the cathode outlet (18). Furthermore, the invention proposes a corresponding device and a use.

Description

The present invention relates generally to the operation of a fuel cell device and in this case in particular to a method and a device for determining the humidity at an inlet of an anode-side fuel path and / or at an outlet of a cathode-side oxidant path. The invention further relates to a use of such methods and apparatuses.A fuel cell device of the type of interest here comprises a fuel cell stack made of fuel cells with respective anodes and cathodes, wherein the fuel cells are typically each plate-shaped, arranged stacked in a stacking direction and interconnected with one another in electrical series connection. The chemical reaction energy of a supplied fuel (e.g. hydrogen) and of a supplied oxidizing agent (e.g. oxygen or air) can thus be converted into electrical energy by an electrochemical reaction. During operation of the fuel cell device, these reactants of the electrochemical reaction, i.e. for example, on the one hand, hydrogen and on the other hand, air, must be supplied to opposite sides of the fuel cells as viewed in the stacking direction.For supplying the fuel cells with the fuel and the oxidant, the fuel cell device comprises an anode-side fuel path and a cathode-side oxidant path, which have a respective inlet and a respective outlet on the outer side of the fuel cell device.Fresh fuel is supplied at the inlet of the anode-side fuel path, which can then flow through the path and in the process is at least partially consumed by the fuel cells. At the outlet of the anode-side fuel path, the fuel thus changed in its composition can be collected continuously, for example, and, after reprocessing (e.g., dehumidifying), can be mixed with the fresh fuel at the inlet. However, in particular when using hydrogen as fuel, it can also be provided, for example, to briefly open the anode-side fuel path at its outlet only from time to time in order, during each such so-called "purging" process, to flush the anode-side fuel path with fresh fuel and to introduce fresh fuel from the inlet.Fresh oxidizing agent is supplied at the inlet of the cathode-side oxidizing agent path, which oxidizing agent can then flow through the path and is at least partially consumed by the fuel cells in the process. At the outlet of the cathode-side oxidant path, the oxidant stream thus changed in its composition can be discharged continuously into the atmosphere, for example, particularly in the case of the use of air as oxidant.With regard to achieving the highest possible efficiency during operation of a fuel cell device, so-called moisture management is of great importance. These are to be understood as meaning measures which either serve for determining a moisture content and / or the spatial distribution of moisture in the fuel cell device, or which, based on the result of such a moisture determination, serve to set an optimum moisture content in the fuel cell device or individual regions thereof.In the following, a determination of the humidity (e.g. relative humidity) at an inlet of the anode-side fuel path that is desirable in this context will be discussed first. Too high a humidity in this region would block the fuel path (e.g. channel structures on anode-side bipolar half plates) in its further course by forming water droplets, which leads to the fuel cells lacking fuel (e.g. hydrogen) on the anode side as reaction partner for the oxidizing agent (e.g. oxygen) flowing in from the cathode side. Consequently, the oxidizing agent does not react with hydrogen and also may adversely corrode, for example, carbon as a carrier of a catalyst in the fuel cells, resulting in premature aging of the fuel cells. In order to avoid this, it must be ensured that the relative air humidity at the inlet of the anode-side fuel path, also referred to below as "anode inlet", is significantly below 100% with the inclusion of a safety factor. However, in fuel cell devices currently in use, there is usually no possibility of regulating the relative humidity in the anode-side fuel path, also referred to below as "anode path". Rather, a certain moisture content is established via operation and, for example, dry hydrogen having a relative humidity of 0% is metered in at the anode inlet. The anode-side moisture then comes about essentially via what is known as "water drag", i.e. a diffusion of water out into the anode path via the membrane of the fuel cells. It can be assumed here that a relative humidity of, for example, approximately 60-70% is established on the anodes, wherein such a humidity value can, however, vary alternately if, for example, dry fuel is brought back into the anode path after a purging process. Depending on the load profile of the operation, however, peak values of 90% and more of relative humidity can also be achieved.In view of this, it would be desirable to more accurately estimate the humidity at the anode inlet. Such a determination of the humidity at the inlet of the anode-side fuel path can then advantageously be used, for example, for optimizing the operation of the fuel cells. By means of an improved moisture determination, degradation of the cells can also be advantageously reduced, which leads to an increased service life of the fuel cell device. A further advantage of determining the moisture is that, with the aid of the additional information about the actual moisture content, a purging of the anode-side fuel path according to requirements is made possible, which results in a reduction in the fuel consumption.Furthermore, a determination of the humidity (e.g. relative humidity) at the outlet of the cathode-side oxidant path that is desirable for humidity management will be discussed below, also referred to below as "cathode outlet". In the prior art, the approach has already been followed to estimate the relative humidity in the cathode-side fuel path, also referred to below as "cathode path", via a physical model. However, the actual humidity in the region of the cathode outlet can thus only be determined relatively roughly, since the accuracy of such a humidity model is limited. Ideally, a fuel cell device is operated at 100% relative humidity at the cathode outlet, since at this operating point the cell stack supplies the highest voltage. In this case, the oxidant stream at the cathode outlet is completely saturated with water, but water droplets which would block the cathode path do not yet form in the course of the cathode path (for example channel structures on cathode-side bipolar half plates). Too high a humidity would promote precisely this water droplet formation, which has a negative effect on the voltage supplied by the cell stack. Too low a humidity in the cathode path would dry out the membrane of the fuel cells, whereby the membrane resistance increases and the power level of the fuel cell stack likewise decreases.In view of this, it would be desirable to estimate the humidity at the cathode outlet more accurately. Such a determination of the humidity at the cathode outlet can likewise advantageously be used, for example, for optimizing the operation or optimized regulation of the operation of the fuel cells. By means of an improved moisture determination, among other things, the efficiency can be improved, whereby the fuel consumption during operation can be reduced. Furthermore, a more accurate humidity control at the cathode outlet is associated with a reduction in the degrading effect due to drying out of the cells, which leads to an increased service life of the fuel cell device.It is an object of the present invention to show a novel way by which, in a fuel cell device, the humidity at the inlet of the anode-side fuel path (anode path) and / or at the outlet of the cathode-side oxidant path (cathode path) can be determined.According to a first aspect of the invention, this object is achieved by a method according to claim 1. The dependent claims relate to advantageous further developments. The method according to the invention comprises the following steps:detecting at least one operating parameter relating to the instantaneous operating state of the fuel cell stack,detecting a flow rate and / or a pressure at at least one point in the course of the anode-side fuel path and / or a flow rate and / or a pressure at least one point in the course of the cathode-side oxidant path,performing an electrochemical impedance spectroscopy on the fuel cell stack in order to acquire a number of n complex impedance values for a number of n different excitation frequencies, wherein n is in the range from 2 to 10, preferably in the range from 3 to 6,evaluating the detected variables using a regression model determined in advance for the fuel cell device in order to determine an estimated value for the humidity at the inlet of the anode-side fuel path and / or an estimated value for the humidity at the outlet of the cathode-side oxidant path.With the method according to the invention, a very precise determination (estimation) of the humidity in the mentioned regions of the fuel cell device can advantageously take place, which in turn can be used to achieve the advantages mentioned at the beginning in connection with both the service life and the efficiency of the fuel cell device during its operation.In an advantageous embodiment of the method, the fuel cell device can be operated or is operated with hydrogen as fuel and air as oxidizing agent. Such a fuel cell device can be used, in particular, for example in the automobile sector, for the electrical supply of an on-board power supply system, from which, in turn, for example, an electrical drive device can be supplied.The first important aspect of the invention is the detection of one or more operating parameters relating to the instantaneous operating state of the fuel cell stack.In particular, these may be operating parameters relating to a momentary electrical power in the widest sense, such as, in particular, e.g., the current currently supplied by the fuel cell stack. Alternatively or additionally, in this step, for example, the voltage currently supplied by the fuel cell stack can be detected.It is also possible to detect an instantaneous electrical power in the narrower sense (product of current and voltage) as the operating parameter in this step.In one embodiment, one or more of the aforementioned variables (electrical current intensity, electrical voltage, electrical power, etc.) are detected as operating parameters relating to the instantaneous operating state of the fuel cell stack.It is furthermore essential for the invention that at least one flow rate and / or a pressure at least one point in the course of one of the two paths, i.e. anode-side fuel path (anode path) or cathode-side oxidant path (cathode path), is detected as a further variable. If the humidity at the anode inlet is determined with the method, the at least one further variable should be detected at least in the course of the anode path, and if the humidity at the cathode outlet is determined with the method, this variable should be detected at least in the course of the cathode path.In an embodiment advantageous with regard to the accuracy of the moisture determination, a flow rate and / or a pressure is detected both at at least one point in the course of the anode path and at least one point in the course of the cathode path, namely regardless of whether the moisture at the anode inlet, or at the cathode outlet, or at both of these points is to be determined with the method.For each of the detections "at (at least) one location in the course" of a relevant path, this location can advantageously be provided in particular in the region of the inlet and / or in the region of the outlet of the relevant path.In one embodiment, therefore, the flow rate and / or pressure at the inlet and / or outlet of a path in question is detected. For detecting such a flow rate, a flow rate sensor, for example, may be used. Alternatively, however, this detection can also be based on modeling (e.g. using the results of a pressure measurement in the course of the relevant path).In one embodiment, it is provided that the pressure at the anode inlet, at the anode outlet, at the cathode inlet and at the cathode outlet is detected, that is to say at least these four pressure values are detected.In one embodiment, at least one lambda value relating to the amounts of fuel and oxidant (provided to the electrochemical reaction) is detected as a further variable. In particular, such a lambda value can reflect, for example, the ratio between introduced material flow (flow rate) and material flow necessary for the current operating point.For example, this may be an anode-side lambda value, which is defined as the ratio between the actual fuel flow rate at the inlet of the anode-side fuel path and the fuel flow rate theoretically required at this point with regard to an electric current intensity of the fuel cell stack required in the current operating situation in accordance with the relevant electrochemical reaction.Analogously, a lambda value on the cathode side can be detected, which is defined as the ratio between the actual oxidant flow rate at the inlet of the oxidant path on the cathode side and the oxidant flow rate theoretically required at this point with regard to an electrical current intensity of the fuel cell stack required in the current operating situation according to the respective electrochemical reaction.In one embodiment, the method further comprises a detection of a temperature at at least one point in the course of the anode path and / or a temperature at at least one point in the course of the cathode path (in order to thereby detect the temperature of the relevant flow). With regard to the accuracy of the moisture determination, it is advantageous if the temperature is detected both at at least one point in the course of the anode path and at least one point in the course of the cathode path. In one embodiment, the temperature is detected at the inlet and / or the outlet of a path in question.In one embodiment, it is provided that the temperature at the anode inlet, at the anode outlet, at the cathode inlet and at the cathode outlet is detected, that is to say at least these four temperature values are detected in the course of the two paths.In a more specific embodiment, the method includes sensing current currently supplied by the fuel cell stack (and optionally additionally, e.g., voltage currently supplied by the fuel cell stack), and sensing pressure and temperature at the inlet and outlet of the anode path and at the inlet and outlet of the cathode path.As far as the performance of electrochemical impedance spectroscopy (EIS) on the fuel cell stack, which is furthermore essential for the invention, this method is indeed known from the prior art for characterizing a large number of electrochemical systems, but has hitherto only been considered to be useful with regard to fuel cell stacks for obtaining information relating to the fuel cells as such.However, it has been found that, within the scope of the invention, the results of an impedance spectroscopy in combination with the results of the above-explained detections concerning the instantaneous operating state of the fuel cell stack (such as e.g. electric current intensity, electric voltage, etc.) and concerning the mentioned "hydrodynamic" operating parameters of the flows in the region of the anode-side fuel path and / or the cathode-side oxidant path (flow rate and / or pressure, and preferably also temperature) can be evaluated using a regression model in order to determine an estimated value for the humidity at the inlet of the anode-side fuel path (anode inlet) and / or an estimated value for the humidity at the outlet of the cathode-side oxidant path (cathode outlet).The detection of a plurality of "complex" impedance values provided in the invention is intended to mean that, in the mathematical sense, they are complex-valued or thus two-dimensional variables, and that, in the context of the evaluation, the two components thereof (real part and imaginary part, or equivalently, for example, magnitude and phase angle) must be taken into account.In one embodiment, it is provided that a regression model valid for the fuel cell device has been determined beforehand in an empirical manner and is provided for use in real time when carrying out the method according to the invention.Such empirical determination of the regression model can comprise the following steps: a) providing a fuel cell device of the same construction as the fuel cell device to be operated later and equipping it with a measuring device comprising an impedance spectroscopy device, moisture sensors at the inlet of the anode-side fuel path (anode inlet) and / or at the outlet of a cathode-side oxidant path (cathode outlet), and further sensors for detecting the moisture at the anode inlet and / or cathode outlet and also detecting variables identical to those variables which are detected at the fuel cell device to be operated later (within the scope of the determination method); (Operating) the fuel cell device and the impedance spectroscopy device and (systematically) varying the current supplied by the fuel cell stack (e.g. also by varying the load) and the flow rates of the fuel and of the oxidizing agent and the humidities of the supplied fuel and of the supplied oxidizing agent in order to record a multidimensional data field on the basis of the measurement values acquired therewith for a plurality of different operating situations, including complex impedance values in respective assignment to the relevant excitation frequencies; c) carrying out a regression analysis on the basis of the multidimensional data field in order to obtain a regression model.The regression model obtained in this way describes, to a certain extent, the relationship between, on the one hand, the humidities to be estimated and, on the other hand, the acquisition data to be provided to the model as "input data" according to the determination method for this purpose.Expediently, when determining the regression model (and also when later using the regression model), the two components of the complex impedance values, i.e. for example a real part and an imaginary part, are recorded in the data field as two separate (real-valued) data, in respective assignment to the relevant excitation frequencies.In one embodiment of the method, the detected variables are evaluated in order to determine the estimated humidity value or values (at the anode inlet and / or cathode outlet) using a parameterized regression model comprising, for example, an equation system containing the relevant variables and regression parameters, in order to calculate the estimated humidity value or values therewith.In another embodiment of the method, the detected variables are evaluated in order to determine the estimated humidity value or values using a regression model implemented as a trained neural network. In this embodiment, an empirical determination of the regression model can be provided as already described above, wherein the regression analysis to be carried out in this case is realized by means of training the neural network (using the recorded multidimensional data field as training data). The neural network trained in this way then represents the regression model to be used in carrying out the method according to the invention.In particular, the regression model can be implemented in a program-controlled computer device (e.g. microcontroller), for example in the form of program code including look-up tables or the like, by means of which the aforementioned evaluation is carried out.The program-controlled computer device can be, in particular, advantageously a computer device by means of which the operation of the fuel cell device is also controlled (e.g. a so-called control device for a fuel cell device operable on board a vehicle). The control of this operation can comprise, for example, the actuation or setting of controllable valves which, depending on the specific design of a fuel cell system formed with the fuel cell device, can be arranged in particular in lines for supplying fuel and oxidizing agent to the relevant inlets or in lines for discharging fuel and oxidizing agent from the relevant outlets.A further characteristic of electrochemical impedance spectroscopy, as used in the method according to the invention, is that the number of complex impedance values detected thereby is in the range from 2 to 10, preferably in the range from 3 to 6.This has several advantages, especially since it is possible to carry out the method according to the invention during operation of the fuel cell device, because impedance spectroscopy can be carried out very quickly, especially if it is restricted to at most 6 impedance values to be detected (and accordingly at most 6 excitation frequencies). In an embodiment advantageous in this regard, it is provided, for example, that the duration of the performance of the electrochemical impedance spectroscopy is in the range from 0.1 to 2 s. This is in turn of interest in particular when using the invention in the automotive sector (for the electrical supply of an on-board power supply system of a vehicle), in which fuel cell operation is generally controlled relatively dynamically and in this case often an uninterrupted operational readiness is required. A further advantage of the restriction to at most 10 and in particular at most 6 complex impedance values to be detected is that the aforementioned evaluation of the detected variables for determining the humidity is thus simplified, i.e. can be carried out in a resource-saving and nevertheless rapid manner, for example in a program-controlled computer device used for this purpose (e.g. on board a vehicle).In this connection, it has advantageously been found that this relatively small number of impedance values (and accordingly excitation frequencies) is sufficient in practice to enable a sufficiently accurate moisture determination. This applies in particular if the excitation frequencies (adapted to the respective application) are selected in a favorable manner. In most cases, it is favorable, for example, if the excitation frequencies are in a range whose lower limit is at least 5 Hz, in particular at least 10 Hz, and / or whose upper limit is at most 1 kHz, in particular at most 0.5 kHz.In a development of the empirical determination of the regression model explained further above, it is provided that the impedance spectroscopy device is operated for excitation with a number of excitation frequencies (and a corresponding recording of a corresponding number of complex impedance values) which clearly exceeds the number required for the regression model (e.g. by a factor in the range from 2 to 10). Then, for example, by means of a statistical evaluation of the (correspondingly enlarged) data field, a selection can be made from these excitation frequencies in order to identify excitation frequencies which are particularly relevant with regard to the accuracy of the moisture determination method and to take them into account in the desired number (in the range from 2 to 10, in particular 3 to 6) for the generation of the regression model to be used later. In other words, within the scope of the invention, particularly advantageous excitation frequencies (at which the impedance values detected at the fuel cell stack are particularly "meaningful") can likewise be determined empirically.In one embodiment of the method, an excitation signal of the electrochemical impedance spectroscopy is applied between an anode connection and a cathode connection of the fuel cell stack.As regards the measurement signal ("response signal") resulting from the excitation, this can be tapped, for example, over the entire stack, i.e. between the anode connection and the cathode connection of the fuel cell stack, within the scope of the invention. However, it can also be provided that the response signal is tapped between an anode connection and a cathode connection of an individual fuel cell. It is also possible to pick up the response signal at a group ("cell cluster") of a plurality of fuel cells which are directly successive in the fuel cell stack. In order to realize in particular the latter two variant embodiments, it is possible, for example, to provide a cell monitoring device in the fuel cell device, by means of which cell monitoring device operating states of individual fuel cells and / or of at least one fuel cell cluster are monitored (for example by tapping off and evaluating and / or further communicating the cell voltage(s)), and to equip this cell monitoring device with means for detecting the response signal, for example by filtering out the response signal (at the corresponding excitation frequencies), and for recording the complex impedance values resulting for the excitations (after evaluating the respective response signals).According to a further aspect of the invention, the object mentioned at the beginning is achieved by a device for determining the humidity at an inlet of an anode-side fuel path and / or at an outlet of a cathode-side oxidant path of a fuel cell device, wherein the fuel cell device comprises a fuel cell stack made of fuel cells having respective anodes and cathodes and the anode-side fuel path and the cathode-side oxidant path having respective inlets and outlets, and wherein the device comprises:a detection device for detecting at least one operating parameter relating to the instantaneous operating state of the fuel cell stack and for detecting a flow rate and / or a pressure (and preferably also a temperature) at at least one point in the course of the anode-side fuel path and / or a flow rate and / or a pressure (and preferably also a temperature) at least one point in the course of the cathode-side oxidant path,an impedance spectroscopy device for performing an electrochemical impedance spectroscopy on the fuel cell stack and for detecting a number of n complex impedance values for a number of n different excitation frequencies, wherein n is in the range from 2 to 10,an evaluation device for evaluating the detected variables using a regression model determined in advance for the fuel cell device and implemented in the evaluation device for determining an estimated value for the moisture at the inlet of the anode-side fuel path and / or an estimated value for the moisture at the outlet of the cathode-side oxidant path.The embodiments and special configurations described here for the method according to the invention can also be provided, individually or in any combination, analogously as embodiments or special configurations of the device according to the invention, and vice versa.For example, in the device according to the invention, the detection device can be configured to detect variables or different combinations of such variables, as have already been described above for the method according to the invention. The detection device can accordingly have, for example, sensors for measuring the current currently supplied by the fuel cell stack, for measuring the voltage currently supplied by the fuel cell stack and / or for measuring at least one temperature in the region of the fuel cell stack (in particular at the inlets and / or outlets of the anode path and / or of the cathode path), and furthermore have, for example, sensors for measuring a flow rate and / or a pressure at at least one point in the course of the anode path and / or at least one point in the course of the cathode path. Furthermore, the device can be designed, for example, for detecting at least one lambda value, for example by means of a calculation of the respective lambda value (for example anode-side lambda value and / or cathode-side lambda value).Such a lambda value can be calculated from values which are detected, on the one hand, for example by flow sensors for measuring the relevant flow rate (of fuel and / or oxidant) and, on the other hand, for example, by sensors for measuring electrical variables relating to the instantaneous operating point of the fuel cell stack. To detect a certain flow rate, calculations or models of the flow rate can also be used instead of using flow sensors. The latter can be carried out in particular, for example, on the basis of the detections of at least one pressure (and optionally also a temperature) at the inlet and / or outlet of the relevant path (anode path, cathode path).Sensors of this type (for supporting or realizing the detection device) are generally present for the most part anyway in fuel cell devices of the type of interest here, for example as peripheral components of a control device (e.g. control device in a vehicle), by means of which the operation of the fuel cell device is controlled and / or monitored. In this case, such sensors can thus also be used simultaneously for implementing the device according to the invention.The evaluation device for evaluating the detected variables can be implemented, for example, as a program-controlled computer device (e.g., microcontroller), for example, in the form of program code including look-up tables or the like, wherein this can be, for example, a computer device by means of which the operation of the fuel cell device is also controlled. This evaluation is carried out using the regression model determined beforehand for the fuel cell device in order to determine the estimated humidity value or values.In one embodiment, the impedance spectroscopy device is at least partially likewise formed by a or the computer device (e.g. microcontroller) of the aforementioned type. In particular, in this way, for example, a frequency generator for generating an excitation signal used for impedance spectroscopy can be implemented, which is applied here (for example amplified via a driver device) between the anode connection and the cathode connection of the fuel cell stack. Modern microcontroller modules often have powerful peripheral modules, using which the frequency generator can be realized (without generating any processor or system bus load noteworthy for this purpose). Furthermore, the computer device can be used, for example, to implement a detection and evaluation device for the measurement signal to be detected during impedance spectroscopy in order to determine the provided number of complex impedance values (for the corresponding number of different excitation frequencies). This can also be accomplished, for example, by the computer device (e.g., microcontroller) (possibly, e.g., using peripheral modules such as an A / D converter module and further signal-processing modules of the microcontroller periphery).According to a further aspect of the invention, a use of a moisture determination method of the type described here and / or a moisture determination device of the type described here for determining the moisture at an inlet of an anode-side fuel path (anode inlet) and / or at an outlet of a cathode-side oxidant path (cathode outlet) of a fuel cell device arranged on board a vehicle is proposed.In one embodiment of this use, it is provided that if the fuel cell device is operating at a point in time provided for carrying out the determination method, the determination of the humidity is carried out during this operation of the fuel cell device. The humidity is preferably determined within a time period in the range from 0.1 to 2 s.In another embodiment of the use, it is provided that if the fuel cell device is in operation at a point in time provided for carrying out the determination method, first a short interruption of this "normal" operation is effected and instead a short-term switch is made to a special "diagnostic mode" of the fuel cell device, in which the determination of the humidity is then carried out, for example within a time period in the range from 0.1 to 5 s. If, in this diagnostic mode, certain operating parameters or other variables are set to permanently predefined values (for example, a current supplied by the fuel cell stack is set to zero by opening a switch), the detection and evaluation of the relevant variable(s) is advantageously dispensed with.In one specific embodiment of the use, it is provided that points in time for carrying out the determination method are provided from time to time, in particular, for example, periodically, during operation of the vehicle. In addition, it can be advantageously provided, for example, that the result of the determination method is used within the scope of a control of the operation of the fuel cell device, for example in order thereby to optimize the operation of the fuel cells, for example with regard to efficiency, service life, etc., and / or in order thereby, for example, to optimize a specification of times for carrying out "cleaning" processes for the anode path. A result of the determination of the humidity(s) according to the invention can be used, for example, as an additional controlled variable when presetting the times of the cleaning processes.The invention will be described further below with reference to exemplary embodiments with reference to the attached drawings. They are each schematically: FIG. 1 shows a fuel cell system having a fuel cell device and a device for moisture determination according to an exemplary embodiment, FIG. 2 shows a fuel cell according to an exemplary embodiment, for use in a fuel cell stack of a fuel cell device, FIG. 3 is a diagram illustrating flows of fuel and oxidant in respective paths of a fuel cell device, FIG. 4 shows a flow chart of a method for determining the humidity, and FIG. 5 is a diagram illustrating a regression model for humidity determination according to an embodiment.FIG. 1 shows a fuel cell system 1 comprising a fuel cell device 10 having a fuel cell stack 11 formed from a plurality of plate-shaped fuel cells 60 (FIG. 2 ). The fuel cells 60 are arranged in a stacked manner orthogonally to their plate planes (transverse directions x, y) in a stacking direction z and are interconnected with one another via their respective anodes and cathodes in electrical series connection.During operation of the fuel cell device 10, the chemical reaction energy of a supplied fuel, in the example hydrogen, and of a supplied oxidizing agent, in the example air, can be converted into electrical energy by an electrochemical reaction using the fuel cell stack 11.During operation of the fuel cell device 10, the hydrogen and the air must be supplied to flat sides of the fuel cells 60 that are opposite one another as viewed in the stacking direction z. For supplying the fuel cells 60 with the two reactants hydrogen and air, the fuel cell device 10 further comprises an anode-side fuel path 12 having an inlet 13 and an outlet 14 and a cathode-side oxidant path 16 having an inlet 17 and an outlet 18.During operation of the fuel cell device 10, hydrogen flows through the fuel cell stack 11 via the inlet 13 and leaves the fuel cell device 10 via the outlet 14, whereas air flows into the fuel cell stack 11 via the inlet 17 and leaves the fuel cell device 10 via the outlet 18 after flowing through the fuel cell stack 11. The paths 12, 16 drawn as dashed lines in FIG. 1, also referred to below as anode path 12 and cathode path 16, only symbolise the respective connections between the inlets and outlets, that is to say not the actual course of these paths 12, 16 (branched in each case and passing through the individual fuel cells 60).FIG. 2 shows in more detail the structure of an individual fuel cell 60 of the fuel cell stack 11 in the fuel cell device 10 from FIG. 1, wherein the structure shown in FIG. 2 is to be understood merely as an example. In this example, it is a proton exchange membrane (PEM) type fuel cell. The fuel cell 60 has been stacked in the stacking direction z:an anode-side bipolar half plate 61 having a channel structure 62 for guiding the fuel (hydrogen),an anode-side gas diffusion layer 63 (e.g., carbon nonwoven fabric),a membrane electrode unit 64 having an electrolyte membrane 65 (which is electrically nonconductive but proton-conductive) and electrode layers 66, 67 which are arranged on both sides thereof in the stacking direction z and are coated with a catalyst 71 (e.g. platinum or palladium) and form an anode and a cathode for the electrochemical reaction of the fuel with the oxidizing agent (air or oxygen contained therein),a cathode-side gas diffusion layer 68 (e.g. carbon nonwoven fabric), anda cathode-side bipolar half plate 69 having a channel structure 70 for guiding the oxidizing agent.The channel structures 62 and 70, respectively, formed on the anode-side bipolar half plate 61 and the cathode-side bipolar half plate 69 in the example of FIG. 2 represent respective sections of the aforementioned paths 12, 16 for supplying the fuel cell 60 with hydrogen on the one hand and air on the other hand. The half plates 61 and 69 also function as the anode and the cathode of the fuel cell 60, and in the fuel cell stack 11, paired anodes and cathodes of fuel cells 60 adjacent to each other are connected to each other (series electric connection). An anode (bipolar half plate) located at one end of the stack 11 and a cathode (bipolar half plate) located at the other end of the fuel cell stack 11 then simultaneously form an anode and a cathode of the fuel cell stack 11, which are electrically connected to anode and cathode electrical terminals (not shown) of the fuel cell device 10, respectively, at which the electrical power of the fuel cell device 10 is provided.The fuel cell system 1 illustrated in FIG. 1 further comprises a fuel tank 2 for storing the fuel (here: hydrogen), from which gaseous hydrogen can be metered into the anode path 12 via and a controllable inlet valve 4 at the inlet 13, also referred to below as anode inlet 13. Optionally, a heating device 3 shown in the figure, for example, can also be provided in this hydrogen supply path. Furthermore, in the fuel cell system 1, a controllable outlet valve 5 is provided at the outlet 14, also referred to below as anode outlet 14, which outlet valve is briefly opened from time to time during operation in order to flush the anode path 12 with fresh hydrogen from the tank 2 or to refill it at least partially from the anode inlet 13 during such a so-called "purging" process (flushing process). Expediently, in a "normal" operating mode, only a partial purging takes place, whereas, for example, a complete purging process can also occur when the fuel cell system 1 is "started up".Furthermore, the fuel cell system 1 comprises a controllable compressor unit 6, by means of which the oxidizing agent (here air) can be metered into the cathode path 16 during operation via a humidifying device 7 at the inlet 17, also referred to below as cathode inlet 17. At the outlet 18, also referred to below as cathode outlet 18, the air is discharged into the atmosphere.By means of a control unit ST, all controllable components (heating device 3, valves 4, 5, etc.) provided in the fuel cell system 1 for the operation of the fuel cell device 10 are controlled.FIG. 3 is a schematic illustration of the entirety of the fuel cells 60 or of the fuel stack 11 and of the anode path 12 and of the cathode path 16. As illustrated in FIG. 3, fresh hydrogen (H 2) is supplied at the anode inlet 13, but this hydrogen is partly consumed by the electrochemical reaction taking place in the fuel cells 60 when the anode path 12 flows through it (protons migrate through the membrane electrode units 64 in the fuel cells 60), with the result that the hydrogen content gradually decreases toward the anode outlet 14. Furthermore, in the course of the anode outlet 14, there is an increase in a content of nitrogen (N2) which, as symbolized in FIG. 3, diffuses from the cathode path 16 through which air flows through into the anode path 12 through the fuel cells 60. Finally, an increase in the humidity (H2O) also occurs in the course of the anode outlet 14, because water diffuses from the fuel cells 60 (to be operated in humidified fashion) into the anode path 12 ("water drag"). In order to compensate for the aforementioned hydrogen consumption and to avoid a greater accumulation of nitrogen (N2), which in combination with the hydrogen concentration thus reduced would be harmful for the catalyst located in the fuel cells 60 (cf. 71 in FIG. 2 ), a so-called "purging" is carried out at predetermined time intervals, in which the fuel changed in its composition at the anode outlet 14 is discharged and fresh fuel is introduced from the anode inlet 13. Too high a humidity of the hydrogen supplied in the example at the anode inlet 13 would clog the anode path 12 in its further course by the formation of water droplets. In order to avoid this, it must be ensured that the relative humidity at the anode inlet 13 is more or less significantly below 100%. In this connection, it is desirable to be able to determine the humidity (e.g. relative humidity) at the anode inlet 13. Instead of metering in dry hydrogen at the anode inlet 13 with a relative humidity of 0% as usual hitherto, the fuel could then be supplied at this point (for example by means of an adjustable moistening of the fuel) in an optimally conditioned manner, in order to thus optimize the operation of the fuel cell device. Furthermore, by using such information, the times and / or time periods for carrying out the purging or the temporal purging intervals for achieving a reduced fuel consumption can be specified more as required.FIG. 3 further illustrates the supply of fresh air at the cathode inlet 17, the composition of which (about 20% O2, about 80% N2) is changed during the flow through the cathode path 16, but by the electrochemical reaction taking place in the fuel cells 60. In this reaction, since oxygen (O2) is consumed and water (H2O) is formed, the humidity (H2O) gradually increases toward the cathode outlet 18. A relative humidity of 100% at the cathode outlet 18 is generally optimum in order, on the one hand, to maintain a required humidity in the fuel cells 60 and, on the other hand, to avoid formation of water droplets in the cathode path 16, which would clog it. Therefore, it would also be desirable to be able to determine the humidity at the cathode outlet 18 more accurately in order to be able to optimize the operation of the fuel cell device in question.FIG. 4 shows a flow diagram of a method for determining the humidity, which can be used in a fuel cell device of the type described here and comprises the following steps in this example:Step S 1: Detection of an electric current intensity currently supplied by the fuel cell stack,Step S 2: detecting a pressure and a temperature at the anode inlet and the anode outlet and the cathode inlet and the cathode outlet, Step S 3: performing electrochemical impedance spectroscopy on the fuel cell stack to detect real parts and imaginary parts of three complex impedance values for three excitation frequencies applied successively to the fuel stack,Step S 4: Evaluation of the detected quantities using a regression model determined in advance to determine an estimated value for the humidity at the anode inlet and / or an estimated value for the humidity at the cathode outlet.FIG. 5 illustrates a regression model M usable in the invention, which delivers an estimated value Ha for the humidity at the anode inlet and / or an estimated value Hk for the humidity at the cathode outlet from a number N of input data d1, d2,..., dN. The data d 1, d 2,..., dN can correspond here, for example, to the values of the variables detected in steps S 1 to S 3 of FIG. 4 (current intensity, pressures, temperatures, real parts and imaginary parts of a plurality of complex impedance values). In an advantageous embodiment of the regression model M, this is implemented as a trained neural network.Such a method for determining the humidity at the anode inlet and / or the humidity at the cathode outlet, e.g. as described above with reference to FIGS. 4 and 5, is also provided in the fuel cell system 1 illustrated in FIG. 1 and is carried out here by means of the control unit ST, which for this purpose has, functionally considered, a detection device 30, impedance spectroscopy device 40 and an evaluation device 50.By means of the detection device 30, in combination with sensory devices (not shown in FIG. 1 ) provided in the region of the fuel cell device 10, the operating parameters of the fuel cell stack 11 provided according to the determination method and further variables (e.g. pressures and temperatures at the inlets and outlets 13, 14, 17, 18) are detected. This detection of operating parameters is symbolized in FIG. 1 by an arrow 30 a.By means of the impedance spectroscopy device 40, an electrochemical impedance spectroscopy (EIS) is carried out on the fuel cell stack 11. For this purpose, an excitation signal (in the form of an alternating voltage or an alternating current) is generated and applied between anode connection and cathode connection of the fuel cell device 10. It is essential within the scope of the invention that this excitation is carried out for a limited number of "n" different excitation frequencies, wherein "n" is in the range from 2 to 10, preferably from 3 to 6. In this excitation, for example, a frequency generator can be used which successively generates "n" excitation signals with mutually different frequencies, which are accordingly applied successively to the fuel cell stack. At the same time, the impedance spectroscopy device 40 detects a measurement signal ("response signal") resulting from the excitation, which measurement signal is tapped off at the anode connection and cathode connection of the fuel cell device 10 as a measurement voltage signal in the example. A resulting phase shift between excitation and response signal is examined, for example, with the aid of a Fourier analysis, and the real part and imaginary part of the cell stack intrinsic impedance are calculated for each of the n excitation frequencies. The basic operation of electrochemical impedance spectroscopy is as such well known in the art. In the specific embodiment within the scope of the invention, therefore, it is also possible, for example, to advantageously resort to methods and details established in this field. The excitation of the fuel cell stack 11 and the detection of the "response" measurement signal produced thereby are symbolized in FIG. 1 by a double arrow 40 a.The evaluation device 50 serves for evaluating the quantities previously detected by the detection device 30 and the impedance spectroscopy device 40. In this case, a regression model, which is determined in advance for the fuel cell device 10 e.g. empirically and implemented in the evaluation device 50, is used in order to calculate an estimated value for the humidity at the anode inlet 13 and / or an estimated value for the humidity at the cathode outlet 18.List of reference characters1 Fuel cell system 2 Fuel tank 3 Heater 4 Inlet valve 5 Outlet valve 6 Compressor unit 7 Humidifier 10 Fuel cell device 11 Fuel cell stack 12 Anode-side fuel path (anode path) 13 Inlet (anode inlet) 14 Outlet (anode outlet) 16 Cathode-side oxidant path (cathode path) 17 Inlet (cathode inlet) 18 Outlet (cathode outlet) ST Controller 30 Detector 40 Impedance spectroscopy device 50 Evaluation device d 1 to dN Data (from detections) M Regression model Ha Humidity estimate (at anode inlet) Hk Humidity estimate (at cathode outlet) 60 Fuel cell 61 Anode-side bipolar half plate 62 Channel structure (for fuel) 63 Gas diffusion sheet 64 Membrane electrode unit 65 Electrolyte membrane 66 Anode (electrode layer) 67 cathode (electrode layer) 68 gas diffusion layer 69 cathode-side bipolar half plate 70 channel structure (for oxidizing agent) 71 catalyst

Claims

Method for determining the moisture at an inlet (13) of an anode-side fuel path (12) and / or at an outlet (18) of a cathode-side oxidant path (16) of a fuel cell device (10), which comprises a fuel cell stack (11) made of fuel cells (60) with respective anodes (66) and cathodes (67) and also the anode-side fuel path (12) and the cathode-side oxidant path (16) with respective inlets (13, 17) and outlets (14, 18), wherein the method comprises the following steps: - acquisition (S1) of at least one operating parameter relating to the current operating state of the fuel cell stack (11), detecting (S2) a flow rate and / or a pressure at at least one location in the course of the anode-side fuel path (12) and / or a flow rate and / or a pressure at least one location in the course of the cathode-side oxidant path (16), carrying out (S3) an electrochemical impedance spectroscopy on the fuel cell stack (11) in order to detect a number of n complex impedance values for a number of n different excitation frequencies, wherein n is in the range from 2 to 10, evaluating (S4) the detected variables (d1, d2,... dN) using a regression model (M) determined beforehand for the fuel cell device (10), to determine an estimate (Ha) for the humidity at the inlet (13) of the anode-side fuel path (12) and / or an estimate (Hk) for the humidity at the outlet (18) of the cathode-side oxidant path (16).The method of claim 1, wherein the fuel cell device (10) is operable or operated with hydrogen as fuel and air as oxidant.Method according to one of the preceding claims, wherein an electrical current intensity and / or an electrical voltage of the fuel cell stack (11) is detected as operating parameter relating to the instantaneous operating state of the fuel cell stack (11).Method according to any of the preceding claims, wherein the flow rate and / or the pressure are detected at the inlet (13, 17) and / or at the outlet (14, 18) of the relevant path (12, 16).Method according to one of the preceding claims, wherein at least one lambda value relating to the amounts of fuel and oxidant is detected.Method according to one of the preceding claims, wherein a temperature at at least one point, in particular the inlet (13) and / or the outlet (14), is detected in the course of the anode-side fuel path (12) and / or a temperature at at least one point, in particular the inlet (17) and / or the outlet (18), is detected in the course of the cathode-side oxidant path (16).Method according to one of the preceding claims, wherein an excitation signal of the electrochemical impedance spectroscopy is applied between an anode connection and a cathode connection of the fuel cell stack (11).Method according to one of the preceding claims, wherein the evaluation of the variables for determining the estimated value or values (Ha, Hk) is carried out using a regression model (M) implemented as a trained neural network.Device (30, 40, 50) for determining the humidity at an inlet (13) of an anode-side fuel path (12) and / or at an outlet (18) of a cathode-side oxidant path (16) of a fuel cell device (10), which comprises a fuel cell stack (11) made of fuel cells (60) with respective anodes (66) and cathodes (67) and also the anode-side fuel path (12) and the cathode-side oxidant path (16) with respective inlets (13, 17) and outlets (14, 18), wherein the device (30, 40, 40, 18), 50) comprises: - a detection device (30) for detecting at least one operating parameter relating to the instantaneous operating state of the fuel cell stack (11) and for detecting a flow rate and / or a pressure at at least one point in the course of the anode-side fuel path (12) and / or a flow rate and / or a pressure at at least one point in the course of the cathode-side oxidant path (16), - an impedance spectroscopy device (40) for carrying out an electrochemical impedance spectroscopy on the fuel cell stack (11) and for detecting a number of n complex impedance values for a number of n different excitation frequencies, wherein n is in the range from 2 to 10, - an evaluation device (50) for evaluating the detected variables (d1, d1, d2,... dN) using a regression model (M) determined beforehand for the fuel cell device ( 10) and implemented in the evaluation device ( 50) for determining an estimated value (Ha) for the moisture at the inlet ( 13) of the anode-side fuel path ( 12) and / or an estimated value (Hk) for the moisture at the outlet ( 18) of the cathode-side oxidant path ( 16).Use of a method according to one of Claims 1 to 8 or of a device (30, 40, 50) according to Claim 9 for determining the moisture at an inlet (13) of an anode-side fuel path (12) and / or at an outlet (18) of a cathode-side oxidant path (16) of a fuel cell device (10) arranged on board a vehicle, in particular during operation of this fuel cell device (10).

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