Determination of the moisture 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 moisture at an anode inlet (13) and / or a cathode outlet (18) of a fuel cell device (10) that comprises a fuel cell stack (11) of fuel cells (60) and an anode path (12) and a cathode path (16) having 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 in the course of the anode path (12) and / or a flow rate and / or a pressure in the course of 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 predetermined regression model (M) in order to determine an estimated value (Ha) for the moisture at the anode inlet (13) and / or an estimated value (Hk) for the moisture at the cathode outlet (18). The invention also relates to a corresponding device and use thereof.

Description

[0001] Description

[0002] Determination of the humidity at an inlet of an anode-side fuel path and / or at an outlet of a cathode-side oxidizer path of a fuel cell device

[0003] The present invention relates generally to the operation of a fuel cell device and, in particular, to a method and a device for determining the moisture content at an inlet of an anode-side fuel path and / or at an outlet of a cathode-side oxidizer path. The invention further relates to the use of such methods and devices.

[0004] A fuel cell system of the type of interest here comprises a fuel cell stack consisting of fuel cells, each with its own anode and cathode. The fuel cells are typically plate-shaped, stacked in a single direction, and connected in series. This allows the chemical reaction energy of a supplied fuel (e.g., hydrogen) and a supplied anode to be harnessed.

[0005] The oxidizing agent (e.g., oxygen or air) is converted into electrical energy through an electrochemical reaction. During operation of the

[0006] In fuel cell systems, these reactants of the electrochemical reaction, e.g. hydrogen on one side and air on the other, must be introduced on opposite sides of the fuel cells when viewed in the stacking direction.

[0007] For the supply of 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 outlet on the outside of the fuel cell device.

[0008] Fresh fuel is supplied to the inlet of the anode-side fuel path, which then flows through the path and is at least partially consumed by the fuel cells. At the outlet of the anode-side fuel path, the fuel, thus altered in its composition, can be continuously collected and, after reprocessing (e.g., dehumidification), mixed with the fresh fuel at the inlet. However, particularly when using hydrogen as fuel, it may also be possible, for example, to briefly open the anode-side fuel path at its outlet only from time to time, in order to collect the fuel during each such so-called "recharge."

[0009] "Purging" process, to flush the anode-side fuel path with fresh fuel and to introduce fresh fuel from the inlet.

[0010] Fresh oxidant is supplied to the inlet of the cathode-side oxidant path, which then flows through the path and is at least partially consumed by the fuel cells. At the outlet of the cathode-side oxidant path, the oxidant stream, now with a modified composition, can be continuously released into the atmosphere, particularly when air is used as the oxidant.

[0011] With a view to achieving the highest possible efficiency in the operation of a fuel cell system, so-called humidity management is of great importance. This includes measures that either serve to determine the humidity level and / or the spatial distribution of moisture within the fuel cell system, or that, based on the results of such a humidity determination, aim to establish an optimal humidity level in the fuel cell system or individual areas thereof.

[0012] The following section will first address the desirable determination of humidity (e.g., relative humidity) at an inlet of the anode-side fuel path. Excessive humidity in this area would clog the fuel path (e.g., channel structures on anode-side bipolar half-plates) further along its length due to the formation of water droplets. This would result in a lack of fuel (e.g., hydrogen) on the anode side of the fuel cell, which would then be a reaction partner for the oxidant (e.g., oxygen) flowing in from the cathode side. Consequently, the oxidant would not react with hydrogen and could also adversely affect components such as carbon, which serves as a catalyst support in the fuel cell, leading to premature aging of the fuel cell.To avoid this, it must be ensured that the relative humidity at the inlet of the anode-side fuel path, hereinafter also referred to as the "anode inlet," is significantly below 100%, including a safety factor. However, in most currently used fuel cell systems, there is no way to regulate the relative humidity in the anode-side fuel path, hereinafter also referred to as the "anode path." Instead, a certain moisture content is established during operation, and, for example, dry hydrogen with a relative humidity of 0% is added at the anode inlet. The anode-side humidity then arises primarily through so-called "water drag," i.e., diffusion of water across the fuel cell membrane into the anode path. It can be assumed that a relative humidity of, for example, approximately [value missing] is present on the anodes.The relative humidity is typically regulated to 60-70%, although this value can vary, for example, when dry fuel is reintroduced into the anode path after a purging process. Depending on the operating load profile, peak values ​​of 90% and higher relative humidity can also be reached.

[0013] Against this background, it would be desirable to estimate the humidity at the anode inlet more accurately. Such a determination of the humidity at the inlet of the anode-side fuel path can then be advantageously used, for example, to optimize the operation of the fuel cells. Improved humidity measurement also allows for a reduction in cell degradation, leading to an increased service life of the fuel cell system. A further advantage of humidity measurement is that the additional information about the actual moisture content enables demand-based purging of the anode-side fuel path, resulting in a reduction in fuel consumption.

[0014] Furthermore, the following section will discuss a method for determining the humidity (e.g., relative humidity) at the outlet of the cathode-side oxidizer path, hereinafter also referred to as the "cathode outlet," which is desirable for humidity management. Prior art approaches have already been used to estimate the relative humidity in the cathode-side fuel path, hereinafter also referred to as the "cathode path," using a physical model. However, this method only allows for a relatively rough determination of the actual humidity in the area of ​​the cathode outlet, as the accuracy of such a humidity model is limited. Ideally, a fuel cell system is operated at 100% relative humidity at the cathode outlet, since the cell stack delivers the highest voltage at this operating point. In this case, the oxidizer flow at the cathode outlet is completely saturated with water; however, condensation forms along the cathode path (e.g.,Channel structures on the cathode-side bipolar half-plates do not yet contain water droplets that would clog the cathode path. Excessive humidity would promote precisely this water droplet formation, negatively impacting the voltage delivered by the cell stack. Conversely, insufficient humidity in the cathode path would dry out the fuel cell membrane, increasing membrane resistance and consequently reducing the power output of the fuel cell stack.

[0015] Against this background, 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 also be advantageous, for example, for optimizing the operation or control of the fuel cell. Improved humidity measurement can, among other things, improve efficiency, thereby reducing fuel consumption during operation. Furthermore, more precise humidity control at the cathode outlet reduces the degrading effect caused by cell drying, leading to an increased service life of the fuel cell system.

[0016] It is an object of the present invention to demonstrate a novel way in which the moisture content at the inlet of the anode-side fuel path (anode path) and / or at the outlet of the cathode-side oxidizing agent path (cathode path) can be determined in a fuel cell device.

[0017] According to a first aspect of the invention, this problem is solved by a method according to claim 1. The dependent claims relate to advantageous embodiments. The method according to the invention comprises the following steps:

[0018] Recording at least one operating parameter relating to the current operating state of the fuel cell stack,

[0019] Determining a flow rate and / or pressure at at least one point along the anode-side fuel path and / or a flow rate and / or pressure at at least one point along the cathode-side oxidant path, performing electrochemical impedance spectroscopy on the fuel cell stack to determine a number of n complex impedance values ​​for a number of n different excitation frequencies, wherein n is in the range of 2 to 10, preferably in the range of 3 to 6,

[0020] Evaluation of the recorded quantities using a regression model determined in advance for the fuel cell device, in order to determine an estimate for the humidity at the inlet of the anode-side fuel path and / or an estimate for the humidity at the outlet of the cathode-side oxidant path.

[0021] The method according to the invention advantageously allows for a very precise determination (estimation) of the moisture in the aforementioned areas of the fuel cell device, which in turn can be used to achieve the aforementioned advantages in connection with both the service life and the efficiency of the fuel cell device in its operation.

[0022] 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, for example, in the automotive sector for the electrical supply of an on-board electrical system, from which, in turn, an electric drive system can be supplied, for example.

[0023] Essential to the invention is, firstly, the acquisition of one or more operating parameters relating to the current operating state of the fuel cell stack.

[0024] In particular, this can involve operating parameters relating to instantaneous electrical power in the broadest sense, such as, for example, the current currently supplied by the fuel cell stack. Alternatively or additionally, the voltage currently supplied by the fuel cell stack can be recorded in this step.

[0025] It is also possible to record an instantaneous electrical power in the narrower sense (product of current and voltage) as an operating parameter in this step. In one embodiment, one or more of the aforementioned quantities (electric current, electrical voltage, electrical power, etc.) are recorded as operating parameters relating to the current operating state of the fuel cell stack.

[0026] Furthermore, it is essential to the invention that at least one additional parameter, a flow rate and / or pressure, is measured at at least one point along one of the two paths, i.e., the anode-side fuel path (anode path) or the cathode-side oxidizer path (cathode path). If the method is used to determine the humidity at the anode inlet, this at least one additional parameter should be measured at least along the anode path, and if the method is used to determine the humidity at the cathode outlet, this parameter should be measured at least along the cathode path.

[0027] In an embodiment advantageous with regard to the accuracy of the moisture determination, a flow rate and / or a pressure is detected at at least one point along the anode path as well as at least one point along the cathode path, regardless of whether the method is intended to determine the moisture at the anode inlet, at the cathode outlet, or at both of these points.

[0028] For each of the recordings "at (at least) one point in the course" of a relevant path, this point may advantageously be provided in particular in the area of ​​the inlet and / or in the area of ​​the outlet of the relevant path.

[0029] In one embodiment, the flow rate and / or pressure at the inlet and / or outlet of a given path is therefore detected. For example, a flow rate sensor positioned at the appropriate location can be used to detect such a flow rate. Alternatively, this detection can also be based on a model (e.g., using the results of a pressure measurement along the path).

[0030] In one embodiment, the pressure at the anode inlet, anode outlet, cathode inlet, and cathode outlet is measured; that is, at least these four pressure values ​​are recorded. In another embodiment, at least one lambda value relating to the quantities of fuel and oxidant supplied to the electrochemical reaction is also measured. In particular, such a lambda value can, for example, reflect the ratio between the input mass flow rate and the mass flow rate required for the current operating point.

[0031] For example, this could be an anode-side lambda value, which is defined as the ratio between the actual

[0032] Fuel flow rate at the inlet of the anode-side fuel path and the theoretically required fuel flow rate at this point with regard to the electric current of the fuel cell stack required in the current operating situation according to the relevant electrochemical reaction.

[0033] Similarly, a cathode-side lambda value can be determined, which is defined as the ratio between the actual oxidant flow rate at the inlet of the cathode-side oxidant path and the theoretically required oxidant flow rate at this point with regard to an electric current of the fuel cell stack required in the current operating situation according to the electrochemical reaction in question.

[0034] In one embodiment, the method further comprises measuring the temperature at at least one point along the anode path and / or at least one point along the cathode path (in order to measure the temperature of the flow in question). For the accuracy of the humidity determination, it is advantageous if the temperature is measured at at least one point along both the anode path and the cathode path. In one embodiment, the temperature is measured at the inlet and / or outlet of the respective path.

[0035] In one embodiment, the temperature at the anode inlet, anode outlet, cathode inlet, and cathode outlet is measured; that is, at least these four temperature values ​​are recorded along the two paths. In a more specific embodiment, the method includes measuring the current currently supplied by the fuel cell stack (and optionally, for example, the voltage currently supplied by the fuel cell stack), and measuring the pressure and temperature at the inlet and outlet of the anode path and at the inlet and outlet of the cathode path.

[0036] Regarding the further essential aspect of the invention of performing electrochemical impedance spectroscopy (EIS) on the fuel cell stack, this method is known from the prior art for characterizing a variety of electrochemical systems, but has so far only been considered useful for obtaining information about the fuel cells themselves with regard to fuel cell stacks.

[0037] However, it has been found that, within the scope of the invention, the results of impedance spectroscopy in combination with the results of the above-explained measurements concerning the current operating state of the fuel cell stack (such as electric current, electric voltage, etc.) as well as concerning the aforementioned "hydrodynamic" operating parameters of the flows in the area 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 estimate for the humidity at the inlet of the anode-side fuel path (anode inlet) and / or an estimate for the humidity at the outlet of the cathode-side oxidant path (cathode outlet).

[0038] The acquisition of several "complex" impedance values ​​provided for in the invention is intended to mean that, in a mathematical sense, these are complex-valued or thus two-dimensional quantities, and that both their components (real part and imaginary part, or equivalently, e.g., magnitude and phase angle) must be taken into account in the evaluation.

[0039] In one embodiment, it is provided that a regression model valid for the fuel cell device has been determined empirically in advance and is made available for use in real time during the execution of the method according to the invention.Such an empirical determination of the regression model can comprise the following steps: a) Providing a fuel cell device identical in construction to the fuel cell device to be operated later and equipping it with a measuring device comprising an impedance spectroscopy device, humidity 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 humidity at the anode inlet and / or cathode outlet, as well as for detecting quantities identical to those measured at the fuel cell device to be operated later (within the framework of the determination procedure); b) 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 oxidant and the moisture content of the supplied fuel and oxidant, in order to record a multidimensional data field based on the measured values ​​thus acquired for a multitude of different operating situations, including complex impedance values ​​in their respective assignment to the relevant excitation frequencies; c) Performing a regression analysis on the multidimensional data field to obtain a regression model.

[0040] The resulting regression model describes, in a sense, the relationship between the moisture levels to be estimated on the one hand and the data to be provided to the model as "input data" according to the determination procedure on the other.

[0041] Advantageously, when determining the regression model (as well as in its subsequent use), the two components of the complex impedance values, e.g., a real part and an imaginary part, are included in the data field as two separate (real-valued) data points, each assigned to the respective excitation frequencies. In one embodiment of the method, the evaluation of the acquired quantities to determine the humidity estimate(s) (at the anode inlet and / or cathode outlet) is carried out using a parameterized regression model comprising, for example, a system of equations containing the relevant quantities and regression parameters, in order to calculate the humidity estimate(s).

[0042] In another embodiment of the method, the recorded parameters are evaluated to determine the estimated moisture value(s) using a regression model implemented as a trained neural network. In this embodiment, an empirical determination of the regression model, as described above, can be provided, wherein the regression analysis to be performed is carried out by 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.

[0043] 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 lookup tables or the like, by means of which the aforementioned evaluation is carried out.

[0044] The program-controlled computer system can be particularly advantageously a computer system that also controls the operation of the fuel cell system (e.g., a so-called control unit for a fuel cell system that can be operated on board a vehicle). The control of this operation can, for example, include the actuation or adjustment of controllable valves, which, depending on the specific design of a fuel cell system equipped with the fuel cell unit, can be arranged, in particular, in lines for supplying fuel and oxidizer to the respective inlets or in lines for removing fuel and oxidizer from the respective outlets.

[0045] Another special feature of electrochemical impedance spectroscopy, as used in the method according to the invention, is that the number of complex impedance values ​​detected is in the range of 2 to 10, preferably in the range of 3 to 6. This offers several advantages, first and foremost the possibility of carrying out the method according to the invention during the ongoing operation of the fuel cell device, because, in particular, when limited to a maximum of 6 impedance values ​​to be detected (and correspondingly a maximum of 6 excitation frequencies), the impedance spectroscopy can be performed very quickly. In a particularly advantageous embodiment, for example, the duration of the electrochemical impedance spectroscopy is in the range of 0.1 to 2 s.This is particularly interesting when the invention is used in the automotive sector (for the electrical supply of a vehicle's electrical system), where fuel cell operation is typically controlled relatively dynamically and uninterrupted operational readiness is often required. A further advantage of limiting the number of complex impedance values ​​to a maximum of 10, and especially a maximum of 6, is that the aforementioned evaluation of the measured parameters for determining humidity is simplified, meaning it can be carried out efficiently and quickly, for example, in a program-controlled computer system used for this purpose (e.g., on board a vehicle).

[0046] In this context, it has been advantageously shown that this relatively small number of impedance values ​​(and corresponding excitation frequencies) is sufficient in practice to enable a sufficiently accurate determination of humidity. This is particularly true if the excitation frequencies are chosen appropriately (adapted to the specific application). In most cases, for example, it is advantageous if the excitation frequencies lie within 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.

[0047] In a further development of the empirical determination of the regression model described above, it is planned to operate the impedance spectroscopy device for excitation with a number of excitation frequencies (and corresponding recording of a corresponding number of complex impedance values) that significantly exceeds the number required for the regression model (e.g., by a factor of 2 to 10). Subsequently, a selection can be made from these excitation frequencies, for example, by means of a statistical evaluation of the (correspondingly enlarged) data field, in order to identify excitation frequencies that are particularly relevant with regard to the accuracy of the moisture determination method and to include them in the desired number (in the range of 2 to 10, especially 3 to 6) for the generation of the regression model to be used later.In other words, particularly advantageous excitation frequencies (at which the impedance values ​​recorded at the fuel cell stack are particularly "meaningful") can also be empirically determined within the scope of the invention.

[0048] In one embodiment of the method, an excitation signal for electrochemical impedance spectroscopy is applied between an anode terminal and a cathode terminal of the fuel cell stack.

[0049] Regarding the measurement signal ("response signal") resulting from the excitation, this can, for example, be tapped across the entire stack, i.e., between the anode and cathode terminals of the fuel cell stack, according to the invention. However, it is also possible to tap the response signal between an anode and cathode terminal of a single fuel cell. Furthermore, it is conceivable to tap the response signal at a group ("cell cluster") of several fuel cells directly adjacent to one another in the fuel cell stack. To implement the latter two embodiments in particular, it is conceivable, for example, to provide a cell monitoring device in the fuel cell assembly, by means of which the operating states of individual fuel cells and / or at least one fuel cell cluster are monitored (e.g.,by tapping and evaluating and / or communicating the cell voltage(s)), and equipping this cell monitoring device with means for capturing the response signal, e.g. by filtering out the response signal (at the corresponding excitation frequencies), and for recording the complex impedance values ​​resulting for the excitations (after evaluation of the respective response signals).

[0050] According to a further aspect of the invention, the aforementioned problem is solved by a device for determining the moisture 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 of fuel cells with respective anodes and cathodes, as well as the anode-side fuel path and the cathode-side oxidant path with respective inlets and outlets.and wherein the device comprises: a detection device for detecting at least one operating parameter relating to the current 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 at least one point in the course of the cathode-side oxidant path, an impedance spectroscopy device for performing 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 of 2 to 10,an evaluation device for evaluating the recorded quantities using a regression model previously determined for the fuel cell device and implemented in the evaluation device 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 oxidizer path.

[0051] The embodiments and special configurations described here for the method according to the invention can, individually or in any combination, also be provided in an analogous manner as embodiments or special configurations of the device according to the invention, and vice versa.

[0052] For example, in the device according to the invention, the detection device can be configured to detect quantities or various combinations of such quantities, as already described above for the method according to the invention. Accordingly, the detection device can, for example, include 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 the cathode path), and further, for example, include sensors for measuring a flow rate and / or a pressure at at least one point along the anode path and / or at least one point along the cathode path. Furthermore, the device can, for example,must be designed to detect at least one lambda value, for example by calculating the respective lambda value (e.g., anode-side lambda value and / or cathode-side lambda value).

[0053] Such a lambda value can be calculated from values ​​obtained, for example, from flow sensors measuring the relevant flow rate (of fuel and / or oxidizer) and, for example, from sensors measuring electrical quantities relating to the current operating point of the fuel cell stack. Instead of using flow sensors, calculations or models of the flow rate can also be used to determine a specific flow rate. The latter can be carried out, in particular, based on measurements of at least one pressure (and possibly also one temperature) at the inlet and / or outlet of the relevant path (anode path, cathode path).

[0054] Advantageously, such sensors (for supporting or implementing the detection device) are generally already present in fuel cell systems of the type of interest here, for example as peripheral components of a control unit (e.g., a control unit in a vehicle) by means of which the operation of the fuel cell system is controlled and / or monitored. In this case, such sensors can therefore also be used to implement the device according to the invention.

[0055] The evaluation unit for analyzing the recorded parameters can be implemented, for example, as a program-controlled computer (e.g., a microcontroller) in the form of program code including lookup tables or similar. This computer could, for example, also be used to control the operation of the fuel cell system. This evaluation is performed using the regression model previously determined for the fuel cell system to calculate the estimated moisture value(s).

[0056] In one embodiment, the impedance spectroscopy device is at least partially implemented by a computer device (e.g., a microcontroller) of the type mentioned above. In particular, a frequency generator for generating an excitation signal used for impedance spectroscopy can be implemented in this way. This signal is applied (e.g., amplified via a driver device) between the anode and cathode terminals of the fuel cell stack. Modern microcontroller components often have powerful peripheral modules that can be used to implement the frequency generator (without generating significant processor or system bus load). Furthermore, the computer device can, for example,A data acquisition and evaluation unit for the measurement signal to be acquired during impedance spectroscopy must be implemented in order to determine the intended number of complex impedance values ​​(for the corresponding number of different excitation frequencies). This can also be accomplished, for example, by the computer system (e.g., a microcontroller) (possibly using peripheral modules such as an A / D converter module and other signal processing modules of the microcontroller peripherals).

[0057] According to a further aspect of the invention, the use of a moisture determination method of the type described herein and / or a moisture determination device of the type described herein for determining the moisture at an inlet of an anode-side fuel path (anode inlet) and / or at an outlet of a cathode-side oxidizing agent path (cathode outlet) of a fuel cell device arranged on board a vehicle is proposed.

[0058] In one embodiment of this application, it is provided that, if the fuel cell device is in operation at a time scheduled for carrying out the determination method, the moisture content is determined during this operation of the fuel cell device. The moisture content is preferably determined within a time period in the range of 0.1 to 2 seconds.

[0059] In another embodiment of the application, if the fuel cell device is in operation at a time designated for carrying out the determination procedure, this "normal" operation is first briefly interrupted, and the fuel cell device is instead switched to a special "diagnostic mode" in which the moisture determination is then carried out, e.g., within a period of 0.1 to 5 seconds. If, during this diagnostic mode, certain operating parameters or other quantities are set to fixed predetermined values ​​(e.g., by opening a switch that sets the current supplied by the fuel cell stack to zero), then the acquisition and evaluation of the relevant quantity(ies) is advantageously unnecessary.

[0060] In one embodiment of the application, it is provided that, during vehicle operation, time points are scheduled, particularly periodically, for carrying out the determination procedure. Furthermore, it can be advantageously provided, for example, that the result of the determination procedure is used within the framework of controlling the operation of the fuel cell system, for example, to optimize the operation of the fuel cells with regard to efficiency, service life, etc., and / or to optimize, for example, the timing of purging operations for the anode path. A result of the moisture determination according to the invention can, for example, be used as an additional control variable when setting the times of the purging operations.

[0061] The invention is further described below with reference to exemplary embodiments and the accompanying drawings. These schematically represent:

[0062] Fig. 1 shows a fuel cell system with a fuel cell device and a device for determining moisture according to an exemplary embodiment.

[0063] Fig. 2 shows a fuel cell according to an exemplary embodiment, for use in a fuel cell stack of a fuel cell device,

[0064] Fig. 3 is a representation illustrating the flows of fuel and oxidant in respective paths of a fuel cell device,

[0065] Fig. 4 shows a flowchart of a method for determining moisture content, and

[0066] Fig. 5 shows a representation illustrating a regression model for determining moisture according to an exemplary embodiment.

[0067] Fig. 1 shows a fuel cell system 1 comprising a fuel cell assembly 10 with a fuel cell stack 11, which is formed from a plurality of plate-shaped fuel cells 60 (Fig. 2). The fuel cells 60 are arranged orthogonally to their plate planes (transverse directions x, y) in a stacking direction z and are connected to each other in an electrical series connection via their respective anodes and cathodes.

[0068] In the operation of the fuel cell device 10, the chemical reaction energy of a supplied fuel, in the example hydrogen, and a supplied oxidizing agent, in the example air, can be converted into electrical energy by means of an electrochemical reaction using the fuel cell stack 11.

[0069] During operation of the fuel cell device 10, the hydrogen and air must be supplied to opposite flat sides of the fuel cells 60 when viewed in the stacking direction z. For this supply of the fuel cells 60 with the two reactants hydrogen and air, the fuel cell device 10 further comprises an anode-side fuel path 12 with an inlet 13 and an outlet 14, as well as a cathode-side oxidant path 16 with an inlet 17 and an outlet 18.

[0070] During operation of the fuel cell assembly 10, hydrogen flows through the inlet 13, through the fuel cell stack 11, and exits the fuel cell assembly 10 through the outlet 14, whereas air flows into the fuel cell stack 11 through the inlet 17 and, after passing through the fuel cell stack 11, exits the fuel cell assembly 10 through the outlet 18. The paths 12 and 16, shown as dashed lines in Fig. 1 and subsequently referred to as the anode path 12 and cathode path 16, merely symbolize the respective connections between the inlets and outlets, and not the actual course of these paths 12 and 16 (which branch out and pass through the individual fuel cells 60).

[0071] Fig. 2 shows in more detail the structure of a single fuel cell 60 of the fuel cell stack 11 in the fuel cell assembly 10 of Fig. 1, whereby the structure shown in Fig. 2 is only to be understood as an example. In this example, it is a proton exchange membrane (PEM) type fuel cell. The fuel cell 60 comprises, stacked in the stacking direction z: an anode-side bipolar half-plate 61 with a channel structure 62 for guiding the fuel (hydrogen), an anode-side gas diffusion layer 63 (e.g., carbon fleece), a membrane electrode unit 64 comprising an (electrically non-conductive, but proton-conducting) electrolyte membrane 65, and, in the stacking direction z, electrode layers 66, 67 arranged on both sides thereof and coated with a catalyst 71 (e.g., platinum or palladium), which form an anode and a cathode for the electrochemical reaction of the fuel with the oxidizing agent (air or hydrogen).forming (containing oxygen), a cathode-side gas diffusion layer 68 (e.g. carbon fleece), and a cathode-side bipolar half-plate 69 with a channel structure 70 for guiding the oxidizing agent.

[0072] The channel structures 62 and 70 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. The half-plates 61 and 69 also function as the anode and cathode of the fuel cell 60, with anodes and cathodes of adjacent fuel cells 60 connected in pairs within the fuel cell stack 11 (electrical series 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 simultaneously form an anode and a cathode of the fuel cell stack 11, which are electrically connected to (not shown) electrical anodes and cathodes, respectively.

[0073] cathode terminals of the fuel cell device 10 are connected, at which the electrical power of the fuel cell device 10 is provided.

[0074] The fuel cell system 1 shown 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 a controllable inlet valve 4 at the inlet 13, hereinafter also referred to as the anode inlet 13. Optionally, a heating device 3, for example, shown in the figure, can also be provided in this hydrogen supply path. Furthermore, a controllable outlet valve 5 is provided in the fuel cell system 1 at outlet 14, hereinafter also referred to as anode outlet 14. This valve is briefly opened from time to time during operation to purge the anode path 12 with fresh hydrogen from tank 2 during such a so-called "purging" process, or at least to refill it partially from the anode inlet 13. Advantageously, only partial purging takes place in a "normal" operating mode, whereas, for example,A complete purging process can also occur during the "start-up" of the fuel cell system 1.

[0075] Furthermore, the fuel cell system 1 comprises a controllable compressor unit 6, by means of which, during operation, the oxidizing agent (here: air) can be metered into the cathode path 16 via a humidification device 7 at the inlet 17, hereinafter also referred to as the cathode inlet 17. At the outlet 18, hereinafter also referred to as the cathode outlet 18, the air is released into the atmosphere.

[0076] All controllable components (heating unit 3, valves 4, 5 etc.) provided in the fuel cell system 1 are controlled by means of a control unit ST for the operation of the fuel cell unit 10.

[0077] Fig. 3 is a schematic representation of the entire fuel cell assembly 60, or fuel stack 11, as well as the anode path 12 and the cathode path 16. As shown in Fig. 3, fresh hydrogen (H₂) is supplied at the anode inlet 13. However, as it flows through the anode path 12, some of this hydrogen is consumed by the electrochemical reaction taking place in the fuel cells 60 (protons migrate through the membrane electrode assemblies 64 in the fuel cells 60), so that the hydrogen content gradually decreases towards the anode outlet 14. Furthermore, the nitrogen (N₂) content increases towards the anode outlet 14. As symbolized in Fig. 3, nitrogen diffuses from the air-filled cathode path 16 through the fuel cells 60 and into the anode path 12.Finally, as the hydrogen flows towards the anode outlet 14, the humidity (H₂O) also increases because water from the (humidified) fuel cells 60 diffuses into the anode path 12 ("water drag"). To compensate for the aforementioned hydrogen consumption and to prevent a larger accumulation of nitrogen (N₂), which, in combination with the resulting reduced hydrogen concentration, would be harmful to the catalyst located in the fuel cells 60 (see 71 in Fig. 2), a so-called "purging" process is carried out at predetermined intervals. During this process, the fuel, whose composition has changed at the anode outlet 14, is discharged, and fresh fuel is introduced from the anode inlet 13. Excessive humidity of the hydrogen supplied at the anode inlet 13 would clog the anode path 12 further along its length due to the formation of water droplets.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 context, it is desirable to be able to determine the humidity (e.g., relative humidity) at the anode inlet 13. Instead of metering dry hydrogen with a relative humidity of 0% at the anode inlet 13, as is currently the practice, the fuel could then be optimally conditioned at this point (e.g., by means of adjustable fuel humidification) to optimize the operation of the fuel cell system. Furthermore, using such information, the timing and / or duration of purging, or the purging intervals, can be more precisely determined to achieve reduced fuel consumption.

[0078] Figure 3 further illustrates the supply of fresh air at the cathode inlet 17, the composition of which (approx. 20% O2, approx. 80% N2) is altered as it flows through the cathode path 16 by the electrochemical reaction taking place in the fuel cells 60. Since oxygen (O2) is consumed and water (H2O) is formed in this reaction, the humidity (H2O) gradually increases towards the cathode outlet 18. A relative humidity of 100% at the cathode outlet 18 is generally optimal to maintain the required humidity in the fuel cells 60 and to prevent the 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 precisely in order to optimize the operation of the fuel cell device in question.

[0079] Fig. 4 shows a flowchart of a method for determining moisture content, which can be used in a fuel cell device of the type described here and in this example comprises the following steps:

[0080] Step S1: Recording the current intensity currently supplied by the fuel cell stack,

[0081] Step S2: Measurement of pressure and temperature at the anode inlet and outlet, and at the cathode inlet and outlet; Step S3: Performance of electrochemical impedance spectroscopy at the

[0082] Fuel cell stack to detect real and imaginary parts of three complex impedance values ​​for three successive excitation frequencies applied to the fuel stack,

[0083] Step S4: Evaluation of the recorded quantities using a pre-determined regression model to determine an estimate for the humidity at the anode inlet and / or an estimate for the humidity at the cathode outlet.

[0084] Fig. 5 illustrates a regression model M usable in the invention, which provides 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 d1, d2, ..., dN can, for example, correspond to the values ​​of the quantities recorded in steps S1 to S3 of Fig. 4 (current, pressures, temperatures, real parts and imaginary parts of several complex impedance values). In an advantageous embodiment of the regression model M, it is implemented as a trained neural network.

[0085] Such a method for determining the moisture at the anode inlet and / or the moisture 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 shown in Fig. 1 and is carried out by means of the control unit ST, which functionally comprises a detection device 30, an impedance spectroscopy device 40 and an evaluation device 50.

[0086] By means of the detection device 30, in combination with sensor devices provided in the area of ​​the fuel cell device 10 (not shown in Fig. 1), the operating parameters of the fuel cell stack 11 and other quantities (e.g.

[0087] Pressures and temperatures at the inlets and outlets 13, 14, 17, 18) are recorded. This recording of operating parameters is symbolized by an arrow 30a in Fig. 1.

[0088] Electrochemical impedance spectroscopy (EIS) is performed on the fuel cell stack 11 using the impedance spectroscopy device 40. For this purpose, an excitation signal (in the form of an alternating voltage or an alternating current) is generated and applied between the anode and cathode terminals of the fuel cell device 10. A key feature of the invention is that this excitation is performed for a limited number of "n" different excitation frequencies, where "n" is in the range of 2 to 10, preferably from 3 to 6. For this excitation, a frequency generator can be used, for example, which successively generates "n" excitation signals with different frequencies, which are then applied sequentially to the fuel cell stack.Simultaneously, the impedance spectroscopy device 40 acquires a measurement signal ('response signal') resulting from the excitation, which in this example is tapped as a measurement voltage signal at the anode and cathode terminals of the fuel cell device 10. A resulting phase shift between the excitation and response signals is investigated, for example, using Fourier analysis, and the real and imaginary parts of the cell stack's intrinsic impedance are calculated for each of the n excitation frequencies. The basic operating principle of electrochemical impedance spectroscopy is well known from the prior art. Therefore, in the specific embodiment within the scope of the invention, established methods and details from this field can be advantageously used. The excitation of the fuel cell stack 11 and the acquisition of the resulting 'response' measurement signal are symbolized in Fig. 1 by a double arrow 40a.

[0089] The evaluation unit 50 serves to evaluate the quantities previously recorded by the acquisition unit 30 and the impedance spectroscopy unit 40. A regression model, previously determined empirically for the fuel cell unit 10, for example, and implemented in the evaluation unit 50, is used 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.

[0090] Reference symbol list

[0091] 1 Fuel cell system

[0092] 2 fuel tank

[0093] 3 Heating system

[0094] 4 Inlet valve

[0095] 5 exhaust valve

[0096] 6 compressor units

[0097] 7 Humidification device

[0098] 10 Fuel cell equipment

[0099] 11 fuel cell stacks

[0100] 12 anode side fuel path (anode path)

[0101] 13 Inlet (anode inlet)

[0102] 14 Outlet (anode outlet)

[0103] 16. Cathode-side oxidizing agent pathway (cathode pathway)

[0104] 17 Entrance (cathode entrance)

[0105] 18 Outlet (cathode outlet)

[0106] ST control unit

[0107] 30 Recording device

[0108] 40 Impedance spectroscopy equipment

[0109] 50 Evaluation unit d1 to dN data (from recordings)

[0110] M Regression model

[0111] Ha is the estimated value for humidity (at the anode inlet).

[0112] Hk Estimated value for humidity (at the cathode outlet)

[0113] 60 Fuel cell

[0114] 61 anode side bipolar half plate

[0115] 62 Channel structure (for fuel)

[0116] 63 Gas diffusion layer

[0117] 64 Membrane Electrode Unit

[0118] 65 Electrolyte membrane

[0119] 66 Anode (electrode layer)

[0120] 67 Cathode (electrode layer)

[0121] 68 Gas diffusion layer

[0122] 69 cathode-side bipolar half-plate

[0123] 70 Channel structure (for oxidizing agents)

[0124] 71 Catalyst

Claims

Patent claims 1. 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 oxidizer path (16) of a fuel cell device (10), which comprises a fuel cell stack (11) of fuel cells (60) with respective anodes (66) and cathodes (67) as well as the anode-side fuel path (12) and the cathode-side oxidizer 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 ), Detection (S2) of a flow rate and / or pressure at at least one point in the course of the anode-side fuel path (12) and / or of a flow rate and / or pressure at at least one point in the course of the cathode-side oxidizer path (16), Performing (S3) an electrochemical impedance spectroscopy on the fuel cell stack (11) to acquire a number of n complex impedance values ​​for a number of n different excitation frequencies, where n is in the range of 2 to 10, Evaluation (S4) of the recorded quantities (d1 , d2, ... dN) using a regression model (M) determined in advance for the fuel cell device (10) to determine 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 oxidizing agent path (16).

2. Method according to claim 1, wherein the fuel cell device (10) is operable or operated with hydrogen as fuel and air as oxidizing agent.

3. Method according to one of the preceding claims, wherein the operating parameter relating to the current operating state of the fuel cell stack (11) an electric current and / or an electric voltage of the fuel cell stack (11) is detected.

4. Method according to one of the preceding claims, wherein the flow rate and / or the pressure at the inlet (13, 17) and / or at the outlet (14, 18) of the relevant path (12, 16) are detected.

5. Method according to one of the preceding claims, wherein at least one lambda value relating to the quantities of fuel and oxidizing agent is recorded.

6. Method according to one of the preceding claims, wherein a temperature is detected at at least one location, in particular the inlet (13) and / or the outlet (14), in the course of the anode-side fuel path (12) and / or a temperature is detected at at least one location, in particular the inlet (17) and / or the outlet (18), in the course of the cathode-side oxidizing agent path (16).

7. Method according to one of the preceding claims, wherein an excitation signal of electrochemical impedance spectroscopy is applied between an anode terminal and a cathode terminal of the fuel cell stack (11).

8. Method according to one of the preceding claims, wherein the evaluation of the quantities to determine the estimated value(s) (Ha, Hk) is carried out using a regression model (M) implemented as a trained neural network.

9. 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 oxidizer path (16) of a fuel cell device (10), which comprises a fuel cell stack (11) of fuel cells (60) with respective anodes (66) and cathodes (67) as well as the anode-side fuel path (12) and the cathode-side oxidizer path (16) with respective inlets (13, 17) and outlets (14, 18), wherein the device (30, 40, 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 along the anode-side fuel path (12) and / or a flow rate and / or a pressure at at least one point along the cathode-side oxidant path (16), an impedance spectroscopy device (40) for performing electrochemical impedance spectroscopy on the fuel cell stack (11) and for recording a number of n complex impedance values ​​for a number of n different excitation frequencies, where n is in the range of 2 to 10, an evaluation device (50) for evaluating the recorded quantities (d1 , d2, ... dN) using a regression model (M) previously determined for the fuel cell device (10) and implemented in the evaluation device (50) to determine an estimated value (Ha) for the humidity at the inlet (13) of the anode-side fuel path (12) and / or an estimated value (Hk) for the humidity at the outlet (18) of the cathode-side oxidizing pathway (16).

10. Use of a method according to any 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 oxidizing agent 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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