Method for diagnosing the health status of a fuel cell, method for maintaining a fuel cell, vehicle configured to implement these methods

Abstract

A diagnostic method is proposed for assessing the health of a fuel cell (2) fitted to a motor vehicle (1) and configured to supply, via a DC-DC voltage converter (7) controlled by a control unit (8), a high-voltage network (3) of the motor vehicle, comprising the following steps implemented by the control unit: - a step of controlling the DC-DC converter so that the fuel cell generates a sinusoidal current, - a step of measuring a characteristic of the impedance between the terminals of the fuel cell during the generation of the sinusoidal current, - a step of comparing the measured characteristic of the impedance to a reference value, and - a step of establishing the diagnosis based on the result of the comparison.A maintenance procedure is also proposed, including the implementation of the diagnostic procedure, and a motor vehicle comprising a control unit configured to implement this procedure. Figure for the abstract: Fig. 1.

Description

Title of the invention: Method for diagnosing the health status of a fuel cell, method for maintaining a fuel cell, vehicle configured to implement these methods. Technical field of the invention

[0001] The present invention relates generally to fuel cells.

[0002] It relates more particularly to a method for diagnosing the health status of a fuel cell equipping a motor vehicle, as well as a method for maintaining a fuel cell. It also relates to a motor vehicle configured to implement one or both of these methods. Prior art

[0003] Fuel cells are electrochemical devices configured to produce an electric current from the oxidation and reduction reactions of fuels. They comprise several identical cells in which the oxidation and reduction reactions take place.

[0004] In particular, so-called "proton exchange membrane fuel cells" (PEMFCs, for "Proton Exchange Membrane Fuel Cells", according to the usual Anglo-Saxon acronym) comprise cells which each have a symmetrical structure comprising, from a central layer forming a proton exchange membrane: a catalytic layer, a diffusion layer and an electrode.

[0005] As a general rule, such a fuel cell uses dihydrogen as a reducing fuel, which, injected into each cell through a first electrode (which then forms the anode), produces a reduction reaction, and dioxygen contained in the air as an oxidizing fuel, which, injected into each cell through a second electrode (which then forms the cathode), produces an oxidation reaction.

[0006] The cells of such a fuel cell are very sensitive to air pollutants. They are particularly sensitive to sulfur dioxide (SO2) and nitrogen dioxide (NO2). These pollutants accumulate in the layers of each cell, especially in the catalytic layer, thus reducing their electrochemical active surface area (ECSA). The reduction of the electrochemical active surface area leads to a decrease in oxidation and reduction reactions within the cell and therefore results in a decrease in the overall efficiency of the fuel cell.

[0007] This progressive deterioration in fuel cell performance leads to increased maintenance costs and a reduction in the cell's lifespan. This deterioration is all the more rapid when the fuel cell is used in areas where the air is polluted (particularly in urban areas).

[0008] This deterioration is, however, reversible, provided that the concentration of pollutants in the cells is not too high. Solutions exist for regenerating the fuel cell. However, these solutions require immobilizing the vehicle in a workshop or garage, possibly involving the removal of the fuel cell. Presentation of the invention

[0009] In order to remedy the aforementioned drawbacks of the prior art, the present invention proposes a diagnostic method and a maintenance method which can be implemented on-board in the vehicle and which do not require the addition of vehicle-specific equipment.

[0010] According to a first aspect, a method for diagnosing the health status of a fuel cell equipping a motor vehicle and configured to supply, via a DC-DC voltage converter controlled by a control unit, a high-voltage network of the motor vehicle, comprising the following steps implemented by the control unit: a step of controlling the DC-DC converter so that the fuel cell generates a sinusoidal current, a step of measuring an impedance characteristic between the terminals of the fuel cell during the generation of the sinusoidal current, a step of comparing the measured impedance characteristic to a reference value, and a step of establishing the diagnosis based on the result of the comparison.

[0011] Thus, thanks to the invention, it is possible to determine the presence of pollutants in the fuel cell and deduce its condition. Implementing this process using components embedded in the vehicle advantageously allows the process to be carried out on-board and thus avoids immobilizing the vehicle in a garage or workshop. Finally, using means conventionally present in the vehicle, in this case the DC-DC converter and its control unit, makes the process executable by existing vehicles and avoids the need for additional equipment.

[0012] Other advantageous and non-limiting features of the diagnostic method according to the invention, taken individually or in all technically possible combinations, are as follows: - the fuel cell generating a power supply current for a high-voltage network of the motor vehicle; the control stage involves superimposing the sinusoidal current onto the power supply current, - the high-voltage network including a battery of accumulators, it is planned during the fuel cell control stage to maintain the supply current at a constant value, a surplus current being supplied by the battery of accumulators if the high-voltage network requires a surplus current and a surplus current being absorbed by the battery of accumulators if the high-voltage network requires a reduction in current, - The impedance characteristic is the real part of the impedance measured for a predetermined frequency value. - the predetermined frequency value is less than or equal to 1000 Hz, and preferably less than or equal to 0.1 Hz.

[0013] The invention also proposes a method for maintaining a fuel cell which equips a motor vehicle and which is configured to supply, via a DC-DC voltage converter controlled by a control unit, a high-voltage network of the motor vehicle, comprising a diagnostic method according to the invention and, according to the diagnosis established, a fuel cell regeneration step implemented by a control unit.

[0014] Other advantageous and non-limiting features of the maintenance method according to the invention, taken individually or in all technically possible combinations, are as follows: - the regeneration step includes a step of controlling the DC-DC converter so that the fuel cell generates a voltage ramp, a step of determining a characteristic value of the current response of the fuel cell to the voltage ramp, then a step of comparing the characteristic value to a reference value and, depending on the result of the comparison, a repetition of the regeneration step or an interruption of the process, - the interruption of the process takes place either when the number of reiterations of the regeneration step reaches a predetermined value while the result of the comparison is significant of a failure of the regeneration, or when the result of the comparison is significant of a success of the regeneration. - the high-voltage network comprising a battery of accumulators configured to absorb the current generated by the fuel cell, the process includes, prior to the stage of controlling the cell, a stage of measuring the state of charge of the battery of accumulators and an interruption of the process if the measured state of charge is greater than a predetermined threshold. - the regeneration step is implemented if the ratio between the measured value and the reference value is greater than or equal to a predetermined threshold.

[0015] The invention also proposes a motor vehicle equipped with a fuel cell configured to power a high-voltage network of the motor vehicle and comprising a DC-DC voltage converter controlled by a control unit and configured to adapt the voltage supplied by the fuel cell to a voltage required by the high-voltage network, the control unit and the DC-DC voltage converter being configured to implement the diagnostic and / or maintenance process according to the invention.

[0016] Of course, the various features, variants, and embodiments of the invention can be combined with one another in various ways, provided they are not incompatible or mutually exclusive. Detailed description of the invention

[0017] The following description, in relation to the attached drawings, given by way of non-limiting examples, will make it clear what the invention consists of and how it can be implemented.

[0018] On the attached drawings:

[0019] [Fig-1] is a schematic representation, from an electrical point of view, of a motor vehicle according to the invention;

[0020] [Fig.2] is a block diagram representing the operation of a control unit of a DC-DC converter of the motor vehicle of [Fig.1];

[0021] [Fig.3] is a block diagram representing the operation of a module of the control unit for an impedance measurement of the fuel cell.

[0022] Fig. 1 represents the electrical architecture of a motor vehicle 1, for example a land vehicle, such as a car, a bus or a truck.

[0023] The motor vehicle 1 is equipped with a fuel cell 2 configured to provide electrical power to a high-voltage network 3 of the motor vehicle 1. In particular, the fuel cell 2 is configured to power a battery accumulator 4 of the high-voltage network 3. It is this battery accumulator 4 that powers the various current-consuming equipment of the high-voltage network 3.

[0024] Among these equipment, the high-voltage network 3 includes in particular an electric traction machine 5. It is coupled to the high-voltage network 3 via a traction inverter 6 which is configured to adapt the voltage delivered by the high-voltage network 3 (in particular here, a direct voltage) to the voltage required by the electric traction machine 5 (in particular here, an alternating voltage).

[0025] The fuel cell 2 is here of the proton exchange membrane fuel cell (PEMFC) type. It comprises a plurality of cells which, when supplied with fuel, produce a current i0 and a voltage u0 across the terminals of the fuel cell 2.

[0026] The fuel cell 2 is here coupled to the high-voltage network 3 via a DC-DC converter 7 (hereinafter referred to as the "DC-DC" converter, for "Direct Current - Direct Current," according to common Anglo-Saxon terminology). The DC-DC converter 7 is configured to modulate the electrical voltage delivered by the fuel cell 2, so as to adapt it to the voltage required by the high-voltage network 3. Since the voltage produced by the fuel cell 2 is lower than the voltage required by the high-voltage network 3, the DC-DC converter 7 acts as a voltage booster. The DC-DC converter typically comprises a plurality of switching cells, each containing several power switches, each of which contains several transistors.

[0027] The DC-DC converter 7 is controlled here by a control unit 8. This control unit 8 comprises a processor and memory (or a programmable logic circuit), as well as various input and output interfaces. Through its input interfaces, the control unit 8 is adapted to receive instructions and measurements of the current intensity i0 supplied by the fuel cell 2 and the voltage u0 across the terminals of the fuel cell 2. Through its output interfaces, it is adapted to control the DC-DC converter 7. Through its memory, it stores a computer application, consisting of computer programs comprising instructions whose execution by the processor enables the control unit to implement the maintenance procedure described below. In the variant where it includes programmable logic, its logic gates are programmed to implement this procedure.

[0028] The fuel cell 2 is further coupled to the DC-DC converter 7 via a filter 9. This filter 9 is a passive bandpass filter comprising a combination of inductors and capacitors. This filter 9 is a passive filter of order greater than or equal to 1. It filters the frequencies resulting from the switching of the transistors of the DC-DC converter 7. Its cutoff frequency is five to ten times higher than the switching frequency of the transistors of the DC-DC converter.

[0029] Fig. 2 is a block diagram illustrating the operation of the control unit 8.

[0030] The control unit 8 is configured to acquire a current intensity setpoint ic to be delivered by the fuel cell 2, which is representative of the required current via the high-voltage network 3. For example, this current setpoint ic varies according to the driving power demanded by the driver of the motor vehicle 1, i.e., for example, according to the pressure exerted by the driver on the accelerator pedal. The control unit 8 is also configured to acquire a measurement of the current i0 and a measurement of the voltage u0.

[0031] A first summing component 10 calculates the difference between the setpoint current intensity to be delivered ic and the measured current intensity i0. The measured current intensity i0 can, however, be filtered beforehand. For this purpose, a first low-pass filter 11 is provided to remove the high-frequency components of the measured current intensity i0. The cutoff frequency of the first low-pass filter 11 depends, in particular, on the sampling frequency of the control unit 8 and the bandwidth of the sensor used to measure the current. For example, it is between 2 kHz and 3 kHz, for example, 2.5 kHz.

[0032] At the output of this first summing component 10, a correction block 12 allows a voltage difference to be obtained. This correction block 12 is, for example, of the proportional-integral type.

[0033] A second summing component 13 calculates the sum of this voltage difference and the measured voltage u0. This measured voltage u0 can, however, be filtered beforehand. A second low-pass filter 14 is provided for this purpose, allowing the high-frequency components of the measured voltage to be removed. The cutoff frequency of the second low-pass filter 14 depends, in particular, on the sampling frequency of the control unit 8 and the bandwidth of the sensor used for voltage measurement. For example, it is between 2 kHz and 6 kHz, for example, 5 kHz.

[0034] This yields a voltage setpoint uc that the DC-DC converter 7 must apply to the input (i.e., supply to the filter 9). The difference between the voltage of the fuel cell 2 and the voltage of the DC-DC converter 7 is applied to the known impedance of the filter 9, thus controlling the current drawn from the fuel cell 2. The voltage of the high-voltage network 3 is imposed by the storage battery 4. This voltage provides the duty cycle n to be used to modulate the input voltage of the DC-DC converter 7 (using pulse-width modulation). This allows for a very responsive setpoint.

[0035] During operation, the layers of the fuel cell 2 cells accumulate pollutants (notably sulfur dioxide SO2 and nitrogen dioxide NO2). These pollutants impair the proper functioning of the fuel cell 2. It is therefore important that this pollution be detected before it becomes irreversible and that it can be easily removed. The control unit 8 and the DC- converter The DC 7 described above are therefore specifically programmed and configured to implement, regularly and in an embedded manner, a fuel cell maintenance process 2.

[0036] For the purposes of implementing this method, the control unit 8 is configured to generate a diagnostic current setpoint id. This diagnostic current id is a sinusoidal current with a low amplitude relative to the current setpoint ic, typically less than 10% of the maximum current setpoint ic and, for example, equal to 1% of the maximum current setpoint ic. A diagnostic current id amplitude of 1 ampere is suitable, for example. The control unit 8 is configured to modulate the frequency of the diagnostic current id, which is therefore variable, as will be described below. This diagnostic current setpoint id is superimposed (its intensity is summed) on the difference between the current setpoint ic and the measured current i0.The amplitude of the diagnostic current id is preferably chosen so that the sum of the diagnostic current id and the current to be delivered ic is positive. This avoids the generation of a reverse current.

[0037] The control unit 8 further includes an impedance measurement module 15, illustrated in [Fig.3], which is configured to determine the impedance of the fuel cell 2.

[0038] This impedance measurement module 15 includes a divider component 16, a first bandpass filter 17, and a second bandpass filter 18. The first bandpass filter 17 is centered on the frequency of the diagnostic current id and is configured to filter the current i0. The second bandpass filter 18 is also centered on the frequency of the diagnostic current id and is configured to filter the voltage u0. The bandwidths of the first bandpass filter 17 and the second bandpass filter 18 are equal to twice the center frequency of the band (here, the frequency of the diagnostic current id). The divider component 16 is configured to establish the ratio between the filtered voltage u0 and the filtered current i0. In other words, the impedance measurement module 15 is configured to measure the voltage response of the fuel cell 2 to the diagnostic current id and to deduce the impedance of the fuel cell 2.

[0039] The impedance measurement module 15 is configured in particular to distinguish the real part Re and the imaginary part Im of the impedance and to store them separately in its memory.

[0040] Finally, the memory of the control unit 8 contains reference data. In particular, it contains initial reference data for diagnostic purposes. This data is representative of the impedance of a fuel cell reference cell, identical to fuel cell 2, but free of pollutants (a new cell of the same model). This includes, for example, data representative of a Nyquist diagram established by electrochemical impedance spectroscopy of the reference cell.

[0041] The memory also includes second reference data for regeneration purposes. This data is representative of a bias curve (a current-voltage characteristic) of the reference fuel cell.

[0042] The first reference data and the second reference data are for example stored in computer memory during the manufacture of motor vehicle 1.

[0043] The maintenance process implemented by the control unit 8 and the DC-DC converter 7 described above has two aspects.

[0044] It comprises, on the one hand, diagnostic steps aimed at determining the health status of the fuel cell 2. This health status depends on the amount of pollutant in the cells of the fuel cell 2. The maintenance process comprises, on the other hand, regeneration (or decontamination) steps for the fuel cell 2, that is to say, steps aimed at restoring the fuel cell 2 to a good health status. In particular, it aims to eliminate at least some of the pollutants from the cells. This process is described below.

[0045] The diagnostic steps can be implemented at any time by the control unit 8 and the DC-DC converter 7, in particular during the driving phases of the motor vehicle 1 or when parked.

[0046] They include a determination of the impedance of the fuel cell 2 by electrochemical impedance spectroscopy, a comparison of the impedance determined to the first reference values, and a determination of the state of health of the fuel cell 2 according to the result of the comparison.

[0047] Determining the impedance of the fuel cell 2 involves sweeping the frequency of the diagnostic current id within a defined frequency range. Here, the frequency sweep extends from 0.1 Hz to 1000 Hz. The frequency of the diagnostic current id varies, for example, on a logarithmic basis at a rate of ten steps per decade. Each frequency value is held for a defined duration, for example, a duration that depends on the frequency. A duration greater than or equal to three periods of the setpoint current, for example, prevents transient regimes from interfering with the measurements.

[0048] For each frequency value of the diagnostic current id, the control unit 8 and the DC-DC converter 7 measure an impedance value of the fuel cell 2. The impedance values ​​(in particular, the real and imaginary parts of the impedance) are recorded in the memory of the control unit 8. It is thus obtained representative data from a Nyquist diagram obtained by electrochemical impedance spectroscopy.

[0049] Since the impedance measurement is performed using the current i0 and the voltage u0, specifically here by the impedance measurement module 15, it is important that the average value of these quantities remain stable during the diagnostic procedure. Thus, in this example, if the current setpoint ic varies, this variation is compensated for by the battery 4. For example, if the current setpoint ic increases, the battery 4 supplies the additional current required by the high-voltage network 3, and if the current setpoint ic decreases, it absorbs the excess current supplied by the fuel cell 2. Therefore, the values ​​of current i0 and voltage u0 remain constant during the diagnostic procedure.

[0050] Here, the reference value to which the measured impedance is compared is taken from the initial reference data. This is therefore the impedance value of a pollutant-free fuel cell.

[0051] In particular, the actual part Re of the measured impedance (the resistance of the fuel cell 2) for a determined frequency value is compared here to the actual part of the reference impedance for that same frequency value. Preferably, the determined frequency value is a low value relative to the frequency range, for example, a frequency in the lower half of the frequency range. Here, the determined frequency value is the lowest frequency in the frequency range (0.1 Hz in this example).

[0052] The comparison here involves establishing a ratio between the measured value and the reference value. Thus, if the fuel cell 2 is new (no pollutants), then the result of the comparison is equal to 1.

[0053] If the comparison result is below a predetermined threshold, it will be indicated that the health status of fuel cell 2 is good (absence of pollution or an acceptable quantity of pollutants). If the comparison result is above the predetermined threshold, it will be indicated that the health status of fuel cell 2 is poor (presence of pollutants or a quantity of pollutants exceeding an acceptable value). In this example, the predetermined threshold is greater than 1, preferably greater than or equal to 1.2. Typically, it could be equal to 1.2 or 1.5.

[0054] If the comparison is significant in the good health of the fuel cell 2, then it is not necessary to regenerate the fuel cell 2. The maintenance process ends.

[0055] If the result of the comparison is indicative of poor health of the fuel cell 2, then it is necessary, or at least recommended, to regenerate the fuel cell 2. The maintenance process then continues with a regeneration step of fuel cell 2.

[0056] This regeneration step can be implemented immediately after the diagnostic steps. However, it is preferably implemented when the motor vehicle 1 is stationary, and preferably parked (ignition off). Therefore, if the diagnostic steps were implemented during a driving phase, then the regeneration step is suspended until the next time the motor vehicle 1 is parked. Furthermore, the implementation of the regeneration step can be conditional upon validation by the driver of the motor vehicle 1. Thus, if the diagnostic steps indicate a poor condition of the fuel cell 2, a message can be sent to the driver (for example, via a human-machine interface of the motor vehicle, such as a touchscreen on the dashboard). This message may, for example, ask the driver to confirm the start of the regeneration step.

[0057] The regeneration step includes a control of the fuel cell 2 so that it generates a current which removes pollutants from the cells by electro-oxidation.

[0058] To ensure the absorption of this current, the implementation of the regeneration of the fuel cell 2 is conditional upon a sufficiently low state of charge of the storage battery 4. For example, regeneration is implemented only if the state of charge of the storage battery 4 is less than or equal to 75%. For example, if the state of charge of the storage battery 4 is greater than 75%, the implementation of regeneration is suspended until the state of charge of the storage battery 4 is less than or equal to 75%.

[0059] The regeneration step includes a step of controlling the fuel cell 2 so that it generates a voltage ramp u0. It is the current generated by the fuel cell 2 in response to this voltage ramp that has the effect of removing the pollutants from the cells of the fuel cell 2, and which is absorbed here by the storage battery 4.

[0060] In order to protect the electrodes (typically made of carbon) from corrosion, the voltage ramp u0 preferably has low voltage values ​​u0. For example, the voltage ramp u0 has voltage values ​​u0 less than IV per cell of the fuel cell 2. It extends, for example, between 0.5 volt and 0.8 volt per cell of the fuel cell 2, over a period of 15 minutes.

[0061] The regeneration step also includes a step for measuring the current response of the fuel cell 2 to the voltage ramp. Preferably, several characteristic values ​​of this response are measured. For example, a current value is measured and saved in the memory of the control unit 8 for each voltage value u0. This gives us a bias curve (a current-voltage characteristic) of the fuel cell 2.

[0062] Next, the measured current values ​​are compared to reference values, in this case, the second set of reference data stored in the memory of the control unit 8 (here, the bias curve of a new fuel cell). In other words, a comparison is made here between at least one characteristic of the bias curve of the fuel cell 2 and at least one characteristic of the bias curve of a new fuel cell.

[0063] For example, the comparison involves establishing an average value of the differences between the measured current values ​​and the corresponding reference current values.

[0064] The regeneration step further includes an indication step, based on the result of the comparison, of whether the regeneration was successful or failed. For example, if the established average value is less than a predetermined threshold (if the difference between the two polarization curves is small), then it is indicated that the regeneration was successful. If the established average value is greater than or equal to the predetermined threshold (if the difference between the two polarization curves is large), then it is indicated that the regeneration was unsuccessful.

[0065] If the regeneration is successful, the maintenance procedure ends. If the regeneration fails, the regeneration step is repeated. If the number of reiterations of the regeneration steps reaches a predetermined value (for example, four iterations), the regeneration is indicated as a failure, and the maintenance procedure ends. This failure can then be displayed on the vehicle's touchscreen and stored in the memory of the control unit 8, so that it is accessible to the operator responsible for the next maintenance of the vehicle in the workshop.

[0066] The invention is not limited to the methods of implementation and embodiment described above in relation to figures 1 to 3.

[0067] In particular, it would be possible to carry out only the diagnostic steps, without seeking to regenerate the fuel cell, even if the result of the diagnosis is significant of a poor state of health.

[0068] Furthermore, it has been described previously, for diagnostic purposes, a measurement of a characteristic of the fuel cell impedance and a comparison of this characteristic to a reference value. Since the real part of the impedance is particularly sensitive to the presence of pollutants, it is preferable to choose it as the impedance characteristic. However, the invention is not limited to this impedance characteristic, but covers embodiments employing other impedance characteristics, for example, the imaginary part of the impedance (the reactance). In this case, it is preferable to consider only Reactance values ​​corresponding to diagnostic signal frequencies below 1 kHz. Furthermore, the characteristic impedance value could be obtained by combining the real and imaginary parts of the impedance.

[0069] Furthermore, a determination, by electrochemical impedance spectroscopy, of data representative of a Nyquist diagram has been described above. Realizing the complete Nyquist diagram is advantageous because it contains data that can be used for other applications, particularly other maintenance applications. That being said, it would be entirely possible, within the scope of the invention, to measure the impedance characteristic by realizing only a portion of the Nyquist diagram. For example, it would be possible to generate the diagnostic current setpoint with a fixed frequency and to measure only the characteristic of the chosen impedance (for example, the resistance of the fuel cell) in response to this current.In such a case, it would also be possible that the initial reference data represent only a part of the Nyquist diagram or that they include a single reference value, for example here the resistance of a reference battery for the fixed frequency of the diagnostic current.

[0070] Furthermore, a fuel cell has been described that powers a current-consuming device (here, an electric traction machine) via a battery. However, within the scope of the invention, it would be possible for the fuel cell to directly power a current-consuming device without using a battery. For example, the fuel cell could be connected in parallel with the battery; a DC-DC converter would then be placed between the battery and the high-voltage network.

[0071] As previously described, the battery absorbs excess current or supplies excess current to maintain the average current supplied by the battery at a stable value during the diagnostic process. This advantageously prevents measurement errors. However, the invention is not limited to this embodiment and covers variations in which the process is interrupted if the setpoint current to be delivered varies too greatly.

[0072] The regeneration described above involves generating a voltage ramp and establishing an average value for the differences between the measured response current values ​​and the corresponding reference current values. The comparison is not limited to this method. It would, for example, be possible to compare a single current value to a single reference value for the same voltage value.

[0073] Finally, a diagnostic current id, which is superimposed on the difference between the setpoint current to be delivered ic and the current i0, has been described previously. It has been noted that, in order to avoid the generation of a reverse current, it is preferable for the sum of the current to be delivered and the diagnostic current to be positive. This condition must also be met when the diagnostic procedure is carried out while the motor vehicle is stationary (the equipment of the motor vehicle's high-voltage network then consumes a very low current). In this case, the fuel cell produces a charging current for the storage battery such that the current to be delivered ic is greater than the amplitude of the diagnostic current ic, so as to avoid the generation of a reverse current.

Claims

Demands

1. A method for diagnosing the health status of a fuel cell (2) fitted to a motor vehicle (1) and configured to supply, via a DC-DC voltage converter (7) controlled by a control unit (8), a high-voltage network (3) of the motor vehicle (1), comprising the following steps implemented by the control unit (8): - a step of controlling the DC-DC converter (7) so that the fuel cell (2) generates a sinusoidal current (id), - a step of measuring a characteristic of the impedance between the terminals of the fuel cell (2) during the generation of the sinusoidal current (id), - a step of comparing the measured characteristic of the impedance to a reference value, and - a step of establishing the diagnosis based on the result of the comparison.

2. A method according to claim 1, wherein the fuel cell (2) generates a supply current (i0) from a high-voltage network (3) of the motor vehicle (1), the control step comprises a superposition of the sinusoidal current (id) to the supply current (i0

3. h Method according to claim 2, wherein the high-voltage network (3) includes a battery of accumulators (4), it is provided during the control step of the fuel cell (2) to maintain the supply current (i0) at a constant value, a surplus current being supplied by the battery of accumulators (4) if the high-voltage network (3) requires a surplus current and a surplus current being absorbed by the battery of accumulators (4) if the high-voltage network (3) requires a reduction in current.

4. A method according to any one of claims 1 to 3, wherein the impedance characteristic is the real part of the impedance measured for a predetermined frequency value.

5. A method according to claim 4, wherein the predetermined frequency value is less than or equal to 1000 Hz.

6. Method of maintaining a fuel cell (2) which equips a motor vehicle (1) and which is configured to supply, via a DC-DC voltage converter (7) controlled by a control unit (8), a high-voltage network of the motor vehicle, comprising: - a diagnostic method according to any one of claims 1 to 5 and - according to the established diagnosis, a fuel cell regeneration step (2) implemented by the control unit (8).

7. A method according to claim 6, wherein the regeneration step comprises: - a step of controlling the DC-DC voltage converter (7) so that the fuel cell (2) generates a voltage ramp, - a step of determining a characteristic value of the current response of the fuel cell (2) to the voltage ramp, then - a step of comparing the characteristic value to a reference value, and - depending on the result of the comparison, a repetition of the regeneration step or an interruption of the process.

8. Maintenance method according to claim 7, wherein the interruption of the process takes place either when the number of reiterations of the regeneration step reaches a predetermined value while the result of the comparison is significant of a failure of the regeneration, or when the result of the comparison is significant of a success of the regeneration.

9. Maintenance method according to claim 7 or 8, wherein the high-voltage network (3) includes a battery of accumulators (4) configured to absorb the current (i0) generated by the fuel cell (2), the method includes, prior to the step of controlling the fuel cell (2), a step of measuring the state of charge of the battery of accumulators (4) and an interruption of the method if the measured state of charge is greater than a predetermined threshold.

10. Maintenance method according to any one of claims 7 to 9, wherein the regeneration step is implemented if the ratio between the measured value and the reference value is greater than or equal to a predetermined threshold.

11. Motor vehicle equipped with a fuel cell (2) configured to power a high-voltage network (3) of the motor vehicle (1) and comprising a DC-DC voltage converter (7) controlled by the control unit (8) and configured to adapt the voltage (u0) supplied by the fuel cell (2) to a voltage required by the high-voltage network (3), the control unit (8) and the DC-DC voltage converter (7) being configured to implement the diagnostic method according to any one of claims 1 to 10.

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