State determinations by means of correlations between mass flow and current for fuel cells

Abstract

The invention relates to a method for examining at least one electrochemical cell, in particular at least one fuel cell (101), preferably of a fuel cell stack (100), comprising: - applying a periodically changing signal (S) of an electrical quantity (I, U) to the at least one electrochemical cell, - capturing a periodically varying mass response signal (n) from the at least one fuel cell (101), - evaluating a state of the electrochemical cell according to the capturing.

Description

[0001] Description

[0002] title

[0003] State determination using mass flow-current correlations in fuel cells

[0004] The invention relates to a method for examining at least one electrochemical cell, in particular a fuel cell, e.g., a PEM fuel cell, preferably a fuel cell stack. Furthermore, the invention relates to a corresponding computer program product. Furthermore, the invention relates to a corresponding control unit for carrying out a corresponding method. Furthermore, the invention relates to a corresponding electrochemical cell with a corresponding control unit.

[0005] State of the art

[0006] Existing fuel cell systems and battery systems are expensive, and their reliability and durability are challenging, especially for mobile applications. In addition, field experience and data regarding failure statistics and system-relevant aging functions are still limited. This often leads to the oversizing of expensive system components in today's systems.

[0007] Controlling the various system components in fuel cell systems (such as the stack, compressor, cooling circuit, and battery) using an operating strategy requires precise knowledge of the component states. With regard to the fuel cell stack, for example, knowledge of temperatures, humidity, pressure, voltage and current, as well as flow conditions, is important for estimating derived variables such as aging and performance readiness. System-relevant states include:

[0008] I) State of Health (ageing state) (e.g. can be determined via the parameters internal resistance, efficiency, maximum power, capacity),

[0009] II) Operating state “State of Operation” (e.g. can be determined via the variables temperature, current power, internal resistance, state of charge).

[0010] An established method in electrochemistry that can be applied up to the vehicle level is electrochemical impedance spectroscopy (EIS). In this method, an alternating current (galvanostatic testing, particularly preferred for fuel cells) is superimposed on a base load current, and the complex, frequency-dependent impedance is calculated from this, together with the alternating voltage response. AC voltage excitation while recording the alternating current response is also possible. The AC or DC signals can be excited either individually or as multisine waves. Impedance analyses based on noise signals from loads or background noise on the object are also conceivable as a starting point, with subsequent Fourier analysis of the signals.

[0011] Disclosure of the invention

[0012] According to the first aspect, the present invention provides: a method for examining at least one electrochemical cell (can be referred to as a cell for short), in particular a fuel cell, for example a PEM fuel cell, preferably a fuel cell stack (can be referred to as a stack for short), for example a PEM fuel cell stack, having the features of the independent method claim. According to the second aspect, the present invention provides: a computer program product having the features of the independent product claim. According to the third aspect, the present invention provides: a control unit having the features of the independent device claim. According to the fourth aspect, the present invention provides: an electrochemical cell having the features of the independent device claim.Features and details that are described in connection with different embodiments and / or aspects of the invention naturally also apply in connection with the other embodiments and / or aspects and vice versa, so that with regard to the disclosure of the individual embodiments and / or aspects, reference is or can always be made mutually.

[0013] The invention provides: a method for examining at least one electrochemical cell, in particular a fuel cell, for example a PEM fuel cell, preferably a fuel cell stack, for example a PEM fuel cell stack.

[0014] The procedure includes the following steps / actions:

[0015] Applying a periodically changing signal of an electrical quantity (e.g. current or voltage) to the at least one electrochemical cell,

[0016] Detecting a periodically varying mass response signal (e.g. mass flow, molar flow or reactant concentration) from the at least one fuel cell,

[0017] Assessing a condition (e.g. a state of health / aging condition “State of Health” and / or an operating condition “State of Operation”) of the electrochemical cell depending on the detection.

[0018] The method enables a novel analytical approach for investigating mass response signals after passage through an electrochemical cell, which arise in response to periodically changing electrical signals, e.g., an AC current signal. This provides access to parameters from the operation of fuel cells and their stacks that would otherwise only be possible through indirect derivation. In this case, these are the flow time, the flow velocity, and also a measure of the

[0019] Uniformity of the flow velocity. The invention thus contributes to the extended determination of the condition with regard to the operation and / or aging of an electrochemical cell, in particular a fuel cell stack.

[0020] The core of the invention is the efficient access to quantitative and qualitative flow velocity indications using a type of generalized impedance function. Flow velocities and / or flow times cannot otherwise be determined directly within the electrochemical cell or within the stack zone, and even otherwise, this can only be achieved with rather complex (cost, installation space, etc.) and sometimes inaccurate sensor technology.

[0021] The analysis principle can be implemented in a variety of ways, depending on the gas to be analyzed and / or the derived fluctuation variable, such as:

[0022] • Optical sensors (e.g. via optical transmission, absorption, scattering effects),

[0023] • Density-related,

[0024] • Pressure-related,

[0025] • Flow velocity related,

[0026] • Mass spectrometry.

[0027] The method can provide that a periodic electrical load (U-AC, I-AC) can be imposed on a base load of the stack (U-DC, l-DC) (e.g. via a sine generator).

[0028] It is conceivable that an imposed periodic signal, e.g. AC sine signals, can be provided, for example, via additive AC sources and / or via power electronics to convert the application currents / voltages.

[0029] AC generation can preferably occur over several selected frequencies or a frequency band. The frequencies can be selected, for example, depending on an expected flow time (in the range of the period duration = flow time of a reactant through the cell). This can be done over, for example, ±1 decade.

[0030] The sinusoidal signal should preferably be applied during phases of constant DC load to minimize the load background as a disturbance. For the example of a sinusoidal current excitation in addition to the DC signal, this could mean:

[0031] I = Io* sin wt, dn / dt » (dno / dnt) * sin (wt + (p + a), with dn / dt - molar current behind the cell [mol / time], dnO / dnt - amplitude of the molar current behind the cell [mol / time],

[0032] (p - phase shift to the current, a - phase shift to the measuring point.

[0033] As a result of the superimposed current sine on a DC base signal, a periodically varying mass response signal (e.g. mass flow, molar flow and / or concentration signal of reactants or concentration signal of passive gas backgrounds) will appear at the output of the stack.

[0034] At the end of the active cell region, a periodically varying mass response signal will occur, which can represent a more or less sinusoidal output signal depending on the applied periodically changing signal, e.g. depending on the current amplitude, current frequency, cell current density distribution and channel routing.

[0035] The mass response signal may have a phase shift (p) to the current signal. If the detection of the mass response signal is carried out at a certain distance from the cell output, an additive phase shift a may occur due to the propagation time.

[0036] The amplitude of the mass response signal can be frequency-dependent. The frequency dependence can be determined, in particular, by the net depletion during passage through the stack and, in part, by a "washing out" of the signal via propagation time paths to the detector location. In a certain "ideal case," for example, when the period of the current I corresponds exactly to an integer multiple of the passage time (or

[0037] flow time), a minimum mass amplitude (the so-called "main minimum") at the detector can be expected. This can be explained as follows: This allows additive and subtractive depletion of the reactant to ideally balance each other out.

[0038] The main minimum and, if applicable, additional minima can then be searched for in the frequency spectrum of the amplitudes, and their "depth" and position in the spectrum can be evaluated. From this, flow time and / or local flow rates or flow velocities can be derived.

[0039] The required pressure level and stoichiometries, as well as flow channel data, are typically known to the stack operator or can be estimated. However, even without much of this data, changes (such as minimum amplitude height and frequency) during operation and over aging can be evaluated and used to determine the condition.

[0040] Furthermore, a generalized impedance quantity - mass response signal amplitude / current amplitude - can also be taken into account, e.g. plotted in the form of a Bode plot.

[0041] Furthermore, phase information of the two reference quantities can also be taken into account. Using phase information, evaluations can be performed directly in the Nyquist plot, e.g., for flow times away from the stack, including paths between the stack / cell and the analysis unit. a) For example, an impedance quantity can be determined based on sinusoidal signals:

[0042] I) Determination of the mass response signal amplitude (e.g. using frequency analysis and / or fitting),

[0043] II) Determination of the current amplitude (usually given by an AC source),

[0044] III) Calculating mass response signal amplitude / current amplitude. b) For example, an impedance value can be determined based on sinusoidal signals:

[0045] I) Determination of an averaged and per period calculated

[0046] M axi ma I m essw e rtes ,

[0047] II) Determination of an averaged measurement value calculated per period,

[0048] III) Determination of an averaged maximum value of the current signal calculated for each period,

[0049] IV) Determination of an averaged and periodically calculated mean value of the current signal.

[0050] (nmax, average naverage, average) / (lmax, average I average, average) , with n - mass response signal (e.g. mass flow, molar flow or reactant concentration).

[0051] Furthermore, the following quantities can be compared and used to assess the condition of the electrochemical cell.

[0052] Ratios of maximum values ​​and / or mean values, even without the application of AC currents or voltages; e.g., across sensitively filtered frequencies or frequency bands during operation.

[0053] Magnitude values ​​and / or phase values ​​(with sufficient prior knowledge, the magnitude information is often sufficient to detect characteristic states)

[0054] The sampling frequency of the mass-related signals and current signals can be determined according to sampling theory.

[0055] On the one hand, it is conceivable in the method that a periodically changing current signal is applied to the at least one electrochemical cell as a periodically changing signal of an electrical quantity. On the other hand, it is conceivable in the method that a periodically changing voltage signal is applied to the at least one electrochemical cell as a periodically changing signal of an electrical quantity. If an electrical current or an electrical voltage applied to the stack changes, the mass response signal changes. The ratios of the signal values ​​of the excitation signal and the mass response signal can thus be obtained and taken into account when assessing the state of the electrochemical cell.

[0056] Furthermore, it is conceivable in the method that a mass flow and / or a molar flow is detected as a periodically varying mass response signal from the at least one fuel cell. Furthermore, it is conceivable in the method that a concentration signal of a reactant, in particular an oxygen-containing or fuel-containing reactant, is detected as a periodically varying mass response signal from the at least one fuel cell. Furthermore, it is conceivable in the method that a concentration signal of a passive gas background is detected as a periodically varying mass response signal from the at least one fuel cell. In this way, a characteristic chemical variable (mass flow, molar flow, or reactant concentration) can be investigated, which varies depending on the excitation signal of an electrical variable.

[0057] Advantageously, when applying a periodically changing signal of an electrical quantity to the at least one electrochemical cell, signals of different excitation frequencies can be applied to the at least one electrochemical cell. In this way, multiple maximum values ​​and / or averages can be determined for both the excitation signal and the response signal, enabling a precise investigation of the state of the electrochemical cell.

[0058] Furthermore, it can be provided that the periodically varying mass response signal is sampled using frequencies that are determined as a function of the excitation frequencies when a periodically changing signal of an electrical quantity is applied. This enables targeted sampling of the mass response signal in expected frequency ranges. Furthermore, it can be provided that, when assessing a state of the electrochemical cell, a period of the periodically changing signal is taken into account in relation to a flow time through the at least one fuel cell by a reactant. This makes it possible to utilize the knowledge that a period of the excitation signal that represents an integer multiple of a flow time results in local minima. In this way, conclusions about the flow time can be drawn directly.If no minima or maxima can be seen in the spectrum of the mass response signal, this may be a sign of cell aging.

[0059] Furthermore, it can be provided that if a period of the periodically changing signal results in an integer multiple of the flow time through the at least one fuel cell through a reactant, then a mass flow response approximately equal to zero or a minimal mass flow response is expected. This can facilitate the evaluation of the mass response signal. In this way, a direct relationship to flow time can be found. The expected relationships to flow velocity can also be established.

[0060] Furthermore, when assessing the state of the electrochemical cell, a period in the frequency spectrum of the periodically varying mass response signal can be examined for maxima and minima. This provides a simple evaluation of the mass response signal, which can be performed, for example, using established sampling techniques.

[0061] Advantageously, when assessing the condition of the electrochemical cell, the aging state and / or operating state of the electrochemical cell can be assessed. Advantageously, when assessing the condition of the electrochemical cell, flow times and / or flow velocities can be determined.

[0062] In principle, it may be advantageous to take into account correlations, impedances, amplitude differences and / or phase shifts between the periodically changing signal and the periodically varying mass response signal when assessing the state of the electrochemical cell.

[0063] According to the second aspect, the present invention provides a computer program product comprising instructions that, when executed by a computer, cause the computer to perform a method that can be executed as described above. The computer program product according to the invention achieves the same advantages as those described above in connection with the method according to the invention. These advantages are incorporated herein by reference.

[0064] According to the third aspect, the present invention provides a control unit comprising a computing unit and a memory unit in which a code is stored which, when at least partially executed by the computing unit, carries out a method that can proceed as described above. The control unit according to the invention achieves the same advantages as those described above in connection with the method according to the invention. These advantages are incorporated herein by reference in their entirety.

[0065] Advantageously, the control unit can be implemented at least partially in a gas analysis unit for an oxygen-containing or fuel-containing reactant. In this way, existing analysis units in the system can be utilized to carry out the method, which can proceed as described above.

[0066] Preferably, the gas analysis unit can be arranged adjacent to a stack outlet for an oxygen-containing or fuel-containing reactant. In this way, the resolution of fluctuations in the periodically varying mass response signal can be increased.

[0067] Furthermore, it is conceivable that the gas analysis unit may comprise a sensor for detecting the periodically varying mass response signal from the at least one fuel cell. The sensor can be implemented in a variety of ways, depending on the gas to be analyzed and / or the fluctuation to be detected, e.g., as:

[0068] • an optical sensor (e.g. via optical transmission, absorption, scattering effects),

[0069] • a density-related sensor,

[0070] • a pressure-related sensor,

[0071] • a flow velocity-related sensor,

[0072] • a sensor that uses the principle of mass spectrometry.

[0073] Furthermore, it is conceivable that the gas analysis unit can be designed to control an AC source in such a way as to apply the periodically changing signal as an electrical quantity to the at least one electrochemical cell. Preferably, the gas analysis unit can be designed to control the AC source in such a way as to apply the periodically changing signal via a plurality of selected excitation frequencies, which can preferably be determined as a function of the flow time through the at least one fuel cell by an oxygen-containing or fuel-containing reactant, advantageously by the gas analysis unit itself.

[0074] According to the fourth aspect, the present invention provides an electrochemical cell, in particular a fuel cell, preferably a PEM fuel cell, comprising a control unit, which can be designed as described above. The cell according to the invention achieves the same advantages as those described above in connection with the method according to the invention. These advantages are incorporated herein by reference. All advantages described above and below with respect to one aspect of the present disclosure apply equally to all other aspects of the present disclosure, and vice versa.

[0075] Preferred embodiments:

[0076] The invention and its further developments, as well as their advantages, are explained in more detail below with reference to the accompanying drawings. They show schematically:

[0077] Fig. 1 shows an exemplary representation of a fuel cell stack,

[0078] Fig. 2 exemplary functions of a periodically changing signal of a stack current as excitation signal and a periodically varying mole current as mass response signal,

[0079] Fig. 3 exemplary functions for a stack current / time curve for a run through time = AC period duration and also for a run through time =

[0080] 1.5 x AC period,

[0081] Fig. 4 Example AC stack current / time curve for a current period significantly smaller than the cell or stack cycle time, and

[0082] Fig. 5 shows an exemplary ideal frequency response of a mass flow-related amplitude signal (here shown as output signal level minus input signal level).

[0083] In the different figures, identical parts of the invention are always provided with the same reference numerals, which is why they are usually only described once.

[0084] Figures 1 to 3 serve to explain a method which was developed for examining at least one electrochemical cell, in particular a fuel cell, for example a PEM fuel cell, preferably a fuel cell stack, for example a PEM fuel cell stack.

[0085] The procedure includes the following steps / actions:

[0086] Applying a periodically changing signal S of an electrical quantity (e.g. current I or voltage U) to the at least one electrochemical cell, detecting a periodically varying mass response signal n (e.g. mass flow dm / dt, molar flow dn / dt or reactant concentration m%) from the at least one fuel cell 101, assessing a state (e.g. a state of health / aging state “state of health” and / or an operating state “state of operation”) of the electrochemical cell depending on the detection.

[0087] The method provides a novel analytical method for investigating mass response signals after passage through an electrochemical cell, which arise in response to periodically changing electrical signals, e.g., an AC current signal. This provides access to parameters from the operation of fuel cells and their stacks that could otherwise only be determined indirectly. In this case, these are the flow time ti_, the flow velocity, and, if applicable, a measure of the uniformity of the flow velocity. Thus, the method can contribute to the extended condition determination with regard to the operation and / or aging of an electrochemical cell 100, in particular a fuel cell stack.

[0088] In this way, quantitative and qualitative conclusions about the flow velocity indications can be drawn using a kind of generalized impedance function. Flow velocities and / or flow times ti_ cannot otherwise be determined directly within the electrochemical cell or within the stack zone.

[0089] The schematic structure of the measuring arrangement is shown in Fig. 1 using the example of a fuel cell stack (or simply called a stack). The stack with the stacked electrochemical cells 100 has a fuel H2 and an air supply, each indicated by an inlet (H2 in, air in) and an outlet (H2 out, air out). A gas analysis unit 11, 21 for the anode gas flow and / or cathode gas flow can be mounted at the outlet (H2 out, air out), preferably close to the stack to increase the resolution of concentration fluctuations in the outgoing gas.

[0090] The gas analysis unit 11, 21 can comprise a sensor and signal processing / conversion depending on the principle and requirements. The analysis principle can be implemented in a variety of ways, depending on the gas to be analyzed and / or the derived fluctuation variable, such as:

[0091] • Optical sensors (e.g. via optical transmission, absorption, scattering effects),

[0092] • Density-related,

[0093] • Pressure-related,

[0094] • Flow velocity related,

[0095] • Mass spectrometry.

[0096] The method can provide that a periodic electrical load (U-AC, I-AC) can be imposed on a base load of the stack (U-DC, l-DC) (e.g. via a sine generator).

[0097] It is conceivable that an imposed periodic signal, e.g. AC sine signals, can be provided, for example, via additive AC sources and / or via power electronics to convert the application currents / voltages.

[0098] AC generation can preferably occur over several selected frequencies f. The frequencies f can, for example, be dependent on an expected flow time ti_ (in the range of period duration = flow time ti_) – and this over, for example, ±1 decade.

[0099] The sinusoidal signal should preferably be applied during phases of constant DC load to minimize the load background as a disturbance. For the example of a sinusoidal current excitation in addition to the DC signal, this could mean:

[0100] I = Io* sin wt, dn / dt » (dno / dnt) * sin (wt + (p + a), with dn / dt - molar current behind the cell [mol / time], dnO / dnt - amplitude of the molar current behind the cell [mol / time],

[0101] (p - phase shift to the current, a - phase shift to the measuring point.

[0102] As a result of the superimposed current sine on a DC base signal, a periodically varying mass response signal n (e.g. mass flow dm / dt, molar flow dn / dt and / or concentration signal m% of reactants or concentration signal of passive gas backgrounds) will appear at the output of the stack.

[0103] At the end of the active cell region, a periodically varying mass response signal dn / dt will be established, which can represent a more or less sinusoidal output signal depending on the applied periodically changing signal, e.g. depending on the current amplitude, current frequency, cell current density distribution and channel routing.

[0104] The mass response signal dn / dt may have a phase shift (p) to the current signal. If the detection of the mass response signal dn / dt is carried out at a certain distance from the cell output, an additive phase shift a may occur due to the propagation time.

[0105] The amplitude dno / dnt of the mass response signal dn / dt can be frequency-dependent. The frequency dependence can be determined, in particular, by the net depletion during passage through the stack (see Figs. 2, 3, and 4) and, on the other hand, by a "washing out" of the signal via propagation paths to the detector location.

[0106] Figure 2 shows an exemplary signal S for current I over time t and a response signal n - molar flow nd / dt as a function of the channel length XL. ti_ is the flow time. A signal curve for the resulting minimum amplitude of the mass response signal n or the molar flow dn / dt or the concentration signal m% of a depleted medium (e.g., fuel H2, oxygen-containing gas mixture, or air O2). The dashed line corresponds to the case without an AC signal.

[0107] In a certain “ideal case”, when, for example, the period of the current I corresponds exactly to an integer multiple of the transit time, a minimum mass amplitude (so-called “main minimum”) at the detector can be expected (see also Fig. 3 and especially Fig. 5).

[0108] This can be explained as follows: This allows additive depletion and subtractive depletion of the reactant to ideally balance each other out.

[0109] Fig. 3 shows an exemplary curve for the current I over time t for the flow time ti_ = AC period and also for 1.5 x AC period.

[0110] Figure 4 shows an example of the current I versus time t for the flow time ti_ > AC period. Only a small portion remains uncompensated. Therefore, the "lows" or "maxima" in the frequency spectrum of Figure 5 become smaller the higher the excitation frequency f.

[0111] Fig. 5 shows an example of a basic, ideal frequency response of a mass flow-related amplitude signal A (here shown as output signal level minus input signal level).

[0112] As Fig. 5 suggests, a main minimum and, if necessary, further minima can be searched for in the frequency spectrum of the amplitudes A, and their "depth" and position in the spectrum can be evaluated. From this, the flow time ti_ and / or local flow rates or flow velocities can then be derived.

[0113] The required pressure level and stoichiometries, as well as flow channel data, are typically known to the stack operator or can be estimated. However, even without much of this data, changes (such as minimum amplitude height and frequency) during operation and over aging can be evaluated and used to determine the condition.

[0114] Furthermore, a generalized impedance quantity - mass response signal amplitude / current amplitude - can also be taken into account, e.g. plotted in the form of a Bode plot.

[0115] Furthermore, phase information of the two reference quantities can also be taken into account. Using phase information, evaluations can be performed directly in the Nyquist plot, for example, on flow times away from the stack, including paths between the stack / cell and the analysis unit. a) For example, an impedance quantity can be determined based on sinusoidal signals:

[0116] I) Determination of the mass response signal amplitude (e.g. using frequency analysis and / or fitting),

[0117] II) Determination of the current amplitude (usually given by an AC source),

[0118] III) Calculating the mass response signal amplitude / current amplitude. b) For example, an impedance value can be determined based on sinusoidal signals:

[0119] I) Determination of an averaged and per period calculated M axi ma I measured value,

[0120] II) Determination of an averaged measurement value calculated per period,

[0121] III) Determination of an averaged maximum value of the current signal calculated per period, IV) Determination of an averaged mean value of the current signal calculated per period.

[0122] (rimax, average n itel, average) / (lmax, average I average, average) , with n - mass response signal (e.g. mass flow, molar flow or reactant concentration).

[0123] Furthermore, the following quantities can be compared and used to assess the condition of the electrochemical cell.

[0124] Ratios of maximum values ​​and / or mean values, even without the application of AC currents or voltages; e.g., across sensitively filtered frequencies or frequency bands during operation.

[0125] Magnitude values ​​and / or phase values ​​(with sufficient prior knowledge, the magnitude information is often sufficient to detect characteristic states).

[0126] The sampling frequency of the mass-related signals and current signals can be determined according to sampling theory.

[0127] The above description of the figures describes the present invention solely by way of example. Of course, individual features of the embodiments can be freely combined with one another, provided this is technically feasible, without departing from the scope of the invention.

Claims

Claims 1 . Method for examining at least one electrochemical cell, in particular at least one fuel cell (101), preferably a fuel cell stack (100), comprising: - applying a periodically changing signal (S) of an electrical quantity (I, U) to the at least one electrochemical cell, - detecting a periodically varying mass response signal (n) from the at least one fuel cell (101), - Assessing a state of the electrochemical cell depending on the detection.

2. The method according to claim 1, wherein a periodically changing current signal (I) is applied to the at least one electrochemical cell as a periodically changing signal (S) of an electrical quantity (I, U), and / or wherein a periodically changing voltage signal (U) is applied to the at least one electrochemical cell as a periodically changing signal (S) of an electrical quantity (I, U).

3. Method according to one of the preceding claims, wherein a mass flow and / or a molar flow is detected as a periodically varying mass response signal (n) from the at least one fuel cell (101), and / or wherein a concentration signal of a reactant, in particular oxygen-containing or fuel-containing reactant, is detected as a periodically varying mass response signal (n) from the at least one fuel cell (101), and / or wherein a concentration signal of a passive gas background is detected as a periodically varying mass response signal (n) from the at least one fuel cell (101).

4. Method according to one of the preceding claims, wherein when a periodically changing signal (S) of an electrical quantity (I, U) is applied to the at least one electrochemical cell, signals (Sf) of different excitation frequencies (f) are applied to the at least one electrochemical cell.

5. Method according to one of the preceding claims, wherein the periodically varying mass response signal (n) is sampled using frequencies which are determined as a function of the excitation frequencies (f) upon application of a periodically changing signal (S) of an electrical quantity (I, U).

6. Method according to one of the preceding claims, wherein, when assessing a state of the electrochemical cell, a period of the periodically changing signal (S) is taken into account in relation to a flow time (ti_) of the at least one fuel cell (101) through a reactant.

7. Method according to one of the preceding claims, if a period of the periodically changing signal (S) in relation to a flow time (ti_) of the at least one fuel cell (101) through a reactant results in an integer multiple, then a mass flow response approximately equal to zero and / or a minimum mass flow response is expected.

8. Method according to one of the preceding claims, wherein, when assessing a state of the electrochemical cell, a period in the frequency spectrum of the periodically varying mass response signal (n) is examined for maxima and minima.

9. Method according to one of the preceding claims, wherein when assessing a state of the electrochemical cell, an aging state and / or operating state of the electrochemical cell is / are assessed, and / or wherein when assessing a state of the electrochemical cell, flow times (ti_) and / or flow velocities are / are determined.

10. Method according to one of the preceding claims, wherein correlations, impedances, amplitude differences and / or phase shifts between the periodically changing signal (S) and the periodically varying mass response signal (n) are taken into account when assessing a state of the electrochemical cell.

11. A computer program product comprising instructions which, when executed by a computer, cause the computer to perform a method according to any one of the preceding claims.

12. Control unit, comprising a computing unit and a memory unit in which a code is stored which, when at least partially executed by the computing unit, carries out a method according to one of the preceding claims 1 to 10.

13. Control unit according to the preceding claim, wherein the control unit is at least partially implemented in a gas analysis unit (11, 21) for an oxygen-containing or fuel-containing reactant 14. Control unit according to the preceding claim, wherein the gas analysis unit (11, 21) is arranged adjacent to a stack outlet for an oxygen-containing or fuel-containing reactant, and / or wherein the gas analysis unit (11, 21) has a sensor for detecting the periodically varying mass response signal (n) from the at least one fuel cell (101), and / or wherein the gas analysis unit (11, 21) is designed to control an AC source in such a way as to apply the periodically changing signal (S) at an electrical quantity (I, U) to the at least one electrochemical cell, preferably via a plurality of selected excitation frequencies (f), which are preferably determinable as a function of a flow-through time (ti_) of the at least one fuel cell (101) by an oxygen-containing or fuel-containing reactant.

15. Electrochemical cell, in particular a fuel cell, preferably a PEM fuel cell, comprising a control unit according to the preceding claim.

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