Method and device for detecting the state of an electrochemical system

Abstract

The invention relates to a method for detecting the state of an electrochemical system (1), in particular a fuel cell, by means of electrochemical impedance spectroscopy, wherein the electrochemical system (1) is excited by a current signal (I(t)) and a voltage response (V(t)) of the electrochemical system (1) is detected, wherein a plurality of predetermined sinusoidal signals (F1, F2, F3, F4) following a respective profile and each having a predetermined individual frequency (f1, f2, f3, f4), a predetermined amplitude (a1, a2, a3, a4) and a predetermined initial phase (φ1, φ2, φ3, φ4) are superimposed to form the current signal (I(t)), the respective impedances (Z1, Z2, Z3, Z4) for the individual frequencies (f1, f2, f3, f4) are determined from the current signal (I(t)) and the associated voltage response (V(t)), and the state of the electrochemical system (1) is determined from the impedances (Z1, Z2, Z3, Z4) of the individual frequencies (f1, f2, f3, f4).

Description

[0001] Method and device for detecting the state of an electrochemical system

[0002] Description:

[0003] The invention relates to a method and a device for detecting the state of an electrochemical system, in particular a fuel cell, wherein the method and the device can particularly preferably also be used in vehicles. State detection of electrochemical systems by so-called electrochemical impedance spectroscopy has long been known in the art. In this case, the impedance, i.e. the alternating current resistance, of such systems is detected as a function of the frequency of an alternating voltage or an alternating current, and the state of the system is deduced from the impedance, since the impedance allows conclusions to be drawn about the processes taking place in the system and, for example, also about the resistance of an electrolyte used in the system.

[0004] Methods for detecting the state of electrochemical systems are known, for example, from the documents CN 111007404 A, CN 105449241 A and US 2016 / 0149240 A1, whereby different, for example triangular or segmented signals at different frequencies are used for this purpose.

[0005] However, it is sometimes not possible to record the state during regular operation of the electrochemical system, and in addition, the state recording is comparatively time-consuming and energy-intensive.

[0006] The invention is therefore based on the object of overcoming the aforementioned disadvantages and providing a method and a device by means of which the state of an electrochemical system, in particular a fuel cell, can be detected quickly and efficiently.

[0007] This problem is solved by the combination of features according to patent claim 1 as well as by the features of the independent claim.

[0008] According to the invention, a method for detecting the state of an electrochemical system, in particular a fuel cell and more particularly a PEMFC, using electrochemical impedance spectroscopy is therefore proposed. As is usual with electrochemical impedance spectroscopy, the electrochemical system is generally excited with a current signal, and a voltage response of the electrochemical system is recorded. However, according to the present invention, a plurality of predetermined sinusoidal signals, each following a respective course and each having a predetermined individual frequency, a predetermined amplitude, and a predetermined initial phase, are superimposed on the current signal. After the electrochemical system has been excited with the current signal or the current signal has been impressed on the system, the system generates a voltage response associated with the current signal, which is then recorded.The respective impedances for the individual frequencies are determined from the current signal and the corresponding voltage response, so that an impedance is known for each individual frequency. Once the impedances have been determined, the state of the electrochemical system can be determined from the impedances of the individual frequencies.

[0009] This and how the state of the system can be determined from the impedances is well known in the art and will therefore not be explained in detail. It is sufficient to note that the impedances can be used to draw conclusions about the state of the system and, in the case of a fuel cell, for example, about the state of a membrane or electrolyte used.

[0010] The method proposed according to the invention allows several impedances for several frequencies to be determined simultaneously, which has several advantages.

[0011] For example, instead of several consecutive measurements, a single measurement of the voltage response is sufficient, resulting in a correspondingly shorter measurement time. This leads to energy and cost savings. Consequently, disruptions to the load supply by the system or fuel cell are also fewer and shorter.

[0012] The sinusoidal signals, including their individual frequencies, amplitudes, and initial phases, can also be freely determined, thus taking specific parameters of the electrochemical system into account. In principle, the number of sinusoidal signals can also be freely determined, as long as it exceeds two, since at least two sinusoidal signals must always be superimposed. Since only one voltage response needs to be processed, the computational load for determining the impedances is also reduced, allowing the entire process to be performed on low-performance computing units, further reducing costs as well as space and energy consumption.

[0013] According to an advantageous variant of the method, the individual frequencies of the sinusoidal signals are different from one another. The amplitudes as well as the initial phases of the sinusoidal signals can also be different from one another or, alternatively, the same.

[0014] For the superposition of the sinusoidal signals, it is preferably provided that they are added together to form the current signal. The resulting current signal can then be normalized to a predetermined value range, whereby normalization is understood to mean adjusting or scaling the amplitude to a predetermined range.

[0015] According to a further development of the method, the current signal is transformed from its time domain to its frequency domain, preferably by Fourier transformation and more preferably by fast Fourier transformation (FFT). In addition, a respective excitation magnitude, which corresponds to the magnitude of the amplitude, as well as a respective associated excitation phase or excitation initial phase can then be determined for each individual frequency.

[0016] If the values ​​of the individual frequencies, the respective amplitudes, and thus also the excitation magnitudes, as well as the excitation phases or initial phases of the sinusoidal signals, are already known, this is not absolutely necessary, as these values ​​can then be stored, for example, in an evaluation unit mentioned below. Alternatively, these values ​​can be determined once using the aforementioned transformation into the frequency domain and then stored, so this is not necessary for every state determination.

[0017] Furthermore, the voltage response can also be transformed from its time domain to its frequency domain, preferably using Fourier transformation and even more preferably using fast Fourier transformation (FFT). In addition, a respective response magnitude, which in turn corresponds to the magnitude of the amplitude, as well as a corresponding response phase or initial response phase, can be determined for each individual frequency. Using FFT, this can be done almost in real time.

[0018] From the preferably previously known individual frequencies with the associated excitation magnitudes and excitation phases and the response magnitudes and response phases determined from the voltage response and assignable to the individual frequencies, an associated impedance can then be determined for each of the individual frequencies, in particular through the phase shifts.

[0019] Furthermore, the impedances can be plotted as a Nyuist plot or alternatively or additionally as a Bode plot, so that the state of the electrochemical system can be evaluated or conclusions can be drawn about its state based on this.

[0020] It is particularly preferred that the electrochemical system is excited with the current signal and the voltage response of the electrochemical system is detected while the electrochemical system supplies a load with electrical energy.

[0021] A further aspect of the invention relates to a device for detecting the state of an electrochemical system, in particular a fuel cell, by means of electrochemical impedance spectroscopy, which is preferably designed to carry out the method according to the invention. The device has a signal generator which, for exciting the electrochemical system or the fuel cell, is connected to the electrochemical system or the fuel cell and is designed to superimpose a plurality of predetermined sinusoidal signals following a respective course, each with a predetermined individual frequency and a predetermined amplitude, to form a current signal for exciting the electrochemical system and / or to excite the electrochemical system with such a current signal.Furthermore, a measuring device which is designed to detect a voltage response generated by the excitation with the current signal by the electrochemical system, as well as an evaluation device which is designed to determine the respective impedances for the individual frequencies from the current signal and the associated voltage response and to determine the state of the electrochemical system from the impedances of the individual frequencies, are part of the device.

[0022] As already described, the evaluation device does not necessarily have to evaluate the current signal itself or transform it into the frequency domain. Rather, the current signal can be transformed into its frequency domain and retrieved by the evaluation device or stored in the evaluation device, so that the excitation magnitudes and excitation phases associated with the individual frequencies are immediately known.

[0023] Furthermore, the evaluation device is preferably designed to transform the voltage response from its time domain into its frequency domain and thereby to determine the response magnitudes and response phases belonging to the individual frequencies.

[0024] The features disclosed above can be combined in any way, as long as this is technically possible and they do not contradict each other. Other advantageous developments of the invention are characterized in the subclaims or are presented in more detail below, together with the description of the preferred embodiment of the invention, with reference to the figures. They show:

[0025] Fig. 1 shows a first system with a device for detecting the state of a fuel cell;

[0026] Fig. 2 shows a second system with a device for detecting the state of a fuel cell;

[0027] Fig. 3 schematic addition of a plurality of predetermined sinusoidal signals to a current signal;

[0028] Fig. 4 schematic recording and evaluation of a voltage response;

[0029] Fig. 5 Procedure for determining the state of an electrochemical system.

[0030] The figures are schematic examples. Identical reference numerals in the figures indicate identical functional and / or structural features.

[0031] Figure 1 shows a schematic and simplified structure of a system comprising a fuel cell 1 or an electrochemical system 1 and a load 2, which is supplied with electrical energy by the fuel cell 1. According to the invention, a device 10 for detecting the state of the fuel cell 1 is also provided, which allows conclusions to be drawn, in particular, about the state of an electrolyte used or a membrane used in the fuel cell 1.

[0032] Such a device 10 essentially comprises a signal generator 11, a measuring device 12, and an evaluation device 13. The signal generator 11 serves to excite the fuel cell 1 with a current signal l(t), so that the current signal l(t) induces a voltage response V(t) in the fuel cell 1, which is detected by the measuring device 12 and evaluated by the evaluation device 13.

[0033] The excitation of the fuel cell 1 by the signal generator 11 can take place during regular operation of the fuel cell 1, i.e. while the latter supplies the load 2 with electrical energy.

[0034] As explained below, the current signal l(t) is a superposition or addition of several sinusoidal signals F1, F2, F3, F4, wherein the signal generator can be designed to add or superimpose the individual sinusoidal signals F1, F2, F3, F4 itself or simply to generate the current signal l(t) already predetermined thereby and to feed it into the fuel cell 1.

[0035] Using the example of a drive train of a vehicle, the load 2 is shown in more detail in Figure 2, whereby the description for Figure 1 applies to the fuel cell 1 and the device 10 for detecting the state of the fuel cell 1.

[0036] In a vehicle, the load 2 can therefore be composed, for example and not exclusively, of an inverter 3, a motor 4, a DC / DC converter 5 and a battery 6.

[0037] Figure 3 shows a simplified illustration of the addition of the individual sinusoidal signals F1, F2, F3, F4 with their respective frequencies f1, f2, f3, f4 to the current signal l(t) serving as the input signal. The current signal l(t) can then be transformed into the frequency domain, preferably via FFT, and the magnitudes Mi, ie the amount of the amplitudes a1, a2, a3, a4, as well as the initial phases epi of the individual sinusoidal signals F1, F2, F3, F4 for the respective frequencies f1, f2, f3, f4 can be determined. If the sinusoidal signals F1, F2, F3, F4 or their frequencies f1, f2, f3, f4, amplitudes a1, a2, a3, a4 and initial phases <p1 , <p2, <p3, <p4 ohnehin bekannt, ist die Transformation in den Frequenzbereich und die Ermittlung dieser Werte nicht notwendig und kann entfallen.

[0038] Figure 4 shows a schematic representation of an important aspect of the method, with the individual steps being explained in more detail using the sequence shown in Figure 5 as an example.

[0039] Referring to Figure 4, in step G, the fuel cell 1 is excited with the current signal l(t). Subsequently, in step H, the voltage response V(t) is measured or determined, which is then transformed into its frequency domain in step J, preferably in near real time, via FFT.

[0040] This will determine the individual response magnitudes Mo and the response phases <po für die einzelnen Frequenzen f1 , f2, f3, f4 ermittelt.

[0041] From the excitation magnitudes Mi, the excitation phases epi, the response magnitudes Mo and the response phases cpo, the impedances Z1, Z2, Z3, Z4 are then determined as complex values ​​in step L.

[0042] A complete yet exemplary process flow is shown in Figure 5. According to this, the process comprises the following steps:

[0043] A Selection of the type and number of sinusoidal signals with a respective amplitude a, frequency f and initial phase cp, ie for example for four sinusoidal signals:

[0044] F1 with [a1 , f 1 , <p1 ]

[0045] F2 with [a2, f2, q>2]

[0046] F3 with [a3, f3, <p3]

[0047] F4 with [a4, f4, <p4]; B Erzeugen der einzelnen Sinussignale

[0048] F1 (t) = a1 ■ sin(2iT f1 + q>1 )

[0049] F2(t) = a2 ■ sin(2iT ■ f2 + q>2)

[0050] F3(t) = a3 ■ sin(2rr ■ f3 + <p3)

[0051] F4(t) = a4 ■ sin(2rr ■ f4 + <p4)

[0052] C Generating the current signal l(t) directly or by addition l(t) = [F1(t) + F2(t) + F3(t) + F4(t)] / 4

[0053] D If the current signal l(t) has already been converted into the frequency domain, particularly via FFT, or the relevant values ​​(frequencies f 1 , f2, f3, f4; excitation magnitudes Mi; excitation phases epi) are already available, continue with step G, otherwise step E

[0054] E Transformation of the current signal l(t) into the frequency domain, in particular by fast Fourier transformation (FFT)

[0055] F Determination of the complex values ​​of the current signal l(t) for the individual frequencies f1 , f2, f3, f4

[0056] G Excitation of fuel cell 1 with the current signal l(t)

[0057] H Measuring or recording the voltage response V(t)

[0058] J Transformation of the voltage response V(t) into the frequency domain, in particular by fast Fourier transformation (FFT)

[0059] K Determination of the complex values ​​of the voltage response V(t) for the individual frequencies f1, f2, f3, f4

[0060] V1 = |V1 |eje1 , V2 = |V2|e je2 , V3 = |V3|eJ e3 , I4 = |V4|eJ e4 L Determine the impedances for each of the individual frequencies f1, f2, f3, f4

[0061] M Optional immediate evaluation or evaluation via Nyquist plot (Z1 (f1 ), Z2(f2), Z3(f3), Z4(f4))

[0062] Such a Nyquist plot 7 is shown as an example and in addition in Figure 5, wherein it can be seen that the plot allows direct conclusions to be drawn about an equivalent circuit diagram 8 of the fuel cell 1 or about the fuel cell 1. The Nyquist plot 7 can also be considered independently of the exemplary process sequence, and the process sequence can be considered independently of the Nyquist plot 7.

Claims

Patent claims 1 . Method for detecting the state of an electrochemical system (1 ), in particular a fuel cell, by means of electrochemical impedance spectroscopy, wherein the electrochemical system (1 ) is excited with a current signal (l(t)) and a voltage response (V(t)) of the electrochemical system (1 ) is detected, characterized in that a plurality of predetermined sinusoidal signals (F1, F2, F3, F4) following a respective course with a respective predetermined individual frequency (f 1, f2, f3, f4), a respective predetermined amplitude (a1, a2, a3, a4) and a respective predetermined initial phase ( <p1 , <p2, cp3, <p4) zu dem Stromsignal (l(t)) überlagert werden, aus dem Stromsignal (l(t)) und der zugehörigen Spannungsantwort (V(t)) die jeweiligen Impedanzen (Z1 , Z2, Z3, Z4) für die Einzelfrequenzen (f 1 , f2, f3, f4) bestimmt werden und aus den Impedanzen (Z1 , Z2, Z3, Z4) der Einzelfrequenzen (f 1 , f2, f3, f4) der Zustand des elektrochemischen Systems (1 ) bestimmt wird.

2. Method according to claim 1, wherein the individual frequencies (f 1 , f2, f3, f4) of the sinusoidal signals (F1 , F2, F3, F4) are different from one another and / or the amplitudes (a1 , a2, a3, a4) of the sinusoidal signals (F1 , F2, F3, F4) are the same or different from one another and / or the initial phases ( <p1 , <p2, cp3, <p4) der Sinussignale (F1 , F2, F3, F4) gleich oder zueinander verschieden sind.

3. Method according to claim 1 or 2, wherein the sinusoidal signals (F1, F2, F3, F4) are combined with each other to form the current signal (l(t)) is added and / or the current signal (l(t)) is normalized to a predetermined range of values.

4. Method according to one of the preceding claims, wherein the current signal (l(t)) is transformed from its time domain into its frequency domain and / or a respective excitation magnitude (Mi) and a respective associated excitation phase (epi) is determined for each individual frequency (f1, f2, f3, f4).

5. Method according to one of the preceding claims, wherein the voltage response (V(t)) is transformed from its time domain into its frequency domain and / or a respective response magnitude (Mo) and a respective associated response phase (epo) are determined for each individual frequency (f 1 , f2, f3, f4).

6. Method according to the two preceding claims, wherein the respective impedance (Z1, Z2, Z3, Z4) is determined from a respective excitation magnitude (Mi), a respective response magnitude (Mo), a respective excitation phase (epi) and a respective response phase (epo) for each individual frequency (f1, f2, f3, f4).

7. Method according to one of the preceding claims, wherein the impedances (Z1, Z2, Z3, Z4) are plotted as a Nyuist plot and / or Bode plot and / or the state of the electrochemical system (1) is evaluated therefrom.

8. Method according to one of the preceding claims, wherein the electrochemical system (1 ) is excited with the current signal (l(t)) and the voltage response (V(t)) of the electrochemical system (1 ) is recorded while the electrochemical system tem (1 ) supplies a load (2) with electrical energy.

9. Device (10) for detecting the state of an electrochemical system (1), in particular a fuel cell, by means of electrochemical impedance spectroscopy, with a signal generator (11) which is connected to the electrochemical system (1) for exciting the electrochemical system (1) and is designed to superimpose a plurality of predetermined sinusoidal signals (F1, F2, F3, F4) following a respective course, each with a predetermined individual frequency (f1, f2, f3, f4) and a predetermined amplitude (a1, a2, a3, a4) to form a current signal (l(t)) for exciting the electrochemical system (1) and / or to excite the electrochemical system with such a current signal (l(t)), with a measuring device (12) which is designed to detect a voltage response (V(t)) generated by the electrochemical system (1) by the excitation with the current signal (l(t)), and with a Evaluation device (13) which is designedto determine the respective impedances for the individual frequencies from the current signal (l(t)) and the associated voltage response (V(t)) and to determine the state of the electrochemical system (1) from the impedances (Z1, Z2, Z3, Z4) of the individual frequencies (f1, f2, f3, f4).

10. Device according to the preceding claim, wherein the current signal (l(t)) is transformed into its frequency domain and can be called up by the evaluation device (13) or stored in the evaluation device (13) and / or the evaluation device (13) is designed to transform the voltage response (V(t)) from its time domain into its frequency domain. * * * * *

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