A method for improving the accuracy of online testing of the electrochemical impedance of a fuel cell

Abstract

The application provides a method for improving the accuracy of on-line electrochemical impedance measurement of a fuel cell, comprising the following steps: continuously applying a high-low frequency superimposed excitation current to the fuel cell during the operation of the fuel cell, and then calculating the excitation current required for measuring the electrochemical impedance according to the real-time obtained response voltage. When the electrochemical impedance spectrum needs to be measured, the electrochemical impedance excitation current determined according to the response voltage is injected into the fuel cell, and finally the electrochemical impedance spectrum of the fuel cell is obtained. The application determines the excitation amplitude required for measuring the electrochemical impedance spectrum by monitoring the voltage response amplitude of the high-low frequency excitation in real time, thereby avoiding the state change of the fuel cell caused by excessive current excitation and the insufficient accuracy of the electrochemical impedance spectrum caused by the too small excitation current.

Description

A method to improve the accuracy of online electrochemical impedance spectroscopy for fuel cells Technical Field

[0001] This invention relates to the field of proton exchange membrane fuel cell technology, and discloses a method for improving the accuracy of online testing of the electrochemical impedance of fuel cells. Background Technology

[0002] Fuel cells, as a novel power source with high conversion efficiency, low noise, and zero pollution, have received widespread attention in recent years. As a power source, frequent load changes are unavoidable, and current fuel cells are prone to system failures such as flooding and dry membrane failure under frequent load variations. Prolonged exposure to such failures can severely impact the service life of fuel cells. Therefore, online monitoring of fuel cells is crucial. Among numerous online testing methods, electrochemical impedance spectroscopy (EIS) stands out due to its non-invasive nature and comprehensive information. Unlike voltage-excited impedance spectroscopy measurements for non-power source systems, EIS measurements for fuel cells require an excitation current to obtain the results, which is usually obtained empirically during testing. Generally, for EIS, the voltage change amplitude needs to be controlled around 10 mVpp. If the voltage amplitude is too high, the tested system may easily deviate from its current state, violating the requirements of non-invasive measurement; if the voltage amplitude is too low, it often approaches the measurement accuracy of the detection system, thus reducing the accuracy of EIS testing. Therefore, selecting a suitable excitation current amplitude is particularly important for fuel cells. However, the voltage response amplitude of a fuel cell varies under different operating conditions with the same excitation current, making it difficult to select the appropriate excitation current amplitude.

[0003] Existing patents CN 115144762 A and CN 114264881 A only use a pair of high-frequency and low-frequency values ​​to reflect the working state of the fuel cell, thus losing the advantage of rich information from electrochemical impedance spectroscopy. CN 109459465A and CN105449241B use excitation sources with other waveforms to obtain electrochemical impedance spectra, which makes data processing more difficult and requires certain computing resources. Summary of the Invention

[0004] In response to the technical problems mentioned in the background section, this invention provides a method to improve the accuracy of online testing of electrochemical impedance spectroscopy in fuel cells. This invention utilizes a superimposed excitation current of high and low frequencies as a probe, leveraging its advantages of fast detection speed and simple data processing to quickly determine the appropriate excitation current amplitude for measuring the electrochemical impedance spectroscopy, thereby controlling the response voltage within a reasonable range and increasing the accuracy of the fuel cell electrochemical impedance spectroscopy.

[0005] The technical means employed in this invention are as follows:

[0006] A method for improving the accuracy of online electrochemical impedance spectroscopy in fuel cells includes the following steps:

[0007] During fuel cell operation, a superimposed high- and low-frequency excitation current is applied to the fuel cell in real time through the load to obtain its response voltage, and the excitation current required to measure electrochemical impedance is calculated based on the response voltage. When it is necessary to measure electrochemical impedance, the superimposed high- and low-frequency excitation is converted into electrochemical impedance excitation. At the same time, current signals and voltage signals are collected by voltage monitoring devices and current detection devices, respectively, and the current signals and voltage signals are processed by corresponding signal conditioning circuits and corresponding analog-to-digital conversion units. After the data acquisition is completed, the electrochemical impedance spectrum of the fuel cell is obtained through the data processing unit.

[0008] Furthermore, the excitation current I HL Determined by the formula:

[0009]

[0010] Among them, A i Indicate; f h Indicates the high-frequency component; t represents; This indicates phase shift in the high-frequency portion; This indicates a phase shift in the low-frequency portion.

[0011] Furthermore, the excitation current required to measure the electrochemical impedance is calculated based on the response voltage:

[0012] First, the excitation amplitude A0 is obtained from the polarization curve in the database. Then, the excitation current is injected into the fuel cell, and the response voltage U0 of the fuel cell is obtained. Finally, it is determined whether this response voltage is within the required response voltage U0 range. n Within the range; if not, then the required response voltage range U. n median U n,mid According to the formula A1=A0(U n,mid After adjusting / U0), the excitation amplitude A1 is regained. Then, it is determined whether the response voltage U1 obtained from the new excitation current is within the range of the response voltage U. n Within the specified range; after repeated iterations, the excitation amplitude A that meets the conditions is finally obtained. f The excitation current I obtained by superimposing high and low frequencies is obtained. HL ;

[0013] Where U0 represents the initial response voltage, U n Indicates the required response voltage range, A1 represents the adjusted amplitude of the superimposed high and low frequency excitation current, U n,mid Indicates the midpoint of the required response voltage range, A fThis indicates the final amplitude of the superimposed high and low frequency excitation current after adjustment.

[0014] Furthermore, the waveform of the electrochemical impedance excitation current is determined according to the formula:

[0015]

[0016] The waveform is composed of multiple superimposed sine waves, where A is based on A f And the formula A = A f / n obtains; where I AC f represents the excitation current of electrochemical impedance. i This represents the frequency value of the i-th superimposed sine wave. This represents the phase offset value of the i-th sine wave, and n represents the number of sine waves that need to be superimposed.

[0017] Furthermore, the analog-to-digital conversion unit is equipped with two analog-to-digital converters; the analog-to-digital converters are for synchronous acquisition, with a sampling frequency of 10kHz-10MHz.

[0018] Furthermore, the data processing unit performs Fourier transform on the measured excitation current and voltage to obtain the current and voltage values ​​in the complex domain at each frequency. Then, it divides the obtained voltage and complex current in the complex domain to obtain the impedance value corresponding to each frequency. Connecting the impedance values ​​at each frequency yields the electrochemical impedance spectrum.

[0019] Furthermore, the polarization curve is the curve of load current versus fuel cell voltage measured under different operating conditions during fuel cell commissioning.

[0020] Furthermore, the current operating current is found in the polarization curve, and a value centered on the corresponding voltage point is found at U. n Voltage upper and lower limits U of width n,h and U n,l Then find U n,h and U n,l The corresponding current I n,h and I n,l Finally, through the formula A0 = (I n,l -I n,h A0 is obtained by ) / 2.

[0021] Furthermore, the response voltage U for a single fuel cell n The value range is 3-20mV; the required response voltage for a fuel cell stack composed of multiple fuel cells is determined by formula U. n,s =NU n Obtain; among which, U n,sThis indicates the required voltage for the fuel cell stack, and N represents the total number of fuel cell stacks.

[0022] Furthermore, the frequency to be varied in the electrochemical impedance is selected from both values, where the high-frequency f is... h The value range is 500-30kHz, and the low frequency f l The value range is 5-100Hz.

[0023] Compared with the prior art, the present invention has the following advantages:

[0024] Unlike conventional methods that use empirical values ​​to calculate the electrochemical impedance excitation current, this invention uses the superimposed excitation current of high and low frequencies as a probe to calculate the amplitude of the electrochemical impedance excitation current in real time during fuel cell operation.

[0025] This invention makes the entire testing process of electrochemical impedance spectroscopy traceable, ensuring that the excitation current amplitude required for electrochemical impedance spectroscopy is within an appropriate range. It avoids the problem of the fuel cell state deviating from the original detection point due to excessive excitation current amplitude, and also avoids the problem of insufficient accuracy of electrochemical impedance spectroscopy caused by weak fuel cell response due to excessive excitation current amplitude. Attached Figure Description

[0026] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0027] Figure 1 is a schematic diagram of the test system for measuring the electrochemical impedance spectroscopy of a fuel cell power system according to the present invention.

[0028] Figure 2 is a flowchart of the process of obtaining high and low frequency superimposed current excitation in this invention.

[0029] Figure 3 is a schematic diagram illustrating the principle of obtaining the amplitude of high and low frequency superimposed current excitation in this invention.

[0030] In Figure 1, 101 is the fuel cell or fuel cell stack under test, 102 is the voltage monitoring device, 103 is the voltage signal conditioning circuit, 104 is the voltage signal analog-to-digital conversion unit, 105 is the current signal conditioning circuit, 106 is the current signal analog-to-digital conversion unit, 107 is the current detection transposer, 108 is the fuel cell load, 109 is the data processing unit, 110 is the high- and low-frequency superimposed excitation current, 111 is the response voltage of the high- and low-frequency superimposed excitation current, 112 is the electrochemical impedance excitation current, and 113 is the calculated electrochemical impedance spectrum.

[0031] In Figure 2, 201 is the step for obtaining the amplitude of the superimposed high- and low-frequency excitation current, 202 is the step for obtaining the superimposed high- and low-frequency excitation current, 203 is the step for verifying the amplitude of the superimposed high- and low-frequency excitation current, 204 is the step for adjusting the superimposed high- and low-frequency excitation current, 205 is the step for adjusting the superimposed high- and low-frequency excitation current, 206 is the step for determining the amplitude of the superimposed high- and low-frequency excitation current, and 207 is the step for calculating the electrochemical impedance excitation current.

[0032] In Figure 3, 301 represents the polarization curve data under the current operating conditions in the database, and 302 represents the current operating current point. Detailed Implementation

[0033] To enable those skilled in the art to better understand the present invention, the technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of the present invention.

[0034] It should be noted that the terms "first," "second," etc., in the specification, claims, and accompanying drawings of this invention are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such data can be interchanged where appropriate so that the embodiments of the invention described herein can be implemented in orders other than those illustrated or described herein. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover a non-exclusive inclusion; for example, a process, method, system, product, or apparatus that comprises a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to such processes, methods, products, or apparatus.

[0035] As shown in Figures 1-3, this invention provides a method for improving the accuracy of online testing of electrochemical impedance in fuel cells, comprising the following steps:

[0036] During fuel cell operation, a superimposed high- and low-frequency excitation current is applied to the fuel cell in real time through the load to obtain its response voltage, and the excitation current required to measure electrochemical impedance is calculated based on the response voltage. When it is necessary to measure electrochemical impedance, the superimposed high- and low-frequency excitation is converted into electrochemical impedance excitation. At the same time, current signals and voltage signals are collected by voltage monitoring devices and current detection devices, respectively, and the current signals and voltage signals are processed by corresponding signal conditioning circuits and corresponding analog-to-digital conversion units. After the data acquisition is completed, the electrochemical impedance spectrum of the fuel cell is obtained through the data processing unit.

[0037] In a preferred embodiment, the excitation current I HL Determined by the formula:

[0038]

[0039] Among them, A i Indicate; f h Indicates the high-frequency component; t represents; This indicates phase shift in the high-frequency portion; This indicates a phase shift in the low-frequency portion.

[0040] Preferably, the step of calculating the excitation current required to measure the electrochemical impedance based on the response voltage is as follows:

[0041] First, the excitation amplitude A0 is obtained from the polarization curve in the database. Then, the excitation current is injected into the fuel cell, and the response voltage U0 of the fuel cell is obtained. Finally, it is determined whether this response voltage is within the required response voltage U0 range. n Within the range; if not, then the required response voltage range U. n median U n,mid According to the formula A1=A0(U n,mid After adjusting / U0), the excitation amplitude A1 is regained. Then, it is determined whether the response voltage U1 obtained from the new excitation current is within the range of the response voltage U. n Within the specified range; after repeated iterations, the excitation amplitude A that meets the conditions is finally obtained. f The excitation current I obtained by superimposing high and low frequencies is obtained. HL ;

[0042] Where U0 represents the initial response voltage, U n Indicates the required response voltage range, A1 represents the adjusted amplitude of the superimposed high and low frequency excitation current, U n,mid Indicates the midpoint of the required response voltage range, A f This indicates the final amplitude of the superimposed high and low frequency excitation current after adjustment.

[0043] In a preferred embodiment, the waveform of the electrochemical impedance excitation current is determined according to the formula:

[0044]

[0045] The waveform is composed of multiple superimposed sine waves, where A is based on A f And the formula A = A f / n obtains; where I AC f represents the excitation current of electrochemical impedance. iThis represents the frequency value of the i-th superimposed sine wave. This represents the phase offset value of the i-th sine wave, and n represents the number of sine waves that need to be superimposed.

[0046] In a preferred embodiment, the analog-to-digital conversion unit is provided with two analog-to-digital converters; the analog-to-digital converters are for synchronous acquisition, and the sampling frequency is 10kHz-10MHz.

[0047] In a preferred embodiment, the data processing unit performs a Fourier transform on the measured excitation current and voltage to obtain the current and voltage values ​​in the complex domain at each frequency. Then, it divides the obtained voltage and complex current in the complex domain to obtain the impedance value corresponding to each frequency. Connecting the impedance values ​​at each frequency yields the electrochemical impedance spectrum.

[0048] In a preferred embodiment, the polarization curve is the curve of load current versus fuel cell voltage measured under different operating conditions during fuel cell commissioning.

[0049] In a preferred embodiment, the current operating current is found in the polarization curve, and a voltage point centered on the corresponding voltage point is found with respect to U. n Voltage upper and lower limits U of width n,h and U n,l Then find U n,h and U n,l The corresponding current I n,h and I n,l Finally, through the formula A0 = (I n,l -I n,h A0 is obtained by ) / 2.

[0050] Preferably, the response voltage U for a single fuel cell n The value range is 3-20mV; the required response voltage for a fuel cell stack composed of multiple fuel cells is determined by formula U. n,s =NU n Obtain; among which, U n,s This indicates the required voltage for the fuel cell stack, and N represents the total number of fuel cell stacks.

[0051] Preferably, in this application, the frequency to be varied in the electrochemical impedance is a value selected from both, wherein the high-frequency f h The value range is 500-30kHz, and the low frequency f l The value range is 5-100Hz. As a preferred embodiment, in this application, the fuel cell can be a single fuel cell or a fuel cell stack composed of multiple cells connected in series.

[0052] Example 1

[0053] This invention increases the accuracy of electrochemical impedance spectroscopy by adding high- and low-frequency superimposed excitation before testing electrochemical impedance spectroscopy, and using the response voltage and excitation amplitude as important bases for determining the excitation amplitude of electrochemical impedance spectroscopy.

[0054] For a 100-cell fuel cell stack with a rated current of 300A, the electrochemical impedance of the stack and the 20th sub-cell were tested at a load current of 100A. The corresponding voltage for this load current in the database was 75V. According to claim 8, the required response voltage U... n The voltage range is 3-20mV, and in this embodiment, the response voltage range is selected as 8-15mV, therefore the corresponding U... n,s It is 0.8-1.5V.

[0055] Determine the initial value A0 of the amplitude of the superimposed high and low frequency excitation current. According to claim 7, the midpoint of the required response voltage is 75V, and the upper and lower voltage limits U... n,h and U n,l The voltages are 74.65V and 75.35V respectively, and the corresponding I in the database is... n,h and I n,l The values ​​are 96A and 101.5A respectively, so the initial amplitude of the superimposed high and low frequency excitation current is 2.75A.

[0056] The high-frequency and low-frequency components of the superimposed high- and low-frequency excitation current are determined. In this embodiment, the high-frequency frequency is selected as 1 kHz and the low-frequency frequency as 50 Hz, with a phase shift of zero. This yields the initial waveform I of the superimposed high- and low-frequency excitation current. HL =2.75[sin(2000πt)+sin(100πt)].

[0057] An initial high- and low-frequency superimposed excitation current is injected into the fuel cell stack 101. The response voltage U0 = 1V of the current excitation is obtained through the voltage monitoring device 102, the voltage signal conditioning circuit 103, and the voltage signal analog-to-electrical conversion unit 104. Since it is within the predetermined voltage response range, A0 = A f =2.75A.

[0058] According to claim 8, the required electrochemical impedance excitation current is determined. In this embodiment, an excitation signal consisting of 30 superimposed sinusoidal signals will be injected into the fuel cell stack. According to the formula, the waveform of the excitation current signal is as follows:

[0059] When it is necessary to measure the electrochemical impedance spectroscopy, first adjust and maintain the inspection device 102 at the measurement of the total voltage of the fuel cell stack, and then remove the high and low frequency superimposed excitation current I. HL Replace with electrochemical impedance excitation current IAC The voltage monitoring device 102 and the current detection device 107 successively transmit the voltage and current signals to their respective signal conditioning circuits 103 and 105 and analog-to-digital conversion units 104 and 106, and finally enter the data processing unit 109.

[0060] In this embodiment, the sampling frequency of the digital-to-analog converter is 20 kHz.

[0061] Perform Fourier transforms on the measured excitation current and stack voltage to obtain the current and voltage values ​​in the complex domain at each frequency. Then, divide the obtained voltage and complex current in the complex domain to obtain the impedance value corresponding to each frequency. Connect the impedance values ​​of each frequency to obtain the electrochemical impedance spectrum 113. Adjust the voltage monitoring device to the voltage of the 20th section to be measured and maintain the monitoring. Obtain the electrochemical impedance spectrum of the 20th section according to the above steps.

[0062] After the test, adjust the voltage monitoring device to normal operation, change the electrochemical impedance excitation current on the load to a superimposed high and low frequency excitation current, and maintain A. f Real-time updates.

[0063] The sequence numbers of the above embodiments of the present invention are for descriptive purposes only and do not represent the superiority or inferiority of the embodiments. In the above embodiments of the present invention, the descriptions of each embodiment have their own emphasis; parts not described in detail in a certain embodiment can be referred to in the relevant descriptions of other embodiments. It should be understood that the disclosed technical content in the several embodiments provided in this application can be implemented in other ways.

[0064] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for improving the accuracy of online testing of electrochemical impedance spectroscopy in fuel cells, characterized in that, Includes the following steps: During fuel cell operation, a superimposed high- and low-frequency excitation current is applied to the fuel cell in real time through the load to obtain its response voltage. The excitation current required to measure electrochemical impedance is calculated based on the response voltage. When electrochemical impedance needs to be measured, the superimposed high- and low-frequency excitation is converted into an electrochemical impedance excitation. Simultaneously, current and voltage signals are collected by a voltage monitoring device and a current detection device, respectively. The analog current and analog voltage signals are then processed by corresponding signal conditioning circuits and analog-to-digital conversion units to obtain digital signal data of voltage and current. After data acquisition, the electrochemical impedance spectrum of the fuel cell is obtained through a data processing unit. The calculation of the excitation current required to measure electrochemical impedance based on the response voltage involves: first, obtaining the excitation amplitude A0 from the polarization curve in the database; injecting the excitation current into the fuel cell and obtaining the fuel cell's response voltage U0; and then determining whether the response voltage is within the required response voltage U0. n Within the range; if not, then the required response voltage range U. n median According to the formula After adjustment, the excitation amplitude A1 is regained, and then it is determined whether the response voltage U1 obtained from the new excitation current is within the range of the response voltage U. n Within the desired range; after repeated iterations, the excitation amplitude of the response voltage within the desired response voltage range is finally obtained. The excitation current of high and low frequency superposition is obtained. ;in U0 represents the initial response voltage, U n This indicates the required response voltage range, and A1 represents the adjusted amplitude of the superimposed high and low frequency excitation current. This represents the midpoint of the required response voltage range. This indicates the final amplitude of the superimposed high and low frequency excitation current after adjustment.

2. The method for improving the accuracy of online testing of electrochemical impedance spectroscopy in fuel cells according to claim 1, characterized in that, The excitation current Determined by the formula: ;in, This indicates the amplitude of the superimposed high and low frequency excitation. Indicates the frequency of the high-frequency component; express; This indicates phase shift in the high-frequency portion; This indicates a phase shift in the low-frequency range; Indicates low frequency.

3. The method for improving the accuracy of online testing of electrochemical impedance spectroscopy in fuel cells according to claim 1, characterized in that, The waveform of the electrochemical impedance excitation current is based on the formula: ； The waveform is composed of multiple superimposed sine waves, where A is based on A f and formula Obtain; among them Indicates the excitation current of electrochemical impedance. This represents the frequency value of the i-th superimposed sine wave. This represents the phase offset value of the i-th sine wave, and n represents the number of sine waves that need to be superimposed.

4. The method for improving the accuracy of online testing of electrochemical impedance spectroscopy in fuel cells according to claim 1, characterized in that, The analog-to-digital conversion unit is equipped with two analog-to-digital converters; the analog-to-digital converters are for synchronous acquisition, and the sampling frequency is 10kHz-10MHz.

5. The method for improving the accuracy of online testing of electrochemical impedance spectroscopy in fuel cells according to claim 1, characterized in that, The data processing unit performs Fourier transform on the measured excitation current and voltage to obtain the current and voltage values ​​in the complex domain at each frequency. Then, it divides the obtained voltage and complex current in the complex domain to obtain the impedance value corresponding to each frequency. Connecting the impedance values ​​at each frequency yields the electrochemical impedance spectrum.

6. The method for improving the accuracy of online testing of electrochemical impedance spectroscopy in fuel cells according to claim 1, characterized in that, The polarization curve is the curve of load current versus fuel cell voltage measured under different operating conditions during fuel cell commissioning.

7. The method for improving the accuracy of online testing of electrochemical impedance spectroscopy in fuel cells according to claim 1, characterized in that, Find the current operating current in the polarization curve, and find the voltage point centered at U. n Voltage upper and lower limits U of width n,h and U n,l Then find U n,h and U n,l The corresponding current I n,h and I n,l Finally, through the formula A0 was obtained.

8. The method for improving the accuracy of online testing of electrochemical impedance spectroscopy in fuel cells according to claim 1, characterized in that, The response voltage U for a single-cell fuel cell n The value range is 3-20mV; the required response voltage for a fuel cell stack composed of multiple fuel cells is determined by formula U. n,s =NU n Obtain; among which, U n,s This indicates the required voltage for the fuel cell stack, and N represents the total number of fuel cell stacks.

9. The method for improving the accuracy of online testing of electrochemical impedance spectroscopy in fuel cells according to claim 1, characterized in that, The frequency to be varied in the electrochemical impedance is selected from two values, among which the high frequency is... The value range is 500-30kHz, low frequency. The value range is 5-100 Hz.

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