A method for analyzing dynamic performance attenuation of a fuel cell based on electrochemical impedance spectrum

Abstract

The application discloses a fuel cell dynamic performance attenuation analysis method based on electrochemical impedance spectroscopy, considers the establishment and fitting process of the impedance spectrum equivalent circuit of low-frequency inductive process, and comprises the following steps: establishing the impedance spectrum equivalent circuit model considering the low-frequency inductive process through the relaxation time distribution, and determining the initial value fitting method of the inductive part circuit through the negative peak area in the peak of the relaxation time distribution; using the quantification method of the influence of the water combination of the ionomer on the dynamic response, the quantification index and calculation method of the influence of the platinum oxide on the dynamic response; finally obtaining the analysis results of the membrane resistance, the proton transport resistance and the influence of the platinum oxide on the dynamic performance attenuation. The application is used for the dynamic performance attenuation behavior analysis, so as to make up for the deficiency of the prior art in the analysis of the dynamic response, mainly identifies the relationship between the internal membrane resistance, the proton transport resistance and the current density based on the electrochemical impedance spectroscopy, and quantifies the influence of the water combination of the ionomer and the platinum catalyst oxidation on the voltage dynamic response.

Description

Technical Field

[0001] This invention belongs to the field of fuel cell technology, and in particular relates to a method for analyzing the dynamic performance degradation of fuel cells based on electrochemical impedance spectroscopy. Background Technology

[0002] Clean and renewable energy is a crucial approach to solving energy and climate issues. Proton exchange membrane fuel cells (PEMFCs) offer advantages such as being pollution-free, highly efficient, and quick to start, making them a significant alternative to traditional internal combustion engines. However, the voltage of PEMFCs varies with power demands and operating conditions, complicating health assessment. Furthermore, fuel cells experience performance degradation and dynamic response changes during long-term operation, leading to control and management system failures and ultimately impacting the safe and reliable operation of the system. Therefore, it is essential to investigate the relationship between dynamic response and internal physical process degradation, which is crucial for optimized management and lifespan extension.

[0003] Revealing the relationship between the dynamic response characteristics of fuel cells and their internal physical processes, especially determining the quantitative relationships of the mechanisms leading to dynamic response, is crucial for health assessment and lifespan extension. Typically, the dynamic response of a fuel cell is divided into two scenarios: loading and unloading. During current density loading, the main characteristic is a rapid voltage drop to a minimum value, followed by a slow increase to a stable value. Similarly, during unloading, the voltage rapidly increases to a maximum value, followed by a slow decrease to a stable value. This phenomenon in dynamic response is caused by slow accompanying processes during load changes, mainly including: ionomer hydration dynamics in the membrane and catalyst layers, oxygen transport in the diffusion layer, and the formation and reduction of platinum oxide. Existing studies mostly focus on one mechanism at a time, without combining experimental data to explore the quantification of the contribution of each mechanism and its decay behavior. Current research methods for dynamic response mainly include: numerical simulation, experimental analysis, and single-frequency impedance monitoring. Numerical models can reveal the dynamic changes in fuel cell voltage and internal state, but model building requires numerous assumptions. Numerical simulation methods require prior knowledge of the physical equations involved in the dynamic response, making modeling complex and time-consuming.

[0004] Experimental analysis primarily investigates the influence of external factors (temperature, pressure, humidity, stoichiometry, etc.) on dynamic voltage, but cannot reflect internal state changes. Experimental testing methods generally measure the dynamic response of voltage during load variations under different operating conditions, but cannot reflect changes in internal polarization processes.

[0005] Multiple single-frequency tests during a dynamic process can monitor changes in internal polarization during the dynamic response; however, studies have shown that changes in the internal state shift the characteristic frequency of the impedance spectrum, making fixed-frequency impedance methods somewhat limited. The fixed-frequency impedance method may affect the accuracy of internal polarization assessment due to changes in the characteristic frequency. Summary of the Invention

[0006] To address the aforementioned issues, this invention proposes a dynamic performance degradation analysis method for fuel cells based on electrochemical impedance spectroscopy. This method is used to analyze dynamic performance degradation behavior and overcomes the shortcomings of existing technologies in analyzing dynamic responses. It mainly uses electrochemical impedance spectroscopy to determine the relationship between internal membrane resistance and proton transport resistance and current density, and quantifies the influence of ionomer hydration and platinum catalyst oxidation on voltage dynamic response.

[0007] To achieve the above objectives, the technical solution adopted by this invention is: a method for analyzing the dynamic performance degradation of fuel cells based on electrochemical impedance spectroscopy, comprising the following steps: Step 1: Test the dynamic response data of the fuel cell voltage; Step 2: Calculate the fuel cell polarization curve based on the results of Step 1, and test the electrochemical impedance spectroscopy representing the activation, ohmic, and concentration regions based on the polarization curve. Step 3: Conduct long-term durability tests on the fuel cell. Repeat steps 1 to 2 after each preset time interval of durability test to obtain voltage dynamic response data and impedance spectrum data at different times. The working current density of the impedance spectrum test is kept consistent with that in step 2. Step 4: Calculate the relaxation time distribution function of the impedance spectrum; Step 5: Establish an equivalent circuit model based on the number of peaks corresponding to the relaxation time distribution function; Step 6: Calculate the resistance value corresponding to each peak based on the relaxation time distribution function; Step 7: Based on the polarization curve calculated in Step 2, calculate the differential resistance at the corresponding current density as the impedance value point with a frequency of 0. Use the polarization process resistance value corresponding to each peak calculated in Step 5 as the initial value for equivalent circuit fitting to perform impedance spectrum fitting, and obtain the membrane resistance and proton transport resistance in the catalyst layer. Step 8: Select a new current density and repeat steps 4 to 7 until the membrane resistance and catalyst layer proton transport resistance values ​​are obtained at all current densities. Step 9: Fit the functional relationships between membrane resistance and proton transport resistance and current density, respectively, and calculate the contributions of membrane resistance and proton transport resistance to the dynamic response under different current densities. Step 10: Based on the test data from Step 1, calculate the overall dynamic response index Tr for different current densities to eliminate the influence of load variation on the dynamic response. Step 11: Subtract the contributions of membrane resistance and proton transport resistance from the total dynamic response index to obtain the contribution of platinum oxide dynamic change to the dynamic response. Step 12: Select data from different times in Step 3 and repeat Steps 4 to 11 until the quantification results of the contribution of each dynamic response under different decay states are obtained. Step 13: Plot the relationship between the contribution of membrane resistance and proton resistance to dynamic response and time at the rated current density of the fuel cell, and analyze the influence of ionomer decay in the membrane and catalyst layer on the dynamic response. Step fourteen: Plot the contribution of platinum oxide to the dynamic response at different times against voltage, analyze the dynamic performance degradation caused by platinum oxide, and obtain the dynamic response change caused by platinum oxide.

[0008] Furthermore, the fuel cell current density was gradually increased from an open circuit to near the limiting current density, and then gradually decreased from the maximum current density back to an open circuit, with each current density maintained for 60 seconds or more to obtain complete dynamic response data.

[0009] Furthermore, the polarization curve of the fuel cell is a voltage-current relationship curve of the fuel cell, where the voltage at each current density point is the average value of the voltage data recorded in the last 20 seconds at that current density step; then, based on the polarization curve, the electrochemical impedance spectroscopy representing the activation, ohmic, and concentration regions is tested respectively, with at least one current density point selected in each region, and the impedance spectrum test frequency range is 10 kHz-0.1 Hz.

[0010] Furthermore, the relaxation time distribution function of the impedance spectrum at a certain moment and a certain current density is calculated using the following formula: ; In the formula, R0 is the ohmic resistance, Z pol(ω) Let γ be the polarization impedance, γ(lnτ) be the relaxation time distribution function, ω be the angular frequency, and T be the relaxation time. Furthermore, an equivalent circuit model is established based on the number of peaks corresponding to the relaxation time distribution function. Each process is represented by a resistor Ri connected in parallel with a constant-phase element Qi; the low-frequency inductive process is represented by a negative resistor connected in parallel with a negative capacitor, which is equivalent to a decaying resistor.

[0011] Furthermore, the resistance value corresponding to each peak is calculated based on the relaxation time distribution function: ; In the formula, R p To specify the resistance during the polarization process, τ u and τ lThese represent the upper and lower limits of the peak time constant for the relaxation time distribution corresponding to the specified polarization process; for the two low-frequency processes, the calculated peak area is negative, and T is the relaxation time.

[0012] Furthermore, the contributions of film resistance and proton transport resistance to the dynamic response under different current densities are calculated using the following formula: ; In the formula, Δ R n In the change of current density Δ i Time film resistance ( R 0) or proton transport resistance ( R pt The change in ).

[0013] Furthermore, the overall dynamic response index Tr is calculated for different current densities: ; In the formula, ∆V is the change in voltage from transient to steady state when the current density changes by Δi.

[0014] The contribution of platinum oxide's dynamic change to the dynamic response is obtained by subtracting the contributions of membrane resistance and proton transport resistance from the total dynamic response index, using the following formula: ; In the formula, T PtO T represents the contribution of platinum oxide to the voltage dynamic response. PEM T contributes to the membrane resistance ion This contributes to the proton resistance.

[0015] The beneficial effects of adopting this technical solution are: The method based on impedance spectroscopy in this invention is simple to model and fit parameters, and can identify the membrane and proton transport resistance under different current densities, thereby calculating and quantifying the influence of ionomer hydration on the dynamic response.

[0016] This invention uses complete electrochemical impedance spectroscopy data, and the frequency range of the impedance data can cover the variation of characteristic frequencies.

[0017] Compared with traditional models, the impedance spectrum model established in this invention takes into account the low-frequency inductive part, which can more accurately identify the resistance of each physical process.

[0018] This invention provides a quantitative analysis of the dynamic response of fuel cells under different degradation states, especially the quantification of the contribution of platinum oxide to the voltage dynamic response and degradation behavior, which are contents that have not been studied and explored in depth in the prior art. Attached Figure Description

[0019] Figure 1This is a schematic diagram of the process for analyzing the dynamic performance degradation of fuel cells based on electrochemical impedance spectroscopy according to the present invention. Figure 2 This is a relaxation time distribution diagram considering the inductive process in an embodiment of the present invention; Figure 3 This is a circuit model diagram of the impedance spectrum equivalent circuit in an embodiment of the present invention; Figure 4 This is a schematic diagram of equivalent circuit fitting and experimentally measured impedance spectrum in an embodiment of the present invention; Figure 5 This is a fitted membrane resistance and proton transport resistance versus current density diagram in an embodiment of the present invention; Figure 6 This is a schematic diagram of the dynamic voltage response of the fuel cell under loading conditions in an embodiment of the present invention; Figure 7 This is a schematic diagram of the dynamic voltage response of the fuel cell under full load in an embodiment of the present invention; Figure 8 This is a schematic diagram illustrating the dynamic performance degradation analysis of ionomer hydration in an embodiment of the present invention. Figure 9 This is a schematic diagram illustrating the dynamic performance degradation analysis of platinum oxide in an embodiment of the present invention. Detailed Implementation

[0020] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described below with reference to the accompanying drawings.

[0021] In this embodiment, see Figure 1 As shown, this invention proposes a method for analyzing the dynamic performance degradation of fuel cells based on electrochemical impedance spectroscopy, including the following steps: Step 1: Test the dynamic response data of the fuel cell voltage; The fuel cell current density was gradually increased from open circuit to near the limit current density, and then gradually decreased from the maximum current density back to open circuit, with each current density held for 60 seconds or more to obtain complete dynamic response data.

[0022] Step 2: Calculate the fuel cell polarization curve based on the results of Step 1, and test the electrochemical impedance spectroscopy representing the activation, ohmic, and concentration regions based on the polarization curve. The polarization curve of the fuel cell is the voltage-current relationship curve of the fuel cell, where the voltage at each current density point is the average value of the voltage data recorded in the last 20 seconds at that current density step. Then, based on the polarization curve, the electrochemical impedance spectroscopy representing the activation, ohmic, and concentration regions is tested respectively. At least one current density point is selected in each region, and the impedance spectrum test frequency range is 10 kHz-0.1 Hz.

[0023] Step 3: Conduct long-term durability tests on the fuel cell. Repeat steps 1 and 2 after each preset time interval of durability test to obtain voltage dynamic response data and impedance spectrum data at different times. The working current density of the impedance spectrum test is kept consistent with that in step 2.

[0024] Step 4: Calculate the relaxation time distribution function of the impedance spectrum; The relaxation time distribution function of the impedance spectrum at a given current density at a given moment is calculated using the following formula: ; In the formula, R0 is the ohmic resistance, Z pol(ω) Let ω be the polarization impedance, γ(lnτ) be the relaxation time distribution function, ω be the angular frequency, and T be the relaxation time. In this formula, an ohmic resistor can be viewed as an RC resistor connected in series with numerous RC parallel elements; therefore, the formula is capacitive. To identify low-frequency inductive processes, the relaxation time distribution function is set to be negative in the low-frequency range. like Figure 2 As shown in the figure, the Ppt peak (~104 Hz) represents the proton transport process, the Pct peak (~700 Hz) represents the charge transfer process, the Pmt peak (~46 Hz) represents the mass transport process, and the Pind,1 peak (~2 Hz) and Pind,2 peak (~0.1 Hz) represent two inductive processes.

[0025] Step 5: Establish an equivalent circuit model based on the number of peaks corresponding to the relaxation time distribution function; like Figure 3 As shown, each process is represented by a resistor R. i With constant phase element Q i Parallel connection representation. Low-frequency inductive processes are represented by a negative resistor and a negative capacitor connected in parallel, which is equivalent to a decaying resistor.

[0026] Step 6: Calculate the resistance value corresponding to each peak based on the relaxation time distribution function; The resistance value corresponding to each peak is calculated based on the relaxation time distribution function: ; In the formula, R p To specify the resistance during the polarization process, τ u and τ l These represent the upper and lower limits of the peak time constant for the relaxation time distribution corresponding to the specified polarization process; for the two low-frequency processes, the calculated peak area is negative, and T is the relaxation time.

[0027] Step 7: Based on the polarization curve calculated in Step 2, calculate the differential resistance at the corresponding current density as the impedance value point with a frequency of 0. Use the polarization process resistance value corresponding to each peak calculated in Step 5 as the initial value for equivalent circuit fitting to perform impedance spectrum fitting, and obtain the membrane resistance and proton transport resistance in the catalyst layer, such as... Figure 4 As shown.

[0028] Step 8: Select a new current density and repeat steps 4 through 7 until the membrane resistance and catalyst layer proton transport resistance values ​​are obtained for all current densities.

[0029] Step nine: Fit the functional relationships between membrane resistance and proton transport resistance and current density, as follows: Figure 5 As shown; the contributions of membrane resistance and proton transport resistance to the dynamic response under different current densities are calculated using the following formula: ; In the formula, Δ R n In the change of current density Δ i Time film resistance ( R 0) or proton transport resistance ( R pt The change in ).

[0030] Step 10: Based on the test data from Step 1, calculate the overall dynamic response index Tr for different current densities to eliminate the influence of load variation on the dynamic response. The dynamic response of the fuel cell voltage is as follows: Figure 6 and Figure 7 As shown; Calculate the overall dynamic response index Tr for different current densities: ; In the formula, ∆V is the change in voltage from transient to steady state when the current density changes by Δi.

[0031] Step 11: Subtract the contributions of membrane resistance and proton transport resistance from the total dynamic response index to obtain the contribution of platinum oxide dynamic change to the dynamic response. The formula is as follows: ; In the formula, T PtO T represents the contribution of platinum oxide to the voltage dynamic response. PEM T contributes to the membrane resistance ion This contributes to the proton resistance.

[0032] Step 12: Select data from different times in Step 3 and repeat Steps 4 to 11 until the quantification results of the contribution of each dynamic response under different decay states are obtained.

[0033] Step thirteen: Plot the relationship between the contributions of membrane resistance and proton resistance to the dynamic response at the fuel cell's rated current density and time, as shown below. Figure 8 As shown, the influence of ionomer attenuation in the membrane and catalyst layer on the dynamic response is analyzed; Step fourteen: Plot the contribution of platinum oxide to the dynamic response at different times against voltage, and analyze the dynamic performance degradation caused by platinum oxide, such as... Figure 9 As shown, the dynamic response change caused by platinum oxide is obtained.

[0034] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed. The scope of protection of this invention is defined by the appended claims and their equivalents.

Claims

1. A method for analyzing the dynamic performance degradation of fuel cells based on electrochemical impedance spectroscopy, characterized in that, Including the following steps: Step 1: Test the dynamic response data of the fuel cell voltage; Step 2: Calculate the fuel cell polarization curve based on the results of Step 1, and test the electrochemical impedance spectroscopy representing the activation, ohmic, and concentration regions based on the polarization curve. Step 3: Conduct long-term durability tests on the fuel cell. Repeat steps 1 to 2 after each preset time interval of durability test to obtain voltage dynamic response data and impedance spectrum data at different times. The working current density of the impedance spectrum test is kept consistent with that in step 2. Step 4: Calculate the relaxation time distribution function of the impedance spectrum; Step 5: Establish an equivalent circuit model based on the number of peaks corresponding to the relaxation time distribution function; Step 6: Calculate the resistance value corresponding to each peak based on the relaxation time distribution function; Step 7: Based on the polarization curve calculated in Step 2, calculate the differential resistance at the corresponding current density as the impedance value point with a frequency of 0. Use the polarization process resistance value corresponding to each peak calculated in Step 5 as the initial value for equivalent circuit fitting to perform impedance spectrum fitting, and obtain the membrane resistance and proton transport resistance in the catalyst layer. Step 8: Select a new current density and repeat steps 4 to 7 until the membrane resistance and catalyst layer proton transport resistance values ​​are obtained at all current densities. Step 9: Fit the functional relationships between membrane resistance and proton transport resistance and current density, respectively, and calculate the contributions of membrane resistance and proton transport resistance to the dynamic response under different current densities. Step 10: Based on the test data from Step 1, calculate the overall dynamic response index Tr for different current densities to eliminate the influence of load variation on the dynamic response. Step 11: Subtract the contributions of membrane resistance and proton transport resistance from the total dynamic response index to obtain the contribution of platinum oxide dynamic change to the dynamic response. Step 12: Select data from different times in Step 3 and repeat Steps 4 to 11 until the quantification results of the contribution of each dynamic response under different decay states are obtained. Step 13: Plot the relationship between the contribution of membrane resistance and proton resistance to dynamic response and time at the rated current density of the fuel cell, and analyze the influence of ionomer decay in the membrane and catalyst layer on the dynamic response. Step fourteen: Plot the contribution of platinum oxide to the dynamic response at different times against voltage, analyze the dynamic performance degradation caused by platinum oxide, and obtain the dynamic response change caused by platinum oxide.

2. The method for analyzing the dynamic performance degradation of fuel cells based on electrochemical impedance spectroscopy according to claim 1, characterized in that, The fuel cell current density was gradually increased from open circuit to near the limit current density, and then gradually decreased from the maximum current density back to open circuit, with each current density held for 60 seconds or more to obtain complete dynamic response data.

3. The method for analyzing the dynamic performance degradation of fuel cells based on electrochemical impedance spectroscopy according to claim 1, characterized in that, The polarization curve of the fuel cell is the voltage-current relationship curve of the fuel cell, where the voltage at each current density point is the average value of the voltage data recorded in the last 20 seconds at that current density step; then, based on the polarization curve, the electrochemical impedance spectroscopy representing the activation, ohmic, and concentration regions is tested respectively, with at least one current density point selected in each region, and the impedance spectrum test frequency range is 10kHz-0.1Hz.

4. The method for analyzing the dynamic performance degradation of a fuel cell based on electrochemical impedance spectroscopy according to claim 1, characterized in that, The relaxation time distribution function of the impedance spectrum at a given current density at a given moment is calculated using the following formula: In the formula, R0 is the ohmic resistance, Z pol(ω) Let ω be the polarization impedance, γ(lnτ) be the relaxation time distribution function, ω be the angular frequency, and T be the relaxation time.

5. The method for analyzing the dynamic performance degradation of fuel cells based on electrochemical impedance spectroscopy according to claim 1, characterized in that, An equivalent circuit model is established based on the number of peaks corresponding to the relaxation time distribution function. Each process is represented by a resistor Ri connected in parallel with a constant-phase element Qi; the low-frequency inductive process is represented by a negative resistor connected in parallel with a negative capacitor, which is equivalent to a decaying resistor.

6. The method for analyzing the dynamic performance degradation of fuel cells based on electrochemical impedance spectroscopy according to claim 1, characterized in that, The resistance value corresponding to each peak is calculated based on the relaxation time distribution function: In the formula, R p To specify the resistance during the polarization process, τ u and τ l These represent the upper and lower limits of the peak time constant for the relaxation time distribution corresponding to the specified polarization process; for the two low-frequency processes, the calculated peak area is negative, and T is the relaxation time.

7. The method for analyzing the dynamic performance degradation of a fuel cell based on electrochemical impedance spectroscopy according to claim 1, characterized in that, The contributions of film resistance and proton transport resistance to the dynamic response under different current densities are calculated using the following formula: In the formula, ΔR n The membrane resistance (R0) or proton transport resistance (R) is the resistance of the membrane when the current density changes by Δi. pt The change in ).

8. The method for analyzing the dynamic performance degradation of a fuel cell based on electrochemical impedance spectroscopy according to claim 1, characterized in that, Calculate the overall dynamic response index Tr for different current densities: In the formula, ΔV is the change in voltage from transient to steady state when the current density changes by Δi.

9. The method for analyzing the dynamic performance degradation of a fuel cell based on electrochemical impedance spectroscopy according to claim 1, characterized in that, The contribution of platinum oxide's dynamic change to the dynamic response is obtained by subtracting the contributions of membrane resistance and proton transport resistance from the total dynamic response index, using the following formula: T PtO =T r -T PEM -T ion In the formula, T PtO T represents the contribution of platinum oxide to the voltage dynamic response. PEM T contributes to the membrane resistance ion This contributes to the proton resistance.

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