Analysis method of fuel cell catalyst layer structure

Abstract

The invention discloses an analysis method for a catalyst layer structure of a fuel cell, which comprises the following steps: S1, preparing a test cell, applying an alternating current disturbance voltage under a set condition, and collecting complex impedance data of a full frequency band to obtain a Nyquist diagram; s2, establishing a mapping relationship between key features of different frequency bands of the Nyquist graph and a catalyst layer structure, and judging structural features of the catalyst layer according to the measured Nyquist graph; and S3, constructing an equivalent circuit model, fitting the equivalent circuit model with the measured Nyquist graph, and quantifying the structural characteristics of the catalyst layer through fitting parameters. According to the method, full-band complex impedance data and catalyst layer structural features are associated through systematic steps, quantitative analysis is achieved by combining equivalent circuit fitting, a clear mapping relation between the structure and the atlas is established, key features such as ionomer distribution, carbon skeleton agglutination and hydrogen cross permeation in the thickness direction of the catalyst layer can be comprehensively and accurately recognized, and the method has a good application prospect. And a standardized and quantifiable technical path is provided for the analysis of the catalyst layer structure.

Description

Technical Field

[0001] The present invention relates to the technical field of fuel cells, and in particular to a method for analyzing the structure of a fuel cell catalyst layer. Background Art

[0002] The catalyst layer is the core area of ​​proton transport and charge conversion in fuel cells. Its microstructure (such as ionomer distribution, carbon skeleton morphology, etc.) directly affects the proton migration path and resistance characteristics. Therefore, accurate analysis of the catalyst layer structure is the key to optimizing fuel cell performance.

[0003] Currently, the main methods for measuring the internal resistance and catalyst layer structure of fuel cells include the current interruption method and the electrochemical impedance spectroscopy (EIS). The current interruption method calculates the polarization resistance by momentarily cutting off the current at a constant operating current and using the voltage jump. While simple to operate and capable of quickly acquiring data, it only measures the polarization resistance and cannot separate the reaction resistance from the capacitance component. Furthermore, the method is susceptible to interference from the load cable inductance and voltage noise, resulting in limited measurement accuracy.

[0004] The electrochemical impedance spectroscopy (EIS) method can separate polarization resistance, reaction resistance, and capacitance components by measuring the phase shift between voltage and current, with higher measurement accuracy and stronger anti-interference ability. However, this method still has many key problems in the analysis of catalyst layer structure: (1) The impedance spectrum is complex, and there is a lack of clear mapping relationship between different frequency bands and the specific structural characteristics of the catalyst layer (such as the distribution position and proportion of ionomers). As a result, the Nyquist plot relies on empirical interpretation and cannot quantify the distribution ratio of ionomers on the membrane side and the gas diffusion layer (GDL) side; (2) It is difficult to decouple the influence of the catalyst layer thickness direction (Z direction) and the in-plane direction (XY direction) on the spectrum, which limits the quantitative reconstruction of the microstructure; (3) In-plane heterogeneity is often masked by the fuzzy features of the 45° and 90° transition zones in the Nyquist plot. Due to the lack of a standardized identification method, it is easily misjudged as a general electrode response and cannot provide a basis for production process evaluation. Summary of the Invention

[0005] In view of the shortcomings of the existing technology, the purpose of the present invention is to develop a catalytic layer structure analysis method based on AC impedance spectroscopy, so as to quantitatively analyze the microstructure and distribution of the catalytic layer, and provide a basis for fuel cell process optimization and performance improvement.

[0006] To achieve the above object, a method for analyzing the structure of a fuel cell catalyst layer comprises the following steps: S1. Prepare the test battery, apply AC disturbance voltage under set conditions, collect complex impedance data of the full frequency band, and obtain the Nyquist plot; S2. Establish a mapping relationship between the key features of different frequency bands of the Nyquist plot and the structure of the catalyst layer. The 45° line deviation in the first frequency band from 1kHz to 100Hz corresponds to the ionomer distribution along the thickness of the catalyst layer. The 90° line deviation in the second frequency band from 100Hz to 10Hz corresponds to the carbon skeleton agglomeration of the catalyst layer. The arc in the third frequency band from 10Hz to 1Hz corresponds to hydrogen cross-penetration. Determine the structural characteristics of the catalyst layer based on the measured Nyquist plot. S3. Construct an equivalent circuit model and fit it with the measured Nyquist plot to quantify the structural characteristics of the catalytic layer through fitting parameters.

[0007] The method of the present invention associates the full-band complex impedance data with the structural characteristics of the catalytic layer through systematic steps, and combines it with equivalent circuit fitting to achieve quantitative analysis, establishing a clear mapping relationship between structure and spectrum. It can comprehensively and accurately identify key characteristics such as the distribution of polymers in the thickness direction of the catalytic layer, carbon skeleton agglomeration, and hydrogen cross-penetration, providing a standardized and quantifiable technical path for the analysis of the catalytic layer structure.

[0008] Furthermore, in step S2, the impedance phase angle of the first frequency band is calculated using the formula θ=arctan(|Z'' / Z'|), where θ is the impedance phase angle, Z'' is the imaginary part of the impedance, Z' is the real part of the impedance, and arctan() is the inverse tangent function; when θ=45°, it is determined that the ionomer is uniformly distributed in the thickness direction of the catalyst layer; when θ>45°, it is determined that the ionomer is concentrated on the proton exchange membrane side in the thickness direction of the catalyst layer; and when θ<45°, it is determined that the ionomer is concentrated on the gas diffusion layer side in the thickness direction of the catalyst layer. This step achieves quantitative determination of the ionomer distribution in the thickness direction of the catalyst layer, solving the problem that traditional methods cannot distinguish whether the ionomer is concentrated on the proton exchange membrane or the gas diffusion layer side. It can clearly determine the symmetry and distribution differences of the proton conduction path, providing a precise structural basis for optimizing ionomer distribution and reducing proton transmission resistance.

[0009] Furthermore, in step S2, when the second frequency band deviates from the 90° line or bends, it is determined that carbon skeleton agglomeration exists in the catalyst layer. This step clarifies the correspondence between deviation or bending of the 90° line in the second frequency band and carbon skeleton agglomeration, and can accurately identify structural degradation characteristics such as a reduction in the three-phase interface and reduced charge transfer efficiency caused by carbon carrier aggregation in the catalyst layer. This solves the problem that traditional methods have difficulty in intuitively determining the uniformity of carbon skeleton distribution.

[0010] Furthermore, in step S2, when an arc structure appears in the third frequency band, hydrogen cross-permeation is determined to exist in the catalytic layer. This step, based on the arc structure in the third frequency band, can intuitively identify the side reaction caused by hydrogen passing through the proton exchange membrane and entering the cathode, thus overcoming the limitation of traditional methods in quantifying the impact of cross-permeation side reactions.

[0011] Furthermore, in step S3, Z-view software is used for modeling and fitting. Taking advantage of its intuitive interface, high parameter extraction efficiency, and clear fitting results, the convenience and accuracy of fitting the equivalent circuit model and the Nyquist diagram are improved.

[0012] Furthermore, in step S3, the equivalent circuit model for the first frequency band is composed of a proton resistor. This equivalent circuit model can accurately characterize the impedance characteristics of the membrane resistance and the proton resistance distribution along the thickness of the catalyst layer corresponding to the first frequency band, eliminating irrelevant interference factors and providing a targeted circuit model foundation for analyzing the ionomer distribution along the thickness of the catalyst layer.

[0013] Furthermore, in step S3, the equivalent circuit model for the second frequency band is expanded upon that for the first frequency band by adding multiple parallel R-CPE units, each consisting of a proton resistor and a constant phase element in series. This equivalent circuit model effectively characterizes the structural heterogeneity within the catalytic layer (in the XY direction) and the impedance characteristics caused by carbon skeleton aggregation. By integrating multiple parallel units, the proton resistance and charge accumulation characteristics of different regions are reflected, achieving multi-dimensional quantification of the complex structural characteristics of the second frequency band.

[0014] Furthermore, in step S3, the equivalent circuit model for the third frequency band is expanded upon that for the second frequency band by adding a polarization resistor in parallel with the constant phase element to the R-CPE unit. This equivalent circuit model accurately captures the polarization impedance characteristics of the hydrogen cross-permeation side reaction and incorporates the impedance characteristics of this slow polarization process, providing reliable circuit model support for quantitatively assessing the degree of cross-permeation.

[0015] Furthermore, in the equivalent circuit model, the impedance calculation formula of the constant phase element is as follows:

[0016] Where Z is the impedance of the constant phase element, Y0 is the admittance parameter of the constant phase element, j is the imaginary unit, ω is the angular frequency, and α is the dispersion coefficient.

[0017] By clarifying the impedance calculation formula of the constant phase element and incorporating the diffusion coefficient α as a quantitative indicator into the equivalent circuit model, the quantitative characterization of the catalytic layer structure is achieved.

[0018] Furthermore, in step S1, the conditions set include: passing an inert gas through the cathode side of the test cell, a test temperature of 60-80°C, and a relative humidity of 10%-20%. Passing an inert gas through the cathode side of the test cell isolates oxygen, eliminating electrochemical reaction interference, and ensuring that the impedance signal only reflects the physical structure of the catalytic layer; the test temperature is close to actual operating conditions to ensure the proton conduction activity of the ionomer; and low humidity conditions increase proton resistance, enhance structural signals, and suppress inductive interference.

[0019] Furthermore, in step S1, the conditions set include: a bias voltage of 0.2 to 1 V, and an AC perturbation voltage amplitude not exceeding 10% of the bias voltage. The bias voltage puts the electrode in a non-reactive state, isolating the structural and reaction signals; limiting the AC perturbation voltage amplitude ensures a linear response of the system and reduces nonlinear distortion. These conditions work together to ensure that the impedance data accurately characterizes the structural characteristics of the catalyst layer.

[0020] Compared with the prior art, the present invention has the following beneficial effects: (1) Establishing a mapping relationship between the Nyquist diagram and the catalytic layer structure: For the first time, the microstructural characteristics of the catalytic layer in the thickness direction (Z direction) and the in-plane direction (XY direction) are systematically correlated with the responses of different frequency bands of the Nyquist diagram, and a quantitative mapping relationship between the physical structure and the spectral characteristics is established. This provides a standardized and quantifiable technical path for the analysis of the catalytic layer structure and provides a direct quantitative basis for process optimization and formulation improvement.

[0021] (2) The proton resistance distribution state can be quantitatively determined to support structural optimization decisions: With the help of spectrum angle analysis, equivalent circuit fitting and other means, it can be clearly determined whether the ionomer distribution is concentrated on the proton exchange membrane side or the gas diffusion layer side, and its symmetry and asymmetry can be quantitatively characterized, providing a quantitative basis for the optimization of the catalyst layer preparation process.

[0022] (3) Identification of complex phenomena such as in-plane heterogeneity and cross-penetration: The characteristic changes in the Nyquist diagram can be used to identify structural or interface problems such as uneven distribution of ionomers in the plane and hydrogen cross-penetration, providing an intuitive visualization basis for failure mechanism analysis and quality control.

[0023] (4) Good adaptability and anti-interference ability: By adopting strategies such as introducing inert gas into the cathode, setting bias voltage, and low humidity conditions, the interference of active reactions and the influence of water management factors are effectively eliminated, so that the test results can more truly reflect the structural characteristics of the catalytic layer, improve the accuracy and repeatability of the test, and are suitable for a variety of membrane electrode systems and working conditions.

[0024] (5) The method is easy to implement and has application value: the analytical method can be implemented on a conventional electrochemical workstation and can be used with analytical software such as Z-view to complete fitting and judgment. It does not require special hardware or consumables and has good portability and engineering potential, making it suitable for both research experiments and industrial testing. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 It is a mapping relationship diagram between the Nyquist diagram and the catalyst layer structure in a specific embodiment of the present invention.

[0026] Figure 2 This is the Nyquist diagram measured in Example 1 of the present invention.

[0027] Figure 3 This is the Nyquist diagram measured in Example 2 of the present invention.

[0028] Figure 4 This is the Nyquist diagram measured in Example 3 of the present invention. DETAILED DESCRIPTION

[0029] To make the above-mentioned objects, features, and advantages of the present invention more clearly understood, specific embodiments of the present invention are described in detail below with reference to the accompanying drawings. It should be noted that the following embodiments are only intended to illustrate the implementation methods and typical parameters of the present invention, and are not intended to limit the parameter ranges described in the present invention. Reasonable variations derived therefrom are still within the scope of protection of the claims of the present invention.

[0030] A specific embodiment of the present invention provides a method for analyzing the structure of a fuel cell catalyst layer, which quantitatively analyzes the microstructure and distribution of the catalyst layer by constructing a mapping between the physical structure and the Nyquist graph characteristics, combining specific measurement conditions and equivalent circuit modeling.

[0031] The analytical method includes the following steps: Step S1: Obtain the Nyquist plot of the test battery.

[0032] The specific process involves preparing the test battery, applying an AC perturbation voltage under specified conditions, and collecting complex impedance data across the entire frequency range to generate a Nyquist plot. This step uses the AC impedance method to measure the phase shift between voltage and current, which can be used to determine the absolute value of the resistance and its components, thereby separating the various resistance types and obtaining a variety of information.

[0033] In a specific embodiment, in order to analyze the structure of the catalyst layer, an inert gas, such as N2, Ar, etc., is introduced into the cathode side. A bias voltage is applied in an environment where no electrode reaction occurs. The catalyst layer is analyzed based on the frequency characteristics. The main purpose of introducing an inert gas into the cathode side is to avoid cathode electrode reaction. Specifically, when an inert gas is introduced, an oxygen reduction reaction will not occur on the cathode of the fuel cell, and therefore no current will be generated. The measured electrochemical impedance signal can accurately reflect the charge transfer characteristics, electrical conductivity, ion conductivity and other physical properties of the catalyst layer without being interfered with by the oxygen reduction reaction.

[0034] In a specific embodiment, the test temperature was set at 60-80°C and the relative humidity was 10%-20%. Testing under lower humidity conditions can increase the proton resistance of the catalyst layer, making the structure-related impedance components more prominent in the Nyquist plot, thereby effectively suppressing the relative interference of the inductance term. This condition is particularly suitable for high-area single cells and low equivalent weight (EW) ionomer systems, helping to improve the accuracy of structure fitting and parameter extraction.

[0035] In a specific embodiment, a bias voltage is applied, typically set to 0.2-1V, to help keep the electrodes in a non-reactive state and separate the structural and reactive signals. The AC perturbation voltage amplitude does not exceed 10% of the bias voltage to ensure a linear response of the system and reduce nonlinear distortion.

[0036] Step S2: Establishing a mapping relationship between key features of the Nyquist plot at different frequency bands and the catalyst layer structure. This step establishes a clear mapping relationship between structure and spectrum, enabling comprehensive and accurate identification of key features such as ionomer distribution, carbon skeleton aggregation, and hydrogen cross-permeation through the thickness of the catalyst layer.

[0037] Combine Figure 1 As shown in the Nyquist diagram, the relationship between the high-frequency, medium-frequency and low-frequency responses and the catalyst layer structure is as follows: (A) The first frequency band (1 kHz to 100 Hz) corresponds to (a) the ionomer distribution along the thickness of the catalytic layer.

[0038] In the Nyquist plot, the real part (horizontal axis) represents resistance, and the imaginary part (vertical axis) represents reactance. The degree of deviation of the first frequency band from the 45° line corresponds to the distribution of ionomers along the thickness of the catalyst layer. The impedance phase angle is calculated using the formula θ=arctan(|Z'' / Z'|), where θ is the impedance phase angle, Z'' is the imaginary part of the impedance, and Z' is the real part of the impedance. The specific mapping relationship is as follows: When θ = 45°, the real and imaginary parts of the impedance increase at the same rate, indicating that the impedance encountered by the current through the proton exchange membrane is equal to the impedance encountered through the gas diffusion layer. The corresponding structural characteristics are uniform ionomer distribution through the thickness of the catalyst layer and symmetrical conduction paths.

[0039] When θ>45°, the real part decreases and the imaginary part increases. This is because the proton resistance on the PEM side is low and the conductivity is good, allowing protons to quickly move to the electrode interface, making it easier for charge to accumulate and form a capacitor. The corresponding structural feature is that the ionomer is concentrated on the PEM side and cannot effectively penetrate to the GDL side.

[0040] When θ < 45°, the real part increases and the imaginary part decreases. This is due to high proton resistance on the PEM side, poor conductivity, and insufficient charge accumulation. The corresponding structural feature is that the ionomer is distributed closer to the GDL side, resulting in insufficient proton channels on the PEM side.

[0041] (B) The second frequency band (100 Hz to 10 Hz) corresponds to (b) the carbon skeleton aggregation of the catalytic layer.

[0042] The electron channels formed by the carbon support can be equivalent to resistive elements, while the ionomer covering its surface forms a proton conduction path, exhibiting capacitive behavior at the electrode / electrolyte interface. This interfacial capacitance is often equated to pure capacitance or constant phase element (CPE), reflecting the charge accumulation capacity and the integrity of the reaction interface.

[0043] When the proton resistor is connected in series with the constant phase element, it appears as a straight line (90°) perpendicular to the horizontal axis in the Nyquist diagram. However, when the carbon carrier aggregates, the ionomer penetration will be restricted, resulting in a reduction in the three-phase interface, weakening the interfacial capacitance, and the inability to effectively accumulate charge. Therefore, the response in the second frequency band is no longer an ideal capacitive behavior. At this time, the interfacial reaction is subject to the dual limitations of the electron conduction path and the formation of capacitance, and the impedance response exhibits a non-ideal behavior consisting of a resistor and a capacitor in series. This series structure causes the mid-frequency region of the Nyquist diagram to be unable to maintain a vertical rise feature close to 90°, but instead exhibits a certain degree of deviation and bending, reflecting the structural degradation characteristics of the decreased three-phase interface density and reduced charge transfer efficiency in the catalytic layer.

[0044] (C) The third frequency band (10 Hz to 1 Hz) corresponds to (c) hydrogen cross-permeation.

[0045] In fuel cells, the impedance response in the low-frequency region typically corresponds to slower electrochemical behavior dominated by polarization processes. When hydrogen cross-permeation occurs, some hydrogen is not fully oxidized at the anode, but instead passes through the proton exchange membrane into the cathode, where it reacts with oxygen to form water. This side reaction introduces a new polarization process. Because this process is slow and has significant hysteresis, it appears as an arc structure in the third frequency band in the Nyquist plot, representing the additional impact of this cross-permeation side reaction on the overall system impedance. Therefore, the presence of the arc in the third frequency band can be used as a basis for judging the severity of hydrogen cross-permeation.

[0046] Step S3: Construct an equivalent circuit model.

[0047] The equivalent circuit model is fitted with the measured Nyquist diagram through software, and the structural characteristics of the catalytic layer are quantified by fitting parameters, providing a standardized and quantifiable technical path for the analysis of the catalytic layer structure.

[0048] In the specific embodiment, Z-view software is used for modeling and fitting. It has the advantages of intuitive interface, high parameter extraction efficiency, clear fitting results, etc., which facilitates rapid judgment of fitting goodness and assists in structural analysis. The equivalent circuit models corresponding to different frequency bands are as follows: The equivalent circuit model for the first frequency band consists of a proton resistor. The variation in proton resistance caused by the uneven distribution of ionomers can be simulated using an equivalent circuit element—a constant phase element. This equivalent circuit model accurately characterizes the impedance characteristics of the proton resistance distribution across the thickness of the catalyst layer corresponding to the first frequency band, eliminating irrelevant interference factors and providing a targeted circuit model foundation for analyzing the distribution of ionomers across the thickness of the catalyst layer.

[0049] The equivalent circuit model for the second frequency band expands upon that of the first band by adding multiple parallel R-CPE units, each consisting of a proton resistor and a constant phase element in series. This equivalent circuit model effectively characterizes the structural heterogeneity within the catalytic layer (XY directions) and the impedance characteristics caused by carbon skeleton aggregation. By integrating multiple parallel units, the proton resistance and charge accumulation characteristics of different regions are reflected, achieving multi-dimensional quantification of the complex structural characteristics of the second frequency band.

[0050] The equivalent circuit model for the third frequency band expands upon the equivalent circuit model for the second frequency band by adding a polarization resistor in parallel with the constant phase element in the R-CPE unit. This equivalent circuit model accurately captures the polarization impedance characteristics caused by the hydrogen cross-permeation side reaction.

[0051] In the above steps, the theoretical impedance curve of the equivalent circuit model is fitted with the measured Nyquist diagram through Z-view software. After the fitting is completed, the software will output the specific parameter values ​​of each component in the model.

[0052] In the above equivalent circuit model, the impedance calculation formula of the constant phase element is as follows:

[0053] Where Z is the impedance of the constant phase element, Y0 is the admittance parameter of the constant phase element, j is the imaginary unit, ω is the angular frequency, and α is the dispersion coefficient.

[0054] By clarifying the impedance calculation formula of the constant phase element and incorporating the diffusion coefficient α as a quantitative indicator into the equivalent circuit model, the quantitative characterization of the catalytic layer structure is achieved.

[0055] The above analysis method can be implemented on a conventional electrochemical workstation and used with analysis software such as Z-view to complete fitting and judgment. It does not require special hardware or consumables and has good portability and engineering potential.

[0056] The technical solutions and effects of the present invention are illustrated below through specific embodiments.

[0057] Example 1

[0058] This example analyzes the structure of the fuel cell catalyst layer, and the specific steps are as follows: (1) The test platform is a circular 13cm 2 A standard test cell with electrode area, equipped with a suitable gas flow path and humidification system. During assembly, the seal should be complete and the contact should be consistent to avoid impedance deviation caused by non-structural factors.

[0059] (2) Set the impedance test conditions to eliminate interference from the active electrode reaction and extract only the structural characteristic parameters. Test temperature: 80°C; gas humidification conditions: anode / cathode dew point of 40°C; bias voltage: 0.5V; AC disturbance voltage amplitude: ±50mV; frequency sweep range: 50kHz to 0.1Hz.

[0060] (3) Use an electrochemical workstation with frequency response analysis function to apply small signal AC perturbations to the battery, collect current / voltage responses, and obtain Nyquist plots, such as Figure 2 shown.

[0061] (4) The structure of the catalyst layer was analyzed based on the mapping relationship between the key features of the Nyquist plot at different frequency bands and the structure of the catalyst layer. The impedance phase angle of the first frequency band was 38°, corresponding to the structural feature that the ionomer distribution was closer to the gas diffusion layer side; the second frequency band deviated by 90°, indicating that the catalyst layer had carbon skeleton agglomeration; the third frequency band had an arc structure, indicating that the catalyst layer had hydrogen cross-permeation.

[0062] (5) Use Z-view software to fit the measured Nyquist plot data to the equivalent circuit model.

[0063] The basic model was selected based on the characteristics of the first frequency band: the proton resistor was used as the core structure of the first frequency band. The proton resistor is often used to simulate the constant phase element CPE1. After importing the Nyquist plot data, the diffusion coefficient of CPE1 was obtained by automatic iterative fitting through the software. The α1 value is less than 0.5, which is consistent with the 38° phase angle (<45°) of the first frequency band in step (4), and quantitatively reflects that the ionomer distribution is closer to the gas diffusion layer side.

[0064] An expanded model for the second frequency band characteristics was developed: Based on the first-band model, two parallel R-CPE units (each connected in series with a proton resistor and a constant phase element) were added. The fitted diffusion coefficient of CPE2, α2, was 0.65 (deviation from 1), indicating that the deviation from the 90° line in the second frequency band is caused by carbon skeleton aggregation. This aggregation restricts ionomer penetration, leading to greater differences in proton resistance between regions and decreased charge accumulation capacity.

[0065] Further optimize the model for the arc characteristics of the third frequency band: in the R-CPE unit of the second frequency band model, add a polarization resistor R in parallel with CPE2 c =1.2Ω・cm 2 The fitting results show that the polarization resistance parameters corresponding to the arc in the third frequency band are significant, and R c The value is positively correlated with the arc angle, which quantitatively characterizes the polarization intensity of the hydrogen cross-permeation side reaction.

[0066] Example 2

[0067] This example verifies the ability of the equivalent circuit model to identify structural heterogeneity within the catalytic layer (in the XY direction). The specific steps are as follows: (1) Two groups of comparison samples were selected, one group was a uniform catalytic layer sample without in-plane distribution (prepared by optimizing the spraying process to ensure that the in-plane distribution of the ionomer and the carbon skeleton was consistent), and the other group was a non-uniform catalytic layer sample with in-plane distribution (prepared by controlling the difference in spraying rate to simulate the uneven local ionomer coating).

[0068] (2) Set the impedance test conditions: test temperature 80℃, anode / cathode dew point 40℃, bias voltage 0.5V, AC disturbance voltage amplitude ±50mV, frequency scanning range 50kHz to 0.1Hz, and N2 is introduced into the cathode side to eliminate interference from the active reaction.

[0069] (3) The Nyquist plots of the two groups of samples were collected using an electrochemical workstation with FRA function. The results are as follows: Figure 3 As shown, (I) is the Nyquist diagram of the sample without in-plane distribution, and (II) is the Nyquist diagram of the sample with in-plane distribution, observing the boundary characteristics of the 45° line and the 90° line.

[0070] (4) Use Z-view software to construct an equivalent circuit model. In the first frequency band, the membrane resistance R mem On the basis of CPE in series, three R-CPE units in parallel are added. Each R-CPE unit consists of a proton resistor R i It is connected in series with the constant phase element CPE, and the impedance data of the two groups of samples are fitted.

[0071] (5) Analysis results: In the Nyquist plot of the sample without in-plane distribution, the boundary between the 45° line and the 90° line is clear, the difference in the parameters of the three R-CPE units obtained by fitting is small, and the difference in the diffusion coefficients of the three constant phase elements is less than 0.05, indicating that the proton resistance and charge accumulation characteristics of each region in the plane are consistent; in the Nyquist plot of the sample with in-plane distribution, the boundary between the 45° line and the 90° line is fuzzy, the difference in the parameters of the three R-CPE units obtained by fitting is significant, and the difference in the diffusion coefficients α of the three constant phase elements fluctuates in a range of more than 0.1, reflecting that there are differences in the distribution of ionomers and the dispersion of carbon skeletons in different regions in the plane.

[0072] Example 3

[0073] This example verifies the influence of the inductance component on the impedance spectrum and the suppression effect of low humidification conditions on inductance interference.

[0074] (1) Two groups of identical MEA samples (low EW ionomer system, catalytic layer thickness 5 μm) were selected to ensure that the catalytic layer structures of the two groups of samples were consistent.

[0075] (2) Two groups of test conditions were set: the group without inductance interference: test temperature 80℃, anode / cathode dew point 40℃, bias voltage 0.5V, AC disturbance voltage amplitude ±50mV, frequency sweep range 50kHz to 0.1Hz, N2 was passed into the cathode side; the group with inductance interference: anode / cathode dew point 60℃, and the rest of the conditions were the same as those of the group without inductance.

[0076] (3) The Nyquist plots of the two groups of samples were collected using an electrochemical workstation with FRA function. The results are as follows: Figure 4 As shown, focus on the features near the high-frequency end.

[0077] (4) Result analysis: The high-frequency end of the group with inductive interference deviates from the real axis and shows a clear upward bending trend, which is manifested as the imaginary part offset caused by inductive reactance. This phenomenon will affect the accurate identification of membrane resistance, and part of the proton resistance of the catalyst layer may be mistakenly included in the membrane resistance. In addition, the phase offset caused by inductive reactance will also increase the angle of the line segment in the impedance spectrum, which is originally about 45°, resulting in misjudgment of the proton resistance distribution in the thickness direction of the catalyst layer, seriously interfering with the judgment and analysis of the in-plane structural distribution. By moderately reducing the humidification conditions and increasing the proton resistance value of the catalyst layer, the structural related impedance component will be more prominent in the Nyquist diagram, thereby effectively suppressing the relative interference of the inductive term.

[0078] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the above embodiments, or make equivalent replacements for some or all of the technical features therein. However, these modifications or replacements 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 analyzing the structure of a fuel cell catalyst layer, characterized in that: The following steps are involved: S1. Prepare the test battery, apply AC disturbance voltage under set conditions, collect complex impedance data of the full frequency band, and obtain the Nyquist plot; S2. Establish a mapping relationship between the key features of different frequency bands of the Nyquist plot and the structure of the catalyst layer. The 45° line deviation in the first frequency band from 1kHz to 100Hz corresponds to the ionomer distribution along the thickness of the catalyst layer. The 90° line deviation in the second frequency band from 100Hz to 10Hz corresponds to the carbon skeleton agglomeration of the catalyst layer. The arc in the third frequency band from 10Hz to 1Hz corresponds to hydrogen cross-penetration. Determine the structural characteristics of the catalyst layer based on the measured Nyquist plot. S3. Construct an equivalent circuit model and fit it with the measured Nyquist plot, and quantify the structural characteristics of the catalytic layer through fitting parameters.

2. The method for analyzing the fuel cell catalyst layer structure according to claim 1, characterized in that: In step S2, the impedance phase angle of the first frequency band is calculated by the formula θ=arctan(|Z'' / Z'|), where θ is the impedance phase angle, Z'' is the imaginary part of the impedance, and Z' is the real part of the impedance; when θ=45°, it is determined that the ionomer is uniformly distributed in the thickness direction of the catalytic layer; when θ>45°, it is determined that the ionomer is concentrated on the proton exchange membrane side in the thickness direction of the catalytic layer; and when θ<45°, it is determined that the ionomer is concentrated on the gas diffusion layer side in the thickness direction of the catalytic layer.

3. The method for analyzing the fuel cell catalyst layer structure according to claim 1, wherein: In step S2, when the second frequency band deviates from the 90° line or is bent, it is determined that carbon skeleton agglomeration exists in the catalytic layer.

4. The method for analyzing the fuel cell catalyst layer structure according to claim 1, characterized in that: In step S2, when an arc structure appears in the third frequency band, it is determined that hydrogen cross-permeation exists in the catalytic layer.

5. The method for analyzing the fuel cell catalyst layer structure according to claim 1, characterized in that: In step S3, Z-view software is used for modeling and fitting.

6. The method for analyzing the fuel cell catalyst layer structure according to claim 5, characterized in that: In step S3, the equivalent circuit model of the first frequency band is composed of a proton resistor.

7. The method for analyzing the fuel cell catalyst layer structure according to claim 6, characterized in that: In step S3, the equivalent circuit model of the second frequency band is expanded on the basis of the equivalent circuit model of the first frequency band: a plurality of parallel R-CPE units are added, and the R-CPE unit is composed of a proton resistor and a constant phase element connected in series.

8. The method for analyzing the fuel cell catalyst layer structure according to claim 7, characterized in that: In step S3, the equivalent circuit model of the third frequency band is expanded on the basis of the equivalent circuit model of the second frequency band: a polarization resistor connected in parallel with the constant phase element is added to the R-CPE unit.

9. The method for analyzing the fuel cell catalyst layer structure according to any one of claims 1 to 8, characterized in that: In step S1, the setting conditions include: introducing an inert gas into the cathode side of the test cell, the test temperature being 60-80° C., and the relative humidity being 10%-20%.

10. The method for analyzing the fuel cell catalyst layer structure according to claim 9, characterized in that: In step S1, the setting conditions include: the bias voltage is 0.2-1V, and the AC disturbance voltage amplitude does not exceed 10% of the bias voltage.

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