Evaluation and life prediction method of fuel cell metal pole plate coating

Abstract

The invention discloses an evaluation and life prediction method for a fuel cell metal pole plate coating, and belongs to the technical field of fuel cell testing. The method is used for predicting the service life under the condition of simulating the working condition of the fuel cell, and comprises the following steps: firstly, determining the corrosion potential of a fuel cell metal pole plate coating through potentiodynamic scanning; the method comprises the following steps: taking a corrosion potential as a starting point, slowly increasing the potential in an operation environment close to a cathode of a fuel cell, continuously scanning until electrochemical corrosion occurs or the potential reaches 1.6-3.0 V (vs.RHE), and finally measuring the contact resistance change of the sample under the pressure of 1.4 MPa for predicting the service life of a pole plate coating in the operation environment of the fuel cell. The attenuation of the pole plate coating in the long-time operation process of the fuel cell is estimated through the change of the contact resistance of the pole plate, including the calculation of the corrosion rate of the coating in the operation environment of the fuel cell and the like, so that the prediction result of the service life is obtained.

Description

Technical Field

[0001] The present invention belongs to the life prediction technology of fuel cells, and specifically relates to the detection of metal plate coatings of fuel cells, and in particular to an evaluation and life prediction method of metal plate coatings of fuel cells. Background Art

[0002] Proton exchange membrane fuel cells (PEMFCs) have a wide range of applications in the field of new energy due to their zero emissions, high power density and high energy conversion efficiency. Due to breakthroughs in technology and costs in key components and batteries, the fuel cell vehicle industry is already on the verge of industrialization. Generally speaking, the life of a fuel cell vehicle must be more than 5,000 hours to meet its application requirements. The reliability and life of the coating are the biggest obstacles to the commercialization of fuel cells.

[0003] The reliability and life prediction and impact research of fuel cell stacks is a very complex issue. It is closely related to the main materials of fuel cells, such as bipolar plates, membrane electrodes, and seals. It is also affected by operating conditions and system control logic. The internal working environment of the fuel cell is weakly acidic, contains hydrofluoric acid HF, a proton membrane decay product, and is high in humidity and high in temperature. Metal materials are very susceptible to corrosion when serving in such an environment. In order to increase the service life of the metal plate, a coating is usually applied to the surface of the metal plate to improve the stability of the metal plate. This is why a stable, corrosion-resistant coating is crucial to the reliability and life of the metal plate. Therefore, the stability of the metal plate coating material and process determines the life of the metal bipolar plate, and ultimately determines the life of the fuel cell stack.

[0004] Surface treatment is crucial to the durability and reliability of coatings on metal plates (such as SS316L). This includes increasing the roughness of the metal surface to provide a larger surface area for subsequent coatings, chemically treating the metal surface to enhance the bonding between the subsequent coating and the substrate and reduce the risk of delamination, and removing oil and oxides from the metal surface to reduce interference with subsequent coatings. In addition, the uniformity of the surface and thickness of the metal substrate is also very important, including identifying and avoiding pinhole defects to avoid becoming the starting point of pitting or mechanical failure. Finally, a well-adhered barrier layer is established on the surface to protect the underlying metal substrate from the effects of the corrosive environment, which can be achieved by designing a primer or passivation treatment of the substrate surface.

[0005] There are several methods for coating the surface of metal plates. The first is precious metal coatings, including platinum, gold or palladium coatings, which have excellent corrosion resistance and conductivity, but are expensive. The second is metal nitrides, commonly used are titanium nitride (TiN) and chromium nitride (CrN), which have good corrosion resistance and are more cost-effective than precious metal coatings. The third is metal carbides, such as titanium carbide (TiC) and tungsten carbide (WC) coatings, which are often used because of their hardness and corrosion resistance. The fourth category is conductive metal oxides, including coatings such as tin oxide (SnO2) and indium tin oxide (ITO), which have good conductivity and corrosion resistance. The fifth category is carbon-based coatings, namely graphite and amorphous carbon coatings, which are commonly used because of their excellent conductivity and corrosion resistance. However, this type of coating has a thicker thickness to achieve long-term durability. The sixth category is composite coatings, which combine different materials, such as metal / metal composite composites, to take advantage of the conductivity, corrosion resistance and cost advantages of each material, as well as the interaction between each material to achieve corrosion resistance. The seventh category is thermal spraying, which is divided into technologies such as atmospheric pressure plasma spraying (APS), vacuum plasma spraying (VPS) and high-velocity oxygen fuel (HVOF) spraying. These methods can produce dense, well-adhered coatings. Finally, there are physical vapor deposition (PVD) and chemical vapor deposition (CVD) methods, which can be used to deposit thin, uniform coatings with excellent adhesion and controlled composition.

[0006] There are many methods for evaluating the life of existing metal plate coatings, which are mainly evaluated through the following channels. The first is corrosion resistance testing, which uses techniques such as setting electrode potential polarization combined with electrochemical impedance spectroscopy (EIS) to evaluate corrosion resistance. These methods can provide in-depth understanding of the ability of the coating to withstand the corrosive environment commonly encountered in fuel cell operation; the second is the interfacial contact resistance (ICR) method. Maintaining a low interfacial contact resistance (ICR) is essential for efficient conductivity. The ICR is usually measured using a four-point probe method. This method helps to evaluate the extent to which the coating maintains electrical contact over time, which is essential for the overall efficiency of the fuel cell; the third is mechanical durability testing. The mechanical robustness of the coating is evaluated through tests such as scratch testing and nanoindentation. These tests evaluate the ability of the coating to withstand mechanical stress during operation to ensure that it does not degrade or peel off under physical strain; further, accelerated aging tests are used to simulate long-term operating conditions to predict the service life of the coating. This type of test includes thermal cycling, exposure to humidity and corrosive agents to collect data on the performance of the coating over a long period of time; the last is field testing, which collects data on coating attenuation under actual working conditions, which helps to establish a predictive model. Among the above methods, the electrochemical corrosion resistance test can provide electrochemical signals that change over time, which is convenient for implementing rapid life testing and formulation evaluation of metal plate coatings, and has advantages over the other methods mentioned above in terms of execution efficiency.

[0007] In fact, corrosion resistance tests are often used in current patent documents to evaluate the life of metal plate coatings. For example, Chinese patent CN111257212B uses a single constant potential (1.5V) to perform offline accelerated testing to quickly evaluate the life of the coating. However, the potential faced by the coating in the actual operation of the fuel cell is a constantly changing dynamic process, so this constant potential test method cannot truly evaluate the performance of the plate. For example, Chinese patent CN114976135B uses 0.67V (vs.RHE) and 1.43V (vs.RHE) to simulate the corrosion resistance of metals under normal fuel cell working conditions and simulated fuel cell start-stop working conditions. Although it is more reasonable than patent CN111257212B to select different potentials for constant potential testing corresponding to different working conditions, the simulated working conditions are single and cannot objectively reflect the dynamic changes of the battery during battery operation. For example, patent CN114486714B uses a gradient increase and gradient decrease from 0.6V to 1.6V as a method to accelerate plate corrosion. Although the introduction of variable potential is one step closer to the actual working conditions than CN111257212B and CN 114976135B, it assumes that the corrosion current integral is related to the mass lost due to corrosion of the metal plate. This assumption is unreasonable because there are many factors that affect the corrosion current, and it reflects the kinetic changes, and the residence time of each potential point in the patent is only 1 to 3 seconds, so the short-cycle corrosion current integral cannot reflect the corrosion resistance of metal materials. Patent CN114018797B determines the concentration, temperature and duration of the corrosion solution in the offline corrosion test experiment according to the concentration, temperature and working condition length of the corrosion solution inside the battery under actual operating conditions, and uses the actual operation of the fuel cell and the national standard GB / T 38914-2020 as the experimental test conditions, which restores the actual operating environment and potential changes of the fuel cell to the greatest extent. However, the patent evaluates the stability of the coating based on the characterization and contact resistance of the coating after the corrosion test, and does not define the standard for coating stability.

[0008] As mentioned above, if a fuel cell is to be commercialized, its life span must reach at least 5000 hours. If each coating material and process is subjected to online testing of a real stack (actual product) for more than 5000 hours, it will be time-consuming and expensive. Therefore, if the quick and convenient characteristics of the offline method can be combined with the real and reliable characteristics of the online testing method, it will be a good way to solve the problem of rapid testing of metal plate coatings. Patent application CN 114858696 A establishes a mapping relationship between the results of offline accelerated testing and online stack testing, so as to predict the online life of a new coating using offline accelerated life testing. However, the assumption of this method is not valid, because there are multiple factors in the attenuation of the stack voltage, such as the failure of the battery seal, carbon paper, catalyst, and proton membrane, so it is unreasonable to simply attribute the failure of the stack to the failure of the plate coating, and the established mapping relationship will not be universal.

[0009] The development of metal fuel cells has now reached a stage where the pursuit of performance has shifted to the pursuit of reliability, which is a key step in the industrialization of fuel cells. For a fuel cell metal stack whose performance design is mature but whose coating life, one of the reliability indicators, needs to be continuously verified, a method for quickly and accurately testing the life of different coatings on metal plates is urgently needed. Summary of the invention

[0010] The present invention aims to provide a method for evaluating and predicting the life of a fuel cell metal plate coating. The method can predict the coating reliability and coating life of the plate, including the ability to detect different coatings, and can solve the defects existing in the comprehensive fuel cell stack life test used in the prior art.

[0011] Technical solution: A method for evaluating and predicting the life of a fuel cell metal plate coating. The method predicts the life under conditions that simulate the working conditions of a fuel cell. The method is as follows:

[0012] Firstly, the corrosion potential of the fuel cell metal plate coating was determined by potentiodynamic scanning with a potential scanning range of -0.6V-1.2V and a scanning rate of 0.5-1.5mV / s;

[0013] Secondly, starting from the corrosion potential, the potential is slowly increased in a fuel cell operating environment, and the scanning speed is 6-10nV / s until electrochemical corrosion occurs or the potential reaches 1.6-3.0V. Then, the contact resistance change of the above samples is measured under a pressure of 1.4MPa to predict the life of the metal plate coating in the fuel cell operating environment.

[0014] The attenuation of the plate coating during long-term operation of the fuel cell is estimated again by measuring the change in the plate contact resistance after the electrochemical scan, including the calculation of the corrosion rate of the coating in the fuel cell operating environment:

[0015] ΔICR=ICR t -ICR 0

[0016] ICR t is the corrosion tested contact resistance, ICR 0 is the contact resistance of the coated plate in the initial state;

[0017] Then, a rapid life test of the fuel cell stack is carried out, and the change rate of the contact resistance at the cathode plate outlet over time ΔΩ is recorded during the test, and the functional relationship between ΔΩ and the test time is established through data fitting:

[0018] ΔΩ=f(t),

[0019] ΔΩ is the change in cathode outlet coating contact resistance during the fuel cell life test, t is the time required for the contact resistance to change, where the functional relationship of ΔΩ=f(t) is a linear, exponential, or polynomial fit, and the sum of squares of the fitting residuals is greater than 0.9.

[0020] If the time taken for the fuel cell stack performance to drop to 90% of the initial performance (i.e., the threshold condition set for the end of the stack life) is T, the increase value of the cathode plate outlet coating contact resistance at this time is ΔΩ T ;

[0021] Compare ΔΩ T and ΔICR, if ΔICR≤ΔΩ T , the plate coating life is greater than T, that is, the performance and quality of the plate coating meet the design requirements; if ΔICR>ΔΩ T , the life of the coating is estimated by the relationship ΔΩ=f(t) in the rapid life test of the stack.

[0022] Furthermore, the test method can also be used to evaluate the reliability of different coatings on the same metal plate substrate, and the reliability and lifespan of different coatings can be ranked according to the different attenuation rates corresponding to the coating materials.

[0023] Furthermore, the corrosion potential is tested by potentiodynamic scanning, that is, the metal plate sample is used as the working electrode, Ag / AgCl is used as the reference electrode, and the platinum electrode is used as the auxiliary electrode in a H2SO4 electrolyte solution containing 0.1ppm HF and pH=3 at a temperature of 90°C. Before starting the test, air is introduced at a flow rate of 10mL / min for 30min. During the test, continuous ventilation is maintained on the liquid surface to obtain the polarization curve of the metal plate coating; the potential at which the corrosion current is zero is the corrosion potential;

[0024] The measured polarization curve is fitted by Tafel, and the current corresponding to the intersection of the Tafel straight line is the corrosion current of the metal sample.

[0025] Furthermore, in order to quickly evaluate the reliability and life of the coating, the present invention establishes a coating electrochemical corrosion test method, that is, the simulated environment of the fuel cell is set to 110-120°C, the pressure is 1.5-2.0 bara, and the composition of the corrosion medium of the fuel cell is a 1M H2SO4 solution containing 2.0ppm HF;

[0026] The electrochemical corrosion test unit consists of three electrodes, of which the metal plate of the coating to be tested is the working electrode with a working area of ​​1cm 2 , Ag / AgCl electrode is used as reference electrode, platinum electrode is used as auxiliary electrode, and the potential change of working condition is realized by controlling the potential in the test by electrochemical workstation.

[0027] Furthermore, the coating electrochemical corrosion test includes placing the electrochemical test unit in a cylindrical high-pressure container with an inner wall of inert Teflon material, and controlling the stability of temperature and pressure during the test process through a PLC controller and temperature and pressure sensors;

[0028] Before starting the test, air was introduced at a flow rate of 10 mL / min for 30 minutes. During the test, continuous ventilation was maintained above the liquid level to simulate the most severe cathode operating conditions of the fuel cell plate.

[0029] Furthermore, the method also includes scanning with a dynamic potential to achieve accelerated polarization of the metal plate coating for evaluating the physicochemical stability of the coating;

[0030] The reliability of more than one coating is ranked by potential kinetic polarization. When a coating structure degrades during scanning, that is, the slope of corrosion current change exceeds a predetermined threshold during the potential scanning process, the scanning process will stop after the slope of corrosion current change returns to below the predetermined threshold. The conclusion and ranking of coating reliability are obtained by calculating the corrosion electricity and recording and analyzing its potential scanning spectrum.

[0031] This method also includes the following offline testing method during the contact resistance test process:

[0032] The metal plate sample is placed between two pieces of carbon paper to form a sample sandwich, and then the sample sandwich is placed between two gold-plated copper electrodes to form a test electrode. Finally, the test electrode is pressurized to 1.4Mpa under a test press, and the voltage change observed by the contact resistance meter at a current of 2A is removed by the voltage difference between the gas diffusion layer and the wire, and then combined with the area of ​​the sample to calculate the contact resistance;

[0033] The metal plate sample is prepared in the following manner:

[0034] Take out several groups of cathode metal plates from the fuel cell stack, confirm the cathode outlet of the plate, and then cut to obtain relatively flat plate material.

[0035] Furthermore, the carbon paper is a porous material having a similar property to a Freudenberg gas diffusion layer.

[0036] Furthermore, the metal plate samples are cut by laser cutting equipment, and the power of the laser cutting is controlled at 100W-500W.

[0037] The method includes predicting the life of a fuel cell stack based on different coating lives, determining idle, benchmark and rated road conditions according to the requirements of loading and unloading time and number in the vehicle cycle, and determining the cycle period for cycle testing. When the voltage decay at the rated current reaches a set minimum threshold, the life test is stopped and the number of cycles and the voltage decay at idle and benchmark currents are recorded.

[0038] Beneficial effects: The method provided by the present invention has the following significant effects and substantial characteristics:

[0039] (1) The present invention separates the attenuation mode test and life prediction of the coating and the fuel cell stack. Compared with the prior art, the life prediction is essentially the sum of the attenuation of multiple materials, including catalysts, carriers, proton membranes, plate design, coatings, stack operating range and control strategies, etc. The present invention can accurately obtain the impact of the coating on the life of the fuel cell stack.

[0040] (2) Compared with the existing national standard GB / T 38914-2020 for the life test method of fuel cell stacks, the present invention can improve the test efficiency, accelerate some operations in the test link, and more quickly obtain the increase of contact resistance in the actual stack life test, thereby quickly inferring the life range of the plate coating, and completing the coating life test within 1-2 days.

[0041] (3) The present invention can be used to evaluate the corrosion resistance of various metal plate coatings by testing and analyzing the effect of metal plate coatings on the life of fuel cell stacks, and plays an important role in promoting the rapid screening of coating materials and processes. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 It is a schematic diagram of the test flow of the method of the present invention;

[0043] Figure 2 is a schematic diagram of the sampling position of the metal plate sample;

[0044] Figure 3 It is a schematic diagram of the off-line contact resistance test method;

[0045] Figure 4 is the life test result of the fuel cell stack in the embodiment;

[0046] Figure 5 It is the attenuation trend of voltage at different currents in the battery stack life test given in the embodiment;

[0047] Figure 6 This is the growth trend of the contact resistance of the sample at the cathode outlet with the number of cycles given in the example. DETAILED DESCRIPTION

[0048] To illustrate the technical solution disclosed in the present invention in detail, a further introduction is given below in conjunction with the accompanying drawings.

[0049] In combination with the content mentioned in the background technology, the present invention provides a rapid evaluation method and life prediction method for plate coatings based on the electrochemical principle of metal material corrosion, and is associated with the change in contact resistance of the plate coating of the metal plate during the rapid life test. This method is a comprehensive evaluation of coating quality, coating process and coating corrosion resistance. By performing potential scanning on various coating samples based on the same metal substrate exposed to similar fuel cell operating temperature, pressure and electrolyte composition, observing the change in corrosion current density, obtaining corrosion potential and pitting potential, and comparing different coatings; at the same time, according to the change in contact resistance of different coating samples before and after the corrosion test, the contact resistance change of the coated metal plate during the rapid life test of the stack is compared, and the life of the coating during the actual measurement process is inferred.

[0050] We understand that in addition to the metal plate substrate, surface pretreatment process and coating method, the external factors that affect the corrosion resistance and stability of the coating mainly come from the operating environment of the fuel cell plate. The first is the pH value of the solution that the stack plate contacts. The acidic environment will accelerate corrosion; the second is the anion concentration, such as fluoride ions, chloride ions, sulfate ions, etc. Generally, the greater the ion concentration, the greater the possibility of corrosion or pitting of the plate coating. The third is the operating temperature, pressure and humidity. High temperature, high pressure and high humidity may cause the mechanical stability of the surface coating to weaken and the surface hydrophobicity to decay. But the biggest influence is the voltage and current applied to the metal plate coating during the operation of the fuel cell. The present invention firstly tests and theoretically analyzes the influence of a large number of metal plates and their coatings on the life of the fuel cell stack to know that when the voltage exceeds a certain threshold (generally 0.6 to 1.2V), the coating may decompose and cause corrosion. The degradation of the coating exposes the underlying substrate to a corrosive environment, the corrosion of the substrate as an anode is accelerated, and the metal atoms lose electrons and enter the solution in the form of ions; high current density accelerates the electrochemical reaction to cause the coating to degrade faster. This provides a basis for the implementation of the preliminary solution of the present invention and ensures that the error between the test and the actual life is smaller and closer to the actual situation.

[0051] Combination Figure 1 As shown, the evaluation method for electrochemical coating of the present invention can be divided into three main steps:

[0052] Step 1: determine the corrosion potential of the coating by fast potentiodynamic scanning, with a scanning rate of 1 mV / s and a potential scanning range of -0.6 V to 1.2 V;

[0053] Step 2: Accelerated life test, starting from the corrosion potential, slowly increasing the potential in a fuel cell operating environment, with a scan rate of 6-10nV / s, until electrochemical corrosion occurs or the potential reaches 1.6-3.0V;

[0054] Step 3: Measure the contact resistance change of the above samples under a pressure of 1.4 MPa to predict the life of the plate coating in the fuel cell operating environment.

[0055] Specifically, the scanning range of the corrosion potential includes the potential intervals of the cathode reaction and the anode reaction. The cathode (reduction) reaction zone is at a negative potential. The more negative the potential, the greater the current density. In the anode (oxidation) reaction zone, the more positive the potential, the greater the current density. At the turning point of the cathode and anode reaction zones, the cathode and anode current densities are equal, the net current is zero, and the coating is in an electrochemical equilibrium state. The potential at this time is the corrosion potential.

[0056] The more positive the corrosion potential is, the better the corrosion resistance of the coating is. The corrosion current density at the corrosion potential is directly related to the corrosion rate. The higher the corrosion current value is, the higher the corrosion rate is at the corrosion potential. The potentiodynamic test is carried out with the sample as the working electrode, Ag / AgCl as the reference electrode, and a platinum electrode (Pt) as the auxiliary electrode at a temperature of 90°C, a H2SO4 electrolyte solution containing 0.1ppm HF and pH=3. Before starting the test, air is introduced at a flow rate of 10mL / min for 30min. The test process is maintained with continuous ventilation on the liquid surface to simulate the actual operating environment of the fuel cell cathode. The linear potential scan rate of the sample is 1mV / s, and the potential scan range is -0.6V~1.2V (vs.RHE). After the corrosion potential scan test, there should be no obvious corrosion changes on the sample surface. The measured polarization curve is Tafel fitted, and the current corresponding to the intersection of the Tafel straight line is the corrosion current of the sample. According to the requirements of metal plate coating design, the corrosion current under normal corrosion potential is less than 1μA / cm2, which meets the quality requirements of metal plate coating of a certain country's Ministry of Energy.

[0057] The accelerated life test of the coating is based on more severe test conditions than those in the existing literature, so as to identify the stability of the coating sample in the fuel cell environment in a short time. The simulated environment of the fuel cell is 110-120℃, 1.5-2.0bara, and the composition of the corrosion medium (electrolyte) of the fuel cell is 1M H2SO4 solution containing 2.0ppm HF. The electrochemical test unit is composed of three electrodes, in which the metal plate of the coating to be tested is the working electrode (working area is 1cm2), the Ag / AgCl electrode is the reference electrode, and the platinum electrode is the auxiliary electrode. The change of the working potential is achieved by controlling the potential during the test by the electrochemical workstation. The electrochemical test unit is placed in a cylindrical high-pressure container with an inert Teflon material inner wall, and the temperature and pressure stability during the test are controlled by the PLC controller and the temperature and pressure sensors. Before starting the test, air is passed at a flow rate of 10mL / min for 30min, and the test process is kept continuously ventilated above the liquid surface. To further explain, the cathode reaction of the fuel cell is to introduce air, first the electrolyte is filled with air, and then the air circulation is maintained. The electrochemical scan here simulates the cathode reaction; the assumption here is that the cathode is the weakest link, and when the cathode life is reached, the coating life ends.

[0058] The present invention further considers that when the voltage decay under the rated current reaches the set minimum threshold, the life test of the fuel cell stack ends, that is, the stack reaches the end of its life. At the same time, the metal plate coating also reaches the end of its service life. The contact resistance value of the metal plate coating at this time point is the indicator of its end of life.

[0059] The degradation of coating materials in corrosive media can be divided into uniform corrosion and pitting corrosion. Uniform corrosion is the process of gradual degradation of the physical or chemical structure of the entire coating surface at high potential, and the thickness of the coating decreases until it disappears; while pitting corrosion is caused by defects such as pits or slits in the coating process, which cause local breakdown of the coating at high potential, resulting in local corrosion, and then causing the pits to grow over time. The present invention determines the corrosion potential and pitting potential of the coating by potential scanning and observation of corrosion current under the above simulated fuel cell working conditions.

[0060] The electrochemical and corresponding potentiodynamic accelerated polarization methods used in the present invention do not distinguish between uniform corrosion and pitting corrosion, because uniform corrosion corresponds to the anti-corrosion performance of the coating, while pitting corrosion corresponds to the state of the entire coating process based on the anti-corrosion performance of the coating. The evaluation criteria are based on the level of the corrosion potential and the corresponding corrosion current. The higher the corrosion potential or pitting potential, the higher the stability of the coating material and process; the lower the corrosion current density and the integral over time (electricity integral) at the same potential, the higher the stability of the coating material and process. In order to avoid the influence of double-layer capacitance and mass transfer on the potential scan in the potential dynamic polarization test, the potential scan range of the present invention takes the corrosion potential of the coating as the starting point, the fixed potential of 1.6-3.0V (vs RHE) as the end point, and the scanning speed is 6-10nV / s.

[0061] The purpose of the potentiodynamic accelerated polarization scan is to evaluate the physical and chemical stability of the coating and to rank the reliability of various coatings through potentiodynamic polarization. When a coating structure degrades during the scan, that is, the slope of the corrosion current change exceeds a predetermined threshold during the potential scan, the scan process will stop after the slope of the corrosion current change returns to below the predetermined threshold, and the potential scan spectrum will be recorded and analyzed to draw conclusions and rankings on the reliability of the coating.

[0062] Furthermore, it is considered to use the change of the contact resistance of the electrode plate to estimate the attenuation of the coating during the long-term operation of the fuel cell. It is observed that the attenuation of the fuel cell electrode plate coating mostly occurs at the cathode outlet. This is because the cathode is the oxygen reduction reaction zone, and the corrosion experienced by the coating is greater than the hydrogen oxidation reaction zone of the anode; in addition, the cathode outlet of the electrode plate experiences the highest reaction temperature and pressure changes in the fuel cell stack. The present invention calculates the corrosion rate of the coating in the fuel cell operating environment by comparing the contact resistance of the sample before and after the accelerated corrosion experiment ΔICR=ICR t -ICR 0 , where ICR t is the contact resistance after rapid corrosion test, ICR 0is the contact resistance of the coated plate in the initial state. Then, according to the change of the contact resistance at the cathode plate outlet over time during the actual fuel cell life test, a fitting curve of contact resistance and measured time is established, ΔΩ=f(t), where ΔΩ is the change value of the cathode outlet coating contact resistance during the fuel cell life test, and t is the time required for the contact resistance to change. If the plate substrate used in the accelerated corrosion life test and the fuel cell accelerated life test is the same, different coating materials will correspond to different attenuation rates, thereby obtaining different coating lifespans.

[0063] The test method for metal plate samples and offline contact resistance is as follows:

[0064] Combination Figure 2 When collecting metal plate samples for offline contact resistance, first take out multiple groups of cathode metal plates in the fuel cell stack, then confirm the cathode outlet of the plate, and collect about 1cm by laser cutting. 2 The relatively flat plate material (see the cathode outlet in the figure below) is finally tested for offline contact resistance using the above test method.

[0065] Combination Figure 3 As shown, firstly, the area of ​​1cm 2 The metal plate sample is sandwiched between two pieces of carbon paper (gas diffusion layer, H23C6) to form a sample sandwich; then the sample sandwich is sandwiched between two gold-plated copper electrodes to form a test electrode. Finally, the test electrode is pressurized to 1.4Mpa under the test press, and the voltage change observed by the contact resistance meter under 2A current is deducted from the voltage difference between the gas diffusion layer and the wire, and then combined with the area of ​​the sample, the contact resistance is calculated.

[0066] Combination Figure 4-6 As shown in the figure, the fuel cell stack life test method refers to the national standard GB / T 38914-2020. According to the requirements of the time and number of loading and unloading in the vehicle cycle, the idle, benchmark and rated road conditions are determined. Each cycle is one hour, and each cycle represents one day of actual vehicle operation. Figure 4 When the voltage decay at rated current reaches 10%, stop the life test and record the number of cycles and the voltage decay at idle speed and reference current.

[0067] Figure 5 The attenuation trend of the voltage at different currents in the battery stack life test of this embodiment is shown. Figure 6 The growth trend of the contact resistance of the sample at the cathode outlet in this embodiment with the number of cycles is shown. The corrosion potential and corrosion current density of different samples are shown in the following table:

[0068]

[0069] From the analysis of the above table, it can be seen that the corrosion potentials of #2 and #5 are higher, while the corrosion potentials of S1 and S2 coatings are lower and the corrosion current density is larger.

Claims

1. A method for evaluating and predicting the life of a fuel cell metal plate coating, characterized in that: This method is to predict the life under the condition of simulating the working condition of the fuel cell, and the method is as follows: S1. First, the corrosion potential of the fuel cell metal plate coating is determined by a dynamic potentiodynamic scan with an electrochemical potential scanning range of -0.6V-1.2V and a scanning rate of 0.5-1.5mV / s; S2, secondly, starting from the corrosion potential, slowly increasing the potential under the simulated fuel cell cathode operating environment, and continuing to scan the sample in step S1 at a scanning rate of 6 to 10 nV / s until the corrosion current exceeds a predetermined threshold or the potential reaches 1.6 V to 3.0 V; S3, measuring the contact resistance change of the sample scanned by electrochemical potentiodynamics in step S2 under a pressure of 1.4 MPa, to predict the life of the metal plate coating in the fuel cell operating environment; S4. The attenuation of the plate coating during the long-term operation of the fuel cell is estimated by the change in the resistance of the plate contact resistance, including the calculation of the corrosion rate of the coating in the fuel cell operating environment, that is, the contact resistance growth rate: ΔICR=ICR t -ICR 0 ICR t is the corrosion tested contact resistance, ICR 0 is the contact resistance of the coated plate in the initial state; S5. Perform a rapid life test on the fuel cell stack according to the predetermined working conditions, monitor the attenuation of the fuel cell stack performance during the life test, and obtain a curve of the contact resistance Ω at the cathode plate outlet versus time t, that is, a fitting curve of the contact resistance change and the measured time: ΔΩ=f(t) ΔΩ is the change in contact resistance of the cathode outlet coating during the fuel cell stack life test, and t is the time required to achieve the change in contact resistance; The functional relationship of ΔΩ=f(t) is a linear, exponential, or polynomial fit, and the sum of squares of the fitting residuals is greater than 0.9; S6. If the time taken for the fuel cell stack performance to drop to the end-of-life condition is T, the increase value of the cathode plate outlet coating contact resistance at this time is ΔΩ T ; Compare ΔΩ T and ΔICR, if ΔICR≤ΔΩ T , the plate coating life is greater than T, that is, the performance and quality of the plate coating meet the design requirements of the stack; If ΔICR>ΔΩ T , then the life of the coating is estimated by the relationship of ΔΩ=f(t) in the rapid life test of the battery stack; S7. Repeat steps S1-S4 to obtain the corrosion potential of the known coating on the metal plate and the change in contact resistance after electrochemical scanning, and establish a coating reliability evaluation standard based on corrosion potential and contact resistance, which becomes an evaluation method for metal plates with unknown coatings.

2. The method for evaluating and predicting the life of a fuel cell metal plate coating according to claim 1, characterized in that: The test method can also be used to test different coatings on the same metal plate substrate. The coating materials will correspond to different decay rates ΔICR, thereby predicting the life of the coating materials.

3. The method for evaluating and predicting the life of a fuel cell metal plate coating according to claim 1, characterized in that: The potentiodynamic corrosion potential scan in step S1 is performed with the metal plate as the working electrode, including sampling to obtain the metal plate sample as the working electrode, and then using Ag / AgCl as the reference electrode and a platinum electrode as the auxiliary electrode in a H2SO4 electrolyte solution at a temperature of 90°C, containing 0.1ppm HF, and pH=3. Before starting the test, air is introduced at a flow rate of 10-30mL / min for 10-60min. During the test, continuous ventilation is maintained on the liquid surface to simulate the actual fuel cell operating conditions, and a polarization curve for the metal plate coating is obtained; The measured polarization curve was fitted by Tafel, and the potential when the corrosion current was zero was the corrosion potential of the coating, and the current corresponding to the intersection of the Tafel straight line was the corrosion current of the coating sample.

4. The method for evaluating and predicting the life of a fuel cell metal plate coating according to claim 1, characterized in that: Step S2: In the coating electrochemical corrosion test, the simulated environment of the fuel cell is set to 110-120° C., the pressure is 1.5-2.0 bara, and the composition of the corrosion medium of the fuel cell is 1M H2SO4 solution containing 2.0 ppm HF; The electrochemical corrosion test unit consists of three electrodes, of which the metal plate of the coating to be tested is the working electrode with a working area of ​​1cm 2 , Ag / AgCl electrode is used as reference electrode, platinum electrode is used as auxiliary electrode, and the potential change of working condition is realized by controlling the potential in the test by electrochemical workstation.

5. The method for evaluating and predicting the life of a fuel cell metal plate coating according to claim 4, characterized in that: The coating electrochemical corrosion test involves placing the electrochemical test unit in a cylindrical high-pressure container with an inert Teflon material inner wall, and controlling the temperature and pressure stability during the test process through a PLC controller and temperature and pressure sensors; Before starting the test, air was introduced at a flow rate of 10-60 mL / min for 10-60 min, and continuous ventilation was maintained above the liquid level during the test.

6. The method for evaluating and predicting the life of a fuel cell metal plate coating according to claim 1, characterized in that: The step S7 of the method further includes scanning with a dynamic potential to achieve accelerated polarization of the metal plate coating, so as to evaluate the physicochemical stability of the coating; The reliability of more than one coating is ranked by potentiodynamic polarization. When a coating structure degrades during scanning, that is, the slope of corrosion current change exceeds a predetermined threshold during the potential scanning process, the scanning process will stop after the slope of corrosion current change returns to below the predetermined threshold, and the potential scanning spectrum will be recorded and analyzed. The corrosion charge is obtained by integrating the corrosion current over time, thereby drawing conclusions and rankings on the reliability of the coating.

7. The method for evaluating and predicting the life of a fuel cell metal plate coating according to claim 1, characterized in that: In the contact resistance test process, the following offline test method is also used: The metal plate sample is placed between two pieces of carbon paper to form a sample sandwich, and then the sample sandwich is placed between two gold-plated copper electrodes to form a test electrode. Finally, the test electrode is pressurized to 1.4Mpa under a test press, and the voltage change observed by the contact resistance meter at a current of 2A is removed by the voltage difference between the gas diffusion layer and the wire, and then combined with the area of ​​the sample to calculate the contact resistance; The metal plate sample is prepared in the following manner: Take out several groups of cathode metal plates from the fuel cell stack, confirm the cathode outlet of the plate, and then cut to obtain relatively flat plate material.

8. The method for evaluating and predicting the life of a fuel cell metal plate coating according to claim 7, characterized in that: The carbon paper is a porous material having a shape like a Freudenberg gas diffusion layer.

9. The method for evaluating and predicting the life of a fuel cell metal plate coating according to claim 7, characterized in that: The metal plate samples are cut by laser equipment, and the power of laser cutting is controlled between 100W-500W.

10. The method for evaluating and predicting the life of a fuel cell metal plate coating according to claim 1, characterized in that: Step S5 of the method includes predicting the life of the fuel cell stack based on different coating lives, including determining the idle, reference and rated road conditions according to the requirements of the time and number of loading and unloading in the vehicle cycle, and also includes determining the cycle period for cycle testing. When the voltage decay at the rated current reaches the set minimum threshold, the life test is stopped and the number of cycles and the voltage decay at idle and reference current are recorded.

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