Method for judging cold start capacity of electric pile

Abstract

The invention discloses a method for judging the cold start capacity of a galvanic pile, which comprises the following steps: S1, stacking a plurality of membrane electrodes respectively to form a galvanic pile which adopts the same bipolar plate and auxiliary component; s2, activating the galvanic piles, testing a polarization curve, and recording performance data of each galvanic pile; s3, placing the galvanic pile in a constant-temperature environment, introducing inert gas with uniform humidity for purging, normalizing the water-containing state of a membrane electrode, monitoring the membrane resistance in real time by using high-frequency impedance, and judging that the purging is completed when the high-frequency impedance value rises from stable to finally tends to be constant; s4, the swept stack is placed in a low-temperature environment to be subjected to freezing and heat preservation treatment, the freezing temperature ranges from-10 DEG C to-30 DEG C, and the freezing time ranges from 4 h to 8 h; and S5, performing low-temperature cold start on the pile, and recording data from start to pile failure.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of fuel cells, and in particular to a method for determining the cold start-up capability of a fuel cell stack. BACKGROUND

[0002] The problem of starting up a fuel cell at low temperatures has been an important technical bottleneck restricting its large-scale popularization and application. In the early development process of the fuel cell stack, the research and development focus is often on the output performance and durability of the membrane electrode, and less attention is paid to its start-up capability in low temperature environments. With the advancement of industrialization, different manufacturers often need to conduct a large number of cold start-up tests when verifying the performance of their fuel cell stack products. Such tests not only have a long cycle and high cost, but also have relatively strict requirements for equipment and environmental conditions. Especially during the cold start-up verification in the system integration stage, a large low-temperature environmental chamber and a complex cooling circuit are often needed, and if the low-temperature start-up performance of the fuel cell stack is insufficient, it may not only lead to the failure of the vehicle system, but also pose a safety risk to the equipment.

[0003] In view of the above technical problems, some research has attempted to indirectly evaluate the cold start-up capability of the fuel cell stack by monitoring the hydration state of the fuel cell stack. For example, Chinese Patent Application Publication No. CN119764490A, entitled "Method for judging low-temperature cold start-up purging degree of fuel cell stack", discloses a method for judging the water content state in the fuel cell stack by combining high-frequency resistance change and voltage monitoring. The method connects the fuel cell stack to an electrochemical impedance spectroscopy (EIS) device, and under the conditions of temperature rise and set back pressure, fuel gas is introduced, a small current is loaded in combination with high-flow purging, and the high-frequency impedance curve of the fuel cell stack is monitored in real time. When the impedance value and the voltage curve tend to be stable, it is judged that the purging is completed, which is used as a basis for judging the preparation state of the cold start-up of the fuel cell stack.

[0004] Although this method introduces impedance spectrum monitoring means, it can to some extent reflect the water content state in the fuel cell stack, but its technical goal mainly focuses on the cold start-up preparation of a single fuel cell stack, that is, how to ensure that the fuel cell stack has basic feasibility for cold start-up after purging is completed, which belongs to a functional verification scheme after the fuel cell stack system is built. There is a lack of a horizontal evaluation mechanism for the performance differences of different fuel cell stacks, and only a method for judging whether the purging is completed or not is provided, and a systematic evaluation framework reflecting the water storage capacity or cold start-up bearing capacity of the fuel cell stack has not been established. SUMMARY

[0005] The purpose of the present application is to overcome the defects existing in the prior art, provide a method for determining the cold start ability of the stack, which can carry out comparative cold start test under uniform initial conditions for the stacks formed by various types of membrane electrode assemblies in the early stage of system integration, and quantitatively analyze the water storage capacity and cold start bearing capacity of the stack in the low temperature environment by combining the voltage response changes in the actual operation process, so as to establish a set of horizontal performance evaluation system suitable for the material research and development stage.

[0006] The technical solution for achieving the purpose of the present application is: a method for determining the cold start ability of the stack, comprising the following steps:

[0007] S1, a plurality of membrane electrodes are stacked to form a stack, and the stack uses the same bipolar plate and auxiliary components;

[0008] S2, the stack is activated and polarization curve test is carried out, and the performance data of each stack is recorded;

[0009] S3, the stack is placed in a constant temperature environment, and inert gas with uniform humidity is introduced for purging, the water content of the membrane electrode is normalized, and high frequency impedance is used to monitor the membrane resistance in real time, when the high frequency impedance value is from flat to rising and finally tends to be constant, it is judged that the purging is completed;

[0010] S4, the stack after purging is placed in a low temperature environment for freezing and insulation treatment, the freezing temperature is-10 to-30 DEG C, and the freezing time is 4 to 8 hours;

[0011] S5, the stack is started at low temperature, and the loading time from starting to stack failure is recorded.

[0012] Further, in step S3, the water content λ of the membrane electrode and the relative humidity a of the purging gas satisfy the following quantitative relationship: λ = 0.043 + 17.81a - 39.85a 2 + 36a 3 , and the λ value is controlled between 2 and 5.

[0013] Further, in step S3, the purging process includes two stages: the first stage is a rapid dehydration stage, the duration is 5 to 10 minutes, and the liquid water in the flow channel and the diffusion layer is removed; the second stage is a slow equilibrium stage, the duration is 0.5 to 3 hours, and the water content of the membrane electrode and the relative humidity of the gas are completely balanced.

[0014] Further, in step S3, the purging temperature is controlled to be 45 to 65 DEG C, the purging gas flow is set according to the current density of 0.25 to 0.8 A / cm 2 , and the anode metering ratio and the cathode metering ratio are (1.8 to 6):(5 to 10).

[0015] Optionally, in step S3, the purge gas flow rate is 0.5 A / cm 2 After calculation under the current density of , the anode stoichiometric ratio and the cathode stoichiometric ratio are set to 3:8.

[0016] Furthermore, in step S3, the inert gas is nitrogen or helium.

[0017] Furthermore, in step S4, the freezing and heat preservation treatment is: placing the purged battery stack in a freezing chamber for static cooling, and the freezing chamber is a controllable temperature constant temperature device with a temperature control accuracy of no more than ±1°C.

[0018] Furthermore, in step S5, the low-temperature cold start mode is a constant current mode or a constant voltage mode.

[0019] Optionally, in step S5, the starting current density is set to 0.05-0.1 A / cm 2 , record the time from the start of loading to the time when the single cell voltage drops to 0.1V, obtain the voltage change curve with time, and use this time as the cold start capability evaluation index.

[0020] Optionally, in step S5, the starting voltage is set to 0.6-0.8V, the current change curve with time is recorded, the integral value of the current with time from the start of loading to the current dropping to 0A is calculated, and the integral value is used as an evaluation index of the cold start capability.

[0021] After adopting the above technical solution, the present invention has the following positive effects:

[0022] (1) The present invention can perform a low-temperature cold start test on a stack formed by assembling different types of membrane electrodes through a unified test process, and record the loading time for the voltage to drop to the cutoff value as an evaluation index. The cold start capability of the stack can be quickly determined in a simple manner. The operation is convenient, the determination process is clear, and the stack has good practicality.

[0023] (2) The present invention advances the screening process of the cold start capability of the fuel cell stack to the material development stage, avoiding the waste of resources caused by repeated cold start verification of the entire stack at the system level, effectively reducing the research and development time during the cold start of the fuel cell stack and the system level, and reducing and saving the overall research and development costs.

[0024] (3) During the test, the present invention introduces a constant temperature environment and a purge gas with uniform humidity to treat the fuel cell stack, which can accurately control the initial water content of different types of membrane electrodes, eliminate the interference of non-structural factors such as human operation differences, environmental disturbances and pre-operating conditions, and is more conducive to reflecting the differences in the characteristics of the membrane electrodes themselves, ensuring the objectivity and scientific nature of the comparison results.

[0025] (4) The application adopts quantitative voltage response with respect to time as a characterization means, and can further combine the loading current and time parameters to calculate the actual water capacity of the stack, establish a quantitative evaluation system corresponding to the strength of the cold start ability, and help to form a scientific and systematic stack performance screening and optimization process.

[0026] (5) The method of the application has strong universality, is suitable for various structural forms and performance levels of membrane electrodes and stack design schemes, has good engineering popularization prospect and application adaptation ability, and provides an effective decision support tool for the early development of fuel cell stacks. BRIEF DESCRIPTION OF DRAWINGS

[0027] Figure 1 is a single piece average high frequency impedance change diagram of the stack purging process of the application;

[0028] Figure 2 is a stack cold start process diagram of the application. DETAILED DESCRIPTION

[0029] The technical solutions of the application will be described in detail below in combination with the drawings and examples, and the drawings are only used for illustrative description and do not constitute a limitation on the protection scope of the application.

[0030] In order to accurately evaluate the cold start ability of the fuel cell stack in the material development stage, the embodiment proposes a technical process based on unified hydration state control and cold start process evaluation, which is used to screen membrane electrode materials with excellent low temperature start-up performance in the early experimental stage. The method proposed by the application will be described below in combination with a specific experimental process.

[0031] The embodiment aims to establish a standardized process that can be used for comparative evaluation of the cold start ability of fuel cell stacks, and the key is to control the key variables and unify the operating conditions, so that different types of membrane electrodes can be evaluated for cold start ability under comparable conditions.

[0032] First, step S1 is performed, and a plurality of membrane electrodes are respectively stacked to form stack samples. The membrane electrodes can be sourced from different manufacturers or have different structural parameters (such as catalyst layer thickness, Nafion load, microporous layer structure, etc.), representing various research and development stages or supplier schemes. During the stacking process, the same specification of bipolar plates and auxiliary components are used to ensure that the structural conditions except the membrane electrodes remain consistent. The auxiliary components include standard components such as compression structure, sealing ring, current collector plate, etc.

[0033] In this embodiment, three representative membrane electrodes are selected for testing, which are: MEA-A: a commercial standard membrane electrode, representing a traditional cost control scheme; MEA-B: a high water capacity membrane electrode, with stronger water storage capacity; MEA-C: a membrane electrode suitable for low humidity working conditions, with excellent dry state conductivity.

[0034] The three membrane electrodes are assembled to form stacks 1, 2 and 3, respectively. Except for the membrane electrode, the remaining structure and stack parameters are kept consistent to ensure the fairness of the comparison test.

[0035] The purpose of the stack assembly step is to control the variables, so that the membrane electrode is the only changing factor in the test process. If different types of bipolar plates or auxiliary structures are used, the differences in their thermal conductivity, flow field arrangement, and even sealing pressure may significantly affect the ice formation behavior or water distribution inside the stack, making it impossible to attribute the cold start performance to the membrane electrode itself.

[0036] Then step S2 is performed, and the three stacks are sequentially activated. The activation process uses a multi-stage constant current loading mode to make each cell of the stack reach a stable working state, thereby significantly reducing performance fluctuations and effectively avoiding confusion between insufficient activation and low temperature-induced performance degradation in subsequent tests. The specific steps are as follows:

[0037] (1) Initial small current activation: In the initial stage, a current density of 0.2 A / cm 2 is used to run, and the cell voltage is monitored until the voltage fluctuation is less than or equal to ±5 mV, indicating preliminary stability.

[0038] (2) Stepwise current increase: The current density is increased step by step by 0.4 A / cm 2 , 0.6 A / cm 2 , and 0.8 A / cm 2 , with each step lasting 20-30 minutes; until the voltage drop rate is less than or equal to 1 mV / min.

[0039] (3) Rated current stable operation: At a rated current density of 1.0 A / cm 2 , stable operation is performed for 2-4 hours, so that the voltage fluctuation range does not exceed ±2% of the rated voltage.

[0040] (4) If the voltage fluctuation range exceeds ±2% of the rated voltage, it indicates that the performance is still unstable, and the above steps can be repeated for 3-5 times of charge and discharge cycles.

[0041] (5) Until the voltage change rate is less than or equal to 0.5 mV / min, the activation process is terminated.

[0042] Through this process, the catalytic layer in the membrane electrode can gradually establish an effective charge transport channel, while the proton membrane is fully hydrated and forms a complete hydration structure, thereby improving the overall conductivity and reaction uniformity of the stack.

[0043] After activation is completed, to further verify the activation effect and record the stack performance parameters, continue to perform the polarization curve test. This test is completed by setting the current density point by point and recording the steady-state voltage, i.e. the constant current method, the specific steps are as follows:

[0044] (1) In the low current area (0-0.2A / cm 2 ), load point by point with a step size of 0.05A / cm 2 , each point lasts for 3-5min, until the voltage tends to be stable with a change rate less than or equal to 1mV / min. This area mainly observes the activation polarization characteristics of the stack, reflecting the activity degree of the catalytic layer in the membrane electrode and the interface reaction efficiency;

[0045] (2) In the medium current area (0.2-1.0A / cm 2 ), load with a step size of 0.1A / cm 2 , each point lasts for 2-3min, the slope of the test results in this stage directly reflects the ohmic polarization characteristics of the stack, i.e. the linear voltage drop caused by factors such as electron and proton conduction in the stack and contact resistance. The greater the ohmic polarization, the greater the energy loss caused by the membrane resistance, contact resistance or electrode conductivity;

[0046] (3) In the high current area (greater than 1.0A / cm 2 ), the step size can be increased to 0.2A / cm 2 , until the stack voltage drops to 0.6V, this stage is used to identify the concentration polarization effect, i.e. the gas mass transfer limitation caused by insufficient supply of reaction gas or liquid water blocking the diffusion channel. Concentration polarization will cause a sharp drop in voltage under high current conditions, forming a "knee point" on the polarization curve, which is an important indicator of the performance decline of the fuel cell.

[0047] During the entire test process, key operating parameters such as current density, voltage, temperature, humidity, gas flow, etc. need to be recorded synchronously to ensure the comprehensiveness and repeatability of the data. To improve the accuracy of the test, it can be repeated 1-2 times after the test to verify the data stability, and if necessary, supplemented by electrochemical impedance spectroscopy (EIS) to further analyze the composition of ohmic impedance and mass transfer impedance. The final polarization curve can comprehensively reflect the performance changes of the stack under different loads, serving as the basis data for performance comparison before and after cold start.

[0048] After stack activation is complete, step S3 is performed, placing each stack in a constant temperature environment at 50°C and commencing a purge operation under uniform conditions. The purge gas used is partially humidified nitrogen, an inert gas. Nitrogen is passed through both the anode and cathode of the stack to be purged. Nitrogen does not react with the membrane electrode, thus preventing the introduction of additional moisture, which helps maintain the controllability of the membrane hydration process.

[0049] The relative humidity RH of the purge gas is set based on the quantitative requirements of membrane hydration state control. The water content λ of the membrane electrode and the relative humidity a of the purge gas satisfy the following quantitative relationship:

[0050] λ=0.043+17.81a-39.85a 2 +36a 3 ,

[0051] In this example, the temperature is set at 50°C and the target λ is set to 3. Substituting this into the above formula yields a corresponding purge gas humidity RH of 37.1%. This allows for uniform initial water content across different membrane electrode models, eliminating the effects of varying purge conditions, internal stack variations, and operator control, facilitating comparison.

[0052] To eliminate the influence of the difference in water storage capacity of the membrane electrode itself, the purge needs to be continued until the membrane water content reaches thermodynamic equilibrium with the lambda corresponding to the set RH. The flow rate of the purge gas is set at a stoichiometric ratio of 3:8 between the anode and cathode, and the stoichiometric ratio is 0.5A / cm 2 The theoretical gas consumption at the current density is converted and determined to ensure that the flux meets the water migration requirements without drying out the membrane. This flow rate setting is based on engineering experience and ensures sufficient gas exchange rate while suppressing membrane mechanical fatigue and flow field disturbance.

[0053] The entire purge process is divided into two stages according to the characteristics of moisture migration:

[0054] Stage 1: Rapid Dehydration: Lasting 10 minutes, a high-flow purge gas is used to remove liquid water from the flow channels and the pores of the gas diffusion layer (GDL). This dehydration process relies primarily on physical processes such as airflow scouring and thermal evaporation. While the water migration rate is relatively fast, most of the water exists as a free liquid and has not yet penetrated the proton exchange membrane structure, resulting in minimal changes in HFR.

[0055] Phase 2: Slow Equilibrium: This lasts for 2 hours. As free water gradually drains away, bound water in the membrane begins to migrate outward via diffusion and desorption. This process is dominated by the relative humidity (RH) of the gas. As the water content decreases, the membrane's proton conductivity gradually decreases, and the HFR value gradually increases. When the HFR trend stabilizes, it indicates that the membrane water content and the relative humidity of the gas have reached thermodynamic equilibrium, reaching the set λ value.

[0056] During the whole purging process, the stack is tested on-line by electrochemical impedance spectroscopy (EIS) device to extract high frequency resistance (HFR) value in real time, so as to monitor the change of hydration state of membrane electrode. HFR is usually taken at a frequency above 1000 Hz, which represents the ion conduction resistance in the proton membrane, and this resistance is highly sensitive to the water content in the membrane: the higher the hydration degree of the membrane, the lower the HFR value; otherwise, the higher the HFR value.

[0057] Therefore, during the continuous purging process, if it is observed that the HFR value rises significantly over time and eventually stabilizes, it can be inferred that the water content in the membrane and the gas humidity have reached a balanced state, so as to determine that the purging process is completed. Figure 1 That is, the single-piece average high-frequency resistance change graph of the stack purging process collected by the present application, in which the horizontal axis is the purging time (unit: seconds), and the vertical axis is the high-frequency resistance per unit area (unit: mΩ·cm 2 ). It can be seen from the graph that the HFR curve experiences three typical stages: the slow rising stage in the early stage is 0-100 s, the rapid rising stage in the middle stage is 100-300 s, and the balance platform stage is stable after 300 s.

[0058] Specifically, when the HFR enters the platform and the fluctuation amplitude is less than a set threshold (such as ±2%), it can be considered that the initial water content state of the membrane electrode has been regulated, and the subsequent cold start experiment after stopping purging can be carried out in this state, ensuring the consistency of the initial conditions of each type of stack.

[0059] Then, step S4 is performed to transfer the completed purging stack to a low-temperature environment for freezing treatment. The freezing environment is realized by a constant-temperature freezing warehouse with a temperature control accuracy of not less than ±1℃, and the freezing temperature is set at -20℃. The stack is cooled and kept at this temperature for 5 h to ensure that the core area and the surface layer reach a thermodynamic equilibrium state. This process aims to simulate the working conditions in a typical cold region application scenario, so as to make the residual water in the membrane electrode completely freeze, thereby evaluating the start-up reliability of the stack in the actual cold environment. In a fuel cell, the ice crystals generated by freezing can block the hydrated proton channels in the proton membrane, the pores in the catalyst layer and the gas diffusion layer, which seriously affects the ion conduction and gas diffusion processes, so this step can effectively amplify the structural differences in the low-temperature frost resistance of different membrane electrodes, which is beneficial to subsequent identification and evaluation.

[0060] After completing the freezing and keeping, step S5 is performed to apply a constant current of 0.1 A / cm 2 to the stack for low-temperature cold start test, and in this embodiment, the start-up current density is set to 0.1 A / cm 2 , and the loading time is recorded until the single-cell voltage of the stack inside drops to 0.1 V due to icing or membrane failure.

[0061] The loading time is the core index of the application for evaluating the cold start ability of the membrane electrode. Since the water content of each stack is initially consistent under the same purging humidity and freezing conditions, the freezing dynamic inside the stack is mainly dominated by the water storage capacity of the membrane electrode, that is, the longer the reaction duration that the stack can withstand under the frozen state, the stronger the adsorption and retention capacity of the membrane electrode to water, the higher the cold start bearing capacity, and the better the performance of the material itself in adapting to the cold environment.

[0062] Figure 2 The voltage change curves of the various types of stacks in the cold start process of the application can be seen. It can be seen that the three stacks all experienced a significant stage evolution process under the same loading conditions.

[0063] The first stage has not started loading current, only the reaction gas is passed. Since no substantial electrochemical reaction occurs, the voltage in the open circuit stage is maintained at 0.95V, indicating that the membrane electrode structure is complete and the gas supply is normal.

[0064] The second stage is the initial response stage of loading, the current density is 0.1A / cm 2 The voltage decreases slightly and enters a short platform zone during the cold start. With the running time, water accumulates in the cathode diffusion layer and the catalyst layer, and ice appears in some areas, the mass transfer channel is blocked but not completely blocked, and the duration of this stage reflects the carrying capacity of the membrane electrode structure to water and ice, that is, its water storage capacity.

[0065] The third stage is the critical point where the voltage drops to 0.1V, when the ice crystals inside the membrane electrode reach the limit, the proton channel and the reaction site are completely blocked, the polarization is serious, the reaction fails, and the output voltage of the stack collapses rapidly.

[0066] The fourth stage shows that the voltage quickly rises to near the open circuit voltage level, indicating that the test stops loading, the system enters the open circuit state, and the original potential is restored.

[0067] From the curve comparison, it can be seen that the time of different types of stacks maintaining in the platform zone is obviously different. Among them, the 2nd stack formed by MEA-B high water storage type membrane electrode maintains the longest time, and the 3rd stack formed by MEA-C suitable for low humidity working conditions has the shortest maintenance time, which shows that the 2nd membrane electrode design has better performance in water storage and ice blocking resistance, can maintain effective mass transfer and reaction for a longer time, has stronger cold start ability. Therefore, the time experienced from the cold start loading to the voltage drop to 0.1V can be used as a key index to judge the cold start performance of the membrane electrode.

[0068] By comparing the time of different stacks from loading to shutdown, not only the quantitative evaluation of the cold start ability of the membrane electrode can be realized, but also the material selection and optimization can be carried out in the early stage of system development, avoiding resource waste and risk accumulation.

[0069] In another embodiment of the present application, the low-temperature cold start test is performed in a constant voltage mode. The stack after the purging and freezing process is placed in a low-temperature environment of-20℃, and a constant voltage of 0.6V is set. The output current-time curve I(t) of the stack from the starting time t1 to the ending time t2 when the current drops to 0A is recorded. The total charge Q is calculated by integrating the curve, i.e.:

[0070]

[0071] wherein Q: the total charge output by the stack during the low-temperature cold start process, in coulombs (C); I(t): the current density of the stack at any time, in amperes per square centimeter (A / cm2). 2

[0072] In actual tests, since the collected data is discrete, the integral can be solved in a numerical approximation form:

[0073] wherein Ii and Ii+1 are the current density values at the i-th and i+1-th sampling points, ti and ti+1 are the corresponding time stamps, and the unit of the integral result is A·s / cm2(equivalent to C / cm2). i i+1 i i+1 2 2

[0074] The integral value Q is a comprehensive index reflecting the ability of the membrane electrode to continuously support electrochemical reactions in a low-temperature environment. If the output current of the stack is maintained within a stable range for a long time, the integral area Q value is large, indicating that the membrane electrode structure has good water storage capacity, anti-freezing performance and low-temperature conductivity. If the current rapidly decreases or even cannot be maintained, the Q value is small, indicating that the membrane structure is prone to polarization failure or ice blockage in low temperature, and the cold start ability is poor.

[0075] The above-described specific embodiments further illustrate the purpose, technical solutions and beneficial effects of the present application. It should be understood that the above-described embodiments are merely specific embodiments of the present application and are not intended to limit the present application. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present application shall be included in the protection scope of the present application.​​​​​​​

Claims

1. A method for determining the cold start capability of a fuel cell stack, characterized in that: The steps include: S1. Stacking several types of membrane electrode respectively to form a stack, wherein the stack uses the same bipolar plates and auxiliary components; S2. activating the battery stack, performing polarization curve testing, and recording performance data of each battery stack; S3. Placing the stack in a constant temperature environment, passing an inert gas of uniform humidity for purging, normalizing the water content of the membrane electrode, and using high-frequency impedance to monitor the membrane resistance in real time. When the high-frequency impedance value increases from stable to constant, the purging is determined to be complete. S4, placing the purged battery stack in a low-temperature environment for freezing and heat preservation treatment, wherein the freezing temperature is -10 to -30°C and the freezing time is 4 to 8 hours; S5. Perform a low-temperature cold start on the fuel cell stack and record data from the start-up to the failure of the fuel cell stack.

2. The method for determining the cold start capability of a fuel cell stack according to claim 1, characterized in that: In step S3, the water content λ of the membrane electrode and the relative humidity a of the purge gas satisfy the following quantitative relationship: λ = 0.043 + 17.81a - 39.85a 2 +36a 3 , the λ value is controlled between 2 and 5.

3. The method for determining the cold start capability of a fuel cell stack according to claim 1, wherein: In step S3, the purge process includes two stages: the first stage is a rapid dehydration stage, which lasts for 5 to 10 minutes and removes liquid water in the flow channel and the diffusion layer; the second stage is a slow equilibrium stage, which lasts for 0.5 to 3 hours and completely balances the water content of the membrane electrode with the relative humidity of the gas.

4. The method for determining the cold start capability of a fuel cell stack according to claim 1, wherein: In step S3, the purge temperature is controlled at 45-65°C, and the purge gas flow rate is 0.25-0.8 A / cm 2 After calculation under the current density of , the anode stoichiometric ratio and the cathode stoichiometric ratio are (1.8~6):(5~10).

5. The method for determining the cold start capability of a fuel cell stack according to claim 4, characterized in that: In step S3, the purge gas flow rate is 0.5A / cm 2 After calculation under the current density of , the anode stoichiometric ratio and the cathode stoichiometric ratio are set to 3:

8.

6. The method for determining the cold start capability of a fuel cell stack according to claim 1, characterized in that: In step S3, the inert gas is nitrogen or helium.

7. The method for determining the cold start capability of a fuel cell stack according to claim 1, characterized in that: In step S4, the freezing and heat preservation treatment is: placing the purged battery stack in a freezing chamber for static cooling. The freezing chamber is a controllable and constant temperature device with a temperature control accuracy of no more than ±1°C.

8. The method for determining the cold start capability of a fuel cell stack according to claim 1, characterized in that: In step S5, the low-temperature cold start mode is a constant current mode or a constant voltage mode.

9. The method for determining the cold start capability of a fuel cell stack according to claim 8, characterized in that: In step S5, when the low temperature cold start mode is a constant current mode, the starting current density is set to 0.05-0.1 A / cm 2 , record the time from the start of loading to the time when the single cell voltage drops to 0.1V, obtain the voltage change curve with time, and use this time as the cold start capability evaluation index.

10. The method for determining the cold start capability of a fuel cell stack according to claim 8, characterized in that: In step S5, when the low-temperature cold start mode is a constant voltage mode, the starting voltage is set to 0.6-0.8V, the current change curve with time is recorded, and the integral value of the current with time from the start of loading to the current dropping to 0A is calculated, and the integral value is used as an evaluation index of the cold start capability.

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