Online evaluation system and online evaluation method for cold start capability of fuel cell

Abstract

The invention relates to an on-line evaluation system and an on-line evaluation method for the cold start-up capability of a fuel cell. The on-line evaluation system comprises a fuel cell stack, an on-line evaluation system and a DCDC, the on-line evaluation system specifically comprises an acquisition unit used for acquiring the voltage of a fuel cell stack and the current of a DCDC input end; the impedance spectrum measurement unit is used for carrying out impedance calculation; the impedance spectrum analysis unit is used for calculating polarization process information of impedance; the excitation signal unit is used for receiving an instruction of the impedance spectrum measurement unit and controlling the amplitude and the frequency of alternating current excitation superposed on the DCDC direct current; the storage unit internally comprises a maximum water capacity database; and the control unit is used for extracting the maximum water capacity database information, obtaining the maximum water capacity of the current state in combination with the calculated polarization process information, and further transmitting a control signal to the fuel cell stack. Compared with the prior art, the polarization process information can be acquired in real time, and the cold start capability of the fuel cell can be acquired and evaluated on line in combination with the maximum water capacity database.

Description

Technical Field

[0001] The present invention belongs to the technical field of fuel cells, and in particular relates to an online evaluation system and an online evaluation method for cold start capability of a fuel cell. Background Art

[0002] Proton exchange membrane fuel cells have attracted much attention as a clean, efficient, environmentally adaptable and energy-dense energy conversion technology, and are widely considered to be a promising solution for future low-carbon energy. However, for fuel cell vehicles to be commercialized, they need to overcome many technical bottlenecks such as durability, cost, and cold start capability. Regarding the issues related to low-temperature operation, for the cold start problem related to fuel cell vehicles, the US Department of Energy (DOE) requires that the fuel cell should not be damaged after repeated start-stop at -40°C, and that it can be started without auxiliary preheating at -30°C.

[0003] As the core indicator of cold start of fuel cells, cold start capability is defined in this article as the maximum water capacity of the fuel cell during low-temperature start. The water capacity of the fuel cell is mainly reflected in the water absorption of the catalyst layer and the proton exchange membrane. Therefore, the liquid water remaining after the fuel cell vehicle is shut down will reduce the water capacity of the catalyst layer and the membrane. At present, the shutdown purge of fuel cell vehicles is mostly loaded rapid purge. How to evaluate the water content of the catalyst layer and the proton exchange membrane after the rapid purge, and then online evaluate the cold start capability of the fuel cell, is to lay the foundation for the optimization of shutdown purge operation parameters and the development of low-temperature cold start strategies.

[0004] CN111199110A discloses a method and system for predicting the low-temperature start-up performance of a fuel cell: by establishing a low-temperature start-up model for a fuel cell, inputting the parameters and conditions to be predicted into the low-temperature start-up model for the fuel cell, and then obtaining the low-temperature start-up performance and internal state distribution of the fuel cell to be predicted. The invention is to perform simulation calculations on a variety of parameters including internal geometric structures and various physical and chemical properties obtained in advance. The simulation calculation model has a certain applicable limitation range and is mainly used in the development and design stage of fuel cells to provide development directions for designers.

[0005] CN111709149A discloses a method for evaluating the cold start capability of a fuel cell stack: by setting the maximum cell voltage deviation and cathode stoichiometry of the fuel cell stack, the heating power and water production rate of the stack are obtained under the maximum cell voltage deviation, thereby calculating the water heating value of the stack, and then dividing it by the heat capacity of the stack to obtain the cold start capability index of the stack. The larger the value, the stronger the cold start capability. This invention is a method for evaluating the cold start capability under the specific strategy of oxygen starvation, and does not consider the influence of residual liquid water from shutdown purge, which has certain limitations.

[0006] CN117471329A discloses a method for consistently and rapidly evaluating the cold start capability of a fuel cell: by setting different gas humidities for balanced purge, the high-frequency impedance values ​​of different fuel cells under different humidities are compared to evaluate the cold start capability. This invention is a relatively mature method for evaluating the cold start capability of MEA, but it is not suitable for the rapid purge conditions of fuel cell vehicles. First, rapid purge cannot achieve precise control of humidity; second, the rapid purge time is short and the purge of each component is uneven, making it difficult to characterize the water content inside the fuel cell through high-frequency impedance.

[0007] At present, the evaluation of the cold start capability of fuel cells is mainly based on model prediction, high-frequency impedance and other methods, which have limitations such as the inability to evaluate online or incomplete information, making it difficult to achieve online evaluation of the cold start capability of fuel cells after shutdown and rapid purge. Summary of the invention

[0008] The purpose of the present invention is to provide a fuel cell cold start capability online evaluation system and online evaluation method in order to overcome the defect of the prior art that it is difficult to realize the online evaluation of the cold start capability of the fuel cell after shutdown and rapid purge.

[0009] The purpose of the present invention can be achieved by the following technical solutions:

[0010] The present invention provides an online evaluation system for cold start capability of a fuel cell, which comprises a fuel cell stack, an online evaluation system and a DCDC (direct current to direct current voltage regulator converter); the DCDC is connected to the fuel cell stack and is used to control the output power of the fuel cell stack;

[0011] The online evaluation system specifically includes:

[0012] A collection unit, used to collect the voltage of the fuel cell stack and the current signal of the DCDC input terminal;

[0013] An impedance spectrum measurement unit, used for receiving data collected by the collection unit and performing impedance calculation;

[0014] An impedance spectrum analysis unit is used to receive the impedance calculated by the impedance spectrum measurement unit and calculate the polarization process information of the impedance through DRT;

[0015] The excitation signal unit is used to receive the instruction of the impedance spectrum measurement unit, control the amplitude and frequency of the AC excitation superimposed on the DC of the DCDC, and transmit the AC excitation signal to the DCDC;

[0016] A storage unit, which internally includes a maximum water holding capacity database;

[0017] The control unit is used to extract the maximum water holding capacity database information, and obtain the maximum water holding capacity of the current state in combination with the calculated polarization process information, and then transmit the control signal to the fuel cell stack.

[0018] Furthermore, the impedance spectrum measurement unit calculates the impedance by using methods such as FFT algorithm, wavelet transform or s transform.

[0019] Furthermore, the polarization process information calculated by the impedance spectrum analysis unit includes activation resistance, mass transfer resistance and ohmic resistance.

[0020] Furthermore, the maximum water holding capacity database includes a correspondence between polarization process information and maximum water holding capacity values.

[0021] The present invention also provides an online evaluation method using the online evaluation system for cold start capability of a fuel cell, comprising the following steps:

[0022] S1: Perform low-temperature shutdown and purge according to the preset shutdown and purge operating conditions;

[0023] S2: Regularly measure impedance spectrum;

[0024] S3: Perform DRT fitting on the impedance spectrum to obtain polarization process information;

[0025] S4: obtaining a maximum water holding capacity database;

[0026] S5: according to the polarization process information obtained in S3, searching the maximum water capacity corresponding to the maximum water capacity database, and comparing it with the water capacity threshold;

[0027] S6: If the maximum water holding capacity value is greater than the water holding capacity threshold, the purge is stopped; if the maximum water holding capacity value is less than the water holding capacity threshold, the process returns to S1 and continues until the maximum water holding capacity value is greater than the water holding capacity threshold.

[0028] Furthermore, step S2 specifically includes the following steps:

[0029] S21: Wide spectrum measurement of fuel cell impedance spectrum under steady-state conditions;

[0030] S22: Refer to S3 to perform DRT fitting on the impedance spectrum to obtain polarization process information;

[0031] S23: Analyze the characteristic frequency distribution range and logarithmic frequency interval of each polarization process of the fuel cell.

[0032] S24: Determine the frequency range and minimum multiple frequency points of impedance spectrum measurement;

[0033] S25: Performing KK verification on the impedance spectrum data of the selected measurement frequency point;

[0034] S26: If the residual error of the KK calibration is less than 1%, a fast impedance spectrum measurement under low temperature shutdown purge is performed with reference to the parameters of the measurement frequency point;

[0035] If the residual does not meet the requirement, the process returns to S21 until the impedance spectrum measurement parameters are confirmed.

[0036] Furthermore, step S3 specifically includes the following steps:

[0037] S31: setting the fitting parameters of the DRT fitting method, including characteristic frequency resolution and regularization parameters;

[0038] S32: Calculate the characteristic frequency and peak area of ​​each DRT characteristic peak;

[0039] S33: Identify the polarization process of DRT characteristic peaks.

[0040] Furthermore, in step S33, the characteristic frequency at 1-10 Hz is the mass transfer process, 20-100 Hz is the charge transfer process, above 200 Hz is the anode and catalyst layer proton transfer process, and the high frequency intercept is the Ohmic process.

[0041] Furthermore, step S4 specifically includes the following steps:

[0042] S41: Conducting an orthogonal test of shutdown purge under different purge operation conditions;

[0043] S42: Referring to S2 and S3, impedance spectrum measurement and polarization process analysis are performed at the end of shutdown purge;

[0044] S43: After the purge is completed, low-temperature storage is performed to fully freeze the fuel cell system;

[0045] S44: Carry out a constant temperature cold start test under low current and record the load time;

[0046] S45: Calculate the maximum water holding capacity under the operating conditions;

[0047] S46: Establishing a mapping relationship between each polarization resistance and the maximum water holding capacity.

[0048] Furthermore, in step S41, the operating conditions include purge temperature, purge flow rate and purge time.

[0049] Furthermore, in step S44, the current density of the small current is not greater than 150 mA / cm 2 .

[0050] Furthermore, in step S44, the loading time is the time during which the loading is continued at a low current until the startup fails due to the entire catalyst layer being frozen.

[0051] Compared with the prior art, the present invention has the following beneficial effects:

[0052] (1) The cold start capability online evaluation system and method of the present invention obtains polarization process information in real time and combines it with the maximum water capacity database to online obtain and evaluate the cold start capability of the fuel cell, laying a foundation for parameter optimization of shutdown purge operation and development of low-temperature cold start strategy.

[0053] (2) The cold start capability online evaluation system of the present invention can quickly measure the impedance spectrum of the fuel cell stack during the shutdown purge process, obtain the characteristic frequency and logarithmic interval of each polarization process of the fuel cell through the DRT method, and select the frequency range and multiple points of the fast impedance spectrum based on the characteristic frequency, thereby realizing the rapid measurement of the impedance spectrum and avoiding the time-varying impedance measurement during the shutdown process.

[0054] (3) The cold start capability online evaluation system of the present invention can obtain online all-round kinetic polarization process information of the fuel cell stack, and provide physically meaningful polarization process parameters for the online evaluation method through the DRT quantitative analysis method. The parameters are used to identify the water content in different components of the fuel cell stack during the shutdown purge process, thereby avoiding the limitation that high-frequency impedance can only characterize the water content of the proton exchange membrane, effectively realizing the online characterization of the water content during the purge process, and providing a theoretical basis for obtaining the maximum water holding capacity during the shutdown purge process.

[0055] (4) The cold start capability online evaluation method of the present invention establishes a maximum water holding capacity database to lay a data foundation for online acquisition of cold start capability during shutdown: the impedance spectra under different shutdown purges are measured experimentally, and the polarization resistance values ​​after purge are analyzed; after shutdown, the equipment is stored at low temperature, and then the maximum cold start capability is calculated through the constant temperature cold start time under low electrical density, thereby establishing a database between polarization resistance and maximum cold start capability.

[0056] (5) The cold start capability online evaluation system of the present invention can be universally applied to current vehicle-mounted fuel cell systems to obtain a proton exchange membrane fuel cell system with better cold start capability, and has broad prospects for industrial application. BRIEF DESCRIPTION OF THE DRAWINGS

[0057] Figure 1 It is a structural schematic diagram of the online evaluation system for cold start capability of a fuel cell of the present invention.

[0058] Figure 2 It is a schematic flow chart of the online evaluation method of the cold start capability of a fuel cell according to the present invention.

[0059] Figure 3 for Figure 2 Specific flow diagram of step S2 in FIG.

[0060] Figure 4 for Figure 2 Specific flow chart of step S3 in FIG.

[0061] Figure 5 for Figure 2 Specific flow chart of step S4 in FIG.

[0062] Figure 6 This is a diagram of actual test results of Example 3 of the present invention.

[0063] Description of the markings in the figure:

[0064] 10-Fuel cell stack;

[0065] 20-online evaluation system, 201-collection unit, 202-impedance spectrum measurement unit, 203-impedance spectrum analysis unit, 204-excitation signal unit, 205-storage unit, 206-control unit;

[0066] 30-DCDC. DETAILED DESCRIPTION

[0067] The present invention is described in detail below in conjunction with the accompanying drawings and specific embodiments. This embodiment is implemented based on the technical solution of the present invention, and provides a detailed implementation method and specific operation process, but the protection scope of the present invention is not limited to the following embodiments.

[0068] It should be noted that the terms "first", "second", etc. in the specification and claims of the present invention and the above-mentioned drawings are used to distinguish similar objects, and are not necessarily used to describe a specific order or sequence. It should be understood that the data used in this way can be interchanged where appropriate, so that the embodiments of the present invention described herein can be implemented in an order other than those illustrated or described herein. In addition, the terms "including" and "having" and any variations thereof are intended to cover non-exclusive inclusions, for example, a process, method, system, product or device that includes a series of steps or units is not necessarily limited to those steps or units that are clearly listed, but may include other steps or units that are not clearly listed or inherent to these processes, methods, products or devices.

[0069] Embodiment 1:

[0070] This embodiment provides a fuel cell cold start capability online evaluation system, including a fuel cell stack 10, an online evaluation system 20 and a DCDC 30. The DCDC 30 is connected to the fuel cell stack 10, and controls the output power of the fuel cell stack 10 through constant current or constant voltage.

[0071] The online evaluation system 20 of this embodiment is as follows Figure 1 As shown, it specifically includes a collection unit 201, an impedance spectrum measurement unit 202, an impedance spectrum analysis unit 203, an excitation signal unit 204, a storage unit 205 and a control unit 206. Among them, the acquisition unit 201 is used to collect the voltage of the fuel cell stack 10 and the current signal at the input end of the DCDC30; the impedance spectrum measurement unit 202 is used to receive the data collected by the acquisition unit 201 and perform impedance calculation through methods such as FFT algorithm, wavelet transform or s transform; the impedance spectrum analysis unit 203 is used to receive the impedance calculated by the impedance spectrum measurement unit 202 and calculate the polarization process information of the impedance through DRT, and the polarization process information includes activation resistance, mass transfer resistance and ohmic resistance, etc.; the excitation signal unit 204 is used to receive the instruction of the impedance spectrum measurement unit 202, control the amplitude and frequency of the AC excitation superimposed on the DC of DCDC30, and transmit the AC excitation signal to DCDC30; the storage unit 205 includes a maximum water capacity database, and the maximum water capacity database includes the corresponding relationship between the polarization process information and the maximum water capacity value; the control unit 206 is used to extract the maximum water capacity database information, and obtain the maximum water capacity of the current state in combination with the calculated polarization process information, and then transmit the control signal to the fuel cell stack 10.

[0072] Embodiment 2:

[0073] This embodiment provides an online evaluation method for the cold start capability of a fuel cell based on the online evaluation system for the cold start capability of a fuel cell described in Embodiment 1, such as Figure 2 As shown, the specific steps include:

[0074] S1: Perform low-temperature shutdown and purge according to the preset shutdown and purge operating conditions;

[0075] S2: Regularly measure impedance spectrum;

[0076] S3: Perform DRT fitting on the impedance spectrum to obtain polarization process information;

[0077] S4: obtaining a maximum water holding capacity database;

[0078] S5: according to the polarization process information obtained in S3, searching the maximum water capacity corresponding to the maximum water capacity database, and comparing it with the water capacity threshold;

[0079] S6: If the maximum water holding capacity value is greater than the water holding capacity threshold, the purge is stopped; if the maximum water holding capacity value is less than the water holding capacity threshold, the process returns to S1 and continues until the maximum water holding capacity value is greater than the water holding capacity threshold.

[0080] Wherein, step S2 specifically includes the following steps:

[0081] S21: Wide spectrum measurement of fuel cell impedance spectrum under steady-state conditions;

[0082] S22: Refer to S3 to perform DRT fitting on the impedance spectrum to obtain polarization process information;

[0083] S24: Determine the frequency range and minimum multiple frequency points of impedance spectrum measurement;

[0084] S25: Performing KK verification on the impedance spectrum data of the selected measurement frequency point;

[0085] S26: If the residual error of the KK calibration is less than 1%, a fast impedance spectrum measurement under low temperature shutdown purge is performed with reference to the parameters of the measurement frequency point;

[0086] Wherein, step S3 specifically includes the following steps:

[0087] S31: setting the fitting parameters of the DRT fitting method, including characteristic frequency resolution and regularization parameters;

[0088] S32: Calculate the characteristic frequency and peak area of ​​each DRT characteristic peak;

[0089] S33: Identify the polarization process of the DRT characteristic peak. The characteristic frequency of 1-10 Hz is the mass transfer process, 20-100 Hz is the charge transfer process, and above 200 Hz is the anode and catalyst layer proton transfer process. The high frequency intercept is the Ohmic process.

[0090] Wherein, step S4 specifically includes the following steps:

[0091] S41: conducting an orthogonal test of shutdown purge under different purge operating conditions, the operating conditions including purge temperature, purge flow rate and purge time;

[0092] S42: Referring to S2 and S3, impedance spectrum measurement and polarization process analysis are performed at the end of shutdown purge;

[0093] S43: After the purge is completed, low-temperature storage is performed to fully freeze the fuel cell system;

[0094] S44: Carry out a constant temperature cold start test under low current and record the load time; the current density of the low current is not greater than 150mA / cm 2The loading time is the time when a small current is continuously loaded until the catalyst layer is completely frozen, resulting in startup failure;

[0095] S45: Calculate the maximum water holding capacity under the operating conditions;

[0096] S46: Establishing a mapping relationship between each polarization resistance and the maximum water holding capacity.

[0097] Embodiment 3:

[0098] This embodiment provides a specific fuel cell cold start capability online evaluation system and online evaluation method. Figure 1 As shown, first, the reaction gas and temperature control required by the fuel cell stack 10 are provided through its peripheral auxiliary systems (such as air intake pressure and flow control through air compressor and back pressure valves, hydrogen pressure and flow control through proportional valves or hydrogen injectors and circulation pumps or ejectors, and real-time temperature control through water pumps and radiators).

[0099] DCDC30 is connected to the fuel cell stack 10, and the output power of the fuel cell stack 10 is controlled by flow or constant pressure. Secondly, when starting the online evaluation method of the cold start capability of the fuel cell stack 10, the excitation signal unit 204 of the online evaluation system 20 receives the instruction of the impedance spectrum measurement unit, controls the amplitude and frequency point of the AC excitation superimposed on the DC of DCDC30, and injects the AC excitation of the fuel cell stack 10. The voltage of the fuel cell stack 10 and the total current of the input end of DCDC30 are synchronously collected by the acquisition unit 201 of the online evaluation system 20. The voltage and current signals collected by the acquisition unit 201 of the online evaluation system 20 are sent to the impedance spectrum measurement unit 202 for impedance calculation, and the impedance of the fuel cell stack 10 is calculated by the FFT algorithm, and sent to the impedance spectrum analysis unit 203 of the online evaluation system 20. The impedance spectrum analysis unit 203 calculates the polarization process information contained in the impedance by the DRT method, and then sends the above-analyzed polarization resistance information to the control unit 206 of the online evaluation system 20. The control unit 206 extracts the information of the maximum water capacity database in the storage unit 205 of the online evaluation system 20, thereby obtaining the current maximum water capacity according to the polarization process information, and controls whether to terminate the current shutdown purge operation according to the water capacity threshold, thereby realizing intelligent control of low-temperature shutdown purge, and laying the foundation for parameter optimization of low-temperature shutdown purge and cold start control strategy.

[0100] Figure 2 to Figure 5 Schematic diagram of the online evaluation method of the cold start capability of a fuel cell based on electrochemical impedance spectroscopy (EIS) of this embodiment.

[0101] The flow chart of the online evaluation method of fuel cell cold start capability based on electrochemical impedance spectroscopy is as follows: Figure 2 As shown. First, low-temperature shutdown purge is performed according to the preset shutdown purge operating conditions. During the shutdown purge, a rapid impedance spectrum measurement is performed at regular intervals (such as 30 seconds). The impedance spectrum is fitted by the DRT method to obtain the polarization process information contained in the impedance spectrum, such as activation resistance, mass transfer resistance, and ohmic resistance. According to the maximum water capacity database obtained offline, the database is the correspondence between the polarization process information and the maximum water capacity value. According to the polarization process information obtained in real time, the maximum water capacity corresponding to the current polarization process information value is found, and the maximum water capacity value is compared with the preset water capacity threshold. If the water capacity value is greater than the water capacity threshold, the purge is stopped and the purge process ends; if the water capacity value is less than the water capacity threshold, the above process is repeated until the calculated water capacity value is greater than the water capacity threshold.

[0102] Since the state of the fuel cell stack during the shutdown purge process is non-steady, that is, it is a time-varying system, in order to meet the time-invariant requirement of impedance spectrum analysis, it is necessary to achieve rapid measurement of the impedance spectrum during the shutdown process, so that the impedance spectrum measurement during the shutdown process is considered to be a "time-invariant" system measurement. Based on this, the electrochemical impedance spectrum measurement flow chart during the shutdown purge process designed in this embodiment is as follows: Figure 3 As shown. First, a wide spectrum measurement of the impedance spectrum of the fuel cell is performed under steady-state conditions, such as 10kHz~0.1Hz, and the impedance spectrum is fitted by the DRT method to calculate the polarization process information contained in the impedance spectrum; the characteristic frequency distribution range and logarithmic frequency interval of each polarization process of the fuel cell are analyzed, according to the required polarization process information, such as ohmic resistance, activation resistance and mass transfer resistance. According to the characteristic frequency range of the above process, the frequency range of the impedance spectrum measurement is determined, and the minimum number of multiple frequency points is determined according to the minimum logarithmic frequency interval, so as to avoid unnecessary measurement frequency points, especially low-frequency measurement points that affect the measurement time. Furthermore, the impedance spectrum data of the selected measurement frequency point is subjected to KK verification to determine whether the residual of the KK verification is less than 1%. If the requirements are met, the fast impedance spectrum measurement under low-temperature shutdown purge is performed according to the impedance spectrum measurement frequency point; if the requirements are not met, the superposition amplitude of the AC impedance spectrum is adjusted, and the above steps are repeated.

[0103] In order to achieve the accurate analysis of the above-mentioned fast impedance spectrum, the impedance spectrum analysis method based on DRT is designed as follows Figure 4As shown. First, the DRT method needs to be improved and the parameters need to be optimized. The two key parameters of the DRT method include the characteristic frequency resolution ΔIogf and the regularization parameter λ. The selection principles of these two parameters are as follows: to avoid the problem of DRT characteristic peak overlap, the ΔIogf of DRT should be less than half of the minimum logarithmic characteristic frequency interval of the system. In order to improve the accuracy, ΔIogf is usually set to less than 1 / 5 of the minimum logarithmic frequency interval; the regularization parameter λ is used to optimize overfitting and underfitting problems, that is, to suppress the "pseudo-peak" problem caused by noise or the problem of reduced characteristic time constant identification caused by underfitting, so the λ selection can be set from large to small. When the fitting residual deviation under adjacent λ values ​​is less than a certain threshold, the value is selected as the regularization parameter λ. After fitting, the characteristic frequency and peak area of ​​each DRT characteristic peak need to be calculated, and the polarization process of each peak is assigned according to the characteristic frequency and peak area of ​​each DRT peak. Generally, the characteristic frequency of 1-10 Hz is the mass transfer process, 20-100 Hz is the charge transfer process, above 200 Hz is the anode and catalyst layer proton transfer process, and the high-frequency intercept is the Ohmic process.

[0104] Finally, in order to achieve online acquisition of the maximum water holding capacity, the database establishment method of the designed maximum water holding capacity is as follows: Figure 5 As shown. Design different purge operating conditions, such as different purge temperatures, purge flows, and purge times, and carry out shutdown purge orthogonal tests. Perform a rapid impedance spectrum measurement before the end of each orthogonal test, and use DRT to analyze the polarization process information of the impedance spectrum, including ohmic resistance, activation resistance, and mass transfer resistance. After the purge is completed, perform low-temperature storage for at least 12 hours to ensure that the fuel cell system is fully frozen. After the freezing is completed, use a small current (such as 100mA / cm 2 A constant temperature cold start test was conducted under the following operating conditions: a small current was continuously loaded until the catalyst layer was completely frozen, resulting in a startup failure. The loading time of the small current was recorded, and the maximum water production of the fuel cell stack under different purge conditions, that is, the maximum water capacity, was calculated based on the time. A mapping relationship between each polarization resistance and the maximum water capacity was established, thus forming an online database containing the maximum water capacity.

[0105] The impedance spectrum and the analyzed polarization resistance values ​​during the low-temperature shutdown process in the actual test of this embodiment vary with the purge time as shown in FIG. Figure 6 shown. Figure 6 (a) shows the trend of cell voltage change during the purge process. From the overall trend, the cell voltage drop process can be divided into three main stages. In stage 1, the cell voltage remains almost unchanged for a short period of time after the start of the purge. In stage 2, the cell voltage drops rapidly and tends to be flat. In stage 3, the cell voltage drops sharply. Figure 6(b) and (c) are the fast impedance measurement results and their DRT distribution. Figure 6 It can be concluded from (b) that as the purge time increases, the Nyquist diagram quickly shifts to the right and the diameter of the arc gradually increases, corresponding to stage 2. Figure 6 From the DRT distribution in (c), it can be seen that with the increase of purge time, the DRT peaks near 500 Hz and 100 Hz increase rapidly. After the 5th fast EIS measurement, other peaks begin to appear near 10 Hz, and in the 15th fast EIS measurement, the peak at this location increases rapidly, corresponding to stage 3.

[0106] Figure 6 (d) The distribution of polarization resistances for improved DRT calculation. First, in stage 1, R ohm,ne and R ion , R conc , R ct and R an It is almost unchanged, indicating that stage 1 is the liquid water removal stage, because the removal of liquid water alone has little effect on each polarization resistance.

[0107] Secondly, in stage 2, R ohm,ne and R ion Since the purge has little effect on the contact resistance and electronic resistance, R ohm,ne and R ion They also represent the changes in the water content of the proton exchange membrane (PEM) and the catalyst layer (CL) ionomer. conc The slow increase in this stage also verifies that only part of the membrane is dried in this stage and does not have a significant impact on oxygen transmission. an The change of R ohm,ne The changes in R an Rapidly increases to a steady-state value and maintains this value. The reason is that no product water is generated at the anode, so the purge efficiency is high. After the liquid water is removed, it can quickly reach a state of humidity equilibrium with the purge gas. Although there is water diffusion from the cathode to the anode, the impact of this process is relatively small compared to large-flow purge. R ct It is almost unchanged at this stage, indicating that partial film drying has little effect on the oxygen concentration on the CL surface.

[0108] Then, in stage 3, R conc and R ohm,ne Both have a significant increase, R conc The increase is much greater than R ohm,ne After entering stage 3, R conc increased by 200%; and R ohm,neThe increase was 40%. This was mainly due to the drying of the membrane under the spine, which led to an increase in ohmic resistance. The drying of the CL ionomer caused a significant increase in oxygen transmission resistance, which in turn led to a significant increase in concentration overpotential. ct There is also a slight increase in this stage, mainly due to the sharp increase in oxygen transfer resistance, which leads to a decrease in oxygen concentration on the CL surface. The superposition of these factors leads to a sharp drop in the cell voltage in stage 3, and the increase in mass transfer resistance caused by the increase in oxygen transfer resistance dominates.

[0109] In summary, combined with the changes in each polarization resistance, the changing law of the water state in each component during the shutdown on-load purge process is revealed. Compared with the HFR method, the fast EIS method more comprehensively and clearly explains the changing law of the water state of each component during the shutdown process. Therefore, the changing law of the water state during the shutdown on-load purge process can be summarized into three stages: Stage 1, the liquid water removal stage of the porous medium (DM) under the channel; Stage 2, the stage of the combined action of the CL ionomer and PEM drying under the flow channel and the liquid water removal of the DM under the flow channel ridge; Stage 3, the CL ionomer and PEM drying stage under the flow channel ridge.

[0110] The above description of the embodiments is to facilitate the understanding and use of the invention by those skilled in the art. It is obvious that those skilled in the art can easily make various modifications to these embodiments and apply the general principles described herein to other embodiments without creative work. Therefore, the present invention is not limited to the above embodiments, and improvements and modifications made by those skilled in the art based on the disclosure of the present invention without departing from the scope of the present invention should be within the scope of protection of the present invention.

Claims

1. A fuel cell cold start capability online evaluation system, comprising a fuel cell stack (10), characterized in that: It also includes an online evaluation system (20) and a DCDC (30); the DCDC (30) is connected to the fuel cell stack (10) and is used to control the output power of the fuel cell stack (10); The online evaluation system (20) specifically comprises: A collection unit (201) for collecting the voltage of the fuel cell stack (10) and the current signal of the input end of the DCDC (30); An impedance spectrum measurement unit (202), used for receiving data collected by the collection unit (201) and performing impedance calculation; An impedance spectrum analysis unit (203) is used to receive the impedance calculated by the impedance spectrum measurement unit (202) and calculate the polarization process information of the impedance through DRT; An excitation signal unit (204) is used to receive instructions from the impedance spectrum measurement unit (202), control the amplitude and frequency of the AC excitation superimposed on the DC of the DCDC (30), and transmit the AC excitation signal to the DCDC (30); A storage unit (205), which internally includes a maximum water holding capacity database; The control unit (206) is used to extract the maximum water holding capacity database information, and obtain the maximum water holding capacity of the current state in combination with the calculated polarization process information, and then transmit the control signal to the fuel cell stack (10).

2. A fuel cell cold start capability online evaluation system according to claim 1, characterized in that: The impedance spectrum measuring unit (202) calculates impedance by using FFT algorithm, wavelet transform or s transform.

3. A fuel cell cold start capability online evaluation system according to claim 1, characterized in that: The polarization process information calculated by the impedance spectrum analysis unit (203) includes activation resistance, mass transfer resistance and ohmic resistance.

4. A fuel cell cold start capability online evaluation system according to claim 1, characterized in that: The maximum water holding capacity database includes the correspondence between polarization process information and maximum water holding capacity values.

5. An online evaluation method using the online evaluation system for cold start capability of a fuel cell according to any one of claims 1 to 4, characterized in that: The following steps are involved: S1: Perform low-temperature shutdown and purge according to the preset shutdown and purge operating conditions; S2: Regularly measure impedance spectrum; S3: Perform DRT fitting on the impedance spectrum to obtain polarization process information; S4: obtaining a maximum water holding capacity database; S5: according to the polarization process information obtained in S3, searching the maximum water capacity corresponding to the maximum water capacity database, and comparing it with the water capacity threshold; S6: If the maximum water holding capacity value is greater than the water holding capacity threshold, the purge is stopped; if the maximum water holding capacity value is less than the water holding capacity threshold, the process returns to S1 and continues until the maximum water holding capacity value is greater than the water holding capacity threshold.

6. An online evaluation method according to claim 5, characterized in that: Step S2 specifically includes the following steps: S21: Wide spectrum measurement of fuel cell impedance spectrum under steady-state conditions; S22: Refer to S3 to perform DRT fitting on the impedance spectrum to obtain polarization process information; S23: Analyze the characteristic frequency distribution range and logarithmic frequency interval of each polarization process of the fuel cell; S24: Determine the frequency range and minimum multiple frequency points of impedance spectrum measurement; S25: Performing KK verification on the impedance spectrum data of the selected measurement frequency point; S26: If the residual error of the KK calibration is less than 1%, a fast impedance spectrum measurement under low temperature shutdown purge is performed with reference to the parameters of the measurement frequency point; If the residual does not meet the requirement, the process returns to S21 until the impedance spectrum measurement parameters are confirmed.

7. An online evaluation method according to claim 5, characterized in that: Step S3 specifically includes the following steps: S31: setting the fitting parameters of the DRT fitting method, including characteristic frequency resolution and regularization parameters; S32: Calculate the characteristic frequency and peak area of ​​each DRT characteristic peak; S33: Identify the polarization process of DRT characteristic peaks.

8. An online evaluation method according to claim 7, characterized in that: In step S33, the characteristic frequency of 1-10 Hz is the mass transfer process, 20-100 Hz is the charge transfer process, above 200 Hz is the anode and catalyst layer proton transfer process, and the high frequency intercept is the Ohmic process.

9. An online evaluation method according to claim 5, characterized in that: Step S4 specifically includes the following steps: S41: Conducting an orthogonal test of shutdown purge under different purge operation conditions; S42: Referring to S2 and S3, impedance spectrum measurement and polarization process analysis are performed at the end of shutdown purge; S43: After the purge is completed, low-temperature storage is performed to fully freeze the fuel cell system; S44: Carry out a constant temperature cold start test under low current and record the load time; S45: Calculate the maximum water holding capacity under the operating conditions; S46: Establishing a mapping relationship between each polarization resistance and the maximum water holding capacity.

10. An online evaluation method according to claim 9, characterized in that: In step S41, the operating conditions include purge temperature, purge flow rate and purge time; In step S44, the current density of the small current is not greater than 150 mA / cm 2 ; In step S44, the loading time is the time during which the loading is continued at a low current until the catalyst layer is completely frozen, resulting in startup failure.

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