Judgment method for low-temperature cold start purging degree of fuel cell stack

Abstract

The invention belongs to the technical field of fuel cells, and provides a method for judging the low-temperature cold start purging degree of a fuel cell stack, and the method comprises the following steps: S01, connecting a to-be-purged stack with an EIS, then activating the to-be-purged stack, and carrying out the performance calibration of the to-be-purged stack; s02, raising the temperature of the to-be-purged galvanic pile to 65-80 DEG C, keeping the constant temperature at 65-80 DEG C, and then carrying out backpressure setting on the to-be-purged galvanic pile; s03, fuel gas is introduced into the to-be-purged electric pile, and then the to-be-purged electric pile is subjected to current loading; and S04, when the average voltage in the inspection voltage diagram and the impedance value in the high-frequency impedance spectrum change trend diagram tend to be constant, recording the purging time, and ending purging. According to the method and the device, the purging degree can be accurately judged when the electric pile is purged, the purging time of the electric pile is locked, and residual water caused by over-dry purging or insufficient purging degree of the electric pile is avoided.

Description

Technical Field

[0001] The present invention belongs to the technical field of fuel cells, and in particular relates to a method for determining the degree of low-temperature cold start purge of a fuel cell stack. Background Art

[0002] There are many key factors restricting the further commercialization and promotion of fuel cell vehicles. Among them, the cold start problem of fuel cells caused by cold regions and climate environment is one of the main obstacles to the commercialization of fuel cells.

[0003] During the cold start process, the output performance of the fuel cell will decay to a certain extent. The main reasons for this decay are: during the startup of the battery, the water generated by the electrochemical reaction is discharged from the CL to the outer GDL and flow channel and freezes due to the low temperature in the area. The ice covers the reaction active sites of the catalyst in the CL and blocks the gas channels in the GDL and flow channel, thus affecting the normal progress of the electrochemical reaction. Moreover, ice will destroy the internal structure of the fuel cell, causing the catalytic layer to fall off and the diffusion layer to break. At the same time, the formation of ice will aggravate the increase in contact resistance, which will also increase the overall impedance of the battery. These factors will eventually lead to the decay of output performance.

[0004] It can be seen that the key to the success of a cold start lies in water management; the lower the water content of the fuel cell stack in the initial stage of a cold start, the greater the success rate of the cold start. The most effective way to reduce the water content of the fuel cell stack is to purge the stack appropriately. However, there is no intuitive and effective criterion for judging the degree of purge in the existing relevant literature; excessive purge of the membrane electrode will lead to increased mechanical attenuation of its proton membrane; insufficient purge will result in a certain amount of water remaining in the membrane, leading to the risk of freezing. How to quickly and accurately judge whether the purge has reached the requirement is a technical problem. At present, most of the existing technologies use a large flow of nitrogen or air for a certain period of time to purge, and it is impossible to directly or indirectly see the water content status in the stack, and there is no specific value for the degree of purge as a criterion. Summary of the invention

[0005] An embodiment of the present invention provides a method for determining the degree of purging during a low-temperature cold start of a fuel cell stack, aiming to solve the problems that the prior art cannot directly or indirectly see the water content status in the stack and that the degree of purging has no specific numerical value as a criterion for judgment.

[0006] In order to achieve the above object, the present invention provides a method for determining the degree of purge during cold start of a fuel cell stack, comprising the following steps:

[0007] S01, connecting the battery stack to be purged to an EIS (Electrochemical Impedance Spectroscopy), then activating the battery stack to be purged, and performing performance calibration on the battery stack to be purged;

[0008] S02, raising the temperature of the fuel cell stack to be purged to 65° C. to 80° C., maintaining a constant temperature at 65° C. to 80° C., and then setting a back pressure on the fuel cell stack to be purged;

[0009] S03, introducing fuel gas into the fuel cell stack to be purged, and then applying current to the fuel cell stack to be purged;

[0010] S04. When the average voltage in the inspection voltage diagram and the impedance value in the high-frequency impedance spectrum change trend diagram tend to be constant, record the purge time and end the purge.

[0011] As a preferred embodiment, in step S01,

[0012] The EIS is preferably an EIS with 1A disturbance; the high-frequency impedance frequency of the EIS is preferably 1000 Hz.

[0013] The activation is achieved by the following method: gradient loading is performed on the battery stack to be purged, the slope of each loading is preferably 10A / s to 50A / s, each gradient is constant current for 2min to 10min, and after loading to the rated current density, the constant current is maintained for 1h; the above steps are repeated 3 to 6 times.

[0014] The performance calibration is achieved by the following method: performing a voltage transformer polarization test on the fuel cell stack to be purged to obtain a polarization curve; and performing performance calibration on the fuel cell stack to be purged based on the performance shown by the polarization curve.

[0015] As a preferred embodiment, in step S02,

[0016] The back pressure is set to match the temperature.

[0017] The back pressure is preferably 50 kPa to 100 kPa.

[0018] As a preferred implementation, in step S03,

[0019] The humidity of the fuel gas is preferably 0% RH to 40% RH.

[0020] The fuel gas is hydrogen and air; air is introduced into the cathode inlet of the fuel cell stack to be purged, and hydrogen is introduced into the anode inlet of the fuel cell stack to be purged.

[0021] The current density of the hydrogen is preferably 0.5 A / cm 2~1.0A / cm 2 The current density of the air is preferably 0.5A / cm 2 ~1.0A / cm 2 .

[0022] According to the stoichiometric ratio, the supply flow rate of the anode of the fuel cell stack to be purged is 1.6 to 1.8 of the stoichiometric ratio of the corresponding current; the supply flow rate of the cathode of the fuel cell stack to be purged is 1.8 to 2.0 of the stoichiometric ratio of the corresponding current.

[0023] The current load follows the principle of small electric density and large flow rate, and the current density of the current load is preferably 0.05A / cm 2 ~0.1A / cm 2 .

[0024] As a preferred implementation, in step S04,

[0025] The purge time is preferably 5 min to 10 min.

[0026] When purging the battery stack of the same model and power in the future, quantitative purging can be performed based on the purging time and average voltage value obtained from the previous experiment, without the need to connect the high-frequency impedance of the auxiliary equipment.

[0027] In the embodiment of the present application, the fuel cell stack maintains a constant temperature of 65°C to 80°C during the purging process. This temperature state can accelerate the evaporation of residual liquid water in the stack and shorten the purging time. During the purging process, the fuel cell stack maintains a small current load and a large flow rate purge. This purging method can quickly reduce the water content in the fuel cell stack, avoid excessive water freezing in a sub-zero environment and cause damage to the membrane electrode, and effectively prevent excessive water loss of the membrane electrode, which affects the performance of the fuel cell stack.

[0028] This application is based on the inversely proportional relationship between high-frequency impedance and the corresponding water content, and the proportional relationship between the hydration state of the proton membrane and the performance of the stack. The water content in the fuel cell stack during purging is judged by high-frequency impedance and stack voltage, thereby confirming the purging time of the fuel cell stack. The method of this application is conducive to accurately judging the degree of purging during low-temperature cold storage, cold start or stack shutdown purging of the fuel cell stack, locking the purging time of the stack, and avoiding the stack from being purged too dry or insufficiently purged, resulting in excess water remaining. Moreover, when purging the fuel cell stack of the same model and power again in the future, the purging standard of the stack can be directly determined based on the voltage value and purging time obtained previously, thereby reducing the need for impedance equipment as a basis for auxiliary judgment, which not only saves the cost of use, but also simplifies the difficulty of operation. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the structures shown in these drawings without paying creative work.

[0030] Figure 1 A schematic diagram of the relationship between the impedance value and the purge time in the impedance spectrum of an embodiment of the present invention;

[0031] Figure 2 A schematic diagram of the corresponding relationship between the purge time and the high-frequency impedance spectrum according to an embodiment of the present invention;

[0032] Figure 3 Schematic diagram of the corresponding relationship between the purge time and the average voltage of the fuel cell stack according to an embodiment of the present invention.

[0033] The realization of the purpose, functional features and advantages of the present invention will be further explained in conjunction with embodiments and with reference to the accompanying drawings. DETAILED DESCRIPTION

[0034] The technical solutions in the embodiments of the present invention are described clearly and completely below. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0035] It should be noted that if the embodiments of the present invention involve directional indications (such as up, down, left, right, front, back, top, bottom...), the directional indications are only used to explain the relative position relationship, movement status, etc. between the components in a certain specific posture. If the specific posture changes, the directional indication will also change accordingly.

[0036] In this application, unless otherwise clearly specified and limited, the terms "installed", "connected", "connected", "fixed" and the like should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediate medium, it can be the internal connection of two elements or the interaction relationship between two elements, unless otherwise clearly defined. For ordinary technicians in this field, the specific meanings of the above terms in this application can be understood according to specific circumstances.

[0037] It should be noted that when an element is referred to as being "fixed to" or "disposed on" another element, it may be directly on the other element or there may be an intermediate element. When an element is considered to be "connected to" another element, it may be directly connected to the other element or there may be an intermediate element at the same time.

[0038] In addition, if there are descriptions involving "first", "second", etc. in the embodiments of the present invention, the descriptions of "first", "second", etc. are only used for descriptive purposes and cannot be understood as indicating or suggesting their relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined as "first" and "second" may explicitly or implicitly include at least one of the features. In addition, the technical solutions between the various embodiments can be combined with each other, but they must be based on the ability of ordinary technicians in the field to implement them. When the combination of technical solutions is contradictory or cannot be implemented, it should be deemed that such a combination of technical solutions does not exist and is not within the scope of protection required by the present invention.

[0039] Generally, the water in the fuel cell stack is mainly distributed in the flow channel, diffusion layer and proton membrane, among which:

[0040] 1) In the proton membrane, due to the water absorption characteristics of the proton exchange membrane, water is hidden inside the membrane in the form of hydration;

[0041] 2) The gas diffusion layer is usually porous carbon paper or carbon cloth. Liquid water is adsorbed in the porous structure due to surface tension. In addition, the carbon material itself can adsorb a small amount of liquid water.

[0042] 3) The plates are covered with flow channels for gas flow. The flow channels are where the most water is stored, mainly large particles of water droplets, which are also the easiest to blow away;

[0043] The forms of combination between various components in the fuel cell stack and liquid water are different, resulting in different degrees of difficulty in blowing away the liquid water in different components during the purge process.

[0044] In the examples of this application, we use electrochemical impedance spectroscopy and stack voltage to describe this process. Electrochemical impedance spectroscopy is a common method for studying proton exchange membrane fuel cells, which can characterize the various impedance states of the current electrochemical reaction of the fuel cell stack online. Among them, high-frequency impedance is the main means of characterizing the water content of the fuel cell.

[0045] The fuel cell stack can be divided into three stages during the purge: slow rise stage, rapid rise stage and equilibrium stage. The corresponding high-frequency impedance spectrum is as follows: Figure 1 As shown; the meanings are as follows:

[0046] a. Slow rising stage: the water inside the flow channel is purged;

[0047] b. Rapid rising stage: the moisture in the diffusion layer and proton membrane is swept;

[0048] c. Equilibrium stage: At this time, the water production in the membrane and the water content carried away by the gas form a balance.

[0049] like Figure 1 As shown, when the impedance spectrum monitors that the high-frequency impedance value is approaching equilibrium, that is, the rapid rise stage is about to enter the equilibrium stage, it means that the water content inside the fuel cell stack is appropriate. At this time, the fuel cell stack purge time and voltage value are recorded to end the purge.

[0050] In the embodiments of the present application, the settings of the parameter ranges can be set according to actual needs, and the present application solution can be implemented as long as it is within the scope required by the present application.

[0051] Specifically, the embodiment of the present application provides a method for determining the degree of cold start purge of a fuel cell stack, comprising the following steps:

[0052] S01, connecting the battery stack to be purged to an EIS (Electrochemical Impedance Spectroscopy), then activating the battery stack to be purged, and performing performance calibration on the battery stack to be purged;

[0053] S02, raising the temperature of the fuel cell stack to be purged to 65° C. to 80° C., maintaining a constant temperature at 65° C. to 80° C., and then setting a back pressure on the fuel cell stack to be purged;

[0054] S03, introducing fuel gas into the fuel cell stack to be purged, and then applying current to the fuel cell stack to be purged;

[0055] S04. When the average voltage in the inspection voltage diagram and the impedance value in the high-frequency impedance spectrum change trend diagram tend to be constant, record the purge time and end the purge.

[0056] As a preferred embodiment, in step S01,

[0057] The EIS is preferably an EIS with 1A disturbance; the high-frequency impedance frequency of the EIS is preferably 1000 Hz.

[0058] The activation is achieved by the following method: gradient loading is performed on the battery stack to be purged, the slope of each loading is preferably 10A / s to 50A / s, each gradient is constant current for 2min to 10min, and after loading to the rated current density, the constant current is maintained for 1h; the above steps are repeated 3 to 6 times.

[0059] The performance calibration is achieved by the following method: performing a voltage transformer polarization test on the fuel cell stack to be purged to obtain a polarization curve; and performing performance calibration on the fuel cell stack to be purged based on the performance shown by the polarization curve.

[0060] In the embodiments of the present application, the transformer polarization test may be performed using conventional test methods and test conditions.

[0061] As a preferred embodiment, in step S02,

[0062] The back pressure is set to match the temperature.

[0063] The back pressure is preferably 50 kPa to 100 kPa.

[0064] As a preferred implementation, in step S03,

[0065] The humidity of the fuel gas is preferably 0% RH to 40% RH.

[0066] The fuel gas is hydrogen and air; air is introduced into the cathode inlet of the fuel cell stack to be purged, and hydrogen is introduced into the anode inlet of the fuel cell stack to be purged.

[0067] The current density (i.e., electric density) of the hydrogen is preferably 0.5 A / cm 2 ~1.0A / cm 2 The current density (i.e., electric density) of the air is preferably 0.5A / cm 2 ~1.0A / cm 2 .

[0068] According to the stoichiometric ratio, the supply flow rate of the anode of the fuel cell stack to be purged is 1.6 to 1.8 of the stoichiometric ratio of the corresponding current; the supply flow rate of the cathode of the fuel cell stack to be purged is 1.8 to 2.0 of the stoichiometric ratio of the corresponding current.

[0069] The current load follows the principle of small electric density and large flow rate, and the current density of the current load is preferably 0.05A / cm 2 ~0.1A / cm 2 .

[0070] As a preferred implementation, in step S04,

[0071] The purge time is preferably 5 min to 10 min.

[0072] When purging the battery stack of the same model and power in the future, quantitative purging can be performed based on the purging time and average voltage value obtained from the previous experiment, without the need to connect the high-frequency impedance of the auxiliary equipment.

[0073] In one embodiment, a method for determining the degree of low temperature storage purge of a fuel cell stack includes the following steps:

[0074] S01. Connect the EIS to the battery stack to be purged, fully activate the battery stack to be purged and calibrate its performance (refer to the above method for activation and performance calibration);

[0075] S02, the stack temperature of the fuel cell to be purged is controlled at 70°C, and the back pressure is set to 67 kPa for the anode and 57 kPa for the cathode;

[0076] S03, according to 1.0A / cm 2 The electric density and metering ratio of 1.8 supply hydrogen flow rate; according to 1.0A / cm 2 The electric density and metering ratio of 2.0 supply air flow rate; 0.1A / cm 2 Load current;

[0077] S04. Observe the high-frequency impedance spectrum change trend and the average voltage change trend. When the average voltage and impedance value tend to be stable, record the purge time and end the purge. The results are as follows: Figure 2 and Figure 3 As shown. Figure 2 and Figure 3 It can be seen that the high-frequency impedance diagram shows the correspondence between the water content and impedance inside the battery stack and the voltage. The impedance spectrum change is divided into three stages: slow rise period, rapid rise period and equilibrium period.

[0078] In the embodiment of the present application, the fuel cell stack maintains a constant temperature of 65°C to 80°C during the purging process. This temperature state can accelerate the evaporation of residual liquid water in the stack and shorten the purging time. During the purging process, the fuel cell stack maintains a small current load and a large flow rate purge. This purging method can quickly reduce the water content in the fuel cell stack, avoid excessive water freezing in a sub-zero environment and cause damage to the membrane electrode, and effectively prevent excessive water loss of the membrane electrode, affecting the performance of the fuel cell stack.

[0079] This application is based on the inversely proportional relationship between high-frequency impedance and the corresponding water content, and the proportional relationship between the hydration state of the proton membrane and the performance of the stack. The water content in the fuel cell stack during purging is judged by high-frequency impedance and stack voltage, thereby confirming the purging time of the fuel cell stack. The method of this application is conducive to accurately judging the degree of purging during low-temperature cold storage, cold start or stack shutdown purging of the fuel cell stack, locking the purging time of the stack, and avoiding the stack from being purged too dry or insufficiently purged, resulting in excess water remaining. Moreover, when purging the fuel cell stack of the same model and power again in the future, the purging standard of the stack can be directly determined based on the voltage value and purging time obtained previously, thereby reducing the need for impedance equipment as a basis for auxiliary judgment, which not only saves the cost of use, but also simplifies the difficulty of operation.

[0080] In the description of this specification, the description with reference to the terms "an embodiment", "example", etc. means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representation of the above terms does not necessarily refer to the same embodiment or example.

[0081] In addition, it should be understood that although this specification is described according to implementation methods, not every implementation method contains only one independent technical solution. This narrative method of the specification is only for the sake of clarity. Those skilled in the art should regard the specification as a whole. The technical solutions in each embodiment may also be appropriately combined to form other implementation methods that can be understood by those skilled in the art.

[0082] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for determining the degree of purge during cold start of a fuel cell stack, characterized in that: The steps include: S01, connecting the battery stack to be purged to the EIS, then activating the battery stack to be purged, and performing performance calibration on the battery stack to be purged; S02, raising the temperature of the fuel cell stack to be purged to 65° C. to 80° C., maintaining a constant temperature at 65° C. to 80° C., and then setting a back pressure on the fuel cell stack to be purged; S03, introducing fuel gas into the fuel cell stack to be purged, and then applying current to the fuel cell stack to be purged; S04. When the average voltage in the inspection voltage diagram and the impedance value in the high-frequency impedance spectrum change trend diagram tend to be constant, record the purge time and end the purge.

2. The method for determining the degree of purge during cold start of a fuel cell stack according to claim 1, characterized in that: In step S01, the EIS is an EIS with 1A disturbance; and the high-frequency impedance frequency of the EIS is 1000 Hz.

3. The method for determining the degree of purge during cold start of a fuel cell stack according to claim 1, characterized in that: In step S01, the activation is achieved by the following method: gradient loading is performed on the battery stack to be purged, the slope of each loading is preferably 10A / s to 50A / s, each gradient is constant current for 2min to 10min, and after loading to the rated current density, the constant current is maintained for 1h; the above steps are repeated 3 to 6 times.

4. The method for determining the degree of purge during cold start of a fuel cell stack according to claim 1, characterized in that: In step S01, the performance calibration is achieved by the following method: performing a voltage transformer polarization test on the fuel cell stack to be purged to obtain a polarization curve; and performing performance calibration on the fuel cell stack to be purged based on the performance shown by the polarization curve.

5. The method for determining the degree of purge during cold start of a fuel cell stack according to claim 1, characterized in that: In step S02, the back pressure is set to match the temperature; the back pressure is 50 kPa to 100 kPa.

6. The method for determining the degree of purge during cold start of a fuel cell stack according to claim 1, characterized in that: In step S03, the humidity of the fuel gas is 0% RH to 40% RH.

7. The method for determining the degree of purge during cold start of a fuel cell stack according to claim 1, characterized in that: In step S03, the fuel gas is hydrogen and air; air is introduced into the cathode inlet of the fuel cell stack to be purged, and hydrogen is introduced into the anode inlet of the fuel cell stack to be purged.

8. The method for determining the degree of purge during cold start of a fuel cell stack according to claim 1, characterized in that: In step S03, the current density of the hydrogen is 0.5A / cm 2 ~1.0A / cm 2 ; The current density of the air is 0.5A / cm 2 ~1.0A / cm 2 .

9. The method for determining the degree of purge during cold start of a fuel cell stack according to claim 1, characterized in that: In step S03, according to the stoichiometric ratio, the supply flow rate of the anode of the battery stack to be purged is 1.6 to 1.8 of the stoichiometric ratio of the corresponding current; the supply flow rate of the cathode of the battery stack to be purged is 1.8 to 2.0 of the stoichiometric ratio of the corresponding current; The current density of the current loading is 0.05A / cm 2 ~0.1A / cm 2 .

10. The method for determining the degree of purge during cold start of a fuel cell stack according to claim 1, characterized in that: In step S04, the purging time is 5 minutes to 10 minutes.

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