Fuel cell start-stop method based on current distribution optimization

Abstract

The application relates to a fuel cell start-stop method based on current distribution optimization, and relates to the technical field of fuel cell start-stop strategies, which can more accurately determine the purging time during the shutdown of a fuel cell stack, perform purging in the shortest possible time, ensure that the water in the fuel cell stack is dried as much as possible, and reduce the attenuation of the fuel cell stack, and the technical points are as follows: after the fuel cell stack enters an idle state, a certain amount of reaction gas is supplied to the fuel cell stack to perform on-load purging; the supply of the reaction gas is stopped according to the collected current distribution state of the fuel cell stack; the output current of the fuel cell stack is reduced to zero; a discharging resistor is connected between the positive and negative electrodes of the fuel cell stack to discharge the fuel cell stack to consume the residual reaction gas in the negative and positive electrodes of the fuel cell stack; and the discharging resistor is disconnected to complete the shutdown when the voltage between the positive and negative electrodes of the fuel cell stack decreases to a set value.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of fuel cell start-stop strategy, and particularly relates to a fuel cell shutdown strategy based on current distribution optimization. BACKGROUND

[0002] A fuel cell is a power generation device that converts chemical energy in fuel and oxidant into electrical energy through electrochemical reactions, and has the advantages of high energy conversion efficiency, no pollution and low noise. With the popularization of its commercialization process, the market has higher requirements for the output power of fuel cells, so fuel cells with large active areas have become a development trend. However, the fuel cell involves multiple physical fields such as water, gas, heat, electricity and force inside the fuel cell, and the performance of the fuel cell is determined by the combined action of these physical fields. However, these physical fields are usually not evenly distributed, especially in a larger active area, so it is more likely to cause uneven decay in different regions inside the stack. In addition, the fuel cell start-stop process is prone to hydrogen-air interface, high potential at the cathode, and reaction gas starvation, thereby accelerating the decay of the stack. In view of the above problems, the fuel cell start-stop strategy based on current distribution optimization has become an important technical means to alleviate the decay of the battery.

[0003] In the process of starting the stack, in order to avoid a long open circuit state, hydrogen (fuel) and air (oxidant) need to be quickly loaded after being introduced to reduce the cathode potential and shorten the high potential duration. However, if the loading rate is too fast, it is easy to cause local gas shortage inside the stack, thereby causing reverse polarity and other phenomena. If the loading rate is too slow, it will lead to a long duration of high potential at the cathode, accelerating the decay of the stack. Therefore, by optimizing the current distribution in the active area of the stack during the start-up loading process, the current distribution is within a reasonable range while the duration of high potential at the cathode is as short as possible.

[0004] In the process of stopping the stack, first of all, the excess water generated in the stack needs to be blown out of the stack by using reaction gas to avoid corrosion and icing of the bipolar plate due to residual water, thereby causing the decay of the stack. However, if the amount of purge gas is small, it is not enough to fully remove the residual moisture, and a long purging time is required, which reduces the convenience of product use. If the amount of purge gas is too large, it is easy to blow the inlet of the stack too dry, causing mechanical damage to the membrane electrode. Therefore, in the process of stopping, no matter what amount of gas is used for purging, the purging time should be accurately grasped, and the time should be avoided to be too short, which will cause the residual moisture in the stack to be fully removed, thereby causing the decay of the stack. However, the time should also be avoided to be too long, which will cause damage to the membrane electrode, thereby causing the decay of the stack. Therefore, it is of great significance to select the appropriate purging time. SUMMARY

[0005] To solve the above problems, in order to realize more accurate determination of purging time during stack shutdown, purging in the shortest time, and possible stack decay reduction while drying the water in the stack as much as possible.

[0006] In the first aspect, according to the fuel cell shutdown method based on current distribution optimization in some embodiments of the present application, the method comprises

[0007] The stack is set to an idle state, and a certain amount of reaction gas is supplied to the stack for on-load purging of the stack;

[0008] The supply of gas is stopped according to the current distribution state inside the stack;

[0009] The output current of the stack is reduced to zero, and a discharge resistor is connected between the positive and negative electrodes of the stack to discharge the stack to consume the residual reaction gas in the anode and cathode; when the voltage between the positive and negative electrodes of the stack drops below a set value, the discharge resistor is disconnected to complete the shutdown.

[0010] According to the fuel cell shutdown method based on current distribution optimization in some embodiments of the present application, the step of stopping the supply of gas according to the current distribution state comprises stopping the on-load purging and the supply of gas when the relative standard deviation of the current distribution tends to be stable.

[0011] According to the fuel cell shutdown method based on current distribution optimization in some embodiments of the present application, if a certain gas measurement ratio is determined to be a first gas measurement ratio, the on-load purging and the supply of gas are stopped when the relative standard deviation of the collected current distribution reaches a standard deviation threshold value;

[0012] If a certain gas measurement ratio is determined to be a second gas measurement ratio, the on-load purging and the supply of gas are stopped when the relative standard deviation of the current distribution tends to be stable;

[0013] Wherein, the first gas measurement ratio is greater than the second gas measurement ratio.

[0014] According to the fuel cell shutdown method based on current distribution optimization in some embodiments of the present application, the gas measurement ratio is the ratio of the purging gas amount of the stack to the intake amount of the stack in the idle state;

[0015] The second gas measurement ratio is in the range of 1.5-10.0, and the first gas measurement ratio is in the range of 10.0-15.0.

[0016] The standard deviation threshold value of the current distribution is 60.0%.

[0017] According to the fuel cell shutdown method based on current distribution optimization in some embodiments of the present application, the stack is set to an idle state, and the temperature of the stack is reduced to a certain value.

[0018] According to the fuel cell shutdown method based on current distribution optimization in some embodiments of the application, the resistance value of the discharge resistor is 0.01-20 times of the internal resistance of the stack; and the voltage drop to the set value is 0.8 V.

[0019] In the second aspect, according to the fuel cell start-stop method based on current distribution optimization in some embodiments of the application, the start-stop method includes a start method and a shutdown method, and the shutdown method includes any one of the methods in claims 1-6; wherein the start method includes

[0020] starting the water pump;

[0021] heating the stack to a set temperature;

[0022] passing a large amount of air into the cathode of the stack to fill the cathode cavity of the stack with air;

[0023] passing a large amount of hydrogen into the anode of the stack to fill the anode cavity of the stack with hydrogen;

[0024] performing load pulling at a certain rate to make the stack enter an idle state.

[0025] According to the fuel cell start-stop method based on current distribution optimization in some embodiments of the application, the start-stop method includes a start method and a shutdown method, wherein in the start method, the flow rates of the large amount of air and the large amount of hydrogen are 1.5-15.0 times of the gas flow rate in the idle state of the stack.

[0026] According to the fuel cell start-stop method based on current distribution optimization in some embodiments of the application, the start-stop method includes a start method and a shutdown method, wherein in the start method, when the open circuit voltage of the stack is above 0.9 V, load pulling is performed, and the rate of the load pulling is 0.01-0.5 A / (cm 2 s).

[0027] In the third aspect, according to the stack in some embodiments of the application, the stack is characterized in that any one of the shutdown method or the start-stop method is implemented.

[0028] Beneficial effects: the application optimizes the purging time determination basis through the current distribution law in the active area of the stack during the load purging process, realizes that the current distribution is in a reasonable range, discharges more water as much as possible, and shortens the purging time as much as possible, and reduces the possibility of stack attenuation.

[0029] The application connects a discharge resistor between the positive electrode and the negative electrode, realizes rapid consumption of residual gas in the anode and the cathode, and thus shortens the high potential duration caused by residual gas in the anode and the cathode. BRIEF DESCRIPTION OF DRAWINGS

[0030] In order to make the technical solution of the embodiments of the present application or the prior art clearer, the drawings needed in the embodiments or prior art description will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present application, and other drawings can be obtained by those skilled in the art without creative labor on the basis of these drawings.

[0031] Figure 1 Flow chart for starting the method of the embodiment of the present application;

[0032] Figure 2 Flow chart for stopping the method of the embodiment of the present application;

[0033] Figure 3 Current distribution trend chart during the load purging process of the embodiment of the present application;

[0034] Figure 4 Current distribution state change trend chart during the load purging process of the embodiment of the present application;

[0035] Figure 5 Influence of different purging gas amounts on current distribution uniformity during the load purging process of the embodiment of the present application.

[0036] Figure 6 Influence of different stopping purging strategies on stack attenuation. DETAILED DESCRIPTION

[0037] In order to make those skilled in the art better understand the present application, the technical solutions in the embodiments of the present application will be described clearly and completely below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only some of the embodiments of the present application, not all. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor should be within the scope of protection of the present application.

[0038] Embodiment: This embodiment takes a proton exchange membrane fuel cell as an example. Specifically, the active area of the stack is 372cm 2 , the working temperature is 75℃, the reaction gas humidity is 30%~100%RH during normal operation, and the reaction gas is dry gas during the load purging process. The fuel cell start-stop method based on current distribution optimization proposed in the present application includes two processes: starting and stopping.

[0039] As Figure 1As shown, during startup, the water pump is first started to heat the fuel cell stack to the set temperature. Then, a large volume of air is introduced into the cathode of the fuel cell stack, filling the cathode cavity with air. Next, a large volume of hydrogen is rapidly introduced into the anode. Once the anode cavity is filled with hydrogen, a load is applied at a certain rate to bring the fuel cell stack into idle mode. Furthermore, a large volume of air is introduced during startup, with a flow rate 1.5 to 15.0 times the air volume corresponding to the idle current. Similarly, a large volume of hydrogen is introduced during startup, with a flow rate 1.5 to 15.0 times the air volume corresponding to the idle current. Furthermore, the fuel cell stack is loaded at a certain rate during startup. Loading begins when all open-circuit voltages of the fuel cell stack are above 0.9V. To ensure the current distribution is within a reasonable range, the load application rate is 0.01 to 0.5 A / (cm²). 2 s).

[0040] like Figure 2 As shown, during shutdown, the current is first reduced to the idle current, then the anode and cathode gas flow rates are increased to 10 times the gas flow rate corresponding to the idle current. Simultaneously, the humidification function is turned off, allowing dry gas to directly enter the fuel cell stack for on-load purging. During this process, as... Figure 3 As shown, the current distribution during the loaded purging process of the fuel cell stack changes over time. It can be seen that as the purging time increases, the current concentration area gradually shifts from the left side (cathode inlet side) to the right side (cathode outlet side). This is because a large amount of dry gas dries out the cathode inlet side, increasing the internal resistance, causing the current to tend towards the relatively more humid outlet side. It was from this that the inventors unexpectedly discovered that the current distribution reflects the water distribution within the fuel cell stack.

[0041] Figure 4 The graph shows the trend of the relative standard deviation of the current distribution over time during the process. It can be seen that as the purging time increases, the consistency of the current distribution gradually deteriorates, and then stabilizes after about 200 seconds.

[0042] Therefore, it can be seen that the current distribution stabilizes after approximately 200 seconds, indicating that the water generated by the reaction within the fuel cell stack and the water removed by the reactant gas purge have reached a dynamic equilibrium. At this point, continuing to purge the fuel cell stack with the current airflow will not significantly reduce the amount of water inside the stack. Furthermore, if the airflow is too high, continued purging could potentially dry out the fuel cell stack inlet, leading to mechanical damage to the membrane electrode assembly (MEA) and fuel cell degradation. Conversely, if the airflow is too low, reaching this equilibrium would require a longer purging time, and continued purging would further reduce the product's ease of use.

[0043] Therefore, by Figures 3-4 It can be seen that the stability of the current distribution of the collected stack can be used as a basis for determining the current stop of the gas purge. And according to the current distribution to judge the stop of the gas purge, at this time, the water produced in the stack and the water removed by the gas purge reach a balance, which is the critical point of reducing the water content in the stack, that is, the purge has reached the best water removal effect. At this time, stopping the purge can avoid the stack degradation caused by large air volume and longer time purge and the reduction of convenience caused by small air volume and longer time purge.

[0044] Figure 5 For the load purge process with different gas metering ratios, it can be seen that the larger the gas metering ratio, the more uneven the current distribution, and the shorter the time it takes to stabilize. Therefore, although in the ideal state, according to the above content, the relative standard deviation of the current distribution tends to be stable as the criterion for determining the stop of the purge. However, although a large gas metering ratio can reach this stability faster than a small gas metering ratio, considering that a large gas metering ratio purge beyond a certain time will cause membrane electrode damage and lead to stack degradation, therefore, for a large gas purge, the relative standard deviation of the current distribution tends to be stable as the criterion for determining the stop of the purge, but when the standard deviation reaches a certain threshold, the purge is ended, which can avoid stack degradation as much as possible and still achieve good water removal effect.

[0045] As shown above, Figure 2 In the shutdown process, first, the stack is unloaded to an idle state, and the stack temperature is reduced to a set value, then the flow rates of hydrogen and air are increased to a set value for load purge, and the load purge is ended when the current distribution consistency tends to be stable, and the stack output current is quickly unloaded to 0, then the supply of reaction gas is stopped, and a discharge resistor is connected to discharge the stack, quickly consuming the residual reaction gas in the cathode and anode, and when the stack voltage drops to a certain range (such as below 0.8V, or below 0.3V), the discharge resistor can be disconnected. Further, in the shutdown process, the amount of gas for purging the cathode and anode is increased, and the gas flow rate is 1.5-15.0 times the gas amount corresponding to the idle current. Further, in the shutdown process, the current is unloaded to 0 after the end of load purge, and in the load purge process, the current distribution uniformity (measured by parameters such as current distribution range, variance, standard deviation, and relative standard deviation) shows an increasing trend, and tends to be stable as the basis for determining the end of load purge. Further, in the shutdown process, a discharge resistor is connected to consume residual reaction gas, and the resistance value of the connected discharge resistor is 0.01-20 times the internal resistance of the stack.

[0046] In another scheme, according to the different gas metering ratios used in the shutdown process, it is determined whether to use the relative standard deviation of the current distribution to tend to be stable as the criterion for determining the stop of the purge, or to use the standard deviation to reach a certain threshold as the criterion for determining the stop of the purge. In some examples, asFigure 5 As shown, a gas metering ratio of 10 is used as the dividing line. If the gas metering ratio is lower than 10, the relative standard deviation of the current distribution area tends to stabilize as the purging stop mark. If the gas metering ratio is greater than 10, the standard deviation reaches a certain threshold as the purging stop mark.

[0047] This invention, from the perspective of current distribution, provides more detailed internal current distribution information for optimizing the start-up and shutdown process of fuel cells, making up for the shortcomings of the current method that relies solely on high-frequency resistance as a judgment criterion. Moreover, the current distribution information can help to gain a deeper understanding of the fuel cell start-up and shutdown process and help to alleviate the stack degradation phenomenon.

[0048] like Figure 6 As shown, when the fuel cell adopts a conventional shutdown purging strategy (i.e., without current distribution optimization), after multiple shutdown purgings, the cell performance decreases from 0.66V to 0.62V, with a degradation rate of 0.23mV / h. However, when a shutdown purging strategy based on current distribution optimization is adopted, the cell performance decreases from 0.68V to 0.65V, with a degradation rate of 0.12mV / h, which is significantly lower than the former. This demonstrates that the shutdown purging strategy based on current distribution optimization plays an important role in mitigating fuel cell degradation.

[0049] It should be noted that the terms "first," "second," etc., in the specification, claims, and accompanying drawings of this invention are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such data can be interchanged where appropriate so that the embodiments of the invention described herein can be implemented in orders other than those illustrated or described herein. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover a non-exclusive inclusion; for example, a process, method, system, product, or apparatus that comprises a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to such processes, methods, products, or apparatus.

Claims

1. A fuel cell shutdown method based on current distribution optimization, characterized by, Comprising The stack is set to idle state, and a certain amount of reaction gas is supplied to the stack to perform load purging on the stack; The supply of gas is stopped according to the current distribution state inside the stack; The output current of the stack is reduced to zero, and a discharge resistor is connected between the positive and negative electrodes of the stack to discharge the stack to consume the residual reaction gas in the anode and cathode; when the voltage between the positive and negative electrodes of the stack drops below a set value, the discharge resistor is disconnected to complete shutdown; In the step of stopping the supply of gas according to the current distribution state, when the relative standard deviation of the current distribution tends to be stable, the load purging and the supply of gas are stopped; If a certain gas measurement ratio is judged to be a first gas measurement ratio, when the relative standard deviation of the collected current distribution reaches a standard deviation threshold, the load purging and the supply of gas are stopped; If a certain gas measurement ratio is judged to be a second gas measurement ratio, when the relative standard deviation of the current distribution tends to be stable, the load purging and the supply of gas are stopped; The first gas measurement ratio is greater than the second gas measurement ratio. The gas measurement ratio is the ratio of the purging gas amount of the stack to the gas amount of the stack in idle state; the second gas measurement ratio is in the range of 1.5-10.0; and the first gas measurement ratio is in the range of 10.0-15.

0.

2. The current distribution optimization based fuel cell shutdown method according to claim 1, characterized in that, The standard deviation threshold of the current distribution is 60.0%.

3. A fuel cell shutdown method based on current distribution optimization according to any of claims 1-2, characterized in that, The stack is set to idle state, and the temperature of the stack is reduced to a certain value.

4. A fuel cell shutdown method based on current distribution optimization according to any of claims 1-2, characterized in that, The resistance value of the discharge resistor is 0.01-20 times of the internal resistance of the stack; and the voltage drop to the set value is 0.8 V.

5. A fuel cell start-stop method based on current distribution optimization, characterized by, The method comprises a starting method and a shutdown method, the shutdown method comprises any one of the methods in claims 1-4; wherein the starting method comprises Starting the water pump; Setting the temperature of the stack to a target value; Passing a large amount of air into the cathode of the stack to fill the cathode cavity of the stack with air; Passing a large amount of hydrogen into the anode of the stack to fill the anode cavity of the stack with hydrogen; Performing load pulling at a certain rate to make the stack enter idle state.

6. The current distribution optimization based fuel cell start-stop method according to claim 5, wherein, In the starting method, the flow rate of the large amount of air and the large amount of hydrogen is 1.5-15.0 times of the gas amount in the idle state of the stack.

7. The current distribution optimization based fuel cell start-stop method according to claim 5, wherein, In the starting method, when the open circuit voltage of the stack is above 0.9 V, the load is pulled, and the rate of pulling the load is 0.01~ 0.5A / (cm 2 s).

8. A stack, characterized by The method is used to implement any one of the methods in claims 1-7.

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