Fuel cell starting and stopping method based on current distribution optimization

Abstract

The invention discloses a fuel cell starting and stopping method based on current distribution optimization, relates to the technical field of fuel cell starting and stopping strategies, and aims to achieve the purposes of more accurately determining purging time in the shutdown process of an electric pile, performing purging in a time as short as possible, ensuring that water in the electric pile is dried as far as possible, and reducing attenuation of the electric pile. According to the technical key points, after the galvanic pile enters an idling state, a certain amount of reaction gas is supplied to the galvanic pile, and on-load purging is carried out on the galvanic pile; judging to stop gas supply according to the acquired current distribution state in the galvanic pile; reducing the load of the output current of the electric pile to zero; connecting a discharge resistor between the anode and the cathode of the stack, so that the stack is electrified to consume residual reaction gas in the anode and the cathode of the stack; and when the voltage between the positive electrode and the negative electrode of the electric pile is reduced to a set value, the discharge resistor is disconnected to finish shutdown.

Description

Technical Field

[0001] The present invention relates to the technical field of fuel cell start-stop strategies, and in particular to a fuel cell shutdown strategy based on current distribution optimization.

[0002] Background

[0003] A fuel cell is a power generation device that directly converts the chemical energy in fuel and oxidant into electrical energy through electrochemical reactions. It has the advantages of high energy conversion rate, no pollution and low noise. With the promotion of its commercialization process, the market has put forward higher requirements for the output power of fuel cells, so fuel cells with large active areas have become a development trend. However, fuel cells involve multiple physical fields such as water, gas, heat, electricity, and force. The combined action of these physical fields determines the power generation performance of the battery. However, these physical fields are usually difficult to distribute evenly, especially in a larger active area, so it is more likely to cause uneven attenuation in different areas of the stack. In addition, the start-stop process of the fuel cell is prone to hydrogen-air interface, high cathode potential, and reaction gas starvation, which accelerates the attenuation of the stack. In response to the above problems, the study of fuel cell start-stop strategies based on current distribution optimization has become an important technical means to alleviate battery attenuation.

[0004] During the start-up of the stack, in order to avoid a long open circuit state, it is necessary to quickly load after introducing hydrogen (fuel) and air (oxidant) to reduce the cathode potential and shorten the duration of high potential. 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 cause the cathode high potential to last for a long time, accelerating the stack attenuation. Therefore, by optimizing the current distribution in the active area of ​​the stack during the startup loading process, the current distribution is kept within a reasonable range while minimizing the duration of the cathode high potential.

[0005] During the shutdown process of the battery stack, the first thing to do is to use the reaction gas to blow the excess water generated in the battery stack out of the battery stack to avoid corrosion and icing of the bipolar plates caused by residual water, which will lead to the attenuation of the battery stack. However, if the purge gas volume is small, it is not enough to fully remove the residual water, and a longer purge time is required, which reduces the convenience of the product; if the purge gas volume is too large, it is easy to purge the inlet of the battery stack too dry, causing mechanical damage to the membrane electrode. It can be seen that during the shutdown process, no matter what gas volume is used for purge, the purge time should be accurately grasped, and the time should be avoided to be too short, which will cause the residual water in the battery stack to be fully removed, resulting in the attenuation of the battery stack. But it should also be avoided that the time is too long, which will cause damage to the membrane electrode and then cause the attenuation of the battery stack. Therefore, it is of great significance to choose the appropriate purge time. Summary of the invention

[0006] In order to solve the above problems, the purge time can be determined more accurately during the shutdown of the fuel cell stack, and the purge can be performed in the shortest possible time, so as to dry the water in the fuel cell stack as much as possible and reduce the possibility of fuel cell stack attenuation.

[0007] In a first aspect, according to some embodiments of the present application, the fuel cell shutdown method based on current distribution optimization includes setting the fuel cell stack to an idle state, supplying a certain amount of reactant gas to the fuel cell stack, and performing load purge on the fuel cell stack;

[0008] Stop gas supply based on the current distribution state inside the fuel cell stack;

[0009] Reduce the output current of the battery stack to zero; connect the discharge resistor between the positive and negative electrodes of the battery stack to discharge the battery stack to consume the residual reactant gas in the anode and cathode; when the voltage between the positive and negative electrodes of the battery stack drops below the set value, disconnect the discharge resistor 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 gas supply is determined according to the current distribution state, including stopping the loaded purge and gas supply 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 metering ratio is determined to be a first gas metering ratio, when the relative standard deviation of the collected current distribution reaches a standard deviation threshold, the on-load purge and gas supply are stopped;

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

[0013] The first gas stoichiometric ratio is greater than the second gas stoichiometric ratio.

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

[0015] The range of the first gas stoichiometric ratio is 1.5 to 10.0; the range of the second gas stoichiometric ratio is 10.0 to 15.0.

[0016] The standard deviation threshold 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 stack temperature is reduced to a certain value.

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

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

[0020] Start the water pump;

[0021] Heat the battery stack to a set temperature;

[0022] Introduce a large amount of air into the cathode of the stack so that the cathode cavity of the stack is filled with air;

[0023] Introduce an atmospheric amount of hydrogen into the anode of the stack so that the anode cavity of the stack is filled with hydrogen;

[0024] The load is pulled at a certain rate to make the battery stack enter the idle state.

[0025] According to the fuel cell start-stop method based on current distribution optimization in some embodiments of the present application, including a start-up method and a shutdown method, wherein in the start-up method, the flow rate of atmospheric air and atmospheric hydrogen introduced is 1.5 to 15.0 times the amount of air introduced in the idle state of the fuel cell stack.

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

[0027] On the third aspect, the fuel cell stack in some embodiments of the present application is characterized in that it is used to implement any shutdown method or start-stop method.

[0028] Beneficial effect: The present invention optimizes the basis for determining the purge time through the law of current distribution within the active area of ​​the stack during the loaded purge process, thereby achieving current distribution within a reasonable range while discharging as much water as possible and shortening the purge time as much as possible, thereby reducing the possibility of stack attenuation.

[0029] The present invention connects a discharge resistor between the positive and negative electrodes to achieve rapid consumption of residual gas in the anode and cathode, thereby shortening the duration of high potential caused by the residual gas in the anode and cathode. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] 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 some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative labor.

[0031] Figure 1 A flowchart of starting the execution of the method according to an embodiment of the present invention;

[0032] Figure 2 A flowchart of a shutdown method according to an embodiment of the present invention;

[0033] Figure 3 This is a graph showing the current distribution change trend during the on-load purge process of an embodiment of the present invention;

[0034] Figure 4 This is a trend diagram of the current distribution state change during the on-load purge process of an embodiment of the present invention;

[0035] Figure 5 The figure shows the influence of different purge gas volumes on the current distribution uniformity during the on-load purge process of the embodiment of the present invention.

[0036] Figure 6 The effect of different shutdown purge strategies on stack degradation. DETAILED DESCRIPTION

[0037] In order to enable those skilled in the art to better understand the scheme of the present invention, the technical scheme in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. 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 should fall within the scope of protection of the present invention.

[0038] Example: This example takes a proton exchange membrane fuel cell as an example. Specifically, the active area of ​​the fuel cell stack is 372 cm 2 The working temperature is 75°C, the humidity of the reaction gas is 30% to 100% RH during normal operation, and the reaction gas is dry gas during the loaded purge process. The fuel cell start-stop method based on current distribution optimization proposed by the present invention includes two processes: start-up and shutdown.

[0039] like Figure 1As shown, during the startup process, the water pump is first started to heat the stack to the set temperature, and then a large amount of air is introduced into the cathode of the stack to fill the cathode cavity of the stack with air, and then a large amount of hydrogen is quickly introduced into the anode. After the hydrogen fills the anode cavity of the stack, it is loaded at a certain rate to put the stack into an idle state. Furthermore, a large amount of air is introduced during the startup process, and its gas flow rate is 1.5 to 15.0 times the gas volume corresponding to the idle current. Furthermore, a large amount of hydrogen is introduced during the startup process, and its gas flow rate is 1.5 to 15.0 times the gas volume corresponding to the idle current. Furthermore, during the startup process, the stack is loaded at a certain rate. When all patrol open-circuit voltages of the stack are above 0.9V, loading begins. To ensure that the current distribution is within a reasonable range, the loading rate is 0.01 to 0.5A / (cm 2 *s).

[0040] like Figure 2 As shown in the figure, during the shutdown process, the current is first reduced to the idle current, and then the anode and cathode gas flow rates are increased to 10 times the gas volume corresponding to the idle current. At the same time, the humidification function is turned off, and dry gas is directly allowed to enter the stack for load purging. Figure 3 As shown in the figure, the current distribution changes with time during the purge process of the stack. It can be seen that as the purge time increases, the current concentration area gradually moves from the left side (cathode inlet side) to the right side (cathode outlet side). The reason is that a large amount of dry gas blows the cathode inlet side too dry, the internal resistance increases, and the current tends to the relatively more humid outlet side. It is also from this that the inventor accidentally discovered that the distribution of current reflects the distribution of water in the stack.

[0041] Figure 4 The figure is a trend chart of the change of the relative standard deviation of the current distribution over time in this process. It can be seen that with the extension of the purge time, the consistency of the current distribution gradually deteriorates and tends to be stable after about 200 seconds.

[0042] It can be seen that after about 200s, the current distribution area is stable, indicating that the water generated by the reaction in the stack and the water removed by the reaction gas purge have reached a dynamic balance. That is, at this time, if the current air volume of the reaction gas is continued to be used to purge the stack, the current distribution of the stack has basically stabilized, and the water generated by the reaction in the stack and the water removed by the reaction gas purge have reached a dynamic balance, so that the amount of water in the stack is basically stable. Therefore, continuing to purge the stack with the current air volume of the reaction gas will not further reduce the amount of water inside the stack. On this basis, if the air volume is relatively large, continuing to purge will easily blow the entrance of the stack too dry, resulting in mechanical damage to the membrane electrode and causing the stack to attenuate. If the air volume is small, it takes a long time to reach this balance, and continuing to purge will further reduce the convenience of the product.

[0043] Therefore, by Figure 3-4 It can be seen that the stability of the current distribution of the collected stack can be used as a basis for judging whether to stop the current gas supply purge. The gas supply purge is stopped based on the current distribution. At this time, the water produced inside the stack and the water removed by the gas supply purge are balanced, which is the critical point for reducing the water volume in the stack. That is, the purge has achieved the best water removal effect. Stopping the purge at this time can avoid the attenuation of the stack caused by the long-term purge with large air volume and the reduction of convenience caused by the long-term purge with small air volume.

[0044] Figure 5 For the loaded purge process with different gas stoichiometric ratios, it can be seen that the larger the gas stoichiometric ratio, the more uneven the current distribution is, and the shorter the time it takes to stabilize. Therefore, although under ideal conditions, according to the above content, the stabilization of the relative standard deviation of the current distribution should be used as a sign to judge the end of the purge. However, although a large gas stoichiometric ratio can reach this stability faster than a small gas stoichiometric ratio, considering that purging with a large gas stoichiometric ratio for more than a certain period of time will cause damage to the membrane electrode and cause the stack to decay, therefore, for large gas volume purges, the stabilization of the relative standard deviation of the current distribution area is usually not used as a sign to judge the end of the purge, but the purge is terminated when the standard deviation reaches a certain threshold, which can minimize the decay of the stack and still achieve a good water removal effect.

[0045] From the above, such as Figure 2 As shown, during the shutdown process, the stack is first reduced to the idle state, and the stack temperature is reduced to the set value, and then the flow rate of hydrogen and air is increased to the set value, and the on-load purge is performed. After the current distribution consistency tends to be stable, the on-load purge is terminated, and the stack output current is quickly reduced to 0, and then the supply of the reactant gas is stopped, and the discharge resistor is connected at the same time to discharge the stack, and the residual reactant gas in the anode and cathode is quickly consumed. After the stack voltage drops to a certain range (such as below 0.8V, or below 0.3V), the discharge resistor is disconnected. Furthermore, the anode and cathode purge gas volume is increased during the shutdown process, and the gas flow rate is 1.5 to 15.0 times the gas volume corresponding to the idle current. Furthermore, the current is reduced to 0 at the end of the on-load purge during the shutdown process. During the on-load purge process, the current distribution uniformity (measured by parameters such as the current distribution range, variance, standard deviation, and relative standard deviation) shows an increasing trend, and it is used as a basis for judging the end of the on-load purge when it tends to be stable. Furthermore, during the shutdown process, a discharge resistor is connected to consume the residual reaction gas, and the resistance value of the connected discharge resistor is 0.01 to 20 times the internal resistance of the fuel cell stack.

[0046] In another solution, the gas metering ratio used in the shutdown process is used to determine whether the relative standard deviation of the current distribution area tends to be stable as the sign of judging the purge stop, or when the standard deviation reaches a certain threshold as the sign of judging the purge stop. In some examples, such as Figure 5As shown, with the gas metering ratio of 10 as the dividing line, when the gas metering ratio is lower than 10, the stabilization of the relative standard deviation of the current distribution area can be used as a sign to judge whether the purge should be stopped. When the gas metering ratio is greater than 10, the standard deviation reaching a certain threshold can be used as a sign to judge whether the purge should be stopped.

[0047] From the perspective of current distribution, the present invention provides more detailed internal current distribution information for optimizing the startup and shutdown process of the fuel cell, making up for the current deficiency of relying solely on high-frequency resistance as the basis for judgment. In addition, the current distribution information can provide assistance for a deeper understanding of the start-stop process of the fuel cell and help alleviate the attenuation phenomenon of the fuel cell stack.

[0048] like Figure 6 As shown in the figure, when the fuel cell adopts the conventional shutdown purge strategy (that is, the current distribution is not optimized), after multiple shutdown purges, the battery performance decays from 0.66V to 0.62V, and the decay rate is 0.23mV / h; while when the shutdown purge strategy based on current distribution optimization is adopted, the battery performance drops from 0.68V to 0.65V, and the decay rate is 0.12mV / h, which is much lower than the former. It can be seen that the shutdown purge strategy based on current distribution optimization plays an important role in alleviating the attenuation of fuel cells.

[0049] 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.

Claims

1. A fuel cell shutdown method based on current distribution optimization, characterized in that: include The stack is set to idle state, a certain amount of reactant gas is supplied to the stack, and the stack is purged under load; Stop gas supply based on the current distribution state inside the fuel cell stack; Reduce the output current of the battery stack to zero; connect the discharge resistor between the positive and negative electrodes of the battery stack to discharge the battery stack to consume the residual reactant gas in the anode and cathode; when the voltage between the positive and negative electrodes of the battery stack drops below the set value, disconnect the discharge resistor to complete the shutdown.

2. The fuel cell shutdown method based on current distribution optimization according to claim 1, characterized in that: In the step, it is determined to stop the gas supply according to the current distribution state, including stopping the loaded purge and gas supply when the relative standard deviation of the current distribution tends to be stable.

3. The fuel cell shutdown method based on current distribution optimization according to claim 2, characterized in that: If a certain gas metering ratio is determined to be the first gas metering ratio, when the relative standard deviation of the collected current distribution reaches a standard deviation threshold, the on-load purge and gas supply are stopped; If a certain gas metering ratio is determined to be the second gas metering ratio, when the relative standard deviation of the current distribution tends to be stable, the on-load purge and gas supply are stopped; The first gas stoichiometric ratio is greater than the second gas stoichiometric ratio.

4. The fuel cell shutdown method based on current distribution optimization according to claim 1, characterized in that: The gas stoichiometric ratio is the ratio of the purge gas volume of the fuel cell stack to the intake air volume of the fuel cell stack at idle speed; The range of the first gas stoichiometric ratio is 1.5 to 10.0; the range of the second gas stoichiometric ratio is 10.0 to 15.

0. The standard deviation threshold of the current distribution is 60.0%.

5. A fuel cell shutdown method based on current distribution optimization according to any one of claims 1 to 4, characterized in that: The stack is set to idle state to reduce the stack temperature to a certain value.

6. A fuel cell shutdown method based on current distribution optimization according to any one of claims 1 to 4, characterized in that: The resistance value of the discharge resistor is 0.01 to 20 times of the internal resistance of the battery stack; the voltage drops to a set value of 0.8V.

7. A fuel cell start-stop method based on current distribution optimization, characterized in that: It includes a startup method and a shutdown method, and the shutdown method includes any method in claims 1-6; wherein the startup method includes Start the water pump; Set the stack temperature to a target value; Introduce a large amount of air into the cathode of the stack so that the cathode cavity of the stack is filled with air; Introduce an atmospheric amount of hydrogen into the anode of the stack so that the anode cavity of the stack is filled with hydrogen; The load is pulled at a certain rate to make the battery stack enter the idle state.

8. The fuel cell start-stop method based on current distribution optimization according to claim 7, wherein: In the starting method, the flow rate of atmospheric air and atmospheric hydrogen is 1.5 to 15.0 times the amount of gas introduced when the stack is in the idle state.

9. The fuel cell start-stop method based on current distribution optimization according to claim 7, wherein: In the startup method, when the open circuit voltage of the battery stack is above 0.9V, the load is pulled at a rate of 0.01 to 0.5A / (cm 2 *s).

10. A battery stack, characterized in that: Used to implement any one of the methods of claims 1 to 9.

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