Fuel cell shutdown purging method

Abstract

The present invention relates to the field of proton exchange membrane fuel cells, and specifically to a fuel cell shutdown purging method. In the first stage of the method of the present invention, the anode hydrogen tail valve is closed, the hydrogen circulation pump is opened, and air is passed into the fuel cell cathode for purging with an air compressor, and hydrogen is introduced at the same time. The hydrogen is circulated through the hydrogen circulation pump to purge the fuel cell anode, and liquid water on the electrode surface, anode flow channel and pipeline is removed under the action of the water separator; in the second stage, the hydrogen circulation pump is closed, and hydrogen is continued to be introduced. At the same time, the cathode continues to purge air with an air compressor, and the free water inside the electrode is removed under the action of the pressure difference and the cathode air purge, and the hydrogen on the anode side is emptied and the purge is ended. The entire purge process of the method of the present invention does not consume a large amount of hydrogen, does not use air to purge the anode for a long time, avoids battery attenuation caused by residual hydrogen, and avoids corrosion caused by using air to purge the anode for a long time; no additional nitrogen cylinder is required, and no improvement is required to the existing fuel cell system.

Description

Technical Field

[0001] The present invention relates to the field of proton exchange membrane fuel cells, and in particular to a fuel cell shutdown purge method. Background Art

[0002] A proton exchange membrane fuel cell (PEMFC) is an energy conversion device that operates on electrochemical principles. The principle is that hydrogen and oxygen react to produce water. Therefore, for every two moles of electrons transferred by the fuel cell, one mole of water is produced. Transferring electrons is the process of performing work, so a large amount of water is generated during fuel cell operation. Although water plays a role in wetting the membrane electrodes within the electrodes, increasing proton conduction rate, and reducing internal resistance, excess water must be drained from the cell before shutting down, especially in low-temperature environments. Freezing of water in low-temperature environments can damage the electrode structure, causing irreversible performance degradation and severely impacting the performance and lifespan of the fuel cell.

[0003] Currently, the main method for removing water from fuel cells is to shut down and purge the system. This raises two basic issues: which gas to choose for the purge and how to determine the purge degree. Regarding the first issue, if the reaction gas is used directly for the purge, the purge time can range from tens of seconds to tens of minutes, and the purge gas volume is large, which can easily lead to hydrogen waste. Furthermore, directly using air to purge the anode may create a hydrogen-air interface and cause corrosion. Therefore, some patents use nitrogen to purge the anode (Chinese patents CN110137536A and CN107946614B), but this undoubtedly requires the installation of a nitrogen cylinder in the existing fuel cell system, increasing the complexity and cost of the system and having low practical application value. Additionally, some patents propose using methods such as deoxygenating air (Chinese patent CN110854415A), online nitrogen generation (Chinese patent CN103915642B), or tail-end nitrogen collection (Chinese patent CN111029620B) to obtain nitrogen-rich and oxygen-reduced air for anode purge, which also greatly increases the complexity and cost of the fuel cell system. Regarding the second issue, Chinese patent CN105161741B uses the internal and external temperature of the fuel cell as a criterion for determining whether the purge is effective, while Chinese patent CN108470924A uses the detection of liquid water as a criterion. However, these criteria are difficult to accurately determine the amount of residual water inside the cell and lack practicality. Therefore, existing purge methods still need to be improved. Summary of the Invention

[0004] The purpose of the present invention is to provide a method for fuel cell shutdown purging to address the shortcomings of the existing technology. The method of the present invention can achieve efficient shutdown purging of fuel cells without wasting hydrogen or making any modifications to the original system, and has strong practicality.

[0005] In order to achieve the above-mentioned object of the invention, the present invention adopts the following technical solutions:

[0006] The present invention provides a fuel cell shutdown purge method, wherein the purge is divided into two stages:

[0007] In the first stage, the anode hydrogen tail valve is closed, the hydrogen circulation pump is turned on, and air is introduced into the fuel cell cathode through an air compressor for purge. At the same time, hydrogen is introduced. The hydrogen is circulated through the hydrogen circulation pump to purge the fuel cell anode, and the liquid water on the electrode surface, anode flow channel and pipeline is removed by the water separator.

[0008] In the second stage, the hydrogen circulation pump is turned off and hydrogen continues to be introduced. At the same time, the cathode continues to be purged with air by the air compressor. Under the action of the pressure difference and the cathode air purge, the free water inside the electrode is removed, the hydrogen on the anode side is emptied and the purge is ended.

[0009] In the above technical solution, further, in the first stage, the internal resistance of the fuel cell is monitored in real time, and when the change in the internal resistance of the fuel cell does not exceed 5% to 10% within 30 consecutive seconds, the second stage is entered.

[0010] In the above technical solution, further, in the second stage, the internal resistance of the fuel cell is monitored in real time, and when the internal resistance of the fuel cell changes by no more than 1% to 5% within 30 consecutive seconds, the hydrogen on the anode side is exhausted.

[0011] In the above technical solution, further, the hydrogen on the anode side is emptied, that is, the hydrogen tail valve is opened, air is quickly introduced into the anode to empty the hydrogen inside the anode cavity, and the purge is ended.

[0012] In the above technical solution, further, after the hydrogen is introduced into the second stage, the anode back pressure value reaches 30-150kPa.

[0013] In the above technical solution, further, the rotational speed of the air compressor and the hydrogen circulation pump is determined by the rated power of the fuel cell system.

[0014] In the above technical solution, further, the internal resistance of the fuel cell is measured by an impedance meter or an ohmmeter.

[0015] Compared with the prior art, the present invention has the following beneficial effects:

[0016] (1) The purging method of the present invention closes the anode tail exhaust (i.e., hydrogen tail exhaust) during the entire purging process. In the first stage of the purging process, only a small amount of hydrogen is used, and the built-in hydrogen circulation system and water separator are used to remove excess liquid water on the electrode surface and inside the flow channel. In the second stage, the free water inside the electrode is removed under the pressure difference between the anode and cathode. The entire purging process does not require the consumption of a large amount of hydrogen, which can effectively prevent fuel waste.

[0017] (2) The purge method of the present invention does not use air to purge the anode for a long time. Air is only quickly introduced during the anode emptying stage to avoid battery degradation caused by residual hydrogen and corrosion caused by long-term air purge of the anode.

[0018] (3) The present invention uses only hydrogen for anode purging, and does not require additional nitrogen bottles or any improvements to the existing fuel cell system. It is easy to operate and highly practical. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 This is a schematic diagram of the fuel cell shutdown purge system of the present invention.

[0020] Figure 2 This is a graph showing the change of the internal resistance of the fuel cell over time during the shutdown and purge process of the fuel cell of the present invention.

[0021] Figure 3 This is a performance comparison chart before and after ten low-temperature parking operations in Example 1 of the present invention. DETAILED DESCRIPTION

[0022] The present invention is further described below with reference to specific examples, but is not intended to limit the present invention in any way.

[0023] Example 1

[0024] like Figure 1 The figure shows a schematic diagram of an existing fuel cell system. The fuel cell system used in the specific implementation of the present invention includes but is not limited to Figure 1 The system shown, Figure 1 The method of the present invention does not change the existing fuel cell system and is based on the existing fuel cell system.

[0025] To simplify the explanation, the fuel cell system in this example is a simplified fuel cell system, which mainly includes a fuel cell stack, a hydrogen circuit and an air circuit. The hydrogen circuit includes a hydrogen cylinder for storing hydrogen, a hydrogen circulation circuit and a hydrogen tail exhaust valve in sequence. A hydrogen circulation pump and a water separator are provided in the hydrogen circulation circuit. The air circuit mainly includes an air compressor and an air side tail exhaust valve. When the fuel cell receives the shutdown signal, it starts to shut down and purge according to the following steps: the air compressor passes air into the fuel cell cathode at a certain speed for purge. At the same time, the anode hydrogen tail valve is closed, a certain amount of hydrogen is passed, and the hydrogen circulation pump is operated at a certain speed. Under the action of the water separator, liquid water on the electrode surface, anode flow channel and pipeline is removed. When it is monitored that the internal resistance of the fuel cell changes by no more than 10% within 30 seconds, the hydrogen circulation pump is turned off, and a small amount of hydrogen is continued to be passed to make the anode back pressure reach the set value. At the same time, the cathode is continued to be purged with air. Under the action of the pressure difference and the cathode air purge, the free water inside the electrode is removed until the internal resistance of the fuel cell changes by no more than 1% within 30 seconds. Then, the hydrogen tail valve is opened, air is quickly passed into the anode to empty the hydrogen inside the anode, and the purge is ended.

[0026] Attachment Figure 2 The method of the present invention is used to shut down and purge the fuel cell. As can be seen from the figure, in the first stage, the liquid water in the anode flow channel, electrode surface and pipeline is mainly removed by the hydrogen circulation pump and water separator. Due to the high initial water content, the increase in the battery internal resistance is small. When it is detected that the fuel cell internal resistance changes by no more than 10% within 30 seconds (see Appendix Figure 2 The change in battery internal resistance from 3min to 3min 30s is 9.94%), indicating that the liquid water on the electrode surface has been basically removed. Subsequently, the anode purge enters the second stage, the hydrogen circulation pump is turned off, a small amount of hydrogen is introduced and the anode is given a back pressure of 50kPa. Since the cathode is at normal pressure, the pressure difference between the cathode and the anode can effectively remove the free water inside the cathode and anode electrodes. This part of water contributes greatly to the internal resistance of the fuel cell, so the internal resistance rises more in this stage. When it is detected that the internal resistance changes by no more than 1% within 30s (see Appendix Figure 2 The battery internal resistance changed by 0.96% during the 7min 30s to 8min purge. After that, the internal resistance of the stack did not change significantly when the purge was continued, and a platform area appeared, indicating that most of the free water inside the electrode had been removed. At this point, the water content inside the fuel cell has been reduced to a relatively safe level.

[0027] The battery purged by this method was sealed and placed in a -30℃ environmental chamber for 12 hours. After being taken out and purged, it was sealed and placed in a -30℃ environmental chamber for 12 hours. This operation was repeated 10 times. After the repetition was completed, the battery performance was tested. The results are shown in the attached figure. Figure 3As shown in the figure, after 10 times of parking at -30℃ using this strategy, the battery still maintains its original performance without any attenuation, indicating that this method has strong practicality and reliability.

Claims

1. A fuel cell shutdown purge method, characterized in that: The method comprises the following stages: in the first stage, closing the anode hydrogen tail valve, starting the hydrogen circulation pump, and using an air compressor to pass air into the fuel cell cathode for purging. At the same time, hydrogen is passed in, and the hydrogen is circulated through the hydrogen circulation pump to purge the fuel cell anode, and liquid water on the electrode surface, anode flow channel and pipeline is removed by the action of a water separator; In the second stage, the hydrogen circulation pump is turned off and hydrogen continues to be introduced. At the same time, the cathode continues to be purged with air by the air compressor. Under the action of the pressure difference and the cathode air purge, the free water inside the electrode is removed, the hydrogen on the anode side is emptied and the purge is completed; In the first stage, when the internal resistance of the fuel cell does not change by more than 5% to 10% within 30 consecutive seconds, the second stage begins. In the second stage, when the internal resistance of the fuel cell does not change by more than 1% to 5% within 30 seconds, the hydrogen on the anode side is exhausted.

2. The method according to claim 1, characterized in that The second stage is to exhaust the hydrogen on the anode side, that is, to open the hydrogen tail valve, quickly introduce air into the anode to exhaust the hydrogen inside the anode, and then end the purge.

3. The method according to claim 1, characterized in that After hydrogen is introduced into the second stage, the anode back pressure reaches 30-150 kPa.

4. The method according to claim 1, wherein The speed at which the air compressor and the hydrogen circulation pump operate is determined by the rated power of the fuel cell system.

5. The method according to claim 1, wherein The internal resistance of the fuel cell is measured by an impedance meter or an ohmmeter.

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