Preparation method and application of bipolar plate with insulated non-reaction region

Abstract

The invention aims to solve the problem that the non-reaction region insulating layer of the bipolar plate has poor inhibition on galvanic pile crystallization precipitation, and solves the problems that the non-reaction region insulating layer of the bipolar plate is easy to separate and fall off and has poor bonding strength. The invention discloses a preparation method of a bipolar plate with an insulated non-reaction region. The preparation method comprises the following steps: S1, polishing the non-reaction region of the bipolar plate to obtain a rough bipolar plate; and S2, coating a non-reaction region of the rough bipolar plate with a uniformly mixed perfluoro-2-methyl-2-pentene liquid and a photoinitiator, and then carrying out ultraviolet radiation, plane hot pressing and pressure maintaining cooling on the non-reaction region to obtain the bipolar plate with the insulated non-reaction region. According to the invention, by using an ultraviolet irradiation mode, not only can the perfluoro-2-methyl-2-pentene monomer be polymerized, but also part of fluororesin molecules in the fluorine-containing bipolar plate can be broken, and then the fluororesin molecules are chemically bonded with the perfluoro-2-methyl-2-pentene polymer, so that the bonding strength between the insulating layer and the fluorine-containing bipolar plate is improved, and the service life of the fluorine-containing bipolar plate is prolonged. And the reliability of the galvanic pile is ensured.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of all-vanadium redox flow battery, in particular to a preparation method of bipolar plate with non-reaction area insulation and its application, which is suitable for the insulation treatment method of non-reaction area of fluorine-containing bipolar plate for all-vanadium redox flow battery stack. BACKGROUND

[0002] All-vanadium redox flow battery has the advantages of long cycle life, easy scaling, fast response, free site selection and other battery systems, and has been successfully applied in many large-scale solar energy storage and wind power storage devices, large-scale emergency power supply systems and peak load shifting of power systems. The bipolar plate is one of the key materials of all-vanadium redox flow battery, which mainly plays the role of isolating the positive and negative electrodes of adjacent two cells in the stack and conducting current. The electrolyte used in traditional all-vanadium redox flow battery is sulfuric acid system. In recent years, in order to further improve the energy density of all-vanadium redox flow battery, mixed acid (sulfuric acid + hydrochloric acid) electrolyte system has been developed. Therefore, the bipolar plate made of fluorine-containing material resistant to mixed acid corrosion has emerged. Generally, the substrate of fluorine-containing bipolar plate is mixed and molded by polyvinylidene fluoride (PVDF) and expanded graphite, which has high electrical conductivity and mechanical properties.

[0003] In the stack of all-vanadium redox flow battery, the bipolar plate covers the reaction area and the non-reaction area of the cell. In the reaction area, the bipolar plate and the carbon felt electrode are tightly pressed together, and the electrons are conducted to the carbon felt through the bipolar plate for electrochemical reaction. In the non-reaction area, the bipolar plate is in contact with the electrode frame with flow channel. Since there are few electrochemical reaction sites in this area, during the long-time flow of electrolyte, the electrically conductive surface of the bipolar plate at the flow channel opening is prone to electric leakage, which leads to the precipitation and deposition of part of vanadium ions, which will block the flow channel opening and affect the performance of the battery. Therefore, insulating the non-reaction area of the bipolar plate in the stack is the best solution to the precipitation and deposition of electrolyte in the non-reaction area.

[0004] Chinese patent 201310438952.7 fixes polyethylene, polypropylene, polyvinyl chloride, glass fiber reinforced polyethylene, hot melt film or engineering plastic on the bipolar plate by injection molding, molding, hot melt forming or coating. This method uses non-fluorine type materials, which cannot meet the corrosion resistance requirements of mixed acid electrolyte. The corrosion-resistant insulating material is combined with the bipolar plate body in a pure physical way, which has low bonding strength and is prone to swelling and delamination under electrolyte conditions. Moreover, the method provided by this patent cannot meet the corrosion resistance requirements of the non-electrode area of fluorine-containing bipolar plate, and the bonding strength cannot be further improved, which cannot guarantee the reliability of the stack.

[0005] The Chinese patent 202110064585.3 provides a method for insulating the surface edge of a bipolar plate, which polymerizes fluorine-containing acrylic ester monomers and cross-linking agent divinylbenzene under gamma ray irradiation to obtain a fluorine-containing plastic film with a thickness of 50 microns. This method uses expensive gamma ray irradiation technology, which is not conducive to industrialization and cost reduction. In addition, the obtained fluorine-containing plastic film has a thickness of up to 50 microns, which is also not conducive to industrialization and cost reduction. SUMMARY

[0006] The present application aims to solve the problem of poor inhibition of crystallization and precipitation of the bipolar plate non-reaction area insulating layer, and the problem of easy layer peeling and poor adhesion strength of the bipolar plate non-reaction area insulating layer.

[0007] To solve the above problems, the present application is realized by the following technical scheme: A method for preparing a non-reaction area insulated bipolar plate, comprising the following steps: S1, polishing the non-reaction area of the bipolar plate to obtain a rough bipolar plate; S2, coating the non-reaction area of the rough bipolar plate with mixed perfluoro-2-methyl-2-pentene liquid and photoinitiator, then performing ultraviolet irradiation, planar hot pressing, and pressure holding cooling to obtain a non-reaction area insulated bipolar plate.

[0008] In step S1, the roughness Ra of the rough bipolar plate is 3.2-25.

[0009] In step S2, the molar ratio of photoinitiator to perfluoro-2-methyl-2-pentene liquid is 1-5:100.

[0010] In step S2, the photoinitiator is any one of 2,4,6(trimethylbenzoyl) diphenyl phosphine oxide, 2-methyl-1-[4-methylthiophenyl]-2-morpholinyl-1-propanone.

[0011] In step S2,

[0012] In step S2, the wavelength of ultraviolet irradiation is 200-400 nm.

[0013] In step S2, the planar hot pressing temperature is 120-145°C.

[0014] In step S2, the non-reaction area of the rough bipolar plate is coated to a thickness of 5-10 microns.

[0015] In step S2, the ultraviolet intensity is 0.7-1.5 mW / cm 2 , and the ultraviolet irradiation time is 12-24h.

[0016] The application of the non-reactive region insulated bipolar plate prepared by the aforementioned method in a vanadium redox flow battery.

[0017] Preferably, the pressure of the planar hot pressing in step S2 is 20 MPa to 22 MPa. Preferably, the pressure holding pressure in step S2 is 20 MPa to 22 MPa.

[0018] Preferably, the temperature is cooled to room temperature (25°C).

[0019] The purpose of polishing the surface of the bipolar plate is to give it a certain roughness, which is beneficial to the subsequent bonding with the perfluoro-2-methyl-2-pentene film and improves the bonding strength. The roughness is controlled within a certain range to ensure the bonding strength between the two materials. Polishing methods such as sandpaper and laser can be used, and the method is not limited, as long as the roughness grade is achieved. It is worth noting that the surface roughness grade of the non-reactive area of ​​the fluorinated bipolar plate is determined according to the People's Republic of China National Standard GB / T 14234-93 "Surface Roughness of Plastic Parts". The conditions for UV irradiation curing are 0.7-1.5 mW / cm². 2 The irradiation time is 12-24 hours; the ultraviolet irradiation wavelength is 200nm-400nm (full band) monochromatic light. The specific wavelength used is selected according to the irradiation instrument and is not specifically limited here. As long as the above-mentioned irradiation dose is met, the requirements of this invention will be met.

[0020] The use of perfluoro-2-methyl-2-pentene as a coating material in step S2 is primarily due to its good compatibility with the fluorinated bipolar plate substrate, its ease of penetration into the plate, improved adhesion, resistance to delamination, and good resistance to mixed acid corrosion. The purpose of using a UV photoinitiator in this invention is that it decomposes into free radicals under UV irradiation. These free radicals not only initiate the polymerization of the perfluoro-2-methyl-2-pentene monomer with double bonds but also partially break the fluororesin molecular chains in the fluorinated bipolar plate, further initiating polymerization and bonding with the perfluoro-2-methyl-2-pentene monomer. This improves the bonding strength between the perfluoro-2-methyl-2-pentene polymer coating layer and the bipolar plate body, preventing delamination. During the above-mentioned irradiation polymerization process, care should be taken to protect the electrode reaction zone of the bipolar plate to prevent the resin material in the electrode reaction zone from being degraded by irradiation molecular chain breakage, which would affect the liquid resistance and other properties of the bipolar plate electrode reaction zone.

[0021] Regarding coating thickness, since the perfluoro-2-methyl-2-pentene polymer itself is an insulating material, its insulation properties will not change significantly within a thickness range of ≥5μm. Considering material saving and ease of hot pressing, a thickness of 5-10μm is preferred, and further optimization is possible, which will not be specifically mentioned here. If the coating thickness is less than 5μm, the thin coating may cause electrical breakdown, and it is difficult to control the coating uniformity, which may lead to conductivity and leakage in some areas.

[0022] The hot pressing process further strengthens the bond between the perfluoro-2-methyl-2-pentene polymer film on the surface of the non-reactive zone and the fluorinated bipolar plate. Cooling to room temperature under pressure allows the bipolar plate to cool down synchronously, avoiding deformation caused by temperature stress. Ultraviolet photoinitiators are small molecules that decompose into free radicals under ultraviolet light irradiation with wavelengths typically between 200 and 400 nm, such as 2,4,6-(trimethylbenzoyl)diphenylphosphine oxide (TPO) and 2-methyl-1-[4-methylthiophenyl]-2-morpholino-1-propanone (907).

[0023] The molar ratio of UV photoinitiator to perfluoro-2-methyl-2-pentene monomer is 1-5:100. If the proportion of UV photoinitiator is too low, it will be insufficient to initiate the polymerization of a sufficient amount of perfluoro-2-methyl-2-pentene monomer and the polymerization between the perfluoro-2-methyl-2-pentene polymer and the bipolar fluorinated resin. If the proportion of UV photoinitiator is too high, too many free radicals will be formed after UV irradiation, which will reduce the molecular weight and mechanical strength of the perfluoro-2-methyl-2-pentene polymer. Therefore, this ratio is limited.

[0024] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. This invention uses ultraviolet irradiation to induce the polymerization of perfluoro-2-methyl-2-pentene monomers and to break some fluororesin molecules in the fluorinated bipolar plate, which then chemically bond with the perfluoro-2-methyl-2-pentene polymer. This improves the bonding strength between the insulation layer and the fluorinated bipolar plate and ensures the reliability of the fuel cell stack.

[0025] 2. This invention provides a simple and easy-to-implement method for insulating the non-reactive region at the edge of a bipolar plate, which is suitable for large-scale industrial applications and is inexpensive.

[0026] 3. The fluorine-containing bipolar plate with non-reactive zone insulation prepared by the method described in this invention can be applied to the vanadium redox flow battery stack. While ensuring its resistance to mixed acid corrosion, it can avoid the risk of electrolyte crystallization and precipitation in the non-reactive zone, which helps to ensure the long-term operating life of the stack. Attached Figure Description

[0027] Figure 1This is a schematic diagram of the molecular structure of the perfluoro-2-methyl-2-pentene monomer.

[0028] Figure 2 This is a schematic diagram of the surface structure of a bipolar plate.

[0029] Figure 3 This is a photograph of a fluorinated bipolar plate with insulation in the non-reactive region.

[0030] Figure 4 This is a comparison of electrolyte crystallization at the flow channel opening between the uninsulated non-reactive zone (left) and the insulated non-reactive zone (right).

[0031] Figure 5 This is a photograph of the coating peeling off the surface of the bipolar plate. Detailed Implementation

[0032] To better understand the present invention, the following embodiments further illustrate its content, but the content of the present invention is not limited to the following embodiments. The following embodiments describe in more detail a method for insulating the non-reactive region on the surface of a fluorinated bipolar plate according to the present invention, and these embodiments are given by way of illustration, but these embodiments do not limit the scope of the present invention. Unless otherwise specified, the experimental methods used in the present invention are conventional methods, and the experimental equipment, materials, reagents, etc. used can be purchased from chemical companies.

[0033] The bipolar plates used in all embodiments of this invention are fluorine-containing molded bipolar plates purchased from Weihai Nanhai Carbon Materials Co., Ltd. A structural schematic diagram is attached. Figure 2 Its polyvinylidene fluoride (PVDF) content is approximately 15 wt%.

[0034] The bipolar plates used in the normal bipolar plate stack were purchased from Weihai Nanhai Carbon Materials Co., Ltd.

[0035] 2,4,6-(trimethylbenzoyl)diphenylphosphine oxide, abbreviated as TPO; 2-Methyl-1-[4-methylthiophenyl]-2-morpholino-1-propanone is abbreviated as 907.

[0036] Unless otherwise specified, the photoinitiator and perfluoro-2-methyl-2-pentene monomer are thoroughly mixed at a stirring speed of 150-200 rpm for 20-30 minutes. The molecular structure of the perfluoro-2-methyl-2-pentene monomer is shown below. Figure 1 .

[0037] Example 1 S1: Polish the non-reactive area on the surface of the fluorine-containing bipolar plate until the roughness grade Ra is 3.2; S2: After thoroughly mixing the TPO photoinitiator with the perfluoro-2-methyl-2-pentene monomer (molar ratio 1:100), the mixture is evenly coated onto the polished non-reactive area using an automatic coating machine. (See...) Figure 3 As shown, it was then cured by irradiation under 265nm wavelength ultraviolet light with an irradiation intensity of 0.7mW / cm². 2 The irradiation time was 24 hours, resulting in a uniform surface film with a thickness of 5.1 μm. The non-reactive region of the bipolar plate was subjected to planar hot pressing at 120°C and 20MPa in a vacuum hot press, so that the surface of the non-reactive region of the bipolar plate and the surface of the normal reactive region were on the same smooth surface. Then, it was naturally cooled to room temperature under pressure.

[0038] The non-reactive region insulation of the fluorinated bipolar plate prepared in this embodiment is obtained.

[0039] Example 2 The roughness in Example 1 was changed to 12.5, while other conditions remained unchanged. This resulted in a fluorinated bipolar plate with insulation in the non-reactive region prepared in this example.

[0040] Example 3 The roughness in Example 1 was changed to 25, while other conditions remained unchanged. This resulted in a fluorinated bipolar plate with insulation in the non-reactive region prepared in this example.

[0041] Example 4 S1: Polish the non-reactive area on the surface of the fluorine-containing bipolar plate until the roughness grade Ra is 6.4; S2: After thoroughly mixing the TPO photoinitiator and perfluoro-2-methyl-2-pentene monomer at a molar ratio of 2.4:100, the mixture is uniformly coated onto the polished non-reactive area using an automatic coating machine. Then, it is irradiated and cured under 312nm ultraviolet light at an intensity of 0.86mW / cm². 2 The irradiation time was 18 hours, resulting in a uniform surface film with a thickness of 5.4 μm. The non-reactive region of the bipolar plate was subjected to planar hot pressing at 130°C (pressure 20 MPa) in a vacuum hot press, so that the surface of the non-reactive region and the surface of the normal reactive region of the bipolar plate were on the same smooth surface. It was then naturally cooled to room temperature under holding pressure. This yielded the fluorinated bipolar plate with an insulated non-reactive region prepared in this embodiment.

[0042] Example 5 S1: Polish the non-reactive area on the surface of the fluorine-containing bipolar plate until the roughness grade Ra is 6.4; S2: After thoroughly mixing 907 photoinitiator and perfluoro-2-methyl-2-pentene monomer at a molar ratio of 5:100, the mixture is uniformly coated onto the polished non-reactive area using an automatic coating machine. Then, it is irradiated and cured under 365nm ultraviolet light at an intensity of 1.5mW / cm². 2 The irradiation time was 12 hours, resulting in a uniform surface film with a thickness of 5.9 μm. The non-reactive region of the bipolar plate was subjected to planar hot pressing at 145°C and 20 MPa in a vacuum hot press, ensuring that the surface of the non-reactive region and the surface of the normal reactive region were on the same smooth surface. The plate was then naturally cooled to room temperature under pressure. This yielded the fluorinated bipolar plate with an insulated non-reactive region prepared in this embodiment.

[0043] The fluorinated bipolar plates with non-reactive region insulation prepared in Examples 1-5 all have good electrical insulation properties, with an insulation resistance greater than 1.0 megohm, which meets the insulation resistance requirements of low-voltage electrical devices and satisfies the insulation requirements of the non-reactive region of the bipolar plate.

[0044] In addition, to verify the effect of insulation in the non-reactive region of the bipolar plate on inhibiting crystal precipitation in the fuel cell stack, the bipolar plate prepared by the method in Example 1 was selected as the experimental object. A 10kW fuel cell stack was used for stacking experiments, and a normal bipolar plate stack with no insulation in the non-reactive region was used as a comparison. The charge-discharge mode was 10kW constant power charging to the stack OCV=1.50V, followed by constant power discharging to OCV=1.25V, and cyclic testing was performed. Twenty 10kW experimental fuel cell stacks were assembled. After 500 cycles, the fuel cell stacks were disassembled and their condition was checked. See [link to relevant documentation]. Figure 4 The results showed that in a normal bipolar stack without insulation in the reaction zone, 19 electrolyte crystals were deposited at the flow channel inlet in the non-reaction zone. However, in the bipolar stack using insulation in the non-reaction zone of Example 1, only one electrolyte crystal was deposited at the flow channel inlet, and this was only a slight precipitation. This demonstrates that insulation in the non-reaction zone of the bipolar stack has a significant effect on suppressing electrolyte precipitation in the non-reaction zone.

[0045] Comparative Example 1 In step S1, the non-reactive area on the surface of the fluorinated bipolar plate is not polished, while the other steps are carried out as in Example 1, to obtain the bipolar plate with insulation in the non-reactive area as described in Comparative Example 1.

[0046] Comparative Example 2 In Example 1, step S2 replaces the photoinitiator with the thermal decomposition initiator azobisisobutyronitrile (AIBN), and initiates the polymerization of perfluoro-2-methyl-2-pentene monomer at 70 degrees Celsius (without using ultraviolet light). The thermal decomposition time is 3 minutes. Other steps are still carried out as in Example 1, and the non-reactive region insulated bipolar plate described in Comparative Example 2 is obtained.

[0047] Comparative Example 3 In Example 1, step S2 is not hot-pressed, and the other steps are still carried out as in Example 1, resulting in the bipolar plate with non-reactive region insulation as described in Comparative Example 3.

[0048] The bipolar plates obtained in Examples 1-5 and Comparative Examples 1-3 were immersed in a mixed acid electrolyte at 50°C and 70% SOC, and the coating condition was observed. (See also...) Figure 5 The results showed that within a week, Comparative Examples 1-3 experienced coating delamination and peeling, while Examples 1-5 did not experience delamination or peeling for approximately six months. This fully demonstrates that the pre-coating polishing process, the UV-induced polymerization process, and the post-coating hot-pressing step are all highly beneficial for improving the adhesion strength of the coating on the plate. The physicochemical properties of the mixed acid electrolyte at 70% SOC are shown in Table 1.

[0049] Table 1 Physicochemical properties of mixed acid electrolyte at 70% SOC

[0050] Here, 70% SOC means the battery is 70% charged, and 0% SOC means the battery is fully discharged.

[0051] The above description is merely a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A method for preparing a bipolar plate with insulation in the non-reactive region, characterized in that, Includes the following steps: S1. Grind the non-reactive area of ​​the bipolar plate to obtain a rough bipolar plate; S2. After coating the non-reactive area of ​​the rough bipolar plate with a uniformly mixed perfluoro-2-methyl-2-pentene liquid and a photoinitiator, the non-reactive area is subjected to ultraviolet irradiation, planar hot pressing, and pressure holding cooling to obtain a bipolar plate with an insulated non-reactive area.

2. The method for preparing a non-reactive region insulated bipolar plate according to claim 1, characterized in that, The roughness Ra of the rough bipolar plate in step S1 is 3.2~25.

3. The method for preparing a non-reactive region insulated bipolar plate according to claim 1, characterized in that, In step S2, the photoinitiator: perfluoro-2-methyl-2-pentene liquid ratio is 1-5:100 (molar ratio).

4. The method for preparing a non-reactive region insulated bipolar plate according to claim 1, characterized in that, In step S2, the photoinitiator is either 2,4,6-(trimethylbenzoyl)diphenylphosphine oxide or 2-methyl-1-[4-methylthiophenyl]-2-morpholino-1-propanone.

5. The method for preparing a non-reactive region insulated bipolar plate according to claim 1, characterized in that, In step S2.

6. The method for preparing a non-reactive region insulated bipolar plate according to claim 1, characterized in that, The wavelength of ultraviolet irradiation in step S2 is 200~400nm.

7. The method for preparing a non-reactive region insulated bipolar plate according to claim 1, characterized in that, The planar hot pressing temperature in step S2 is 120-145℃.

8. The method for preparing a non-reactive region insulated bipolar plate according to claim 1, characterized in that, In step S2, the coating thickness of the non-reactive region of the rough bipolar plate is 5-10 micrometers.

9. The method for preparing a non-reactive region insulated bipolar plate according to claim 1, characterized in that, In step S2, the ultraviolet intensity is 0.7-1.5 mW / cm². 2 The duration of ultraviolet irradiation is 12-24 hours.

10. The application of a non-reactive region insulated bipolar plate prepared by the method of any one of claims 1 to 9 in a vanadium redox flow battery.

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