Crosslinked fluorine-containing ion exchange membrane, and preparation method and application thereof

Abstract

The application belongs to the field of ion exchange membranes, and discloses a crosslinked fluorine-containing ion exchange membrane and a preparation method and application thereof. Styrene sulfonic acid and 4-vinylbenzyl pentafluorophenol ether are pre-polymerized in an inert gas atmosphere, and divinylbenzene is added in the inert gas atmosphere, and the mixture is poured into a smooth horizontal tank for in-situ crosslinking polymerization reaction. Solvents in the system are evaporated and removed to obtain a pre-prepared ion exchange membrane. After sufficient sulfonation, the ion exchange membrane is washed with 0.5-1 mol / L dilute sulfuric acid and deionized water in sequence until the washing liquid is neutral, and is naturally dried at room temperature. The raw materials are widely sourced and inexpensive, the cost is effectively controlled, the preparation process is relatively simple, the conditions are mild, and the application is suitable for large-scale industrial production. The application has good mechanical properties, high vanadium battery efficiency, excellent durability or corrosion resistance, and can replace the existing ion exchange membrane to be applied to the field of flow battery energy storage.

Description

Technical Field

[0001] This invention relates to the field of ion exchange membranes, and in particular to a cross-linked fluorine-containing ion exchange membrane, its preparation method and application, which is suitable for ion exchange membranes used in flow batteries, especially ion exchange membranes used in vanadium batteries. Background Technology

[0002] Ion exchange membranes used in vanadium redox flow batteries need to withstand the strong acidity, strong oxidizing properties, and chloride ion-containing environment of the electrolyte. Therefore, the corrosion resistance of the ion exchange membranes is required to be high. Currently, the main type of ion exchange membrane used is the perfluorosulfonic acid ion exchange membrane (such as those produced by DuPont). The high electronegativity of fluorine atoms in perfluorosulfonic acid resin molecules (e.g., series) contributes to its superior corrosion resistance. However, the complex synthesis process and high production cost of perfluorosulfonic acid resins result in high prices for perfluorosulfonic acid ion exchange membranes, significantly increasing the manufacturing cost of vanadium redox flow battery stacks. Therefore, developing low-cost ion exchange membrane materials with excellent corrosion resistance is a key research focus in this field. Researchers have turned to non-fluorine ion exchange membranes; however, even the high-performance PBI and SPEEK ion exchange membranes described in the literature do not meet the expected lifespan requirements (typically 20 years) of vanadium redox flow batteries in electrolytes over extended periods. This underscores the unparalleled role of fluorine in the durability and corrosion resistance of ion exchange membranes.

[0003] Since fluorine-containing components are required in membrane-forming resins, and resin costs need to be reduced, developing a low-cost ion exchange membrane material containing fluorine and possessing excellent performance has become one of the important research directions in this field. Summary of the Invention

[0004] To overcome the shortcomings of existing ion exchange membrane materials in terms of durability and chemical corrosion resistance, the present invention involves: prepolymerizing a fluorinated monomer with a styrene structure and a styrene sulfonic acid monomer, followed by introducing a crosslinking agent, divinylbenzene, and carrying out the polymerization reaction in a smooth horizontal glass bath. After drying the solvent, a pre-prepared ion exchange membrane is obtained. This pre-prepared ion exchange membrane is then further sulfonated to obtain the fluorinated ion exchange membrane with a crosslinked structure of the present invention. Both the monomer and the crosslinking agent, divinylbenzene, used in this invention have styrene structural units with similar polymerization rates, facilitating copolymerization and resulting in a uniform distribution of fluorinated structural units. Furthermore, the sulfonation of the benzene rings in the molecular structure of the obtained pre-prepared ion exchange membrane increases the ion exchange capacity, which helps improve the proton conductivity of the ion exchange membrane and reduce membrane resistance. In addition, the presence of the fluorinated structure effectively protects the hydrocarbon backbone of the ion exchange membrane molecules, giving the ion exchange membrane obtained by this invention excellent durability and chemical corrosion resistance.

[0005] The technical solution of this invention is as follows:

[0006] A method for preparing a cross-linked fluorine-containing ion exchange membrane includes the following steps:

[0007] (1) Using styrene sulfonic acid monomer and 4-vinylbenzylpentafluorophenol ether (VBFP) as monomers, prepolymerization was carried out by solution polymerization in an inert gas atmosphere to obtain a prepolymer solution;

[0008] (2) Add the crosslinking agent divinylbenzene to the prepolymer solution in an inert gas atmosphere, pour it into a smooth horizontal tank, and carry out the in-situ crosslinking polymerization reaction until the reaction is completed.

[0009] (3) Evaporate the solvent in the system to obtain the pre-made ion exchange membrane;

[0010] (4) The pre-made ion exchange membrane is placed in an excess of sulfonating agent for full sulfonation, and then washed with 0.5-1 mol / L dilute sulfuric acid and deionized water in sequence until the pH value of the washing solution is neutral. It is then air-dried at room temperature to obtain the cross-linked fluorine-containing ion exchange membrane of the present invention.

[0011] Furthermore, in step (1), the molar ratio of styrene sulfonic acid monomer to 4-vinylbenzyl pentafluorophenol ether (VBFP) monomer is (0.5-5):1. It is worth noting that the proportion of VBFP in the monomer needs to be controlled within the above range because when the proportion of VBFP is low, the corrosion resistance of the final ion exchange membrane will be greatly affected. When the proportion of VBFP is high, the final ion exchange membrane will be affected by the fluorine-containing structure in the subsequent sulfonation process, resulting in a decrease in the degree of sulfonation.

[0012] Furthermore, the molar ratio of divinylbenzene to the total molar ratio of styrene sulfonic acid monomer and 4-vinylbenzyl pentafluorophenol ether (VBFP) monomer is (1-5):20.

[0013] Furthermore, the inert gas in steps (1) and (2) is a gas that does not participate in the chemical reaction, such as high-purity nitrogen or argon, and is not specifically limited in this invention.

[0014] Furthermore, the reaction conditions for the polymerization reaction described in step (2) can be determined based on the actual situation of initiating solution polymerization. It can be initiated by azo initiators, redox initiators, gamma-ray irradiation, etc., and the specific type is not limited.

[0015] It is worth noting that the in-situ crosslinking polymerization reaction described in step (2) is completed by judging the amount of residual double bonds after the reaction as <1%.

[0016] The method for detecting the residual hydrogen in the double bonds is as follows: Accurately weigh one-tenth of the total mass of styrene sulfonic acid monomer and 4-vinylbenzylpentafluorophenol ether (VBFP) monomer, using 1,3,5-trioxane solid as an internal standard (not participating in the polymerization reaction), add it to the solution before the reaction, stir until fully dissolved, and then add one drop of the reaction solution to 0.6 mL of d-DMSO for 1H NMR analysis. After the reaction is complete, take another drop of the reaction solution and repeat the process. 1 1H NMR analysis was performed. Since the molar amount of the internal standard 1,3,5-trioxane remained constant before and after the reaction, while the double bond hydrogen content decreased, the residual double bond hydrogen was calculated by comparing the integrated areas of the double bond proton absorption peaks before and after the reaction. Other methods can also be used to determine the end of the reaction; these only indicate that the monomers in the system have completely participated in the reaction, and are not limited here.

[0017] Furthermore, the temperature of the solvent in the evaporation system described in step (3) is 100-150℃, and the evaporation temperature can be selected according to the actual solvent used.

[0018] Furthermore, the sulfonating agent is one of concentrated sulfuric acid, chlorosulfonic acid, or fuming sulfuric acid.

[0019] Another object of the present invention is to request protection for the cross-linked fluorinated ion exchange membrane prepared by the above method.

[0020] The third objective of this invention is to claim protection for the application of the cross-linked fluorine-containing ion exchange membrane, which can be used in all vanadium redox flow battery systems and, theoretically, in other flow battery systems, can improve the membrane's durability in vanadium battery systems.

[0021] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0022] (1) This invention provides a novel cross-linked fluorine-containing ion exchange membrane and its preparation method. The raw materials are widely available and inexpensive, effectively controlling costs. The preparation process is relatively simple and the conditions are mild, making it suitable for large-scale industrial production.

[0023] (2) The cross-linked fluorine-containing ion exchange membrane prepared by the present invention has good mechanical properties, high vanadium battery efficiency, and excellent durability or corrosion resistance, and can replace the existing ion exchange membrane in the field of flow battery energy storage. Detailed Implementation

[0024] 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 cross-linked fluorine-containing ion exchange membrane and its preparation method 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.

[0025] The synthesis method of the 4-vinylbenzylpentafluorophenol ether (VBFP) monomer used in this invention can be found in the supporting information in Angew. Chem. Int. Ed. 2005, 44, 5262–5265.

[0026] The thickness of the ion exchange membrane was measured using a digital micrometer screw gauge, with 50 values ​​measured at different locations for each sample and the average value calculated.

[0027] The tensile strength and elongation at break of the ion exchange membrane were tested in accordance with the standard GB / T 1040.3-2006 "Determination of tensile properties of plastics - Part 3: Test conditions for films and sheets". The membrane was cut into strips with a width of 10 mm and an initial clamping distance of 50 mm, and the test was conducted at a tensile rate of 200 mm / min.

[0028] Performance testing conditions for vanadium redox flow batteries with ion exchange membranes: at a current density of 80 mA / cm² 2 Charge-discharge experiments were conducted under the following conditions: charging to 1.55V and discharging to 1.00V. Graphite carbon felt produced by Liaoyang Jingu Carbon Materials Co., Ltd. was used as the reaction electrode, with an effective working area of ​​48 cm². 2 The positive and negative electrode electrolytes are V0 and V0, respectively. 2+ / VO2 + and V 2+ / V 3+The battery operates at a temperature of 37°C and contains a sulfuric acid solution.

[0029] In the following embodiments, the ethanol / water mixed solvent is not mandatory and can be replaced with other solvents, provided that the monomers and crosslinking agents in the system can be completely dissolved. Here, the use of an ethanol / water mixed solvent (mass ratio of 50:50) is merely an example. When using an ethanol / water mixed solvent (mass ratio of 50:50), the total mass ratio of the monomers (styrene sulfonic acid + VBFP) to the ethanol / water mixed solvent is (1-2):10. The ratio may vary when using different solvents, but it will not significantly affect the final effect of the embodiment, so it is not a limitation.

[0030] In the following examples, the thermally decomposable initiator azobisisobutyronitrile is used as a representative example, and its typical dosage is one-tenth of the total molar amount of the polymerizing monomers.

[0031] Example 1

[0032] (1) Dissolve 9.2g (0.05mol) styrene sulfonic acid monomer and 30g (0.1mol) VBFP monomer in 400g ethanol / water mixed solvent (mass ratio of 50:50), add 2.46g (0.015mol) of thermally decomposable initiator azobisisobutyronitrile, and prepolymerize at 70℃ for 3.5h in a nitrogen atmosphere to obtain a prepolymer solution;

[0033] (2) Continue to add 1.95 g (0.015 mol) of crosslinking agent divinylbenzene to the above prepolymer solution in a nitrogen atmosphere, then pour it into a smooth horizontal tank and continue the in-situ crosslinking polymerization reaction at 65 °C until the reaction is completed.

[0034] (3) Evaporate the solvent in the system at 120℃ to obtain the pre-made ion exchange membrane;

[0035] (4) The pre-made ion exchange membrane is placed in excess concentrated sulfuric acid for full sulfonation, and then washed with 0.5-1 mol / L dilute sulfuric acid and deionized water in sequence until the pH value of the washing solution is neutral. It is then air-dried at room temperature to obtain the ion exchange membrane with a thickness of 50±3 μm as described in the example.

[0036] In this embodiment, the molar ratio of styrene sulfonic acid monomer to VBFP monomer is 0.5:1, and the molar ratio of crosslinking agent divinylbenzene to the total monomer is 2:20.

[0037] Example 2

[0038] The difference between this embodiment and Example 1 is that the mass of styrene sulfonic acid monomer is 46g (0.25mol), making the molar ratio of styrene sulfonic acid monomer to VBFP monomer 2.5:1. The azobisisobutyronitrile initiator and the crosslinking agent divinylbenzene are increased according to the proportions described in Example 1.

[0039] Example 3

[0040] The difference between this embodiment and Example 1 is that the mass of styrene sulfonic acid monomer is 92g (0.50mol), making the molar ratio of styrene sulfonic acid monomer to VBFP monomer 5:1. The azobisisobutyronitrile initiator and the crosslinking agent divinylbenzene are increased according to the proportions described in Example 1.

[0041] Example 4

[0042] The difference between this embodiment and Embodiment 2 is that the ratio of the crosslinking agent divinylbenzene to the total molar amount of the monomer is 1:20, while the rest is the same as in Embodiment 2.

[0043] Example 5

[0044] The difference between this embodiment and Embodiment 2 is that the ratio of the crosslinking agent divinylbenzene to the total molar amount of the monomer is 5:20, while the rest is the same as in Embodiment 2.

[0045] Comparative Example 1

[0046] The difference between this comparative example and Example 2 is that the crosslinking agent divinylbenzene is not added.

[0047] Comparative Example 2

[0048] The difference between this comparative example and Example 2 is that no sulfonating agent is added to sulfonate the pre-prepared ion exchange membrane. Instead, the pre-prepared ion exchange membrane is directly washed sequentially with 0.5-1 mol / L dilute sulfuric acid and deionized water until the pH of the washing solution is neutral, and then air-dried at room temperature to obtain the ion exchange membrane with a thickness of 50±3 μm as described in the example.

[0049] Comparative Example 3

[0050] This comparative example was prepared according to Example 4 in Chinese Patent CN 112909277 A.

[0051] The cross-linked fluorine-containing ion exchange membranes prepared in Examples 1-5 of this invention, the ion exchange membranes prepared in Comparative Examples 1-3, and the commercially available Nafion 212 ion exchange membrane and Celtec PBI ion exchange membrane (BASF, Germany) were used as examples for performance testing of vanadium redox flow batteries. The test results are shown in Table 1.

[0052] Table 1. Performance data of the membranes prepared in Examples 1-5 and Comparative Examples 1-3, as well as commercially available Nafion 212 ion exchange membranes and Celtec PBI ion exchange membranes (BASF, Germany).

[0053]

[0054] As shown in Table 1, the ion exchange membrane prepared by this invention has good tensile strength, i.e., good mechanical properties. This is because the ion exchange membrane synthesized by this invention uses divinylbenzene as a crosslinking agent for chemical reinforcement, which greatly helps to improve the mechanical properties of the ion exchange membrane. Its tensile strength can be directly compared with the data of Examples 2, 4, and 5 and Comparative Example 1. As can be seen from Examples 1-3, as the proportion of VBFP decreases, the coulombic efficiency of the membrane decreases while the voltage efficiency increases, indicating that the proportion of VBFP in the membrane has a significant impact on the final membrane performance. From the data on the number of cycles required for a 2% decrease in voltage efficiency, the ion exchange membrane prepared by this invention has strong durability, which is basically comparable to that of the Nafion 212 perfluorosulfonic acid ion exchange membrane and far exceeds that of non-fluorinated membranes (such as the Celtec PBI membrane and the membrane of Comparative Example 3), fully demonstrating that the ion exchange membrane prepared by this invention has excellent durability and corrosion resistance.

[0055] The preparation process of perfluorosulfonic acid resin (Nafion212 raw material) monomers and resin raw materials is complex and demanding, and only a few companies worldwide have the production capacity. The complexity of the preparation process itself and the industry's monopoly have led to the high cost of perfluorosulfonic acid resin and the ion exchange membranes made from it.

[0056] The preparation conditions for non-fluorinated or partially fluorinated materials are relatively milder than those for perfluorinated materials. Furthermore, the monomers and raw materials used in this invention are widely available or can be prepared by relatively simple synthesis methods, and the technical route is mature, thus having a significant cost advantage.

[0057] 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 cross-linked fluorine-containing ion exchange membrane, characterized in that, Includes the following steps: (1) Using styrene sulfonic acid monomer and 4-vinylbenzyl pentafluorophenol ether as monomers, prepolymerization was carried out by solution polymerization in an inert gas atmosphere to obtain a prepolymer solution; (2) Add the crosslinking agent divinylbenzene to the prepolymer solution in an inert gas atmosphere, pour it into a smooth horizontal tank, and carry out the in-situ crosslinking polymerization reaction until the reaction is completed; (3) Evaporate the solvent in the system to obtain the pre-made ion exchange membrane; (4) The pre-made ion exchange membrane is placed in an excess of sulfonating agent for full sulfonation, and then washed with 0.5-1 mol / L dilute sulfuric acid and deionized water in sequence until the pH value of the washing solution is neutral. It is then air-dried at room temperature to obtain the cross-linked fluorine-containing ion exchange membrane.

2. The method for preparing the cross-linked fluorine-containing ion exchange membrane as described in claim 1, characterized in that, In step (1), the molar ratio of styrene sulfonic acid monomer to 4-vinylbenzylpentafluorophenol ether monomer is (0.5-5):

1.

3. The method for preparing the cross-linked fluorine-containing ion exchange membrane as described in claim 1, characterized in that, The molar ratio of divinylbenzene to the total molar ratio of styrene sulfonic acid monomer and 4-vinylbenzyl pentafluorophenol ether monomer is (1-5):

20.

4. The method for preparing the cross-linked fluorine-containing ion exchange membrane as described in claim 1, characterized in that, In steps (1) and (2), the inert gas is a gas that does not participate in the chemical reaction.

5. The method for preparing the cross-linked fluorine-containing ion exchange membrane as described in claim 1, characterized in that, The in-situ crosslinking polymerization reaction described in step (2) is considered complete by measuring the residual amount of double bond hydrogens after the reaction as <1%.

6. The method for preparing the cross-linked fluorine-containing ion exchange membrane as described in claim 5, characterized in that, The method for detecting the residual hydrogen in the double bonds is as follows: Accurately weigh one-tenth of the total mass of styrene sulfonic acid monomer and 4-vinylbenzylpentafluorophenol ether monomer, using 1,3,5-trioxane solid as an internal standard (not participating in the polymerization reaction), add it to the solution before the reaction, stir until fully dissolved, and then add one drop of the reaction solution to 0.6 mL. d - Do in DMSO 1 H NMR analysis; after the reaction is complete, take another drop of the reaction solution and repeat the process. 1 H NMR nuclear magnetic resonance test.

7. The method for preparing the cross-linked fluorine-containing ion exchange membrane as described in claim 1, characterized in that, In step (3), the solvent in the system is removed by evaporation at a temperature of 100-150℃.

8. The method for preparing the cross-linked fluorine-containing ion exchange membrane as described in claim 1, characterized in that, The sulfonating agent is one of concentrated sulfuric acid, chlorosulfonic acid, or fuming sulfuric acid.

9. The ion exchange membrane prepared by the method for preparing the cross-linked fluorine-containing ion exchange membrane as described in claim 1.

10. The application of the ion exchange membrane prepared by the method for preparing the cross-linked fluorine-containing ion exchange membrane as described in claim 1 in a flow battery system.

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