A covalent organic framework composite membrane and a preparation method and application thereof

Abstract

The present application relates to the technical field of all-vanadium redox flow battery diaphragm, discloses a kind of covalent organic framework composite membrane and its preparation method and application.Preparation method includes the following steps: preparation covalent organic framework@template agent powder;Preparation covalent organic framework nanosheet casting solution;Covalent organic framework-perfluorosulfonic acid composite membrane is prepared by blade coating method.The present application can efficiently prepare covalent organic framework nanosheet dispersion liquid using template agent.The prepared composite membrane has staggered stacking channel structure, reduces effective pore size, improves the ion selectivity and vanadium resistance of composite membrane.In addition, covalent organic frameworks with different charge properties can also be used to enhance the coulomb efficiency or voltage efficiency of vanadium battery respectively.

Description

Technical Field

[0001] This invention relates to the field of vanadium redox flow battery separator technology, and particularly to a covalent organic framework composite membrane, its preparation method, and its application. Background Technology

[0002] In a vanadium redox flow battery system, the separator is one of the core components. Its main functions include: isolating the positive and negative electrolytes to prevent cross-contamination of vanadium ions of different valence states, which could lead to self-discharge; selectively conducting protons to form a closed charge loop to maintain current conduction; and regulating water flux to balance changes in electrolyte volume at both electrodes and prevent battery failure. The performance of the separator directly affects battery efficiency, lifespan, and cost, making it a key focus of vanadium battery research and a crucial factor for technological breakthroughs.

[0003] Currently, the most commercially successful vanadium battery separator is the perfluorosulfonic acid ion exchange membrane. Taking Nafion membranes as an example, they use a perfluorocarbon chain (-CF2-CF2-) as the core framework, with perfluoroether side chains connected to the ends of the main chain, and sulfonic acid groups (-SO3H) at the ends. In an aqueous environment, the sulfonic acid groups dissociate into fixed negative charges (-SO3H). - ) and free H + This forms a continuous network of hydrophilic ion clusters (micelle channels), constituting the proton conduction channel. Perfluorosulfonic acid membranes possess advantages such as good chemical stability, high proton conductivity, and good mechanical properties. However, they also have disadvantages such as high vanadium ion permeability and high cost, limiting their further development in vanadium battery applications. The high vanadium ion permeability of perfluorosulfonic acid membranes is mainly due to the fact that the size of the micelle channels is still significantly larger than the hydration diameter of vanadium ions. Therefore, to improve vanadium barrier properties, suitable porous materials and modification methods are needed to reduce the effective pore size of the membrane.

[0004] Covalent organic frameworks (COFs) are novel crystalline porous nanomaterials whose pore sizes can be continuously tunable at the nanoscale through monomer design. Their fully covalent structure endows them with high chemical stability, and they have already seen initial applications in adsorption, catalysis, sensing, and separation. However, COFs are typically present in powder form, and the variety of monomers suitable for preparing separation membranes is limited. Furthermore, the preparation methods for COF membranes mainly focus on solvothermal growth methods, which require high temperatures, pressures, and caustic organic solvents, making direct application to the modification of perfluorosulfonic acid membranes difficult. Therefore, there is a need to develop a simple and effective method to combine COFs with perfluorosulfonic acid membranes to prepare COF-perfluorosulfonic acid composite membranes with high vanadium barrier properties and high proton conductivity. Summary of the Invention

[0005] This invention provides a covalent organic framework composite membrane, its preparation method, and its application to solve the aforementioned technical difficulties. First, inorganic template materials with layered or tubular structures (such as hydrotalcite, barium titanate, graphene, sodium chloride crystals, etc.) are screened. The template material is added to a mixed dispersion (i.e., reaction solution) of amine monomers, aldehyde monomers, and a catalyst. After conformal growth under solvothermal conditions, a thin layer of covalent organic framework with the same layered or tubular structure is obtained on the template material surface. Selecting template materials with different structures yields covalent organic framework structures with different morphologies. Further, a suitable method is used to remove the template material or separate it from the covalent organic framework thin layer, resulting in a layered covalent organic framework nanosheet dispersion. The dispersion is centrifuged, dialyzed, and concentrated by evaporation to obtain a casting solution with a high concentration of covalent organic framework nanosheets. This solution is then coated onto the surface of a perfluorosulfonic acid membrane. After heat treatment and cross-linking, a covalent organic framework-perfluorosulfonic acid composite ion exchange membrane is obtained.

[0006] To achieve the above objectives, the present invention provides the following technical solution:

[0007] In a first aspect, the present invention discloses a method for preparing a covalent organic framework composite membrane, comprising the following steps:

[0008] Step S1: Add monomer 1, monomer 2, and catalyst to the template agent dispersion to react and obtain covalent organic framework@template agent powder;

[0009] Step S2: Remove or separate the template agent from the covalent organic framework@template agent powder to obtain pure covalent organic framework powder, and disperse the covalent organic framework powder in ethanol and perform rotary evaporation to obtain a concentrated solution;

[0010] Step S3: The concentrated liquid is coated onto the surface of the perfluorosulfonic acid membrane and subjected to heat treatment to obtain a covalent organic framework-perfluorosulfonic acid composite membrane.

[0011] In one implementation, in step S1:

[0012] The mass ratio of monomer 1, monomer 2, catalyst and template agent dispersion is (1~3):(1~5):(1~2):(1000~3000).

[0013] In one implementation, in step S1:

[0014] The monomer 1 is selected from at least one of 1,3,5-tris(4-formylphenyl)benzene, 2,4,6-trialdehyde phenol, 2,4,6-tris(4-aldehydephenyl)-1,3,5-triazine, trialdehyde phloroglucinol and 2,4,6-trialdehyde pyridine;

[0015] The monomer 2 is selected from at least one of ethidium bromide, guanidine triaminohydrochloride, 4,4'-diaminobiphenyl, 4,4-diaminobiphenyl-2,2-disulfonic acid and 2,5-diaminobenzenesulfonic acid;

[0016] The catalyst is selected from at least one of acetic acid, p-toluenesulfonic acid, ferric trifluoromethanesulfonate, tris(pentafluorophenyl)borane, and trichloroacetic acid.

[0017] Monomer 1 and monomer 2 undergo a Schiff base reaction, generating a thin-layered, highly crystalline imine covalent organic framework in the presence of a catalyst. This framework provides the structural basis for conformal growth on the template surface and for the integrity of the framework after exfoliation. Ethidium bromide and guanidine triaminohydrochloride are positively charged amine monomers, 4,4-diaminobiphenyl-2,2-disulfonic acid and 2,5-diaminobenzenesulfonic acid are negatively charged amine monomers, and 4,4'-diaminobiphenyl is an electrically neutral amine monomer.

[0018] In one implementation, in step S1:

[0019] The template agent dispersion is prepared by dispersing 10-20 parts by weight of the template agent in 1000-3000 parts by weight of a solvent; wherein,

[0020] The template agent is selected from at least one of aluminum magnesium hydrotalcite, graphene, tubular barium titanate crystals, and sodium chloride crystals;

[0021] The solvent is selected from at least one of trimethylbenzene, dioxane, N,N-dimethylformamide, glycerol, and N-methylpyrrolidone.

[0022] Aluminum magnesium hydrotalcite and graphene have typical layered structures, barium titanate crystals have cylindrical or tubular structures, and sodium chloride crystals have hexahedral structures. By selecting template agents with different structures, covalent organic frameworks with different structures can be obtained on the template agent surface. However, after removing the template agent, a covalent organic framework layered structure, i.e., covalent organic framework nanosheets, can be obtained in all cases.

[0023] In one implementation, in step S1:

[0024] The reaction includes sequential ultrasonic treatment, air removal, sealing, high-temperature reaction, filtration, washing, and drying; wherein the high-temperature reaction is carried out at a temperature of 100~150℃ for 24~48h.

[0025] In one implementation, in step S2:

[0026] The method of removal or separation is determined by the type of template agent, and the removal or separation includes:

[0027] If the template agent is hydrotalcite or barium titanate, the covalent organic framework@template agent powder is ground, placed in a Buchner funnel, filtered and washed with concentrated hydrochloric acid, and the filter residue is dried.

[0028] If the template agent is sodium chloride crystal, then the covalent organic framework@template agent powder is ground, placed in a Buchner funnel, filtered and washed with pure water, and the filter residue is dried.

[0029] If the template agent is graphene, the covalent organic framework@template agent powder is placed in pure water, ultrasonically treated, centrifuged, and dried after precipitation.

[0030] In one implementation method

[0031] The mass ratio of the pure covalent organic framework powder to the ethanol is 1:2000;

[0032] The concentration of the covalent organic framework in the concentrate is 1.2~1.8 g / L.

[0033] In one implementation, in step S3:

[0034] The thickness of the coating is 10~50μm;

[0035] The heat treatment temperature is 60~80℃ and the time is 5~30min.

[0036] Secondly, the present invention discloses a covalent organic framework composite membrane, which is prepared by the covalent organic framework composite membrane preparation method described above.

[0037] Thirdly, this invention discloses an application of a covalent organic framework composite membrane, wherein the covalent organic framework composite membrane prepared by the above-described preparation method or the covalent organic framework composite membrane described above is applied to a vanadium redox flow battery.

[0038] The beneficial effects of this invention are as follows:

[0039] This invention can prepare covalent organic frameworks with different structures based on the morphology of different template agents, thereby improving the film-forming properties and processability of covalent organic frameworks.

[0040] The covalent organic framework-perfluorosulfonic acid composite membrane prepared by this invention has a staggered stacked pore structure, which reduces the effective pore size and improves the ion selectivity of the composite membrane and the coulombic efficiency of the battery.

[0041] This invention can use covalent organic frameworks with different charge properties: if a positively charged covalent organic framework is used, the vanadium barrier property can be improved through charge repulsion, thereby further improving the coulombic efficiency of the battery; if a negatively charged covalent organic framework containing sulfonic acid groups is used, the conductivity of the membrane can be improved, thereby improving the voltage efficiency of the battery. Attached Figure Description

[0042] Figure 1 This is a schematic diagram of the template method for preparing covalent organic framework nanosheets according to the present invention;

[0043] Figure 2 The images are scanning electron microscope (SEM) images of the synthesis solutions after the reaction in Example 1 and Comparative Example 2 of this invention.

[0044] Figure 3 This is a transmission electron microscope (TEM) image of covalent organic framework nanosheets in Example 2 of the present invention;

[0045] Figure 4 The images shown are SEM and TEM images of the covalent organic framework-perfluorosulfonic acid composite membrane in Example 3 of this invention. Detailed Implementation

[0046] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. However, it should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of the invention. Furthermore, descriptions of well-known structures and technologies are omitted in the following description to avoid unnecessarily obscuring the concept of the invention.

[0047] A method for preparing a covalent organic framework-polymer hybrid porous membrane includes the following steps:

[0048] S1. Preparation of covalent organic framework@template powder: Disperse 10-20 parts by weight of the template agent in 1000-3000 parts by weight of the solvent to obtain a template agent dispersion. Add 1-3 parts by weight of monomer 1, 1-5 parts by weight of monomer 2, and 1-2 parts by weight of catalyst to the template agent dispersion. After ultrasonic treatment, obtain a templated covalent organic framework synthesis solution. Remove air from the synthesis solution and seal it. React at 100-150℃ for 24-48 hours. After filtration, washing, and drying, the covalent organic framework@template powder is obtained. The template agent is selected from one of the following: hydrated magnesium aluminate carbonate (aluminum magnesium hydrotalcite), graphene, tubular barium titanate crystals, and sodium chloride crystals; the solvent is selected from one of the following: mesitylene, dioxane, N,N-dimethylformamide, glycerol, and N-methylpyrrolidone; monomer 1 is an organic small molecule containing multiple aldehyde groups, selected from one of the following: 1,3,5-tris(4-formylphenyl)benzene, 2,4,6-trialdehyde phenol, 2,4,6-tris(4-aldehyde phenyl)-1,3,5-triazine, trialdehyde phloroglucinol, and 2,4,6-trialdehyde pyridine; monomer 2 is an organic small molecule containing multiple amino groups, selected from one of the following: ethidium bromide, triaminoguanidine hydrochloride, p-phenylenediamine, 4,4'-diaminobiphenyl, and 2,5-diaminobenzenesulfonic acid; the catalyst is selected from one of the following: acetic acid, p-toluenesulfonic acid, ferric trifluoromethanesulfonate, tris(pentafluorophenyl)borane, and trichloroacetic acid.

[0049] S2. Remove or separate the template agent from the covalent organic framework@template agent powder to obtain pure covalent organic framework powder, disperse it in ethanol, and rotary evaporate to obtain a concentrated solution; wherein, the method of removal or separation is determined by the type of template agent, and the removal or separation includes: if the template agent is hydrotalcite or barium titanate, grind the covalent organic framework@template agent powder, place it in a Buchner funnel, filter and wash it with concentrated hydrochloric acid, and dry the filter residue; if the template agent is sodium chloride crystal, grind the covalent organic framework@template agent powder, place it in a Buchner funnel, filter and wash it with pure water, and dry the filter residue; if the template agent is graphene, place the covalent organic framework@template agent powder in pure water, sonicate, centrifuge, precipitate and dry; the mass ratio of the pure covalent organic framework powder to ethanol is 1:2000; the concentration of covalent organic framework in the concentrated solution is 1.2~1.8 g / L.

[0050] S3. Preparation of covalent organic framework-perfluorosulfonic acid composite membrane by blade coating: The concentrated covalent organic framework solution from S2 is coated onto the surface of the perfluorosulfonic acid membrane using a blade. After heat treatment and crosslinking, the covalent organic framework-perfluorosulfonic acid composite membrane is obtained. The height of the blade is 10~50 μm; the heat treatment temperature is 60~80℃, and the heat treatment time is 5~30 min.

[0051] The mechanism involved in this invention is as follows:

[0052] 1. Selection and function of template agents. For example... Figure 1 As shown, this invention selects various template agents with different morphologies, such as lamellar, cylindrical (tubular), and cubic shapes, and grows covalent organic framework layered structures on their surfaces using a solvothermal synthesis method. After removing the template agent, covalent organic framework nanosheets are obtained. The template agent surface, after treatment with a siloxane coupling agent, possesses a large number of hydroxyl groups, allowing monomers to be grafted and fixed onto the template agent surface. This restricts the Schiff base reaction for synthesizing the covalent organic framework to occur on the template agent surface, rather than in the synthesis solution. Therefore, compared to traditional solvothermal reactions, there are fewer particulate powders in the synthesis solution, and the product is mainly concentrated on the template agent surface, appearing in a thin layer, improving monomer utilization and increasing the yield of covalent organic framework nanosheets. After removing the template agent by physical or chemical means, the covalent organic framework layers on the template agent surface are released into the solution, resulting in a covalent organic framework nanosheet dispersion.

[0053] 2. Preparation of covalent organic framework composite membranes by casting solution coating method. Generally, covalent organic frameworks have poor dispersibility in solution, making it impossible to prepare separation membranes using common coating methods. In this invention, an ethanol solution of low-concentration covalent organic framework nanosheets is evaporated and concentrated to obtain a high-concentration dispersion (0.8~1.8 g / L). This dispersion can then be coated onto the surface of a commercial perfluorosulfonic acid ion exchange membrane to prepare a composite membrane.

[0054] 3. Covalent Organic Framework-Perfluorosulfonic Acid Composite Membrane. This composite membrane has a two-layer structure: a covalent organic framework separation layer on the upper surface and a perfluorosulfonic acid base membrane below. The covalent organic framework separation layer has uniform channels of 1-3 nm, while the perfluorosulfonic acid membrane has micellar channels of 3-5 nm. These two types of channels are stacked and staggered, reducing the effective pore size of the composite membrane, which improves the barrier to vanadium ions and enhances the coulombic efficiency and capacity retention of vanadium batteries. Furthermore, using positively charged or negatively charged (sulfonic acid groups) monomers to prepare the composite membrane can further enhance the coulombic efficiency and voltage efficiency of the battery, respectively.

[0055] Example 1: The following technical solution is adopted.

[0056] S1. 20g of aluminum magnesium hydrotalcite was dispersed in 1000g of mesitylene to obtain a template agent dispersion. 3g of 1,3,5-tris(4-formylphenyl)benzene, 5g of ethidium bromide, and 1g of acetic acid were added to the template agent dispersion. After ultrasonic treatment, a templated covalent organic framework synthesis solution was obtained. The air in the synthesis solution was removed and sealed. The reaction was carried out at 150℃ for 48h. After filtration, washing, and drying, the covalent organic framework@template agent powder was obtained.

[0057] S2. Grind the covalent organic framework@template agent powder, place it in a Buchner funnel, filter and wash it with concentrated hydrochloric acid, dry the filter residue to obtain pure covalent organic framework powder, disperse the pure covalent organic framework powder with ethanol at a mass ratio of 1:2000, and rotary evaporate to obtain a concentrated solution with a concentration of 1.5 g / L.

[0058] S3. Use a scraper to coat the covalent organic framework concentrate from S2 onto the surface of the perfluorosulfonic acid membrane. The coating thickness is 20 μm. After cross-linking by heat treatment at 80℃ for 15 min, the covalent organic framework-perfluorosulfonic acid composite membrane can be obtained.

[0059] Example 2: The following technical solution is adopted.

[0060] S1. 15g of graphene was dispersed in 1500g of dioxane to obtain a template agent dispersion. 2.5g of 2,4,6-trialdehyde phenol, 4g of triaminoguanidine hydrochloride, and 1.2g of p-toluenesulfonic acid were added to the template agent dispersion. After ultrasonic treatment, a templated covalent organic framework synthesis solution was obtained. Air in the synthesis solution was removed and the solution was sealed. The reaction was carried out at 140℃ for 24h. After filtration, washing, and drying, the covalent organic framework@template agent powder was obtained.

[0061] S2. Place the covalent organic framework@template powder in pure water, sonicate, centrifuge, precipitate and dry to obtain pure covalent organic framework powder. Disperse the pure covalent organic framework powder with ethanol at a mass ratio of 1:2000, and rotary evaporate to obtain a concentrated solution with a concentration of 1.2 g / L.

[0062] S3. Using a scraper, the concentrated covalent organic framework solution in S2 is coated onto the surface of the perfluorosulfonic acid membrane to a thickness of 30 μm. After cross-linking by heat treatment at 75°C for 30 min, the covalent organic framework-perfluorosulfonic acid composite membrane is obtained.

[0063] Example 3: The following technical solution is adopted.

[0064] S1. 10g of tubular barium titanate crystals were dispersed in 2000g of N,N-dimethylformamide to obtain a template agent dispersion. 2g of 2,4,6-tris(4-aldehydephenyl)-1,3,5-triazine, 3g of 4,4'-diaminobiphenyl, and 1.5g of ferric trifluoromethanesulfonate were added to the template agent dispersion. After ultrasonic treatment, a templated covalent organic framework synthesis solution was obtained. Air in the synthesis solution was removed and the solution was sealed. The reaction was carried out at 130℃ for 36h. After filtration, washing, and drying, the covalent organic framework@template agent powder was obtained.

[0065] S2. Grind the covalent organic framework@template agent powder, place it in a Buchner funnel, filter and wash it with concentrated hydrochloric acid, dry the filter residue to obtain pure covalent organic framework powder, disperse the pure covalent organic framework powder with ethanol at a mass ratio of 1:2000, and rotary evaporate to obtain a concentrated solution with a concentration of 1.8 g / L.

[0066] S3. Use a scraper to coat the covalent organic framework concentrate in S2 onto the surface of the perfluorosulfonic acid membrane. The coating thickness is 40 μm. After cross-linking by heat treatment at 70℃ for 5 min, the covalent organic framework-perfluorosulfonic acid composite membrane can be obtained.

[0067] Example 4: The following technical solution is adopted.

[0068] S1. Disperse 15g of sodium chloride crystals in 2500g of glycerol to obtain a template agent dispersion. Add 1.5g of trialdehyde phloroglucinol, 2g of 4,4-diaminobiphenyl-2,2-disulfonic acid, and 1.7g of tris(pentafluorophenyl)borane to the template agent dispersion. After ultrasonic treatment, obtain a templated covalent organic framework synthesis solution. Remove air from the synthesis solution and seal it. React at 115℃ for 24h. After filtration, washing, and drying, obtain the covalent organic framework@template agent powder.

[0069] S2. Grind the covalent organic framework@template agent powder, place it in a Buchner funnel, filter and wash it with pure water, dry the filter residue to obtain pure covalent organic framework powder, disperse the pure covalent organic framework powder with ethanol at a mass ratio of 1:2000, and perform rotary evaporation to obtain a concentrated solution with a concentration of 1.4 g / L.

[0070] S3. Using a scraper, the concentrated covalent organic framework solution from S2 is coated onto the surface of the perfluorosulfonic acid membrane to a thickness of 50 μm. After cross-linking by heat treatment at 65°C for 20 min, the covalent organic framework-perfluorosulfonic acid composite membrane is obtained.

[0071] Example 5: The following technical solution is adopted.

[0072] S1. 20g of sodium chloride crystals were dispersed in 3000g of N-methylpyrrolidone to obtain a template agent dispersion. 1g of 2,4,6-trialdehydepyridine, 1g of 2,5-diaminobenzenesulfonic acid, and 2g of trichloroacetic acid were added to the template agent dispersion. After ultrasonic treatment, a templated covalent organic framework synthesis solution was obtained. The air in the synthesis solution was removed and sealed. The reaction was carried out at 100℃ for 48h. After filtration, washing, and drying, the covalent organic framework@template agent powder was obtained.

[0073] S2. Grind the covalent organic framework@template agent powder, place it in a Buchner funnel, filter and wash it with pure water, dry the filter residue to obtain pure covalent organic framework powder, disperse the pure covalent organic framework powder with ethanol at a mass ratio of 1:2000, and perform rotary evaporation to obtain a concentrated solution with a concentration of 1.6 g / L.

[0074] S3. Using a scraper, the concentrated covalent organic framework solution from S2 is coated onto the surface of the perfluorosulfonic acid membrane to a thickness of 50 μm. After cross-linking by heat treatment at 65°C for 20 min, the covalent organic framework-perfluorosulfonic acid composite membrane is obtained.

[0075] Comparative Example 1: The following technical solution is adopted.

[0076] In Example 1, the positively charged monomer ethidium bromide was replaced with neutral p-phenylenediamine to prepare an electrically neutral covalent organic framework with similar pore size. Other steps were the same as in Example 1.

[0077] Comparative Example 2: The following technical solution is adopted.

[0078] Without using the template agent aluminum-magnesium hydrotalcite from Example 1, the prepared covalent organic framework powder was ultrasonically dispersed in ethanol to obtain a nanosheet dispersion. Other film-forming steps were the same as in Example 1.

[0079] The characterization tests employed in this invention are as follows:

[0080] 1. Microscopic morphology characterization: A covalent organic framework dispersion was dropped onto a clean anodic aluminum oxide (AAO) film surface, dried, and then characterized by SEM (ZEISS Gemini SEM360) and TEM (JEOL JEM-2100). The polymer-covalent organic framework hybrid composite film was characterized by SEM and TEM of its surface and cross-section.

[0081] 2. Battery performance testing: The membranes from Examples 1-5 and Comparative Examples 1-2 were assembled into battery stacks for testing. Coulombic efficiency, voltage efficiency, and energy efficiency were tested and recorded under the same operating conditions. The test results are shown in Table 1.

[0082] Table 1 Battery performance test results

[0083]

[0084] The preparation process of the covalent organic framework-perfluorosulfonic acid composite membranes in Examples 1-5 is as follows: Figure 1 As shown. By Figure 2It can be seen that the morphology of the covalent organic frameworks in the synthesis solutions of Example 1 and Comparative Example 2 are completely different: Example 1 used layered hydrotalcite as a template, and after removing the template agent, covalent organic framework nanosheets with a thin-layer structure were obtained; Comparative Example 2 did not use a template agent, and large-sized covalent organic framework bulks were generated in the synthesis solution, which could not be directly used for membrane preparation and required additional exfoliation methods. Figure 3 It can be seen that the synthesized covalent organic framework nanosheets have a relatively thin thickness and a large lateral dimension. This indicates that the template method provided in this invention can improve the film-forming properties and processability of covalent organic frameworks. Figure 4 It can be seen that there are no obvious defects on the surface of the composite membrane. The cross-sectional TEM image shows a complete, continuous, and uniformly thick covalent organic framework separation layer on the surface of the perfluorosulfonic acid-based membrane, with a thickness of about 15 μm.

[0085] Performance test results show that the composite films prepared using template agents in Examples 1-5 maintained a capacity retention rate of over 76% after 300 cycles in vanadium battery testing, while the capacity retention rate of Comparative Example 2, prepared without template agents, was less than 40%. This indicates that the template method can obtain high-purity, high-concentration covalent organic framework nanosheet dispersions, which can be composited with perfluorosulfonic acid-based films to obtain a pore structure with misaligned stacking, reducing the effective pore size of the composite film and improving the ion selectivity of the composite film and the coulombic efficiency and stability of the battery. Comparative Example 1 replaced the positively charged monomer ethidium bromide in Example 1 with neutral p-phenylenediamine to prepare an electrically neutral covalent organic framework with similar pore size. Test results show that the coulombic efficiency of the battery decreased by nearly 1% after using the neutral covalent organic framework, and the capacity retention rate also decreased significantly. Furthermore, Examples 4 and 5 used covalent organic frameworks containing sulfonic acid groups; compared to Examples 4 and 5, Comparative Example 1 also showed a voltage efficiency decrease of approximately 2%. The above two points illustrate that the present invention can use covalent organic frameworks with different charge properties: if a cationic covalent organic framework is used, the vanadium barrier property can be improved through charge repulsion, thereby further improving the coulombic efficiency of the battery; if an anionic covalent organic framework containing sulfonic acid groups is used, the conductivity of the membrane can be improved, thereby improving the voltage efficiency of the battery.

[0086] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions or improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing a covalent organic framework composite membrane, characterized in that, The preparation method comprises the following steps: Step S1: monomer 1, monomer 2, catalyst are added to the template dispersing liquid to react, and a covalent organic framework@template powder is obtained; the monomer 1 is selected from at least one of 1,3,5-tri(4-formylphenyl)benzene, 2,4,6-trialdehyde phenol, 2,4,6-tri(4-aldehyde phenyl)-1,3,5-triazine, tri-aldehyde resorcinol and 2,4,6-trialdehyde pyridine; the monomer 2 is selected from at least one of ethidium bromide, guanidine triaminohydrochloride, 4,4'-diaminobiphenyl, 4,4-diaminobiphenyl-2,2-disulfonic acid and 2,5-diaminobenzenesulfonic acid; the template is selected from at least one of aluminum magnesium hydrotalcite, graphene, tubular barium titanate crystal and sodium chloride crystal; Step S2: the template in the covalent organic framework@template powder is removed or separated to obtain a pure covalent organic framework powder, and the covalent organic framework powder is dispersed in ethanol, treated by rotary evaporation to obtain a concentrated liquid; Step S3: the concentrated liquid is scraped on the surface of a perfluorosulfonic acid membrane and heat treated to obtain a covalent organic framework-perfluorosulfonic acid composite membrane.

2. The production method according to claim 1, wherein In the step S1: The mass ratio of the monomer 1, the monomer 2, the catalyst and the template dispersing liquid is (1-3):(1-5):(1-2):(1000-3000).

3. The production method according to claim 1 or 2, characterized by, In the step S1: The catalyst is selected from at least one of acetic acid, p-toluenesulfonic acid, iron trifluoromethanesulfonate, tris(pentafluorophenyl)borane and trichloroacetic acid.

4. The production method according to claim 1, wherein In the step S1: The template dispersing liquid is prepared by dispersing 10-20 mass parts of the template into 1000-3000 mass parts of a solvent; wherein, The solvent is selected from at least one of mesitylene, dioxane, N,N-dimethylformamide, glycerol and N-methylpyrrolidone.

5. The production method according to claim 1, wherein In the step S1: The reaction comprises ultrasonic treatment, air discharge, sealing, high-temperature reaction, filtration, washing and drying in sequence; wherein, the temperature of the high-temperature reaction is 100-150 DEG C, and the time is 24-48 h.

6. The production method according to claim 1, wherein In the step S2: The removal or separation method is determined by the type of the template, and the removal or separation comprises: If the template is hydrotalcite or barium titanate, the covalent organic framework@template powder is ground and placed in a Buchner funnel, washed by suction filtration with concentrated hydrochloric acid, and the filter residue is dried; If the template is sodium chloride crystal, the covalent organic framework@template powder is ground and placed in a Buchner funnel, washed by suction filtration with pure water, and the filter residue is dried; If the template is graphene, the covalent organic framework@template powder is placed in pure water, ultrasonically treated, centrifuged, and the precipitate is dried.

7. The preparation method of claim 1, wherein: The mass ratio of the pure covalent organic framework powder to the ethanol is 1:2000; The concentration of the covalent organic framework in the concentrated liquid is 1.2-1.8 g / L.

8. The production method according to claim 1, wherein In the step S3: The thickness of the scraping is 10-50 μm; The temperature of the heat treatment is 60-80 DEG C, and the time is 5-30 min.

9. A covalent organic framework composite membrane, characterized in that, The covalent organic framework composite film is prepared by the preparation method of any one of claims 1-8.

10. Use of a covalent organic framework composite membrane, characterized in that The covalent organic framework composite film prepared by the preparation method of any one of claims 1-8 or the covalent organic framework composite film of claim 9 is applied to a full vanadium redox flow battery.

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