A fuel cell composite membrane and preparation method thereof

Abstract

The present invention belongs to the field of membrane materials, and relates to a composite membrane and a preparation method thereof. The composite membrane is formed by adding a casting solution to the surface of a polyolefin porous membrane, and the casting solution is compounded by imidazolium salt crystals, polyvinyl benzyl chloride and a cross-linking agent, and the synthesis of imidazolium salt crystals uses a low-temperature vacuum method to fix the material formed by 4,5-disubstituted imidazolium salts as raw materials in a metal organic framework crystalline material; the preparation of the composite membrane is to form a casting solution by blending polyvinyl benzyl chloride, a cross-linking agent and imidazolium salt crystals, and the casting solution is added to the surface of the porous membrane, and then a release film is laid to form a "sandwich" structure, and the composite membrane after cross-linking is subjected to quaternization and alkalization treatment by heating and cross-linking to obtain an ultra-thin composite membrane. The composite membrane of the present invention has the advantages of high degree of cross-linking, high mechanical strength, long service life and slow degradation rate, and the cross-linked structure makes the composite membrane have a better fixing effect on the active group, and improves the ion conductivity of the composite membrane.

Description

Technical Field

[0001] The invention relates to a composite film and a preparation method thereof, belonging to the field of film materials. Background Art

[0002] In the existing energy system, the main fossil fuels are facing increasingly serious sustainable development and environmental problems. The world urgently needs efficient and clean energy conversion and storage equipment. Fuel cell technology has the advantages of high energy efficiency and low harmful emissions, and has attracted widespread attention in the past few decades. Proton exchange membrane fuel cells (PEMFCs) have been initially applied, but they still face the cost of precious metal catalysts and the life of the battery. Anion exchange membrane fuel cells (AEMFCs) are considered to be another potential more promising approach, especially in the fields of portable, fixed and backup power. Compared with PEMFCs working under acidic conditions, AEMFCs working under alkaline conditions have the following advantages: (1) the oxygen reduction reaction kinetics are faster under alkaline conditions; (2) non-precious metal catalysts can be used; (3) the corrosion problem is less severe under alkaline conditions. However, the commercialization of AEMFCs is severely restricted by the lack of high-performance anion exchange membrane (AEM). In addition, the alkaline anion exchange membrane is the core component of the alkaline anion exchange membrane fuel cell. It plays a dual role of conducting ions and blocking fuel. Its performance directly determines the performance and life of the fuel cell.

[0003] Chinese invention patent CN101844042A discloses a method for preparing an anion exchange membrane based on ionic liquids, which includes adding imidazole-type ionic liquids, acrylic acid ester monomers, solvents, and initiators into a reactor, polymerizing by heating, and separating to obtain a polymer, and forming the obtained polymer into a membrane by a phase inversion method to obtain an anion exchange membrane. This method avoids the use of highly toxic chloromethyl ether in the traditional anion exchange membrane preparation process, making the preparation process simpler and safer. However, since the anion exchange membrane prepared by this method is an aliphatic polymer main chain, it will swell excessively during use, and its dimensional stability and mechanical strength are poor. Zhang et al. grafted styrene trimethyl chloride (VBTAC) to polyvinylidene fluoride (PVDF) to obtain a quaternary ammonium functionalized anion exchange membrane, which has a mild synthesis route and its strength decreases after alkali treatment (3 days in 3M NaOH at 60°C) [FXZhang, et al. J. Power Sources 196, 2011, 3099–3103]. The alkaline anion membrane is prepared by cross-linking, which can effectively reduce the swelling and water absorption of the alkaline anion exchange membrane. However, the cross-linking density should not be too high, otherwise it will increase the stress inside the membrane, hinder the rotation of the molecular chain, and make the dry membrane brittle.

[0004] Therefore, in the preparation process of AEMs, the influencing factors that need to be considered are very complex. Not only the contradiction between mechanical properties and electrochemical properties, but also the relationship between membrane thickness and ion conductivity must be considered. So how to explore a green, simple, high conductivity, low cost, excellent mechanical properties and good alkali resistance alkaline anion membrane is an urgent problem that needs to be solved. Summary of the invention

[0005] The purpose of the present invention is to provide a high-strength ultra-thin composite membrane, which has the advantages of ensuring high chemical stability of the composite membrane while reducing the thickness of the membrane and improving the ion conductivity of the composite membrane.

[0006] The above technical purpose of the present invention is achieved through the following technical solutions:

[0007] On one hand, the present invention provides a composite membrane, which is formed by injecting a casting solution onto the surface of a polyolefin porous membrane, wherein the casting solution is composited with imidazolium salt crystals, polyvinyl benzyl chloride (PVBC) and a crosslinking agent, and the imidazolium salt crystals are: a substance formed with 4,5-disubstituted imidazolium salt as a raw material fixed in a metal organic framework crystal material; the 4,5-disubstituted imidazolium salt is a 1-butyl C4, C5 disubstituted imidazolium salt or a 1-hexyl C4, C5 disubstituted imidazolium salt.

[0008] In the above technical solution, further, the raw material of the imidazolium salt crystal also includes p-chloromethylstyrene.

[0009] In the above technical solution, further, the metal organic framework crystal material is a cubic structure with micropores, and the pore size of the metal organic framework crystal material is 100-1000nm; the metal organic framework crystal material is one or more of MIL-101 (Fe), MIL-101 (Cr), MIL-53 (Cr), MIL-53 (Fe), ZIF-6, ZIF-8, ZIF-10;

[0010] The cross-linking agent is one or more of N,N,N′,N′-tetramethylmethanediamine (TMMDA), N,N,N′,N′-tetramethylethylenediamine (TMEDA), and N,N,N′,N′-tetramethyl-1,6-hexanediamine (TMHDA);

[0011] The polyolefin porous membrane is any one of a polyethylene porous membrane and a polypropylene porous membrane. The polyolefin porous membrane has a thickness of 5-10 μm, a porosity of 45-80%, and an average pore size of 0.1-0.3 μm.

[0012] In the above technical solution, further, the imidazolium salt crystal is synthesized by a low-temperature vacuum method, and the C4 and C5 substituted imidazolium salts are reacted with other raw materials to be "fixed" in the crystal material of the metal organic framework material with a microporous cubic structure, comprising the following steps:

[0013] (1) placing a metal organic framework crystal material having a microporous cubic structure in a Schlenk tube equipped with a constant pressure funnel, and continuously evacuating the metal organic framework crystal material to place it in a vacuum state;

[0014] (2) under low temperature environment, mixing a mixture of 4,5-disubstituted imidazolium salt and p-chloromethylstyrene with a metal organic framework crystal material, stirring and reacting, so that the 4,5-disubstituted imidazolium salt reacts with p-chloromethylstyrene;

[0015] (3) After the reaction is completed, centrifugation is performed, and the precipitate is dried, and the vinyl group in the chloromethylstyrene is self-crosslinked to obtain imidazolium salt crystals.

[0016] In the above technical scheme, further, the vacuum degree of the vacuum state in step (1) is -0.6 to -1.0 MPa; the low-temperature reaction temperature in step (2) is -20 to -10°C, the stirring time is 48-96h, and the molar ratio of the metal organic framework crystal material to the 4,5-disubstituted imidazolium salt to p-chloromethylstyrene is 1:2:0.5-5; the precipitate drying temperature in step (3) is 60-100°C, and the drying time is 4-8h; step (1) and step (2) are under the same vacuum conditions.

[0017] In the above technical solution, further, the C4 and C5 substituents in the 1-butyl C4, C5 disubstituted imidazolium salt or the 1-hexyl C4, C5 disubstituted imidazolium salt are R1 and R2, respectively, and the R1 and R2 are any one of methyl, ethyl, propyl, butyl, isopropyl, and tert-butyl. The structures of the 1-butyl C4, C5 disubstituted imidazolium salt or the 1-hexyl C4, C5 disubstituted imidazolium salt are as follows:

[0018] Another aspect of the present invention provides a method for preparing the aforementioned composite membrane, comprising the following membrane-making steps:

[0019] A. Weigh polyvinyl benzyl chloride (PVBC) and add it to a high boiling point solvent, and stir at room temperature to obtain a PVBC solution; the high boiling point solvent is preferably one of: N-methylpyrrolidone, N,N-dimethylformamide, tetrahydrofuran, and dimethyl sulfoxide.

[0020] B. Weigh a cross-linking agent and add it to the polyvinyl benzyl chloride (PVBC) solution prepared in step A, and stir at room temperature to obtain a mixed solution;

[0021] C. Weigh imidazolium salt crystals, add them to the mixed solution in step B, and stir to form a film casting solution;

[0022] D. Lay the ultra-thin polyolefin porous membrane on the release polyester film, add the casting liquid to the surface of the porous membrane, and then lay a layer of release polyester film on the surface of the casting liquid to form a "sandwich" structure, use a flat plate to drive out bubbles, and scrape and smoothen to obtain a composite layer of the "sandwich" structure;

[0023] E. placing the composite layer of the "sandwich" structure prepared in step D between graphite plates, placing it under a hydraulic press, heating and hot pressing it, and then taking it out to obtain a composite film, and peeling the prepared composite film from the graphite plate;

[0024] F. Soaking the composite membrane prepared in step E in a quaternary ammonium aqueous solution for quaternization treatment, and then rinsing with deionized water;

[0025] G. Soak the composite membrane prepared in step F in an alkaline solution, then wash it with deionized water for more than 5 times, and dry it to obtain a high-strength ultra-thin composite membrane.

[0026] In the above technical solution, further, in step A, the concentration of the PVBC solution is 0.1-0.5 g / mL; in step B, the mass ratio of the crosslinking agent to PVBC is 0.1-0.5:1; in step C, the amount of the imidazolium salt crystal added is 2-10% of the mass of PVBC.

[0027] In the above technical scheme, further, in step A and step B, the room temperature is 25°C, and the stirring and dissolving time is 24-48h; the stirring time in step B is 30-60s; the stirring time in step C is: 24-48h, and the heating temperature in step E is 80-100°C, the heating time is 1-2h, and the pressure is 1-3MPa.

[0028] In the above technical scheme, further, the quaternization solution in step F is any one of a 20-40wt% triethylamine aqueous solution or a 20-40wt% trimethylamine aqueous solution; the quaternization time is 48-72h; the alkaline solution is one of sodium hydroxide or potassium hydroxide, the concentration of the alkaline solution is 0.1-2mol / L, the soaking time is 24-48h, and the drying temperature is 50-60°C.

[0029] Beneficial Effects

[0030] 1. The composite membrane prepared by the present invention uses imidazolium salt, metal organic framework crystal material, polyvinyl benzyl chloride (PVBC), crosslinking agent, etc. as functional groups and crosslinking groups. The imidazolium salt enters the micropore aperture of the metal organic framework crystal material to form a micropore aperture channel; first, the formation of a stable channel can orderly transfer ions, and secondly, the imidazolium salt molecules are fixed in the metal organic framework material as ion transport groups and will not be lost. The prepared composite membrane has an orderly ion transport channel and can stably transport ions.

[0031] In addition, the present invention adopts a polyolefin porous membrane as the supporting layer of the composite membrane. Since the polyolefin porous membrane itself is a thin film and has micropores, the above-mentioned functional groups can be filled into the micropores, thereby improving the mechanical strength of the composite membrane while reducing the thickness of the membrane, further improving the ion conductivity of the composite membrane, and reducing the impedance. The polyolefin porous membrane can undergo a cross-linking reaction with materials such as a cross-linking agent due to the presence of its own terminal olefins, thereby further improving the stability of the composite membrane.

[0032] 2. The composite membrane prepared by the present invention is prepared by adding a cross-linking agent after the polymers are mixed to form a casting liquid, and cross-linking is carried out by heat initiation. The prepared composite membrane has a high degree of cross-linking and high mechanical strength, which prolongs the service life of the composite membrane and slows down the degradation rate of the composite membrane. The cross-linked structure enables the composite membrane to have a better solid effect on the active groups, thereby improving the ion conductivity of the composite membrane. In addition, the cross-linking agent added in the process of preparing the composite membrane of the present invention has the characteristic of easy cross-linking, and the functional group formed after the nitrogen atom is cross-linked can still become a provider of quaternary ammonium groups, which not only increases the stability and strength of the membrane, but also increases the ion transmission of the composite membrane.

[0033] 3. In the process of heat cross-linking of the present invention, graphite plates are used for covering and hot pressing. Since the internal structure of the graphite plates is gradually compressed during the pressure process, it plays a certain supporting role in the film making process. Specifically, the graphite plates are covered on both sides of the film. During the hot pressing process, the buffering effect of the graphite plates relieves the force on both sides of the film, so that the polymer compound and the base film can be more densely combined, preventing the instantaneous excessive pressure from damaging the film structure, and improving the air tightness of the film.

[0034] 4. The composite membrane prepared by the present invention uses an alkane group (butyl or hexyl in 4,5-disubstituted imidazolium salt) as the part connected to the quaternary ammonium group. The alkane group is used as an electron-donating group to form a stable compound effect, so that the chemical barrier of the hydroxide ion attacking the active site is increased, and the degradation effect on the membrane is effectively reduced; in addition, the alkane group itself has good stability in an alkaline environment and is not easily chemically degraded. In addition, the present invention uses the prepared imidazolium salt crystal as an additive, so that the metal organic framework material can fix the functional material, on the one hand, preventing the loss of single-component compounds with the use of the membrane, and at the same time, a single component such as a metal organic framework material does not have an ion transport function, and the combination of multiple components can make advantageous use of the physical and chemical properties of different materials. DETAILED DESCRIPTION

[0035] The present invention is described in further detail below.

[0036] Embodiment 1:

[0037] (1) Weigh 6.99 g (0.01 mol) of MIL-101(Fe) and place it in a Schlenk tube equipped with a constant pressure funnel. Continuously evacuate the tube to maintain a vacuum degree of -1.0 MPa and lower the temperature of the Schlenk tube to -10°C.

[0038] (2) Weigh 3.04 g (0.02 mol) of 1-hexyl-4,5-dimethylimidazolium salt and 1.525 g (0.01 mol) of p-chloromethylstyrene, inject the mixture into the Schlenk tube of step (1), and continue stirring for 96 h to allow 1-hexyl-4,5-dimethylimidazolium salt and p-chloromethylstyrene to react fully;

[0039] (3) After the reaction is completed, the precipitate is centrifuged and then dried at 60° C. for 8 h to allow the vinyl groups to self-crosslink and form imidazolium salt crystals;

[0040] (4) At 25°C, 10 g of polyvinylbenzyl chloride was dissolved in 100 mL of N-methylpyrrolidone and stirred for 24 h to obtain a polymer solution with a concentration of 0.1 g / mL. Then, 1 g of N,N,N′,N′-tetramethylmethanediamine was added and stirred for 30 s.

[0041] (5) Weigh 0.2 g of imidazolium salt crystals and add them to step (4), stir for 24 h until uniform, and obtain a casting solution for later use;

[0042] (6) Laying a polyethylene porous film with a thickness of 10 μm and a porosity of 80% on the surface of the release polyester film, then pouring the casting liquid obtained in step (5) on the surface of the polyethylene porous film, and then laying a layer of release polyester film on the surface of the casting liquid to form a "sandwich" structure, using a flat plate to drive out bubbles, and scraping and leveling;

[0043] (7) placing the "sandwich" structure composite layer obtained in step (6) between graphite plates, and placing it under a hydraulic press at a pressure of 1 MPa and a temperature of 80° C. for 2 h, taking it out, and peeling off the prepared composite film;

[0044] (8) The composite membrane prepared in step (7) is placed in a 20% trimethylamine aqueous solution for quaternization treatment for 48 hours, removed, and repeatedly rinsed with deionized water for 10 times, and then immersed in a 0.1 mol / L sodium hydroxide solution to fully alkalize for 48 hours, and then taken out and rinsed off the alkaline solution on the surface of the membrane with deionized water, and then thoroughly dried at 60°C to obtain a high-strength ultra-thin composite membrane.

[0045] Embodiment 2:

[0046] (1) Weigh 2.3 g (0.01 mol) of ZIF-8 and place it in a Schlenk tube equipped with a constant pressure funnel. Continuously evacuate the tube to maintain a vacuum degree of -0.6 MPa, and lower the temperature of the Schlenk tube to -20 °C.

[0047] (2) Weigh 3.04 g (0.02 mol) of 1-butyl-4,5-dimethylimidazolium salt and 1.525 g (0.01 mol) of p-chloromethylstyrene, inject the mixture into the Schlenk tube of step (1), and continue stirring for 48 h to allow 1-butyl-4,5-dimethylimidazolium salt and p-chloromethylstyrene to react fully;

[0048] (3) After the reaction is completed, the precipitate is centrifuged and then dried at 100° C. for 4 h to allow the vinyl groups to self-crosslink and form imidazolium salt crystals;

[0049] (4) At 25°C, 50 g of polyvinylbenzyl chloride was dissolved in 100 mL of N-methylpyrrolidone and stirred for 48 h to obtain a polymer solution with a concentration of 0.5 g / mL. Then, 25 g of N,N,N′,N′-tetramethylethylenediamine was added and stirred for 60 s.

[0050] (5) Weigh 5 g of imidazolium salt crystals and add them to step (4), stir for 24 h until uniform, and obtain a casting solution for later use;

[0051] (6) laying a polypropylene porous membrane with a thickness of 5 μm and a porosity of 45% on the surface of the release polyester membrane, then pouring the casting liquid obtained in step (5) on the surface of the polypropylene porous membrane, and then laying a layer of release polyester film on the surface of the casting liquid to form a "sandwich" structure, using a flat plate to drive out bubbles, and scraping and smoothing;

[0052] (7) placing the "sandwich" structure composite layer obtained in step (6) between graphite plates, and placing it under a hydraulic press at a pressure of 3 MPa and a temperature of 100° C. for 1 h, taking it out, and peeling off the prepared composite film;

[0053] (8) The composite membrane prepared in step (7) is placed in a 40% triethylamine aqueous solution for quaternization treatment for 24 hours, taken out, and repeatedly rinsed with deionized water for 10 times, and then immersed in a 2 mol / L potassium hydroxide solution to fully alkalize for 24 hours, and then taken out and rinsed off the alkaline solution on the surface of the membrane with deionized water, and then thoroughly dried at 50°C to obtain a high-strength ultra-thin composite membrane.

[0054] Embodiment 3:

[0055] (1) Weigh 7.19 g (0.01 mol) of MIL-101(Cr) and place it in a Schlenk tube equipped with a constant pressure funnel. Continuously evacuate the tube to maintain a vacuum degree of -0.8 MPa and lower the temperature of the Schlenk tube to -15°C.

[0056] (2) Weigh 3.04 g (0.02 mol) of 1-hexyl-4,5-diethylimidazolium salt and 1.525 g (0.01 mol) of p-chloromethylstyrene, inject the mixture into the Schlenk tube of step (1), and continue stirring for 36 h to allow 1-hexyl-4,5-diethylimidazolium salt to react fully with p-chloromethylstyrene;

[0057] (3) After the reaction is completed, the precipitate is centrifuged and then dried at 80° C. for 6 h to allow the vinyl groups to self-crosslink and form imidazolium salt crystals;

[0058] (4) At 25°C, 20 g of polyvinylbenzyl chloride was dissolved in 100 mL of N-methylpyrrolidone and stirred for 36 h to obtain a polymer solution with a concentration of 0.2 g / mL. Then, 4 g of N,N,N′,N′-tetramethyl-1,6-hexanediamine was added and stirred for 40 s.

[0059] (5) Weigh 1.2 g of imidazolium salt crystals and add them to step (4), stir for 24 h until uniform, and obtain a casting solution for later use;

[0060] (6) Laying a polyethylene porous film with a thickness of 8 μm and a porosity of 60% on the surface of the release polyester film, then pouring the casting liquid obtained in step (5) on the surface of the polypropylene porous film, and then laying a layer of release polyester film on the surface of the casting liquid to form a "sandwich" structure, using a flat plate to drive out bubbles, and scraping and smoothing;

[0061] (7) placing the "sandwich" structure composite layer obtained in step (6) between graphite plates, and placing it under a hydraulic press at a pressure of 2 MPa and a temperature of 90° C. for 1.5 h, taking it out, and peeling off the prepared composite film;

[0062] (8) The composite membrane prepared in step (7) is placed in a 30% triethylamine aqueous solution for quaternization treatment for 36 hours, removed, and repeatedly rinsed with deionized water for 10 times, and then immersed in a 1 mol / L potassium hydroxide solution to fully alkalize for 36 hours, and then removed and the alkaline solution on the membrane surface is rinsed off with deionized water, and then thoroughly dried at 55°C to obtain a high-strength ultra-thin composite membrane.

[0063] Embodiment 4:

[0064] (1) Weigh 6.99 g (0.01 mol) of MIL-101(Fe) and place it in a Schlenk tube equipped with a constant pressure funnel. Continuously evacuate the tube to maintain a vacuum degree of -1.0 MPa and lower the temperature of the Schlenk tube to -10°C.

[0065] (2) Weigh 3.04 g (0.02 mol) of 1-hexyl-4,5-dimethylimidazolium salt and 0.7625 g (0.005 mol) of p-chloromethylstyrene, inject the mixture into the Schlenk tube of step (1), and continue stirring for 96 h to allow 1-hexyl-4,5-dimethylimidazolium salt to react fully with p-chloromethylstyrene;

[0066] (3) After the reaction is completed, the precipitate is centrifuged and then dried at 60° C. for 8 h to allow the vinyl groups to self-crosslink and form imidazolium salt crystals;

[0067] (4) At 25°C, 10 g of polyvinylbenzyl chloride was dissolved in 100 mL of N-methylpyrrolidone and stirred for 24 h to obtain a polymer solution with a concentration of 0.1 g / mL. Then, 1 g of N,N,N′,N′-tetramethylmethanediamine was added and stirred for 30 s.

[0068] (5) Weigh 0.2 g of imidazolium salt crystals and add them to step (4), stir for 24 h until uniform, and obtain a casting solution for later use;

[0069] (6) Laying a polyethylene porous film with a thickness of 10 μm and a porosity of 80% on the surface of the release polyester film, then pouring the casting liquid obtained in step (5) on the surface of the polyethylene porous film, and then laying a layer of release polyester film on the surface of the casting liquid to form a "sandwich" structure, using a flat plate to drive out bubbles, and scraping and leveling;

[0070] (7) placing the "sandwich" structure composite layer obtained in step (6) between graphite plates, and placing it under a hydraulic press at a pressure of 1 MPa and a temperature of 80° C. for 2 h, taking it out, and peeling off the prepared composite film;

[0071] (8) The composite membrane prepared in step (7) is placed in a 20% trimethylamine aqueous solution for quaternization treatment for 48 hours, removed, and repeatedly rinsed with deionized water for 10 times, and then immersed in a 0.1 mol / L sodium hydroxide solution to fully alkalize for 48 hours, and then taken out and rinsed off the alkaline solution on the surface of the membrane with deionized water, and then thoroughly dried at 60°C to obtain a high-strength ultra-thin composite membrane.

[0072] Embodiment 5:

[0073] (1) Weigh 6.99 g (0.01 mol) of MIL-101(Fe) and place it in a Schlenk tube equipped with a constant pressure funnel. Continuously evacuate the tube to maintain a vacuum degree of -1.0 MPa and lower the temperature of the Schlenk tube to -10°C.

[0074] (2) Weigh 3.04 g (0.02 mol) of 1-hexyl-4,5-dimethylimidazolium salt and 7.625 g (0.05 mol) of p-chloromethylstyrene, inject the mixture into the Schlenk tube of step (1), and continue stirring for 96 h to allow 1-hexyl-4,5-dimethylimidazolium salt and p-chloromethylstyrene to react fully;

[0075] (3) After the reaction is completed, the precipitate is centrifuged and then dried at 60° C. for 8 h to allow the vinyl groups to self-crosslink and form imidazolium salt crystals;

[0076] (4) At 25°C, 10 g of polyvinylbenzyl chloride was dissolved in 100 mL of N-methylpyrrolidone and stirred for 24 h to obtain a polymer solution with a concentration of 0.1 g / mL. Then, 1 g of N,N,N′,N′-tetramethylmethanediamine was added and stirred for 30 s.

[0077] (5) Weigh 0.2 g of imidazolium salt crystals and add them to step (4), stir for 24 h until uniform, and obtain a casting solution for later use;

[0078] (6) Laying a polyethylene porous film with a thickness of 10 μm and a porosity of 80% on the surface of the release polyester film, then pouring the casting liquid obtained in step (5) on the surface of the polyethylene porous film, and then laying a layer of release polyester film on the surface of the casting liquid to form a "sandwich" structure, using a flat plate to drive out bubbles, and scraping and leveling;

[0079] (7) placing the "sandwich" structure composite layer obtained in step (6) between graphite plates, and placing it under a hydraulic press at a pressure of 1 MPa and a temperature of 80° C. for 2 h, taking it out, and peeling off the prepared composite film;

[0080] (8) The composite membrane prepared in step (7) is placed in a 20% trimethylamine aqueous solution for quaternization treatment for 48 hours, removed, and repeatedly rinsed with deionized water for 10 times, and then immersed in a 0.1 mol / L sodium hydroxide solution to fully alkalize for 48 hours, and then taken out and rinsed off the alkaline solution on the surface of the membrane with deionized water, and then thoroughly dried at 60°C to obtain a high-strength ultra-thin composite membrane.

[0081] Comparative Example 1:

[0082] (1) A mixture of 3.04 g (0.02 mol) of 1-hexyl-4,5-dimethylimidazolium salt and 1.525 g (0.01 mol) of p-chloromethylstyrene was injected into a Schlenk tube and stirred for 96 h to allow the 1-hexyl-4,5-dimethylimidazolium salt and p-chloromethylstyrene to react fully;

[0083] (2) After the reaction is completed, the precipitate is centrifuged and then dried at 60° C. for 8 h to allow the vinyl groups to self-crosslink to form an imidazolium salt compound;

[0084] (3) At 25°C, 10 g of polyvinylbenzyl chloride was dissolved in 100 mL of N-methylpyrrolidone and stirred for 24 h to obtain a polymer solution with a concentration of 0.1 g / mL. Then, 1 g of N,N,N′,N′-tetramethylmethanediamine was added and stirred for 30 s.

[0085] (4) Weigh 0.2 g of an imidazolium salt compound and add it to step (3), stir evenly, and obtain a casting solution for later use;

[0086] (5) Laying a polyethylene porous film with a thickness of 10 μm and a porosity of 80% on the surface of the release polyester film, then pouring the casting liquid obtained in step (4) on the surface of the polyethylene porous film, and then laying a layer of release polyester film on the surface of the casting liquid to form a "sandwich" structure, using a flat plate to drive out bubbles, and scraping and smoothing;

[0087] (6) placing the "sandwich" structure composite layer obtained in step (5) between graphite plates, and placing it under a hydraulic press at a pressure of 1 MPa and a temperature of 80° C. for 2 h, taking it out, and peeling off the prepared composite film;

[0088] (7) The composite membrane prepared in step (6) is placed in a 20% trimethylamine aqueous solution for quaternization treatment for 48 hours, removed, and repeatedly rinsed with deionized water for 10 times, and then immersed in a 0.1 mol / L sodium hydroxide solution to fully alkalize for 48 hours, and then taken out and rinsed off the alkaline solution on the membrane surface with deionized water, and then thoroughly dried at 60°C to obtain a high-strength ultra-thin composite membrane.

[0089] Comparative Example 2:

[0090] (1) Weigh 6.99 g (0.01 mol) of MIL-101(Fe) and place it in a Schlenk tube equipped with a constant pressure funnel. Continuously evacuate the tube to maintain a vacuum degree of -1.0 MPa and lower the temperature of the Schlenk tube to -10°C.

[0091] (2) Weigh 3.04 g (0.02 mol) of 1-hexyl-4,5-dimethylimidazolium salt and 1.525 g (0.01 mol) of p-chloromethylstyrene, inject the mixture into the Schlenk tube of step (1), and continue stirring for 96 h to allow 1-hexyl-4,5-dimethylimidazolium salt and p-chloromethylstyrene to react fully;

[0092] (3) After the reaction is completed, the precipitate is centrifuged and then dried at 60° C. for 8 h to allow the vinyl groups to self-crosslink and form imidazolium salt crystals;

[0093] (4) At 25°C, 10 g of polyvinyl benzyl chloride was dissolved in 100 mL of N-methylpyrrolidone and stirred for 24 h to obtain a polymer solution with a concentration of 0.1 g / mL;

[0094] (5) Weigh 0.2 g of imidazolium salt crystals and add them to step (4), stir evenly to obtain a casting solution for later use;

[0095] (6) Laying a polyethylene porous film with a thickness of 10 μm and a porosity of 80% on the surface of the release polyester film, then pouring the casting liquid obtained in step (5) on the surface of the polyethylene porous film, and then laying a layer of release polyester film on the surface of the casting liquid to form a "sandwich" structure, using a flat plate to drive out bubbles, and scraping and leveling;

[0096] (7) placing the "sandwich" structure composite layer obtained in step (6) between graphite plates, and placing it under a hydraulic press at a pressure of 1 MPa and a temperature of 80° C. for 2 h, taking it out, and peeling off the prepared composite film;

[0097] (8) The composite membrane prepared in step (7) is placed in a 20% trimethylamine aqueous solution for quaternization treatment for 48 hours, removed, and repeatedly rinsed with deionized water for 10 times, and then immersed in a 0.1 mol / L sodium hydroxide solution to fully alkalize for 48 hours, and then taken out and rinsed off the alkaline solution on the surface of the membrane with deionized water, and then thoroughly dried at 60°C to obtain a high-strength ultra-thin composite membrane.

[0098] Comparative Example 3:

[0099] (1) Weigh 6.99 g (0.01 mol) of MIL-101(Fe) and place it in a Schlenk tube equipped with a constant pressure funnel. Continuously evacuate the tube to maintain a vacuum degree of -1.0 MPa and lower the temperature of the Schlenk tube to -10°C.

[0100] (2) Weigh 3.04 g (0.02 mol) of 1-hexyl-4,5-dimethylimidazolium salt and 1.525 g (0.01 mol) of p-chloromethylstyrene, inject the mixture into the Schlenk tube of step (1), and continue stirring for 96 h to allow 1-hexyl-4,5-dimethylimidazolium salt and p-chloromethylstyrene to react fully;

[0101] (3) After the reaction is completed, the precipitate is centrifuged and then dried at 60° C. for 8 h to allow the vinyl groups to self-crosslink and form imidazolium salt crystals;

[0102] (4) At 25°C, 10 g of polyvinylbenzyl chloride was dissolved in 100 mL of N-methylpyrrolidone and stirred for 24 h to obtain a polymer solution with a concentration of 0.1 g / mL. Then, 1 g of N,N,N′,N′-tetramethylmethanediamine was added and stirred for 30 s.

[0103] (5) Weigh 0.2 g of imidazolium salt crystals and add them to step (4), stir evenly to obtain a casting solution for later use;

[0104] (6) laying a layer of release polyester film on the glass plate, pouring the casting liquid obtained in step (5) on the release polyester film, and then laying another layer of release polyester film on the surface of the casting liquid to form a "sandwich" structure, using a flat plate to drive out bubbles, and scraping and smoothing;

[0105] (7) placing the "sandwich" structure composite layer obtained in step (6) between graphite plates, and placing it under a hydraulic press at a pressure of 1 MPa and a temperature of 80° C. for 2 h, taking it out, and peeling off the prepared composite film;

[0106] (8) The composite membrane prepared in step (7) is placed in a 20% trimethylamine aqueous solution for quaternization treatment for 48 hours, removed, and repeatedly rinsed with deionized water for 10 times, and then immersed in a 0.1 mol / L sodium hydroxide solution to fully alkalize for 48 hours, and then taken out and rinsed off the alkaline solution on the surface of the membrane with deionized water, and then thoroughly dried at 60°C to obtain a high-strength ultra-thin composite membrane.

[0107] Comparative Example 4:

[0108] (1) Weigh 6.99 g (0.01 mol) of MIL-101(Fe) and place it in a Schlenk tube equipped with a constant pressure funnel. Continuously evacuate the tube to maintain a vacuum degree of -1.0 MPa and lower the temperature of the Schlenk tube to -10°C.

[0109] (2) Weigh 3.04 g (0.02 mol) of 1-hexyl-4,5-dimethylimidazolium salt and 1.525 g (0.01 mol) of p-chloromethylstyrene, inject the mixture into the Schlenk tube of step (1), and continue stirring for 96 h to allow 1-hexyl-4,5-dimethylimidazolium salt and p-chloromethylstyrene to react fully;

[0110] (3) After the reaction is completed, the precipitate is centrifuged and then dried at 60° C. for 8 h to allow the vinyl groups to self-crosslink and form imidazolium salt crystals;

[0111] (4) At 25°C, 10 g of polyvinylbenzyl chloride was dissolved in 100 mL of N-methylpyrrolidone and stirred for 24 h to obtain a polymer solution with a concentration of 0.1 g / mL. Then, 1 g of N,N,N′,N′-tetramethylmethanediamine was added and stirred for 30 s.

[0112] (5) Weigh 0.2 g of imidazolium salt crystals and add them to step (4), stir evenly to obtain a casting solution for later use;

[0113] (6) Laying a polyethylene porous film with a thickness of 10 μm and a porosity of 80% on the surface of the release polyester film, then pouring the casting liquid obtained in step (5) on the surface of the polyethylene porous film, and then laying a layer of release polyester film on the surface of the casting liquid to form a "sandwich" structure, using a flat plate to drive out bubbles, and scraping and leveling;

[0114] (7) drying the "sandwich" structure composite layer obtained in step (6), and peeling off the prepared composite film;

[0115] (8) The composite membrane prepared in step (7) is placed in a 20% trimethylamine aqueous solution for quaternization treatment for 48 hours, removed, and repeatedly rinsed with deionized water for 10 times, and then immersed in a 0.1 mol / L sodium hydroxide solution to fully alkalize for 48 hours, and then taken out and rinsed off the alkaline solution on the surface of the membrane with deionized water, and then thoroughly dried at 60°C to obtain a high-strength ultra-thin composite membrane.

[0116] Comparative Example 5:

[0117] (1) At 25°C, 10 g of polyvinylbenzyl chloride was dissolved in 100 mL of N-methylpyrrolidone and stirred for 24 h to obtain a polymer solution with a concentration of 0.1 g / mL. Then, 1 g of N,N,N′,N′-tetramethylmethanediamine was added and stirred for 30 s.

[0118] (2) Weigh 6.99 g (0.01 mol) of MIL-101 (Fe) and add it to step (1), stir evenly to obtain a casting solution for later use;

[0119] (3) Laying a polyethylene porous film with a thickness of 10 μm and a porosity of 80% on the surface of the release polyester film, then pouring the casting liquid obtained in step (2) on the surface of the polyethylene porous film, and then laying a layer of release polyester film on the surface of the casting liquid to form a "sandwich" structure, using a flat plate to drive out bubbles, and scraping and leveling;

[0120] (4) placing the "sandwich" structure composite layer obtained in step (3) between graphite plates, and placing it under a hydraulic press at a pressure of 1 MPa and a temperature of 80° C. for 2 h, taking it out, and peeling off the prepared composite film;

[0121] (5) The composite membrane prepared in step (4) is placed in a 20% trimethylamine aqueous solution for quaternization treatment for 48 hours, removed, and repeatedly rinsed with deionized water for 10 times, and then immersed in a 0.1 mol / L sodium hydroxide solution to fully alkalize for 48 hours, and then taken out and rinsed off the alkaline solution on the membrane surface with deionized water, and then thoroughly dried at 60°C to obtain a high-strength ultra-thin composite membrane.

[0122] Comparative Example 6:

[0123] (1) Weigh 6.99 g (0.01 mol) of MIL-101(Fe) and place it in a Schlenk tube equipped with a constant pressure funnel. Continuously evacuate the tube to maintain a vacuum degree of -1.0 MPa and lower the temperature of the Schlenk tube to -10°C.

[0124] (2) Weigh 1.525 g (0.01 mol) of the mixed solution of p-chloromethylstyrene and inject it into the Schlenk tube in step (1), and continue stirring for 96 h;

[0125] (3) At 25°C, 10 g of polyvinylbenzyl chloride was dissolved in 100 mL of N-methylpyrrolidone and stirred for 24 h to obtain a polymer solution with a concentration of 0.1 g / mL. Then, 1 g of N,N,N′,N′-tetramethylmethanediamine was added and stirred for 30 s.

[0126] (4) Weighing the compound prepared in step (2) and adding it to step (3), stirring evenly to obtain a casting solution for later use;

[0127] (5) Laying a polyethylene porous film with a thickness of 10 μm and a porosity of 80% on the surface of the release polyester film, then pouring the casting liquid obtained in step (4) on the surface of the polyethylene porous film, and then laying a layer of release polyester film on the surface of the casting liquid to form a "sandwich" structure, using a flat plate to drive out bubbles, and scraping and smoothing;

[0128] (6) placing the "sandwich" structure composite layer obtained in step (5) between graphite plates, and placing it under a hydraulic press at a pressure of 1 MPa and a temperature of 80° C. for 2 h, taking it out, and peeling off the prepared composite film;

[0129] (7) The composite membrane prepared in step (6) is placed in a 20% trimethylamine aqueous solution for quaternization treatment for 48 hours, removed, and repeatedly rinsed with deionized water for 10 times, and then immersed in a 0.1 mol / L sodium hydroxide solution to fully alkalize for 48 hours, and then taken out and rinsed off the alkaline solution on the membrane surface with deionized water, and then thoroughly dried at 60°C to obtain a high-strength ultra-thin composite membrane.

[0130] Comparative Example 7:

[0131] (1) Weigh 6.99 g (0.01 mol) of MIL-101(Fe) and place it in a Schlenk tube equipped with a constant pressure funnel. Continuously evacuate the tube to maintain a vacuum degree of -1.0 MPa and lower the temperature of the Schlenk tube to -10°C.

[0132] (2) Weigh 3.04 g (0.02 mol) of 1-hexyl-4,5-dimethylimidazolium salt and 0.38125 g (0.0025 mol) of p-chloromethylstyrene, inject the mixture into the Schlenk tube of step (1), and continue stirring for 96 h to allow 1-hexyl-4,5-dimethylimidazolium salt to react fully with p-chloromethylstyrene;

[0133] (3) After the reaction is completed, the precipitate is centrifuged and then dried at 60° C. for 8 h to allow the vinyl groups to self-crosslink and form imidazolium salt crystals;

[0134] (4) At 25°C, 10 g of polyvinylbenzyl chloride was dissolved in 100 mL of N-methylpyrrolidone and stirred for 24 h to obtain a polymer solution with a concentration of 0.1 g / mL. Then, 1 g of N,N,N′,N′-tetramethylmethanediamine was added and stirred for 30 s.

[0135] (5) Weigh 0.1 g of imidazolium salt crystals and add them to step (4), stir evenly to obtain a casting solution for later use;

[0136] (6) Laying a polyethylene porous film with a thickness of 10 μm and a porosity of 80% on the surface of the release polyester film, then pouring the casting liquid obtained in step (5) on the surface of the polyethylene porous film, and then laying a layer of release polyester film on the surface of the casting liquid to form a "sandwich" structure, using a flat plate to drive out bubbles, and scraping and leveling;

[0137] (7) placing the "sandwich" structure composite layer obtained in step (6) between graphite plates, and placing it under a hydraulic press at a pressure of 1 MPa and a temperature of 80° C. for 2 h, taking it out, and peeling off the prepared composite film;

[0138] (8) The composite membrane prepared in step (7) is placed in a 20% trimethylamine aqueous solution for quaternization treatment for 48 hours, removed, and repeatedly rinsed with deionized water for 10 times, and then immersed in a 0.1 mol / L sodium hydroxide solution to fully alkalize for 48 hours, and then taken out and rinsed off the alkaline solution on the surface of the membrane with deionized water, and then thoroughly dried at 60°C to obtain a high-strength ultra-thin composite membrane.

[0139] Comparative Example 8:

[0140] (1) Weigh 6.99 g (0.01 mol) of MIL-101(Fe) and place it in a Schlenk tube equipped with a constant pressure funnel. Continuously evacuate the tube to maintain a vacuum degree of -1.0 MPa and lower the temperature of the Schlenk tube to -10°C.

[0141] (2) Weigh 3.04 g (0.02 mol) of 1-hexyl-4,5-dimethylimidazolium salt and 9.15 g (0.06 mol) of p-chloromethylstyrene, inject the mixture into the Schlenk tube of step (1), and continue stirring for 96 h to allow 1-hexyl-4,5-dimethylimidazolium salt and p-chloromethylstyrene to react fully;

[0142] (3) After the reaction is completed, the precipitate is centrifuged and then dried at 60° C. for 8 h to allow the vinyl groups to self-crosslink and form imidazolium salt crystals;

[0143] (4) At 25°C, 10 g of polyvinylbenzyl chloride was dissolved in 100 mL of N-methylpyrrolidone and stirred for 24 h to obtain a polymer solution with a concentration of 0.1 g / mL. Then, 1 g of N,N,N′,N′-tetramethylmethanediamine was added and stirred for 30 s.

[0144] (5) Weigh 0.5 g of imidazolium salt crystals and add them to step (4), stir evenly to obtain a casting solution for later use;

[0145] (6) Laying a polyethylene porous film with a thickness of 10 μm and a porosity of 80% on the surface of the release polyester film, then pouring the casting liquid obtained in step (5) on the surface of the polyethylene porous film, and then laying a layer of release polyester film on the surface of the casting liquid to form a "sandwich" structure, using a flat plate to drive out bubbles, and scraping and leveling;

[0146] (7) placing the "sandwich" structure composite layer obtained in step (6) between graphite plates, and placing it under a hydraulic press at a pressure of 1 MPa and a temperature of 80° C. for 2 h, taking it out, and peeling off the prepared composite film;

[0147] (8) The composite membrane prepared in step (7) is placed in a 20% trimethylamine aqueous solution for quaternization treatment for 48 hours, removed, and repeatedly rinsed with deionized water for 10 times, and then immersed in a 0.1 mol / L sodium hydroxide solution to fully alkalize for 48 hours, and then taken out and rinsed off the alkaline solution on the surface of the membrane with deionized water, and then thoroughly dried at 60°C to obtain a high-strength ultra-thin composite membrane.

[0148] Compared with Examples 1-3, no metal organic framework crystalline material is added in Comparative Example 1, no cross-linking agent is added in Comparative Example 2, no ultra-thin polyolefin porous membrane is added in Comparative Example 3, and no graphite plate heating cross-linking is used in Comparative Example 4. Comparative Examples 5 and 6 lack imidazolium salt materials, respectively, and no imidazolium salt crystals are formed. The alkaline anion exchange membranes of the embodiments and comparative examples are tested for conductivity and tensile strength, and the results are shown in Table 1. It can be seen from Table 1 that the conductivity and tensile strength of the exchange membrane prepared by the present invention are both relatively high, and the effects are better than those of the comparative examples. This is because imidazolium salts are introduced into AEMs in two forms of "fixation" by covalent cross-linking and metal organic framework, respectively, to increase the number of functional groups in the membrane, and to construct regular and orderly ion transfer channels with the help of regular lattice structures, thereby reducing ion transfer resistance; polyvinyl benzyl chloride (PVBC) and a cross-linking agent are used for cross-linking reaction to form a skeleton structure, thereby improving the stability and mechanical strength of the ion exchange membrane.

[0149] In Comparative Example 1, no metal organic framework crystal material was added, and imidazolium salt was directly added in the preparation of the anion exchange membrane. As the operation time increases, the imidazolium salt will be lost, resulting in a decrease in its performance;

[0150] In Comparative Example 2, no crosslinking agent was added. When the graphite plate was heated and pressurized, the composite membrane was only dried without forming a crosslinked structure. On the one hand, its mechanical strength and stability were reduced. On the other hand, the crosslinking agent contained functional groups, which reduced the ion conductivity of the composite membrane.

[0151] In Comparative Example 3, no polyolefin porous membrane was used as a support, and the prepared composite membrane had high electrochemical performance, but the mechanical strength and stability were the worst.

[0152] In Comparative Example 4, the graphite plate was not heated and cross-linked under pressure, and the prepared composite membrane had a low degree of cross-linking, resulting in the loss of functionalized compounds such as imidazolium salt crystal materials, thereby reducing its performance. Moreover, after an air tightness test (pressure difference method), the pressure drop of the membrane prepared in Comparative Example 4 was reduced from 0.5 MPa to 0.2 MPa in 60 minutes, while the pressure drop of the membrane prepared in Example 1 was reduced from 0.5 MPa to 0.49 MPa in 60 minutes, indicating that the air tightness structure of the membrane prepared in Comparative Example 4 is worse than that of Example 1.

[0153] The composite membranes prepared in Comparative Examples 5 and 6 have poor performance due to the lack of ion transporting compounds.

[0154] In Comparative Example 7, the amount of p-chloromethylstyrene added is small, and in Comparative Example 8, the amount of p-chloromethylstyrene added is too large. The electrochemical performance and mechanical strength of the composite membrane finally prepared are poor in comprehensive evaluation, and may only meet one of the requirements, while the requirements of the other are seriously reduced.

[0155] Table 1 Conductivity and tensile strength of anion exchange membrane

[0156] Case <![CDATA[Conductivity / S﹒cm -1 > Tensile strength / MPa Example 1 0.068 32 Example 2 0.071 34 Example 3 0.069 33 Example 4 0.056 29.2 Example 5 0.065 32.1 Comparative Example 1 0.021 28 Comparative Example 2 0.063 13 Comparative Example 3 0.075 2.2 Comparative Example 4 0.065 15.7 Comparative Example 5 0.034 15 Comparative Example 6 0.030 16 Comparative Example 7 0.023 19 Comparative Example 8 0.055 14

[0157] The anion exchange membranes prepared in the examples of the present invention and the comparative examples were immersed in 1.0 mol / L sodium hydroxide solution to test the alkali stability, and the results are shown in Table 2. During the preparation of AEMs, the alkali stability of AEMs that were not covalently cross-linked was significantly poor, indicating that the cross-linking treatment had a great influence on the alkali tolerance of the composite membrane.

[0158] Table 2 Anion exchange membrane quality residual rate test

[0159]

[0160]

[0161] This specific embodiment is merely an explanation of the present invention and is not a limitation of the present invention. After reading this specification, those skilled in the art may make non-creative modifications to the present embodiment as needed. However, as long as they are within the scope of the claims of the present invention, they are protected by the patent law.

Claims

1. A composite film, characterized in that: The composite membrane is formed by injecting a casting solution onto the surface of a polyolefin porous membrane, wherein the casting solution is composed of imidazolium salt crystals, polyvinyl benzyl chloride (PVBC) and a crosslinking agent, wherein the imidazolium salt crystals are: a substance formed by using 4,5-disubstituted imidazolium salt as a raw material and fixed in a metal organic framework crystal material; the 4,5-disubstituted imidazolium salt is a 1-butyl C4, C5 disubstituted imidazolium salt or a 1-hexyl C4, C5 disubstituted imidazolium salt; The raw materials of the imidazolium salt crystals also include p-chloromethylstyrene; The molar ratio of the metal organic framework crystal material, 4,5-disubstituted imidazolium salt and p-chloromethylstyrene is 1:2:0.5-5.

2. The composite membrane according to claim 1, characterized in that: The metal organic framework crystal material is a cubic structure with micropores, and the pore size of the metal organic framework crystal material is 100-1000nm; the metal organic framework crystal material is one or more of MIL-101 (Fe), MIL-101 (Cr), MIL-53 (Cr), MIL-53 (Fe), ZIF-6, ZIF-8, and ZIF-10; The cross-linking agent is one or more of N,N,N′,N′-tetramethylmethanediamine (TMMDA), N,N,N′,N′-tetramethylethylenediamine (TMEDA), and N,N,N′,N′-tetramethyl-1,6-hexanediamine (TMHDA); The polyolefin porous membrane is any one of a polyethylene porous membrane and a polypropylene porous membrane. The polyolefin porous membrane has a thickness of 5-10 μm, a porosity of 45-80%, and an average pore size of 0.1-0.3 μm.

3. The composite membrane according to claim 2, characterized in that: The imidazolium salt crystal is synthesized by a low temperature vacuum method, comprising the following steps: (1) placing the metal organic framework crystal material in a vacuum state; (2) under low temperature conditions, mixing a mixture of 4,5-disubstituted imidazolium salt and p-chloromethylstyrene with a metal organic framework crystal material, and stirring to react; (3) Centrifugation and drying to obtain imidazolium salt crystals.

4. The composite membrane according to claim 3, characterized in that: The vacuum degree of the vacuum state in step (1) is -0.6 to -1.0 MPa; the low-temperature reaction temperature in step (2) is -20 to -10°C, and the stirring time is 48 to 96 hours; the precipitate drying temperature in step (3) is 60 to 100°C, and the drying time is 4 to 8 hours; steps (1) and (2) are in the same vacuum state.

5. The composite membrane according to claim 1, characterized in that: The C4 and C5 substituents in the 1-butyl C4, C5 disubstituted imidazolium salt or the 1-hexyl C4, C5 disubstituted imidazolium salt are R1 and R2, respectively, and the R1 and R2 are any one of methyl, ethyl, propyl, butyl, isopropyl, and tert-butyl. The structures of the 1-butyl C4, C5 disubstituted imidazolium salt or the 1-hexyl C4, C5 disubstituted imidazolium salt are as follows: 1-Butyl C4, C5 disubstituted imidazolium salt 1-hexyl C4, C5 disubstituted imidazolium salt.

6. The method for preparing the composite film according to any one of claims 1 to 5, characterized in that: The film-making process includes the following steps: A. Weigh polyvinyl benzyl chloride (PVBC) and add it to a high boiling point solvent, and stir at room temperature to obtain a PVBC solution; the high boiling point solvent is one of N-methylpyrrolidone, N,N-dimethylformamide, tetrahydrofuran, and dimethyl sulfoxide; B. adding a crosslinking agent to the polyvinyl benzyl chloride (PVBC) solution of step A, stirring at room temperature to obtain a mixed solution; C. Take imidazolium salt crystals, add them to the mixed solution in step B, and stir to form a film casting solution; D. Lay the polyolefin porous membrane on the release polyester film, add the casting liquid to the surface of the porous membrane, and then lay a layer of release polyester film on the surface of the casting liquid to drive out bubbles and scrape and smooth it to obtain a composite layer with a "sandwich" structure; E. placing the composite layer of the "sandwich" structure prepared in step D between graphite plates, taking it out after heating and hot pressing to obtain a composite film, and peeling the prepared composite film from the graphite plates; F. Soaking the composite membrane prepared in step E in a quaternary ammonium aqueous solution for quaternary ammonium treatment, and then rinsing with deionized water; G. Soak the composite membrane prepared in step F in an alkaline solution, then wash it with deionized water for more than 5 times, and dry it to obtain a high-strength ultra-thin composite membrane.

7. The preparation method according to claim 6, characterized in that: The concentration of the PVBC solution in step A is 0.1-0.5 g / mL; the mass ratio of the crosslinking agent to PVBC in step B is 0.1-0.5:1; and the amount of the imidazolium salt crystal added in step C is 2-10% of the mass of the PVBC.

8. The preparation method according to claim 6, characterized in that: In step A, the room temperature is 25° C., and the stirring and dissolving time is 24-48 h; in step B, the stirring time is 30-60 s; in step C, the stirring time is 24-48 h; in step E, the heating temperature is 80-100° C., the heating time is 1-2 h, and the pressure is 1-3 MPa.

9. The preparation method according to claim 6, characterized in that: The quaternization solution in step F is any one of a 20-40wt% triethylamine aqueous solution or a 20-40wt% trimethylamine aqueous solution; the quaternization time is 48-72h; the alkaline solution is one of sodium hydroxide or potassium hydroxide, the concentration of the alkaline solution is 0.1-2mol / L, the soaking time is 24-48h, and the drying temperature is 50-60°C.

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