Method for preparing proton exchange membrane based on ePTFE

Abstract

The invention belongs to the technical field of proton exchange membrane fuel cells, and particularly relates to a preparation method of an expanded polytetrafluoroethylene (ePTFE) reinforced fluorine-containing copolymer proton exchange membrane. According to the preparation method, ePTFE is taken as a reinforcing phase, in-situ polymerization is carried out on a perfluorovinyl sulfonyl fluoride monomer, fluoroolefin and a vinyl ether monomer, the polymer membrane is prepared through free radical polymerization reaction under the heating or illumination condition, and the polymer membrane subjected to hydrolytic acidification can be directly used as a proton exchange membrane. The method has the advantages of simplicity and convenience in operation, mild reaction conditions, low cost and the like, the prepared proton exchange membrane is good in mechanical property, high in proton conductivity and good in stability, and a new way is provided for synthesis of the proton exchange membrane.

Description

Technical Field

[0001] This invention belongs to the field of proton exchange membranes for fuel cells, specifically relating to a method for preparing a fluoropolymer proton exchange membrane reinforced with expanded polytetrafluoroethylene (ePTFE). Background Technology

[0002] Currently, approximately 80% of global energy consumption still relies on traditional fossil fuels such as coal, oil, and natural gas. This energy structure not only poses the threat of resource depletion but also triggers severe environmental pollution and carbon emissions problems, becoming a focal point of common concern for countries worldwide. To address climate change and promote a green transformation of the energy structure, developing efficient and clean renewable energy technologies has become an urgent priority. Among numerous energy conversion devices, proton exchange membrane fuel cells (PEMFCs) based on hydrogen energy systems stand out due to their high energy conversion efficiency, environmental friendliness, mild operating conditions, and rapid start-up. Nature ,2001, 414 (345) Its outstanding advantages have attracted widespread attention from the scientific research and industry communities.

[0003] The technological development of PEMFCs can be traced back to the 1960s. At that time, General Electric (GE) pioneered the development of proton exchange membrane fuel cell systems and collaborated with NASA to apply them to manned space missions under the Gemini program. Journal of the Electrochemical Society ,1960, 107 (131), which opened up the engineering applications of PEMFC in specialized fields. In 1962, DuPont developed the landmark perfluorosulfonic acid proton exchange membrane—Nafion. ® This membrane, while maintaining high proton conductivity, significantly improved chemical stability and mechanical strength, greatly extending the lifespan of PEMFCs and laying the foundation for subsequent technological development. Subsequently, numerous international companies and research institutions, including 3M, Solvay, Gore, Fumatech, and AGC, invested in the research and improvement of proton exchange membranes, resulting in significant progress in PEMFC technology.

[0004] However, after more than sixty years of development, DuPont Nafion is still the leading manufacturer of proton exchange membranes for commercial application. ® The market is dominated by perfluorosulfonic acid (PFMS) series and their derivatives, with relatively simple structural types. While these PFMS membranes possess excellent proton conductivity and stability, their synthesis process is complex, relying on perfluorinated monomers and demanding polymerization conditions, resulting in high manufacturing costs and limiting the widespread adoption of PEMFCs in civilian applications. Furthermore, the proton conductivity of PFMS membranes decreases significantly at high temperatures (80 °C), restricting the flexibility of battery operating conditions.

[0005] To overcome the limitations of perfluorosulfonic acid membranes, researchers have proposed several alternatives and are gradually exploring commercial applications, mainly including partially fluorinated proton exchange membranes, fluorine-free proton exchange membranes, and composite proton exchange membranes. Journal of Membrane Science ,2003, 226 Partially fluorinated proton exchange membranes reduce costs by decreasing fluorine content while retaining some fluorine atoms to improve corrosion resistance. Regarding fluorine-free membranes, researchers have sulfonated high-performance engineering plastics such as polyaryletherketone (PEEK), polyphenylene sulfide (PPS), and polyimide (PI) to introduce sulfonic acid groups, thereby endowing them with proton conductivity. These materials exhibit excellent thermal stability and mechanical strength, and are relatively inexpensive, showing promising application prospects. However, limited by the efficiency of the sulfonation reaction and the uniformity of group distribution, the proton conductivity of these membranes is generally lower than that of perfluorosulfonic acid membranes. Furthermore, the introduction of hydrophilic sulfonic acid groups can easily lead to excessive swelling of the membrane in the hydrated state, resulting in decreased dimensional stability and affecting the long-term durability of the battery.

[0006] Therefore, developing novel proton exchange membranes that combine high performance, high stability, and low cost has become a key issue in promoting the further development of PEMFC technology. Summary of the Invention

[0007] In view of this, and in view of the problems existing in the prior art, the first technical objective of the present invention is to provide a fluorinated copolymer with high proton conductivity.

[0008] To achieve the above objectives, the present invention adopts the following technical solution: A proton-conducting fluoropolymer has the unit structure shown in formula (1): The fluoroolefin comonomer can be tetrafluoroethylene, trifluorochloroethylene, vinylidene fluoride, perfluoromethyl vinyl ether, perfluoro-n-propyl vinyl ether, or perfluoro-n-octyl vinyl ether; R 3 It is n-propyl, n-octyl, phenyl, benzyl, p-methoxyphenyl, p-cyanophenyl, etheroxy; R 4 It refers to ether chains, fluorinated ether chains, and alkane chains containing reactive sites with 1 to 10 carbon units. The repeating unit x is a positive integer from 20 to 80, y is a positive integer from 20 to 80, n is a positive integer from 40 to 160, and m is a positive integer from 2 to 20.

[0009] Furthermore, the perfluorovinyl ether sulfonyl fluoride monomer in the comonomer is preferably a perfluorovinyl ether sulfonyl fluoride monomer, and its structural formula is shown in formula (2): Furthermore, the fluoroolefin and vinyl ether monomers in the comonomer are preferably fluoroolefin and vinyl ether monomers, and their structural formulas are shown in formula (3): Among them, R 3 The preferred monomers are n-propyl, n-octyl, phenyl, benzyl, p-methoxyphenyl, p-cyanophenyl, and etheroxy, where n is a positive integer from 2 to 9.

[0010] Furthermore, the divinyl crosslinking agent in the comonomer is preferably a divinyl crosslinking agent, and its structural formula is shown in formula (4): Among them, R 4 Preferred carbon units are divinyl ether chains, fluorinated ether chains, and alkane chains with a carbon number of 1 to 10.

[0011] Furthermore, the preparation method specifically includes the following steps: (1) Mix the reactants evenly in a glass bottle and remove oxygen from the reaction system by bubbling or freezing-vacuuming-venting. (2) The polymerization reaction is started by heat or light, and the reaction time is 6 to 48 hours; (3) After the reaction is completed, the obtained polymer is precipitated and washed several times with a preferred organic solvent. Then it is soaked in a 5-50% lithium hydroxide aqueous solution for hydrolysis for 2-48 hours. After hydrolysis, the polymer is thoroughly rinsed with deionized water and then acidified in a 1-2 mol / L sulfuric acid aqueous solution for 1-24 hours. After acidification, it is repeatedly soaked and rinsed with deionized water 4-8 times. Finally, the polymer is placed in a vacuum drying oven at 60-100 °C and dried for 6-24 hours to obtain the fluorinated copolymer.

[0012] Furthermore, the reaction raw materials include preferred perfluorovinyl ether sulfonyl fluoride monomers, preferred fluoroolefin monomers, preferred vinyl ether monomers, preferred divinyl ether crosslinking agents, preferred initiators, and preferred solvents.

[0013] Furthermore, the preferred molar ratio of the perfluorovinyl ether sulfonyl fluoride monomer, fluoroolefin monomer, vinyl ether monomer, and divinyl ether crosslinking agent is 1:1:1:0.1~0.4; Further, in step (1), the thermal initiator is one or more combinations of azo compounds, organic peroxides, and persulfates; the photoinitiator is one or more combinations of acetophenones, acylphosphine oxides, and thioxanthones; and the solvent is one or more combinations of dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, n-butyl carbonate, ethylene carbonate, ethyl acetate, tert-butyl acetate, anisole, and acetonitrile.

[0014] Further, in step (1), the preferred initiator content is 0.5 to 5 mol of the preferred perfluoroolefin ether sulfonyl fluoride monomer, and the preferred perfluorovinyl ether sulfonyl fluoride monomer concentration in the preferred solvent is 0.2 to 2 mol / L.

[0015] Furthermore, the thermal initiation temperature is 30~80 °C, the wavelength of the light source for the photoreaction is 380~700 nm, and the light intensity is 5~50 W cm⁻¹. -2 .

[0016] The second technical objective of this invention is to provide a method for preparing an ePTFE-reinforced fluorinated copolymer proton exchange membrane.

[0017] The fluorinated copolymer obtained by the first technical objective can serve as the base phase of a composite proton exchange membrane to provide proton conduction capability, while expanded polytetrafluoroethylene (ePTFE) serves as the reinforcing phase to provide mechanical properties.

[0018] To achieve the above objectives, the present invention adopts the following technical solution: A composite proton exchange membrane is obtained by in-situ polymerization on expanded polytetrafluoroethylene (ePTFE) using a free radical copolymerization reaction of preferred perfluorovinyl ether sulfonyl fluoride monomers, preferred fluoroolefins, preferred vinyl ether monomers, and preferred divinyl ether crosslinking agent comonomers.

[0019] Compared with the prior art, the present invention has the following beneficial effects: This invention discloses a method for preparing a proton exchange membrane based on ePTFE. The membrane uses a preferred perfluorovinyl ether sulfonyl fluoride monomer, fluoroolefin, vinyl ether monomer, and divinyl ether crosslinking agent as the core comonomer system. It is prepared by in-situ polymerization of ePTFE base membrane via thermal or photo-initiated free radical copolymerization. The perfluorovinyl ether sulfonyl fluoride monomer has excellent chemical stability and proton conduction sites, the fluoroolefin provides mechanical properties and thermal stability, the vinyl ether monomer can balance polymerization activity and proton conduction assistance, the divinyl crosslinking agent can construct a three-dimensional crosslinking network to improve membrane structural stability, and ePTFE provides good dimensional stability and mechanical properties. Through the synergistic effect of each component, the fluorinated proton exchange membrane has good proton conductivity and mechanical stability.

[0020] This preparation process has significant advantages: the operation does not require complex pre-forming or post-processing steps, and only requires three core operations: raw material mixing, uniform coating, and initiation, making the process simple and easy to scale up; the reaction conditions are mild, requiring no high temperature or high pressure environment, and no special corrosion-resistant or high temperature-resistant equipment, which not only reduces production energy consumption but also reduces equipment investment costs; moreover, the selected comonomers are all industrial-grade and readily available raw materials, which do not require multiple complex modification synthesis steps, and the raw material utilization rate in the in-situ polymerization process exceeds 90%, further controlling the overall raw material cost.

[0021] From the perspective of product performance, the prepared fluorinated copolymer, due to the fluorocarbon backbone and proton-conducting active groups (such as sulfonic acid groups or ether oxygen bonds) contained in its molecular structure, can construct a highly efficient proton conduction channel, exhibiting excellent proton conduction ability under conditions of 80 ℃ and 100% relative humidity. As a reinforcing substrate, the ePTFE membrane itself possesses outstanding mechanical properties (tensile strength up to 30~40 MPa, elongation at break up to 250%) and dimensional stability (linear swelling rate ≤5% within the range of 40~90% relative humidity), effectively resisting the mechanical stress and humidity fluctuations during fuel cell operation. Furthermore, this invention avoids the interlayer delamination problem and material loss problem that are prone to occur in traditional composite membranes through in-situ crosslinking composite process. Attached Figure Description

[0022] To clearly illustrate the technical solutions, molecular structures, and preparation process details of the embodiments of the present invention, the following is a brief description of the drawings used in the embodiments of the present invention: The following drawings are only some embodiments of the present invention, and their purpose is to intuitively show the molecular structure of the fluorinated copolymer, the microstructure of the ePTFE reinforced composite film, and the key steps of the in-situ polymerization process; those skilled in the art can also obtain other drawings based on the provided drawings without creative effort.

[0023] Figure 1 This is a flowchart of the copolymer synthesis route for Example 1.

[0024] Figure 2 This is a flowchart of the copolymer synthesis route for Example 4.

[0025] Figure 3 This is a schematic diagram illustrating the preparation of an ePTFE-reinforced fluorinated copolymer proton exchange membrane.

[0026] Figure 4 The attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectra of the composite film and copolymer before and after application example 1 are shown.

[0027] Figure 5 The results are for the characterization of the energy dispersive spectrometer (EDS) in Application Example 1.

[0028] Figure 6The results of small-angle X-ray scattering (SAXS) characterization are shown in Application Example 1.

[0029] Figure 7 This is a real-life photo of the water contact angle test and composite membrane used in Application Example 1.

[0030] Figure 8 Thermogravimetric analysis (TGA) and its differential curve are shown in Application Example 2.

[0031] Figure 9 The stress-strain curve is shown in the tensile test diagram for application example 2.

[0032] Figure 10 This is a graph showing the relationship between temperature and proton conductivity in application example 3.

[0033] Figure 11 This is a diagram verifying the durability of the composite membrane after immersion at 80 ℃. Detailed Implementation

[0034] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0035] The term "embodiment" used herein, as an example, is not necessarily to be construed as superior to or better than other embodiments. Performance testing in the embodiments of this application, unless otherwise specified, employs conventional testing methods in the art. It should be understood that the terminology used in this application is merely for describing particular implementations and is not intended to limit the scope of this disclosure.

[0036] Unless otherwise stated, the technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains; other experimental methods and technical means not specifically mentioned herein refer to experimental methods and technical means commonly used by one of ordinary skill in the art.

[0037] To better illustrate the content of this application, numerous specific details are provided in the following detailed embodiments. Those skilled in the art should understand that this application can be implemented even without certain specific details. In the embodiments, some methods, means, instruments, and devices well known to those skilled in the art are not described in detail in order to highlight the main points of this application.

[0038] Without conflict, the technical features disclosed in the embodiments of this application can be combined arbitrarily, and the resulting technical solution belongs to the content disclosed in the embodiments of this application.

[0039] This invention discloses an expanded polytetrafluoroethylene (ePTFE) reinforced fluorinated proton exchange membrane, which is prepared by in-situ copolymerization of an ePTFE base membrane. The copolymerization system comprises a preferred perfluorovinyl ether sulfonyl fluoride monomer, a fluoroolefin, a vinyl ether monomer, and a divinyl crosslinking agent. The perfluorovinyl ether sulfonyl fluoride monomer exhibits excellent chemical stability and provides proton conduction sites; the fluoroolefin helps improve the membrane's mechanical properties and thermal stability; the vinyl ether monomer maintains good polymerization activity while also assisting in proton conduction; and the divinyl crosslinking agent can construct a three-dimensional crosslinking network, thereby enhancing the membrane's structural stability. The synergistic effect of these components results in a fluorinated proton exchange membrane with both excellent proton conductivity and mechanical stability.

[0040] To better understand the present invention, the following embodiments are provided for further detailed description of the invention, but they should not be construed as limiting the invention. Any non-essential improvements and adjustments made by those skilled in the art based on the above description are also considered to fall within the protection scope of the present invention.

[0041] Part 1: Synthesis of Fluorinated Copolymers Example 1 Thermally initiated polymerization was used to prepare a perfluoro(4-methyl-3,6-dioxa-7-octene) sulfonyl fluoride-vinylidene fluoride-n-propyl vinyl ether-1,4-butanediol divinyl ether copolymer. Perfluoro(4-methyl-3,6-dioxa-7-octene)sulfonyl fluoride, n-propyl vinyl ether, and 1,4-butanediol divinyl ether were added to a reactor in a molar ratio of 5:5:5:1 and mixed. The mixture was then dissolved in 5 mL of diethyl carbonate, with 2 mmol of perfluoro(4-methyl-3,6-dioxa-7-octene)sulfonyl fluoride added. 2 mmol of vinylidene fluoride monomer was added, followed by the addition of benzoyl peroxide at a molar ratio of 200:1 to the initiator. The mixture was heated to 70 °C to initiate polymerization, and the reaction was allowed to proceed for 24 hours. Nuclear magnetic resonance fluorine spectroscopy showed a monomer conversion rate of 98% for the perfluorovinyl ether sulfonyl fluoride. After the reaction, unreacted monomers and solvents were removed by washing with methanol. The product was then transferred to a vacuum drying oven at 60 °C and dried to obtain a milky white viscous solid (structure shown in Figure 1). Figure 1 (As shown).

[0042] Example 2 Photopolymerization was used to prepare a crosslinked copolymer of perfluoro(4-methyl-3,6-dioxa-7-octene) sulfonic acid-perfluoron-octyl vinyl ether-n-butyl vinyl ether-1,4-butanediol divinyl ether. Perfluoro(4-methyl-3,6-dioxa-7-octene)sulfonyl fluoride, perfluoron-octyl vinyl ether, n-butyl vinyl ether, and 1,4-butanediol divinyl ether were added to a reactor in a molar ratio of 5:5:5:1 and mixed. The mixture was then dissolved in 5 mL of diethyl carbonate. The amount of perfluoro(4-methyl-3,6-dioxa-7-octene)sulfonyl fluoride added was 2 mmol. 0.01 mmol of benzoin dimethyl ether was added at a molar ratio of perfluoro(4-methyl-3,6-dioxa-7-octene)sulfonyl fluoride to initiator of 200:1. After thorough stirring, the mixture was deoxygenated under light irradiation at a wavelength of 400-700 nm and an intensity of 15 W cm⁻¹. -2 The reaction was carried out overnight under white LED light irradiation. The conversion rate of perfluorinated monomers reached 93% by nuclear magnetic resonance fluorine spectroscopy. After the reaction was completed, unreacted monomers and solvents were removed by washing with methanol, and the product was transferred to a vacuum drying oven at 60 °C to dry, yielding a milky white viscous solid.

[0043] The milky white viscous solid was then immersed in a 5% (w / w) lithium hydroxide aqueous solution for hydrolysis for 6 hours. After hydrolysis, it was thoroughly rinsed with deionized water and then acidified in a 1 mol / L sulfuric acid aqueous solution for 6 hours. After acidification, it was repeatedly immersed and rinsed with deionized water 4 to 8 times. Finally, it was placed in a vacuum drying oven at 60 °C for 24 hours to obtain the target fluorinated copolymer.

[0044] Part Two: Preparation of ePTFE-Reinforced Fluorinated Copolymers Example 3 Photopolymerization was used to prepare a perfluoro(4-methyl-3,6-dioxa-7-octene) sulfonyl fluoride-perfluoron-n-propyl vinyl ether-n-butyl vinyl ether-1,4-butanediol divinyl ether crosslinked copolymer composite film with ePTFE as the reinforcing phase. Perfluoro(4-methyl-3,6-dioxa-7-octene)sulfonyl fluoride, perfluoro-n-propyl vinyl ether, n-butyl vinyl ether, and 1,4-butanediol divinyl ether were added to a reactor in a molar ratio of 5:5:5:1 and mixed. The mixture was then dissolved in 5 mL of diethyl carbonate, with 2 mmol of perfluoro(4-methyl-3,6-dioxa-7-octene)sulfonyl fluoride added. 0.01 mmol of benzoin dimethyl ether was added to the mixture at a molar ratio of 200:1 (perfluoro(4-methyl-3,6-dioxa-7-octene)sulfonyl fluoride to initiator) and mixed thoroughly to obtain a mixed solvent. A pre-cut ePTFE (10 cm × 10 cm × 50 μm) was placed in a mold, and then 1000 μL of the mixed solvent was added to the mold. After the solution fully wetted the membrane, it was irradiated with light at a wavelength of 400-700 nm and an intensity of 15 W / cm². -2The polymerization was initiated by an LED light. After 12 hours of reaction, the composite membrane was removed, thoroughly washed with dichloromethane, and dried in a vacuum oven at 60 °C for 12 hours to obtain the composite membrane.

[0045] Example 4 The composite membrane obtained in Example 3 was immersed in a 5% (w / w) lithium hydroxide aqueous solution for hydrolysis for 6 hours. After hydrolysis, it was thoroughly rinsed with deionized water, and then acidified in a 1 mol / L sulfuric acid aqueous solution for 6 hours. After acidification, it was repeatedly immersed and rinsed with deionized water 4-8 times. Finally, the composite membrane was placed in a vacuum drying oven at 60 °C for 24 hours to obtain the target composite proton exchange membrane (structure as shown). Figure 2 (As shown).

[0046] Part 3: Structural and performance characterization of ePTFE-reinforced fluorinated copolymer proton exchange membranes Application Example 1: Characterization of the Chemical Structure and Surface Properties of Composite Membranes The composite membrane obtained in Example 4 was subjected to attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), energy dispersive spectroscopy (EDS), small-angle X-ray scattering (SAXS), and water contact angle measurements. ATR-FTIR results... Figure 4 This indicates that the composite membrane at 1470 cm⁻¹... -¹ A characteristic absorption peak at 1060 cm⁻¹ was observed, attributed to the S=O asymmetric stretching vibration of the sulfonyl fluoride group. After alkali treatment, the characteristic peak of the sulfonyl fluoride essentially disappeared, while a distinct characteristic absorption peak of the sulfonate group appeared at 1060 cm⁻¹, indicating that the sulfonyl fluoride had been successfully converted into a sulfonic acid group. Figure 5 The EDS surface scan results showed that the polymer was uniformly distributed in the ePTFE matrix without obvious agglomeration. Figure 6 A clear ion cluster scattering signal was observed in the SAXS spectrum, reflecting a microphase separation structure within the membrane. Water contact angle measurement. Figure 7 Further evidence shows that the composite membrane changes from hydrophobic to hydrophilic after treatment, consistent with the introduction of sulfonic acid groups.

[0047] Application Example 2: Evaluation of Thermal Stability and Mechanical Properties of Composite Membranes The thermal stability and mechanical strength of the composite membrane were evaluated using thermogravimetric analysis (TGA) and tensile testing. TGA results. Figure 8 The results show that the 3% thermogravimetric temperature of the composite membrane is 310 °C, which is higher than the conventional operating temperature range of proton exchange membranes, indicating that it has good thermal stability; its differential thermogravimetric (DTG) curve further confirms the successful loading of the polymer in the ePTFE skeleton. Figure 9Tensile test results show that the composite membrane has a Young's modulus comparable to that of commercial Nafion 117, a breaking strength superior to Nafion 117, and an elongation at break of 120%, indicating that it maintains good flexibility while possessing sufficient rigidity, and can meet the mechanical performance requirements of proton exchange membranes in practical applications.

[0048] Application Example 3: Testing of Proton Conductivity and Durability of Composite Membranes According to the national standard GB / T 20042.3-2022, the proton conductivity of the composite membrane was tested. A 2 cm × 3 cm sample was fixed using a special fixture, and impedance spectra (frequency range: 1~2 × 10⁻⁶) were acquired using a CHI660 electrochemical workstation under different temperature and humidity conditions. 6 (Hz, perturbation voltage: 10 mV), and the proton conductivity is calculated using the corresponding formula. Figure 10 The results showed that the proton conductivity reached 87 mS / cm under conditions of 100% relative humidity and room temperature; it further increased to 207 mS / cm at 80 °C. Furthermore, after the composite membrane was continuously immersed in hot water at 80 °C for 180 hours, as... Figure 11 The results show that its proton conductivity retention rate is still as high as 97%, demonstrating excellent long-term hydrothermal stability.

[0049] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for preparing a proton exchange membrane based on ePTFE, characterized in that, It has the structural formula shown in equation (1): 。 2. A method for preparing the fluoropolymer as described in claim 1, characterized in that, The method is based on the free radical copolymerization reaction of perfluorovinyl ether sulfonyl fluoride monomers, fluoroolefins, vinyl ethers, and divinyl ether crosslinking agent comonomers.

3. The preparation method according to claim 2, characterized in that, The structural formula of the perfluorovinyl ether sulfonyl fluoride monomer is shown in formula (2): 。 4. The preparation method according to claim 2, wherein the fluoroolefin monomer and the vinyl ether monomer have the structural formulas shown in formula (3): in, The comonomers of fluoroolefins can be tetrafluoroethylene, trifluorochloroethylene, vinylidene fluoride, perfluoromethyl vinyl ether, perfluoron-propyl vinyl ether, and perfluoron-octyl vinyl ether; R 3 It is n-propyl, n-octyl, phenyl, benzyl, p-methoxyphenyl, p-cyanophenyl, etheroxy; R 4 It refers to ether chains, fluorinated ether chains, and alkane chains containing reactive sites with 1 to 10 carbon units; the repeating unit x is a positive integer from 20 to 80, y is a positive integer from 20 to 80, n is a positive integer from 40 to 160, and m is a positive integer from 2 to 20.

5. The preparation method according to claim 2, wherein the structural formula of the divinyl crosslinking agent is shown in formula (4): Where R 4 It consists of divinyl ether chains, perfluorinated chains, and alkane chains with 2 to 10 carbon units.

6. The preparation method according to any one of claims 2-5, characterized in that, The preparation method specifically includes the following steps: (1) Mix the reactants evenly in a glass bottle and remove oxygen from the reaction system by bubbling or freezing-vacuuming-venting. (2) Under a nitrogen atmosphere, the liquid reaction raw materials are evenly coated on the ePTFE membrane. After standing for 5 minutes to allow the solution to fully wet the membrane, the membrane is transferred to the mold. Add the fluoroolefin monomer in an amount equal to the vinyl ether, and start polymerization by heating or irradiation. The reaction time is 6 to 48 hours. (3) After the reaction is completed, the composite membrane is washed several times with organic solvent and then soaked in 5-50 wt% lithium hydroxide aqueous solution for hydrolysis reaction for 2-48 hours. After being thoroughly rinsed with deionized water, it is acidified with 1-2 mol / L sulfuric acid aqueous solution for 1-24 hours. Then it is repeatedly soaked and rinsed with deionized water 4-8 times and vacuum dried at 60-100 ℃ for 6-24 hours to obtain ePTFE-reinforced fluorinated copolymer proton exchange membrane.

7. The preparation method according to claim 6, characterized in that, The reaction raw materials include perfluorovinyl ether sulfonyl fluoride monomer, fluoroolefin, vinyl ether, divinyl ether crosslinking agent, initiator and solvent; the molar ratio of perfluorovinyl ether sulfonyl fluoride monomer, fluoroolefin, vinyl ether and divinyl ether crosslinking agent is 1:1:1:0.1~0.4, and the molar ratio of perfluorovinyl ether sulfonyl fluoride monomer and initiator is 1:0.005~0.

05.

8. The preparation method according to claim 6, characterized in that, In step (1), the thermal initiator is one or more combinations of azo compounds, organic peroxides, and persulfates; the photoinitiator is one or more combinations of acetophenones, acylphosphine oxides, and thioxanthones; and the solvent used is one or more combinations of dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, n-butyl carbonate, ethylene carbonate, ethyl acetate, tert-butyl acetate, anisole, and acetonitrile.

9. The preparation method according to claim 6, characterized in that, In step (2), the ePTFE film thickness is 30~300μm, the thermal initiation temperature is 30~80℃, the light source wavelength for the photoreaction is 380~700 nm, and the light intensity is 5~50 W cm⁻¹. -2 .

10. The preparation method according to claim 6, characterized in that, The initiator content is 0.5~5 mol of perfluoroolefin ether monomer and the concentration of perfluorovinyl ether sulfonyl fluoride monomer is 0.2~2 mol / L.

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