Preparation method of sulfonated expanded polytetrafluoroethylene composite membrane

Abstract

The invention discloses a preparation method of a sulfonated expanded polytetrafluoroethylene composite membrane, which comprises the following steps: preparing a cellulose-based sulfonation modifier as a sulfonation modification material, combining the cellulose-based sulfonation modifier with an ePTFE base band, and embedding the cellulose-based sulfonation modifier into a pore structure of the PTFE base band to prepare a sulfonated composite ePTFE membrane. And preparing the composite proton exchange membrane by taking the sulfonated composite ePTFE membrane as a reinforcing layer. Through the mode, the expanded polytetrafluoroethylene is sulfonated, the mass transfer capacity of ePTFE is improved, the sulfonated ePTFE is used as a reinforcing layer to prepare the composite proton exchange membrane, and the performance of the composite membrane is enhanced.

Description

Technical Field

[0001] The invention relates to the technical field of composite proton exchange membrane fuel cells, in particular to a method for preparing a sulfonated expanded polytetrafluoroethylene composite membrane. Background Art

[0002] As traditional fossil energy is gradually depleting, new energy is developing rapidly. Proton exchange membrane fuel cells (PEMFC) can directly convert hydrogen energy into electrical energy, have commercial potential value, and are gaining more and more attention. As one of the core components of fuel cells, the performance of proton exchange membrane directly affects the entire fuel cell system.

[0003] Proton exchange membranes are generally based on polytetrafluoroethylene as the skeleton, and the side chains of polytetrafluoroethylene are sulfonic acid groups. The function of proton exchange membranes mainly depends on the sulfonic acid groups on the side chains, which is attributed to the fact that the sulfonic acid groups cluster in the membrane system to form hydrophilic regions. When the membrane is in a water environment, the hydrophilic regions formed by the sulfonic acid groups form a space for proton mass transfer. The richer the sulfonic acid group clusters, the wider the distribution of the hydrophilic regions formed, the higher the proton conduction rate, and the faster the mass transfer performance of the membrane. That is, the sulfonic acid groups play a role in transferring protons during use, so the sulfonic acid groups have a greater impact on the performance.

[0004] Composite proton exchange membranes have stronger durability and service life and can be widely used in the field of fuel cells. Expanded polytetrafluoroethylene (ePTFE) is the main supporting material for composite proton exchange membranes. Thanks to the high strength, chemical stability and mechanical toughness of ePTFE, the commercialization of composite membranes has been greatly promoted.

[0005] However, expanded polytetrafluoroethylene is a hydrophobic substance without other active groups such as sulfonic acid groups. It has low activity and poor affinity for proton transfer, which is not conducive to proton transfer. In the three-layer structure of the composite proton exchange membrane, the ePTFE in the middle layer is undoubtedly the shortcoming of the performance, which makes the mass transfer capacity of the composite membrane not excellent. Therefore, it is extremely important to improve the mass transfer capacity of ePTFE.

[0006] Due to the strong chemical inertness of expanded polytetrafluoroethylene, it is difficult to directly "graft" sulfonic acid groups onto the bulk structure of expanded polytetrafluoroethylene using conventional methods, resulting in obstacles in improving the mass transfer performance of ePTFE. Summary of the invention

[0007] The main technical problem solved by the present invention is to provide a method for preparing a sulfonated expanded polytetrafluoroethylene composite membrane, in which a cellulose-based sulfonating agent with sulfonic acid groups is filled into the pore structure of ePTFE, and then the sulfonic acid groups are modified on the ePTFE. This is a method for sulfonating ePTFE similar to "film pasting". The sulfonated ePTFE is used as a reinforcing layer to prepare a composite proton exchange membrane, thereby enhancing the performance of the composite membrane.

[0008] In order to solve the above technical problems, a technical solution adopted by the present invention is: to provide a method for preparing a sulfonated expanded polytetrafluoroethylene composite membrane, which is carried out according to the following steps: S1, preparation of sulfonated modifier: Precursor and 3-mercaptopropyltriethoxysilane (MPTMS) were added to ethanol solvent, heated and mixed evenly, then microcrystalline cellulose was added, 0.05M HCL solution was added dropwise to adjust the pH to maintain at 1.8-2.2, and the temperature was raised to 80°C and refluxed for 12 hours. Then, the centrifugal speed is controlled at 2500-3200 rpm and the duration of the centrifugation is 20-45 min. The solid is filtered out by centrifugation and then freeze-dried. The freezing temperature range is -5--10°C and the freeze-drying time is 40-48 hours. Sufficient hydrogen peroxide was added to the freeze-dried solid product at a mass ratio of 1:40 to oxidize the thiol group (-SH) grafted on the MPTMS modifier to a sulfonic acid group (-SO 3 H), then filtering, washing and drying to obtain a cellulose-based sulfonated modifier; S2, preparation of ePTFE base tape: The ePTFE raw material resin and the lubricating agent are put into a pulverizer for pulverization, mixing and stirring, and the mixed raw material resin is placed in a molded blank, and the blank is pressed to preform an ePTFE base tape, and the pressure is adjusted to 5-12MPa, and the pressing time is 45-75 min. The base tape is then left to stand for aging, and the aging temperature is 60-80°C, and the aging time is 24-30h. After aging, subsequent sulfonation modification is performed; S3, ePTFE membrane sulfonation bonding: The pre-prepared cellulose-based sulfonated modifier is evenly coated on the surface of the matured ePTFE base tape, and then a roller press is used for rolling treatment. The rolling temperature is 150-180°C and the rolling time is 1-2 hours. After rolling, the base tape is cured by microwave radiation. The microwave radiation temperature is controlled to be 80-100°C and the microwave radiation time is controlled to be 20-30 minutes. The base tape is then extruded and calendered into a thin sheet through a calendering machine, the calendering roller temperature is controlled at 50~100℃, the calendering roller speed is controlled at 10~30mm / min, and then deoiled in a high-temperature oven at 160~250℃. Then, the film is stretched horizontally and vertically at a temperature of 200-400°C. After the stretching, the thickness of the ePTFE film is 6-14 μm. Finally, the film is sintered at a high temperature of 300-400°C to form a sulfonated composite ePTFE film. S4, Composite proton exchange membrane preparation: The sulfonated composite ePTFE membrane was used as the reinforcement layer and the perfluorosulfonic acid dispersion was evenly applied on both sides of the sulfonated composite ePTFE membrane by the cast film method. The membrane was initially dried at a temperature of 50-75°C and then transferred to an oven for drying and curing. The curing temperature range was 80-120°C and the curing time was 1-4 hours. A composite proton exchange membrane with a thickness of 9-20 μm was obtained.

[0009] In a preferred embodiment of the present invention, the precursor in step S1 uses tetraethyl orthosilicate, tetrabutyl titanate or cerium oxide.

[0010] In a preferred embodiment of the present invention, the mass ratio of the precursor, 3-mercaptopropyltriethoxysilane and ethanol in step S1 is 1:(1-10):(10-20).

[0011] In a preferred embodiment of the present invention, in step S1, the mass proportion of microcrystalline cellulose in the precursor and 3-mercaptopropyltriethoxysilane is 5-12%.

[0012] In a preferred embodiment of the present invention, the mass proportion of the lubricating agent described in step S2 in the ePTFE raw material resin and the lubricating agent is 5-50%.

[0013] In a preferred embodiment of the present invention, the lubricating agent is an isoparaffin compound, naphtha, silicone oil or paraffin, wherein the isoparaffin compound can be Mobil Isopar M series products.

[0014] In a preferred embodiment of the present invention, the mass proportion of the cellulose-based sulfonated modifier in step S3 in the ePTFE base tape is 15-30%.

[0015] In a preferred embodiment of the present invention, in step S3, the transverse and longitudinal tensile strength is ≥35 MPa, the stretching temperature is 300-350°C, and the sintering temperature is 350-360°C.

[0016] In a preferred embodiment of the present invention, the thickness of the ePTFE membrane after the transverse and longitudinal stretching in step S3 is 8-10 um, and the thickness of the finished composite proton exchange membrane in step S4 is 10-12 um.

[0017] In a preferred embodiment of the present invention, the composite proton exchange membrane in step S4 is dried and cured by gradient temperature increase, first drying at 80°C for 1 hour to remove residual solvent, then increasing the temperature to 100°C and curing for 2 hours, and then increasing the temperature again to 120°C and curing for 2 hours.

[0018] The beneficial effects of the present invention are: using nanoparticles such as sulfonated silicon dioxide and sulfonated titanium dioxide as sulfonation modifiers, and using microcrystalline cellulose as the sulfonation modifier base to prepare a cellulose-based sulfonation modifier; at the same time, the raw material resin is combined with the prepared cellulose-based sulfonation modifier, and the mass transfer capacity of the sulfonated ePTFE is significantly enhanced. The composite proton exchange membrane prepared with this as the support layer makes up for the shortcomings of the mass transfer capacity. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following briefly introduces the drawings required for describing the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative work, among which: Figure 1 These are electron microscope scanning images of the sulfonated composite ePTFE in Example 1: Figure a is an electron microscope scanning image of the sulfonated silicon membrane, Figure b is an electron microscope scanning image of the sulfonated composite ePTFE membrane, and Figure c is an electron microscope scanning image of the ePTFE membrane; Figure 2 is the infrared test image of the sulfonated modifier in Example 1; Figure 3 is the XPS test graph of S 2p of the cellulose-based sulfonated modifier in Example 1; DETAILED DESCRIPTION

[0020] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Generally, the components of the embodiments of the present invention described and shown in the drawings here can be arranged and designed in various different configurations.

[0021] Therefore, the following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the invention claimed for protection, but merely represents selected embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0022] The function of proton exchange membrane mainly depends on the sulfonic acid group of the side chain. The mechanism of mass transfer of sulfonic acid group is mainly attributed to the clustering of sulfonic acid groups in the membrane system to form hydrophilic areas. When the membrane is in a water environment, the hydrophilic areas formed by sulfonic acid groups form a spatial place for proton mass transfer. The richer the clustering of sulfonic acid groups, the wider the distribution of the formed hydrophilic areas, the higher the proton conduction rate, and the faster the mass transfer performance of the membrane.

[0023] On the other hand, there is no such sulfonic acid hydrophilic region in expanded polytetrafluoroethylene, so the proton conduction lacks driving force and the rate cannot be improved. However, the pore structure of ePTFE is used to provide a transmission channel for protons, and the modified substance rich in sulfonic acid groups is modified in this pore structure, which can ultimately improve the mass transfer performance.

[0024] Based on the above mechanism, the present invention uses nanoparticles of sulfonated silicon dioxide, sulfonated titanium dioxide, etc. as sulfonated modifiers, and microcrystalline cellulose as the base of the sulfonated modifier. Cellulose has the characteristics of hydrophilicity, high strength, good toughness, and branched shape. It is stretched and pressed together with the ePTFE raw resin during the film-forming process to form an interlocking and interfused overall structure. The raw resin is combined with the prepared sulfonated modifier during the film-forming process to enhance the interface effect; the chemical bonding between the sulfonated modifier and the expanded polytetrafluoroethylene structure is enhanced by controlling high temperature and microwave radiation. The modified ePTFE is finally prepared into a composite proton exchange membrane by conventional methods such as casting.

[0025] The following examples are used to verify the performance of the sulfonated expanded polytetrafluoroethylene composite membrane of the present invention. Embodiment 1:

[0026] S1, preparation of sulfonated modifier: Take 5g of tetraethyl orthosilicate and 10g of MPTMS and add them to 70g of ethanol solution, heat at 60℃ and stir for 2h to mix evenly, then add 1.3g of microcrystalline cellulose, continue to stir at 60℃ until mixed evenly, add 0.05M HCl solution to adjust the pH to 2, heat to 80℃ and reflux for 12h, then centrifuge at 3000rpm for 30min to filter out the solid, freeze-dry at -10℃ for 45h, add hydrogen peroxide to the freeze-dried product at a mass ratio of 1:40 for oxidation for 24h, then filter and wash the product, and dry to obtain a cellulose-based sulfonated modifier; S2, Preparation of ePTFE tape Mix the additive IsoparM and 10g of ePTFE resin into a grinder and grind and mix. Control the amount of IsoparM added to account for 20% of the total mass of IsoparM and ePTFE resin. Then the mixed raw resin is placed in the molding body, and the embryo is pressed to preform the ePTFE base tape, the pressure is adjusted to 10MPa, the pressing time is 60min, and then it is left to mature at a temperature of 70℃ and a time of 26h; S3, ePTFE membrane sulfonation bonding The pre-prepared cellulose-based sulfonated modifier is evenly coated on the surface of the matured ePTFE base tape. The mass proportion of the cellulose-based sulfonated modifier in the ePTFE resin base tape is 22%. Then, a roller press was used for rolling treatment, with a rolling temperature of 160°C and a rolling time of 2 hours. After rolling, the base tape was cured by microwave radiation, with a microwave radiation temperature of 92°C and a microwave radiation time of 25 minutes. The base tape is then extruded and calendered into a thin sheet through a calendering machine, the calendering roller temperature is controlled at 80°C and the calendering roller speed is controlled at 20mm / min, and then deoiled in a high-temperature oven at 200°C. Then it is stretched horizontally and vertically, with a tensile strength of ≥35Mpa and a stretching temperature of 300-350°C. Finally, it is sintered at a high temperature of 350-360°C to form a sulfonated composite ePTFE membrane. S4, Composite proton exchange membrane preparation: The sulfonated composite ePTFE membrane was used as the reinforcement layer. The perfluorosulfonic acid dispersion was evenly applied on both sides of the sulfonated composite ePTFE membrane by the cast film method. The membrane was initially dried at 60°C and then transferred to an oven for drying and curing. A gradient heating method was adopted: first, it was dried at 80°C for 1h to remove the residual solvent, then the temperature was raised to 100°C and cured for 2h, and then the temperature was raised to 120°C again and cured for 2h to obtain a composite proton exchange membrane. Embodiment 2:

[0027] S1, preparation of sulfonated modifier: Take 8g of tetrabutyl titanate and 10g of MPTMS and add them to 80g of ethanol solution, heat at 60℃ and stir for 2h to mix evenly, then add 1.0g of microcrystalline cellulose, continue to stir at 60℃ until mixed evenly, add 0.05M HCl solution to adjust the pH to 2, heat to 80℃ and reflux for 12h, then centrifuge at 3000rpm for 30min to filter out the solid, freeze-dry at -10℃ for 45h, add hydrogen peroxide to the freeze-dried product at a mass ratio of 1:40 for oxidation for 24h, then filter and wash the product, and dry to obtain a cellulose-based sulfonated modifier; S2, Preparation of ePTFE tape Mix the additive IsoparM and 10g of ePTFE resin into a grinder and grind and mix. Control the amount of IsoparM added to account for 20% of the total mass of IsoparM and ePTFE resin. Then the mixed raw resin is placed in the molding body, and the embryo is pressed to preform the ePTFE base tape, the pressure is adjusted to 10MPa, the pressing time is 60min, and then it is left to mature at a temperature of 70℃ and a time of 26h; S3, ePTFE membrane sulfonation bonding The pre-prepared cellulose-based sulfonated modifier is evenly coated on the surface of the matured ePTFE base tape. The mass proportion of the cellulose-based sulfonated modifier in the ePTFE resin base tape is 15%. Then, a roller press was used for rolling treatment, with a rolling temperature of 160°C and a rolling time of 2 hours. After rolling, the base tape was cured by microwave radiation, with a microwave radiation temperature of 80°C and a microwave radiation time of 20 minutes. The base tape is then extruded and calendered into a thin sheet through a calendering machine, the calendering roller temperature is controlled at 60°C and the calendering roller speed is controlled at 20mm / min, and then deoiled in a high-temperature oven at 200°C. Then it is stretched horizontally and vertically, with a tensile strength of ≥35Mpa and a stretching temperature of 300-350°C. Finally, it is sintered at a high temperature of 350-360°C to form a sulfonated composite ePTFE membrane. S4, Composite proton exchange membrane preparation: The sulfonated composite ePTFE membrane was used as the reinforcement layer. The perfluorosulfonic acid dispersion was evenly applied on both sides of the sulfonated composite ePTFE membrane by the cast film method. The membrane was initially dried at 60°C and then transferred to an oven for drying and curing. A gradient heating method was adopted: first, it was dried at 80°C for 1h to remove the residual solvent, then the temperature was raised to 100°C and cured for 2h, and then the temperature was raised to 120°C again and cured for 2h to obtain a composite proton exchange membrane. Embodiment three:

[0028] S1, preparation of sulfonated modifier: Take 5g of cerium oxide nanoparticles and 25g of MPTMS and add them to 100g of ethanol solution, heat at 60℃ and stir for 2h to mix evenly, then add 3.5g of microcrystalline cellulose, continue to stir at 60℃ until mixed evenly, add 0.05M HCl solution to adjust the pH to 2, heat to 80℃ and reflux for 12h, then centrifuge at 3000rpm, centrifuge for 30min to filter out the solid, and freeze-dry at -10℃ for 45h. The freeze-dried product is added with hydrogen peroxide at a mass ratio of 1:40 for oxidation for 24h, then the product is filtered and washed, and dried to obtain a cellulose-based sulfonated modifier; S2, Preparation of ePTFE tape Mix the additive IsoparM and 10g of ePTFE resin into a grinder and grind and mix. Control the amount of IsoparM added to account for 20% of the total mass of IsoparM and ePTFE resin. Then the mixed raw resin is placed in the molding body, and the embryo is pressed to preform the ePTFE base tape, the pressure is adjusted to 10MPa, the pressing time is 60min, and then it is left to mature at a temperature of 70℃ and a time of 26h; S3, ePTFE membrane sulfonation bonding The pre-prepared cellulose-based sulfonated modifier is evenly coated on the surface of the matured ePTFE base tape, and the mass proportion of the cellulose-based sulfonated modifier in the ePTFE resin base tape is 30%. Then, a roller press was used for rolling treatment, with a rolling temperature of 160°C and a rolling time of 2 hours. After rolling, the base tape was cured by microwave radiation, with a microwave radiation temperature of 100°C and a microwave radiation time of 30 minutes. The base tape is then extruded and calendered into a thin sheet through a calendering machine, the calendering roller temperature is controlled at 60°C and the calendering roller speed is controlled at 20mm / min, and then deoiled in a high-temperature oven at 200°C. Then it is stretched horizontally and vertically, with a tensile strength of ≥35Mpa and a stretching temperature of 300-350°C. Finally, it is sintered at a high temperature of 350-360°C to form a sulfonated composite ePTFE membrane. S4, Composite proton exchange membrane preparation: The sulfonated composite ePTFE membrane was used as the reinforcement layer. The perfluorosulfonic acid dispersion was evenly applied on both sides of the sulfonated composite ePTFE membrane by the cast film method. The membrane was initially dried at 60°C and then transferred to an oven for drying and curing. A gradient heating method was adopted: first, it was dried at 80°C for 1h to remove the residual solvent, then the temperature was raised to 100°C and cured for 2h, and then the temperature was raised to 120°C again and cured for 2h to obtain a composite proton exchange membrane.

[0029] Example 4, Example 5, Example 6, and Example 7 were carried out according to the experimental method of Example 1, except that the amount of MPTMS was changed to prepare materials with different degrees of sulfonation. The film forming process parameters and experimental conditions were the same as those of Example 1. The specific amounts are shown in Table 1 below.

[0030] Table 1:

[0031] Here, TEOS stands for tetraethyl orthosilicate, and MPTMS stands for 3-mercaptopropyltriethoxysilane.

[0032] Comparative Example 1: Comparative Example 1 is an ePTFE membrane prepared without adding a cellulose-based sulfonated modifier and a composite proton exchange membrane. Comparative Example 1 and Example 1 were tested and compared with each other in terms of various properties.

[0033] Test results: 1. Characterization of sulfonated morphology 1) Scanning electron microscope Figure 1 This is a scanning electron microscope image of the sulfonated composite ePTFE in Example 1: Figure a is an electron microscope scanning image of a sulfonated silicon membrane, Figure b is an electron microscope scanning image of a sulfonated composite ePTFE membrane, and Figure c is an electron microscope scanning image of an ePTFE membrane; from Figure 1 It can be observed that the sulfonated layer based on cellulose microcrystals and the original ePTFE membrane have been embedded together to form a composite membrane structure. The main body is still ePTFE, and the sulfonated layer can modify the mass transfer effect of ePTFE.

[0034] 2) Infrared test First, the sulfonated materials were subjected to infrared testing. Pure silicon dioxide (SiO 2 ), sulfuric acid solution, and the sulfonated modifier (sulfonated silicon) in Example 1 were subjected to infrared testing. Figure 2 As shown, by comparing the three, we can find that: Sulfonic acid group (-SO 3 The strong stretching vibration absorption of H appears at 1000, 820 and 592 cm -1 The deformation vibration of OSO causes the 820 and 592 cm -1 The asymmetric and symmetric stretching vibrations of OSO cause absorption peaks at 1240 and 1100 cm －1 The absorption peak at .

[0035] The sulfonated modifiers have a higher affinity to silica in the range of 500 to 900 cm -1 The characteristic peaks of sulfonic acid groups are shown, and compared with the sulfuric acid solution, the characteristic peaks of silicon dioxide are obvious, which indicates that the sulfonic acid groups are successfully grafted onto SiO 2 middle.

[0036] 3) XPS test Figure 3 The XPS test results of sulfonated ePTFE in Example 1 show that: The peak at around 170 eV is the S2p peak of S, which is consistent with the description in the literature and corresponds to the main chemical state of sulfur being sulfate type (HSO 3 - ), the S content in the test results is 10%. 2. Membrane performance test 1) The conductivity of the composite proton exchange membranes prepared in Examples 1 to 7 and Comparative Example 1 was measured by impedance testing. The conductivity test results are shown in Table 2 below.

[0037] Table 2:

[0038] in conclusion: Compared with comparative example 1, the composite proton membrane after sulfonation exhibits better conductivity. In Examples 4 to 7, as the content of the sulfonation reagent MPTMS increases, the conductivity after membrane formation also increases. The possible reason is that the increase in the dosage of MPTMS increases the degree of sulfonation, indicating that the degree of sulfonation is positively correlated with the conductivity.

[0039] The beneficial effects of the method for preparing the sulfonated expanded polytetrafluoroethylene composite membrane of the present invention are: Sulfonated ePTFE introduces sulfonic acid groups, which significantly enhances its mass transfer capacity. The composite proton exchange membrane is prepared using the sulfonated ePTFE as a reinforcement layer. The electrochemical test shows that the conductivity performance is better than that of the unhybridized comparison membrane. The chemical bonding method allows the sulfonic acid groups to solidify in the internal layer structure of ePTFE, thereby directly improving the mass transfer performance of ePTFE.

[0040] The above descriptions are merely embodiments of the present invention and are not intended to limit the patent scope of the present invention. Any equivalent structure or equivalent process transformation made using the contents of the present invention specification, or directly or indirectly applied in other related technical fields, are also included in the patent protection scope of the present invention.

Claims

1. A method for preparing a sulfonated expanded polytetrafluoroethylene composite membrane, characterized in that: The method proceeds as follows: S1, preparation of sulfonated modifier: Precursor and 3-mercaptopropyltriethoxysilane were added to ethanol solvent, heated and mixed evenly, then microcrystalline cellulose was added, 0.05M HCL solution was added dropwise to adjust the pH to maintain at 1.8-2.2, and the temperature was raised to 80°C for reflux reaction for 12 hours. Then, the centrifugal speed is controlled at 2500-3200 rpm and the duration of the centrifugation is 20-45 min. The solid is filtered out by centrifugation and then freeze-dried. The freezing temperature range is -5--10°C and the freeze-drying time is 40-48 hours. After freeze-drying, a sufficient amount of hydrogen peroxide is added to the solid product at a mass ratio of 1:40 for oxidation, and then filtered, washed and dried to obtain a cellulose sulfonated modifier; S2, preparation of ePTFE base tape: The ePTFE raw material resin and the lubricating agent are put into a pulverizer for pulverization, mixing and stirring, and the mixed raw material resin is placed in a molded blank, and the blank is pressed to preform an ePTFE base tape, and the pressure is adjusted to 5-12MPa, and the pressing time is 45-75 min. The base tape is then left to stand for aging, and the aging temperature is 60-80°C, and the aging time is 24-30h. After aging, subsequent sulfonation modification is performed; S3, ePTFE membrane sulfonation bonding: The pre-prepared cellulose-based sulfonated modifier is evenly coated on the surface of the matured ePTFE base tape, and then a roller press is used for rolling treatment. The rolling temperature is 150-180°C and the rolling time is 1-2 hours. After rolling, the base tape is cured by microwave radiation. The microwave radiation temperature is controlled to be 80-100°C and the microwave radiation time is controlled to be 20-30 minutes. The base tape is then extruded and calendered into a thin sheet through a calendering machine, the calendering roller temperature is controlled at 50~100℃, the calendering roller speed is controlled at 10~30mm / min, and then deoiled in a high-temperature oven at 160~250℃. Then, the film is stretched horizontally and vertically at a temperature of 200-400°C. After the stretching, the thickness of the ePTFE film is 6-14 μm. Finally, the film is sintered at a high temperature of 300-450°C to form a sulfonated composite ePTFE film. S4, Composite proton exchange membrane preparation: The sulfonated composite ePTFE membrane was used as the reinforcement layer and the perfluorosulfonic acid dispersion was evenly applied on both sides of the sulfonated composite ePTFE membrane by the cast film method. The membrane was initially dried at a temperature of 50-75°C and then transferred to an oven for drying and curing. The curing temperature range was 80-120°C and the curing time was 1-4 hours. A composite proton exchange membrane with a thickness of 9-20 μm was obtained.

2. The method for preparing a sulfonated expanded polytetrafluoroethylene composite membrane according to claim 1, characterized in that: The precursor in step S1 uses tetraethyl orthosilicate, tetrabutyl titanate or cerium oxide.

3. The method for preparing a sulfonated expanded polytetrafluoroethylene composite membrane according to claim 1, characterized in that: In step S1, the mass ratio of the precursor, 3-mercaptopropyltriethoxysilane, and ethanol is 1:(1-10):(10-20).

4. The method for preparing a sulfonated expanded polytetrafluoroethylene composite membrane according to claim 1, characterized in that: In step S1, the mass proportion of microcrystalline cellulose in the precursor and 3-mercaptopropyltriethoxysilane is 5-12%.

5. The method for preparing a sulfonated expanded polytetrafluoroethylene composite membrane according to claim 1, characterized in that: The mass proportion of the lubricating agent described in step S2 in the ePTFE raw material resin and the lubricating agent is 5-50%.

6. The method for preparing a sulfonated expanded polytetrafluoroethylene composite membrane according to claim 5, characterized in that: The lubricating agent is an isoparaffin compound, naphtha, silicone oil or paraffin, wherein the isoparaffin compound can be Mobil Isopar M series products.

7. The method for preparing a sulfonated expanded polytetrafluoroethylene composite membrane according to claim 1, characterized in that: The mass proportion of the cellulose-based sulfonated modifier described in step S3 in the ePTFE base tape is 15-30%.

8. The method for preparing a sulfonated expanded polytetrafluoroethylene composite membrane according to claim 1, characterized in that: In step S3, the transverse and longitudinal tensile strengths are ≥35 MPa, the stretching temperature is 400-450°C, and the sintering temperature is 450-460°C.

9. The method for preparing a sulfonated expanded polytetrafluoroethylene composite membrane according to claim 1, characterized in that: The thickness of the ePTFE membrane after the horizontal and vertical stretching in step S3 ranges from 5 to 15 um, and the thickness of the finished composite proton exchange membrane in step S4 ranges from 8 to 20 um.

10. The method for preparing a sulfonated expanded polytetrafluoroethylene composite membrane according to claim 1, characterized in that: The composite proton exchange membrane described in step S4 is dried and cured by gradient temperature increase, first drying at 80° C. for 1 h to remove residual solvent, then the temperature is increased to 100° C. and cured for 2 h, and then the temperature is increased to 120° C. and cured for 2 h.

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