Ultrathin composite proton exchange membrane and its preparation method and application

Abstract

The present invention discloses an ultra-thin composite proton exchange membrane, a preparation method and application thereof. The composite proton exchange membrane adopts an ultra-thin, high-strength and high-porosity expanded polytetrafluoroethylene (ePTFE) membrane as a skeleton, and is prepared by filling perfluorosulfonic acid resin raw materials and additives in ePTFE. The proton exchange membrane obtained by the "gradient filling-predrying-molding" method of the present invention has important advantages such as ultra-thin thickness (can be as low as 4 microns), high mechanical strength (can be as high as 40MPa or more), high proton conductivity, complete filling, low hydrogen permeability, etc. The membrane electrode prepared using the ultra-thin proton exchange membrane prepared by the present invention shows excellent performance, and at the same time can greatly reduce the amount of perfluorosulfonic acid resin used, and there is no need to perform complex hydrophilic treatment on ePTFE, effectively reducing the cost of fuel cells, and solving the problem of poor performance of membrane electrodes under low humidity, which is of great significance for promoting large-scale commercialization of fuel cells.

Description

Technical Field

[0001] The present invention belongs to the technical field of fuel cell proton exchange membrane preparation, and relates to an ultra-thin composite proton exchange membrane and a preparation method and application thereof. Background Art

[0002] Proton exchange membrane fuel cells (PEMFCs) offer key advantages such as high energy conversion efficiency, zero emissions, and fast startup, and hold broad application prospects in transportation, communications, aviation / aerospace, and submarine applications. Performance and cost are the two main technical barriers to the commercialization of PEMFCs in current PEM preparation and application. Because the commonly used perfluorosulfonic acid resin (PFSR) material has low mechanical strength, thin pure PFSR membranes are difficult to use in fuel cell operating environments. Therefore, porous substrate materials such as expanded polytetrafluoroethylene (ePTFE) and polyvinylidene fluoride (PVDF) are often introduced to enhance the mechanical properties of PEMs. Expanded polytetrafluoroethylene (ePTFE)-reinforced PEMs are currently the most widely used type of PEM, offering key advantages such as thinness, high mechanical strength, and excellent dimensional stability. Research has shown that as a solid electrolyte, the thinner the PEM thickness, the lower the impedance of proton transfer and the better the battery performance. Ultrathin PEMs and ultrathin membrane electrodes are key to achieving low-cost, high-performance membrane electrodes in the future. The current lowest commercial proton exchange membrane thickness is only 8μm, and the performance of the ultra-thin proton exchange membranes proposed so far is unsatisfactory when applied to fuel cells. How to prepare ultra-thin proton exchange membranes with stable and excellent performance, simple preparation methods and high performance for fuel cells is a key issue that limits the commercialization of fuel cells. For proton exchange membranes reinforced with porous ePTFE, the technical challenge faced in reducing the thickness to prepare ultra-thin proton exchange membranes is to ensure the high mechanical strength and low gas permeability of the proton exchange membrane while optimizing the molding method to ensure that the perfluorosulfonic acid resin is completely filled in the micropores of the ePTFE. The commonly used filling methods for perfluorosulfonic acid resins are dipping, coating or spraying, and the solvent in the casting solution is removed at a certain temperature and vacuum to form the composite proton exchange membrane. As the solvent evaporates, pores often appear in the porous membrane, which reduces the conductivity of the proton exchange membrane and affects the performance and durability of the proton exchange membrane in the fuel cell. Since perfluorosulfonic acid resin has very poor compatibility with ePTFE, methods are often used to optimize the hydrophilicity of the ePTFE surface, such as plasma treatment, modification and coating. However, the decrease in the pore size and porosity of the porous membrane will affect the performance of the proton exchange membrane, which is particularly obvious in ultra-thin proton exchange membranes. The complex pretreatment steps also hinder its application in large-scale production. In addition, ultra-thin proton exchange membranes used in fuel cells are susceptible to chemical corrosion caused by free radical attacks on the main chain and side chains of the perfluorosulfonic acid resin. In order to improve the durability of the ultra-thin proton exchange membrane, metal oxide particles and organic antioxidants are added to the perfluorosulfonic acid resin polymer as free radical scavengers to reduce the performance degradation of the fuel cell proton exchange membrane.

[0003] Chinese patent application CN112940343A discloses an enhanced perfluorosulfonic acid composite proton exchange membrane and its production process. The process involves impregnating ePTFE with a perfluorosulfonic acid resin membrane-forming solution and forming the membrane using a stepped temperature program in a vertical furnace. The composite membrane is stretched in the direction of the resin to remove the solvent, effectively avoiding the effects of gravity on the slurry on both sides of the ePTFE and improving the thickness stability and uniformity of the composite membrane. However, the single-shot molding method used in this invention cannot guarantee that the perfluorosulfonic acid resin layer will completely fill the gaps when the solvent evaporates. This increases the gas permeability of the composite membrane, and poses durability issues due to hydrogen permeation when used in fuel cells.

[0004] Chinese patent application CN111916807A discloses an ultrathin reinforced composite proton exchange membrane, preparation method, and application. The composite proton exchange membrane comprises a hydrophilic polytetrafluoroethylene substrate, impregnated with a mixed solution of perfluorosulfonic acid resin and polyvinyl alcohol, preformed by atmospheric drying, hot-extruded perfluorosulfonic acid resin membranes at both ends and roll-laminated for lamination, and finally subjected to biaxial stretching at a specific temperature. This can improve the pore size and porosity shrinkage of the polytetrafluoroethylene nanofiber membrane caused by drying and other treatments. The rolled perfluorosulfonic acid resin layer can fill the pores enlarged by stretching, further improving the proton conductivity of the composite proton exchange membrane. However, the polyvinyl alcohol used in this invention only serves to enhance the hydrophilicity of the cast membrane and has no proton conductivity effect. Furthermore, the impregnated perfluorosulfonic acid resin and rolled perfluorosulfonic acid resin layers are difficult to fully bond, resulting in the formation of pores. This results in poor performance and durability of the proton exchange membrane when used in membrane electrode applications.

[0005] Chinese patent application CN113861502A discloses a method for preparing a porous framework for proton exchange membranes and a composite proton exchange membrane. The porous framework is a polyimide-coated porous polytetrafluoroethylene sheet made by impregnating and stretching it with polyamic acid and then heating and dehydrating it. This porous framework exhibits excellent dimensional stability, enhances the hydrophilicity of the porous framework, and improves the compatibility of the porous framework material with the proton exchange membrane resin. However, after the hydrophilic treatment, the porous framework still contains voids that cannot be filled with perfluorosulfonic acid resin. Furthermore, the polyimide layer, which lacks proton transport capability, occupies some of the voids in the porous framework, impairing the transport of protons between the anode and cathode in the fuel cell, resulting in reduced performance of fuel cells fabricated with this proton exchange membrane.

[0006] Improving fuel cell performance by reducing the thickness of the proton exchange membrane has been the most important approach to improving fuel cell performance internationally for many years. However, reducing the thickness will lead to a decrease in the mechanical strength of the proton exchange membrane and an increase in hydrogen leakage. How to ensure that the proton exchange membrane maintains good mechanical strength and low hydrogen leakage while reducing the thickness has always been a challenging problem in this field.

[0007] In addition, there are a large number of capillary pores in ePTFE, which is a supporting and reinforcing material. How to make the resin fully fill these pores while keeping the ultra-thin proton exchange membrane flat and uniform has always been a challenging technical problem.

[0008] To address the current problems with the preparation and molding of ultra-thin proton exchange membranes, the present invention employs a novel "gradient filling-predrying-molding" method, assisted by ultrasound-assisted filling technology, to produce an ultra-thin composite proton exchange membrane with a thickness as low as 4 μm, excellent mechanical strength and dimensional stability. This membrane is then used in the preparation of ultra-thin film electrodes with excellent electrochemical properties. Different types of free radical scavengers are added to the ultra-thin proton exchange membrane to effectively extend the resistance to chemical corrosion and improve the durability of the proton exchange membrane. This invention not only addresses the key challenges of the mechanical, electrochemical, and durability of ultra-thin proton exchange membranes, but also significantly reduces the cost of proton exchange membranes, which is of great significance for promoting the large-scale commercialization of fuel cells. Summary of the Invention

[0009] Ultra-thin composite proton exchange membranes are of great significance for reducing the cost and improving the performance of fuel cell membrane electrodes. In view of the problems existing in the background technology, the purpose of the present invention is to provide a method for preparing an ultra-thin proton exchange membrane with expanded polytetrafluoroethylene (ePTFE) as a porous skeleton reinforcement, so as to solve the problems existing in the current ultra-thin proton exchange membrane molding and solve the technical problem that the current ultra-thin proton exchange membrane has poor performance in fuel cell applications. The ultra-thin composite proton exchange membrane described in the present invention has the characteristics of thin thickness (effectively reducing the internal resistance of the membrane electrode), high proton conductivity, dense filling of perfluorosulfonic acid resin, and high mechanical strength. The membrane electrode prepared using this proton exchange membrane has the important advantages of high polarization curve performance and a wide range of humidity. The use of the ultra-thin proton exchange membrane of the present invention can effectively reduce the cost of fuel cells while ensuring the high performance of fuel cells. At the same time, the "gradient filling-predrying-molding" method proposed in the present invention solves the problem of poor performance durability caused by incomplete filling in the molding of composite proton exchange membranes. The present invention solves an important problem facing the commercialization of fuel cells and is of great significance for promoting the large-scale commercialization of fuel cells.

[0010] To achieve the above objectives, the technical solution provided by the present invention is:

[0011] A "gradient filling-predrying-molding" preparation method for an expanded polytetrafluoroethylene-reinforced ultrathin proton exchange membrane comprises an ePTFE porous membrane as a support and a perfluorosulfonic acid resin. The perfluorosulfonic acid resin is uniformly filled in the ePTFE pores, resulting in a smooth and flat surface. The resulting ultrathin proton exchange membrane has a thickness of 4-12 microns and a strength exceeding 40 MPa.

[0012] The preparation method comprises the following steps:

[0013] Step 1: Degreasing and cleaning of expanded polytetrafluoroethylene: Use organic solvent to immerse and clean the ePTFE membrane, and after drying, fix it on a special hollow tooling mold for use;

[0014] Step 2: Preparation of a filling casting solution: After washing, acid exchange, and drying the perfluorosulfonic acid resin raw material, an organic solvent is added, the raw material is placed in a high-pressure reactor, and heated and dissolved to obtain a perfluorosulfonic acid resin solution. When the perfluorosulfonic acid resin solution is used as the raw material, no pretreatment or dissolution process is required. Specifically, the perfluorosulfonic acid resin solution is taken, an organic solvent and an additive are added, and low-temperature ultrasonication is performed to obtain a filling casting solution.

[0015] Step 3: Multiple coating and filling of the casting film with gradient solution: A certain amount of low-concentration casting solution is evenly coated on both sides of the ePTFE porous membrane, and ultrasonic treatment is performed to allow the casting solution to fully fill the micropores of the ePTFE porous membrane. After pre-drying to 10μm-50μm, a high-concentration casting solution is used for a second coating and pre-dried. The "coating-pre-drying" step can be repeated repeatedly until the calculated amount of casting solution is coated;

[0016] Step 4: The proton exchange membrane prepared above is dried under normal pressure and then subjected to vacuum high-temperature heat treatment to obtain an ultra-thin composite proton exchange membrane.

[0017] In the present invention, the thickness of the proton exchange membrane is as thin as 4 μm, while its mechanical strength is still as high as 40 MPa, and its performance is better than the currently available ultra-thin proton exchange membrane.

[0018] Preferably, in step 1, the ePTFE porous skeleton has a thickness of 3-10 μm and a porosity of 70-90%.

[0019] Preferably, in step 1, the cleaning refers to placing the ePTFE skeleton in an organic solvent for ultrasonic cleaning, and the organic solvent includes one or more of methanol, ethanol, isopropanol, and acetone.

[0020] Preferably, in step 2, the perfluorosulfonic acid resin raw material includes one or more of perfluorosulfonic acid resin particles (such as DuPont NafionR resin, 3M PFSR resin), perfluorosulfonic acid resin dispersion (such as DuPont Nafion D2020), and proton exchange membrane (such as DuPont Nafion117, DuPont Nafion211, DuPont Nafion212, etc.) scraps.

[0021] Preferably, the pretreatment in step 2 is to place the perfluorosulfonic acid resin raw material in water, H2O2 solution, and H2SO4 solution in sequence and stir them, wherein the concentration of H2O2 solution is 1%-20%, the concentration of H2SO4 solution is 0.1mol / L-1mol / L, the temperature is 60℃-100℃, and the treatment time is 30min-4h.

[0022] Preferably, in step 2, the organic solvent includes one or more of methanol, ethanol, isopropanol, dimethyl sulfoxide, N,N-dimethylformamide, N,N-dimethylacetamide, and N-methylpyrrolidone.

[0023] Preferably, in step 2, the dissolution temperature is 100°C-200°C, and the dissolution time is 2h-48h.

[0024] Preferably, in step 2, the low boiling point solvent includes one or more of methanol, ethanol, isopropanol, and acetone, the temperature is -5-30°C, the concentration of the casting solution ranges from 1% to 25%, and the time of the low temperature ultrasound is 2-12 hours.

[0025] Preferably, in step 2, the additives used to prepare the casting solution include free radical scavengers, surfactants, enhancers, etc.; the free radical scavengers include nano CeO2, MnO2, CrO2, CoO2, Al2O3, dihydromyricetin, epicatechin, quercetin, etc., and the addition amount is 0.1-1wt%, the surfactants include sodium dodecylbenzene sulfonate, polyvinyl alcohol, sodium bile acid, etc., and the addition amount is 0.01-5wt%; the enhancers include PVDF powder and PTFE powder, and the addition amount is 1-15wt%.

[0026] Preferably, in step three, the ultrasonic temperature range is -5-50°C, the pre-drying temperature is 10-50°C, the pre-drying time is 1 min-2 h, and the thickness after pre-drying is 10 μm-50 μm.

[0027] Preferably, in step 4, the atmospheric pressure drying temperature is 20°C-80°C, the atmospheric pressure drying time is 30min-10h, the vacuum heat treatment temperature is 100°C-180°C, the vacuum heat treatment time is 30min-24h, and the vacuum degree is 0.1-1MPa.

[0028] In the present invention, the ultrathin composite proton exchange membrane is applied to hydrogen fuel cells, methanol fuel cells or hydrogen production by water electrolysis.

[0029] In the present invention, the ultrathin composite proton exchange membrane has good electrical conductivity and mechanical strength. The performance of a membrane electrode made using the ultrathin proton exchange membrane prepared by the present invention is better than that of a membrane electrode made using existing commercial proton exchange membranes.

[0030] The present invention discloses an expanded polytetrafluoroethylene-reinforced ultrathin composite proton exchange membrane with a thickness of 4-12 μm. The ePTFE pores are densely and uniformly filled with a perfluorosulfonic acid resin, resulting in a smooth surface. This ultrathin composite proton exchange membrane is suitable for use in fuel cells and exhibits high mechanical strength, high proton conductivity, a wide humidity range, and low internal resistance.

[0031] The membrane electrode prepared by using an expanded polytetrafluoroethylene reinforced ultra-thin composite proton exchange membrane as a proton exchange membrane in a fuel cell has excellent single cell performance.

[0032] The present invention focuses on the filling and preparation method of ultra-thin composite proton exchange membranes, aiming to utilize the "gradient filling-predrying-molding" method, without the need for hydrophilic pretreatment of the expanded polytetrafluoroethylene porous membrane, so that the perfluorosulfonic acid resin is tightly filled into the ePTFE pores, reducing the generation of pores, improving the performance and durability of the membrane electrode, greatly improving the utilization rate of the perfluorosulfonic acid resin, and effectively reducing costs.

[0033] Compared with the prior art, the present invention has the advantages of:

[0034] 1. The "gradient filling-predrying-molding" method provided by this invention enables the preparation of ultra-thin composite proton exchange membranes with a thickness as low as 4 μm. The membranes are completely filled, uniformly flat, and have high mechanical strength (up to 40 MPa or more). When used in fuel cell membrane electrodes, they exhibit excellent performance, reducing the proton transport resistance between the anode and cathode of the fuel cell membrane electrode, lowering the internal resistance of the membrane electrode, and enhancing water diffusion and transport between the anode and cathode of the membrane electrode, effectively improving the performance of the membrane electrode in low-humidity operating environments.

[0035] 2. The ultra-thin composite proton exchange membrane provided by the present invention is densely and uniformly filled, and the proton transport material is tightly bonded to the porous framework, effectively inhibiting hydrogen permeation between the anode and cathode, and suppressing the generation of free radicals that cause chemical attenuation of the proton exchange membrane. The ultra-thin thickness improves mechanical strength, and the "gradient filling-predrying-molding" method facilitates the preparation of universal membrane electrodes and their application in actual fuel cell operating conditions.

[0036] 3. The ultra-thin composite proton exchange membrane provided by the present invention has a smooth surface and uniform thickness, which is conducive to full contact between the cathode and anode catalytic materials of the membrane electrode and improves the overall performance of the membrane electrode;

[0037] 4. The ultra-thin composite proton exchange membrane provided by the present invention has enhanced mechanical properties and good flexibility, is resistant to stretching and bending, and can provide good reliability for the preparation of ultra-thin film electrodes;

[0038] 5. The ultra-thin composite proton exchange membrane provided by the present invention has good heat resistance, which can ensure the high durability of the membrane electrode;

[0039] 6. The ultra-thin composite proton exchange membrane provided by the present invention has enhanced durability. The addition of free radical scavengers significantly reduces the chemical corrosion of free radicals on the perfluorosulfonic acid resin side, extending the service life of the proton exchange membrane in the fuel cell.

[0040] 7. The present invention is pollution-free during the preparation process, is easy to operate, has low equipment requirements, and the molding process is easy to master and control, which is conducive to the application in large-scale production of ultra-thin proton exchange membranes. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1 This is a scanning electron microscope (SEM) image of the ePTFE of Example 1 of the present invention;

[0042] Figure 2 This is a scanning electron microscope (SEM) image of the surface of the ePTFE-reinforced ultrathin composite proton exchange membrane of Example 1 of the present invention;

[0043] Figure 3 This is a scanning electron microscope (SEM) image of a cross section of the ePTFE-reinforced ultrathin composite proton exchange membrane of Example 1 of the present invention;

[0044] Figure 4 This is a scanning electron microscope (SEM) image of the surface of the proton exchange membrane of Comparative Example 1 of the present invention;

[0045] Figure 5 This is a scanning electron microscope (SEM) image of a cross section of the proton exchange membrane of Comparative Example 2 of the present invention;

[0046] Figure 6 This is a graph showing the tensile strength of the ePTFE-reinforced ultrathin composite proton exchange membrane of Example 1 of the present invention;

[0047] Figure 7 Polarization curves of H2-Air fuel cells using Example 1 of the present invention, Comparative Example 1, and commercial proton exchange membranes;

[0048] Figure 8 Polarization curves of H2-Air fuel cells using Example 2 of the present invention, Comparative Example 2, and a commercial proton exchange membrane;

[0049] Figure 9 Polarization curves of the H2-Air fuel cell at different humidity levels in Example 1 of the present invention;

[0050] Figure 10 These are the OCV accelerated durability test curves of Example 3 and Comparative Example 3 of the present invention. DETAILED DESCRIPTION

[0051] The present invention will be further described in detail below with reference to specific examples, but the embodiments of the present invention are not limited thereto. For process parameters not particularly noted, conventional techniques may be used.

[0052] Example 1

[0053] (1) Cut 3 μm thick ePTFE into 5 cm x 5 cm squares, ultrasonically treat with acetone, ethanol, and isopropanol for 24 h, and then fix the squares on a frame mold and dry them.

[0054] (2) Dupont Nafion211 membrane scraps were placed in water, 5% H2O2 solution, and 0.5 mol / L H2SO4 solution in turn, stirred at 80℃ for 1 hour, and vacuum dried at 0.1 MPa and 60℃ for 24 hours; 3 g of perfluorosulfonic acid resin raw material and 3.82 mL of isopropanol were weighed and dissolved in 25.6 mL of N,N-dimethylacetamide (DMAC), sealed and stirred at room temperature for 30 minutes; transferred to a high-temperature reactor and dissolved at 120℃ for 6 hours to obtain a slightly yellowish transparent viscous slurry; acetone was added and stirred in an ice bath to prepare casting solutions with perfluorosulfonic acid resin mass fractions of 2.5%, 5%, and 10%, respectively;

[0055] (3) A casting solution with a mass fraction of 2.5% perfluorosulfonic acid resin is evenly coated on both sides of the ePTFE porous membrane, and the coating thickness on both sides is 80 μm; ultrasonic treatment is performed at 30°C for 15 minutes to allow the casting solution to fully fill the ePTFE pores, and pre-dried at 40°C for 10 minutes to a total wet film thickness of 50 μm; a casting solution with a mass fraction of 5% perfluorosulfonic acid resin is evenly coated on both sides of the ePTFE porous membrane, and the coating thickness on both sides is 40 μm; ultrasonic treatment is performed at 30°C for 15 minutes to fully mix the casting solution, and pre-dried at 40°C for 5 minutes to a total wet film thickness of 50 μm; a casting solution with a mass fraction of 10% perfluorosulfonic acid resin is evenly coated on both sides of the ePTFE porous membrane, and the coating thickness on both sides is 16 μm; ultrasonic treatment is performed at 30°C for 15 minutes to fully mix the casting solution;

[0056] (4) The wet membrane was dried at 45°C under normal pressure for 6 h, heat treated at 150°C under a vacuum of 0.1 MPa for 10 h, and cooled to room temperature to obtain an ultrathin composite proton exchange membrane with a thickness of 4.5 μm.

[0057] The ultra-thin composite proton exchange membrane was prepared as an ultra-thin film electrode: Pt / C catalyst, an appropriate amount of 5% Nafion solution and isopropanol were weighed to prepare a catalyst slurry, wherein the anode catalyst loading was 0.1 mg Pt / cm 2 , the cathode catalyst loading is 0.2mgPt / cm 2 After ultrasonic dispersion, the slurry is evenly sprayed onto the above-mentioned ultra-thin composite proton exchange membrane to prepare a membrane electrode.

[0058] The above membrane electrode was assembled into a fuel cell and the battery performance test was carried out. The conditions of the battery performance test were as follows: the anode fuel was hydrogen, the cathode oxidant was air, the battery temperature was 80°C, the relative humidity of the anode and cathode was 100%, the anode and cathode back pressure was 200 kPa, and no humidification device was used for the anode and cathode during the test without humidification conditions.

[0059] SEM image of ePTFE membrane Figure 1 ; The planar SEM image of the ePTFE reinforced ultrathin proton exchange membrane prepared by this method is shown in Figure 2 ; The SEM image of its cross section is shown in Figure 3 ; Its tensile strength capacity is shown in Figure 4 ;

[0060] Example 2

[0061] (1) Cut 5 μm thick ePTFE into 10 cm x 10 cm squares, ultrasonically treat with acetone and methanol for 24 h, and then fix the four sides on a frame mold and dry them;

[0062] (2) Using a 5% mass fraction of perfluorosulfonic acid resin dispersion as the raw material, evaporate the solvent and vacuum dry at 60°C for 48 hours; weigh 3g of perfluorosulfonic acid resin raw material and 3.8mL of ethanol in 25.3mL of N,N-dimethylformamide (DMF), seal and stir at room temperature for 30 minutes; transfer to a high-temperature reactor and dissolve at 130°C for 6 hours to obtain a slightly yellowish transparent viscous slurry; add isopropyl alcohol and stir in an ice bath to prepare casting solutions with perfluorosulfonic acid resin mass fractions of 2.5% and 5%, respectively;

[0063] (3) A casting solution with a mass fraction of 2.5% perfluorosulfonic acid resin was evenly coated on both sides of the ePTFE porous membrane to a coating thickness of 80 μm; ultrasonic treatment was performed at 30°C for 15 minutes to allow the casting solution to fully fill the ePTFE pores, and pre-dried at 40°C for 10 minutes to a total wet film thickness of 50 μm; a casting solution with a mass fraction of 5% perfluorosulfonic acid resin was evenly coated on both sides of the ePTFE porous membrane to a coating thickness of 88 μm; ultrasonic treatment was performed at 30°C for 15 minutes to allow the casting solution to be fully mixed;

[0064] (4) The wet membrane was dried at 40°C under normal pressure for 6 h, heat treated at 140°C under a vacuum of 0.1 MPa for 10 h, and cooled to room temperature to obtain an ultrathin composite proton exchange membrane with a thickness of 6 μm.

[0065] The above catalyst was used to prepare a membrane electrode, and the fuel cell performance test method was the same as in Example 1.

[0066] Example 3

[0067] Except for the following changes, the rest is the same as Example 1;

[0068] (1) Using 5 μm 80% porosity ePTFE as the porous membrane;

[0069] (2) The raw material source was changed to 3 g of perfluorosulfonic acid resin particles DuPont Nafion R, the solvent was adjusted to methanol and dimethyl sulfoxide, the dissolution temperature was adjusted to 110 ° C, and the time was 12 h; 1% dihydromyricetin was added as an organic free radical scavenger, the solvent was adjusted to isopropanol, the mixing temperature was adjusted to 10 ° C, and the concentration of the casting solution was prepared to be 1% and 2.5%;

[0070] (3) 1% perfluorosulfonic acid casting solution was applied once, and 2.5% perfluorosulfonic acid casting solution was applied three times. The ultrasonic temperature was adjusted to 20°C, ultrasonication was performed for 30 minutes, and the pre-drying temperature was adjusted to 45°C. The wet film thickness after pre-drying was 80 μm.

[0071] (4) The atmospheric pressure drying temperature was adjusted to 60°C, the drying time was 8 hours, the vacuum heat treatment temperature was 140°C, the heat treatment time was 12 hours, and the proton exchange membrane thickness was 8 μm.

[0072] The membrane electrode was prepared for an accelerated durability test of open circuit voltage. The test conditions were as follows: hydrogen was used as the anode, air was used as the cathode oxidant, the battery temperature was 80°C, the relative humidity of the anode and cathode was 30%, the electronic load was in an open circuit state, the cathode was switched to nitrogen every 24 hours, and the relative humidity of the two electrodes was increased to 100% and maintained for 1 hour before entering the next cycle.

[0073] Comparative Example 1:

[0074] The same method is used, but the difference from Example 1 is that a casting solution with a mass fraction of 2.5% perfluorosulfonic acid resin is directly coated on both sides of the ePTFE at one time, dried after ultrasonic treatment, and directly subjected to high-temperature heat treatment for molding. The total mass of the perfluorosulfonic acid resin used is the same as that of Example 1.

[0075] Comparative Example 2:

[0076] The same method is used, but the difference from Example 2 is that in step 4, each time the casting solution is applied, it is thoroughly dried at room temperature before applying the next layer of casting solution: a casting solution with a mass fraction of 2.5% of perfluorosulfonic acid resin is evenly coated on both sides of the ePTFE porous membrane, and the coating thickness on both sides is 80 μm; ultrasonic treatment is performed at 30°C for 15 minutes to allow the casting solution to fully fill the ePTFE channels, and dried at 40°C for 6 hours; a casting solution with a mass fraction of 5% of perfluorosulfonic acid resin is evenly coated on both sides of the ePTFE porous membrane, and the coating thickness on both sides is 88 μm; ultrasonic treatment is performed at 30°C for 15 minutes; the total mass of perfluorosulfonic acid resin used is the same as that in Example 1.

[0077] Comparative Example 3:

[0078] The same method is adopted, but the difference from Example 3 is that no free radical scavenger is added.

[0079] Figure 1 This is a scanning electron microscope (SEM) image of the ePTFE of Example 1 of the present invention;

[0080] Figure 2 This is a scanning electron microscope (SEM) image of the surface of the ePTFE-reinforced ultrathin composite proton exchange membrane of Example 1 of the present invention. The perfluorosulfonic acid resin layer is completely covered, the surface is smooth and flat, and no ePTFE is exposed.

[0081] Figure 3 This is a scanning electron microscope (SEM) image of a cross-section of the ePTFE-reinforced ultrathin composite proton exchange membrane of Example 1 of the present invention. The perfluorosulfonic acid resin layer is uniformly and densely filled, there are no pores or defects in the proton exchange membrane, and the overall thickness is uniform.

[0082] Figure 4 This is a scanning electron microscope (SEM) image of the surface of the proton exchange membrane of Comparative Example 1 of the present invention. The perfluorosulfonic acid resin layer is obviously underfilled, and the ePTFE is exposed, which is not conducive to proton exchange transmission and contact with the cathode and anode catalyst layers in the membrane electrode.

[0083] Figure 5 This is a scanning electron microscope (SEM) image of a cross-section of the proton exchange membrane of Comparative Example 2 of the present invention. The perfluorosulfonic acid resin is unevenly filled in the ePTFE, and there are obvious cross sections between the perfluorosulfonic acid resin layers.

[0084] Figure 6 This is a graph showing the tensile strength of the ePTFE-reinforced ultrathin composite proton exchange membrane of Example 1 of the present invention;

[0085] Figure 7 The fuel cell polarization curves of Example 1 of the present invention, Comparative Example 1, and a commercial proton exchange membrane show that the performance of the ultra-thin proton exchange membrane prepared by the filling and molding method of the present invention is significantly improved compared with the commercial Gore 12μm proton exchange membrane, with the maximum power density increased by 28.7%. The conventional coating and drying method used in Comparative Example 1 cannot produce an ultra-thin proton exchange membrane uniformly filled with perfluorosulfonic acid resin.

[0086] Figure 8 The fuel cell polarization curves of Example 2 of the present invention, Comparative Example 2, and a commercial proton exchange membrane show that the performance of the ultra-thin proton exchange membrane prepared by the filling molding method of the present invention is significantly improved compared with the commercial Gore 12μm proton exchange membrane, with the current density at 0.7V increased by 25%. The gradient concentration coating and drying method used in Comparative Example 2 has defects in filling the ePTFE without hydrophilic treatment, and different perfluorosulfonic acid resin layers cannot be tightly bonded, affecting the performance of the ultra-thin proton exchange membrane.

[0087] Figure 9 The fuel cell polarization curves under different humidity conditions of Example 1 of the present invention are shown. The proton exchange membrane prepared by this method can still maintain good membrane electrode performance without external humidification, effectively broadening the humidity application range of ultra-thin proton exchange membranes.

[0088] Figure 10 This is the OCV accelerated durability test curve of Example 3 of the present invention and Comparative Example 3. After adding dihydromyricetin as a free radical scavenger, the OCV of the proton exchange membrane did not significantly attenuate after 216 hours of durability test, and the durability was significantly improved compared with the proton exchange membrane without adding free radical scavengers.

[0089] Therefore, the filling and molding method provided by the present invention can simply and effectively prepare low-cost, high-performance ultra-thin proton exchange membranes, effectively solving the problems of complex preparation procedures of ultra-thin proton exchange membranes and low performance when applied to fuel cells.

[0090] Example 4

[0091] Except for the following changes, the rest is the same as Example 1;

[0092] (1) Using 5 μm 90% porosity ePTFE as the porous membrane;

[0093] (2) The source of perfluorosulfonic acid resin raw material was changed to 3 g of DuPont Nafion 117 proton exchange membrane scraps, the solvent was adjusted to ethanol, isopropanol, N,N-dimethylformamide, the dissolution temperature was adjusted to 125 °C, and the dissolution time was adjusted to 4 h;

[0094] 0.05% polyvinyl alcohol was added as a surfactant and 0.5% CeO2 nanoparticles were added as a free radical scavenger; the solvent was adjusted to acetone and ethanol, and the concentrations of the casting solution were prepared to be 2% and 5%;

[0095] (3) 2% perfluorosulfonic acid casting solution was applied twice, and 5% perfluorosulfonic acid casting solution was applied once. The ultrasonic temperature was adjusted to 30°C, ultrasonication was performed for 10 minutes, and the pre-drying temperature was adjusted to 50°C. The wet film thickness after pre-drying was 50 μm.

[0096] (4) The atmospheric pressure drying temperature was adjusted to 80°C, the drying time was 4 hours, the vacuum heat treatment temperature was 145°C, the heat treatment time was 8 hours, and the proton exchange membrane thickness was 6 μm.

[0097] Consistent with Example 3, the proton exchange membrane ePTFE is completely filled without pores and defects. When used as a proton exchange membrane, the membrane electrode exhibits excellent performance, good airtightness, and improved durability of the ultra-thin proton exchange membrane.

[0098] Example 5

[0099] Except for the following changes, the rest is the same as Example 1;

[0100] (1) 8 μm 85% porosity ePTFE was used as the porous membrane;

[0101] (2) N-methylpyrrolidone was used as the solvent, the dissolution temperature was adjusted to 130°C, and the dissolution time was 4 h; the solvent was adjusted to acetone and isopropanol; 0.035% sodium dodecylbenzenesulfonate was added as a surfactant, and 0.5% MnO2 nanoparticles were added as a free radical scavenger

[0102] (3) 2.5% perfluorosulfonic acid casting solution was applied once, and 5% perfluorosulfonic acid casting solution was applied three times. The ultrasonic temperature was adjusted to 40°C, ultrasonication was performed for 5 minutes, and the pre-drying temperature was adjusted to 30°C. The wet film thickness after pre-drying was 60 μm.

[0103] (4) The atmospheric pressure drying temperature was adjusted to 50°C for 10 h, the vacuum heat treatment temperature was 155°C for 10 h, and the proton exchange membrane thickness was 10 μm.

[0104] Consistent with Examples 1 and 2, the proton exchange membrane obtained by filling and molding has high proton conductivity and low swelling rate. It is used as a proton exchange membrane to prepare a membrane electrode with excellent performance. Tests show that the durability of the proton exchange membrane is effectively improved.

[0105] Example 6

[0106] Except for the following changes, the rest is the same as Example 2;

[0107] (1) 3 μm 90% porosity ePTFE was used as the porous membrane;

[0108] (2) The resin raw material source was changed to 10% perfluorosulfonic acid resin dispersion, and the solvent was not evaporated; the solvent was adjusted to methanol and N, N-dimethylacetamide, the dissolution temperature was adjusted to 130°C, and the dissolution time was 6 hours; the solvent was adjusted to ethanol, and the concentration of the casting solution was 1% and 5%;

[0109] (3) 1% perfluorosulfonic acid casting solution was applied twice, and 5% perfluorosulfonic acid casting solution was applied twice. The ultrasonic temperature was adjusted to 20°C, ultrasonication was carried out for 30 minutes, and the pre-drying temperature was adjusted to 40°C. The wet film thickness after pre-drying was 60 μm.

[0110] (4) The atmospheric pressure drying temperature was adjusted to 65°C, the drying time was 4 h, the vacuum heat treatment temperature was 155°C, the heat treatment time was 10 h, and the proton exchange membrane thickness was 6 μm.

[0111] Consistent with Examples 1 and 2, the thickness of the filled and molded proton exchange membrane is uniform, the perfluorosulfonic acid resin is tightly bonded to the ePTFE, and it is used as a proton exchange membrane to prepare a membrane electrode with excellent performance. Performance tests show that the tensile strength of the proton exchange membrane is significantly improved.

[0112] Example 7

[0113] Except for the following changes, the rest is the same as Example 2;

[0114] (1) 6 μm 90% porosity ePTFE was used as the porous membrane;

[0115] (2) The solvent was adjusted to a mixed solvent of acetone, N,N-dimethylformamide, and N,N-dimethylacetamide, the dissolution temperature was adjusted to 120°C, and the dissolution time was 10 h; the solvent was adjusted to ethanol, and the concentration was 1%, 2.5%,

[0116] 5% casting solution;

[0117] (3) 1% perfluorosulfonic acid casting solution was applied once, 2.5% perfluorosulfonic acid casting solution was applied once, and 5% perfluorosulfonic acid casting solution was applied once. The ultrasonic temperature was adjusted to 10°C, ultrasonication was performed for 5 minutes, and the pre-drying temperature was adjusted to 35°C. The wet film thickness after pre-drying was 50 μm.

[0118] (4) The atmospheric pressure drying temperature was adjusted to 35°C, and the drying was carried out for 24 hours. The vacuum heat treatment temperature was set to 150°C, the heat treatment time was set to 15 hours, and the proton exchange membrane thickness was set to 12 μm.

[0119] Consistent with Examples 1 and 2, the proton exchange membrane ePTFE is completely filled without pores or defects. When used as a proton exchange membrane, the membrane electrode exhibits excellent performance under different humidity conditions, effectively broadening the humidity application range of fuel cells.

[0120] The above embodiments of the present invention are merely examples for the purpose of clearly illustrating the present invention and are not intended to limit the embodiments of the present invention. Those skilled in the art will appreciate that other variations or modifications may be made based on the above description. It is not necessary and impossible to enumerate all embodiments here. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention are intended to be included within the scope of protection of the claims of the present invention.

Claims

1. A method for preparing an ultrathin composite proton exchange membrane, characterized in that: It uses an ultra-thin, high-porosity expanded polytetrafluoroethylene membrane as a skeleton, which is formed by gradient filling with perfluorosulfonic acid resin, pre-drying, and molding. The perfluorosulfonic acid resin is densely filled, and the resulting ultra-thin proton exchange membrane has a thickness of 4-12 microns and a strength of more than 40MPa. The preparation method comprises the following steps: Step 1: Degreasing and cleaning of expanded polytetrafluoroethylene: Use organic solvent to immerse and clean the ePTFE membrane, and after drying, fix it on a hollow tooling mold for later use; Step 2: Preparation of a filling casting solution: After washing, acid exchange, and drying the perfluorosulfonic acid resin raw material, an organic solvent is added, the raw material is placed in a high-pressure reactor, and heated to dissolve to obtain a perfluorosulfonic acid resin solution. When the perfluorosulfonic acid resin solution is used as the raw material, no pretreatment or dissolution process is required. The perfluorosulfonic acid resin solution is taken, an organic solvent and an additive are added, and the mixture is stirred in an ice bath to obtain a filling casting solution. Step 3: Multiple coating and filling of the cast film with gradient solution: A certain amount of low-concentration casting solution is evenly coated on both sides of the ePTFE porous membrane, and ultrasonic treatment is performed to allow the casting solution to fully fill the micropores of the ePTFE porous membrane. After pre-drying to 10μm-50μm, a high-concentration casting solution is used for a second coating and pre-drying. The "coating-pre-drying" step is repeated until the calculated amount of casting solution is coated; the pre-drying temperature is 10-50℃, and the pre-drying time is 1min-2h; Step 4: After the obtained proton exchange membrane is dried under normal pressure, it is subjected to vacuum high-temperature heat treatment to obtain an ultra-thin composite proton exchange membrane; the vacuum high-temperature heat treatment temperature is 100°C-180°C.

2. The method for preparing an ultra-thin composite proton exchange membrane according to claim 1, characterized in that: In step 1, the ePTFE membrane has a thickness of 3-10 μm and a porosity of 70-90%; The cleaning refers to placing the ePTFE membrane in an organic solvent for ultrasonic cleaning, and the organic solvent includes one or more of methanol, ethanol, isopropanol, and acetone.

3. The method for preparing an ultra-thin composite proton exchange membrane according to claim 1, wherein: In step 2, the perfluorosulfonic acid resin raw material includes one or more of perfluorosulfonic acid resin particles, perfluorosulfonic acid resin dispersion, and proton exchange membrane scraps; the perfluorosulfonic acid resin particles include DuPont NafionR resin or 3M PFSR resin; the perfluorosulfonic acid resin dispersion includes DuPont Nafion D2021; and the proton exchange membrane includes DuPont Nafion117, DuPont Nafion211, or DuPont Nafion212.

4. The method for preparing an ultra-thin composite proton exchange membrane according to claim 1, wherein: In step 2, the perfluorosulfonic acid resin raw material is washed and acid-exchanged, which means that the solid perfluorosulfonic acid resin raw material is placed in water, H2O2 solution, and H2SO4 solution in sequence for cleaning and impregnation, wherein the concentration of H2O2 solution is 1%-20%, the concentration of H2SO4 solution is 0.1mol / L-1mol / L, the temperature is 60℃-100℃, and the treatment time is 30min-4h.

5. The method for preparing an ultra-thin composite proton exchange membrane according to claim 1, characterized in that: In step 2, the organic solvent used to dissolve the solid perfluorosulfonic acid resin includes one or more of methanol, ethanol, isopropanol, dimethyl sulfoxide, N,N-dimethylformamide, N,N-dimethylacetamide, and N-methylpyrrolidone; the dissolution temperature for dissolving the solid perfluorosulfonic acid resin is 100°C-200°C, and the dissolution time is 2h-48h.

6. The method for preparing an ultra-thin composite proton exchange membrane according to claim 1, characterized in that: In step 2, the low boiling point solvent used to prepare the casting solution includes a mixture of one or more of ethanol, isopropanol, n-propanol, and acetone, the temperature is -5-30° C., and the concentration of the casting solution ranges from 1% to 25%.

7. The method for preparing an ultra-thin composite proton exchange membrane according to claim 1, characterized in that: In step 2, the additives used to prepare the casting solution include free radical scavengers, surfactants or enhancers; the free radical scavengers include nano-CeO2, MnO2, CrO2, CoO2, Al2O3, dihydromyricetin, epicatechin or quercetin, and the addition amount is 0.1-1wt%; the surfactant is sodium dodecylbenzenesulfonate, polyvinyl alcohol or sodium bile acid, and the addition amount is 0.01-5 wt%; the enhancer is PVDF powder, PTFE powder, and the addition amount is 1-15 wt%.

8. The method for preparing an ultra-thin composite proton exchange membrane according to claim 1, characterized in that: In step three, an ultrasonic method is used to promote the diffusion and filling of the resin in the ePTFE capillary pores; the ultrasonic temperature range is -5-50°C, and the thickness after pre-drying is 10μm-50μm.

9. The method for preparing an ultra-thin composite proton exchange membrane according to claim 1, characterized in that: In step 4, the atmospheric pressure drying temperature is 20° C.-80° C., the atmospheric pressure drying time is 30 min-10 h, the vacuum heat treatment time is 30 min-24 h, and the vacuum degree is 0.1-1 MPa.

10. The ultra-thin composite proton exchange membrane prepared by the preparation method according to any one of claims 1 to 9 is applied to hydrogen fuel cells, methanol fuel cells or hydrogen production by water electrolysis.

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