A proton exchange membrane and a method for preparing the same

Abstract

This invention relates to a proton exchange membrane and its preparation method, comprising the following steps: preparing a composite functional unit, including: dispersing nano-ferroelectric materials in a covalent organic framework; or introducing a precursor for forming the nano-ferroelectric materials into the covalent organic framework and growing the nano-ferroelectric materials in situ within the covalent organic framework; mixing the composite functional unit with a film-forming matrix material to form a casting solution, wherein the mixing method is physical blending or in-situ polymerization of the composite functional unit with monomers for forming the film-forming matrix material; and molding and drying the casting solution to obtain the proton exchange membrane. By combining nano-ferroelectric materials with a covalent organic framework to construct a composite functional unit, and then introducing it into the proton exchange membrane matrix material, the proton dissociation energy can be reduced and the proton conduction rate can be increased; at the same time, cation shuttle is suppressed, improving the overall stability of the membrane, such as temperature resistance, moisture resistance, and swelling resistance.

Description

Technical Field

[0001] This invention relates to the field of membrane materials technology, specifically to a proton exchange membrane and its preparation method. Background Technology

[0002] As a core component in electrochemical devices such as fuel cells, flow batteries, and water electrolysis, the performance of proton exchange membranes directly determines the energy conversion efficiency, power density, and cycle life of these devices. Currently, existing proton exchange membrane technologies generally face two major technical challenges, and there is a difficult trade-off between them: On the one hand, the problem of low ionic conductivity is quite prominent. Due to factors such as insufficient separation of hydrophilic and hydrophobic phases, poor continuity of proton transport channels, and insufficient active sites for proton conduction, the resistance in the proton conduction process is large, which leads to increased internal resistance of the device and decreased energy conversion efficiency. Especially under harsh conditions such as high temperature and low humidity, the conductivity is significantly reduced or even fails. On the other hand, cation shuttle phenomenon is severe. Due to the disordered macroporous structure within the membrane, insufficient electrostatic repulsion, and the driving effect of the high concentration and potential difference between the positive and negative electrodes during operation, Li + Na + Fe 2+ / Fe 3+ Zn 2+ Cations are prone to transmembrane migration, which can lead to a series of problems such as loss of active substances in the electrolyte, cross-contamination between positive and negative electrodes, deterioration of membrane structure and deactivation of catalyst.

[0003] The two major bottlenecks mentioned above severely restrict the further development of electrochemical devices in terms of power density improvement, lifespan extension, and industrial application. Existing single-dimensional modification strategies (such as optimizing only the pore structure or enhancing only the electrostatic repulsion) are insufficient to achieve synergistic optimization of "high ionic conductivity" and "high cation selectivity".

[0004] The above background information is provided only to aid in understanding the concept and technical solution of this application. It does not necessarily belong to the prior art of this application, nor does it necessarily provide technical guidance. In the absence of clear evidence that the above information was disclosed before the filing date of this application, the above background information should not be used to evaluate the novelty and inventiveness of this application. Summary of the Invention

[0005] The purpose of this invention is to provide a proton exchange membrane that can suppress cation shuttle while improving ionic conductivity and a method for preparing the same.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows: This invention provides a method for preparing a proton exchange membrane, comprising the following steps: The method for preparing a composite functional unit includes: dispersing a nano-ferroelectric material in a covalent organic framework; or introducing a precursor for forming the nano-ferroelectric material into the covalent organic framework and growing the nano-ferroelectric material in situ within the covalent organic framework, wherein the covalent organic framework is grafted with oxygen-containing acidic functional groups. The composite functional unit is mixed with a film-forming matrix material to form a casting solution. The mixing method is physical blending, or the composite functional unit is polymerized in situ with a monomer used to form the film-forming matrix material. The casting solution is shaped and dried to obtain the proton exchange membrane.

[0007] This invention combines nano-ferroelectric materials with controllable polarization electric field characteristics with covalent organic frameworks (COFs) with ordered and controllable channels to construct nano-ferroelectric / COF composite functional units. These units are then introduced into proton exchange membrane (PEM) matrix materials, forming a dual-dimensional synergistic modification system of "structural regulation + energy drive," fundamentally resolving the trade-off between "conductivity and cation selectivity." In this innovative system, COF, as a static functional carrier, has a precisely controllable pore size (e.g., controlled to 0.3~0.4 nm), forming ordered and continuous proton transport channels while blocking cations from crossing the membrane through size sieving. The nano-ferroelectric material, as a dynamic control unit, possesses a spontaneous ferroelectric polarization effect, generating a stable and controllable polarization electric field. This field provides directional electrostatic driving force for proton conduction, reducing proton dissociation energy and increasing proton conduction rate; it also constructs a strong electrostatic repulsion barrier, further inhibiting cation shuttle. The two work together to achieve the dual effects of "rapid proton conduction and precise cation blocking," while improving the overall stability of the membrane, such as temperature resistance, humidity resistance, and swelling resistance, providing a brand-new design concept and preparation path for the next generation of high-performance proton exchange membranes.

[0008] In some embodiments, the nano-ferroelectric material is selected from BaTiO3, BiFeO3, KNaNbO3, and PbZr. x Ti 1-x One or more of O3, where 0 < x < 1.

[0009] In this invention, the nano-ferroelectric materials can be prepared by sol-gel method, hydrothermal synthesis method, or microemulsion method, or can be obtained commercially. Furthermore, the nano-ferroelectric materials can be modified with surfactants (such as PVP, used in amounts of 3%~5% of the nano-ferroelectric material's mass); or, by reacting a grafting agent with the nano-ferroelectric materials to graft active groups (such as -NH2, -CHO) onto them, thereby reducing the agglomeration of the nano-ferroelectric materials and improving their compatibility with COF.

[0010] In this invention, the precursors for forming the nano-ferroelectric materials are conventional precursors in the art and are not specifically limited. For example: the precursors for forming BiFeO3 are bismuth nitrate and ferric nitrate; the precursors for forming KNaNbO3 are potassium carbonate, sodium carbonate, and niobium pentoxide; the precursors for forming PbZr... x Ti 1-x The precursors of O3 are lead nitrate, zirconium oxychloride, and tetrabutyl titanate.

[0011] In this invention, the particle size of the nano-ferroelectric material is a conventional particle size in the art. As an example, the particle size of the nano-ferroelectric material is 1 nm to 100 nm.

[0012] In some embodiments, the content of the nano-ferroelectric material is 1% to 7% based on the total solid mass of the casting solution, preferably 2% to 5%, such as 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, and 5%.

[0013] In some embodiments, the mass ratio of the nano-ferroelectric material to the covalent organic framework is 1:(0.2~1), preferably 1:(0.4~0.8), such as 1:0.4, 1:0.5, 1:0.6, 1:0.7, 1:0.8.

[0014] In some embodiments, the method for preparing the covalent organic framework includes: polymerizing an aromatic monomer with a p-phenylenediamine monomer in the presence of a solvent and a catalyst. Wherein, when the p-phenylenediamine monomer does not carry the oxygen-containing acidic functional group, the method further includes: reacting the product of the polymerization reaction with a reagent carrying the oxygen-containing acidic functional group to graft the oxygen-containing acidic functional group onto the product of the polymerization reaction.

[0015] In this invention, the oxygen-containing acidic functional groups include, but are not limited to, -SO3H and -COOH.

[0016] In this invention, the aromatic monomer is a conventional monomer in the art. As an example, the aromatic monomer includes one or more of terephthalaldehyde and 2,4,6-triformylphloroglucinol.

[0017] In this invention, the p-phenylenediamine monomers are conventional monomers in the art. As an example, the p-phenylenediamine monomers include one or more of p-phenylenediamine and p-phenylenediamine-2,5-disulfonic acid.

[0018] Further, the molar ratio of the aromatic monomer to the p-phenylenediamine monomer is 1:(1.0~1.5), preferably 1:(1.1~1.5), such as 1:1.1, 1:1.2, 1:1.3, 1:1.4, 1:1.5.

[0019] In this invention, the catalyst is a catalyst conventionally used in the art. As an example, the catalyst is acetic acid.

[0020] Furthermore, the amount of the catalyst is 0.1 mol% to 5 mol% of the total molar amount of the aromatic monomer and the p-phenylenediamine monomer, preferably 0.5 mol% to 2 mol%.

[0021] In this invention, the solvent is a solvent conventionally used in the art. As an example, the solvent is a mixture of 1,4-dioxane, mesitylene, and / or water.

[0022] Furthermore, the polymerization reaction temperature is 80℃~140℃, more preferably 100℃~140℃, and even more preferably 110℃~130℃.

[0023] In some embodiments, the film-forming matrix material is selected from fluorinated or non-fluorinated matrices. The fluorinated matrix includes perfluorosulfonic acid ion exchange resins (such as Nafion resin); the non-fluorinated matrix includes one or more of polyimide, polybenzimidazole, and sulfonated polyetherketone.

[0024] In this invention, the molding process can employ conventional film-forming processes in the art, such as solvent casting, melt extrusion, or hot pressing.

[0025] In this invention, the drying process can employ conventional drying techniques in the art, such as vacuum drying, hot air drying, or infrared drying. Taking hot air drying as an example, the preferred drying temperature is 50℃~70℃, and the preferred drying time is 10h~14h, to allow the solvent to evaporate slowly and initially form a solid film.

[0026] In some embodiments, the preparation method further includes heat treatment of the dried proton exchange membrane, wherein the peak temperature of the heat treatment is 100°C to 140°C, preferably 110°C to 130°C.

[0027] In some specific implementations, the heat treatment is performed using programmed temperature rise, which includes: holding at a first temperature, and then gradually increasing the temperature to the peak temperature, wherein the temperature rise in each step is 10°C to 30°C.

[0028] Furthermore, the first temperature is 70℃~90℃, and the temperature is maintained at the first temperature for 1h~3h, preferably 1.5h~2.5h.

[0029] Furthermore, the holding time for each heating step is 1h to 3h, preferably 1.5h to 2.5h.

[0030] In some embodiments, the preparation method further includes: first immersing the proton exchange membrane in hydrogen peroxide, and then cleaning it; then immersing the cleaned proton exchange membrane in an acidic solution, and then washing it until neutral.

[0031] In some embodiments, the preparation method further includes subjecting the proton exchange membrane to an external electric field polarization treatment. This external electric field polarization treatment can regulate the formation of stable spontaneous polarization in the nano-ferroelectric material.

[0032] Furthermore, the polarization treatment voltage is 5kV / mm ~ 20kV / mm, the time is 10min ~ 30min, and the temperature is 100℃ ~ 200℃.

[0033] Furthermore, after the external electric field polarization treatment, the proton exchange membrane is annealed (preferably at 80℃~120℃ for 1h~2h) to eliminate internal stress, stabilize the polarization state, improve the membrane structure density and interfacial compatibility, and finally obtain a high-quality nano-ferroelectric material / COF composite material modified proton exchange membrane.

[0034] The present invention also provides a proton exchange membrane prepared by the preparation method described above.

[0035] The proton exchange membrane of the present invention has the following advantages: (1) High cation rejection rate: Through the modification of nano ferroelectric / COF composite functional units, the cation rejection rate can be >99.5%, which inhibits cation shuttle from the source, thereby effectively avoiding problems such as cross-contamination between positive and negative electrodes, loss of active materials and deactivation of catalysts, eliminating safety hazards such as device short circuit and sudden performance drop caused by membrane degradation, and ensuring the safety and stability of electrochemical devices during operation.

[0036] (2) Long service life: The fully covalently bonded framework structure of COF and the high stability of nano-ferroelectric materials can significantly improve the mechanical strength (tensile strength > 25 MPa), swelling resistance (swelling rate < 15.5%) and chemical stability of the proton exchange membrane; at the same time, the strong interfacial bonding between the composite functional unit and the matrix material can effectively reduce the decay of proton conductivity and cation selectivity. Tests show that the conductivity of this proton exchange membrane decreases by < 5% after 1000 hours of continuous operation, which can extend the cycle life of electrochemical devices that rely on this membrane by 2 to 3 times, and significantly reduce the maintenance and replacement costs of the devices.

[0037] (3) Good compatibility: The composite functional unit is universal and can be compatible with various mainstream proton exchange membrane substrates such as fluorine and non-fluorine, without the need for major modifications to the existing membrane substrate preparation process; at the same time, the composite membrane can be adapted to the wide operating conditions of -20℃~180℃, relative humidity 0~100%, pH=1~14, and can match the operating parameters of different types of electrochemical devices such as fuel cells, flow batteries, and water electrolysis, with strong compatibility and wide applicability.

[0038] (4) Wide range of applications: The proton exchange membrane can be widely used in various electrochemical energy devices, including: proton exchange membrane fuel cells (PEMFC, suitable for new energy vehicles, portable power generation equipment, etc.), flow batteries (vanadium redox flow batteries, iron-zinc flow batteries, etc., suitable for large-scale energy storage systems), proton exchange membrane water electrolysis devices (PEMWE, suitable for green hydrogen production), and can also be extended to niche fields such as micro fuel cells and electrolysis oxygen production, with broad market application prospects.

[0039] Due to the application of the above technical solution, the present invention has the following advantages compared with the prior art: This invention combines nano-ferroelectric materials with covalent organic frameworks to construct nano-ferroelectric / COF composite functional units, and then introduces them into the matrix material of proton exchange membranes. This can reduce the proton dissociation energy and increase the proton conduction rate; at the same time, it can inhibit cation shuttle and improve the overall stability of the membrane, such as temperature resistance, humidity resistance, and swelling resistance. Detailed Implementation

[0040] In this invention, unless the context explicitly requires otherwise, the numerical range referred to as "numerical value A to numerical value B" refers to the range including the endpoints A and B. The numerical range referred to as "above" or "below" refers to the numerical range including the stated number. "Optional" or "optional" indicates that certain substances, components, execution steps, application conditions, etc., may or may not be used, and there is no limitation on the manner of use.

[0041] The present invention will be further described below with reference to embodiments. However, the present invention is not limited to the following embodiments. The implementation conditions used in the embodiments can be further adjusted according to different requirements of specific applications, and the implementation conditions not specified are conventional conditions in the industry. The technical features involved in the various embodiments of the present invention can be combined with each other as long as they do not conflict with each other.

[0042] Unless otherwise specified, the reagents and instruments used in the following examples and comparative examples are all commercially available products, or can be prepared with reference to existing technologies.

[0043] Example 1: A BaTiO3@TpPa-SO3H / Nafion composite proton exchange membrane is prepared by the following steps: (1) Weigh the raw materials for later use. The raw materials include: Proton exchange membrane substrate material: perfluorosulfonic acid resin (Nafion) dispersion (DuPont D520, 5wt% aqueous alcohol solution), 100 g (equivalent to 5g solid content).

[0044] Nano-ferroelectric material: Barium titanate nanoparticles (BaTiO3, BTO), average particle size 100 nm, tetragonal phase (with ferroelectric properties), 0.15 g.

[0045] Covalent organic framework (COF) monomers: 2,4,6-tricarboxymethyl phloroglucinol (Tp), 0.1 g; p-phenylenediamine-2,5-disulfonic acid (Pa-SO3H), 0.15 g. These monomers provide proton conduction sites.

[0046] Solvents and reagents: 1,4-dioxane, 50 mL (for COF synthesis); mesitylene, 10 mL (for COF synthesis as a structure-directing agent); glacial acetic acid, 0.002 mL (6 M, as a COF synthesis catalyst); N,N-dimethylformamide (DMF), 50 mL (for washing); deionized water, large quantity; anhydrous ethanol, 150 mL.

[0047] (2) Synthesis of sulfonated COF (TpPa-SO3H): In a round-bottom flask equipped with a magnetic stir bar, 0.1 g Tp and 0.15 g Pa-SO3H were dissolved in a mixed solvent consisting of 50 mL 1,4-dioxane and 10 mL mesitylene. The solution was sonicated for 30 minutes to ensure complete dissolution and dispersion of the monomers. 0.002 mL of 6M glacial acetic acid was slowly added to the clear solution, and the mixture was stirred at room temperature (25℃±5℃) for 10 minutes. The reaction flask was then sealed and placed in an oil bath at 120°C under reflux for 72 hours. After the reaction was completed, the mixture was allowed to cool naturally to room temperature, yielding a deep red precipitate. The precipitate was collected by centrifugation (8000 rpm, 10 minutes) and washed three times each with DMF and anhydrous ethanol to remove unreacted monomers and solvent. Finally, the product was dried in a vacuum oven at 80°C for 12 hours to obtain a deep red powder, namely sulfonic acid functionalized COF (TpPa-SO3H), which was then ground for later use.

[0048] (3) Preparation of nano-ferroelectric material / COF composite material (BTO@TpPa-SO3H): 0.15 g of BaTiO3 nanoparticles were dispersed in 30 mL of anhydrous ethanol and sonicated for 1 hour to obtain a uniform suspension A. 0.1 g of TpPa-SO3H powder synthesized in step (2) was weighed and added to suspension A. The mixture was sonicated for another 2 hours with mechanical stirring (500 rpm) to ensure that the BaTiO3 nanoparticles and COF powder were fully physically mixed and adhered to each other. Subsequently, the mixed suspension was stirred and evaporated at 60°C to remove most of the ethanol. The resulting wet solid material was dried in a vacuum drying oven at 60°C for 6 hours. Finally, the dried block was gently ground to obtain a light yellowish-brown BTO@TpPa-SO3H composite powder.

[0049] (4) Preparation and film formation of composite casting solution: 100 g (equivalent to 5 g solids content) of Nafion D520 dispersion was placed in a beaker, and the BTO@TpPa-SO3H composite powder prepared above (total amount 0.25 g, accounting for approximately 4.76 wt% of the final membrane dry weight; of which BaTiO3 accounts for approximately 2.86% of the membrane dry weight and COF accounts for approximately 1.90% of the membrane dry weight) was added. Then, a mixed solvent of 20 mL anhydrous ethanol and 10 mL deionized water was added to adjust the overall viscosity and dispersibility. The mixture was then placed in an ice-water bath and treated with an ultrasonic cell disruptor (300 W power, 2 seconds of sonication followed by 3 seconds of pause) for 30 minutes to ensure that the composite filler was highly uniformly dispersed in the Nafion solution. Finally, the beaker was placed in an 80°C water bath and slowly evaporated and concentrated under magnetic stirring (300 rpm) until the solution became viscous and slightly fluid (approximately 3-5 hours). This process removed some of the solvent and promoted the initial interaction between the filler and the Nafion molecular chains.

[0050] (5) Casting and post-treatment: The viscous composite casting solution obtained in step (4) was poured onto a clean 20cm×20cm flat glass mold placed horizontally. A glass scraper (with a pre-set gap of 500 μm) was used to scrape the solution at a uniform speed to form a uniform wet film. Then, the scraped wet film, along with the glass plate, was transferred to a forced-air drying oven and dried at 60°C for 12 hours to allow the solvent to evaporate slowly and form a preliminary solid film. After the preliminary drying, the film was carefully peeled off from the glass plate and sandwiched between two polytetrafluoroethylene plates. The film was then placed in a vacuum drying oven and the temperature was programmed to rise: first, the temperature was raised to 80°C and kept at that temperature for 2 hours; then, the temperature was raised to 100°C and kept at that temperature for 2 hours; finally, the temperature was raised to 120°C and kept at that temperature for 2 hours to completely remove residual solvent and promote the microstructure reorganization of Nafion.

[0051] After heat treatment, the membrane is removed and subjected to a standard "activation-protonation" treatment ("activation-protonation" treatment: the membrane is immersed in a 3 wt% hydrogen peroxide (H2O2) aqueous solution in an 80°C water bath for 1 hour to oxidize and remove organic impurities; then rinsed with a large amount (approximately 200~300 mL) of 80°C deionized water, and then immersed in a 0.5 M sulfuric acid (H2SO4) solution in an 80°C water bath for 1 hour to ensure that all sulfonic acid groups are protonated). + The film was then rinsed repeatedly with plenty of 80°C deionized water until neutral and stored in deionized water for later use. A uniform, translucent, 50 ± 5 μm pale yellow composite film was obtained, in which BaTiO3 nanoparticles were partially coated or closely contacted by TpPa-SO3H COF and uniformly dispersed in the Nafion matrix.

[0052] Example 2: A BaTiO3@TpPa-SO3H / Nafion composite proton exchange membrane, the preparation method of which is basically the same as that of Example 1, except that: after step (5), the composite proton exchange membrane is further subjected to external electric field polarization and annealing treatment as follows: Remove the composite proton exchange membrane stored in deionized water and allow it to drain naturally to remove surface moisture. The membrane was placed in a uniform external electric field polarization device with electrode plates and subjected to external electric field polarization treatment under the conditions of electric field strength of 10 kV / mm, polarization temperature of 120℃ and polarization time of 20 min. After polarization, the electric field is kept constant, the temperature is cooled to room temperature, and then the film is placed in a vacuum drying oven and annealed at 100°C for 1 h to stabilize the polarization orientation of the nano-ferroelectric material, eliminate internal stress and improve the stability of the film structure. After annealing, the membrane was naturally cooled to room temperature and then placed back into deionized water for storage, finally yielding a polarization-enhanced BaTiO3@TpPa-SO3H / Nafion composite proton exchange membrane.

[0053] Example 3: This example is basically the same as Example 1, except that an equal amount of Pb(Zr) is used. 0.52 Ti 0.48 Replace BaTiO3 with PZT (Purchased from Aladdin Reagent Co., Ltd.).

[0054] Example 4: This example is basically the same as Example 1, except that the amount of BaTiO3 nanoparticles used is different.

[0055] In this embodiment, the amount of BaTiO3 nanoparticles used is 0.05g, that is, BaTiO3 accounts for about 1% of the dry weight of the membrane.

[0056] Example 5: This example is basically the same as Example 1, except that the amount of BaTiO3 nanoparticles used is different.

[0057] In this embodiment, the amount of BaTiO3 nanoparticles used is 0.27g, that is, BaTiO3 accounts for about 5% of the dry weight of the membrane.

[0058] Example 6: This example is basically the same as Example 1, except that the amount of BaTiO3 nanoparticles used is different.

[0059] In this embodiment, the amount of BaTiO3 nanoparticles used is 0.38g, that is, BaTiO3 accounts for about 7% of the dry weight of the membrane.

[0060] Example 7: This example is basically the same as Example 1, except that the p-phenylenediamine monomer used to prepare COF is different.

[0061] In this embodiment, some of the sulfonated monomer Pa-SO3H is replaced with an equimolar amount of the non-sulfonated monomer p-phenylenediamine (Pa) to reduce the degree of sulfonation of COF; wherein, the amount of Tp is still 0.1 g, the amount of Pa-SO3H is 0.075 g, the amount of p-phenylenediamine (Pa) is 0.042 g, and the molar ratio of Tp to p-phenylenediamine monomers is approximately 1:1.4.

[0062] Example 8: This example is basically the same as Example 1, except that the peak temperature of heat treatment in step (5) is different.

[0063] In this embodiment, the temperature is programmed to rise to 80°C and then kept at that temperature for 2 hours; then rise to 100°C and keep at that temperature for 4 hours. There is no higher temperature range, meaning the peak temperature of the heat treatment is 100°C.

[0064] Example 9: This example is basically the same as Example 1, except that the peak temperature of heat treatment in step (5) is different.

[0065] In this embodiment, the temperature is programmed as follows: first, the temperature is raised to 80°C and kept at that temperature for 2 hours; then, the temperature is raised to 100°C and kept at that temperature for 2 hours; finally, the temperature is raised to 140°C and kept at that temperature for 2 hours, that is, the peak temperature of the heat treatment is 140°C.

[0066] Comparative Example 1: This comparative example provides a Nafion membrane, the preparation method of which is roughly the same as that of Example 1, except that: BTO@TpPa-SO3H composite powder is not added, but 100 g (equivalent to 5 g of solid content) of Nafion D520 dispersion is placed in a beaker, a mixed solvent of 20 mL of anhydrous ethanol and 10 mL of deionized water is added, and the mixture is placed in an 80°C water bath and slowly evaporated and concentrated under magnetic stirring (300 rpm) until the solution becomes viscous and exhibits slight fluidity (about 3 to 5 hours). Then, a pure Nafion membrane is prepared according to the process of step (5) of Example 1, and the final membrane thickness is 50±5 μm.

[0067] Comparative Example 2: This comparative example provides a TpPa-SO3H / Nafion composite membrane, the preparation method of which is roughly the same as that of Example 1, except that: no nano-ferroelectric material is added. That is: 0.1g of TpPa-SO3H powder obtained in step (2) of Example 1 is directly mixed with 100g (equivalent to 5g of solid content) of Nafion D520 dispersion, the casting solution and film formation are prepared according to the process of step (4) of Example 1, and the composite membrane is made according to the process of step (5) of Example 1, with a final membrane thickness of 50±5 μm.

[0068] Comparative Example 3: This comparative example is basically the same as Example 1, except that an equal amount of SrTiO3 (STO, cubic phase, not ferroelectric) is used to replace BaTiO3. The other process parameters are the same as in Example 1, and the final film thickness is 50±5 μm.

[0069] Performance testing: 1. Proton conductivity: Tested according to Chinese National Standard GB / T 20042.3-2022 "Proton Exchange Membrane Fuel Cells Part 3: Test Methods for Proton Exchange Membranes". Test conditions: 80℃, 100%RH.

[0070] 2. Water swelling rate: Tested according to the method specified in Chinese National Standard GB / T 20042.3-2022. Dimensional stability is evaluated by measuring the rate of change of the transverse dimensions of the membrane sample.

[0071] The tests were conducted in accordance with the Chinese National Standard GB / T 20042.3-2022 "Proton Exchange Membrane Fuel Cells - Part 3: Test Methods for Proton Exchange Membranes".

[0072] 3. Tensile strength: The test shall be conducted in accordance with the method specified in Chinese National Standard GB / T 20042.3-2022 to characterize its mechanical properties.

[0073] 4. Vanadium ion permeability: Tested in accordance with Chinese industry standard NB / T 42080-2023 "General technical conditions and test methods for ion conduction membranes for all-vanadium redox flow batteries".

[0074] The performance test results of the above embodiments and comparative examples are shown in Table 1.

[0075] Table 1

[0076] Comparing the experimental results of Example 1, Comparative Example 1, and Comparative Example 2, it is evident that different material systems have significantly different effects on the overall performance of the proton exchange membrane. Compared to the system without functional fillers, the introduction of sulfonated COF alone improved the membrane's proton conductivity, water swelling rate, and vanadium blocking performance. This indicates that the structural characteristics of sulfonated COF itself can initially optimize membrane performance. Further composite with ferroelectric materials based on the introduction of sulfonated COF resulted in a more significant improvement in all membrane properties. This comparative result reveals a synergistic effect mechanism between the two. The polarization electric field of the ferroelectric material can significantly promote proton transport and synergistically limit the excessive swelling of the membrane polymer chains with sulfonated COF, resulting in better dimensional stability of the membrane in a wet state.

[0077] Comparing the experimental results of Example 1 and Example 2, it can be seen that external electric field polarization treatment has a certain regulatory effect on the performance of proton exchange membranes. Example 1 did not undergo external electric field polarization, while Example 2, based on Example 1, added external electric field polarization and annealing treatment. Its proton conductivity slightly decreased to 0.156 S / cm, water swelling ratio slightly decreased to 12.6%, while tensile strength increased to 33.8 MPa, and vanadium ion permeability decreased to 0.65 × 10⁻⁶. -6 m 2 / min. This indicates that external electric field polarization treatment can further stabilize the polarization orientation of ferroelectric materials, enhance the mechanical properties and vanadium blocking properties of the film, and although it has a slight impact on proton conductivity, it improves the overall service performance of the film, verifying the rationality of external electric field polarization as the preferred step.

[0078] Comparing the experimental results of Examples 1, 3, and 3, it can be seen that the intrinsic polarization intensity of different nanoparticles and their interfacial compatibility with the PEM matrix affect the performance of the proton exchange membrane. Among various nanoparticles, BaTiO3 (BTO) with a tetragonal phase structure exhibits better overall performance. Tetragonal BaTiO3 possesses strong ferroelectricity, and its polarization electric field has a more significant orientation effect on protons; in addition, tetragonal BaTiO3 has good compatibility with Nafion. Based on the synergistic effect of the above polarization effect and interfacial compatibility, the composite membrane is superior in both proton conductivity and vanadium blocking performance. Pb(Zr 0.52 Ti 0.48PZT (PZT) has performance close to BTO, but due to its lead content, it is slightly inferior in terms of environmental friendliness and application safety. In contrast, cubic SrTiO3 does not possess ferroelectric properties. When introduced into Nafion membranes as a nanofiller, its role is mainly limited to physical filling, with limited improvement on membrane performance.

[0079] Comparing the experimental results of Examples 1, 4 to 6, it can be seen that the content of ferroelectric material in the membrane affects the performance of the proton exchange membrane. When the content of ferroelectric material in the membrane is too low (less than 1% of the membrane dry weight), the synergistic effect is insufficient; when the content of ferroelectric material in the membrane is too high (more than 7% of the membrane dry weight), nanoparticles are prone to agglomeration, which destroys the membrane uniformity, leading to a decrease in conductivity, an increase in swelling rate, and an increase in vanadium ion permeability. Therefore, the preferred content of ferroelectric material in the membrane is 1% to 7% of the membrane dry weight, and more preferably 2% to 5%.

[0080] Comparing the experimental results of Examples 1 and 7, it is evident that the degree of COF sulfonation affects the performance of the proton exchange membrane. The sulfonic acid group (-SO3H) can serve as a proton hopping site, constructing additional proton transport channels. The degree of COF sulfonation determines its ability to provide proton hopping sites. High-sulfonation COF provides a denser proton transport path and exhibits a stronger synergistic effect with the polarization field of ferroelectric materials, thereby significantly improving membrane conductivity and other properties. Therefore, sulfonated COF with a high degree of sulfonation is preferred.

[0081] Comparing the experimental results of Examples 1, 8, and 9, it is evident that the heat treatment temperature (peak temperature) affects the performance of the proton exchange membrane. When the heat treatment temperature is too low (e.g., below 100°C), the polymer chain rearrangement is insufficient, resulting in a loose membrane structure and poor performance. When the heat treatment temperature is too high (e.g., above 140°C), it may cause localized polymer decomposition, damaging the structural integrity of the membrane. In this case, the performance gain from further increasing the temperature is no longer significant. Therefore, the preferred heat treatment temperature is 100°C to 140°C. Within this temperature range, the polymer can fully rearrange to form a good ion cluster structure and achieve a tight interfacial bond with the nano-ferroelectric material / COF composite material. Furthermore, considering process stability and optimal performance, the preferred heat treatment temperature is 110°C to 130°C.

[0082] In addition, the present invention further studies the proton conductivity decay rate, surface resistance growth rate, and energy efficiency and energy efficiency decay of the proton exchange membranes obtained in Example 1, Comparative Example 1, and Comparative Example 2, and the results are shown in Table 2.

[0083] Table 2

[0084] The test method for proton conductivity decay rate is as follows: First, the proton exchange membrane to be tested is cut to a specified size. Referring to the Chinese National Standard GB / T 20042.3-2022 "Proton Exchange Membrane Fuel Cells Part 3: Test Methods for Proton Exchange Membranes", its initial conductivity under standard test conditions (e.g., 80℃, 100% relative humidity) is measured and recorded as σ0 (unit: S / cm). Subsequently, the same sample is immersed in an accelerated aging test solution, which is a 3% hydrogen peroxide (H2O2) solution at 80℃ (containing 40ppm Fe). 2+ This was used to simulate the highly oxidizing free radical environment generated during the operation of a vanadium redox flow battery. The immersion time was 500 hours. After 500 hours of treatment, the sample was removed, cleaned with deionized water, and dried. Its conductivity was then measured again according to the method described in GB / T 20042.3-2022 and denoted as σ. t The proton conductivity attenuation rate (D) is calculated according to the following formula: D = (σ0 - σ) t ) / σ0 × 100%.

[0085] The smaller the decay rate, the better the durability of the proton exchange membrane in an acidic oxidizing environment.

[0086] Method for measuring sheet resistivity growth rate: A single cell is assembled from the proton exchange membrane to be tested, an anode and cathode gas diffusion layer, and a catalyst layer. The anode and cathode gas diffusion layers and catalyst layer are made of conventional materials available in the art. The assembled single cell is connected to an electrochemical workstation. It is stabilized for 30 minutes at a cell temperature of 80°C and a relative humidity (RH) of 100% for the gas (e.g., hydrogen / nitrogen). Electrochemical impedance spectroscopy (EIS) is used for scanning, with a frequency range of 1 kHz to 10 kHz and a perturbation voltage amplitude of 10 mV. The intersection of the high-frequency impedance spectrum with the real axis is recorded; this intersection value is the high-frequency resistance HFR0 (unit: Ω). The initial sheet resistivity ASR0 (unit: Ω•cm) is calculated according to the following formula. 2 ): ASR0 = HFR0 × A, where A is the effective activation area of ​​a single cell (unit: cm²). 2 ).

[0087] The above-mentioned single cell was continuously operated for 500 hours in an acidic oxidation environment. The specific parameters of the acidic oxidation environment were: temperature 80℃, air / nitrogen gas introduced at the anode, air introduced at the cathode, and gas humidity 100%. After 500 hours of operation, the surface resistivity was measured again under the above test conditions and formula, and recorded as ASR. t The surface resistivity growth rate R is calculated according to the following formula. ASR : RASR =(ASR) t – ASR0) / ASR0 × 100%.

[0088] The lower the growth rate, the stronger the proton exchange membrane's ability to maintain low interfacial resistance under long-term acidic oxidative conditions.

[0089] Test method for the energy efficiency of vanadium redox flow batteries: The proton exchange membrane to be tested is assembled into a single cell. Both the positive and negative electrodes are carbon felt electrodes, and the current collector is a graphite plate. The electrolyte for both the positive and negative electrodes is 1.5 mol / L V. 3+ / V 4+ A mixed solution of +3.0 mol / L H2SO4 was prepared. Before testing, the electrolyte was circulated between the storage tank and the battery using a circulation pump. Test conditions: 25℃ ± 2℃; constant current charge / discharge; current density 80 mA / cm². 2 The charging cutoff condition is a voltage of 1.55 V; the discharging cutoff condition is a voltage of 0.8 V; at least three complete charge-discharge cycles must be performed consecutively. Record the charging energy (E) of the third cycle. c (Unit: Wh), discharge energy (E) d (Unit: Wh). Calculate the energy efficiency EE0 using the following formula: Energy efficiency EE0 = (E d / E c ) × 100%.

[0090] Calculate the energy efficiency decay rate ΔEE after 500 cycles using the following formula: ΔEE = (EE0 – EE) 500 ) / EE0× 100%, EE 500 The energy efficiency for the 500th cycle.

[0091] In summary, compared with the comparative example, the composite membrane of the present invention maintains a lower rate of conductivity decay and a lower rate of sheet resistance increase, indicating that it can maintain stable ion transport capability during long-term operation. Simultaneously, its energy efficiency is highest in the initial state and exhibits the smallest decay over time. These technical effects demonstrate that the composite membrane of the present invention performs excellently in suppressing performance degradation and maintaining energy conversion efficiency, thereby endowing the corresponding electrochemical device with higher operational safety and a longer cycle life.

[0092] The present invention has been described in detail above, with the aim of enabling those skilled in the art to understand and implement the invention. However, this description should not be construed as limiting the scope of protection of the invention. All equivalent changes or modifications made in accordance with the spirit and essence of the invention should be included within the scope of protection of the invention.

Claims

1. A method for preparing a proton exchange membrane, characterized in that, Includes the following steps: The method for preparing a composite functional unit includes: dispersing a nano-ferroelectric material in a covalent organic framework; or introducing a precursor for forming the nano-ferroelectric material into the covalent organic framework and growing the nano-ferroelectric material in situ within the covalent organic framework, wherein the covalent organic framework is grafted with oxygen-containing acidic functional groups. The composite functional unit is mixed with a film-forming matrix material to form a casting solution. The mixing method is physical blending, or the composite functional unit is polymerized in situ with a monomer used to form the film-forming matrix material. The casting solution is shaped and dried to obtain the proton exchange membrane.

2. The method for preparing a proton exchange membrane according to claim 1, characterized in that, The nano-ferroelectric material is selected from BaTiO3, BiFeO3, KNaNbO3, and PbZr. x Ti 1-x One or more of O3, wherein 0 < x < 1; and / or, The particle size of the nano-ferroelectric material is 1 nm to 100 nm.

3. The method for preparing a proton exchange membrane according to claim 1 or 2, characterized in that, Based on the total solid mass of the casting solution, the content of the nano-ferroelectric material is 1% to 7%.

4. The method for preparing a proton exchange membrane according to claim 1, characterized in that, The mass ratio of the nano-ferroelectric material to the covalent organic framework is 1:(0.2~1).

5. The method for preparing a proton exchange membrane according to claim 1 or 4, characterized in that, The method for preparing the covalent organic framework includes: polymerizing an aromatic monomer with a p-phenylenediamine monomer in the presence of a solvent and a catalyst; Wherein, when the p-phenylenediamine monomer does not carry the oxygen-containing acidic functional group, the method further includes: reacting the product of the polymerization reaction with a reagent carrying the oxygen-containing acidic functional group to graft the oxygen-containing acidic functional group onto the product of the polymerization reaction.

6. The method for preparing a proton exchange membrane according to claim 5, characterized in that, The aromatic monomers include one or more of terephthalaldehyde and 2,4,6-tricarboxymethyl phloroglucinol; and / or The p-phenylenediamine monomers include one or more of p-phenylenediamine and p-phenylenediamine-2,5-disulfonic acid; and / or The molar ratio of the aromatic monomer to the p-phenylenediamine monomer is 1:(1.0-1.5); and / or, The catalyst is acetic acid, and the amount of the catalyst used is 0.1 mol% to 5 mol% of the total molar amount of the aromatic monomer and the p-phenylenediamine monomer; and / or, The solvent is a mixture of 1,4-dioxane and mesitylene and / or water; and / or, The polymerization reaction is carried out at a temperature of 80℃ to 140℃.

7. The method for preparing a proton exchange membrane according to claim 1, characterized in that, The film-forming matrix material is selected from fluorine-based or non-fluorine-based matrices. The fluorine matrix includes perfluorosulfonic acid type ion exchange resin; The non-fluorinated matrix includes one or more of polyimide, polybenzimidazole, and sulfonated polyetherketone.

8. The method for preparing a proton exchange membrane according to claim 1, characterized in that, The preparation method further includes heat treatment of the proton exchange membrane, wherein the peak temperature of the heat treatment is 100℃~140℃.

9. The method for preparing a proton exchange membrane according to claim 1, characterized in that, The preparation method further includes: first immersing the proton exchange membrane in hydrogen peroxide, and then cleaning it; then immersing the cleaned proton exchange membrane in an acidic solution, and then washing it until neutral.

10. The proton exchange membrane prepared by any one of claims 1 to 9.

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