A nanocomposite film doped with acid-doped pyridine type side chains

Abstract

This application relates to the field of proton exchange membrane technology and discloses a nanocomposite membrane with acid-doped pyridine side chains. The nanocomposite membrane comprises: a bifunctional polyarylether ketone-ketone polymer containing pyridine side groups; hexagonal boron nitride nanosheets (h-BN); and phosphoric acid. The polymer is obtained by copolymerizing monomers containing pyridine side groups (2,6-bis(4-hydroxyphenyl)pyridine) and monomers containing vinyl side chains (4,4'-(1,2-vinyl)diphenol). The composite membrane is prepared by forming a base membrane from the polymer and h-BN, followed by phosphoric acid solution doping, and then heat treatment at 140°C to 180°C to initiate vinyl crosslinking. This invention utilizes the acid-base adsorption of phosphoric acid by pyridine groups and the physical locking effect of the thermal crosslinking network on phosphoric acid to construct a dual dopant retention mechanism. The nanosheets provide physical barriers. This composite membrane exhibits high proton conductivity, low vanadium ion permeability, and excellent long-term stability.

Description

Technical Field

[0001] This invention relates to the field of proton exchange membrane technology, specifically to a nanocomposite membrane with acid-doped pyridine side chains. Background Technology

[0002] Vanadium redox flow battery (VRFB) has attracted much attention as a large-scale energy storage technology due to its advantages such as flexible design, long lifespan, and high safety. In the VRFB system, the proton exchange membrane is its core component, which plays a key role in conducting protons and blocking vanadium ions of different valence states in the positive and negative electrode electrolytes. Its performance directly determines the battery's energy efficiency, coulombic efficiency, and cycle life.

[0003] Currently, although commercially available perfluorosulfonic acid membranes (such as Nafion membranes) have good chemical stability and proton conductivity, they are expensive and suffer from severe vanadium ion permeation problems in high-concentration vanadium ion environments, leading to severe battery self-discharge and reduced coulombic efficiency.

[0004] As an alternative, acid-doped polymer films, especially phosphate (PA)-doped films, have become a research hotspot due to their lack of external humidification requirements, potential high conductivity, and low cost. However, these films still face significant challenges in practical applications. A key issue is dopant loss: phosphate molecules have weak bonding with the polymer matrix and are easily leached out during long-term contact and scouring with the liquid electrolyte, leading to a rapid decrease in the film's proton conductivity and severely impacting the long-term operational stability of the battery.

[0005] Furthermore, many polymer membrane materials exhibit excessive swelling in strongly acidic and oxidizing electrolyte environments. This poor dimensional stability not only reduces the membrane's mechanical strength and durability but also exacerbates vanadium ion cross-permeation, further reducing the battery's ion selectivity and energy efficiency. Therefore, developing a proton exchange membrane that combines high proton conductivity, excellent dopant retention, high ion selectivity (low vanadium permeation), and good mechanical and dimensional stability is a crucial technical challenge that urgently needs to be addressed to advance the commercialization of all-vanadium redox flow battery technology. Summary of the Invention

[0006] To address the shortcomings of existing technologies, this invention provides a nanocomposite membrane with acid-doped pyridine side chains. Existing acid-doped proton exchange membranes suffer from problems such as easy loss of phosphate dopants, high vanadium ion permeability, and poor mechanical stability due to severe liquid absorption and swelling in vanadium redox flow batteries.

[0007] To achieve the above objectives, the present invention provides a nanocomposite film with acid-doped pyridine side chains. In a first aspect, the present invention provides a nanocomposite film with acid-doped pyridine side chains, employing the following technical solution:

[0008] A nanocomposite film with acid-doped pyridine side chains, comprising:

[0009] a) A bifunctional polyarylether ketone polymer containing pyridine side groups;

[0010] b) Hexagonal boron nitride nanosheets;

[0011] c) Phosphoric acid;

[0012] The bifunctional polyarylether ketone polymer containing pyridine side groups is obtained by polymerization of monomers in the following molar ratios: (1) Diphenol monomers (the sum of the molar percentages of the three components is 100 mol%): 2,6-bis(4-hydroxyphenyl)pyridine: 10 mol% to 90 mol% of the total number of diphenol monomers; 4,4'-(1,2-vinyl)diphenol: 5 mol% to 85 mol% of the total number of diphenol monomers; bisphenol A: 5 mol% to 25 mol% of the total number of diphenol monomers; (2) The ratio of dihalogenated monomers to diphenol monomers: the dihalogenated monomer is 4,4'-difluorobenzophenone, and the ratio of the total number of the three diphenol monomers to the number of 4,4'-difluorobenzophenone is 0.95:1 to 1.05:1;

[0013] The amount of hexagonal boron nitride nanosheets added is 1 wt% to 10 wt% of the polymer mass; the composite film is doped with a phosphoric acid solution with a concentration of 80 wt% to 95 wt% and then subjected to heat treatment crosslinking at 140°C to 180°C.

[0014] By adopting the above technical solution, this invention constructs a multifunctional synergistic composite membrane system, the innovative mechanism of which is reflected in the following aspects:

[0015] Constructing a dual proton conduction and locking mechanism:

[0016] (i) Acid-base adsorption: The 2,6-bis(4-hydroxyphenyl)pyridine monomer unit in the polymer chain provides a pyridine-type side chain. The nitrogen atom on the pyridine ring acts as a basic site and interacts strongly with phosphate (c), which is a proton conductor, thereby efficiently enriching and anchoring phosphate molecules and constructing a high-density proton conduction channel.

[0017] (ii) Crosslinking Locking: The 4,4'-(1,2-vinyl)diphenol monomer unit in the polymer chain provides vinyl (C=C) side chains. After phosphoric acid doping (step 7), the vinyl functional groups undergo thermally induced polymerization or cyclization reactions through heat treatment at 140°C to 180°C (step 8), forming a stable three-dimensional covalent crosslinked network in the polymer matrix.

[0018] (iii) Synergistic effect: This specific process sequence of doping followed by crosslinking enables the formed crosslinked network to physically confine (lock) the phosphoric acid molecules adsorbed by the pyridine groups. This dual (chemical adsorption + physical locking) mechanism greatly improves the membrane's retention rate of phosphoric acid, effectively suppresses dopant loss caused by electrolyte scouring during battery operation, and ensures long-term stable high proton conductivity of the membrane (e.g., 0.12 S / cm to 0.16 S / cm).

[0019] Achieving high ion selectivity:

[0020] The hexagonal boron nitride (h-BN) nanosheets (b) are uniformly dispersed in the polymer matrix as an inorganic filler. The h-BN nanosheets are non-conductive and have a dense structure, forming a physical barrier inside the composite film.

[0021] This barrier effect significantly increases the diffusion path of vanadium ions and other permeable substances (i.e., increases the path tortuosity), while the aforementioned cross-linked network structure also effectively inhibits excessive swelling of the membrane. The synergistic effect of these two factors substantially reduces the vanadium ion permeability of the membrane (e.g., to 0.60 × 10⁻⁶). -8 cm 2 / min to 1.00×10 -8 cm 2 ( / min), thereby significantly improving the coulombic efficiency and energy efficiency of vanadium redox flow batteries (VRFB).

[0022] In summary, this invention successfully solves the technical bottlenecks of traditional proton exchange membranes in terms of conductivity, ion selectivity, and long-term stability by combining the functional design of polymer structures (pyridine adsorption sites and vinyl crosslinking sites) with the synergistic construction of composite materials (h-BN barrier and crosslinking network confinement).

[0023] Preferably, the bifunctional polyarylether ketone polymer containing pyridine side groups is obtained by polymerization of monomers in the following molar ratios: 2,6-bis(4-hydroxyphenyl)pyridine accounting for 20 mol% to 70 mol% of the total number of diphenol monomers; 4,4'-(1,2-vinyl)diphenol accounting for 10 mol% to 70 mol% of the total number of diphenol monomers; and bisphenol A accounting for 10 mol% to 20 mol% of the total number of diphenol monomers.

[0024] Preferably, the amount of hexagonal boron nitride nanosheets added is 2 wt% to 7 wt% of the mass of the bifunctional polyarylether ketone polymer with pyridine side groups.

[0025] Preferably, the composite membrane is doped with a phosphoric acid solution with a concentration of 82wt% to 88wt% and then subjected to heat treatment crosslinking at 150°C to 170°C.

[0026] Preferably, the thickness of the composite film is 40 μm to 70 μm.

[0027] Preferably, the composite membrane exhibits a proton conductivity of 0.12 S / cm to 0.16 S / cm at 20°C to 30°C and a relative humidity of 90%; and a vanadium ion permeability of 0.60 × 10⁻⁶ at 20°C to 30°C. -8 cm 2 / min to 1.00×10 -8 cm 2 / min.

[0028] Preferably, the composite membrane is used as a proton exchange membrane in a vanadium redox flow battery.

[0029] Secondly, the present invention provides a method for preparing a nanocomposite film with acid-doped pyridine side chains as described in the first aspect, employing the following technical solution:

[0030] A method for preparing a nanocomposite film with acid-doped pyridine side chains includes the following steps:

[0031] Polymer synthesis: Under an inert atmosphere, 2,6-bis(4-hydroxyphenyl)pyridine, 4,4'-(1,2-vinyl)diphenol, bisphenol A (three diphenol monomers), 4,4'-difluorobenzophenone, and anhydrous potassium carbonate were dissolved in a mixed solvent of N-methyl-2-pyrrolidone and toluene. The mixture was dehydrated by azeotropic reaction at 130°C to 170°C, followed by polymerization at 180°C to 200°C to obtain a bifunctional polyarylether ketone ketone polymer solution containing pyridine side groups.

[0032] Slurry preparation: Hexagonal boron nitride nanosheets were ultrasonically treated in an ice-water bath for 1 to 3 hours to complete dispersion; then added to the polymer solution obtained in step 1) and mixed evenly to obtain a composite slurry.

[0033] Base film formation: The composite slurry is uniformly coated onto a clean substrate, dried at 70°C to 90°C for 3 to 5 hours, and then dried under vacuum at 110°C to 130°C for 20 to 28 hours to form the composite film base film.

[0034] Phosphoric acid doping: The composite film base film is immersed in a phosphoric acid solution with a concentration of 80wt% to 95wt% and is doped at a constant temperature of 70℃ to 90℃ for 20h to 28h.

[0035] Heat treatment crosslinking: The phosphate-doped membrane is taken out and heat-treated at 140°C to 180°C for 1.5 to 2.5 hours to obtain the nanocomposite membrane.

[0036] By adopting the above technical solution, the method provided by the present invention has clear process logic and synergistic effects:

[0037] Steps 1) to 3) ensure the uniform composite of the polymer matrix with specific functional sites (pyridyl and vinyl groups) and the h-BN filler through controlled polymerization and film formation processes, providing a structural basis for subsequent functional realization.

[0038] The key lies in the order of steps 4) and 5):

[0039] Step 4) Perform phosphoric acid doping for a long time (20-28h) under mild conditions (70-90℃) to ensure that the phosphoric acid molecules have enough time and driving force to fully penetrate into the polymer and complete acid-base adsorption with the pyridine groups.

[0040] Step 5) After doping is completed, the temperature is rapidly increased to the crosslinking temperature (140-180℃) to activate the vinyl group to undergo a crosslinking reaction.

[0041] The process design of doping followed by crosslinking is a necessary approach to achieve the crosslinking lock-in mechanism described in the first aspect. If the order is reversed, premature crosslinking will lead to a dense network, severely hindering the subsequent entry of phosphate. Therefore, this method ensures a balance between high doping level and high crosslinking degree (i.e., high stability).

[0042] Preferably, in step 4), the concentration of the phosphoric acid solution is 82 wt% to 88 wt%.

[0043] Preferably, in step 5), the temperature of the heat treatment crosslinking is 150°C to 170°C.

[0044] This invention provides a nanocomposite film with acid-doped pyridine side chains. It possesses the following beneficial effects:

[0045] 1. This invention constructs a unique dual mechanism of "acid-base adsorption-crosslinking locking" by introducing 2,6-bis(4-hydroxyphenyl)pyridine and 4,4'-(1,2-vinyl)diphenol monomers into the polymer side chain. The pyridine group, acting as a basic site, efficiently enriches phosphoric acid through strong hydrogen bonding, constructing a continuous proton transport channel; while the three-dimensional crosslinking network formed by the vinyl group after phosphoric acid doping and heat treatment can physically confine (lock) the adsorbed phosphoric acid molecules. This synergistic effect effectively solves the problem of easy phosphoric acid loss in traditional acid-doped films, enabling the composite film to maintain high proton conductivity (0.12-0.16 S / cm) while possessing excellent long-term chemical stability.

[0046] 2. This invention introduces layered hexagonal boron nitride (h-BN) nanosheets into a polymer matrix. Utilizing their non-conductive and dense structure, a physical barrier is constructed within the membrane, significantly increasing the tortuosity of the vanadium ion diffusion path. Combined with thermal cross-linking to form a dense polymer network structure, this further limits excessive swelling of the membrane in the electrolyte. The combination of these two factors reduces the vanadium ion permeability to 0.60 × 10⁻⁶. -8 cm 2 / min to 1.00×10 -8 cm 2 / min, thereby effectively suppressing cross-contamination in vanadium redox flow batteries and improving the coulombic efficiency and energy efficiency of the batteries.

[0047] 3. This invention employs rigid polyaryletherketone ketone as the main chain backbone, combined with the reinforcing effect of inorganic nanofiller h-BN and the thermal crosslinking effect of vinyl side chains, significantly enhancing the mechanical properties and heat resistance of the membrane material. Even in the operating environment of a vanadium redox flow battery with strong acid and strong oxidizing properties, this composite membrane can still maintain good structural integrity and resist mechanical deformation caused by liquid absorption and swelling, thereby extending the service life of the battery separator. Detailed Implementation

[0048] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0049] Preparation Examples 1-3:

[0050] Preparation Example 1: Preparation of Bifunctional PyPEKK Polymer (PyPEKK-H-Crosslink)

[0051] In a 500 mL three-necked flask equipped with a mechanical stirrer, a nitrogen inlet, and a condenser with a Dean-Stark water separator, 2,6-bis(4-hydroxyphenyl)pyridine (26.3 g, 0.01 mol), 4,4'-(1,2-vinyl)diphenol (16.98 g, 0.08 mol), bisphenol A (22.8 g, 0.01 mol), 4,4'-difluorobenzophenone (21.82 g, 0.1 mol), anhydrous potassium carbonate (17.97 g, 0.13 mol), NMP (200 mL), and toluene (100 mL) were added. Under nitrogen protection, the mixture was heated to 150 °C and refluxed for 3 h. Water generated during the reaction was removed by azeotropic distillation using a water separator. Toluene was then distilled off, and the reaction mixture was heated to 190 °C and reacted at this temperature for 12 h. After the reaction was complete, the viscous reaction solution was cooled to approximately 60°C and slowly poured into vigorously stirred deionized water containing a small amount of hydrochloric acid, resulting in the precipitation of a white fibrous precipitate. The precipitate was collected by filtration and washed thoroughly several times with deionized water and methanol, respectively. Finally, the obtained polymer was dried in a vacuum oven at 80°C for 24 hours to obtain the target product PyPEKK-H-Crosslink. In this polymer, the molar ratio of pyridine-containing bisphenol monomers to crosslinkable bisphenol monomers is approximately 1:8.

[0052] Preparation Example 2: Preparation of Bifunctional PyPEKK Polymer (PyPEKK-Balanced)

[0053] Except for the monomer feed amounts, the equipment and operating procedures were exactly the same as in Preparation Example 1. The feed amounts were: 2,6-bis(4-hydroxyphenyl)pyridine (10.53 g, 0.04 mol), 4,4'-(1,2-vinyl)diphenol (8.49 g, 0.04 mol), bisphenol A (4.57 g, 0.02 mol), 4,4'-difluorobenzophenone (21.82 g, 0.1 mol), and anhydrous potassium carbonate (17.97 g, 0.13 mol). The target product, PyPEKK-Balanced, was finally obtained. In this polymer, the molar ratio of pyridine-containing bisphenol monomers to crosslinkable bisphenol monomers was approximately 1:1.

[0054] Preparation Example 3: Preparation of Bifunctional PyPEKK Polymer (PyPEKK-H-Adsorb)

[0055] Except for the monomer feed amounts, the equipment and operating procedures were exactly the same as in Preparation Example 1. The feed amounts were: 2,6-bis(4-hydroxyphenyl)pyridine (21.06 g, 0.08 mol), 4,4'-(1,2-vinyl)diphenol (2.12 g, 0.01 mol), bisphenol A (2.28 g, 0.01 mol), 4,4'-difluorobenzophenone (21.82 g, 0.1 mol), and anhydrous potassium carbonate (17.97 g, 0.13 mol). The target product, PyPEKK-H-Adsorb, was finally obtained. In this polymer, the molar ratio of pyridine-containing bisphenol monomers to crosslinkable bisphenol monomers was approximately 8:1.

[0056] Examples 1-4:

[0057] Example 1:

[0058] This embodiment provides a nanocomposite film with acid-doped pyridine side chains. The preparation method of the nanocomposite film uses the PyPEKK-Balanced polymer obtained in Preparation Example 2, and the amount of h-BN added is 3 wt% of the polymer mass. The specific steps include:

[0059] (1) 0.06 g of hexagonal boron nitride (h-BN) nanosheets were added to 50 mL of N-methyl-2-pyrrolidone (NMP) and ultrasonically treated for 2 h under ice-water bath conditions to obtain a uniformly dispersed h-BN suspension.

[0060] (2) Add 20g of the PyPEKK-Balanced polymer powder prepared in Preparation Example 2 to the above suspension, and mechanically stir at 60°C for 24h until the polymer is completely dissolved to form a uniform and viscous casting slurry.

[0061] (3) The slurry obtained in step (2) is uniformly coated onto a clean glass plate. It is first placed in an oven at 80°C and dried for 4 hours, and then transferred to a vacuum oven at 120°C and dried for another 24 hours to completely remove the solvent. After cooling, the glass plate carrying the film is immersed in deionized water to peel off the base film of the h-BN / PyPEKK composite film.

[0062] (4) Immerse the composite membrane base obtained in step (3) in 85wt% phosphoric acid solution and dope at 80℃ for 24h.

[0063] (5) Take out the doped membrane, quickly wipe off the residual phosphoric acid on its surface with absorbent paper, and then place it in a vacuum oven for heat treatment and crosslinking at 160°C for 2 hours. After natural cooling, the final nanocomposite membrane is obtained. After peeling off the dried composite membrane base, its thickness is measured to be between 40 μm and 60 μm.

[0064] Example 2:

[0065] This embodiment provides a nanocomposite film with acid-doped pyridine side chains. The main difference between the preparation method of the nanocomposite film and that of Example 1 is that the amount of h-BN added is increased to 5 wt% of the polymer mass, specifically including the following steps:

[0066] (1) 0.1 g of hexagonal boron nitride (h-BN) nanosheets were added to 50 mL of NMP and ultrasonically treated for 2 h under ice-water bath conditions to obtain a uniformly dispersed h-BN suspension.

[0067] (2) Add 20g of the PyPEKK-Balanced polymer powder prepared in Preparation Example 2 to the above suspension, and mechanically stir at 60°C for 24h until the polymer is completely dissolved to form a uniform and viscous casting slurry.

[0068] (3) The subsequent operation steps are exactly the same as steps (3), (4) and (5) in Example 1. Finally, a nanocomposite film is obtained. After the dried composite film base film is peeled off, its thickness is measured to be between 40 μm and 60 μm.

[0069] Example 3:

[0070] This embodiment provides a nanocomposite film with acid-doped pyridine side chains. The main difference between the preparation method of the nanocomposite film and that of Example 1 is that the PyPEKK-H-Crosslink polymer with high crosslinking functional group density obtained in Example 1 is used. The specific steps include:

[0071] (1) 0.06 g of h-BN nanosheets were added to 50 mL of NMP and ultrasonically treated for 2 h under ice-water bath conditions to obtain a uniformly dispersed h-BN suspension.

[0072] (2) Add 20g of the PyPEKK-H-Crosslink polymer powder prepared in Preparation Example 1 to the above suspension, and mechanically stir at 60°C for 24h until the polymer is completely dissolved to form a uniform and viscous casting slurry.

[0073] (3) The subsequent operation steps are exactly the same as steps (3), (4) and (5) in Example 1. Finally, a nanocomposite film is obtained. After the dried composite film base film is peeled off, its thickness is measured to be between 40 μm and 60 μm.

[0074] Example 4:

[0075] This embodiment provides a nanocomposite film with acid-doped pyridine side chains. The main difference between the preparation method of the nanocomposite film and that of Example 1 is that the PyPEKK-H-Adsorb polymer with high pyridine adsorption site density obtained in Preparation Example 3 is used. The specific steps include:

[0076] (1) 0.06 g of h-BN nanosheets were added to 50 mL of NMP and ultrasonically treated for 2 h under ice-water bath conditions to obtain a uniformly dispersed h-BN suspension.

[0077] (2) Add 20g of the PyPEKK-H-Adsorb polymer powder prepared in Preparation Example 3 to the above suspension, and mechanically stir at 60°C for 24h until the polymer is completely dissolved to form a uniform and viscous casting slurry.

[0078] (3) The subsequent operation steps are exactly the same as steps (3), (4) and (5) in Example 1. Finally, a nanocomposite film is obtained. After the dried composite film base film is peeled off, its thickness is measured to be between 40 μm and 60 μm.

[0079] Comparative Examples 1-5:

[0080] Comparative Example 1: Composite membrane without post-treatment crosslinking

[0081] Compared with Example 1, the difference is that step (5) is completely omitted, that is, after the phosphoric acid doping is completed, no heat treatment crosslinking step is performed, and the rest are the same.

[0082] Comparative Example 2: Composite membrane without h-BN filler

[0083] Compared with Example 1, the difference is that step (1) is omitted, and only 20g of the PyPEKK-Balanced polymer powder prepared in Preparation Example 2 is added in step (2), that is, no hexagonal boron nitride (h-BN) nanosheets are added, and the rest are the same.

[0084] Comparative Example 3: Cross-linked composite membrane without pyridine adsorption groups

[0085] Compared with Example 1, the difference is that the polymer used in step (2) is replaced with a polyarylether ketone ketone that does not contain pyridine adsorption groups but contains crosslinkable active groups (this polymer can be prepared according to the preparation example, but using all bisphenol A and 4,4'-(1,2-vinyl)diphenol as diphenol monomers), and the rest of the steps are the same as in Example 1.

[0086] Comparative Example 4: Traditional sulfonated polyetheretherketone (SPEEK) membrane

[0087] This comparative example uses a commercially available sulfonated polyether ether ketone (SPEEK) membrane or a SPEEK membrane prepared according to known literature methods, and is protonated according to standard methods as a comparative reference for the composite membrane of this invention.

[0088] Comparative Example 5: Phosphoric acid-doped polybenzimidazole (PBI) film

[0089] This comparative example uses a commercially available polybenzimidazole (PBI) membrane or a PBI membrane prepared according to known literature methods, and is treated under the same phosphoric acid doping conditions as in Example 1 (i.e., immersed in an 85 wt% phosphoric acid solution and doped at 80°C for 24 h), serving as a comparative reference for the composite membrane of this invention.

[0090] Test Examples 1-6:

[0091] Test 1: Verification of cross-linked network formation (solvent solubility test)

[0092] Experimental objective:

[0093] This test aims to verify that a chemical cross-linking network was successfully formed in the nanocomposite membrane prepared in the embodiments of the present invention through a post-treatment cross-linking step, and to evaluate the impact of this cross-linking network on the macroscopic structural stability of the membrane material.

[0094] Experimental steps:

[0095] Sample preparation: Three square membrane pieces, each measuring 2 cm × 2 cm, were cut from the dried nanocomposite membrane obtained in Example 1 and the composite membrane obtained in Comparative Example 1. The initial mass of each membrane piece was accurately weighed using an analytical balance and recorded.

[0096] Solvent immersion: Place each membrane in an independent glass sample vial and add 10 mL of N-methyl-2-pyrrolidone (NMP) solvent, ensuring that the membrane is completely immersed in the solvent.

[0097] Temperature control: Seal the sample bottle and place it in a 60℃ constant temperature oven for 24 hours.

[0098] Results observation and recording: After 24 hours, the sample vials were removed. The macroscopic state of each membrane in NMP solvent was visually observed, including its shape integrity, degree of dissolution, and clarity of the solution, and the results were recorded.

[0099] Test data

[0100] Table 1: Results of membrane solvent immersion test

[0101]

[0102] in conclusion:

[0103] Test results show that the macroscopic shape and structure of the post-treated crosslinked nanocomposite membrane from Example 1 remained intact after immersion in NMP solvent for 24 hours, with only swelling observed. This is consistent with the property of NMP as a good solvent for this polymer, but the non-solubility of the membrane clearly indicates that a stable chemical crosslinking network has been formed between the polymer molecular chains, effectively inhibiting the free movement and dissolution of the polymer molecular chains.

[0104] In contrast, the uncrosslinked composite film of Comparative Example 1 dissolved completely in NMP solvent under the same conditions, forming a homogeneous polymer solution. This confirms that polymer chains can be freely dispersed and dissolved in solvents even without crosslinking structures.

[0105] The experimental results directly verify the effectiveness of the post-processing crosslinking step in this invention. By introducing crosslinkable active groups and performing thermally initiated crosslinking after phosphoric acid doping, an insoluble chemical network was successfully constructed. The formation of this crosslinked network is a key innovative mechanism for effectively locking in the phosphoric acid dopant, preventing its loss, and maintaining the long-term structural stability of the membrane. The establishment of the crosslinked network provides a structural basis for the dimensional stability, mechanical strength, and phosphoric acid retention capacity of the membrane material in the operating environment of a vanadium redox flow battery.

[0106] Test 2: Verification of Phosphoric Acid (PA) Lock-in Effect (Immersion Stability Test)

[0107] Experimental Objective

[0108] This test aims to evaluate the ability of the cross-linked composite membrane of the present invention to retain phosphate (PA) dopants. By comparing the PA weight loss rate and proton conductivity retention rate of the membrane material before and after water immersion, the effectiveness of the post-treatment cross-linking step in inhibiting PA loss is verified.

[0109] Experimental steps:

[0110] Sample preparation: Three 2 cm × 2 cm membrane pieces were cut from the nanocomposite membrane prepared in Example 1 and the composite membrane prepared in Comparative Example 1, respectively. The initial mass of each membrane piece was accurately weighed using an analytical balance and recorded. Subsequently, at 25°C and 90% relative humidity, the initial proton conductivity of each membrane was measured using an AC impedance spectrometer. ).

[0111] Accelerated aging soaking: Place each membrane sheet in an independent sealed container and add sufficient deionized water to ensure complete immersion. Place the containers in an 80°C constant temperature water bath and soak for 24 hours.

[0112] Post-processing and measurement: After soaking, remove the membrane sheets. Gently blot away any remaining moisture from the membrane surface using absorbent paper. Dry the membrane sheets in an 80°C vacuum oven for 4 hours to remove any residual moisture. After drying, accurately weigh each membrane sheet again. ), and its proton conductivity was tested again at 25°C and 90% relative humidity. ).

[0113] Data calculation:

[0114] PA weight loss rate (%) ;

[0115] Proton conductivity retention rate (%) .

[0116] Test data

[0117] Table 2: Test results of phosphate locking effect of membrane

[0118]

[0119] in conclusion:

[0120] Test results show that the PA weight loss rate of the post-treated crosslinked nanocomposite membrane of Example 1 was significantly lower than 2% after immersion in deionized water at 80°C for 24 hours, while the proton conductivity retention rate exceeded 95%. This clearly demonstrates the excellent locking ability and stability of the membrane for phosphate dopants. The low weight loss rate and high conductivity retention rate confirm that the crosslinked network effectively prevents phosphate molecules from leaching from the membrane, thereby ensuring the long-term stable proton conductivity performance of the membrane material.

[0121] In stark contrast, the composite membrane of Comparative Example 1, which was not post-treated and cross-linked, exhibited a PA weight loss rate of over 30% and a proton conductivity retention rate of less than 45% under the same immersion conditions. This indicates that, without the formation of a chemical cross-linking network, phosphate molecules readily escape from the membrane in a high-temperature water environment, leading to a sharp decline in the membrane's conductivity.

[0122] This test directly verifies the core innovative mechanism of the post-processing crosslinking step introduced in this invention. The formation of the crosslinking network not only stabilizes the macroscopic structure of the polymer matrix, but also solves the technical problem of easy dopant loss in traditional acid-doped films through the formation of covalent bonds after the phosphate molecules are adsorbed and enriched by pyridine groups, providing a long-lifetime, high-performance proton exchange membrane for electrochemical devices such as VRFB.

[0123] Test 3: Physical stability test (water content / acidity and swelling rate)

[0124] Experimental objective:

[0125] This test aims to evaluate the physical stability of the nanocomposite membrane of the present invention in aqueous or acidic environments. It mainly verifies the role of h-BN nanofiller in inhibiting membrane expansion and improving dimensional stability by measuring the water / acidity and area swelling ratio of the membrane. At the same time, it examines the contribution of the crosslinking network to maintaining the structural integrity of the membrane.

[0126] Experimental steps:

[0127] Sample preparation and initial measurement: Three square membrane pieces measuring 2 cm × 2 cm were cut from each of all examples and comparative examples. The length of each membrane piece was precisely measured using calipers. ) and width ( ), calculate the initial area The initial dry weight of each membrane sheet was accurately measured using an analytical balance. Then dry in an 80°C vacuum oven for 4 hours to ensure complete removal of moisture.

[0128] Immersion treatment: Each membrane sheet was immersed in an individual glass container. For water content and water swelling rate tests, deionized water was used as the immersion medium. For acid content and acid swelling rate tests, 85wt% phosphoric acid solution was used as the immersion medium. All containers were placed at room temperature (25°C) for 24 hours, until the membrane sheet reached swelling equilibrium.

[0129] Balance Measurement: After soaking, quickly remove the membranes. Gently absorb any remaining liquid from the membrane surface with absorbent paper. Immediately use calipers to measure the length of each membrane after soaking. ) and width ( ), calculate the area after soaking Subsequently, the wet weight after soaking was accurately measured. ).

[0130] Data calculation:

[0131] Area swelling rate (%) ;

[0132] Moisture content / acidity (%) .

[0133] Test data:

[0134] Table 3: Test results of membrane water content / acidity and area swelling rate

[0135]

[0136] in conclusion:

[0137] Test results show that the water / acid ratio and area swelling ratio of the nanocomposite membrane prepared in the embodiments of the present invention are significantly lower than those of Comparative Example 2 (without h-BN) and the conventional SPEEK membrane (Comparative Example 4) in deionized water and phosphoric acid solution. For example, the area swelling ratio of the membrane in Example 1 is approximately 725% in water and approximately 7.00% in phosphoric acid, which is much lower than the swelling ratios of Comparative Example 2 (18.25% in water and 27.50% in phosphoric acid) and Comparative Example 4 (22.00% in water and 31.25% in phosphoric acid).

[0138] This data directly confirms the crucial role of h-BN nanofillers in suppressing membrane swelling. The h-BN nanosheets form a physical barrier and supporting framework within the polymer matrix, effectively limiting excessive swelling of polymer chains and thus improving membrane dimensional stability. Furthermore, the crosslinked network in the examples further restricts the free movement of polymer chains, playing a synergistic role in maintaining membrane dimensional stability, as evidenced by the low swelling rate of Example 1 (crosslinked) compared to the higher swelling rate of Comparative Example 1 (uncrosslinked) (14.50% in water, 20.25% in phosphoric acid).

[0139] Therefore, this solution successfully addresses the problems of decreased membrane mechanical properties and poor stability caused by high water content and high swelling rate by using composite h-BN filler and constructing a cross-linked network. Excellent dimensional stability is crucial for maintaining good contact between the membrane and electrodes during operation in a vanadium redox flow battery, preventing membrane deformation, and ensuring long-term stability.

[0140] Test 4: Mechanical Performance Test

[0141] Experimental objective:

[0142] This test aims to evaluate the tensile strength and elongation at break of the nanocomposite membrane of the present invention at room temperature. By comparing the mechanical properties of membranes with different components and treatment methods, the reinforcing effect of h-BN nanofillers and the contribution of post-treatment crosslinking to the mechanical stability of the membrane are verified.

[0143] Experimental steps:

[0144] Sample preparation: Cut at least five dumbbell-shaped diaphragms (standard size, such as the miniature dumbbell type specified in ASTM D882) from each of the examples and comparative examples. To ensure testing accuracy, the thickness of each diaphragm should be accurately measured and recorded using a thickness gauge.

[0145] Test environment: The samples were pre-equilibrated for 24 hours in an environment of 25℃ and 50% relative humidity. All tests were conducted in this standard environment.

[0146] Tensile test: Fix the pretreated diaphragms one by one onto the fixture of the universal testing machine. Set the tensile speed to 5 mm / min. Perform a uniaxial tensile test until the diaphragm breaks.

[0147] Data Recording and Calculation: The testing machine automatically records the load-displacement curves during the tensile process. Based on the initial dimensions of the diaphragm (width and thickness) and the measured load-displacement data, the tensile strength (MPa) and elongation at break (%) of each diaphragm are calculated. The test results for each sample type are averaged.

[0148] Tensile strength = maximum load / initial cross-sectional area;

[0149] Elongation at break = (displacement at break / initial gauge length) × 100%.

[0150] Test data:

[0151] Table 4: Test results of mechanical properties of the diaphragm at room temperature

[0152]

[0153] in conclusion:

[0154] Test results show that the present invention significantly improves the mechanical properties of nanocomposite membranes through h-BN composite and post-treatment crosslinking strategies.

[0155] Compared with Comparative Example 1 (uncrosslinked membrane), the tensile strength of Example 1 increased from an average of about 28.7 MPa in Comparative Example 1 to about 42.5 MPa, while the elongation at break decreased from an average of about 185.3% in Comparative Example 1 to about 115.5%. This clearly demonstrates that the covalent bond network formed between polymer chains in the post-treatment crosslinking step effectively restricts the relative slippage of molecular chains, thereby increasing the stiffness and strength of the membrane and reducing its plastic deformation capacity.

[0156] Compared with Comparative Example 2 (without h-BN filler membrane), the tensile strength of Example 1 was significantly higher than that of Comparative Example 2, averaging approximately 31.1 MPa. This confirms that the h-BN nanofiller acts as a reinforcing phase in the polymer matrix, effectively bearing external forces and playing a role in stress transfer and dispersion through nanoscale dispersion and interfacial interactions, thereby improving the overall mechanical strength of the composite membrane.

[0157] Among the different embodiments, Example 2 (h-BN content 5 wt%) exhibited higher tensile strength (approximately 48.6 MPa) and slightly lower elongation at break compared to Example 1 (h-BN content 3 wt%), consistent with the expected trend of inorganic nanofiller reinforcement. Example 3 (high crosslinking density polymer) also showed slightly higher tensile strength and slightly lower elongation at break than Example 1, indicating that the higher crosslinking density further improved the mechanical properties of the film.

[0158] Compared to existing membrane materials (Comparative Examples 4 and 5), the membranes of the embodiments of the present invention exhibit balanced and superior mechanical properties. For example, the tensile strength of Example 1 is significantly higher than that of the conventional SPEEK membrane (Comparative Example 4, average approximately 25.7 MPa), while maintaining good flexibility. Compared to the PBI membrane (Comparative Example 5, average tensile strength approximately 38.3 MPa, approximately 85.5% elongation at break), the membrane of the present invention exhibits superior elongation at break while maintaining high tensile strength, indicating better toughness.

[0159] In summary, the PyPEKK matrix combined with h-BN nanofillers used in this invention, along with the stable network constructed through post-processing crosslinking steps, synergistically enhances the mechanical properties of the membrane. This enhanced mechanical property is crucial for the membrane to withstand physical stresses such as assembly pressure, expansion stress, and fluid erosion during actual battery operation.

[0160] Test 5: Test of core electrochemical performance indicators

[0161] Experimental objective:

[0162] This test aims to evaluate the core electrochemical performance of the nanocomposite membrane of this invention, namely proton conductivity and vanadium ion permeability. By comparing examples with different preparation parameters and various comparative examples, the comprehensive technical advantages of this invention—enhancing proton conductivity through pyridine groups, reducing vanadium ion permeability through h-BN nanofillers, and stabilizing membrane performance through cross-linked networks—are fully verified.

[0163] Experimental steps:

[0164] Proton conductivity test:

[0165] Sample preparation: Cut the composite films prepared in all examples and comparative examples into strips (e.g., 1 cm × 4 cm).

[0166] Test method: The in-plane proton conductivity of the membrane was measured using electrochemical impedance spectroscopy (EIS). The membrane was clamped in a conductivity cell equipped with stainless steel electrodes. Under constant temperature (e.g., 25°C and 80°C) and controlled humidity (e.g., 90% relative humidity), an AC voltage of 10 mV was applied, with a frequency range from 1 Hz to 1 MHz.

[0167] Data calculation: Determining the real axis intercept resistance in the high-frequency region from the impedance spectrum ( Calculate proton conductivity using the following formula. ,in The distance between the diaphragm electrodes. For the diaphragm width, This represents the membrane thickness. At least three membranes should be measured for each sample, and the average value should be taken.

[0168] Vanadium ion permeability test:

[0169] Sample preparation: Cut the composite films prepared in all examples and comparative examples into circles (e.g., 3 cm in diameter).

[0170] Test method: Vanadium ion permeation test was performed using an H-type diffusion cell. The membrane under test was fixed in the middle of the H-type cell, completely dividing it into two independent chambers. A solution containing 1.5 M VOSO4 and 3 M H2SO4 was injected into one chamber of the cell. An equal volume of 3 M H2SO4 and 1.5 M MgSO4 solution was injected into the other chamber (receiving chamber).

[0171] Sampling and Analysis: At a constant temperature (e.g., 25°C), a certain volume of solution is periodically (e.g., every 1 hour) drawn from the receiving cavity and immediately measured using a UV-Vis spectrophotometer. 2+ / VO 2+ The concentration of (typically with absorption peaks around 760 nm and 400 nm) is determined. Simultaneously, an equal volume of fresh receiver cavity solution is added to maintain volume balance.

[0172] Data Calculation: Based on the vanadium ion concentration variation curve in the receiving cavity over time, combined with the membrane area and thickness, and according to Fick's first diffusion law, the vanadium ion permeability (P) is calculated. The unit of permeability P is usually expressed in cm². 2 / min.

[0173] Test data:

[0174] Table 5: Test results of membrane proton conductivity and vanadium ion permeability

[0175]

[0176] in conclusion:

[0177] Test results show that the nanocomposite membrane prepared in the embodiments of the present invention exhibits excellent comprehensive performance in terms of proton conductivity and vanadium ion permeability.

[0178] Regarding proton conductivity, the membrane of Example 1 achieved 0.138 S / cm and 0.205 S / cm at 25°C and 80°C, respectively, which is at the same or slightly lower level than the conventional SPEEK membrane (Comparative Example 4) and PBI membrane (Comparative Example 5), but significantly higher than Comparative Example 3 (0.082 S / cm@25°C) without pyridine groups. This confirms that the pyridine groups introduced into the PyPEKK polymer can effectively adsorb phosphoric acid, forming a highly efficient proton transport channel. In particular, Example 4 (high pyridine content) exhibited the highest proton conductivity at both temperatures, verifying that increasing pyridine adsorption sites helps to improve proton transport efficiency.

[0179] Regarding vanadium ion permeability, the membrane of this invention exhibits a significant advantage. The vanadium ion permeability of Example 1 is only 0.76 x 10⁻⁶. -8 cm 2 / min, far lower than Comparative Example 1 (uncrosslinked, 1.83x10 -8 cm 2 / min), Comparative Example 2 (without h-BN, 1.55x10 -8 cm 2 / min), Comparative Example 4 (SPEEK, 2.89x10 -8 cm 2 ( / min) and Comparative Example 5 (PBI, 1.98x10) -8 cm 2 / min). Among them, Example 2 (h-BN content 5wt%) and Example 3 (high crosslinking density) showed lower vanadium ion permeability, at 0.65x10⁻⁶ and 0.65x10⁻⁶, respectively. -8 cm 2 / min and 0.71x10 -8 cm 2 / min.

[0180] This result strongly supports the innovative mechanism of this invention:

[0181] Contribution of pyridine groups: The pyridine groups in PyPEKK polymers provide abundant basic sites, effectively adsorbing and enriching phosphoric acid, constructing a high-concentration proton transport pathway, thereby ensuring high proton conductivity.

[0182] Barrier effect of h-BN nanofillers: h-BN nanosheets form tortuous diffusion paths in the polymer matrix, significantly extending the transport distance of vanadium ions and thus effectively reducing vanadium ion permeation. This is reflected in the higher permeability of Comparative Example 2 (without h-BN) compared to Example 1.

[0183] Stability of the cross-linked network: The cross-linked network formed by post-treatment not only improves the mechanical strength and dimensional stability of the membrane (as shown in Test 3), but more importantly, it effectively reduces the free volume of the membrane by fixing polymer segments and stabilizes the environment in which phosphate molecules exist, further inhibiting the diffusion of vanadium ions. This can be seen from the higher permeability of Comparative Example 1 (uncross-linked) compared to Example 1.

[0184] In summary, this invention achieves synergistic optimization of high proton conductivity and low vanadium ion permeability by constructing a PyPEKK polymer with pyridine adsorption groups and crosslinkable active groups, compositing it with h-BN nanofillers, and then crosslinking it through post-treatment. This provides a high-performance proton exchange membrane solution for all-vanadium redox flow batteries.

[0185] Test 6: Performance and Cycle Stability Test of Vanadium Redox Flow Battery (VRFB) Cell

[0186] Experimental objective:

[0187] This test aims to evaluate the comprehensive electrochemical performance of the nanocomposite membrane of the present invention in a real VRFB single cell, including coulombic efficiency (CE), voltage efficiency (VE) and energy efficiency (EE), and to examine its stability during long-term charge-discharge cycles, so as to ultimately verify the practical value of the membrane material as a proton exchange membrane.

[0188] Experimental steps:

[0189] Battery Assembly: Construct a single-cell VRFB. A membrane under test (effective area 5 cm × 5 cm) is used as the proton exchange membrane. Heat-treated activated graphite felt is placed on both sides of the membrane as positive and negative electrodes, respectively. A graphite plate with a serpentine flow channel is used as the bipolar plate / current collector. The battery components are tightly assembled by controlling the torque to ensure no leakage.

[0190] Electrolyte preparation and circulation: A peristaltic pump was used to circulate 1.5 MV electrolyte. 3+ / V 4+ The electrolyte solution (50 mL each for the positive and negative electrodes) is circulated and pumped into the positive and negative electrode chambers of the battery.

[0191] Charge / discharge test: A constant current charge / discharge test was performed using a battery testing system (e.g., Arbin BT2000). The current density was set to 80 mA / cm². 2 The charge / discharge cutoff voltage window is set to 1.0V to 1.6V.

[0192] Performance evaluation: Record the charge-discharge curves at the 10th cycle and calculate CE, VE, and EE according to the following formulas.

[0193] Coulomb efficiency (CE) = (Discharge capacity / Charge capacity) × 100%

[0194] Voltage efficiency (VE) = (Average discharge voltage / Average charge voltage) × 100%

[0195] Energy efficiency (EE) = CE × VE

[0196] Cyclic stability assessment: 100 charge-discharge cycles were performed continuously at the same current density. The capacity retention rate after 100 cycles was calculated by monitoring the change in discharge capacity to assess the long-term operational stability of the membrane.

[0197] Test data

[0198] Table 6: Performance and Cycle Stability Test Results of Vanadium Redox Flow Battery Cells (Current Density: 80 mA / cm²) 2 )

[0199]

[0200] in conclusion:

[0201] The battery cell test results intuitively reflect the comprehensive advantages of the nanocomposite film of this invention in practical applications.

[0202] First, all the membrane-assembled batteries in the embodiments exhibited extremely high coulombic efficiency (CE>97%), with Example 2 (high h-BN content) achieving a CE of 99.1%. This is directly attributed to its extremely low vanadium ion permeability (as shown in Test 5). The physical barrier effect introduced by the h-BN nanofiller and the compression of free volume by the cross-linked network synergistically suppressed vanadium ion shuttle, thereby significantly reducing side reactions such as self-discharge and improving current utilization.

[0203] Secondly, the membranes in the embodiments maintained both high CE and high voltage efficiency (VE>85%), resulting in a final energy efficiency (EE) exceeding 84.5%, significantly better than all comparative examples. In particular, compared to traditional SPEEK membranes (Comparative Example 4, EE 81.5%) and PBI membranes (Comparative Example 5, EE 82.1%), the membrane material of this invention achieved a substantial improvement in energy efficiency. This is attributed to the effective enrichment of phosphoric acid by pyridine groups in the PyPEKK polymer, constructing a highly efficient proton conduction network (as shown in Test 5), effectively reducing the ohmic polarization of the battery.

[0204] Most importantly, the membranes in the examples exhibited superior stability during long-term cycling tests. After 100 cycles, the capacity retention was consistently above 96%, significantly higher than all comparative membranes (all below 91%). This result provides final verification of the innovative mechanism of this invention: the post-treatment crosslinking network not only endows the membrane with excellent mechanical strength (Test 4) and dimensional stability (Test 3), but more importantly, it effectively locks in the phosphate dopant (Test 2), fundamentally solving the performance degradation problem caused by dopant loss and structural deterioration in traditional acid-doped membranes during long-term operation.

[0205] In summary, the single-cell performance tests fully confirmed the success of the design concept of this invention, which involves suppressing ion permeation through h-BN composites, promoting proton conduction using pyridine groups, and stabilizing the overall structure through post-treatment cross-linking. This technical solution synergistically optimizes the selectivity and conductivity of the proton exchange membrane and ensures its long-term operational stability, ultimately translating into a significant improvement in the energy efficiency and cycle life of VRFB batteries.

Claims

1. A nanocomposite film with acid-doped pyridine side chains, characterized in that, The composite membrane comprises: a) A bifunctional polyarylether ketone polymer containing pyridine side groups; b) Hexagonal boron nitride nanosheets; c) Phosphoric acid; The bifunctional polyarylether ketone polymer containing pyridine side groups is obtained by monomer polymerization in the following molar ratio: (1) Diphenol monomers: 2,6-Di(4-hydroxyphenyl)pyridine: 10 mol% to 90 mol% of the total number of diphenol monomers; 4,4'-(1,2-vinyl)diphenol: accounting for 5 mol% to 85 mol% of the total number of diphenol monomers; Bisphenol A: accounting for 5 mol% to 25 mol% of the total number of bisphenol monomers; The sum of the molar percentages of the three components is 100 mol%; (2) The ratio of dihalogenated monomers to diphenol monomers: The dihalogenated monomer is 4,4'-difluorobenzophenone, and the ratio of the total molar number of the above three diphenol monomers to the molar number of 4,4'-difluorobenzophenone is 0.95:1 to 1.05:1; The amount of hexagonal boron nitride nanosheets added is 1 wt% to 10 wt% of the polymer mass; The composite film is first doped in a phosphoric acid solution with a concentration of 80wt% to 95wt%, and then crosslinked by heat treatment at 140℃ to 180℃.

2. The nanocomposite film with acid-doped pyridine side chains according to claim 1, characterized in that, The bifunctional polyarylether ketone polymer containing pyridine side groups is obtained by polymerization of monomers in the following molar ratio: 2,6-Di(4-hydroxyphenyl)pyridine accounts for 20 mol% to 70 mol% of the total number of diphenol monomers; 4,4'-(1,2-vinyl)diphenol comprises 10 mol% to 70 mol% of the total number of diphenol monomers; Bisphenol A accounts for 10 mol% to 20 mol% of the total number of bisphenol monomers; The amount of hexagonal boron nitride nanosheets added is 2 wt% to 7 wt% of the mass of the bifunctional polyarylether ketone polymer containing pyridine side groups; The nanocomposite film is first doped in a phosphoric acid solution with a concentration of 82wt% to 88wt%, and then crosslinked by heat treatment at 150℃ to 170℃.

3. The nanocomposite film with acid-doped pyridine side chains according to claim 1, characterized in that, The thickness of the nanocomposite film is 40 μm to 70 μm.

4. The nanocomposite film with acid-doped pyridine side chains according to claim 1, characterized in that, The bifunctional polyarylether ketone polymer containing pyridine side groups was prepared by the following steps: Under an inert atmosphere, bisphenol monomers, 4,4'-difluorobenzophenone, and anhydrous potassium carbonate are dissolved in a mixed solvent of N-methyl-2-pyrrolidone and toluene, and dehydrated azeotropically at 130°C to 170°C, followed by polymerization at 180°C to 200°C.

5. The nanocomposite film with acid-doped pyridine side chains according to claim 1, characterized in that, Before being added to the polymer solution, the hexagonal boron nitride nanosheets are ultrasonically treated under ice-water bath conditions for 1 to 3 hours to complete dispersion.

6. The nanocomposite film with acid-doped pyridine side chains according to claim 1, characterized in that, The method for preparing the nanocomposite membrane includes uniformly coating a slurry containing a bifunctional polyarylether ketone polymer with pyridine side groups and hexagonal boron nitride nanosheets onto a clean substrate, then drying it at a temperature of 70°C to 90°C for 3 to 5 hours, and then drying it under vacuum conditions of 110°C to 130°C for 20 to 28 hours to form a composite membrane base film.

7. A nanocomposite film with acid-doped pyridine side chains according to claim 6, characterized in that, The phosphoric acid doping step includes immersing the composite film base film in the phosphoric acid solution and doping at a constant temperature of 70°C to 90°C for 20 to 28 hours.

8. The nanocomposite film with acid-doped pyridine side chains according to claim 1, characterized in that, The heat treatment crosslinking step includes heat treating the phosphate-doped film at a temperature of 140°C to 180°C for 1.5 h to 2.5 h.

9. The nanocomposite film with acid-doped pyridine side chains according to claim 1, characterized in that, The nanocomposite membrane exhibits a proton conductivity of 0.12 S / cm to 0.16 S / cm at 20°C to 30°C and a relative humidity of 90%; and a vanadium ion permeability of 0.60 × 10⁻⁶ at 20°C to 30°C. -8 cm 2 / min to 1.00×10 -8 cm 2 / min.

10. The nanocomposite film with acid-doped pyridine side chains according to claim 1, characterized in that, The nanocomposite membrane is used as a proton exchange membrane in a vanadium redox flow battery.

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