A method for preparing a crosslinked sulfonated polyimide film

Abstract

The application discloses a preparation method of a crosslinked sulfonated polyimide film, and comprises the following steps: synthesizing 3,5-bis(4-aminophenoxy)benzoic acid through nucleophilic substitution and reduction reaction; synthesizing 4,4'-(1,1'-bis(4-aminophenoxy))octafluorobiphenyl through nucleophilic substitution and reduction reaction; taking 3,5-bis(4-aminophenoxy)benzoic acid and 4,4'-(1,1'-bis(4-aminophenoxy))octafluorobiphenyl as raw materials, and preparing a sulfonated polyimide polymer containing carboxyl groups through high-temperature polycondensation reaction; dissolving the sulfonated polyimide polymer containing carboxyl groups in an organic solvent, and adding polyvinyl alcohol and 4-dimethylaminopyridine to react; after the reaction is completed, casting into a film, and after washing and acidification, the crosslinked sulfonated polyimide film is obtained. The crosslinked sulfonated polyimide film prepared by the method has good performance, and has a good application prospect in a vanadium redox flow battery.

Description

Technical Field

[0001] This invention belongs to the field of battery separator technology, and more specifically, this invention relates to a method for preparing a cross-linked sulfonated polyimide membrane. Background Technology

[0002] With rapid socio-economic development, people's demand for energy is increasing daily. The shortage of non-renewable resources (such as coal, oil, and natural gas) and the pollution caused by their overuse have become hot topics of common concern worldwide. At the same time, severe energy shortages will become a bottleneck restricting the sustainable economic development and improvement of people's living standards in various countries. Therefore, the development and utilization of green renewable energy (such as solar, tidal, and wind power) has received high attention from researchers worldwide. However, these new renewable energy sources are affected by external factors such as time, temperature, season, and day / night cycles, leading to significant instability and discontinuity in the power generation process, making it difficult to directly connect to the grid to achieve continuous and stable power output. Equipping the grid with efficient energy storage systems is an important means to solve the non-steady-state characteristics of renewable energy power generation, and a key technology for resolving the power system's supply and demand imbalance, ensuring the rational utilization of new energy sources, and the stable operation of efficient and intelligent power grids. Therefore, the development and research of efficient and green energy storage and conversion systems is imperative.

[0003] In 1985, the vanadium redox flow battery (VRFB), proposed by Professor M. Skyllas-Kazacos et al. of the University of New South Wales, has attracted much attention from researchers due to its advantages such as safety, adjustable capacity and power, high-current non-destructive deep discharge, long service life, and no environmental pollution. As one of the key components of the VRFB, the separator mainly functions in the following two aspects: (1) preventing the cross-penetration of vanadium ions of different valence states in the positive and negative electrode electrolytes, thereby avoiding short circuits inside the battery, suppressing the self-discharge phenomenon of the battery, improving the coulombic efficiency of the battery, and extending the service life of the battery; (2) constructing channels for charge carrier ions inside the battery, allowing the H+ ions to balance the charge. + SO4 2- Specific ions pass through to achieve current transfer, forming a complete closed circuit in the battery and ensuring charge balance between the two electrodes. Therefore, an ideal separator should possess characteristics such as high ionic conductivity, low vanadium ion permeability, excellent electro / chemical stability, good mechanical properties, and low cost.

[0004] Currently, the widely used separator in vanadium redox flow batteries is the perfluorosulfonic acid membrane, such as the Nafion series membrane manufactured by DuPont, which possesses advantages such as high proton conductivity, excellent mechanical strain, and superior chemical stability. However, the Nafion series membrane has a very high vanadium ion permeability, which leads to easy cross-permeation of vanadium ions in the positive and negative electrode electrolytes during charge and discharge, resulting in severe self-discharge of the battery. Furthermore, the Nafion series membrane is expensive (600–800 USD / m³). -2 The vanadium redox flow battery membrane (VRB) accounts for 40% of the entire VRB system, significantly limiting its large-scale commercial application. Therefore, there is an urgent need to develop a new type of separator for VRBs that can replace the Nafion membrane.

[0005] Sulfonated polyimide (SPI) membranes have attracted much attention in recent years due to their excellent film-forming properties and low vanadium ion permeability, coupled with their reasonable price and flexible structural design. WJXu et al. prepared a novel porous cross-linked polyimide membrane (PCrPI) with ultra-high chemical stability using β-cyclodextrin as a template. The PCrPI membrane possesses a unique cross-linking structure, and the imidazole groups exhibiting the Donnan effect effectively hinder the permeation of vanadium ions. Simultaneously, the imidazole groups and porous structure also promote ion transport and effectively reduce sheet resistivity. (40-200 mA / cm²) -2 At current densities of 60-300 mA, PCrPI films exhibit higher coulombic efficiency (96.3-99.6%) and energy efficiency (67.9-88.9%) than commercially available Nafion 212 films. Furthermore, the 2250 charge-discharge cycles of the PCrPI films demonstrate their excellent chemical stability, placing them at the top level compared to previously reported SPI films. The authors also explained the superior durability of the PCrPI-10 films through theoretical calculations of natural bond orbital charges and binding energies. JCLi et al. prepared PFSPI-PAA-x films by crosslinking hydrophilic polyacrylic acid (PAA) with polyfluoroSPI (PFSPI). The optimal PFSPI-PAA-25 film showed a slightly lower sheet resistivity than the Nafion 212 film and significantly better vanadium blocking performance. -2 At current densities of [specific values ​​missing], the PFSPI-PAA-25 film exhibits coulombic efficiency of 97.3-99.9% and energy efficiency of 90.7-73.6%, both superior to the Nafion 212 film. Simultaneously, its peak power density is 451.4 mW / cm². -2 It is still higher than the 427.9 mW cm⁻¹ of the Nafion 212 membrane. -2 .

[0006] However, the current research on SPI / PI membranes in vanadium redox flow batteries is still in its early stages, and its shortcomings are mainly in the following two aspects: (1) In the strong acid and strong oxidizing electrolyte environment, the chemical stability of SPI / PI membranes is weak, mainly because the carbonyl group in its imide ring is easily attacked, which leads to the breakage of the imide bond to generate polyamic acid, and the amino group is subsequently oxidized, which seriously affects its service life in vanadium redox flow batteries; (2) The lack of obvious hydrophilic and hydrophobic phase separation structure, the insufficient strength of the hydrogen bond network, and the insufficient smoothness of the proton transport channel result in low proton conductivity of SPI / PI membranes, which is not conducive to obtaining higher voltage efficiency in vanadium redox flow batteries. Summary of the Invention

[0007] One object of the present invention is to solve at least the above-mentioned problems and / or defects, and to provide at least the advantages described below.

[0008] To achieve these objectives and other advantages according to the present invention, a cross-linked sulfonated polyimide film is provided, the chemical structural formula of which is as follows:

[0009]

[0010] A method for preparing a crosslinked sulfonated polyimide film includes the following steps:

[0011] Step 1: Synthesize 3,5-bis(4-aminophenoxy)benzoic acid via nucleophilic substitution and reduction reactions;

[0012] Step 2: Synthesize 4,4'-(1,1'-bis(4-aminophenoxy))octafluorobiphenyl via nucleophilic substitution and reduction reactions;

[0013] Step 3: Using 3,5-bis(4-aminophenoxy)benzoic acid and 4,4'-(1,1'-bis(4-aminophenoxy))octafluorobiphenyl as raw materials, a sulfonated polyimide polymer containing carboxyl groups is prepared by high-temperature polycondensation reaction.

[0014] Step 4: Dissolve the carboxyl-containing sulfonated polyimide polymer in an organic solvent, add polyvinyl alcohol and 4-dimethylaminopyridine to react; after the reaction is complete, cast into a film, wash and acidify to obtain a crosslinked sulfonated polyimide film.

[0015] Preferably, in step one, the method for synthesizing 3,5-bis(4-aminophenoxy)benzoic acid includes the following steps:

[0016] Step I: Take 15-30 parts by weight of 3,5-dihydroxybenzoic acid, 31-63 parts by weight of p-chloronitrobenzene, and 30-60 parts by weight of anhydrous potassium carbonate. Measure 20-40 parts by volume of toluene and 100-200 parts by volume of organic solvent and add them sequentially to the reaction vessel. Stir at room temperature for 1-2 hours, then heat to 120-140℃ and react for 12-24 hours. After the reaction is complete, pour the mixture into a mixed solvent of ethanol and deionized water. After acidification, a yellow precipitate will form. After washing and drying, 3,5-bis(4-nitrophenoxy)benzoic acid is obtained.

[0017] Step II: Add 17-35 parts by weight of 3,5-bis(4-nitrophenoxy)benzoic acid, 1-2 parts by weight of activated carbon, and 0.2-0.4 parts by weight of ferric chloride, along with 75-150 parts by volume of anhydrous ethanol, to the reaction vessel in sequence. Stir at room temperature for 1-2 hours, then raise the temperature to 70-80°C and slowly add 25-50 parts by volume of hydrated hydrazine. React for 12-24 hours. After the reaction is complete, filter and collect the filtrate. Pour the filtrate into deionized water, and after acidification, a white precipitate will form. After washing and drying, 3,5-bis(4-aminophenoxy)benzoic acid is obtained.

[0018] Preferably, in step two, the method for synthesizing 4,4'-(1,1'-bis(4-aminophenoxy))octafluorobiphenyl includes the following steps:

[0019] Step I: Take 8-17 parts by weight of decafluorobiphenyl, 6.5-14 parts by weight of p-nitrophenol, and 10-22 parts by weight of anhydrous potassium carbonate. Measure 10-20 parts by volume of toluene and 100-200 parts by volume of organic solvent and add them sequentially to the reaction vessel. Stir at room temperature for 1-2 hours, then heat to 80-100℃ and react for 12-24 hours to obtain a mixture. After the reaction is complete, pour the mixture into deionized water, and a solid will precipitate. Wash, filter, and dry the solid to obtain the intermediate product 4,4'-(1,1'-bis(4-nitrophenoxy))octafluorobiphenyl.

[0020] Step II: 13–27 parts by weight of 4,4'-(1,1'-bis(4-nitrophenoxy))octafluorobiphenyl, 2–4 parts by weight of activated carbon, 0.3–0.8 parts by weight of ferric chloride, and 150–200 parts by volume of anhydrous ethanol were added sequentially to a reaction vessel and stirred at 80–90°C for 0.5–1 h. Then the temperature was lowered to 70°C, and 30–60 parts by volume of hydrated hydrazine were added dropwise. The reaction was allowed to proceed for 12–24 h. After the reaction was completed, the mixture was filtered and the filtrate was collected. The filtrate was poured into deionized water, and a solid precipitated out. The solid was washed, filtered, and dried to obtain the product 4,4'-(1,1'-bis(4-aminophenoxy))octafluorobiphenyl.

[0021] Preferably, in step two, the high-temperature polycondensation reaction includes: under nitrogen protection, mixing 2,2'-bis(sulfonated)benzidine, m-methylphenol, and triethylamine, and heating to 60-80°C while stirring until the solid dissolves; then adding 4,4'-(1,1'-bis(4-aminophenoxy))octafluorobiphenyl and 3,5-bis(4-aminophenoxy)benzoic acid and stirring until the solid dissolves; finally adding 1,4,5,8-naphthalenetetracarboxylic dianhydride and benzoic acid, and raising the temperature to 80-100°C for 1-5 hours, then raising the temperature to 180-200°C for 15-25 hours; after cooling the reaction system to 90-100°C, adding a diluent; and when the reaction system cools to 50°C, pouring the resulting viscous solution into a precipitant to obtain a solid; washing, filtering, and drying the solid yields a carboxyl-containing sulfonated polyimide polymer.

[0022] Preferably, the molar ratio of 4,4'-(1,1'-bis(4-aminophenoxy))octafluorobiphenyl, 3,5-bis(4-aminophenoxy)benzoic acid, 2,2'-bissulfonic acid benzidine, 1,4,5,8-naphthalenetetracarboxylic dianhydride, and benzoic acid is 1.0–2.0:1.0–2.0:2.0–4.0:4.0–8.0:8.0–16.0; and the molar ratio of triethylamine to 2,2'-bissulfonic acid benzidine is 1.0–3.0:1 The volume ratio of m-methylphenol to triethylamine is 70.0–3.0; the volume ratio of m-methylphenol to triethylamine is 15.0–30.0:2.5–5.5; the precipitant is one or more of acetone, methanol, and ethanol; the detergent is one or more of acetone, methanol, ethanol, or deionized water; the volume ratio of the precipitant to the viscous solution is 5.0–10.0:1.0.

[0023] Preferably, step four includes: mixing a carboxyl-containing sulfonated polyimide polymer with an organic solvent, heating to 60–80°C and stirring until dissolved, then adding a catalyst 4-dimethylaminopyridine and a crosslinking agent polyvinyl alcohol, and raising the stirring temperature to 100–120°C, reacting for 12–24 hours to obtain a casting solution, pouring the casting solution onto a dry and clean glass plate to cast a film; then drying the glass plate at 100–120°C for 12–24 hours, and acidifying to obtain a crosslinked sulfonated polyimide film;

[0024] In step four, the detergent is any one of methanol, ethanol, or deionized water, and the acidifying reagent is 1.0–3.0 mol / L. -1 Aqueous solution of sulfuric acid.

[0025] Preferably, the organic solvent is any one of N,N-dimethylacetamide, N,N-dimethylformamide, dimethyl sulfoxide, and N-methylpyrrolidone;

[0026] The thickness of the crosslinked sulfonated polyimide film is 40–55 μm.

[0027] Preferably, to improve the efficiency of the cross-linked sulfonated polyimide film in the application of vanadium-sulfur liquid batteries, the cross-linked sulfonated polyimide film after washing and acidification undergoes post-treatment. The specific method includes: placing the cross-linked sulfonated polyimide film in a vacuum environment and irradiating it with a high-energy electron beam. The electron beam irradiation energy is 2.8–4.5 MeV, the electron beam current intensity is 10–25 mA, and the irradiation dose is 50–600 kGy; immersing the electron beam-irradiated cross-linked sulfonated polyimide film in 10% hydrogen peroxide for 30–60 min, and then immersing it in 1.0–3.0 mol L... -1 The sulfuric acid aqueous solution is subjected to secondary acidification for 40-55 minutes, then washed with deionized water and dried in a nitrogen atmosphere at a temperature of 120-150℃.

[0028] An application of a crosslinked sulfonated polyimide membrane, wherein the crosslinked sulfonated polyimide membrane is used in the field of vanadium redox flow batteries as a separator for vanadium redox flow batteries.

[0029] The present invention has at least the following beneficial effects:

[0030] (1) The cross-linked sulfonated polyimide membrane of the present invention, due to the introduction of polyfluorinated monomers, is conducive to the construction of mass-transferable hydrogen bond network. In addition, the strong electronegativity of fluorine atoms can reduce the electron cloud density of polyimide polymer chains, which is beneficial to improving chemical stability.

[0031] (2) The cross-linked sulfonated polyimide membrane of the present invention, due to the introduction of the hydrophilic cross-linking agent polyvinyl alcohol, is beneficial to improve the hydrophilicity of the membrane, thereby optimizing the proton transfer capability of the membrane.

[0032] (3) The cross-linked sulfonated polyimide film of the present invention, due to the presence of cross-linked structure, enables the polyimide polymer to form a three-dimensional network structure, which is beneficial to the improvement of mechanical and chemical properties.

[0033] (4) The cross-linked sulfonated polyimide membrane of the present invention, due to the presence of hydrophobic polyfluorinated polyimide main chain and hydrophilic polyvinyl alcohol side chain, is conducive to forming a hydrophilic-hydrophobic microphase separation structure, thereby further improving the proton transfer capability of the membrane.

[0034] Other advantages, objectives and features of the present invention will become apparent in part from the following description, and in part from those skilled in the art through study and practice of the invention. Attached Figure Description

[0035] Figure 1 The synthesis flow chart of 4,4'-(1,1'bis(4-aminophenoxy))octafluorobiphenyl of the present invention is shown below.

[0036] Figure 2 This is a flowchart illustrating the synthesis of 3,5-bis(4-aminophenoxy)benzoic acid according to the present invention.

[0037] Figure 3 This is a flowchart illustrating the synthesis process of the crosslinked sulfonated polyimide membrane of the present invention.

[0038] Figure 4 The infrared spectrum of the crosslinked sulfonated polyimide film of the present invention is shown below.

[0039] Figure 5 This is the 1H NMR spectrum of the crosslinked sulfonated polyimide membrane of the present invention;

[0040] Figure 6 A comparison of the coulombic efficiency of all-vanadium redox flow batteries using the crosslinked sulfonated polyimide membrane prepared in Example 1 of this invention and the commercial Nafion 212 membrane;

[0041] Figure 7 Comparison of coulombic efficiency of all-vanadium redox flow batteries using crosslinked sulfonated polyimide membranes prepared in Examples 1 and 8 of this invention. Detailed Implementation

[0042] The present invention will now be described in further detail with reference to the accompanying drawings, so that those skilled in the art can implement it based on the description.

[0043] It should be understood that terms such as “having,” “comprising,” and “including” as used herein do not exclude the presence or addition of one or more other elements or combinations thereof.

[0044] Example 1:

[0045] like Figure 3 As shown, a method for preparing a cross-linked sulfonated polyimide film includes the following steps:

[0046] Step 1: Under nitrogen protection, add 2 mmol of 2,2'-disulfonic acid benzidine, 2 mmol of triethylamine, and 30 mL of m-cresol to a 250 mL three-necked flask equipped with a reflux condenser. Heat to 60 °C and stir until completely dissolved. Then add 1 mmol of 4,4'-(1,1'-bis(4-aminophenoxy))octafluorobiphenyl and 1 mmol of 3,5-bis(4-aminophenoxy)benzoic acid to the 250 mL three-necked flask and continue stirring until completely dissolved. Finally, add... Add 4 mmol of 1,4,5,8-naphthalenetetracarboxylic dianhydride and 8 mmol of benzoic acid to a flask, and heat to 80°C and stir for 4.5 h; then heat to 180°C and react for 18 h; after cooling the reaction system to below 90°C, add 10 mL of m-cresol to a 250 mL three-necked flask, and continue to cool the system to 50°C; while stirring, pour the reaction system into 100 mL of acetone as a precipitant to obtain a solid. After washing, filtering, and drying the solid, the product, a sulfonated polyimide polymer containing carboxyl groups, is obtained.

[0047] Step 2: Add 2.00g of the carboxyl-containing sulfonated polyimide polymer obtained in Step 1 to a 250mL three-necked flask equipped with a reflux condenser, and add 20mL of dimethyl sulfoxide. Stir at 80℃ until the polymer is completely dissolved. Then, add 0.3g of polyvinyl alcohol and 0.02g of 4-dimethylaminopyridine to the three-necked flask, and heat to 100℃ to react for 24h. After the reaction is completed, pour the casting solution onto a clean glass plate and dry it in an oven at 120℃ for 12h to obtain a triethylamine crosslinked sulfonated polyimide membrane.

[0048] Step 3: Immerse the triethylamine crosslinked sulfonated polyimide membrane obtained in Step 2 in ethanol for 24 hours to remove unreacted monomers and residual solvent, then place it in 1.0 mol L... -1 The cross-linked sulfonated polyimide membrane was soaked in sulfuric acid solution for 24 hours for protonation; finally, it was washed four times with deionized water to obtain the cross-linked sulfonated polyimide membrane; the thickness of the obtained cross-linked sulfonated polyimide membrane was 50 μm; the structural formula of the prepared cross-linked sulfonated polyimide membrane was:

[0049]

[0050] like Figure 1 As shown, the method for synthesizing 4,4'-(1,1'-bis(4-aminophenoxy))octafluorobiphenyl in step one includes the following steps:

[0051] Step I: Take 8.35 g of decafluorobiphenyl, 6.96 g of p-nitrophenol and 10.35 g of anhydrous potassium carbonate, and measure 10 mL of toluene and 100 mL of N,N-dimethylacetamide and add them to a three-necked flask in sequence; stir at room temperature for 1 h, and then heat to 80 °C and react for 12 h; after the reaction is completed, pour the mixture into deionized water, and a solid will precipitate; wash, filter and dry the solid to obtain the intermediate product 4,4'-(1,1'bis(4-nitrophenoxy))octafluorobiphenyl;

[0052] Step II: 13.50 g of 4,4'-(1,1'-bis(4-nitrophenoxy))octafluorobiphenyl, 2.00 g of activated carbon, 0.37 g of ferric chloride, and 150 mL of anhydrous ethanol were added sequentially to a three-necked flask and stirred at 80 °C for 0.5 h. Then the temperature was lowered to 70 °C, and 30 mL of hydrated hydrazine was added dropwise. The reaction was allowed to proceed for 12 h. After the reaction was completed, the mixture was filtered and the filtrate was collected. The filtrate was poured into deionized water, and a solid precipitated. The solid was washed, filtered, and dried to obtain the product 4,4'-(1,1'-bis(4-aminophenoxy))octafluorobiphenyl.

[0053] like Figure 2 As shown, the method for synthesizing 3,5-bis(4-aminophenoxy)benzoic acid in step one includes the following steps:

[0054] Step I: Take 15.41 g of 3,5-dihydroxybenzoic acid, 31.51 g of p-chloronitrobenzene, and 55.29 g of anhydrous potassium carbonate. Measure 20 mL of toluene and 200 mL of N,N-dimethylacetamide and add them sequentially to a three-necked flask. Stir at room temperature for 1 h, then heat to 120 °C and react for 24 h. After the reaction is complete, pour into a mixed solvent of ethanol and deionized water. After acidification, a yellow precipitate will form. After washing and drying, 3,5-bis(4-nitrophenoxy)benzoic acid is obtained.

[0055] Step II: 35.00 g of 3,5-bis(4-nitrophenoxy)benzoic acid, 1.45 g of activated carbon, and 0.37 g of ferric chloride were added sequentially to a three-necked flask along with 150 mL of anhydrous ethanol. The mixture was stirred at room temperature for 1 hour, then heated to 70°C, and 50 mL of hydrated hydrazine was slowly added dropwise. The reaction was allowed to proceed for 12 hours. After the reaction was complete, the mixture was filtered and the filtrate was collected. The filtrate was poured into deionized water, and after acidification, a white precipitate formed. After washing and drying, 3,5-bis(4-aminophenoxy)benzoic acid was obtained.

[0056] The infrared spectrum of the crosslinked sulfonated polyimide film prepared in this embodiment is as follows: Figure 4 As shown, the proton NMR spectrum is as follows: Figure 5 As shown; the coulombic efficiency of the crosslinked sulfonated polyimide membrane prepared in this embodiment was compared with that of the commercial Nafion membrane in a vanadium redox flow battery, and the results are as follows. Figure 6As shown, the crosslinked sulfonated polyimide membrane prepared in this embodiment has a higher coulombic efficiency in an all-vanadium redox flow battery than the commercial Nafion membrane.

[0057] Example 2:

[0058] A method for preparing a crosslinked sulfonated polyimide film, comprising the following steps:

[0059] Step 1: Under nitrogen protection, add 4 mmol of 2,2'-disulfonic acid benzidine, 4 mmol of triethylamine, and 60 mL of m-cresol to a 250 mL three-necked flask equipped with a reflux condenser. Heat to 60 °C and stir until completely dissolved. Then add 2 mmol of 4,4'-(1,1'-bis(4-aminophenoxy))octafluorobiphenyl and 2 mmol of 3,5-bis(4-aminophenoxy)benzoic acid to the 250 mL three-necked flask and continue stirring until completely dissolved. Finally, add... 8 mmol of 1,4,5,8-naphthalenetetracarboxylic dianhydride and 16 mmol of benzoic acid were added to a flask, and the mixture was heated to 80 °C and stirred for 4.5 h. The temperature was then increased to 180 °C and the reaction was allowed to proceed for 18 h. After the reaction system was cooled to below 90 °C, 20 mL of m-cresol was added to a 250 mL three-necked flask, and the system was further cooled to 50 °C. The reaction system was then poured into 200 mL of acetone precipitant while stirring to obtain a solid. The solid was washed, filtered, and dried to obtain the product, a sulfonated polyimide polymer containing carboxyl groups.

[0060] Step 2: Add 1.00g of the carboxyl-containing sulfonated polyimide polymer obtained in Step 1 to a 250mL three-necked flask equipped with a reflux condenser, and add 10mL of dimethyl sulfoxide. Stir at 80℃ until the polymer is completely dissolved. Then, add 0.15g of polyvinyl alcohol and 0.01g of 4-dimethylaminopyridine to the three-necked flask, and heat to 100℃ to react for 24h. After the reaction is completed, pour the casting solution onto a clean glass plate and dry it in an oven at 120℃ for 12h to obtain a triethylamine crosslinked sulfonated polyimide membrane.

[0061] Step 3: Immerse the triethylamine crosslinked sulfonated polyimide membrane obtained in Step 2 in ethanol for 24 hours to remove unreacted monomers and residual solvent, then place it in 1.0 mol L... -1 The cross-linked sulfonated polyimide membrane was soaked in sulfuric acid solution for 24 hours to undergo protonation; finally, it was washed 4 times with deionized water to obtain the cross-linked sulfonated polyimide membrane; the thickness of the obtained cross-linked sulfonated polyimide membrane was 50 μm.

[0062] In step one, the synthesis method of 4,4'-(1,1'-bis(4-aminophenoxy))octafluorobiphenyl includes the following steps:

[0063] Step I: Take 8.35 g of decafluorobiphenyl, 6.96 g of p-nitrophenol and 10.35 g of anhydrous potassium carbonate, and measure 10 mL of toluene and 100 mL of N,N-dimethylacetamide and add them to a three-necked flask in sequence; stir at room temperature for 1 h, and then heat to 80 °C and react for 12 h; after the reaction is completed, pour the mixture into deionized water, and a solid will precipitate; wash, filter and dry the solid to obtain the intermediate product 4,4'-(1,1'bis(4-nitrophenoxy))octafluorobiphenyl;

[0064] Step II: 13.50 g of 4,4'-(1,1'-bis(4-nitrophenoxy))octafluorobiphenyl, 2.00 g of activated carbon, 0.37 g of ferric chloride, and 150 mL of anhydrous ethanol were added sequentially to a three-necked flask and stirred at 80 °C for 0.5 h. Then the temperature was lowered to 70 °C, and 30 mL of hydrated hydrazine was added dropwise. The reaction was allowed to proceed for 12 h. After the reaction was completed, the mixture was filtered and the filtrate was collected. The filtrate was poured into deionized water, and a solid precipitated. The solid was washed, filtered, and dried to obtain the product 4,4'-(1,1'-bis(4-aminophenoxy))octafluorobiphenyl.

[0065] In step one, the method for synthesizing 3,5-bis(4-aminophenoxy)benzoic acid includes the following steps:

[0066] Step I: Take 15.41 g of 3,5-dihydroxybenzoic acid, 31.51 g of p-chloronitrobenzene, and 55.29 g of anhydrous potassium carbonate. Measure 20 mL of toluene and 200 mL of N,N-dimethylacetamide and add them sequentially to a three-necked flask. Stir at room temperature for 1 h, then heat to 120 °C and react for 24 h. After the reaction is complete, pour into a mixed solvent of ethanol and deionized water. After acidification, a yellow precipitate will form. After washing and drying, 3,5-bis(4-nitrophenoxy)benzoic acid is obtained.

[0067] Step II: 35.00 g of 3,5-bis(4-nitrophenoxy)benzoic acid, 1.45 g of activated carbon, and 0.37 g of ferric chloride were added sequentially to a three-necked flask along with 150 mL of anhydrous ethanol. The mixture was stirred at room temperature for 1 hour, then heated to 70°C, and 50 mL of hydrated hydrazine was slowly added dropwise. The reaction was allowed to proceed for 12 hours. After the reaction was complete, the mixture was filtered and the filtrate was collected. The filtrate was poured into deionized water, and after acidification, a white precipitate formed. After washing and drying, 3,5-bis(4-aminophenoxy)benzoic acid was obtained.

[0068] Example 3:

[0069] A method for preparing a crosslinked sulfonated polyimide film, comprising the following steps:

[0070] Step 1: Under nitrogen protection, add 3 mmol of 2,2'-disulfonic acid benzidine, 3 mmol of triethylamine, and 40 mL of m-cresol to a 250 mL three-necked flask equipped with a reflux condenser. Heat to 60 °C and stir until completely dissolved. Then, add 1.5 mmol of 4,4'-(1,1'-bis(4-aminophenoxy))octafluorobiphenyl and 1.5 mmol of 3,5-bis(4-aminophenoxy)benzoic acid to the 250 mL three-necked flask and continue stirring until completely dissolved. Finally, add 3 mmol of 2,2'-disulfonic acid benzidine, 3 mmol of triethylamine, and 40 mL of m-cresol to the 250 mL three-necked flask. 6 mmol of 1,4,5,8-naphthalenetetracarboxylic dianhydride and 12 mmol of benzoic acid were added to a necked flask, and the temperature was raised to 80 °C and stirred for 4.5 h; then the temperature was raised to 180 °C and the reaction was carried out for 18 h; after the reaction system was cooled to below 90 °C, 10 mL of m-cresol was added to a 250 mL three-necked flask, and the system was further cooled to 50 °C; while stirring, the reaction system was poured into 150 mL of acetone as a precipitant to obtain a solid. After washing, filtering and drying the solid, the product, a sulfonated polyimide polymer containing carboxyl groups, was obtained.

[0071] Step 2: Add 1.00g of the carboxyl-containing sulfonated polyimide polymer obtained in Step 1 to a 250mL three-necked flask equipped with a reflux condenser, and add 10mL of dimethyl sulfoxide. Stir at 80℃ until the polymer is completely dissolved. Then, add 0.15g of polyvinyl alcohol and 0.01g of 4-dimethylaminopyridine to the three-necked flask, and heat to 100℃ to react for 24h. After the reaction is completed, pour the casting solution onto a clean glass plate and dry it in an oven at 120℃ for 12h to obtain a triethylamine crosslinked sulfonated polyimide membrane.

[0072] Step 3: Immerse the triethylamine crosslinked sulfonated polyimide membrane obtained in Step 2 in ethanol for 24 hours to remove unreacted monomers and residual solvent, then place it in 1.0 mol L... -1 The cross-linked sulfonated polyimide membrane was soaked in sulfuric acid solution for 24 hours to undergo protonation; finally, it was washed 4 times with deionized water to obtain the cross-linked sulfonated polyimide membrane; the thickness of the obtained cross-linked sulfonated polyimide membrane was 50 μm.

[0073] In step one, the synthesis method of 4,4'-(1,1'-bis(4-aminophenoxy))octafluorobiphenyl includes the following steps:

[0074] Step I: Take 8.35 g of decafluorobiphenyl, 6.96 g of p-nitrophenol and 10.35 g of anhydrous potassium carbonate, and measure 10 mL of toluene and 100 mL of N,N-dimethylacetamide and add them to a three-necked flask in sequence; stir at room temperature for 1 h, and then heat to 80 °C and react for 12 h; after the reaction is completed, pour the mixture into deionized water, and a solid will precipitate; wash, filter and dry the solid to obtain the intermediate product 4,4'-(1,1'bis(4-nitrophenoxy))octafluorobiphenyl;

[0075] Step II: 13.50 g of 4,4'-(1,1'-bis(4-nitrophenoxy))octafluorobiphenyl, 2.00 g of activated carbon, 0.37 g of ferric chloride, and 150 mL of anhydrous ethanol were added sequentially to a three-necked flask and stirred at 80 °C for 0.5 h. Then the temperature was lowered to 70 °C, and 30 mL of hydrated hydrazine was added dropwise. The reaction was allowed to proceed for 12 h. After the reaction was completed, the mixture was filtered and the filtrate was collected. The filtrate was poured into deionized water, and a solid precipitated. The solid was washed, filtered, and dried to obtain the product 4,4'-(1,1'-bis(4-aminophenoxy))octafluorobiphenyl.

[0076] In step one, the method for synthesizing 3,5-bis(4-aminophenoxy)benzoic acid includes the following steps:

[0077] Step I: Take 15.41 g of 3,5-dihydroxybenzoic acid, 31.51 g of p-chloronitrobenzene, and 55.29 g of anhydrous potassium carbonate. Measure 20 mL of toluene and 200 mL of N,N-dimethylacetamide and add them sequentially to a three-necked flask. Stir at room temperature for 1 h, then heat to 120 °C and react for 24 h. After the reaction is complete, pour into a mixed solvent of ethanol and deionized water. After acidification, a yellow precipitate will form. After washing and drying, 3,5-bis(4-nitrophenoxy)benzoic acid is obtained.

[0078] Step II: 35.00 g of 3,5-bis(4-nitrophenoxy)benzoic acid, 1.45 g of activated carbon, and 0.37 g of ferric chloride were added sequentially to a three-necked flask along with 150 mL of anhydrous ethanol. The mixture was stirred at room temperature for 1 hour, then heated to 70°C, and 50 mL of hydrated hydrazine was slowly added dropwise. The reaction was allowed to proceed for 12 hours. After the reaction was complete, the mixture was filtered and the filtrate was collected. The filtrate was poured into deionized water, and after acidification, a white precipitate formed. After washing and drying, 3,5-bis(4-aminophenoxy)benzoic acid was obtained.

[0079] Example 4:

[0080] A method for preparing a crosslinked sulfonated polyimide film, comprising the following steps:

[0081] Step 1: Under nitrogen protection, add 1 mmol of 2,2'-disulfonic acid benzidine, 1 mmol of triethylamine, and 20 mL of m-cresol to a 250 mL three-necked flask equipped with a reflux condenser. Heat to 60 °C and stir until completely dissolved. Then, add 0.5 mmol of 4,4'-(1,1'-bis(4-aminophenoxy))octafluorobiphenyl and 0.5 mmol of 3,5-bis(4-aminophenoxy)benzoic acid to the 250 mL three-necked flask and continue stirring until completely dissolved. Finally, add 250 mL of m-cresol to the flask. 2 mmol of 1,4,5,8-naphthalenetetracarboxylic dianhydride and 4 mmol of benzoic acid were added to a necked flask, and the temperature was raised to 80 °C and stirred for 4.5 h; then the temperature was raised to 180 °C and the reaction was carried out for 18 h; after the reaction system was cooled to below 90 °C, 10 mL of m-cresol was added to a 250 mL three-necked flask, and the system was further cooled to 50 °C; while stirring, the reaction system was poured into 100 mL of acetone as a precipitant to obtain a solid. After washing, filtering and drying the solid, the product, a sulfonated polyimide polymer containing carboxyl groups, was obtained.

[0082] Step 2: Add 1.00g of the carboxyl-containing sulfonated polyimide polymer obtained in Step 1 to a 250mL three-necked flask equipped with a reflux condenser, and add 10mL of dimethyl sulfoxide. Stir at 80℃ until the polymer is completely dissolved. Then, add 0.15g of polyvinyl alcohol and 0.01g of 4-dimethylaminopyridine to the three-necked flask, and heat to 100℃ to react for 24h. After the reaction is completed, pour the casting solution onto a clean glass plate and dry it in an oven at 120℃ for 12h to obtain a triethylamine crosslinked sulfonated polyimide membrane.

[0083] Step 3: Immerse the triethylamine crosslinked sulfonated polyimide membrane obtained in Step 2 in ethanol for 24 hours to remove unreacted monomers and residual solvent, then place it in 1.0 mol L... -1 The cross-linked sulfonated polyimide membrane was soaked in sulfuric acid solution for 24 hours to undergo protonation; finally, it was washed 4 times with deionized water to obtain the cross-linked sulfonated polyimide membrane; the thickness of the obtained cross-linked sulfonated polyimide membrane was 50 μm.

[0084] In step one, the synthesis method of 4,4'-(1,1'-bis(4-aminophenoxy))octafluorobiphenyl includes the following steps:

[0085] Step I: Take 8.35 g of decafluorobiphenyl, 6.96 g of p-nitrophenol and 10.35 g of anhydrous potassium carbonate, and measure 10 mL of toluene and 100 mL of N,N-dimethylacetamide and add them to a three-necked flask in sequence; stir at room temperature for 1 h, and then heat to 80 °C and react for 12 h; after the reaction is completed, pour the mixture into deionized water, and a solid will precipitate; wash, filter and dry the solid to obtain the intermediate product 4,4'-(1,1'bis(4-nitrophenoxy))octafluorobiphenyl;

[0086] Step II: 13.50 g of 4,4'-(1,1'-bis(4-nitrophenoxy))octafluorobiphenyl, 2.00 g of activated carbon, 0.37 g of ferric chloride, and 150 mL of anhydrous ethanol were added sequentially to a three-necked flask and stirred at 80 °C for 0.5 h. Then the temperature was lowered to 70 °C, and 30 mL of hydrated hydrazine was added dropwise. The reaction was allowed to proceed for 12 h. After the reaction was completed, the mixture was filtered and the filtrate was collected. The filtrate was poured into deionized water, and a solid precipitated. The solid was washed, filtered, and dried to obtain the product 4,4'-(1,1'-bis(4-aminophenoxy))octafluorobiphenyl.

[0087] In step one, the method for synthesizing 3,5-bis(4-aminophenoxy)benzoic acid includes the following steps:

[0088] Step I: Take 15.41 g of 3,5-dihydroxybenzoic acid, 31.51 g of p-chloronitrobenzene, and 55.29 g of anhydrous potassium carbonate. Measure 20 mL of toluene and 200 mL of N,N-dimethylacetamide and add them sequentially to a three-necked flask. Stir at room temperature for 1 h, then heat to 120 °C and react for 24 h. After the reaction is complete, pour into a mixed solvent of ethanol and deionized water. After acidification, a yellow precipitate will form. After washing and drying, 3,5-bis(4-nitrophenoxy)benzoic acid is obtained.

[0089] Step II: 35.00 g of 3,5-bis(4-nitrophenoxy)benzoic acid, 1.45 g of activated carbon, and 0.37 g of ferric chloride were added sequentially to a three-necked flask along with 150 mL of anhydrous ethanol. The mixture was stirred at room temperature for 1 hour, then heated to 70°C, and 50 mL of hydrated hydrazine was slowly added dropwise. The reaction was allowed to proceed for 12 hours. After the reaction was complete, the mixture was filtered and the filtrate was collected. The filtrate was poured into deionized water, and after acidification, a white precipitate formed. After washing and drying, 3,5-bis(4-aminophenoxy)benzoic acid was obtained.

[0090] Example 5:

[0091] A method for preparing a crosslinked sulfonated polyimide film, comprising the following steps:

[0092] Step 1: Under nitrogen protection, add 2 mmol of 2,2'-disulfonic acid benzidine, 2 mmol of triethylamine, and 60 mL of m-cresol to a 250 mL three-necked flask equipped with a reflux condenser. Heat to 60 °C and stir until completely dissolved. Then, add 1 mmol of 4,4'-(1,1'-bis(4-aminophenoxy))octafluorobiphenyl and 1 mmol of 3,5-bis(4-aminophenoxy)benzoic acid to the 250 mL three-necked flask and continue stirring until completely dissolved. Finally, add 25 mL of benzidine benzidine to a 250 mL three-necked flask. Add 4 mmol of 1,4,5,8-naphthalenetetracarboxylic dianhydride and 8 mmol of benzoic acid to a 0 mL three-necked flask; then heat to 180 °C and react for 24 h; after cooling the reaction system to below 90 °C, add 10 mL of m-cresol to a 250 mL three-necked flask, and continue to cool the system down to 50 °C; while stirring, pour the reaction system into 100 mL of anhydrous ethanol as a precipitant to obtain a solid. After washing, filtering, and drying the solid, the product, a sulfonated polyimide polymer containing carboxyl groups, is obtained.

[0093] Step 2: Add 1.00g of the carboxyl-containing sulfonated polyimide polymer obtained in Step 1 to a 250mL three-necked flask equipped with a reflux condenser, and add 10mL of dimethyl sulfoxide. Stir at 80℃ until the polymer is completely dissolved. Then, add 0.15g of polyvinyl alcohol and 0.01g of 4-dimethylaminopyridine to the three-necked flask, and heat to 100℃ to react for 24h. After the reaction is completed, pour the casting solution onto a clean glass plate and dry it in an oven at 120℃ for 12h to obtain a triethylamine crosslinked sulfonated polyimide membrane.

[0094] Step 3: Immerse the triethylamine crosslinked sulfonated polyimide membrane obtained in Step 2 in ethanol for 24 hours to remove unreacted monomers and residual solvent, then place it in 1.0 mol L... -1 The cross-linked sulfonated polyimide membrane was soaked in sulfuric acid solution for 24 hours to undergo protonation; finally, it was washed 4 times with deionized water to obtain the cross-linked sulfonated polyimide membrane; the thickness of the obtained cross-linked sulfonated polyimide membrane was 50 μm.

[0095] In step one, the synthesis method of 4,4'-(1,1'-bis(4-aminophenoxy))octafluorobiphenyl includes the following steps:

[0096] Step I: Take 8.35 g of decafluorobiphenyl, 6.96 g of p-nitrophenol and 10.35 g of anhydrous potassium carbonate, and measure 10 mL of toluene and 100 mL of N,N-dimethylacetamide and add them to a three-necked flask in sequence; stir at room temperature for 1 h, and then heat to 80 °C and react for 12 h; after the reaction is completed, pour the mixture into deionized water, and a solid will precipitate; wash, filter and dry the solid to obtain the intermediate product 4,4'-(1,1'bis(4-nitrophenoxy))octafluorobiphenyl;

[0097] Step II: 13.50 g of 4,4'-(1,1'-bis(4-nitrophenoxy))octafluorobiphenyl, 2.00 g of activated carbon, 0.37 g of ferric chloride, and 150 mL of anhydrous ethanol were added sequentially to a three-necked flask and stirred at 80 °C for 0.5 h. Then the temperature was lowered to 70 °C, and 30 mL of hydrated hydrazine was added dropwise. The reaction was allowed to proceed for 12 h. After the reaction was completed, the mixture was filtered and the filtrate was collected. The filtrate was poured into deionized water, and a solid precipitated. The solid was washed, filtered, and dried to obtain the product 4,4'-(1,1'-bis(4-aminophenoxy))octafluorobiphenyl.

[0098] In step one, the method for synthesizing 3,5-bis(4-aminophenoxy)benzoic acid includes the following steps:

[0099] Step I: Take 15.41 g of 3,5-dihydroxybenzoic acid, 31.51 g of p-chloronitrobenzene, and 55.29 g of anhydrous potassium carbonate. Measure 20 mL of toluene and 200 mL of N,N-dimethylacetamide and add them sequentially to a three-necked flask. Stir at room temperature for 1 h, then heat to 120 °C and react for 24 h. After the reaction is complete, pour into a mixed solvent of ethanol and deionized water. After acidification, a yellow precipitate will form. After washing and drying, 3,5-bis(4-nitrophenoxy)benzoic acid is obtained.

[0100] Step II: 35.00 g of 3,5-bis(4-nitrophenoxy)benzoic acid, 1.45 g of activated carbon, and 0.37 g of ferric chloride were added sequentially to a three-necked flask along with 150 mL of anhydrous ethanol. The mixture was stirred at room temperature for 1 hour, then heated to 70°C, and 50 mL of hydrated hydrazine was slowly added dropwise. The reaction was allowed to proceed for 12 hours. After the reaction was complete, the mixture was filtered and the filtrate was collected. The filtrate was poured into deionized water, and after acidification, a white precipitate formed. After washing and drying, 3,5-bis(4-aminophenoxy)benzoic acid was obtained.

[0101] Example 6:

[0102] A method for preparing a crosslinked sulfonated polyimide film, comprising the following steps:

[0103] Step 1: Under nitrogen protection, add 4 mmol of 2,2'-disulfonic acid benzidine, 4 mmol of triethylamine, and 60 mL of m-cresol to a 250 mL three-necked flask equipped with a reflux condenser. Heat to 60 °C and stir until completely dissolved. Then add 2 mmol of 4,4'-(1,1'-bis(4-aminophenoxy))octafluorobiphenyl and 2 mmol of 3,5-bis(4-aminophenoxy)benzoic acid to the 250 mL three-necked flask and continue stirring until completely dissolved. Finally, add... 8 mmol of 1,4,5,8-naphthalenetetracarboxylic dianhydride and 16 mmol of benzoic acid were added to a flask, and the mixture was heated to 80 °C and stirred for 4.5 h. The temperature was then increased to 180 °C and the reaction was allowed to proceed for 18 h. After the reaction system was cooled to below 90 °C, 20 mL of m-cresol was added to a 250 mL three-necked flask, and the system was further cooled to 50 °C. The reaction system was then poured into 200 mL of acetone precipitant while stirring to obtain a solid. The solid was washed, filtered, and dried to obtain the product, a sulfonated polyimide polymer containing carboxyl groups.

[0104] Step 2: Add 1.00g of the carboxyl-containing sulfonated polyimide polymer obtained in Step 1 to a 250mL three-necked flask equipped with a reflux condenser, and add 10mL of dimethyl sulfoxide. Stir at 80℃ until the polymer is completely dissolved. Then, add 0.10g of polyvinyl alcohol and 0.01g of 4-dimethylaminopyridine to the three-necked flask, and heat to 100℃ to react for 24h. After the reaction is completed, pour the casting solution onto a clean glass plate and dry it in an oven at 120℃ for 12h to obtain a triethylamine crosslinked sulfonated polyimide membrane.

[0105] Step 3: Immerse the triethylamine crosslinked sulfonated polyimide membrane obtained in Step 2 in ethanol for 24 hours to remove unreacted monomers and residual solvent, then place it in 1.0 mol L... -1 The cross-linked sulfonated polyimide membrane was soaked in sulfuric acid solution for 24 hours to undergo protonation; finally, it was washed 4 times with deionized water to obtain the cross-linked sulfonated polyimide membrane; the thickness of the obtained cross-linked sulfonated polyimide membrane was 50 μm.

[0106] In step one, the synthesis method of 4,4'-(1,1'-bis(4-aminophenoxy))octafluorobiphenyl includes the following steps:

[0107] Step I: Take 8.35 g of decafluorobiphenyl, 6.96 g of p-nitrophenol and 10.35 g of anhydrous potassium carbonate, and measure 10 mL of toluene and 100 mL of N,N-dimethylacetamide and add them to a three-necked flask in sequence; stir at room temperature for 1 h, and then heat to 80 °C and react for 12 h; after the reaction is completed, pour the mixture into deionized water, and a solid will precipitate; wash, filter and dry the solid to obtain the intermediate product 4,4'-(1,1'bis(4-nitrophenoxy))octafluorobiphenyl;

[0108] Step II: 13.50 g of 4,4'-(1,1'-bis(4-nitrophenoxy))octafluorobiphenyl, 2.00 g of activated carbon, 0.37 g of ferric chloride, and 150 mL of anhydrous ethanol were added sequentially to a three-necked flask and stirred at 80 °C for 0.5 h. Then the temperature was lowered to 70 °C, and 30 mL of hydrated hydrazine was added dropwise. The reaction was allowed to proceed for 12 h. After the reaction was completed, the mixture was filtered and the filtrate was collected. The filtrate was poured into deionized water, and a solid precipitated. The solid was washed, filtered, and dried to obtain the product 4,4'-(1,1'-bis(4-aminophenoxy))octafluorobiphenyl.

[0109] In step one, the method for synthesizing 3,5-bis(4-aminophenoxy)benzoic acid includes the following steps:

[0110] Step I: Take 15.41 g of 3,5-dihydroxybenzoic acid, 31.51 g of p-chloronitrobenzene, and 55.29 g of anhydrous potassium carbonate. Measure 20 mL of toluene and 200 mL of N,N-dimethylacetamide and add them sequentially to a three-necked flask. Stir at room temperature for 1 h, then heat to 120 °C and react for 24 h. After the reaction is complete, pour into a mixed solvent of ethanol and deionized water. After acidification, a yellow precipitate will form. After washing and drying, 3,5-bis(4-nitrophenoxy)benzoic acid is obtained.

[0111] Step II: 35.00 g of 3,5-bis(4-nitrophenoxy)benzoic acid, 1.45 g of activated carbon, and 0.37 g of ferric chloride were added sequentially to a three-necked flask along with 150 mL of anhydrous ethanol. The mixture was stirred at room temperature for 1 hour, then heated to 70°C, and 50 mL of hydrated hydrazine was slowly added dropwise. The reaction was allowed to proceed for 12 hours. After the reaction was complete, the mixture was filtered and the filtrate was collected. The filtrate was poured into deionized water, and after acidification, a white precipitate formed. After washing and drying, 3,5-bis(4-aminophenoxy)benzoic acid was obtained.

[0112] Example 7:

[0113] A method for preparing a crosslinked sulfonated polyimide film, comprising the following steps:

[0114] Step 1: Under nitrogen protection, add 4 mmol of 2,2'-disulfonic acid benzidine, 4 mmol of triethylamine, and 60 mL of m-cresol to a 250 mL three-necked flask equipped with a reflux condenser. Heat to 60 °C and stir until completely dissolved. Then add 2 mmol of 4,4'-(1,1'-bis(4-aminophenoxy))octafluorobiphenyl and 2 mmol of 3,5-bis(4-aminophenoxy)benzoic acid to the 250 mL three-necked flask and continue stirring until completely dissolved. Finally, add... 8 mmol of 1,4,5,8-naphthalenetetracarboxylic dianhydride and 16 mmol of benzoic acid were added to a flask, and the mixture was heated to 80 °C and stirred for 4.5 h. The temperature was then increased to 180 °C and the reaction was allowed to proceed for 18 h. After the reaction system was cooled to below 90 °C, 20 mL of m-cresol was added to a 250 mL three-necked flask, and the system was further cooled to 50 °C. The reaction system was then poured into 200 mL of acetone precipitant while stirring to obtain a solid. The solid was washed, filtered, and dried to obtain the product, a sulfonated polyimide polymer containing carboxyl groups.

[0115] Step 2: Add 1.00g of the carboxyl-containing sulfonated polyimide polymer obtained in Step 1 to a 250mL three-necked flask equipped with a reflux condenser, and add 10mL of dimethyl sulfoxide. Stir at 80℃ until the polymer is completely dissolved. Then, add 0.20g of polyvinyl alcohol and 0.01g of 4-dimethylaminopyridine to the three-necked flask, and heat to 100℃ to react for 24h. After the reaction is completed, pour the casting solution onto a clean glass plate and dry it in an oven at 120℃ for 12h to obtain a triethylamine crosslinked sulfonated polyimide membrane.

[0116] Step 3: Immerse the triethylamine crosslinked sulfonated polyimide membrane obtained in Step 2 in ethanol for 24 hours to remove unreacted monomers and residual solvent, then place it in 1.0 mol L... -1 The cross-linked sulfonated polyimide membrane was soaked in sulfuric acid solution for 24 hours to undergo protonation; finally, it was washed 4 times with deionized water to obtain the cross-linked sulfonated polyimide membrane; the thickness of the obtained cross-linked sulfonated polyimide membrane was 50 μm.

[0117] In step one, the synthesis method of 4,4'-(1,1'-bis(4-aminophenoxy))octafluorobiphenyl includes the following steps:

[0118] Step I: Take 8.35 g of decafluorobiphenyl, 6.96 g of p-nitrophenol and 10.35 g of anhydrous potassium carbonate, and measure 10 mL of toluene and 100 mL of N,N-dimethylacetamide and add them to a three-necked flask in sequence; stir at room temperature for 1 h, and then heat to 80 °C and react for 12 h; after the reaction is completed, pour the mixture into deionized water, and a solid will precipitate; wash, filter and dry the solid to obtain the intermediate product 4,4'-(1,1'bis(4-nitrophenoxy))octafluorobiphenyl;

[0119] Step II: 13.50 g of 4,4'-(1,1'-bis(4-nitrophenoxy))octafluorobiphenyl, 2.00 g of activated carbon, 0.37 g of ferric chloride, and 150 mL of anhydrous ethanol were added sequentially to a three-necked flask and stirred at 80 °C for 0.5 h. Then the temperature was lowered to 70 °C, and 30 mL of hydrated hydrazine was added dropwise. The reaction was allowed to proceed for 12 h. After the reaction was completed, the mixture was filtered and the filtrate was collected. The filtrate was poured into deionized water, and a solid precipitated. The solid was washed, filtered, and dried to obtain the product 4,4'-(1,1'-bis(4-aminophenoxy))octafluorobiphenyl.

[0120] In step one, the method for synthesizing 3,5-bis(4-aminophenoxy)benzoic acid includes the following steps:

[0121] Step I: Take 15.41 g of 3,5-dihydroxybenzoic acid, 31.51 g of p-chloronitrobenzene, and 55.29 g of anhydrous potassium carbonate. Measure 20 mL of toluene and 200 mL of N,N-dimethylacetamide and add them sequentially to a three-necked flask. Stir at room temperature for 1 h, then heat to 120 °C and react for 24 h. After the reaction is complete, pour into a mixed solvent of ethanol and deionized water. After acidification, a yellow precipitate will form. After washing and drying, 3,5-bis(4-nitrophenoxy)benzoic acid is obtained.

[0122] Step II: 35.00 g of 3,5-bis(4-nitrophenoxy)benzoic acid, 1.45 g of activated carbon, and 0.37 g of ferric chloride were added sequentially to a three-necked flask along with 150 mL of anhydrous ethanol. The mixture was stirred at room temperature for 1 hour, then heated to 70°C, and 50 mL of hydrated hydrazine was slowly added dropwise. The reaction was allowed to proceed for 12 hours. After the reaction was complete, the mixture was filtered and the filtrate was collected. The filtrate was poured into deionized water, and after acidification, a white precipitate formed. After washing and drying, 3,5-bis(4-aminophenoxy)benzoic acid was obtained.

[0123] Studies have shown that the coulombic efficiency of the crosslinked sulfonated polyimide membranes prepared in Examples 1-7 in vanadium redox flow batteries can be further improved. In order to further improve the coulombic efficiency of the crosslinked sulfonated polyimide membranes in vanadium redox flow batteries, the crosslinked sulfonated polyimide membranes prepared in Example 1 were subjected to the post-processing processes of Examples 8-10.

[0124] Example 8:

[0125] The preparation method of the crosslinked sulfonated polyimide film provided in this embodiment differs from that in Example 1 in step three. After washing with deionized water four times, the crosslinked sulfonated polyimide film is placed in a vacuum environment and irradiated with a high-energy electron beam. The electron beam irradiation energy is 2.8 MeV, the electron beam current intensity is 10 mA, and the irradiation dose is 50 kGy. The electron beam-irradiated crosslinked sulfonated polyimide film is then immersed in 10% hydrogen peroxide for 30 min, and then immersed in 1.0 mol / L... -1 The sulfuric acid aqueous solution was subjected to secondary acidification for 40 minutes, then washed with deionized water and dried in a nitrogen atmosphere at 120°C to obtain a cross-linked sulfonated polyimide film. The remaining steps in this embodiment are the same as in Example 1.

[0126] The crosslinked sulfonated polyimide film prepared in this embodiment was compared with the crosslinked sulfonated polyimide film prepared in Example 1, and the results are as follows: Figure 7 As shown, the crosslinked sulfonated polyimide membrane prepared in this embodiment has a higher coulombic efficiency in the all-vanadium redox flow battery than that in Example 1.

[0127] Example 9:

[0128] The preparation method of the crosslinked sulfonated polyimide film provided in this embodiment differs from that in Example 1 in step three. After washing with deionized water four times, the crosslinked sulfonated polyimide film is placed in a vacuum environment and irradiated with a high-energy electron beam. The electron beam irradiation energy is 3.2 MeV, the electron beam current intensity is 20 mA, and the irradiation dose is 150 kGy. The electron beam-irradiated crosslinked sulfonated polyimide film is then immersed in 10% hydrogen peroxide for 45 min, and then immersed in 2.0 mol L... -1 The sulfuric acid aqueous solution was subjected to secondary acidification for 50 minutes, then washed with deionized water and dried in a nitrogen atmosphere at 140°C to obtain a cross-linked sulfonated polyimide film. The remaining steps in this embodiment are the same as in Example 1.

[0129] Example 10:

[0130] The preparation method of the crosslinked sulfonated polyimide film provided in this embodiment differs from that in Example 1 in step three. After washing with deionized water four times, the crosslinked sulfonated polyimide film is placed in a vacuum environment and irradiated with a high-energy electron beam. The electron beam irradiation energy is 4.5 MeV, the electron beam current intensity is 25 mA, and the irradiation dose is 600 kGy. The electron beam-irradiated crosslinked sulfonated polyimide film is then immersed in 10% hydrogen peroxide for 60 min, and then immersed in 3.0 mol L... -1The sulfuric acid aqueous solution was subjected to secondary acidification for 55 minutes, then washed with deionized water and dried in a nitrogen atmosphere at 150°C to obtain a cross-linked sulfonated polyimide film. The remaining steps in this embodiment are the same as in Example 1.

[0131] The number of devices and processing scale described herein are for the purpose of simplifying the description of the invention. Applications, modifications, and variations of the invention will be readily apparent to those skilled in the art.

[0132] Although embodiments of the present invention have been disclosed above, they are not limited to the applications listed in the specification and embodiments. They can be applied to various fields suitable for the present invention. For those skilled in the art, other modifications can be easily made. Therefore, without departing from the general concept defined by the claims and their equivalents, the present invention is not limited to the specific details and illustrations shown and described herein.

Claims

1. A method for preparing a cross-linked sulfonated polyimide film, characterized in that, The chemical structural formula of the crosslinked sulfonated polyimide film is: ， The preparation method of crosslinked sulfonated polyimide film includes the following steps: Step 1: Synthesize 3,5-bis(4-aminophenoxy)benzoic acid via nucleophilic substitution and reduction reactions; Step 2: Synthesize 4,4'-(1,1'-bis(4-aminophenoxy))octafluorobiphenyl via nucleophilic substitution and reduction reactions; Step 3: Using 3,5-bis(4-aminophenoxy)benzoic acid and 4,4'-(1,1'-bis(4-aminophenoxy))octafluorobiphenyl as raw materials, a sulfonated polyimide polymer containing carboxyl groups is prepared by high-temperature polycondensation reaction. Step 4: Dissolve the carboxyl-containing sulfonated polyimide polymer in an organic solvent, add polyvinyl alcohol and 4-dimethylaminopyridine to react; after the reaction is complete, cast into a film, wash and acidify, and then perform post-treatment on the washed and acidified crosslinked sulfonated polyimide film. The specific methods include: placing the crosslinked sulfonated polyimide film in a vacuum environment and irradiating it with a high-energy electron beam. The electron beam irradiation energy is 2.8~4.5MeV, the electron beam current intensity is 10~25mA, and the irradiation dose is 50~600kGy; immerse the electron beam-irradiated crosslinked sulfonated polyimide film in 10% hydrogen peroxide for 30~60 min, and then immerse it in 1.0~3.0 mol L... -1 The sulfuric acid aqueous solution is subjected to secondary acidification for 40-55 minutes, then washed with deionized water, and dried in a nitrogen atmosphere at a temperature of 120-150℃ to obtain the cross-linked sulfonated polyimide film. In step one, the method for synthesizing 3,5-bis(4-aminophenoxy)benzoic acid includes the following steps: Step I: Take 15-30 parts by weight of 3,5-dihydroxybenzoic acid, 31-63 parts by weight of p-chloronitrobenzene, and 30-60 parts by weight of anhydrous potassium carbonate. Measure 20-40 parts by volume of toluene and 100-200 parts by volume of organic solvent and add them sequentially to the reaction vessel. Stir at room temperature for 1-2 hours, then raise the temperature to 120-140℃ and react for 12-24 hours. After the reaction is complete, pour the mixture into a mixed solvent of ethanol and deionized water. After acidification, a yellow precipitate will form. After washing and drying, 3,5-bis(4-nitrophenoxy)benzoic acid is obtained. Step II: Add 17-35 parts by weight of 3,5-bis(4-nitrophenoxy)benzoic acid, 1-2 parts by weight of activated carbon, and 0.2-0.4 parts by weight of ferric chloride, along with 75-150 parts by volume of anhydrous ethanol, to the reaction vessel. Stir at room temperature for 1-2 hours, then heat to 70-80°C and slowly add 25-50 parts by volume of hydrated hydrazine. React for 12-24 hours. After the reaction is complete, filter and collect the filtrate. Pour the filtrate into deionized water, and after acidification, a white precipitate will form. Wash and dry the precipitate to obtain 3,5-bis(4-aminophenoxy)benzoic acid. In step three, the high-temperature polycondensation reaction includes: under nitrogen protection, mixing 2,2'-bis(sulfonated) benzidine, m-methylphenol, and triethylamine, and heating to 60-80°C while stirring until the solid dissolves; then adding 4,4'-(1,1'-bis(4-aminophenoxy))octafluorobiphenyl and 3,5-bis(4-aminophenoxy)benzoic acid and stirring until the solid dissolves; finally adding 1,4,5,8-naphthalenetetracarboxylic dianhydride and benzoic acid, and heating the temperature to 80-100°C for 1-5 hours, then heating to 180-200°C for 15-25 hours. After cooling the reaction system to 90-100°C, adding a diluent; and when the reaction system cools to 50°C, pouring the resulting viscous solution into a precipitant to obtain a solid; washing, filtering, and drying the solid yields a carboxyl-containing sulfonated polyimide polymer.

2. The method for preparing the crosslinked sulfonated polyimide film according to claim 1, characterized in that, In step two, the synthesis method of 4,4'-(1,1'-bis(4-aminophenoxy))octafluorobiphenyl includes the following steps: Step I: Take 8-17 parts by weight of decafluorobiphenyl, 6.5-14 parts by weight of p-nitrophenol, and 10-22 parts by weight of anhydrous potassium carbonate. Measure 10-20 parts by volume of toluene and 100-200 parts by volume of organic solvent and add them sequentially to the reaction vessel. Stir at room temperature for 1-2 hours, then heat to 80-100℃ and react for 12-24 hours to obtain a mixture. After the reaction is complete, pour the mixture into deionized water, and a solid will precipitate. Wash, filter, and dry the solid to obtain the intermediate product 4,4'-(1,1'-bis(4-nitrophenoxy))octafluorobiphenyl. Step II: 13-27 parts by weight of 4,4'-(1,1'-bis(4-nitrophenoxy))octafluorobiphenyl, 2-4 parts by weight of activated carbon, 0.3-0.8 parts by weight of ferric chloride, and 150-200 parts by volume of anhydrous ethanol were added sequentially to a reaction vessel and stirred at 80-90°C for 0.5-1 h. Then the temperature was lowered to 70°C, and 30-60 parts by volume of hydrated hydrazine were added dropwise. The reaction was allowed to proceed for 12-24 h. After the reaction was completed, the mixture was filtered and the filtrate was collected. The filtrate was poured into deionized water, and a solid precipitated out. The solid was washed, filtered, and dried to obtain the product 4,4'-(1,1'-bis(4-aminophenoxy))octafluorobiphenyl.

3. The method for preparing the crosslinked sulfonated polyimide film according to claim 1, characterized in that, In step three, the molar ratio of 4,4'-(1,1'-bis(4-aminophenoxy))octafluorobiphenyl, 3,5-bis(4-aminophenoxy)benzoic acid, 2,2'-bissulfonic acid benzidine, 1,4,5,8-naphthalenetetracarboxylic dianhydride, and benzoic acid is 1.0~2.0:1.0~2.0:2.0~4.0:4.0~8.0:8.0~16.0; the molar ratio of triethylamine to 2,2'-bissulfonic acid benzidine is 1.0~3.0:1.

0. ~3.0; the volume ratio of m-methylphenol to triethylamine is 70.0~140.0:2.5~5.5; the diluent is m-methylphenol, and the volume ratio of the diluent to triethylamine is 15.0~30.0:2.5~5.5; the precipitant is one or more of acetone, methanol, and ethanol; the detergent used for washing is one or more of acetone, methanol, ethanol, or deionized water; the volume ratio of the precipitant to the viscous solution is 5.0~10.0:1.

0.

4. The method for preparing the crosslinked sulfonated polyimide film according to claim 1, characterized in that, Step four includes: mixing a carboxyl-containing sulfonated polyimide polymer with an organic solvent, heating the mixture to 60-80°C and stirring until dissolved, then adding a catalyst 4-dimethylaminopyridine and a crosslinking agent polyvinyl alcohol, and raising the stirring temperature to 100-120°C and reacting for 12-24 hours to obtain a casting solution. The casting solution is then poured onto a dry and clean glass plate for casting into a film. Subsequently, the glass plate is dried at 100-120°C for 12-24 hours and acidified to obtain a crosslinked sulfonated polyimide film. In step four, the detergent is any one of methanol, ethanol, or deionized water, and the acidifying reagent is 1.0~3.0 mol / L. -1 Aqueous solution of sulfuric acid.

5. The method for preparing the crosslinked sulfonated polyimide film according to claim 1, characterized in that, The organic solvent is any one of N,N-dimethylacetamide, N,N-dimethylformamide, dimethyl sulfoxide, and N-methylpyrrolidone.

6. The method for preparing the crosslinked sulfonated polyimide film according to claim 1, characterized in that, The thickness of the cross-linked sulfonated polyimide film is 40~55μm.

7. An application of a crosslinked sulfonated polyimide film, wherein the crosslinked sulfonated polyimide film is prepared by the method for preparing the crosslinked sulfonated polyimide film according to any one of claims 2-6, characterized in that, The cross-linked sulfonated polyimide membrane is used in the field of vanadium redox flow batteries as a separator for vanadium redox flow batteries.

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