A method for preparing a branched sulfonated polyimide film

Abstract

The application discloses a preparation method of a branched sulfonated polyimide film, which comprises the following steps: preparing 1,3,5-tri(4-naphthoxy-1,8-dioic acid) benzene trianhydride, using the 1,3,5-tri(4-naphthoxy-1,8-dioic acid) benzene trianhydride as raw material, and adding 4,4'-diamino diphenyl, 2,2'-disulfonic acid benzidine, triethylamine and benzoic acid to obtain a triethylamine salt type branched sulfonated polyimide film; soaking the triethylamine salt type branched sulfonated polyimide film in anhydrous ethanol, then soaking the triethylamine salt type branched sulfonated polyimide film in a sulfuric acid aqueous solution, and then washing the triethylamine salt type branched sulfonated polyimide film with deionized water to obtain a branched sulfonated polyimide film, and storing the branched sulfonated polyimide film. The branched sulfonated polyimide film prepared by the application has good performance and is suitable for a full 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 branched sulfonated polyimide membrane. Background Technology

[0002] With societal development, human demand for energy is rising significantly, while environmental pollution caused by the massive consumption of fossil fuels is becoming increasingly severe. Therefore, the large-scale development and application of renewable energy is a crucial pathway for energy transition and low-carbon economic development worldwide. However, renewable energy sources, such as wind and solar power, are characterized by discontinuity, instability, and uncontrollability, making it difficult to maintain a balance between power generation and load. Their large-scale grid connection significantly impacts the stability and reliability of the power system. Therefore, large-scale energy storage technologies, especially long-term energy storage technologies, are urgently needed to achieve peak shaving and valley filling in the power grid, thereby improving the grid's capacity to absorb renewable energy generation and solving problems such as wind and solar curtailment. Vanadium redox flow batteries, with their advantages of high safety, long cycle life, recyclable electrolyte, and high cost-effectiveness over their lifecycle, are considered one of the large-scale energy storage technologies with promising prospects for large-scale commercial application.

[0003] Vanadium redox flow batteries (VRFBs) mainly consist of electrodes, a separator, and an electrolyte. The separator, as a key material in VRFBs, serves to prevent short circuits by blocking contact between the positive and negative electrodes, prevent cross-mixing of the electrolytes to avoid side reactions, and simultaneously transfer charge carriers to form a complete battery circuit. The selectivity, conductivity, stability, and cost of the separator are directly related to the efficiency, lifespan, and techno-economic viability of VRFBs. Currently, the most commonly used membrane in VRFBs is the perfluorosulfonic acid ion exchange membrane (e.g., Nafion membrane), which has advantages such as high stability and strong proton conductivity. However, due to the high vanadium ion permeability of Nafion membranes, vanadium ions easily cross-permeate between the two electrode liquids during VRFB charging and discharging, leading to a series of problems such as decreased battery efficiency, severe self-discharge, and shortened cycle life. Furthermore, the high cost of Nafion membranes also limits their large-scale commercial application in VRFBs. Therefore, developing novel VRFB separator materials with low vanadium ion permeability, excellent chemical / electrochemical stability, and low cost is of significant research importance and commercial value.

[0004] Due to the aforementioned limitations of Nafion membranes, sulfonated aromatic polymer membranes, which possess higher mechanical properties, thermal stability, good chemical stability, reasonable cost, and tunable chemical structure, have gradually become a research hotspot for VRFB separators in recent years. Currently developed sulfonated aromatic polymer membranes mainly include sulfonated polyether ether ketone (SPEEK) membranes, sulfonated polysulfone (SPSF) membranes, sulfonated polybenzimidazole (SPBI) membranes, and sulfonated polyimide (SPI) membranes. Among these sulfonated aromatic polymer membranes, the six-membered ring sulfonated polyimide membrane exhibits excellent heat resistance, high mechanical strength, good film-forming properties, and low vanadium ion permeability, making it a promising alternative to Nafion membranes. However, sulfonated polyimide membranes face two bottlenecks in VRFB applications: (1) In strong acid and oxidizing electrolyte environments, the chemical stability of sulfonated polyimide membranes is weak, severely affecting their lifespan in the battery; (2) Low proton conductivity hinders the battery from achieving higher voltage efficiency. Therefore, there is an urgent need to significantly improve the chemical stability and proton conductivity of sulfonated polyimide films.

[0005] Currently, to improve the stability of sulfonated polyimide films in VRFB, researchers typically employ the following three methods: (1) Combining sulfonated polyimide films with other functional materials. Liu Jun et al. used a ring-opening grafting method to graft sulfonyl groups into aminated multi-walled carbon nanotubes to obtain functional sulfonated aminated multi-walled carbon nanotubes (s-MWCNTs), which were then introduced into branched sulfonated polyimide in different proportions to prepare a series of branched sulfonated polyimide / s-MWCNTs composite films. Experimental results show that the branched sulfonated polyimide / s-MWCNTs composite films have superior vanadium blocking ability and proton selectivity in the range of 80–160 mA cm⁻¹. -2 Compared with Nafion 212 membrane, VRFB can achieve higher coulombic efficiency (96.0%-92.5%) and energy efficiency (79.7%-69.5%). (2) Crosslinking of sulfonated polyimide polymer. Xu Wenjie et al. prepared hydroxyl-containing polyfluorosulfonated polyimide polymer by high-temperature polymerization, and then reacted it with polystyrene acid as a crosslinking agent to prepare covalently crosslinked polyfluorosulfonated polyimide membranes with different polyacrylic acid contents. The experimental results showed that the vanadium ion permeability of the covalently crosslinked polyfluorosulfonated polyimide membrane was (5.9~6.77×10). -9 cm 2 min -1 Two orders of magnitude lower than Nafion 212 membrane, the best covalently cross-linked polyfluorosulfonated polyimide membrane (0.15 Ωcm) 2 The sheet resistivity is also lower than that of the Nafion 212 film (0.16 Ωcm). 2Furthermore, the coulombic efficiency and energy efficiency of VRFB using the optimal covalently crosslinked polyfluorinated sulfonated polyimide membrane are higher than those of the battery using Nafion 212. (3) Designing branched monomers to obtain branched sulfonated polyimide membranes. Long Jun et al. prepared branched anhydrides containing five-membered rings and prepared bibranched sulfonated polyimide membranes. Compared with linear sulfonated polyimide membranes, this membrane has higher proton conductivity and lower vanadium permeability. In addition, the branched structure has a unique three-dimensional network structure, which can overcome the shortcomings of membrane chemical stability and proton conductivity. To this end, in recent years, the research group has tried to prepare branched sulfonated polyimide polymers by designing branched monomers to obtain branched sulfonated polyimide membrane materials with better chemical stability. Summary of the Invention

[0006] 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.

[0007] To achieve these objectives and other advantages according to the present invention, a branched sulfonated polyimide film is provided, characterized in that the branched sulfonated polyimide film has the following structural formula:

[0008]

[0009] A method for preparing a branched sulfonated polyimide film includes the following steps:

[0010] Step 1: Under nitrogen protection, m-cresol, 2,2'-bis(sulfonic acid) benzidine and triethylamine are added to a reactor and stirred at a certain temperature until dissolved. Then, 4,4'-diaminodiphenyl is added and stirred until dissolved. Next, 1,3,5-tris(4-naphthoxy-1,8-dicarboxylic acid) benzoic acid is added and stirred for a certain time to obtain a casting solution. The casting solution is poured onto a dry and clean glass plate to cast a film. The glass plate is then dried under a temperature gradient for a certain time to obtain a triethylamine salt type branched sulfonated polyimide film.

[0011] Step 2: Soak the triethylamine salt type branched sulfonated polyimide membrane in anhydrous ethanol for a certain period of time, then soak it in a sulfuric acid aqueous solution of a certain concentration, and then wash it with deionized water 5 to 10 times to obtain the branched sulfonated polyimide membrane. Store it in deionized water.

[0012] Preferably, in step one, the molar ratio of 2,2'-bis(sulfonic acid) benzidine, 1,3,5-tris(4-naphthoxy-1,8-dicarboxylic acid) benzoic acid, 4,4'-diaminodiphenyl and benzoic acid is 0.3-1.2:1:0.3-1.2:3.

[0013] Preferably, in step one, the volume ratio of m-cresol to triethylamine is 30-90:1-3; and the ratio of triethylamine to 2,2'-disulfonic acid benzidine is such that for every 1-3 mmol of 2,2'-disulfonic acid benzidine added, the volume of triethylamine added is 1.0-3.0 mL.

[0014] Preferably, in step one, m-cresol is replaced by one or a mixture of two or more of N,N-dimethylformamide, N,N-dimethylacetamide, dimethyl sulfoxide, and N-methylpyrrolidone.

[0015] Preferably, in step one, the thickness of the film formed by casting the film solution on the glass plate is 40–100 μm.

[0016] Preferably, in step two, anhydrous ethanol is replaced with one or a mixture of two or more of methanol, acetone, and isopropanol; and deionized water is replaced with distilled water or ultrapure water.

[0017] Preferably, in step one, m-cresol, 2,2'-disulfonic acid benzidine and triethylamine are added to the reactor and stirred until the temperature of dissolution is 60-90°C; 1,3,5-tris(4-naphthoxy-1,8-dicarboxylic acid) benzoic acid is added and stirred for 15-20 hours; the glass plate is dried for 15 hours under different temperature gradients, and the range of different temperature gradients is 80-150°C.

[0018] In step two, the triethylamine salt-type branched sulfonated polyimide membrane is immersed in anhydrous ethanol for 24–36 hours, and the concentration of the sulfuric acid aqueous solution is 1.0–3.0 mol / L. -1 The soaking time in sulfuric acid aqueous solution is 24-48 hours.

[0019] Preferably, in step one, the preparation method of 1,3,5-tris(4-naphthoxy-1,8-dicarboxylic acid) phenyltrihydric anhydride includes: placing NaOH in methanol and stirring at room temperature until the solid dissolves; adding phloroglucinol; purging with nitrogen gas at room temperature; stirring for 1 hour; then slowly heating to 80°C and reacting for 4-6 hours until all methanol is distilled off to obtain a powdery solid; wherein the mass-to-volume ratio of NaOH, methanol, and phloroglucinol is 2.400 g: 60.0 mL: 2.522 g; cooling to room temperature; adding 4-bromo-1,8-naphthic anhydride and N,N-dimethylacetamide; attaching a constant-pressure dropping funnel containing toluene to the mouth of the reaction vessel; purging with nitrogen gas at room temperature and stirring for 1 hour; heating to 120°C and then adding toluene dropwise; heating to 140°C and reacting for 4 hours; distilling off the toluene and then heating again. The reaction was carried out at 165–170 °C for 20 h. After the reaction was completed, the mixture was cooled to room temperature, poured into acetone for precipitation, and stirred rapidly for 1 h. The mass-to-volume ratio of resorcinol, 4-bromo-1,8-naphthyl anhydride, N,N-dimethylacetamide, toluene, and acetone was 2.522 g: 16.624 g: 50.0 mL: 30.0 mL: 200.0 mL. The solid was filtered and collected, washed thoroughly with water and acetone in sequence, and dried in a vacuum oven at 120 °C for 12 h to obtain crude 1,3,5-tris(4-naphthoxy-1,8-dicioic acid)benzene trihydric anhydride. The crude 1,3,5-tris(4-naphthoxy-1,8-dicioic acid)benzene trihydric anhydride was recrystallized from acetic anhydride to obtain pure 1,3,5-tris(4-naphthoxy-1,8-dicioic acid)benzene trihydric anhydride.

[0020] Preferably, in order to improve the coulombic efficiency and energy efficiency of the branched sulfonated polyimide membrane in VRFB applications, the branched sulfonated polyimide membrane after washing and acidification in step two is post-treated. The specific method includes: placing the branched sulfonated polyimide membrane 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 branched sulfonated polyimide membrane 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℃.

[0021] An application of a branched sulfonated polyimide membrane, wherein the branched sulfonated polyimide membrane is used as a diaphragm in VRFB.

[0022] Compared with existing VRFB technology using sulfonated polyimide films, the present invention has at least the following advantages:

[0023] (1) The purpose of this invention is to overcome the shortcomings of weak chemical stability and proton conductivity of sulfonated polyimide membranes used in VRFB. A sulfonated diamine monomer with mass transfer capability (branched trihydride monomer) and a non-sulfonated diamine monomer are used as raw materials to prepare a sulfonated polyimide membrane containing branched nodes. The preparation route is as follows: Figure 1 As shown, the branched sulfonated polyimide membrane obtained can significantly improve the interaction between molecular chains and the free volume, thereby enhancing its chemical stability, proton conductivity, and application potential in the VRFB field. Furthermore, by changing the ratio of the sulfonated diamine monomer 2,2'-bissulfonated benzidine and the non-sulfonated diamine monomer 4,4'-diaminodiphenyl, the degree of sulfonation of the membrane can be effectively controlled, thus specifically addressing the problems of weak chemical stability and low proton conductivity of the sulfonated polyimide membrane. The synthesis process of the branched trihydric anhydride monomer 1,3,5-tris(4-naphthoxy-1,8-dicarboxylic acid)phenyltrihydric anhydride required for preparing the branched sulfonated polyimide membrane in this invention is as follows: Figure 2 As shown.

[0024] (2) The key physicochemical properties of the branched sulfonated polyimide membrane prepared by the present invention are as follows: vanadium ion permeability (0.71~3.02×10⁻⁶). -7 cm 2 min -1 It is nearly an order of magnitude lower than commercially available Nafion 212 membranes; proton conductivity (0.65–2.79 × 10⁻⁶) -2 S cm -1 The results show that the branched sulfonated polyimide membrane provided by this invention can better meet the application requirements of VRFB. The proton selectivity is also superior to that of Nafion 212 membrane; the stability is stronger than most reported sulfonated aromatic polymer membrane materials for VRFB; and the cost is only one-quarter of the price of Nafion 212 membrane.

[0025] 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

[0026] Figure 1 This is the preparation route for the branched sulfonated polyimide membrane in this invention;

[0027] Figure 2 This is the synthetic route for 1,3,5-tris(4-naphthoxy-1,8-dicarboxylic acid) phenyltrihydric anhydride in this invention;

[0028] Figure 3 The infrared spectrum of 1,3,5-tris(4-naphthoxy-1,8-dicarboxylic acid) phenyltrihydride of the present invention is shown below.

[0029] Figure 4 The 1H NMR spectrum of 1,3,5-tris(4-naphthoxy-1,8-dicarboxylic acid) phenyltrihydride of the present invention;

[0030] Figure 5 The infrared spectrum of the branched sulfonated polyimide film of the present invention is shown below.

[0031] Figure 6 The 1H NMR spectrum of the branched sulfonated polyimide membrane of the present invention is shown below.

[0032] Figure 7 A comparison of the open-circuit voltage curves of VRFB using the branched sulfonated polyimide membrane prepared in Example 5 of this invention and the commercial Nafion 212 membrane.

[0033] Figure 8 A comparison graph showing the VRFB efficiency of the branched sulfonated polyimide membrane prepared using Example 5 of the present invention and the commercial Nafion 212 membrane;

[0034] Figure 9 A comparison graph showing the efficiency of VRFB using branched sulfonated polyimide films prepared in Examples 5 and 8 of this invention. Detailed Implementation

[0035] 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.

[0036] 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.

[0037] Example 1:

[0038] like Figure 1 As shown, a method for preparing a branched sulfonated polyimide film includes the following steps:

[0039] Step 1: Under nitrogen protection, add 0.6 mmol of 2,2'-bis(sulfonic acid) benzidine, 0.6 mL of triethylamine, and 30.0 mL of m-cresol to a flask and stir at 80°C until the monomer is completely dissolved. Then add 2.4 mmol of 4,4'-diaminodiphenyl and continue stirring until the solid is completely dissolved. Next, add 2.0 mmol of 1,3,5-tris(4-naphthoxy-1,8-dicarboxylic acid) phenyltrihydride and 6.0 mmol of benzoic acid and stir for 15 h to obtain a branched sulfonated polyimide casting solution. Pour the casting solution onto a dry and clean glass plate to cast a film and dry at 80°C for 12 h; then dry at 100°C, 120°C, and 150°C for 1 h each to obtain a triethylamine salt type branched sulfonated polyimide film. Figure 2As shown, the synthetic steps of 1,3,5-tris(4-naphthoxy-1,8-dicarboxylic acid)benzenetricarbin are as follows:

[0040] 2.400 g of NaOH was placed in 60.0 mL of methanol and stirred at room temperature until the solid was completely dissolved. Then, 2.522 g of phloroglucinol was added, and nitrogen gas was introduced at room temperature. After stirring for 1 h, the temperature was slowly raised to 80 °C and reacted for 4 h until all the methanol was evaporated to obtain a powdery solid. After cooling to room temperature, 16.624 g of 4-bromo-1,8-naphthalene anhydride and 50.0 mL of N,N-dimethylacetamide were added. A constant-pressure dropping funnel containing 30.0 mL of toluene was placed on a three-necked flask, and nitrogen gas was introduced at room temperature and stirred for 1 h. After heating to 120 °C, toluene was added dropwise, and the temperature was raised to 140 °C and reacted for 4 h. After the toluene was evaporated, the temperature was raised to 165 °C and reacted for 20 h. After the reaction was completed and cooled to room temperature, the solid was precipitated in approximately 200.0 mL of acetone and stirred rapidly for 1 h. The solid was then filtered and collected, washed thoroughly with water and acetone sequentially, and dried in a vacuum oven at 120 °C for 12 h to obtain crude 1,3,5-tris(4-naphthoxy-1,8-dicylic acid)benzenetricarbin. The crude product was recrystallized from acetic anhydride to obtain purified 1,3,5-tris(4-naphthoxy-1,8-dicylic acid)benzenetricarbin, the infrared spectrum of which is shown below. Figure 3 As shown, the proton NMR spectrum is as follows: Figure 4 As shown.

[0041] Step 2: Immerse the obtained triethylamine salt-type branched sulfonated polyimide membrane in anhydrous ethanol for 24 hours to remove residual solvent and unreacted monomers; then, immerse it in 1.0 mol L... -1 The protonation process was completed in sulfuric acid solution for 24 hours. Finally, the membrane was washed 6 times with deionized water to obtain a branched sulfonated polyimide membrane with a thickness of 52 μm.

[0042] Example 2:

[0043] A method for preparing a branched sulfonated polyimide film includes the following steps:

[0044] Under nitrogen protection, 0.9 mmol of 2,2'-disulfonic acid benzidine, 0.9 mL of triethylamine, and 30.0 mL of m-cresol were added to a flask and stirred at 80 °C until the monomer was completely dissolved. Then, 2.1 mmol of 4,4'-diaminodiphenyl was added and stirring continued until the solid was completely dissolved. Next, 2.0 mmol of 1,3,5-tris(4-naphthoxy-1,8-dicarboxylic acid) phenyltrihydride and 6.0 mmol of benzoic acid were added and the mixture was stirred for 15 h to obtain a branched sulfonated polyimide casting solution. The casting solution was poured onto a dry and clean glass plate to form a film, which was dried at 80 °C for 12 h. The film was then dried at 100 °C, 120 °C, and 150 °C for 1 h each to obtain a triethylamine salt type branched sulfonated polyimide film. The synthesis method of 1,3,5-tris(4-naphthoxy-1,8-dicarboxylic acid) phenyltrihydride was the same as in Example 1.

[0045] Step 2: Immerse the obtained triethylamine salt-type branched sulfonated polyimide membrane in anhydrous ethanol for 24 hours to remove residual solvent and unreacted monomers; then, immerse it in 1.0 mol L... -1 The protonation process was completed in sulfuric acid solution for 24 hours; finally, it was washed 6 times with deionized water to obtain the branched sulfonated polyimide membrane; the thickness of the obtained branched sulfonated polyimide membrane was 52 μm.

[0046] Example 3:

[0047] A method for preparing a branched sulfonated polyimide film includes the following steps:

[0048] Under nitrogen protection, 1.2 mmol of 2,2'-disulfonic acid benzidine, 1.2 mL of triethylamine, and 30.0 mL of m-cresol were added to a flask and stirred at 80 °C until the monomer was completely dissolved. Then, 1.8 mmol of 4,4'-diaminodiphenyl was added and stirring continued until the solid was completely dissolved. Next, 2.0 mmol of 1,3,5-tris(4-naphthoxy-1,8-dicarboxylic acid) phenyltrihydride and 6.0 mmol of benzoic acid were added and the mixture was stirred for 15 h to obtain a branched sulfonated polyimide casting solution. The casting solution was poured onto a dry and clean glass plate and cast into a film, which was dried at 80 °C for 12 h. The film was then dried at 100 °C, 120 °C, and 150 °C for 1 h each to obtain a triethylamine salt type branched sulfonated polyimide film. The synthesis method of 1,3,5-tris(4-naphthoxy-1,8-dicarboxylic acid) phenyltrihydride was the same as in Example 1.

[0049] Step 2: Immerse the obtained triethylamine salt-type branched sulfonated polyimide membrane in anhydrous ethanol for 24 hours to remove residual solvent and unreacted monomers, then immerse it in 2.0 mol L... -1The protonation process is completed in sulfuric acid solution for 24 hours. Finally, the membrane is washed 6 times with deionized water to obtain a branched sulfonated polyimide membrane. The thickness of the obtained branched sulfonated polyimide membrane can be controlled at 51 μm.

[0050] Example 4:

[0051] A method for preparing a branched sulfonated polyimide film includes the following steps:

[0052] Under nitrogen protection, 1.5 mmol of 2,2'-disulfonic acid benzidine, 1.5 mL of triethylamine, and 45.0 mL of m-cresol were added to a flask and stirred at 80 °C until the monomer was completely dissolved. Then, 1.5 mmol of 4,4'-diaminodiphenyl was added and stirring continued until the solid was completely dissolved. Next, 2.0 mmol of 1,3,5-tris(4-naphthoxy-1,8-dicarboxylic acid) phenyltrihydride and 6.0 mmol of benzoic acid were added and the mixture was stirred for 17 h to obtain a branched sulfonated polyimide casting solution. The casting solution was poured onto a dry and clean glass plate to cast a film and dried at 80 °C for 12 h. The film was then dried at 100 °C, 120 °C, and 150 °C for 1 h each to obtain a triethylamine salt type branched sulfonated polyimide film. The synthesis method of 1,3,5-tris(4-naphthoxy-1,8-dicarboxylic acid) phenyltrihydride was the same as in Example 1.

[0053] Step 2: Immerse the obtained triethylamine salt-type branched sulfonated polyimide membrane in anhydrous ethanol for 24 hours to remove residual solvent and unreacted monomers, then immerse it in 3.0 mol L... -1 The protonation process is completed in sulfuric acid solution for 24 hours. Finally, the membrane is washed 6 times with deionized water to obtain a branched sulfonated polyimide membrane. The thickness of the obtained branched sulfonated polyimide membrane can be controlled at 49 μm.

[0054] Example 5:

[0055] A method for preparing a branched sulfonated polyimide film includes the following steps:

[0056] Step 1: Under nitrogen protection, add 1.8 mmol of 2,2'-disulfonic acid benzidine, 1.8 mL of triethylamine, and 60.0 mL of m-cresol to a flask and stir at 80°C until the monomer is completely dissolved. Then add 1.2 mmol of 4,4'-diaminodiphenyl and continue stirring until the solid is completely dissolved. Next, add 2.0 mmol of 1,3,5-tris(4-naphthoxy-1,8-dicarboxylic acid) phenyltrihydride and 6.0 mmol of benzoic acid and stir for 20 h to obtain a branched sulfonated polyimide casting solution. Pour the casting solution onto a dry and clean glass plate to cast a film and dry it at 80°C for 12 h. Then dry it at 100°C, 120°C, and 150°C for 1 h each to obtain a triethylamine salt type branched sulfonated polyimide film. The synthesis method of 1,3,5-tris(4-naphthoxy-1,8-dicarboxylic acid) phenyltrihydride is the same as in Example 1.

[0057] Step 2: Immerse the obtained triethylamine salt-type branched sulfonated polyimide membrane in anhydrous ethanol for 24 hours to remove residual solvent and unreacted monomers; then, immerse it in 1.0 mol L... -1 The protonation process was completed in sulfuric acid solution for 24 hours. Finally, the membrane was washed six times with deionized water to obtain the branched sulfonated polyimide membrane. The thickness of the obtained branched sulfonated polyimide membrane was 50 μm. The infrared spectrum of the branched sulfonated polyimide membrane is shown below. Figure 5 As shown, the proton NMR spectrum is as follows: Figure 6 As shown.

[0058] Example 6:

[0059] A method for preparing a branched sulfonated polyimide film includes the following steps:

[0060] Step 1: Under nitrogen protection, add 2.1 mmol of 2,2'-disulfonic acid benzidine, 2.1 mL of triethylamine, and 60.0 mL of N,N-dimethylformamide to a flask and stir at 80°C until the monomer is completely dissolved. Then add 0.9 mmol of 4,4'-diaminodiphenyl and continue stirring until the solid is completely dissolved. Next, add 2.0 mmol of 1,3,5-tris(4-naphthoxy-1,8-dicarboxylic acid) phenyltrihydride and 6.0 mmol of benzoic acid and stir for 20 h to obtain a branched sulfonated polyimide casting solution. Pour the casting solution onto a dry and clean glass plate to cast a film and dry it at 80°C for 12 h. Then dry it at 100°C, 120°C, and 150°C for 1 h each to obtain a triethylamine salt type branched sulfonated polyimide film. The synthesis method of 1,3,5-tris(4-naphthoxy-1,8-dicarboxylic acid) phenyltrihydride is the same as in Example 1.

[0061] Step 2: Immerse the obtained triethylamine salt-type branched sulfonated polyimide membrane in anhydrous ethanol for 24 hours to remove residual solvent and unreacted monomers; then, immerse it in 1.0 mol L... -1 The protonation process was completed in sulfuric acid solution for 24 hours. Finally, the membrane was washed 6 times with deionized water to obtain a branched sulfonated polyimide membrane with a thickness of 51 μm.

[0062] Example 7:

[0063] A method for preparing a branched sulfonated polyimide film includes the following steps:

[0064] Step 1: Under nitrogen protection, add 2.4 mmol of 2,2'-bis(2,2'-disulfonic acid) benzidine, 2.4 mL of triethylamine, and 60.0 mL of N-methylpyrrolidone to a flask and stir at 80°C until the monomer is completely dissolved. Then add 0.6 mmol of 4,4'-diaminodiphenyl and continue stirring until the solid is completely dissolved. Next, add 2.0 mmol of 1,3,5-tris(4-naphthoxy-1,8-dicarboxylic acid) phenyltrihydride and 6.0 mmol of benzoic acid and stir for 20 h to obtain a branched sulfonated polyimide casting solution. Pour the casting solution onto a dry and clean glass plate to cast a film and dry it at 80°C for 12 h. Then dry it at 100°C, 120°C, and 150°C for 1 h each to obtain a triethylamine salt type branched sulfonated polyimide film. The synthesis method of 1,3,5-tris(4-naphthoxy-1,8-dicarboxylic acid) phenyltrihydride is the same as in Example 1.

[0065] Step 2: Immerse the obtained triethylamine salt-type branched sulfonated polyimide membrane in anhydrous ethanol for 24 hours to remove residual solvent and unreacted monomers; then, immerse it in 1.0 mol L... -1 The protonation process was completed in sulfuric acid solution for 24 hours. Finally, the membrane was washed 6 times with deionized water to obtain a branched sulfonated polyimide membrane with a thickness of 51 μm.

[0066] Example 8:

[0067] A method for preparing a branched sulfonated polyimide film includes the following steps:

[0068] Step 1: Under nitrogen protection, add 1.8 mmol of 2,2'-disulfonic acid benzidine, 1.8 mL of triethylamine, and 60.0 mL of m-cresol to a flask and stir at 80°C until the monomer is completely dissolved. Then add 1.2 mmol of 4,4'-diaminodiphenyl and continue stirring until the solid is completely dissolved. Next, add 2.0 mmol of 1,3,5-tris(4-naphthoxy-1,8-dicarboxylic acid) phenyltrihydride and 6.0 mmol of benzoic acid and stir for 20 h to obtain a branched sulfonated polyimide casting solution. Pour the casting solution onto a dry and clean glass plate to cast a film and dry it at 80°C for 12 h. Then dry it at 100°C, 120°C, and 150°C for 1 h each to obtain a triethylamine salt type branched sulfonated polyimide film. The synthesis method of 1,3,5-tris(4-naphthoxy-1,8-dicarboxylic acid) phenyltrihydride is the same as in Example 1.

[0069] Step 2: Immerse the obtained triethylamine salt-type branched sulfonated polyimide membrane in anhydrous ethanol for 32 hours to remove residual solvent and unreacted monomers, then immerse it in 2.0 mol L... -1 The protonation process was completed in sulfuric acid solution for 32 hours. The membrane was then washed six times with deionized water. Next, the branched sulfonated polyimide membrane was placed in a vacuum environment and irradiated with a high-energy electron beam at an energy of 2.8 MeV, a beam current of 10 mA, and a dose of 120 kGy. After irradiation, the branched sulfonated polyimide membrane was immersed in 10% hydrogen peroxide solution for 50 minutes, followed by immersion in 1.0 mol L... -1 The sulfuric acid aqueous solution is used for secondary acidification for 40 min, followed by washing with deionized water and drying in a nitrogen atmosphere at 120℃ to obtain a branched sulfonated polyimide film. The thickness of the obtained branched sulfonated polyimide film can be controlled at 50 μm.

[0070] The branched sulfonated polyimide film prepared in this embodiment was applied to VRFB, and at 100 mA cm⁻¹... -2 The coulombic efficiency of the battery at the current density is 97.5% and the energy efficiency is 86%, both of which are higher than those of the battery using the Nafion 212 film, and also better than the branched sulfonated polyimide films prepared in Examples 1 to 7.

[0071] The m-cresol solvent in each embodiment can be replaced with one or a mixture of two or more of N,N-dimethylformamide, N,N-dimethylacetamide, dimethyl sulfoxide, and N-methylpyrrolidone; anhydrous ethanol can be replaced with one or a mixture of two or more of methanol, acetone, and isopropanol; and deionized water can be replaced with distilled water or ultrapure water.

[0072] In the above embodiments, except for 1,3,5-tris(4-naphthoxy-1,8-dicarboxylic acid) benzotrihydride (i.e., TNDPAD), which was synthesized in our laboratory, all other raw materials were commercially available products.

[0073] The branched sulfonated polyimide films prepared in Examples 1-7 were subjected to the following performance tests, using the following methods:

[0074] (1) Vanadium ion permeability test:

[0075] A branched sulfonated polyimide membrane was sandwiched between two diffusion cells as a separator, with 1.0 mol L⁻¹ water added to each side of the membrane. -1 VO 2+ +2.0 mol L -1 H2SO4 solution and 1.0 mol L -1 MgSO4+ 2.0 mol L -1 A solution of H2SO4 was prepared; at regular intervals, sample solutions were taken from the right cell of the membrane, and VO was measured using a UV-Vis spectrophotometer. 2+ The absorbance of ions is used to calculate the VO passing through the membrane at that moment using a standard curve. 2+ Ion concentration; after each test, pour the sample solution back into the right-side cell; the formula for calculating the vanadium ion permeability of the diaphragm is as follows:

[0076]

[0077] Where V is the volume of the solution on the left and right sides of the membrane (cm³). 3 );C t The VO in the solution on the right side of the membrane at time t (min) 2+ Ion concentration (mol L) -1 );C L It is the VO in the solution on the left side of the membrane 2+ Initial concentration of ions (mol L) -1 A is the effective area of ​​the diaphragm (cm²). 2 ); t is time (min); P is the VO of the diaphragm. 2+ Ion permeability (cm) 2 min -1 );

[0078] The vanadium ion permeability of the branched sulfonated polyimide membranes in Examples 1-7 is shown in Table 1:

[0079] (2) Proton conductivity test:

[0080] First, the membrane was dried at 40°C for 12 hours, and then its thickness was measured. Subsequently, the dried membrane was subjected to 1.0 mol / L... - 1Soak in H2SO4 solution for 24 hours. Finally, use 1.0 mol L... -1 Using H2SO4 solution as the medium, an H-type electrolytic cell and an electrochemical workstation were used to perform electrochemical impedance spectroscopy tests on the testing device. The proton conductivity (σ) of the diaphragm was calculated using the following formula:

[0081]

[0082] Where σ is the proton conductivity of the membrane (S cm⁻¹). -1 R0 and R1 are the impedances (Ωcm) of the conductivity cell with and without a membrane, respectively. 2 A is the effective area of ​​the diaphragm (cm²) 2 ), where d is the thickness of the diaphragm (cm).

[0083] The proton conductivity of the branched sulfonated polyimide films in Examples 1-7 is shown in Table 1:

[0084] (3) Proton selectivity:

[0085] The proton selectivity (PS) of a membrane is defined as the ratio of proton conductivity to vanadium ion permeability. The proton selectivity can be used to evaluate the overall performance of the membrane. The calculation formula is as follows:

[0086]

[0087] The proton selectivity of the branched sulfonated polyimide films of Examples 1-7 is shown in Table 1:

[0088] Table 1

[0089]

[0090] In the above embodiments, the amount of substance can be converted into mass; the mass unit can be gram or kilogram.

[0091] As can be seen from Table 1, the vanadium ion permeability of the branched sulfonated polyimide membranes prepared in Examples 1-7 is (0.71-3.02×10⁻⁶). -7 cm 2 min -1 All of these values ​​are lower than those of Nafion 212 membrane (7.31 × 10⁻⁶). -7 cm 2 min -1 This demonstrates that the prepared branched sulfonated polyimide membrane possesses excellent vanadium-blocking ability, which can be attributed to the three-dimensional network structure of the branched membrane, effectively resisting the migration of vanadium ions. In summary, the branched membrane can effectively prevent the cross-permeation of vanadium ions.

[0092] To further evaluate the vanadium blocking properties of the branched sulfonated polyimide film and Nafion 212 film prepared in Example 5, the VRFB self-discharge behavior of the branched sulfonated polyimide film and Nafion 212 film prepared in Example 5 was studied, and the results are as follows: Figure 7 As shown, the open-circuit voltage of the VRFB using the branched sulfonated polyimide film prepared in Example 5 and the Nafion 212 film exhibited similar trends, decreasing slowly before reaching 1.2V and then rapidly dropping to 0.8V. The VRFB using the branched sulfonated polyimide film prepared in Example 5 maintained an open-circuit voltage above 0.8V for approximately 60 hours, about five times that of the cell using the Nafion 212 film (12.4 hours). The results indicate that the branched sulfonated polyimide film exhibits stronger vanadium blocking capability compared to the Nafion 212 film.

[0093] Meanwhile, the branched sulfonated polyimide membrane prepared in Example 5 exhibited the highest proton selectivity (1.11 × 10⁻⁶) among all membranes. 5 S min cm -3 ), approximately 0.44 × 10⁻⁶ for Nafion 212 membrane. 5 S min cm -3 The performance of the branched sulfonated polyimide membrane prepared in Example 5 was approximately three times that of the standard membrane, therefore, the membrane was selected for testing battery performance and compared with that of a battery using the commercially available Nafion 212 membrane. VRFBs were assembled using the branched sulfonated polyimide membrane prepared in Example 5 of this invention and the commercially available Nafion 212 membrane to verify the membrane's battery performance. 1.7 mol L of [a specific chemical compound] was placed in both the positive and negative electrode reservoirs. -1 VO 3.5+ +4.7 mol L -1 40.0 mL of H₂SO₄ solution was used to deliver the electrolyte into the battery via a magnetic pump. The battery was then subjected to constant current charge-discharge testing (current density 300–60 mA / cm²) using a Xinwei battery testing system (CT-4008T-5V / 12A-204n-F). -2 The voltage range is 0.8–1.65V. The coulombic efficiency, energy efficiency, and voltage efficiency can be calculated using the following formulas:

[0094] Coulomb efficiency = Discharge capacity / Charge capacity × 100%

[0095] Energy efficiency = Discharge energy / Charge energy × 100%

[0096] Voltage efficiency = Energy efficiency / Coulomb efficiency × 100%

[0097] The results for coulomb efficiency and energy efficiency are as follows: Figure 8As shown. The coulombic efficiency (99.3-94.7%) of the VRFB using the branched sulfonated polyimide membrane prepared in Example 5 is higher than that of the battery using the Nafion 212 membrane (97.7-90.4%). This result indicates that the branched sulfonated polyimide membrane prepared in Example 5 has low vanadium ion permeability. Furthermore, energy efficiency is the most important indicator for measuring the energy conversion and storage capacity of VRFB. The branched sulfonated polyimide membrane prepared in Example 5 achieves efficiency in the range of 300–60 mA / cm². -2 Compared to Nafion 212 membranes (68.9-85.6%), branched sulfonated polyimide membranes (RSM) can achieve higher energy efficiency values ​​(69.5-88.0%) for VRFB, a result consistent with the proton selectivity results of both. The results indicate that branched sulfonated polyimide membranes have excellent prospects for VRFB applications. A comparison of the VRFB efficiencies of the branched sulfonated polyimide membranes prepared in Examples 5 and 8 is provided. Figure 9 As shown, Example 8 further improves the energy efficiency of the branched sulfonated polyimide membrane.

[0098] 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.

[0099] 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 branched sulfonated polyimide film, characterized in that, Includes the following steps: Step 1: Under nitrogen protection, m-cresol, 2,2'-bis(sulfonic acid) benzidine and triethylamine are added to a reactor and stirred at a certain temperature until dissolved. Then, 4,4'-diaminodiphenyl ether is added and stirred until dissolved. Next, 1,3,5-tris(4-naphthoxy-1,8-dicarboxylic acid) benzoic acid is added and stirred for a certain time to obtain a casting solution. The casting solution is poured onto a dry and clean glass plate to cast a film. The glass plate is then dried under a temperature gradient for a certain time to obtain a triethylamine salt-type branched sulfonated polyimide film. Step 2: Immerse the triethylamine salt-type branched sulfonated polyimide membrane in anhydrous ethanol for a certain period of time, then immerse it in a sulfuric acid aqueous solution of a certain concentration, and then wash it with deionized water 5-10 times to obtain the branched sulfonated polyimide membrane. Post-treatment of the branched sulfonated polyimide membrane includes: placing the branched sulfonated polyimide membrane 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; immerse the electron beam-irradiated branched sulfonated polyimide membrane in 10% hydrogen peroxide for 30-60 minutes, and then immerse it in 1.0-3.0 mol L... -1 The sulfuric acid aqueous solution was subjected to secondary acidification for 40-55 minutes, then washed with deionized water, dried in a nitrogen atmosphere at 120-150℃, and then stored in deionized water. In step one, the molar ratio of 2,2'-bis(sulfonic acid) benzidine, 1,3,5-tris(4-naphthoxy-1,8-dicarboxylic acid) benzoic acid, 4,4'-diaminodiphenyl ether, and benzoic acid is 0.3~1.2:1:0.3~1.2:3; The structural formula of the branched sulfonated polyimide membrane is: 。 2. The method for preparing the branched sulfonated polyimide film according to claim 1, characterized in that, In step one, the volume ratio of m-cresol to triethylamine is 30~90:1~3; the ratio of triethylamine to 2,2'-disulfonic acid benzidine is such that for every 1~3 mmol of 2,2'-disulfonic acid benzidine added, the volume of triethylamine added is 1.0~3.0 mL.

3. The method for preparing the branched sulfonated polyimide film as described in claim 1, characterized in that, In step one, m-cresol is replaced by one or a mixture of two or more of N,N-dimethylformamide, N,N-dimethylacetamide, dimethyl sulfoxide, and N-methylpyrrolidone.

4. The method for preparing the branched sulfonated polyimide film as described in claim 1, characterized in that, In step one, the thickness of the film formed by casting the solution on the glass plate is 40~100μm.

5. The method for preparing the branched sulfonated polyimide film according to claim 1, characterized in that, In step two, anhydrous ethanol is replaced with one or a mixture of two or more of methanol, acetone, and isopropanol; deionized water is replaced with distilled water or ultrapure water.

6. The method for preparing the branched sulfonated polyimide film according to claim 1, characterized in that, In step one, m-cresol, 2,2'-disulfonic acid benzidine and triethylamine are added to the reactor and stirred until the dissolution temperature is 60~90℃. 1,3,5-tris(4-naphthoxy-1,8-dicarboxylic acid) benzoic acid is added and stirred for 15~20h. The glass plate is dried for 15h under different temperature gradients, and the range of different temperature gradients is 80~150℃. In step two, the triethylamine salt-type branched sulfonated polyimide membrane is immersed in anhydrous ethanol for 24-36 hours, and the concentration of the sulfuric acid aqueous solution is 1.0-3.0 mol / L. -1 The soaking time in sulfuric acid aqueous solution is 24~48h.

7. The method for preparing the branched sulfonated polyimide film according to claim 1, characterized in that, In step one, the preparation method of 1,3,5-tris(4-naphthoxy-1,8-dicarboxylic acid) phenyltrihydric anhydride includes: placing NaOH in methanol and stirring at room temperature until the solid dissolves; adding phloroglucinol; purging with nitrogen gas at room temperature; stirring for 1 hour; then slowly heating to 80°C and reacting for 4-6 hours until all methanol is evaporated to obtain a powdery solid, wherein the mass-to-volume ratio of NaOH, methanol, and phloroglucinol is 2.400 g: 6.0 mL: 2.522 g; cooling to room temperature; adding 4-bromo-1,8-naphthic anhydride and N,N-dimethylacetamide; attaching a constant-pressure dropping funnel containing toluene to the mouth of the reaction vessel; purging with nitrogen gas at room temperature and stirring for 1 hour; heating to 120°C; then adding toluene dropwise; and heating to 140°C and reacting for 4 hours. The toluene was distilled off and the temperature was raised to 165-170℃ for 20 h. After the reaction was completed, the mixture was cooled to room temperature and precipitated in acetone, with rapid stirring for 1 h. The mass-to-volume ratio of resorcinol, 4-bromo-1,8-naphthyl anhydride, N,N-dimethylacetamide, toluene, and acetone was 2.522 g : 16.624 g : 50.0 mL : 30.0 mL : 200.0 mL. The solid was filtered and collected, washed thoroughly with water and acetone in sequence, and dried in a vacuum oven at 120℃ for 12 h to obtain crude 1,3,5-tris(4-naphthoxy-1,8-dicioic acid) phenyltrihydric anhydride. The crude 1,3,5-tris(4-naphthoxy-1,8-dicioic acid) phenyltrihydric anhydride was recrystallized from the crude product with acetic anhydride to obtain pure 1,3,5-tris(4-naphthoxy-1,8-dicioic acid) phenyltrihydric anhydride.

8. An application of a branched sulfonated polyimide membrane, wherein the branched sulfonated polyimide membrane is prepared by the method for preparing the branched sulfonated polyimide membrane according to any one of claims 1-7, and the branched sulfonated polyimide membrane is used as a separator in a vanadium redox flow battery.

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