Sulfonated fluorine-containing polyimide-metal organic framework composite membrane as well as preparation method and application thereof

Abstract

The invention discloses a sulfonated fluorine-containing polyimide-metal organic framework composite membrane as well as a preparation method and application thereof, and belongs to the technical field of all-vanadium redox flow battery diaphragms. The method comprises the following steps: synthesizing sulfonated fluorine-containing polyimide FSPI by using a high-temperature one-step method; preparing a metal organic framework MIL-101 (Cr) by adopting a hydrothermal method; the preparation method comprises the following steps: preparing a metal organic framework alpha-Fe2O3 / MIL-101 (Cr) by adopting a hydrothermal method; and preparing the sulfonated fluorine-containing polyimide-metal organic framework composite membrane FSPI / alpha-Fe2O3 / MIL-101 (Cr) by adopting a tape casting method. An ion transmission channel is regulated and controlled by introducing a metal organic framework alpha-Fe2O3 / MIL-101 (Cr) into sulfonated fluorine-containing polyimide. And the regulated ion transmission channel is larger than hydrated protons (lt, 0.24 nm) and smaller than hydrated vanadium ions (gt, 0.6 nm), so that the ion selectivity of the sulfonated fluorine-containing polyimide diaphragm is improved. The composite membrane prepared by the invention has high proton conductivity, good vanadium resistance and excellent ion selectivity. When the prepared composite membrane is used in the all-vanadium redox flow battery, the cycling stability of the battery is improved.

Description

Technical Field

[0001] This invention belongs to the field of vanadium redox flow battery separators, specifically, it relates to a sulfonated fluorinated polyimide-metal-organic framework composite membrane for vanadium redox flow batteries, its preparation method and application. Background Technology

[0002] Developing large-scale energy storage systems is crucial for promoting the development of renewable energy technologies utilizing clean energy sources such as solar and wind power. Among various energy storage technologies, vanadium redox flow batteries (VRBs) are expected to become a viable option for large-scale applications due to their environmental friendliness, long cycle stability, and high capacity and power. The proton exchange membrane (PEM), as the core component of the VRB, not only plays a key role in blocking the electrolytes at the positive and negative electrodes but also constructs an efficient proton transport channel, thus playing a decisive role in battery performance. To achieve high battery efficiency, improving the proton conductivity and ion selectivity of the membrane is essential.

[0003] Nafion perfluorosulfonic acid membrane developed by DuPont ® Nafion membranes are widely used due to their excellent chemical stability and superior proton conductivity. However, the ion channels (3-5 nm) in Nafion membranes are significantly larger than the diameters of hydrated protons (<0.24 nm) and hydrated vanadium ions (>0.6 nm), resulting in lower ion selectivity. Furthermore, their high cost further limits their large-scale application. Therefore, developing new membrane materials with superior performance and low cost has become an urgent priority.

[0004] Sulfonated polyimides have attracted much attention due to their excellent chemical stability and outstanding mechanical properties. Achieving high proton conductivity in all-vanadium redox flow batteries typically requires a high degree of sulfonation (DS). However, excessively high DS can exacerbate vanadium ion permeation. Therefore, achieving a balance between improving proton conductivity and inhibiting vanadium ion permeation has become a core issue in membrane development. Ion selectivity (defined as the ratio of proton conductivity to vanadium ion permeability), as a core parameter for quantifying this balance, directly reflects the proton exchange membrane's ability to screen for protons and vanadium ions. Summary of the Invention

[0005] To address the above problems, this invention provides a sulfonated fluorinated polyimide-metal-organic framework composite membrane, its preparation method, and its applications. The purpose of this invention is to regulate ion transport channels by introducing a metal-organic framework into sulfonated fluorinated polyimide. This regulates the ion transport channels to be larger than hydrated protons (<0.24 nm) and smaller than hydrated vanadium ions (>0.6 nm), thereby improving the ion selectivity of the sulfonated fluorinated polyimide membrane.

[0006] To address the aforementioned technical problems, the present invention adopts the following technical solution: The purpose of this invention is to provide a method for preparing a sulfonated fluorinated polyimide-metal-organic framework composite film for vanadium redox flow batteries, characterized by comprising the following steps: Step 1: In an inert gas atmosphere, m-cresol and triethylamine are added to BDSA in sequence. After they are completely dissolved, TFMB and NTDA are added, followed by benzoic acid. The mixture is stirred at 70℃-90℃ and then heated to 170℃-190℃ while continuing to stir. After the reaction is complete, acetone is poured in for precipitation and washing. The mixture is then dried under vacuum to obtain sulfonated fluorinated polyimide. Step 2: Add Cr(NO3)3·9H2O, terephthalic acid and hydrofluoric acid to deionized water, disperse by ultrasonication, and then perform hydrothermal reaction at 210℃-230℃. After the reaction is completed, cool and wash with N,N-dimethylformamide and anhydrous ethanol by centrifugation in sequence, and dry under vacuum to obtain MIL-101(Cr). Step 3: Add MIL-101(Cr) and FeCl3·6H2O to deionized water, disperse by ultrasonication at room temperature, and hydrothermally react at 130℃-150℃. After the reaction is completed, cool and wash successively with N,N-dimethylformamide and anhydrous ethanol by centrifugation, and dry under vacuum to obtain α-Fe2O3 / MIL-101(Cr). Step 4: Add α-Fe2O3 / MIL-101(Cr) and sulfonated fluorinated polyimide to a dimethyl sulfoxide solution and stir the reaction at 80°C. After the reaction is complete, cool to room temperature, form a film, vacuum dry it, and after it is completely dry, immerse it in sulfuric acid solution for protonation treatment. Rinse with deionized water to complete the process. Further specifying, in step 1, the mass ratio of BDSA, TFMB, NTDA, m-cresol, triethylamine and benzoic acid is (150-60):(5-8):(50-70):(540-550):(210-220):(210-220); preferably: 54:6:60:545:216:216.

[0007] Further specifying, in step 1, vacuum drying is performed at 100°C for 12 hours.

[0008] Further specifying, in step 2, the ratio of Cr(NO3)3·9H2O, terephthalic acid, hydrofluoric acid and deionized water is 2 mmol: 2 mmol: 1 ml: 14 ml.

[0009] Further specifying, in step 2, drying is performed in a vacuum oven at 60°C for 8 hours.

[0010] Further specifying, in step 3, the ratio of MIL-101(Cr), FeCl3·6H2O and deionized water is (0.1-0.2)g:(0.2-0.4)g:(20-40)ml; preferably: 0.15g:0.3g:30ml.

[0011] To further specify, in step 3, drying is performed in a vacuum oven at 60°C for 8 hours. Further specifying, in step 4, the mass ratio of α-Fe2O3 / MIL-101(Cr), sulfonated fluorinated polyimide and dimethyl sulfoxide is 1:(100-200):(2020-4060).

[0012] Further specifying, in step 4, drying is performed in a vacuum oven at 80°C for 24 hours.

[0013] Further specifying, in step 4, the concentration of the sulfuric acid solution is 1 mol / L.

[0014] To further specify, in step 1, the inert gas is nitrogen.

[0015] In step 4, the film is formed by solution casting and casting.

[0016] Another objective of this invention is to provide a sulfonated fluorinated polyimide-metal-organic framework composite film prepared by any of the above methods, with a thickness of 5~50 μm, wherein α-Fe2O3 / MIL-101(Cr) is a metal-organic framework with a heterojunction structure.

[0017] In addition, the use of sulfonated fluorinated polyimide-metal-organic framework composite membranes prepared by any of the above methods is also provided, specifically their application in vanadium redox flow batteries.

[0018] This invention employs a high-temperature one-step method, selecting 2,2'-benzidine disulfonic acid (BDSA) as the sulfonated diamine monomer, 2,2'-bis(trifluoromethyl)diaminobiphenyl (TFMB) as the non-sulfonated fluorinated diamine monomer, and 1,4,5,8-naphthalenetetracarboxylic anhydride (NTDA) as the dianhydride monomer to synthesize the sulfonated fluorinated polyimide FSPI. A metal-organic framework (MOF) MIL-101(Cr) is prepared using a hydrothermal method. An MOF α-Fe₂O₃ / MIL-101(Cr) is then prepared using a hydrothermal method. Finally, a sulfonated fluorinated polyimide-MOF composite membrane FSPI / α-Fe₂O₃ / MIL-101(Cr) is prepared using a casting method. The hydroxyl groups on the surface of α-Fe₂O₃ / MIL-101(Cr) and the sulfonic acid groups in FSPI provide abundant sites for proton transport. Simultaneously, the hydroxyl and sulfonic acid groups form acid-base pairs, constructing a more continuous proton transport channel. In addition, the size sieving effect of α-Fe2O3 / MIL-101(Cr) effectively inhibits the cross-contamination of vanadium ions.

[0019] Compared with the prior art, the present invention has the following beneficial effects: This invention modulates the ion transport channel by introducing a metal-organic framework α-Fe₂O₃ / MIL-101(Cr) into sulfonated fluorinated polyimide. The modulated ion transport channel is larger than hydrated protons (<0.24 nm) and smaller than hydrated vanadium ions (>0.6 nm), thereby improving the ion selectivity of the sulfonated fluorinated polyimide membrane.

[0020] The composite membrane prepared by the method of this invention has high proton conductivity, good vanadium blocking performance and superior ion selectivity.

[0021] The composite membrane prepared by the method of this invention is used in vanadium redox flow batteries, which improves battery performance.

[0022] For a deeper understanding of the features and technical content of this invention, please refer to the accompanying detailed description and drawings. It should be noted that the drawings are provided for illustrative purposes only and are not intended to limit the scope of the invention. Attached Figure Description

[0023] Figure 1 The images show the morphology of the metal-organic framework α-Fe2O3 / MIL-101(Cr) of the present invention, where (a) is a SEM image, and (b) and (c) are TEM images. Figure 2 The images are scanning electron microscope (SEM) images of the sulfonated fluorinated polyimide separator and its composite membrane of the present invention, wherein (a) is a cross-section of the sulfonated fluorinated polyimide separator, (b) is a cross-section of the FSPI / α-Fe2O3 / MIL-101(Cr)-0.5 composite membrane, (c) is a cross-section of the FSPI / α-Fe2O3 / MIL-101(Cr)-1 composite membrane, and (d) is a cross-section of the FSPI / α-Fe2O3 / MIL-101(Cr)-1.5 composite membrane; Figure 3 (a) is a nitrogen adsorption isotherm diagram of the metal-organic framework α-Fe2O3 / MIL-101(Cr) of the present invention. Figure 3 (b) is a diagram showing the aperture distribution; Figure 4 The performance of the all-vanadium redox flow battery in Example 3 of this invention (current density 80 mA cm⁻¹) -2 Cyclic stability plot; Figure 5 The performance capacity (current density 80 mA cm⁻¹) of the all-vanadium redox flow battery of Example 3 of the present invention is shown. -2 Retention Rate Chart Detailed Implementation The present invention will be described in detail below with reference to specific embodiments. These embodiments will help those skilled in the art to further understand the present invention, but should not be considered as limiting the present invention. It should be noted that those skilled in the art can make several modifications and improvements without departing from the concept of the present invention. These all fall within the protection scope of the present invention.

[0024] In the examples below, 2,2'-benzidine disulfonic acid (BDSA, 75%), 2,2'-bis(trifluoromethyl)diaminobiphenyl (TFMB, 98%), 1,4,5,8-naphthalenetetracarboxylic anhydride (NTDA, 96%), and benzoic acid ( 99%), m-cresol (>99%), triethylamine (99.5%), chromium(III) nitrate nonahydrate (Cr(NO3)3·9H2O, 99%), terephthalic acid (PTA, 99%), ferric chloride hexahydrate (FeCl3·6H2O, 99%), and N,N-dimethylformamide (DMF, 99.8%) were all purchased from Shanghai Mailin Biotechnology Co., Ltd. Concentrated sulfuric acid (98%) and hydrofluoric acid (HF, AR) were provided by Cirron Scientific Co., Ltd.; dimethyl sulfoxide (DMSO, (99%) was provided by Tianjin Fuyu Fine Chemical Co., Ltd.; anhydrous ethanol was purchased from Tianjin Tianli Chemical Reagent Co., Ltd.

[0025] Example 1 (1) Preparation of sulfonated fluorinated polyimide FSPI Under a nitrogen atmosphere, 3.72 g BDSA, 109 ml m-cresol, and 4.37 g triethylamine were sequentially added to a three-necked flask. After the monomers were completely dissolved, 0.38 g TFMB and 3.22 g NTDA were added, followed by 5.28 g benzoic acid. The mixture was mechanically stirred at 3000 rpm for 2 hours at 80 °C, then heated to 180 °C and stirred for another 16 hours. After the reaction was complete, the viscous liquid was poured into 500 ml acetone for precipitation and washing. Finally, the mixture was dried under vacuum at 100 °C for 12 hours.

[0026] (2) Preparation of metal-organic framework MIL-101(Cr) 1.6 g Cr(NO3)3·9H2O, 0.66 g terephthalic acid, and 0.2 ml hydrofluoric acid were added to 28 ml deionized water and dispersed by ultrasonication at 40 kHz for 30 min. Subsequently, the mixture was transferred to a stainless steel pressure reactor with a polytetrafluoroethylene liner and hydrothermally reacted at 220 °C for 8 h. After the reaction was complete, the mixture was cooled, and the product was washed sequentially by centrifugation with N,N-dimethylformamide and anhydrous ethanol. Finally, it was dried in a vacuum oven at 60 °C for 8 h.

[0027] (3) Preparation of metal-organic framework α-Fe2O3 / MIL-101(Cr) 0.15 g of MIL-101(Cr) and 0.3 g of FeCl3·6H2O were added to 30 mL of deionized water and dispersed using ultrasonication at 40 kHz for 30 min at room temperature. The mixture was then transferred to a polytetrafluoroethylene-lined autoclave and hydrothermally reacted at 140 °C for 24 h. After cooling, the product was washed sequentially by centrifugation with N,N-dimethylformamide and anhydrous ethanol, and finally dried in a vacuum oven at 60 °C for 8 h.

[0028] (4) Preparation of sulfonated fluorinated polyimide-metal-organic framework composite membrane FSPI / α-Fe2O3 / MIL-101(Cr)-0.5 0.01 g of α-Fe₂O₃ / MIL-101(Cr) and 2 g of sulfonated fluorinated polyimide were added to 40.2 g of dimethyl sulfoxide solution. The mixture was stirred at 80 °C for 8 h. After cooling to room temperature, the solution was poured into a petri dish and dried in a vacuum oven at 80 °C for 24 h. The completely dried septum was then immersed in 1 mol / L sulfuric acid solution for 24 h for protonation treatment, and then rinsed with deionized water to remove residual solution.

[0029] Example 2 The difference between this embodiment and embodiment 1 lies in the specific operation of preparing the composite membrane in step (4). The preparation methods of steps (1), (2) and (3) are exactly the same as those in embodiment 1.

[0030] Step (4) Preparation of sulfonated fluorinated polyimide-metal-organic framework composite membrane FSPI / α-Fe2O3 / MIL-101(Cr)-1 0.02 g of α-Fe₂O₃ / MIL-101(Cr) and 2 g of sulfonated fluorinated polyimide were added to 40.4 g of dimethyl sulfoxide solution. The mixture was stirred at 80 °C for 8 h. After cooling to room temperature, the solution was poured into a petri dish and dried in a vacuum oven at 80 °C for 24 h. The completely dried septum was then immersed in 1 mol / L sulfuric acid solution for 24 h for protonation treatment, and then rinsed with deionized water to remove residual solution.

[0031] Example 3 The difference between this embodiment and embodiment 1 lies in the specific operation of preparing the composite membrane in step (4). The preparation methods of steps (1), (2) and (3) are exactly the same as those in embodiment 1.

[0032] Step (4) Preparation of sulfonated fluorinated polyimide-metal-organic framework composite membrane FSPI / α-Fe2O3 / MIL-101(Cr)-1.5 0.03 g of α-Fe₂O₃ / MIL-101(Cr) and 2 g of sulfonated fluorinated polyimide were added to 40.6 g of dimethyl sulfoxide solution. The mixture was stirred at 80 °C for 8 h. After cooling to room temperature, the solution was poured into a petri dish and dried in a vacuum oven at 80 °C for 24 h. The completely dried septum was then immersed in 1 mol / L sulfuric acid solution for 24 h for protonation treatment, and then rinsed with deionized water to remove residual solution.

[0033] Comparative Example 1 (1) Preparation of sulfonated fluorinated polyimide FSPI Under a nitrogen atmosphere, 3.72 g BDSA, 109 ml m-cresol, and 4.37 g triethylamine were sequentially added to a three-necked flask. After the monomers were completely dissolved, 0.38 g TFMB and 3.22 g NTDA were added, followed by 5.28 g benzoic acid. The reaction was mechanically stirred at 80 °C for 2 h, then heated to 180 °C and stirred for another 16 h. After the reaction was complete, the viscous liquid was poured into 500 ml acetone for precipitation and washing. Finally, the mixture was dried under vacuum at 100 °C for 12 h.

[0034] (4) Preparation of sulfonated fluorinated polyimide membrane FSPI 2 g of sulfonated fluorinated polyimide was added to 40 g of dimethyl sulfoxide solution. The mixture was stirred at 80 °C for 8 h. After cooling to room temperature, the solution was poured into a petri dish and dried in a vacuum oven at 80 °C for 24 h. The completely dried septum was then immersed in 1 mol / L sulfuric acid solution for 24 h for protonation treatment, and then rinsed with deionized water to remove residual solution.

[0035] The following tests were performed on α-Fe2O3 / MIL-101(Cr): (1) Morphological characteristics Scanning electron microscopy and transmission electron microscopy were used to test α-Fe₂O₃ / MIL-101(Cr). Figure 1 As shown, α-Fe2O3 / MIL-101(Cr) consists of two parts: spherical α-Fe2O3 and octahedral MIL-101(Cr). α-Fe2O3 binds to MIL-101(Cr) through interfacial interactions. In particular, a clear interface exists between α-Fe2O3 and MIL-101(Cr), and the lattice fringe spacing of α-Fe2O3 is 0.24 nm, corresponding to the (110) crystal plane, indicating an interfacial interaction between the two.

[0036] (2) BET test The adsorption capacity and pore size of the crystal were investigated using N2 adsorption experiments. Figure 3As shown, the isotherm exhibits Type IV characteristics, with a hysteresis loop appearing at P / P0 = 0.1, confirming that α-Fe₂O₃ / MIL-101(Cr) is a porous material. α-Fe₂O₃ / MIL-101(Cr) has a specific surface area of ​​1124.91 m² / g, indicating a high specific surface area that provides more active sites and promotes proton transport. Notably, the pore size of α-Fe₂O₃ / MIL-101(Cr) is 0.54 nm, exceeding the diameter of a proton (<0.24 nm) but smaller than the diameter of a vanadium ion (>0.6 nm), forming a precise "size difference" that enhances ion selectivity.

[0037] The diaphragms of Examples 1-3 and Comparative Example 1 were tested below: (1) Morphological characteristics The diaphragms of Examples 1-3 and Comparative Example 1 were subjected to scanning electron microscopy. Figure 2 As shown, the FSPI membrane without a metal-organic framework has a smooth and uniform surface. In the FSPI / α-Fe₂O₃ / MIL-101(Cr) composite membrane, cracks gradually appear as the α-Fe₂O₃ / MIL-101(Cr) content increases. Specifically, the FSPI / α-Fe₂O₃ / MIL-101(Cr)-1.5 composite membrane exhibits significant MOF particle aggregation, leading to localized cracking and noticeable pores.

[0038] (2) Proton conductivity test The diaphragm was tested using an electrochemical workstation, with a test frequency range of 100 MHz to 2 × 10⁻⁶. 6 Hz. The surface resistance value can be obtained according to formula (1).

[0039]

[0040] In equation (1), S is the effective area of ​​the membrane, and r1 and r2 are the measured resistance values ​​with and without the membrane, respectively. According to equation (2), the proton conductivity (δ) of the membrane can be obtained.

[0041]

[0042] In equation (2), L is the distance between the two stages and R is the surface resistance.

[0043] (3) Vanadium ion permeability test The diaphragm was placed between two containers of equal volume. 20 ml of a mixed solution of 1.5 mol / L VOSO4 and 3 mol / L H2SO4, and a mixed solution of 1.5 mol / L MgSO4 and 3 mol / L H2SO4, were added to the left and right containers, respectively. Then, the concentration of VOSO4 in the right container was measured using a UV-Vis spectrophotometer.2+ The concentration of vanadium ions. Finally, the vanadium ion permeability (P) of the membrane is calculated according to formula (3).

[0044]

[0045] In equation (3), C0 represents the concentration of vanadium ions in the left chamber, C(t) represents the concentration of vanadium ions in the right chamber at a specific moment, V represents the volume of the solution in the right chamber, A represents the effective area of ​​the diaphragm, and L represents the thickness of the membrane. The concentration of vanadium ions in the left chamber changes very little, remaining basically constant at 1.5 mol / L.

[0046] (4) Ion selectivity Ion selectivity (defined as the ratio of proton conductivity to vanadium ion permeability) is the core parameter for quantifying this balance relationship, and directly reflects the proton exchange membrane's ability to screen protons and vanadium ions.

[0047] The test results of proton conductivity, vanadium ion permeability and ion selectivity of the sulfonated fluorinated polyimide membrane and its composite membrane of the present invention are shown in Table 1.

[0048] Table 1:

[0049] As shown in Table 1, the prepared composite membrane exhibits significantly improved proton conductivity, vanadium ion permeability, and ion selectivity compared to the pure sulfonated fluorinated polyimide membrane. Among all composite membranes, the composite membrane of Example 1 shows the highest ion selectivity. This is because the hydroxyl groups on the surface of α-Fe₂O₃ and the sulfonic acid groups in FSPI provide abundant sites for proton transport. Simultaneously, the hydroxyl and sulfonic acid groups form acid-base pairs, constructing a more continuous proton transport channel. Furthermore, the size sieving effect of α-Fe₂O₃ / MIL-101(Cr) effectively suppresses vanadium ion cross-contamination.

[0050] (5) Single cell test The diaphragm is sandwiched between two membranes with a thickness of 4.35 mm and an effective area of ​​9 cm². 2 Between the graphite felt electrodes. Two graphite plates are used as current collectors. The positive and negative electrode electrolytes are 30 ml of 6.5 mol / L V... 3.5+ Solution (dissolved in 4.7 mol / L sulfuric acid). Before testing, nitrogen gas was injected into the negative electrode electrolyte for 10 min to purge air. During the test, the electrolyte flow rate was maintained at 100 ml / min. A Newway system was used at 40–160 mA cm⁻¹. -2 Battery performance was tested at a specific current density. The upper limit of the charging voltage was 1.65V, and the lower limit of the discharging voltage was 0.8V.

[0051] Based on its excellent ion selectivity, Example 1 was selected for single-cell testing. As shown in the figure, at 80 mAcm -2 After undergoing 250 charge-discharge cycles at a given current density, the energy efficiency remained at 95%, demonstrating excellent cycle stability. Furthermore, after 100 charge-discharge cycles, the battery still retained 45.3% of its capacity.

[0052] The specific embodiments of the present invention have been described in detail above. It should be noted that the present invention is not limited to the specific embodiments described above. Various modifications or alterations can be made by those skilled in the art without departing from the scope of protection defined by the claims, and all such modifications or alterations fall within the scope of the present invention.

Claims

1. A method for preparing a sulfonated fluorinated polyimide-metal-organic framework composite membrane, characterized in that, Includes the following steps: Step 1: In an inert gas atmosphere, m-cresol and triethylamine are added to BDSA in sequence. After they are completely dissolved, TFMB and NTDA are added, followed by benzoic acid. The mixture is stirred at 70-90°C and then heated to 170-190°C while continuing to stir. After the reaction is complete, acetone is poured in for precipitation and washing. The mixture is then dried under vacuum to obtain sulfonated fluorinated polyimide. Step 2: Add Cr(NO3)3·9H2O, terephthalic acid and hydrofluoric acid to deionized water, disperse by ultrasonication, and then perform hydrothermal reaction at 210-230℃. After the reaction is completed, cool and wash with N,N-dimethylformamide and anhydrous ethanol by centrifugation in sequence, and dry under vacuum to obtain MIL-101(Cr). Step 3: Add MIL-101(Cr) and FeCl3·6H2O to deionized water, disperse by ultrasonication at room temperature, and hydrothermally react at 130-150℃. After the reaction is completed, cool and wash successively with N,N-dimethylformamide and anhydrous ethanol by centrifugation, and dry under vacuum to obtain α-Fe2O3 / MIL-101(Cr). Step 4: Add α-Fe2O3 / MIL-101(Cr) and sulfonated fluorinated polyimide to a dimethyl sulfoxide solution and stir the reaction at 80°C. After the reaction is complete, cool to room temperature and vacuum dry. After complete drying, immerse in sulfuric acid solution for protonation treatment and rinse with deionized water to complete the process.

2. The method according to claim 1, characterized in that, The mass ratio of BDSA, TFMB, NTDA, m-cresol, triethylamine and benzoic acid is (150-60): (5-8): (50-70): (540-550): (210-220): (210-220).

3. The method according to claim 1, characterized in that, The ratio of Cr(NO3)3·9H2O, terephthalic acid, hydrofluoric acid and deionized water is (1-3) mmol: (1-3) mmol: (1-3) ml: (13-15) ml.

4. The method according to claim 1, characterized in that, The ratio of MIL-101(Cr), FeCl3·6H2O and deionized water is (0.1-0.2) g : (0.2-0.4) g : (20-40) ml.

5. The method according to claim 1, characterized in that, The mass ratio of α-Fe2O3 / MIL-101(Cr), sulfonated fluorinated polyimide and dimethyl sulfoxide is 1:(100-200):(2020-4060).

6. The method according to claim 1, characterized in that, The inert gas is nitrogen.

7. The method according to claim 1, characterized in that, Films were prepared using solution casting and casting methods.

8. The method according to claim 1, characterized in that, The concentration of the sulfuric acid solution is 1-3 mol / L.

9. A sulfonated fluorinated polyimide-metal-organic framework composite film prepared by the method of any one of claims 1-8, having a thickness of 5~50 μm, wherein α-Fe2O3 / MIL-101(Cr) is a metal-organic framework with a heterojunction structure.

10. A sulfonated fluorinated polyimide-metal-organic framework composite membrane prepared by the method of any one of claims 1-8 is used as a separator in a vanadium redox flow battery.

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