Preparation method for casting ultrathin asymmetric bipolar membrane layer by layer

Abstract

The invention discloses a preparation method of a layer-by-layer casting ultrathin asymmetric bipolar membrane, and belongs to the technical field of membrane separation. The invention aims to solve the problems that the total thickness of the traditional bipolar membrane is small, the water decomposition rate is low, and the conversion efficiency of different salts is obviously different. The asymmetric bipolar membrane is prepared by using a layer-by-layer casting method, and the thickness of the anion exchange layer and the cation exchange layer of the membrane is regulated, so that ion leakage is avoided, and the product purity is remarkably improved; the anion-cation exchange layer is prepared from a polymer with charge, and the adhesion capacity of the membrane is greatly improved by virtue of electrostatic attraction through the characteristic of opposite charge of the anion-cation exchange layer, so that the membrane is prevented from swelling in water. According to the invention, the anion and cation exchange membranes are adhered together through electrostatic adsorption force by virtue of the characteristic of opposite charges, the adhesion strength of the membrane is enhanced by virtue of the chemical compatibility of SPPSU and QPPO, the membrane has good mechanical property, and the service time and frequency of the membrane are improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of membrane separation, and particularly relates to a preparation method of a layer-by-layer cast ultra-thin asymmetric bipolar membrane. Background Art

[0002] With the acceleration of the industrialization process, the total amount of industrial wastewater has been continuously rising. Its components often contain low-value by-products, and direct discharge is likely to cause ecological imbalance of water bodies. Driven by the concept of sustainable development, efficient wastewater treatment and resource recycling have become the core demands in the field of environmental protection. Due to its advantages such as low energy consumption and high water recovery rate, electrodialysis technology is regarded as an optimal solution for industrial water treatment. As a key component of the electrodialysis system, a bipolar membrane (BPM) is composed of an anion exchange layer (AEL) and a cation exchange layer (CEL). At its interface, water molecules can be catalytically dissociated into H + and OH - , thereby realizing in-situ preparation of acid / alkali and directional conversion of salts. Research shows that the hydrolysis rate at the BPM interface can reach 7 times that of the free solution, and its performance mainly depends on the interlayer adhesion strength, ion exchange group density, and interfacial ion transport efficiency. Driven by an external electric field, the dissociated H + and OH - can migrate to the corresponding electrodes respectively, forming an efficient ion current.

[0003] However, the total thickness of traditional BPM is usually 100–300 μm (50–150 μm for a single layer), and there are defects such as insufficient hydrolysis rate and significant fluctuations in salt conversion efficiency. Theoretical research shows that reducing the membrane thickness can effectively improve hydrolysis kinetics and enhance acid-base recovery rate. Among them, optimizing the single-layer thickness is a cost-controllable improvement path, which can not only balance the asymmetric co-ion flux but also reduce the membrane resistance, thereby systematically improving the performance of BPM. Therefore, developing a new type of ultra-thin asymmetric bipolar membrane material with simple preparation process, ultra-thin thickness, and long-term stability has become the core breakthrough point to promote the technological growth of the electrodialysis system. Summary of the Invention

[0004] The purpose of the present invention is to solve the problems of the large total thickness of traditional bipolar membranes, low water decomposition rate, and significant differences in the conversion efficiency of different salts, and to provide a preparation method of a layer-by-layer cast ultra-thin asymmetric bipolar membrane.

[0005] The present invention uses a layer-by-layer casting method to prepare an asymmetric bipolar membrane. By regulating the thickness of the anion exchange layer (AEL) and the cation exchange layer (CEL), ion leakage is avoided, and the product purity is significantly improved; charged polymers are used to prepare the anion and cation exchange layers. Due to the opposite charge characteristics of the anion and cation exchange layers, the adhesion ability of the membrane is greatly improved by electrostatic attraction, avoiding swelling in water.

[0006] A preparation method of a layer-by-layer cast ultra-thin asymmetric bipolar membrane, which is specifically completed according to the following steps:

[0007] I. Preparation of the cation exchange layer:

[0008] ①. Sulfonation reaction:

[0009] Dissolve polysulfone in concentrated sulfuric acid, heat and stir for a period of time, then pour the mixture into cold water for precipitation to obtain a precipitated substance; wash the precipitated substance with deionized water until neutral, and dry it under vacuum to obtain sulfonated polysulfone;

[0010] ②. Preparation of the cation exchange layer:

[0011] Dissolve sulfonated polysulfone in N-methylpyrrolidone to obtain a sulfonated polysulfone solution; uniformly coat the sulfonated polysulfone solution on a glass plate, and then dry it to form a cation exchange layer on the glass plate, obtaining a glass plate containing a cation exchange layer;

[0012] II. Preparation of the anion exchange layer:

[0013] ①. Bromination reaction: Dissolve polyphenylene oxide in chlorobenzene to obtain a polyphenylene oxide solution; add a brominating agent and an initiator to the polyphenylene oxide solution, carry out a hydrothermal reaction for a period of time, after the reaction is completed, cool to room temperature and pour it into methanol for precipitation, and vacuum dry the obtained solid substance to obtain brominated polyphenylene oxide;

[0014] ②. Quaternization reaction: Dissolve brominated polyphenylene oxide in N-methylpyrrolidone to obtain a brominated polyphenylene oxide solution; add N-methylmorpholine to the brominated polyphenylene oxide solution, carry out a quaternization reaction for a period of time under stirring conditions, after the reaction is completed, cool to room temperature and pour it into methanol for precipitation, and vacuum dry it to obtain quaternized polyphenylene oxide;

[0015] ③. Preparation of the bipolar membrane: Dissolve quaternized polyphenylene oxide in N-methylpyrrolidone to obtain a quaternized polyphenylene oxide solution; coat the quaternized polyphenylene oxide solution on the glass plate containing the cation exchange layer, and then dry it to form an anion exchange layer on the cation exchange layer, obtaining a glass plate containing a bipolar membrane;

[0016] III. Post-treatment of the bipolar membrane:

[0017] Soak the glass plate containing the bipolar membrane in an HCl solution or an NaOH solution for a period of time, take it out and then rinse it with deionized water until neutral to obtain a layer-by-layer cast ultra-thin asymmetric bipolar membrane.

[0018] The main principle of the present invention:

[0019] The present invention prepares an asymmetric bipolar membrane with cation and anion exchange layers through a layer-by-layer solution casting technique. By reducing the thickness of the anion exchange layer, the migration of OH- can be accelerated, and at the same time, the co-ion leakage of H + / OH - is balanced, minimizing ion leakage to improve the performance in terms of product purity and recovery rate. Due to the opposite charges of the two, the bipolar membrane has good stability and strong adhesion. The bipolar membrane interface forms high-density functional groups through electrostatic interaction, which can catalyze the decomposition of water into H + and OH - , and the rate is increased by 7 times compared to the free solution. The chemical compatibility of sulfonated polysulfone (SPPSU) and quaternized polyphenylene oxide (QPPO) enhances the interlayer adhesion and inhibits the swelling of the membrane in water.

[0020] Advantages of the present invention:

[0021] 1. The present invention simplifies the preparation process through the layer-by-layer casting method, without the need for additional adhesives or high-temperature treatment, avoiding material waste and environmental pollution;

[0022] 2. Due to the opposite charge characteristics of the present invention, the cation and anion exchange membranes are adhered together by electrostatic adsorption force, and the adhesion strength of the membrane is enhanced by relying on the chemical compatibility of SPPSU and QPPO. It has good mechanical properties and improves the service time and number of times of the membrane;

[0023] 3. By reducing the thickness of the anion exchange membrane, the present invention improves the migration rate of OH- migration, reduces ion leakage, and improves product purity. Description of the Drawings

[0024] Figure 1 is the scanning electron microscope image of the layer-by-layer cast ultra-thin asymmetric bipolar membrane prepared in Example 1. The left figure in the figure is the surface image of the membrane, and the right figure is the cross-sectional view of the membrane;

[0025] Figure 2 is the performance graph of converting NaCl, NaNO3, NH4Cl, CH3COONa, and HCOONa into corresponding acids and bases of the layer-by-layer cast ultra-thin asymmetric bipolar membrane prepared in Example 1;

[0026] Figure 3 is the oxidation stability graph of the layer-by-layer cast ultra-thin asymmetric bipolar membrane prepared in Example 1 after cyclic use. Detailed Embodiments

[0027] Detailed Embodiment 1: A preparation method of a layer-by-layer cast ultra-thin asymmetric bipolar membrane in this embodiment is specifically completed according to the following steps:

[0028] I. Preparation of the cation exchange layer:

[0029] ①. Sulfonation reaction:

[0030] Dissolve polyphenylsulfone in concentrated sulfuric acid, heat and stir for a period of time, then pour the mixture into cold water for precipitation to obtain a precipitated substance; wash the precipitated substance with deionized water until neutral, and dry it under vacuum to obtain sulfonated polyphenylsulfone;

[0031] ②. Preparation of cation exchange layer:

[0032] Dissolve sulfonated polyphenylsulfone in N-methylpyrrolidone to obtain a sulfonated polyphenylsulfone solution; uniformly coat the sulfonated polyphenylsulfone solution on a glass plate, and then dry it to form a cation exchange layer on the glass plate, obtaining a glass plate with a cation exchange layer;

[0033] II. Preparation of anion exchange layer:

[0034] ①. Bromination reaction: Dissolve polyphenylene oxide in chlorobenzene to obtain a polyphenylene oxide solution; add a brominating agent and an initiator to the polyphenylene oxide solution, carry out a hydrothermal reaction for a period of time, after the reaction is completed, cool to room temperature and pour it into methanol for precipitation, and vacuum dry the obtained solid substance to obtain brominated polyphenylene oxide;

[0035] ②. Quaternization reaction: Dissolve brominated polyphenylene oxide in N-methylpyrrolidone to obtain a brominated polyphenylene oxide solution; add N-methylmorpholine to the brominated polyphenylene oxide solution, and carry out a quaternization reaction for a period of time under stirring conditions. After the reaction is completed,

[0036] Cool to room temperature and pour it into methanol for precipitation, and vacuum dry it to obtain quaternized polyphenylene oxide;

[0037] ③. Preparation of bipolar membrane: Dissolve quaternized polyphenylene oxide in N-methylpyrrolidone to obtain a quaternized polyphenylene oxide solution; coat the quaternized polyphenylene oxide solution onto the glass plate with a cation exchange layer, and then dry it to form an anion exchange layer on the cation exchange layer, obtaining a glass plate with a bipolar membrane;

[0038] III. Post-treatment of bipolar membrane:

[0039] Immerse the glass plate with a bipolar membrane in HCl solution or NaOH solution for a period of time, take it out and then rinse it with deionized water until neutral to obtain a layer-by-layer cast ultra-thin asymmetric bipolar membrane.

[0040] Specific Embodiment 2: The difference between this embodiment and Specific Embodiment 1 is that the mass ratio of the polyphenylsulfone described in Step 1 to the volume of concentrated sulfuric acid is (4 g - 6 g): 200 mL; the mass fraction of the concentrated sulfuric acid described in Step 1 is 98%. Other steps are the same as those in Specific Embodiment 1.

[0041] Specific Embodiment 3: The difference between this embodiment and one of Specific Embodiments 1 or 2 is as follows: In Step 1, the temperature for heating and stirring is 45°C to 60°C, and the time for heating and stirring is 3.5 h to 8 h; in Step 1, the temperature for vacuum drying is 50°C to 60°C, and the time for vacuum drying is 24 h to 48 h. Other steps are the same as those in Specific Embodiment 1 or 2.

[0042] Specific Embodiment 4: The difference between this embodiment and one of Specific Embodiments 1 to 3 is as follows: In Step 1②, the mass fraction of the sulfonated polysulfone solution is 5% to 10%; in Step 1②, the temperature for drying is 50°C to 60°C; in Step 1②, the thickness of the cation exchange layer is 10 μm to 12 μm. Other steps are the same as those in Specific Embodiments 1 to 3.

[0043] Specific Embodiment 5: The difference between this embodiment and one of Specific Embodiments 1 to 4 is as follows: In Step 2①, the mass fraction of the polyphenylene oxide solution is 3% to 10%; in Step 2①, the brominating agent is N-bromosuccinimide; in Step 2①, the initiator is azobisisobutyronitrile. Other steps are the same as those in Specific Embodiments 1 to 4.

[0044] Specific Embodiment 6: The difference between this embodiment and one of Specific Embodiments 1 to 5 is as follows: In Step 2①, the volume ratio of the mass of the brominating agent to the volume of the polyphenylene oxide solution is (0.1 g to 0.15 g):(90 mL to 100 mL); in Step 2①, the volume ratio of the mass of the initiator to the volume of the polyphenylene oxide solution is (0.1 g to 0.15 g):(90 mL to 100 mL). Other steps are the same as those in Specific Embodiments 1 to 5.

[0045] Specific Embodiment 7: The difference between this embodiment and one of Specific Embodiments 1 to 6 is as follows: In Step 2①, the temperature for the hydrothermal reaction is 135°C to 140°C, and the time for the hydrothermal reaction is 3 h to 4 h; in Step 2①, the temperature for vacuum drying is 70°C to 90°C, and the time is 4 h to 6 h. Other steps are the same as those in Specific Embodiments 1 to 6.

[0046] Specific Embodiment 8: The difference between this embodiment and one of Specific Embodiments 1 to 7 is as follows: In Step 2②, the mass fraction of the brominated polyphenylene oxide solution is 3% to 7%; in Step 2②, the volume ratio of the mass of N-methylmorpholine to the volume of the brominated polyphenylene oxide solution is (5 g to 6 g):(90 mL to 100 mL); in Step 2②, the temperature for the quaternization reaction is 20°C to 40°C, and the time is 12 h to 18 h; in Step 2②, the temperature for vacuum drying is 60°C to 70°C, and the time is 2 h to 4 h. Other steps are the same as those in Specific Embodiments 1 to 7.

[0047] Embodiment 9: The difference between this embodiment and any one of Embodiments 1 to 8 is as follows: the mass fraction of the quaternized polyphenylene ether solution described in step 2③ is 3% - 7%; the drying temperature described in step 2③ is 60°C - 70°C; the thickness of the anion exchange layer described in step 2③ is 5μm - 6μm. Other steps are the same as those in Embodiments 1 to 8.

[0048] Embodiment 10: The difference between this embodiment and any one of Embodiments 1 to 9 is as follows: the concentration of the HCl solution described in step 3 is 0.1mol / L; the concentration of the NaOH solution described in step 3 is 0.1mol / L; the soaking time described in step 3 is 20h - 24h. Other steps are the same as those in Embodiments 1 to 9.

[0049] The following examples are used to verify the beneficial effects of the present invention:

[0050] Example 1: A preparation method of a layer-by-layer cast ultra-thin asymmetric bipolar membrane is specifically completed according to the following steps:

[0051] I. Preparation of the cation exchange layer:

[0052] ①. Sulfonation reaction:

[0053] Dissolve 5g of polyphenylsulfone (PPSU) in 200mL of concentrated sulfuric acid with a mass fraction of 98%, heat and stir at 60°C for 6h, then pour the mixture into cold water for precipitation to obtain a precipitated substance; wash the precipitated substance with deionized water until neutral, and then vacuum dry at 60°C for 24h to obtain sulfonated polyphenylsulfone (SPPSU);

[0054] ②. Preparation of the cation exchange layer:

[0055] Dissolve the sulfonated polyphenylsulfone (SPPSU) obtained in step I① in N-methylpyrrolidone to obtain a sulfonated polyphenylsulfone solution with a mass fraction of 10%; uniformly coat the sulfonated polyphenylsulfone solution on a glass plate, and then dry at 60°C to form a cation exchange layer (CEL) on the glass plate, obtaining a glass plate containing the cation exchange layer;

[0056] The thickness of the cation exchange layer described in step I② is about 10 - 12μm;

[0057] II. Preparation of the anion exchange layer:

[0058] ①. Bromination reaction: Dissolve 5 g of polyphenylene oxide (PPO) in chlorobenzene to obtain a 5% polyphenylene oxide solution by mass; add 0.1 g of brominating agent and 0.1 g of initiator to the 5% polyphenylene oxide solution by mass, and carry out hydrothermal reaction at 140 °C for 4 h. After the reaction is completed, cool to room temperature and pour into methanol for precipitation. Vacuum dry the obtained solid substance at 80 °C for 5 h to obtain brominated polyphenylene oxide (BPPO);

[0059] The brominating agent described in step ②① is N-bromosuccinimide;

[0060] The initiator described in step ②① is azobisisobutyronitrile;

[0061] ②. Quaternization reaction: Dissolve the brominated polyphenylene oxide (BPPO) obtained in step ②① in N-methylpyrrolidone to obtain a 5% brominated polyphenylene oxide solution by mass; add 0.197 g of N-methylmorpholine to the 5% brominated polyphenylene oxide solution by mass, and carry out quaternization reaction under stirring at 30 °C for 16 h. After the reaction is completed, cool to room temperature and pour into methanol for precipitation. Vacuum dry at 60 °C for 3 h to obtain quaternized polyphenylene oxide (QPPO);

[0062] ③. Preparation of bipolar membrane: Dissolve the quaternized polyphenylene oxide (QPPO) in N-methylpyrrolidone to obtain a 5% quaternized polyphenylene oxide solution by mass; coat the 5% quaternized polyphenylene oxide solution by mass onto a glass plate containing a cation exchange layer, and then dry at 60 °C to form an anion exchange layer (AEL) on the cation exchange layer to obtain a glass plate containing a bipolar membrane;

[0063] The thickness of the anion exchange layer (AEL) described in step ②③ is about 5 - 6 μm;

[0064] III. Post-treatment of bipolar membrane:

[0065] Immerse the glass plate containing the bipolar membrane in a 0.1 mol / L HCl solution for 24 h, take it out and then rinse with deionized water until neutral to obtain a layer-by-layer cast ultra-thin asymmetric bipolar membrane (BPM).

[0066] Figure 1 Figure is the scanning electron microscope image of the layer-by-layer cast ultra-thin asymmetric bipolar membrane prepared in Example 1. The left figure in the figure is the surface image of the membrane, and the right figure is the cross-sectional view of the membrane;

[0067] From Figure 1 the left figure in it can be seen that: the materials are uniformly dispersed on the entire membrane surface; from Figure 1 the right figure in it can be seen that: the upper side is the anion exchange layer (AEL) with a thickness of about 5 μm, and the lower side is the cation exchange layer (CEL) with a thickness of about 10 μm.

[0068] In this experiment, the acid-base conversion performance of the layer-by-layer cast ultra-thin asymmetric bipolar membrane prepared in Example 1 for various salts (NaCl, NaNO3, NH4Cl, CH3COONa, HCOONa) was evaluated through a bipolar membrane electrodialysis (BMED) system. A four-chamber structure (salt chamber, acid chamber, alkali chamber, electrode chamber) was built, and a layer-by-layer cast ultra-thin asymmetric bipolar membrane and a commercial homogeneous ion exchange membrane (IP-AEM / IP-CEM) were configured. The electrode used a mixed metal oxide (Pt-Ru-Rh) coated titanium tantalum material. 500 mL of a 0.2 mol / L target salt solution (such as NaCl) was injected into the salt chamber, 500 mL of deionized water was injected into each of the acid chamber and the alkali chamber, and a 0.05 mol / mL Na2SO4 solution was circulated in the electrode chamber to inhibit side reactions. Under the conditions of a constant voltage of 6 V and a temperature of 25 °C, a peristaltic pump was used to continuously circulate at a constant flow rate of 60 mL min -1 from the outlet to the inlet. At equilibrium, a sample from the alkali chamber was taken every unit time and titrated with a 0.1 mol HCl solution, and the OH - concentration was measured. The calculation formula for the hydrolysis dissociation flux (J) in BMED is as follows:

[0069]

[0070] where dC OH- / dt is the change in the concentration of OH - (molm -3 ) with time (s). Here, we considered the influence of the change in the concentration of OH - (NaOH) on the hydrolysis dissociation rate. V is the total volume of the electrolyte solution (0.5×10 -3 m 3 ), and A is the area of the layer-by-layer cast ultra-thin asymmetric bipolar membrane (50 cm 2 ).

[0071] Figure 2 Performance diagrams for converting NaCl, NaNO3, NH4Cl, CH3COONa, and HCOONa of the layer-by-layer cast ultra-thin asymmetric bipolar membrane prepared in Example 1 into the corresponding acids and bases;

[0072] From Figure 2 it can be seen that: the layer-by-layer cast ultra-thin asymmetric bipolar membrane has a rapid hydrolysis dissociation rate for salt solutions of NaCl, NaNO3, and NH4Cl, maintaining above 33 mol m -2 s -1 This is because the ionic radii of these salt solutions are relatively short, and ionization and electrodialysis are prone to occur. However, for CH3COONa and HCOONa, due to the presence of long C chains, the hydrolysis dissociation rate is relatively low (20.4 mol m -2 s -1, HCOONa is 25.2 mol m -2 s -1 ).

[0073] Immerse the ultrathin asymmetric bipolar membrane prepared by layer-by-layer casting in Example 1 in an aqueous solution of FeSO4 and H2O2 at 80 °C (where the mass fraction of H2O2 is 3% and the concentration of FeSO4) for 4 hours, take it out and dry it, and then test the mass loss (%) continuously and repeat the measurement of the mass loss (%) of the material 10 times to test the oxidation stability of the membrane, as shown in Figure 3 shown;

[0074] Figure 3 is the oxidation stability performance diagram of the ultrathin asymmetric bipolar membrane prepared by layer-by-layer casting in Example 1 for recycling;

[0075] From Figure 3 it can be seen that: under the ten-cycle test, the weight loss of the material is only 4.9%, which proves that the material has good cycle stability performance.

[0076] Test of water dissociation flux:

[0077] Replace the salt chamber with 0.5 L of deionized water. The calculation formula for the water dissociation flux is as follows:

[0078]

[0079] Water splittingrate (gh -1 ) is the water dissociation flux, 0.5 L is the volume of deionized water, and 18.02 is the molar mass of deionized water.

[0080] The experimental results show that: the water dissociation flux of the ultrathin asymmetric bipolar membrane prepared by layer-by-layer casting in Example 1 is 1.836 gh -1 .

Claims

1. A preparation method of a layer-by-layer cast ultra-thin asymmetric bipolar membrane, characterized in that The preparation method is specifically completed according to the following steps: I. Preparation of the cation exchange layer: ①. Sulfonation reaction: Dissolve polysulfone in concentrated sulfuric acid, heat and stir for a period of time, then pour the mixture into cold water for precipitation to obtain a precipitated substance; wash the precipitated substance with deionized water until neutral, and dry it under vacuum to obtain sulfonated polysulfone; ②. Preparation of the cation exchange layer: Dissolve sulfonated polysulfone in N-methylpyrrolidone to obtain a sulfonated polysulfone solution; uniformly coat the sulfonated polysulfone solution on a glass plate, and then dry it to form a cation exchange layer on the glass plate, obtaining a glass plate with a cation exchange layer; II. Preparation of the anion exchange layer: ①. Bromination reaction: Dissolve polyphenylene ether in chlorobenzene to obtain a polyphenylene ether solution; add a brominating agent and an initiator to the polyphenylene ether solution, carry out a hydrothermal reaction for a period of time, after the reaction is completed, cool to room temperature and pour it into methanol for precipitation, and vacuum dry the obtained solid substance to obtain brominated polyphenylene ether; ②. Quaternization reaction: Dissolve brominated polyphenylene ether in N-methylpyrrolidone to obtain a brominated polyphenylene ether solution; add N-methylmorpholine to the brominated polyphenylene ether solution, and carry out a quaternization reaction under stirring for a period of time, after the reaction is completed, cool to room temperature and pour it into methanol for precipitation, and vacuum dry to obtain quaternized polyphenylene ether; ③. Preparation of the bipolar membrane: Dissolve quaternized polyphenylene ether in N-methylpyrrolidone to obtain a quaternized polyphenylene ether solution; Coat the quaternized polyphenylene ether solution on the glass plate with a cation exchange layer, and then dry it to form an anion exchange layer on the cation exchange layer, obtaining a glass plate with a bipolar membrane; III. Post-treatment of the bipolar membrane: Immerse the glass plate with the bipolar membrane in an HCl solution or an NaOH solution for a period of time, take it out and then rinse it with deionized water until neutral to obtain a layer-by-layer cast ultra-thin asymmetric bipolar membrane.

2. The preparation method of a layer-by-layer cast ultra-thin asymmetric bipolar membrane according to claim 1, characterized in that In step I, the mass ratio of the polysulfone to the volume of the concentrated sulfuric acid is (4 g - 6 g): 200 mL; the mass fraction of the concentrated sulfuric acid in step I is 98%.

3. The preparation method of a layer-by-layer cast ultra-thin asymmetric bipolar membrane according to claim 1, characterized in that In step I, the temperature of the heating and stirring is 45° - 60°C, and the heating and stirring time is 3.5 h - 8 h; the temperature of the vacuum drying in step I is 50°C - 60°C, and the vacuum drying time is 24 h - 48 h.

4. The preparation method of a layer-by-layer cast ultra-thin asymmetric bipolar membrane according to claim 1, characterized in that In step I②, the mass fraction of the sulfonated polysulfone solution is 5% - 10%; the drying temperature in step I② is 50°C - 60°C; the thickness of the cation exchange layer in step I② is 10 μm - 12 μm.

5. The preparation method of a layer-by-layer cast ultra-thin asymmetric bipolar membrane according to claim 1, characterized in that In step II①, the mass fraction of the polyphenylene ether solution is 3% - 10%; the brominating agent in step II① is N-bromosuccinimide; the initiator in step II① is azobisisobutyronitrile.

6. The preparation method of a layer-by-layer cast ultra-thin asymmetric bipolar membrane according to claim 1, characterized in that In step II①, the mass ratio of the brominating agent to the volume of the polyphenylene ether solution is (0.1 g - 0.15 g): (90 mL - 100 mL); the mass ratio of the initiator to the volume of the polyphenylene ether solution in step II① is (0.1 g - 0.15 g): (90 mL - 100 mL).

7. The preparation method of a layer-by-layer cast ultra-thin asymmetric bipolar membrane according to claim 1, characterized in that The temperature of the hydrothermal reaction described in Step 2① is 135°C to 140°C, and the time of the hydrothermal reaction is 3h to 4h; the temperature of the vacuum drying described in Step 2① is 70°C to 90°C, and the time is 4h to 6h.

8. The preparation method of a layer-by-layer cast ultra-thin asymmetric bipolar membrane according to claim 1, characterized in that The mass fraction of the polyphenylene oxide bromide solution described in Step 2② is 3% to 7%; the mass ratio of N-methylmorpholine to the volume of the polyphenylene oxide bromide solution described in Step 2② is (5g to 6g):(90mL to 100mL); the temperature of the quaternization reaction in Step 2② is 20°C to 40°C, and the time is 12h to 18h; the temperature of the vacuum drying described in Step 2② is 60°C to 70°C, and the time is 2h to 4h.

9. The preparation method of a layer-by-layer cast ultra-thin asymmetric bipolar membrane according to claim 1, characterized in that The mass fraction of the quaternized polyphenylene oxide solution described in Step 2③ is 3% to 7%; the temperature of the drying described in Step 2③ is 60°C to 70°C; the thickness of the anion exchange layer described in Step 2③ is 5μm to 6μm.

10. The preparation method of a layer-by-layer cast ultra-thin asymmetric bipolar membrane according to claim 1, characterized in that The concentration of the HCl solution described in Step 3 is 0.1mol / L; the concentration of the NaOH solution described in Step 3 is 0.1mol / L; the soaking time described in Step 3 is 20h to 24h.

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