A method for preparing an inorganic-organic composite bipolar membrane material

Abstract

This invention provides a method for preparing an inorganic-organic bipolar membrane material. A suitable amount of polydopamine-coated halloysite nanotubes are added to a sodium alginate membrane solution using a solution mixing method, followed by loading with phosphotungstic acid. The mixture is then cast onto a glass plate, dried, and hot-pressed with an anion exchange membrane to form an organic-inorganic composite bipolar membrane material. Characterization and electrochemical testing show that when the phosphotungstic acid content reaches 10%, it not only improves the mechanical strength and chemical stability of the bipolar membrane but also maximizes the ion mobility to 0.8173, enhances ion exchange capacity, and reduces the transmembrane voltage drop to a minimum of 1.188V. This bipolar membrane preparation method is simple, and the materials are readily available. Therefore, this invention effectively overcomes the various shortcomings of existing technologies and has high practical value.

Description

Technical Field

[0001] This invention belongs to the field of materials and electrochemical energy conversion technology, and specifically relates to a method for preparing an inorganic-organic composite bipolar membrane material. Background Technology

[0002] With the rapid development of science and technology and the increasing awareness of environmental protection, efficient and environmentally friendly electrochemical separation technology has become a research hotspot. Bipolar membranes (BPMs), as a special type of ion exchange membrane, can achieve the dissociation of water molecules during electrolysis, simultaneously generating H+ and OH- ions. This characteristic makes them highly promising for applications in water electrolysis for hydrogen production, acid-base recovery, and other electrochemical processes. However, traditional bipolar membrane materials still have many shortcomings in terms of proton conductivity, mechanical strength, stability, and environmental adaptability, limiting their performance in practical applications. Theoretically, the threshold voltage for water dissociation in a bipolar membrane is 0.83V, but in reality, the actual water separation voltage drop in bipolar membranes is usually higher than this value. Therefore, improving water dissociation efficiency and reducing the required voltage is one of the key research focuses.

[0003] Phosphotungstic acid (PWA) is a typical inorganic acid. Belonging to the class of polyoxometalates (POMs), it is composed of oxides of tungsten and phosphorus, exhibiting strong acidity and unique chemical properties. Structurally, PWA consists of a central tungsten-oxygen octahedron and a phosphorus-oxygen tetrahedron, a structure that endows it with high thermal stability and acidity. Due to its strong acidity, catalytic activity, and good thermodynamic stability, it can improve the proton conductivity of cation exchange membranes, and is widely used in catalysis, electrochemistry, and materials science. Using phosphotungstic acid-modified cation exchange membranes not only reduces dependence on expensive materials, thus lowering membrane costs, but also enhances the chemical stability of cation exchange membranes, making them more durable in harsh chemical environments.

[0004] Halloysite nanotubes (HNTs) are inexpensive and readily available, and are considered the most likely alternative to carbon nanotubes. However, due to the lack of active groups on the surface of HNTs, their van der Waals forces are weak, hindering their dispersion. Therefore, surface polymerization matrices are needed to improve the performance of HNTs. Polydopamine (PDA) is a functional polymer compound formed by the self-polymerization of dopamine (DA). It can modify the carrier surface through electrostatic interactions between cationic polyelectrolytes, thereby improving material properties. Phosphotungstic acid (PWA), with its strong acidity and excellent proton conductivity, is a key component for improving the water dissociation capability of bipolar films. Halloysite nanotubes (DHNTs) encapsulated with polydopamine can form acid-base pairs with PWA, thereby immobilizing PWA and inhibiting its loss with water.

[0005] Therefore, this study prepared a bipolar membrane material loaded with inorganic acids for fuel cells and water electrolysis for hydrogen production, using polydopamine-encapsulated halloysite nanotubes as phosphotungstic acid loading sites. Polydopamine (PDA) is a functional polymer compound formed by the self-polymerization of dopamine (DA). PDA has good adhesion and can stably fix halloysite nanotubes on the membrane surface. It can modify the carrier surface through electrostatic interactions between cationic polyelectrolytes, thereby improving material performance and enhancing the overall mechanical and chemical stability of the structure. Phosphotungstic acid (PWA), with its strong acidity and excellent proton conductivity, is a key component for improving the water dissociation capability of the bipolar membrane. Polydopamine-encapsulated halloysite nanotubes (DHNTs) can form acid-base pairs with PWA, thereby fixing PWA and inhibiting its loss with water. Using sodium alginate (SA) and chitosan (CS), two biodegradable materials, as the substrate of the exchange membrane makes the bipolar membrane more environmentally friendly and meets the development needs of green energy technology. The unique self-humidification mechanism of the bipolar membrane helps simplify the water management of fuel cells. In bipolar membrane fuel cells, water is generated at the interface layer. This self-humidification mechanism can reduce dependence on external humidification systems, thereby reducing system costs and complexity. Summary of the Invention

[0006] In view of the shortcomings of the prior art, the purpose of this invention is to prepare an inorganic-organic bipolar membrane material to solve the problem of high transmembrane voltage drop that is common in the prior art. At the same time, this invention will also provide a method for preparing a bipolar membrane with stable ion transport.

[0007] To achieve the above and other related objectives, the technical solution of the present invention is as follows:

[0008] This invention provides an inorganic-organic composite bipolar membrane, the bipolar membrane comprising a cation exchange membrane and an anion exchange membrane; the cation exchange membrane is prepared by loading phosphotungstic acid onto halloysite nanotubes coated with polydopamine, wherein the halloysite nanotubes coated with polydopamine are formed by the self-polymerization of dopamine in a solution containing halloysite nanotubes.

[0009] Step 1: Add HNTs to an appropriate amount of deionized water and stir using a magnetic stirrer to ensure uniform dispersion of HNTs in the water. Sonicate the resulting HNTs suspension to further enhance dispersibility. Add an appropriate amount of dopamine (DA) to the HNTs suspension and adjust the pH of the suspension to approximately 8 using Tris-HCl buffer to promote the self-polymerization reaction of dopamine. At room temperature, maintain the mixed suspension of HNTs and DA under stirring for 48 hours to allow dopamine to self-polymerize on the HNTs surface to form a polydopamine coating.

[0010] Step 2: Prepare a cation exchange membrane solution using a certain amount of SA. Weigh an appropriate amount of the polydopamine-coated halloysite nanotubes obtained in Step 1 and mix them with PWA. After ultrasonic dispersion, continue stirring at a certain temperature for 24 hours, followed by vacuum degassing. Cast the viscous solution onto a flat plexiglass plate and evaporate the solvent at room temperature. After drying, peel the membrane off the plate and crosslink it with an acetone-water mixture for several hours. Prepare a chitosan membrane solution by mixing chitosan and an aqueous acetic acid solution. Separately, weigh polyvinyl alcohol (PVA) and prepare an aqueous solution. Dissolve it by stirring at 90°C for 2 hours, add it to the above CS solution, mix and stir, and degas under vacuum to obtain a viscous membrane solution. Cast the solution onto a flat plexiglass plate and dry it at 40°C for 12 hours. After drying, immerse the membrane in NaOH solution for 12 hours, and then wash it with deionized water until neutral. Dry the surface moisture of the cation exchange membrane and anion exchange membrane with filter paper, stack them, and heat the two membrane layers at 80°C and apply appropriate pressure for 20 minutes using a hot press. After the membranes cool down, they are ready.

[0011] This invention discloses a composite bipolar membrane material prepared according to the above method and its application method. From the perspective of materials, sodium alginate and chitosan are used as the matrix. Both are natural polymer materials that are widely available and renewable, biodegradable, and environmentally friendly. Both of these materials have good film-forming properties and can form a uniform and stable membrane structure.

[0012] This invention discloses an innovative method of using phosphotungstic acid to modulate the structure of sodium alginate cation exchange membranes, promoting the dissociation of water in the intermediate layer of the bipolar membrane. The adhesive and protective effects of polydopamine can enhance the stability of halloysite nanotubes, while improving the overall chemical stability and mechanical strength of the cation exchange membrane. The introduction of phosphotungstic acid can improve the conductivity of the membrane, thereby increasing proton conductivity and reducing transmembrane voltage drop. This is very important for applications such as fuel cells that require efficient electron transport. Attached Figure Description

[0013] Figure 1 The image shown is a scanning electron microscope image of the sodium alginate cation exchange membrane disclosed in Embodiment 2 of the present invention.

[0014] Figure 2 The image shown is a scanning electron microscope image of the sodium alginate cation exchange membrane disclosed in Comparative Example 1 of this invention.

[0015] Figure 3 The image shown is a scanning electron microscope image of the sodium alginate cation exchange membrane disclosed in Comparative Example 2 of this invention. Detailed Implementation

[0016] The preparation method of the inorganic-organic composite bipolar membrane of the present invention will be described in detail below with reference to the embodiments and accompanying drawings. However, the present invention is not limited to the following embodiments.

[0017] Example 1: A certain amount of SA was prepared into an aqueous solution with a concentration of 4wt%, 0.2g of DHNTs powder was added and stirred evenly, then 5% of PWA (relative to the mass of SA) was added, ultrasonicated for 10min, and stirred continuously at 60℃ for 24h, followed by vacuum degassing.

[0018] The viscous solution was cast onto a flat plexiglass plate, and the solvent was evaporated at room temperature (303 K). After drying, the membrane was peeled off the plate and crosslinked in a solution containing a 7:3 acetone-water mixture. 2 mL of glutaraldehyde was added, and the crosslinking reaction was catalyzed with 2 mL of HCl. After crosslinking, the membrane was repeatedly washed several times in deionized water to remove trace amounts of unreacted glutaraldehyde and residual HCl. 3.0 g of CS was weighed and dissolved in a 2.0 wt% aqueous acetic acid solution to prepare a 3.0 wt% chitosan-acetic acid solution.

[0019] Separately, 4.0 g of PVA was weighed and prepared into a 4.0 wt% aqueous solution, which was stirred and dissolved at 90 °C for 2 h, and then added to the above CS solution. The mixture was stirred for 6 h, then degassed under vacuum to obtain a viscous film solution, which was cast onto a flat plexiglass plate and dried at 40 °C for 12 h. After drying, the film was immersed in a 4 wt% NaOH solution for 12 h, and then washed with deionized water until neutral.

[0020] The cation exchange membrane and anion exchange membrane were dried with filter paper, stacked, and then subjected to a hot press at 80°C with moderate pressure for 20 minutes. After cooling, the membranes were ready. Testing showed that at a current density of 70.77 mA / cm², the transmembrane voltage drop of the bipolar membrane was 2.988 V, and the tensile strength reached 29.5 MPa.

[0021] Example 2: A certain amount of SA was prepared into an aqueous solution with a concentration of 4wt%, 0.2g of DHNTs powder was added and stirred evenly, then 10% of PWA (relative to the mass of SA) was added, ultrasonicated for 10min, and stirred continuously at 60℃ for 24h, followed by vacuum degassing.

[0022] The viscous solution was cast onto a flat plexiglass plate, and the solvent was evaporated at room temperature (303 K). After drying, the membrane was peeled off the plate and crosslinked in a solution containing a 7:3 acetone-water mixture. 2 mL of glutaraldehyde was added, and the crosslinking reaction was catalyzed with 2 mL of HCl. After crosslinking, the membrane was repeatedly washed several times in deionized water to remove trace amounts of unreacted glutaraldehyde and residual HCl. 3.0 g of CS was weighed and dissolved in a 2.0 wt% aqueous acetic acid solution to prepare a 3.0 wt% chitosan-acetic acid solution.

[0023] Separately, 4.0 g of PVA was weighed and prepared into a 4.0 wt% aqueous solution, which was stirred and dissolved at 90 °C for 2 h, and then added to the above CS solution. After stirring for 6 h, the solution was degassed under vacuum to obtain a viscous film solution, which was then cast onto a flat plexiglass plate and dried at 40 °C for 12 h.

[0024] After drying, the membrane was immersed in a 4 wt% NaOH solution for 12 hours, and then washed with deionized water until neutral.

[0025] The cation exchange membrane and anion exchange membrane were dried with filter paper, stacked, and then pressed together at 80°C using a hot press.

[0026] The membrane was heated and moderately pressurized for 20 minutes, and then allowed to cool. Testing showed that at a current density of 70.77 mA / cm², the transmembrane voltage drop of the bipolar membrane was 1.188 V, and the tensile strength reached 37.4 MPa.

[0027] Example 3: A certain amount of SA was prepared into an aqueous solution with a concentration of 4wt%, 0.2g of DHNTs powder was added and stirred evenly, then 15% of PWA (relative to the mass of SA) was added, ultrasonicated for 10min, and stirred continuously at 60℃ for 24h, followed by vacuum degassing.

[0028] The viscous solution was cast onto a flat plexiglass plate, and the solvent was evaporated at room temperature (303 K). After drying, the membrane was peeled off the plate and crosslinked in a solution containing a 7:3 acetone-water mixture. 2 mL of glutaraldehyde was added, and the crosslinking reaction was catalyzed with 2 mL of HCl. After crosslinking, the membrane was repeatedly washed several times in deionized water to remove trace amounts of unreacted glutaraldehyde and residual HCl. 3.0 g of CS was weighed and dissolved in a 2.0 wt% aqueous acetic acid solution to prepare a 3.0 wt% chitosan-acetic acid solution. Separately, 4.0 g of PVA was weighed and dissolved in a 4.0 wt% aqueous solution at 90 °C for 2 h, and then added to the CS solution. After stirring for 6 h and vacuum degassing, a viscous membrane solution was obtained, which was cast onto a flat plexiglass plate and dried at 40 °C for 12 h. After drying, the membrane was immersed in a 4 wt% NaOH solution for 12 h, and then washed with deionized water until neutral.

[0029] The cation exchange membrane and anion exchange membrane were dried with filter paper, stacked, and then subjected to a hot press at 80°C with moderate pressure for 20 minutes. After cooling, the membranes were ready. Testing showed that at a current density of 70.77 mA / cm², the transmembrane voltage drop of the bipolar membrane was 2.065 V, and the tensile strength reached 34.6 MPa.

[0030] Comparative Example 1: A certain amount of SA was prepared into a 4 wt% aqueous solution. 0.2 g of DHNTs powder was added and stirred until homogeneous. Phosphotungstic acid was not added. After sonication for 10 min, the solution was continuously stirred at 60 °C for 24 h, followed by vacuum degassing. This viscous solution was cast onto a flat plexiglass plate, and the solvent was evaporated at room temperature (303 K). After drying, the membrane was peeled off the plate and crosslinked in a solution containing a 7:3 acetone-water mixture. 2 mL of glutaraldehyde was added, and the crosslinking reaction was catalyzed with 2 mL of HCl. After crosslinking, the membrane was repeatedly washed several times in deionized water to remove trace amounts of unreacted glutaraldehyde and residual HCl. 3.0 g of CS was weighed and dissolved in a 2.0 wt% aqueous acetic acid solution to prepare a 3.0 wt% chitosan-acetic acid solution. Separately, 4.0 g of PVA was weighed and prepared into a 4.0 wt% aqueous solution, stirred at 90 °C for 2 h, and added to the above CS solution. Stir for 6 hours, then degas under vacuum to obtain a viscous film solution. Cast the solution onto a flat plexiglass plate and dry at 40°C for 12 hours. After drying, immerse the film in a 4 wt% NaOH solution for 12 hours, then wash with deionized water until neutral.

[0031] The cation exchange membrane and anion exchange membrane were dried with filter paper, stacked, and then subjected to a hot press at 80°C with moderate pressure for 20 minutes. After cooling, the membranes were ready. Testing showed that at a current density of 70.77 mA / cm², the transmembrane voltage drop of the bipolar membrane was 4.975 V, and the tensile strength reached 26.4 MPa.

[0032] Comparative Example 2: A certain amount of SA was prepared into a 4 wt% aqueous solution and stirred continuously at 60°C for 24 h, followed by vacuum degassing. This viscous solution was cast onto a flat plexiglass plate, and the solvent was evaporated at room temperature (303 K). After drying, the membrane was peeled off the plate and crosslinked in a solution containing a 7:3 acetone-water mixture. 2 mL of glutaraldehyde was added, and the crosslinking reaction was catalyzed with 2 mL of HCl. After crosslinking, the membrane was repeatedly washed several times in deionized water to remove trace amounts of unreacted glutaraldehyde and residual HCl. 3.0 g of CS was weighed and dissolved in a 2.0 wt% aqueous acetic acid solution to prepare a 3.0 wt% chitosan-acetic acid solution. Separately, 4.0 g of PVA was weighed and prepared into a 4.0 wt% aqueous solution, stirred and dissolved at 90°C for 2 h, and added to the above CS solution. After stirring for 6 h and vacuum degassing, a viscous membrane solution was obtained, which was cast onto a flat plexiglass plate and dried at 40°C for 12 h. After drying, the membrane was immersed in a 4 wt% NaOH solution for 12 hours, and then washed with deionized water until neutral. The cation exchange membrane and anion exchange membrane were blotted dry with filter paper, stacked, and subjected to a hot press at 80°C with moderate pressure for 20 minutes. After cooling, the membrane was ready. Testing showed that at a current density of 70.77 mA / cm², the transmembrane voltage drop of the bipolar membrane was 5.4 V, and the tensile strength reached 15.2 MPa.

[0033] The above embodiments are merely illustrative of the principles and effects of the present invention and are not intended to limit the invention. Any person skilled in the art can modify or alter the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or alterations made by those skilled in the art without departing from the spirit and technical concept disclosed in the present invention should still be covered by the claims of the present invention.

Claims

1. A method for preparing an inorganic-organic composite bipolar membrane, characterized in that, Follow these steps: Step 1: Add HNTs to an appropriate amount of deionized water and stir using a magnetic stirrer to ensure uniform dispersion of HNTs in the water. Sonicate the resulting HNTs suspension to further enhance dispersibility. Add an appropriate amount of dopamine (DA) to the HNTs suspension and adjust the pH of the suspension to approximately 8 using Tris-HCl buffer to promote the self-polymerization reaction of dopamine. At room temperature, maintain the mixed suspension of HNTs and DA under stirring for 48 hours to allow dopamine to self-polymerize on the HNT surface to form a polydopamine (PDA) coating. Step 2: Prepare a cation exchange membrane solution by taking a certain amount of SA. Weigh an appropriate amount of the polydopamine-coated halloysite nanotubes obtained in Step 1 and stir them evenly with PWA. After ultrasonic dispersion, stir continuously at a certain temperature for 24 hours and then degas under vacuum. Cast the viscous solution onto a flat plexiglass plate and evaporate the solvent at room temperature. After drying, peel the membrane off the plate and crosslink it with an acetone-water mixture for several hours. Prepare a chitosan membrane solution by stirring and mixing chitosan and acetic acid aqueous solution. Separately, weigh PVA and prepare an aqueous solution, stir and dissolve it at 90°C for 2 hours, add it to the above CS solution, mix and stir, degas under vacuum to obtain a viscous membrane solution, cast it onto a flat plexiglass plate, and dry it at 40°C for 12 hours. After drying, immerse the membrane in NaOH solution for 12 hours, and then wash it with deionized water until neutral. Blot the surface moisture of the cation exchange membrane and anion exchange membrane with filter paper, stack them, and heat the two membrane layers at 80°C and apply moderate pressure for 20 minutes using a hot press. After the membrane cools down, it is ready.

2. The method for preparing an inorganic-organic composite bipolar membrane according to claim 1, characterized in that, In step 1, the concentration of the HNTs suspension is 10 g / L.

3. The method for preparing an inorganic-organic composite bipolar membrane according to claim 1, characterized in that, In step 1, the pH of the suspension was precisely adjusted to 8.5 using Tris-HCl buffer.

4. The method for preparing an inorganic-organic composite bipolar membrane according to claim 2, characterized in that, In step 2, the concentration of the SA solution is 4 wt%.

5. The method for preparing an inorganic-organic composite bipolar membrane according to claim 2, characterized in that, Using phosphotungstic acid to improve the membrane structure results in a more compact sodium alginate anion exchange membrane.

6. The method for preparing an inorganic-organic composite bipolar membrane according to claim 2, characterized in that, The amount of phosphotungstic acid added during stirring is 5%-15% (relative to the mass of SA).

7. The method for preparing an inorganic-organic composite bipolar membrane according to claim 2, characterized in that, When using acetone-water crosslinking, 2 mL of glutaraldehyde and 2 mL of HCl need to be added to catalyze the crosslinking.

8. The method for preparing an inorganic-organic composite bipolar membrane according to claim 2, characterized in that, In step 2, after drying, the membrane is immersed in a 4 wt% NaOH solution for 12 hours, and then washed with deionized water until neutral.

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