Preparation and application of a high-selectivity composite ion-conducting membrane

Abstract

The application discloses a preparation and application of a high-selectivity composite ion conductive membrane, and a preparation method thereof is as follows: dissolving PVDF powder in an organic solvent, adding a pore-forming agent, immersing the obtained membrane in water after casting the membrane, taking out the membrane and drying the membrane to obtain a porous substrate; mixing an HPA solution and a MXene dispersion liquid, adjusting the pH to be acidic, and obtaining a MXene-HPA composite after heating and stirring; dispersing sulfonated polyether ether ketone and the MXene-HPA composite in an organic solvent to obtain a third mixed liquid, coating the third mixed liquid on the surface of the porous substrate, and drying to obtain an intermediate; and immersing the intermediate in a PEI cross-linking liquid to obtain the composite ion conductive membrane. The composite ion conductive membrane prepared on the basis of SPEEK has a low vanadium permeation rate, and meanwhile, the problem of low proton conduction capacity of the SPEEK membrane is overcome. The all-vanadium redox flow battery assembled on the basis of the composite ion conductive membrane has excellent cycle capacity retention.

Description

Technical Field

[0001] This invention relates to the preparation and application of an ion-conducting membrane, and more particularly to the preparation of a highly selective composite ion-conducting membrane and its application in an all-vanadium redox flow battery. Background Technology

[0002] Sulfonated polyether ether ketone (SPEEK) is a commonly used ion exchange membrane material, attracting attention in the fuel cell field due to its excellent chemical and thermal stability. However, when SPEEK membranes are used as ion exchange membranes in vanadium redox flow batteries, vanadium ions easily permeate the SPEEK membrane, migrating from one electrolyte chamber to another, resulting in high vanadium permeability. High vanadium permeability directly leads to numerous problems in the flow battery, including capacity decay, increased self-discharge rate, and significant reductions in coulombic and energy efficiency. Furthermore, although SPEEK membranes possess some proton conductivity, their ionic conductivity is still lower than other dedicated ion exchange membranes (such as Nafion membranes). SPEEK membranes also suffer from insufficient mechanical strength and poor selectivity. These problems severely hinder the large-scale application of SPEEK membranes in vanadium redox flow batteries, making the development of a highly selective composite ion-conducting membrane based on SPEEK suitable for vanadium redox flow batteries a pressing technical challenge. Summary of the Invention

[0003] Objective of this invention: The objective of this invention is to provide a method for preparing a highly selective composite ion-conducting membrane, solving the problem of how to prepare a highly selective composite ion-conducting membrane with low vanadium permeability. Another objective of this invention is to propose the application of the highly selective composite ion-conducting membrane in the preparation of all-vanadium redox flow batteries, solving the problem of how to prepare all-vanadium redox flow batteries.

[0004] Technical solution: The preparation method of a highly selective composite ion-conducting membrane according to the present invention includes the following steps:

[0005] (1) Dissolve PVDF powder in an organic solvent, add a pore-forming agent and mix to obtain a first mixture. Cast the first mixture into a film and immerse it in water for conversion. After conversion, take out the film and dry it to obtain a porous substrate.

[0006] (2) Dissolve phosphotungstic acid in an aqueous ethanol solution to obtain HPA solution, disperse MXene in an aqueous ethanol solution to obtain MXene dispersion, mix HPA solution and MXene dispersion to obtain a second mixture, adjust the pH of the second mixture to acidic, heat and stir the reaction, centrifuge to collect the precipitate, dry the precipitate to obtain MXene-HPA complex.

[0007] (3) Disperse sulfonated polyether ether ketone and MXene-HPA complex in an organic solvent to obtain a third mixture, coat the third mixture onto the surface of a porous substrate, and dry it to obtain an intermediate;

[0008] (4) The intermediate is immersed in PEI crosslinking solution for reaction. After the reaction, the intermediate is taken out and subjected to vacuum heat treatment to obtain composite ion-conducting membrane.

[0009] This invention anchors HPA (heteropolyacid) onto the surface of MXene through electrostatic adsorption and hydrogen bonding. MXene has a two-dimensional layered structure with certain spacing between its layers. When MXene and HPA (heteropolyacid) form a complex, HPA can embed into the interlayer spaces of MXene, further adjusting the interlayer distance. This interlayer structure physically hinders the permeation of vanadium ions, requiring them to diffuse through narrow channels between the layers, increasing the difficulty of permeation. Simultaneously, the layered structure of the MXene-HPA complex provides a large specific surface area, making the adsorption and diffusion process of vanadium ions between the layers more complex, lengthening the permeation path of vanadium ions, and thus reducing the amount of vanadium ions permeating.

[0010] Preferably, in step (1), the organic solvent includes at least one of N-methyl-2-pyrrolidone, dimethylacetamide, dimethyl sulfoxide, sulfolane, and hexafluoroisopropanol, and the pore-forming agent is LiCl.

[0011] Preferably, in step (1), the mass ratio of PVDF powder, organic solvent and pore-forming agent is 10-15:85-90:3-7, and the thickness of the porous substrate is 10-50 mm.

[0012] Preferably, in step (2), the volume ratio of ethanol to water in the ethanol-water solution is 1-2:1-2, the concentration of phosphotungstic acid in the HPA solution is 0.05-0.15M, and the molar ratio of phosphotungstic acid to MXene is 1-4:1-4.

[0013] Preferably, in step (2), the pH value of the second mixture is adjusted to 2-4, the heating and stirring reaction conditions are heating to 50-70℃ and stirring reaction for 6-24 hours, and the drying method is vacuum drying.

[0014] Preferably, in step (3), the organic solvent includes at least one of dimethylacetamide, N-methyl-2-pyrrolidone, and dimethyl sulfoxide.

[0015] Preferably, in step (3), the mass ratio of the sulfonated polyether ether ketone, MXene-HPA composite, and organic solvent is 8-12:1-2:80-120, and the coating amount of the third mixture on the porous substrate surface is 10-50 g / m². 2The drying temperature is 50-70℃.

[0016] Preferably, in step (4), the PEI crosslinking solution is an aqueous solution of polyethyleneimine with a concentration of 2-5 wt% and a pH of 8-10.

[0017] Preferably, in step (4), the reaction time is at least 20 min, and the vacuum heat treatment conditions are 60-100℃ vacuum heat treatment for 1-3 h.

[0018] In another aspect, this invention discloses the application of the composite ion-conducting membrane prepared by the above method in the preparation of an all-vanadium redox flow battery.

[0019] Beneficial effects: Compared with the prior art, the present invention has the following significant advantages:

[0020] The composite ion-conducting membrane based on SPEEK in this invention exhibits low vanadium permeability, as low as 1.3 × 10⁻⁶. -6 cm 2 ·h -1 It also overcomes the problem of low proton conductivity in SPEEK membranes, achieving a proton conductivity of 63 mS·cm. -1 This composite ion-conducting membrane also possesses high strength, making it a promising candidate for application in vanadium redox flow batteries. Vanadium redox flow batteries assembled based on this composite ion-conducting membrane exhibit excellent cycle capacity retention. Attached Figure Description

[0021] Figure 1 This is a TEM image of the composite ion-conducting membrane prepared in Example 1. Detailed Implementation

[0022] The technical solution of the present invention will be further described below with reference to the accompanying drawings.

[0023] Example 1: A method for preparing a highly selective composite ion-conducting membrane is as follows:

[0024] (1) PVDF powder, N-methyl-2-pyrrolidone and LiCl were weighed in a mass ratio of 10:90:5. PVDF powder was dissolved in N-methyl-2-pyrrolidone at 160℃. LiCl was then added and mixed to obtain the first mixture. The first mixture was cast into a film and then immersed in deionized water for conversion at room temperature for 10h. After conversion, the film was taken out and dried to obtain a porous substrate with a thickness of 50mm. The porosity of the porous substrate was about 70% and the average pore size was 147nm.

[0025] (2) An ethanol-water solution was obtained by mixing ethanol and water at a volume ratio of 1:1. Phosphotungstic acid was dissolved in the ethanol-water solution to obtain an HPA solution with a phosphotungstic acid concentration of 0.1M. Titanium carbide MXene (CAS No.:12363-89-2) was dispersed in the ethanol-water solution to obtain an MXene dispersion with a MXene concentration of 0.1M. The HPA solution and the MXene dispersion were mixed at a volume ratio of 2:1 to obtain a second mixture. The pH of the second mixture was adjusted to 3. The mixture was then heated to 60°C and stirred for 12 hours. After centrifugation, the precipitate was collected. The precipitate was then dried under vacuum to obtain the MXene-HPA complex.

[0026] (3) Dissolve sulfonated polyether ether ketone with a sulfonation degree of 70% in dimethylacetamide at a mass ratio of 1:9. Then add MXene-HPA complex at a mass ratio of 10:1 to sulfonated polyether ether ketone. After ultrasonic dispersion for 2 hours, a third mixture is obtained. The coating amount is 30 g / m. 2 The third mixture was coated onto the surface of a porous substrate using a doctor blade coating method, and dried at 60°C to obtain an intermediate.

[0027] (4) The intermediate was immersed in the PEI crosslinking solution and reacted for 30 min. The intermediate was then removed and heat-treated under vacuum at 80 °C for 2 h to obtain the composite ion-conducting membrane. The PEI crosslinking solution was a 3 wt% aqueous solution of polyethyleneimine with a pH of 9.

[0028] Example 2: A method for preparing a highly selective composite ion-conducting membrane is as follows:

[0029] (1) PVDF powder, dimethylacetamide and LiCl were weighed in a mass ratio of 15:85:3. PVDF powder was dissolved in dimethylacetamide at 140℃. LiCl was then added and mixed to obtain the first mixture. The first mixture was cast into a film and then immersed in deionized water for conversion at room temperature for 10h. After conversion, the film was taken out and dried to obtain a porous substrate with a thickness of 50mm. The porosity of the porous substrate was about 68% and the average pore size was 153nm.

[0030] (2) An ethanol-water solution was obtained by mixing ethanol and water at a volume ratio of 2:1. Phosphotungstic acid was dissolved in the ethanol-water solution to obtain an HPA solution with a phosphotungstic acid concentration of 0.05M. Titanium carbide MXene (CAS No.:12363-89-2) was dispersed in the ethanol-water solution to obtain an MXene dispersion with a MXene concentration of 0.1M. The HPA solution and the MXene dispersion were mixed at a volume ratio of 2:1 to obtain a second mixture. The pH of the second mixture was adjusted to 2. The mixture was then heated to 50°C and stirred for 24 hours. After centrifugation, the precipitate was collected. The precipitate was then dried under vacuum to obtain the MXene-HPA complex.

[0031] (3) Dissolve sulfonated polyether ether ketone with a sulfonation degree of 80% in dimethylacetamide at a mass ratio of 1:9. Then add MXene-HPA complex at a mass ratio of 8:1 (sulfonated polyether ether ketone to MXene-HPA complex). After ultrasonic dispersion for 2 hours, a third mixture is obtained. The coating amount is 10 g / m. 2 The third mixture was coated onto the surface of a porous substrate using a doctor blade coating method, and dried at 50°C to obtain an intermediate.

[0032] (4) The intermediate was immersed in the PEI crosslinking solution and reacted for 20 min. The intermediate was then removed and heat-treated under vacuum at 60 °C for 3 h to obtain the composite ion-conducting membrane. The PEI crosslinking solution was a 2 wt% aqueous solution of polyethyleneimine with a pH of 8.

[0033] Example 3: A method for preparing a highly selective composite ion-conducting membrane is as follows:

[0034] (1) PVDF powder, dimethyl sulfoxide and LiCl were weighed in a mass ratio of 12:88:7. PVDF powder was dissolved in dimethyl sulfoxide at 100°C. LiCl was then added and mixed to obtain the first mixture. The first mixture was cast into a film and then immersed in deionized water for conversion at room temperature for 10 hours. After conversion, the film was taken out and dried to obtain a porous substrate with a thickness of 50 mm. The porosity of the porous substrate was about 73% and the average pore size was 166 nm.

[0035] (2) An ethanol-water solution was obtained by mixing ethanol and water at a volume ratio of 1:2. Phosphotungstic acid was dissolved in the ethanol-water solution to obtain an HPA solution with a phosphotungstic acid concentration of 0.15M. Titanium carbide MXene (CAS No.:12363-89-2) was dispersed in the ethanol-water solution to obtain an MXene dispersion with a MXene concentration of 0.075M. The HPA solution and the MXene dispersion were mixed at a volume ratio of 2:1 to obtain a second mixture. The pH of the second mixture was adjusted to 4. The mixture was then heated to 70°C and stirred for 6 hours. After centrifugation, the precipitate was collected. The precipitate was then dried under vacuum to obtain the MXene-HPA complex.

[0036] (3) Dissolve sulfonated polyether ether ketone with a sulfonation degree of 60% in N-methyl-2-pyrrolidone at a mass ratio of 1:9. Then add MXene-HPA complex at a mass ratio of 6:1 to sulfonated polyether ether ketone. After ultrasonic dispersion for 2 hours, a third mixture is obtained. The coating amount is 10 g / m. 2 The third mixture was coated onto the surface of a porous substrate using a doctor blade coating method, and dried at 70°C to obtain an intermediate.

[0037] (4) The intermediate was immersed in the PEI crosslinking solution and reacted for 40 min. The intermediate was then removed and heat-treated under vacuum at 100 °C for 1 h to obtain the composite ion-conducting membrane. The PEI crosslinking solution was a 5 wt% aqueous solution of polyethyleneimine with a pH of 10.

[0038] Example 4: A method for preparing a highly selective composite ion-conducting membrane is as follows:

[0039] (1) PVDF powder, sulfolane and LiCl were weighed in a mass ratio of 14:86:5. PVDF powder was dissolved in sulfolane at 130°C. LiCl was then added and mixed to obtain the first mixture. The first mixture was cast into a film and then immersed in deionized water for conversion at room temperature for 10 hours. After conversion, the film was taken out and dried to obtain a porous substrate with a thickness of 50 mm. The porosity of the porous substrate was about 64% and the average pore size was 123 nm.

[0040] (2) An ethanol-water solution was obtained by mixing ethanol and water at a volume ratio of 1:1. Phosphotungstic acid was dissolved in the ethanol-water solution to obtain an HPA solution with a phosphotungstic acid concentration of 0.05M. Titanium carbide MXene (CAS No.:12363-89-2) was dispersed in the ethanol-water solution to obtain an MXene dispersion with a MXene concentration of 0.4M. The HPA solution and the MXene dispersion were mixed at a volume ratio of 2:1 to obtain a second mixture. The pH of the second mixture was adjusted to 3. The mixture was then heated to 60℃ and stirred for 12 hours. The precipitate was then centrifuged and dried under vacuum to obtain the MXene-HPA complex.

[0041] (3) Dissolve sulfonated polyether ether ketone with a sulfonation degree of 70% in dimethyl sulfoxide at a mass ratio of 1:9. Then add MXene-HPA complex at a mass ratio of 10:1 to sulfonated polyether ether ketone. After ultrasonic dispersion for 2 hours, a third mixture is obtained. The coating amount is 50 g / m. 2 The third mixture was coated onto the surface of a porous substrate using a doctor blade coating method, and dried at 60°C to obtain an intermediate.

[0042] (4) The intermediate was immersed in the PEI crosslinking solution and reacted for 30 min. The intermediate was then removed and heat-treated under vacuum at 90 °C for 2 h to obtain the composite ion-conducting membrane. The PEI crosslinking solution was a 3 wt% aqueous solution of polyethyleneimine with a pH of 9.

[0043] Comparative Example 1: Everything else is the same as in Example 1, except that:

[0044] In step (3), without adding the MXene-HPA complex, the dimethylacetamide solution of sulfonated polyether ether ketone is directly coated onto the surface of the porous substrate.

[0045] Comparative Example 2: Everything else is the same as in Example 1, except that:

[0046] In step (3), the MXene-HPA composite is replaced with titanium carbide MXene (CAS No.:12363-89-2).

[0047] Comparative Example 3: Everything else is the same as in Example 1, except that:

[0048] In step (3), the MXene-HPA complex is replaced with phosphotungstic acid.

[0049] Comparative Example 4: Everything else is the same as in Example 1, except that:

[0050] Replace phosphotungstic acid in step (2) with phosphotomolybdic acid.

[0051] Comparative Example 5: Everything else is the same as in Example 1, except that:

[0052] To prepare the MXene-HPA complex, phosphotungstic acid and MXene were directly added to a dimethylacetamide solution of sulfonated polyether ether ketone.

[0053] Comparative Example 6: Everything else is the same as in Example 1, except that:

[0054] In step (3), the porous substrate is replaced with a PVDF membrane of the same material and thickness.

[0055] The composite ion-conducting membranes prepared in Examples 1-4 and Comparative Examples 1-6 were subjected to the following performance tests:

[0056] Vanadium permeability testing method: Diffusion cell method. A diffusion cell apparatus is used, with a composite ion-conducting membrane placed between two compartments. One compartment contains a solution containing vanadium ions, while the other contains a blank solution or a specific receiving liquid. Under specific temperature and stirring conditions, vanadium ions are allowed to diffuse through the membrane. After a period of time, the concentration of vanadium ions in the receiving liquid is determined using analytical methods such as atomic absorption spectrometry (AAS) or inductively coupled plasma optical emission spectrometry (ICP-OES). The amount of vanadium ions that permeate through a unit area of ​​the membrane per unit time, i.e., the vanadium permeability, is then calculated.

[0057] Proton conductivity testing method: DC polarization method. A stable DC voltage is applied between two electrodes, causing protons to conduct through the membrane and form a current. The steady-state current through the membrane is measured, and the proton conductivity is calculated using Ohm's law, taking into account the membrane area and thickness as well as the applied voltage. This method requires careful attention to avoid the influence of electrode reactions on the measurement results. Appropriate electrode materials and electrolytes are typically selected, and thorough pretreatment is performed.

[0058] The cycle capacity retention test method simulates battery cycle testing: A composite ion-conducting membrane is applied to a vanadium redox flow battery simulation system to assemble a battery. Charge-discharge cycles are performed at a certain current density, and the capacity during each charge-discharge cycle is recorded (calculated by measuring charge-discharge time and current). After 100 cycles, the battery capacity retention rate is calculated. The formula is: Capacity retention rate = (Cn / C1) * 100%, where Cn is the capacity of the nth cycle and C1 is the capacity of the first cycle.

[0059] Tensile strength testing method using a universal testing machine: The composite ion-conducting membrane is prepared into a standard-sized specimen, typically rectangular or dumbbell-shaped. The specimen is mounted on the fixture of the universal testing machine, and a tensile force is applied at a constant rate until the specimen breaks. The testing machine automatically records the force-displacement curve during the tensile process, and the maximum tensile force F at specimen breakage is obtained from the curve. Then, based on the initial cross-sectional area S of the specimen, the tensile strength is calculated using the formula: Tensile Strength = F / S.

[0060] The results are as follows:

[0061] Table 1 Performance test results of different ion-conducting membranes

[0062]

[0063]

[0064] Table 1 shows that neither phosphotungstic acid nor MXene alone, or a direct mixture of the two, can effectively suppress the increase in vanadium permeability, and both exhibit low proton conductivity. Only the MXene-HPA composite, obtained by anchoring phosphotungstic acid to the MXene surface through electrostatic adsorption and hydrogen bonding, can effectively reduce the vanadium permeability of the ion-conducting membrane while simultaneously improving the proton conductivity of the sPEEK membrane. It also demonstrates excellent ion selectivity and effectively enhances the cycle capacity of the all-vanadium redox flow battery. Furthermore, Comparative Example 6 indicates that the vanadium permeability suppression effect of the MXene-HPA composite also depends on the numerous pores in the porous substrate membrane.

Claims

1. A method for preparing a highly selective composite ion-conducting membrane, characterized in that, Includes the following steps: (1) Dissolve PVDF powder in an organic solvent, add a pore-forming agent and mix to obtain a first mixture. Cast the first mixture into a film and immerse it in water for conversion. After conversion, take out the film and dry it to obtain a porous substrate. (2) Dissolve phosphotungstic acid in an aqueous ethanol solution to obtain HPA solution, disperse MXene in an aqueous ethanol solution to obtain MXene dispersion, mix HPA solution and MXene dispersion to obtain a second mixture, adjust the pH of the second mixture to acidic, heat and stir the reaction, centrifuge to collect the precipitate, dry the precipitate to obtain MXene-HPA complex. (3) Disperse sulfonated polyether ether ketone and MXene-HPA complex in an organic solvent to obtain a third mixture, coat the third mixture onto the surface of a porous substrate, and dry it to obtain an intermediate; (4) The intermediate is immersed in PEI crosslinking solution for reaction. After the reaction, the intermediate is taken out and subjected to vacuum heat treatment to obtain composite ion-conducting membrane.

2. The method for preparing the highly selective composite ion-conducting membrane according to claim 1, characterized in that, In step (1), the organic solvent includes at least one of N-methyl-2-pyrrolidone, dimethylacetamide, dimethyl sulfoxide, sulfolane, and hexafluoroisopropanol, and the pore-forming agent is LiCl.

3. The method for preparing the highly selective composite ion-conducting membrane according to claim 1, characterized in that, In step (1), the mass ratio of PVDF powder, organic solvent and pore-forming agent is 10-15:85-90:3-7, and the thickness of the porous substrate is 10-50 mm.

4. The method for preparing the highly selective composite ion-conducting membrane according to claim 1, characterized in that, In step (2), the volume ratio of ethanol to water in the ethanol-water solution is 1-2:1-2, the concentration of phosphotungstic acid in the HPA solution is 0.05-0.15M, and the molar ratio of phosphotungstic acid to MXene is 1-4:1-4.

5. The method for preparing the highly selective composite ion-conducting membrane according to claim 1, characterized in that, In step (2), the pH value of the second mixture is adjusted to 2-4, the heating and stirring reaction conditions are heated to 50-70℃ and stirred for 6-24 hours, and the drying method is vacuum drying.

6. The method for preparing the highly selective composite ion-conducting membrane according to claim 1, characterized in that, In step (3), the organic solvent includes at least one of dimethylacetamide, N-methyl-2-pyrrolidone, and dimethyl sulfoxide.

7. The method for preparing the highly selective composite ion-conducting membrane according to claim 1, characterized in that, In step (3), the mass ratio of the sulfonated polyether ether ketone, MXene-HPA composite, and organic solvent is 8-12:1-2:80-120, and the coating amount of the third mixture on the porous substrate surface is 10-50 g / m². 2 The drying temperature is 50-70℃.

8. The method for preparing the highly selective composite ion-conducting membrane according to claim 1, characterized in that, In step (4), the PEI crosslinking solution is an aqueous solution of polyethyleneimine with a concentration of 2-5 wt% and a pH of 8-10.

9. The method for preparing the highly selective composite ion-conducting membrane according to claim 1, characterized in that, In step (4), the reaction time is at least 20 min, and the vacuum heat treatment conditions are 60-100℃ vacuum heat treatment for 1-3 h.

10. The application of the composite ion-conducting membrane prepared by the preparation method according to any one of claims 1-9 in the preparation of an all-vanadium redox flow battery.

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