MOF / polyamide double-composite membrane as well as preparation method and application thereof

Abstract

The invention discloses preparation of an MOF / polyamide dual-composite membrane and application of the MOF / polyamide dual-composite membrane in an energy conversion and storage device of an alkaline medium, which comprises the following steps: 1) growing an MOF membrane in situ on a porous substrate by utilizing an immiscible solvent interface self-assembly, so that the coverage degree and the uniformity of an MOF material on the porous substrate are greatly improved; and 2) introducing a polyamide coating on the substrate on which the MOF film is grown by utilizing interfacial polymerization to fill the defects of the MOF layer. The compact skin layer of the composite membrane is composed of MOF and polyamide, so that on one hand, the ion selectivity of the membrane material is ensured; on the other hand, the MOF material has a regular and through porous structure and can selectively transmit ions, so that the membrane material keeps relatively high ion conductivity. The method is simple in preparation and easy to amplify, and the process is easy to regulate and control. The prepared MOF / polyamide dual-composite membrane is applied to an energy conversion and storage device based on an alkaline medium and can endow a battery reactor with good performance.

Description

Technical Field

[0001] This invention belongs to the field of energy storage and hydrogen energy technology, specifically relating to a method for preparing a MOF / polyamide dual composite membrane and its application in an energy conversion and storage device based on an alkaline medium. Background Technology

[0002] Metal-organic frameworks (MOFs) are a class of crystalline porous materials with periodic network structures, formed by the self-assembly of inorganic metals (metal ions or metal clusters) and organic ligands. As representatives of novel porous materials, MOFs are diverse and possess advantages such as high porosity, good chemical stability, tunable pore structure, and ease of functionalization, leading to their wide application in gas separation, electrochemical energy storage, adsorption, catalysis, sensing, and biomedicine.

[0003] In recent years, utilizing energy storage technology to store and stably output renewable energy has become a strategic direction for addressing fossil fuel shortages and environmental pollution crises, achieving large-scale utilization of renewable energy, and promoting energy structure transformation. Among numerous electrochemical energy storage technologies, aqueous flow batteries based on alkaline systems offer advantages such as low cost, abundant resources, high energy density, and high safety, making them promising candidates for energy storage. Furthermore, hydrogen energy serves as an excellent bridge, with key advantages including: efficient hydrogen-to-electricity conversion via fuel cells; relatively high hydrogen energy density; and relatively easy storage; and the potential for large-scale application of hydrogen-to-electricity conversion. The ion-conducting membrane, a crucial component in these energy conversion and storage devices, is primarily responsible for transferring charge carriers and forming the internal circuit of the device; simultaneously, it isolates the positive and negative electrodes, preventing cross-penetration of active materials or gases. Its performance and cost determine the performance, cost, and reliability of the energy conversion and storage device. Traditional commercial perfluorosulfonic acid cation exchange membranes (Nafion membranes) have low ionic conductivity in energy conversion and storage devices, which is detrimental to improving battery device performance. While traditional quaternary ammonium anion exchange membranes have high ionic conductivity, the quaternary ammonium groups on the membrane are susceptible to OH groups. - The attack of ions leads to poor alkali resistance and stability of the membrane. Non-fluorinated porous membranes without ion exchange groups have gradually become the development trend of ion conduction membranes for energy conversion and storage devices in recent years. However, how to construct continuous ion transport channels in porous ion conduction membranes and break the "tradeoff" effect of ion selectivity and ion conductivity of porous ion conduction membranes is a major challenge in achieving high-performance and low-cost membrane materials. Summary of the Invention

[0004] To address the aforementioned problems, this invention provides a MOF / polyamide dual composite membrane and its preparation method. The dense skin of this type of composite membrane is composed of MOF material and polyamide. On one hand, by introducing polyamide to fill defects in the MOF layer, the ion selectivity of the membrane material is ensured; on the other hand, the regular and interconnected porous structure of the MOF material itself promotes ion transport, enabling the membrane material to maintain high ion conductivity. Applying it to energy conversion and storage devices can endow them with excellent device performance.

[0005] To achieve the above objectives, the present invention provides the following technical solution:

[0006] On one hand, the present invention provides a MOF / polyamide dual composite membrane for alkaline systems, the composite membrane comprising a porous support layer and a dense separation layer, the dense separation layer comprising a metal-organic framework (MOF) material and an aromatic polyamide (PA).

[0007] On the other hand, the present invention provides a method for preparing the above-mentioned MOF / polyamide dual composite film, the specific preparation steps of which are as follows:

[0008] A porous substrate with low ion selectivity is placed in a solution containing metal salt A required for synthesizing MOF materials and left to stand for 12-24 hours. Then, the porous substrate filled with solution A is transferred to a solution containing organic ligand B required for synthesizing MOF materials to grow MOF membranes. The solvents of solutions A and B are immiscible, and a uniform MOF membrane is formed in situ on the surface of the porous substrate.

[0009] Subsequently, the substrate with the MOF layer was placed in a solution containing precursor C required for the synthesis of polyamide. After a certain period of time, it was transferred to a solution containing precursor D required for the synthesis of polyamide for interfacial polymerization. The solvents of solutions C and D are also immiscible. By introducing polyamide to fill the defects in the MOF layer, a MOF / polyamide dual composite film was prepared.

[0010] The thickness of the dense separation layer containing the MOF and polyamide layers is 3–5 μm.

[0011] The porous substrate with low ion selectivity (pore size of 10-500 nm, porosity of 50%-90%, and thickness of 50-100 μm) can be any one of commercially available Daramic, polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polysulfone (PSF), polyethersulfone-sulfonated polyether ether ketone (PES-SPEEK), polyethersulfone-polyvinylpyrrolidone (PES-PVP), or polyethersulfone-polyethylene glycol (PES-PEG) porous ion-conducting membranes.

[0012] The metal salt A is one or more of copper acetate, copper fluorosilicate, zinc acetate, zinc nitrate, zinc chloride, cobalt nitrate, cobalt chloride, and zirconium chloride; the solvent of solution A is water.

[0013] Preferably, the metal salt A is any one of copper acetate, zinc nitrate, and zirconium chloride.

[0014] The organic ligand B is one or more of 1,3,5-benzenetricarboxylic acid, terephthalic acid, 2-hydroxyterephthalic acid, pyrazine, pyrazine dicarboxylate, imidazole, and 2-methylimidazolium; the solvent of solution B is one or more of n-hexane, n-heptane, n-octanol, isobutanol, and chloroform.

[0015] Preferably, when the metal salt A is copper acetate, the organic ligand B is 1,3,5-benzenetricarboxylic acid; when the metal salt A is zinc nitrate, the organic ligand B is 2-methylimidazole; when the metal salt A is zirconium chloride, the organic ligand B is terephthalic acid.

[0016] The precursor C is one or more of amine organic compounds such as pyridine, piperazine, m-phenylenediamine, and polyethyleneimine; the solvent of the C solution is water.

[0017] The precursor D is one or more of the following acyl chloride organic compounds: 1,3,5-benzenetrisulfonyl chloride (TMC), terephthaloyl chloride (TPC), 1,5-naphthalenedisulfonyl chloride (NDSC), and 1,3,6-naphthalenedisulfonyl chloride (NTSC); the solvent of the D solution is one or more of the following: n-hexane, n-heptane, n-octanol, isobutanol, and chloroform.

[0018] The concentration of the MOF material metal salt A solution is 20-200 mg / mL (preferably 35-100 mg / mL), and the concentration of the MOF material organic ligand B solution is 40-400 mg / mL (preferably 70-200 mg / mL).

[0019] The growth time of the MOF membrane is 6–24 h (preferably 12–18 h);

[0020] The concentration of the polyamide precursor C solution is 1.0–10.0 wt. / v.% (preferably 2.0–5.0 wt. / v.%); the concentration of the polyamide precursor D solution is 0.1–1.0 wt. / v.% (preferably 0.2–0.5 wt. / v.%).

[0021] The time for immersing the substrate with the MOF layer in solution C is 1 to 10 minutes (preferably 3 to 5 minutes);

[0022] The polyamide layer undergoes interfacial polymerization in solution D for 10–120 s (preferably 30–60 s).

[0023] The above preparation method was used to obtain a MOF / polyamide dual composite membrane.

[0024] On the other hand, the present invention provides an application of the aforementioned MOF / polyamide dual composite membrane in an energy conversion and storage device in an alkaline medium.

[0025] Furthermore, the MOF / polyamide dual composite membrane is used as a separator in the device; the energy conversion and storage device based on alkaline medium includes alkaline zinc-iron flow batteries, alkaline all-iron flow batteries, alkaline quinone-based flow batteries, or alkaline organic / iron flow batteries, alkaline zinc-iron / nickel mixed flow batteries, or alkaline medium water electrolysis hydrogen production devices.

[0026] The MOF / polyamide dual composite membrane is used in energy conversion and storage devices in alkaline media, with the dense layer side facing the negative electrode or cathode of the device.

[0027] Compared with the prior art, the present invention has the following advantages:

[0028] 1. This invention utilizes the self-assembly of immiscible solvent interfaces to grow an in-situ MOF film on a porous substrate, which greatly improves the coverage and uniformity of MOF materials on the porous substrate; and solves the problems of poor interfacial compatibility and easy detachment between solid matrix materials and polymers.

[0029] 2. This invention fills the defects in the MOF layer by introducing polyamide, which on the one hand ensures the ion selectivity of the membrane material; on the other hand, the regular and interconnected porous structure of the MOF material itself can provide continuous and rapid ion transport channels, so that the membrane material maintains a high ion conductivity, breaking the "tradeoff" effect of ion selectivity and ion conductivity of traditional porous ion conduction membranes, and endowing the energy conversion and storage device of alkaline medium with good device performance.

[0030] 3. The MOF / polyamide dual composite membrane prepared by this invention has MOF layer and polyamide layer structure that can be effectively controlled according to precursor type and concentration, solvent composition, reaction time, etc., providing a simple, easy-to-operate and scalable membrane fabrication process; it broadens the types and application range of membrane materials for energy conversion and storage devices in alkaline media. Attached Figure Description

[0031] Figure 1 SEM images of the upper and lower surfaces of Comparative Examples 3(a), (c) and Examples 1(b), (d) of the present invention;

[0032] Figure 2 The alkaline zinc-iron flow battery assembled with the membranes prepared in Comparative Example 3 and Example 1 of this invention operates at 80 mA / cm². -2Battery performance tested under current density conditions. Detailed Implementation

[0033] The alkaline zinc-iron flow battery performance test conditions used in the following examples are as follows: both positive and negative electrodes use carbon felt as electrodes, with an effective area of ​​9 cm². 2 The positive electrode electrolyte is 0.4 mol / L. -1 Na₄Fe(CN)₆ + 1.0 mol L -1 The KOH aqueous solution was used as the electrolyte, with a volume of 40 mL; the negative electrode electrolyte was 0.1 mol / L. -1 ZnO + 1.2 mol L -1 The NaOH aqueous solution has a volume of 80 mL; the battery uses a constant current charge / discharge mode at 80 mA cm⁻¹. -2 Under the operating current density, charging takes approximately 25 minutes. Discharging is performed with voltage as the cutoff condition, at 80 mA cm⁻¹. -2 Discharged to 0.1V under the current density conditions.

[0034] The following embodiments are further illustrations of the present invention, but are not intended to limit the scope of the invention.

[0035] Comparative Example 1

[0036] Commercially available porous PTFE membranes with pore size ~0.45μm, thickness 80±5μm, and porosity ~83.0%.

[0037] Comparative Example 2

[0038] Prepare 100 mL of 2 wt. / v.% m-phenylenediamine / aqueous solution and 100 mL of 0.2 wt. / v.% TMC / n-hexane solution. Immerse a commercial porous PTFE membrane (pore size ~0.45 μm, thickness 80±5 μm, porosity ~83.0%) in the m-phenylenediamine / aqueous solution for 3 min. After wiping off the moisture on the membrane surface, transfer it to the TMC / n-hexane solution for interfacial polymerization for 1 min to obtain a PTFE / polyamide (PA) composite membrane.

[0039] Comparative Example 3

[0040] Prepare 100 mL of 50 mg / mL Zn(NO3)2·6H2O / water solution and 100 mL of 100 mg / mL 2-methylimidazole / n-octanol solution. Immerse a commercial porous PTFE membrane (pore size ~0.45 μm, thickness 80±5 μm, porosity ~83.0%) in the Zn(NO3)2·6H2O / water solution for 12 h. After drying the membrane surface, transfer it to the 2-methylimidazole / n-octanol solution and allow it to stand for 18 h to obtain a PTFE / ZIF-8 composite membrane. SEM images clearly show that the ZIF-8 layer is uniformly covered on the surface of the commercial porous PTFE membrane. Figure 1 a) Assemble an alkaline zinc-iron flow battery using the membrane material prepared above, and run it at 80 mA cm⁻¹. -2 Under high current density conditions, charge-discharge tests were conducted on the battery. Due to defects in the ZIF-8 layer, the composite membrane exhibited low ion selectivity, resulting in a coulombic efficiency of only 95.4%. Figure 2 ).

[0041] Comparative Example 4

[0042] 5g of Zn(NO3)2·6H2O and 10g of 2-methylimidazole were dissolved in 100mL of water and mixed and stirred to obtain a homogeneous precursor solution. 60mL of the precursor solution was transferred to a polytetrafluoroethylene (PTFE) reactor. A commercially available porous PTFE membrane (pore size ~0.45μm, thickness 80±5μm, porosity ~83.0%) was then placed vertically in the reactor and reacted at 80℃ for 18h. A PTFE / ZIF-8 composite membrane was obtained via a hydrothermal method. An alkaline zinc-iron flow battery was assembled using the membrane material prepared above, and the results were obtained at 80mA / cm². -2 Under the current density conditions, the battery was charged and discharged. Due to the uneven growth and low coverage of the ZIF-8 layer on the porous PTFE substrate, the composite membrane had low ion selectivity, and the battery coulombic efficiency was only 92.4%.

[0043] Comparative Example 5

[0044] Prepare 100 mL of 50 mg / mL Zn(NO3)2·6H2O / water solution and 100 mL of 100 mg / mL 2-methylimidazole / n-octanol solution, and simultaneously prepare 10 mL of 1 wt% PiP-HCO3. - A (piperidinyl polymer) / ethanol dispersion was used as a binder. Commercially available porous PTFE membranes (pore size ~0.45 μm, thickness 80±5 μm, porosity ~83.0%) were immersed in a Zn(NO3)2·6H2O / water solution for 12 h, dried, and then transferred to a 2-methylimidazole / n-octanol solution for 18 h. The membranes were then repeatedly washed three times with alternating ethanol and deionized water to remove residual precursors and organic solvents. Finally, 10 mL of 1wt% PiP-HCO3 was added. - An ethanol dispersion was sprayed onto the surface of a PTFE / ZIF-8 membrane to fill defects in the ZIF-8 layer, resulting in a PTFE / ZIF-8 / PiP dual composite membrane. An alkaline zinc-iron flow battery was assembled using the membrane material prepared above, and its performance was tested at 80 mA / cm². -2Under the current density conditions, the battery was charged and discharged. By introducing piperidine polymer as a binder to fill the defects of ZIF-8 layer, the ion selectivity of the membrane material can be improved to a certain extent. However, the spraying method may result in uneven coating or failure to completely and accurately fill the defects of ZIF-8 layer, and the coulombic efficiency of the battery is only 95.6%.

[0045] Comparative Example 6

[0046] Prepare 100 mL of 50 mg / mL Zn(NO3)2·6H2O aqueous solution and 100 mL of 100 mg / mL 2-methylimidazole / n-octanol solution; simultaneously, dissolve 4.5 g of polyamide resin (nylon resin) in 85.5 g of DMAc (N,N-dimethylacetamide), stir slowly at room temperature for 24 h, and then let stand to remove bubbles to obtain a casting solution with a solid content of 5 wt%. Commercially available porous PTFE membranes (pore size ~0.45 μm, thickness 80±5 μm, porosity ~83.0%) were immersed in a Zn(NO3)2·6H2O / water solution for 12 h, dried, and then transferred to a 2-methylimidazole / n-octanol solution for 18 h. The membranes were then repeatedly washed three times with alternating ethanol and deionized water to remove residual precursors and organic solvents. A PA / DMAc casting solution (10 μm thick) was then applied to the PTFE / ZIF-8 membrane surface using a doctor blade to fill defects in the ZIF-8 layer. The membranes were dried at 50 °C to obtain a PTFE / ZIF-8 / PA dual composite membrane. An alkaline zinc-iron flow battery was assembled using the membrane material prepared above, and the results were obtained at 80 mA cm⁻¹. -2 Under the current density conditions, the battery was charged and discharged. Due to the introduction of a third layer (dense polyamide layer) on the surface of the ZIF-8 layer, the ZIF-8 pores, which are the intermediate layer, were blocked, and a continuous ion transport path could not be provided, resulting in a low battery voltage efficiency of only 78%.

[0047] Example 1

[0048] Prepare 100 mL of 50 mg / mL Zn(NO3)2·6H2O / aqueous solution, 100 mL of 100 mg / mL 2-methylimidazole / n-octanol solution, 100 mL of 2 wt. / v.% m-phenylenediamine / aqueous solution, and 100 mL of 0.2 wt. / v.% TMC / n-hexane solution;

[0049] Commercial porous PTFE membranes (pore size ~0.45 μm, thickness 80±5 μm, porosity ~83.0%) were immersed in Zn(NO3)2·6H2O / aqueous solution for 12 h. After drying the membrane surface, they were transferred to a 2-methylimidazolium / n-octanol solution and allowed to stand for 18 h. The membrane was then washed three times alternately with ethanol and deionized water to remove residual precursors and organic solvents, yielding a PTFE / ZIF-8 membrane. The PTFE / ZIF-8 membrane was then immersed in m-phenylenediamine / aqueous solution for 3 min, dried, and transferred to a TMC / n-hexane solution for interfacial polymerization for 1 min, resulting in a PTFE / ZIF-8 / PA dual composite membrane. The ZIF-8 / PA dense separation layer on the upper surface of the PTFE membrane had a thickness of 4±0.5 μm. SEM observation showed that the surface of the PTFE / ZIF-8 / PA dual composite membrane was relatively smooth and dense. Figure 1 b). Assemble an alkaline zinc-iron flow battery using the membrane material prepared above, and test it at 80 mA cm⁻¹. -2 Charge-discharge tests were conducted on the battery at a current density of [value missing]. The introduction of polyamide to fill defects in the ZIF-8 layer improved the ion selectivity of the membrane material. Simultaneously, the regular and interconnected porous structure of ZIF-8 itself provides continuous and rapid ion transport channels, enabling the membrane material to maintain high ion conductivity. The battery achieved a coulombic efficiency of 97.4%, a voltage efficiency of 90.4%, and an energy efficiency of 88.0%. Figure 2 ).

[0050] Example 2

[0051] Preparation of polyethersulfone-sulfonated polyetheretherketone membranes (PES-SPEEK membranes) as porous substrates:

[0052] 12.0 g of PES (polyethersulfone) and SPEEK (sulfonated polyether ether ketone) resin (PES:SPEEK = 95 wt%: 5 wt%) was dissolved in 36 g of DMAc (N,N-dimethylacetamide) solvent. The mixture was slowly stirred at room temperature for 24 h, and then allowed to stand to remove bubbles, yielding a casting solution with a solid content of 25 wt%. The PES-SPEEK / DMAc casting solution was coated onto a clean and flat glass plate using a doctor blade (200 μm thick). The plate was then immersed in water for phase inversion (temperature below 20℃, humidity below 20%RH) until the film naturally separated from the glass plate, resulting in a porous PES-SPEEK substrate with a pore size of 50–100 nm, a thickness of 80 ± 5 μm, and a porosity of ~75%.

[0053] Prepare 100 mL of 50 mg / mL Zn(NO3)2·6H2O / water solution, 100 mL of 100 mg / mL 2-methylimidazole / n-octanol solution, 100 mL of 2 wt. / v.% m-phenylenediamine / water solution, and 100 mL of 0.2 wt. / v.% TMC / n-hexane solution.

[0054] The PES-SPEEK porous substrate was immersed in Zn(NO3)2·6H2O / aqueous solution for 12 h, the surface moisture was wiped off, and the substrate was transferred to a 2-methylimidazolium / n-octanol solution for 18 h of static reaction. The substrate was then washed three times alternately with ethanol and deionized water to remove residual precursors and organic solvents, yielding a PES-SPEEK / ZIF-8 membrane. This PES-SPEEK / ZIF-8 membrane was then immersed in m-phenylenediamine / aqueous solution for 3 min, the surface moisture was wiped off, and the membrane was transferred to a TMC / n-hexane solution for interfacial polymerization for 1 min, resulting in a PES-SPEEK / ZIF-8 / PA dual composite membrane. The thickness of the dense ZIF-8 / PA separation layer on the porous substrate was 4 ± 0.5 μm. An alkaline zinc-iron flow battery was assembled using the membrane material prepared above, and the results were obtained at 80 mA cm⁻¹. -2 The battery was charged and discharged under the specified current density conditions, and the battery coulombic efficiency was 97.6%, voltage efficiency was 89.9%, and energy efficiency was 87.7%.

[0055] Examples 3-8

[0056] The membrane preparation process and conditions are the same as in Example 1, with the differences shown in Table 1;

[0057] Comparative Examples 7-9

[0058] The membrane preparation process and conditions are the same as in Example 2, with the differences shown in Table 1;

[0059] Table 1. Process parameters and 80mA cm for each embodiment and comparative example. -2 Performance of alkaline zinc-iron flow batteries at operating current density (process parameters and battery testing conditions are the same as in Example 1 unless otherwise specified)

[0060]

[0061]

[0062] The implementation data from Comparative Examples 1-3, 7-9, and 1-2 show that introducing only one polyamide layer or MOF layer on a porous substrate with low ion selectivity does not significantly improve the performance of zinc-iron flow batteries due to unavoidable defects. The implementation data from Comparative Examples 4-6 and Example 1 show that the present invention utilizes the self-assembly of immiscible solvent interfaces to grow MOF membranes in situ on porous substrates. Compared with the traditional hydrothermal method (Comparative Example 4), this method can greatly improve the coverage and uniformity of MOF materials on porous substrates. Furthermore, compared with spraying (Comparative Example 5) or blade coating (Comparative Example 6), the introduction of polyamide through interfacial polymerization on the substrate where the MOF membrane is grown to fill defects in the MOF layer and prepare a MOF / polyamide dual composite membrane improves the ion selectivity of the membrane material. On the other hand, the regular and interconnected porous structure of the MOF material itself provides continuous ion transport channels, enabling the membrane material to maintain high ion conductivity. This breaks the "trade-off" effect between ion selectivity and ion conductivity in traditional porous ion-conducting membranes, endowing the alkaline flow battery system with excellent battery performance.

[0063] The implementation data from Examples 1-8 above demonstrate that controlling the type of porous substrate, MOF layer growth time, and the types of precursors and solvents can effectively regulate the density of the membrane, enabling the development of high-performance, low-cost porous ion-conducting membranes and further improving the performance of alkaline flow zinc-iron batteries. Different porous substrates exhibit varying degrees of adhesion to MOF materials; selecting a substrate with minimal rigidity difference from the MOF material is more conducive to the growth and adhesion of the MOF layer on the membrane surface, improving the coverage and uniformity of the MOF material on the porous substrate. Simultaneously, extending the reaction time also facilitates the growth of a denser and more uniform MOF layer, enhancing the ion selectivity of the membrane material. The various types of precursors and solvents listed in this invention can effectively prepare MOF / polyamide dual composite membranes, imparting good battery performance to alkaline zinc-iron flow batteries. This indicates that the membrane fabrication process is universal, simple, easy to operate, and scalable, broadening the types and application range of membrane materials for alkaline energy conversion and storage devices.

Claims

1. A method for preparing a MOF / polyamide dual composite membrane, characterized in that: The composite membrane includes a porous support layer and a dense separation layer, wherein the dense separation layer comprises a metal-organic framework (MOF) material and an aromatic polyamide (PA). The method for preparing the composite membrane includes the following steps: A porous substrate with low ion selectivity is placed in a solution containing metal salt A required for synthesizing MOF materials and left to stand for 12-24 hours. Then, the porous substrate filled with solution A is transferred to a solution containing organic ligand B required for synthesizing MOF materials to grow MOF membranes. The solvents of solutions A and B are immiscible, and a uniform MOF membrane is formed in situ on the surface of the porous substrate. Subsequently, the substrate with the MOF layer was placed in a solution containing precursor C required for the synthesis of polyamide. After a certain period of time, it was transferred to a solution containing precursor D required for the synthesis of polyamide for interfacial polymerization. The solvents of solutions C and D are also immiscible. By introducing polyamide to fill the defects in the MOF layer, a MOF / polyamide dual composite film was prepared.

2. The preparation method according to claim 1, characterized in that, The thickness of the dense separation layer is 3–5 μm; The porous substrate with low ion selectivity has a pore size of 10–500 nm, a porosity of 50%–90%, and a thickness of 50–100 μm; it includes any one or more of Daramic, polysulfone, polyvinylidene fluoride, polytetrafluoroethylene, polyethersulfone-sulfonated polyetheretherketone, polyethersulfone-polyethylene glycol, and polyethersulfone-polyvinylpyrrolidone porous ion-conducting membranes.

3. The preparation method according to claim 1, characterized in that, The metal salt A is one or more of copper acetate, copper fluorosilicate, zinc acetate, zinc nitrate, zinc chloride, cobalt nitrate, cobalt chloride, and zirconium chloride; the solvent of solution A is water. Preferably, the metal salt A is any one or more of copper acetate, zinc nitrate, and zirconium chloride.

4. The preparation method according to claim 1, characterized in that, The organic ligand B is one or more of 1,3,5-benzenetricarboxylic acid, terephthalic acid, 2-hydroxyterephthalic acid, pyrazine, pyrazine dicarboxylate, imidazole, and 2-methylimidazolium. The solvent for solution B is one or more of the following: n-hexane, n-heptane, n-octanol, isobutanol, and chloroform. Preferably, when the metal salt A is copper acetate, the organic ligand B is 1,3,5-benzenetricarboxylic acid; when the metal salt A is zinc nitrate, the organic ligand B is 2-methylimidazole; when the metal salt A is zirconium chloride, the organic ligand B is terephthalic acid.

5. The preparation method according to claim 1, characterized in that, The precursor C is one or more of pyridine, piperazine, m-phenylenediamine, and polyethyleneimine; the solvent of the C solution is water.

6. The preparation method according to claim 1, characterized in that, The precursor D is one or more of pyromellitic trisulfonyl chloride, terephthaloyl chloride, 1,5-naphthalenedisulfonyl chloride, and 1,3,6-naphthalenedisulfonyl chloride; The solvent for solution D is one or more of the following: n-hexane, n-heptane, n-octanol, isobutanol, and chloroform.

7. The preparation method according to any one of claims 1 to 6, characterized in that, The concentration of the MOF material metal salt A solution is 20–200 mg / mL (preferably 35–100 mg / mL), and the concentration of the MOF material organic ligand B solution is 40–400 mg / mL (preferably 70–200 mg / mL); the growth time of the MOF membrane is 6–24 h (preferably 12–18 h). The concentration of the polyamide precursor C solution is 1.0–10.0 wt. / v.% (preferably 2.0–5.0 wt. / v.%); the concentration of the polyamide precursor D solution is 0.1–1.0 wt. / v.% (preferably 0.2–0.5 wt. / v.%); the immersion time of the substrate film with the MOF layer in the C solution is 1–10 min (preferably 3–5 min); the interfacial polymerization time of the polyamide layer in the D solution is 10–120 s (preferably 30–60 s).

8. A MOF / polyamide dual composite membrane prepared by the preparation method according to any one of claims 1 to 7.

9. The application of the MOF / polyamide dual composite membrane according to claim 8 in an energy conversion and storage device in an alkaline medium.

10. The application according to claim 9, characterized in that, The MOF / polyamide dual composite membrane is used as a separator in the device; the energy conversion and storage device based on alkaline medium includes alkaline zinc-iron flow batteries, alkaline all-iron flow batteries, alkaline quinone-based flow batteries, or alkaline organic / iron flow batteries, alkaline zinc-iron / nickel mixed flow batteries, or alkaline medium water electrolysis hydrogen production devices.

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