Composite membrane for flow battery as well as preparation method and application of composite membrane

Abstract

The invention discloses a composite membrane for a flow battery as well as a preparation method and application of the composite membrane. Comprising a porous selection layer and a porous support layer, the porous selection layer is made of a covalent organic framework material; the porous supporting layer is made of organic polymer resin; and the thickness of the composite film is 34-75 microns. The porous composite membrane prepared from the double-layer porous membrane can more accurately screen protons and ions, so that the battery efficiency of a flow battery (represented by an all-vanadium flow battery) is improved. In addition, the membrane does not contain ion exchange groups, has excellent chemical stability, and provides a new direction for the design of the ultra-high ion selectivity membrane of the flow battery. The preparation method is simple and easy to implement, and mass production is easy to realize. And the COF selection layer is not easy to fall off, good in flexibility and excellent in mechanical property. When the material is applied to a flow battery, the material has excellent ion selectivity in respective systems, so that the material has relatively high coulombic efficiency.

Description

Technical Field

[0001] This application relates to a composite membrane for flow batteries, its preparation method and application, and belongs to the field of flow battery materials. Background Technology

[0002] With economic development, the demand for energy is increasing, and the environmental problems caused by the massive consumption of fossil fuels are becoming increasingly prominent. Large-scale utilization of renewable energy and achieving energy diversification have become important strategies for energy security and sustainable development for countries worldwide. However, the discontinuity and instability of renewable energy sources such as wind and solar power make it difficult to directly utilize the electricity they generate. Therefore, utilizing energy storage technology to achieve a continuous supply of renewable energy is key to solving these problems. Vanadium redox flow batteries (VFBs), due to their flexible design (separate capacity and power design), good safety, and long design life, have become one of the most promising technologies in the large-scale energy storage market.

[0003] Ion-conducting membranes are a crucial component of flow batteries. On one hand, they conduct charge-balancing ions to form the internal electrical circuit of the battery; on the other hand, they isolate the positive and negative electrolytes, preventing ion cross-permeation that could lead to capacity decay. Their physicochemical properties directly affect the performance of flow batteries. Currently, the most widely used ion-conducting membrane in flow batteries is the commercially available perfluorosulfonic acid ion exchange membrane (Nafion). Nafion membranes possess excellent chemical stability and proton conductivity; however, their high ion permeability and high cost limit their further development.

[0004] Subsequently, non-fluorinated ion exchange membranes have been extensively studied, and commonly used non-fluorinated ion exchange membrane materials are based on aromatic polymers with high mechanical stability and low cost. Among all polymers, sulfonated polyether ether ketone (SPEEK) has attracted widespread attention due to its simple preparation and low cost. However, the ion exchange groups on the SPEEK backbone affect its chemical stability requirements for long-term cycling life under strong acid and strong oxidizing conditions.

[0005] Covalent organic frameworks (COFs) are a new class of organic porous materials with uniformly arranged pores. COFs are linked by stable covalent bonds, exhibiting high surface area, good thermal stability, and chemical stability. COFs offer flexibility in building block selection and precise control and tunability of nanopores, allowing for the screening of hydrogen and vanadium ions using COFs with appropriate pore sizes, thereby achieving high ion selectivity in ion-conducting membranes.

[0006] Traditional methods for preparing composite membranes, such as spraying, spin coating, pressure-assisted or vacuum-assisted filtration, suffer from problems such as poor mechanical properties (coatings are prone to peeling off) and interfacial resistance between different materials. Therefore, it is necessary to develop new methods to prepare composite membranes with good flexibility and mechanical properties. Summary of the Invention

[0007] This application provides a method for preparing a covalent organic framework selective layer to regulate porous composite membranes for flow batteries. A covalent organic framework selective layer is self-deposited onto a polymer under gravity, thereby forming a covalent organic framework composite membrane. The composite membrane prepared in this application consists of two porous membrane layers, exhibiting higher ion selectivity and conductivity. The assembled flow battery demonstrates excellent battery efficiency, which is of great significance for further improving the performance of flow batteries, especially all-vanadium redox flow batteries.

[0008] According to one aspect of this application, a composite membrane for flow batteries is provided.

[0009] The composite membrane includes a porous selective layer and a porous support layer;

[0010] The porous support layer has a pore size of 80–120 nm and a porosity of 60–70%.

[0011] The porous selective layer has a pore size of 0.5–3 nm and a porosity of 60–70%.

[0012] The porous selective layer is composed of a covalent organic framework material;

[0013] The porous support layer is composed of organic polymer resin.

[0014] The thickness of the composite membrane is 34–75 μm;

[0015] The thickness of the selective layer is 4–15 μm;

[0016] The thickness of the support layer is 30–60 μm.

[0017] Optionally, the building monomers of the covalent organic framework material are benzidine and trialdehyde phloroglucinol.

[0018] The organic polymer resin is selected from at least one of polyethersulfone, polyacrylonitrile, polyimide, polyetherketone, polytetrafluoroethylene, polyvinylidene fluoride, and polybenzimidazole.

[0019] According to another aspect of this application, a method for preparing the above-described composite membrane for flow batteries is provided, comprising the following steps:

[0020] A mixed solution containing organic polymer resin and COF microspheres was poured onto a substrate, immersed in water, cured, and Fe3O4 was etched away to obtain the composite film for flow batteries.

[0021] The preparation method of the COF microspheres includes:

[0022] Fe3O4 nanoclusters were mixed with an organic solvent, benzidine and trialdehyde resorcinol, and subjected to a hydrothermal reaction to obtain a solid product. The solid product was then mixed with an organic solvent, reacted, and dried to obtain COF microspheres.

[0023] The Fe3O4 nanoclusters have a particle size of 15–25 nm.

[0024] Fe3O4 nanoclusters are mixed with organic solvent, benzidine and trialdehyde resorcinol and subjected to a hydrothermal reaction. While benzidine and trialdehyde resorcinol are polymerizing, rearrangement and crosslinking occur in the organic solvent.

[0025] The organic solvent is selected from at least one of tetrahydrofuran, dichlorobenzene, and n-butanol.

[0026] The temperature of the heating reaction is 40–60°C.

[0027] Optionally, the temperature of the heating reaction is any value among 40°C, 50°C, and 60°C, or a range between any two.

[0028] The heating reaction takes 8 to 12 hours.

[0029] Optionally, the heating reaction time is any value among 2h, 9h, 10h, 11h, and 12h, or a range between any two.

[0030] The reaction temperature is 100–120°C.

[0031] Optionally, the temperature of the reaction is any value among 100°C, 110°C, and 120°C, or a range between any two.

[0032] The reaction time is 48–72 hours.

[0033] Optionally, the reaction time is any value among 48h, 60h, and 72h, or a range between any two.

[0034] Optionally, the covalent organic framework material microspheres are obtained through the following steps:

[0035] (1) Ferric chloride hexahydrate, ammonium acetate, and sodium citrate were dissolved in ethylene glycol, with a mass ratio of ferric chloride hexahydrate, ammonium acetate, and sodium citrate of (0.5–1.5):(2–4):(0.1–0.5). The concentration of the solution was 5–25% g / mL. The mixture was stirred at 120–180 °C for 1–3 h, followed by hydrothermal reaction at 150–250 °C for 12–24 h, and then cooled to room temperature. The mixture was repeatedly washed with ethanol and deionized water. Finally, it was dried at 60–80 °C for 12 h to obtain Fe3O4 nanoclusters with a particle size of 15–25 nm.

[0036] (2) The Fe3O4 nanoclusters obtained in step (1) were dispersed in tetrahydrofuran and sonicated for 10–30 min. Then benzidine and trialdehyde phloroglucinol were added, and the mixture was hydrothermally heated at 40–60 °C for 8–12 h, followed by rotary evaporation at room temperature to obtain a solid product. The solid product was then added to a mixed solvent of dichlorobenzene and n-butanol and reacted at 100–120 °C for 48–72 h. After repeated washing with methanol and dichloromethane, the mixture was dried at 60–80 °C for 12 h to obtain covalent organic framework material microspheres (COF microspheres).

[0037] Among them, Fe3O4 nanoclusters can be purchased commercially directly.

[0038] In the mixed solution containing organic polymer resin and COF microspheres, the content of COF microspheres is 2 to 10 wt% of the organic polymer resin.

[0039] Optionally, in the mixed solution containing organic polymer resin and COF microspheres, the content of COF microspheres is any value among 2 wt%, 5 wt%, and 10 wt% of the organic polymer resin, or any value between two of them.

[0040] The mixed solution containing organic polymer resin and COF microspheres contains a solvent;

[0041] The solvent is selected from at least one of dimethyl sulfoxide (DMSO), N,N'-dimethylacetamide (DMAc), N-methylpyrrolidone (NMP), and N,N'-dimethylformamide (DMF).

[0042] In the organic solvent solution of the organic polymer resin, the concentration of the organic polymer resin is 10-60 wt%.

[0043] Optionally, in the organic solvent solution of the organic polymer resin, the concentration of the organic polymer resin is any value or a range between 10wt%, 20wt%, 30wt%, 40wt%, 50wt%, and 60wt%.

[0044] Optionally, in the organic solvent solution of the organic polymer resin, the concentration of the organic polymer resin is 15-25 wt%.

[0045] In the organic solvent solution of the covalent organic framework material microspheres, the concentration of the covalent organic framework material microspheres is 1-4 wt%.

[0046] Optionally, the concentration of the covalent organic framework material microspheres in the organic solvent solution is 1.5 to 3.6 wt%.

[0047] The mass ratio of the organic solvent solution of the organic polymer resin to the organic solvent solution of the covalent organic framework material microspheres is 3 to 2:1.

[0048] The etching conditions are: immersion in hydrochloric acid solution at 30–50°C for 12–24 hours.

[0049] Specifically, the etching conditions are: immersion in 3M hydrochloric acid solution at 50°C for 24 hours.

[0050] Optionally, the following steps are included:

[0051] Organic polymer resin is dissolved in an organic solvent and stirred at a temperature of 10–80°C for 2–48 hours to prepare a polymer casting solution.

[0052] COF microspheres were ultrasonically dispersed in an organic solvent for 10–20 min and then added to a polymer casting solution. The mixture was ultrasonically dispersed for 10–20 min to form a mixed solution.

[0053] Pour the mixed solution onto a plate and allow the solvent to evaporate for 0–60 seconds. Then immerse the film in deionized water at room temperature to allow it to fully cure. The curing time is 2–10 minutes.

[0054] Optionally, the obtained composite membrane for the flow battery is immersed in methanol for 12–24 hours, then transferred to hydrochloric acid solution and immersed at 30–50°C for 12–24 hours, and finally immersed in water.

[0055] According to another aspect of this application, an application of the above-described composite membrane for a flow battery is provided, the flow battery including, but not limited to, vanadium redox flow batteries, zinc / cerium redox flow batteries, vanadium / bromine redox flow batteries or iron / chromium redox flow batteries.

[0056] It is particularly suitable for vanadium redox flow batteries.

[0057] The beneficial effects that this application can produce include:

[0058] 1. This application provides an application of a porous membrane with a covalent organic framework selective layer in flow batteries. This porous composite membrane consists of a porous selective layer and a porous support layer. The porous composite membrane prepared from the bilayer porous membrane can achieve more precise sieving of protons and ions, thereby improving the battery efficiency of flow batteries (represented by vanadium redox flow batteries). Furthermore, this membrane does not contain ion exchange groups and exhibits excellent chemical stability, providing a new direction for the design of ultra-high ion selectivity membranes for flow batteries.

[0059] 2. The preparation methods of the ion-conducting membrane and COF provided in this application are simple and easy to implement, and can be easily mass-produced.

[0060] 3. The COF selective layer of the ion-conducting membrane provided in this application is not easily detached, has good flexibility, and possesses excellent mechanical properties.

[0061] 4. The ion-conducting membrane prepared in this application is used in flow batteries, which enable it to have excellent ion selectivity in its respective system, and thus have high coulombic efficiency. Attached Figure Description

[0062] Figure 1 The sheet resistance of the composite film obtained in Comparative Example 1 and the composite films obtained in Examples 1, 2, and 3 is shown.

[0063] Figure 2 The composite membrane obtained in Comparative Example 1 and the composite membranes obtained in Examples 1, 2, and 3 were compared at 80 mA cm⁻¹. 2 The battery efficiency of the assembled vanadium redox flow battery.

[0064] Figure 3 The composite membrane obtained in Comparative Example 1 and the composite membrane obtained in Example 2 were compared at 80 mA cm⁻¹. 2 Discharge capacity retention of the assembled vanadium redox flow battery.

[0065] Figure 4 This is a schematic diagram of the structure of the composite membranes obtained in Examples 1, 2, and 3 of this application. Detailed Implementation

[0066] The present application is described in detail below with reference to the embodiments, but the present application is not limited to these embodiments.

[0067] Unless otherwise specified, all raw materials used in the embodiments of this application were purchased through commercial channels.

[0068] Example 1

[0069] 1.35 g of ferric chloride hexahydrate, 3.85 g of ammonium acetate, and 0.40 g of sodium citrate were dissolved in 50.00 mL of ethylene glycol and stirred at 160 °C for 1 h to form a homogeneous solution. The solution was then subjected to hydrothermal reaction at 200 °C for 20 h, followed by cooling to room temperature. The solution was washed several times with ethanol and deionized water. Finally, it was dried at 60 °C for 12 h to obtain Fe3O4 nanoclusters with a particle size of 15–25 nm.

[0070] 50.00 mg of Fe3O4 nanoclusters were dispersed in 50.00 mL of tetrahydrofuran and sonicated for 20 min. Then, 50.00 mg of benzidine and 30.00 mg of trialdehyde resorcinol were added, and the mixture was hydrothermally heated at 50 °C for 10 h, followed by rotary evaporation at room temperature to obtain a solid product. The solid product was then added to a mixed solvent of 4.50 mL dichlorobenzene and 0.50 mL n-butanol, and reacted at 120 °C for 72 h. The mixture was then repeatedly washed with methanol and dichloromethane, and finally dried at 60 °C for 12 h to obtain COF microspheres.

[0071] 10.20 g of polybenzimidazole (PBI) was dissolved in 49.80 g of DMAc, stirred at room temperature for 48 hours, and allowed to stand to obtain the polymer casting solution.

[0072] 0.30 g of COF microspheres were ultrasonically dispersed in 20.00 g of DMAc for 20 min and then added to the polymer casting solution. The mixture was ultrasonically dispersed for 20 min to form a mixed solution.

[0073] The mixed solution is spread evenly on a glass plate and then immersed in deionized water at room temperature to allow the membrane to fully solidify, thus obtaining a composite membrane.

[0074] The prepared composite film was transferred to a 3M hydrochloric acid solution and immersed at 50°C for 24 hours to etch away Fe3O4, finally obtaining the composite film (selective layer thickness of 4μm and support layer thickness of 40μm).

[0075] The resulting composite membrane has a support layer with a pore size of 80–120 nm and a porosity of 60–70%. The selective layer has a pore size of 0.5–3 nm and a porosity of 60–70%.

[0076] The membrane-assembled battery prepared in this embodiment has a coulombic efficiency of 93.8%, a voltage efficiency of 89.2%, and an energy efficiency of 83.7%.

[0077] Example 2

[0078] 1.35 g of ferric chloride hexahydrate, 3.85 g of ammonium acetate, and 0.40 g of sodium citrate were dissolved in 50.00 mL of ethylene glycol and stirred at 160 °C for 1 h to form a homogeneous solution. The solution was then subjected to hydrothermal reaction at 200 °C for 20 h, followed by cooling to room temperature. The solution was washed several times with ethanol and deionized water. Finally, it was dried at 60 °C for 12 h to obtain Fe3O4 nanoclusters with a particle size of 15–25 nm.

[0079] 50.00 mg of Fe3O4 nanoclusters were dispersed in 50.00 mL of tetrahydrofuran and sonicated for 20 min. Then, 50.00 mg of benzidine and 30.00 mg of trialdehyde resorcinol were added, and the mixture was hydrothermally heated at 50 °C for 10 h, followed by rotary evaporation at room temperature to obtain a solid product. The solid product was then added to a mixed solvent of 4.50 mL dichlorobenzene and 0.50 mL n-butanol, reacted at 120 °C for 72 h, and repeatedly washed with methanol and dichloromethane. Finally, the mixture was dried at 60 °C for 12 h to obtain COF microspheres.

[0080] 10.20 g of polybenzimidazole (PBI) was dissolved in 49.80 g of DMAc, stirred at room temperature for 48 hours, and allowed to stand to obtain the polymer casting solution.

[0081] 0.51 g of COF microspheres were ultrasonically dispersed in 20.00 g of DMAc for 20 min and then added to the polymer casting solution. The mixture was ultrasonically dispersed for 20 min to form a mixed solution.

[0082] The mixed solution is spread evenly on a glass plate and then immersed in deionized water at room temperature to allow the membrane to fully solidify, thus obtaining a composite membrane.

[0083] The prepared composite film was transferred to a 3M hydrochloric acid solution and immersed at 50°C for 24 hours to etch away Fe3O4, finally obtaining the composite film (selective layer thickness of 9 μm and support layer thickness of 40 μm).

[0084] The resulting composite membrane has a support layer with a pore size of 80–120 nm and a porosity of 60–70%. The selective layer has a pore size of 0.5–3 nm and a porosity of 60–70%.

[0085] The membrane-assembled battery prepared in this embodiment has a coulombic efficiency of 99.2%, a voltage efficiency of 88.8%, and an energy efficiency of 88.1%.

[0086] Example 3

[0087] 1.35 g of ferric chloride hexahydrate, 3.85 g of ammonium acetate, and 0.40 g of sodium citrate were dissolved in 50.00 mL of ethylene glycol and stirred at 160 °C for 1 h to form a homogeneous solution. The solution was then subjected to hydrothermal reaction at 200 °C for 20 h, followed by cooling to room temperature. The solution was washed several times with ethanol and deionized water. Finally, it was dried at 60 °C for 12 h to obtain Fe3O4 nanoclusters with a particle size of 15–25 nm.

[0088] 50.00 mg of Fe3O4 nanoclusters were dispersed in 50.00 mL of tetrahydrofuran and sonicated for 20 min. Then, 50.00 mg of benzidine and 30.00 mg of trialdehyde resorcinol were added, and the mixture was hydrothermally heated at 50 °C for 10 h, followed by rotary evaporation at room temperature to obtain a solid product. The solid product was then added to a mixed solvent of 4.50 mL dichlorobenzene and 0.50 mL n-butanol, reacted at 120 °C for 72 h, and repeatedly washed with methanol and dichloromethane. Finally, the mixture was dried at 60 °C for 12 h to obtain COF microspheres.

[0089] 10.20 g of polybenzimidazole (PBI) was dissolved in 49.80 g of DMAc, stirred at room temperature for 48 hours, and allowed to stand to obtain the polymer casting solution.

[0090] 0.71 g of COF microspheres were ultrasonically dispersed in 20.00 g of DMAc for 20 min and then added to the polymer casting solution. The mixture was ultrasonicated for 20 min to form a mixed solution.

[0091] The mixed solution is spread evenly on a glass plate and then immersed in deionized water at room temperature to allow the membrane to fully solidify, thus obtaining a composite membrane.

[0092] The prepared composite film was transferred to a 3M hydrochloric acid solution and immersed at 50°C for 24 hours to etch away Fe3O4, finally obtaining the composite film (selective layer thickness of 12 μm and support layer thickness of 40 μm).

[0093] The resulting composite membrane has a support layer with a pore size of 80–120 nm and a porosity of 60–70%. The selective layer has a pore size of 0.5–3 nm and a porosity of 60–70%.

[0094] The membrane-assembled battery prepared in this embodiment has a coulombic efficiency of 99.4%, a voltage efficiency of 87.2%, and an energy efficiency of 86.7%.

[0095] Comparative Example 1

[0096] 10.20 g of polybenzimidazole (PBI) was dissolved in 49.80 g of DMAc, stirred at room temperature for 48 hours, and allowed to stand to obtain the polymer casting solution.

[0097] The polymer casting solution was spread evenly on a glass plate, the solvent was allowed to evaporate for 5 seconds, and then the film was immersed in deionized water at room temperature to completely solidify the film, thus obtaining the composite film.

[0098] The membrane-assembled battery prepared in this comparative example showed a coulombic efficiency of 81.8%, a voltage efficiency of 90.3%, and an energy efficiency of 73.9%.

[0099] Comparative Example 2

[0100] 10.20 g of polybenzimidazole (PBI) was dissolved in 49.80 g of DMAc, stirred at room temperature for 48 hours, and allowed to stand to obtain the polymer casting solution.

[0101] 50.00 mg benzidine and 30.00 mg phloroglucinol were dissolved in 50.00 mL tetrahydrofuran. The mixture was hydrothermally heated at 50 °C for 10 h, followed by rotary evaporation at room temperature to obtain a solid product. The solid product was then added to a mixed solvent of 4.50 mL dichlorobenzene and 0.50 mL n-butanol, reacted at 120 °C for 72 h, and repeatedly washed with methanol and dichloromethane. Finally, it was dried at 60 °C for 12 h to obtain the COF material. 1.0 g of the COF material was dissolved in 20 g DMAc and stirred until homogeneous to obtain a COF dispersion.

[0102] The polymer casting solution and COF dispersion were mixed evenly to form a uniform polymer solution, which was then spread evenly on a glass plate. The glass plate was then transferred to a water bath containing 25°C deionized water and completely immersed. Since the COF material does not contain Fe3O4 nanoclusters, it cannot form an effective density difference with the polymer. Therefore, the support layer and the selective layer cannot be effectively separated by gravity. The prepared membrane material is a mixed-phase porous membrane, which cannot achieve effective and rapid separation of protons and vanadium ions, thus adversely affecting mass transfer. After assembling a battery using the prepared membrane material, the coulombic efficiency of the battery was tested to be 93.5%, the voltage efficiency was 78.7%, and the energy efficiency was 73.6%, which is much lower than the energy efficiency of Examples 1-3, indicating that the overall performance of the battery is low.

[0103] Comparative Example 3

[0104] 1.35 g of ferric chloride hexahydrate, 3.85 g of ammonium acetate, and 0.40 g of sodium citrate were dissolved in 50.00 mL of ethylene glycol. The solution was stirred at 160 °C for 1 h to form a homogeneous solution. The solution was then subjected to hydrothermal reaction at 200 °C for 20 h, followed by cooling to room temperature. The solution was washed several times with ethanol and deionized water. Finally, it was dried at 60 °C for 12 h to obtain Fe3O4 nanoclusters.

[0105] 50.00 mg of Fe3O4 nanoclusters were dispersed in 50.00 mL of tetrahydrofuran and sonicated for 20 min. Then, 50.00 mg of benzidine and 30.00 mg of trialdehyde resorcinol were added, and the mixture was hydrothermally heated at 50 °C for 10 h, followed by rotary evaporation at room temperature to obtain a solid product. The solid product was then added to a mixed solvent of 4.50 mL dichlorobenzene and 0.50 mL n-butanol, reacted at 120 °C for 72 h, and repeatedly washed with methanol and dichloromethane. Finally, the mixture was dried at 60 °C for 12 h to obtain COF microspheres.

[0106] 10.20 g of polybenzimidazole (PBI) was dissolved in 49.80 g of DMAc, stirred at room temperature for 48 hours, and allowed to stand to obtain the polymer casting solution.

[0107] 0.1 g of COF microspheres were ultrasonically dispersed in 10.00 g of DMAc for 20 min and then added to the polymer casting solution. The mixture was ultrasonically dispersed for 20 min to form a mixed solution.

[0108] The mixed solution was spread evenly on a glass plate, the solvent was allowed to evaporate for 5 seconds, and then the mixture was immersed in deionized water at room temperature to allow the membrane to fully solidify, thus obtaining the composite membrane.

[0109] The prepared composite membrane was transferred to a 3M hydrochloric acid solution and immersed at 50°C for 24 hours to etch away Fe3O4, finally obtaining a composite membrane (the thickness of the porous selective layer is 1 μm and the thickness of the porous support layer is 40 μm).

[0110] The surface resistivity of the film prepared in this comparative example was measured to be 422 mΩ·cm. 2 After assembling the battery, the coulombic efficiency, voltage efficiency, and energy efficiency of the battery were tested to be 92.5%, 89.2%, and 82.5%, which are far lower than the energy efficiency of Examples 1 to 3. This indicates that the selective layer thickness is too thin, resulting in a decrease in the overall performance of the battery.

[0111] Comparative Example 4

[0112] 1.35 g of ferric chloride hexahydrate, 3.85 g of ammonium acetate, and 0.40 g of sodium citrate were dissolved in 50.00 mL of ethylene glycol and stirred at 160 °C for 1 h to form a homogeneous solution. The solution was then subjected to hydrothermal reaction at 200 °C for 20 h, followed by cooling to room temperature. The solution was washed several times with ethanol and deionized water. Finally, it was dried at 60 °C for 12 h to obtain Fe3O4 nanoclusters with a particle size of 15–25 nm.

[0113] 50.00 mg of Fe3O4 nanoclusters were dispersed in 50.00 mL of tetrahydrofuran and sonicated for 20 min. Then, 50.00 mg of benzidine and 30.00 mg of trialdehyde resorcinol were added, and the mixture was hydrothermally heated at 50 °C for 10 h, followed by rotary evaporation at room temperature to obtain a solid product. The solid product was then added to a mixed solvent of 4.50 mL dichlorobenzene and 0.50 mL n-butanol, reacted at 120 °C for 72 h, and repeatedly washed with methanol and dichloromethane. Finally, the mixture was dried at 60 °C for 12 h to obtain COF microspheres.

[0114] 10.20 g of polybenzimidazole (PBI) was dissolved in 49.80 g of DMAc, stirred at room temperature for 48 hours, and allowed to stand to obtain the polymer casting solution.

[0115] 1.23 g of COF microspheres were ultrasonically dispersed in 20.00 g of DMAc for 20 min and then added to the polymer casting solution. The mixture was ultrasonically dispersed for 20 min to form a mixed solution.

[0116] The mixed solution is spread evenly on a glass plate and then immersed in deionized water at room temperature to allow the membrane to fully solidify, thus obtaining a composite membrane.

[0117] The prepared composite film was transferred to a 3M hydrochloric acid solution and immersed at 50°C for 24 hours to etch away Fe3O4, finally obtaining the composite film (selective layer thickness of 18 μm and support layer thickness of 40 μm).

[0118] The surface resistivity of the film prepared in this comparative example was tested to be 714 mΩ·cm. 2 After assembling the battery, the coulombic efficiency was measured to be 99.5%, the voltage efficiency to be 83.7%, and the energy efficiency to be 83.5%, which is much lower than the energy efficiency of Examples 1-3. This indicates that the thickness of the selective layer is too thick, which leads to a decrease in the overall performance of the battery.

[0119] Test case

[0120] A vanadium redox flow battery was assembled using the prepared membrane material, wherein the catalyst layer was an activated carbon felt, the bipolar plates were graphite plates, and the effective membrane area was 36 cm². 2 The current density is 80 mA cm⁻¹ -2 The vanadium ion concentration in the electrolyte is 1.50 mol / L. -1 The concentration of H2SO4 is 3 mol L. -1 .

[0121] Table 1

[0122] <![CDATA[VO 2+ Permeability (cm) 2 s -1 )]]> Comparative Example 1 <![CDATA[1.20×10 -8 ]]> Comparative Example 2 <![CDATA[3.30×10 -8 ]]> Comparative Example 3 <![CDATA[9.12×10 -9 ]]> Comparative Example 4 <![CDATA[0.35×10 -9 ]]> Example 1 <![CDATA[4.78×10 -9 ]]> Example 2 <![CDATA[1.67×10 -9 ]]> Example 3 <![CDATA[0.95×10 -9 ]]>

[0123] As shown in Table 1, Comparative Example 1 exhibits high vanadium ion permeability, while the permeability of Examples 1, 2, and 3 decreases with increasing selective layer thickness. These results indicate that the porous selective layer of the composite membrane can effectively suppress vanadium ions. 2+ The permeability of Example 2 is an order of magnitude lower than that of Comparative Example 1, resulting in excellent ion selectivity and coulombic efficiency of the battery.

[0124] from Figure 1 As can be seen, Comparative Example 1 exhibits a low sheet resistance due to its excellent pore structure and thin pore walls. The sheet resistance of Examples 1, 2, and 3 increases with the increase of the porous selective layer thickness, while the sheet resistance of Examples 1 and 2 remains close to that of Comparative Example 1, demonstrating that the COF selective layer has little additional impact on proton conduction, resulting in high proton conductivity and battery voltage efficiency.

[0125] The well-developed pore structure of the porous support layer provides the composite membrane with high ion flux. The nanoscale pores of the porous selective layer effectively block most of the vanadium ion permeation, while proton conduction is not significantly affected. Simultaneously, the amino groups on the COF pore walls protonate into amine groups under acidic conditions, which can form a hydrogen bond network with water molecules. The formation and breaking of these hydrogen bonds accelerates proton transfer. Therefore, the composite membrane achieves both high proton conductivity and ion selectivity.

[0126] from Figure 2It can be seen that the battery in Comparative Example 1 exhibits a lower coulombic efficiency (CE) and a better voltage efficiency (VE), mainly due to its higher vanadium ion permeability and lower sheet resistivity. The batteries in Examples 1, 2, and 3 have significantly higher CEs than Comparative Example 1. With increasing selective layer thickness, the CE of the batteries in Examples 1, 2, and 3 increases from 92.7% to 99.8%. This indicates that a thicker selective layer provides better vanadium ion blocking. Correspondingly, with increasing selective layer thickness, the VE of the batteries in Examples 1, 2, and 3 also decreases. Example 2 exhibits the best battery energy efficiency (EE), far exceeding that of Comparative Example 1.

[0127] from Figure 3 The cycling performance of Comparative Example 1 and Example 2 was demonstrated. After 50 cycles, the discharge capacity retention rate of Example 2 was 70%, higher than that of Comparative Example 1 (47%). The capacity loss per cycle of the battery using Example 2 was 0.6%, while the capacity decay per cycle of the battery using Comparative Example 1 was 1.06%. This is due to the VO of Example 2... 2+ The low permeability resulted in less capacity decay in Example 2. Low capacity decay is beneficial for long-term battery operation and also indicates good membrane stability.

[0128] The above description is merely a few embodiments of this application and is not intended to limit this application in any way. Although this application discloses preferred embodiments as described above, it is not intended to limit this application. Any changes or modifications made by those skilled in the art without departing from the scope of the technical solution of this application using the disclosed technical content are equivalent to equivalent implementation cases and fall within the scope of the technical solution.

Claims

1. A composite membrane for use in flow batteries, characterized in that, The composite membrane includes a porous selective layer and a porous support layer; The porous support layer has a pore size of 80–120 nm and a porosity of 60–70%. The porous selective layer has a pore size of 0.5–3 nm and a porosity of 60–70%. The porous selective layer is composed of a covalent organic framework material; The porous support layer is composed of organic polymer resin.

2. The composite membrane for flow batteries according to claim 1, characterized in that, The thickness of the composite membrane is 34–75 μm; The thickness of the selective layer is 4–15 μm; The thickness of the support layer is 30–60 μm.

3. The composite membrane for flow batteries according to claim 1, characterized in that, The building blocks of the covalent organic framework material are benzidine and trialdehyde phloroglucinol.

4. The composite membrane for flow batteries according to claim 1, characterized in that, The organic polymer resin is selected from at least one of polyethersulfone, polyacrylonitrile, polyimide, polyetherketone, polytetrafluoroethylene, polyvinylidene fluoride, and polybenzimidazole.

5. A method for preparing a composite membrane for a flow battery according to any one of claims 1 to 4, characterized in that, Includes the following steps: A mixed solution containing organic polymer resin and COF microspheres was poured onto a substrate, immersed in water, cured, and Fe3O4 was etched away to obtain the composite film for flow batteries. The preparation method of the COF microspheres includes: Fe3O4 nanoclusters were mixed with an organic solvent, benzidine and trialdehyde resorcinol, and subjected to a hydrothermal reaction to obtain a solid product. The solid product was then mixed with an organic solvent, reacted, and dried to obtain COF microspheres. The Fe3O4 nanoclusters have a particle size of 15–25 nm.

6. The preparation method according to claim 5, characterized in that, The temperature of the hydrothermal reaction is 40–60°C; The hydrothermal reaction time is 8–12 hours; The reaction temperature is 100–120°C; The reaction time is 48–72 hours.

7. The preparation method according to claim 5, characterized in that, In the mixed solution containing organic polymer resin and COF microspheres, the content of COF microspheres is 2 to 10 wt% of the organic polymer resin.

8. The preparation method according to claim 5, characterized in that, The mixed solution containing organic polymer resin and COF microspheres contains a solvent; The solvent is selected from at least one of dimethyl sulfoxide, N,N'-dimethylacetamide, N-methylpyrrolidone, and N,N'-dimethylformamide.

9. The preparation method according to claim 5, characterized in that, The etching conditions are: immersion in hydrochloric acid solution at 30–50°C for 12–24 hours.

10. The application of the composite membrane for a flow battery according to any one of claims 1 to 4, characterized in that, The flow batteries include vanadium redox flow batteries, zinc / cerium flow batteries, vanadium / bromine flow batteries, or iron / chromium flow batteries.

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