Preparation method and application of double-skin porous membrane for flow battery

Abstract

The invention relates to the technical field of flow batteries, in particular to a preparation method and application of a double-skin porous membrane for a flow battery. The preparation method comprises the following steps: S1, mixing agarose with water, stirring and heating to obtain gel, standing the gel, defoaming, and blade-coating a glass plate with the gel with a blade-coating thickness of 50-500 [mu] m; s2, heating the glass plate coated with the gel at 70 to 85 DEG C for 2 to 4 minutes; s3, dissolving resin in a solvent to obtain a film preparation solution; and S4, blade-coating the gel film with the film preparation solution with the blade-coating thickness of 50-200 [mu] m, staying in air for 2-30 min, and then immersing in water for at least 10 min to obtain the double-skin-layer porous film. A structure with compact skin layers on two sides and a porous supporting layer in the middle is obtained through gel-induced phase separation in combination with a non-solvent immersion phase separation method, and the structure remarkably reduces membrane pollution and relieves active ion intercrossing.

Description

Technical Field

[0001] The present invention relates to the technical field of flow batteries, and in particular, to a preparation method and application of a double-skin porous membrane for a flow battery. Background Art

[0002] With the development of the economy, the demand for energy is increasing day by day, and the environmental problems caused by the large consumption of fossil energy are becoming increasingly prominent. Large-scale utilization of renewable energy and realization of energy diversification have become important strategies for energy security and sustainable development in various countries around the world. However, renewable energy sources such as wind energy and solar energy are discontinuous and unstable, making the generated electric energy difficult to be directly utilized. Energy storage technologies, especially flow battery technologies, have become the key to solving this problem due to their advantages such as flexible design, high safety, and long lifespan. The ion-conducting membrane in a flow battery, as one of the core materials, not only needs to isolate the positive and negative electrolyte solutions to prevent cross-mixing, but also needs to conduct charge-balancing ions to form an internal electrical circuit of the battery.

[0003] Currently, the most widely used ion-conducting membrane in flow batteries is the commercial perfluorosulfonic acid ion-exchange membrane (Nafion). However, the production process of the Nafion membrane is relatively complex and expensive, which limits its further development. In order to reduce costs, non-fluorinated ion-exchange membranes have been widely studied. However, it contains ion-exchange groups, which results in insufficient stability in the strongly acidic and strongly oxidizing environment of a flow battery, and it is prone to degradation, which will further affect the performance and cycle life of the battery. In order to fundamentally solve the problem of poor stability of the ion-exchange membrane, porous membranes without ion-exchange groups have been proposed and further studied and developed.

[0004] Traditional porous membranes are mainly prepared by the immersion phase inversion method, and their structure consists of a dense skin layer and a macroporous support layer. Among them, the macroporous support layer ensures the rapid passage of ions and provides mechanical properties for the porous membrane. The skin layer is thin and dense, realizing the selective permeation of ions in a flow battery. However, the porous membranes prepared by the current process have only one skin layer, resulting in an obvious "Trade-off" effect (i.e., the rise and fall of selectivity and conductivity) between its selectivity and conductivity. In addition, the side of the macroporous support layer is easily filled with electrolyte solution and active ions that are not easy to pass through the membrane, leading to the problems of mutual penetration of positive and negative active ions and membrane fouling.

[0005] Therefore, it is urgent to effectively design and optimize the structure of the porous membrane, innovate the preparation process, break the traditional porous membrane structure, and then break the "Trade-off" effect between the ion selectivity and ion conductivity of the porous membrane, and alleviate the problems of mutual penetration of active ions and membrane fouling. This is of great significance for promoting the commercialization and industrialization development of flow batteries. Summary of the Invention

[0006] The object of the present invention is to provide a preparation method and application of a double - skin porous membrane for a flow battery. The present invention forms a lower skin layer by causing phase inversion of a part of the film - forming solution close to the gel side by doctor - blading the film - forming solution on a thin gel layer, and then quickly transferring it to a non - solvent to induce phase inversion of another part of the film - forming solution to form an upper skin layer, obtaining a structure with dense skin layers on both sides and a porous support layer in the middle. In addition, by regulating the experimental parameters of the above two processes, independent regulation of the upper and lower skin layers can be achieved to suit different flow battery systems. The porous membrane structure of the present invention greatly improves the ion selectivity of the porous membrane while ensuring ion conductivity, greatly reduces membrane fouling, improves mechanical properties, and limits membrane swelling, which is of great significance for promoting the development of flow batteries in the field of large - scale energy storage.

[0007] In order to achieve the above object, the technical solution of the present invention is as follows:

[0008] On the one hand, the present invention provides a preparation method of a double - skin porous membrane for a flow battery, and the method comprises the following steps:

[0009] S1. Mix agarose and water, stir and heat to obtain a gel. After standing the gel to remove bubbles, doctor - blade it on a glass plate, and the doctor - blading thickness is 50 - 500 μm;

[0010] S2. Heat the glass plate with the gel bladed on it at 70 - 85 °C for 2 - 4 min;

[0011] S3. Dissolve the resin in a solvent to obtain a film - forming solution;

[0012] S4. Doctor - blade the film - forming solution prepared in step S3 on the gel film prepared in step S2, with a doctor - blading thickness of 50 - 200 μm, stay in the air for 2 - 30 min, and then immerse it in water for at least 10 min to obtain the double - skin porous membrane.

[0013] The present invention first uses gel - induced phase separation to optimize the structure of the lower skin layer of the porous membrane by regulating the water content and thickness in the gel, and then combines the non - solvent - induced phase separation method to regulate its upper skin layer by changing the non - solvent.

[0014] Since the surface of the gel film is very smooth and the film - forming solution cannot be cast on the gel surface, the present invention solves the problem of doctor - blading the film - forming solution on the gel by heating the gel. This is because as the heating time of the gel increases, the water inside decreases, the surface friction increases, and the adhesion of the film - forming solution increases.

[0015] In the above - mentioned technical solution, further, in step S1, the mass ratio of the agarose to the water is 1:5 - 1:15, preferably 1:6.

[0016] In the above technical solution, further, in step S1, the heating temperature is 100-250 °C, preferably 220 °C, and the heating time is 8-15 min, preferably 9 min.

[0017] In the above technical solution, further, in step S1, the scraping thickness is 200 μm.

[0018] In the above technical solution, further, in step S2, the heating time is 2 min.

[0019] In the above technical solution, further, in step S3, the resin is one or two of polybenzimidazole, sulfonated polyether ether ketone, polyacrylonitrile, polyethersulfone, polyvinylidene fluoride, polyvinylpyrrolidone; the solvent is one of dimethylacetamide, dimethylformamide or N-methylpyrrolidone.

[0020] In the above technical solution, further, in step S3, in the film-forming solution, the mass fraction of the resin is 5 wt.% - 45 wt.%.

[0021] In the above technical solution, further, in step S4, the scraping thickness is 80 μm.

[0022] In the above technical solution, further, in step S4, the residence time in air is 14 min.

[0023] On the other hand, the present invention provides an application of the double-cortex porous membrane prepared by the above preparation method in a flow battery.

[0024] The flow system includes a vanadium redox flow battery, an iron / chromium flow battery, an iron / titanium flow battery, a zinc / manganese flow battery or a zinc / iron flow battery, but is not limited to these flow batteries.

[0025] The beneficial effects of the present invention are as follows:

[0026] 1. The present invention prepares the lower cortex by gel-induced phase separation, and at the same time combines the non-solvent immersion phase separation method to obtain the upper cortex, obtaining a structure with two dense cortices on both sides and a porous support layer in the middle. This structure significantly reduces membrane fouling and alleviates the intermixing of active ions.

[0027] 2. The present invention can realize the separate regulation of the pore structure of the double cortex by optimizing the gel and non-solvent phase compositions. The pore structure of the double cortex can be regulated according to the application scenario, so that the porous membrane has appropriate flux, conductivity and selectivity.

[0028] 3. The double-cortex porous membrane of the present invention is applied in a flow battery and has excellent energy efficiency. Due to the addition of a dense cortex layer, while ensuring high conductivity, the ion selectivity is greatly improved, breaking the Trade-off effect. In addition to the application in flow batteries, it is expected to be applied in fields such as sewage treatment, seawater desalination, and oil-water separation.

[0029] 4. The preparation scheme of the present invention is simple, applicable to a variety of film-forming polymer materials, easy to operate, with controllable morphology, low cost, and easy for large-scale production. Description of the Drawings

[0030] Figure 1 SEM top surface and bottom surface morphology diagrams of the double-cortex porous membranes prepared in Example 12 and Example 13. a is the top surface of Example 13, b is the bottom surface of Example 13, c is the top surface of Example 12, and d is the bottom surface of Example 12;

[0031] Figure 2 SEM cross-sectional morphology diagrams of the double-cortex porous membranes prepared in Example 12 and Example 13. a is Example 13, and b is Example 12. Detailed Description of the Invention

[0032] The following examples can enable those of ordinary skill in the art to understand the present invention more comprehensively, but do not limit the present invention in any way.

[0033] Unless otherwise specified, the materials used in the embodiments of the present invention can be obtained through commercial channels or prepared according to conventional methods well-known to those skilled in the art.

[0034] Example 1

[0035] S1. Take 3 g of agarose and dissolve it in 27 g of water. Stir and heat it with a magnetic stirrer at 220 °C for 9 min, let it stand to remove bubbles, and scrape the gel on a glass plate with a scraper with a scraping thickness of 50 μm. Store it in an incubator at 3 °C for 2 min;

[0036] S2. Heat the glass plate coated with the gel on a hot plate at 80 °C for 2 min;

[0037] S3. Take 4.5 g of polybenzimidazole (PBI) and dissolve it in 25.5 g of dimethylacetamide. Stir at 25 °C for 10 h to obtain a film-forming solution;

[0038] S4. Pour the film-forming solution prepared in step S3 onto the gel film prepared in step S2, scrape it with a scraper at a scraping distance of 80 μm, stay in the air for 2 min for phase inversion to prepare the lower cortex, and then immerse it in water for 10 min to prepare the upper cortex to complete phase inversion and obtain a double-cortex porous membrane.

[0039] Example 2

[0040] Example 2 uses the same preparation method as Example 1, except that in step S2, the heating time is 4 min.

[0041] Comparative Example 1

[0042] Comparative Example 1 uses the same preparation method as Example 1, except that in step S2, the heating time is 1 min.

[0043] Comparative Example 2

[0044] Comparative Example 2 uses the same preparation method as Example 1, except that in step S2, the heating time is 6 min.

[0045] The double - layer porous membranes prepared in Examples 1 - 2 and Comparative Examples 1 - 2 are assembled into single cells, and the performance is tested by constant - current charge - discharge under the condition of a working current of 200 mA·cm -2 The electrode area of the single cell is 9 cm 2 , and 30 mL of electrolyte (vanadium ion concentration is 1.50 mol L -1 , H2SO4 concentration is 3 mol L -1 ) is added to each of the positive and negative electrode liquid storage tanks. In Examples 1 - 2 and Comparative Examples 1 - 2, by adjusting the heating time on the hot stage in step S2, the moisture in the gel is reduced, the surface friction is increased, thereby increasing the adhesion of the film - forming solution. At the same time, an overly long heating time will cause the moisture in the gel to completely volatilize, making it unable to support the gel side to complete the induced phase separation. The test results are shown in Table 1.

[0046] Example 3

[0047] S1. Take 3 g of agarose and dissolve it in 27 g of water. Stir and heat it with a magnetic stirrer at 220 °C for 9 min, let it stand to remove bubbles, scrape the gel on a glass plate with a scraper with a scraping thickness of 200 μm, and store it in a constant - temperature oven at 3 °C for 2 min;

[0048] S2. Transfer the glass plate coated with the gel to an 80 °C hot stage and heat it for 2 min;

[0049] S3. Take 4.5 g of polybenzimidazole (PBI) and dissolve it in 25.5 g of dimethylacetamide, stir at 25 °C for 10 h to obtain a film - forming solution;

[0050] S4. Pour the film - forming solution prepared in step S3 onto the gel film prepared in step S2, scrape it with a scraper at a scraping distance of 80 μm, stay in the air for 2 min for phase inversion to prepare the lower skin layer, and then immerse it in water for 10 min to prepare the upper skin layer, complete the phase inversion, and obtain a double - layer porous membrane.

[0051] Examples 4 - 7

[0052] Examples 4 - 7 adopt the same preparation method as Example 1, except that in step S1, the gel scraping thicknesses are 100μm, 300μm, 400μm, and 500μm respectively.

[0053] Comparative Example 3

[0054] Comparative Example 3 adopts the same preparation method as Example 3, except that in step S1, the gel scraping thickness is 30μm.

[0055] Comparative Example 4

[0056] Comparative Example 3 adopts the same preparation method as Example 3, except that in step S1, the gel scraping thickness is 600μm.

[0057] The double - skin porous membranes prepared in Examples 3 - 7 and Comparative Examples 3 - 4 are assembled into single cells, and the performance tests are carried out by constant - current charge - discharge under the condition of a working current of 200 mA·cm -2 The single - cell electrode area is 9 cm 2 , and 30 mL of electrolyte (vanadium ion concentration is 1.50 mol L -1 , H2SO4 concentration is 3 mol L -1 ) are added to each of the positive and negative electrode liquid storage tanks. In Examples 3 - 7 and Comparative Examples 3 - 4, by adjusting the thickness of the gel film in step S1, the longitudinal structure of the porous membrane is optimized, and the test results are shown in Table 2.

[0058] Example 8

[0059] S1. Take 3 g of agarose and dissolve it in 27 g of water. Stir and heat it with a magnetic stirrer at 220 °C for 9 min, let it stand to remove bubbles, and scrape the gel on a glass plate with a scraper with a scraping thickness of 200μm, and store it in a constant - temperature oven at 3 °C for 2 min;

[0060] S2. Heat the glass plate with the scraped gel on a hot plate at 80 °C for 2 min;

[0061] S3. Take 4.5 g of polybenzimidazole (PBI) and dissolve it in 25.5 g of dimethylacetamide, stir at 25 °C for 10 h to obtain a film - forming solution;

[0062] S4. Pour the film - forming solution prepared in step S3 onto the gel film prepared in step S2, scrape it with a scraper at a scraping distance of 50μm, stay in the air for 2 min for phase inversion to prepare the lower skin layer, and then immerse it in water for 10 min to prepare the upper skin layer, complete the phase inversion, and obtain a double - skin porous membrane.

[0063] Examples 9 - 11

[0064] The same preparation method as in Example 8 was adopted, except that in step S4, the scraping thicknesses of the film-forming solution were 120 μm, 150 μm, and 200 μm, respectively.

[0065] The double-layered porous membranes prepared in Examples 8-11 were assembled into single cells, and performance tests were carried out by constant current charge and discharge under the condition of a working current of 200 mA·cm -2 The electrode area of the single cell was 9 cm 2 , and 30 mL of electrolyte solution (vanadium ion concentration was 1.50 mol L -1 , and H2SO4 concentration was 3 mol L -1 ) were added to each of the positive and negative electrode liquid storage tanks. In Examples 8-11, by adjusting the scraping thickness of the film-forming solution in step S4, it was explored whether the double-layered porous membranes with different thicknesses all had excellent performance. The test results are shown in Table 3.

[0066] Example 12

[0067] S1. Dissolve 3 g of agarose in 27 g of water, stir and heat with a magnetic stirrer at 220 °C for 9 min, let it stand to remove bubbles, scrape the gel on a glass plate with a scraper with a scraping thickness of 200 μm, and store it in a constant temperature oven at 3 °C for 2 min;

[0068] S2. Heat the glass plate coated with the gel on a hot plate at 80 °C for 2 min;

[0069] S3. Dissolve 4.5 g of polybenzimidazole (PBI) in 25.5 g of dimethylacetamide, stir at 25 °C for 10 h to obtain a film-forming solution;

[0070] S4. Pour the film-forming solution prepared in step S3 onto the gel film prepared in step S2, scrape it with a scraper at a scraping distance of 80 μm, stay in the air for 14 min for phase inversion to prepare the lower layer, and then immerse it in water for 10 min to prepare the upper layer, complete the phase inversion, and obtain a double-layered porous membrane.

[0071] Examples 13-17

[0072] Examples 13-17 adopted the same preparation method as in Example 12, except that in step S4, the reaction time on the gel was changed, which were 6 min, 8 min, 10 min, 18 min, and 30 min, respectively.

[0073] The double-layered porous membranes prepared in Examples 12-17 were assembled into single cells, and performance tests were carried out by constant current charge and discharge under the condition of a working current of 200 mA·cm -2 The electrode area of the single cell was 9 cm 2, 30 mL of electrolyte solution (vanadium ion concentration is 1.50 mol L -1 , and H2SO4 concentration is 3 mol L -1 ) were added to the positive and negative electrolyte storage tanks respectively. In Examples 12 - 17, by changing the reaction time on the gel in Step S4, the regulation results of the pore size and skin layer thickness of the porous membrane were explored, and the test results are shown in Table 4.

[0074] The double - skin porous membrane prepared in Example 12 was assembled into a single cell, and the performance test was carried out by constant - current charge - discharge under the condition of a working current of 40 mA·cm -2 in the zinc / iron flow battery system. The electrode area of the single cell is 9 cm 2 , the positive electrolyte is 3 mol L -1 KOH and 0.4 mol L -1 Na4[Fe(CN)6]·12H2O, and the negative electrolyte is 8 mol L -1 NaOH and 0.2 mol L - 1 ZnO. The test results are shown in Table 6.

[0075] The double - skin porous membrane prepared in Example 12 was assembled into a single cell, and the performance test was carried out by constant - current charge - discharge under the condition of a working current of 80 mA·cm -2 in the iron / titanium flow battery system. The electrode area of the cell is 9 cm 2 , the positive electrolyte is 1 mol L -1 FeSO4 and 3 mol L -1 H2SO4, 1 mol L -1 TiOSO4 and 3 mol L -1 H2SO4. The test results are shown in Table 6.

[0076] Example 18

[0077] S1. Take 3 g of agarose and dissolve it in 27 g of water, stir and heat it with a magnetic stirrer at 220 °C for 9 min, let it stand to remove bubbles, scrape the gel on a glass plate with a scraper with a scraping thickness of 200 μm, and store it in an incubator at 3 °C for 2 min;

[0078] S2. Heat the glass plate coated with the gel on a hot plate at 80 °C for 2 min;

[0079] S3. Take 2.25 g of polyvinylpyrrolidone (PVP) and 6.75 g of polyacrylonitrile (PAN) and dissolve them in 21 g of dimethylacetamide, stir at 25 °C for 10 h to obtain a membrane - forming solution;

[0080] S4. Pour the film-forming solution prepared in step S3 onto the gel film prepared in step S2, and use a scraper to scrape it at a scraping distance of 80 μm, and let it stay in the air for 14 min for phase inversion to prepare the lower skin layer, and then immerse it in water for 10 min to prepare the upper skin layer, complete the phase inversion, and obtain a double-skin porous membrane.

[0081] Example 19

[0082] Example 19 uses the same preparation method as Example 18, except that in step S3, 2.25 g of sulfonated polyether ether ketone (SPEEK) and 6.75 g of polyethersulfone (PES) are dissolved in 21 g of dimethylacetamide to obtain a film-forming solution.

[0083] Example 20

[0084] Example 20 uses the same preparation method as Example 18, except that in step S3, 2.25 g of polyvinylpyrrolidone (PVP) and 6.75 g of polyethersulfone (PES) are dissolved in 21 g of dimethylacetamide to obtain a film-forming solution.

[0085] Example 21

[0086] Example 21 uses the same preparation method as Example 18, except that in step S3, 2.25 g of sulfonated polyether ether ketone (SPEEK) and 6.25 g of polyacrylonitrile (PAN) are dissolved in 21 g of dimethylacetamide to obtain a film-forming solution.

[0087] Example 22

[0088] Example 22 uses the same preparation method as Example 18, except that in step S3, 4.5 g of polyvinylidene fluoride (PVDF) is dissolved in 25.5 g of dimethylacetamide to obtain a film-forming solution.

[0089] Assemble the double-skin porous membranes prepared in Examples 18 - 22 into single cells, and perform performance tests under constant current charge and discharge at a working current of 200 mA·cm -2 for the all-vanadium redox flow single cell. The electrode area of the single cell is 9 cm 2 , and 30 mL of electrolyte (vanadium ion concentration is 1.50 mol L -1 , H2SO4 concentration is 3 mol L -1 ) is added to each of the positive and negative electrode liquid storage tanks. In Examples 18 - 22, by changing the type of resin in step S3, it is used to investigate whether the double-skin porous membranes prepared from different resins all have good battery performance. The test results are shown in Table 5.

[0090] Table 1

[0091] Coulombic efficiency (%) Voltage efficiency (%) Energy efficiency (%) Example 1 92.9 79.8 74.1 Example 2 91.9 80.4 73.8 Comparative Example 1 89.1 80.2 71.4 Comparative Example 2 89.6 80.9 72.4

[0092] Table 2

[0093] Coulombic efficiency (%) Voltage efficiency (%) Energy efficiency (%) Example 3 95.2 80.9 77.0 Example 4 93.4 80.9 75.5 Example 5 95 80.3 76.2 Example 6 94.1 80 75.2 Example 7 92.1 79.5 73.2 Comparative Example 3 89.1 80.2 71.4 Comparative Example 4 89.4 80.1 71.6

[0094] Table 3

[0095] Coulombic efficiency (%) Voltage efficiency (%) Energy efficiency (%) Example 8 94.5 81.1 76.6 Example 9 96 80.7 77.5 Example 10 96.2 80.5 77.4 Example 11 96.3 80.4 77.4

[0096] Table 4

[0097] Coulombic efficiency (%) Voltage efficiency (%) Energy efficiency (%) Example 13 97.4 80.1 78 Example 14 98.9 79.6 78.7 Example 15 99.2 79.8 79.2 Example 12 99.8 80.4 80.2 Example 16 99.9 80.2 80.1 Example 17 99.9 80.1 80.0 Example 3 95.2 80.9 77.0

[0098] Table 5

[0099] Coulombic efficiency (%) Voltage efficiency (%) Energy efficiency (%) Example 18 99.1 80.1 79.2 Example 19 99.3 80.2 79.6 Example 20 99.5 80.4 79.9 Example 21 99.5 80.7 80.2 Example 22 99.1 80.3 79.5 Example 12 99.9 80.4 80.2

[0100] Table 6

[0101] Coulombic efficiency (%) Voltage efficiency (%) Energy efficiency (%) Zinc / iron flow battery system 99.5 80.1 77.2 Iron / titanium flow battery system 96.1 79.6 76.8 All-vanadium flow battery system 99.9 80.4 80.2

[0102] From Figure 1 It can be seen that after reacting on the gel for different times and then performing non-solvent induced phase separation, upper and lower skin layers will be formed. From Figure 2 It can be seen that in Example 13, the phase transition time on the gel is short, and its upper skin layer is thicker than the lower skin layer. In Example 12, it can be seen that after a longer phase transition time on the gel, its lower skin layer becomes thicker and the upper skin layer becomes thinner.

[0103] It can be seen from Table 1 that if the heating time of the gel is insufficient, not only the coating of the membrane-forming solution on the gel is uneven, but also excessive moisture will cause rapid phase transition on the gel side, resulting in insufficient denseness of the skin layer structure and affecting the formation of the lower skin layer. However, too long a heating time will cause the moisture in the gel to completely volatilize, making it unable to support the induced phase separation on the gel side. Therefore, the coulombic efficiency of Comparative Examples 1 and 2 is low, while that of Examples 1 and 2 is improved. Thus, precise control of the heating time is crucial for ensuring the quality of the membrane layer.

[0104] As can be seen from Table 2, with the decrease in the thickness of the gel film, the rate of phase inversion slows down. When the lower skin layer is formed, the casting solution has not yet solidified. It is found that with the increase in the thickness of the gel film, the content of the non-solvent is relatively high, which will increase the rate of gel phase inversion, resulting in the complete solidification of the porous membrane after gel phase inversion and the inability to form the upper skin layer anymore, proving that it is very important to select an appropriate thickness of the gel film. The formation of the upper and lower skin layers, the improvement of the selectivity of the porous membrane, and the reduction of the interpenetration of vanadium ions lead to an increase in the Coulomb efficiency. However, an overly thin gel film will affect the generation of the lower skin layer and reduce the ion selectivity of the porous membrane. There is a "Trade-off" effect between the ionic conductivity and ion selectivity of the porous membrane. However, with the formation of the double-skin layer structure, the synergistic effect of the two skin layers on both sides will greatly improve the selectivity of the porous membrane, but it will not have any impact on the conductivity. Through comprehensive comparison of the energy efficiency of the liquid flow single cell, when the thickness of the gel film is 200 μm, the prepared double-skin layer porous membrane has the best performance, optimizing the double-skin layer structure of the porous membrane.

[0105] As can be seen from Table 3, double-skin layer porous membranes with different thicknesses will cause slight changes in the Coulomb efficiency and voltage efficiency. However, as long as the membrane thickness is maintained within an appropriate range, its performance still exhibits excellent characteristics. This indicates that under specific application conditions, by optimizing the membrane thickness, the adaptability of the membrane material can be improved, thus providing broad possibilities for its expansion in practical applications. Further research on the thickness optimization and regulation of the membrane material will help reveal how the microstructural characteristics affect the macroscopic properties. This is not only of great significance for improving the Coulomb efficiency and voltage efficiency of the membrane, but also can promote the development of materials science and engineering technology, providing new ideas for the design of high-performance and multifunctional membrane materials.

[0106] As can be seen from Table 4, in the case of no significant change in the voltage efficiency, the significant increase in the Coulomb efficiency indicates that by adjusting the gel phase inversion time, it will not only affect the structure of the lower skin layer, but also have a certain effect on the upper skin layer, thereby changing the structural characteristics of the overall porous membrane and improving its comprehensive performance. Specifically, the change in the gel phase inversion time can directly regulate the porosity, pore size, and structural uniformity of the lower skin layer. Thus, it indirectly affects the structure formation of the upper skin layer and the functional performance of the whole membrane. In addition, the interaction between the lower skin layer and the upper skin layer will also lead to the adjustment of the interfacial characteristics due to the change in the gel phase inversion process, thereby optimizing the battery performance of the porous membrane. This dual regulation effect of the phase inversion time on the upper and lower skin layers indicates that when adjusting the phase separation and film-forming conditions of the gel, the synergistic effect of the double-layer structure needs to be comprehensively considered. By precisely controlling this parameter, not only can the structural characteristics be optimized, but also a theoretical basis and technical support can be provided for the design of porous membranes to meet different application requirements.

[0107] As can be seen from Table 5, the Coulombic efficiency of the porous membranes prepared from different resins is higher than that of the single-skin porous membranes prepared from the same resins reported in the existing literature, indicating that the preparation method of this double-skin porous membrane is applicable to a variety of values and can effectively improve the Coulombic efficiency of the porous membrane.

[0108] As can be seen from Table 6, the double-skin porous membrane is also applicable to the zinc / iron flow battery system and the iron / titanium flow battery system, bringing good ion selectivity and ion conductivity to them.

[0109] In summary, based on the gel-induced phase separation method and the immersion-induced phase separation method, through the improvement and ingenious combination of the two methods, a new porous membrane structure, the double-skin porous membrane, is prepared. By reasonably regulating the preparation parameters of the "gel-induced phase separation method" and the "immersion-induced phase separation method", the regulation and optimization of the transverse / longitudinal surface structure of the porous membrane are realized, enabling it to have excellent ion conductivity while greatly improving ion selectivity, and making the flow battery assembled with it have excellent performance. At the same time, the preparation scheme of the present invention is simple, which can promote the improvement of the performance of the flow battery and facilitate its further development in the field of large-scale energy storage.

[0110] The above embodiments are only the preferred embodiments of the present invention and are not intended to limit the implementation manner. The protection scope of the present invention should be defined by the scope of the claims. Other different forms of changes or modifications can be made based on the above description. The obvious changes or modifications derived therefrom are still within the protection scope of the present invention.

Claims

1. A method for preparing a double-skin porous membrane for a flow battery, characterized in that: The method comprises the following steps: S1. Mix agarose and water, stir and heat to obtain a gel, let the gel stand to remove bubbles, and then scrape it on a glass plate with a scraping thickness of 50-500 μm; S2, heating the glass plate coated with the gel at 70-85°C for 2-4 minutes; S3, dissolving the resin in a solvent to obtain a film-forming solution; S4, scraping the membrane-making solution prepared in step S3 onto the gel film prepared in step S2, with a scraping thickness of 50-200 μm, leaving it in the air for 2-30 minutes, and then immersing it in water for at least 10 minutes to obtain the double-skinned porous membrane.

2. The preparation method according to claim 1, characterized in that: In step S1, the mass ratio of agarose to water is 1:5-1:

15.

3. The preparation method according to claim 1, characterized in that: In step S1, the heating temperature is 100-250°C, and the heating time is 8-15 minutes.

4. The preparation method according to claim 1, characterized in that: In step S1, the coating thickness is 200 μm.

5. The preparation method according to claim 1, characterized in that: In step S2, the heating time is 2 minutes.

6. The preparation method according to claim 1, characterized in that: In step S3, the resin is one or two of polybenzimidazole, sulfonated polyetheretherketone, polyacrylonitrile, polyethersulfone, polyvinylidene fluoride, and polyvinylpyrrolidone; and the solvent is one of dimethylacetamide, dimethylformamide, or N-methylpyrrolidone.

7. The preparation method according to claim 1, characterized in that: In step S3, the mass fraction of the resin in the membrane-forming solution is 5wt.%-45wt.%.

8. Use of a double-skinned porous membrane prepared by the preparation method according to any one of claims 1 to 7 in a liquid flow battery.

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