Charge gradient cation exchange membrane, preparation method thereof and salinity gradient energy power generation system

Abstract

The invention provides a charge gradient cation exchange membrane, a preparation method thereof and a salinity gradient energy power generation system. The charge gradient cation exchange membrane comprises a first membrane layer, a second membrane layer and a middle membrane layer arranged between the first membrane layer and the second membrane layer, the first membrane layer comprises first sulfonated polyetheretherketone, and the second membrane layer comprises second sulfonated polyetheretherketone; the sulfonation degree of the first sulfonated polyetheretherketone is greater than that of the second sulfonated polyetheretherketone; the middle film layer comprises a carbon nanotube-metal organic framework composite material. The charge gradient cation exchange membrane is applied to a salinity gradient energy power generation system and has good ion transmission characteristics and energy conversion efficiency.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of charge gradient cation exchange membrane, in particular to a charge gradient cation exchange membrane and a preparation method thereof, and a salinity gradient power generation system. BACKGROUND

[0002] Salinity gradient power (SGP) is a new type of renewable energy, which is a chemical energy form of ocean energy. The salinity gradient power mainly exists at the junction of river water and seawater. Unlike light energy, wind energy and wave energy, which can change significantly depending on the climate, the salinity gradient power is less affected by the climate. At present, the salinity gradient power has been recognized as a non-polluting and sustainable energy source, and is known as the "blue energy".

[0003] Reverse electrodialysis (RED) is a technology for extracting and utilizing the natural salinity gradient power. The concentration difference between two solutions of different concentrations causes ions to migrate from the side with high osmotic pressure to the side with low osmotic pressure through an ion exchange membrane, thereby forming an additional electromotive force. The energy is then converted into electrical energy through the oxidation-reduction reaction of the electrodes at both ends of the reverse electrodialysis. The pore size of the traditional anion and cation exchange membrane is comparable to the size of the solution ions, and the thickness is generally a few hundred microns. Under the influence of the double disadvantages of mass transfer and membrane resistance, the ion flux is low. In addition, the concentration polarization phenomenon existing at the membrane-solution interface also weakens the ion transport performance. These shortcomings reduce the energy conversion efficiency of the reverse electrodialysis system. SUMMARY

[0004] In view of this, the present application provides a charge gradient cation exchange membrane and a preparation method thereof, and a salinity gradient power generation system, aiming to improve the energy conversion efficiency of the salinity gradient power generation system.

[0005] To solve the above technical problems, one technical solution adopted by the present application is to provide a charge gradient cation exchange membrane for salinity gradient power conversion, comprising a first membrane layer, a second membrane layer, and an intermediate membrane layer arranged between the first membrane layer and the second membrane layer; the first membrane layer comprises a first sulfonated polyether ether ketone, and the second membrane layer comprises a second sulfonated polyether ether ketone; the sulfonation degree of the first sulfonated polyether ether ketone is greater than the sulfonation degree of the second sulfonated polyether ether ketone; and the intermediate membrane layer comprises a carbon nanotube-metal organic framework composite material.

[0006] According to an embodiment of the present application, the sulfonation degree of the first sulfonated polyether ether ketone is in the range of 50% to 70%; and / or, the sulfonation degree of the second sulfonated polyether ether ketone is in the range of 50% to 70%.

[0007] According to an embodiment of the present application, the carbon nanotube-metal organic framework composite material comprises a carbon nanotube-zirconium amino phthalate composite material.

[0008] According to an embodiment of the present application, the thickness of the first film layer is in the range of 5-15 microns; and / or, the thickness of the second film layer is in the range of 5-15 microns; and / or, the thickness of the intermediate film layer is in the range of 5-15 microns.

[0009] A second technical solution adopted by the present application is to provide a preparation method of the charge gradient cation exchange membrane for salt gradient energy conversion according to any one of the above, comprising: adding a first sulfonated polyether ether ketone to a first solvent to obtain a first casting solution; adding a second sulfonated polyether ether ketone to a second solvent to obtain a second casting solution; adding a carbon nanotube-metal organic framework composite material to a third solvent to obtain a third casting solution; sequentially coating the first casting solution, the third casting solution and the second casting solution on a substrate to form a thin film preform; heat treating the thin film preform at a first preset temperature for a first preset time to obtain a charge gradient cation exchange membrane; the charge gradient cation exchange membrane comprises a first film layer, a second film layer and an intermediate film layer arranged between the first film layer and the second film layer, the first film layer is formed by the first casting solution, the second film layer is formed by the second casting solution, and the intermediate film layer is formed by the third casting solution; wherein the sulfonation degree of the first sulfonated polyether ether ketone is greater than the sulfonation degree of the second sulfonated polyether ether ketone.

[0010] According to an embodiment of the present application, the first preset temperature is in the range of 40-80 DEG C; and / or, the first preset time is in the range of 6-24 hours.

[0011] According to an embodiment of the present application, the first solvent comprises at least one of N, N-dimethylformamide, N, N-dimethylacetamide and N-methylpyrrolidone; and / or, the second solvent comprises at least one of N, N-dimethylformamide, N, N-dimethylacetamide and N-methylpyrrolidone; and / or, the third solvent comprises at least one of N, N-dimethylformamide, N, N-dimethylacetamide and N-methylpyrrolidone.

[0012] According to an embodiment of the present application, the mass percentage of the first sulfonated polyether ether ketone is 15-20 wt% based on the total mass of the first casting solution; and / or, the mass percentage of the second sulfonated polyether ether ketone is 15-20 wt% based on the total mass of the second casting solution.

[0013] According to an embodiment of the present application, the preparation method of the carbon nanotube-metal organic framework composite material comprises: dispersing carbon nanotubes in a fourth solvent to obtain a first dispersion; adding ZrCl4 and HAc to the first dispersion to obtain a first mixture; adding amino terephthalic acid to a fifth solvent to obtain a second dispersion; mixing the first mixture and the second dispersion to obtain a precursor solution, and transferring the precursor solution to an autoclave to react at a second preset temperature for a second preset time to obtain the carbon nanotube-metal organic framework composite material.

[0014] According to an embodiment of the present application, the fourth solvent comprises at least one of N,N-dimethylformamide, N,N-dimethylacetamide, and N-methylpyrrolidone; and / or, the fifth solvent comprises at least one of N,N-dimethylformamide, N,N-dimethylacetamide, and N-methylpyrrolidone.

[0015] According to an embodiment of the present application, the mass ratio of the carbon nanotube powder and the fourth solvent is (0.025-0.1):1; and / or, the mass ratio of the carbon nanotube powder, ZrCl4, and HAc is (1-4):25:150; and / or, the mass ratio of the amino terephthalic acid and ZrCl4 is (0.8-1.2):1.

[0016] The third technical solution adopted in the present application is to provide a salt difference energy power generation system, comprising: the charge gradient cation exchange membrane of any one of the above or the charge gradient cation exchange membrane prepared by the preparation method of any one of the above; a silver / silver chloride electrode with a salt bridge, a salt solution with a first concentration, and a salt solution with a second concentration; wherein the charge gradient cation exchange membrane is arranged between the salt solution with the first concentration and the salt solution with the second concentration; the first concentration is less than the second concentration.

[0017] According to an embodiment of the present application, the first membrane layer of the charge gradient cation exchange membrane is arranged on the side of the intermediate membrane layer close to the salt solution with the first concentration; and the second membrane layer of the charge gradient cation exchange membrane is arranged on the side of the intermediate membrane layer close to the salt solution with the second concentration.

[0018] According to an embodiment of the present application, the first concentration is in the range of 0.001 mol·L -1 -0.1 mol·L -1 ; and / or, the second concentration is in the range of 0.5 mol·L -1 -1 mol·L -1 .

[0019] According to an embodiment of the present application, the salt solution comprises at least one of potassium chloride solution, sodium chloride solution, and lithium chloride solution.

[0020] The beneficial effects of the present application are: the embodiments of the present application provide a charge gradient cation exchange membrane, the exchange membrane comprising a first membrane layer, a second membrane layer and an intermediate membrane layer arranged between the first membrane layer and the second membrane layer; the first membrane layer comprises a first sulfonated polyether ether ketone, and the second membrane layer comprises a second sulfonated polyether ether ketone; the sulfonation degree of the first sulfonated polyether ether ketone is greater than the sulfonation degree of the second sulfonated polyether ether ketone; the intermediate membrane layer comprises a carbon nanotube-metal organic framework composite material. The present application can reduce the charge polarization phenomenon by regulating the difference in sulfonation degree of the first membrane layer and the second membrane layer to form a cation exchange membrane with a charge gradient, and the first membrane layer with high sulfonation degree can improve the cation selectivity, and the second membrane layer with low sulfonation degree can reduce the membrane resistance, thereby synergistically reducing the mass transfer resistance of cations. At the same time, the intermediate membrane layer can inhibit the swelling phenomenon of the first membrane layer and the second membrane layer caused by the presence of sulfonic acid groups, and has superior conductivity, forming an efficient cation migration channel. The charge gradient cation exchange membrane is applied to a salt differential energy power generation system, and has good ion transmission characteristics and energy conversion efficiency. BRIEF DESCRIPTION OF DRAWINGS

[0021] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the drawings needed in the embodiment description will be briefly introduced as follows. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can be obtained by those skilled in the art without creative labor. Among them:

[0022] Figure 1 is a structural schematic diagram of the charge gradient cation exchange membrane provided by the embodiments of the present application;

[0023] Figure 2 is a flowchart of the preparation method of the charge gradient cation exchange membrane provided by the embodiments of the present application;

[0024] Figure 3 is a flowchart of the preparation method of the carbon nanotube-metal organic framework composite material provided by the embodiments of the present application;

[0025] Figure 4 is a schematic diagram of a salt differential energy power generation system provided by the embodiments of the present application;

[0026] Figure 5 is a scanning electron microscope diagram of the charge gradient cation exchange membrane provided by the embodiment 1 of the present application; wherein, Figure 5 (a) of is a surface schematic diagram of the charge gradient cation exchange membrane, Figure 5 (b) of is a cross-sectional schematic diagram of the charge gradient cation exchange membrane;

[0027] Figure 6is a schematic diagram of the corresponding relationship between the sulfonation degree and ion exchange capacity of the membrane layer provided by the examples and comparative examples of the present application;

[0028] Figure 7 is a schematic diagram of the total thickness, open circuit voltage and energy conversion efficiency of the membrane of the charge gradient cation exchange membrane of Example 1, Example 3, Example 6 and Example 7 of the present application under 50 times salt difference;

[0029] Figure 8 is a graph of the power density and external resistance relationship of the charge gradient cation exchange membrane provided by the examples of the present application in 50 times different kinds of salt solution;

[0030] Figure 9 is a comparative diagram of the water absorption, thickness swelling and area swelling of the cation exchange membrane provided by Example 1 and Comparative Example 1 of the present application. DETAILED DESCRIPTION

[0031] In order to make the above-mentioned purposes, features and advantages of the present application more obvious and easy to understand, the specific embodiments of the present application will be described in detail below with reference to the accompanying drawings. It can be understood that the specific embodiments described herein are only used to explain the present application, but not to limit the present application. In addition, it should be noted that, in order to facilitate the description, only the parts related to the present application are shown in the drawings, and not all the structures. Based on the examples in the present application, all other examples obtained by those skilled in the art without creative labor are within the scope of protection of the present application.

[0032] In this paper, the "example" means that the specific features, structures or characteristics described in conjunction with the example can be included in at least one example of the present application. The phrase appears at various places in the specification does not necessarily refer to the same example, nor is it an independent or alternative example that is not mutually exclusive with other examples. Those skilled in the art explicitly and implicitly understand that the examples described herein can be combined with other examples.

[0033] Please refer to Figure 1 , Figure 1 is a schematic diagram of the structure of the charge gradient cation exchange membrane provided by the examples of the present application.

[0034] The first aspect of the present application provides a charge gradient cation exchange membrane for salt difference energy conversion, referring to Figure 1 The exchange membrane comprises a first membrane layer, a second membrane layer and an intermediate membrane layer arranged between the first membrane layer and the second membrane layer; the first membrane layer comprises a first sulfonated polyether ether ketone, and the second membrane layer comprises a second sulfonated polyether ether ketone; the sulfonation degree of the first sulfonated polyether ether ketone is greater than the sulfonation degree of the second sulfonated polyether ether ketone; the intermediate membrane layer comprises carbon nanotube-metal organic framework composite material.

[0035] In the embodiments of the present application, the charge gradient cation exchange membrane for salt gradient energy conversion has a first membrane layer, an intermediate membrane layer and a second membrane layer arranged in sequence to form a "sandwich" structure of the exchange membrane.

[0036] The first membrane layer comprises a first sulfonated polyether ether ketone, and the second membrane layer comprises a second sulfonated polyether ether ketone, and the sulfonation degree of the first sulfonated polyether ether ketone is higher than that of the second sulfonated polyether ether ketone. Polyether ether ketone (PEEK) is a semi-crystalline and highly stable polymer, which is low in price and has excellent thermal stability and chemical stability. The sulfonation of PEEK obtains sulfonated polyether ether ketone (SPEEK), which can improve the hydrophilicity and ion transport performance of PEEK while maintaining the original properties of PEEK. Because the sulfonic acid group (-SO3H) will dissociate in aqueous solution, the membrane layer will have a negative charge, thereby selectively allowing cations to pass through.

[0037] The first sulfonated polyether ether ketone of the first membrane layer has a high sulfonation degree, which can provide more negative charge sites and stronger attraction to cations, thereby effectively promoting the rapid migration of cations. The second sulfonated polyether ether ketone of the second membrane layer has a low sulfonation degree, and the negative charge sites of the second membrane layer are relatively few. This makes the cations not excessively aggregated after entering the second membrane layer from the intermediate membrane layer due to the strong charge attraction in the membrane, thereby ensuring the stable migration of the cations. In addition, the second membrane layer with low sulfonation degree can block the reverse osmosis of anions from the high salt concentration side to the low salt concentration side to some extent, thereby improving the selectivity of the exchange membrane and ensuring the efficient performance of the salt gradient energy conversion process.

[0038] By adjusting the difference in sulfonation degree between the first membrane layer (high sulfonation degree polyether ether ketone) and the second membrane layer (low sulfonation degree polyether ether ketone), a cation exchange membrane with a charge gradient is formed, which can reduce the charge polarization phenomenon. The first membrane layer with high sulfonation degree improves the selectivity of cations, and the second membrane layer with low sulfonation degree reduces the membrane resistance, thereby synergistically reducing the mass transfer resistance of cations.

[0039] The intermediate film layer comprises a carbon nanotube-metal organic framework composite material. The sulfonic acid groups in the sulfonated polyether ether ketone (SPEEK) have strong hydrophilicity and are easy to absorb water when in contact with an aqueous solution, thereby causing the film to swell. The metal organic framework material in the intermediate film layer has a regular pore structure and a large specific surface area, and can adsorb part of the water molecules, reducing the direct contact of the sulfonic acid groups with water, thereby inhibiting the swelling of the first film layer and the second film layer caused by the presence of the sulfonic acid groups, and enabling the cation exchange membrane to maintain stable structure and performance during the salinity gradient energy conversion process. Further, the carbon nanotube has excellent electrical properties and can form an efficient electron conduction channel. After the carbon nanotube and the metal organic framework are compounded, the two materials synergistically act, not only improving the electrical conductivity of the intermediate film layer itself, but also interacting with the first film layer and the second film layer on both sides to promote the migration of cations, thereby improving the electrical conductivity of the entire cation exchange membrane and ensuring efficient transmission of cations during the salinity gradient energy conversion process.

[0040] The intermediate film layer inhibits the swelling phenomenon, maintains the structural stability of the cation exchange membrane, and avoids performance degradation and shortened service life caused by swelling. At the same time, the improved electrical conductivity helps to reduce the membrane resistance and energy loss, enabling the salinity gradient energy conversion process to be carried out efficiently and stably. In addition, the carbon nanotube-metal organic framework composite material itself has good chemical stability and mechanical properties, and can withstand the erosion of the solution and various external forces during the long-term salinity gradient energy conversion process, thereby ensuring the long-term stable operation of the entire system.

[0041] The cation exchange membrane with the "sandwich" structure having a charge gradient is used for salinity gradient energy conversion in the embodiments of the present application, which is conducive to promoting the transmission of cations and thereby improving the energy conversion efficiency of the salinity gradient energy.

[0042] According to an embodiment of the present application, the sulfonation degree of the first sulfonated polyether ether ketone is in the range of 50% to 70%; and / or, the sulfonation degree of the second sulfonated polyether ether ketone is in the range of 50% to 70%.

[0043] In the embodiments of the present application, the sulfonation degree of the first sulfonated polyether ether ketone and / or the sulfonation degree of the second sulfonated polyether ether ketone is in the above range, so that the hydrophilicity of the first film layer and / or the second film layer is moderate. On the one hand, the appropriate hydrophilicity enables the first film layer and / or the second film layer to better contact with the aqueous solution, which is conducive to the transmission of cations in the membrane; on the other hand, it avoids excessive swelling of the membrane caused by too high hydrophilicity, ensures the structural stability of the membrane in the aqueous solution, and enables the membrane to maintain good performance in long-term use; and on the third hand, it avoids poor selectivity caused by too low sulfonation degree.

[0044] The sulfonation degree of the first sulfonated polyether ether ketone can be 50%, 55%, 56%, 57%, 59%, 62%, 65%, 68%, 70%, or the like, or a range formed by any two of the above values, such as 50% to 59%, 59% to 68%, 68% to 70%, 55% to 62%, 57% to 65%, and the like.

[0045] The sulfonation degree of the second sulfonated polyether ether ketone can be 50%, 54%, 55%, 56%, 58%, 61%, 64%, 67%, 70%, or the like, or a range formed by any two of the above values, such as 50% to 58%, 58% to 67%, 67% to 70%, 54% to 61%, 56% to 67%, and the like.

[0046] According to an embodiment of the present application, the carbon nanotube-metal organic framework composite material includes a carbon nanotube-zirconium aminophthalate composite material.

[0047] In the embodiment of the present application, the zirconium aminophthalate as a metal organic framework material has high chemical stability and can resist the corrosion of an electrolyte environment such as a salt solution. Meanwhile, the carbon nanotube also has good chemical stability. After the two are compounded, in the salt differential energy conversion process, in the face of different concentrations of salt solutions and other possible chemical substances, the structure and performance can be kept stable and are not prone to chemical reactions to cause material failure, thereby ensuring the long-term stable operation of the cation exchange membrane. The carbon nanotube-metal organic framework composite material is selected from the above materials, raw materials are abundant, preparation and optimization conditions are simple, and the material has a wide range of applications.

[0048] According to an embodiment of the present application, the thickness of the first membrane layer is in the range of 5 μm to 15 μm; and / or, the thickness of the second membrane layer is in the range of 5 μm to 15 μm; and / or, the thickness of the intermediate membrane layer is in the range of 5 μm to 15 μm.

[0049] In the embodiment of the present application, the thickness of the first membrane layer and / or the second membrane layer is in the above range, which ensures high ion exchange efficiency and is conducive to improving the salt differential energy conversion efficiency. If the membrane layer is too thin, it can not form a sufficient charge gradient and it is difficult to play a significant inhibitory role on charge polarization; and if the membrane layer is too thick, it can increase the path length of ion transmission and cause the ion migration resistance to increase.

[0050] The thickness of the intermediate membrane layer is in the above range, which effectively suppresses the swelling phenomenon of the first membrane layer and / or the second membrane layer caused by the presence of sulfonic acid groups.

[0051] It should be noted that the thickness of the first membrane layer, the second membrane layer, and the intermediate membrane layer can be the same or different, which is not limited in the present application.

[0052] The thickness of the first membrane layer can be 5 μm, 6 μm, 8 μm, 10 μm, 12 μm, 14 μm, 15 μm, or the range between any two of the above values, for example, 5 μm to 10 μm, 10 μm to 15 μm, 6 μm to 12 μm, etc.

[0053] The thickness of the second membrane layer can be 5 μm, 5.5 μm, 7 μm, 10 μm, 12 μm, 13 μm, 15 μm, or the range between any two of the above values, for example, 5 μm to 12 μm, 12 μm to 15 μm, 7 μm to 13 μm, etc.

[0054] The thickness of the intermediate membrane layer can be 5 μm, 6.5 μm, 9.5 μm, 11 μm, 12.5 μm, 13.9 μm, 15 μm, or the range between any two of the above values, for example, 5 μm to 11 μm, 11 μm to 15 μm, 9.5 μm to 13.9 μm, etc.

[0055] The second technical solution adopted in the present application is to provide a preparation method of the charge gradient cation exchange membrane for salt differential energy conversion according to any one of the above. Figure 2 , Figure 2 The flowchart of the preparation method of the charge gradient cation exchange membrane provided by the embodiments of the present application is shown in FIG. 1. The preparation method comprises the following steps:

[0056] S10: adding the first sulfonated polyether ether ketone into the first solvent to obtain a first casting solution.

[0057] According to an embodiment of the present application, the first solvent comprises at least one of N,N-dimethylformamide, N,N-dimethylacetamide, and N-methylpyrrolidone.

[0058] The first solvent is selected from the above-mentioned solvents, has good solubility for the sulfonated polyether ether ketone particles, and can form a uniform and stable first casting solution with the sulfonated polyether ether ketone. Moreover, the above-mentioned solvents have moderate volatility, and the solvent can gradually volatilize during the film forming process to form a solid film.

[0059] According to an embodiment of the present application, the mass percentage of the first sulfonated polyether ether ketone based on the total mass of the first casting solution comprises 15 wt% to 20 wt%.

[0060] The mass percentage of the first sulfonated polyether ether ketone in the total mass of the first casting solution is within the above range, which can make the prepared membrane have an appropriate amount of sulfonic acid groups, thereby providing appropriate ion exchange sites, promoting the transmission of cations in the membrane, and improving the ion exchange efficiency. If the content of the sulfonated polyether ether ketone is too low, the ion exchange sites of the membrane are insufficient, which cannot fully exchange with cations, thereby limiting the ion transmission rate and reducing the salinity gradient energy conversion efficiency. If the content of the sulfonated polyether ether ketone is too high, although the ion exchange sites increase, the ion concentration in the membrane may be too high, which causes mutual interference between ions, and is also not conducive to the efficient transmission of ions. The mass percentage of the first sulfonated polyether ether ketone in the total mass of the first casting solution can be 15 wt%, 16 wt%, 17 wt%, 18 wt%, 19 wt%, 20 wt%, or a range formed by any two of the above values, for example, 15 wt%-17 wt%, 17 wt%-20 wt%, 16 wt%-19 wt%, etc.

[0061] S20: adding the second sulfonated polyether ether ketone into the second solvent to obtain a second casting solution.

[0062] According to an embodiment of the present application, the second solvent includes at least one of N,N-dimethylformamide, N,N-dimethylacetamide, and N-methylpyrrolidone.

[0063] The second solvent has good solubility for the sulfonated polyether ether ketone particles, and can form a uniform and stable second casting solution with the sulfonated polyether ether ketone. Moreover, the above solvents have moderate volatility, and the solvent can gradually volatilize during the film forming process to form a solid film.

[0064] According to an embodiment of the present application, the mass percentage of the second sulfonated polyether ether ketone in the total mass of the second casting solution includes 15 wt%-20 wt%.

[0065] The mass percentage of the second sulfonated polyether ether ketone in the total mass of the second casting solution is within the above range, which has the same technical effect as that when the mass percentage of the first sulfonated polyether ether ketone is within 15 wt%-20 wt%, and will not be described here. The mass percentage of the second sulfonated polyether ether ketone in the total mass of the second casting solution can be 15 wt%, 16.5 wt%, 17.2 wt%, 18.5 wt%, 19.5 wt%, 20 wt%, or a range formed by any two of the above values, for example, 15 wt%-17.2 wt%, 17.2 wt%-20 wt%, 16.5 wt%-19.5 wt%, etc.

[0066] S30: adding the carbon nanotube-metal organic framework composite material into a third solvent to obtain a third casting solution.

[0067] According to an embodiment of the present application, the third solvent comprises at least one of N,N-dimethylformamide, N,N-dimethylacetamide, and N-methylpyrrolidone.

[0068] The third solvent has good solubility to the carbon nanotube-metal organic framework composite material, and can form a uniform and stable third casting solution with the carbon nanotube-metal organic framework composite material. Moreover, the above solvent has moderate volatility, and can gradually volatilize during the film forming process to form a solid film.

[0069] S40: sequentially coating the first casting solution, the third casting solution, and the second casting solution on the substrate to form a thin film pre-product.

[0070] S50: heat treating the thin film pre-product at a first preset temperature for a first preset time length to obtain a charge gradient cation exchange membrane; the charge gradient cation exchange membrane comprises a first film layer, a second film layer, and an intermediate film layer arranged between the first film layer and the second film layer, the first film layer is formed by the first casting solution, the second film layer is formed by the second casting solution, and the intermediate film layer is formed by the third casting solution; wherein the sulfonation degree of the first sulfonated polyether ether ketone is greater than the sulfonation degree of the second sulfonated polyether ether ketone.

[0071] In the embodiments of the present application, the first casting solution, the third casting solution, and the second casting solution are solidified by heat treating the thin film pre-product at a first preset temperature for a first preset time length through a thermal phase inversion method to form a cation exchange membrane with a “sandwich” structure.

[0072] In some embodiments, the thickness of the coating tool and the phase inversion temperature are changed to obtain charge gradient cation exchange membranes with different thicknesses.

[0073] According to an embodiment of the present application, the first preset temperature is in the range of 40°C to 80°C; and / or, the first preset time length is in the range of 6h to 24h.

[0074] In the embodiments of the present application, a lower phase inversion temperature leads to the formation of a large pore structure, and as the temperature increases, the pore size of the membrane is converted to the nanometer level, which is beneficial to improving the selectivity of the composite membrane to cations.

[0075] The first preset temperature can be 40°C, 45°C, 50°C, 65°C, 70°C, 75°C, 80°C, or a range formed by any two of the above values, such as 40°C to 65°C, 65°C to 80°C, 65°C to 75°C, etc.

[0076] The first preset time length is in the above range, so that the reaction in the membrane is more sufficient, the interaction between the molecular chains is more stable, thereby improving the mechanical strength and chemical stability of the membrane. At the same time, it is also beneficial to the formation of a more uniform distribution of ion exchange groups in the membrane, the optimization of the ion transmission channel of the membrane, and the improvement of the ion exchange efficiency and selectivity of the membrane.

[0077] The first preset time length can be 6h, 8h, 10h, 15h, 18h, 21h, 24h, or a range composed of any two of the above values, for example, 6h-15h, 15h-24h, 6h-18h, etc.

[0078] According to an embodiment of the present application, referring to Figure 3 , Figure 3 FIG. 1 is a flowchart of a preparation method of a carbon nanotube-metal organic framework composite material according to an embodiment of the present application. The preparation method of the carbon nanotube-metal organic framework composite material comprises the following steps:

[0079] S301: dispersing carbon nanotubes (CNT) in a fourth solvent to obtain a first dispersion liquid.

[0080] According to an embodiment of the present application, the fourth solvent comprises at least one of N,N-dimethylformamide, N,N-dimethylacetamide, and N-methylpyrrolidone.

[0081] The fourth solvent has good solubility to the carbon nanotubes, and can form a uniform and stable first dispersion liquid with the carbon nanotubes. Similarly, in the subsequent reaction, the carbon nanotubes (CNT) can be uniformly distributed in the reaction system, avoiding local accumulation.

[0082] According to an embodiment of the present application, the mass ratio of the carbon nanotube powder and the fourth solvent is (0.025-0.1):1.

[0083] The mass ratio of the carbon nanotube powder and the fourth solvent is in the above range, the carbon nanotubes are more uniformly dispersed in the fourth solvent, and a suitable mass ratio helps the carbon nanotubes to better combine with other materials in the subsequent preparation of the carbon nanotube-metal organic framework composite material and the like.

[0084] The mass ratio of the carbon nanotube powder and the fourth solvent can be 0.025:1, 0.028:1, 0.03:1, 0.038:1, 0.045:1, 0.05:1, 0.06:1, 0.075:1, 0.08:1, 0.09:1, 0.1:1, or a range composed of any two of the above values, for example, 0.025:1-0.045:1, 0.045:1-0.08:1, 0.08:1-0.1:1, 0.038:1-0.075:1, etc.

[0085] S302: adding zirconium chloride (ZrCl4) and acetic acid (HAc) into the first dispersion liquid to obtain a first mixed liquid.

[0086] ZrCl4 acts as a zirconium metal source and preliminarily coordinates with the surface groups of the carbon nanotubes (CNT) to form a pre-assembly structure.

[0087] HAc regulates the hydrolysis rate of Zr 4+ The HAc regulates the hydrolysis rate of Zr, inhibits too fast precipitation, and promotes uniform nucleation.

[0088] The oxygen-containing functional groups (-COOH, -OH) on the surface of the carbon nanotubes (CNT) in the first dispersion liquid can act as nucleation sites to induce the epitaxial growth of ZrCl4 and HAc along the surface of the carbon nanotubes (CNT).

[0089] According to an embodiment of the present application, the mass ratio of the carbon nanotube powder to ZrCl4 and HAc is (1-4):25:150.

[0090] The mass ratio of the carbon nanotube powder to ZrCl4 and HAc in the above range is beneficial to the better crystallization and nucleation of the metal organic framework composite, and the octahedral structure with high crystallinity and stable framework is obtained.

[0091] The mass ratio of the carbon nanotube powder to ZrCl4 and HAc can be 1:25:150, 1.5:25:150, 2:25:150, 2.6:25:150, 3:25:150, 4:25:150, etc., or a range composed of any two of the above values, such as 1:25:150-2.6:25:150, 2.6:25:150-4:25:150, 1.5:25:150-3:25:150, 2:25:150-4:25:150, etc.

[0092] S303: adding amino terephthalic acid (NH2-BDC) into the fifth solvent to obtain a second dispersion liquid.

[0093] Amino terephthalic acid is an important organic ligand for building metal organic frameworks. After being uniformly dispersed in the fifth solvent, it is beneficial to its full contact with ZrCl4 and other substances in the subsequent reaction and reaction, thereby forming metal organic frameworks with specific structure and performance.

[0094] According to an embodiment of the present application, the fifth solvent includes at least one of N,N-dimethylformamide, N,N-dimethylacetamide, and N-methylpyrrolidone.

[0095] According to an embodiment of the present application, the mass ratio of the amino terephthalic acid to ZrCl4 is (0.8-1.2):1.

[0096] The mass ratio of the amino terephthalic acid to ZrCl4 can be 0.8:1, 0.85:1, 0.9:1, 1:1, 1.2:1, etc., or a range composed of any two of the above values, such as 0.8:1-1:1, 1:1-1.2:1, 0.85:1-0.9:1, etc.

[0097] S304: mixing the first mixed solution and the second dispersion solution to obtain a precursor solution, and transferring the precursor solution to an autoclave to react at a second preset temperature for a second preset time duration, so as to obtain the carbon nanotube-metal organic framework composite material.

[0098] In the mixing process of the first mixed solution and the second dispersion solution, the autoclave provides a high-temperature and high-pressure closed reaction environment, which is conducive to accelerating the chemical reaction, making the coordination reaction between ZrCl4 and amino terephthalic acid more complete, and promoting the growth and crystallization of the metal organic framework. At the same time, the carbon nanotube and the metal organic framework can be better combined to form a stable composite structure.

[0099] The second preset temperature ranges from 120°C to 140°C, and the second preset time duration ranges from 24 hours to 36 hours.

[0100] By controlling the second preset temperature and the second preset time duration within the above ranges, the growth rate, crystal structure and morphology of the metal organic framework can be accurately controlled, thereby affecting the performance of the carbon nanotube-metal organic framework composite material.

[0101] In some embodiments, after the precursor solution is transferred to the autoclave and reacts at the second preset temperature for the second preset time duration in step S304, the reaction solution needs to be repeatedly centrifuged with DMF (N,N-dimethylformamide) and methanol, and then dried at 40°C to 80°C for 12 hours to 24 hours to obtain the carbon nanotube-metal organic framework composite material. Here, the DMF (N,N-dimethylformamide) is used to remove the unspent NH2-BDC and free Zr6O4(OH)4 clusters after the reaction. 4+ Methanol is used to mix with DMF, and methanol has stronger volatility, which can gradually replace DMF, facilitating subsequent drying; methanol can also dissolve part of the organic by-products (such as acetic acid derivatives), thereby improving the purity of the product; methanol can also wash and remove part of the residual unstable coordination DMF molecules, making the framework of the metal organic framework more solid.

[0102] According to an embodiment of the present application, the preparation method of the sulfonated polyether ether ketone particles in step S10 and / or step S20 comprises:

[0103] S101: A certain amount of polyether ether ketone (PEEK) powder is dissolved and dispersed in a certain volume of concentrated sulfuric acid, the reaction temperature is 40°C to 60°C, and the reaction time is controlled for 4 to 10 hours to obtain a sulfonated polyether ether ketone solution with different sulfonation degrees.

[0104] The sulfonation reaction is carried out in concentrated sulfuric acid, and the longer the reaction time, the higher the sulfonation degree. In the embodiments of the present application, the sulfonation degree needs to be controlled within a certain range, and too high sulfonation degree will cause the swelling property of the ion exchange membrane to be too high, and the performance of the ion exchange membrane to be poor.

[0105] The ratio of the mass of the polyether ether ketone (PEEK) powder to the volume of the concentrated sulfuric acid is 2:25 to 1:25.

[0106] S102: Obtain a sulfonated polyether ether ketone preform by an ice-water bath phase inversion method, and dry it at 60-80°C for 12-36 hours to obtain a sulfonated polyether ether ketone particle

[0107] Pour the sulfonated polyether ether ketone solution obtained in step S101 into an ice-water bath (0-5°C), and the temperature of the sulfonated polyether ether ketone solution is rapidly reduced, the concentration of sulfuric acid is diluted, so that the sulfonation reaction is terminated, and the sulfonation degree of the sulfonated polyether ether ketone is fixed.

[0108] The principle of the ice-water bath phase inversion method is: by rapidly changing the mixing state of the solvent (concentrated sulfuric acid) and the non-solvent (water), the sulfonated polyether ether ketone is precipitated from the solution to form solid particles. Among them, after water as a non-solvent is mixed with sulfuric acid, the solubility of sulfonated polyether ether ketone (SPEEK) is suddenly reduced, the polymer chain is aggregated, and the sulfonated polyether ether ketone (SPEEK) particles are precipitated.

[0109] Reference Figure 4 , Figure 4 is a schematic diagram of a salt difference energy power generation system provided by the embodiments of the present application, and the third technical solution adopted by the present application is: providing a salt difference energy power generation system, comprising: the charge gradient cation exchange membrane of any one of the above or the charge gradient cation exchange membrane prepared by the preparation method of any one of the above; silver / silver chloride electrode with salt bridge, salt solution with first concentration and salt solution with second concentration; wherein the charge gradient cation exchange membrane is arranged between the salt solution with the first concentration and the salt solution with the second concentration; the first concentration is less than the second concentration.

[0110] In the embodiments of the present application, the cations in the salt solution will diffuse from the salt solution with the second concentration to the salt solution with the first concentration. The cation exchange membrane only allows cations to pass through, wherein the cation exchange membrane has a charge gradient, which can improve the cation transmission rate while reducing the cation mass transfer resistance, thereby improving the energy conversion efficiency of the salt difference energy power generation system. On the side of the low-concentration salt solution, a reduction reaction occurs on the surface of the silver / silver chloride electrode, which accepts the cations conducted from the exchange membrane and generates electrons at the same time, and the electrons flow to the electrode on the side of the high-concentration salt solution through the external circuit. On the side of the high-concentration salt solution, an oxidation reaction occurs on the surface of the electrode, so that the cations in the salt solution obtain electrons to generate the corresponding metal or other substances, thereby forming a complete loop and realizing the conversion of salt difference energy to electric energy.

[0111] According to an embodiment of the present application, the first membrane layer of the charge gradient cation exchange membrane is arranged on the side of the intermediate membrane layer close to the salt solution with the first concentration; and the second membrane layer of the charge gradient cation exchange membrane is arranged on the side of the intermediate membrane layer close to the salt solution with the second concentration.

[0112] In the embodiment of the present application, the first sulfonated polyether ether ketone of the first membrane layer has a high sulfonation degree, strong hydrophilicity and large ion exchange capacity. Arranging it on the side close to the low-concentration (first concentration) salt solution is conducive to the rapid migration of cations migrated from the high-concentration salt solution into the low-concentration salt solution, provides a directional channel for the transmission of cations, and reduces the mass transfer resistance of the system. Due to the high sulfonation degree of the first membrane layer, more sulfonic acid groups can be provided as ion exchange sites, making it easier for cations to bind to sulfonic acid groups and pass through the first membrane layer, thereby improving the transmission rate of cations. The second sulfonated polyether ether ketone of the second membrane layer has a lower sulfonation degree and relatively weak ion exchange capacity, and is arranged on the side close to the high-concentration salt solution (second concentration). This can slow down the penetration speed of cations in the high-concentration salt solution to a certain extent. In this way, cations on the high-concentration side will not rush into the membrane too quickly, which helps to maintain the stability and gradient of ion transmission in the membrane, enabling ions to be more orderly transmitted from the high-concentration salt solution side to the low-concentration salt solution side, thereby improving the ion transmission efficiency and power generation performance.

[0113] The charge gradient of the cation exchange membrane cooperates with the concentration gradient of the salt solution to guide the directional movement of cations more efficiently, reduce the reverse diffusion and disordered movement of ions, thereby improving the selectivity and efficiency of ion transmission, and ultimately improving the power generation efficiency of the salt difference power generation system.

[0114] According to an embodiment of the present application, the first concentration is in the range of 0.001 mol·L -1 ~ 0.1 mol·L -1 ; and / or, the second concentration is in the range of 0.5 mol·L -1 ~ 1 mol·L -1 .

[0115] In the embodiment of the present application, the first concentration is in the above range, and the second concentration is in the above range, thereby forming a significant concentration difference between the two. According to the Nernst equation, a larger concentration difference can generate a larger chemical potential difference, providing a strong driving force for the diffusion of cations from the high-concentration (second concentration) salt solution to the low-concentration (second concentration) salt solution, prompting more cations to pass through the charge gradient cation exchange membrane, thereby increasing the ion current and improving the power generation efficiency.

[0116] According to an embodiment of the present application, the salt solution includes at least one of a potassium chloride solution, a sodium chloride solution and a lithium chloride solution.

[0117] In the embodiments of the present application, the salt solution selected from the above range has high chemical stability under general temperature, pH value and other conditions, and is not easy to decompose or undergo other chemical reactions. In the salt differential energy power generation system, it can withstand certain changes in operating conditions, such as temperature fluctuations and changes in solution concentration, and will not affect the power generation performance due to its own chemical changes, ensuring the reliability and long-term operation stability of the power generation system.

[0118] In order to make the technical problems, technical solutions and beneficial effects solved by the embodiments of the present application more clear, the following will be further described in detail in combination with the embodiments and drawings. Obviously, the described embodiments are only part of the embodiments of the present application, not all. The following description of at least one exemplary embodiment is actually only illustrative, but not as any limitation on the present application and its application. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative labor belong to the scope of protection of the present application.

[0119] The features and performances of the present application will be further described in detail in combination with the embodiments below.

[0120] Embodiment 1

[0121] (1) Preparation of the first sulfonated polyether ether ketone: 2g of PEEK powder was weighed and dispersed into 50mL of concentrated sulfuric acid, the reaction temperature was 50℃, the reaction time was 8 hours, and the solution after reaction was phase-inverted by ice water bath to generate SPEEK material, which was dried at 60℃ for 24 hours to obtain the first sulfonated polyether ether ketone with a sulfonation degree of 67.9%.

[0122] (2) Preparation of the second sulfonated polyether ether ketone: 2g of PEEK powder was weighed and dispersed into 50mL of concentrated sulfuric acid, the reaction temperature was 50℃, the reaction time was 6 hours, and the solution after reaction was phase-inverted by ice water bath to generate SPEEK material, which was dried at 60℃ for 24 hours to obtain the second sulfonated polyether ether ketone with a sulfonation degree of 59.9%.

[0123] (3) Preparation of carbon nanotube-metal organic framework composite material (CNT / UiO-66-NH2): 0.025 g of CNT powder was dispersed in 25 mL of DMF to obtain a first dispersion liquid, 0.63 g of ZrCl4 and 3.4 mL of acetic acid (HAc) were added to the first dispersion liquid respectively, and ultrasonic dispersion was performed for 30 minutes to obtain a first mixed liquid; 0.51 g of NH2-BDC was ultrasonically dispersed in 50 mL of DMF for 30 minutes to obtain a second dispersion liquid; the first mixed liquid was mixed with the first dispersion liquid, and ultrasonic mixing was performed for 60 minutes to obtain a precursor liquid; the precursor liquid was transferred to a hydrothermal kettle, and reaction was performed at 120°C for 24 hours; after reaction, the solution was repeatedly centrifuged with DMF and methanol to obtain a CNT / UiO-66-NH2 nanocomposite material, which was dried at 60°C for 12 hours to obtain a carbon nanotube-metal organic framework composite material.

[0124] (4) Preparation of a cation exchange membrane: 1.5 g of the first sulfonated polyether ether ketone was dissolved in 8.5 g of DMF to obtain a first casting solution with a content of 15 wt%;

[0125] 1.5 g of the second sulfonated polyether ether ketone was dissolved in 8.5 g of DMF to obtain a second casting solution with a content of 15 wt%;

[0126] 0.075 g of CNT / UiO-66-NH2 carbon nanotube-metal organic framework composite material was ultrasonically dispersed in 8.5 g of DMF to obtain a third casting solution;

[0127] The first casting solution, the third casting solution, and the second casting solution were successively cast on a glass plate by using an automatic film casting machine to form a thin film preform; the charge gradient cation exchange membrane comprises a first film layer, a second film layer, and an intermediate film layer arranged between the first film layer and the second film layer, the first film layer is formed by the first casting solution, the second film layer is formed by the second casting solution, and the intermediate film layer is formed by the third casting solution. The cation exchange membrane with a "sandwich" structure was cured by a thermal induced phase separation method, the thickness of the second film layer was 5 μm, the thickness of the intermediate film layer was 10 μm, the thickness of the first film layer was 15 μm, and the charge gradient cation exchange membrane was obtained by heat curing at 60°C for 12 hours.

[0128] (5) Application of cation exchange membrane in salt differential energy power generation: the salt type in the salt differential energy power generation system was a potassium chloride (KCl) solution, the low salt solution concentration was 0.01 mol·L -1 , the high salt solution concentration was 0.5 mol·L -1 , and the external resistance was 10000 Ω.

[0129] Example 2

[0130] Similar to Example 1, the difference is that:

[0131] The step (1) of this example is changed to: the reaction time is 7 hours, and a first sulfonated polyether ether ketone with a sulfonation degree of 63.5% is obtained.

[0132] The step (2) of this example is changed to: the reaction time is 4 hours, and a second sulfonated polyether ether ketone with a sulfonation degree of 56.2% is obtained.

[0133] Example 3

[0134] Similar to Example 1, except that:

[0135] The step (4) of this example is changed to: the second film layer thickness is 5 μm, the intermediate film layer thickness is 10 μm, and the first film layer thickness is 10 μm,

[0136] Example 4

[0137] Similar to Example 1, except that:

[0138] The step (1) of this example is changed to: the reaction time is 10 hours, and a first sulfonated polyether ether ketone with a sulfonation degree of 72.4% is obtained.

[0139] Example 5

[0140] Similar to Example 1, except that:

[0141] The step (5) of this example is changed to: the salt type in the salt differential energy power generation system is changed to a sodium chloride (NaCl) solution.

[0142] Example 6

[0143] Similar to Example 1, except that:

[0144] The step (4) of this example is changed to: the second film layer thickness is 20 μm, the intermediate film layer thickness is 5 μm, and the first film layer thickness is 10 μm.

[0145] Example 7

[0146] Similar to Example 1, except that:

[0147] The step (4) of this example is changed to: the second film layer thickness is 2 μm, the intermediate film layer thickness is 3 μm, and the first film layer thickness is 3 μm.

[0148] Comparative Example 1

[0149] Similar to Example 1, except that:

[0150] The step (4) of this comparative example is changed to: the second film layer thickness is 15 μm, the intermediate film layer is not set, and the first film layer thickness is 15 μm.

[0151] Comparative Example 2

[0152] Similar to Example 1, except that:

[0153] Step (2) of this comparative example was changed to a reaction time of 2 hours to obtain a second sulfonated polyether ether ketone with a degree of sulfonation of 35.4%.

[0154] Comparative Example 3

[0155] Similar to Example 1, except that:

[0156] Step (1) of this comparative example was changed to a reaction time of 6 hours to obtain a first sulfonated polyether ether ketone with a degree of sulfonation of 59.9%.

[0157] Comparative Example 4

[0158] Similar to Example 1, except that:

[0159] Step (1) of this comparative example was changed to a reaction time of 6 hours to obtain a first sulfonated polyether ether ketone with a degree of sulfonation of 59.9%.

[0160] Step (2) of this example was changed to a reaction time of 8 hours to obtain a second sulfonated polyether ether ketone with a degree of sulfonation of 67.9%.

[0161] The cation exchange membranes 1-11 obtained in Examples 1-7 and Comparative Examples 1-4 above were tested for performance, and the results are shown in Tables 1 and 2.

[0162] Test Method: The salt differential energy generation system in the steps of the above examples and comparative examples was a closed system, and the ion transport characteristics (I-V curve) of the prepared cation exchange membranes were tested by an electrochemical workstation (Chenhua CHI700E). The energy conversion efficiency of the cation exchange membranes was investigated by a method of connecting a benchtop multimeter and a variable resistor box, and the resistance (R L ) of the variable resistor box was 10000Ω.

[0163] The formulas involved in this part are shown in (1-8):

[0164]

[0165] The data that can be directly measured by the benchtop multimeter are E diff , E redox in formula (1), and I in formula (4).

[0166] The data that can be directly measured by the electrochemical workstation are Voc in formula (1).

[0167] The data that can be directly measured by the variable resistor box are R L in formula (4).

[0168] wherein, in formula (1), E diff , V OC , and E redox represent a permeation voltage (V), an open circuit voltage (V), and a redox potential (V), respectively;

[0169] In formula (2), t + represents a cation transference number, C H represents a high ion concentration (mol·L -1 ), and C L represents a low ion concentration (mol·L -1 ); R, T, Z, F, and γ represent a universal gas constant (J·mol -1 ·K -1 ), a temperature (K), a charge number, a Faraday constant (C·mol -1 ), and an ion activity coefficient, respectively; γ H represents a high ion activity coefficient, and γ L represents a low ion activity coefficient.

[0170] In formula (3), η represents an energy conversion efficiency (%);

[0171] In formula (4), P represents an output power density, I, R L , and S represent an ion current (A), an external resistance (Ω), and a test area (m -2 ), respectively;

[0172] In formula (5), WU% represents a water uptake, W wet and W dry represent weights (g) of a membrane under wet and dry conditions, respectively;

[0173] In formula (6), Thickness SR% represents a thickness swelling rate, T wet and T dry represent thicknesses (m) of a membrane under wet and dry conditions, respectively;

[0174] In formula (7), Area SR% represents an area swelling rate, A wet and A dry represent areas (m -2 ) of a membrane under wet and dry conditions, respectively;

[0175] In formula (8), IEC represents an ion exchange capacity (mmol·g -1 ), DS is a sulfonation degree of SPEEK, and M0=288 g·mol -1 .

[0176]

[0177]

[0178]

[0179] From the above Table 1 and Table 2, it can be seen that the osmotic voltage, ion selectivity, energy conversion efficiency and output power density of Examples 1-7 are all higher than those of Comparative Examples 1-4, which indicates that the use of the charge gradient cation exchange membrane having a "sandwich" structure and the sulfonation degree of the first sulfonated polyether ether ketone in the first membrane layer being greater than that of the second sulfonated polyether ether ketone in the second membrane layer is conducive to promoting the transport of cations, thereby improving the energy conversion efficiency of the salinity gradient energy.

[0180] Among them, the ion selectivity and energy conversion efficiency of Examples 1-3 are better than that of Example 4, which may be due to the sulfonation degree of the first sulfonated polyether ether ketone in the first membrane layer of Example 4 exceeding 70%, which easily leads to the swelling of the ion exchange membrane being high, and the performance of the ion exchange membrane being poor.

[0181] The ion selectivity and energy conversion efficiency of Examples 1-4 are better than that of Example 6, which may be due to the thickness of the membrane layer of Example 6 being thicker, which easily increases the path length of ion transport, leading to an increase in ion migration resistance.

[0182] The ion selectivity and energy conversion efficiency of Examples 1-4 are better than that of Example 7, which may be due to the thickness of the membrane layer of Example 7 being thinner, which is not easy to form a sufficient charge gradient, and it is difficult to play a significant inhibitory role on charge polarization.

[0183] The ion selectivity and energy conversion efficiency of Example 1 are better than those of Example 5, because Example 1 uses a salt solution KCl, and Example 5 uses a salt solution NaCl, and the diffusion coefficient of K ions is greater than that of Na ions, and the faster the same-valence cations diffuse, the higher the efficiency of charge separation.

[0184] Referring to Figure 5 , Figure 5 is a scanning electron microscope schematic diagram of the charge gradient cation exchange membrane provided by Example 1 of the present application, wherein, Figure 5 (a) of is a surface schematic diagram of the charge gradient cation exchange membrane, and the granular feeling of the middle membrane layer can be seen through the surface layer, which represents the successful introduction of the carbon nanotube-metal organic framework composite material layer; Figure 5 (b) of is a cross-sectional schematic diagram of the charge gradient cation exchange membrane, the upper layer is the first membrane layer with a higher sulfonation degree, and the lower layer is the second membrane layer with a lower sulfonation degree, which can confirm that the prepared ion exchange membrane has a multi-layer structure.

[0185] Referring to Figure 6 , Figure 6is a schematic diagram of the relationship between the sulfonation degree of the membrane layer and the ion exchange capacity provided by the embodiments and comparative examples of the present application. It can be concluded that the higher the sulfonation degree, the greater the ion exchange capacity.

[0186] Referring to Figure 7 , Figure 7 is a schematic diagram of the total thickness, open circuit voltage and energy conversion efficiency of the charge gradient cation exchange membrane of Example 1, Example 3, Example 6 and Example 7 of the present application at 50 times the salt difference. It can be concluded that the total thickness of the cation exchange membranes of Example 1 and Example 3 is within the range of the embodiments provided by the present application, and the open circuit voltage and energy conversion efficiency are higher than those of Example 6 and Example 7.

[0187] Referring to Figure 8 , Figure 8 is a schematic diagram of the power density and external resistance relationship of the charge gradient cation exchange membrane provided by the embodiments of the present application in 50 times different kinds of salt solution. It can be concluded that the power density of the salt difference power generation system using potassium chloride, sodium chloride and lithium chloride as the salt solution is superior to that of the system using calcium chloride and magnesium chloride.

[0188] Referring to Figure 9 , Figure 9 is a comparative diagram of the water absorption, thickness swelling and area swelling of the cation exchange membrane provided by Example 1 and Comparative Example 1 of the present application. It can be concluded that the water absorption, thickness swelling and area swelling of Example 1 are all lower than those of Comparative Example 1, indicating that the use of the intermediate membrane layer provided by the embodiments of the present application can alleviate the swelling of the membrane.

[0189] The above is only an embodiment of the present application, and does not limit the patent scope of the present application, and any equivalent structure or equivalent process transformation using the content of the specification and drawings of the present application, or direct or indirect application in other related technical fields, are also included in the patent protection scope of the present application.

Claims

1. A charge gradient cation exchange membrane for salinity power conversion, characterized by, The first membrane layer comprises a first sulfonated polyether ether ketone, the second membrane layer comprises a second sulfonated polyether ether ketone, the sulfonation degree of the first sulfonated polyether ether ketone is greater than the sulfonation degree of the second sulfonated polyether ether ketone, and the intermediate membrane layer comprises a carbon nanotube-metal organic framework composite material.

2. The charge gradient cation exchange membrane according to claim 1, wherein, The sulfonation degree of the first sulfonated polyether ether ketone is in a range of 50% to 70%, and / or the sulfonation degree of the second sulfonated polyether ether ketone is in a range of 50% to 70%.

3. The charge gradient cation exchange membrane of claim 1, wherein, The carbon nanotube-metal organic framework composite material comprises a carbon nanotube-zirconium aminobenzenedicarboxylate composite material.

4. The charge gradient cation exchange membrane of claim 1, wherein, The thickness of the first membrane layer is in a range of 5 μm to 15 μm, and / or the thickness of the second membrane layer is in a range of 5 μm to 15 μm, and / or the thickness of the intermediate membrane layer is in a range of 5 μm to 15 μm.

5. A method for producing a charge gradient cation exchange membrane for salt concentration energy conversion according to any one of claims 1 to 4, characterized by, The method comprises: adding the first sulfonated polyether ether ketone into a first solvent to obtain a first casting solution; adding the second sulfonated polyether ether ketone into a second solvent to obtain a second casting solution; adding the carbon nanotube-metal organic framework composite material into a third solvent to obtain a third casting solution; sequentially coating the first casting solution, the third casting solution and the second casting solution on a substrate to form a thin film preparation; heat treating the thin film preparation at a first preset temperature for a first preset time length to obtain the charge gradient cation exchange membrane; the charge gradient cation exchange membrane comprises a first membrane layer, a second membrane layer and an intermediate membrane layer arranged between the first membrane layer and the second membrane layer, the first membrane layer is formed by the first casting solution, the second membrane layer is formed by the second casting solution, and the intermediate membrane layer is formed by the third casting solution; The sulfonation degree of the first sulfonated polyether ether ketone is greater than the sulfonation degree of the second sulfonated polyether ether ketone.

6. The preparation method according to claim 5, characterized in that, The first preset temperature is in a range of 40°C to 80°C, and / or the first preset time length is in a range of 6 h to 24 h.

7. The production method according to claim 5 or 6, characterized by, The first solvent comprises at least one of N,N-dimethylformamide, N,N-dimethylacetamide and N-methylpyrrolidone, and / or the second solvent comprises at least one of N,N-dimethylformamide, N,N-dimethylacetamide and N-methylpyrrolidone, and / or the third solvent comprises at least one of N,N-dimethylformamide, N,N-dimethylacetamide and N-methylpyrrolidone.

8. The preparation method according to claim 5, characterized in that, The mass percentage of the first sulfonated polyether ether ketone is 15 wt% to 20 wt% based on the total mass of the first casting solution, and / or the mass percentage of the second sulfonated polyether ether ketone is 15 wt% to 20 wt% based on the total mass of the second casting solution.

9. The preparation method according to claim 5, characterized in that, The preparation method of the carbon nanotube-metal organic framework composite material comprises: dispersing carbon nanotubes in a fourth solvent to obtain a first dispersion liquid; adding ZrCl4 and HAc into the first dispersion liquid to obtain a first mixed liquid; adding amino terephthalic acid into a fifth solvent to obtain a second dispersion liquid; adding ZrCl4 and HAc into the first dispersion liquid to obtain a first mixed liquid; The first mixed solution and the second dispersion liquid are mixed to obtain a precursor liquid, and the precursor liquid is transferred to an autoclave to react at a second preset temperature for a second preset time length, so as to obtain the carbon nanotube-metal organic framework composite material.

10. The method of claim 9, wherein, The fourth solvent includes at least one of N,N-dimethylformamide, N,N-dimethylacetamide and N-methylpyrrolidone; and / or, the fifth solvent includes at least one of N,N-dimethylformamide, N,N-dimethylacetamide and N-methylpyrrolidone.

11. The production method according to claim 9 or 10, characterized by, The mass ratio of the carbon nanotube powder and the fourth solvent is (0.025-0.1):1; and / or, The mass ratio of the carbon nanotube powder, ZrCl4 and HAc is (1-4):25:150; and / or, The mass ratio of the amino terephthalic acid and the ZrCl4 is (0.8-1.2):

1.

12. A salinity power generation system, characterized by, Comprise: The charge gradient cation exchange membrane according to any one of claims 1-4 or prepared by the preparation method according to any one of claims 5-11; A silver / silver chloride electrode with a salt bridge, a salt solution with a first concentration, and a salt solution with a second concentration; The charge gradient cation exchange membrane is arranged between the salt solution with the first concentration and the salt solution with the second concentration; the first concentration is less than the second concentration.

13. The salt gradient power generation system according to claim 12, characterized in that, The first membrane layer of the charge gradient cation exchange membrane is arranged on one side of the intermediate membrane layer close to the salt solution with the first concentration; and the second membrane layer of the charge gradient cation exchange membrane is arranged on one side of the intermediate membrane layer close to the salt solution with the second concentration.

14. The salinity power generation system of claim 12, wherein, said first concentration is in the range of 0.001 mol L -1 ~ 0.1 mol L -1 ; and / or, said second concentration is in the range of 0.5 mol L -1 ~ 1 mol L -1 .

15. The salinity power generation system of claim 12, wherein, The salt solution includes at least one of a potassium chloride solution, a sodium chloride solution and a lithium chloride solution.

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