A ZIF / SPEEK composite diaphragm for a flow battery and a preparation method thereof

Abstract

The present invention discloses a ZIF / SPEEK composite diaphragm for a flow battery and a preparation method thereof. The composite diaphragm uses an alkali metal ion type sulfonated polyether ether ketone as a base film, and the sulfonation degree is 60% to 70%. The filler includes ZIF-8-X and ZIF-90-X. The ZIF-8-X material is ZIF-8-SO3H, ZIF-8-COOH or ZIF-8-OH, and the ZIF-90-X material is ZIF-90-SO3H, ZIF-90-COOH or ZIF-90-OH. The total doping amount of ZIF-8-X and ZIF-90-X is 1 to 5%. The present invention has excellent ion selectivity and ion conductivity, good chemical stability and mechanical properties, is applicable to various flow battery systems, and has good battery performance.

Description

Technical Field

[0001] The present invention belongs to the technical field of flow batteries, and relates to a ZIF / SPEEK composite separator for flow batteries and a preparation method thereof. Background Art

[0002] Due to the gradual reduction of traditional fossil energy resources and the resulting environmental pollution, renewable energy has attracted more and more attention. However, renewable energy is intermittent and unstable, and needs to be used in combination with large-scale energy storage technologies. Flow batteries have very broad application prospects in the field of large-scale energy storage due to their advantages such as long cycle life, flexible design, safety and reliability, and rapid response. The separator is one of the key materials of flow batteries, which has the functions of preventing cross-contamination of positive and negative electrolyte solutions and completing the circuit loop. An ideal composite separator needs to have the following characteristics: high proton conductivity, high ion selectivity, high chemical stability, low cost, etc. At present, the separator materials mainly used in domestic and foreign demonstration projects are the Nafion series membranes of DuPont Company in the United States (the Nafion membrane is a cationic composite membrane of polytetrafluoroethylene), which are expensive and have serious ion penetration, restricting the commercial application of flow batteries in the field of large-scale energy storage. Therefore, many researchers have conducted extensive research on composite separators for flow batteries. Although the performance of the separator in some aspects has been effectively improved, these separators still have the disadvantages of high cost, poor comprehensive performance, complex preparation process and inapplicability to multiple flow battery systems, and cannot meet the requirements of commercial application of flow batteries. Therefore, preparing a composite separator with low cost, long service life, excellent comprehensive performance and applicable to multiple systems is an urgent problem to be solved to promote the large-scale application of flow batteries. Summary of the Invention

[0003] In order to solve the above problems, the present invention provides a ZIF / SPEEK composite separator for flow batteries, which has excellent ion selectivity and ion conductivity, good chemical stability and mechanical properties, is applicable to multiple flow battery systems, and has good battery performance, solving the problems existing in the prior art.

[0004] Another object of the present invention is to provide a preparation method of a ZIF / SPEEK composite separator for flow batteries.

[0005] The technical solution adopted by the present invention is a ZIF / SPEEK composite diaphragm for a flow battery. The composite diaphragm uses an alkali metal ion type sulfonated polyether ether ketone as the base film, and the sulfonation degree is 60% - 70%; the filler includes ZIF-8-X and ZIF-90-X. The ZIF-8-X material is ZIF-8-SO3H, ZIF-8-COOH or ZIF-8-OH, and the ZIF-90-X material is ZIF-90-SO3H, ZIF-90-COOH or ZIF-90-OH. The total doping amount of ZIF-8-X and ZIF-90-X is 1% - 5%.

[0006] Further, it includes the following steps:

[0007] S1, prepare sulfonated polyether ether ketone with a sulfonation degree of 60% - 70%;

[0008] S2, prepare ZIF-8-X; ZIF-8-X is ZIF-8-SO3H, ZIF-8-COOH or ZIF-8-OH;

[0009] S3, prepare ZIF-90-X; the ZIF-90-X material is ZIF-90-SO3H, ZIF-90-COOH or ZIF-90-OH;

[0010] S4, add ZIF-8-X and ZIF-90-X into the organic solution of sulfonated polyether ether ketone, carry out light-shielding treatment under the constant temperature condition of 40 - 100°C, perform ultrasonic treatment, and configure a mixed solution with the total mass fraction of ZIF-8-X and ZIF-90-X being 1% - 5%. The mixed solution is formed into a film by the doctor blade method.

[0011] Further, in the said S4, the added masses of ZIF-8-X and ZIF-90-X are the same.

[0012] Further, in the said S1, to prepare sulfonated polyether ether ketone with a sulfonation degree of 60% - 70%, specifically: carry out a sulfonation reaction on polyether ether ketone powder and concentrated sulfuric acid. The mass-volume ratio of polyether ether ketone powder to concentrated sulfuric acid is 1g:6 - 15mL; stir and react under the constant temperature condition of 40 - 100°C for 1 - 12h; after the reaction ends, wash the reaction product with deionized water until it is neutral, dry it in the air, and then put it into an oven at 60 - 100°C and dry it for 24 - 48h to obtain dry sulfonated polyether ether ketone with a sulfonation degree of 60% - 70%.

[0013] Further, S1 also includes subjecting the dried sulfonated polyether ether ketone to ionization treatment, specifically: placing the dried sulfonated polyether ether ketone in a 0.5 - 2M alkali solution, soaking it at a constant temperature of 40 - 60°C for 2 - 4h, then placing it in deionized water for soaking and washing until neutral, air-drying, and then drying it at 60 - 100°C for 24 - 48h to obtain the dried potassium ion-type or sodium ion-type sulfonated polyether ether ketone.

[0014] Further, the ZIF-8-X and ZIF-90-X are obtained by grafting hydrophilic groups -SO3H, -COOH or -OH onto ZIF-8 or ZIF-90 through diazo reaction and diazo coupling reaction under room temperature conditions.

[0015] Further, in S2, the preparation method of ZIF-8-X: Disperse ZIF-8 in deionized water, then sequentially add a substance containing a hydrophilic group with the same amount of substance as ZIF-8 and 0.4wt% - 0.6wt% concentrated HCl, and the molar ratio of the substance containing a hydrophilic group to concentrated HCl is 1:2 - 1:4; stir the resulting suspension at room temperature, and then dropwise add a NaNO2 solution with the same amount of substance as ZIF-8; heat the mixture, after the reaction is completed, filter the suspension through a dialysis bag and wash it with deionized water; collect the residue in the dialysis bag and dry it to obtain; the substance containing a hydrophilic group is one of sulfamic acid, glycine or hydroxylamine, and ZIF-8-SO3H, ZIF-8-COOH or ZIF-8-OH are correspondingly obtained.

[0016] Further, in S3, the preparation method of ZIF-90-X: Disperse ZIF-90 in deionized water, then sequentially add a substance containing a hydrophilic group with the same amount of substance as ZIF-90 and 0.4wt% - 0.6wt% concentrated HCl, and the molar ratio of the substance containing a hydrophilic group to concentrated HCl is 1:2 - 1:4; stir the resulting suspension at room temperature, and then dropwise add a NaNO2 solution with the same amount of substance as ZIF-90; heat the mixture, after the reaction is completed, filter the suspension through a dialysis bag and wash it with deionized water; collect the residue in the dialysis bag and dry it to obtain; the substance containing a hydrophilic group is one of sulfamic acid, glycine or hydroxylamine, and ZIF-90-SO3H, ZIF-90-COOH or ZIF-90-OH are correspondingly obtained.

[0017] Further, in S4, the preparation of the organic solution of the alkali metal ion-type sulfonated polyether ether ketone: Add the dried alkali metal ion-type sulfonated polyether ether ketone to a high-boiling organic solvent, and the mass ratio of the alkali metal ion-type sulfonated polyether ether ketone to the high-boiling organic solvent is 1 / 5 - 1 / 32; under the heating condition of 40 - 120°C, stir for 0.5 - 1.5h.

[0018] Further, in S4, the high-boiling organic solvent is dimethyl sulfoxide solution, dimethylformamide, dimethylacetamide or pyrrolidone.

[0019] The beneficial effects of the present invention are as follows:

[0020] 1. The present invention prepares a ZIF / SPEEK composite membrane by mixing a ZIF material with high porosity and an SPEEK matrix with low cost and high proton conductivity. The variable pore size of the ZIF-8 material endows the composite membrane with adjustable ion sieving ability. The addition of the ZIF-90 material with a stable framework structure effectively reduces the breathing effect and pore blocking effect, further improves the ion selectivity of the membrane, and functionalizes the ZIF material. The grafted hydrophilic groups not only improve the compatibility between ZIF and the matrix, but also improve the proton conductivity of the membrane. By doping ZIF materials with high stability, high porosity and high hydrophilicity, while ensuring high proton conductivity of the membrane, the ion selectivity of the membrane is improved, and the chemical stability of the membrane is enhanced to improve the efficiency and cycle stability of the flow battery. This method for preparing the composite membrane will provide a new approach for the preparation of commercial composite membrane materials for flow batteries and is expected to become a method for preparing commercial composite membranes for flow batteries.

[0021] 2. The present invention alkali-treats the matrix material, and the prepared composite membrane is a sodium ion type or potassium ion type composite membrane, which can provide abundant ion-carrying groups, improve the ion conduction performance. The composite membrane has excellent ion conductivity, extremely low active ion permeability and excellent flow battery performance; and due to the controllability of the alkali treatment, the prepared composite membrane can adapt to redox flow batteries of various systems and has a very wide applicability to flow batteries, which helps to promote the commercial development of various flow batteries.

[0022] 3. In the preparation process of the composite membrane of the present invention, the raw materials used are low-cost and environmentally friendly, the equipment used is easy to operate, the preparation process is very simple, and the membrane preparation method (doctor blade method) used is conducive to the batch preparation of the membrane, which has the characteristics of industrial practicality and helps to promote the development of commercial composite membrane materials for flow batteries and the commercial production of flow batteries.

[0023] In summary, the addition of ZIF in the present invention can ensure the high ionic conductivity of the composite separator while reducing the permeability of active ions, improving ion selectivity, and enhancing the chemical stability and mechanical properties of the composite separator. The ZIF / SPEEK composite separator prepared by the doctor blade method in the present invention has high ionic conductivity, excellent ion selectivity, good chemical stability and mechanical properties, high capacity retention rate, high efficiency, low self-discharge rate, small polarization, low preparation cost, and is applicable to various flow battery systems in the flow battery. The ZIF / SPEEK composite separator has superior capacity retention rate and cycle stability in the application of flow batteries. Therefore, the composite separator prepared by this method is expected to be industrialized in various flow battery systems. Brief Description of the Drawings

[0024] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0025] Figure 1 It is the SEM diagram of Example 2.

[0026] Figure 2 It is the EDX diagram of Example 2.

[0027] Figure 3 It is the comparison diagram of charge-discharge curves of Example 2.

[0028] Figure 4 It is the comparison diagram of discharge capacities of Example 2.

[0029] Figure 5 It is the Coulomb efficiency diagram of Example 2.

[0030] Figure 6 It is the charge-discharge curve of Example 3.

[0031] Figure 7 It is the comparison diagram of discharge capacities of Example 3.

[0032] Figure 8 It is the Coulomb efficiency diagram of Example 3.

[0033] Figure 9 It is the cycle performance diagram of Example 4.

[0034] Figure 10 It is the cycle performance diagram of Example 5.

[0035] Figure 11 It is the cycle performance diagram of Example 6.

[0036] Figure 12 It is the cycle performance graph of Example 7.

[0037] Figure 13 It is the cycle performance graph of Example 8.

[0038] Figure 14 It is the cycle performance graph of Example 9.

[0039] Figure 15 It is the cycle performance graph of Example 10.

[0040] Figure 16 It is the cycle performance graph of Example 11.

[0041] Figure 17 It is the cycle performance graph of Example 12.

[0042] Figure 18 It is the cycle performance graph of Example 13.

[0043] Figure 19 It is the cycle performance graph of Example 15.

[0044] Figure 20 It is the discharge capacity comparison graph of Example 14.

[0045] Figure 21 It is the Coulomb efficiency graph of Example 14.

[0046] Figure 22 It is the energy efficiency graph of Example 14. Detailed implementation manners

[0047] Next, in combination with the embodiments of the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0048] Example 1

[0049] A ZIF / SPEEK composite membrane for a flow battery, the raw materials including: a base membrane, a ZIF-90-X material, and a ZIF-8-X material; wherein the base membrane is a sulfonated polyether ether ketone matrix material; the sulfonation degree of the sulfonated polyether ether ketone is 60% - 70%; the ZIF-8-X material is ZIF-8-SO3H, ZIF-8-COOH or ZIF-8-OH, and the doping amount is 1% - 5%; the ZIF-90-X material is ZIF-90-SO3H, ZIF-90-COOH or ZIF-90-OH, and the doping amount is 1% - 5%; the ZIF / SPEEK composite membrane is a sodium ion type or potassium ion type composite membrane, and its thickness range is 40 - 120 μm.

[0050] In some embodiments, the added masses of ZIF-8-X and ZIF-90-X are the same, each being half; the variable pore size of the ZIF-8 material endows the composite membrane with adjustable ion sieving ability, while the addition of the ZIF-90 material with a stable framework structure effectively reduces the breathing effect and pore blocking effect, further improving the ion selectivity of the membrane. The combined action of the two materials ensures high proton conductivity of the membrane while improving the ion selectivity of the membrane and enhancing the chemical stability of the membrane, so as to improve the efficiency and cycle stability of the flow battery.

[0051] Example 2

[0052] A preparation method of a ZIF / SPEEK composite membrane for a flow battery, comprising the following steps:

[0053] S1, Preparation of SPEEK:

[0054] Perform a sulfonation reaction on PEEK powder (particle size of 15 μm) and 98 wt% concentrated sulfuric acid, and the mass-volume ratio of the PEEK powder to the concentrated sulfuric acid is 1 / 13 (g / mL). React at 60 °C and a stirring speed of 700 r / min for 2 h. After the reaction, wash the reaction product with deionized water until neutral, dry it at room temperature, and then place it in an oven at 60 °C for drying for 24 h to obtain dry SPEEK. Measure the sulfonation degree of the SPEEK to be 60%. Place the dry SPEEK in a 1 M NaOH solution, soak it at a constant temperature of 40 °C for 4 h, then place it in deionized water for soaking and washing until neutral, dry it at room temperature (25 °C), and then dry it at 60 °C for 24 h to obtain dry SPEEK-Na.

[0055] When the composite membrane needs to be ionized, treating it after the composite membrane is prepared will damage the membrane structure and reduce the comprehensive performance of the membrane. However, pre-ionizing the membrane matrix material before preparing the membrane can effectively avoid the damage of the ionized solution to the membrane structure and effectively ensure the comprehensive performance of the membrane.

[0056] The SPEEK material used for preparing the separator is treated with NaOH solution. Sodium hydroxide is used as the supporting electrolyte in the zinc-iron flow battery. The separator prepared from the sodium-ionized SPEEK material is beneficial for the transmission of Na in the zinc-iron flow battery + .

[0057] S2. Preparation of ZIF-8: Weigh 5.190 g of 2-methylimidazole and dissolve it in 160 mL of methanol solution. Weigh 2.346 g of Zn(NO3)2·6H2O and dissolve it in 160 mL of methanol solution. After mixing the two solutions, stir magnetically at room temperature for 1 h, centrifuge and wash with methanol solution, and dry in a vacuum drying oven at 90 °C for 12 h to obtain the ZIF-8 material

[0058] Preparation of ZIF-8-SO3H:

[0059] Disperse ZIF-8 in deionized water, and then sequentially add sulfanilic acid and 0.5 wt% concentrated HCl. The molar ratio of sulfanilic acid to concentrated HCl is 1:4. Stir the obtained suspension at room temperature for 30 minutes, then dropwise add NaNO2 solution. The molar ratio of ZIF-8, sulfanilic acid, and NaNO2 added is 1:1:1. Heat the mixture at 60 °C. After the reaction is completed, filter the suspension through a 500 kDa dialysis bag and wash with deionized water. Collect the residue in the dialysis bag and dry it in an oven at 100 °C to obtain ZIF-8-SO3H

[0060] Preparation of ZIF-8-COOH:

[0061] Disperse ZIF-8 in deionized water, and then sequentially add glycine and 0.4 wt% concentrated HCl. The molar ratio of glycine to concentrated HCl is 1:2. Stir the obtained suspension at room temperature for 30 minutes, then dropwise add NaNO2 solution. The molar ratio of ZIF-8, glycine, and NaNO2 added is 1:1:1. Heat the mixture at 60 °C. After the reaction is completed, filter the suspension through a 500 kDa dialysis bag and wash with deionized water. Collect the residue in the dialysis bag and dry it in an oven at 100 °C to obtain ZIF-8-COOH

[0062] Preparation of ZIF-8-OH:

[0063] Disperse ZIF-8 in deionized water, then sequentially add hydroxylamine and 0.6 wt% concentrated HCl, with the molar ratio of hydroxylamine to concentrated HCl being 1:3; stir the resulting suspension at room temperature for 30 minutes, then add a NaNO2 solution dropwise, with the molar ratio of ZIF-8, hydroxylamine, and NaNO2 being 1:1:1; heat the mixture at 60 °C, and after the reaction is complete, filter the suspension through a 500 kDa dialysis bag and wash it with deionized water; collect the residue in the dialysis bag and dry it in an oven at 100 °C to obtain ZIF-8-OH.

[0064] S3. Preparation of ZIF-90: Dissolve 0.845 g of imidazole-2-carboxaldehyde in 50 ml of DMF, stir it in a 60 °C water bath for 1.5 h and filter it through a membrane (0.22 μm) to obtain an orange-yellow clear solution. Subsequently, dissolve 0.482 g of zinc acetate dihydrate (with a ligand to coordination center molar ratio of 4:1) in 50 ml of DMF, quickly pour the resulting zinc acetate solution into the imidazole-2-carboxaldehyde solution, and stir it at room temperature (25 °C) for 6 h to obtain a yellowish-white suspension. Then, centrifuge the resulting suspension and wash it repeatedly 3 times with DMF and methanol, and dry it overnight in a vacuum oven (60 °C) to obtain the ZIF-90 material.

[0065] Preparation of ZIF-90-SO3H:

[0066] Disperse ZIF-90 in deionized water, then sequentially add sulfanilic acid and 0.5 wt% concentrated HCl, with the molar ratio of sulfanilic acid to concentrated HCl being 1:4. Stir the resulting suspension at room temperature for 30 minutes. Then add a NaNO2 solution dropwise, with the molar ratio of ZIF-90, sulfanilic acid, and NaNO2 being 1:1:1. Heat the mixture at 60 °C, and after the reaction is complete, filter the suspension through a 500 kDa dialysis bag and wash it with deionized water. Collect the residue in the dialysis bag and dry it in an oven at 100 °C, and then obtain ZIF-90-SO3H.

[0067] Preparation of ZIF-90-COOH:

[0068] Disperse ZIF-90 in deionized water, then sequentially add glycine and 0.4 wt% concentrated HCl, with the molar ratio of glycine to concentrated HCl being 1:2; stir the resulting suspension at room temperature for 30 minutes, then add a NaNO2 solution dropwise, with the molar ratio of ZIF-90, glycine, and NaNO2 being 1:1:1; heat the mixture at 60 °C, and after the reaction is complete, filter the suspension through a 500 kDa dialysis bag and wash it with deionized water; collect the residue in the dialysis bag and dry it in an oven at 100 °C to obtain ZIF-90-COOH.

[0069] Preparation of ZIF-90-OH:

[0070] Disperse ZIF-90 in deionized water, then sequentially add hydroxylamine and 0.6 wt% concentrated HCl. The molar ratio of hydroxylamine to concentrated HCl is 1:3. Stir the resulting suspension at room temperature for 30 minutes, and then add a NaNO2 solution dropwise. The molar ratio of ZIF-90, hydroxylamine, and NaNO2 added is 1:1:1. Heat the mixture at 60 °C. After the reaction is completed, filter the suspension through a 500 kDa dialysis bag and wash it with deionized water. Collect the residue in the dialysis bag and dry it in an oven at 100 °C to obtain ZIF-90-OH.

[0071] S4. Preparation of ZIF / SPEEK-Na composite membrane:

[0072] (1) Preparation of SPEEK-Na solution: Add 1.7 g of dry SPEEK-Na to 50 mL of dimethyl sulfoxide solution (the mass ratio of SPEEK to the high-boiling organic solvent dimethyl sulfoxide is 1:5), and stir at 60 °C for 1.0 h under heating conditions.

[0073] (2) Add 0.025 g of ZIF-8-SO3H and 0.025 g of ZIF-90-SO3H to the mixed solution in step (1), perform light-shielding treatment, and stir at a constant temperature of 60 °C for 3 h, then perform ultrasonic treatment to prepare a ZIF / SPEEK-Na mixed solution with a ZIF mass percentage of 3%. Among them, the mass percentages of ZIF-8-SO3H and ZIF-90-SO3H are 1.5% respectively.

[0074] The grafted hydrophilic groups (-SO3H) improve the compatibility between ZIF and the matrix, and these groups are conducive to proton transport, further improving the proton conductivity of the membrane. The grafted hydrophilic groups X of ZIF-8-X and ZIF-90-X can be different.

[0075] (3) Pour the ZIF / SPEEK-Na mixed solution obtained in step (2) onto a horizontal glass plate of a doctor blade heating coater, and form a film by the doctor blade method. Dry it at 80 °C for 10 h and then at 100 °C for 5 h to obtain a ZIF / SPEEK-Na composite membrane. In this composite ion-selective membrane, the content of ZIF is 3 wt%. The doctor blade method is selected, which effectively avoids the problems of low preparation efficiency and large equipment floor area of the traditional solution casting method, improves the production yield of the membrane preparation, thereby reducing the preparation cost, and can effectively promote the industrialization process.

[0076] In this example, the obtained ZIF / SPEEK-Na composite diaphragm has a thickness of 65 μm. The composite diaphragm is uniform and dense in texture, without the phenomenon of ZIF particle dissolution, and has good mechanical properties. It is assembled into a zinc-iron redox flow battery for battery testing.

[0077] Assembly of zinc-iron redox flow battery:

[0078] (1) Preparation of positive electrolyte: Prepare a ferricyanide solution with deionized water, and add a supporting electrolyte to prepare a positive iron solution. The ferricyanide is sodium ferrocyanide with a molar concentration of 0.1 M, and the supporting electrolyte is sodium hydroxide with a molar concentration of 1 M.

[0079] (2) Preparation of negative electrolyte: Prepare an aqueous zinc salt solution with deionized water, and add a supporting electrolyte to prepare a negative zinc solution. The zinc salt is zinc bromide with a molar concentration of 0.2 M, and the supporting electrolyte is sodium hydroxide with a molar concentration of 4 M.

[0080] (3) Preparation of diaphragm: Immerse the ZIF-8-X / SPEEK-Na composite diaphragm in 1 M NaCl;

[0081] (4) Use graphite felt or carbon felt as the positive and negative electrodes of the battery;

[0082] (5) Assemble the prepared positive electrolyte, negative electrolyte, diaphragm, positive and negative electrodes into a zinc-iron flow battery.

[0083] The relevant performance data of this example are as follows:

[0084] Figure 1 is the scanning electron microscope image of the ZIF / SPEEK-Na composite diaphragm, Figure 2 is the EDX spectrum of the ZIF / SPEEK-Na composite diaphragm. As Figure 1 、 Figure 2 shown, the ZIF material has been successfully incorporated into the ZIF / SPEEK-Na composite diaphragm, and the ZIF material can be evenly distributed in the ZIF / SPEEK-Na composite diaphragm.

[0085] Figure 3 is the comparison chart of charge-discharge curves of zinc-iron flow batteries equipped with Nafion 212 membrane and ZIF-8 / SPEEK-Na composite diaphragm. As Figure 3 shown, at room temperature, compared with the zinc-iron flow battery equipped with Nafion 212 membrane, the zinc-iron flow battery equipped with this ZIF / SPEEK-Na composite diaphragm has less polarization. As Figure 4 shown, the capacity of the zinc-iron flow battery equipped with Nafion 212 membrane decays rapidly, while the zinc-iron flow battery equipped with ZIF / SPEEK-Na composite diaphragm has a higher capacity retention rate, and asFigure 5 As shown, it can operate stably at a relatively high Coulombic efficiency, indicating that the addition of ZIF-8 improves the ionic conductivity while enhancing the ionic selectivity and chemical stability of the separator. At the same time, the cost of this composite membrane is much lower than that of Nafion 212 membrane. The ZIF / SPEEK-Na composite separator prepared in this example has good prospects for industrial application in zinc-iron flow batteries.

[0086] Example 3

[0087] The difference from Example 2 is that:

[0088] The ZIF / SPEEK-K composite separator used in this example conducts potassium ions. Among them, in S1, the dry SPEEK is ionized with a 1M KOH solution. Potassium chloride is used as the supporting electrolyte in the iron-sulfur flow battery. The separator prepared from the potassium-ionized SPEEK material is beneficial for the transmission of K + ;

[0089] Weigh 0.025 g of ZIF-8-SO3H and 0.025 g of ZIF-90-SO3H and add them to the SPEEK-K mixed solution. The potassium-ionized ZIF / SPEEK-K composite separator with a ZIF mass fraction of 3% is prepared by using the remaining steps in Example 2. Among them, the mass percentages of ZIF-8-SO3H and ZIF-90-SO3H are 1.5% respectively.

[0090] In this example, the obtained ZIF / SPEEK-K composite separator has a thickness of 65 μm. The composite ion exchange membrane has a uniform texture and no phenomenon of ZIF particle dissolution. It is installed in an iron-sulfur redox flow battery for testing.

[0091] Assemble an iron-sulfur redox flow battery:

[0092] (1) Preparation of the positive electrolyte: Prepare a ferricyanide solution with deionized water, and add a supporting electrolyte to prepare a positive iron solution. Among them, the ferricyanide is potassium ferricyanide with a molar concentration of 0.1 M, and the supporting electrolyte is potassium chloride with a molar concentration of 2 M.

[0093] (2) Preparation of the negative electrolyte: Prepare an aqueous sulfide solution with deionized water, and add a supporting electrolyte to prepare a negative sulfur solution. Among them, the sulfide is potassium sulfide with a molar concentration of 2 M, and the supporting electrolyte is potassium chloride with a molar concentration of 1 M.

[0094] (3) Preparation of the separator: Immerse the ZIF / SPEEK-K composite separator in 1M KCl;

[0095] (4) Use graphite felt or carbon felt as the positive and negative electrodes of the battery;

[0096] (5) Assemble the prepared positive electrolyte, negative electrolyte, separator, positive and negative electrodes into an iron-sulfur flow battery.

[0097] The relevant performance data of this example are as follows:

[0098] Figure 6 Figure for comparing charge-discharge curves of an iron-sulfur flow battery equipped with a Nafion 212 membrane and a ZIF / SPEEK-K composite separator. As Figure 3 shown, at room temperature, compared with the iron-sulfur flow battery equipped with a Nafion 212 membrane, the iron-sulfur flow battery equipped with this ZIF / SPEEK-K composite separator has less polarization. As Figure 7 shown, the capacity of the iron-sulfur flow battery equipped with a Nafion 212 membrane decays rapidly, while the iron-sulfur flow battery equipped with a ZIF / SPEEK-K composite separator has a higher capacity retention rate. And as Figure 8 shown, it can operate stably at a higher Coulomb efficiency, indicating that the addition of ZIF improves the ion conductivity while enhancing the ion selectivity and chemical stability of the separator. At the same time, the cost of this composite membrane is much lower than that of the Nafion 212 membrane. The ZIF / SPEEK-K composite separator prepared in this example has good prospects for industrial application in iron-sulfur flow batteries.

[0099] Example 4:

[0100] The difference from Example 2 is that:

[0101] In this example, the SPEEK used has a higher sulfonation degree. It reacts for 2.5 h at 60 °C and a stirring speed of 700 r / min to obtain SPEEK with a sulfonation degree of 70% (under other unchanged conditions, increasing the stirring time can increase the reaction time between PEEK and concentrated sulfuric acid and improve the grafting rate of sulfonic acid groups on PEEK. The sulfonation degree of SPEEK can be adjusted to 70% by controlling the extension of time). The ZIF / SPEEK-Na composite separator is prepared by using the remaining steps in Example 2.

[0102] In this example, the obtained ZIF / SPEEK-Na composite separator has a thickness of 75 μm, the composite separator has a uniform and dense texture, no ZIF particle dissolution phenomenon, and good mechanical properties. It is installed in a zinc-iron redox flow battery for testing.

[0103] Figure 9 Figure for the cycle performance of a zinc-iron flow battery equipped with the ZIF / SPEEK-Na composite separator prepared in this example. As Figure 9As shown, the zinc-iron flow battery equipped with the ZIF / SPEEK-Na composite separator prepared in this example can stably cycle at a high Coulomb efficiency and maintain a high capacity retention rate, indicating that the ZIF / SPEEK-Na composite separator prepared in this example has high ion selectivity and can stably cycle in the zinc-iron flow battery.

[0104] Example 5:

[0105] The difference from Example 3 is that:

[0106] The SPEEK used in this example has a higher sulfonation degree. It reacts for 2.5 h under the conditions of 60 °C and a stirring speed of 700 r / min to obtain SPEEK with a sulfonation degree of 70%. The ZIF / SPEEK-K composite separator is prepared by using the remaining steps in Example 3.

[0107] In this example, the obtained ZIF / SPEEK-K composite separator has a thickness of 75 μm. The composite separator is uniform and dense in texture, without the phenomenon of ZIF particle dissolution, and has good mechanical properties. It is installed in an iron-sulfur redox flow battery for testing.

[0108] Figure 10 Figure Figure 10 As shown, the iron-sulfur flow battery equipped with the ZIF / SPEEK-K composite separator prepared in this example can stably cycle at a high Coulomb efficiency and maintain a high capacity retention rate, indicating that the ZIF / SPEEK-K composite separator prepared in this example has high ion selectivity and can stably cycle in the iron-sulfur flow battery.

[0109] Example 6:

[0110] The difference from Example 2 is that:

[0111] In the ZIF / SPEEK mixed solution prepared in this example, the mass percentage of ZIF added is 1% (0.017 g of ZIF-8-SO3H and 0.017 g of ZIF-90-SO3H), and the ZIF / SPEEK-Na composite separator with a ZIF content of 1 wt% is obtained. It is installed in a zinc-iron redox flow battery for testing. The mass percentages of ZIF-8-SO3H and ZIF-90-SO3H are 0.5% and 0.5% respectively.

[0112] Figure 11 Figure Figure 11As shown, the zinc-iron flow battery equipped with the ZIF / SPEEK-Na composite separator prepared in this example can stably cycle at a high Coulombic efficiency and maintain a high capacity retention rate, indicating that the ZIF / SPEEK-Na composite separator prepared in this example has high ion selectivity and can stably cycle in the zinc-iron flow battery.

[0113] Example 7:

[0114] The difference from Example 3 is as follows:

[0115] For the ZIF / SPEEK mixed solution prepared in this example, the mass percentage of ZIF added is 1% (0.0085 g of ZIF-8-SO3H and 0.0085 g of ZIF-90-SO3H), and a ZIF / SPEEK-K composite separator with a ZIF content of 1 wt% is obtained. It is installed in an iron-sulfur redox flow battery for testing. Among them, the mass percentages of ZIF-8-SO3H and ZIF-90-SO3H are 0.5% and 0.5% respectively.

[0116] Figure 12 Figure for the cycling performance of the iron-sulfur flow battery equipped with the ZIF / SPEEK-K composite separator prepared in this example, as Figure 12 As shown, the iron-sulfur flow battery equipped with the ZIF / SPEEK-K composite separator prepared in this example can stably cycle at a high Coulombic efficiency and maintain a high capacity retention rate, indicating that the ZIF / SPEEK-K composite separator prepared in this example has high ion selectivity and can stably cycle in the iron-sulfur flow battery.

[0117] Example 8:

[0118] The difference from Example 2 is as follows:

[0119] For the ZIF / SPEEK mixed solution prepared in this example, the mass percentage of ZIF added is 5% (0.0042 g of ZIF-8-SO3H and 0.0042 g of ZIF-90-SO3H), and a ZIF / SPEEK-Na composite separator with a ZIF content of 5 wt% is obtained. It is installed in a zinc-iron redox flow battery for testing. Among them, the mass percentages of ZIF-8-SO3H and ZIF-90-SO3H are 0.25% and 0.25% respectively.

[0120] Figure 13 Figure for the cycling performance of the zinc-iron flow battery equipped with the ZIF / SPEEK-Na composite separator prepared in this example, as Figure 13As shown, the zinc-iron redox flow battery equipped with the ZIF / SPEEK-Na composite separator prepared in this example can stably cycle at a high Coulombic efficiency and maintain a high capacity retention rate, indicating that the ZIF / SPEEK-Na composite separator prepared in this example has high ion selectivity and can stably cycle in the zinc-iron redox flow battery.

[0121] Example 9:

[0122] The difference from Example 3 is as follows:

[0123] For the ZIF / SPEEK mixed solution prepared in this example, the mass percentage of ZIF added is 5% (0.0042 g of ZIF-8-SO3H and 0.0042 g of ZIF-90-SO3H), and the ZIF / SPEEK-K composite separator with a ZIF content of 5 wt% is obtained. It is installed in an iron-sulfur redox flow battery for testing. Among them, the mass percentages of ZIF-8-SO3H and ZIF-90-SO3H are 0.25% and 0.25% respectively.

[0124] Figure 14 Figure 13 is the cycle performance diagram of the iron-sulfur redox flow battery equipped with the ZIF / SPEEK-K composite separator prepared in this example. As Figure 14 shown, the iron-sulfur redox flow battery equipped with the ZIF / SPEEK-K composite separator prepared in this example can stably cycle at a high Coulombic efficiency and maintain a high capacity retention rate, indicating that the ZIF / SPEEK-K composite separator prepared in this example has high ion selectivity and can stably cycle in the iron-sulfur redox flow battery.

[0125] Example 10:

[0126] The difference from Example 2 lies in the preparation of ZIF-8-COOH and ZIF-90-COOH:

[0127] 0.5 g of the original ZIF-8 was immersed in 200 g of HNO3 / H2SO4 solution (volume ratio 1:3), and then ultrasonic treatment was carried out for 1 hour. The suspension was refluxed in a water bath at 80 °C for 12 h, then diluted with deionized water, filtered through a 0.22 μm membrane to neutrality, and dried in a vacuum oven at 40 °C for 12 h to obtain the ZIF-8-COOH material.

[0128] 0.5 g of the original ZIF-90 was immersed in 200 g of HNO3 / H2SO4 solution (volume ratio 1:3), and then ultrasonic treatment was carried out for 1 hour. The suspension was refluxed in a water bath at 80 °C for 12 h, then diluted with deionized water, filtered through a 0.22 μm membrane to neutrality, and dried in a vacuum oven at 40 °C for 12 h to obtain the ZIF-90-COOH material.

[0129] The ZIF / SPEEK mixed solution prepared in this example, with the added ZIF being ZIF-8-COOH material and ZIF-90-COOH, yielded the ZIF / SPEEK-Na composite separator. The ZIF / SPEEK-Na composite separator prepared in this example was installed in a zinc-iron redox flow battery for testing.

[0130] Figure 15 Figure for the cycling performance of the zinc-iron flow battery equipped with the ZIF / SPEEK-Na composite separator prepared in this example, as Figure 16 shown, the zinc-iron flow battery equipped with the ZIFX / SPEEK-Na composite separator prepared in this example can stably cycle at a relatively high Coulombic efficiency and maintain a relatively high capacity retention rate, indicating that the ZIF / SPEEK-Na composite separator prepared in this example has high ion selectivity and can stably cycle in a zinc-iron flow battery.

[0131] Example 11:

[0132] The difference from Example 3 lies in the preparation of ZIF-8-OH and ZIF-90-OH:

[0133] Disperse 1 g of the original ZIF-8 in 50 mL of an aqueous potassium hydroxide solution (2.0 M). After ultrasonic treatment for 1 hour, transfer the suspension to the Teflon liner of a stainless-steel reaction autoclave and heat it at 180 °C for 2 hours. Then dilute it with deionized water, filter it through a 0.22 μm membrane to neutrality, and dry it in a vacuum oven at 40 °C for 12 h to obtain the ZIF-8-OH material.

[0134] Disperse 1 g of the original ZIF-90 in 50 mL of an aqueous potassium hydroxide solution (2.0 M). After ultrasonic treatment for 1 hour, transfer the suspension to the Teflon liner of a stainless-steel reaction autoclave and heat it at 180 °C for 2 hours. Then dilute it with deionized water, filter it through a 0.22 μm membrane to neutrality, and dry it in a vacuum oven at 40 °C for 12 h to obtain the ZIF-90-OH material.

[0135] The ZIF / SPEEK mixed solution prepared in this example, with the added ZIF being ZIF-8-OH and ZIF-90-OH materials, yielded the ZIF / SPEEK-K composite separator. The ZIF / SPEEK-K composite separator prepared in this example was installed in an iron-sulfur redox flow battery for testing.

[0136] Figure 16 Figure for the cycling performance of the iron-sulfur flow battery equipped with the ZIF / SPEEK-K composite separator prepared in this example, as Figure 16As shown, the iron-sulfur flow battery equipped with the ZIF / SPEEK-K composite separator prepared in this embodiment can stably cycle at a high Coulombic efficiency and maintain a high capacity retention rate, indicating that the ZIF / SPEEK-K composite separator prepared in this embodiment has high ion selectivity and can stably cycle in the iron-sulfur flow battery.

[0137] In Examples 6-11, the thickness of the prepared ZIF-SPEEK composite separator was the same as that in Example 3, and the thickness of the obtained ZIF-SPEEK-K composite separator was 65 μm.

[0138] In the embodiments of the present invention, the mass ratio of SPEEK to high-boiling organic solvent is 1 / 5 to 1 / 30, and stirring is carried out for 0.5 to 1.5 h under the condition of constant temperature at 40 to 120 °C, which affects the sulfonation degree of SPEEK, material cost, preparation process cost, equipment cost, and safety hazards. If the mass ratio of SPEEK to high-boiling organic solvent is less than 1 / 5, the initially added PEEK cannot be dispersed in concentrated sulfuric acid, and PEEK cannot react with concentrated sulfuric acid, which will affect the sulfonation degree of SPEEK. In addition, due to the inability of PEEK to be dispersed, PEEK forms clusters in concentrated sulfuric acid. When the SPPEK solution mixed with concentrated sulfuric acid is poured into deionized water, the prepared SPEEK contains a large amount of PEEK powder lumps and even cannot be formed. If the mass ratio of SPEEK to high-boiling organic solvent is greater than 1 / 30, too much concentrated sulfuric acid is used. First, a large amount of concentrated sulfuric acid will be wasted, the material cost will be higher, and the safety hazard will be higher. Moreover, the acid content in deionized water will be higher, requiring higher processes and equipment for subsequent water recovery treatment, higher usage of deionized water, and further increased costs.

[0139] Under the condition that other temperatures remain unchanged, temperature can affect, and even control, the degree of sulfonation reaction of PEEK to a certain extent. When the sulfonation temperature decreases, in order to ensure the sulfonation degree of SPEEK, the reaction time of PEEK with concentrated sulfuric acid needs to be increased, that is, the reaction time is increased. When the sulfonation temperature increases, in order to control the sulfonation degree, the reaction time of PEEK with concentrated sulfuric acid needs to be reduced, that is, the stirring time is reduced. And the longer the stirring time of PEEK in concentrated sulfuric acid and the higher the stirring temperature, the higher the requirements for the reaction equipment, and the safety hazards, process costs such as electricity consumption, and labor custody costs will all increase accordingly. Therefore, while ensuring the sulfonation degree, it is necessary to control the temperature and time of the reaction of PEEK in sulfuric acid within a certain range.

[0140] In the embodiments of the present invention, ZIF-8-OH and ZIF-90-OH are added to a mixed solution of SPEEK and a high-boiling organic solvent, and stirred in the dark under a constant temperature condition of 40 to 100 °C. The stirring temperature will affect the dissolution of SPEEK solid, the dispersion of ZIF materials in the high-boiling organic solvent, and the time required for the successful preparation of adding ZIF-8-OH and ZIF-90-OH to the mixed solution of SPEEK and the high-boiling organic solvent. When the stirring temperature is less than 40 °C, the dissolution and dispersion rates of the three materials in the organic solvent are slower, and the stirring time needs to be increased, thereby increasing the process cost and labor cost. When the stirring temperature is greater than 100 °C, the organic solvent is likely to volatilize, which easily leads to an increase in the viscosity of the mixed solution and affects the subsequent diaphragm preparation effect (when the viscosity of the mixed solution is too high, it is easy to cause the situation of being unable to be pushed away, uneven thickness, and formation of holes during diaphragm preparation). The volatilization of the organic solvent requires further equipment for recovering and treating the organic solvent, further increasing the cost.

[0141] Example 12,

[0142] A preparation method of a ZIF / SPEEK composite diaphragm for a flow battery, which is different from Example 2 in steps S1 and S4.

[0143] Among them, S1, Preparation of SPEEK:

[0144] Perform a sulfonation reaction on PEEK powder (particle size of 15 μm) and 98 wt% concentrated sulfuric acid. The mass-volume ratio of PEEK powder to concentrated sulfuric acid is 1 / 15 (g / mL). React at 40 °C and a stirring speed of 700 r / min for 12 h. After the reaction, wash the reaction product with deionized water until neutral, dry it at room temperature, and then put it in an 80 °C oven and dry it for 36 h to obtain dry SPEEK. Measure the sulfonation degree of SPEEK to be 60%. Immerse the dry SPEEK in a 0.5 M NaOH solution and soak it for 3 h under a constant temperature condition of 50 °C, then place it in deionized water and soak and wash it until neutral, dry it at room temperature, and then dry it at 70 °C for 36 h to obtain dry SPEEK-Na.

[0145] S4, Preparation of ZIF / SPEEK-Na composite diaphragm:

[0146] (1) Preparation of SPEEK-Na solution: Dissolve the dried ionized SPEEK in a high-boiling organic solvent. The mass ratio of SPEEK to the high-boiling organic solvent is 1 / 32; under a heating condition of 80 °C, stir and process for 0.5 h;

[0147] (2) Add 0.025 g of ZIF-8-SO3H and 0.025 g of ZIF-90-COOH to the mixed solution in step (1), perform light-shielding treatment, and stir at a constant temperature of 40 °C for 5 h. After ultrasonic treatment, prepare a ZIF / SPEEK-Na mixed solution with a ZIF mass percentage of 3%. Among them, the mass percentage of ZIF-90-COOH is 1.5%, and the mass percentage of ZIF-8-SO3H is 1.5%.

[0148] (3) Pour the ZIF / SPEEK-Na mixed solution obtained in step (2) onto a horizontal glass plate of a doctor blade heating coating machine, and form a film by the doctor blade method. Dry at 60 °C for 20 h and then dry at 110 °C for 6 h to obtain a ZIF / SPEEK-Na composite diaphragm. In this composite ion-selective membrane, the content of ZIF is 3 wt%.

[0149] Figure 17 It is a cyclic performance diagram of an iron-sulfur flow battery equipped with the ZIF / SPEEK-Na composite diaphragm prepared in this example, as Figure 17 shown. The iron-sulfur flow battery equipped with the ZIF / SPEEK-Na composite diaphragm prepared in this example can stably cycle at a relatively high Coulomb efficiency and maintain a relatively high capacity retention rate, indicating that the ZIF / SPEEK-K composite diaphragm prepared in this example has high ion selectivity and can stably cycle in the iron-sulfur flow battery.

[0150] Example 13,

[0151] A preparation method of a ZIF / SPEEK composite diaphragm for a flow battery, which is different from Example 2 in steps S1 and S4.

[0152] Among them, S1, preparation of SPEEK:

[0153] Perform a sulfonation reaction on PEEK powder (particle size of 15 μm) and 98 wt% concentrated sulfuric acid. The mass-volume ratio of PEEK powder to concentrated sulfuric acid is 1 / 6 (g / mL). React at 100 °C and a stirring speed of 700 r / min for 1 h. After the reaction, wash the reaction product with deionized water until neutral, dry at room temperature, and then dry in an oven at 100 °C for 48 h to obtain dry SPEEK. Measure the sulfonation degree of SPEEK to be 70%. Immerse the dry SPEEK in 2 M NaOH solution and soak at a constant temperature of 60 °C for 2 h, then place it in deionized water for soaking and washing until neutral, dry at room temperature, and then dry at 100 °C for 48 h to obtain dry SPEEK-Na.

[0154] S4, preparation of ZIF / SPEEK-Na composite diaphragm:

[0155] (1) Preparation of SPEEK-Na solution: Dissolve the dried ionized SPEEK in a high-boiling organic solvent with a mass ratio of SPEEK to high-boiling organic solvent of 1 / 15; under the heating condition of 100 °C, stir for 0.5 h;

[0156] (2) Add 0.025 g of ZIF-8-SO3H and 0.025 g of ZIF-90-OH to the mixed solution in step (1), carry out light-shielding treatment, and stir at a constant temperature of 100 °C for 5 h, then after ultrasonic treatment, prepare a ZIF / SPEEK-Na mixed solution with a ZIF mass percentage of 3%.

[0157] (3) Pour the ZIF / SPEEK-Na mixed solution obtained in step (2) onto a horizontal glass plate of a doctor blade heating coating machine and form a film by the doctor blade method. Dry at 100 °C for 15 h and then at 140 °C for 5.5 h to obtain a ZIF / SPEEK-Na composite diaphragm. In this composite ion-selective membrane, the content of ZIF is 3 wt%.

[0158] Figure 18 Figure showing the cycling performance of the iron-sulfur flow battery equipped with the ZIF / SPEEK-Na composite diaphragm prepared in this example, as Figure 18 shown, the iron-sulfur flow battery equipped with the ZIF / SPEEK-Na composite diaphragm prepared in this example can stably cycle at a relatively high Coulomb efficiency and maintain a relatively high capacity retention rate, indicating that the ZIF / SPEEK-K composite diaphragm prepared in this example has high ion selectivity and can stably cycle in the iron-sulfur flow battery.

[0159] In the embodiment of the present invention, the high-boiling organic solvent is dimethyl sulfoxide, dimethylformamide, dimethylacetamide or pyrrolidone, and dimethyl sulfoxide has relatively low toxicity.

[0160] Example 14:

[0161] The difference from Example 2 is that:

[0162] The ZIF / SPEEK composite proton exchange membrane used in this example conducts protons. Among them, in S1, sulfonate the PEEK powder (particle size of 15 μm) with 98 wt% concentrated sulfuric acid, and the mass-volume ratio of PEEK powder to concentrated sulfuric acid is 1 / 13 (g / mL). React at 60 °C and a stirring speed of 700 r / min for 2 h. After the reaction, wash the reaction product with deionized water until neutral, dry at room temperature, and then put it into an oven at 60 °C to dry for 24 h to obtain dry SPEEK. Measure the sulfonation degree of SPEEK to be 60%. The dried SPEEK is not subjected to ionization treatment.

[0163] Weigh 0.025 g of ZIF-8-SO3H and 0.025 g of ZIF-90-SO3H and add them to the SPEEK mixed solution. Use the remaining steps in Example 2 to prepare a ZIF / SPEEK composite proton exchange membrane with a ZIF mass fraction of 3%. The mass percentages of ZIF-8-SO3H and ZIF-90-SO3H are 1.5% respectively.

[0164] In this example, the thickness of the obtained ZIF / SPEEK composite diaphragm is 65 μm. The composite proton exchange membrane has a uniform texture and no phenomenon of ZIF particle dissolution. It is installed in a vanadium redox flow battery for testing.

[0165] Assemble a vanadium redox flow battery:

[0166] (1) Electrolyte preparation: Dissolve vanadyl sulfate in sulfuric acid solution with a molar concentration of 1.5 M. The supporting electrolyte is sulfuric acid with a molar concentration of 3 M.

[0167] (2) Diaphragm preparation: Immerse the ZIF / SPEEK composite proton exchange membrane in deionized water;

[0168] (3) Use graphite felt or carbon felt as the positive and negative electrodes of the battery;

[0169] (4) Take 30 ml and 15 ml of the electrolyte as the positive and negative electrolyte of the vanadium flow battery respectively. Perform a pre-charging treatment on the battery, and then take out 15 ml of the positive electrolyte for battery testing.

[0170] The relevant performance data of this example are as follows:

[0171] Figure 20 It is a comparison chart of the capacity decay rate of vanadium flow batteries equipped with Nafion 212 membrane and ZIF / SPEEK composite proton exchange membrane. The capacity of the vanadium flow battery equipped with Nafion 212 membrane decays rapidly, while the vanadium flow battery equipped with ZIF / SPEEK composite proton exchange membrane has a higher capacity retention rate. As Figure 21 shown, the vanadium flow battery equipped with ZIF / SPEEK composite proton exchange membrane can stably cycle at a higher Coulomb efficiency. And as Figure 22 shown, the vanadium flow battery equipped with ZIF / SPEEK composite proton exchange membrane has a high energy efficiency, indicating that the addition of ZIF improves the proton conductivity while improving the ionic selectivity and chemical stability of the diaphragm. At the same time, the cost of this composite proton exchange membrane is much lower than that of Nafion 212 membrane. The ZIF / SPEEK composite proton exchange membrane prepared in this example has good prospects for industrial application in vanadium flow batteries.

[0172] Example 15:

[0173] The difference from Example 3 is that the ZIF material used in this example is ZIF-8-SO3H. Weigh 0.05 g of ZIF-8-SO3H and add it to the SPEEK-K mixed solution. The remaining steps in Example 3 are used to prepare a ZIF / SPEEK-K composite diaphragm with a mass fraction of 3% of potassium-ionized ZIF-8-SO3H.

[0174] In this example, the obtained ZIF / SPEEK-K composite diaphragm has a thickness of 65 μm. The composite ion exchange membrane has a uniform texture and no phenomenon of ZIF particle dissolution. It is installed in an iron-sulfur redox flow battery for testing.

[0175] Figure 19 Figure for the cycling performance of the iron-sulfur flow battery equipped with the ZIF / SPEEK-K composite diaphragm prepared in this example, as Figure 19 shown, the Coulomb efficiency cycle capacity retention rate of the iron-sulfur flow battery equipped with the ZIF / SPEEK-K composite diaphragm prepared in this example indicates that the cycling stability of the ZIF / SPEEK-K composite diaphragm prepared in this example in the iron-sulfur flow battery needs to be improved. Only adding ZIF-8 material to the matrix diaphragm has a poor effect on improving the performance of the composite diaphragm.

[0176] The requirements for the diaphragm in the field of flow batteries are to be able to block the shuttling of positive and negative active substances and transfer salt ions such as protons, potassium ions, and sodium ions. Therefore, the diaphragm used in flow batteries needs to select materials with good hydrophilicity, high ion selectivity, and strong mechanical properties. So in this invention, sulfonated polyether ether ketone material (SPEEK) with high proton conductivity (high sulfonation degree) is selected to prepare the diaphragm. However, due to the high sulfonation degree of the SPEEK material, its mechanical properties are reduced. Therefore, it is hoped to improve the chemical and mechanical properties of the diaphragm by adding porous materials with high stability and high specific surface area; in this invention, ZIF-90 and ZIF-8 porous materials are added at the same time, which improves the compatibility between the ZIF and the SPEEK matrix material, enables the ZIF porous material to be evenly mixed into the matrix material, and reduces the agglomeration of the ZIF material. Adding more hydrophilic groups improves the proton conductivity of the diaphragm; meets the development needs of flow batteries and breaks through the "bottleneck" problem of diaphragms used in flow batteries.

[0177] In the embodiment of the present invention, the SPEEK matrix with high sulfonation degree and high proton conductivity is mixed with the ZIF material with high porosity and high stability, overcoming the problems that the SPEEK diaphragm has a high sulfonation degree, high proton conductivity, but low mechanical properties and large ion permeability; while the SPEEK has a low sulfonation degree, high mechanical properties and chemical stability, but low proton conductivity; and it is impossible to achieve a balance in the comprehensive performance of sulfonation degree and mechanical properties, proton conductivity and ion selectivity. While ensuring the high proton conductivity of the diaphragm, the mechanical properties, chemical stability, and ion selectivity of the diaphragm are improved.

[0178] In the matrix material, the variable pore size of the ZIF-8 material endows the composite separator with adjustable ion sieving ability. However, since the energy required for the ligand rotation of the ZIF-8 material is only 0.36 kcal / mol, this causes its pore cage to always be in a state that can be expanded. During this process, the polymer chains are very likely to insert into the pore cage structure of the porous material, forming dead zones, rendering some fillers ineffective, and reducing the ion sieving effect of the separator. The breathing effect and the problem of the pore structure being blocked by polymer chains result in the SPPEK composite separator doped only with the ZIF-8 material having poor ion selectivity. The shuttling of the active substances on the positive and negative electrodes of the battery leads to battery failure and reduces the cycle stability of the battery. The ZIF-90 material has a large specific surface area and a stable framework structure, and has a pore structure with a pore size of about . At present, the synthesis process of ZIF-90 has been relatively mature. The addition of the ZIF-90 material can effectively reduce the breathing effect and the pore blocking effect, and further improve the ion selectivity of the separator.

[0179] Functionalize the ZIF material. The grafted hydrophilic groups not only improve the compatibility between the ZIF and the matrix, but also improve the proton conductivity of the separator.

[0180] In the embodiments of the present invention, ZIF-90 and ZIF-8 are added simultaneously. ZIF materials generally have the advantage of high porosity. However, in addition to having a high porosity and high chemical stability, ZIF-8 also has an adjustable pore radius. The adjustable pore radius of ZIF-8 can bring a wider applicability to the separator. However, due to its breathing effect, it gives the separator a low ion selectivity. The ZIF-90 material with a stable structure framework and a mature synthesis process can improve the influence of the ZIF-8 material on the ion sieving property of the separator and improve the ion selectivity. In order to further improve the compatibility between the ZIF material and the matrix separator and the salt ion transport rate of the separator, in the embodiments of the present invention, under normal temperature conditions, -SO3H, -COOH or -OH on sulfamic acid (glycine, hydroxylamine) is grafted onto the ZIF-8 material through diazo reaction and diazo coupling reaction to achieve the functionalization treatment of ZIF-8.

[0181] The above are only the preferred embodiments of the present invention and are not intended to limit the protection scope of the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention are included in the protection scope of the present invention.

Claims

1. A ZIF / SPEEK composite diaphragm for a flow battery, characterized in that, The composite separator uses alkali metal ion-based sulfonated polyether ether ketone as the base film, with a sulfonation degree of 60% - 70%; the filler includes ZIF-8-X and ZIF-90-X. The ZIF-8-X material is ZIF-8-SO3H, ZIF-8-COOH or ZIF-8-OH, and the ZIF-90-X material is ZIF-90-SO3H, ZIF-90-COOH or ZIF-90-OH. The total doping amount of ZIF-8-X and ZIF-90-X is 1% - 5%, and the added masses of ZIF-8-X and ZIF-90-X are the same.

2. The preparation method of the ZIF / SPEEK composite diaphragm for a flow battery according to claim 1, wherein, It includes the following steps: S1, Prepare sulfonated polyether ether ketone with a sulfonation degree of 60% - 70%; S2, Prepare ZIF-8-X; ZIF-8-X is ZIF-8-SO3H, ZIF-8-COOH or ZIF-8-OH; S3, Prepare ZIF-90-X; the ZIF-90-X material is ZIF-90-SO3H, ZIF-90-COOH or ZIF-90-OH; S4, Add ZIF-8-X and ZIF-90-X into the organic solution of sulfonated polyether ether ketone, and under the condition of constant temperature at 40 - 100°C, perform light-shielding treatment, ultrasonic treatment, and configure a mixed solution with a total mass fraction of ZIF-8-X and ZIF-90-X of 1% - 5%. The mixed solution is formed into a film by the doctor blade method.

3. The preparation method of the ZIF / SPEEK composite diaphragm for a flow battery according to claim 2, characterized in that, In the above S1, to prepare sulfonated polyether ether ketone with a sulfonation degree of 60% - 70%, specifically: perform a sulfonation reaction on polyether ether ketone powder and concentrated sulfuric acid, and the mass-volume ratio of polyether ether ketone powder to concentrated sulfuric acid is 1 g : 6 - 15 mL; stir and react at a constant temperature of 40 - 100°C for 1 - 12 h; after the reaction, wash the reaction product with deionized water until neutral, dry it, and place it in an oven at 60 - 100°C for drying for 24 - 48 h to obtain dry sulfonated polyether ether ketone with a sulfonation degree of 60% - 70%.

4. The preparation method of the ZIF / SPEEK composite diaphragm for a flow battery according to claim 2, wherein, The above S1 also includes subjecting the dried sulfonated polyether ether ketone to ionization treatment, specifically: placing the dried sulfonated polyether ether ketone in a 0.5 - 2 M alkali solution, soaking it at a constant temperature of 40 - 60°C for 2 - 4 h, then placing it in deionized water for soaking and washing until neutral, drying it, and then drying it at 60 - 100°C for 24 - 48 h to obtain dried potassium ion-based or sodium ion-based sulfonated polyether ether ketone.

5. The preparation method of the ZIF / SPEEK composite diaphragm for a flow battery according to claim 2, characterized in that, The above ZIF-8-X and ZIF-90-X are obtained by grafting hydrophilic groups -SO3H, -COOH or -OH onto ZIF-8 or ZIF-90 through diazo reaction and diazo coupling reaction under room temperature conditions.

6. The preparation method of a ZIF / SPEEK composite separator for a flow battery according to claim 2, characterized in that, In S2, the preparation method of ZIF-8-X: Disperse ZIF-8 in deionized water, and then successively add a substance containing a hydrophilic group in an amount equal to that of ZIF-8 and 0.4 wt% - 0.6 wt% of concentrated HCl. The molar ratio of the substance containing a hydrophilic group to concentrated HCl is 1:2 - 1:

4. Stir the resulting suspension at room temperature, and then dropwise add a NaNO2 solution in an amount equal to that of ZIF-8. Heat the mixture. After the reaction is completed, filter the suspension through a dialysis bag and wash it with deionized water. Collect the residue in the dialysis bag and dry it to obtain the product. The substance containing a hydrophilic group is one of sulfamic acid, glycine, or hydroxylamine, and ZIF-8-SO3H, ZIF-8-COOH, or ZIF-8-OH is correspondingly obtained.

7. The preparation method of a ZIF / SPEEK composite separator for a flow battery according to claim 2, characterized in that, In S3, the preparation method of ZIF-90-X: Disperse ZIF-90 in deionized water, and then successively add a substance containing a hydrophilic group in an amount equal to that of ZIF-90 and 0.4 wt% - 0.6 wt% of concentrated HCl. The molar ratio of the substance containing a hydrophilic group to concentrated HCl is 1:2 - 1:

4. Stir the resulting suspension at room temperature, and then dropwise add a NaNO2 solution in an amount equal to that of ZIF-90. Heat the mixture. After the reaction is completed, filter the suspension through a dialysis bag and wash it with deionized water. Collect the residue in the dialysis bag and dry it to obtain the product. The substance containing a hydrophilic group is one of sulfamic acid, glycine, or hydroxylamine, and ZIF-90-SO3H, ZIF-90-COOH, or ZIF-90-OH is correspondingly obtained.

8. The preparation method of a ZIF / SPEEK composite separator for a flow battery according to claim 2, characterized in that In S4, the preparation of the organic solution of alkali metal ion type sulfonated polyether ether ketone: Add the dried alkali metal ion type sulfonated polyether ether ketone to a high-boiling organic solvent. The mass ratio of the alkali metal ion type sulfonated polyether ether ketone to the high-boiling organic solvent is 1 / 5 - 1 / 32. Under the heating condition of 40 - 120 °C, stir for 0.5 - 1.5 h.

9. The preparation method of a ZIF / SPEEK composite diaphragm for a flow battery according to claim 8, characterized in that, In S4, the high-boiling organic solvent is dimethyl sulfoxide solution, dimethylformamide, dimethylacetamide, or pyrrolidone.

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