A covalent organic framework self-supporting membrane and its preparation method and application

Abstract

The present invention relates to a covalent organic framework (COF) self-supporting membrane, its preparation method, and application. The main design and preparation method involves introducing polymer chains with sulfonic acid groups during the preparation process. Crystallization channels and abundant -SO3H groups within the COFs (COFs) backbone form a basic channel for proton transport. Compared with the prior art, the additional -SO3H groups introduced into the polymer chains catalyze the reaction of the COF precursor, helping to increase the crystallinity of the membrane. The chains also fill the gaps between adjacent COF nanoparticles, improving the continuity of proton hopping sites, synergistically constructing a rapid proton transport channel, and enhancing the electrochemical performance of liquid flow batteries. This invention is of great significance in promoting the application of COFs membranes in liquid flow batteries.

Description

Technical Field

[0001] The present invention relates to the technical field of redox flow batteries, and in particular to a covalent organic skeleton self-supporting membrane for aqueous organic flow batteries, and a preparation method and application thereof. Background Art

[0002] The global demand for a balance between energy consumption and carbon neutrality has driven the adoption of renewable energy sources such as solar and wind power. Given the intermittent and unstable nature of renewable energy, cutting-edge energy conversion and storage technologies, such as fuel cells and water electrolysis, are being extensively researched. Among these, redox flow batteries (RFBs) have attracted significant attention due to their tunable capacity, high cycle safety, environmental friendliness, and flexible operation.

[0003] Membranes play a crucial role in RFBs, performing ion selection, blocking crosstalk between active species, and conducting ions to complete circuit connections. They are a key factor influencing overall performance. Ideal permeable membranes should exhibit high proton conductivity and high selectivity between protons and active species in the electrolyte. However, the inherent trade-off between membrane permeability and selectivity poses significant challenges in preparing membranes with ideal performance, sparking extensive research.

[0004] In recent years, porous crystalline materials, including zeolite frameworks and metal-organic frameworks (MOFs), have attracted widespread attention because their abundant intrinsic channels ensure rapid proton conductivity through ordered pore channels and effectively block active components through size exclusion. They are widely used in the performance optimization of liquid flow battery membranes. Among these porous materials, covalent organic frameworks (COFs) have a covalently linked structure, acid resistance, and tunable pore size (0.5-4.7 nm), showing great application prospects in the construction of proton exchange membranes. These characteristics are of great significance for the long-term operation of RFBs and the selection of appropriate frameworks based on the size of active species to achieve diverse applications in various RFB systems.

[0005] Currently, the application of COFs in flow battery membranes primarily focuses on modifying polymer membranes by incorporating COF nanoparticles into matrices. The construction of COF-related membranes for flow batteries inevitably requires the assistance of polymers, as the membranes must possess excellent mechanical properties to withstand the pressure of the electrodes and the impact of the flowing electrolyte. However, the presence of polymers, which often constitute the majority of the membrane, significantly hinders the advantages of COFs. Therefore, to fully utilize the ordered pores and high porosity of COFs, the development of self-supporting COF membranes is desired for successful application in flow batteries. However, due to the insolubility of synthesized COF particles in common solvents and their poor processing properties, the preparation of mechanically strong, self-supporting COF membranes for flow batteries is a significant challenge. Furthermore, the high crystallinity of COFs is typically achieved through harsh synthesis conditions such as high temperature and high pressure to promote the exchange of dynamic covalent bonds, which makes it difficult to form continuous, free-standing COF membranes under such conditions. Therefore, achieving highly crystalline, self-supporting COF membranes is another challenge. Summary of the Invention

[0006] The purpose of the present invention is to overcome the defects of the above-mentioned prior art and to provide a covalent organic framework self-supporting membrane and its preparation method and application. It mainly provides a self-supporting membrane for liquid flow batteries, which uses covalent organic framework material as the main membrane structure and introduces sulfonated polymer chains into it.

[0007] The purpose of the present invention can be achieved by the following technical solutions:

[0008] The first aspect of the present invention provides a covalent organic framework self-supporting membrane, comprising the following steps:

[0009] Step 1: dissolving the ammonia monomer for synthesizing COF in a first solvent to obtain a first pre-reaction solution;

[0010] Step 2: dissolving the aldehyde monomer and sulfonated polymer chain for synthesizing COF in a second solvent to obtain a second pre-reaction solution;

[0011] Step 3: Mix the first pre-reaction liquid and the second pre-reaction liquid to obtain a reaction solution, then heat the reaction solution for reaction, activate, and wash to obtain a covalent organic framework self-supporting membrane.

[0012] Furthermore, the ammonia monomer for synthesizing COF is selected from 2,5-diaminobenzenesulfonic acid and 2,5-diaminobenzene-1,4-disulfonic acid.

[0013] Furthermore, the aldehyde monomer for synthesizing COF is trialdehyde phloroglucinol.

[0014] Furthermore, the first solvent is selected from a mixture of one or more of dimethyl sulfoxide, 1,4-dioxane, and tetrahydrofuran.

[0015] Furthermore, the second solvent is selected from a mixture of one or more of N-methylpyrrolidone, mesitylene, and propylene glycol.

[0016] Furthermore, the sulfonated polymer is selected from one of sulfonated polyetheretherketone, sulfonated polybenzimidazole, and sulfonated polyimide.

[0017] Furthermore, the weight proportion of the sulfonated polymer chain in the covalent organic skeleton self-supporting membrane is 0.1%-5%.

[0018] Furthermore, the heating reaction process in step 3 adopts normal pressure reaction, reaction temperature 60-120 ° C, and reaction time 24-144 h.

[0019] Furthermore, in step 3, the activation process is to immerse the self-supporting membrane in an acid solution, wherein the acid solution is a 0.1-2 M sulfuric acid aqueous solution.

[0020] Furthermore, by introducing sulfonated polymer chains into the self-supporting covalent organic framework membrane, the mechanical strength, crystallinity and proton conductivity of the self-supporting membrane are improved.

[0021] The second aspect of the present invention provides a covalent organic framework self-supporting membrane prepared by the above method, wherein the covalent organic framework self-supporting membrane uses a covalent organic framework material as the main membrane structure, and introduces a sulfonated polymer chain therein, wherein the covalent organic framework material is TpPa-2SO3H or TpPa-SO3H, and the weight proportion of the sulfonated polymer chain in the covalent organic framework self-supporting membrane is 0.1%-5%.

[0022] Furthermore, the thickness of the self-supporting film is 8-20 μm, and the area of the self-supporting film is 4-25 cm 2 .

[0023] A third aspect of the present invention provides a use of the above-mentioned covalent organic framework self-supporting membrane in an aqueous organic liquid flow battery.

[0024] Compared with the prior art, the present invention has the following technical advantages:

[0025] 1) This invention proposes for the first time the application of covalent organic framework self-supporting membranes in aqueous organic flow batteries. The crystalline channels and rich groups in the COFs skeleton give the membranes prepared from this material ideal performance in aqueous organic flow battery systems.

[0026] 2) Furthermore, the -SO3H groups in the sulfonated polymer chains introduced in this invention catalyze the reaction of the COF precursor, promoting reversible bond exchange, which helps increase the crystallinity and more ordered channels of the freestanding membrane. The chains can also penetrate into the gaps between adjacent COF nanoparticles, improving the continuity of proton hopping sites and synergistically optimizing the membrane's proton conductivity and cyclic stability.

[0027] 3) This invention further expands the application of covalent organic frameworks in liquid flow battery membranes, providing new ideas for the development of a new generation of high-performance proton exchange membranes. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 Middle: a) FT-IR spectra of TpPa-SO3H, Tp, and Pa-SO3H. b) FT-IR spectra of TpPa-SO3H / SPEEK-x% membrane and SPEEK. c) Raman spectra of TpPa-SO3H / SPEEK-x% membrane. d) FT-IR spectra of TpPa-SO3H / SPEEK-0% membrane. 13 C solid-state NMR spectrum.

[0029] Figure 2 It's SPEEK 1 H NMR spectrum.

[0030] Figure 3 Middle: a) Digital image of a TpPa-SO3H / SPEEK-0% membrane. b) Surface scanning electron microscopy (SEM) image of a TpPa-SO3H / SPEEK-x% membrane. c) Cross-sectional SEM image of a TpPa-SO3H / SPEEK-x% membrane. d) Water contact angle (WCA) of a TpPa-SO3H / SPEEK-0% membrane.

[0031] Figure 4 Middle: a) X-ray diffraction (XRD) patterns of TpPa-SO3H / SPEEK-x% film and SPEEK. b) X-ray diffraction pattern of TpPa-SO3H / SPEEK-x% powder.

[0032] Figure 5 Middle: a) Swelling ratio (SR) and water absorption (WU) of TpPa-SO3H / SPEEK-x% membrane. b) X-ray diffraction spectrum of TpPa-SO3H / SPEEK-1% membrane after acid immersion.

[0033] Figure 6 Middle: a) Nyquist plot of TpPa-SO3H / SPEEK-x% film. b) Conductivity and impedance plots of TpPa-SO3H / SPEEK-x% film.

[0034] Figure 7 Middle: Curve fitted by Arrhenius equation for TpPa-SO3H / SPEEK-x% membrane

[0035] Figure 8 Middle: TpPa-SO3H / SPEEK-x% membrane at 40-120 mA cm -2 Battery performance diagrams at different current densities: a) CE, b) VE, c) EE.

[0036] Figure 9 Middle: a) TpPa-SO3H / SPEEK-0%, TpPa-SO3H / SPEEK-1% and Nafion212 membrane at 80 mA cm -2 Long-term cycling performance under different current densities. b) SEM image of TpPa-SO3H / SPEEK-1% membrane after cycling test.

[0037] Figure 10 a) Cross-sectional SEM images of TpPa-SO3H / SPEEK-1% membranes with different thicknesses, b) 40-120 mA cm -2 Battery performance diagram under current density. DETAILED DESCRIPTION

[0038] The present invention will be described in detail below with reference to the accompanying drawings and specific embodiments. Any features such as preparation methods, materials, structures or composition ratios not explicitly described in this technical solution are considered to be common technical features disclosed in the prior art.

[0039] For the specific system of aqueous organic flow batteries used in this invention, it is necessary to select COF materials with acid stability, redox stability, and suitable pore sizes. The main crystalline framework of the COF membrane can establish proton transport channels, and the nanometer-sized pores can effectively screen active substances in the electrolyte. At the same time, it is necessary to select sulfonated polymer chains with high compatibility with the COF membrane and effective regulation of the COF crystal structure and add them to the COF membrane to further optimize the proton transport path, build a continuous hydrogen bond network for proton hopping, and enhance the membrane's proton conductivity.

[0040] The COF material used for the membrane host of the present invention is selected from TpPa-2SO3H (NUS-10) or TpPa-SO3H (NUS-9).

[0041] The sulfonated polymer chain added in the present invention is selected from sulfonated polyetheretherketone, sulfonated polybenzimidazole or sulfonated polyimide.

[0042] Under optimal conditions, the COF material used in this invention utilizes the TpPa-SO3H structure, characterized by: 1. Vertically penetrating channels create a fast proton transport pathway; 2. The backbone is connected by ketene bonds, resulting in excellent stability; 3. The theoretical pore size is smaller than the size of the active material in the electrolyte, facilitating complete isolation of the active material; and 4. The backbone is rich in sulfonic acid functional groups, which facilitate the formation of a hydrogen bonding network and promote rapid proton transfer. The TpPa-2SO3H structure shares these four characteristics with TpPa-SO3H, offering similar feasibility and performance.

[0043] The present invention uses polyetheretherketone (PEEK) as the polymer chain. It has a stable chemical structure and can maintain long-term operation in acidic environments. It is also easy to sulfonate, which facilitates precise control of the amount of sulfonic acid groups introduced.

[0044] Example

[0045] Preparation of SPEEK

[0046] PEEK was placed in an oven at 80 °C for 12 h to completely remove adsorbed water. Dry PEEK (10 g) and concentrated sulfuric acid (100 g) were then added simultaneously to a three-necked flask and mechanically stirred at 50 °C for 10 h. After the reaction, the solution was slowly poured into deionized water, and the resulting precipitate was rinsed several times with deionized water until the pH reached 7. Finally, the resulting SPEEK product was collected and dried at 50 °C for 12 h for subsequent use.

[0047] Preparation of TpPa-SO3H / SPEEK membrane

[0048] A TpPa-SO3H / SPEEK free-standing membrane was prepared by a tape casting method. 0.013 g of Tp monomer was dissolved in 0.5 mL of NMP (N-methylpyrrolidone). Similarly, 0.017 g of Pa-SO3H monomer (2,5-diaminobenzenesulfonic acid) and a certain amount of SPEEK (sulfonated polyetheretherketone) were dissolved in 0.5 mL of DMSO (dimethyl sulfoxide). Ultrasonic treatment was performed for 30 minutes to uniformly disperse the monomers in the solvent. The two monomer solutions were mixed and ultrasonicated for 30 minutes to thoroughly mix the monomers. The solution was then filtered and collected using a syringe filter to remove impurities and ensure that the subsequent membrane was defect-free. A certain amount of the solution was then added dropwise onto a glass plate and heated in an oven at 60 °C for 6 days. The membrane was then immersed in deionized water, peeled from the glass plate, and rinsed with NMP and deionized water to obtain a TpPa-SO3H / SPEEK-x% membrane (x% is the mass ratio of SPEEK:m). SPEEK / m(Tp+Pa-SO3H) Finally, the membrane was immersed in 2 M sulfuric acid solution for 24 h for activation, then washed repeatedly with deionized water and stored in water.

[0049] Preparation of TpPa-SO3H / SPEEK powder

[0050] TpPa-SO3H / SPEEK powder was prepared by a solvothermal method. Tp (0.063 g) and Pa-SO3H (0.084 g) monomers were dispersed in a solvent mixture of mesitylene (1.2 mL) and 1,4-dioxane (0.3 mL) and sonicated for 30 min. The two solutions were then mixed and sonicated for 10 min. Then, 6 M acetic acid (0.6 mL) and SPEEK (0.147 x% g) were added to form a reaction mixture. The mixture was sonicated for 10 min and heated in an oven at 60 °C for 72 h. The resulting precipitate was filtered, washed with 1,4-dioxane, and dried at 120 °C to obtain the corresponding product.

[0051] Test Method

[0052] Swelling rate and water absorption rate

[0053] To calculate the water uptake (WU) and swelling ratio (SR) of the membrane, the wet membrane was prepared by immersing the membrane in deionized water for 24 h and wiping the membrane surface with filter paper before measuring the mass. The dry membrane was prepared by vacuum drying at room temperature for 24 h. WU and SR were calculated using the following formulas:

[0054]

[0055]

[0056] Where W and d are the mass and length of the membrane, respectively.

[0057] Proton transfer rate

[0058] The impedance spectroscopy test was performed on a CHI 660E electrochemical workstation to obtain the impedance value (R, Ω) of the film. The sample was sandwiched between two silver sheets and the test frequency was between 0.1 and 10 6 Hz, AC amplitude is 0.5 V. Proton conductivity (Ω,Scm -1 ) is calculated as follows:

[0059]

[0060] Where L (cm) is the length between the two electrodes, A (cm 2 ) is the cross-sectional area of the membrane.

[0061] Membrane surface resistance ( Ω cm -2 ) can be calculated by the following formula:

[0062]

[0063] Single cell performance

[0064] The performance of the membrane was evaluated using a homemade redox flow battery system. For a single cell assembly, the test membrane (with an effective area of 4 cm2) was sandwiched between a carbon felt electrode and a graphite plate. -2 ) and then clamped with the shell. The carbon felt is activated by pretreatment at 400 °C for 12 h, and its compression rate can reach about 20% when assembled in the component. The positive electrode electrolyte is 0.2 M 1,2-dihydroxybenzene-3,5-disulfonic acid (BQDS) dissolved in 2 M H2SO4 solution, and the negative electrode electrolyte is 0.2 anthraquinone-2-sulfonic acid (AQS) dissolved in 2 M H2SO4 solution. The redox active substances (1,2-dihydroxybenzene-3,5-disulfonic acid disodium salt and anthraquinone-2-disulfonic acid sodium salt) are obtained in advance by ion exchange to obtain their respective acid forms, thereby obtaining higher solubility. Before the test, the tank was purged with argon for 2 h and the electrolyte was heated at 10 mA cm -2 The charge and discharge cycles were performed at a constant current density of 40, 60, 80, 100, 120, and 140 mA cm -2 The charge and discharge tests were carried out at cut-off voltages of 0.01 and 1.1 V. The cycle stability test was carried out at a current density of 80 mA cm -2 , tested at the same cut-off voltage. Use the formula to calculate the coulombic efficiency (CE), energy efficiency (EE) and voltage efficiency (VE).

[0065]

[0066]

[0067]

[0068] Where I, V, and t represent current, voltage, and time, respectively. The subscripts c and d represent the charge and discharge processes, respectively.

[0069] Membrane characterization

[0070] The TpPa-SO3H / SPEEK membrane was synthesized by condensing the precursor trialdehyde (Tp) and diamine (Pa-SO3H) and adding a certain proportion of SPEEK. The chemical structure of the membrane was characterized by Fourier transform infrared spectroscopy (FT-IR). Figure 1 As shown in a, the C=O (1639 cm -1)、O=CH (2890 cm -1 ) and NH (3335,3424 cm -1 ) stretching band, the characteristic stretching peaks of C=C and CN bands in TpPa-SO3H film appear at 1570 and 1225 cm -1 This indicates the formation of β-ketoenamine structure and the complete reaction or removal of monomers. Similarly, the films prepared by adding SPEEK all showed specific stretching vibration peaks ( Figure 1 b), verifying the successful synthesis of the same structure. In addition, Raman spectroscopy also confirmed its chemical structure ( Figure 1 c), respectively, from 1390 cm -1 Stretching vibration of the secondary amine group (=C-NH), 1597 cm -1 C=C of the benzene ring and 1658 cm -1 The C=O of the β-ketoamine structure is represented. Figure 1 d, solid state 13 The C-NMR spectrum is another proof of the structure formation, which shows the typical signal of carbonyl carbon at 183.5 ppm. 1 The sulfonation degree was calculated to be 78.5% by H NMR spectrum. Figure 2 However, in the above characterization, the characteristic peaks belonging to SPEEK could not be clearly observed in the TpPa-SO3H / SPEEK membrane, which may be because the content of SPEEK (0.1-5 wt%) was too small to be detected.

[0071] Macroscopic characteristics of the membrane such as Figure 3 As shown in a, TpPa-SO3H / SPEEK-1% is dark green with a side length of about 5 cm. Importantly, the membrane has excellent flexibility, which is crucial for mass production and preservation of the membrane. The microstructure of the membrane was detected by SEM, and it was found that the membrane surface was dense and continuous ( Figure 3 b). Cross-sectional SEM image ( Figure 3 c) shows that the membrane thickness is around 15 μm. A thinner thickness is conducive to rapid proton transfer. The morphology of TpPa-SO3H / SPEEK-1% is smoother than that of TpPa-SO3H / SPEEK-0%, indicating that the polymer chains added with SPEEK fill the grain boundary defects of COF nanoparticles, which is beneficial for blocking the active substances in the electrolyte.

[0072] Since the electrolyte of this flow battery is aqueous, a water contact angle test was performed to evaluate the hydrophilicity of the membrane. Figure 3As shown in Figure d, the water contact angle of TpPa-SO3H / SPEEK-0% is 54.4°. The small water contact angle reveals the highly hydrophilic nature of the membrane, which is due to the abundant -SO3H groups in the skeleton that facilitate the adsorption of abundant water molecules, thereby facilitating the formation of transport channels. Since SPEEK is rich in aromatic rings, it is relatively hydrophobic (water contact angle of 86.9°). Therefore, with increasing SPEEK content, the water contact angle of the membrane shows an upward trend, but the overall change is limited (~4°). At the same time, it can be seen that the water permeation rate of the membrane is slow, which further proves its dense structure.

[0073] The crystal structure of TpPa-SO3H / SPEEK film was determined by X-ray diffraction. Figure 4 a It can be seen that the XRD spectra of all films have a characteristic peak at 5.1°, which is related to the diffraction of the (100) crystal plane, indicating the formation of a one-dimensional nanochannel. At the same time, the diffraction peak at about 27° originates from the (001) crystal plane, indicating a π-π stacking structure. In comparison, the peak of amorphous SPEEK is weaker, so it is difficult to observe its diffraction peak in the film. Compared with the (001) plane, as the amount of SPEEK added increases, the (100) peak intensity of the TpPa-SO3H / SPEEK film increases, indicating that the crystallinity of the film is improved. At the same time, TpPa-SO3H / SPEEK powder was synthesized to further evaluate the effect of SPEEK on the crystallization process. Similarly, as Figure 4 As shown in Figure b, when the addition ratio of SPEEK reaches 1%, the crystallinity of the powder increases significantly. As the addition amount of SPEEK increases, the crystallinity of the powder remains basically unchanged. This proves that the appropriate amount of SPEEK can control the crystallization process, thereby making the pores of the skeleton more orderly, which is conducive to proton transfer and screening of active substances.

[0074] Membrane properties

[0075] The structural stability of the membrane was evaluated by testing its swelling rate characteristics. Figure 5 As shown in Figure 5, the SR of TpPa-SO3H / SPEEK-0% is about 2%. Such a low value is attributed to the rigidity of the COF framework, which helps to maintain the H + After adding 1% SPEEK, the SR of the membrane increased slightly to 2.5% due to the flexible chains of SPEEK. When the amount of SPEEK added increased, the SR rose to 4.5%. The higher swelling rate is not conducive to the membrane maintaining its nanostructure during long-term operation. Therefore, the amount of SPEEK added should be controlled within an appropriate range. The porous structure of COF is conducive to water retention, which can be verified by water absorption evaluation. Figure 5In the experiment, as the SPEEK ratio increased from 0% to 5%, the water absorption value of the membrane increased from 22.9% to 34.1%. This phenomenon is due to the optimized crystallization process that creates more porous channels in the membrane. Importantly, the presence of abundant water in the nanochannels helps to improve the conductivity of protons. In addition, the membrane operates in an acidic environment, so the acid stability of the membrane is of great significance. Figure 5 As shown in Figure b, the membrane still maintains its crystal structure after being immersed in 2 M H2SO4 for 72 h. When the immersion time is extended to 240 h, the crystallinity is still partially retained. The material exhibits excellent resistance to acid decomposition.

[0076] Proton transport performance is one of the important parameters of proton conducting membranes. The above characterization shows that the physical and chemical properties of the membrane changed after SPEEK treatment. Therefore, the impact of these differences on proton transport behavior was further explored. The resistance of the TpPa-SO3H / SPEEK membrane ( Figure 6 a) and calculate the corresponding conductivity. The conductivity of TpPa-SO3H / SPEEK-0% membrane at 100% RH and 25°C is 0.061 S cm -1 , close to Nafion 212 (0.059 S cm -1 ). The higher conductivity is due to the fact that the abundant -SO3H groups in the COF skeleton can promote the formation of hydrogen bond networks and accelerate the transfer of protons. Figure 6 As shown in b, after adding 1% SPEEK to the pure membrane, the conductivity is higher (0.073 S cm -1 ), the resistance is lower, which may be due to the increase in crystallization leading to more orderly mass transfer channels. When the proportion of SPEEK continues to increase to 5%, the conductivity drops to 0.054cm -1 , the resistance increases. This decrease in proton conductivity may be due to the addition of too much SPEEK, which blocks the crystalline nanochannels. In addition, the activation energy (Ea) of proton transfer in the membrane was tested to further explore the proton transfer mechanism in the membrane. Figure 7As shown in the figure, according to the Arrhenius equation, the Ea of proton transport in the TpPa-SO3H / SPEEK-0%, TpPa-SO3H / SPEEK-1%, and TpPa-SO3H / SPEEK-5% membranes are 0.135 eV, 0.128 eV, and 0.142 eV, respectively, all less than 0.4 eV, indicating that proton conduction within the membranes is primarily a Grotthuss mechanism. In this mechanism, proton transport occurs between water molecules via the formation and breakage of hydrogen bonds. Therefore, the tortuosity of the transfer channel and the density or spatial contact distance of the acid groups have a significant impact on the hopping efficiency. In the TpPa-SO3H / SPEEK-1% membrane, the enhanced crystallinity of the TpPa-SO3H backbone effectively shortens the overall transport distance. Furthermore, the addition of SPEEK introduces more acid groups. These optimizations contribute to a synergistic reduction in the energy barrier for proton transport.

[0077] Performance Testing

[0078] The single cell performance of TpPa-SO3H / SPEEK membrane was tested to further evaluate its electrochemical relevance. In addition, commercial Nafion 212 was also tested. Figure 8 As shown in a, at 40 mA cm -2 At low current densities, the CE of the TpPa-SO3H / SPEEK battery is greater than 97%, and the CE values of different membranes are close at different current densities, indicating that the different membranes have little difference in their blocking effect on BQDS / AQS. During the test, due to the shortened charge and discharge time and the reduced electrolyte crossover, the CE increases with the increase of current density. The battery equipped with TpPa-SO3H / SPEEK-1% membrane shows the highest VE ( Figure 8 b), this is because the membrane has the highest proton conductivity and the lowest surface resistance. Due to the concentration polarization and large resistance, when the current density increases, the VE of the battery will decrease. EE is a combination of CE and VE, reflecting the comprehensive performance of the membrane-based battery. Figure 8 c shows the EE (at 40 mA cm) of the cell equipped with TpPa-SO3H / SPEEK-1% membrane. -2 0% at 40 mA cm) is significantly higher than the EE of the cell equipped with TpPa-SO3H / SPEEK-0% membrane (81.0% at 40 mA cm -2 The results show that the appropriate amount of SPEEK can effectively improve the performance of the membrane. In addition, the change trends of EE and VE are consistent with the increase of SPEEK, indicating that proton conductivity plays a crucial role in the overall performance of the membrane.

[0079] The cell assembled with TpPa-SO3H / SPEEK and Nafion 212 membrane was tested at 80 mA cm -2 The cycling performance was evaluated under Figure 9 a). Due to the continuous flushing of the electrolyte and the harsh redox environment, the EE of the TpPa-SO3H / SPEEK-0% membrane dropped rapidly after about 40 cycles, proving that the cycling stability of the pure membrane needs to be improved. In contrast, the TpPa-SO3H / SPEEK-1% membrane has better stability, showing an EE higher than that of Nafion 212 after 100 cycles. This improvement is attributed to the introduction of SPEEK into the membrane. In addition, the increase in crystallinity means that the structure of the membrane is more ordered, so destroying the membrane structure requires overcoming a higher energy barrier. Therefore, their synergistic effect ensures the long-term operation of the battery equipped with the TpPa-SO3H / SPEEK-1% membrane. The membrane was characterized after the cycling test to verify the stability of the membrane. SEM surface and cross-sectional images ( Figure 9 b) shows the intact morphology of the membrane.

[0080] Under the condition of SPEEK ratio of 1%, the effect of film thickness on its performance was investigated. Figure 10 As shown in a, the film thickness changes from 8 μm to 20 μm with the increase of the casting solution volume. The cell using the thinnest film exhibits the lowest CE (at 40-140 mA cm -2 90.4%-98.3%)( Figure 10 b), indicating that the membrane's barrier effect on active substances in the electrolyte is weak. As membrane thickness increases, the membrane's barrier capacity strengthens, and CE also improves. However, as membrane thickness increases from 16 μm to 20 μm, CE does not increase further, while VE decreases significantly. This indicates that within this thickness range, the proton transfer barrier is significantly increased, but the barrier effect on BQDS or AQS molecules is not significantly improved. Therefore, based on the combined effects on CE and VE, the EE of cells equipped with a 16 μm membrane is higher than that of cells with membranes of other thicknesses.

[0081] The above description of the embodiments is intended to facilitate understanding and use of the invention by those skilled in the art. It will be apparent that those skilled in the art can readily make various modifications to these embodiments and apply the general principles described herein to other embodiments without requiring inventive effort. Therefore, the present invention is not limited to the above-described embodiments. Improvements and modifications made by those skilled in the art based on the disclosure of the present invention, without departing from the scope of the present invention, should be within the scope of protection of the present invention.

Claims

1. A method for preparing a covalent organic framework self-supporting membrane, characterized in that: The following steps are involved: Step 1: dissolving the ammonia monomer for synthesizing COF in a first solvent to obtain a first pre-reaction solution; Step 2: dissolving the aldehyde monomer and sulfonated polymer chain for synthesizing COF in a second solvent to obtain a second pre-reaction solution; Step 3: mixing the first pre-reaction liquid and the second pre-reaction liquid to obtain a reaction solution, then heating the reaction solution for reaction, activating, and washing to obtain a covalent organic framework self-supporting membrane; The first solvent is selected from a mixture of one or more of dimethyl sulfoxide, 1,4-dioxane, and tetrahydrofuran; The second solvent is selected from a mixture of one or more of N-methylpyrrolidone, mesitylene, and propylene glycol; The sulfonated polymer is selected from one of sulfonated polyetheretherketone, sulfonated polybenzimidazole, and sulfonated polyimide; The -SO3H groups in the introduced sulfonated polymer chains catalyzed the reaction of the COF precursor and promoted reversible bond exchange, which helped increase the crystallinity and more ordered channels of the free-standing membranes. The sulfonated polymer chains penetrated into the gaps between adjacent COF nanoparticles, improving the continuity of proton hopping sites.

2. The method for preparing a covalent organic framework self-supporting membrane according to claim 1, characterized in that: The weight proportion of the sulfonated polymer chains in the covalent organic framework self-supporting membrane is 0.1% to 5%.

3. The method for preparing a covalent organic framework self-supporting membrane according to claim 1, characterized in that: The heating reaction process in step 3 adopts normal pressure reaction, reaction temperature 60-120 ° C, and reaction time 24-144 h.

4. The method for preparing a covalent organic framework self-supporting membrane according to claim 1, characterized in that: In step 3, the activation process is to immerse the self-supporting membrane in an acid solution, wherein the acid solution is a 0.1-2 M sulfuric acid aqueous solution.

5. The method for preparing a covalent organic framework self-supporting membrane according to claim 1, characterized in that: By introducing sulfonated polymer chains into the self-supporting covalent organic framework membrane, the mechanical strength, crystallinity and proton conductivity of the self-supporting membrane are improved.

6. A covalent organic framework self-supporting membrane prepared by the method according to any one of claims 1 to 5, characterized in that: The covalent organic framework self-supporting membrane uses a covalent organic framework material as the main membrane structure, and introduces a sulfonated polymer chain therein. The covalent organic framework material is TpPa-2SO3H or TpPa-SO3H, and the weight proportion of the sulfonated polymer chain in the covalent organic framework self-supporting membrane is 0.1%-5%.

7. Use of the covalent organic framework self-supporting membrane as claimed in claim 6 in an aqueous organic liquid flow battery.

Images