Flow battery composite membrane materials based on Cu-CDs / N-CDs / graphene, their preparation methods and applications

Abstract

This invention discloses a Cu-CDs / N-CDs / graphene-based composite membrane material for flow batteries, its preparation method, and its application, belonging to the field of electrochemical energy storage technology. The method includes: pretreating a microporous membrane, immersing it in a graphene solution, and drying it to obtain a graphene-modified microporous membrane; adding sodium copper chlorophyllin to anhydrous ethanol, ultrasonically dissolving it, performing a hydrothermal reaction, and then post-treating it to prepare a Cu-CDs solution for storage; dissolving ascorbic acid and ethylenediamine in deionized water, performing a hydrothermal reaction, and then post-treating it to prepare an N-CDs solution for storage; adding the Cu-CDs solution dropwise to the positive electrode side of the graphene-modified microporous membrane, and adding the N-CDs solution dropwise to the negative electrode side of the graphene-modified microporous membrane, thus obtaining the Cu-CDs / N-CDs / graphene-based composite membrane material for flow batteries. Modifying the positive electrode side with Cu-CDs solution can effectively alleviate the problems of bromine permeation and sluggish bromine kinetics; modifying the negative electrode side with N-CDs solution can further effectively alleviate the problem of zinc dendrite growth.

Description

Technical Field

[0001] This invention belongs to the field of electrochemical energy storage technology, specifically relating to Cu-CDs / N-CDs / graphene-based flow battery composite membrane materials, their preparation methods, and applications. Background Technology

[0002] Zinc-bromine flow batteries, as an important electrochemical energy storage technology, have broad application prospects in the field of large-scale energy storage due to their advantages such as high energy density, low cost, and long cycle life. However, their industrialization has long been limited by the following core bottlenecks: Cathode: (1) Active bromine species (Br3) - / Br2) penetrates the membrane and triggers self-discharge, resulting in a sharp drop in coulombic efficiency (usually <85%) and continuous capacity decay; (2) positive electrode bromine / bromine (Br2 / Br - The conversion reaction is slow, involving multiple steps and requiring high activation energy. Typically, a complexing agent is added to the positive electrode to complex Br2 and improve its solubility, but this further complicates the Br2 reaction pathway. The slow Br2 reaction becomes a bottleneck in overall battery performance, resulting in high polarization voltage, significantly reducing voltage efficiency and power density, leading to low energy efficiency, limited power density, and low coulombic efficiency. The negative electrode has an uneven electric field distribution, making it prone to zinc dendrite formation during zinc deposition. This dendrites can easily puncture the separator, causing a short circuit, threatening safety, and limiting cycle life (<500 cycles). These bottlenecks directly contribute to the industrialization difficulties of zinc-bromine flow batteries.

[0003] Traditional zinc-bromine flow battery separators suffer from large pore sizes, weak bromine barrier capabilities, and an inability to suppress zinc dendrites. While ceramic membranes offer strong bromine barrier properties, they suffer from high brittleness and low ionic conductivity. Furthermore, existing patented MOF-based bromine barrier materials primarily rely on pore size sieving mechanisms and lack chemisorption mechanisms, further limiting their bromine barrier effectiveness and application scope.

[0004] Therefore, there is an urgent need to develop novel membrane materials that combine high selectivity, high catalytic performance, high ion conductivity, and strong mechanical / chemical stability. Developing a Cu-CDs / N-CDs / graphene-based composite membrane material for flow batteries, along with its preparation method and applications, is of great significance for improving the performance of zinc-bromine flow batteries and promoting their practical application. Summary of the Invention

[0005] In order to overcome the shortcomings of the prior art, the present invention aims to provide a flow battery composite membrane material based on Cu-CDs / N-CDs / graphene, its preparation method and application, so as to solve the technical problems of bromine permeation, slow bromine kinetics and zinc dendrite growth in flow batteries.

[0006] To achieve the above objectives, the present invention employs the following technical solution:

[0007] This invention discloses a method for preparing a flow battery composite film material based on Cu-CDs / N-CDs / graphene, comprising:

[0008] After pretreatment, the microporous membrane was immersed in a graphene solution and dried to obtain a graphene-modified microporous membrane.

[0009] Sodium copper chlorophyll was added to anhydrous ethanol, dissolved by sonication, and after a first hydrothermal reaction, a first filtration, a first dialysis purification, and a first vacuum drying, Cu-CDs were obtained and prepared into Cu-CDs solution for storage.

[0010] Ascorbic acid and ethylenediamine were dissolved in deionized water, and after a second hydrothermal reaction, a second filtration, a second dialysis purification, and a second vacuum drying, N-CDs were obtained and prepared into an N-CDs solution for storage.

[0011] A graphene-modified microporous membrane was laid flat on a vacuum adsorption stage. A Cu-CDs solution was slowly added dropwise to the positive electrode side of the graphene-modified microporous membrane. After standing and a third vacuum drying, an N-CDs solution was slowly added dropwise to the negative electrode side of the graphene-modified microporous membrane. After standing and a fourth vacuum drying, a flow battery composite membrane material based on Cu-CDs / N-CDs / graphene was obtained.

[0012] Preferably, the pretreatment conditions include: ultrasonically cleaning the microporous membrane with ethanol or deionized water for 15-30 minutes to remove surface impurities and contaminants, and then drying it in a vacuum drying oven at 40-60°C for 12-24 hours.

[0013] Preferably, the graphene solution has a mass fraction of 5% to 25%; the soaking time is 30 to 60 minutes; and the drying conditions are: vacuum drying at 60 to 100°C for 10 to 24 hours.

[0014] Preferably, the ratio of sodium copper chlorophyllin to anhydrous ethanol is (80~120) g: (80~120) mL.

[0015] Preferably, the conditions for the first hydrothermal reaction include: reaction at 120~200℃ for 5~8h; the conditions for the first filtration include: filtration with a 0.22μm polytetrafluoroethylene membrane; the conditions for the first dialysis purification include: dialysis purification through a 1000Da cellulose dialysis bag for 12~36h; the conditions for the first vacuum drying include: drying at 60~100℃ for 10~24h; and the storage temperature of the Cu-CDs solution is 2~5℃.

[0016] Preferably, the molar ratio of ascorbic acid to ethylenediamine is 3:(10~12).

[0017] Preferably, the conditions for the second hydrothermal reaction include: reaction at 180~220℃ for 8~12h; the conditions for the second dialysis purification include: dialysis purification via a 1000Da cellulose dialysis bag for 12~36h; the conditions for the second vacuum drying include: drying at 60~100℃ for 10~24h; and the storage temperature of the N-CDs solution is 2~5℃.

[0018] Preferably, the mass concentration of the Cu-CDs solution is 5.0~25.0 mg / mL; the mass concentration of the N-CDs solution is 5.0~25.0 mg / mL.

[0019] The adsorption pressure of the vacuum adsorption stage is -0.10 to -0.06 MPa; the conditions for the third vacuum drying include drying at 60 to 100℃ for 10 to 24 hours; the conditions for the fourth vacuum drying include drying at 60 to 100℃ for 10 to 24 hours.

[0020] The present invention also discloses a flow battery composite membrane material based on Cu-CDs / N-CDs / graphene, which is prepared by the above-mentioned preparation method of the flow battery composite membrane material based on Cu-CDs / N-CDs / graphene.

[0021] The present invention also discloses the application of the Cu-CDs / N-CDs / graphene-based flow battery composite membrane material prepared by the above-mentioned method in the preparation of zinc-bromine flow batteries. The coulombic efficiency of the zinc-bromine flow battery is 95.5%~96.1%; the voltage efficiency is 86.0%~86.9%; and the energy efficiency is 82.13%~83.51%.

[0022] Compared with the prior art, the present invention has the following beneficial effects:

[0023] This invention discloses a method for preparing a Cu-CDs / N-CDs / graphene-based composite membrane material for flow batteries. First, a microporous membrane is modified with a graphene solution to form a graphene-modified microporous membrane. Then, copper-doped carbon dots (Cu-CDs) and nitrogen-doped carbon dots (N-CDs) are synthesized using a hydrothermal method. The positive electrode side of the graphene-modified microporous membrane is modified with a Cu-CDs solution to form a Cu-CDs / graphene-modified microporous membrane, which effectively mitigates bromine permeation and the sluggish bromine kinetics. Next, the negative electrode side of the Cu-CDs / graphene-modified microporous membrane is modified with an N-CDs solution to form a Cu-CDs / N-CDs / graphene-based composite membrane material for flow batteries, which further effectively mitigates zinc dendrite growth. Graphene possesses a highly uniform microporous structure. Allowing small ions to pass through, it can directly block Br2 and Br3. - The graphene physically blocks the bromine permeation pathway, thus achieving bromine inhibition, reducing side reactions, improving coulombic efficiency, and significantly reducing capacity loss. Graphene exhibits good chemical stability in acidic electrolyte environments, is not easily oxidized or degraded by bromine, and its stable framework ensures that its sieve channels remain intact during long-term operation, continuously exerting its bromine inhibition effect and extending its lifespan. Graphene has a highly conductive surface, which can improve the conductivity of Br-... - The increased Br2 redox reaction rate improves battery charge / discharge efficiency. Graphene's high conductivity homogenizes the deposition current, preventing zinc dendrite growth caused by localized high current density. Graphene's mechanical strength effectively prevents zinc dendrites from piercing the separator, preventing short circuits and improving the lifespan of the microporous separator. Cu-CDs exhibit excellent catalytic performance, providing more active sites; the Cu in this material... + / Cu 2+ Redox couples can significantly accelerate Br - The conversion of / Br2 reduces polarization. Cu-CDs surfaces possess a large number of negatively charged -COO2. - It can electrostatically repel Br - / Br3 - Cu + / Cu 2+ Redox couples can suppress Br3 -The generation of N-CDs reduces the bromine permeation source at its root, significantly reducing side reactions and improving coulombic efficiency. Based on the homogenized electric field distribution of graphene, N-CDs can further homogenize the electric field. The tiny gaps between N-CDs form a "nanoreactor," limiting the diffusion space of zinc deposition and forcing zinc to grow uniformly in three dimensions, forming a dense deposition layer rather than zinc dendrites. The ultra-small size and high dispersibility of N-CDs allow them to closely adhere to the membrane surface, forming a nanoscale uniform coating. Their abundant nitrogen-containing functional groups, such as pyridine nitrogen and pyrrole nitrogen, have zinc affinity and can adsorb Zn. 2+ It guides ions to distribute uniformly at the nanoscale, fundamentally reducing zinc dendrite nucleation sites.

[0024] Furthermore, without pretreatment, the surface of the microporous membrane may be adhered to residual release agents, dust, or organic matter from the processing. These contaminants can hinder the uniform adsorption of the subsequent graphene solution, leading to localized defects in the composite membrane material. Ultrasonic cleaning with ethanol or deionized water can thoroughly remove impurities from the micropores of the membrane. The vacuum drying process, completed at a gentle temperature, avoids the shrinkage and deformation of the membrane caused by traditional high-temperature drying, while maintaining the integrity of the membrane's original porous structure, thus providing a foundation for the efficient loading of subsequent graphene modification layers.

[0025] Furthermore, after pretreatment, the microporous membrane is immersed in a graphene solution with a mass fraction of 5%–25% for 30–60 minutes to ensure that the graphene is uniformly dispersed and adheres to the membrane surface and the interior of the pores. Subsequently, vacuum drying is performed. Through the synergistic effect of temperature and vacuum conditions, the graphene layer adheres tightly to the membrane substrate, forming a stable modified layer. In this process, the concentration of the graphene solution and the immersion time jointly regulate the thickness and coverage of the coating, while the drying conditions ensure the structural stability and pore connectivity of the graphene modified layer.

[0026] Furthermore, in the preparation of Cu-CDs, the copper element in sodium copper chlorophyllin salt combines with the carbon framework through coordination to form a quantum dot structure, while anhydrous ethanol, as the sole solvent, avoids interference from other impurities. The precursor can be completely dissolved to form a homogeneous solution, while ensuring that the concentration of copper element in the reaction system is sufficient to form catalytically active Cu-CDs.

[0027] Furthermore, after hydrothermal reaction of sodium copper chlorophyllin in ethanol, undecomposed macromolecular residues are removed by filtration through a polytetrafluoroethylene membrane. Subsequent long-term purification using a cellulose dialysis bag eliminates small-molecule byproducts. The vacuum drying stage avoids structural damage to the carbon quantum dots by controlling the temperature range, and the final Cu-CDs solution maintains its dispersion and catalytic activity under low-temperature storage conditions. This preparation process ensures that the carbon quantum dots have a uniform size distribution and stable surface properties through precise control of reaction parameters and purification conditions.

[0028] Furthermore, a suitable molar ratio of ascorbic acid to ethylenediamine ensures sufficient reaction between the carbon and nitrogen sources to form uniform nitrogen-doped carbon quantum dots. Deionized water ensures complete dissolution of the reactants while preventing quantum dot aggregation due to excessive concentration. During the hydrothermal reaction, the carboxylic acid groups of ascorbic acid and the amino groups of ethylenediamine undergo dehydration condensation to form a carbon core framework. Simultaneously, the amino groups of ethylenediamine are covalently modified on the surface of the carbon quantum dots, forming a zinc-loving pyridine nitrogen structure.

[0029] Furthermore, in a mixed solution of ascorbic acid and ethylenediamine, a hydrothermal reaction promotes the decomposition of the carbon source and nitrogen atom doping, forming N-CDs with abundant nitrogen-containing functional groups. Subsequently, a 1000Da dialysis bag is used for purification to remove unreacted small molecule impurities, ensuring a concentrated particle size distribution of the N-CDs. During vacuum drying, temperature and time are controlled to prevent carbon point agglomeration, maintaining their ultra-small size characteristics. Finally, the N-CDs solution is stored at low temperature to delay oxidation or agglomeration, ensuring the uniform dispersion of the material in subsequent coating processes. This series of process conditions works synergistically to form a high density of pyridine and pyrrole nitrogen functional groups on the surface of the N-CDs, enhancing their adsorption capacity for zinc ions and thus guiding uniform zinc deposition.

[0030] Furthermore, the mass concentrations of Cu-CDs and N-CDs solutions were controlled between 5.0 and 25.0 mg / mL to ensure suitable fluidity, allowing carbon dots to be uniformly loaded onto the graphene-modified membrane surface. During vacuum adsorption, the adsorption pressure was set to -0.10 to -0.06 MPa to promote rapid solution penetration into the membrane pores while preventing graphene layer exfoliation due to excessive pressure. Drying conditions removed residual solvent while maintaining the chemical bonding between carbon dots and graphene, thus forming a stable composite membrane structure.

[0031] This invention also discloses a flow battery composite membrane material based on Cu-CDs / N-CDs / graphene. The composite structure is formed by modifying the surface of a microporous membrane with a graphene layer, and loading a Cu-CDs layer on the positive electrode side and an N-CDs layer on the negative electrode side. The three layers work synergistically to achieve the following functions: the graphene layer physically blocks bromine permeation, and its conductive surface promotes charge transport; the Cu-CDs layer on the positive electrode side reduces polarization by catalyzing the bromine reaction, while simultaneously electrostatically repelling Br. 3- To reduce permeation sources, the N-CDs layer on the negative electrode side inhibits zinc dendrite formation by adsorbing zinc ions and confining the deposition space. The three-layer structure is sequentially loaded under vacuum adsorption conditions to ensure tight interfacial bonding.

[0032] This invention also discloses the application of a Cu-CDs / N-CDs / graphene-based composite membrane material in the preparation of zinc-bromine flow batteries. By modifying the positive and negative electrode sides of a graphene-modified microporous membrane with Cu-CDs and N-CDs solutions respectively, the zinc-bromine flow battery exhibits a coulombic efficiency of 95.5%–96.1%, a voltage efficiency of 86.0%–86.9%, and an energy efficiency of 82.13%–83.51%. The graphene layer directly blocks the penetration of bromine molecules and polybromine ions through a pore size sieving mechanism, cutting off the bromine permeation path, and its chemical stability ensures long-term bromine blocking effect. On the positive electrode side, the negatively charged groups of Cu-CDs electrostatically repel bromine species, and Cu… + / Cu 2+ The redox kinetics of bromine are accelerated by the redox couple, reducing reaction polarization. On the negative electrode side, N-CDs adsorb zinc ions through pyridine and pyrrole nitrogen groups, guiding uniform zinc nucleation within the spatial confinement of the nanoreactor and inhibiting zinc dendrite formation. The functional layers on both sides of the composite membrane work synergistically: on the positive electrode side, bromine permeation is suppressed and the reaction rate is increased; on the negative electrode side, zinc deposition behavior is regulated; and the graphene substrate ensures the overall mechanical strength of the structure. This effectively suppresses bromine permeation during the operation of the zinc-bromine flow battery, reduces positive electrode reaction polarization, and avoids the risk of zinc dendrites piercing the separator by controlling the zinc deposition morphology. While maintaining high ion conductivity, this achieves a synergistic improvement in coulombic efficiency, voltage efficiency, and energy efficiency, extending the battery cycle life. Attached Figure Description

[0033] Figure 1 This is a flowchart of the preparation method of the flow battery composite membrane material based on Cu-CDs / N-CDs / graphene disclosed in this invention. Detailed Implementation

[0034] The technical solution of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0035] Unless otherwise specified, all embodiments and preferred embodiments mentioned herein can be combined to form new technical solutions.

[0036] Unless otherwise specified, all the technical features and preferred features mentioned herein can be combined to form new technical solutions.

[0037] In this invention, unless otherwise specified, percentage (%) or parts refer to weight percentage or parts relative to the composition.

[0038] Unless otherwise specified, the components or preferred components involved in this invention can be combined with each other to form new technical solutions.

[0039] In this invention, unless otherwise specified, the numerical range "a~b" represents a shortened representation of any combination of real numbers between a and b, where a and b are both real numbers. For example, the numerical range "6~22" indicates that all real numbers between "6~22" have been listed in this document, and "6~22" is simply a shortened representation of these numerical combinations.

[0040] The "scope" disclosed in this invention can be in the form of a lower limit and an upper limit, and can be one or more lower limits and one or more upper limits, respectively.

[0041] In this invention, the term "and / or" as used herein refers to any combination of one or more of the associated listed items, as well as all possible combinations, and includes such combinations.

[0042] In this invention, unless otherwise stated, the various reactions or operation steps may be performed sequentially or in a particular order. Preferably, the reaction methods described herein are performed sequentially.

[0043] Unless otherwise stated, the technical and scientific terms used herein have the same meanings as those familiar to those skilled in the art. Furthermore, any methods or materials similar to or equivalent to those described herein may also be used in this invention.

[0044] This invention provides a method for preparing a flow battery composite film material based on Cu-CDs / N-CDs / graphene, comprising the following steps:

[0045] 1) Pretreatment of microporous membranes

[0046] The microporous membrane is cleaned by ultrasonic cleaning with ethanol or deionized water for 15-30 minutes to remove surface impurities and contaminants. It is then dried in a vacuum drying oven at 40-60℃ for 12-24 hours for later use. The microporous membrane is a composite membrane of polypropylene (PP), polyethylene (PE), or polyolefin; its thickness is 0.5-1.0 mm, thickness uniformity is <3%, plasticizer content is <2%, and BET specific surface area is >60-120 m². 2 / g, cumulative pore volume >0.15mL / g, average pore size <20nm.

[0047] 2) Preparation of graphene-modified composite membrane materials

[0048] Graphene solutions with mass fractions of 5%, 10%, 15%, 20%, and 25% were prepared. 10 mL of each solution was taken and a 3 cm × 3 cm microporous membrane was immersed in the solution for 30–60 min. The immersed microporous membrane was then placed in a vacuum drying oven and vacuum dried at 60–100 °C for 10–24 h to obtain the graphene-modified microporous membrane.

[0049] 3) Preparation of Cu-CDs

[0050] 80-120 g of sodium copper chlorophyllin was added to 80-120 mL of anhydrous ethanol and dissolved by sonication. The solution was then transferred to a stainless steel autoclave containing polytetrafluoroethylene (PTFE) and heated in a forced-air drying oven at 120-200 °C for 5-8 h. The reaction product was then filtered through a 0.22 μm PTFE membrane to obtain a yellow-green solution, which was then purified by dialysis using a 1000 Da cellulose dialysis bag for 12-36 h. After vacuum drying at 60-100 °C for 10-24 h, solid Cu-CDs were obtained and stored at 2-5 °C for long-term storage. Cu-CDs solutions of 5.0 mg / mL, 15 mg / mL, and 25 mg / mL were prepared and stored at 4 °C.

[0051] 4) Preparation of N-CDs

[0052] Accurately weigh a certain mass of ascorbic acid and ethylenediamine according to the ratio of n(ascorbic acid):n(ethylenediamine) = 3:(10~12), and place them in 15~25 mL of deionized water. Stir and dissolve. Transfer the solution to a 50 mL high-pressure reactor and heat at 180~220℃ for 8~12 h. After the reactor cools to room temperature, filter to remove large particulate impurities, and then dialyze through a 1000 Da cellulose dialysis bag for 12~36 h to purify. Obtain a brownish-black N-CDs solution, which is then vacuum dried to obtain solid N-CDs. Store the solid N-CDs at 2~5℃ for a long time, and prepare N-CDs solutions of 5.0 mg / mL, 15 mg / mL, and 25 mg / mL, and store them in a refrigerator at 4℃.

[0053] 5) Preparation of Cu-CDs / graphene-modified composite film materials

[0054] The prepared graphene-modified composite membrane material was laid flat on a vacuum adsorption stage with an adsorption pressure of -0.10 to -0.06 MPa. 5 mL of Cu-CDs solution with concentrations of 5 mg / mL, 15 mg / mL, and 25 mg / mL was slowly added dropwise to the positive electrode side of the graphene-modified microporous membrane using a pipette. After standing for 30 min, the membrane was placed in a vacuum drying oven at 60 to 100 °C for 10 to 24 h to obtain the Cu-CDs / graphene-modified microporous membrane.

[0055] 6) Preparation of Cu-CDs / N-CDs / graphene-modified composite film materials

[0056] The prepared Cu-CDs / graphene-modified microporous membrane was laid flat on a vacuum adsorption stage with an adsorption pressure of -0.10 to -0.06 MPa. 5 mL of N-CDs solutions with concentrations of 5 mg / mL, 15 mg / mL, and 25 mg / mL were slowly added dropwise to the negative electrode side of the Cu-CDs / graphene-modified microporous membrane using a pipette. After standing for 30 min, the membrane was dried in a vacuum drying oven at 60 to 100 °C for 10 to 24 h to obtain a Cu-CDs / N-CDs / graphene-based flow battery composite membrane material.

[0057] Figure 1 This is a flowchart of the preparation method of the Cu-CDs / N-CDs / graphene-based composite membrane material for flow batteries disclosed in this invention. As shown in the flowchart, the preparation method of the Cu-CDs / N-CDs / graphene-based composite membrane material for flow batteries includes: pretreating a microporous membrane, immersing it in a graphene solution, and drying it to obtain a graphene-modified microporous membrane; adding sodium copper chlorophyllin to anhydrous ethanol, dissolving it ultrasonically, undergoing hydrothermal reaction, filtering, dialysis purification, and vacuum drying to obtain Cu-CDs, and preparing a Cu-CDs solution for storage; and adding ascorbic acid and... Ethylenediamine was dissolved in deionized water, subjected to hydrothermal reaction, filtered, purified by dialysis, and vacuum dried to obtain N-CDs, which were then prepared into an N-CDs solution for storage. A graphene-modified microporous membrane was laid flat on a vacuum adsorption stage, and a Cu-CDs solution was slowly added dropwise to the positive electrode side of the graphene-modified microporous membrane. After standing and vacuum drying, a Cu-CDs / graphene-modified microporous membrane was obtained. Then, an N-CDs solution was slowly added dropwise to the negative electrode side of the graphene-modified microporous membrane. After standing and vacuum drying, a Cu-CDs / N-CDs / graphene-based flow battery composite membrane material was obtained.

[0058] The membrane materials for zinc-bromine flow batteries have long faced core challenges, including self-discharge caused by bromine permeation at the positive electrode and short circuits due to zinc dendrite growth at the negative electrode. Traditional microporous membranes, due to their uneven pore size distribution, cannot effectively block bromine species permeation and lack a mechanism to inhibit zinc dendrite formation. While ceramic membranes can physically block bromine permeation, their high brittleness and low ionic conductivity make them unsuitable for practical applications. Some improved solutions utilize metal-organic framework materials, but these rely on a single pore size sieving mechanism and do not incorporate chemical adsorption or catalytic functions, thus limiting their bromine blocking effect.

[0059] Research has found that the uniform microporous structure of graphene can construct a physical barrier, but its surface inertness cannot solve the problem of sluggish bromine reaction kinetics. Further investigation revealed that copper-doped carbon dots can accelerate bromine conversion through redox couples, while the nitrogen-containing groups of nitrogen-doped carbon dots can regulate zinc deposition behavior. This led to the following technical concept: constructing a physical barrier layer using graphene as a substrate, introducing copper-doped carbon dots on the positive electrode side to enhance catalytic activity, and introducing nitrogen-doped carbon dots on the negative electrode side to optimize the electric field distribution. The microporous membrane was pretreated and then immersed in a graphene solution, followed by drying to form a graphene-modified layer. Copper-doped and nitrogen-doped carbon dot solutions were prepared separately using a hydrothermal method. The two carbon dot solutions were then loaded onto the positive and negative electrode sides of the membrane using a stepwise vacuum adsorption method, followed by drying to form a composite membrane material.

[0060] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. The components of the embodiments of the present invention described and shown in the accompanying drawings can generally be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0061] Example 1

[0062] A method for preparing a graphene-modified microporous membrane, comprising:

[0063] The microporous membrane was cleaned by ultrasonic cleaning with ethanol for 30 minutes to remove surface impurities and contaminants, and then dried in a vacuum drying oven at 60°C for 24 hours for later use.

[0064] A 5% (w / w) graphene solution was prepared. 10 mL of the solution was used to immerse the pretreated microporous membrane in the solution for 30 min. The immersed microporous membrane was then placed in a vacuum drying oven and dried at 80 °C for 12 h to obtain the graphene-modified microporous membrane.

[0065] Example 2

[0066] A method for preparing a graphene-modified microporous membrane, comprising:

[0067] The microporous membrane was cleaned by ultrasonic cleaning with deionized water for 30 minutes to remove surface impurities and contaminants, and then dried in a vacuum drying oven at 60°C for 24 hours for later use.

[0068] A 10% graphene solution was prepared. 10 mL of the solution was used to immerse the pretreated microporous membrane in the solution for 30 min. The immersed microporous membrane was then placed in a vacuum drying oven and dried at 80 °C for 12 h to obtain the graphene-modified microporous membrane.

[0069] Example 3

[0070] A method for preparing a graphene-modified microporous membrane, comprising:

[0071] The microporous membrane pretreatment steps are the same as in Example 1;

[0072] A 15% (w / w) graphene solution was prepared. 10 mL of the solution was used to immerse the pretreated microporous membrane in the solution for 30 min. The immersed microporous membrane was then placed in a vacuum drying oven and dried at 80 °C for 12 h to obtain the graphene-modified microporous membrane.

[0073] Example 4

[0074] A method for preparing a graphene-modified microporous membrane, comprising:

[0075] The microporous membrane pretreatment steps are the same as in Example 1;

[0076] A 20% graphene solution was prepared. 10 mL of the solution was used to immerse the pretreated microporous membrane in the solution for 30 min. The immersed microporous membrane was then placed in a vacuum drying oven and dried at 80 °C for 12 h to obtain the graphene-modified microporous membrane.

[0077] Example 5

[0078] A method for preparing a graphene-modified microporous membrane, comprising:

[0079] The microporous membrane pretreatment steps are the same as in Example 1;

[0080] A 25% (w / w) graphene solution was prepared. 10 mL of the solution was used to immerse the pretreated microporous membrane in the solution for 30 min. The immersed microporous membrane was then placed in a vacuum drying oven and dried at 80 °C for 12 h to obtain the graphene-modified microporous membrane.

[0081] Example 6

[0082] A method for preparing a Cu-CDs / graphene-modified microporous membrane, comprising:

[0083] The microporous membrane pretreatment steps are the same as in Example 1;

[0084] The preparation process of the graphene-modified microporous membrane is the same as in Example 5;

[0085] 100g of sodium copper chlorophyllin was added to 100mL of anhydrous ethanol and dissolved by sonication. The solution was then transferred to a polytetrafluoroethylene (PTFE) stainless steel autoclave and heated in a 180°C drying oven for 6 hours. The reaction product was then filtered through a 0.22μm PTFE membrane to obtain a yellow-green solution, which was then purified by dialyzing through a 1000Da cellulose dialysis bag for 36 hours. After vacuum drying at 80°C for 12 hours, solid Cu-CDs were obtained and stored at 4°C for long-term storage.

[0086] Prepare a 5.0 mg / mL solution and store it in a refrigerator at 4℃. Spread the prepared graphene-modified composite membrane material on a vacuum adsorption stage with an adsorption pressure of -0.08 MPa. Use a pipette to take 5 mL of 5 mg / mL Cu-CDs solution and slowly add it dropwise to the positive electrode side of the graphene-modified composite membrane material. Let it stand for 30 min and then dry it in a vacuum drying oven at 80℃ for 12 h to obtain a Cu-CDs / graphene-modified microporous membrane.

[0087] Example 7

[0088] A method for preparing a Cu-CDs / graphene-modified microporous membrane, comprising:

[0089] The microporous membrane pretreatment steps are the same as in Example 1;

[0090] The preparation process of the graphene-modified microporous membrane is the same as in Example 5;

[0091] The preparation process of Cu-CDs is the same as in Example 6;

[0092] Prepare a 15.0 mg / mL solution and store it in a refrigerator at 4℃. Spread the prepared graphene-modified composite membrane material on a vacuum adsorption stage with an adsorption pressure of -0.08 MPa. Use a pipette to slowly add 5 mL of 15 mg / mL Cu-CDs solution to the positive electrode side of the graphene-modified composite membrane material. Let it stand for 30 min, and then dry it in a vacuum drying oven at 80℃ for 12 h to obtain a Cu-CDs / graphene-modified microporous membrane.

[0093] Example 8

[0094] A method for preparing a Cu-CDs / graphene-modified microporous membrane, comprising:

[0095] The microporous membrane pretreatment steps are the same as in Example 1;

[0096] The preparation process of the graphene-modified microporous membrane is the same as in Example 5;

[0097] The preparation process of Cu-CDs is the same as in Example 6.

[0098] Prepare a 25.0 mg / mL solution and store it in a refrigerator at 4℃. Spread the prepared graphene-modified composite membrane material on a vacuum adsorption stage with an adsorption pressure of -0.08 MPa. Use a pipette to slowly add 5 mL of 25 mg / mL Cu-CDs solution to the positive electrode side of the graphene-modified composite membrane material. Let it stand for 30 min, and then dry it in a vacuum drying oven at 80℃ for 12 h to obtain a Cu-CDs / graphene-modified microporous membrane.

[0099] Example 9

[0100] A method for preparing a Cu / N-CDs / graphene-based composite membrane material for flow batteries, comprising:

[0101] The microporous membrane pretreatment steps are the same as in Example 1;

[0102] The preparation process of the graphene-modified microporous membrane is the same as in Example 5;

[0103] The preparation process of Cu-CDs is the same as in Example 6;

[0104] The preparation process of the Cu-CDs / graphene modified microporous membrane is the same as in Example 8;

[0105] Ascorbic acid and ethylenediamine were accurately weighed separately at a molar ratio of 3:11 and placed in 20 mL of deionized water. The mixture was stirred and dissolved. The solution was transferred to a 50 mL high-pressure reactor and heated at 180 °C for 10 h. After the reactor cooled to room temperature, large particulate impurities were removed by filtration, and then purified by dialyzing through a 1000 Da cellulose dialysis bag for 36 h. A brownish-black N-CDs solution was obtained, which was dried under vacuum at 80 °C for 12 h to obtain solid N-CDs, which were stored at 4 °C for long-term storage.

[0106] A 5.0 mg / mL N-CDs solution was prepared and stored at 4°C. The prepared Cu-CDs / graphene-modified microporous membrane was laid flat on a vacuum adsorption stage with an adsorption pressure of -0.08 MPa. 5 mL of the 5 mg / mL N-CDs solution was slowly added dropwise to the negative electrode side of the graphene-modified composite membrane material using a pipette. After standing for 30 min, the membrane was dried in a vacuum drying oven at 80°C for 12 h to obtain the Cu-CDs / N-CDs / graphene-based flow battery composite membrane material.

[0107] Example 10

[0108] A method for preparing a flow battery composite membrane material based on Cu-CDs / N-CDs / graphene, comprising:

[0109] The microporous membrane pretreatment steps are the same as in Example 1;

[0110] The preparation process of the graphene-modified microporous membrane is the same as in Example 5;

[0111] The preparation process of Cu-CDs is the same as in Example 6;

[0112] The preparation process of the Cu-CDs / graphene modified microporous membrane is the same as in Example 8;

[0113] The preparation process of N-CDs is the same as in Example 9;

[0114] A 15.0 mg / mL N-CDs solution was prepared and stored at 4°C. The prepared Cu-CDs / graphene-modified microporous membrane was laid flat on a vacuum adsorption stage with an adsorption pressure of -0.08 MPa. 5 mL of a 5 mg / mL N-CDs solution was slowly added dropwise to the negative electrode side of the graphene-modified composite membrane material using a pipette. After standing for 30 min, the membrane was dried in a vacuum drying oven at 80°C for 12 h to obtain the Cu-CDs / N-CDs / graphene-based flow battery composite membrane material.

[0115] Example 11

[0116] A method for preparing a flow battery composite membrane material based on Cu-CDs / N-CDs / graphene, comprising:

[0117] The microporous membrane pretreatment steps are the same as in Example 1;

[0118] The preparation process of the graphene-modified microporous membrane is the same as in Example 5;

[0119] The preparation process of Cu-CDs is the same as in Example 6;

[0120] The preparation process of the Cu-CDs / graphene modified microporous membrane is the same as in Example 8;

[0121] The preparation process of N-CDs is the same as in Example 9;

[0122] A 25.0 mg / mL N-CDs solution was prepared and stored at 4°C. The prepared Cu-CDs / graphene-modified microporous membrane was laid flat on a vacuum adsorption stage with an adsorption pressure of -0.08 MPa. 5 mL of a 5 mg / mL N-CDs solution was slowly added dropwise to the negative electrode side of the graphene-modified composite membrane material using a pipette. After standing for 30 min, the membrane was dried in a vacuum drying oven at 80°C for 12 h to obtain the Cu-CDs / N-CDs / graphene-based flow battery composite membrane material.

[0123] Example 12

[0124] A method for preparing a flow battery composite membrane material based on Cu-CDs / N-CDs / graphene, comprising:

[0125] The microporous membrane was cleaned by ultrasonic cleaning with ethanol for 15 minutes to remove impurities and contaminants from the surface, and then dried in a vacuum drying oven at 40°C for 18 hours for later use.

[0126] A 5% (w / w) graphene solution was prepared. 10 mL of the solution was used to immerse the pretreated microporous membrane in the solution for 45 min. The immersed microporous membrane was then placed in a vacuum drying oven and dried at 60 °C for 24 h to obtain the graphene-modified microporous membrane.

[0127] 80 g of sodium copper chlorophyllin was added to 80 mL of anhydrous ethanol and dissolved by sonication. The solution was then transferred to a polytetrafluoroethylene (PTFE) stainless steel autoclave and heated in a 120 °C oven for 24 h. The reaction product was then filtered through a 0.22 μm PTFE membrane to obtain a yellow-green solution, which was then purified by dialyzing through a 1000 Da cellulose dialysis bag for 36 h. After vacuum drying at 60 °C for 24 h, solid Cu-CDs were obtained and stored at 2 °C for long-term storage.

[0128] Prepare a 5.0 mg / mL solution and store it in a refrigerator at 2℃. Spread the prepared graphene-modified composite membrane material on a vacuum adsorption stage with an adsorption pressure of -0.1 MPa. Use a pipette to slowly add 5 mL of 5 mg / mL Cu-CDs solution to the positive electrode side of the graphene-modified composite membrane material. Let it stand for 30 min, and then dry it in a vacuum drying oven at 60℃ for 24 h to obtain a Cu-CDs / graphene-modified microporous membrane.

[0129] Ascorbic acid and ethylenediamine were accurately weighed according to a ratio of n(ascorbic acid):n(ethylenediamine) = 3:10, and placed in 15 mL of deionized water. The mixture was stirred and dissolved. The solution was transferred to a 50 mL high-pressure reactor and heated at 200 °C for 12 h. After the reactor cooled to room temperature, large particulate impurities were removed by filtration, and then purified by dialyzing through a 1000 Da cellulose dialysis bag for 12 h. A brownish-black N-CDs solution was obtained, which was dried under vacuum at 60 °C for 24 h to obtain solid N-CDs, which were stored at 2 °C for long-term storage.

[0130] A 5.0 mg / mL N-CDs solution was prepared and stored at 2°C. The prepared Cu-CDs / graphene-modified microporous membrane was laid flat on a vacuum adsorption stage with an adsorption pressure of -0.1 MPa. 5 mL of the 5 mg / mL N-CDs solution was slowly added dropwise to the negative electrode side of the graphene-modified composite membrane material using a pipette. After standing for 30 min, the membrane was dried in a vacuum drying oven at 60°C for 24 h to obtain the Cu-CDs / N-CDs / graphene-based flow battery composite membrane material.

[0131] Example 13

[0132] A method for preparing a flow battery composite membrane material based on Cu-CDs / N-CDs / graphene, comprising:

[0133] The microporous membrane was cleaned by ultrasonic cleaning with ethanol for 20 minutes to remove impurities and contaminants from the surface, and then dried in a vacuum drying oven at 50°C for 12 hours for later use.

[0134] A 15% graphene solution was prepared. 10 mL of the solution was used to immerse the pretreated microporous membrane in the solution for 60 min. The immersed microporous membrane was then placed in a vacuum drying oven and dried at 100 °C for 10 h to obtain the graphene-modified microporous membrane.

[0135] 120 g of sodium copper chlorophyllin was added to 120 mL of anhydrous ethanol and dissolved by sonication. The solution was then transferred to a polytetrafluoroethylene (PTFE) stainless steel autoclave and heated in a 200°C drying oven for 12 h. The reaction product was then filtered through a 0.22 μm PTFE membrane to obtain a yellow-green solution, which was then purified by dialyzing through a 1000 Da cellulose dialysis bag for 36 h. After vacuum drying at 100°C for 10 h, solid Cu-CDs were obtained and stored at 5°C for long-term storage.

[0136] Prepare a 25.0 mg / mL solution and store it in a refrigerator at 5℃. Spread the prepared graphene-modified composite membrane material on a vacuum adsorption stage with an adsorption pressure of -0.06 MPa. Use a pipette to take 5 mL of 15 mg / mL Cu-CDs solution and slowly add it dropwise to the positive electrode side of the graphene-modified composite membrane material. Let it stand for 30 min and then dry it in a vacuum drying oven at 100℃ for 10 h to obtain a Cu-CDs / graphene-modified microporous membrane.

[0137] Ascorbic acid and ethylenediamine were accurately weighed separately at a molar ratio of 3:12 and placed in 25 mL of deionized water. The mixture was stirred and dissolved. The solution was transferred to a 50 mL high-pressure reactor and heated at 220 °C for 8 h. After the reactor cooled to room temperature, large particulate impurities were removed by filtration, and then purified by dialyzing through a 1000 Da cellulose dialysis bag for 24 h. A brownish-black N-CDs solution was obtained, which was dried under vacuum at 100 °C for 10 h to obtain solid N-CDs, which were stored at 5 °C for long-term storage.

[0138] A 25.0 mg / mL N-CDs solution was prepared and stored at 5°C. The prepared Cu-CDs / graphene-modified microporous membrane was laid flat on a vacuum adsorption stage with an adsorption pressure of -0.06 MPa. 5 mL of a 15 mg / mL N-CDs solution was slowly added dropwise to the negative electrode side of the graphene-modified composite membrane material using a pipette. After standing for 30 min, the membrane was dried in a vacuum drying oven at 100°C for 10 h to obtain the Cu-CDs / N-CDs / graphene-based flow battery composite membrane material.

[0139] Comparative Example 1

[0140] Comparative Example 1 uses a microporous membrane for battery performance testing, with the membrane pretreatment being the same as in the previous examples. The measured coulombic efficiency was 91.3%, voltage efficiency was 83.2%, and energy efficiency was 75.96%.

[0141] To test the performance of the composite membrane materials for flow batteries in Examples 1-11 and Comparative Example 1, batteries were assembled using conventional electrode materials and electrolyte systems from zinc-bromine flow batteries. The electrodes were commercially available carbon-plastic bipolar plates with an electrode area of ​​9 cm². 2 The electrolyte is ZnBr2. Charge-discharge tests were performed on a battery testing system with a charge-discharge rate of 20 mA / cm². 2 The charging time was 2 hours, and the test temperature was 25°C. The synthesis conditions of the composite membrane materials in different embodiments are shown in Table 1. The electrochemical performance of the zinc-bromine flow battery under different embodiments is compared, and the test results are shown in Table 2.

[0142] Table 1 Synthesis conditions of composite membrane materials in different embodiments

[0143]

[0144] Table 2 Comparison of electrochemical performance of zinc-bromine flow batteries under different embodiment conditions

[0145]

[0146] As can be seen from Table 2, Examples 1-11 all showed better electrochemical performance data than Comparative Example 1, indicating that the CDs / graphene-modified microporous membrane has better electrochemical performance as a zinc-bromine flow battery membrane than the unmodified microporous membrane. This shows that CDs and graphene help improve the energy efficiency, coulombic efficiency, and voltage efficiency of the zinc-bromine flow battery, and that the CDs / graphene-modified microporous membrane has a significant impact on improving bromine barrier capacity, accelerating bromine reaction capacity, and inhibiting zinc dendrite formation.

[0147] Comparing Examples 1-5 and Comparative Example 1, the electrochemical performance data, including parameters such as energy efficiency, voltage efficiency, and coulombic efficiency, were measured when graphene solutions of different mass fractions were used to modify the zinc-bromine flow battery membrane. As can be seen from Table 2, the modification of this microporous membrane with graphene resulted in better electrochemical performance data for the battery, indicating that graphene has a significant impact on improving bromine barrier capacity, accelerating bromine reaction capacity, and inhibiting zinc dendrite formation.

[0148] Compared with Examples 1-5, Example 5 showed better electrochemical performance data, indicating that the composite membrane material made of graphene solution with a higher mass fraction has higher coulombic efficiency, voltage efficiency and energy efficiency. This shows that increasing the mass fraction of graphene solution has a significant impact on improving bromine barrier ability, accelerating bromine reaction ability and inhibiting zinc dendrite formation.

[0149] Comparing Examples 6-8, Example 5, and Comparative Example 1, the electrochemical performance data, including energy efficiency, voltage efficiency, and coulombic efficiency, were measured when the graphene-modified microporous membrane was modified with Cu-CDs solutions of different concentrations and used in a zinc-bromine flow battery. As can be seen from Table 2, modifying the microporous membrane with Cu-CDs / graphene resulted in better electrochemical performance data for the battery, indicating that the addition of Cu-CDs has a significant impact on improving the bromine barrier capacity and accelerating the bromine reaction of the composite material.

[0150] Compared with Examples 6-8, Example 8 showed better electrochemical performance data, indicating that the Cu-CDs / graphene modified microporous membrane prepared with a higher concentration of Cu-CDs solution has higher coulombic efficiency, voltage efficiency and energy efficiency, thus proving that increasing the concentration of Cu-CDs solution has a significant impact on improving bromine inhibition ability and accelerating bromine reaction.

[0151] Comparing Examples 9-11, Example 8, and Comparative Example 1, the electrochemical performance data, including energy efficiency, voltage efficiency, and coulombic efficiency, were measured when the Cu-CDs / graphene-modified microporous membrane was modified with N-CDs solutions of different concentrations and used in a zinc-bromine flow battery. As can be seen from Table 2, modifying the Cu-CDs / graphene-modified microporous membrane with N-CDs resulted in better electrochemical performance data for the battery, indicating that the addition of N-CDs has a significant impact on the ability of the composite material to suppress zinc dendrites.

[0152] Compared with Examples 9-11, Example 11 showed better electrochemical performance data, indicating that the Cu / N-CDs / graphene modified microporous membrane prepared with higher concentration of N-CDs solution has higher coulombic efficiency, voltage efficiency and energy efficiency, thus proving that increasing the N-CDs solution concentration has a significant impact on the ability to suppress zinc dendrites.

[0153] In summary, this invention relates to a Cu-CDs / N-CDs / graphene composite membrane material for flow batteries, its preparation method, and its application. First, the microporous membrane undergoes surface cleaning to remove impurities, followed by immersion in a graphene dispersion to uniformly cover the membrane surface with graphene nanosheets, forming a dense physical barrier. Subsequently, copper-doped and nitrogen-doped carbon dots are synthesized separately, and unreacted substances are removed through dialysis purification. Under vacuum adsorption conditions, the copper-doped carbon dot solution is directionally loaded onto the positive electrode side, utilizing its negative surface charge to repel bromide ions, while the copper active centers accelerate the bromine conversion reaction. After the positive electrode side modification is complete, nitrogen-doped carbon dots are loaded onto the negative electrode side under the same conditions, adsorbing zinc ions through nitrogen-containing groups and guiding uniform deposition. This effectively solves the problem of decreased coulombic efficiency caused by bromine permeation, while inhibiting zinc dendrite growth and extending battery life. The graphene layer provides a stable physical barrier, the copper-doped carbon dots accelerate bromine conversion and reduce side reactions, and the nitrogen-doped carbon dots optimize ion distribution to prevent localized deposition. The synergistic effect of these three elements comprehensively improves the composite membrane's bromine barrier performance, reaction kinetics, and zinc dendrite suppression.

[0154] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing a Cu-CDs / N-CDs / graphene-based flow battery composite membrane material, characterized in that, The application relates to a Cu-CDs / N-CDs / graphene-based flow battery composite membrane material. After pretreatment of the microporous diaphragm, the microporous diaphragm is soaked in a graphene solution, and after drying, a graphene-modified microporous diaphragm is obtained. Sodium copper chlorophyll is added into anhydrous ethanol, ultrasonic dissolution is carried out, Cu-CDs are obtained after first hydrothermal reaction, first filtration, first dialysis purification and first vacuum drying, and a Cu-CDs solution is prepared; Ascorbic acid and ethylenediamine are dissolved in deionized water, N-CDs are obtained after second hydrothermal reaction, second filtration, second dialysis purification and second vacuum drying, and an N-CDs solution is prepared; The graphene-modified microporous diaphragm is laid on a vacuum adsorption table, the Cu-CDs solution is slowly added to the positive electrode side of the graphene-modified microporous diaphragm, and after standing, third vacuum drying is carried out; then the N-CDs solution is slowly added to the negative electrode side of the graphene-modified microporous diaphragm, and after standing, fourth vacuum drying is carried out, thereby obtaining the Cu-CDs / N-CDs / graphene-based flow battery composite membrane material.

2. The method for preparing Cu-CDs / N-CDs / graphene-based flow battery composite membrane material according to claim 1, characterized in that, The pretreatment conditions include that the microporous diaphragm is ultrasonically cleaned with ethanol or deionized water for 15-30 min to remove impurities and pollutants on the surface, and then dried at 40-60 DEG C for 12-24 h.

3. The method for preparing Cu-CDs / N-CDs / graphene-based flow battery composite membrane material according to claim 1, characterized in that, The mass fraction of the graphene solution is 5%-25%, the soaking time is 30-60 min, and the drying conditions are that the graphene solution is dried at 60-100 DEG C under vacuum for 10-24 h.

4. The method for preparing Cu-CDs / N-CDs / graphene-based flow battery composite membrane material according to claim 1, characterized in that, The amount ratio of the sodium copper chlorophyll to the anhydrous ethanol is (80-120) g:(80-120) mL.

5. The method for preparing Cu-CDs / N-CDs / graphene-based flow battery composite membrane material according to claim 1, characterized in that, The first hydrothermal reaction conditions include that the reaction is carried out at 120-200 DEG C for 5-8 h, the first filtration conditions include that the filtration is carried out with a polytetrafluoroethylene membrane with a pore size of 0.22 mu m, the first dialysis purification conditions include that the dialysis purification is carried out with a cellulose dialysis bag with a molecular weight of 1000 Da for 12-36 h, the first vacuum drying conditions include that the vacuum drying is carried out at 60-100 DEG C for 10-24 h, and the storage temperature of the Cu-CDs solution is 2-5 DEG C.

6. The method for preparing Cu-CDs / N-CDs / graphene-based flow battery composite membrane material according to claim 1, characterized in that, The molar ratio of the ascorbic acid to the ethylenediamine is 3:(10-12).

7. The method for preparing Cu-CDs / N-CDs / graphene-based flow battery composite membrane material according to claim 1, characterized in that, The second hydrothermal reaction conditions include that the reaction is carried out at 180-220 DEG C for 8-12 h, the second dialysis purification conditions include that the dialysis purification is carried out with a cellulose dialysis bag with a molecular weight of 1000 Da for 12-36 h, the second vacuum drying conditions include that the vacuum drying is carried out at 60-100 DEG C for 10-24 h, and the storage temperature of the N-CDs solution is 2-5 DEG C.

8. The method for preparing Cu-CDs / N-CDs / graphene-based flow battery composite membrane material according to claim 1, characterized in that, The mass concentration of the Cu-CDs solution is 5.0-25.0 mg / mL, and the mass concentration of the N-CDs solution is 5.0-25.0 mg / mL. The adsorption pressure of the vacuum adsorption table is-0.10--0.06 MPa. The third vacuum drying conditions include that the vacuum drying is carried out at 60-100 DEG C for 10-24 h, and the fourth vacuum drying conditions include that the vacuum drying is carried out at 60-100 DEG C for 10-24 h.

9. A Cu-CDs / N-CDs / graphene-based flow battery composite membrane material, characterized in that, The Cu-CDs / N-CDs / graphene-based flow battery composite membrane material is prepared by the preparation method in any one of claims 1-8.

10. The use of the Cu-CDs / N-CDs / graphene-based composite membrane material for redox flow batteries prepared by the method of any one of claims 1-8 in the preparation of zinc-bromine redox flow batteries, characterized in that, The coulomb efficiency of the zinc-bromine flow battery is 95.5% to 96.1%; the voltage efficiency is 86.0% to 86.9%; and the energy efficiency is 82.13% to 83.51%.

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