Flow battery composite membrane material based on Cu-CDs / N-CDs / graphene, and preparation method and application thereof

Abstract

The invention discloses a Cu-CDs / N-CDs / graphene-based flow battery composite membrane material as well as a preparation method and application thereof, and belongs to the technical field of electrochemical energy storage. The method comprises the following steps: pretreating a microporous diaphragm, soaking the pretreated microporous diaphragm in a graphene solution, and drying the pretreated microporous diaphragm to obtain a graphene-modified microporous diaphragm; the preparation method comprises the following steps: adding sodium copper chlorophyllin into absolute ethyl alcohol, performing ultrasonic dissolution, and performing hydrothermal reaction and post-treatment to prepare a Cu-CDs solution for storage; the preparation method comprises the following steps: dissolving ascorbic acid and ethylenediamine in deionized water, carrying out hydrothermal reaction and post-treatment, and preparing an N-CDs solution for storage; and dropwise adding the Cu-CDs solution to the positive electrode side of the graphene-modified microporous diaphragm, and dropwise adding the N-CDs solution to the negative electrode side of the graphene-modified microporous diaphragm to obtain the Cu-CDs / N-CDs / graphene-based flow battery composite membrane material. The Cu-CDs solution is used for modifying the positive electrode side, so that the problem of bromine permeation and the problem of slow kinetic reaction of bromine can be effectively solved; and the N-CDs solution is used for modifying the negative electrode side, so that the zinc dendrite growth problem can be further effectively alleviated.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of electrochemical energy storage, and particularly relates to a Cu-CDs / N-CDs / graphene-based composite membrane material for flow batteries, a preparation method and application thereof. BACKGROUND

[0002] As an important electrochemical energy storage technology, zinc-bromine flow batteries have a broad application prospect in the field of large-scale energy storage due to high energy density, low cost and long cycle life. However, the industrialization of zinc-bromine flow batteries has been limited for a long time due to the following core bottlenecks: positive electrode: (1) active bromine species (Br3 - / Br2) penetrate the separator to cause self-discharge, resulting in a sharp drop in coulombic efficiency (usually < 85%) and continuous capacity decay; (2) the bromine / bromide (Br2 / Br - ) conversion reaction rate is slow, involving multiple steps and requiring high activation energy. In addition, a complexing agent is usually added to the positive electrode to improve the solubility of Br2, which makes the reaction path of Br2 more complex. The sluggishness of Br2 reaction becomes a short board of the overall battery performance, resulting in high polarization voltage, significantly reduced voltage efficiency and power density, low energy efficiency, limited power density, and low coulombic efficiency. Negative electrode: uneven distribution of electric field, easy formation of zinc dendrites during zinc deposition, easy puncture of the separator to cause short circuit of the battery, threatening safety and limiting cycle life (< 500 times). The above bottlenecks directly lead to the industrialization dilemma of zinc-bromine flow batteries.

[0003] Traditional zinc-bromine flow battery separators have large pore sizes, weak bromine-blocking ability, and cannot inhibit zinc dendrites. Ceramic membranes have strong bromine-blocking ability, but have high brittleness and low ionic conductivity. In addition, the MOF-based bromine-blocking materials disclosed in existing patents mainly rely on pore size sieving mechanism and lack chemical adsorption mechanism, further limiting their bromine-blocking effect and application range.

[0004] Therefore, there is an urgent need to develop a new type of separator material with high selectivity, high catalytic performance, high ionic conductivity, and strong mechanical / chemical stability. The development of a Cu-CDs / N-CDs / graphene-based composite membrane material for flow batteries, a preparation method and application thereof is of great significance to improve the performance of zinc-bromine flow batteries and promote their practical application. SUMMARY

[0005] In order to overcome the shortcomings of the prior art, the purpose of the present application is to provide a Cu-CDs / N-CDs / graphene-based composite membrane material for flow batteries, a preparation method and application thereof, to solve the technical problems of bromine penetration, slow bromine kinetic reaction and zinc dendrite growth in flow batteries.

[0006] In order to achieve the above purpose, the technical scheme adopted by the present application is as follows: The application discloses a preparation method of a flow battery composite membrane material based on Cu-CDs / N-CDs / graphene. After the microporous diaphragm is pretreated, the microporous diaphragm is soaked in a graphene solution, and after drying, a graphene-modified microporous diaphragm is obtained. Sodium copper chlorophyll is added to anhydrous ethanol, ultrasonically dissolved, and after a first hydrothermal reaction, a first filtration, a first dialysis purification and a first vacuum drying, Cu-CDs are obtained, and a Cu-CDs solution is prepared for storage. Ascorbic acid and ethylenediamine are dissolved in deionized water, and after a second hydrothermal reaction, a second filtration, a second dialysis purification and a second vacuum drying, N-CDs are obtained, and an N-CDs solution is prepared for storage. The graphene-modified microporous diaphragm is placed on a vacuum adsorption table, the Cu-CDs solution is slowly added to the positive side of the graphene-modified microporous diaphragm, and after standing and a third vacuum drying, the N-CDs solution is slowly added to the negative side of the graphene-modified microporous diaphragm, and after standing and a fourth vacuum drying, a flow battery composite membrane material based on Cu-CDs / N-CDs / graphene is obtained.

[0007] Preferably, 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 in a vacuum drying box at 40-60 DEG C for 12-24 h.

[0008] Preferably, the mass fraction of the graphene solution is 5%-25%, the soaking time is 30-60 min, and the drying conditions are vacuum drying at 60-100 DEG C for 10-24 h.

[0009] Preferably, the amount ratio of sodium copper chlorophyll to anhydrous ethanol is (80-120) g:(80-120) mL.

[0010] Preferably, 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 0.22 mu m polytetrafluoroethylene membrane, the first dialysis purification conditions include that the dialysis purification is carried out in a 1000 Da cellulose dialysis bag for 12-36 h, the first vacuum drying conditions include that the 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.

[0011] Preferably, the molar ratio of ascorbic acid to ethylenediamine is 3:(10-12).

[0012] Preferably, the second hydrothermal reaction condition comprises 180-220 DEG C reaction for 8-12 hours; the second dialysis purification condition comprises dialysis purification through a 1000 Da cellulose dialysis bag for 12-36 hours; the second vacuum drying condition comprises 60-100 DEG C drying for 10-24 hours; and the N-CDs solution storage temperature is 2-5 DEG C.

[0013] Preferably, 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 to -0.06 MPa; the third vacuum drying condition comprises 60-100 DEG C drying for 10-24 hours; and the fourth vacuum drying condition comprises 60-100 DEG C drying for 10-24 hours.

[0014] The application further discloses a Cu-CDs / N-CDs / graphene-based flow battery composite membrane material prepared by the preparation method.

[0015] The application further discloses application of the Cu-CDs / N-CDs / graphene-based flow battery composite membrane material prepared by the preparation method in preparation of a zinc-bromine flow battery, and the zinc-bromine flow battery has a coulomb efficiency of 95.5-96.1%, a voltage efficiency of 86.0-86.9% and an energy efficiency of 82.13-83.51%.

[0016] Compared with the prior art, the application has the following beneficial effects: The application discloses a preparation method of a Cu-CDs / N-CDs / graphene-based flow battery composite membrane material. First, a microporous diaphragm is modified by a graphene solution to form a graphene-modified microporous diaphragm. Then, copper-doped carbon dots (Cu-CDs) and nitrogen-doped carbon dots (N-CDs) are synthesized by a hydrothermal synthesis method. The Cu-CDs solution is used to modify the positive side of the graphene-modified microporous diaphragm to form a Cu-CDs / graphene-modified microporous diaphragm, which can effectively slow down the bromine permeation problem and the kinetic reaction of bromine. Then, the N-CDs solution is used to modify the negative side of the Cu-CDs / graphene-modified microporous diaphragm to form a Cu-CDs / N-CDs / graphene-based flow battery composite membrane material, which can further effectively slow down the zinc dendrite growth problem. The graphene has a highly uniform microporous structure. Small-sized ions are allowed to pass through, and Br2 and Br3 can be directly blocked.- The penetration of bromine is physically cut off, achieving the function of blocking bromine, reducing side reactions, improving coulomb efficiency, and significantly reducing capacity loss. Graphene exhibits good chemical stability in acidic electrolyte environment and is not easily oxidized and degraded by bromine. The stable framework ensures that the screening channel remains intact during long-term operation, continuously plays a role in blocking bromine, and improves the service life. Graphene has a high conductive surface, which can improve the Br - / Br2 redox reaction rate, and improve the battery charge and discharge efficiency. The high electrical conductivity of graphene can homogenize the deposition current and avoid the growth of zinc dendrites caused by local high current density. The mechanical strength of graphene can effectively block zinc dendrites from penetrating the separator and prevent short circuits, thereby improving the service life of the microporous separator. Cu-CDs have good catalytic performance and can provide more active sites. The Cu + / Cu 2+ redox couple in the material can significantly accelerate the conversion of Br - / Br2 and reduce polarization. The surface of Cu-CDs has a large number of negative charges -COO - , which can electrostatically repel Br - / Br3 - , and the Cu + / Cu 2+ redox couple can inhibit the generation of Br3 - , thereby reducing the source of bromine penetration and having a significant effect on reducing side reactions and improving coulomb efficiency. N-CDs can further homogenize the electric field on the basis of the homogenization of the electric field by graphene. The small gaps between N-CDs form "nanoreactors" that limit the diffusion space of zinc deposition, forcing zinc to grow uniformly in three dimensions and form a dense deposition layer rather than zinc dendrites. The ultra-small size and high dispersity of N-CDs enable them to closely adhere to the surface of the separator and form a nanoscale uniform coating. The abundant nitrogen-containing functional groups on the surface of N-CDs, such as pyridine nitrogen and pyrrole nitrogen, have a zinc affinity that can adsorb Zn 2+ and guide the uniform distribution of ions at the nanoscale, thereby fundamentally reducing the nucleation points of zinc dendrites.

[0017] Further, the surface of the microporous separator may be attached with residual release agents, dust or organic matter during the processing process when the microporous separator is not pretreated. These contaminants can hinder the uniform adsorption of the subsequent graphene solution, resulting in local defects in the composite membrane material. Ultrasonic cleaning with ethanol or deionized water can remove impurities deep inside the microporous separator, and the vacuum drying process is completed at a moderate temperature, which avoids the shrinkage and deformation of the separator caused by traditional high-temperature drying and maintains the integrity of the original porous structure of the separator, providing a basis for efficient loading of the subsequent graphene modification layer.

[0018] Further, the microporous separator is immersed in a graphene solution with a mass fraction of 5% to 25% for 30 to 60 minutes after pretreatment, so that the graphene is uniformly dispersed and attached to the surface and inside the pores of the separator. Subsequently, vacuum drying is performed, and the synergistic effect of temperature and vacuum conditions promotes the graphene layer to closely adhere to the separator substrate, forming a stable modified layer. In this process, the concentration of the graphene solution and the soaking time jointly control the thickness and coverage of the coating, and the drying conditions ensure the structural stability and pore connectivity of the graphene modified layer.

[0019] Further, in the process of preparing Cu-CDs, the copper element of sodium copper chlorophyllin is combined with the carbon skeleton to form a quantum dot structure through coordination, and anhydrous ethanol as a single solvent can avoid 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 Cu-CDs with catalytic activity.

[0020] Further, after the sodium copper chlorophyllin is subjected to a hydrothermal reaction in ethanol, the undecomposed macromolecular residues can be removed by filtering through a polytetrafluoroethylene membrane, and then a cellulose dialysis bag is used for long-term purification to eliminate small molecule byproducts. In the vacuum drying stage, the temperature range is controlled to avoid damage to the carbon quantum dot structure, and the finally obtained Cu-CDs solution maintains its dispersion state and catalytic activity under low-temperature storage conditions. This preparation process ensures that the carbon quantum dots have uniform size distribution and stable surface properties by precisely controlling the reaction parameters and purification conditions.

[0021] Further, the appropriate molar ratio of ascorbic acid to ethylenediamine ensures that the carbon source and nitrogen source fully react to form uniform nitrogen-doped carbon quantum dots. Deionized water can ensure that the reactants are fully dissolved, and can also avoid quantum dot agglomeration due to high concentration. During the hydrothermal reaction, the carboxyl group of ascorbic acid and the amino group of ethylenediamine form a carbon nucleus skeleton through dehydration condensation, and the amino group of ethylenediamine is modified on the surface of the carbon quantum dots through covalent bonds, forming a pyridine nitrogen structure with zinc affinity.

[0022] Further, in the mixed solution of ascorbic acid and ethylenediamine, the carbon source is decomposed and nitrogen atoms are doped through a hydrothermal reaction to form N-CDs with rich nitrogen-containing functional groups; then a 1000 Da dialysis bag is used for purification to remove unreacted small molecule impurities, ensuring that the particle size distribution of N-CDs is concentrated; during the vacuum drying process, the temperature and time are controlled to avoid carbon dot agglomeration, maintaining its ultra-small size characteristics; finally, the N-CDs solution is stored at low temperature to slow down oxidation or agglomeration, ensuring the dispersion uniformity of the material in the subsequent coating process. The synergistic effect of these process conditions enables the formation of a high-density pyridine nitrogen and pyrrole nitrogen functional group on the surface of N-CDs, enhancing its adsorption capacity for zinc ions and guiding the uniform deposition of zinc.

[0023] Further, the mass concentration of the Cu-CDs solution and the N-CDs solution is controlled to be 5.0-25.0 mg / mL, so that the solution has appropriate fluidity, and the carbon dots are uniformly loaded on the surface of the graphene modified diaphragm. In the vacuum adsorption process, the adsorption pressure is set to be-0.10--0.06 MPa, so that the solution can quickly penetrate into the pores of the diaphragm, and the graphene layer is prevented from being peeled off due to the excessively high pressure. The drying condition can remove the residual solvent while maintaining the chemical bonding between the carbon dots and the graphene, so that a stable composite membrane structure is formed.

[0024] The application further discloses a Cu-CDs / N-CDs / graphene-based flow battery composite membrane material. 3- The Cu-CDs layer on the positive electrode side reduces the polarization by catalyzing the bromine reaction, and the N-CDs layer on the negative electrode side inhibits the formation of zinc dendrites by adsorbing zinc ions and limiting the deposition space.

[0025] The application further discloses an application of the Cu-CDs / N-CDs / graphene-based flow battery composite membrane material in preparing a zinc-bromine flow battery. + The Cu-CDs on the positive electrode side accelerate the redox kinetics of bromine by forming an electrostatic repulsion with bromine species, and the N-CDs on the negative electrode side guide the uniform nucleation of zinc by adsorbing zinc ions through pyridine nitrogen and pyrrole nitrogen groups, thereby inhibiting the formation of zinc dendrites. 2+ The composite membrane effectively inhibits the bromine penetration phenomenon in the operation of the zinc-bromine flow battery, reduces the positive electrode reaction polarization, and avoids the risk of zinc dendrites piercing the diaphragm by regulating the zinc deposition morphology. BRIEF DESCRIPTION OF DRAWINGS

[0026] Figure 1 A flow chart of the preparation method of the Cu-CDs / N-CDs / graphene-based flow battery composite membrane material disclosed in the present application. DETAILED DESCRIPTION

[0027] The technical solutions of the present application will be described below in a clear and complete manner. Obviously, the described embodiments are part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of protection of the present application.

[0028] In the present application, all the embodiments and preferred embodiments mentioned in the present application can be combined to form new technical solutions, unless otherwise specified.

[0029] In the present application, all the technical features and preferred features mentioned in the present application can be combined to form new technical solutions, unless otherwise specified.

[0030] In the present application, unless otherwise specified, the percentage (%) or part refers to the percentage by weight or weight part of the composition.

[0031] In the present application, unless otherwise specified, each component or its preferred component involved can be combined to form a new technical solution.

[0032] In the present application, unless otherwise specified, the numerical range "a~b" represents a shorthand notation for any real number combination between a and b, where a and b are both real numbers. For example, the numerical range "6~22" represents that all real numbers between "6~22" have been listed herein, and "6~22" is only a shorthand notation for these numerical combinations.

[0033] The lower limit and upper limit of the range disclosed in the present application can be one or more lower limits and one or more upper limits, respectively.

[0034] In the present application, the term "and / or" used herein means any combination of one or more of the associated listed items and all possible combinations, and includes these combinations.

[0035] In the present application, unless otherwise specified, each reaction or operation step can be performed sequentially or according to the sequence. Preferably, the reaction method herein is performed sequentially.

[0036] Unless otherwise specified, the professional and scientific terms used herein have the same meaning as those familiar to those skilled in the art. In addition, any method or material similar or equivalent to that described can also be applied in the present application.

[0037] The application provides a preparation method of a Cu-CDs / N-CDs / graphene-based flow battery composite membrane material, and comprises the following steps: 1) Pretreatment of microporous diaphragm The microporous diaphragm is subjected to cleaning treatment, and can be subjected to ultrasonic cleaning with ethanol or deionized water for 15-30 min to remove impurities and pollutants on the surface, and then is dried in a vacuum drying box at 40-60°C for 12-24 h for standby use. The microporous diaphragm is a polypropylene (PP) diaphragm, a polyethylene (PE) diaphragm or a polyolefin composite diaphragm; the thickness of the microporous diaphragm is 0.5-1.0 mm, the thickness uniformity is <3%, the plasticizer content is <2%, the BET specific surface area is >60-120 m 2 / g, the cumulative pore volume is >0.15 mL / g, and the average pore size is <20 nm.

[0038] 2) Preparation of graphene-modified composite membrane material Graphene solutions with mass fractions of 5%, 10%, 15%, 20% and 25% are prepared, and each 10 mL of the graphene solution is used to immerse the pretreated 3 cm*3 cm microporous diaphragm in the solution for 30-60 min, and then the immersed microporous diaphragm is placed in a vacuum drying box and dried at 60-100°C for 10-24 h to obtain a graphene-modified microporous diaphragm.

[0039] 3) Preparation of Cu-CDs 80-120 g of sodium copper chlorophyll salt is added to 80-120 mL of anhydrous ethanol and ultrasonically dissolved. Then the solution is transferred to a stainless steel high-pressure reaction kettle made of polytetrafluoroethylene, and heated in a 120-200°C blast drying box, and after 5-8 h, the reaction product is filtered with a 0.22 μm polytetrafluoroethylene membrane to obtain a yellow-green solution, and then purified by dialysis with a 1000 Da cellulose dialysis bag for 12-36 h. After vacuum drying at 60-100°C for 10-24 h, the solid Cu-CDs is stored at 2-5°C for a long time, and 5.0 mg / mL, 15 mg / mL and 25 mg / mL Cu-CDs solutions are prepared and stored in a 4°C refrigerator.

[0040] 4) Preparation of N-CDs According to n (ascorbic acid): n (ethylenediamine) = 3: (10~12), a certain mass of ascorbic acid and ethylenediamine is accurately weighed and placed in 15~25 mL of deionized water, stirred and dissolved. The solution is transferred to a 50 mL high-pressure reaction kettle, heated at 180~220℃ for 8~12h. After the reaction kettle is cooled to room temperature, filter out the large particle impurities, and then purify by 1000 Da cellulose dialysis bag for 12~36h. A brown-black solution of N-CDs is obtained, which is vacuum dried to obtain a solid of N-CDs, which is stored at 2~5℃ for a long time. Prepare 5.0 mg / mL, 15 mg / mL, and 25 mg / mL N-CDs solutions and store them in a 4℃ refrigerator.

[0041] 5) Preparation of Cu-CDs / graphene modified composite membrane material The prepared graphene modified composite membrane material is laid on a vacuum adsorption table with an adsorption pressure of -0.10~-0.06 MPa. 5 mL of 5 mg / mL, 15 mg / mL, and 25 mg / mL Cu-CDs solution is taken with a pipette and slowly added to the positive side of the graphene modified microporous separator. After standing for 30 min, it is placed in a vacuum drying oven at 60~100℃ for 10~24h to obtain a Cu-CDs / graphene modified microporous separator.

[0042] 6) Preparation of Cu-CDs / N-CDs / graphene modified composite membrane material The prepared Cu-CDs / graphene modified microporous separator is laid on a vacuum adsorption table with an adsorption pressure of -0.10~-0.06 MPa. 5 mL of 5 mg / mL, 15 mg / mL, and 25 mg / mL N-CDs solution is taken with a pipette and slowly added to the negative side of the Cu-CDs / graphene modified microporous separator. After standing for 30 min, it is placed in a vacuum drying oven at 60~100℃ for 10~24h to obtain a Cu-CDs / N-CDs / graphene based flow battery composite membrane material.

[0043] Figure 1A flow chart of the preparation method of the Cu-CDs / N-CDs / graphene-based flow battery composite membrane material disclosed in the application; as can be seen from the figure, the preparation method of the Cu-CDs / N-CDs / graphene-based flow battery composite membrane material comprises the following steps: after pretreatment of a microporous separator, the microporous separator is soaked in a graphene solution, and after drying, a graphene-modified microporous separator is obtained; sodium copper chlorophyll is added to anhydrous ethanol, ultrasonically dissolved, subjected to hydrothermal reaction, filtration, dialysis purification, vacuum drying, and then Cu-CDs are obtained, and a Cu-CDs solution is prepared for storage; ascorbic acid and ethylenediamine are dissolved in deionized water, subjected to hydrothermal reaction, filtration, dialysis purification, vacuum drying, and then N-CDs are obtained, and an N-CDs solution is prepared for storage; the graphene-modified microporous separator is laid on a vacuum adsorption table, the Cu-CDs solution is slowly added to the positive electrode side of the graphene-modified microporous separator, and after standing and vacuum drying, a Cu-CDs / graphene-modified microporous separator is obtained; and then the N-CDs solution is slowly added to the negative electrode side of the graphene-modified microporous separator, and after standing and vacuum drying, a Cu-CDs / N-CDs / graphene-based flow battery composite membrane material is obtained.

[0044] The separator material of the zinc-bromine flow battery has long been faced with the core problems of self-discharge caused by positive electrode bromine penetration and short circuit caused by negative electrode zinc dendrite growth. The traditional microporous separator cannot effectively block the penetration of bromine species due to uneven pore size distribution, and lacks a mechanism to inhibit zinc dendrites. Although ceramic membranes can physically block bromine penetration, they are high in brittleness and low in ion conductivity, and are difficult to meet the actual application requirements. Some improved schemes use metal organic framework materials, but rely on a single pore size screening mechanism and do not introduce chemical adsorption or catalytic functions, so the bromine blocking effect is limited.

[0045] Research has found that the uniform microporous structure of graphene can build a physical barrier, but its surface inertness cannot solve the problem of slow bromine reaction kinetics. Further research has found that copper-doped carbon dots can accelerate bromine conversion through redox couples, and nitrogen-doped carbon dots can regulate zinc deposition behavior through nitrogen-containing groups. Therefore, the technical concept is formed: graphene is used as a substrate to build a physical barrier layer, copper-doped carbon dots are introduced on the positive electrode side to improve catalytic activity, and nitrogen-doped carbon dots are introduced on the negative electrode side to optimize the electric field distribution. After pretreatment of a microporous separator, the microporous separator is soaked in a graphene solution, and a graphene modification layer is formed after drying; copper-doped carbon dot and nitrogen-doped carbon dot solutions are prepared by a hydrothermal method; the two kinds of carbon dot solutions are loaded on the positive and negative electrode sides of the separator by a step-by-step vacuum adsorption method, and a composite membrane material is formed after drying.

[0046] In order to make the purposes, technical solutions and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be described clearly and completely below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only some but not all of the embodiments of the present application. The components of the embodiments of the present application described and shown in the drawings herein can be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of the present application provided in the drawings is not intended to limit the scope of the claimed present application, but only represents selected embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative work fall within the scope of protection of the present application.

[0047] Embodiment 1 A preparation method of a graphene modified microporous separator, comprising: The microporous separator is subjected to cleaning treatment, and the porous membrane is ultrasonically cleaned with ethanol for 30 min to remove impurities and pollutants on the surface, and then dried in a vacuum drying box at 60 DEG C for 24 h for standby.

[0048] A graphene solution with a mass fraction of 5% is prepared, 10 mL of which is taken, and the pretreated microporous separator is soaked in the solution for 30 min. The soaked microporous separator is placed in a vacuum drying box and dried at 80 DEG C for 12 h to obtain a graphene modified microporous separator.

[0049] Embodiment 2 A preparation method of a graphene modified microporous separator, comprising: The microporous separator is subjected to cleaning treatment, and the porous membrane is ultrasonically cleaned with deionized water for 30 min to remove impurities and pollutants on the surface, and then dried in a vacuum drying box at 60 DEG C for 24 h for standby.

[0050] A graphene solution with a mass fraction of 10% is prepared, 10 mL of which is taken, and the pretreated microporous separator is soaked in the solution for 30 min. The soaked microporous separator is placed in a vacuum drying box and dried at 80 DEG C for 12 h to obtain a graphene modified microporous separator.

[0051] Embodiment 3 A preparation method of a graphene modified microporous separator, comprising: The microporous separator pretreatment step is the same as that in Embodiment 1; A graphene solution with a mass fraction of 15% is prepared, 10 mL of which is taken, and the pretreated microporous separator is soaked in the solution for 30 min. The soaked microporous separator is placed in a vacuum drying box and dried at 80 DEG C for 12 h to obtain a graphene modified microporous separator.

[0052] Embodiment 4 A method for preparing a graphene modified microporous separator, comprising: The microporous separator pretreatment step is the same as in Example 1; A graphene solution with a mass fraction of 20% is prepared, 10 mL of which is taken and used to soak the pretreated microporous separator for 30 min. The soaked microporous separator is placed in a vacuum drying oven and dried at 80°C for 12 h to obtain a graphene modified microporous separator.

[0053] Example 5 A method for preparing a graphene modified microporous separator, comprising: The microporous separator pretreatment step is the same as in Example 1; A graphene solution with a mass fraction of 25% is prepared, 10 mL of which is taken and used to soak the pretreated microporous separator for 30 min. The soaked microporous separator is placed in a vacuum drying oven and dried at 80°C for 12 h to obtain a graphene modified microporous separator.

[0054] Example 6 A method for preparing a Cu-CDs / graphene modified microporous separator, comprising: The microporous separator pretreatment step is the same as in Example 1; The graphene modified microporous separator preparation process is the same as in Example 5; 100 g of sodium copper chlorophyll salt is added to 100 mL of anhydrous ethanol and ultrasonically dissolved. Then the solution is transferred to a stainless steel high-pressure reaction kettle made of polytetrafluoroethylene, heated in a blast drying oven at 180°C, and after 6 h, the reaction product is filtered with a 0.22 μm polytetrafluoroethylene membrane to obtain a yellow-green solution, which is then purified by dialysis in a 1000 Da cellulose dialysis bag for 36 h. After vacuum drying at 80°C for 12 h, the solid Cu-CDs is obtained and stored at 4°C for a long time.

[0055] A 5.0 mg / mL solution is prepared and stored in a 4°C refrigerator. The prepared graphene modified composite membrane material is placed on a vacuum adsorption table with an adsorption pressure of -0.08 MPa. 5 mL of 5 mg / mL Cu-CDs solution is taken with a pipette and slowly added to the positive side of the graphene modified composite membrane material. After standing for 30 min, it is placed in a vacuum drying oven at 80°C for 12 h to obtain a Cu-CDs / graphene modified microporous separator.

[0056] Example 7 A method for preparing a Cu-CDs / graphene modified microporous separator, comprising: The microporous separator pretreatment step is the same as in Example 1; The graphene modified microporous separator preparation process is the same as in Example 5; The preparation process of Cu-CDs is the same as that in Example 6. Formed into a 15.0 mg / mL solution and stored in a 4°C refrigerator. The prepared graphene-modified composite membrane material was laid on a vacuum adsorption table with an adsorption pressure of-0.08 MPa. 5 mL of the prepared Cu-CDs solution was slowly added dropwise to the positive electrode side of the graphene-modified composite membrane material, and the graphene-modified composite membrane material was dried in a vacuum drying oven at 80°C for 12 h to obtain the Cu-CDs / graphene-modified microporous separator.

[0057] Example 8 A method for preparing a Cu-CDs / graphene-modified microporous separator, comprising: The microporous separator pretreatment step is the same as that in Example 1. The graphene-modified microporous separator preparation process is the same as that in Example 5. The preparation process of Cu-CDs is the same as that in Example 6, Formed into a 25.0 mg / mL solution and stored in a 4°C refrigerator. The prepared graphene-modified composite membrane material was laid on a vacuum adsorption table with an adsorption pressure of-0.08 MPa. 5 mL of the prepared Cu-CDs solution was slowly added dropwise to the positive electrode side of the graphene-modified composite membrane material, and the graphene-modified composite membrane material was dried in a vacuum drying oven at 80°C for 12 h to obtain the Cu-CDs / graphene-modified microporous separator.

[0058] Example 9 A method for preparing a Cu / N-CDs / graphene-based flow battery composite membrane material, comprising: The microporous separator pretreatment step is the same as that in Example 1. The graphene-modified microporous separator preparation process is the same as that in Example 5. The preparation process of Cu-CDs is the same as that in Example 6; The preparation process of Cu-CDs / graphene-modified microporous separator is the same as that in Example 8. Ascorbic acid and ethylenediamine were accurately weighed in a molar ratio of 3:11 and placed in 20 mL of deionized water, stirred and dissolved. The solution was transferred to a 50 mL high-pressure reaction kettle and heated at 180°C for 10 h. After the reaction kettle was cooled to room temperature, the large particle impurities were removed by filtration, and then purified by dialysis in a 1000 Da cellulose dialysis bag for 36 h. A brown-black N-CDs solution was obtained, which was dried at 80°C under vacuum for 12 h to obtain N-CDs solids, which were stored at 4°C for a long time.

[0059] The N-CDs solution was prepared at 5.0 mg / mL and stored in a refrigerator at 4°C. The prepared Cu-CDs / graphene modified microporous separator was placed on a vacuum adsorption table with an adsorption pressure of -0.08 MPa. 5 mL of the N-CDs solution at 5 mg / mL was slowly added to the negative side of the graphene modified composite membrane material using a pipette, and the mixture was left to stand for 30 min. The mixture was then dried in a vacuum drying oven at 80°C for 12 h to obtain a Cu-CDs / N-CDs / graphene-based flow battery composite membrane material.

[0060] Example 10 A method for preparing a Cu-CDs / N-CDs / graphene-based flow battery composite membrane material, comprising: The microporous separator pretreatment step was the same as in Example 1. The graphene modified microporous separator was prepared according to the process in Example 5. The Cu-CDs were prepared according to the process in Example 6. The Cu-CDs / graphene modified microporous separator was prepared according to the process in Example 8. The N-CDs were prepared according to the process in Example 9. The N-CDs solution was prepared at 5.0 mg / mL and stored in a refrigerator at 4°C. The prepared Cu-CDs / graphene modified microporous separator was placed on a vacuum adsorption table with an adsorption pressure of -0.08 MPa. 5 mL of the N-CDs solution at 5 mg / mL was slowly added to the negative side of the graphene modified composite membrane material using a pipette, and the mixture was left to stand for 30 min. The mixture was then dried in a vacuum drying oven at 80°C for 12 h to obtain a Cu-CDs / N-CDs / graphene-based flow battery composite membrane material.

[0061] Example 11 A method for preparing a Cu-CDs / N-CDs / graphene-based flow battery composite membrane material, comprising: The microporous separator pretreatment step was the same as in Example 1. The graphene modified microporous separator was prepared according to the process in Example 5. The Cu-CDs were prepared according to the process in Example 6. The Cu-CDs / graphene modified microporous separator was prepared according to the process in Example 8. The N-CDs were prepared according to the process in Example 9. N-CDs solution was prepared to 25.0 mg / mL and stored in a 4℃ refrigerator. The prepared Cu-CDs / graphene modified microporous separator was placed on a vacuum adsorption table with an adsorption pressure of-0.08 MPa. 5 mL of 5 mg / mL N-CDs solution was slowly added to the negative side of the graphene modified composite membrane material using a pipette, and left to stand for 30 min. The Cu-CDs / N-CDs / graphene based flow battery composite membrane material was obtained by drying in a vacuum drying oven at 80℃ for 12 h.

[0062] Example 12 A method for preparing a Cu-CDs / N-CDs / graphene based flow battery composite membrane material, comprising: The microporous separator was cleaned by ultrasonic cleaning with ethanol for 15 min to remove impurities and contaminants on the surface, and then dried in a vacuum drying oven at 40℃ for 18 h for standby use.

[0063] A graphene solution with a mass fraction of 5% was prepared, and 10 mL was taken. The pretreated microporous separator was immersed in the solution for 45 min, and then the immersed microporous separator was placed in a vacuum drying oven and dried at 60℃ for 24 h to obtain a graphene modified microporous separator.

[0064] 80 g of sodium copper chlorophyll salt was added to 80 mL of anhydrous ethanol and ultrasonically dissolved. The solution was then transferred to a stainless steel high-pressure reaction kettle made of polytetrafluoroethylene, heated in a 120℃ blast drying oven, and after 24 h, the reaction product was filtered with a 0.22 μm polytetrafluoroethylene membrane to obtain a yellow-green solution, which was then purified by dialysis in a 1000 Da cellulose dialysis bag for 36 h. After vacuum drying at 60℃ for 24 h, the solid Cu-CDs was obtained and stored at 2℃ for long-term storage.

[0065] A 5.0 mg / mL solution was prepared and stored in a 2℃ refrigerator. The prepared graphene modified composite membrane material was placed on a vacuum adsorption table with an adsorption pressure of-0.1 MPa. 5 mL of 5 mg / mL Cu-CDs solution was slowly added to the positive side of the graphene modified composite membrane material using a pipette, and left to stand for 30 min. The Cu-CDs / graphene modified microporous separator was obtained by drying in a vacuum drying oven at 60℃ for 24 h.

[0066] Accurately weigh ascorbic acid and ethylenediamine in a molar ratio of n (ascorbic acid): n (ethylenediamine) = 3:10 into 15 mL of deionized water, and stir and dissolve. Transfer the solution into a 50 mL high-pressure reaction kettle, and heat to react at 200°C for 12 h. After the reaction kettle is cooled to room temperature, remove large-particle impurities by filtration, and then purify by dialysis in a 1000 Da cellulose dialysis bag for 12 h. A brown-black solution of N-CDs is obtained, and after vacuum drying at 60°C for 24 h, solid N-CDs is obtained and stored at 2°C for long-term preservation.

[0067] Prepare a solution of N-CDs at 5.0 mg / mL, and store in a refrigerator at 2°C. Place the prepared Cu-CDs / graphene-modified microporous separator on a vacuum adsorption table at an adsorption pressure of -0.1 MPa, and use a pipette to take 5 mL of the solution of N-CDs at 5 mg / mL, and slowly drop onto the negative side of the graphene-modified composite membrane material, and stand for 30 min, and then place in a vacuum drying oven at 60°C for drying for 24 h, to obtain a Cu-CDs / N-CDs / graphene-based flow battery composite membrane material.

[0068] Example 13 A method for preparing a Cu-CDs / N-CDs / graphene-based flow battery composite membrane material, comprising: Wash the microporous separator, and ultrasonically wash the porous membrane with ethanol for 20 min to remove impurities and contaminants on the surface, and then dry in a vacuum drying oven at 50°C for 12 h, and reserve for use.

[0069] Prepare a graphene solution at a mass fraction of 15%, and take 10 mL, and immerse the pretreated microporous separator in the solution for 60 min, and then place the immersed microporous separator in a vacuum drying oven, and dry at 100°C for 10 h, to obtain a graphene-modified microporous separator.

[0070] Add 120 g of sodium copper chlorophyllin to 120 mL of anhydrous ethanol, and ultrasonically dissolve. Then transfer the solution into a stainless steel high-pressure reaction kettle made of polytetrafluoroethylene, and heat in a blast drying oven at 200°C, and after 12 h, filter the reaction product with a 0.22 μm polytetrafluoroethylene membrane, to obtain a yellow-green solution, and then purify by dialysis in a 1000 Da cellulose dialysis bag for 36 h. After vacuum drying at 100°C for 10 h, obtain solid Cu-CDs, and store at 5°C for long-term preservation.

[0071] 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.

[0072] 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.

[0073] 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.

[0074] Comparative Example 1 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%.

[0075] 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 at 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.

[0076] Table 1 Synthesis conditions of composite membrane materials in different embodiments

[0077] Table 2 Comparison of electrochemical performance of zinc-bromine flow batteries under different embodiment conditions

[0078] 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.

[0079] 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.

[0080] 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.

[0081] 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.

[0082] 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.

[0083] 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.

[0084] 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.

[0085] In summary, this invention relates to a Cu-CDs / N-CDs / graphene composite membrane material for flow batteries, its preparation method, and its applications. First, the microporous membrane undergoes surface cleaning to remove impurities, followed by immersion in a graphene dispersion to allow graphene nanosheets to uniformly cover the membrane surface, 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.

[0086] 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 flow battery composite membrane material based on Cu-CDs / N-CDs / graphene, characterized in that, include: After pretreatment, the microporous membrane was immersed in a graphene solution and dried to obtain a graphene-modified microporous membrane. 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 a Cu-CDs solution was prepared. 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. 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.

2. The method for preparing the Cu-CDs / N-CDs / graphene-based composite membrane material for flow batteries according to claim 1, characterized in that, 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 at 40-60°C for 12-24 hours.

3. The method for preparing the flow battery composite film material based on Cu-CDs / N-CDs / graphene according to claim 1, characterized in that, 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.

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

5. The method for preparing the Cu-CDs / N-CDs / graphene-based composite membrane material for flow batteries according to claim 1, characterized in that, 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℃.

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

7. The method for preparing the flow battery composite film material based on Cu-CDs / N-CDs / graphene according to claim 1, characterized in that, 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℃.

8. The method for preparing the Cu-CDs / N-CDs / graphene-based composite membrane material for flow batteries according to claim 1, characterized in that, The Cu-CDs solution has a mass concentration of 5.0~25.0 mg / mL; the N-CDs solution has a mass concentration of 5.0~25.0 mg / mL; 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 60~100℃ for 10~24h; the conditions for the fourth vacuum drying include: drying at 60~100℃ for 10~24h.

9. A flow battery composite membrane material based on Cu-CDs / N-CDs / graphene, characterized in that, The composite membrane material for flow batteries based on Cu-CDs / N-CDs / graphene, as described in any one of claims 1 to 8, was prepared using this method.

10. The application of the Cu-CDs / N-CDs / graphene-based flow battery composite membrane material prepared by the method of any one of claims 1 to 8 in the preparation of zinc-bromine flow batteries, characterized in that... The zinc-bromine flow battery has 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%.

Images