Composite bipolar plate for zinc-bromine flow battery and preparation method of composite bipolar plate

Abstract

The invention belongs to the technical field of electrochemical energy storage, and discloses a composite bipolar plate for a zinc-bromine flow battery and a preparation method thereof.The preparation method comprises the steps that carbon fibers, ketjen black and graphene dispersion liquid are stirred to be uniform, and a conductive carbon composite material is obtained; a binder, a dispersing agent and the conductive carbon composite material are subjected to ball milling, and a bipolar plate prefabricated material is obtained; carrying out melt blending on the bipolar plate prefabricated material, and extruding to obtain a substrate; and spraying the nitrogen-doped porous carbon material on the surface of a substrate, and performing hot pressing. The graphene, the carbon fibers and the ketjen black are stirred and compounded, the carbon fibers and the ketjen black enter the space between graphene sheet layers, and the ketjen black is attached to the surfaces of the carbon fibers and the graphene, so that the structure not only can prevent the graphene from generating a re-stacking effect in the mixing process, but also can form a'point-line-surface 'ternary interpenetrating high-conductivity network, so that the conductivity of the graphene is improved. According to the preparation method disclosed by the invention, the deposition uniformity of zinc is improved, so that the voltage efficiency and the cycling stability of the battery are improved.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of electrochemical energy storage, and particularly relates to a composite bipolar plate for a zinc-bromine flow battery and a preparation method thereof. BACKGROUND

[0002] As an electrochemical energy storage technology with great development potential, the zinc-bromine flow battery has attracted extensive attention from research institutions and industries in the world in the field of large-scale energy storage due to its high energy density (up to 70-85 Wh / kg), long cycle life (>2000 times) and low material cost (electrodes and electrolyte can be completely recycled). In particular, in the application scenarios such as renewable energy grid connection, power grid peak shaving and frequency modulation and distributed energy storage, the zinc-bromine flow battery shows a good commercial application prospect. However, although it has many advantages, the commercialization process of this technology still faces several key technical bottlenecks, and the poor performance of the bipolar plate as a core component of the battery has become one of the main obstacles to its large-scale popularization and application.

[0003] At present, carbon plastic composite materials are generally used as bipolar plates in commercial zinc-bromine flow batteries. This material is mainly prepared by molding or injection molding process from conductive fillers (such as carbon black and natural graphite) and high molecular polymer matrix (commonly including polypropylene, polyvinyl chloride and polyvinylidene fluoride). Although this carbon plastic composite material has the advantages of light weight, good processing performance and strong corrosion resistance, there are still several key problems that seriously affect the performance of the battery to be solved. The conductive performance of the traditional carbon plastic bipolar plate is poor, mainly because the conductive fillers are not uniformly dispersed in the high molecular polymer matrix, so that the conductive network presents a discontinuous state, the current distribution on the electrode surface is uneven, the contact resistance is high, and significant ohmic polarization loss occurs during operation. When the battery works under the condition of large current (>40 mA / cm 2 ), the conductive performance is poor. In addition, during the long-term cycle process, the conductive fillers may fall off from the polymer matrix due to the corrosion of the electrolyte. SUMMARY

[0004] In order to overcome the technical problem of low electrical conductivity of the bipolar plate in the prior art, the application provides a composite bipolar plate for a zinc-bromine flow battery and a preparation method thereof. The composite bipolar plate has high electrical conductivity, high strength, high catalytic performance and excellent bromine fixation performance, which not only solves the problem of poor conductive performance of the bipolar plate, but also optimizes the bromine shuttling effect and reaction kinetics while maintaining the inherent advantages of the carbon plastic bipolar plate, and improves the uniformity of zinc deposition.

[0005] To achieve the above purpose, the technical scheme is as follows: A preparation method of a composite bipolar plate for a zinc-bromine flow battery, comprising the following steps: Carbon fiber, Ketjen black and graphene dispersion were stirred evenly and dried to obtain conductive carbon composite material. The binder, dispersant and conductive carbon composite material were ball-milled to obtain bipolar plate preforms; The bipolar plate preform is melt-blended and then extruded to obtain the substrate. Nitrogen-doped porous carbon material is sprayed onto the surface of a substrate and then hot-pressed to obtain a composite bipolar plate for zinc-bromine flow batteries.

[0006] Furthermore, the concentration of the graphene dispersion is 5-15 wt%; The diameter of graphene sheets is 5~20μm; The carbon fiber is short-cut carbon fiber with a length of <5 mm; Ketjen Black is battery-grade Ketjen Black with a BET specific surface area of ​​1400-1500 m². 2 / g, iodine adsorption capacity is 1050 mg / g, and particle size is 30-40nm.

[0007] Furthermore, by weight, the carbon fiber comprises 15-30 parts, Ketjen black 3-8 parts, graphene 10-15 parts, binder 50-60 parts, and dispersant 5-10 parts.

[0008] Furthermore, the adhesive is polypropylene or polyethylene.

[0009] Furthermore, the particle size of polypropylene and polyethylene is 1-2 mm, the molecular weight is 500,000-800,000, and the crystallinity is 60-65%.

[0010] Furthermore, the dispersant is a titanate coupling agent.

[0011] Furthermore, the melt blending temperature is 180-220 ℃, and the time is 2-5 min; the extrusion pressure is 17-20 MPa, and the temperature is 180-220 ℃.

[0012] Furthermore, the thickness of the nitrogen-doped porous carbon material sprayed onto the substrate surface is 0.5-1 mm.

[0013] Furthermore, the hot pressing pressure is 0.5-0.8 MPa, the temperature is 150-180℃, and the time is 1-3 min.

[0014] A composite bipolar plate for zinc-bromine flow batteries, wherein the tensile strength of the composite bipolar plate for zinc-bromine flow batteries is 38-46 MPa, the elongation at break is 34-43%, and the conductivity is 19-25 mS / cm.

[0015] Compared with the prior art, the beneficial effects of the present invention are as follows: This invention involves stirring and compounding graphene dispersion, carbon fibers, and Ketjen black. The carbon fibers and Ketjen black are incorporated between the graphene sheets, and the black also adheres to the surfaces of the carbon fibers and graphene. This structure not only prevents graphene from re-stacking during mixing but also forms a "point-line-surface" ternary interpenetrating highly conductive network, improving the conductivity of the bipolar plate. The nitrogen-doped porous carbon material in this invention possesses a large specific surface area and mesoporous structure. This large specific surface area and mesoporous structure provide abundant Br2 adsorption sites, increasing the bromine concentration at the electrode / electrolyte interface. Furthermore, the mesoporous structure exhibits optimal bromine molecule transport kinetics. The synergistic effect between the high bromine concentration at the electrode / electrolyte interface and the bromine molecule transport kinetics significantly enhances the bromine redox reaction (…). The kinetic rate of ) can also hinder the growth of polybrominates (such as ) through physical confinement. The migration of ) effectively inhibits the bromine shuttle effect.

[0016] Furthermore, the polypropylene, titanate coupling agent, and conductive carbon composite material are ball-milled to make the conductive carbon composite material more uniformly mixed with the polypropylene and titanate coupling agent, thereby improving the uniformity of the bipolar plate. Melt blending further improves the uniformity of the mixture. As a highly dispersant, the titanate coupling agent can reduce the agglomeration of the conductive carbon composite material during the mixing process with the binder, reduce the problem of excessive local current caused by uneven filler and uneven bipolar plate, thereby improving the uniformity of zinc deposition, inhibiting zinc dendrite formation, and improving the voltage efficiency and cycle stability of the battery.

[0017] Furthermore, graphene, as a typical two-dimensional nanomaterial, possesses extremely high specific surface area and intrinsic mechanical properties, exhibits strong interfacial bonding with binders, and the van der Waals forces between graphene sheets can effectively transfer stress, hindering crack propagation. Therefore, this invention introduces sheet-like graphene into bipolar plate materials, which can improve the flexural strength and fracture toughness of bipolar plates.

[0018] The composite bipolar plate for zinc-bromine flow batteries of the present invention has a tensile strength of 38-46 MPa, an elongation at break of 34-43%, and an electrical conductivity of 19-25 mS / cm. When assembled into a zinc-bromine flow battery and tested, it exhibits an energy efficiency of 96.0%-98.5%, a voltage efficiency of 80.8-83.9%, and a total energy efficiency of 78.0-82.1%. It combines high conductivity, excellent surface uniformity, efficient electrocatalytic performance, and high strength and toughness, laying a material foundation for the stable operation of zinc-bromine flow batteries. Detailed Implementation

[0019] To facilitate understanding of the present invention, it is described more fully below through embodiments. The present invention can be implemented in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided to provide a more thorough and complete understanding of the disclosure of the present invention.

[0020] The present invention discloses a method for preparing a composite bipolar plate for a zinc-bromine flow battery, comprising the following steps: S1. Preparation of bipolar plate preform: Carbon fiber, Ketjen black and graphene dispersion with a concentration of 5-15wt% (solvent is water) are stirred and dried to obtain conductive carbon composite material. The stirring speed was 800 rpm, the time was 60 min, the drying temperature was 70 ℃, and the drying time was 12 h. The graphene sheets have a diameter of 5–20 μm; the carbon fibers are short-cut carbon fibers with a length of <5 mm; the Ketjen black is battery-grade Ketjen black with a BET (Brunauer-Emmett-Teller) specific surface area of ​​1400–1500 m². 2 / g, iodine adsorption capacity is 1050 mg / g, and particle size is 30-40nm.

[0021] After ball milling the binder, dispersant and conductive carbon composite material, a bipolar plate preform is obtained; The composition, by mass, consists of 15-30 parts carbon fiber, 3-8 parts Ketjen black, 10-15 parts graphene, 50-60 parts binder, and 5-10 parts dispersant.

[0022] The ball mill speed is 300-500 rpm, and the ball milling time is 30-60 min.

[0023] The binder is polypropylene or polyethylene. The particle size of polypropylene and polyethylene is 1-2 mm, the molecular weight is 500,000-800,000, and the crystallinity is 60-65%.

[0024] The dispersant is a titanate coupling agent (KR-TTS).

[0025] S2. Bipolar plate preparation: The bipolar plate preform is melt-blended using a micro twin-screw extruder, and then the melt-blended bipolar plate preform is extruded using a T-die. It is then shaped by a three-roll calender (which includes an upper roll, a middle roll, and a lower roll), and then cooled by a water-cooled conveyor belt to obtain a substrate with a thickness of 1 mm. The melt blending temperature is 180-220 ℃, and the time is 2-5 min; the extrusion pressure is 17-20 MPa, and the temperature is 180-220 ℃. S3. Preparation of nitrogen-doped porous carbon material: 400 g of zinc nitrate hexahydrate was dissolved in 300 mL of methanol, and then 10 mL of 10 mg / mL nanocellulose solution was added. After sonication, the mixture was stirred at 800 rpm for 6 hours. Finally, 600 g of 2-methylimidazole was added, and the mixture was stirred for 12 hours. The mixture was centrifuged at 500 rpm for 8 min, washed, and dried at 70 ℃ for 12 h to obtain ZIF-8 / nanocellulose composite material. The ZIF-8 / nanocellulose composite material was placed in a tube furnace and subjected to high-temperature treatment under a nitrogen atmosphere to obtain nitrogen-doped porous carbon material. The high-temperature treatment temperature was 850-1000 ℃, the time was 2-3 h, and the heating rate was 2-10 ℃ / min.

[0026] S4. The nitrogen-doped porous carbon material obtained in step S3 is uniformly sprayed onto the substrate surface with a coating thickness of 0.5-1 mm. After spraying, the floating carbon on the substrate surface is cleaned off, and then the substrate is conveyed to a hot press for hot pressing. A second surface cleaning is then performed to obtain a composite bipolar plate for zinc-bromine flow batteries. The hot pressing pressure is 0.5-0.8 MPa, the temperature is 150-180℃, and the time is 1-3 min.

[0027] A composite bipolar plate for zinc-bromine flow batteries prepared according to the above method has a tensile strength of 38-46 MPa, an elongation at break of 34-43%, and an electrical conductivity of 19-25 mS / cm. When the composite bipolar plate for zinc-bromine flow batteries is assembled into a zinc-bromine flow battery and tested, the coulombic efficiency is 96.0%-98.5%, the voltage efficiency is 80.8-83.9%, and the energy efficiency is 78.0%-82.1%.

[0028] This invention improves the performance of composite bipolar plates by optimizing the material structure, with the following specific advantages: (1) The present invention uses a graphene-carbon fiber-Ketjen black ternary composite system to construct a “point-line-surface” interpenetrating conductive network. The carbon fiber is inserted into the graphene sheet, and the Ketjen black is coated on the carbon fiber and graphene surface to prevent graphene stacking and significantly improve the conductivity of the composite bipolar plate. (2) Titanate coupling agent facilitates the uniform compounding of polypropylene and carbon fiber, improves the uniformity of zinc deposition, and inhibits dendrite formation; (3) The high strength of graphene can enhance the mechanical properties of composite bipolar plates. Graphene's large specific surface area and sp... 2 Hybrid structure enhances interfacial adhesion; (4) Nitrogen-doped porous carbon materials provide Br2 adsorption sites, promoting redox reactions while limiting the migration of polybrominates. This invention synergistically improves the conductivity, mechanical strength, and electrochemical performance of composite bipolar plates for zinc-bromine flow batteries, and solves the key technical bottlenecks of traditional carbon-plastic bipolar plates.

[0029] The present invention will be further illustrated by the following examples.

[0030] Example 1 A method for preparing a composite bipolar plate for a zinc-bromine flow battery, comprising: S1. Preparation of bipolar plate preform: 1875 mL of 8 wt% graphene dispersion, 200 g of carbon fiber and 50 g of Ketjen black were added to a beaker, stirred at 800 rpm for 60 min, and dried in a forced-air oven at 70 ℃ for 12 h to obtain conductive carbon composite material. 500 g of polypropylene particles, 100 g of titanate coupling agent KR-TTS and conductive carbon composite material were ball-milled at 300 rpm for 60 min to obtain bipolar plate preform. S2. Bipolar plate preparation: Using a micro twin-screw extruder, the bipolar plate preform is melt-blended at a preset temperature of 180 ℃ for 5 min. Then, the melt-blended bipolar plate preform is extruded using a T-die at a pressure of 17 MPa and a temperature of 180 ℃. The preform is then shaped using a three-roll calender and cooled by a water-cooled conveyor belt to obtain a substrate with a thickness of 1 mm. S3. Preparation of nitrogen-doped porous carbon material: 400 g of zinc nitrate hexahydrate was dissolved in 300 mL of methanol, and then 10 mL of 10 mg / mL nanocellulose solution was added. After sonication, the mixture was stirred at 800 rpm for 6 hours. Finally, 600 g of 2-methylimidazole was added, and the mixture was stirred for 12 hours. The mixture was centrifuged at 500 rpm for 8 min, washed, and dried at 70 ℃ for 12 h to obtain ZIF-8 / nanocellulose composite material. The ZIF-8 / nanocellulose composite material was placed in a tube furnace and heated to 850 ℃ at a heating rate of 2 ℃ / min under a nitrogen atmosphere for 3 h to obtain nitrogen-doped porous carbon material. S4. The nitrogen-doped porous carbon material obtained in step S3 is uniformly sprayed onto the substrate surface with a spraying thickness of 0.8 mm. After spraying, the floating carbon on the substrate surface is cleaned off, and then conveyed to a hot press for hot pressing into a plate at a temperature of 150°C, a pressure of 0.5 MPa, and a time of 3 min. After hot pressing into a plate, the surface is cleaned a second time to obtain a composite bipolar plate for zinc-bromine flow batteries.

[0031] Example 2 A method for preparing a composite bipolar plate for a zinc-bromine flow battery, comprising: S1. Preparation of bipolar plate preform: 1250 mL of 8 wt% graphene dispersion, 300 g of carbon fiber and 50 g of Ketjen black were added to a beaker, stirred at 800 rpm for 60 min, and dried in a forced-air oven at 70 ℃ for 12 h to obtain conductive carbon composite material. 600 g of polypropylene particles, 50 g of titanate coupling agent KR-TTS and conductive carbon composite material were ball-milled at 500 rpm for 30 min to obtain bipolar plate preform. S2. Bipolar plate preparation: Using a micro twin-screw extruder, the bipolar plate preform is melt-blended at a preset temperature of 200 ℃ for 3 min. Then, the melt-blended bipolar plate preform is extruded using a T-die at a pressure of 19 MPa and a temperature of 200 ℃. The preform is then shaped using a three-roll calender and cooled by a water-cooled conveyor belt to obtain a substrate with a thickness of 1 mm. S3. Preparation of nitrogen-doped porous carbon material: 400 g of zinc nitrate hexahydrate was dissolved in 300 mL of methanol, and then 10 mL of 10 mg / mL nanocellulose solution was added. After sonication, the mixture was stirred at 800 rpm for 6 hours. Finally, 600 g of 2-methylimidazole was added, and the mixture was stirred for 12 hours. The mixture was centrifuged at 500 rpm for 8 min, washed, and dried at 70 ℃ for 12 h to obtain ZIF-8 / nanocellulose composite material. The ZIF-8 / nanocellulose composite material was placed in a tube furnace and heated to 950 ℃ at a heating rate of 2 ℃ / min under a nitrogen atmosphere for 3 h to obtain nitrogen-doped porous carbon material. S4. The nitrogen-doped porous carbon material obtained in step S3 is uniformly sprayed onto the substrate surface with a spray thickness of 0.5 mm. After spraying, the floating carbon on the substrate surface is cleaned off, and then conveyed to a hot press for hot pressing into a plate at a temperature of 160°C, a pressure of 0.7 MPa, and a time of 2 min. After hot pressing into a plate, the surface is cleaned a second time to obtain a composite bipolar plate for zinc-bromine flow batteries.

[0032] Example 3 A method for preparing a composite bipolar plate for a zinc-bromine flow battery, comprising: S1. Preparation of bipolar plate preform: 1250 mL of 8 wt% graphene dispersion, 150 g of carbon fiber and 80 g of Ketjen Black were added to a beaker, stirred at 800 rpm for 60 min, and dried in a forced-air oven at 70 ℃ for 12 h to obtain conductive carbon composite material. 600 g of polypropylene particles, 70 g of titanate coupling agent KR-TTS and conductive carbon composite material were ball-milled at 400 rpm for 50 min to obtain bipolar plate preform. S2. Bipolar plate preparation: Using a micro twin-screw extruder, the bipolar plate preform is melt-blended at a preset temperature of 220 ℃ for 2 min. The melt-blended bipolar plate preform is then extruded using a T-die at a pressure of 20 MPa and a temperature of 200 ℃. The preform is then shaped using a three-roll calender and cooled by a water-cooled conveyor belt to obtain a substrate with a thickness of 1 mm. S3. Preparation of nitrogen-doped porous carbon material: 400 g of zinc nitrate hexahydrate was dissolved in 300 mL of methanol, and then 10 mL of 10 mg / mL nanocellulose solution was added. After sonication, the mixture was stirred at 800 rpm for 6 hours. Finally, 600 g of 2-methylimidazole was added, and the mixture was stirred for 12 hours. The mixture was centrifuged at 500 rpm for 8 min, washed, and dried at 70 ℃ for 12 h to obtain ZIF-8 / nanocellulose composite material. The ZIF-8 / nanocellulose composite material was placed in a tube furnace and heated to 1000 ℃ at a heating rate of 5 ℃ / min under a nitrogen atmosphere for 2 h to obtain nitrogen-doped porous carbon material. S4. The nitrogen-doped porous carbon material obtained in step S3 is uniformly sprayed onto the substrate surface with a coating thickness of 1 mm. After spraying, the floating carbon on the substrate surface is cleaned off, and then the substrate is conveyed to a hot press for hot pressing at a temperature of 180°C, a pressure of 0.8 MPa, and a time of 1 min. After hot pressing, the surface is cleaned a second time to obtain a composite bipolar plate for zinc-bromine flow batteries. Example 4 A method for preparing a composite bipolar plate for a zinc-bromine flow battery, comprising: S1. Preparation of bipolar plate preform: 1625 mL of 8 wt% graphene dispersion, 210 g of carbon fiber and 60 g of Ketjen Black were added to a beaker, stirred at 800 rpm for 60 min, and dried in a forced-air oven at 70 ℃ for 12 h to obtain conductive carbon composite material. 500 g of polypropylene particles, 100 g of titanate coupling agent KR-TTS and conductive carbon composite material were ball-milled at 500 rpm for 30 min to obtain bipolar plate preform. S2. Bipolar plate preparation: Using a micro twin-screw extruder, the bipolar plate preform is melt-blended at a preset temperature of 190 ℃ for 5 min. Then, the melt-blended bipolar plate preform is extruded using a T-die at a pressure of 18 MPa and a temperature of 190 ℃. The preform is then shaped using a three-roll calender and cooled by a water-cooled conveyor belt to obtain a substrate with a thickness of 1 mm. S3. Preparation of nitrogen-doped porous carbon material: 400 g of zinc nitrate hexahydrate was dissolved in 300 mL of methanol, and then 10 mL of 10 mg / mL nanocellulose solution was added. After sonication, the mixture was stirred at 800 rpm for 6 hours. Finally, 600 g of 2-methylimidazole was added, and the mixture was stirred for 12 hours. The mixture was centrifuged at 500 rpm for 8 min, washed, and dried at 70 ℃ for 12 h to obtain ZIF-8 / nanocellulose composite material. The ZIF-8 / nanocellulose composite material was placed in a tube furnace and heated to 950 ℃ at a heating rate of 2 ℃ / min under a nitrogen atmosphere for 3 h to obtain nitrogen-doped porous carbon material. S4. The nitrogen-doped porous carbon material obtained in step S3 is uniformly sprayed onto the substrate surface with a spraying thickness of 1 mm. After spraying, the floating carbon on the substrate surface is cleaned off, and then conveyed to a hot press for hot pressing into a plate at a temperature of 180°C, a pressure of 0.5 MPa, and a time of 3 min. After hot pressing into a plate, the surface is cleaned a second time to obtain a composite bipolar plate for zinc-bromine flow batteries.

[0033] Example 5 A method for preparing a composite bipolar plate for a zinc-bromine flow battery, comprising: S1. Preparation of bipolar plate preform: 1875 mL of 8 wt% graphene dispersion, 250 g of carbon fiber and 50 g of Ketjen Black were added to a beaker, stirred at 800 rpm for 60 min, and dried in a forced-air oven at 70 ℃ for 12 h to obtain conductive carbon composite material. 600 g of polypropylene particles, 50 g of titanate coupling agent KR-TTS and conductive carbon composite material were ball-milled at 400 rpm for 50 min to obtain bipolar plate preform. S2. Bipolar plate preparation: Using a micro twin-screw extruder, the bipolar plate preform is melt-blended at a preset temperature of 200 ℃ for 3 min. Then, the melt-blended bipolar plate preform is extruded using a T-die at a pressure of 19 MPa and a temperature of 200 ℃. The preform is then shaped using a three-roll calender and cooled by a water-cooled conveyor belt to obtain a substrate with a thickness of 1 mm. S3. Preparation of nitrogen-doped porous carbon material: 400 g of zinc nitrate hexahydrate was dissolved in 300 mL of methanol, and then 10 mL of 10 mg / mL nanocellulose solution was added. After sonication, the mixture was stirred at 800 rpm for 6 hours. Finally, 600 g of 2-methylimidazole was added, and the mixture was stirred for 12 hours. The mixture was centrifuged at 500 rpm for 8 min, washed, and dried at 70 ℃ for 12 h to obtain ZIF-8 / nanocellulose composite material. The ZIF-8 / nanocellulose composite material was placed in a tube furnace and heated to 950 ℃ at a heating rate of 3 ℃ / min under a nitrogen atmosphere for 3 h to obtain nitrogen-doped porous carbon material. S4. The nitrogen-doped porous carbon material obtained in step S3 is uniformly sprayed onto the substrate surface with a spraying thickness of 1 mm. After spraying, the floating carbon on the substrate surface is cleaned off, and then conveyed to a hot press for hot pressing into a plate at a temperature of 180°C, a pressure of 0.8 MPa, and a time of 2 min. After hot pressing into a plate, the surface is cleaned a second time to obtain a composite bipolar plate for zinc-bromine flow batteries.

[0034] Example 6 A method for preparing a composite bipolar plate for a zinc-bromine flow battery, comprising: S1. Preparation of bipolar plate preform: 2400 mL of 5 wt% graphene dispersion, 270 g of carbon fiber and 30 g of Ketjen black were added to a beaker, stirred at 800 rpm for 60 min, and dried in a forced-air oven at 70 ℃ for 12 h to obtain conductive carbon composite material. 550 g of polyethylene, 60 g of titanate coupling agent KR-TTS and conductive carbon composite material were ball-milled at 350 rpm for 60 min to obtain bipolar plate preform. S2. Bipolar plate preparation: Using a micro twin-screw extruder, the bipolar plate preform was melt-blended at a preset temperature of 200 ℃ for 3 min. The melt-blended bipolar plate preform was then extruded using a T-die at a pressure of 19 MPa and a temperature of 220 ℃. The preform was then shaped using a three-roll calender and cooled by a water-cooled conveyor belt to obtain a substrate with a thickness of 1 mm. S3. Preparation of nitrogen-doped porous carbon material: 400 g of zinc nitrate hexahydrate was dissolved in 300 mL of methanol, and then 10 mL of 10 mg / mL nanocellulose solution was added. After sonication, the mixture was stirred at 800 rpm for 6 hours. Finally, 600 g of 2-methylimidazole was added, and the mixture was stirred for 12 hours. The mixture was centrifuged at 500 rpm for 8 min, washed, and dried at 70 ℃ for 12 h to obtain ZIF-8 / nanocellulose composite material. The ZIF-8 / nanocellulose composite material was placed in a tube furnace and heated to 900 ℃ at a heating rate of 10 ℃ / min under a nitrogen atmosphere for 3 h to obtain nitrogen-doped porous carbon material. S4. The nitrogen-doped porous carbon material obtained in step S3 is uniformly sprayed onto the substrate surface with a spraying thickness of 0.7 mm. After spraying, the floating carbon on the substrate surface is cleaned off, and then conveyed to a hot press for hot pressing into a plate at a temperature of 170°C, a pressure of 0.6 MPa, and a time of 1 min. After hot pressing into a plate, the surface is cleaned a second time to obtain a composite bipolar plate for zinc-bromine flow batteries.

[0035] Example 7 A method for preparing a composite bipolar plate for a zinc-bromine flow battery, comprising: S1. Preparation of bipolar plate preform: 1000 mL of 15 wt% graphene dispersion, 180 g of carbon fiber and 40 g of Ketjen black were added to a beaker, stirred at 800 rpm for 60 min, and dried in a forced-air oven at 70 ℃ for 12 h to obtain conductive carbon composite material. 520 g of polypropylene particles, 80 g of titanate coupling agent KR-TTS and conductive carbon composite material were ball-milled at 450 rpm for 40 min to obtain bipolar plate preform. S2. Bipolar plate preparation: Using a micro twin-screw extruder with a preset temperature of 210 ℃, the bipolar plate preform is melt-blended for 3 min. Then, the melt-blended bipolar plate preform is extruded using a T-die at a pressure of 17 MPa and a temperature of 210 ℃. It is then shaped by a three-roll calender and cooled by a water-cooled conveyor belt to obtain a substrate with a thickness of 1 mm. S3. Preparation of nitrogen-doped porous carbon material: 400 g of zinc nitrate hexahydrate was dissolved in 300 mL of methanol, and then 10 mL of 10 mg / mL nanocellulose solution was added. After sonication, the mixture was stirred at 800 rpm for 6 hours. Finally, 600 g of 2-methylimidazole was added, and the mixture was stirred for 12 hours. The mixture was centrifuged at 500 rpm for 8 min, washed, and dried at 70 ℃ for 12 h to obtain ZIF-8 / nanocellulose composite material. The ZIF-8 / nanocellulose composite material was placed in a tube furnace and heated to 950 ℃ at a heating rate of 7 ℃ / min under a nitrogen atmosphere for 2 h to obtain nitrogen-doped porous carbon material. S4. The nitrogen-doped porous carbon material obtained in step S3 is uniformly sprayed onto the substrate surface with a spraying thickness of 0.9 mm. After spraying, the floating carbon on the substrate surface is cleaned off, and then conveyed to a hot press for hot pressing into a plate at a temperature of 165°C, a pressure of 0.5 MPa, and a time of 2 min. After hot pressing into a plate, the surface is cleaned a second time to obtain a composite bipolar plate for zinc-bromine flow batteries.

[0036] Comparative Example 1 The difference from Example 1 is that in step S1, 1875 mL of graphene dispersion with a concentration of 8 wt% and 200 g of carbon fiber are added to a beaker, stirred at 800 rpm for 60 min, and dried in a forced-air oven at 70 ℃ for 12 h to obtain a conductive carbon composite material. 500 g of polypropylene particles, 50 g of titanate coupling agent KR-TTS and conductive carbon composite material were ball-milled at 300 rpm for 20 min to obtain bipolar plate preform. The remaining steps are the same as in Example 1.

[0037] Comparative Example 2 The difference from Example 3 is that in step S1, 1250 mL of graphene dispersion with a concentration of 8 wt% and 80 g of Ketjen Black were added to a beaker, stirred at 800 rpm for 60 min, and dried in a forced-air oven at 70 ℃ for 12 h to obtain a conductive carbon composite material. 600 g of polypropylene particles, 50 g of titanate coupling agent KR-TTS and conductive carbon composite material were ball-milled at 300 rpm for 20 min to obtain bipolar plate preform. The remaining steps are the same as in Example 3.

[0038] Comparative Example 3 The difference from Example 2 is that in step S1, 300 g of carbon fiber and 50 g of Ketjen Black are added to a beaker, stirred at 800 rpm for 60 min, and dried in a forced-air oven at 70 °C for 12 h to obtain a conductive carbon composite material. 600 g of polypropylene particles, 50 g of titanate coupling agent KR-TTS and conductive carbon composite material were ball-milled at 300 rpm for 20 min to obtain bipolar plate preform. The remaining steps are the same as in Example 2.

[0039] Comparative Example 4 Commercially available carbon-plastic bipolar plates were purchased from the market and used as bipolar plates for batteries.

[0040] Table 1 compares the key parameters of Examples 1-7 and Comparative Examples 1-3. Table 1 clearly shows the preparation conditions of each example and comparative example.

[0041] Table 1. Composition and proportion of key components in Examples 1-7 and Comparative Examples 1-3

[0042] The comparison results of conductivity and mechanical properties of Examples 1-7 and Comparative Examples 1-4 are shown in Table 2.

[0043] Table 2. Bipolar plate performance of Examples 1-7 and Comparative Examples 1-4

[0044] As shown in Table 2, the conductivity of the composite bipolar plate for zinc-bromine flow batteries decreased when Ketjenblack was not added in Comparative Example 1. The mechanical properties of the composite bipolar plate for zinc-bromine flow batteries gradually decreased in Comparative Example 2 (without carbon fiber) and Example 3 (without graphene). According to the formulation of this invention, the addition of graphene, carbon fiber, Ketjenblack, polypropylene particles, and titanate coupling agent resulted in composite bipolar plates for zinc-bromine flow batteries with superior mechanical and electrical properties compared to commercial carbon-plastic bipolar plates.

[0045] The zinc-bromine flow battery composite bipolar plates prepared in Examples 1-7 and the bipolar plates in Comparative Examples 1-4 were assembled into zinc-bromine flow batteries, and their performance was tested.

[0046] Zinc-bromine flow battery assembly: The prepared composite bipolar plate for zinc-bromine flow batteries was used as the electrode, with the side coated with nitrogen-doped carbon material as the positive electrode and the side without nitrogen-doped carbon material as the negative electrode; the separator was a commercially available microporous membrane; the electrolytes for both the positive and negative electrodes were mixed solutions of zinc bromide, potassium chloride, and N-ethyl-N-methylpyrrolidone bromide, with deionized water as the solvent, zinc bromide concentration of 2 mol / L, potassium chloride concentration of 1 mol / L, and N-ethyl-N-methylpyrrolidone bromide concentration of 0.4 mol / L; Zinc-bromine redox flow battery test: current density 20 mA / cm² 2The charging time was 4 hours and the discharge cutoff voltage was 0.5V. The coulombic efficiency (CE), voltage efficiency (VE), and energy efficiency (EE) of the battery were tested, and the results are shown in Table 3.

[0047] Table 3. Performance comparison of bipolar zinc-bromine flow batteries in Examples 1-7 and Comparative Examples 1-4

[0048] As shown in Table 3, the composite bipolar plate for zinc-bromine flow batteries prepared according to the proportions specified in this invention, with the addition of graphene, carbon fiber, Ketjen black, polypropylene particles, and titanate coupling agent, exhibits a coulombic efficiency of 96.0%-98.5%, a voltage efficiency of 80.8%-83.9%, and an energy efficiency of 78.0%-82.1%, all superior to comparative examples 1-4. This demonstrates that the present invention not only improves the mechanical properties and conductivity of the composite bipolar plate for zinc-bromine flow batteries but also effectively enhances the redox kinetics of bromine, suppresses bromine transport, and mitigates zinc dendrite formation, thereby improving the performance of the zinc-bromine flow battery.

[0049] The above description is only of the preferred embodiment of the present invention and should not be construed as limiting the scope of the claims. The present invention is not limited to the above embodiments, and variations in its specific structure are permitted. All variations made within the scope of the independent claims of the present invention are also within the scope of protection of the present invention.

[0050] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used in this specification is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

Claims

1. A method for preparing a composite bipolar plate for a zinc-bromine flow battery, characterized in that, Includes the following steps: Carbon fiber, Ketjen black and graphene dispersion were stirred evenly and dried to obtain conductive carbon composite material. The binder, dispersant and conductive carbon composite material were ball-milled to obtain bipolar plate preforms; The bipolar plate preform is melt-blended and then extruded to obtain the substrate. Nitrogen-doped porous carbon material is sprayed onto the surface of a substrate and then hot-pressed to obtain a composite bipolar plate for zinc-bromine flow batteries.

2. The method for preparing the composite bipolar plate for a zinc-bromine flow battery according to claim 1, characterized in that, The concentration of the graphene dispersion is 5-15 wt%. The graphene sheets used to prepare the graphene dispersion had a diameter of 5~20μm; The carbon fiber is short-cut carbon fiber with a length of <5 mm; Ketjen Black is battery-grade Ketjen Black with a BET specific surface area of ​​1400-1500 m². 2 / g, iodine adsorption capacity is 1050 mg / g, and particle size is 30-40nm.

3. The method for preparing the composite bipolar plate for a zinc-bromine flow battery according to claim 2, characterized in that, By weight, carbon fiber is 15-30 parts, Ketjen black is 3-8 parts, graphene is 10-15 parts, binder is 50-60 parts, and dispersant is 5-10 parts.

4. The method for preparing the composite bipolar plate for a zinc-bromine flow battery according to claim 1, characterized in that, The adhesive is polypropylene or polyethylene.

5. The method for preparing the composite bipolar plate for a zinc-bromine flow battery according to claim 4, characterized in that, The particle size of polypropylene and polyethylene is 1-2 mm, the molecular weight is 500,000-800,000, and the crystallinity is 60-65%.

6. The method for preparing the composite bipolar plate for a zinc-bromine flow battery according to claim 1, characterized in that, The dispersant is a titanate coupling agent.

7. The method for preparing the composite bipolar plate for a zinc-bromine flow battery according to claim 1, characterized in that, The melt blending temperature is 180-220 ℃ and the time is 2-5 min; the extrusion pressure is 17-20 MPa and the temperature is 180-220 ℃.

8. The method for preparing the composite bipolar plate for a zinc-bromine flow battery according to claim 1, characterized in that, The thickness of the nitrogen-doped porous carbon material sprayed onto the substrate surface is 0.5-1 mm.

9. The method for preparing the composite bipolar plate for a zinc-bromine flow battery according to claim 1, characterized in that, The hot pressing pressure is 0.5-0.8 MPa, the temperature is 150-180℃, and the time is 1-3 min.

10. A composite bipolar plate for a zinc-bromine flow battery prepared by the method according to any one of claims 1-9, characterized in that, The composite bipolar plate used in the zinc-bromine flow battery has a tensile strength of 38-46 MPa, an elongation at break of 34-43%, and a conductivity of 19-25 mS / cm.

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