Cobalt-based composite bipolar plate for flow battery and preparation method thereof, flow battery

Abstract

This application provides a cobalt-based composite bipolar plate for flow batteries, its preparation method, and the flow battery itself. The preparation method includes the following steps: pretreating cobalt-based alloy powder; mixing and stirring the pretreated cobalt-based alloy powder with an organic solvent dispersion of graphene nanosheets at a concentration of 3 mg / mL–10 mg / mL, followed by heat treatment under an inert atmosphere to obtain graphene nanosheet-coated cobalt-based alloy powder; mixing the graphene nanosheet-coated cobalt-based alloy powder with expanded graphite, carbon nanotubes, and carbon black to obtain a mixed filler; melt-blending the mixed filler at a mass ratio of 1:1–7:3 with a polymer matrix powder, extruding and granulating to obtain composite granules; and placing the composite granules into a preheated mold, hot-pressing, and cooling to demold, thereby obtaining the composite bipolar plate. The cobalt-based composite bipolar plate of this application exhibits excellent mechanical and electrical stability in vanadium redox flow battery stacks.

Description

Technical Field

[0001] This application relates to the field of flow batteries, and more particularly to a cobalt-based composite bipolar plate for flow batteries, a method for preparing the same, and the flow battery itself. Background Technology

[0002] Bipolar plates are one of the core components of flow battery stacks, and their performance directly affects the battery's energy efficiency, power density, and lifespan. Therefore, high-performance bipolar plates must possess high conductivity, excellent corrosion resistance, good mechanical strength, and low gas permeability.

[0003] Therefore, there is a need for an improved bipolar plate for flow batteries and its fabrication method, as well as flow batteries. Summary of the Invention

[0004] The following is an overview of the subject matter described in detail herein. This overview is not intended to limit the scope of protection of this application.

[0005] In one aspect, this application provides a method for preparing a cobalt-based composite bipolar plate for a flow battery, comprising the following steps:

[0006] S1: Pretreated cobalt-based alloy powder;

[0007] S2: The pretreated cobalt-based alloy powder is mixed and stirred with an organic solvent dispersion of graphene nanosheets with a concentration of 3-10 mg / mL and then heat-treated under an inert atmosphere to obtain cobalt-based alloy powder coated with graphene nanosheets.

[0008] S3: A mixed filler is obtained by mixing cobalt-based alloy powder coated with graphene nanosheets with expanded graphite, carbon nanotubes and carbon black.

[0009] S4: The mixed filler in a mass ratio of (1:1) to (7:3) is melt-blended with polymer matrix powder, extruded, and then granulated to obtain composite granules; and

[0010] S5: Place the composite granules into a preheated mold, hot press and cool to demold, to obtain the composite bipolar plate;

[0011] The cobalt-based alloy is selected from one or more of Stellite 6, Stellite 12, and Stellite 21; and

[0012] The mixed filler comprises, by weight, 20-50 parts of cobalt-based alloy powder, 1-5 parts of graphene nanosheets, 10-30 parts of expanded graphite, 2-10 parts of carbon nanotubes, and 1-5 parts of carbon black.

[0013] In this application, the cobalt-based alloy may be a commercial cobalt-based alloy from the Stellite® series, such as Stellite 6, Stellite 12, and Stellite 21, for example, purchased from Kennametal Stellite Metals (Shanghai) Co., Ltd.

[0014] In an exemplary embodiment, the organic solvent in the organic solvent dispersion of the graphene nanosheets is N-methylpyrrolidone (NMP) or ethanol.

[0015] In one exemplary embodiment, the D50 particle size of the cobalt-based alloy powder is 15 μm-30 μm.

[0016] In one exemplary embodiment, the expanded graphite has a particle size of 40 μm-220 μm.

[0017] In one exemplary embodiment, the carbon nanotube is a single-walled carbon nanotube or a multi-walled carbon nanotube.

[0018] In one exemplary embodiment, the polymer matrix is ​​polyvinylidene fluoride, ethylene-chlorotrifluoroethylene copolymer, polypropylene, or polyethylene (such as high-density polyethylene).

[0019] In this application, polymers are chosen as the matrix because they offer advantages such as excellent acid and oxidation resistance; they also have well-defined melting points, good flowability, and excellent processing properties, making them ideal for injection molding and compression molding; and they utilize their binding properties to hold the fillers together, forming a structurally stable whole. Other polymer matrices used in the art for preparing bipolar plates may also be employed in this application.

[0020] In an exemplary embodiment, step S1 includes pretreatment of cobalt-based alloy powder by ultrasonic cleaning in an acid solution, followed by washing with deionized water and drying.

[0021] In an exemplary embodiment, in step S1, the acid solution is 5wt%-30wt% dilute nitric acid or 5wt%-30wt% dilute sulfuric acid.

[0022] In an exemplary embodiment, in step S1, the ultrasonic cleaning time is 10 min to 60 min.

[0023] In an exemplary embodiment, in step S1, the drying temperature is 40°C-100°C and the drying time is 2-6 hours.

[0024] In an exemplary embodiment, in step S2, the stirring includes stirring at a speed of 5000 rpm to 10000 rpm.

[0025] In an exemplary embodiment, in step S2, the heat treatment temperature is 100°C-400°C and the heat treatment time is 1-4 hours.

[0026] In an exemplary embodiment, in step S3, the mixing is performed using a mixer with a rotation speed of 100 rpm to 1000 rpm and a mixing time of 20 min to 80 min.

[0027] In one exemplary embodiment, in step S4, extrusion is performed using an extruder with a temperature of 150°C-220°C and a rotation speed of 100rpm-1000rpm.

[0028] In an exemplary embodiment, in step S5, the mold preheating temperature is 150°C-200°C, and the preheating time is 5-15 minutes.

[0029] In an exemplary embodiment, in step S5, the hot pressing pressure is 5-30 MPa and the hot pressing time is 10-40 min.

[0030] In an exemplary embodiment, in step S5, the cooling temperature is 20°C-80°C.

[0031] On the other hand, this application provides a cobalt-based composite bipolar plate for flow batteries, which is prepared by the above method.

[0032] In another aspect, this application provides a flow battery including the aforementioned cobalt-based composite bipolar plate.

[0033] This application uses expanded graphite as the main framework to connect cobalt-based alloy powder encapsulated by graphene nanosheets; carbon nanotubes act as connecting bridges to connect cobalt-based alloy powder encapsulated by graphene nanosheets that are not connected to expanded graphite; carbon black fills the tiny gaps between the cobalt-based alloy powder encapsulated by graphene nanosheets and the carbon nanotubes. The synergy of these materials enables the fabricated composite bipolar plate to achieve extremely low sheet resistivity.

[0034] This application employs graphene nanosheets to encapsulate cobalt-based alloy powder. The graphene nanosheets effectively isolate the electrolyte from direct contact with the cobalt-based alloy powder, preventing corrosion. The cobalt-based alloy exhibits extremely strong corrosion resistance, and its synergistic effect with the graphene's insulating properties results in excellent stability of the cobalt-based composite bipolar plate in vanadium redox flow battery stacks.

[0035] The cobalt-based alloy used in this application has high strength and hardness, which improves the bending strength and creep resistance of the multi-element composite bipolar plate; the carbon nanotubes used in this application enhance the toughness of the material and reduce the brittleness of the bipolar plate.

[0036] Other features and advantages of this application will be set forth in the following description, and will be apparent in part from the description, or may be learned by practicing the application. Other advantages of this application may be realized and obtained by means of the methods described in the description. Detailed Implementation

[0037] To make the objectives, technical solutions, and advantages of this application clearer, the embodiments of this application are described in detail below. It should be noted that, unless otherwise specified, the embodiments and features described in these embodiments can be arbitrarily combined with each other.

[0038] Unless otherwise specified, all materials used in the following examples and comparative examples are commercially available. The dimensions (e.g., particle size) of products such as carbon black, carbon nanotubes, and graphene nanosheets are based on the dimensions of conventional commercially available products, and this application does not limit them.

[0039] Example 1

[0040] Polymer matrix: Polyvinylidene fluoride (PVDF), 30g;

[0041] Conductive mixed filler: 70g, including 45.5g cobalt-based alloy powder (D50=25μm), 17.5g expanded graphite (particle size 150μm), 2.8g multi-walled carbon nanotubes, 2.1g graphene nanosheets, and 2.1g carbon black.

[0042] The preparation method is as follows:

[0043] S1: 45.5g of Stellite 6 alloy powder was ultrasonically cleaned in 10wt% dilute nitric acid solution for 20 minutes, washed with deionized water until neutral, and dried at 40°C for 4 hours.

[0044] S2: The pretreated Stellite 6 alloy powder was mixed with 2.1 g of N-methylpyrrolidone (NMP) dispersion of graphene nanosheets (concentration 4 mg / mL) and treated in a high-speed shear emulsifier (8000 rpm) for 20 minutes; then it was placed in a tube furnace and heat-treated at 350°C for 1 hour under Ar atmosphere protection to obtain cobalt-based alloy powder coated with graphene nanosheets.

[0045] S3: Cobalt-based alloy powder coated with graphene nanosheets, 17.5g of expanded graphite, 2.8g of multi-walled carbon nanotubes, and 2.1g of carbon black were placed in a high-speed mixer and mixed at 1000 rpm for 30 minutes to obtain a mixed filler.

[0046] S4: Add the mixed filler and 30g of polyvinylidene fluoride (PVDF) polymer matrix to a twin-screw extruder (screw speed of 300 rpm), melt-blend at 200°C, extrude and granulate to obtain composite granules.

[0047] S5: Place the composite granules into a mold that has been preheated at 190°C for 10 minutes, then hold it under pressure of 20 MPa for 30 minutes, and then cool it to below 40°C to demold, thus obtaining a cobalt-based composite bipolar plate.

[0048] Example 2

[0049] Polymer matrix: Polyvinylidene fluoride (PVDF), 40g;

[0050] Mixed filler: 60g, including 39g cobalt-based alloy powder (D50=25μm), 15g expanded graphite (particle size 150μm), 2.4g multi-walled carbon nanotubes, 1.8g graphene nanosheets, and 1.8g carbon black.

[0051] Preparation method: Refer to Example 1.

[0052] Example 3

[0053] Polymer matrix: Polyvinylidene fluoride (PVDF), 50g;

[0054] Mixed filler: 50g, including 32.5g cobalt-based alloy powder (D50=25μm), 12.5g expanded graphite (particle size 150μm), 2g multi-walled carbon nanotubes, 1.5g graphene nanosheets, and 1.5g carbon black.

[0055] Preparation method: Refer to Example 1.

[0056] Comparative Example 1 (Traditional Graphite / Polymer Composite Board)

[0057] 40g of polymer matrix PVDF and 60g of conductive filler natural flake graphite (particle size 75μm).

[0058] Preparation process: PVDF and graphite powder are directly mixed and then molded under the same conditions (190°C, 20 MPa).

[0059] Comparative Example 2 (316L stainless steel / polymer composite plate)

[0060] 40g of polymer matrix PVDF and 60g of conductive filler 316L stainless steel powder (D50=25μm).

[0061] Preparation process: 316L stainless steel powder and PVDF powder are directly mixed and then molded under the same conditions (190°C, 20MPa).

[0062] Performance testing:

[0063] The performance of the cobalt-based composite bipolar plates prepared in Examples 1-3 and the bipolar plates prepared in Comparative Examples 1-2 was tested. The contact resistance, bending strength, and corrosion current density of the bipolar plates were tested according to NB / T-42007-2013. Following ASTM A262 (corrosion test method), the samples were immersed in a 40°C, 3 M H2SO4 solution for 500 hours, and the concentration of metal ions in the solution was detected by inductively coupled plasma mass spectrometry. The test results are shown in Table 1 below.

[0064] Table 1

[0065]

[0066] As can be seen from Table 1, compared with Comparative Example 1 (graphite composite plate), the cobalt-based composite bipolar plate of this application has significantly reduced contact resistance, i.e., significantly improved conductivity; significantly improved mechanical strength such as bending strength; compared with Comparative Example 2 (stainless steel composite plate), while maintaining excellent conductivity and mechanical strength, the corrosion current density is significantly reduced, i.e., significantly enhanced corrosion resistance; and the problem of metal ion leaching is completely avoided, enhancing the corrosion resistance of the bipolar plate.

[0067] The cobalt-based composite bipolar plate of this application has good conductivity, excellent corrosion resistance, and superior mechanical strength, and is suitable for long-life, high-reliability flow batteries.

[0068] Although embodiments of this application have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting this application. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of this application.

Claims

1. A method for preparing a cobalt-based composite bipolar plate for flow batteries, characterized in that, Includes the following steps: S1: Pretreated cobalt-based alloy powder; S2: The pretreated cobalt-based alloy powder is mixed and stirred with an organic solvent dispersion of graphene nanosheets at a concentration of 3-10 mg / mL and then heat-treated under an inert atmosphere to obtain cobalt-based alloy powder coated with graphene nanosheets. S3: A mixed filler is obtained by mixing cobalt-based alloy powder coated with graphene nanosheets with expanded graphite, carbon nanotubes and carbon black. S4: The mixed filler in a mass ratio of 1:1 to 7:3 is melt-blended with polymer matrix powder, extruded, and then granulated to obtain composite granules; and S5: Place the composite granules into a preheated mold, hot-press and cool to demold, to obtain the cobalt-based composite bipolar plate; The cobalt-based alloy is selected from one or more of Stellite 6, Stellite 12, and Stellite 21; and The mixed filler, by weight, comprises 20-50 parts of cobalt-based alloy powder, 1-5 parts of graphene nanosheets, 10-30 parts of expanded graphite, 2-10 parts of carbon nanotubes, and 1-5 parts of carbon black.

2. The method according to claim 1, characterized in that, In the organic solvent dispersion of the graphene nanosheets, the organic solvent is N-methylpyrrolidone or ethanol.

3. The method according to claim 1, characterized in that, The D50 particle size of the cobalt-based alloy powder is 15μm-30μm; and / or, The expanded graphite has a particle size of 40μm-220μm; and / or, The carbon nanotubes are single-walled carbon nanotubes or multi-walled carbon nanotubes.

4. The method according to claim 1, characterized in that, The polymer matrix is ​​polyvinylidene fluoride, ethylene-chlorotrifluoroethylene copolymer, polypropylene, or polyethylene.

5. The method according to any one of claims 1-4, characterized in that, In step S1, the pretreatment includes ultrasonic cleaning of the cobalt-based alloy powder in an acid solution, washing with deionized water and drying.

6. The method according to claim 5, characterized in that, The acid solution is 5wt%-30wt% dilute nitric acid or 5wt%-30wt% dilute sulfuric acid; and / or, Ultrasonic cleaning time is 10-60 minutes; and / or, The drying temperature is 40°C-100°C, and the drying time is 2-6 hours.

7. The method according to any one of claims 1-4, characterized in that, In step S2, the stirring includes stirring at a speed of 5000 rpm to 10000 rpm; and / or, In step S2, the heat treatment temperature is 100°C-400°C, and the heat treatment time is 1-4 hours; and / or, In step S3, mixing is performed using a mixer with a rotation speed of 100 rpm to 1000 rpm and a mixing time of 20 min to 80 min; and / or, In step S4, extrusion is performed using an extruder with a temperature of 150-220℃ and a rotation speed of 100rpm-1000rpm.

8. The method according to any one of claims 1-4, characterized in that, In step S5, the mold preheating temperature is 150℃-200℃, and the preheating time is 5min-15min; and / or, In step S5, the hot-pressing pressure is 5-30 MPa, and the hot-pressing time is 10-40 min; and / or, In step S5, the cooling temperature is 20°C-80°C.

9. A cobalt-based composite bipolar plate for flow batteries, characterized in that, The cobalt-based composite bipolar plate is prepared by the method described in any one of claims 1-8.

10. A flow battery, characterized in that, Including the cobalt-based composite bipolar plate as described in claim 9.

Images