Bipolar plate for all-vanadium redox flow battery as well as surface modification method and application of bipolar plate

Abstract

The invention provides a bipolar plate for an all-vanadium redox flow battery as well as a surface modification method and application of the bipolar plate. The surface modification method comprises the following steps: providing the bipolar plate, wherein the composition material of the bipolar plate comprises a resin filler; coating the surface of the bipolar plate with the modified slurry, and then performing hot pressing to form a modified coating; wherein the modified slurry comprises a carbon nanotube, a binder and a solvent. The modified coating is prepared on the surface of the bipolar plate based on the carbon nanotubes, so that the negative influence of a resin-rich layer on the contact resistance between the carbon felt electrode and the bipolar plate is weakened, meanwhile, the proportion of resin in the bipolar plate is not changed, and the negative influence on the mechanical property of the bipolar plate is avoided. In addition, through combination of coating and hot pressing, the interface bonding force of the modified coating is enhanced, and the structural stability is improved. The surface modification method meets the harsh requirements of the bipolar plate of the flow battery on high conductivity, strong combination and super corrosion resistance of the coating, and is one of key technical paths for improving the energy density and the capacity retention ratio of the battery.

Description

Technical Field

[0001] This invention belongs to the field of energy storage battery manufacturing technology, specifically relating to a bipolar plate for vanadium redox flow batteries, its surface modification method, and its application. Background Technology

[0002] Flow batteries, as long-term energy storage devices, offer significant advantages in safety and reliability due to their use of aqueous electrolytes. They are a core solution for addressing the volatility of renewable energy and the demand for long-term energy storage, and are of great importance for replacing traditional fossil fuels with new energy sources. The bipolar plate, a key component of the flow battery, plays a crucial role in supporting and sealing the battery, conducting current, isolating the positive and negative electrolytes, and establishing a corrosion-resistant barrier. Flow channel design on the bipolar plate can further reduce concentration polarization, decrease system pump losses, and thus improve overall system efficiency.

[0003] Currently, flexible graphite plates are commonly used in flow batteries due to their excellent conductivity and corrosion resistance. However, to meet the required mechanical strength of the battery, resin fillers need to be added during the bipolar plate manufacturing process to improve the mechanical strength. The addition of resin fillers, however, reduces the conductivity of the bipolar plate. Furthermore, most bipolar plates are produced using a hot-pressing process. Because resin has relatively low surface energy, it tends to migrate to the surface during hot pressing, inevitably forming a resin-rich layer on the bipolar plate surface, which increases the contact resistance between the carbon felt and the bipolar plate.

[0004] The current mainstream approach is to adjust the ratio of bipolar plate fillers, increase the content of conductive graphite, and reduce the amount of resin to improve the conductivity of the material. However, this approach has the problem that the reduction in resin content will lead to a decrease in the mechanical strength of the bipolar plate, which in turn will reduce the sealing performance and reliability of the bipolar plate, making it impossible to achieve the goal of improving the conductivity of the bipolar plate while ensuring mechanical strength.

[0005] Therefore, how to improve the surface conductivity of bipolar plates without reducing their mechanical strength is a technical problem that urgently needs to be solved. Summary of the Invention

[0006] To address the shortcomings of existing technologies, the present invention aims to provide a bipolar plate for vanadium redox flow batteries, its surface modification method, and its applications. Based on carbon nanotubes, this invention employs a deep coupling strategy of "coating and shaping + hot-pressing and shaping" to prepare a modified coating on the surface of the bipolar plate. This weakens the negative impact of the resin-rich layer on the contact resistance between the carbon felt electrode and the bipolar plate, while maintaining the resin ratio in the bipolar plate, thus avoiding any adverse effects on its mechanical properties. Furthermore, the combination of coating and hot-pressing enhances the interfacial bonding of the modified coating, improving structural stability. This surface modification method exhibits good process compatibility and controllability, meeting the stringent requirements of high conductivity, strong bonding, and ultra-corrosion resistance for coatings in flow battery bipolar plates, and demonstrates promising industrialization prospects. It is one of the key technological pathways to overcome current technological bottlenecks and improve battery energy density and capacity retention.

[0007] To achieve this objective, the present invention adopts the following technical solution:

[0008] In a first aspect, the present invention provides a method for surface modification of a bipolar plate for a vanadium redox flow battery, the surface modification method comprising the following steps:

[0009] A bipolar plate is provided, the constituent materials of which include expanded graphite and resin filler.

[0010] A modified slurry is coated on the surface of the bipolar plate, and then hot-pressed to form a modified coating.

[0011] The modified slurry includes carbon nanotubes, a binder, and a solvent.

[0012] This invention employs a deep coupling strategy of "coating and shaping + hot pressing" based on carbon nanotubes to prepare a modified coating on the surface of a bipolar plate. This weakens the negative impact of the resin-rich layer on the contact resistance between the carbon felt electrode and the bipolar plate, while maintaining the resin ratio in the bipolar plate and thus avoiding any adverse effects on its mechanical properties. Furthermore, the combination of coating and hot pressing enhances the interfacial bonding of the modified coating, improving structural stability. This surface modification method exhibits good process compatibility and controllability, meeting the stringent requirements of flow battery bipolar plates for high conductivity, strong adhesion, and superior corrosion resistance in coatings. It demonstrates promising industrialization prospects and represents one of the key technological pathways to overcome current technological bottlenecks and improve battery energy density and capacity retention.

[0013] The purpose of using carbon nanotubes as conductive fillers in the modified coating in this invention is: 1. to utilize the high conductivity of carbon nanotubes to construct conductive pathways; 2. to utilize carbon nanotubes to pierce the resin and weaken the influence of the resin-rich layer.

[0014] Preferably, the bipolar plate is a flexible graphite plate.

[0015] Preferably, the thickness of the flexible graphite plate is 0.6-2mm, for example, it can be 0.6mm, 0.8mm, 1mm, 1.2mm, 1.4mm, 1.6mm, 1.8mm or 2mm, etc.

[0016] Preferably, the resin filler accounts for 15-25% of the mass of the flexible graphite plate, for example, it can be 15%, 20% or 25%.

[0017] Preferably, the solid content of the modified slurry is 40-70 wt%, for example, it can be 40 wt%, 50 wt%, 60 wt%, or 70 wt%.

[0018] Preferably, the viscosity of the modified slurry is 4000-10000 mPa·s, for example, it can be 4000 mPa·s, 5000 mPa·s, 6000 mPa·s, 7000 mPa·s, 8000 mPa·s, 9000 mPa·s or 10000 mPa·s, etc.

[0019] Preferably, based on the dry basis mass of the modified slurry, the mass fraction of the carbon nanotubes is 80-95%, for example, it can be 80%, 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94% or 95%, etc.

[0020] This invention introduces a suitable mass fraction of carbon nanotubes into the modified slurry. The high conductivity of carbon nanotubes can be used to construct conductive pathways, and the carbon nanotubes can be used to pierce the resin, thereby reducing the negative impact of the resin-rich layer on the contact resistance between the carbon felt electrode and the bipolar plate.

[0021] Preferably, the mass ratio of the carbon nanotubes to the binder is (80-95):(5-20), wherein the carbon nanotubes selected in the range of "80-95" can be, for example, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94 or 95, and the binder selected in the range of "5-20" can be, for example, 5, 10, 15 or 20.

[0022] Preferably, the adhesive comprises any one or a combination of at least two of polyvinylidene fluoride, Nafion solution, or polyaniline (PANI).

[0023] Preferably, the carbon nanotubes include multi-walled carbon nanotubes and / or single-walled carbon nanotubes, with multi-walled carbon nanotubes being more preferred.

[0024] In this invention, multi-walled carbon nanotubes are selected, which, compared with single-walled carbon nanotubes, can more effectively construct a stable and efficient three-dimensional conductive network and significantly improve the structural stability, mechanical strength, interfacial bonding force and conductivity durability of the modified coating.

[0025] Preferably, the number of wall layers of the multi-walled carbon nanotube is 5-15, for example, it can be 5, 10 or 15 layers.

[0026] In this invention, multi-walled carbon nanotubes that meet the above-mentioned number of wall layers are advantageous in ensuring high conductivity while also maintaining good dispersibility and structural stability. If the number of layers is too small, the mechanical strength of the tube wall is insufficient, and it is easily damaged under hot-pressing shear stress; if the number of layers is too large, the tube wall thickens, leading to increased interlayer electron scattering, and the conductivity will approach that of graphite microcrystals and decrease.

[0027] Preferably, the diameter of the multi-walled carbon nanotube is 2-20 nm, for example, it can be 2 nm, 5 nm, 10 nm, 15 nm or 20 nm.

[0028] Preferably, the aspect ratio of the multi-walled carbon nanotube is 100-500, for example, it can be 100, 200, 300, 400 or 500.

[0029] In this invention, multi-walled carbon nanotubes that meet the above aspect ratio are beneficial for constructing a highly interconnected, low percolation threshold three-dimensional conductive network and for enhancing the mechanical integrity of the coating.

[0030] Preferably, the coating method includes a blade coating method.

[0031] Preferably, the coating thickness of the modified slurry is 70-150 μm, for example, it can be 70 μm, 80 μm, 90 μm, 100 μm, 110 μm, 120 μm, 130 μm, 140 μm or 150 μm.

[0032] The present invention limits the coating thickness of the modified slurry to 70-150μm, which helps to improve the surface conductivity of the bipolar plate and reduce the negative impact of the resin-rich layer on the surface of the bipolar plate.

[0033] Preferably, the pressure of the hot pressing is 0.5-1.5 MPa, for example, it can be 0.5 MPa, 0.6 MPa, 0.7 MPa, 0.8 MPa, 0.9 MPa, 1 MPa, 1.1 MPa, 1.2 MPa, 1.3 MPa, 1.4 MPa or 1.5 MPa, etc.

[0034] Preferably, the hot pressing temperature is 120-200℃, for example, it can be 120℃, 130℃, 140℃, 150℃, 160℃, 170℃, 180℃, 190℃ or 200℃.

[0035] Preferably, the hot pressing time is 1-4 minutes, for example, it can be 1 minute, 2 minutes, 3 minutes or 4 minutes.

[0036] This invention employs a specific pressure and a specific hot-pressing temperature for a suitable time to perform hot-pressing treatment. Under multiple parameter constraints, it can precisely control the thickness of the final coating while ensuring the durability of the coating and preventing it from peeling off.

[0037] Preferably, the modified slurry further includes graphene, and the mass ratio of the carbon nanotubes to the graphene is 1:(0.5-1), for example, it can be 1:0.5, 1:0.6, 1:0.7, 1:0.8, 1:0.9 or 1:1, etc.

[0038] In this invention, the introduction of graphene can connect isolated carbon nanotubes, forming a denser and more stable three-dimensional conductive structure; it also makes the in-plane conductivity more uniform; and it helps to improve the density of the modified coating and its adhesion to the substrate. Furthermore, when the two are used in synergy at an appropriate mass ratio, their synergistic effect can be maximized, realizing a qualitative change in the conductive network from "physical mixing" to "functional coupling".

[0039] Preferably, the modified slurry further includes metal oxide nanoparticles, which include manganese dioxide and / or cerium dioxide. Based on the dry basis of the modified slurry, the mass content of the metal oxide nanoparticles is 1-5 wt%, for example, it can be 1 wt%, 2 wt%, 3 wt%, 4 wt%, or 5 wt%.

[0040] In this invention, the introduction of metal oxide nanoparticles helps to further improve the surface conductivity of the bipolar plate. Furthermore, if the metal oxide dissolves and adheres to the carbon felt, it can enhance the carbon felt's resistance to the vanadium ion pair (VO2+). 2+ / VO2 + The reversibility of the reaction is improved, and the polarization overpotential is reduced. Furthermore, the appropriate mass content of metal oxide nanoparticles can provide significant electrocatalytic activity without causing structural damage to the conductive network, thus achieving a balance between conductivity and catalytic activity.

[0041] Preferably, the surface modification method includes the following steps:

[0042] (1) Provide a bipolar plate, wherein the bipolar plate is a flexible graphite plate, and the constituent materials of the flexible graphite plate include expanded graphite and resin filler; the resin filler includes PVDF (polyvinylidene fluoride) and / or epoxy resin; in the flexible graphite plate, the mass ratio of resin filler is 15-25%.

[0043] Multi-walled carbon nanotubes, a binder, and a solvent are mixed to prepare a modified slurry with a viscosity of 4000-10000 mPa·s and a solid content of 40-70 wt%. The modified slurry, based on its dry weight, contains 80-95% multi-walled carbon nanotubes by mass, and the mass ratio of carbon nanotubes to binder is (80-95):(5-20). The binder comprises any one or a combination of at least two of polyvinylidene fluoride, Nafion solution, or polyaniline. The multi-walled carbon nanotubes have 5-15 wall layers, a diameter of 2-20 nm, and an aspect ratio of 100-500. The solvent comprises methylpyrrolidone.

[0044] (2) The modified slurry is coated on the surface of the bipolar plate with a thickness of 70-150 μm, and then hot-pressed at a pressure of 0.5-1.5 MPa and a temperature of 120-200 °C for 1-4 min to form a modified coating.

[0045] In a second aspect, the present invention provides a bipolar plate for a vanadium redox flow battery, wherein the bipolar plate for a vanadium redox flow battery is prepared by the surface modification method described in the first aspect.

[0046] Thirdly, the present invention provides a vanadium redox flow battery, the vanadium redox flow battery comprising a bipolar plate, a carbon felt electrode and a separator, wherein the bipolar plate is the vanadium redox flow battery bipolar plate described in the second aspect.

[0047] For example, the thickness of the carbon felt electrode is 2-4.6 mm, such as 2 mm, 2.5 mm, 3 mm, 3.5 mm, 4 mm, or 4.6 mm. The diaphragm is a proton exchange membrane, and its thickness can be 40-117 μm, such as 40 μm, 50 μm, 60 μm, 70 μm, 80 μm, 90 μm, 100 μm, 110 μm, or 117 μm.

[0048] The numerical range described in this invention includes not only the point values ​​listed above, but also any point values ​​within the numerical ranges not listed above. Due to space limitations and for the sake of brevity, this invention will not exhaustively list all the specific point values ​​included in the range.

[0049] Compared with the prior art, the present invention has the following beneficial effects:

[0050] This invention employs a deep coupling strategy of "coating and shaping + hot pressing" based on carbon nanotubes to prepare a modified coating on the surface of a bipolar plate. This weakens the negative impact of the resin-rich layer on the contact resistance between the carbon felt electrode and the bipolar plate, while maintaining the resin ratio in the bipolar plate and thus avoiding any adverse effects on its mechanical properties. Furthermore, the combination of coating and hot pressing enhances the interfacial bonding of the modified coating, improving structural stability. This surface modification method exhibits good process compatibility and controllability, meeting the stringent requirements of flow battery bipolar plates for high conductivity, strong adhesion, and superior corrosion resistance in coatings. It demonstrates promising industrialization prospects and represents one of the key technological pathways to overcome current technological bottlenecks and improve battery energy density and capacity retention. Attached Figure Description

[0051] Figure 1 The energy efficiency-current density comparison curves of the all-vanadium redox flow battery prepared based on the bipolar plates provided in Example 1 and Comparative Example 1 are shown.

[0052] Figure 2 The curves show the comparison of the cycle performance of the all-vanadium redox flow battery prepared based on the bipolar plates provided in Example 1 and Comparative Example 1. Detailed Implementation

[0053] The technical solution of the present invention will be further illustrated below through specific embodiments. Those skilled in the art should understand that the embodiments described are merely illustrative of the present invention and should not be construed as limiting the invention in any way.

[0054] Example 1

[0055] This embodiment provides a surface modification method for bipolar plates used in vanadium redox flow batteries, the surface modification method comprising the following steps:

[0056] (1) Provide a bipolar plate, wherein the bipolar plate is a flexible graphite plate, and the constituent materials of the flexible graphite plate include expanded graphite and resin filler; the resin filler is PVDF; and the mass percentage of the resin filler in the flexible graphite plate is 18%.

[0057] A modified slurry with a viscosity of 8000 mPa·s and a solid content of 65 wt% was prepared by mixing 0.95 g of multi-walled carbon nanotubes and 0.05 g of binder in a solvent using a vacuum mixer. The slurry, based on its dry weight, contained 95% multi-walled carbon nanotubes and a mass ratio of 95:5 between the multi-walled carbon nanotubes and the binder. The binder was polyvinylidene fluoride (PVDF). The multi-walled carbon nanotubes had 10 wall layers, a diameter of 15 nm, and an aspect ratio of 300. The solvent was methylpyrrolidone.

[0058] (2) Using a coating machine, the modified slurry is scraped onto both sides of the bipolar plate with a coating thickness of 100 μm. Then, a hot press is used to perform hot pressing at a pressure of 1 MPa and a temperature of 180°C for 3 minutes. After the hot pressing is completed, the plate is cooled to room temperature of 25°C to form a modified coating and obtain a surface-modified bipolar plate.

[0059] Example 2

[0060] This embodiment provides a surface modification method for bipolar plates used in vanadium redox flow batteries, the surface modification method comprising the following steps:

[0061] (1) Provide a bipolar plate, wherein the bipolar plate is a flexible graphite plate, and the constituent materials of the flexible graphite plate include conductive graphite and resin filler; the resin filler is PVDF; and the mass percentage of the resin filler in the flexible graphite plate is 18%.

[0062] A mixture of 0.9 g of multi-walled carbon nanotubes and 0.1 g of binder was added to a solvent and mixed evenly to prepare a modified slurry with a viscosity of 9000 mPa·s and a solid content of 70 wt%. The modified slurry, based on its dry weight, contained 90% multi-walled carbon nanotubes by mass, and the mass ratio of multi-walled carbon nanotubes to binder was 90:10. The binder was polyvinylidene fluoride (PVDF). The multi-walled carbon nanotubes had 10 wall layers, a diameter of 15 nm, and an aspect ratio of 500. The solvent was methylpyrrolidone.

[0063] (2) Using a coating machine, the modified slurry is coated on both sides of the bipolar plate with a coating thickness of 70 μm. Then, a hot press is used to perform hot pressing at a pressure of 0.5 MPa and a temperature of 200°C for 2 minutes. After the hot pressing is completed, the plate is cooled to room temperature of 25°C to form a modified coating and obtain a surface-modified bipolar plate.

[0064] Example 3

[0065] This embodiment provides a surface modification method for bipolar plates used in vanadium redox flow batteries, the surface modification method comprising the following steps:

[0066] (1) Provide a bipolar plate, wherein the bipolar plate is a flexible graphite plate, and the constituent materials of the flexible graphite plate include conductive graphite and resin filler; the resin filler is PVDF; and the mass percentage of the resin filler in the flexible graphite plate is 18%.

[0067] A mixture of 0.8 g of multi-walled carbon nanotubes and 0.2 g of binder was added to a solvent and mixed evenly to prepare a modified slurry with a viscosity of 10000 mPa·s and a solid content of 60 wt%. The modified slurry, based on its dry weight, contained 80% multi-walled carbon nanotubes and a mass ratio of 80:20 between the multi-walled carbon nanotubes and the binder. The binder was polyvinylidene fluoride (PVDF). The multi-walled carbon nanotubes had 10 wall layers, a diameter of 10 nm, and an aspect ratio of 100. The solvent was methylpyrrolidone.

[0068] (2) Using a coating machine, the modified slurry is coated on both sides of the bipolar plate with a coating thickness of 150 μm. Then, a hot press is used to perform hot pressing at a pressure of 1.5 MPa and a temperature of 120 °C for 4 min. After the hot pressing is completed, the plate is cooled to room temperature of 25 °C to form a modified coating and obtain a surface-modified bipolar plate.

[0069] Example 4

[0070] The difference between this embodiment and embodiment 1 is that in step (2), the hot pressing time is 2 minutes.

[0071] The remaining surface modification methods and parameters are consistent with those in Example 1.

[0072] Example 5

[0073] The difference between this embodiment and embodiment 1 is that in step (2), the coating thickness is 150 μm.

[0074] The remaining surface modification methods and parameters are consistent with those in Example 1.

[0075] Example 6

[0076] The difference between this embodiment and Embodiment 1 is that the modified slurry further includes graphene oxide, and the mass ratio of the multi-walled carbon nanotubes to graphene oxide is 1:0.75.

[0077] The remaining surface modification methods and parameters are consistent with those in Example 1.

[0078] Example 7

[0079] The difference between this embodiment and Embodiment 1 is that the modified slurry further includes manganese dioxide; based on the dry basis of the modified slurry, the mass content of manganese dioxide is 3 wt%.

[0080] The remaining surface modification methods and parameters are consistent with those in Example 1.

[0081] Example 8

[0082] The difference between this embodiment and Embodiment 1 is that the multi-walled carbon nanotubes are replaced with single-walled carbon nanotubes.

[0083] The remaining surface modification methods and parameters are consistent with those in Example 1.

[0084] Example 9

[0085] The difference between this embodiment and Embodiment 1 is that the aspect ratio of the multi-walled carbon nanotubes is 80.

[0086] The remaining surface modification methods and parameters are consistent with those in Example 1.

[0087] Example 10

[0088] The difference between this embodiment and Embodiment 1 is that the aspect ratio of the multi-walled carbon nanotube is 550.

[0089] The remaining surface modification methods and parameters are consistent with those in Example 1.

[0090] Example 11

[0091] The difference between this embodiment and Embodiment 1 is that the pressure of the hot pressing is 2 MPa.

[0092] The remaining surface modification methods and parameters are consistent with those in Example 1.

[0093] Example 12

[0094] The difference between this embodiment and Embodiment 6 is that the mass ratio of the multi-walled carbon nanotubes to graphene oxide is 1:1.5.

[0095] The remaining surface modification methods and parameters are consistent with those in Example 6.

[0096] Example 13

[0097] The difference between this embodiment and Embodiment 7 is that, based on the dry basis of the modified slurry, the mass content of manganese dioxide is 7wt%.

[0098] The remaining surface modification methods and parameters are consistent with those in Example 7.

[0099] Comparative Example 1

[0100] This comparative example provides a bipolar plate as described in step (1) of Example 1, i.e., without surface modification of the bipolar plate.

[0101] The charge-discharge performance of the all-vanadium redox flow batteries prepared based on the bipolar plates provided in Example 1 and Comparative Example 1 was tested under the following conditions: ambient temperature, ambient pressure, and ambient humidity. The flow batteries were subjected to a charge-discharge rate of 140 mA / cm². 2 160mA / cm2 200mA / cm 2 Constant current charge-discharge test at current density. Test results are as follows: Figure 1 As shown, by Figure 1 It can be seen that the bipolar plate provided in Example 1 operates at 200 mA / cm². 2 Energy efficiency was improved by 4% at current density.

[0102] Cyclic performance tests were conducted on the vanadium redox flow batteries fabricated using the bipolar plates provided in Example 1 and Comparative Example 1. The test conditions were: ambient temperature, ambient pressure, and ambient humidity, with the flow batteries operating at 140 mA / cm². 2 A 100-cycle charge-discharge test was conducted at the specified current density. The test results are as follows: Figure 2 As shown, by Figure 2 It can be seen that the bipolar plate provided in Example 1 has a greater discharge capacity in 100 discharge cycles.

[0103] Comparative Example 2

[0104] The difference between this comparative example and Example 1 is that the multi-walled carbon nanotubes in step (1) are replaced with conductive carbon black.

[0105] The remaining surface modification methods and parameters are consistent with those in Example 1.

[0106] Comparative Example 3

[0107] The difference between this comparative example and Example 1 is that the multi-walled carbon nanotubes in step (1) are replaced with graphite worms.

[0108] The remaining surface modification methods and parameters are consistent with those in Example 1.

[0109] Performance testing

[0110] Assemble the bipolar plates provided in the above embodiments and comparative examples into an all-vanadium redox flow battery. The specific steps include: 1) providing the bipolar plates provided in the above embodiments and comparative examples, providing a carbon felt electrode with a thickness of 3.5 mm, providing a proton exchange membrane (made of Nafion117) with a thickness of 50 μm as a separator, and the vanadium ion electrolyte including 1.7 mol / L of 3.5 valence vanadium ions and 3 mol / L of H2SO4; 2) assembling the above materials into a single cell of the flow battery, with a positive and negative electrode electrolyte volume of 70 mL.

[0111] The assembled vanadium redox flow battery was subjected to charge-discharge tests to obtain its energy efficiency. The test conditions were: the flow battery was subjected to a charge-discharge test at 140 mA / cm² under normal temperature, pressure, and humidity conditions. 2 160mA / cm 2 200mA / cm 2A constant current test was performed at the current density, and the voltage-time curve was recorded. The energy efficiency (%) was calculated using the formula (discharge energy / charge energy) × 100%, yielding 200 mA / cm². 2 Battery energy efficiency at current density.

[0112] At 140mA / cm 2 Long-cycle testing was conducted on the vanadium redox flow battery under constant current conditions to determine the capacity retention rate after 100 cycles.

[0113] The test results are shown in Table 1.

[0114] Table 1

[0115]

[0116] analyze:

[0117] As shown in Table 1, this invention employs a deep coupling strategy of "coating and shaping + hot pressing" based on carbon nanotubes to prepare a modified coating on the surface of the bipolar plate. This weakens the negative impact of the resin-rich layer on the contact resistance between the carbon felt electrode and the bipolar plate, while maintaining the resin ratio in the bipolar plate and thus avoiding any adverse effects on its mechanical properties. Furthermore, the combination of coating and hot pressing enhances the interfacial bonding force of the modified coating, improving structural stability. This surface modification method exhibits good process compatibility and controllability, meeting the stringent requirements of flow battery bipolar plates for high conductivity, strong bonding, and ultra-corrosion resistance in coatings. It also demonstrates promising industrialization prospects and represents one of the key technological pathways to overcome current technological bottlenecks and improve battery energy density and capacity retention.

[0118] A comparison of Examples 1 and 8 shows that if multi-walled carbon nanotubes are replaced with single-walled carbon nanotubes, the overall effect on constructing a stable, efficient, and mechanically superior modified coating is not significant.

[0119] As can be seen from the comparison between Example 1 and Examples 9-10, if the aspect ratio of the multi-walled carbon nanotubes is too small, it is difficult to effectively overlap and form a continuous and efficient three-dimensional conductive network in the coating, which makes it difficult to meet the stringent requirements of the flow battery bipolar plate for high conductivity, strong bonding and ultra-corrosion resistance of the coating, so that the energy density and capacity retention of the battery cannot be improved. If the aspect ratio of the multi-walled carbon nanotubes is too large, the carbon nanotubes are prone to severe entanglement and agglomeration, making it difficult to disperse evenly in the slurry. After coating, local conductive enrichment areas are easily formed, resulting in uneven conductivity of the coating, so that the energy density and capacity retention of the battery cannot be improved.

[0120] As can be seen from the comparison between Example 1 and Example 11, if the pressure of hot pressing is too high, the graphite particles will be excessively broken, thereby destroying the conductive path. At the same time, the broken graphite particles will block the resin flow channel, which will increase the porosity inside the bipolar plate. In addition, higher pressure will increase the process cost.

[0121] As can be seen from the comparison between Example 6 and Example 12, if the mass ratio of multi-walled carbon nanotubes to graphene oxide is too small, that is, if too much graphene oxide is added, the excessive amount of graphene oxide will hinder the penetration of electrolyte and ion transport, and will also cause the viscosity of the slurry to rise sharply, which is not conducive to coating and seriously affects the energy density and capacity retention of the battery.

[0122] As can be seen from the comparison between Example 7 and Example 13, if too much manganese dioxide is added, the excess manganese dioxide will severely disrupt the conductive network, causing the overall resistivity of the coating to increase. Furthermore, the excess nanoparticles are more likely to agglomerate, reducing the density and mechanical strength of the coating and adversely affecting the energy density and capacity retention of the battery.

[0123] As can be seen from the comparison between Example 1 and Comparative Example 1, the surface modification method provided by the present invention can effectively weaken the negative impact of the resin-rich layer on the contact resistance between the carbon felt electrode and the bipolar plate, and will not have an adverse effect on the mechanical properties of the bipolar plate, so that the battery energy density and capacity retention rate are significantly improved.

[0124] As can be seen from the comparison between Example 1 and Comparative Examples 2-3, if conductive carbon black is used to prepare the modified slurry, the capacity retention rate of the vanadium redox flow battery after 100 cycles is poor; if graphite worms are used to prepare the modified slurry, the energy efficiency and capacity retention rate of the vanadium redox flow battery both show a significant downward trend.

[0125] It should be noted that the present invention is illustrated through the above embodiments, but the present invention is not limited to the above process steps, that is, it does not mean that the present invention must rely on the above process steps to be implemented. Those skilled in the art should understand that any improvements to the present invention, equivalent substitutions of the raw materials used in the present invention, additions of auxiliary components, selection of specific methods, etc., all fall within the protection scope and disclosure scope of the present invention.

Claims

1. A surface modification method of a bipolar plate for a vanadium redox flow battery, characterized by, The surface modification method includes the following steps: A bipolar plate is provided, the constituent materials of which include expanded graphite and resin filler; A modified slurry is coated on the surface of the bipolar plate, and then hot-pressed to form a modified coating. The modified slurry includes carbon nanotubes, a binder, and a solvent.

2. The surface modification method according to claim 1, wherein, The bipolar plate is a flexible graphite plate; Preferably, the thickness of the flexible graphite plate is 0.6-2 mm; Preferably, the resin filler accounts for 15-25% of the mass of the flexible graphite plate.

3. The surface modification method according to claim 1 or 2, characterized in that, The solid content of the modified slurry is 40-70 wt%; Preferably, the viscosity of the modified slurry is 4000-10000 mPa·s.

4. The surface modification method according to any one of claims 1 to 3, characterized in that, Based on the dry weight of the modified slurry, the mass fraction of the carbon nanotubes is 80-95%. Preferably, the mass ratio of the carbon nanotubes to the binder is (80-95):(5-20).

5. The surface modification method according to any one of claims 1 to 4, characterized in that, The adhesive comprises any one or a combination of at least two of polyvinylidene fluoride, Nafion solution, or polyaniline; Preferably, the carbon nanotubes include multi-walled carbon nanotubes and / or single-walled carbon nanotubes, and more preferably multi-walled carbon nanotubes; Preferably, the number of wall layers of the multi-walled carbon nanotube is 5-15; Preferably, the diameter of the multi-walled carbon nanotubes is 2-20 nm; Preferably, the aspect ratio of the multi-walled carbon nanotube is 100-500.

6. The surface modification method according to any one of claims 1 to 5, characterized in that, The coating method includes a scraping method; Preferably, the coating thickness of the modified slurry is 70-150 μm; Preferably, the pressure of the hot pressing is 0.5-1.5 MPa; Preferably, the hot pressing temperature is 120-200℃; Preferably, the hot pressing time is 1-4 minutes.

7. The surface modification method according to any one of claims 1 to 6, characterized in that, The modified slurry also includes graphene, and the mass ratio of the carbon nanotubes to the graphene is 1:(0.5-1); Preferably, the modified slurry further includes metal oxide nanoparticles, which include manganese dioxide and / or bismuth oxide; the mass content of the metal oxide nanoparticles is 1-5 wt% based on the dry basis of the modified slurry.

8. The surface modification method according to any one of claims 1 to 7, characterized in that, The surface modification method includes the following steps: (1) A bipolar plate is provided, wherein the bipolar plate is a flexible graphite plate, and the constituent materials of the flexible graphite plate include expanded graphite and resin filler; the resin filler includes PVDF and / or epoxy resin; and the mass percentage of the resin filler in the flexible graphite plate is 15-25%. Multi-walled carbon nanotubes, a binder, and a solvent are mixed to prepare a modified slurry with a viscosity of 4000-10000 mPa·s and a solid content of 40-70 wt%. The modified slurry, based on its dry weight, contains 80-95% multi-walled carbon nanotubes by mass, and the mass ratio of carbon nanotubes to binder is (80-95):(5-20). The binder comprises any one or a combination of at least two of polyvinylidene fluoride, Nafion solution, or polyaniline. The multi-walled carbon nanotubes have 5-15 wall layers, a diameter of 2-20 nm, and an aspect ratio of 100-500. The solvent comprises methylpyrrolidone. (2) coating the modified slurry on the surface of the bipolar plate, with a coating thickness of 70-150 μm, and then hot-pressing at a pressure of 0.5-1.5 MPa and a temperature of 120-200 ℃ for 1-4 min to form a modified coating.

9. A bipolar plate for a vanadium redox flow battery, characterized by The bipolar plate for the all-vanadium redox flow battery is prepared by the surface modification method according to any one of claims 1-8.

10. An all-vanadium redox flow battery characterised in that, The all-vanadium redox flow battery comprises a bipolar plate, a carbon felt electrode and a separator, wherein the bipolar plate is the bipolar plate for the all-vanadium redox flow battery according to claim 9.

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