Heat exchange tube for vanadium flow battery and preparation method and application thereof

Abstract

The application discloses a heat exchange tube for a vanadium liquid flow battery and a preparation method and application thereof, and belongs to the technical field of liquid flow battery heat exchange. The base material of the heat exchange tube for the vanadium liquid flow battery is any one of an industrial pure titanium tube, a titanium alloy tube or a nickel-based alloy tube; the outer surface of the heat exchange tube is covered with a protective layer, and the protective layer is a polymer coating or a metal coating; wherein the outer surface of the polymer coating has a micro-crease structure. The heat exchange tube for the vanadium liquid flow battery has excellent corrosion resistance and thermal cycle stability, avoids the problem that metal ions in the base material are easily dissolved out, leading to ion pollution of an electrolyte, and thus the safety and stability of the vanadium liquid flow battery are effectively improved.

Description

Technical Field

[0001] This invention belongs to the field of heat exchange technology for flow batteries, and specifically relates to a heat exchange tube for vanadium redox flow batteries, its preparation method, and its application. Background Technology

[0002] Vanadium redox flow batteries have become the preferred technology for large-scale energy storage due to their high safety, long cycle life, and strong capacity scalability. Their electrolyte, as the energy storage medium, needs to be maintained within its optimal operating temperature range via a heat exchanger. Currently, titanium tubes are commonly used as heat exchanger materials for vanadium redox flow batteries.

[0003] However, in actual operation, it has been found that although some heat exchange tubes (such as titanium tubes and nickel-based alloy tubes) have excellent corrosion resistance, the surface oxide film will partially dissolve in the high-temperature, high-acidity electrolyte environment, leading to the leaching of ions from the tubes. In particular, the titanium ions leached from titanium tubes will compete with vanadium ions in the electrolyte for coordination, accelerating the precipitation of high-valence vanadium and reducing the stability of the electrolyte. In addition, existing materials such as pure titanium, titanium alloys, and nickel-based alloys still face the risk of pitting corrosion and ion contamination under long-term thermal cycling conditions. Furthermore, the existing heat exchange tubes coated with planar protective layers (such as polytetrafluoroethylene) are prone to temperature fluctuations in the electrolyte environment during the operation of vanadium redox flow batteries. These fluctuations generate alternating thermal stress within the coating, which can easily lead to coating cracking, electrolyte penetration, and titanium ion leaching. Summary of the Invention

[0004] To overcome the shortcomings of existing technologies, the main objective of this invention is to provide a heat exchange tube for vanadium redox flow batteries, its preparation method, and its application. This invention has advantages such as improving the corrosion resistance and thermal cycling stability of existing vanadium redox flow battery heat exchange tubes, avoiding the problem of easy dissolution of substrate metal ions leading to ion contamination of the electrolyte, thereby effectively improving the safety and stability of vanadium redox flow batteries.

[0005] To achieve the above objectives, one aspect of the present invention provides a heat exchange tube for a vanadium redox flow battery, wherein the substrate of the heat exchange tube is any one of industrial pure titanium tube, titanium alloy tube, or nickel-based alloy tube; the outer surface of the heat exchange tube is covered with a protective layer, wherein the protective layer is a polymer coating or a metal coating; wherein the outer surface of the polymer coating has a micro-wrinkled structure.

[0006] Furthermore, the protrusion height of the micro-fold structure is 3-15 μm.

[0007] Furthermore, the thickness of the protective coating is 30-500µm.

[0008] Furthermore, the substrate of the heat exchange tube and the protective layer have a roughened bonding interface.

[0009] Furthermore, the polymer in the polymer coating is selected from one or more of polytetrafluoroethylene, polyethylene, polyvinyl chloride, and polypropylene.

[0010] Furthermore, the metal coating is any one of a platinum coating, a gold coating, a nickel alloy coating, or a silver coating.

[0011] Another aspect of the present invention provides a method for preparing the aforementioned heat exchange tube for a vanadium redox flow battery, comprising the following steps: S1, Surface treatment of the substrate of the vanadium redox flow battery heat exchange tube; S2, a protective layer is applied to the outer surface of the surface-treated heat exchanger tube substrate, including: When the protective layer is a polymer coating, the process includes: coating a wet polymer coating film on the outer surface of the heat exchange tube substrate; and then subjecting the heat exchange tube coated with the wet polymer coating film to thermal cycling sintering to form a wrinkled structure on the surface of the polymer coating. Alternatively, when the protective coating is a metal coating, it includes: applying a metal coating to the surface of the heat exchange tube substrate that has undergone the surface treatment by spraying or dipping, and then performing a high-temperature sintering process to form the protective layer.

[0012] Furthermore, the thermal cycling sintering process includes: S21, preheating: heating from room temperature to 180-200℃ at a rate of 3.0±0.5℃ / min, and holding at that temperature for 10±5min; S22, multiple thermal cycles: heat up to 280-320℃ at 5.0±0.3℃ / min, hold for 8.0±0.5min, then cool down to 180-200℃ at 3.0±0.2℃ / min as one thermal cycle, repeat 3 to 7 times; S23, high-temperature sintering: heat up to 350-380℃ at a rate of 2.0±0.5℃ / min, and hold for 30–60 min; S24, Cooling: Cool to room temperature at a rate of 1.5±0.5℃ / min.

[0013] Another aspect of the present invention provides a heat exchanger for a vanadium redox flow battery, the heat exchanger comprising a housing, a heat exchange tube bundle disposed within the housing, and tube boxes disposed at both ends of the housing, the heat exchange tube bundle comprising a plurality of heat exchange tubes, the heat exchange tubes being the aforementioned heat exchange tubes for a vanadium redox flow battery.

[0014] Compared with the prior art, the present invention has the following beneficial effects: The heat exchange tubing for vanadium redox flow batteries provided by this invention uses titanium or nickel-based alloys as the substrate, taking advantage of their excellent processing performance and basic corrosion resistance; and the protective layer covering its surface plays a crucial role in physical isolation. This protective layer can effectively prevent direct contact between the high-temperature, high-acidity vanadium electrolyte and the substrate, and significantly inhibit ion dissolution.

[0015] Meanwhile, the protective layer of this invention also solves the risk of pitting corrosion and ion contamination caused by the dissolution of the surface oxide film of the substrate under long-term thermal cycling conditions. It avoids the accelerated precipitation of high-valence vanadium due to competitive coordination between titanium ions and vanadium ions in titanium materials, thereby maintaining the stability of the electrolyte, extending the charge and discharge life of the battery, and reducing maintenance costs.

[0016] Furthermore, the polymer coating of the present invention has a micro-wrinkled structure compared to existing polymer coatings. It can act as a buffer unit, absorbing alternating thermal stress through reversible elastic deformation during thermal cycling. This disperses the stress that was originally concentrated at the coating-substrate interface to the entire wrinkled network, fundamentally avoiding cracking caused by stress concentration. The hierarchical morphology of the wrinkles extends the possible penetration path of the electrolyte, while encapsulating potential micro-defects at the bottom of the wrinkle valleys, effectively blocking the diffusion channels of corrosive media to the substrate and reducing the risk of coating failure due to thermal cycling caused by coating cracking.

[0017] The present invention can also use a metal coating on the surface of the heat exchange tube. Although the cost is higher, in certain specific designs or high-requirement scenarios, the use of such a metal coating can not only completely isolate corrosion, but also optimize the potential distribution on the surface of the heat exchanger by utilizing its good conductivity, further suppressing the occurrence of electrochemical corrosion and providing extremely reliable protection.

[0018] Other features and effects of the invention will be set forth in the following description, and will be apparent in part from the description, or may be learned by practicing the invention. Attached Figure Description

[0019] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0020] Figure 1 This is a schematic diagram of the structure of the heat exchanger for the vanadium redox flow battery of the present invention; Figure 2 This is a comparison chart showing the amount of titanium ion deposition in the heat exchange tubes of the vanadium redox flow battery in Example 1 and Comparative Example 1 of the present invention. Detailed Implementation

[0021] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0022] To achieve the above objectives, a first aspect of the present invention provides a heat exchange tube for a vanadium redox flow battery, wherein the substrate of the heat exchange tube is any one of industrial pure titanium tube, titanium alloy tube, or nickel-based alloy tube; the outer surface of the heat exchange tube is covered with a protective layer, wherein the protective layer is a polymer coating or a metal coating; wherein the outer surface of the polymer coating has a micro-wrinkled structure.

[0023] In some preferred embodiments of the present invention, the protrusion height of the micro-pleated structure is 3-15 μm. Based on extensive research, the inventors have found that the protrusion height of the micro-pleated structure has a crucial impact on the performance of the heat exchange tube. When the protrusion is less than 3 μm, the micro-pleats cannot have sufficient geometric undulations, making it difficult to effectively disperse thermal stress; protrusions greater than 15 μm can easily lead to excessively thin coatings in certain areas, especially on pipe surfaces with large curvatures, resulting in protective failure. A further preferred height is 5-12 μm.

[0024] In some preferred embodiments of the present invention, the thickness of the protective coating is 30-500µm. Extensive research has found that: a protective coating thickness below 30µm results in insufficient protective barrier, allowing corrosive media to easily penetrate the coating and cause substrate corrosion, significantly increasing ion precipitation and failing to meet long-term corrosion protection requirements; a thickness greater than 500µm leads to excessive internal stress, making the coating prone to cracking, bulging, and peeling during thermal cycling, increasing manufacturing difficulty and cost, and ultimately reducing overall protective reliability. A further preferred thickness is 50-200µm.

[0025] In some preferred embodiments of the present invention, a roughened bonding interface is formed between the substrate of the heat exchange tube and the protective layer. By roughening the surface of the substrate of the heat exchange tube to introduce a roughened bonding interface, the contact area and mechanical interlocking force between the substrate and the protective layer are greatly increased, thereby significantly improving the adhesion of the protective layer and preventing the protective layer from peeling or flaking under long-term thermal stress and fluid erosion, ensuring the long-term reliability of the protective effect.

[0026] In some preferred embodiments of the present invention, the polymer in the polymer coating may be selected from one or more of polytetrafluoroethylene (PTFE), polyethylene, polyvinyl chloride (PVC), and polypropylene. PTFE material possesses excellent high-temperature resistance, chemical corrosion resistance, and a low coefficient of friction. Using PTFE as a protective layer, the resulting coating, formed after sintering at high temperatures, is dense and stable, perfectly suited to the strong acid and high oxidizing environment of vanadium redox flow batteries, providing long-lasting and stable corrosion protection for the titanium substrate. Polyethylene, PVC, and polypropylene are common engineering plastics with good acid and alkali resistance and relatively low cost. Using such coatings can effectively control the overall manufacturing cost of the heat exchange tube while ensuring a certain level of corrosion resistance, providing diverse options for different budgets and application scenarios.

[0027] In some preferred embodiments of the present invention, the metal coating may be selected from, for example, a platinum coating, a gold coating, a nickel alloy coating, or a silver coating. These metal coatings possess extremely high chemical inertness and electrical conductivity. Although costly, in certain specific designs or demanding scenarios, the use of such metal coatings not only completely isolates corrosion but also utilizes their excellent electrical conductivity to optimize the potential distribution on the heat exchanger surface, further suppressing electrochemical corrosion and providing extremely reliable protection.

[0028] A second aspect of the present invention also provides a method for preparing the aforementioned heat exchange tube for a vanadium redox flow battery, comprising the following steps: S1, Surface treatment of the substrate of the vanadium redox flow battery heat exchange tube; S2, a protective layer is applied to the outer surface of the surface-treated heat exchanger tube substrate, including: When the protective layer is a polymer coating, the process includes: coating a wet polymer coating film on the outer surface of the heat exchange tube substrate; and then subjecting the heat exchange tube coated with the wet polymer coating film to thermal cycling sintering to form a wrinkled structure on the surface of the polymer coating. Alternatively, when the protective coating is a metallic coating, the process includes: applying a metallic coating to the surface of the heat exchange tube substrate that has undergone the surface treatment using a spraying or dipping method, followed by a high-temperature sintering process to form the protective layer. Preferably, the sintering temperature of the high-temperature sintering process is 350~380℃, and the sintering time is 30~60min.

[0029] In some preferred embodiments of the present invention, the thermal cycling sintering process includes: S21, Preheating: Increase the temperature from room temperature to 180-200℃ at a rate of 3.0±0.5℃ / min, and hold for 10±5min; S22, multiple thermal cycles: heating to 280-320℃ at 5.0±0.3℃ / min, holding at that temperature for 8.0±0.5min, and then cooling to 180-200℃ at 3.0±0.2℃ / min constitutes one thermal cycle, repeated 3 to 7 times; more preferably, the number of thermal cycles is 5, the temperature range of the thermal cycles is 200-300℃, the heating rate is 5.0℃ / min, the cooling rate is 3.0℃ / min, and the holding time is 8.0min.

[0030] S23, high-temperature sintering: heat up to 350-380℃ at a rate of 2.0±0.5℃ / min, and hold for 30–60 min; S24, Cooling: Cool to room temperature at a rate of 1.5±0.5℃ / min.

[0031] In the aforementioned thermal cycling sintering process, the temperature is maintained at 180-200℃ to ensure the coating material reaches a highly elastic state. During multiple thermal cycles, the high-temperature range is controlled at 280-320℃ to strictly keep it below the polymer's thermal degradation threshold, preventing material softening, flow, and structural damage. This ensures that the micro-wrinkles are controllable elastic buckling rather than defective cracks. During the complete thermal cycling phase, the polymer coating and the titanium substrate experience periodic alternating interfacial stress due to the difference in their coefficients of thermal expansion. The molecular chains possess the ability to rearrange themselves, providing the necessary conditions for the "deformation memory" of the micro-wrinkled structure. After multiple complete thermal cycles, the polymer molecular chains rearrange themselves along the stress direction and remember the deformation, ultimately locking into a regular, uniform micro-wrinkled network structure during the sintering and solidification process.

[0032] By rationally controlling the parameters of multiple thermal cycles, a micro-fold structure was constructed on the surface of the polymer coating. The stress originally concentrated at the coating-substrate interface was dispersed to the entire fold network, fundamentally avoiding cracking caused by stress concentration. The hierarchical morphology of the folds extended the possible penetration path of the electrolyte, while encapsulating potential micro-defects at the bottom of the folds, effectively blocking the diffusion channels of corrosive media to the substrate.

[0033] In some preferred embodiments of the present invention, the surface treatment includes degreasing, derusting, and roughening of the heat exchanger tube substrate. The roughening process employs sandblasting, where the surface of the heat exchanger tube substrate is vertically blasted under a pressure of 0.4-0.6 MPa, with a blasting distance of 150-200 mm and a treatment time of 30-60 s; after treatment, the surface roughness of the substrate Ra = 3.2-6.3 μm. This roughness range ensures effective mechanical bonding of the protective coating while avoiding localized thinning or stress concentration of the coating due to excessively deep pits.

[0034] Surface treatment is fundamental to forming a high-quality protective layer. Degreasing and rust removal ensure the cleanliness of the substrate surface, preventing impurities from affecting coating adhesion; roughening creates conditions for forming a highly adhesive bonding interface, thereby ensuring that the final heat exchanger has excellent corrosion resistance.

[0035] In some preferred embodiments of the present invention, the thickness of the polymer coating wet film is 320-380 μm.

[0036] A third aspect of the present invention also provides a heat exchanger for a vanadium redox flow battery, such as... Figure 1 As shown, the heat exchanger includes a shell, a heat exchange tube bundle disposed within the shell, and tube boxes disposed at both ends of the shell. The heat exchange tube bundle includes a plurality of heat exchange tubes, and the heat exchange tubes are the aforementioned heat exchange tubes for vanadium redox flow batteries.

[0037] The present invention will be further described in detail below with reference to specific embodiments, which should not be construed as limiting the scope of protection claimed by the present invention.

[0038] Example 1 A heat exchange tube for a vanadium redox flow battery is made of industrial pure titanium (dimensions: outer diameter 9.5 mm, wall thickness 0.5 mm, length 20 mm). The surface of the industrial pure titanium tube is covered with a protective layer (thickness 100 μm). The protective layer is a polytetrafluoroethylene (PTFE) coating, and the outer surface of the PTFE coating has a micro-wrinkled structure (i.e., the PTFE coating forms a continuous undulating protrusion structure along the surface of the heat exchange tube). The average height of the protrusions of the micro-wrinkled structure is 8 μm. There is a roughened bonding interface between the substrate of the heat exchange tube and the protective layer.

[0039] The specific preparation method of the heat exchange tube for the vanadium redox flow battery is as follows: (1) Surface treatment: The above-mentioned industrial pure titanium tubes are surface treated. First, they are degreased with acetone, and then roughened by sandblasting (vertical spraying at a pressure of 0.5MPa, spraying distance of 200mm, treatment time of 45s, and surface roughness of substrate Ra=3.2-6.3μm after treatment). Then, they are rinsed with deionized water and dried. (2) Protective coating: The industrial pure titanium tube with the above surface treatment is coated with a polymer coating wet film (thickness of 300μm), and then subjected to the following thermal cycling sintering to form a polymer coating with a micro-wrinkled structure: (2.1) Preheating: Increase the temperature to 200℃ at a rate of 3.0℃ / min and hold for 10min; (2.2) Multiple thermal cycles: Five complete thermal cycles were performed in the 200℃-300℃ cycle range (heating up 5.0℃ / min, cooling down 3.0℃ / min, and holding at the temperature for 8 min each time); (2.3) High-temperature sintering: Heat to 360℃ at 2.0℃ / min and hold for 45min.

[0040] (2.4) Cooling: Cool to room temperature at a rate of 1.5℃ / min.

[0041] Example 2 A heat exchange tube for a vanadium redox flow battery differs from Example 1 only in that the protective layer is a metal coating (nickel alloy). The metal coating is prepared by spraying a nickel alloy coating onto the surface of a surface-treated heat exchange tube substrate, followed by a high-temperature sintering process at 380°C for 45 minutes.

[0042] Example 3 A heat exchange tube for a vanadium redox flow battery differs from Example 1 only in that the average height of the protrusions in the micro-pleated structure is 2 μm.

[0043] Example 4 A heat exchange tube for a vanadium redox flow battery differs from Example 1 only in that the average height of the protrusions in the micro-pleated structure is 17 μm.

[0044] Example 5 A heat exchange tube for a vanadium redox flow battery differs from Example 1 only in the preparation process of the polytetrafluoroethylene coating via thermal cycling sintering. In this example, the temperature range for multiple thermal cycles during thermal cycling sintering is 200℃-350℃.

[0045] Example 6 A heat exchange tube for a vanadium redox flow battery differs from Example 1 only in the preparation process of the polytetrafluoroethylene coating via thermal cycling sintering. In this example, the thermal cycling sintering involves two thermal cycles.

[0046] Example 7 A heat exchange tube for a vanadium redox flow battery differs from Example 1 only in the preparation process of the polytetrafluoroethylene coating via thermal cycling sintering. In this example, the thermal cycling sintering involves eight thermal cycles.

[0047] Comparative Example 1 A heat exchange tube for a vanadium redox flow battery uses the same industrial pure titanium tube as in Example 1, but without a protective layer on the surface of the industrial pure titanium tube.

[0048] The vanadium redox flow cell of this comparative example was subjected to ion dissolution testing using a heat exchange tube. The experimental steps were as follows: Industrial pure titanium tubes were placed in a positive electrode storage tank containing 30 mL of simulated electrolyte and subjected to a corrosion resistance immersion test at room temperature. The corrosion resistance was tested at 200 mA / cm². 2The charge-discharge cycle was performed at the current density for 24 hours. After the test, the positive electrode electrolyte was collected and analyzed using ICP-MS (inductively coupled plasma mass spectrometry) to detect the content of titanium ions in the solution.

[0049] Experimental results: ICP-MS test results show that, under the same immersion time and environment, compared with the original sample of unimmersed electrolyte (Ti: 0.36 mg / L), the titanium ion deposition of pure titanium tube is 17.01 mg / L, which exceeds the requirement of vanadium electrolyte for impurity element content (<10 ppm). This indicates that pure titanium tube will undergo significant corrosion in the electrolyte environment of vanadium redox flow battery, leading to the dissolution of titanium ions.

[0050] Comparative Example 2 A heat exchange tube for a vanadium redox flow battery differs from Example 1 only in that its polytetrafluoroethylene (PTFE) coating has no wrinkled structure on its surface and is a conventional planar PTFE coating. The coating is subjected to conventional isothermal sintering: holding at 200°C for 10 minutes, then heating to 360°C at a rate of 3.0°C / min and holding at that temperature for 45 minutes before furnace cooling to room temperature.

[0051] Comparative Example 3 A heat exchange tube for a vanadium redox flow battery uses a nickel alloy tube of the same size as in Example 1, but the nickel alloy surface is not provided with a protective layer.

[0052] Ion dissolution tests were conducted on the vanadium redox flow battery heat exchange tubes used in this comparative example. ICP-MS results showed that, compared to the unimmersed electrolyte (Ni: 0.001 mg / L), the nickel ion deposition in the nickel alloy tubes was only 0.037 mg / L, far below the requirement for impurity element content in vanadium electrolytes (<10 ppm). This indicates that the nickel alloy tubes have an extremely low corrosion rate, with almost no nickel ion dissolution, and the electrolyte exhibits good stability. Therefore, nickel-based alloys can be selected as corrosion-resistant heat exchanger materials, effectively solving the corrosion problems existing in current titanium tube heat exchangers.

[0053] Performance testing: (1) Corrosion resistance test was conducted on the heat exchange tubes of the vanadium redox flow batteries in the above embodiments and comparative examples: The heat exchange tubes were placed in a positive electrode tank containing 30 mL of simulated electrolyte, and a corrosion resistance immersion test was conducted at room temperature at 200 mA / cm². 2 The charge-discharge cycle was performed at the current density for 24 hours. After the test, the positive electrode electrolyte was collected and analyzed using ICP-MS (inductively coupled plasma mass spectrometry) to determine the content of titanium ions in the solution. The test results are shown in Table 1.

[0054] (2) The heat exchange tubes of the vanadium redox flow batteries in the above embodiments and comparative examples were subjected to thermal cycling tests (cycle temperature: 0℃-60℃, number of cycles: 100). The thermal cycling test method is as follows: The sample was completely immersed in the electrolyte and placed in a temperature cycling test chamber for testing. The temperature range was 0℃ to 60℃, starting from room temperature (25±2℃), with a cycling temperature range of 0℃ to 50℃. The heating rate was 2℃ / min, the cooling rate was 2℃ / min, the holding time at the high temperature of 60℃ was 30 minutes, and then the holding time at the low temperature of 0℃ was 30 minutes. The number of cycles was 100. After the test, the sample was removed, rinsed with deionized water, dried, and the coating crack area was evaluated. The test results are shown in Table 1.

[0055] Table 1

[0056] from Figure 2 (Comparison of titanium ion deposition amounts between Example 1 and Comparative Example 1) and Table 1 show that, compared to the uncoated titanium tube (Comparative Example 1), the titanium tube coated with a PTFE layer with a micro-wrinkled structure (Example 1) exhibits significantly lower titanium ion deposition amounts under the same immersion time and environment, indicating that the PTFE coating can effectively improve the corrosion resistance of the titanium tube. Combining the comparison results of Example 1 and Comparative Example 1, the uncoated pure titanium tube shows a titanium ion deposition amount as high as 17.01 mg / L in the electrolyte, far exceeding the electrolyte's requirements for impurity elements (<10 ppm); while the titanium tube coated with a PTFE protective layer shows a significantly reduced titanium ion deposition amount under the same conditions. This proves that the protective layer solves the pitting corrosion risk and ion contamination problem caused by the dissolution of the surface oxide film of titanium materials under long-term thermal cycling conditions. It avoids the competitive coordination between titanium ions and vanadium ions, which would accelerate the precipitation of high-valence vanadium, thereby maintaining the stability of the electrolyte, extending the charge and discharge life of the battery, and reducing maintenance costs.

[0057] As can be seen from the performance test results in Table 1, there are significant differences between Example 1 and Comparative Example 2 in terms of corrosion resistance and thermal cycling stability. The titanium ion leaching amount in Example 1 was 1.66 mg / L, while that in Comparative Example 2 was as high as 15.82 mg / L, with the former being only 10.5% of the latter. In terms of thermal cycling stability, the cracked area ratio of the coating in Example 1 was 1.8%, while that in Comparative Example 2 reached 18.7%, with the former being only 9.6% of the latter.

[0058] During the operation of a vanadium redox flow battery, the electrolyte environment surrounding the heat exchange tube experiences temperature fluctuations, which generate alternating thermal stress within the coating. Due to the significant difference in the coefficients of thermal expansion between the titanium substrate and the polytetrafluoroethylene coating, substantial thermal stress is generated at the coating-substrate interface during temperature cycling. The micro-folded structure of Example 1 can absorb alternating thermal stress through reversible elastic deformation during thermal cycling, dispersing the stress originally concentrated at the coating-substrate interface to the entire folded network, fundamentally preventing cracking caused by stress concentration. In contrast, the planar coating of Comparative Example 2 lacks this stress buffering mechanism; thermal stress acts directly on the coating-substrate interface, leading to a gradual decrease in interfacial bonding strength and ultimately cracking.

[0059] Meanwhile, the hierarchical morphology of the micro-folds significantly extends the possible penetration path of the electrolyte, forcing the corrosive medium to penetrate along the folded path rather than in a straight line. The micro-folded structure can encapsulate potential micro-defects at the bottom of the folds, effectively blocking the diffusion channels of the corrosive medium to the substrate. In contrast, the planar coating of Comparative Example 2 has a straight penetration path; once microcracks appear in the coating, the corrosive medium can directly contact the titanium substrate, leading to rapid dissolution of titanium ions.

[0060] Compared with Example 1, Example 2 showed a higher leaching amount and a higher crack area. Since the nickel alloy coating is a rigid planar structure and lacks a micro-wrinkle elastic buffering mechanism, it mainly relies on irreversible plastic deformation to relieve stress during thermal cycling, and is prone to cumulative damage after multiple cycles.

[0061] Compared with Example 1, Example 3 showed a higher dissolution rate and a higher crack area. However, due to the low average height of the protrusions (only 2 μm), the wrinkle height was too low to form an effective stress buffer network. The insufficient wrinkle density resulted in a weak stress dispersion effect. At the same time, the shallow wrinkles could not form an effective interfacial bond with the roughness of the substrate.

[0062] Compared with Example 1, Example 4 showed a higher dissolution rate and a higher crack area. Due to the average height of the protrusions reaching 17 μm, the excessive wrinkles resulted in a significant reduction in the thickness of the top coating, which caused local stress concentration during thermal cycling, becoming the crack initiation point and affecting the protective properties of the coating.

[0063] Compared with Example 1, Example 5 showed a higher dissolution rate and a higher crack area. This was because the thermal cycling sintering temperature was 350°C, which caused the polymer to soften at high temperatures. After cooling, irreversible microcracks were formed, and the wrinkled structure was destroyed.

[0064] Compared with Example 1, Example 6 showed a higher dissolution rate and a higher cracking area. However, due to only two thermal cycles, the arrangement of polymer molecular chain wrinkles was affected, resulting in insufficient wrinkle density and uniformity, which prevented the formation of an effective stress buffer network.

[0065] Compared with Example 1, Example 7 showed a higher dissolution rate and a higher crack area. This was because the number of thermal cycles reached 8, which caused slight fatigue of the polymer molecular chains, a decrease in elastic modulus, and a tendency to generate local stress concentration.

[0066] Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A heat exchange tube for a vanadium redox flow battery, characterized in that, The substrate of the heat exchange tube is any one of industrial pure titanium tube, titanium alloy tube, or nickel-based alloy tube; the outer surface of the heat exchange tube is covered with a protective layer, which is a polymer coating; wherein the outer surface of the polymer coating has a micro-wrinkled structure.

2. The heat exchange tube for a vanadium redox flow battery according to claim 1, characterized in that, The protective layer is a metal coating.

3. The heat exchange tube for a vanadium redox flow battery according to claim 1, characterized in that, The protrusion height of the micro-fold structure is 3-15 μm.

4. The heat exchange tube for a vanadium redox flow battery according to claim 1 or 2, characterized in that, The thickness of the protective coating is 30-500µm.

5. The heat exchange tube for a vanadium redox flow battery according to claim 1 or 2, characterized in that, The heat exchange tube has a roughened interface between its substrate and the protective layer.

6. The heat exchange tube for a vanadium redox flow battery according to claim 1, characterized in that, The polymer in the polymer coating is selected from one or more of polytetrafluoroethylene, polyethylene, polyvinyl chloride, and polypropylene.

7. The heat exchange tube for a vanadium redox flow battery according to claim 2, characterized in that, The metal coating is any one of platinum coating, gold coating, nickel alloy coating or silver coating.

8. A method for preparing a heat exchange tube for a vanadium redox flow battery as described in any one of claims 1 to 7, characterized in that, Includes the following steps: S1, Surface treatment of the substrate of the vanadium redox flow battery heat exchange tube; S2, a protective layer is applied to the outer surface of the surface-treated heat exchanger tube substrate, including: When the protective layer is a polymer coating, the process includes: coating a wet polymer coating film on the outer surface of the heat exchange tube substrate; and then subjecting the heat exchange tube coated with the wet polymer coating film to thermal cycling sintering to form a wrinkled structure on the surface of the polymer coating. Alternatively, when the protective coating is a metal coating, it includes: applying a metal coating to the surface of the heat exchange tube substrate that has undergone the surface treatment by spraying or dipping, and then performing a high-temperature sintering process to form the protective layer.

9. The method for preparing a heat exchange tube for a vanadium redox flow battery according to claim 8, characterized in that, The thermal cycling sintering process includes: S21, Preheating: Increase the temperature from room temperature to 180-200℃ at a rate of 3.0±0.5℃ / min, and hold for 10±5min; S22, multiple thermal cycles: heat up to 280-320℃ at 5.0±0.3℃ / min, hold for 8.0±0.5min, then cool down to 180-200℃ at 3.0±0.2℃ / min as one thermal cycle, repeat 3 to 7 times; S23, high-temperature sintering: heat up to 350-380℃ at a rate of 2.0±0.5℃ / min, and hold for 30–60 min; S24, Cooling: Cool to room temperature at a rate of 1.5±0.5℃ / min.

10. A heat exchanger for a vanadium redox flow battery, characterized in that, The heat exchanger includes a shell, a heat exchange tube bundle disposed within the shell, and tube boxes disposed at both ends of the shell. The heat exchange tube bundle includes a plurality of heat exchange tubes, and the heat exchange tubes are heat exchange tubes for vanadium redox flow batteries as described in any one of claims 1 to 7.

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