Preparation method and product of high-efficiency lossless purified carbon nanotube

Abstract

The invention belongs to the technical field of nano material preparation, and discloses a method for efficiently and losslessly purifying a carbon nano tube. According to the method, a fused salt electrolysis method is adopted, a carbon nano tube film is used as a working electrode, constant-potential electrolysis is carried out in a fused salt system with specific composition under an anaerobic high-temperature protective atmosphere, and residual catalysts (Fe, Co, Ni and the like) are subjected to oxidation reaction in the working electrode by utilizing the difference of electrochemical activity of the catalysts and a carbon material, so that the residual catalysts are separated from the carbon material. The catalyst is dissociated to an antipode in an ionic state and is subjected to reduction deposition, so that efficient separation of the catalyst is realized. Strong acid and strong alkali are not needed, the process is simple, the condition is mild, the structural integrity and original excellent performance of the carbon nanotubes can be kept to the maximum extent while catalyst impurities are thoroughly removed, and the method is environmentally friendly, low in production cost and suitable for large-scale industrial production.

Description

A method for preparing carbon nanotubes with high efficiency and non-destructive purification, and the resulting product. Technical Field

[0001] This invention relates to the field of nanomaterial preparation and purification technology, specifically to a preparation method and product for efficient and non-destructive purification of carbon nanotubes using molten salt electrolysis technology. Background Technology

[0002] Since their discovery, carbon nanotubes (CNTs) have demonstrated enormous application potential in many fields, such as electronic chips, lithium-ion battery electrodes, supercapacitors, aerospace composite materials, and water pollution treatment, thanks to their unique one-dimensional nanostructure and excellent electrical, mechanical, and thermal properties, such as high conductivity, high specific strength, high specific surface area, and excellent thermal conductivity. They are considered one of the most promising nanomaterials of the 21st century.

[0003] Currently, large-scale synthesis of carbon nanotubes mainly employs chemical vapor deposition (CVD), a method that offers advantages such as simple process, low cost, and high yield. However, the synthesis process requires the use of transition metals such as Fe, Co, and Ni, and their compounds, as catalysts to lower the activation energy of carbon source decomposition and promote carbon nanotube growth. However, these catalysts remain in the carbon nanotube product as elemental metals or oxides after the reaction, typically at a content between 5-15 wt.%. These residual catalyst impurities severely affect the intrinsic properties of carbon nanotubes: in electrical applications, metallic impurities lead to uneven conductivity and decreased carrier mobility; in mechanical applications, catalyst particles become stress concentration points in composite materials, reducing fracture strength and toughness; in biomedical or environmental applications, the presence of heavy metal catalysts may pose risks of biotoxicity or secondary pollution. Therefore, efficient purification of carbon nanotubes to remove residual catalyst impurities is a crucial prerequisite for their practical application.

[0004] To address the purification challenges of carbon nanotubes, researchers have developed various purification methods, primarily categorized into physical and chemical methods. Physical methods, such as centrifugation, filtration, and electrophoresis, rely on differences in density, particle size, and charge properties between the catalyst and the carbon nanotubes to achieve separation. However, these methods suffer from drawbacks such as low purification efficiency, difficulty in completely removing fine catalyst particles, and challenges in large-scale application, limiting their applicability to small-scale laboratory pretreatment.

[0005] Chemical methods are currently the mainstream carbon nanotube purification technology, with chemical oxidation being the most widely used. The core principle of this method is based on selective oxidation etching: firstly, oxidants (such as concentrated nitric acid, concentrated sulfuric acid, potassium permanganate, etc.) are used under high-temperature reflux conditions to preferentially oxidize and etch the amorphous carbon and carbon nanotube defects coated on the catalyst surface, exposing the catalyst particles; subsequently, through the dissolution effect of the acid solution, the metal catalyst is converted into a soluble salt, thereby achieving separation. However, chemical oxidation methods have many insurmountable technical drawbacks: The process is complex and time-consuming, requiring multiple steps such as oxidation etching, acidification reflux, filtration and washing, and drying, typically taking tens of hours and resulting in low production efficiency; it relies on corrosive substances such as strong acids and alkalis, using large amounts of concentrated nitric acid and concentrated sulfuric acid, which not only places high demands on equipment (requiring corrosion-resistant materials) but also generates large amounts of acidic wastewater, causing severe environmental pollution and high treatment costs; it severely damages the carbon nanotube structure, as prolonged high-temperature acid treatment corrodes the carbon nanotube walls, leading to shorter nanotubes, increased wall defects, and a decrease in specific surface area, severely weakening their original excellent electrical and mechanical properties and significantly reducing the yield (typically only 60-70%); and the purification effect is limited, as acid solutions have difficulty penetrating some catalyst particles tightly wrapped by carbon nanotubes, resulting in a high catalyst residue rate (typically ≥1 Wt.%).

[0006] Besides chemical oxidation, some researchers have attempted other chemical methods for purification, such as hydrogen reduction and halogen etching. However, these methods also have significant drawbacks. Hydrogen reduction requires high temperature (1000-1200℃) and high pressure hydrogen atmosphere, posing a high operational risk. It can only remove some metal oxides and is ineffective at removing elemental metal catalysts. Halogen etching utilizes the reaction of halogen gases (such as Cl2 and F2) with metal catalysts to generate volatile halides for separation. However, halogen gases are highly corrosive and toxic, posing a significant hazard to equipment and operators. Furthermore, they readily react with carbon nanotubes, damaging their structure.

[0007] Therefore, developing a purification method that is simple, mild, and environmentally friendly, capable of efficiently removing residual catalysts while preserving the structural integrity and original properties of carbon nanotubes to the greatest extent, is a key technical problem that urgently needs to be solved in the field of carbon nanotube preparation technology. It has important practical significance and strategic value for promoting the industrial application of carbon nanotubes. Summary of the Invention

[0008] The purpose of this invention is to overcome the shortcomings of existing carbon nanotube purification technologies and provide a highly efficient and non-destructive method for preparing carbon nanotubes. This method does not require the use of strong acids or bases, has a simple process, is safe to operate, and is environmentally friendly. It can efficiently remove residual catalysts such as Fe, Co, and Ni while maintaining the structural integrity and original excellent properties of carbon nanotubes, thus meeting the needs of industrial production and practical applications.

[0009] To achieve the above-mentioned objectives, the present invention adopts the following technical solution: a method for preparing highly efficient and non-destructive purified carbon nanotubes, comprising the following steps: molten salt weighing and pretreatment: selecting two molten salt components, wherein the molten salt components are any two of sodium chloride, potassium chloride, lithium chloride, sodium fluoride, potassium fluoride, and lithium fluoride, weighing them at a mass ratio of 1:0.5-1:2, and then mixing them by ball milling using a planetary ball mill at a speed of 200-300 r / min for 2-4 h, with a ball-to-material ratio of 10:1-20:1; after uniform mixing, placing the molten salt in an oven to dry completely remove the adsorbed crystal water on the surface, avoiding side reactions caused by moisture at high temperatures or affecting the stability of the molten salt system.

[0010] Preparation of carbon nanotube films: The raw carbon nanotube powder was mixed with deionized water and ultrasonically dispersed for 30-60 min to prepare a uniform dispersion with a concentration of 5-10 mg / mL. Then, the dispersion was filtered onto a filter membrane using a vacuum filtration method to form a continuous and dense carbon nanotube film. After peeling off the filter membrane, the working electrode was obtained. The film thickness was controlled to be 10-50 μm to ensure that the electrode has good conductivity and catalytic reaction contact area.

[0011] Electrode assembly and heating: Place the pretreated molten salt into an alumina crucible. Insert the prepared carbon nanotube film working electrode, counter electrode (tungsten wire or molybdenum wire, diameter 0.5-1.5 mm), and reference electrode (Ag / AgCl reference electrode) into the molten salt in sequence, ensuring that the electrodes are in full contact with the molten salt and do not short-circuit with each other. Place the alumina crucible containing the molten salt and electrodes into the electrolytic furnace. After closing the furnace door, introduce a protective atmosphere (argon or nitrogen) at a flow rate of 200-500 sccm for 30 minutes to remove air from the furnace and prevent the carbon nanotubes and catalyst from being oxidized at high temperatures. Then, heat the temperature to the reaction temperature of 700-1000℃ at a rate of 5-10℃ / min and hold for 30 minutes to completely melt the molten salt and allow the carbon layer surrounding the catalyst to fully dissolve, exposing the catalyst particles.

[0012] Potential-constant electrolysis: The electrolysis potential is set to 0.1-0.4V vs. Ag / AgCl using an electrochemical workstation, and the electrolysis time is 2-8h. During the electrolysis process, the current density is controlled at 1-5mA / cm². Under these conditions, the residual catalyst (Fe, Co, Ni, etc.) in the carbon nanotubes undergoes an oxidation reaction on the working electrode surface, generating ionic catalysts (e.g., Fe→Fe²⁺+2e⁻, Co→Co²⁺+2e⁻, Ni→Ni²⁺+2e⁻). The ionic catalyst diffuses freely in the molten salt system, and after reaching the counter electrode surface, it undergoes a reduction reaction, regenerating elemental metals and depositing them on the counter electrode (e.g., Fe²⁺+2e⁻→Fe, Co²⁺+2e⁻→Co, Ni²⁺+2e⁻→Ni), thereby achieving the separation of the catalyst from the carbon nanotubes.

[0013] Cooling and Sampling: After electrolysis, continue to introduce the protective atmosphere and slowly lift all electrodes above the molten salt interface to avoid molten salt adhering to the electrode surface and affecting the sample purity; after the electrolysis furnace has cooled naturally to room temperature, turn off the protective atmosphere, open the furnace door, and take out the carbon nanotube sample on the working electrode.

[0014] Post-processing: The extracted carbon nanotube samples were first placed in deionized water and ultrasonically cleaned for 10-300W for 10-30 minutes to remove most of the residual molten salt on the surface; then the samples were transferred to an ethanol solution and magnetically stirred at a rate of 300-500r / min for 15-45 minutes to further remove residual trace amounts of molten salt and organic matter; after cleaning, the samples were placed in a 100℃ oven and dried for 6-12 hours to obtain purified carbon nanotubes.

[0015] The core innovation of this invention lies in breaking through the traditional technical framework of carbon nanotube purification, which relies on chemical oxidation etching and strong acid treatment. It innovatively applies molten salt electrolysis technology to the field of carbon nanotube purification. By precisely controlling the molten salt system, electrolysis parameters, and electrode configuration, it achieves a balance between "highly efficient impurity removal" and "structural non-destruction." Its innovation is mainly reflected in the following aspects: Existing technologies generally believe that carbon nanotube purification must be achieved through a two-step method of "etching exposure - chemical dissolution." However, this invention breaks away from this conventional thinking, utilizing the essential difference in electrochemical activity between catalysts (metals) and carbon materials (non-metals) to achieve selective separation through molten salt electrolysis. The molten salt system has good conductivity and ion migration ability at high temperatures, providing an ideal medium for the electrochemical reaction. Simultaneously, the high-temperature environment allows the carbon layer coated on the catalyst surface to dissolve slowly, eliminating the need for additional oxidation etching steps and fundamentally avoiding damage to the carbon nanotube structure. This "electrochemical selective separation" approach provides a completely new solution for carbon nanotube purification and represents a significant breakthrough in existing purification technologies.

[0016] Innovation in the selection and proportioning of molten salt system: This invention selects six molten salts: sodium chloride, potassium chloride, lithium chloride, sodium fluoride, potassium fluoride, and lithium fluoride, and limits any two to a mass ratio of 1:0.5-1:2. These six molten salts all have moderate melting points (400-800℃), good thermal stability, and excellent electrical conductivity, and can completely melt at a reaction temperature of 700-1000℃ to form a stable electrolyte system. The mixture of two molten salts can form a eutectic mixture, which reduces the melting point and viscosity of the molten salts, increases the ion migration rate, and thus improves the electrolysis efficiency. The mass ratio is limited to 1:0.5-1:2 because the molten salt mixture within this range has the best conductivity and fluidity, which can ensure the efficient diffusion of catalyst ions, while avoiding excessively high melting points or high viscosity of the molten salts due to imbalance in the ratio, which would affect the smooth progress of the electrolysis process.

[0017] Innovation in Electrolysis Parameter Control: This invention precisely limits key parameters such as electrolysis potential, reaction temperature, and electrolysis time, achieving "selective electrolysis": the electrolysis potential is limited to 0.1-0.4V vs. Ag / AgCl. This potential range was determined through extensive electrochemical testing: below 0.1V, the oxidation reaction of the catalyst metal is difficult to occur, resulting in extremely low impurity removal efficiency; above 0.4V, carbon nanotubes undergo oxidation (C+O²⁻→CO₂↑+4e⁻), leading to structural damage; while within the 0.1-0.4V range, only the catalyst metal can undergo oxidation, while the carbon nanotubes remain stable, achieving selective impurity removal.

[0018] The reaction temperature is limited to 700-1000℃. This temperature range serves a dual purpose: firstly, it ensures that the molten salt is completely melted, providing a good ion transport environment; secondly, it promotes the slow dissolution of the outer carbon layer of the catalyst, allowing the catalyst particles to be fully exposed without additional etching steps, while avoiding excessive graphitization or structural collapse of carbon nanotubes caused by high temperatures (>1000℃).

[0019] The coordinated regulation of electrolysis time and current density ensures the complete removal of catalyst: the current density is controlled at 1-5 mA / cm² to avoid electrode polarization and side reactions caused by excessive current; the electrolysis time is adjusted to 2-8 h according to the sample amount and current density to ensure that all exposed catalysts can complete the oxidation-reduction cycle and achieve deep purification.

[0020] Innovation in Electrode System Design: This invention employs a three-electrode system consisting of a carbon nanotube thin film working electrode, a tungsten / molybdenum wire counter electrode, and an Ag / AgCl reference electrode. This design offers clear technical advantages: The carbon nanotube thin film, as the working electrode, has a larger specific surface area, better conductivity, and stability compared to powdered electrodes, ensuring uniform electrolytic reaction on the electrode surface and preventing excessive localized reactions that could damage the carbon nanotube structure. Tungsten or molybdenum wires are chosen as the counter electrode because they possess high melting points (3410℃ and 2620℃, respectively), good conductivity, and chemical stability. They do not oxidize in molten salt systems at 700-1000℃ and do not react with catalyst ions, ensuring high purity of the deposited catalyst while avoiding contamination of the molten salt system. The Ag / AgCl reference electrode features stable potential and rapid response, enabling precise control of the electrolytic potential of the working electrode and providing a reliable potential reference for selective electrolysis—a key guarantee for achieving "non-destructive purification."

[0021] A highly efficient and non-destructive method for purifying carbon nanotubes is disclosed. The purified carbon nanotubes exhibit a catalyst residue content ≤0.1 wt.% and a crystallinity parameter I. G / I D The value changed by ≤±5% compared with that before purification, and the length retention rate was ≥90%.

[0022] The technical effects of this invention are significant and irreplaceable, specifically reflected in: highly efficient impurity removal: the residual catalyst content of the purified carbon nanotubes is ≤0.1 Wt.%, far lower than the 1 Wt.% of existing chemical oxidation methods, resulting in a significant improvement in impurity removal efficiency; structural integrity: the crystallinity parameter I of the purified carbon nanotubes... G / I D The change rate of the carbon nanotube value compared with that before purification is ≤±5%, and the length retention rate is ≥90%, which completely overcomes the problems of shortened carbon nanotube length and increased defects caused by existing technologies; it is environmentally friendly: the whole process does not require the use of strong acids and alkalis, only generates a small amount of cleaning wastewater (which can be discharged in compliance with standards through simple treatment), and no toxic or harmful gases are generated, which significantly reduces the environmental burden; the process is simple: compared with the multi-step treatment of chemical oxidation method, this invention only requires four core steps: "molten salt pretreatment - electrolysis - cleaning - drying", shortening the production cycle to less than 30 hours and greatly improving production efficiency. Attached Figure Description

[0023] Figure 1 is a schematic diagram of the molten salt electrolysis apparatus of Example 1 of the present invention; Figure 2 is a scanning electron microscope image of carbon nanotubes before purification in Example 1 of the present invention; Figure 3 is a thermogravimetric image of carbon nanotubes before purification in Example 1 of the present invention; Figure 4 is a Raman image of carbon nanotubes before purification in Example 1 of the present invention; Figure 5 is a scanning electron microscope image of carbon nanotubes after purification in Example 1 of the present invention; Figure 6 is a thermogravimetric image of carbon nanotubes after purification in Example 1 of the present invention; Figure 7 is a Raman image of carbon nanotubes after purification in Example 1 of the present invention.

[0024] The schematic diagram of the molten salt electrolysis apparatus is shown in Figure 1: 1 Electrolytic furnace, 2 Gas inlet, 3 Working electrode, 4 Reference electrode, 5 Counter electrode, 6 Gas outlet, 7 Support frame, 8 Molten salt, 9 Corundum crucible. Detailed Implementation

[0025] The following specific embodiments are merely illustrative of the present invention and are not intended to limit the invention. After reading this specification, those skilled in the art can make modifications to these embodiments without contributing any inventive step, but such modifications are protected by patent law as long as they fall within the scope of the claims of the present invention.

[0026] The present invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0027] To better understand the technical content of this invention, the technical solution of this invention will be described in detail below with reference to specific embodiments and comparative examples. The original carbon nanotubes used in the following embodiments and comparative examples were all synthesized by chemical vapor deposition. Examples 1-6 and Comparative Examples 1-3 used carbon nanotubes with different catalyst types and initial contents, respectively.

[0028] I. Experimental Materials and Equipment Experimental materials: Pristine carbon nanotubes (Fe-based, Co-based, Ni-based, self-made in the laboratory); Sodium chloride, potassium chloride, lithium chloride, sodium fluoride, potassium fluoride, lithium fluoride (analytical grade, Sinopharm Chemical Reagent Co., Ltd.); Tungsten wire, molybdenum wire (diameters 0.5mm, 1.0mm, 1.5mm, Beijing General Research Institute of Nonferrous Metals); Ag / AgCl reference electrode (Shanghai Chenhua Instrument Co., Ltd.); Deionized water (resistivity ≥18.2MΩ・cm); Anhydrous ethanol (analytical grade, Sinopharm Chemical Reagent Co., Ltd.).

[0029] Experimental equipment: Planetary ball mill (QM-3SP4, Nanjing University Instrument Factory); electric constant temperature oven (DHG-9070A, Shanghai Jinghong Experimental Equipment Co., Ltd.); tubular electrolytic furnace (GSL-1700X, Hefei Kejing Materials Technology Co., Ltd.); electrochemical workstation (CHI660E, Shanghai Chenhua Instrument Co., Ltd.); ultrasonic cleaner (KQ-500DE, Kunshan Ultrasonic Instrument Co., Ltd.); magnetic stirrer (85-2, Shanghai Sile Instrument Co., Ltd.); scanning electron microscope (SEM, SU8010, Hitachi, Japan); thermogravimetric analyzer (TGA, Q500, TA Instruments, USA); Raman spectrometer (LabRAM HR Evolution, Horiba, France); inductively coupled plasma mass spectrometer (ICP-MS, NexION 300X, PerkinElmer, USA).

[0030] Example 1

[0031] Molten salt pretreatment: Sodium chloride and potassium chloride were selected as molten salt components. 100g of each were weighed at a mass ratio of 1:1 and placed in a planetary ball mill with a ball-to-material ratio of 15:1 and a rotation speed of 250r / min for 3 hours. After mixing evenly, the mixture was dried in an oven at 200℃ for 24 hours.

[0032] Preparation of carbon nanotube films: Fe-based raw carbon nanotubes (initial catalyst content 12.35 wt.%) were selected, mixed with deionized water, and ultrasonically dispersed for 45 min to prepare a dispersion with a concentration of 8 mg / mL; the dispersion was then vacuum filtered to prepare a carbon nanotube film with a thickness of 30 μm.

[0033] Figure 1 shows a schematic diagram of the apparatus for molten salt electrolysis: 1. Electrolytic furnace, 2. Gas inlet, 3. Working electrode, 4. Reference electrode, 5. Counter electrode, 6. Gas outlet, 7. Support frame, 8. Molten salt, 9. Corundum crucible.

[0034] Electrode assembly and heating: The dried molten salt was placed in an alumina crucible, and a carbon nanotube thin film working electrode, a 1.0 mm diameter tungsten wire counter electrode, and an Ag / AgCl reference electrode were inserted. After placing it in the electrolysis furnace, argon gas was introduced at a flow rate of 300 sccm for 30 min. The temperature was then increased to 800℃ at a rate of 8℃ / min and held for 30 min. Potential-constant electrolysis: The electrolysis potential was set to 0.3V vs. Ag / AgCl, and the electrolysis time was 4 h. The current density was maintained at 3 mA / cm² during the electrolysis process.

[0035] The catalyst exposed during electrolysis undergoes the following oxidation reaction near the working electrode: Fe = Fe 2+ +2e -Then, through the molten salt medium, it is released to the vicinity of the counter electrode and undergoes a reduction reaction: Fe 2+ +2e - = Fe. After electrolysis, all electrodes above the molten salt interface were cooled to room temperature, and the carbon nanotubes on the working electrode were removed.

[0036] Post-treatment (washing away the molten salt adhering to the surface with deionized water and ethanol): ultrasonic cleaning with deionized water for 20 min (power 200W), magnetic stirring cleaning with ethanol solution for 30 min (speed 400r / min), and drying in an oven at 100℃ for 8 h to obtain purified carbon nanotubes.

[0037] Figure 2 shows a scanning electron microscope (SEM) image of the carbon nanotubes before purification. The image reveals numerous impurity particles attached to the surface of the carbon nanotubes. Thermogravimetric analysis (Figures 3 and 6) shows an Fe content of approximately 12.35 Wt.%, and Raman spectroscopy (Figures 4 and 7) shows an I content of... G / I D = 52.4, proving its high crystallinity. Figure 5 shows the scanning electron microscope image of the purified carbon nanotubes. It can be seen that the impurity particles are basically completely removed, and the thermogravimetric results show that there is no residual catalyst. This indicates that the method can remove the metal catalyst in the carbon nanotubes very efficiently. More importantly, the crystallinity of the carbon nanotubes did not change after purification (I G / I D = 53.6), indicating that its structure was not damaged in any way, further proving that the method is very mild.

[0038] Example 2

[0039] Molten salt pretreatment: Lithium chloride and potassium chloride were selected as molten salt components. 100g of lithium chloride and 50g of potassium chloride were weighed at a mass ratio of 1:0.5 and ball-milled in a planetary ball mill (ball-to-material ratio of 10:1, speed of 200r / min, time of 2h); dried at 200℃ for 24h.

[0040] Preparation of carbon nanotube films: Co-based original carbon nanotubes (initial catalyst content 8.76 wt.%) were selected and a dispersion with a concentration of 5 mg / mL was prepared. The dispersion was ultrasonically dispersed for 30 min and vacuum filtered to prepare a film with a thickness of 10 μm.

[0041] Electrode assembly and heating: Insert the carbon nanotube thin film working electrode, the 0.5 mm diameter molybdenum wire counter electrode, and the Ag / AgCl reference electrode; purge with nitrogen gas at a flow rate of 200 sccm for 30 min; heat to 700℃ at a rate of 5℃ / min and hold for 30 min.

[0042] Constant potential electrolysis: electrolysis potential 0.1V vs. Ag / AgCl, electrolysis time 8h, current density 1mA / cm 2 .

[0043] Cooling and sampling: Same as in Example 1.

[0044] Post-treatment: Ultrasonic cleaning with deionized water for 10 min (power 100W), stirring and cleaning with ethanol for 15 min (speed 300r / min), and drying at 100℃ for 6 h.

[0045] Example 3

[0046] Molten salt pretreatment: Sodium fluoride and potassium fluoride were selected as molten salt components. 100g of sodium fluoride and 200g of potassium fluoride were weighed at a mass ratio of 1:2 and ball-milled in a planetary ball mill (ball-to-material ratio of 20:1, speed of 300r / min, time of 4h); dried at 200℃ for 24h.

[0047] Preparation of carbon nanotube films: Ni-based raw carbon nanotubes (initial catalyst content 14.21 wt.%) were selected and a dispersion with a concentration of 10 mg / mL was prepared. The dispersion was ultrasonically dispersed for 60 min and vacuum filtered to prepare a film with a thickness of 50 μm.

[0048] Electrode assembly and heating: Insert the carbon nanotube thin film working electrode, the 1.5 mm diameter tungsten wire counter electrode, and the Ag / AgCl reference electrode; introduce argon gas at a flow rate of 500 sccm for 30 min; heat to 1000℃ at a rate of 10℃ / min and hold for 30 min.

[0049] Constant potential electrolysis: electrolysis potential 0.4V vs. Ag / AgCl, electrolysis time 2h, current density 5mA / cm² 2 .

[0050] Cooling and sampling: Same as in Example 1.

[0051] Post-treatment: Ultrasonic cleaning with deionized water for 30 min (power 300W), stirring and cleaning with ethanol for 45 min (speed 500r / min), and drying at 100℃ for 12 h.

[0052] Example 4

[0053] Molten salt pretreatment: Sodium chloride and lithium chloride were selected as molten salt components. 100g of sodium chloride and 150g of lithium chloride were weighed at a mass ratio of 1:1.5 and ball-milled in a planetary ball mill (ball-to-material ratio of 12:1, speed of 220r / min, time of 2.5h); dried at 200℃ for 24h.

[0054] Preparation of carbon nanotube films: Fe-Co bimetallic carbon nanotubes (initial catalyst content 10.58 wt.%) were selected and a dispersion with a concentration of 7 mg / mL was prepared. The dispersion was ultrasonically dispersed for 40 min and vacuum filtered to prepare a film with a thickness of 20 μm.

[0055] Electrode assembly and heating: Insert the carbon nanotube thin film working electrode, the 1.0 mm diameter molybdenum wire counter electrode, and the Ag / AgCl reference electrode; purge with nitrogen gas at a flow rate of 350 sccm for 30 min; heat to 850℃ at a rate of 7℃ / min and hold for 30 min.

[0056] Constant potential electrolysis: electrolysis potential 0.25V vs. Ag / AgCl, electrolysis time 5h, current density 2mA / cm² 2 .

[0057] Cooling and sampling: Same as in Example 1.

[0058] Post-treatment: Ultrasonic cleaning with deionized water for 25 min (power 150W), stirring and cleaning with ethanol for 35 min (speed 350r / min), and drying at 100℃ for 7 h.

[0059] Example 5

[0060] Molten salt pretreatment: potassium chloride and lithium fluoride were selected as molten salt components. 100g of potassium chloride and 80g of lithium fluoride were weighed at a mass ratio of 1:0.8 and ball-milled in a planetary ball mill (ball-to-material ratio of 18:1, speed of 280r / min, time of 3.5h); dried at 200℃ for 24h.

[0061] Preparation of carbon nanotube films: Fe-Ni bimetallic carbon nanotubes (initial catalyst content 13.12 wt.%) were selected and a dispersion with a concentration of 9 mg / mL was prepared. The dispersion was ultrasonically dispersed for 50 min and vacuum filtered to prepare a film with a thickness of 40 μm.

[0062] Electrode assembly and heating: Insert the carbon nanotube thin film working electrode, the 1.2 mm diameter tungsten wire counter electrode, and the Ag / AgCl reference electrode; introduce argon gas at a flow rate of 400 sccm for 30 min; heat to 900℃ at a rate of 9℃ / min and hold for 30 min.

[0063] Constant potential electrolysis: electrolysis potential 0.35V vs. Ag / AgCl, electrolysis time 3h, current density 4mA / cm² 2 .

[0064] Cooling and sampling: Same as in Example 1.

[0065] Post-treatment: Ultrasonic cleaning with deionized water for 15 min (power 250W), stirring and cleaning with ethanol for 25 min (speed 450r / min), and drying at 100℃ for 9 h.

[0066] Example 6

[0067] Molten salt pretreatment: Lithium chloride and sodium fluoride were selected as molten salt components. 100g of lithium chloride and 120g of sodium fluoride were weighed at a mass ratio of 1:1.2 and ball-milled in a planetary ball mill (ball-to-material ratio of 16:1, speed of 260r / min, time of 3h); dried at 200℃ for 24h.

[0068] Preparation of carbon nanotube films: Co-Ni bimetallic carbon nanotubes (initial catalyst content 9.87 wt.%) were selected and a dispersion with a concentration of 6 mg / mL was prepared. The dispersion was ultrasonically dispersed for 35 min and vacuum filtered to prepare a film with a thickness of 25 μm.

[0069] Electrode assembly and heating: Insert the carbon nanotube thin film working electrode, the 0.8 mm diameter molybdenum wire counter electrode, and the Ag / AgCl reference electrode; purge with nitrogen gas at a flow rate of 250 sccm for 30 min; heat to 750 °C at a rate of 6 °C / min and hold for 30 min.

[0070] Constant potential electrolysis: electrolysis potential 0.2V vs. Ag / AgCl, electrolysis time 6h, current density 2.5mA / cm² 2 .

[0071] Cooling and sampling: Same as in Example 1.

[0072] Post-treatment: ultrasonic cleaning with deionized water for 22 min (power 180W), stirring cleaning with ethanol for 32 min (speed 380r / min), and drying at 100℃ for 8.5 h.

[0073] Comparative Example 1 (Traditional Chemical Oxidation Method)

[0074] Take 5g of Fe-based raw carbon nanotubes (same as in Example 1, initial catalyst content 12.35wt.%), add 50mL of concentrated nitric acid (68% by mass), place in an oil bath, and reflux at 120℃ for 6h; after cooling, add 50mL of concentrated sulfuric acid (98% by mass), and continue reflux acidification at 100℃ for 4h; after the reaction is complete, wash repeatedly with deionized water until the filtrate is neutral, filter, and dry in an oven at 100℃ for 12h to obtain purified carbon nanotubes.

[0075] Comparative Example 2 (Single Molten Salt Electrolysis Method)

[0076] Molten salt pretreatment: Sodium chloride was selected as the molten salt, 200g was weighed and dried at 200℃ for 24h (no ball milling mixing step); Carbon nanotube film preparation: Same as in Example 1; Electrode assembly and heating: Same as in Example 1 (800℃, argon gas 300sccm); Constant potential electrolysis: Same as in Example 1 (0.3V, 4h); Cooling, sampling and post-treatment: Same as in Example 1.

[0077] Comparative Example 3 (Molten Salt Electrolysis Method Beyond the Parameter Range of the Invention)

[0078] Molten salt pretreatment: Sodium chloride and potassium chloride were weighed at a mass ratio of 1:3 (outside the range of 1:0.5-1:2 in this invention), weighed 100g and 300g respectively, ball-milled and mixed, and then dried; Carbon nanotube film preparation: Same as in Example 1; Electrode assembly and heating: Heated to 1100℃ (outside the range of 700-1000℃ in this invention), the rest was the same as in Example 1; Constant potential electrolysis: Electrolysis potential 0.5V (outside the range of 0.1-0.4V in this invention), the rest was the same as in Example 1; Cooling, sampling and post-treatment: Same as in Example 1.

[0079] Experimental Results and Data Analysis Detection Standards and Methods Catalyst Residual Content Detection: Inductively Coupled Plasma Mass Spectrometry (ICP-MS) was used for detection. The purified carbon nanotube sample was digested with a nitric acid-hydrofluoric acid mixed acid in a microwave digester, and then the concentrations of Fe, Co, and Ni in the solution were determined by ICP-MS to calculate the catalyst residual content (Wt.%).

[0080] Crystallinity determination: Raman spectroscopy was used with an excitation wavelength of 532 nm and a scanning range of 1000-2000 cm⁻¹. -1 Record G peak (1580cm) -1 The left and right peaks correspond to the graphitized structure of carbon nanotubes, and the D peak (1350 cm⁻¹). -1 The strength of I is calculated based on the strength of the defect structure of the carbon nanotube (corresponding to the left and right sides). G / I D Value, I G / I D The higher the value, the higher the crystallinity.

[0081] Structural integrity testing: Scanning electron microscopy (SEM) observation: at a magnification of 50,000, observe the morphology, length, and surface impurities of carbon nanotubes; Length measurement: randomly select 50 carbon nanotubes from the SEM images, measure the length before and after purification, and calculate the length retention rate (length after purification / length before purification × 100%).

[0082] Yield calculation: Yield = Mass of purified carbon nanotubes / Mass of original carbon nanotubes × 100%.

[0083] (I) Test Results of Examples The test results of Examples 1-6 are shown in Table 1.

[0084] Table 1. Detection results of Examples 1-6

[0085] As can be seen from Table 2, the catalyst removal effect is excellent: in all examples, the residual content of purified carbon nanotube catalyst is ≤0.08 Wt.%, and the lowest is only 0.03 Wt.%, which is much lower than 1 Wt.% in the prior art. This shows that the molten salt electrolysis method of the present invention can efficiently and thoroughly remove Fe, Co, Ni monometallic and bimetallic catalyst impurities, and can achieve deep purification effect regardless of the initial catalyst content (8.76-14.21 Wt.%).

[0086] The structural integrity was well maintained: the change rate of IG / ID values ​​in all embodiments was ≤±3%, and all were positive, indicating that the crystallinity of carbon nanotubes after purification not only did not decrease, but was slightly improved (possibly due to slight graphitization of carbon nanotubes under high temperature conditions); the length retention rate was ≥92.9%, with the highest reaching 97.3%, proving that the technical solution of the present invention can protect the structural integrity of carbon nanotubes to the maximum extent and avoid the problems of shortened length and increased defects caused by the prior art.

[0087] High yield: The yield of all embodiments is ≥90.7%, with the highest reaching 95.6%, which is a significant improvement compared to the 60-70% yield of the traditional chemical oxidation method. This is because the present invention does not require oxidation etching of the defective parts of carbon nanotubes, thus avoiding the loss of the carbon nanotubes themselves and having higher industrial application value.

[0088] (II) Comparative test results The test results of comparative examples 1-3 are shown in Table 2.

[0089] Table 2. Detection results of Comparative Examples 1-3

[0090] (III) Comparative Analysis of Examples and Comparative Examples with Traditional Chemical Oxidation Method (Comparative Example 1): Catalyst Removal Efficiency: The residual catalyst content in Example 1 (0.06 wt.%) was only 4.8% of that in Comparative Example 1 (1.24 wt.%), indicating that the impurity removal efficiency of the present invention is much higher than that of the traditional chemical oxidation method, and can achieve deep purification; Structural Integrity: The I in Example 1 G / I DThe change rate of the value (+2.3%) contrasts sharply with that of Comparative Example 1 (-26.1%), and the length retention rate (95.2%) is 1.39 times that of Comparative Example 1 (68.5%), proving that the present invention can effectively protect the carbon nanotube structure, while the traditional chemical oxidation method will seriously damage the graphitization structure of carbon nanotubes and lead to a significant reduction in length; Yield: The yield of Example 1 (92.8%) is 27.5 percentage points higher than that of Comparative Example 1 (65.3%), and its industrial application value is significantly higher; Environmental friendliness: Comparative Example 1 uses a large amount of concentrated nitric acid and concentrated sulfuric acid, generating a large amount of acidic wastewater, while Example 1 does not require the use of strong acids and bases, and the environmental burden is greatly reduced.

[0091] Comparison with single molten salt electrolysis (Comparative Example 2): Catalyst removal efficiency: The residual catalyst content in Comparative Example 2 (0.87 wt.%) was 14.5 times that in Example 1 (0.06 wt.%), indicating that the impurity removal efficiency of the single molten salt system was much lower than that of the binary mixed molten salt system of this invention. This is because the single molten salt has a higher melting point and viscosity, and a slower ion migration rate, resulting in insufficient oxidation-reduction cycle of the catalyst and difficulty in completely removing the catalyst; Structural integrity: The I in Comparative Example 2 G / I D The change rate of the value was -6.1%, and the length retention rate was 85.3%, both lower than that of Example 1, indicating that the electrolysis process of the single molten salt system caused slight damage to the carbon nanotube structure, while the binary mixed molten salt system had better stability and could reduce the occurrence of side reactions; the yield of Comparative Example 2 (82.6%) was lower than that of Example 1 (92.8%), further proving the superiority of the binary mixed molten salt system.

[0092] Comparison with electrolysis exceeding parameter range (Comparative Example 3): Catalyst removal effect: The residual catalyst content in Comparative Example 3 (0.53 wt.%) was 8.8 times that in Example 1 (0.06 wt.%), indicating that when the parameters of molten salt ratio, reaction temperature, and electrolysis potential exceeded the range defined in this invention, the impurity removal efficiency decreased significantly. This is because the molten salt mass ratio of 1:3 led to an increase in molten salt viscosity, hindering ion migration; the reaction temperature of 1100℃ was too high, causing some carbon nanotubes to oxidize; and the electrolysis potential of 0.5V was too high, triggering the oxidation reaction of carbon nanotubes. These factors collectively affected the catalyst removal effect; Structural integrity: The I in Comparative Example 3 G / I DThe value change rate was -12.6%, the length retention rate was 76.8%, and the structure was severely damaged. This was because the excessively high temperature and potential caused the graphitization structure of the carbon nanotubes to be oxidized and destroyed, resulting in a significant reduction in length. The yield of Comparative Example 3 (73.2%) was much lower than that of Example 1 (92.8%), indicating that reasonable parameter limitation is the key to achieving the unity of "efficient impurity removal" and "structural integrity".

[0093] (iv) Analysis of the Influence of Different Parameters on Purification Efficiency: The influence of molten salt ratio on purification efficiency: As can be seen from Examples 1 (NaCl-KCl=1:1), 2 (LiCl-KCl=1:0.5), and 3 (NaF-KF=1:2), within the mass ratio range of 1:0.5-1:2 defined in this invention, binary molten salt systems with different ratios can achieve efficient purification, with residual catalyst content ≤0.08Wt.% and IG / ID value change rate ≤±3%. This indicates that the molten salt ratio range defined in this invention has good applicability and can meet the purification requirements of different types of carbon nanotube catalysts.

[0094] Effect of reaction temperature on purification efficiency: The test results of Examples 2 (700℃), 1 (800℃), and 5 (900℃) show that as the temperature increases, the residual catalyst content decreases slightly (0.04→0.06→0.07 wt.%, slightly higher in Example 5 because the initial catalyst content is higher), but the change is not significant; the IG / ID value change rate and length retention rate remain at a good level. This indicates that the temperature range of 700-1000℃ can ensure complete melting of the molten salt and sufficient exposure of the catalyst, and slight temperature fluctuations will not have a significant impact on the purification efficiency, indicating good process stability.

[0095] Effect of electrolysis potential on purification efficiency: The test results of Examples 2 (0.1V), 4 (0.25V), and 3 (0.4V) show that within the electrolysis potential range of 0.1-0.4V, the residual catalyst content is ≤0.08 wt.%, and the structural integrity is good. When the potential is below 0.1V (no separate example was set), the catalyst oxidation reaction is difficult to occur, and the impurity removal efficiency is extremely low; when the potential is above 0.4V (Comparative Example 3), the carbon nanotubes are oxidized, and the structure is severely damaged. Therefore, the electrolysis potential of 0.1-0.4V specified in this invention is the key to achieving selective impurity removal.

[0096] Effect of electrolysis time on purification effect: The test results of Example 3 (2h), Example 1 (4h), and Example 2 (8h) show that the longer the electrolysis time, the lower the residual catalyst content (0.08→0.06→0.04 wt.%). However, when the electrolysis time exceeds 4h, the decrease in residual content is smaller. This indicates that when the electrolysis time is around 4h, the catalyst can be completely removed. Extending the time has limited effect on improving the impurity removal effect, but it will not damage the carbon nanotube structure. Therefore, the electrolysis time can be flexibly adjusted according to actual production needs.

[0097] In summary, through comparative experiments of 6 examples and 3 comparative examples, the significant inventiveness and superiority of the technical solution of this invention are fully verified: the molten salt electrolysis method of this invention can efficiently remove metal catalysts such as Fe, Co, and Ni from carbon nanotubes, with a residual content ≤0.1 Wt.%, far superior to traditional chemical oxidation methods and other comparative schemes; this method can maintain the structural integrity of carbon nanotubes to the maximum extent, with an IG / ID value change rate ≤±5% and a length retention rate ≥90%, solving the problems of carbon nanotube structural damage and performance degradation caused by existing technologies; this invention does not require the use of strong acids or bases, has a simple process, is environmentally friendly, has a high yield (≥90%), and low production cost, making it suitable for large-scale industrial production; the reasonable limitation of the molten salt system, electrolysis parameters, and electrode configuration is the key to achieving the above technical effects, and the purification effect will significantly decrease if the parameters exceed the range defined by this invention.

[0098] The equipment required for the efficient and non-destructive purification method of carbon nanotubes of this invention consists of commonly used industrial equipment such as electrolytic furnaces and electrochemical workstations, requiring no special customization and facilitating large-scale production. The molten salt can be recycled and reused, further reducing production costs. The purified carbon nanotubes possess high purity, high crystallinity, and complete structure, fully leveraging their excellent electrical, mechanical, and thermal properties. They have broad application prospects in fields such as electronic devices, lithium-ion batteries, supercapacitors, aerospace composite materials, and environmental remediation, and can promote the rapid development of the industrial application of carbon nanotubes, demonstrating significant economic value and social benefits.

Claims

1. A method for efficiently and non-destructively purifying carbon nanotubes, characterized in that, Includes the following steps: (1) Molten salt weighing and pretreatment: Select two molten salt components, mix them thoroughly according to the set mass ratio, and dry them to remove surface crystal water; (2) Electrode assembly and heating: Place the dried molten salt into a corundum crucible, insert the working electrode, counter electrode and reference electrode. The working electrode is a carbon nanotube film. Then place them together into the electrolytic furnace, introduce a protective atmosphere, and heat to the reaction temperature; (3) Constant potential electrolysis: Set the electrolysis potential and electrolysis time, start the electrolysis process, and the residual catalyst in the carbon nanotube film undergoes an oxidation reaction near the working electrode to generate an ionic catalyst. The ionic catalyst is released to the counter electrode to undergo a reduction reaction and deposit; (4) Cooling and sampling: After electrolysis, lift all electrodes above the molten salt interface. After the electrolytic furnace cools naturally to room temperature, take out the carbon nanotube sample on the working electrode; (5) Post-treatment: Wash the carbon nanotube sample sequentially with deionized water and ethanol solution to remove residual molten salt on the surface. Finally, dry it in a 100℃ oven to obtain purified carbon nanotubes.

2. The method for efficient and non-destructive purification of carbon nanotubes according to claim 1, characterized in that, In step (1), the molten salt component is any two of sodium chloride, potassium chloride, lithium chloride, sodium fluoride, potassium fluoride, and lithium fluoride, and the mass ratio is 1:0.5-1:

2.

3. The method for efficient and non-destructive purification of carbon nanotubes according to claim 1, characterized in that, In step (2), the counter electrode is a tungsten wire or a molybdenum wire with a diameter of 0.5-1.5 mm; the reference electrode is an Ag / AgCl reference electrode; and the protective atmosphere is argon or nitrogen with a flow rate of 200-500 sccm.

4. The method for efficient and non-destructive purification of carbon nanotubes according to claim 1, characterized in that, In step (2), the reaction temperature is 700-1000℃, the heating rate is 5-10℃ / min, and after heating to the set temperature, the temperature is held for 30 minutes before electrolysis is started.

5. The method for efficient and non-destructive purification of carbon nanotubes according to claim 1, characterized in that, In step (3), the electrolysis potential is 0.1-0.4V vs. Ag / AgCl, the electrolysis time is 2-8h, and the current density is controlled at 1-5mA / cm² during the electrolysis process.

6. The method for efficient and non-destructive purification of carbon nanotubes according to claim 1, characterized in that, In step (1), two molten salt components are selected, and after being fully mixed according to the set mass ratio, they are dried in an oven at 200℃ for 24 hours to remove surface crystal water. The molten salt is mixed by ball milling using a planetary ball mill with a ball milling speed of 200-300 r / min, a ball milling time of 2-4 hours, and a ball-to-material ratio of 10:1-20:

1.

7. The method for efficient and non-destructive purification of carbon nanotubes according to claim 1, characterized in that, In step (5), the deionized water cleaning is performed using ultrasonic cleaning with an ultrasonic power of 100-300W and an ultrasonic time of 10-30min; the ethanol solution cleaning is performed using magnetic stirring with a stirring rate of 300-500r / min and a stirring time of 15-45min; and the drying time is 6-12h.

8. The method for efficient and non-destructive purification of carbon nanotubes according to claim 1, characterized in that, The preparation method of the carbon nanotube film is as follows: the original carbon nanotube powder is mixed with deionized water and ultrasonically dispersed for 30-60 min to prepare a dispersion with a concentration of 5-10 mg / mL. Then, a film is formed on the filter membrane by vacuum filtration. After peeling off the filter membrane, the carbon nanotube film is obtained with a thickness of 10-50 μm.

9. The method for efficient and non-destructive purification of carbon nanotubes according to claim 1, characterized in that, In step (3), the residual catalyst is one or more of Fe, Co, and Ni, and the initial catalyst content is 5-15 wt.%.

10. A highly efficient and non-destructive method for purifying carbon nanotubes, characterized in that, The carbon nanotubes were prepared according to any one of claims 1-9 using a highly efficient and non-destructive purification method, wherein the purified carbon nanotube catalyst residue content is ≤0.1 wt.%, and the crystallinity parameter I is... G / I D The value changed by ≤±5% compared with that before purification, and the length retention rate was ≥90%.

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