Method for manufacturing small carbon nanotubes through carbon dioxide conversion and small carbon nanotubes manufactured thereby

Abstract

The present invention relates to a method for manufacturing small carbon nanotubes through carbon dioxide conversion and small carbon nanotubes manufactured thereby. The method for manufacturing small carbon nanotubes through carbon dioxide conversion according to the present invention enables mass production of carbon nanotubes with excellent quality, allows carbon dioxide, which is a raw material, to be utilized as it is in exhaust gas, thereby omitting separate separation and capture processes, and allows impurities contained in the exhaust gas to be doped into the carbon nanotubes being produced, thereby contributing to improved conductivity. In addition, since the method can be commercialized for mass production, it can be widely applied as a carbon reduction technology. Further, the small carbon nanotubes according to the present invention have a small and uniform diameter, and thus are advantageously applied to related parts and industries that require higher physical properties for carbon nanotubes. Since the small carbon nanotubes are significantly doped with fluorine (F), their conductivity is improved, and thus they can be used as conductive supports in various industrial fields such as semiconductors, displays, and fuel cells. In addition, with carbonyl groups, carboxyl groups, and the like bonded to the surface thereof, the small carbon nanotubes have excellent dispersibility and, when applied to secondary batteries, can suppress formation of a solid electrolyte interphase (SEI) layer.

Description

Method for manufacturing small carbon nanotubes through carbon dioxide conversion and small carbon nanotubes manufactured thereby

[0001] The present invention relates to a method for manufacturing carbon nanotubes, and more specifically, to a method for manufacturing small carbon nanotubes through carbon dioxide conversion and small carbon nanotubes manufactured thereby.

[0002]

[0003] Currently, 85% of energy demand is supplied by fossil fuels. Climate change is accelerating due to carbon dioxide, a greenhouse gas emitted from the use of fossil fuels, and this is causing various environmental problems such as rising sea levels, extreme weather events like heatwaves and heavy snowfall, and ocean acidification. Therefore, the treatment and reduction of carbon dioxide is an urgent task for all countries on Earth.

[0004] Recently, to overcome this situation, technologies that reduce carbon dioxide emissions using carbon capture, utilization, and storage technologies, and furthermore, technologies that convert carbon dioxide into high-value-added products, are being developed. Representative examples include technologies for producing various chemical substances such as formic acid, carbon monoxide, methane, ethylene, and ethanol. However, the listed chemical substances have low economic feasibility due to their low market prices (at the level of $0.4 to $1.1 per kg).

[0005] Meanwhile, carbon nanotubes are cylindrical nanostructured carbon allotropes known for their excellent electrical and mechanical properties, and are therefore attracting attention for their usefulness in fields such as the electronics industry (including semiconductors), the energy industry (including secondary batteries and fuel cells), the structural composite materials industry, the chemical materials and polymer industries, and the medical industry.

[0006] In addition, carbon nanotubes command a high market price of $100 to $6,500 per kg and can generate high added value. Therefore, assuming the same amount of carbon dioxide is converted, it is expected that more than 100 times the profit can be obtained compared to the aforementioned chemicals such as ethanol and methanol, thereby dramatically improving the economic feasibility of carbon dioxide conversion technology.

[0007] Therefore, there is an urgent need for research on technology to mass-produce high-quality, small-sized carbon nanotubes that are eco-friendly—such as by reducing carbon dioxide emissions and mitigating global warming because they are converted from carbon dioxide—and possess high added value and potential for application in various fields.

[0008]

[0009] The present invention has been devised to overcome the aforementioned problems. The first objective of the present invention is to provide a method for manufacturing carbon nanotubes through carbon dioxide conversion that is environmentally friendly and economical, capable of mass-producing high-value-added carbon nanotubes of excellent quality without generating harmful substances from carbon dioxide; capable of utilizing industrial exhaust gas as the raw material (carbon dioxide) to eliminate separate separation and capture processes; capable of converting impurities such as sulfides and nitrides contained in the exhaust gas into a doping form for the manufactured carbon nanotubes to contribute to improved conductivity; and capable of commercializing for mass production purposes due to the ease of large-area and continuous process design, thereby allowing for widespread application as a carbon reduction technology.

[0010] The second problem to be solved by the present invention is to provide an electrolytic cell for manufacturing small carbon nanotubes through carbon dioxide conversion that is environmentally friendly and economical, capable of mass-producing high-value-added carbon nanotubes of excellent quality without generating harmful substances from carbon dioxide; capable of utilizing industrial exhaust gas as the raw material carbon dioxide, thereby eliminating separate separation and capture processes; capable of converting impurities such as sulfides and nitrides contained in the exhaust gas into a doping form for the manufactured carbon nanotubes, thereby contributing to improved conductivity; and capable of commercializing for mass production due to the ease of large-area and continuous process design, thus allowing for widespread application as a carbon reduction technology.

[0011] The third problem to be solved in the present invention is to provide a carbon nanotube that can be manufactured through the above manufacturing method, has a small diameter and has a uniform diameter, which is advantageous for application in related parts and industries requiring higher physical properties for carbon nanotubes, and has improved conductivity because it is significantly doped with fluorine (F), so it can be used as a conductive support in various industrial fields such as semiconductors, displays, and fuel cells, and has excellent dispersibility because carbonyl groups, carboxyl groups, etc. are bonded to the surface, and especially when applied to secondary batteries such as lithium batteries, it can suppress the occurrence of a Solid Electrolyte Interphase (SEI) layer.

[0012]

[0013] The problems that the present invention aims to solve are not limited to those mentioned above, and other unmentioned problems will be clearly understood by those skilled in the art to which the present invention belongs from the description below.

[0014]

[0015] To solve the first problem described above, a method for manufacturing small carbon nanotubes through carbon dioxide conversion is provided, comprising: (1) a step of supplying an electrolyte containing molten carbonate and fluoride salts between an anode and a cathode in an electrolytic section; (2) a step of supplying carbon dioxide into the molten electrolyte; (3) a step of growing carbon nanotubes through seed catalyst particles deposited on the cathode by applying an electric current to the anode and cathode; and (4) a step of collecting the grown carbon nanotubes from the cathode.

[0016] In addition, the carbon dioxide may be in the form of flue gas emitted from a carbon dioxide emission source.

[0017] In addition, the electrolyte may further contain chloride salt at a concentration of 45 to 75 mol% based on the total molar amount of the electrolyte.

[0018] In addition, the above seed catalyst particles allow the anode-derived oxide to be reduced on the cathode.

[0019] According to one embodiment of the present invention, a fluorine anion (F) in the electrolyte due to the fluoride salt - The concentration of ) is 1 to 50 mol%, and the molten electrolyte temperature can be 450 to 800 ℃.

[0020] In addition, the applied current may have a current density of 10 to 500 mA / cm².

[0021] Additionally, the electrolyte comprises lithium chloride (LiCl) and lithium fluoride (LiF), and the number of moles of lithium chloride relative to the total number of moles of the electrolyte may be greater than the number of moles of lithium fluoride.

[0022] In addition, the electrolyte may contain 15 to 25 mol% of lithium carbonate (Li2CO3), 45 to 75 mol% of lithium chloride (LiCl), and 5 to 35 mol% of lithium fluoride (LiF) relative to the total molar amount.

[0023] In addition, the method may further include a step of washing the carbon nanotubes collected in step (4) with acid.

[0024]

[0025] To solve the second problem described above, an electrolytic cell for manufacturing small carbon nanotubes through carbon dioxide conversion is provided, characterized by comprising: an anode; a cathode on which seed catalyst particles, which are the starting point for tip growth of carbon nanotubes, are deposited on the surface; and a molten electrolyte disposed between the anode and the cathode and comprising carbonate and fluoride salts; a heating unit for controlling the temperature of the electrolyte; and a power supply unit for supplying a predetermined current to the anode and the cathode of the electrolytic unit.

[0026] In addition, the temperature of the heating unit may be set so that the temperature of the molten electrolyte is 450 to 800 ℃, and the power supply unit may be set so that the current density of the power supplied to the anode and cathode is 10 to 500 mA / ㎠.

[0027] In addition, the electrolyte may contain 15 to 25 mol% of lithium carbonate (Li2CO3), 45 to 75 mol% of lithium chloride (LiCl), and 5 to 35 mol% of lithium fluoride (LiF) relative to the total molar amount.

[0028] In addition, the seed catalyst particles are deposited on the cathode after a predetermined current is supplied to the anode and cathode, and the average diameter of the seed catalyst particles may be 1 to 50 nm.

[0029]

[0030] To solve the third problem described above, small carbon nanotubes derived from carbon dioxide are provided, which have an average diameter of 50 nm or less, a G / D ratio of 0.3 to 1.5, and are doped with fluorine (F).

[0031] In addition, the small carbon nanotube may contain 80 to 90 at% carbon (C), 5 to 15 at% oxygen (O), and 1 to 10 at% fluorine (F).

[0032] In addition, the small carbon nanotube may include carboxyl groups and / or carbonyl groups.

[0033] In addition, the above-mentioned small carbon nanotube has a multi-walled structure and may be provided with metal particles of a size corresponding to the inner diameter.

[0034] In addition, the metal particle may be provided inside the first wall in the radial direction from the center of the carbon nanotube.

[0035] In addition, the small carbon nanotubes may contain a metal, the metal being at least one type of transition metal, and the metal may be included in an amount of 3 at% or less.

[0036] In addition, the above-described small carbon nanotubes can be manufactured using the manufacturing method described above.

[0037]

[0038] Meanwhile, it is disclosed that the present invention was developed with the support of the following national research and development project.

[0039] [National R&D Project that supported this invention 1]

[0040] [Project ID] 2710076679

[0041] [Assignment No.] 00210114(A2025-0096)

[0042] [Ministry Name] Ministry of Science and ICT

[0043] [Name of Project Management (Specialized) Agency] National Research Foundation of Korea

[0044] [Research Project Name] Outstanding Young Researcher

[0045] [Project Title] Development of All-Solid Carbon Dioxide Conversion Platform Technology for Solid Hydrogen Storage Fuel Production (3 / 4)

[0046] [Name of Project Performing Organization] Kookmin University

[0047] [Research Period] 2025.03.01 ~ 2026.02.28

[0048]

[0049] The method for manufacturing small carbon nanotubes through carbon dioxide conversion and the electrolytic cell for manufacturing small carbon nanotubes through carbon dioxide conversion according to the present invention enable the mass production of high-value-added carbon nanotubes of excellent quality from carbon dioxide without the generation of harmful substances. Furthermore, since the carbon dioxide used as a raw material can be utilized directly from industrial exhaust gas, separate separation and capture processes can be omitted. Additionally, impurities such as sulfides and nitrides contained in the exhaust gas can be converted into a form that dopes into the carbon nanotubes being manufactured, thereby contributing to improved conductivity. Moreover, as it facilitates large-area fabrication and continuous process design, it is commercially viable for mass production, allowing for widespread application as a carbon reduction technology. It is also environmentally friendly and economical.

[0050] Meanwhile, the small carbon nanotubes produced by the above manufacturing method have a small diameter and a uniform diameter, making them advantageous for application in related parts and industries that require higher physical properties for carbon nanotubes. In addition, since they are significantly doped with fluorine (F), their conductivity is enhanced, allowing them to be used as conductive supports in various industrial fields such as semiconductors, displays, and fuel cells. Furthermore, because carbonyl groups and carboxyl groups are bonded to the surface, they exhibit excellent dispersibility and can suppress the formation of a Solid Electrolyte Interphase (SEI) layer when applied to secondary batteries such as lithium batteries.

[0051]

[0052] The effects of the present invention are not limited to the effects described above, and should be understood to include all effects that can be inferred from the composition of the invention described in the description or claims of the present invention.

[0053]

[0054] FIG. 1a is a schematic diagram of an electrolytic reaction occurring in the electrolytic section of a method for manufacturing small carbon nanotubes through carbon dioxide conversion according to a preferred embodiment of the present invention.

[0055] FIG. 1b is a potential-current density graph measured by cyclic voltammetry under conditions of a scan speed of 10 mV / s and a voltage range of 0.2 to -1.5 V (vs. Ag / AgCl) with different anode electrode types in the method for manufacturing small carbon nanotubes through carbon dioxide conversion of the present invention.

[0056] FIG. 1c is a potential-current density graph measured by cyclic voltammetry under conditions of a scan speed of 10 mV / s and a voltage range of 0.2 to -1.5 V (vs. Ag / AgCl) with different cathode electrode types in the method for manufacturing small carbon nanotubes through carbon dioxide conversion of the present invention.

[0057] FIG. 1d is a potential-current density graph measured by cyclic voltammetry under conditions of a scan rate of 10 mV / s and a voltage range of 0.2 to -1.5 V (vs. Ag / AgCl) with varying fluoride salt concentrations in the method for manufacturing small carbon nanotubes through carbon dioxide conversion of the present invention.

[0058] FIGS. 2a to 2d show the current density of 25 mA / cm² when the fluoride salt concentration is 0 mol% (molten electrolyte temperature 550 °C), 10 mol% (molten electrolyte temperature 500 °C), 20 mol% (molten electrolyte temperature 500 °C), and 30 mol% (molten electrolyte temperature 500 °C), respectively, in the method for manufacturing small carbon nanotubes through carbon dioxide conversion according to the present invention. 2 These are the scanning electron microscope (SEM) results (Scale bar: 1 μm) of small carbon nanotubes prepared under conditions of an electrolysis time of 1 hour. Figure 2a shows a molten electrolyte temperature of 550 ℃, and Figures 2b to 2d show a molten electrolyte temperature of 500 ℃.

[0059] FIG. 2e shows the current density of 25 mA / cm² when the fluoride salt concentration is 0 mol% (molten electrolyte temperature 550 °C), 10 mol% (molten electrolyte temperature 500 °C), 20 mol% (molten electrolyte temperature 500 °C), and 30 mol% (molten electrolyte temperature 500 °C), respectively, in the method for manufacturing small carbon nanotubes through carbon dioxide conversion according to the present invention. 2 This is the result of measuring the outer diameter distribution of small carbon nanotubes manufactured under conditions of an electrolysis time of 1 hour.

[0060] FIG. 2f shows the current density of 25 mA / cm² when the fluoride salt concentration is 0 mol% (molten electrolyte temperature 550 °C), 10 mol% (molten electrolyte temperature 500 °C), 20 mol% (molten electrolyte temperature 500 °C), and 30 mol% (molten electrolyte temperature 500 °C), respectively, in the method for manufacturing small carbon nanotubes through carbon dioxide conversion according to the present invention. 2 These are the results of the Faraday efficiency of small carbon nanotubes prepared under conditions of an electrolysis time of 1 hour.

[0061] FIG. 2g shows the current density of 25 mA / cm² when the fluoride salt concentration is 0 mol% (molten electrolyte temperature 550 °C), 10 mol% (molten electrolyte temperature 500 °C), 20 mol% (molten electrolyte temperature 500 °C), and 30 mol% (molten electrolyte temperature 500 °C), respectively, in the method for manufacturing small carbon nanotubes through carbon dioxide conversion according to the present invention. 2 This is the result of the cell potential measured for 1 hour of electrolysis under these conditions.

[0062] FIG. 2h shows a carbon nanotube manufactured through the method for manufacturing small carbon nanotubes by carbon dioxide conversion of the present invention, with a cathode (Ni sheet, 2.25 cm 2 This is a photographic image (scale bar: 1cm) showing the shape attached to ).

[0063] FIGS. 3a to 3c illustrate the method for manufacturing small carbon nanotubes through carbon dioxide conversion according to the present invention, with a fluoride salt concentration of 30 mol%, a molten electrolyte temperature of 500 °C, and a current density of 25 mA / cm². 2 These are the HRTEM (high-resolution transmission electron microscopy) results of carbon nanotubes prepared under conditions of an electrolysis time of 1 hour. Figures 3a and 3b show seed catalyst particles (metal particles) provided inside small carbon nanotubes, and Figure 3c shows lattice fringe images of small carbon nanotubes.

[0064] FIG. 3d shows the method for manufacturing small carbon nanotubes through carbon dioxide conversion according to the present invention, with a fluoride salt concentration of 30 mol%, a molten electrolyte temperature of 500 °C, and a current density of 25 mA / cm². 2 The HAADF STEM (high-angle annular dark-field scanning transmission electron microscopy) (scale bar: 10 nm) results of carbon nanotubes prepared under conditions of an electrolysis time of 1 hour are shown, and Figures 3e to 3h are the EDS (Energy Dispersive Spectroscopy) mapping analysis results for C, Fe, Ni, and Cr, respectively, in the carbon nanotubes of Figure 3d.

[0065] FIG. 3i shows the current density of 25 mA / cm² when the fluoride salt concentration is 0 mol% (molten electrolyte temperature 550 °C), 10 mol% (molten electrolyte temperature 500 °C), 20 mol% (molten electrolyte temperature 500 °C), and 30 mol% (molten electrolyte temperature 500 °C), respectively, in the method for manufacturing small carbon nanotubes through carbon dioxide conversion according to the present invention. 2 This is the result of measuring the inner diameter distribution of small carbon nanotubes manufactured under conditions of an electrolysis time of 1 hour.

[0066] FIG. 3j shows the current density of 25 mA / cm² when the fluoride salt concentration is 0 mol% (molten electrolyte temperature 550 °C), 10 mol% (molten electrolyte temperature 500 °C), 20 mol% (molten electrolyte temperature 500 °C), and 30 mol% (molten electrolyte temperature 500 °C), respectively, in the method for manufacturing small carbon nanotubes through carbon dioxide conversion according to the present invention. 2 This is the result of measuring the size distribution of seed catalyst particles (metal particles) that appear when manufacturing small carbon nanotubes under conditions of an electrolysis time of 1 hour.

[0067] FIG. 3k shows the current density of 25 mA / cm² when the fluoride salt concentration is 0 mol% (molten electrolyte temperature 550 °C), 10 mol% (molten electrolyte temperature 500 °C), 20 mol% (molten electrolyte temperature 500 °C), and 30 mol% (molten electrolyte temperature 500 °C), respectively, in the method for manufacturing small carbon nanotubes through carbon dioxide conversion according to the present invention. 2 These are the results of measuring the outer diameter distribution and wall thickness of small carbon nanotubes manufactured under conditions of an electrolysis time of 1 hour.

[0068] FIGS. 4a to 4d show the method for manufacturing small carbon nanotubes by carbon dioxide conversion according to the present invention, when the molten electrolyte temperature is 500 °C, 550 °C, 600 °C, and 650 °C, respectively, with a fluoride salt concentration of 30 mol% and a current density of 25 mA / cm². 2 This is the SEM result of small carbon nanotubes prepared under conditions of an electrolysis time of 1 hour.

[0069] FIG. 4e shows the molten electrolyte temperature at 30 mol% and a current density of 25 mA / cm² when varying the molten electrolyte temperature in the method for manufacturing small carbon nanotubes through carbon dioxide conversion according to the present invention. 2 These are the results of the Faraday efficiency of small carbon nanotubes prepared under conditions of an electrolysis time of 1 hour.

[0070] FIG. 4f shows the fluoride salt concentration at 30 mol% and the current density at 25 mA / cm² when the molten electrolyte temperature is varied in the method for manufacturing small carbon nanotubes through carbon dioxide conversion according to the present invention. 2 This is the result of measuring the outer diameter distribution of small carbon nanotubes manufactured under conditions of an electrolysis time of 1 hour.

[0071] FIG. 4g shows the fluoride salt concentration at 30 mol% and the current density at 25 mA / cm² when the molten electrolyte temperature is varied in the method for manufacturing small carbon nanotubes through carbon dioxide conversion according to the present invention. 2 This is the result of measuring the size distribution of seed catalyst particles (metal particles) that appear when manufacturing small carbon nanotubes under conditions of an electrolysis time of 1 hour.

[0072] FIG. 4h shows the fluoride salt concentration at 30 mol% and the current density at 25 mA / cm² when the molten electrolyte temperature is varied in the method for manufacturing small carbon nanotubes through carbon dioxide conversion according to the present invention. 2 This is the result of measuring the wall thickness of small carbon nanotubes manufactured under conditions of an electrolysis time of 1 hour.

[0073] Figure 5 shows the HR-XPS (High-Resolution X-ray Photoelectron Spectroscopy) results for small carbon nanotubes through carbon dioxide conversion of the present invention.

[0074]

[0075] Hereinafter, the present invention will be described in detail with reference to the attached drawings so that those skilled in the art can easily implement it. The present invention may be embodied in various forms and is not limited to the embodiments described herein. In the drawings, parts not directly related to the description are omitted to clearly explain the present invention.

[0076] The terms used in this specification are used merely to describe specific embodiments and are not intended to limit the invention. The singular expression includes the plural expression unless the context clearly indicates otherwise. In this application, terms such as “comprising” or “having” should be understood as indicating the presence of the features, numbers, steps, actions, components, or combinations thereof described in the specification, and not as precluding the possibility that one or more other features, numbers, steps, actions, components, or combinations thereof may be present or added.

[0077] Unless otherwise defined, all terms used herein, including technical or scientific terms, have the same meaning as generally understood by those skilled in the art to which the present invention pertains. Terms identical to those defined in commonly used dictionaries should be interpreted in a meaning consistent with their meaning in the context of the relevant technology, and should not be interpreted in an ideal or overly formal sense unless explicitly defined herein.

[0078]

[0079] Hereinafter, in this specification, “small carbon nanotube” means a carbon nanotube with a diameter of 50 nm or less, and its length is not limited.

[0080]

[0081] As mentioned above, there is an urgent need for research on technology to mass-produce high-quality, small-sized carbon nanotubes that are eco-friendly—such as by reducing carbon dioxide emissions and mitigating global warming because they are converted from carbon dioxide—and possess high added value and potential for application in various fields.

[0082]

[0083] Accordingly, the present invention seeks to solve the above-mentioned problem by providing a method for manufacturing small carbon nanotubes through carbon dioxide conversion, comprising: (1) a step of supplying an electrolyte containing molten carbonate and fluoride salts between an anode and a cathode in an electrolytic section; (2) a step of supplying carbon dioxide into the molten electrolyte; (3) a step of growing carbon nanotubes through seed catalyst particles deposited on the cathode by applying current to the anode and cathode; and (4) a step of collecting the grown carbon nanotubes from the cathode.

[0084] Through this, high-value carbon nanotubes of excellent quality can be mass-produced from carbon dioxide without generating harmful substances. Since the carbon dioxide used as a raw material can be utilized directly from industrial exhaust gas, separate separation and capture processes can be omitted. Furthermore, impurities such as sulfides and nitrides contained in the exhaust gas can be converted into doping for the manufactured carbon nanotubes, thereby contributing to improvements in conductivity. Additionally, the method for manufacturing small carbon nanotubes according to the present invention facilitates large-area scaling and continuous process design, making it commercially viable for mass production. Consequently, it can be widely applied as a carbon reduction technology and is both environmentally friendly and economical.

[0085] FIG. 1a is a schematic diagram of an electrolytic reaction occurring in the electrolytic section of a method for manufacturing small carbon nanotubes through carbon dioxide conversion according to a preferred embodiment of the present invention. The method for manufacturing small carbon nanotubes according to the present invention will be explained below through FIG. 1a.

[0086]

[0087] First, as a step (1), an electrolyte containing molten carbonate and fluoride salts is supplied between the anode and cathode in the electrolytic section.

[0088] The electrolyte of the present invention comprises a molten carbonate. Here, the carbonate may be used without limitation as a known electrolyte used for the electrolysis of carbon dioxide based on molten carbonate, and may include, for example, lithium carbonate, which may be more advantageous for achieving the purpose of the present invention. In addition, the carbonate may include lithium carbonate as the main component of the carbonate, but may further include salts such as sodium carbonate, potassium carbonate, barium carbonate, magnesium carbonate, and calcium carbonate, and the present invention is not specifically limited thereto. For example, it may be 5 mol% or more, 10 mol% or more, or 15 mol% or more.

[0089]

[0090] In addition, the electrolyte comprises a molten fluoride salt. Since the fluorine anion of the fluoride salt is stabilized by binding with metal ions derived from the anode, the size of the seed catalyst particles reduced on the cathode can be reduced, and accordingly, the manufacturing method of the present invention can reduce the inner diameter of the carbon nanotube produced, which may be advantageous for realizing small carbon nanotubes. Furthermore, the fluorine anion can strongly interact with the surface of the growing carbon nanotube, adsorbing to the surface or forming covalent bonds, thereby inhibiting the radial growth of the carbon nanotube from the center. The fluoride salt may be, for example, an alkali metal or alkaline earth metal fluoride salt, and specifically, for example, lithium fluoride (lithium fluoride, LiF).

[0091] Referring to FIGS. 1d, 2, 3i, 3j, and 3k, the fluoride anion (F) in the electrolyte due to the fluoride salt is described above. -The concentration of ) can be 1 to 50 mol%, preferably 5 to 35 mol%, and more preferably 25 to 35 mol%. The carbon nanotubes realized through this method are small, have a uniform diameter, and can be significantly doped with fluorine (F), which can suppress the formation of a Solid Electrolyte Interphase (SEI) layer when applied to secondary batteries such as lithium batteries. If the concentration of fluorine anions in the electrolyte is less than 5 mol%, the size of the seed catalyst particles cannot be sufficiently reduced, which may result in an increased inner diameter of the realized carbon body or a thicker wall thickness, and the carbon body produced through this may not be sufficiently doped with fluorine. Additionally, if it is less than 5 mol%, less metal dissolution occurs compared to when it is 5 mol% or more, which may reduce the Faraday efficiency. If the concentration of fluoride anions in the electrolyte exceeds 35 mol%, problems such as corrosion of the anode may occur, and the stability of the electrochemical cell operation may be compromised, such as damage to the electrode contacts. On the other hand, if the concentration of fluoride anions in the electrolyte is in the range of 25 to 35 mol%, the diameter is very small and fluorine is moderately doped, so it can be applied in various fields requiring high physical properties.

[0092]

[0093] In addition, the electrolyte may further include a chloride salt. This ensures that the electrolyte becomes a three-component system, guarantees a wide operating temperature range, enables the miniaturization and uniformity of the diameter of carbon nanotubes, and improves the Faraday efficiency for carbon nanotube production by using low temperatures. As an example, the chloride salt may be any known chloride salt commonly used in electrolytes without limitation, specifically a metal chloride salt, and more specifically a lithium chloride.

[0094] The chloride salt may be included at a concentration of 45 to 75 mol% based on the total molar amount of the electrolyte. If the chloride salt is included at a concentration of less than 45 mol% based on the total molar amount of the electrolyte, it may be difficult to manufacture the small carbon nanotubes and may cause problems in lowering the temperature of the molten salt electrolyte. If the chloride salt is included at a concentration exceeding 75 mol% based on the total molar amount of the electrolyte, it may be difficult to manufacture carbon nanotubes that are small in size and have a uniform diameter as the relative concentration of the fluoride salt decreases, and the carbon nanotubes may not be sufficiently doped with fluorine, making it difficult to suppress the formation of the SEI layer when applied to secondary batteries such as lithium batteries, and there may be additional problems in that they do not have excellent conductivity to be used as a conductive support.

[0095]

[0096] Additionally, referring to FIGS. 2 and 3, the electrolyte may include lithium fluoride (LiF) as a fluoride salt and lithium chloride (LiCl) as a chloride salt, and the number of moles of lithium chloride relative to the total number of moles of the electrolyte may be greater than the number of moles of lithium fluoride. When the number of moles of lithium chloride is greater than the number of moles of lithium fluoride, the operating temperature range of the electrolyte may be lowered, and the Faraday efficiency for carbon nanotube production may be improved.

[0097] According to a preferred embodiment of the present invention, the electrolyte may include lithium carbonate (Li2CO3) as a carbonate, lithium fluoride (LiF) as a fluoride salt, and lithium chloride (LiCl) as a chloride salt, and more preferably may include 15 to 25 mol% of lithium carbonate (Li2CO3), 45 to 75 mol% of lithium chloride (LiCl), and 5 to 35 mol% of lithium fluoride (LiF) relative to the total molar amount, which may be more advantageous for achieving the objective of the present invention.

[0098]

[0099] The above-described electrolyte can be supplied in a molten state between the anode and cathode within the electrolytic section where the electrolytic reaction takes place. At this time, referring to FIG. 4, the fluoride anion (F) in the electrolyte due to the fluoride salt - When the concentration of ) is 1 to 50 mol%, the molten electrolyte temperature may be 450 to 800 °C, and preferably 450 to 630 °C. If the molten electrolyte temperature is below 450 °C, the electrolyte may not melt sufficiently, resulting in a weak carbon nanotube growth reaction, and problems may arise where the electrochemical reaction pattern changes due to uneven control of the electrolyte components. If the molten electrolyte temperature exceeds 630 °C, it may grow in the form of carbon fibers rather than carbon nanotubes (see Fig. 4d), the diameter may become very large (see Figs. 4f and 4g), and the carbon body production efficiency may become very low (see Fig. 4e).

[0100]

[0101] Meanwhile, the electrolytic unit includes an anode and a cathode that are spaced apart, and as an electrochemical environment in which the electrolytic reaction according to the present invention takes place, it may have a conventional structure and mechanism in the industry that performs such a function, and the present invention is not particularly limited thereto.

[0102] At this time, the anode and cathode can be adopted without limitation in terms of material and shape for use in molten carbonate-based carbon dioxide conversion, and the present invention does not specifically limit this. Referring to FIGS. 1b and 1c, for example, the material of the anode may be implemented as a metal or an alloy, and specifically, it may be one metal selected from the group consisting of nickel, chromium, iron, etc., or a mixed metal of two or more, or an alloy containing two or more. In addition, for example, the cathode material may be one metal selected from the group consisting of copper, zinc, iron, etc., or a mixed metal of two or more, or an alloy containing two or more.

[0103]

[0104] Next, as step (2), carbon dioxide is supplied into the molten electrolyte.

[0105] In the present invention, carbon dioxide functions as a carbon source for manufacturing carbon nanotubes. The specific properties of the supplied carbon dioxide, whether in the form of a gas or liquid containing carbon dioxide, are not specifically limited. For example, the supplied carbon dioxide may be obtained by separating and capturing pure carbon dioxide from the atmosphere or the like. Alternatively, the carbon dioxide may be supplied as flue gas containing carbon dioxide directly emitted from emission sources such as various industrial plants; in this case, the flue gas may be flue gas collected from the emission source or flue gas directly supplied via a direct connection to the emission source, and the present invention is not specifically limited thereto.

[0106] In addition, carbon dioxide supplied into the electrolyte may be supplied to the electrolyte within the electrolytic section, or carbon dioxide may be supplied into the molten electrolyte during the process of supplying the molten electrolyte to the electrolytic section and supplied to the electrolytic section together with the molten electrolyte, and the present invention is not particularly limited thereto.

[0107]

[0108] Meanwhile, the steps (1) and (2) described above may be performed in sequence, with one step preceding the other, or simultaneously, and the present invention is not specifically limited thereto.

[0109]

[0110] Next, as step (3), current is applied to the anode and cathode to grow carbon nanotubes using seed catalyst particles deposited on the cathode.

[0111] Specifically, regarding the reaction occurring within the electrolytic section in step (3), if we look at Figure 1a and the reaction equation below,

[0112] Cathode: CO2 + O 2- → CO3 2- (1)

[0113] Cathode: CO3 2- + 4e - → C + 3O 2- (2)

[0114] Cathode: CO3 2- + 2e - → CO + 2O 2- (3)

[0115] Cathode: M n+ + ne - → M (4)

[0116] Anode: 2O 2- → O2+ 4e - (5)

[0117] Anode: M → M n+ + ne - (6)

[0118] Reactions (1) to (4) are reactions that occur at the cathode, in which carbon dioxide supplied into the electrolyte reacts with oxygen ions to produce carbonate ions (Reaction (1)), the carbonate ions are reduced to carbon on the cathode (Reaction (2)), and a reaction to reduce to carbon monoxide (Reaction (3)) occurs competitively along with the reaction to reduce to carbon. In addition, at the cathode, metal ions dissolved at the anode are reduced and grow into seed catalyst particles (Reaction (4)).

[0119] Reactions (5) and (6) are reactions that occur at the anode, where oxygen ions in the electrolyte are generated as oxygen gas on the anode (reaction (5)), and metal dissolution occurs at the metal anode due to the high corrosiveness of fluoride ions (reaction (6)).

[0120] Meanwhile, since a high negative potential is required in the manufacturing method of the present invention, the decomposition of the fluoride salt can be ignored. For example, if the fluoride salt is lithium fluoride, the lithium fluoride is hardly decomposed into Li and F2.

[0121]

[0122] In addition, according to one embodiment of the present invention, the current applied to the cathode and anode may be applied at a current density of 10 to 500 mA / cm², preferably at a level of 15 to 45 mA / cm², and as a specific example, may be 25 mA / cm². If the applied current exceeds a current density of 45 mA / cm², crystallinity is reduced as amorphous carbon is formed on the surface of the carbon nanotubes being produced, in contrast to the excellent selectivity of carbon nanotubes among the products, and the diameter may increase while the non-uniformity of the diameter increases, making it difficult to obtain the desired miniaturized carbon nanotubes with uniform diameter. In addition, if the current density is less than 15 mA / cm², it may be difficult to obtain the desired carbon nanotubes.

[0123]

[0124] Meanwhile, when current is applied to the anode and cathode, seed catalyst particles are deposited on the cathode, and carbon nanotubes can crystallize and grow through the deposited seed catalyst particles.

[0125] Here, the seed catalyst particles may be formed by reducing an oxide of a metal or alloy on a metal or alloy anode on a cathode, thereby avoiding the process inconvenience of separately adding a metal oxide to deposit the seed catalyst particles, and having the advantage of minimizing the diameter non-uniformity between carbon nanotubes caused by the non-uniformity of the seed catalyst particles resulting from the simultaneous deposition of the separately added metal oxide on the cathode.

[0126] Meanwhile, as can be seen in FIGS. 1d, 2, and 3, the present invention facilitates anodic oxidation on a metal anode by means of fluorine anions contained in the electrolyte as described above, and since the anodic oxidation rate on the anode is controlled by controlling the current density, seed catalyst particles serving as growth points for carbon nanotubes can be formed more easily, and the size of the formed seed catalyst particles can be realized more uniformly. Consequently, it may be advantageous to further ensure diameter uniformity among the obtained carbon nanotubes. Under such desirable conditions, the average diameter of the seed catalyst particles deposited on the cathode may preferably be 1 to 50 nm, or as other examples, 1 to 25 nm, 5 to 25 nm, or 5 to 15 nm, which may be advantageous for realizing carbon nanotubes that are more miniaturized and have a uniform diameter (see FIG. 3j). Here, the diameter of the seed catalyst particle is the diameter in the case of a circular shape, and refers to the length of the minor axis in the case of an elliptical or rod-shaped shape, and the length of the minor axis refers to the length of the longest line segment perpendicular to the major axis.

[0127] In addition, as the seed catalyst particle becomes a growth point for the carbon nanotube grown on the cathode, the carbon nanotube can be provided with a metal particle of a size corresponding to the inner diameter, and such a metal particle may be derived from the seed catalyst particle, and it can be confirmed that the short axis of the metal particle provided on the carbon nanotube corresponds to the inner diameter of the carbon nanotube (see FIG. 3c). Preferably, the metal particle may be provided inside the first wall in the radial direction from the center of the carbon nanotube.

[0128]

[0129] Next, as step (4) of the present invention, the grown carbon nanotubes are collected from the cathode.

[0130] The above step (4) may appropriately employ a process for separating and obtaining carbon bodies obtained on the cathode during carbon dioxide conversion based on molten carbonate, and the present invention omits a specific description of said step.

[0131]

[0132] The method for manufacturing carbon nanotubes through carbon dioxide conversion according to one embodiment of the present invention described above can be carried out through an electrolytic cell for manufacturing small carbon nanotubes through carbon dioxide conversion, characterized by comprising, for example, an anode, a cathode on which seed catalyst particles, which are the starting point for tip growth of carbon nanotubes, are deposited on the surface, and an electrolytic unit comprising a molten electrolyte disposed between the anode and the cathode and comprising carbonate and fluoride salts; a heating unit for controlling the temperature of the electrolyte; and a power supply unit for supplying a predetermined current to the anode and the cathode of the electrolytic unit.

[0133]

[0134] First, the electrolytic unit may appropriately employ the electrolytic unit of a conventional electrolytic cell used for molten carbonate-based carbon dioxide conversion, and, as an example, may include a reaction vessel having an internal space for accommodating an electrolyte and where an electrolytic reaction takes place, and an anode and a cathode spaced apart from each other within the reaction vessel. Additionally, the reaction vessel may include at least one inlet and at least one outlet to allow for the inflow and outflow of the electrolyte and the discharge of generated oxygen gas.

[0135] Details regarding the anode, cathode, seed catalyst particles, and electrolyte included in the above-mentioned electrolytic unit that overlap with those described above are omitted.

[0136] The seed catalyst particles included in the above electrolytic unit can be deposited on the cathode after a predetermined current is supplied to the anode and the cathode.

[0137]

[0138] In addition, the electrolytic cell includes a heating unit that controls the temperature of the electrolyte.

[0139] The heating unit described above performs the function of melting the electrolyte supplied into the electrolytic unit and / or melting the electrolyte supplied into the electrolytic unit or controlling the temperature of the melted electrolyte to a set temperature, for example, 450 to 800 ℃, preferably 450 to 630 ℃, and may be a known heating unit for this purpose, and the present invention is not particularly limited thereto.

[0140]

[0141] In addition, the power supply unit supplies a predetermined current to the anode and cathode of the electrolytic unit, and can be used without limitation in the case of a power supply unit employed in a conventional electrolytic cell. In addition, the power supply unit can be controlled such that, for example, the current density of the power supplied to the anode and cathode is 10 to 500 mA / ㎠, preferably 15 to 45 mA / ㎠.

[0142]

[0143] To solve the above-mentioned problem, small carbon nanotubes derived from carbon dioxide are provided, having an average diameter of 50 nm or less, a G / D ratio of 0.3 to 1.5, and doped with fluorine (F). Such small carbon nanotubes can be manufactured through the manufacturing method described above.

[0144] The above-mentioned small carbon nanotubes have a small diameter and a uniform diameter, making them advantageous for application in related parts and industries that require higher physical properties. In addition, since they are significantly doped with fluorine (F), their conductivity is enhanced, allowing them to be used as conductive supports in various industrial fields such as semiconductors, displays, and fuel cells. Furthermore, carbonyl groups and carboxyl groups are bonded to the surface, resulting in excellent dispersibility and the ability to suppress the formation of a Solid Electrolyte Interphase (SEI) layer when applied to secondary batteries such as lithium batteries.

[0145]

[0146] Referring to FIGS. 2e and FIGS. 4f, the average diameter of the small carbon nanotubes may be 50 nm or less, preferably 20 nm or less, and more preferably 15 nm or less. As the average diameter decreases, the number of carbon nanotube strands provided per unit volume can be increased. An increase in the number of carbon nanotube strands provided per unit volume can enhance the effects attributed to carbon nanotubes in applications using carbon nanotubes. For example, when used as a conductive material for secondary batteries, it is possible to manufacture electrodes with high energy density. For another example, when used as an adsorbent material, it is advantageous to achieve high adsorption and oil absorption per unit area and unit volume of the adsorbent. Meanwhile, the carbon nanotubes of the present invention have a small average diameter, so the uniformity of the diameter is excellent.

[0147] Meanwhile, the average diameter of the above-mentioned small carbon nanotubes may be 1 nm or more or 5 nm or more, but is not limited thereto. As the average diameter increases, the durability of the carbon nanotubes increases, and the lifespan of applications using carbon nanotubes can be increased.

[0148]

[0149] The carbon nanotubes of the above-mentioned small carbon nanotubes are 1580±50cm² obtained by Raman spectroscopy using a 633 nm wavelength laser. -1 Maximum peak intensity of the G band at (I G 1330±50 cm for ) -1 Maximum peak intensity of the D band at (I D The ratio of )(I G / I D Alternatively, the average value of G / D can be 0.3 to 1.5. Crystallinity can be confirmed through the G / D ratio; the G band is a peak representing planar sp2 bonds, indicating carbon crystals without structural defects, while the D band is a peak representing sp3 bonds in carbon nanotubes, increasing when sp2 atomic bonds are broken to become sp3 bonds. Since such a D band increases when disorder or defects are generated within the carbon nanotube, the maximum peak intensity of the G band (I G Maximum peak intensity of the D band (I) for ) D The ratio of )(I G / I D The level of crystallinity can be confirmed as ), and I G / I D It can be determined that crystallinity increases as the ratio increases. A small carbon nanotube according to one embodiment of the present invention has a very small average diameter as described above, and the crystallinity of the carbon nanotube implemented can be good as the ratio of G / D is 0.3 to 1.5.

[0150]

[0151] Referring to FIG. 5, the small carbon nanotube may contain 80 to 90 at% carbon (C), 5 to 15 at% oxygen (O), and 1 to 10 at% fluorine (F).

[0152] If the carbon content is less than 80 at%, the small carbon nanotubes may have many defects. On the other hand, if the carbon content exceeds 90 at%, the oxygen content or the fluorine content may decrease, and functional groups such as the carboxyl group and / or carbonyl group described later may decrease, which may reduce dispersibility or decrease the fluorine doping effect.

[0153] If the oxygen content is less than 5 at%, functional groups such as carboxyl groups and / or carbonyl groups described later may be reduced, which may decrease dispersibility. If the oxygen content exceeds 15 at%, impurities or defects in the carbon nanotubes may increase, and their physical properties may decrease.

[0154] If the above fluorine content is less than 1 at%, the doping effect is reduced, so the occurrence of the SEI layer cannot be suppressed when applied to secondary batteries such as lithium batteries. On the other hand, if the above fluorine content exceeds 10 at%, the crystal structure of the carbon nanotube may be damaged due to the excessive fluorine content, electrical properties or chemical stability may be compromised, and mechanical strength may be insufficient.

[0155]

[0156] The above-mentioned small carbon nanotubes may include carboxyl groups and / or carbonyl groups. By including such functional groups, the carbon nanotubes can be dispersed more effectively, and if the carbon nanotubes are applied to a secondary battery such as a lithium battery, they can attract lithium ions more effectively, thereby improving the performance of the secondary battery.

[0157]

[0158] Referring to FIG. 3c, the small carbon nanotube has a multi-walled structure and may be provided with a metal particle of a size corresponding to the inner diameter. Here, the metal particle may be a particle derived from the seed catalyst particle described above.

[0159] The above-mentioned small carbon nanotube may have a multi-walled structure and may be a multi-walled carbon nanotube (MWCNT), and the average number of the multi-walls is not particularly limited as long as it does not exceed the average diameter range of the carbon nanotube. Preferably, the average number of the multi-walls may be 2 to 50, and more preferably, 4 to 25. As the number of multi-walls decreases, the diameter of the carbon nanotube may decrease, which may increase the likelihood of it becoming a small carbon nanotube.

[0160] Referring to FIG. 3k and FIG. 4h, the average wall thickness of the multiwall is not limited as long as it satisfies the average number of multiwalls and the average diameter. Preferably, it may be 0.5 to 18 nm, more preferably 1 to 9 nm, and even more preferably 3 to 5 nm. As the average wall thickness of the multiwall decreases, the diameter of the carbon nanotube may decrease, which increases the likelihood of it becoming a small carbon nanotube.

[0161] Referring to FIG. 3i, the inner diameter of the small carbon nanotube may be the inner diameter of a tube formed by the first wall in the radial direction from the center of the carbon nanotube, and since the size of the inner diameter is not limited as long as it does not exceed the average diameter and wall thickness range of the carbon nanotube described above, the present invention does not specifically limit it thereto. Preferably, it may be 15 nm or less, and more preferably, it may be 1 to 10 nm. As the inner diameter decreases, the diameter of the carbon nanotube may decrease, so the possibility of it becoming a small carbon nanotube may increase.

[0162]

[0163] In addition, the metal particle may be provided within the first wall in the radial direction from the center of the carbon nanotube, but is not limited thereto.

[0164] In addition, the small carbon nanotubes described above contain a metal, wherein the metal is at least one type of transition metal, and the metal may be included in an amount of 3 at% or less. Here, the metal may refer to the metal included in the metal particles described above, but does not refer only to the metal included in the metal particles. Meanwhile, the metal may be included in an amount of 3 at% or less relative to the total particles of the small carbon nanotubes through the acid treatment described above. If the metal content exceeds 3 at%, it may reduce the strength, flexibility, electrical properties, and surface properties of the carbon nanotubes, and when applied to secondary batteries, excessive acid treatment is required to lower the metal content, which may be disadvantageous to the performance of the carbon nanotubes.

[0165]

[0166] The present invention will be explained more specifically through the following examples, but the following examples are not intended to limit the scope of the invention and should be interpreted as being for the purpose of aiding understanding of the invention.

[0167]

[0168] <Example>

[0169] Example 1

[0170] First, lithium carbonate (Li2CO3, 98.0%, Daejeong), lithium chloride (LiCl, 98.0%, Daejeong), and lithium fluoride (LiF, 98.0%, Alfa Aesar) were prepared as electrolytes and mixed in a ratio of 20:50:30 mol%. In addition, a nickel sheet (thickness 0.2 mm) was prepared as the cathode, which is the working electrode, and Inconel 718 (thickness 0.3 mm) was prepared as the anode, which is the counter electrode. Furthermore, the working electrode was prepared by polishing with sandpaper and cleaning with deionized water, acetone, and ethanol, respectively, using ultrasound.

[0171] Subsequently, as shown in Fig. 1, the working electrode and the counter electrode were horizontally positioned in an alumina crucible located in the furnace, and the electrolyte was filled and melted at 650 °C. Then, the electrolyte temperature was changed to the target temperature of 500 °C, and pure carbon dioxide (99.9%) was continuously supplied into the furnace at a flow rate of 100 mL / min. A current of 25 mA / cm² was applied to the working electrode and the counter electrode to grow carbon nanotubes on the cathode surface for 1 hour. After a cooling process, the product was collected from the working electrode. To remove the electrolyte and impurities, the product was washed using ultrasound with diluted hydrochloric acid (1 M), vacuum filtered using filter paper with a diameter of 0.2 μm, and then dried at 60 °C to obtain the product.

[0172]

[0173] Examples 2 to 3

[0174] The product was prepared in the same manner as in Example 1, but in Example 2, the anode was changed to graphite to obtain the product, and in Example 3, the cathode was changed to Inconel 718 and the anode to graphite to obtain the product.

[0175]

[0176] Examples 4 to 5

[0177] The product was prepared in the same manner as in Example 1, but the ratio of lithium carbonate, lithium chloride, and lithium fluoride in the electrolyte was changed from 20:50:30 mol% to 20:60:20 mol% (Example 4) and 20:70:10 mol% (Example 5), respectively, to obtain the product.

[0178]

[0179] Examples 6 to 8

[0180] The product was prepared in the same manner as in Example 1, but the electrolyte temperature was changed from 500 ℃ to 550 (Example 6), 600 (Example 7), and 650 (Example 8), respectively, to obtain the product.

[0181]

[0182] <Comparative Example>

[0183] Comparative Example 1

[0184] The product was prepared in the same manner as in Example 1, but the lithium fluoride in the electrolyte was omitted, the ratio of lithium carbonate to lithium chloride was changed to 20:80 mol%, and the electrolyte temperature was changed to 550 ℃ to obtain the product.

[0185]

[0186] Comparative Examples 2 to 8

[0187] As Comparative Example 2, FloTube9000 from Cnano was purchased and prepared; as Comparative Example 3, NC7000 from Nanocyl was purchased and prepared; and as Comparative Example 4, K-Nanos-100p from Kumho Petrochemical was purchased and prepared. In addition, as Comparative Examples 5 and 6, BT1001M and BT1003M from LG Chem were purchased and prepared, respectively; as Comparative Example 7, NTeC from Chasm was purchased and prepared; and as Comparative Example 8, CNT MR99 from Carbon Nano Tech was purchased and prepared. The above Comparative Examples 2 through 8 were not manufactured through carbon dioxide conversion.

[0188]

[0189] <Experimental Example>

[0190] Experimental Example 1: Measurement of Circulating Voltage and Current

[0191] A reference electrode was introduced into the system to perform cyclic voltammetry in the 3-electrode system measurements. An Ag / AgCl (0.1% AgCl) electrode was used with AgCl (99.5%, Daejeong) and a 1 mm diameter silver (Ag) wire. The Ag / AgCl (0.1% AgCl) electrode was used after filling an alumina tube with a mixture of LiCl:Li2CO3:LiF (55.09:16.50:28.41 mol%) (internal electrolyte). A 1 mm thick silver wire was inserted into the internal electrolyte and sealed in the alumina tube to serve as the reference electrode. All cyclic voltammetry and electrolysis experiments were performed using an electrochemical workstation (Ivium-n-Stat, Ivium Technologies, BV, NL).

[0192] After introducing the above reference electrode into Examples 1 to 5 and Comparative Example 1, the cyclic voltammetry was measured using the electrochemical workstation, and the results are shown in FIGS. 1b to 1d. At this time, the scan speed was 10 mV, and the voltage range was 0.2 to -1.5 V (vs. Ag / AgCl).

[0193]

[0194] Figure 1b shows the results of Examples 1 and 2, comparing graphite and Inconel 718 as anode materials. When graphite was replaced with Inconel 718, the reduction onset potential shifted from approximately -0.9 V to approximately -0.7 V (Ag / AgCl basis), and a metal reduction peak (C2) appeared, indicating that additional metal ions originating from the anode dissolution were reduced at the cathode surface. This process can promote carbon formation through the reduction of carbonates, as evidenced by the C1 peak. During the reverse scan, the anodic reaction started at -0.8 V (Ag / AgCl basis) and increased at higher potentials. Peaks A1 and A2 correspond to the oxidation of deposited carbon (A1) and the oxidation of nickel or dissolved metal to oxides (A2), respectively.

[0195] Figure 1c shows the results of Examples 1 and 3, comparing nickel and Inconel 718 as cathode materials. The Inconel 718 electrode showed almost no C2 peak, which means that metal reduction was minimized compared to the nickel electrode. Consequently, as indicated by the C1 peak, only a small amount of carbonate reduction occurred. During backscan, Inconel 718 exhibited only a small current for the oxidation of carbon (A1) and deposited metal (A2). On the other hand, the nickel electrode started oxidation at an onset potential similar to that of the Inconel 718 electrode, approximately -0.6 V (Ag / AgCl basis), reached the A1 peak at approximately -0.1 V, and continued to increase toward the A2 peak. These results demonstrate that Inconel 718 is capable of carbon reduction while possessing higher oxidation resistance compared to nickel.

[0196] Figure 1d shows the results for Examples 1, 4, and 5 as an evaluation of the effect of LiF addition. Two distinct peaks, C1 and C2, occurred due to an increase in reduction current starting at -0.7 V (based on Ag / AgCl). As the LiF concentration increased to 30 mol% (Example 1), the reduction current at C2 increased, indicating that more metal dissolution occurred at the anode and was subsequently reduced to seed catalyst particles. Additionally, the presence of LiF slightly increased the carbonate reduction current compared to the absence of LiF (Comparative Example 1). This suggests that fluorine-based molten salts can corrode the metal layer of Inconel 718 to promote carbon formation. The dissolved metal diffuses and is reduced at the cathode surface, forming additional seed particles.

[0197]

[0198] Experimental Example 2: Electron Microscope Analysis and EDS (Energy Dispersive Spectroscopy) Mapping Analysis

[0199] For Example 1, Examples 4 to 8 and Comparative Example 1, Scanning Electron Microscope (SEM) images were obtained using a Scanning Electron Microscope (FE-SEM SU8700, Hitachi Ltd., Japan) equipped with Energy Dispersive Spectroscopy (SEM-EDS) analysis. For Example 1, Transmission Electron Microscope (TEM) images were obtained using a Transmission Electron Microscope (JEM-2100F, JEOL Ltd., Japan) equipped with Energy Dispersive Spectroscopy (TEM-EDS) analysis.

[0200] SEM images of Example 1, Examples 4 to 5 and Comparative Example 1 are shown in FIGS. 2a to 2d, the diameter distribution of the carbon products generated through FIGS. 2a to 2d is shown in FIG. 2e and Table 2, and the inner diameter distribution, seed catalyst particle diameter distribution, and average wall thickness of the generated carbon products are shown in FIGS. 3i, FIG. 3j, and FIG. 3k, respectively.

[0201] In addition, the HRTEM (high-resolution transmission electron microscopy) images of Example 1 are shown in Figures 3a to 3c at different magnifications, and the HAADF STEM images and EDS mapping analysis of Example 1 are shown in Figures 3d to 3h.

[0202] Furthermore, SEM images of Example 1, Example 6 to Example 8 are shown in FIGS. 4a to 4d, and the diameter distribution, seed catalyst particle distribution, and average wall thickness of the carbon products generated through FIGS. 4a to 4d are shown in FIGS. 4f, FIG. 4g, and FIG. 4h, respectively.

[0203]

[0204] Referring to Figures 2a through 2e, carbon nanotubes were formed as the average diameter (outer diameter) decreased from approximately 70.7 nm to 32.3 nm, 23.0 nm, and 16.0 nm, respectively, as the LiF concentration increased from 0 mol% (electrolyte temperature was 550 °C) (Comparative Example 1) to 10 mol% (Example 5), 20 mol% (Example 4), and 30 mol% (Example 1) (electrolyte temperature was 500 °C for all). This decrease in size may be due to the fact that the viscosity of the molten salt mixture increases as the LiF concentration increases. When viscosity increases, the movement of dissolved metal ions (e.g., nickel (Ni) ions, iron (F) ions) is hindered, which slows down the growth of seed catalyst particles and ultimately limits the size of the seed catalyst particles, resulting in smaller carbon nanotubes.

[0205] Figures 3a through 3e show that carbon nanotubes grew through nucleation and growth on the surface of metal seed particles. The size of these seed particles was nearly identical to the inner diameter of the carbon nanotubes. Figures 3f through 3h confirm that the seed catalyst particles are composed of Fe and Ni. This indicates that Fe and Ni have the ability to stabilize carbon nanotube growth. The lattice fringe observed in Example 1 is approximately 0.34 nm, which is consistent with the typical lattice fringe of carbon nanotubes. This result demonstrates that the presence of fluorine (F) did not interfere with the stacking process of the carbon nanotubes.

[0206] Referring to Figs. 3i and 3j, the average inner diameter of carbon nanotubes and the seed catalyst particle size decreased significantly when the LiF content increased from 0 mol% (Comparative Example 1) to 10–30 mol% at an electrolyte temperature of 500 °C. Specifically, the seed catalyst particle size decreased from approximately 15.4 nm to a range of 3.13–3.67 nm, and the average inner diameter decreased from approximately 29.0 nm to a range of 2.74–3.11 nm. These results demonstrate that the addition of LiF significantly reduces the seed catalyst particle size, thereby decreasing the average inner diameter of carbon nanotubes; this phenomenon is attributed to metal ions (e.g., Ni, Fe) in the electrolyte [specifically NiF] x and FeF x This is because it is stabilized by oxidation species such as... However, increasing the LiF concentration from 10 mol% did not significantly affect the seed catalyst particle size or average inner diameter, which suggests that there are additional factors affecting carbon nanotube growth.

[0207] Referring to Fig. 3k, the analysis of carbon nanotube wall thickness according to changes in LiF concentration revealed that as the LiF concentration increased from 10 mol% to 30 mol%, the average wall thickness decreased to 14.8 nm, 10.0 nm, and 6.5 nm, respectively, confirming a decrease in the overall average diameter of the carbon nanotubes. This is due to the carbon nanotube surface and F - Due to the strong interaction between, F - This is because it inhibits carbon nanotube growth along the wall growth direction (radial direction from the center) by adsorbing to the surface or forming CF covalent bonds. In summary, it was confirmed that LiF contributes to a reduction in the diameter of carbon nanotubes by significantly reducing the seed catalyst particle size while simultaneously inhibiting the growth of carbon nanotubes in the radial direction.

[0208] Referring to Figures 4a through 4d and Figure 4f, it can be observed that the diameter, structure, and morphology of the carbon products change with increasing electrolyte temperature. In particular, carbon nanotubes were mainly formed at low electrolyte temperatures, and the smallest average diameter of approximately 11.3 nm was observed at 550 °C, which was smaller than that of those produced at 500 °C (16.0 nm). The average diameter of the carbon nanotubes gradually increased at temperatures higher than 550 °C, exhibiting a thick-walled surface similar to amorphous carbon. At an electrolyte temperature of 650 °C, an amorphous carbon structure became the dominant product, which implies that the electrolyte temperature has a significant influence on the structure of the carbon products.

[0209] Referring to Figures 4g and 4h, changes in electrolyte temperature are closely related to the increase in seed catalyst particle size and carbon nanotube wall thickness. As can be seen in Figure 4g, the seed catalyst particle size increased to approximately 2.67, 3.13, 5.88, and 22.4 nm at temperatures of 500, 550, 600, and 650 °C, respectively. This increase may be due to a greater amount of dissolved metal forming the seed catalyst particles, as anode corrosion becomes more severe at high temperatures. Meanwhile, as the electrolyte temperature increased from 500 °C to 550 °C, the carbon nanotube wall thickness decreased from 6.5 nm to 4.2 nm, and then increased again at 600 °C. This is due to fluoride ions (F - This is because the interaction between the ) and the carbon nanotube surface (e.g., adsorption of fluoride ions on the surface or covalent bonding (CF) between fluoride and the surface) is influenced by temperature and peaks at 550 °C, and at higher temperatures (e.g., 600 °C), the wall growth of the carbon nanotubes outweighs the inhibitory effect of fluoride ions. In summary, the electrolyte temperature has a significant effect on the average diameter of the carbon nanotubes and the seed catalyst particle size, and the smallest carbon nanotubes were observed at 550 °C (Example 6).

[0210]

[0211] Experimental Example 3: Evaluation of Faraday efficiency for total product

[0212] Electrochemical impedance spectroscopy (EIS) measurements were performed on the products according to Example 1, Example 4, Example 5 and Comparative Example 1, and the Faraday efficiency for the total measured products was calculated and is shown in Fig. 2f. Likewise, electrochemical impedance spectroscopy (EIS) measurements were performed on the products according to Example 1 and Examples 6 to 8, and the Faraday efficiency for the total measured products was calculated and is shown in Fig. 4e.

[0213]

[0214] Figure 2f shows that the addition of fluoride ions affects not only the diameter of the carbon nanotubes but also the mass of the generated carbon. When the LiF concentration was 0 mol% and the temperature was 550 °C (Comparative Example 1), the Faraday efficiency (FE) of the carbon was approximately 39.7%. On the other hand, as the fluoride concentration increased, the Faraday efficiency of the carbon increased significantly, reaching an optimal value of approximately 82.5% when the LiF was 30 mol% (Example 1). This concentration appears to represent the optimal level for fluoride addition, and the increase in Faraday efficiency with increasing fluoride addition showed a tendency to gradually flatten. In other words, since LiF contributes to providing more seed catalyst particles, the Faraday efficiency increases as the LiF content increases.

[0215] As shown in Fig. 4e, the Faraday efficiency decreased to 82.5%, 73.8%, 46.4%, and 65.5% as the temperature increased from 500 ℃ to 550 ℃, 600 ℃, and 650 ℃, respectively. In other words, the electrolyte temperature has a significant effect on the Faraday efficiency, and the optimal condition for carbon products appears to be around 500 ℃.

[0216]

[0217] Experimental Example 4: Evaluation of Chrono Potentiometry (CP)

[0218] Time-based potentiometric evaluations were performed for Examples 1, 4, 5 and Comparative Example 1 using an electrochemical workstation (Ivium-n-Stat, Ivium Technologies, BV, NL), and the potential monitored for 1 hour is shown in Fig. 2g.

[0219]

[0220] As shown in Fig. 2g, increasing the fluorine concentration reduces the total energy required for CO2 decomposition, resulting in a cell potential of approximately 2.75 V at LiF concentrations of 20 mol% (Example 4) and 30 mol% (Example 1), which is lower than that at 10 mol% (Example 5). However, in the case of 30 mol% LiF (Example 1), voltage instability was observed, which appears to be due to the increased concentration of dissolved metal in the electrolyte affecting ion diffusion and the reduction process occurring at the cathode surface. In the case of Comparative Example 1, the cell potential reached -2.6 V at 550 °C, which is a lower voltage than that of the electrolyte containing LiF. This low potential demonstrates that high temperatures provide additional energy for carbonate reduction.

[0221]

[0222] Experimental Example 5: XPS (X-ray Photoelectron Spectroscopy) Analysis

[0223] High-Resolution X-ray Photoelectron Spectroscopy (HR-XPS) measurements for Example 1 were performed using an ESCALAB250 instrument (Thermo Scientific, USA), and the results are shown in Figure 5 and Table 1.

[0224]

[0225] Example 1 Element Type at%C 1s84.98O 1s9.71F 1s3.22Cr 2p0.89Fe 2p0.62Ni 2p0.57

[0226] Referring to Figure 5 and Table 1, in Example 1, carbon nanotubes were obtained using an electrolyte with a high LiF content, resulting in a high level of fluorine (F) doping content. Due to the high fluorine content, it appears that the formation of the SEI (Solid Electrolyte Interphase) layer can be suppressed when applied to secondary batteries such as lithium batteries. Additionally, the oxygen (O) content is high, which indicates that the carbon nanotubes of Example 1 contain carboxyl or carbonyl groups, and such functional groups can enhance dispersibility. Meanwhile, chromium (Cr), iron (Fe), and nickel (Ni) originate from the seed catalyst particles, and since the total content is maintained at 3 at% or less, it can be confirmed that the performance of the carbon nanotubes will be excellent.

[0227]

[0228] Experimental Example 6: Evaluation of Crystallinity

[0229] For the carbon nanotubes according to Examples 1, 4, 5, 6 and Comparative Example 1, Raman spectra were obtained by Raman spectroscopy using a laser with a wavelength of 633 nm, and the ratio of maximum peak intensity between the D band and the G band calculated therefrom (I G / I D or G / D) is shown in Table 2.

[0230]

[0231] Average Diameter (nm) Crystallinity (G / D Ratio) Example 1 (LiF 30 mol%, 500 ℃) 16.0 0.87 Example 4 (LiF 20 mol%, 500 ℃) 23.0 1.03 Example 5 (LiF 10 mol%, 500 ℃) 32.3 0.92 Example 6 (LiF 30 mol%, 550 ℃) 11.3 - Comparative Example 1 (LiF 0 mol%, 550 ℃) 70.7 0.81 Comparative Example 2 (Cnano, FloTube9000) 17.5 0.7 Comparative Example 3 (Nanocyl, NC7000) 9.5 0.6 Comparative Example 4 (Kumho Petrochemical, K-Nanos-100p) 15 0.8 Comparative Example 5 (LG Chem, BT1001M)101.5 Comparative Example 6 (LG Chem, BT1003M)13- Comparative Example 7 (Chasm, NTeC)101.48 Comparative Example 8 (Carbon Nano Tech, CNT MR99)200.9

[0232] Referring to Table 2, it can be seen that the carbon nanotubes produced by the manufacturing method of the present invention have a very small average diameter compared to existing commercial products and have excellent crystallinity with a G / D ratio of 0.3 to 1.5.

[0233]

[0234] Although embodiments of the present invention have been described above, the spirit of the present invention is not limited to the embodiments presented in this specification. Those skilled in the art who understand the spirit of the present invention may easily propose other embodiments within the scope of the same spirit by adding, changing, deleting, or adding components, and such are also to be considered to fall within the scope of the spirit of the present invention.

Claims

1. (1) A step of supplying an electrolyte containing molten carbonate and fluoride salts between an anode and a cathode in an electrolytic section; (2) A step of supplying carbon dioxide into the molten electrolyte; (3) a step of growing carbon nanotubes by applying current to the anode and cathode using seed catalyst particles deposited on the cathode; and (4) A step of collecting grown carbon nanotubes from the cathode; comprising, Method for manufacturing small carbon nanotubes through carbon dioxide conversion.

2. In Paragraph 1, A method for manufacturing small carbon nanotubes through carbon dioxide conversion, characterized in that the carbon dioxide is in the state of flue gas emitted from a carbon dioxide emission source.

3. In Paragraph 1, A method for manufacturing small carbon nanotubes through carbon dioxide conversion, characterized in that the above electrolyte further contains a chloride salt at a concentration of 45 to 75 mol% based on the total molar amount of the electrolyte.

4. In Paragraph 1, A method for producing small carbon nanotubes through carbon dioxide conversion, characterized in that the above seed catalyst particles are anode-derived oxides reduced on the cathode.

5. In Paragraph 1, Fluorine anions (F) in the electrolyte due to the above fluoride salt - A method for manufacturing small carbon nanotubes through carbon dioxide conversion, characterized in that the concentration of ) is 1 to 50 mol% and the molten electrolyte temperature is 450 to 800 ℃.

6. In Paragraph 1, A method for manufacturing small carbon nanotubes through carbon dioxide conversion, characterized in that the applied current has a current density of 10 to 500 mA / cm².

7. In Paragraph 1, The above electrolyte includes lithium chloride (LiCl) and lithium fluoride (LiF), and A method for manufacturing small carbon nanotubes through carbon dioxide conversion, characterized in that the number of moles of lithium chloride relative to the total number of moles of the electrolyte is greater than the number of moles of lithium fluoride.

8. In Paragraph 1, A method for manufacturing small carbon nanotubes through carbon dioxide conversion, characterized in that the electrolyte comprises 15 to 25 mol% lithium carbonate (Li2CO3), 45 to 75 mol% lithium chloride (LiCl), and 5 to 35 mol% lithium fluoride (LiF) relative to the total molar amount.

9. In Paragraph 1, A method for manufacturing small carbon nanotubes through carbon dioxide conversion, characterized by further including the step of washing the carbon nanotubes collected in step (4) with acid.

10. Electrolytic unit comprising: an anode; a cathode on which seed catalyst particles, which are the starting points for tip growth of carbon nanotubes, are deposited on the surface; and a molten electrolyte disposed between the anode and the cathode and comprising carbonate and fluoride salts; A heating unit for controlling the temperature of the above electrolyte; and An electrolytic cell for manufacturing small carbon nanotubes through carbon dioxide conversion, characterized by including a power supply unit that supplies a predetermined current to the anode and cathode of the above-mentioned electrolytic unit.

11. In Paragraph 10, The temperature of the heating part is set so that the temperature of the molten electrolyte is 450 to 800 ℃, and Electrolytic cell for manufacturing small carbon nanotubes through carbon dioxide conversion, characterized in that the power supply unit is set such that the current density of the power supplied to the anode and cathode is 10 to 500 mA / ㎠.

12. In Paragraph 10, Electrolyte cell for manufacturing small carbon nanotubes through carbon dioxide conversion, characterized in that the above electrolyte comprises 15 to 25 mol% lithium carbonate (Li2CO3), 45 to 75 mol% lithium chloride (LiCl), and 5 to 35 mol% lithium fluoride (LiF) relative to the total molar amount.

13. In Paragraph 10, An electrolytic cell for manufacturing small carbon nanotubes through carbon dioxide conversion, characterized in that the seed catalyst particles are deposited on the cathode after a predetermined current is supplied to the anode and the cathode, and the average diameter of the seed catalyst particles is 1 to 50 nm.

14. The average diameter is 50 nm or less, and The G / D ratio is 0.3 to 1.5, and Fluorine (F) doped, Small carbon nanotubes derived from carbon dioxide. In Article 15.14, Small carbon nanotubes characterized by containing 80 to 90 at% carbon (C), 5 to 15 at% oxygen (O), and 1 to 10 at% fluorine (F).

16. In Paragraph 14, Small carbon nanotubes characterized by containing carboxyl and / or carbonyl groups.

17. In Paragraph 14, A small carbon nanotube having a multi-walled structure and characterized by having a metal particle of a size corresponding to the inner diameter.

18. In Paragraph 17, A small carbon nanotube characterized in that the metal particle is provided inside the first wall in the radial direction from the center of the carbon nanotube.

19. In Paragraph 14, It contains metal, The above metal is at least one type of transition metal, and Small carbon nanotubes characterized by containing the above metal in an amount of 3 at% or less.

20. In Paragraph 14, A small carbon nanotube characterized by being manufactured by the manufacturing method of any one of claims 1 to 9.

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