CARBON NANOTUBE, METHOD AND DEVICE FOR PRODUCING CARBON NANOTUBE

Abstract

Disclosed herein are a carbon nanotube, a method for producing carbon nanotubes, and an apparatus for producing carbon nanotubes. The method comprises maintaining a molten salt bath in a carbon-containing environment; immersing an iron-based cathode, an anode, and a reference electrode of an electrode system into the molten salt bath; and applying a negative potential to the iron-based cathode to form the carbon nanotubes at the cathode. The apparatus for producing carbon nanotubes comprises a molten salt bath maintained in a carbon-containing environment and an electrode system having an iron-based cathode, an anode, and a reference electrode immersed in the molten salt bath, wherein applying a negative potential to the iron-based cathode forms carbon nanotubes at the cathode.The carbon nanotube comprises an iron core-shell structure with an iron nucleating tip.

Description

REFERENCE TO RELATED APPLICATION

[0001] This application claims priority to Singapore Application No. 10202251274R, filed on October 5, 2022, the contents of which are hereby incorporated by reference in their entirety for all purposes. TECHNICAL FIELD

[0002] This application relates to a carbon nanotube, a method for producing carbon nanotubes and an apparatus for producing carbon nanotubes from carbon dioxide. BACKGROUND

[0003] Carbon capture, storage, and utilization has made great strides in mitigating the growing greenhouse effect and improving the carbon cycle. Carbon dioxide can be converted into valuable carbon-rich fuels and chemical feedstocks in the gas and liquid phases using biological, photochemical, electrochemical, and other methods. The molten salt electrochemical CO2 reduction reaction (MSCO2RR) enables the liquid-phase capture of carbon dioxide (CO2) and its electrochemical reduction to valuable solid carbons such as carbon nanotubes (CNTs). However, previous attempts have produced carbon products that are generally non-uniform and represent a mixture of different carbon morphologies, e.g.,Carbon nanotubes were mixed with carbon spheres and graphite sheets, resulting in low selectivity, purity, and yield, and thus problems with product separation, hindering application and commercialization. Furthermore, such carbon products were uncompetitive compared to carbon nanotubes produced using conventional gas-phase techniques such as chemical vapor deposition (CVD), which often require high energy consumption and thus a larger carbon footprint. Therefore, the production of high-quality carbon nanotubes using MSCO2RR is of great importance. SUMMARY

[0004] In one aspect, the present application discloses a method for producing carbon nanotubes. The method comprises maintaining a molten salt bath in a carbon-containing environment; immersing an iron-based cathode, an anode, and a reference electrode of an electrode system into the molten salt bath; and applying a negative potential to the iron-based cathode to form the carbon nanotubes at the cathode.

[0005] In another aspect, the present application discloses an apparatus for producing carbon nanotubes. The apparatus comprises a cell structure with a molten salt bath maintained in a carbon-containing environment and an electrode system with an iron-based cathode, an anode, and a reference electrode immersed in the molten salt bath, wherein applying a negative potential to the iron-based cathode forms carbon nanotubes at the iron-based cathode.

[0006] According to another aspect, the present application discloses an apparatus for producing carbon nanotubes. The apparatus comprises a cell structure with a molten salt bath maintained in a carbon-containing environment, and an electrode system with an iron-based cathode and an anode immersed in the molten salt bath, wherein at least one of the iron-based cathode and the anode is configured as a reference electrode, wherein the application of a cell potential across the iron-based cathode and the anode forms carbon nanotubes at the iron-based cathode.

[0007] In another aspect of the present application, a carbon nanotube is disclosed. The carbon nanotube includes an iron core-shell structure. In some embodiments, the iron core-shell structure includes an iron nucleating tip. In some embodiments, the iron core-shell structure includes one or more carbon nanotube branches. BRIEF DESCRIPTION OF THE DRAWINGS

[0008] Various embodiments of the present disclosure are described below with reference to the following drawings: Fig. 1 is a perspective view of an apparatus for fabricating carbon nanotubes according to embodiments of the present disclosure; Fig. 2 is a detailed view of Fig. 1, which shows the electrochemical reactions in the device; Fig. 3 shows a method for manufacturing carbon nanotubes according to embodiments of the present disclosure; Fig. 4 is a perspective view of an apparatus for manufacturing carbon nanotubes according to other embodiments of the present disclosure; Fig. 5A is a perspective view of an apparatus for fabricating carbon nanotubes according to examples of the disclosure; Fig. 5B is a sectional view of a reactor according to examples of the disclosure; Fig. 5C is a cross-sectional view of a reference electrode according to examples of the disclosure; Fig. Figure 6A shows the Gibbs free energy of all possible decomposition reactions of molten Li2CO3 (melting point: 723°C) as a function of temperature, calculated using HSC Chemistry V9.5 between 723°C and 1123°C with a step size of 20°C; Fig. Figure 6B is a graph showing the Gibbs free energy of the decomposition reaction of M2CO3 (M: Li, Na, K) into M2O and C and the decomposition reaction of M2O into M; Fig. Figure 7 is a diagram showing a relationship between potential and temperature for various reactions based on thermodynamic calculations; Fig. 8 is a diagram showing an XRD pattern of the iron foil cathode according to an example; Fig. Figure 9 shows the cyclic voltammetry (CV) curves of the example of Fig. 8; Fig. 10A to 10G are characteristic scanning electron microscopy (SEM) images of carbon nanotubes prepared with different applied potentials; Fig. Figures 11A to 11D are characteristic scanning electron micrographs of carbon nanotubes prepared at different molten salt bath temperatures; Fig. 12A to 12F are characteristic scanning electron micrographs of carbon nanotubes prepared according to various examples; Fig. 13A to 13F are transmission electron microscope (TEM) images of carbon nanotubes prepared according to various examples; Fig. Figures 14A to 14D show carbon nanotubes prepared with distinct nucleating agents at their tips; Fig. 15A, Fig. 15B and Fig. 15C are SEM and TEM images, respectively, showing that the Fe-containing galvanized steel-catalyzed carbon nanotubes are multi-walled and hollow; Fig. Figure 15D is a Raman spectrum showing that the obtained multi-walled carbon nanotubes (MWCNTs) have a good degree of graphitization with an ID / IG ratio of 0.633; Fig. Figures 16A to 16C show EDS imaging results of the obtained MWCNTs with oxygen signals corresponding to oxygen-containing functional groups formed by the cathodic deoxygenation of CO3 2- formed during the process and the surface modification effect of the molten salt; Fig. Figure 16D shows XPS-O scan analysis of carbon dioxide-derived MWCNTs; Fig. Figure 17A shows the SEM images of commercial MWCNTs used for comparison; Fig. Figure 17B shows the result of the XPS characterization of O-scans of the untreated, commercial MWCNTs from Fig. 17A; Fig. Figure 17C shows the result of XPS characterization of O scans of the functionalized commercial MWCNTs; Fig. Figure 18 shows a comparison of the tensile strength between the commercially available MWCNTs in the untreated state and the MWCNTs prepared according to the present disclosure; Fig. Figure 19A shows cyclic voltammetry (CV) curves measured with a Fe cathode between -1.0 and 0.15 V (vs. Ag / Ag2SO4) at a scan rate of 50 mV / s under Ar atmosphere at 760°C with inert platinum foil as anode; Fig. Figures 19B to 19F show FESEM characterizations of carbon products obtained at different cathodic potentials; Fig. Figures 19G to 19I show the respective XRD patterns, Raman spectra, and C1s XPS spectra of carbon nanotubes obtained by MSCO2RR, which were prepared at -0.78 V (vs. Ag / Ag2SO4) at 760 °C for 30 min; Fig. 20A and Fig. Figure 20B shows the respective cell voltages and the corresponding current densities of the chronoamperometry electrolysis as a function of the applied potential, which was increased from -0.74 to -0.82 V (vs. Ag / Ag2SO4) with a step size of 0.02 V when the electrolysis temperature and time were fixed at 760 °C and 30 minutes; The Fig. 20C to 20E show the respective XRD patterns, Raman spectra and the corresponding calculated degrees of graphitization of the Fig. 20A and Fig. 20B obtained carbon products; Fig. Figure 21 is a photograph of Fe cathodes after electrolysis as a function of the applied potentials (compared to Ag / Ag2SO4); The Fig. show FETEM images of carbon nanotubes prepared at a cathodic potential of -0.78 V (vs. Ag / Ag2SO4) at 760 °C for 30 minutes; Fig. Figure 23A shows the XPS overview scan, where the C : O : Fe atomic concentration ratio determined here is 87.98 : 11.30 : 0.72%; Fig. Figure 23B shows the corresponding XPS O1s spectrum, which was approximated by four Gaussian-Lorentz peaks at 530.3, 532.3, 533.8, and 534.1 eV, which were associated with O=C, OC, C-OH bonds, and adsorbed H2O, respectively; Fig. Figure 23C shows the XPS Fe2p spectrum, which represents the primary XPS region of Fe; Fig. Figure 23D shows the FTIR spectrum of carbon nanotubes prepared at a cathodic potential of -0.78 V (vs. Ag / Ag2SO4) at 760 °C for 30 minutes; Fig. 24A and Fig. Figure 24B shows the respective cell voltages and the corresponding current densities of the chronoamperometry electrolysis as a function of the electrolysis temperature, which was increased from 760 °C to 800 °C in steps of 10 °C, when the cathodic potential and duration were fixed at -0.78 V (vs. Ag / Ag2SO4) and 30 minutes; Fig. 24C to 24E show the respective XRD patterns, Raman spectra and the corresponding calculated degrees of graphitization of the Fig. 24A and Fig. 24B obtained carbon products; Fig. Figures 25A to 25I show the FESEM images of carbon products obtained at -0.78 V (vs. Ag / Ag2SO4) for 30 minutes at 770 °C and under carbon dioxide atmosphere; Fig. Figure 26A shows the online GC analysis of the outlet gas as a function of the electrolysis temperature, which was increased from 760 to 800 °C in steps of 10 °C, when the cathodic potential and duration were fixed at -0.78 V (vs. Ag / Ag2SO4) and 30 minutes; Fig. Figure 26B shows the calculated Faraday efficiency of CO (FE CO ) as a function of the electrolysis temperature increased from 760 °C to 800 °C; The Fig. show the respective FESEM characterizations of carbon products obtained under carbon dioxide atmosphere at different electrolysis times when the cathodic potential and electrolysis temperature were fixed at -0.78 V (vs. Ag / Ag2SO4) and 760 °C; Fig. Figure 27G is a TEM image of carbon nanotubes grown for 15 minutes, taken by measurements at the location corresponding to the HAADF-STEM image in the inset; Fig. 27H to 27J are EDS images of the carbon nanotubes from Fig. 27G; The Fig. show HRTEM images of the carbon nanotubes from Fig. ; Fig. 27M is a TEM diffraction ring pattern of the carbon nanotubes from Fig. 27G; Fig. 27N is the EELS spectrum of carbon nanotubes from Fig. 27G; Fig. Figures 28A to 28F are FETEM images of cathodic carbon products obtained after chronoamperometry electrolysis at -0.78 V (vs. Ag / Ag2SO4) at 760 °C and carbon dioxide atmosphere for various times; Fig. Figures 29A to 29L are FESEM images of the Fe cathode surface after chronoamperometry electrolysis at -0.78 V (vs. Ag / Ag2SO4) at 760 °C and carbon dioxide atmosphere for different times; Fig. 30A to 30E are Fe@C core-shell structures obtained at -0.78 V (vs. Ag / Ag2SO4) at 760 °C for 15 minutes in carbon dioxide atmosphere; Fig. 30F to 30H are the corresponding EDS imaging results; Fig. 30I is the sum spectrum of the Fig. 30E shown Fe@C, where the Cu signal originates from the TEM copper mesh; Fig. Figure 31A is a schematic representation of the catalytic growth mechanism and process of Fe-catalyzed carbon nanotubes; and Fig. Figure 31B shows the Faradaic efficiency of Fe-catalyzed MSCO2RR-derived carbon and evolved CO as a function of the applied cathodic potential (compared to Ag / Ag2SO4) when the electrolysis temperature and time were fixed at 760 °C and 30 min. DETAILED DESCRIPTION

[0009] The following detailed description is made with reference to the accompanying drawings, in which details and embodiments of the present disclosure are shown for illustrative purposes. Features described in connection with one embodiment may be correspondingly applicable to the same or similar features in the other embodiments, even if they are not expressly described in those other embodiments. Additions and / or combinations and / or alternatives as described for a feature in connection with one embodiment may be correspondingly applicable to the same or similar feature in the other embodiments.

[0010] In the context of various embodiments, the articles "a", "an", and "the", "the", "the", "the" used with respect to a feature or element include a reference to one or more of the features or elements.

[0011] In the context of various embodiments, the term "approximately" or "approximately" with respect to a numerical value includes the exact value and a reasonable deviation as generally understood in the relevant technical field, e.g., of 10% of the stated value.

[0012] The term “and / or” as used here includes all combinations of one or more of the listed characteristics.

[0013] The term "potential" can generally refer to an electrical potential applied to a conducting structure, such as an electrode, relative to another conducting structure, such as another electrode. For brevity, the term "potential" can refer to one or more of the terms "electrical potential," "electrical potential energy," "potential difference," etc., as the context indicates.

[0014] The present disclosure relates to devices and methods for producing carbon nanotubes from molten salt in a carbonaceous environment by electrochemical molten salt CO2 reduction reaction (MSCO2RR). For example, the devices and methods can produce carbon nanotubes from carbon dioxide while simultaneously achieving improved material properties such as tensile strength, electrical conductivity, and capacitance of the carbon nanotubes. The produced carbon nanotubes can be used in various applications such as catalyst supports, heavy metal adsorbents, supercapacitors, and lithium-ion batteries. Furthermore, the produced carbon nanotubes can be used more widely as carbon dioxide-negative additives for structural materials, coatings, fibers, etc.The chemical synthesis of carbon nanotubes, molten salt electrochemistry, and sustainability are also described below.

[0015] Compared to carbon nanotubes produced by conventional methods, which often contain a mixture of carbon nanotubes and other carbon "junk" with irregular morphology, the carbon nanotubes produced according to embodiments of the present invention exhibit higher selectivity, purity, quality, and yield of carbon nanotubes. The present disclosure addresses the problem of uncontrolled catalytic growth of carbon nanotubes produced from carbon dioxide in molten salt.Compared with the traditional energy-intensive mass production of carbon nanotubes by CVD, the solution-phase fabrication of carbon nanotubes by MSCO2RR offers a lower carbon footprint and lower energy consumption, enabling sustainable and environmentally friendly material synthesis, reducing the dependence on conventional carbon sources.

[0016] In various embodiments, the use of the molten salt CO2 reduction reaction (MSCO2RR) for the production of carbon nanotubes is described. MSCO2RR is a technology that enables carbon dioxide to be captured and converted into value-added carbon nanostructures, including carbon nanotubes, and is thus green and environmentally friendly. MSCO2RR uses carbon dioxide, a greenhouse gas, as a carbon source for the synthesis of carbon nanostructures in a solution phase, and its anhydrous, molten electrolyte eliminates the hydrogen formation reaction, ensuring a safe, H2-free, CO2-negative carbon synthesis process. Furthermore, the carbon dioxide source can be pure carbon dioxide gas, air, or flue gas, reducing the need for extensive system shielding and sealing and making the process highly scalable compared to CVD.Furthermore, since the entire process consumes no electrolytes, complex or expensive catalysts are not required, making this a cost-effective and economical solution. Furthermore, renewable energy / energy sources can be used to power the electrolysis, as described above.

[0017] Various embodiments of the present disclosure enable direct and rapid conversion of a carbon-containing source, such as carbon dioxide gas, into carbon nanotubes using molten salt in the liquid phase in an electrolysis environment. In some embodiments, by employing a three-electrode system comprising a cathode, an anode, and a reference electrode, the cathode potential determined by thermodynamic calculations for carbon nanotube growth or carbon nanotube fabrication can be precisely controlled. This enables controllable growth of carbon nanotubes from carbon dioxide. The resulting carbon nanotubes can have high purity and a favorable degree of graphitization (~0.24 I). D / I G ) and a Faraday efficiency FE CNT of ~82.59%.

[0018] For ease of understanding and not by way of limitation, a detailed description of various embodiments of an apparatus for producing carbon nanotubes and a method for producing carbon nanotubes is provided below with reference to the accompanying figures. For the sake of brevity, various examples of the apparatus and method for producing carbon nanotubes are described below, but it is understood that the apparatus and method can be used in numerous scenarios and applications.

[0019] Fig. 1 is a schematic representation of an apparatus 100 for producing carbon nanotubes. The apparatus 100 includes a cell structure with a molten salt bath 80 and an electrode system 110. The molten salt bath 80 may be maintained in a carbon-containing environment 81, or in other words, the molten salt is maintained in contact with a carbon source or a carbon compound / material. In some embodiments, the molten salt bath 80 may be maintained in the carbon-containing environment 81 at an elevated temperature. For example, the elevated temperature may be controlled in a range of 300°C to 1000°C, preferably between 740°C and 800°C. In some embodiments, the elevated temperature may be controlled in a range of 723°C to 760°C. In some embodiments, the elevated temperature may be controlled and fixed at 760°C.

[0020] The electrode system 110 may be a three-electrode system including an iron-based cathode 112, an anode 114, and a reference electrode 116 arranged at a distance from each other. The arrangement of the three electrodes 112 / 114 / 116 in the molten salt 80 may include positioning each of the electrodes 112 / 114 / 116 at the same distance from each other. In some examples, the electrodes 112 / 114 / 116 may be arranged vertically parallel or horizontally parallel. The cathode 112, the anode 114, and the reference electrode 116 may each have the shape or form of a foil (like the cathode 112 in Fig. 1), a wire (such as the reference electrode 116 in Fig. 1) or a coil (like the anode 114 in Fig. 1). The shape or form of each of the electrodes 112 / 114 / 116 can be interchangeable and determined based on the specific applications.

[0021] The electrode system 110 may further include a controller 118 that is in signal communication with the cathode 112, the anode 114, and the reference electrode 116. The controller 118 may include a current source or a voltage source. The controller 118 may be configured to apply a negative potential to the cathode 112, the anode 114, and / or the reference electrode 116. In some embodiments, the controller 118 may be configured to apply a precise negative potential to the cathode 112, the anode 114, and / or the reference electrode 116 based on a desired or preset negative potential value.

[0022] When the cathode 112, the anode 114, and the reference electrode 116 are immersed in the molten salt bath 80, the electrode system 100 can be configured to apply a negative potential to the cathode 112 to form carbon nanotubes 82 at the cathode 112. In some embodiments, the negative potential applied to the cathode 112 is in a range of -0.58 V to -0.78 V. In some embodiments, the negative potential applied to the cathode 112 relative to the reference electrode 116 is in a range of -0.58 V to -0.78 V.

[0023] In some embodiments, the three-electrode system 110 is a stable and reliable system for precisely controlling the potential applied to the cathode 112 for the growth of carbon nanotubes, instead of controlling the current or current density applied to the cathode 112. In some embodiments, the potential applied to the cathode 112 can be determined by thermodynamic calculations. This enables the controlled growth of carbon nanotubes from carbon dioxide. The resulting carbon nanotubes exhibit excellent crystallinity, a high degree of graphitization, and a uniform morphology, achieving a yield and purity of nearly 100%. Furthermore, the morphology of the resulting carbon nanotubes is comparable to conventional multi-walled carbon nanotubes produced using CVD technology.

[0024] In some embodiments, the fabricated carbon nanotubes 82 may have an iron core-shell structure. The iron core-shell structure may include an iron nucleating tip. In some embodiments, the iron core-shell structure may include iron in a body of the iron core-shell structure. In some embodiments, the iron core-shell structure may include one or more carbon nanotube branches.

[0025] In some embodiments, the carbonaceous environment 81 may consist of maintaining the molten salt bath 80 in an atmosphere 81 containing a carbonaceous gas, e.g., pure carbon dioxide gas, flue gas, exhaust gas, air, or a carbon dioxide-argon mixed gas, or a mixture / combination thereof. The cell structure may comprise a container 140 for holding the molten salt bath 80. The container 140 may be enclosed such that the molten salt bath 80, together with the carbonaceous gas, is enclosed by the container 140, with a supply of carbonaceous gas being provided through at least one inlet of the cell structure to create the carbonaceous environment. In other embodiments, the container 140 is open, allowing a constant supply of carbonaceous gas from the surrounding atmosphere.

[0026] In alternative embodiments, the carbonaceous environment may consist of the carbonaceous gas being injected directly into the molten salt bath 80. In some embodiments, the carbonaceous gas may be injected into the molten salt bath while bubbling. For example, the carbonaceous gas may be injected into the molten salt bath via a conduit, e.g., a stainless steel or corundum tube, while bubbling.

[0027] In some embodiments, the molten salt bath 80 may contain a salt, such as an alkali metal carbonate, an alkaline earth metal carbonate, a chloride, a fluoride, a metal oxide, and a combination thereof. Specifically, the alkali metal carbonate may include Li2CO3, Na2CO3, K2CO3, Rb2CO3, and Cs2CO3. The alkaline earth metal carbonate may include BeCO3, MgCO3, CaCO3, SrCO3, or BaCO3. The chloride may include LiCl, NaCl, KCl, or CaCl2. The fluoride may include LiF, NaF, and KF. The metal oxide may include Li2O, CaO, GeO2, SnO2, CuO, Fe2O3, Fe3O4, BaGeO3, or Na2SnO3.

[0028] In some examples, the molten salt bath 80 may be held in a container 140 or a crucible. As non-limiting examples, the container 140 may be an alumina crucible, a graphite crucible, or a stainless steel container. In some embodiments, the container 140 may include a heater or be heated to maintain the molten salt bath 80 at an elevated temperature.

[0029] Fig. Figure 2 shows the mechanism of carbon dioxide capture and conversion into carbon nanotubes (CNTs) according to one embodiment. As an example, electrolysis with constant voltage and an electrolysis temperature of less than 900 °C is shown. During the electrolysis reaction, the reduction of CO3 takes place. 2- to carbon nanotubes on the cathode surface (Reaction 1 below). The oxygen formation reaction or oxygen formation by the discharge of released O2- takes place at the anode (reaction 2 below). The consumed CO3 2- is achieved by combining O 2- and gaseous CO2 (Reaction 3 below), the CO2 capture reaction, resulting in a net reduction of CO2 to carbon and O2. The net reaction is thus the electrosplitting of CO2 into solid carbon nanotubes 82 and gaseous O2 (Reaction 4 below). CO3 2- + 4e - → C + 3O 2- Reaction 1: Cathode reaction: 2O 2- → O2 + 4e - Reaction 2: Anode reaction: CO2 capture: O 2- + CO2 → CO3 2- Reaction 3: Net reaction: CO2 - C + O2 Reaction 4:

[0030] The carbon nanotubes prepared according to the present disclosure do not require any additional chemical reactions to functionalize the carbon nanotubes. This is because during the MSCO2RR process, due to the cathodic deoxygenation of CO3 2-and the surface modification effect of the molten salt, the cathodic carbon products (i.e., the carbon nanotubes) can be functionalized during the process. In some embodiments, the carbon nanotubes 82 are functionalized during the process with an oxygen-containing surface functional group or a combination of oxygen-containing surface functional groups. The oxygen-containing surface functional group can include, for example, an epoxy (CO) functional group, a hydroxyl (CO) functional group, and a carboxylate (C=O) functional group. The produced carbon nanotubes can be directly used to improve material properties such as tensile strength, electrical conductivity, and capacitance, which reduces costs and is energy-saving and environmentally friendly.

[0031] With reference to Fig. 3, a method 300 for fabricating carbon nanotubes is disclosed in accordance with various embodiments of the present disclosure. The method 300 includes, in step 310, maintaining a molten salt bath in a carbon-containing environment. The carbon-containing environment may include maintaining the molten salt bath in an atmosphere containing a carbon-containing gas, enclosing the molten salt bath with a carbon-containing gas, and injecting a carbon-containing gas into the molten salt bath. The method 300 may include, in step 320, immersing an iron-based cathode, an anode, and a reference electrode of an electrode system into the molten salt bath; and, in step 330, applying a negative potential to the iron-based cathode to form carbon nanotubes at the cathode.In some embodiments, the negative potential applied to the cathode 112 may be in a range of -0.58 V to -0.78 V.

[0032] In some embodiments, the method 300 may further comprise, in step 340, forming iron clusters on a surface of the iron-based cathode and accumulating carbon on the iron clusters. In some embodiments, the method 300 may also comprise, in step 350, diffusing the iron clusters away from the surface of the cathode to form a corresponding iron core shell structure. In some embodiments, the iron core shell structure may include an iron nucleating tip. In some embodiments, the iron core shell structure may include iron in a body of the iron core shell structure.

[0033] In some embodiments, the method 300 may further comprise functionalizing the carbon nanotubes with an oxygen-containing functional surface group or a combination of oxygen-containing functional surface groups during the method. The oxygen-containing functional surface group may include an epoxy (CO) functional group, a hydroxyl (CO) functional group, and a carboxylate (C=O) functional group. In some embodiments, the molten salt bath may be maintained at an elevated temperature in the range of 300°C to 1000°C. Preferably, the elevated temperature may be in a range between 760°C and 780°C.

[0034] In some embodiments, method 300 further includes pretreating the molten salt bath in a galvanostatic electrolysis system to remove moisture and impurities. The galvanostatic electrolysis system includes a graphite cathode and an inert anode, e.g., a platinum anode. The galvanostatic electrolysis system may, for example, include a graphite rod cathode and an inert platinum foil as the anode. The platinum foil may be polished with sandpaper after each electrolysis for reuse.

[0035] In some embodiments, the negative potential applied to the iron-based cathode 112 may be a negative potential with respect to the anode and / or a negative potential with respect to the reference electrode 116. The reference electrode 116 may help provide a reference potential for regulating the potential applied to the iron-based cathode 112. Therefore, by providing the reference electrode 116, accurate / rapid regulation of the potential applied to the cathode 116 may be achieved.

[0036] In some embodiments, the controller 118 may regulate the negative potential at the cathode 112 within a range. In some embodiments, the potential applied to the cathode 112 is a negative cathode potential relative to the reference electrode for a predetermined duration. For example, the negative cathode potential may be in a range of -6 V to -0.001 V relative to the reference electrode 116, and the predetermined duration may be in a range of 1 second to 200 hours. In some embodiments, the negative cathode potential may be in a range of -0.58 V to -0.78 V relative to the reference electrode 116, and the predetermined duration may be in a range of 30 seconds to 4 hours. As more specific examples, the predetermined duration may be 15 seconds, 30 seconds, 1 minute, 5 minutes, or 20 minutes to form carbon nanotubes 82 at the cathode 112.In some embodiments, the negative cathode potential applied to the cathode 112 may be a constant potential relative to the reference electrode 116, e.g., a fixed potential of -0.64 V relative to the reference electrode 116. In other examples, the constant potential may be -0.58 V, -0.62 V, -0.66 V, -0.70 V, -0.74 V, or -0.78 V relative to the reference electrode 116. In some embodiments, the negative cathode potential may be a stepwise controlled potential relative to the reference electrode, e.g., with a first step at -0.74 V, a next step at -0.76 V, and a final step at -0.78 V relative to the reference electrode 116.

[0037] In some embodiments, the iron-based cathode 112 may be an iron-containing cathode or a pure iron cathode. In some embodiments, the iron-based cathode 112 may be made of iron with a purity of 99.9%. In some embodiments, the cathode 112 may be a polycrystalline iron foil cathode. In other embodiments, the iron-based cathode 112 may be made of an iron alloy, 304 stainless steel, 316 stainless steel, galvanized steel, or a combination thereof.

[0038] In some embodiments, the anode 114 may be made of one material or a combination of materials, such as a metallic anode material, a metallic alloy anode material, a cermet anode material, a ceramic anode material, a non-metallic anode material, and a metal / metal oxide coated anode material. The metallic anode material may include, in particular, nickel, platinum, palladium, copper, iron, iridium, titanium, and gold. The metallic alloy anode material may include Ni 10 Cu 11 Fe, Ni 11 Fe 10Cu, stainless steel 304, and Inconel 718. The cermet anode material can be Ni-TiO2, (1-x)CaTiO3-xNi). The ceramic anode material can be SnO2 and RuO2-TiO2. The non-metallic anode material can be graphite, carbon, and glassy carbon. The metal / metal oxide coated anode material can be Pt-coated Ti, Al2O3-coated Ni, and NiO / Ni2O3 loaded Pt.

[0039] In some embodiments, the reference electrode 116 may be made of an inert conductive material such as platinum, silver, gold, and graphite. In other embodiments, the reference electrode 116 may be made of Ag / Ag2SO4 or Ag / AgCl.

[0040] In Fig. 4 illustrates embodiments of an apparatus 200 for producing carbon nanotubes. Similar to the above embodiments, the apparatus 200 includes a molten salt bath 80 and an electrode system 210. The molten salt bath 80 may be maintained in a carbon-containing environment, or in other words, the molten salt bath is maintained in contact with a carbon source or a carbon compound / material. In some embodiments, the molten salt bath 80 may be maintained in the carbon-containing environment at an elevated temperature. The electrode system 210 may be a two-electrode system including an iron-based cathode 212 and an anode 214 spaced apart from each other. The electrode system 210 may further include a controller 218 in signal communication with the iron-based cathode 212 and the anode 214.The iron-based cathode 212 and the anode 214 may be immersed in the molten salt bath 80. When the iron-based cathode 212 and the anode 214 are immersed in the molten salt bath 80, the controller 218 may be configured to apply a cell potential across the iron-based cathode 212 and the anode 214 to form carbon nanotubes 82 at the cathode 212.

[0041] In some embodiments, either the iron-based cathode 212 or the anode 214, or both, may serve as a reference electrode. For example, the anode 214 may be used as a reference electrode such that the cell potential applied to the iron-based cathode 212 is relative to the reference electrode or the anode 214 to form carbon nanotubes 82. In other examples, where the iron-based cathode 212 is coupled to the anode 214, both the iron-based cathode 212 and the anode 214 function as respective reference electrodes. Therefore, the three-electrode system described in previous embodiments is now a pseudo-three-electrode system that is in a two-electrode configuration. The two-electrode configuration is used to control the cell potential to create carbon nanotubes, alternatively controlling the cathode potential or the potential applied to the iron-based cathode.As an example of the cell potential range, the cell potential may be in a range from 0.001 V to 100 V. In some embodiments, the cell potential may be a constant potential, e.g., a potential fixed at 1.0 V. In some embodiments, the cell potential may be a stepwise regulated potential, e.g., with a first step at 0.30 V, a next step at 0.40 V, and a final step at 1.0 V. Examples

[0042] Fig. 5A and Fig. 5B show an apparatus 400 of the present disclosure according to various examples. The apparatus may include a molten salt reactor 410 with a vertical cracked-tube furnace 420 (operating temperature ≤ 1100°C ± 1°C) and may be flanged to the bottom plate of a glove box 430 having a working atmosphere of argon (99.9995%, Air Liquide). The reactor lid 413 may include tube holes for attaching and securing the electrodes 432 / 434 / 436, the thermocouple 418, and the gas inlet and outlet tubes 419. The electrodes 432 / 434 / 436, the thermocouple 418, and the gas tubes 419 may extend into the chamber of the reactor 410. The electrodes 432 / 434 / 436, the thermocouple 418 and the gas inlet and gas outlet tubes 419 can be inserted into the designated tube holes in the reactor cover 413 and securely sealed by tightening the nut around the Viton O-ring 412.The lid was then sealed to the reactor body with a Viton O-ring 415 and a KF 100 clamp. The H2O and O2 concentrations inside the glove box were both below 0.01 ppm, indicating an excellent inert atmosphere for conducting the experiments. An annular container 417 with circulating cooling water (15~25°C) can be welded to the upper part of the reactor body to reduce the operating temperature at the reactor lid in the glove box 430. Maintaining a water- and oxygen-free atmosphere can prevent the metal electrodes from oxidizing prior to electrolysis. Otherwise, a co- or two-stage reduction could occur at the cathode surface, involving the reduction of metal oxide to metal catalysts, followed or simultaneously by the reduction of CO3. 2-to carbon, which is catalyzed by the just-reduced metal catalyst. This disordered catalytic reduction of CO3 2- to carbon could disturb the morphological evolution of the formed carbon species, which would show an indirect discontinuous catalytic behavior of the metal.

[0043] Fig. 5C shows an Ag / Ag2SO4 reference electrode 450 according to examples of the disclosure. The Ag / Ag2SO4 reference electrode 450 may be a custom-made reference electrode comprising a one-sided closed alumina tube 452 and a silver (Ag) wire 454 extending from an open end of the alumina tube 452 to a closed end of the alumina tube 452. The open end of the alumina tube may be sealed with an epoxy adhesive 456. The reference electrode may further include a molding material 458, such as Li2CO3-Ag2SO4, disposed inside the alumina tube 452. The method for fabricating the Ag / Ag2SO4 reference electrode is as follows. The alumina tube (99%, OD=8 mm, ID=5 mm, L=50 cm) closed at one end can be cleaned with acetone (>99%), ethanol (99%) and deionized water (DI) respectively and dried overnight in an oven at 60 °C before being placed in the glove box.The Ag wire (99.99%, φ=1 mm, L=55 cm) can be wiped with acetone (>99%) and ethanol (99%), respectively, and returned to the glove box after cleaning. The alumina tube can be filled with Li2CO3 and Ag2SO4. 1 g of dehydrated Li2CO3 (≥ 99%, Sigma Aldrich) and 0.1 mmol of Ag2SO4 (99.999%, Alfa Aesar) were weighed, mixed, and filled into the prepared alumina tube. The alumina tube can be shaken sufficiently so that the salt reaches the bottom of the tube. The Ag wire is then inserted into the tube and penetrates deeply into the bottom of the tube. The tube filled with the Ag wire and Li2CO3-Ag2SO4 was heated to 780 °C at a heating rate of 5 °C / min and held there for 30 minutes to melt the Li2CO3-Ag2SO4 electrolyte. It was then cooled to room temperature. The open end of the alumina tube was sealed with epoxy adhesive (Araldite).After the adhesive had cured, the sealed corundum Ag2SO4 tube (. Fig. 5C) was immersed in molten Li2CO3 at 780 °C for 8 hours to activate the alumina membrane and thus achieve ionic conduction across the alumina tube wall / membrane. Finally, the activated corundum-Ag / Ag2SO4 reference electrode was cooled to room temperature and was then ready for use.

[0044] In some examples of the present disclosure, pure Li2CO3 or a salt mixture containing Li2CO3 (e.g., Li2CO3-Na2CO3-K2CO3, Li2CO3-CaCO3-K2CO3, Li2CO3-Ba2CO3) is placed in a container such as an alumina crucible, a graphite crucible, or a stainless steel container. The salt-filled container is kept in an atmosphere such as high-purity carbon dioxide, flue gas, or air and heated to an appropriate electrolysis temperature such as 760°C, 780°C, or 800°C. Three electrodes, a foil-shaped cathode made of pure iron or an Fe-containing alloy, an inert anode in a coil shape, such as a platinum electrode, a platinum-coated titanium electrode, or an SnO2 electrode, and a wire-shaped reference electrode, such as a platinum electrode or a silver electrode, are immersed in the molten salt at a distance from each other. Electrolysis occurs by applying a negative potential to the pure iron cathode, e.g.at a potential of -0.58 V, -0.60 V, -0.62 V, or -0.64 V, relative to the platinum reference electrode for a period of time, e.g., 60 minutes, 90 minutes, or 120 minutes. Afterward, the carbon dioxide-derived solid carbon nanotubes can be collected at the copper cathode. The collected carbon dioxide-derived solid carbon nanotubes are each washed with 1~7 M diluted hydrochloric acid and deionized water and collected by methods such as suction filtration or centrifugation before being dried in an oven for use. In this example, the use of high-purity Li2CO3 salt without other additives and high-purity iron foil without other elements doped therein eliminates adverse interference from other influencing factors during the growth of the carbon nanotubes. As a result, carbon dioxide-derived carbon nanotubes with high yield, purity, selectivity, and Faradaic efficiency are achieved.

[0045] A uniform molten salt such as a Li2CO3 electrolyte can be used instead of a mixed salt (e.g., LiCO3-Na2CO3-K2CO3 and CaCl2-NaCl-CaO), especially a mixed salt containing Na+ and K+. Mixed salts containing Na+ or K+ can eliminate the influence of the reduced alkaline earth metal and / or other metals from the salt on the subsequent catalytic carbon deposits and their morphologies. Fig. Figure 6A shows the results of thermodynamic calculations for the decomposition reactions of molten Li2CO3. The decomposition of liquid Li2CO3 into solid carbon and gaseous O2 required the lowest Gibbs free energy and was thus the thermodynamically most favorable reaction. Fig. Figure 6B shows the thermodynamic calculations for the reduction of Na+ and K+ to alkaline earth metals. The use of a pure transition metal cathode in conjunction with an inert Pt anode instead of alloy electrodes and the omission of catalyst additives such as Co3O4 and NiO help to avoid the different catalytic influence of the different elements on the carbon morphology during electrolysis. This can facilitate the control of carbon growth or the growth of carbon nanotubes by determining the catalytic behavior of the individual elements.

[0046] In an example of the present disclosure relating to the Fig. 6 to 14, 180 grams of Li2CO3 (≥99.0%) were filled into an alumina crucible (99%, OD=75 mm, H=90 mm). The crucible was kept in a carbon dioxide atmosphere (99.995%) and heated to the specified electrolysis temperature, including 760 °C, 780 °C, 800 °C, or 820 °C. Three electrodes, i.e., an iron foil cathode (99.9%, 2 cm*1.5 cm*0.3 mm, 3 cm 2 ), a coil-shaped platinum wire anode (99.99%, Φ 1 mm, 6 cm 2) and a quasi-reference electrode made of platinum wire (99.99%, Φ 1 mm), spaced 3 cm apart, were immersed in the molten Li2CO3. Electrolysis was performed by applying a series of fixed negative potentials (-0.58 to -0.70 V vs. Pt, 0.02 V increments) to the cathode for 90 minutes each. The solid carbon nanotubes obtained from the carbon dioxide were then collected at the iron cathode. The solid carbon nanotubes obtained from the carbon dioxide were each washed with 2 M dilute hydrochloric acid and deionized water, collected by suction filtration (0.4 µm filter membrane), and finally oven-dried overnight at 60 °C for further use.

[0047] The decomposition of Li2CO3 to Li2O, solid carbon and gaseous O2 requires, based on thermodynamic calculations, as in Fig. 7, the lowest potential and lowest energy, which is thermodynamically preferred compared to other possible competing reactions. This means that to achieve high selectivity and high efficiency in the conversion of carbon dioxide into carbon nanotubes with high yield and purity, the cell potential must be higher than the initial potential for carbon nanotube growth, but at the same time lower than the potentials for other competing reactions that would reduce the efficiency. The overall cell potential should therefore be controlled to fall within the hatched area in Fig. 7. To understand the initial cathode potential (relative to the platinum reference electrode) of the carbon deposition and to fine-tune the growth of the carbon nanotubes by tuning the cathode potential within the effective range, cyclic voltammetry (CV) curves were also determined using iron as the cathode, a coiled platinum wire as the anode, and a platinum wire as the quasi-reference electrode. As described in Fig. As shown in Figure 8, the XRD pattern of the iron foil cathode used was confirmed as amorphous iron. The obtained CV curves ( Fig. 9) showed an obvious reduction peak of C1, which exceeded the reduction peak of CO3 2- to carbon nanotubes and provided a position for the applied initial cathode potential that can lead to CNT growth.

[0048] As in Fig. As shown in Figures 10A to 10G, the processed, collected CNT products obtained at a fixed electrolysis temperature of 780 °C and different cathodic potentials were characterized by scanning electron microscopy (SEM). When the applied cathodic potential was increased in 0.02 V increments from -0.58 V to -0.70 V (with respect to the platinum reference electrode), the quality and morphology of the carbon nanotubes derived from carbon dioxide initially increased and then began to decrease. The preferred electrolysis potential for CNT growth in this example is at or around -0.64 V. This potential of -0.64 V is relatively high and can lead to carbon dioxide formation and reduce the overall current efficiency. Starting with the cathodic potential set at -0.64 V, the electrolysis temperature was varied to influence the growth of the carbon nanotubes. As shown in Fig. As can be seen in Figure 11, the quality and morphology of the carbon nanotubes improve when the electrolysis temperature is increased from 760 °C to 820 °C in 20 °C increments. However, the quality of the carbon nanotubes either deteriorates or continues to decline, so that no carbon nanotubes can be found when the temperature is 800 °C or higher. Considering energy consumption, 760 °C may be the preferred or target electrolysis temperature for carbon nanotube growth. More importantly, when electrolysis was carried out with a cathode potential of -0.64 V at 760 °C, a very high yield and purity of nearly 100% of carbon nanotubes obtained from carbon dioxide were achieved ( Fig. 12). The formed carbon nanotubes can be characterized, for example, by TEM, which shows the typical hollow features of carbon nanotubes (as in Fig. 13A to 13B). The carbon nanotubes can also be multi-walled and have good crystallinity (as shown in the Fig. shown). In addition, the CNTs exhibited significant nucleation at their tips ( Fig. 14), suggesting a tip growth mode for iron-catalyzed carbon nanotubes derived from carbon dioxide. EDS images confirmed that these tips contain iron elements, demonstrating the catalytic behavior of iron elements for MSCO2RR.

[0049] In another example of the present disclosure, which relates to Fig. 15A to 17, 160 grams of Li2CO3 (≥99.0%) were filled into an alumina crucible (99%, OD=65 mm, H=90 mm). The crucible was kept in a carbon dioxide atmosphere (99.995%) and heated to the electrolysis temperature of 780 °C. Ferrous galvanized steel (Φ 1 mm, 3 cm 2) was used as the cathode for 90 minutes with molten Li2CO3 as the molten salt. The carbon dioxide-derived solid CNTs were then peeled off the galvanized steel cathode. The collected carbon dioxide-derived solid CNTs were each washed with 2 M diluted hydrochloric acid and deionized water, collected by suction filtration (0.4 µm filter membrane), and finally oven-dried at 60 °C for 8 hours for further use.

[0050] In this example, both the SEM ( Fig. 15A) as well as the TEM images ( Fig. 15B and Fig. 15C) that the carbon nanotubes catalyzed with iron-containing galvanized steel are multi-walled tubes with typical hollow properties. The results of the Raman spectra ( Fig. 15D) showed that the obtained multi-walled carbon nanotubes (MWCNTs) had an appropriate degree of graphitization with an I D / IG -ratio of 0.633. The EDS imaging results ( Fig. 16A to 16C) showed that the obtained multi-walled carbon nanotubes contain oxygen signals originating from oxygen-containing functional groups generated by the cathodic deoxygenation of CO3 2- during the process and the surface modification effect of the molten salt.

[0051] Fig. Figure 16D shows the XPS O-scan analysis of carbon dioxide-derived MWCNTs. Fig. Figure 17A shows the SEM images of commercial MWCNTs used for comparison. The result of the XPS characterization of the O scans of the untreated commercial MWCNTs is shown in Fig. 13B. A typical solution functionalization process can be used to apply epoxy groups to the untreated commercial MWCNTs. Fig. Figure 17C shows the result of XPS characterization of O scans of the functionalized commercial MWCNTs. Fig. 16D, Fig. 17B and Fig. 17C were all approximated for CO peaks (i.e., epoxy and hydroxyl functional groups) and C=O peaks (i.e., carboxylate functional groups). Comparing the approximated XPS results of untreated ( Fig. 17B) and functionalized ( Fig. 17C) commercial MWCNTs, the intensities of the carboxylate peaks (C=O) were approximately the same before and after the functionalization process. This suggests that carboxylate groups are present in commercial MWCNTs when purchased, whereas the intensities of the epoxy / hydroxyl peaks (CO) increased after functionalization of the commercial MWCNTs. Comparing the MWCNTs obtained from carbon dioxide using molten salt ( Fig. 16D) with the functionalized commercial MWCNTs ( Fig. 17C), the molten salt samples exhibit a comparable amount of epoxy and hydroxyl functional groups and slightly increased carboxylate functional groups. This in turn suggests that during the MOCO2RR electrolysis process, the cathodic carbon products are functionalized during the process and provided with surface functional groups such as epoxy (CO), hydroxyl (CO), and carboxylate (C=O) groups, which are directed towards the cathodic deoxygenation of CO3. 2- and the surface modification effect of the molten salt.

[0052] In addition, the molten salt MWCNTs functionalized during the process were compared with commercial MWCNTs in terms of tensile strength. As described in Fig. As shown in Figure 18, the MWCNTs prepared with molten salt and functionalized during the process have slightly better tensile strength than that of the untreated commercial MWCNTs, suggesting that the carbon nanotube preparation methods of the present disclosure are not only a green and cost-effective process but also produce functionalized carbon nanotubes with improved properties. The above examples demonstrate the uniqueness of iron as a catalyst for CNTs prepared from carbon dioxide with molten salt. The synthesized MWCNTs have a yield, purity, and selectivity of nearly 100%, a good degree of graphitization, and good crystallinity.Furthermore, due to the nature of the melt synthesis during the process, the MWCNTs are functionalized with oxygen-containing functional surface groups such as epoxy (CO), hydroxyl (CO) and carboxylate (C=O) functional groups and have improved properties such as tensile strength.

[0053] Several other examples of carbon nanotube fabrication, described in the following section, demonstrate the advantage of using an iron-based cathode for carbon nanotube fabrication. MSCO2RR was performed with a three-electrode configuration, including a pure transition metal foil cathode, an inert platinum foil anode, and a Ag / Ag2SO4 reference electrode ( Fig. 5C) and a pure molten Li2CO3 electrolyte. The Fig. 5A and Fig. The apparatus shown in Figure 5B was used to prevent impurities (such as reduced alkali metals from mixed salts, mixed transition metal catalysts, and reduced metals from pre-oxidized catalysts) from interfering with catalytic growth and carbon morphology. A 99% alumina crucible with an outer diameter (OD) of 75 mm, a height (H) of 105 mm, and a wall thickness of 4 mm was attached to the reactor 410 with a support 442. The cathode and anode were each connected with a Mo lead wire (OD x length (L): 3 mm x 500 mm). The Mo wire was sealed with a 99% alumina tube (OD x inner diameter (ID) x L: 8 mm x 5 mm x 450 mm) open at both ends using alumina adhesive. All Sc to Cu transition metals (elements 21 to 29) with a nominal electrochemically active surface of about 3 cm 2were immersed in the molten salt. The current density was related to the cathodically active surface area. The inert anodes were L-shaped Pt foils (99.99%, 0.2 mm thick) with a nominal electrochemically active surface area of ​​approximately 5 cm 2immersed in the molten salt. The cathode-to-anode active surface area ratio was 3:5. Error calibration between the set temperature point and the actual temperature within the reactor chamber was performed using a K-type thermocouple (TJ36-CAXL-14G-18-CC-XSIB, Omega). The working atmosphere within the sealed reactor could be either high-purity argon (99.9995%, Air Liquide) or carbon dioxide (99.995%, Air Liquide), with both flow rates controlled by a mass flow controller. The argon flow, carbon dioxide flow, and all gas pressures in this work were 50 sccm, 100 sccm, and 1 atm, respectively. The exhaust gases from the molten salt reactor were passed into a fume hood for analysis by gas chromatography or through a bubble counter.

[0054] To synthesize carbon nanotubes using the device, 140 g of Li2CO3 (≥ 99%, Sigma Aldrich) was placed in the alumina crucible and dehydrated at 300 °C under an argon atmosphere for 24 hours. The Li2CO3 was then melted at 780 °C under a carbon dioxide atmosphere and held for 2 hours until completely melted. To further remove moisture and metallic impurities, the completely molten Li2CO3 was dehydrated for 3 hours by applying a potential of -0.6 V (vs. Ag / Ag2SO4) to a graphite rod cathode (99.995%, Φ 5 mm, 3.34 cm). 2 active surface) connected to the L-shaped inert Pt anode (5 cm 2 active surface). To obtain good oxygen evolution reaction (OER) activity, the Pt anode was polished after each electrolysis with silicon carbide sandpaper of grit #2000, #3000, or #5000 (Starcke).

[0055] The carbon dioxide electroreduction process was carried out in purified Li2CO3 using the three-electrode configuration with a pure transition metal cathode, a pure Pt foil anode, and a corundum Ag / Ag2SO4 reference electrode at the specified temperature, potential, and duration. Nine first-row transition metals, including Sc foil (99.95%, 0.3 mm thickness), Ti foil (99.7%, 0.3 mm thickness), V foil (99.99%, 0.3 mm thickness), Cr foil (99.95%, 0.5 mm thickness), Mn flakes (99.99%), Fe foil (99.9%, 0.3 mm thickness), Co foil (99.99%, 0.3 mm thickness), Ni foil (99.95%, 0.3 mm thickness), and Cu foil (99.99%, 0.3 mm thickness), were each used as cathodes for the electrolysis. All transition metal electrodes were stored in the glove box to prevent oxidation.Except for the CV measurements, which were performed under Ar atmosphere to exclude the influence of carbon dioxide, all electrolyses were carried out under carbon dioxide atmosphere.

[0056] After electrolysis was completed, the cathode with the carbon products deposited on it was lifted to the top of the reactor for natural cooling and finally removed from the reactor and glove box for further processing. The cathodic carbon products were peeled off and washed with ~2 M dilute hydrochloric acid (Sigma Aldrich) to remove frozen salts, collected by suction filtration (0.22 µm filter membrane), and finally dried overnight at 60 °C in a vacuum oven prior to characterization. For carbon products deposited within short time periods such as 30 seconds, 5 minutes, 6 minutes, 8 minutes, and 10 minutes, the metal cathodes with the carbon deposited on them were washed with a weak acid such as ~1 M dilute acetic acid (Sigma Aldrich).The weak acid removes frozen salts and, above all, prevents strong acid corrosion on the metal cathode surface, which could impair subsequent FESEM observation.

[0057] Cyclic voltammetry tests, PEIS tests, and chronoamperometry electrolysis based on the three-electrode configuration were performed using a potentiostat (SP300, Biologic) equipped with a booster (2 A, 30 V). The structures and morphologies of the carbon materials derived from carbon dioxide were investigated using a field emission scanning electron microscope (FESEM, JSM-7610 FPlus), a transmission electron microscope (TEM, JEM-2100F), and a scanning transmission electron microscope (STEM, FEI Titan). The elemental composition of the carbon materials was measured using energy-dispersive spectroscopy (EDS) on the TEM. The electron energy loss spectroscopy (EELS) spectrum of the obtained CNTs was recorded using a Gatan Tridiem HR detector in HAADF-STEM mode. Raman analysis was performed using a LabRam HR Evolution (Horiba Scientific) with a 514.53 nm Ar laser excitation source.XRD patterns of carbon products and transition metal foils were recorded using a Bruker system with Cu K-α radiation (λ = 1.5418 Å). X-ray photoelectron spectroscopy analysis of the prepared CNTs was performed using a Thermo Scientific K-Alpha XPS system (monochrome Al K-α radiation, hv = 1486.6 eV). Calibration was performed by setting the C1s peak to 284.8 eV. The functional groups bound to the CNTs were measured using a Cary 660 FTIR spectrometer (Agilent Technologies).

[0058] The results of the above-mentioned studies are presented below, demonstrating the unique suitability of the iron electrocatalyst for producing high-purity and high-yield catalysis for the production of CNTs using MSCO2RR. This may be due to the affinity for carbon and the stability of the first-row transition metal carbides (Sc to Cu, elements 21 to 29). After carbon uptake, elements 21 to 26 (Sc to Fe) formed mixed phases of transition metal and the respective carbide, with the stability of the carbide decreasing across the elements from Sc to Fe. Furthermore, elements 27 to 29 (Co, Ni, and Cu) remained metallic, forming mixed phases of metal and graphite. Therefore, iron exhibited moderate carbon adsorption energy and carbon solubility and was the critical element between stable carbide formation and non-formation (i.e., moderate stability).Based on a screening of the catalytic behavior of Sc to Cu transition metals for carbon dioxide electroreduction in the solution phase of molten Li2CO3, it was found that only iron can catalyze the growth of carbon nanotubes in the solution phase on a large scale. This demonstrates that iron is suitable for achieving uniform carbon nanotubes with high yield. This contrasts with other transition metal elements used, which only catalyze carbon spheres or soot.

[0059] In some examples, parameters for the formation of carbon nanotubes with high yield were demonstrated compared to other carbon growth methods. Cyclic voltammetry (CV) curves were performed with the iron cathode substrate ( Fig. 19A). The pure iron cathode substrate used was polycrystalline and had a ferrite phase (α-Fe) with a body-centered cubic (BCC) structure at 760 °C and 1 atm Ar atmosphere. Fig. Figure 19A shows a pair of observed reduction (C1, around -0.78 V) and oxidation (A1, around -0.28 V) features. The C1 reduction peak around -0.78 V was interpreted as the reduction peak of CO3. 2- to carbon by examining the cathodic potential from -0.70 to -0.85 V (vs. Ag / Ag2SO4). With increasing potential, the carbon deposition (qualified by XRD, Fig. 20C) on the cathode increases from none to a considerable amount, which then decreases to a smaller amount ( Fig. 21). In addition, with increasing number of CV scan cycles, more carbon was deposited on the cathode, which was reduced and oxidized during the scan, leading to gradually increased peak current densities of C1 and A1 ( Fig. 19A). The cathodic potential range between -0.74 and -0.82 V, centered around the C1 peak ( Fig. 20A and Fig. 20B), was further investigated to determine the morphology evolution and the optimal potential for CNT growth. At potentials from -0.74 to -0.78 V, only pure, uniform carbon nanotubes with an average diameter of ~80 nm were observed, with a transformation from 2D spider web to 3D island growth occurring ( Fig. 19B to 19D). At -0.80 to -0.82 V, carbon spheres mixed with carbon nanotubes were observed, indicating lower selectivity and yield of the carbon nanotubes derived from carbon dioxide ( Fig. 19E and Fig. 19F). At -0.85 V, only pure carbon spheres were observed. Thus, three potential ranges for potential-dependent cathodic carbon growth were observed: pure carbon nanotubes, mixed carbon nanotubes and carbon spheres, and pure carbon spheres.

[0060] In some examples, the potential for transitioning from pure carbon nanotube growth to mixed morphology growth was at a potential of -0.78 V, which corresponds to the C1 reduction potential. An increased driving force, such as an increased potential, can enable multiple carbon growth modes. In these examples, a potential window of 40 mV can support the growth of high-purity carbon nanotubes using MSCO2RR. This highlights the importance of potential control for achieving high purity and yield of MSCO2RR-fabricated carbon nanotubes.

[0061] The quality of potential-dependent carbon products, such as carbon nanotubes, was determined by the degree of crystallization and graphitization. The Raman spectroscopic features of carbon materials, namely the D band, the G band, and the 2D peak, were observed at approximately 1350, 1576, and 2700 cm, respectively. -1 observed ( Fig. 20D). A higher I D / I G -ratio indicates a greater presence of disordered or defective carbon structures, indicating a lower degree of graphitization, and vice versa. A stronger 2D peak was indicative of higher quality carbon nanotubes. The XRD peaks at approximately 22° and 26° corresponded to the amorphous and crystalline properties of the carbon, respectively ( Fig. 20C). The presence of the D-band indicates that the carbon nanotubes prepared from MSCO2RR contain structural distortions, defects, and incomplete graphite crystallites (i.e., amorphous portions), which is also reflected by the amorphous XRD peak at ~22°. When the potential increased from -0.74 V to -0.82 V ( Fig. 14C to 14E), the carbon material prepared with MSCO2RR showed an increasing XRD peak intensity around 26° (i.e. increased crystallinity) and a decreasing I D / I G ratio (i.e. increased degree of graphitization). The average I D / I G The ratio could reach a value of only ~0.24 at -0.78 V. Based on selectivity, yield, and quality (e.g., sharpness of the 2D peak), -0.78 V (corresponding to a total cell voltage of ~2.0 V) was the optimal potential for CNT growth at a fixed electrolysis temperature of 760 °C for MSCO2RR during 30-min reactions. Fig. 19G to 19I show XRD, Raman, and C1s-XPS spectra of the optimized CNTs. The TEM characterization ( Fig. 22A to Fig. 22H) confirmed a typical hollow, multi-walled carbon nanotube structure with good yield and quality. XPS analysis of the carbon nanotubes prepared with MSCO2RR revealed only the elements C, O, and Fe ( Fig. 23A to 23C), where the element O is mainly oxygen-containing functional surface groups on the carbon nanotubes, which are characterized by the FTIR spectrum ( Fig. 23D). These were primarily carboxyl groups, which were formed by the surface modification of molten Li2CO3 and the CO3 2- Deoxygenation at the cathode during electrolysis. It was also confirmed that the element Fe is the metallic nucleating agent.

[0062] Temperature is another parameter that affects the morphology, yield, and quality of CNTs produced with MSCO2RR, as temperature alters mass transport and reaction kinetics. In various examples, increasing the temperature from 760 °C to 800 °C resulted in an increase in cell voltage and current densities ( Fig. 24A and Fig. 24B). At 770 °C ( Fig. 25A to 25I), the carbon products obtained were pure carbon nanotubes with an average diameter of ~100 nm, as obtained at 760 °C ( Fig. 19D). However, more and more carbon spheres (CS) were formed, which became increasingly abundant between 780 °C and 800 °C. At a temperature of 800 °C, only pure carbon sphere morphologies were observed. Overall, higher temperatures resulted in a lower Gibbs free energy of the reactions, a stronger driving force, and more energy for the production of more CSs. Fig. show the temperature-dependent XRD patterns, the Raman spectra and the ratio I D / I G of the carbon products. Based on the reduced intensity of the amorphous XRD peak ( Fig. 24°C), higher temperatures can improve the crystallinity of the carbon. At 760 °C, the degree of graphitization was relatively high ( Fig. 24E). In addition, temperature-dependent CO formation was monitored by analyzing the exhaust gas using online gas chromatography. Increasing the temperature generally increased the Faradaic efficiency FE. CO ( Fig. 26A and Fig. 26B), as it lowers the reaction barrier for the competitive CO evolution reaction. Therefore, in some examples, 760 °C is identified as one of the temperatures for Fe-catalyzed MSCO2RR for carbon nanotube growth, which provides high yield and quality of carbon nanotubes with inhibited current efficiency for CO.

[0063] Elucidating the growth mechanism of Fe-catalyzed MSCO2RR to carbon nanotubes helps to understand and control the growth of carbon nanotubes. As described in Fig. 27A to 27F, in one example, during the first 30 seconds of electrolysis, metallic iron particles are formed which act as nucleating agents and emerge from the surface of the iron foil cathode ( Fig. 27A). After 5 minutes, the iron particles had diffused and grown into clusters of approximately 1 µm in size ( Fig. 27B). Each of the iron particles is located in a carbon-encapsulated Fe structure (Fe@C) ( Fig. 28B to 28C), which are characterized by the reduction of CO3 2- to carbon, catalyzed by the iron nucleating agents. After 6 minutes, the Fe@C clusters and the iron nucleating agents diffused out of the iron electrode surface, causing the Fe-catalyzed carbon nanotubes to grow and branch ( Fig. 27C). After 8 to 10 minutes, the diffusion of Fe@C clusters and iron nucleators as well as the catalytic growth and branching of the carbon nanotubes occurred simultaneously ( Fig. 27D and Fig. 27E). After 15 minutes, the Fe-catalyzed carbon nanotubes continued to grow rapidly ( Fig. 27F). It can be seen that the carbon nanotubes have one or more branches.

[0064] As in Fig. 27G to 27J, Fe-catalyzed carbon nanotube tip growth was observed rather than basal growth. The Fe@C led the growth of the carbon nanotubes, although a portion of the Fe catalyst may remain in the bodies of the carbon nanotubes. Fig. show HRTEM images, lattice stripes and the TEM diffraction ring pattern of a Fe@C catalyst particle, confirming the iron core (~0.210 nm lattice stripe) formed by a fully sp 2 -bonded multi-walled carbon shell (~0.352 nm lattice stripes) and the polycrystalline BCC α-phase of the Fe foil cathode used. After electrolysis at 760 °C, the α-phase Fe undergoes no phase changes. Fig. Figure 27N shows the HAADF-STEM and EELS spectrum of a Y-shaped carbon nanotube grown in 15 minutes, illustrating the branching of the carbon nanotube growth. Since only the K ionization edge of the carbon and the L 2,3 -edge of Fe, the carbon nanotubes were catalyzed by metallic iron instead of its oxides. Further EELS spectra and images also confirmed that the carbon nanotubes were catalyzed by metallic iron. Finally, carbon nanotubes with high yield, purity, and quality were obtained at longer growth times of 30 minutes ( Fig. 19D).

[0065] In other examples, which are in the Fig. As shown in Figures 29A to 29L, the electrolysis was carried out at a cathodic potential of -0.78 V (relative to the Ag / Ag2SO4 reference electrode) at 760 °C and under carbon dioxide atmosphere for a period of 30 seconds to 15 minutes. Fig. Figures 30A to 30H show other Fe@C core-shell structures with corresponding EDS imaging and spectral results.

[0066] The generalized growth mechanism of carbon nanotubes produced from carbon dioxide by Fe catalysts is described in Fig. 31A and comprises six steps: (i) formation and (ii) diffusion of Fe nucleating clusters, (iii) carbon accumulation and (iv) encapsulation, (v) initial CNT growth and random diffusion of Fe catalysts, and (vi) branching and rapid growth of the carbon nanotubes. In Fig.Figure 31B shows a comparison of the Faradaic efficiency for various carbon products prepared at different cathodic potentials. By applying the optimal electrolysis potential and temperature of -0.78 V and 760 °C, we can achieve optimal selectivity, purity, and quality of the CNTs with an average FE CNT and FE CO of over 82% and 15% respectively. In addition, an extension of the electrolysis time increased the CO3 2- -reduction to carbon (i.e., CNT growth) and thus increased the CNT mass yield and FE CNT For example, FE CNT after 2 and 4 hours of electrolysis, about 93% and 91% respectively.

[0067] In embodiments of the present disclosure, a breakthrough was achieved in the production of high-quality, homogeneous CNTs with high yield by MSCO2RR. Fe exclusively catalyzed CNT growth due to the moderate carbon adsorption energy, carbon solubility, and moderate stability of iron carbide compared to other carbides. Optimal potential and temperature windows were revealed for various carbon growth modes, resulting in Fe-catalyzed CNTs produced by MSCO2RR with a purity of nearly ~100%, a favorable graphitization degree of ~0.24 I D / I G and FE CNT of ~82.59% were produced.

[0068] Any examples of devices, methods, materials, or products described herein are for illustrative purposes and are not intended to be limiting or exhaustive. Modifications may be made by one skilled in the art without departing from the scope of the claimed invention.

Claims

[1] A process for producing carbon nanotubes, the process comprising: Maintaining a bath of molten salt in a carbonaceous environment; Immersing an iron-based cathode, an anode and a reference electrode of an electrode system into the molten salt bath; and Applying a negative potential to the iron-based cathode to form the carbon nanotubes at the cathode. [2] The method of claim 1, further comprising forming iron clusters on a surface of the iron-based cathode and accumulating carbon on the iron clusters. [3] The method of claim 2, comprising diffusing the iron clusters away from the surface of the iron-based cathode to form a corresponding iron core-shell structure. [4] The method of claim 3, wherein the iron core shell structure includes an iron nucleating tip. [5] The method of claim 3, wherein the iron core shell structure contains iron in a body of the iron core shell structure. [6] The method of claim 1, wherein the iron-based cathode is made exclusively of iron with a purity of 99.9%. [7] The method of claim 1, wherein the negative potential applied to the cathode is in a range of -0.001 V to -6 V. [8] A process according to claim 1, wherein the bath of molten salt is maintained at an elevated temperature of between 300°C and 1000°C. [9] The method of claim 1, further comprising functionalizing the carbon nanotubes during the process with one or a combination of oxygen-containing functional surface groups selected from the group consisting of: an epoxy (CO) functional group, a hydroxyl (CO) functional group, and a carboxylate (C=O) functional group. [10] The method of claim 1, wherein the carbonaceous environment comprises a carbonaceous gas injected into the molten salt bath. [11] The method of claim 1, wherein the carbonaceous environment comprises enclosing the molten salt bath in an atmosphere containing a carbonaceous gas. [12] A method according to any one of claims 10 or 11, wherein the carbon-containing gas is a gas selected from the group consisting of pure carbon dioxide gas, flue gas, exhaust gas, air, carbon dioxide-argon mixed gas and a combination thereof. [13] The method of claim 1, wherein the molten salt bath comprises a molten salt selected from the group consisting of an alkali metal carbonate, an alkaline earth metal carbonate, a chloride, a fluoride, a metal oxide, and a combination thereof. [14] The method of claim 13, wherein the alkali metal carbonate is selected from the group consisting of: Li2CO3, Na2CO3, K2CO3, Rb2CO3, and Cs2CO3. [15] The method of claim 13, wherein the alkaline earth metal carbonate is selected from the group consisting of: BeCO3, MgCO3, CaCO3, SrCO3, and BaCO3. [16] The method of claim 13, wherein the chloride is selected from the group consisting of: LiCl, NaCl, KCl and CaCl2. [17] The method of claim 13, wherein the fluoride is selected from the group consisting of: LiF, NaF, and KF. [18] The method of claim 13, wherein the metal oxide is selected from the group consisting of: Li2O, CaO, GeO2, SnO2, CuO, Fe2O3, Fe3O4, BaGeO3, and Na2SnO3. [19] The method of claim 1, wherein the negative potential at the iron-based cathode is controlled to a constant potential relative to the reference electrode for a predetermined period. [20] A method according to claim 1, wherein the negative potential at the iron-based cathode relative to the reference electrode is controlled stepwise over a predetermined period. [21] A method according to claim 19 or 20, wherein the predetermined duration is in a range of 1 second to 200 hours. [22] The method of claim 1 further comprises pretreating the molten salt bath in a galvanostatic electrolysis system to remove moisture and impurities. [23] Apparatus for producing carbon nanotubes, the apparatus comprising: a cellular structure with a bath of molten salt held in a carbonaceous environment; and an electrode system comprising an iron-based cathode, an anode, and a reference electrode immersed in the molten salt bath, wherein the application of a negative potential to the iron-based cathode forms carbon nanotubes at the iron-based cathode. [24] The device of claim 23, wherein the reference electrode provides a reference potential for controlling the negative potential at the iron-based cathode within a range of -0.001 V to -6 V. [25] The apparatus of claim 23, wherein the cell structure comprises at least one inlet for injecting a carbon-containing gas into the cell structure to create the carbon-containing environment. [26] The device of claim 23, wherein the iron-based cathode is made of an iron-based material selected from the group consisting of: an iron alloy, 304 stainless steel, 316 stainless steel, galvanized steel, and a combination thereof. [27] The device of claim 23, wherein the iron-based cathode is a polycrystalline iron foil cathode. [28] The apparatus of claim 23, wherein the molten salt bath comprises a molten salt selected from the group consisting of an alkali metal carbonate, an alkaline earth metal carbonate, a chloride, a fluoride, a metal oxide, and a combination thereof. [29] Device according to claim 28, wherein the alkali metal carbonate is selected from the group consisting of Li2CO3, Na2CO3, K2CO3, Rb2CO3 and Cs2CO3; the alkaline earth metal carbonate is selected from the group consisting of BeCO3, MgCO3, CaCO3, SrCO3, BaCO3; the chloride is selected from the group consisting of LiCl, NaCl, KCl, CaCl2; the fluoride is selected from the group consisting of LiF, NaF and KF; and the metal oxide is selected from the group consisting of Li2O, CaO, GeO2, SnO2, CuO, Fe2O3, Fe3O4, BaGeO3, Na2SnO3. [30] The device of claim 23, wherein the reference electrode is made of an inert conductive material selected from the group consisting of platinum, gold and graphite, or Ag / Ag2SO4 or Ag / AgCl. [31] The device of claim 23, wherein the anode is made of a material selected from the group consisting of a metal material, a metal alloy material, a cermet material, a ceramic material, a non-metal material, and a metal / metal oxide plated material. [32] Device according to claim 31, wherein the metallic material is selected from the group consisting of: nickel, platinum, palladium, copper, iron, iridium, titanium and gold; the metal alloy material is selected from the group consisting of: Ni 10 Cu 11 Fe, Ni 11 Fe 10 Cu, stainless steel 304 and Inconel 718; the cermet material is selected from the group consisting of: Ni-TiO2 and ((1-x)CaTiO3-xNi); the ceramic material is selected from the group consisting of: SnO2 and RuO2-TiO2; the non-metallic material is selected from the group consisting of: graphite, carbon and glassy carbon; and the metal / metal oxide coated material is selected from the group consisting of: Pt-coated Ti, Al2O3-coated Ni and NiO / Ni2O3-loaded Pt. [33] Apparatus for producing carbon nanotubes, the apparatus comprising: a cellular structure with a bath of molten salt held in a carbonaceous environment; and an electrode system with an iron-based cathode and an anode immersed in the bath of molten salt, wherein at least one of the iron-based cathode and the anode is configured as a reference electrode, wherein applying a cell potential across the iron-based cathode and the anode forms carbon nanotubes at the iron-based cathode. [34] The device of claim 33, wherein the cell potential is in a range of 0.001 V to 100 V. [35] The device of claim 34, wherein the cell potential is controlled to a constant potential for a predetermined period. [36] A device according to claim 34, wherein the cell potential is controlled stepwise over a predetermined period. [37] Apparatus according to any one of claims 35 or 36, wherein the predetermined duration is in a range of 1 second to 200 hours. [38] Carbon nanotube with an iron core-shell structure. [39] The carbon nanotube of claim 38, wherein the iron core shell structure contains an iron nucleating tip. [40] The carbon nanotube of claim 38, wherein the iron core shell structure contains one or more carbon nanotube branches.

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