CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit of priority to the Singapore application no. 10202251274R filed 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 of producing carbon nanotubes and an apparatus for producing carbon nanotubes from carbon dioxide.

BACKGROUND

[0003] Capturing, storing, and utilizing carbon dioxide has made great strides toward mitigating the growing greenhouse effect and augmenting the carbon cycle. Carbon dioxide may be transformed through biological, photochemical, electrochemical, and other methods into valuable gas-phase and liquid-phase multi-carbon fuels and chemical feedstocks. Molten salt electrochemical CO2 reduction reaction (MSCO2RR) enables the liquid-phase capture and electroreduction of carbon dioxide (CO2) into value-added solid-state carbons, such as carbon nanotubes (CNTs). However, previous attempts produced carbon products which were generally non-uniform and were also mixtures of different carbon morphologies, for instance: carbon nanotubes mixed with carbon spheres and graphite sheets, thus resulting in low selectivity, purity, and yield, leading to associated product separation issues, therefore hindering application and commercialization. Further, such carbon products had been uncompetitive when compared to carbon nanotubes produced by conventional gas-phase techniques such as chemical vapor deposition (CVD) methods which often require high energy consumption and hence a higher carbon footprint. Consequently, producing high-quality carbon nanotubes via MSCO2RR holds significant importance.

SUMMARY

[0004] In one aspect, the present application discloses a method of producing carbon nanotubes. The method includes maintaining a molten salt bath in a carbonaceous environment; immersing an iron-based cathode, an anode and a reference electrode of an electrode system in the molten salt bath; and applying a negative potential at 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 includes a cell structure including a molten salt bath maintained in a carbonaceous 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 at 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 includes a cell structure including a molten salt bath maintained in a carbonaceous environment; and an electrode system having 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 respective reference electrode, wherein applying 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 nucleation 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:

[0009] FIG. 1 is a perspective view of an apparatus for producing carbon nanotubes according to embodiments of the present disclosure;

[0010] FIG. 2 is a detailed view of FIG. 1 illustrating the electrochemical reactions in the apparatus;

[0011] FIG. 3 is a method for producing carbon nanotubes according to embodiments of the present disclosure;

[0012] FIG. 4 is a perspective view of an apparatus for producing carbon nanotubes according to other embodiments of the present disclosure;

[0013] FIG. 5 A is perspective view of an apparatus for producing carbon nanotubes according to examples of the disclosure;

[0014] FIG. 5B is a sectional view of a reactor according to examples of the disclosure;

[0015] FIG. 5C is a sectional view of a reference electrode according to examples of the disclosure;

[0016] FIG. 6A is a plot showing Gibbs free energy of all possible decomposition reactions of molten Li2CO3 (melting point: 723 °C) as a function of temperature, calculated by HSC chemistry V9.5 between 723 and 1123°C with a step of 20°C;

[0017] FIG. 6B is a plot showing 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; [0018] FIG. 7 is a plot showing a relationship between potential and temperature for various reactions based on thermodynamic calculations;

[0019] FIG. 8 is a chart showing a XRD pattern of the iron foil cathode according to an example;

[0020] FIG. 9 shows cyclic voltammetry (CV) curves of the example of FIG. 8;

[0021] FIGs. 10A to 10G are scanning electron microscope characterization images of carbon nanotubes produced with different potential applied;

[0022] FIGs. 11A to 11D are scanning electron microscope characterization images of carbon nanotubes produced with different temperature of the molten salt bath;

[0023] FIGs. 12A to 12F are scanning electron microscope characterization images of carbon nanotubes produced according to various examples;

[0024] FIGs. 13 A to 13F are transmission electron microscopes (TEM) images of carbon nanotubes produced according to various examples;

[0025] FIGs. 14A to 14D shows carbon nanotubes as produced with clear nucleation agents at their tips;

[0026] FIGs. 15 A, 15B and 15C are SEM image and TEM images respectively, illustrating the Fe-containing galvanized steel catalyzed carbon nanotubes have multi wall with hollow feature;

[0027] FIG. 15D is a Raman spectra results demonstrating the obtained multi wall carbon nanotubes (MWCNTs) have decent graphitization degree with an ID/IG ratio of 0.633;

[0028] FIGs. 16A to 16 C show EDS mapping results of the obtained MWCNT s with oxygen signals which corresponds to oxygen-containing functional groups formed by the in operando cathodic deoxygenation of CCE ² ' and the surface modification effect of molten salt;

[0029] FIG. 16D shows the XPS O scans analysis of carbon dioxide-derived MWCNTs;

[0030] FIG. 17A shows the SEM images of commercial MWCNTs utilized for comparison; [0031] FIG. 17B shows the XPS characterization result of O scans of the raw commercial

MWCNTs of FIG. 17 A;

[0032] FIG. 17C shows the XPS characterization result of O scans of the functionalized commercial MWCNTs;

[0033] FIG. 18 shows a comparison of tensile strength between the raw commercial MWCNTs and the MWCNTs produced according to the present disclosure;

[0034] FIG. 19A shows cyclic voltammetry (CV) curves were measured with Fe cathode between -1.0 and 0.15 V (vs. Ag/Ag2SO4) at a scan rate of 50 mV/s under an Ar atmosphere at 760°C with inert platinum foil as anode;

[0035] FIGs. 19B to 19F shows FESEM characterizations of carbon products obtained under different cathodic potentials;

[0036] FIGs. 19G to 191 shows the respective XRD pattern, Raman spectra, and Cis XPS spectrum of MSCO ₂ RR-derived carbon nanotubes obtained with -0.78 V (vs. Ag/Ag2SO4) under 760°C for 30 minutes;

[0037] FIGs. 20 A and 20B shows the respective cell voltages and corresponding current densities of chronoamperometry electrolysis as a function of the applied potential increased from -0.74 to -0.82 V (vs. Ag/Ag2SO4) with a step of 0.02 V when the electrolysis temperature and time were fixed at 760°C and 30 minutes;

[0038] FIGs. 20C to 20E shows the respective XRD patterns, Raman spectra, and the corresponding calculated graphitization degrees of carbon products obtained in FIGs. 20A and 20B;

[0039] FIG. 21 is a photograph of post-electrolysis Fe cathodes as a function of applied potentials (vs. Ag/Ag2SO4);

[0040] FIGs. 22A to 22H shows FETEM images of carbon nanotubes produced at a cathodic potential of -0.78 V (vs. Ag/Ag2SO4) under 760°C for 30 minutes; [0041] FIG. 23A shows the XPS survey scan wherein the C: O: Fe atomic concentration ratio determined here was 87.98: 11.30: 0.72%;

[0042] FIG. 23B shows the corresponding XPS Ols spectrum fitted by four Gaussian - Lorentzian peaks centered at 530.3, 532.3, 533.8, and 534.1 eV, which were associated with O=C, O-C, C-OH bonds, and adsorbed H2O respectively;

[0043] FIG. 23C shows the XPS Fe2p spectrum which is the primary XPS region of Fe;

[0044] FIG. 23D shows the FTIR spectrum of carbon nanotubes produced at a cathodic potential of -0.78 V (vs. Ag/Ag2SO4) under 760°C for 30 minutes;

[0045] FIGs. 24A and 24B shows the respective cell voltages and corresponding current densities of chronoamperometry electrolysis as a function of the electrolysis temperature increased from 760°C to 800°C with a step of 10°C, when the cathodic potential and time were fixed at -0.78 V (vs. Ag/Ag2SO4) and 30 minutes;

[0046] FIGs. 24C to 24E shows the respective XRD patterns, Raman spectra, and the corresponding calculated graphitization degrees of carbon products obtained in FIGs. 24A and 24B;

[0047] FIG. 25A to 251 shows the FESEM images of carbon products obtained at -0.78 V (vs. Ag/Ag2SO4) for 30 minutes under 770°C and carbon dioxide atmosphere;

[0048] FIG. 26A shows the online-GC analysis of the outlet gas as a function of the electrolysis temperature increased from 760 to 800°C with a step of 10°C, when the cathodic potential and time were fixed at -0.78 V (vs. Ag/Ag2SO4) and 30 minutes, respectively;

[0049] FIG. 26B shows the calculated Faradaic efficiencies of CO (FECO) as a function of the electrolysis temperature increased from 760°C to 800°C;

[0050] FIGs. 27A to 27F shows the respective FESEM characterizations of carbon products obtained under carbon dioxide atmosphere at various electrolysis duration, when cathodic potential and electrolysis temperature were fixed at -0.78 V (vs. Ag/Ag2SO4) and 760°C, respectively;

[0051] FIG. 27G is a TEM image of carbon nanotubes grown for 15 minutes, which was acquired by taking measurements at the location corresponding to the HAADF-STEM image inset;

[0052] FIGs. 27H to 27J are EDS mappings of the carbon nanotubes of FIG. 27G;

[0053] FIGs. 27K to 27L are HRTEM of the carbon nanotubes of FIG. 27G;

[0054] FIG. 27M is a TEM diffraction ring pattern of the carbon nanotubes of FIG. 27G;

[0055] FIG. 27N is the EELS spectrum of of the carbon nanotubes of FIG. 27G;

[0056] FIGs. 28A to 28F are FETEM images of cathodic carbon products obtained after the chronoamperometry electrolysis at -0.78 V (vs. Ag/Ag2SO4) under 760°C and carbon dioxide atmosphere for various durations;

[0057] FIGs. 29A to 29L are FESEM images of the Fe cathode surface after the chronoamperometry electrolysis at -0.78 V (vs. Ag/Ag2SO4) under 760°C and carbon dioxide atmosphere for various durations;

[0058] FIGs. 30A to 30E are Fe@C core-shell structures obtained @ -0.78 V (vs. Ag/Ag2SO4) under 760°C and carbon dioxide atmosphere for 15 minutes;

[0059] FIGs. 30F to 30H are the corresponding EDS mapping results;

[0060] FIG. 301 is the sum spectrum of the Fe@C shown in FIG. 30E, wherein the Cu signal originated from the TEM copper mesh;

[0061] FIG. 31A is a schematic diagram of the catalytic growth mechanism and process of Fe-catalyzed carbon nanotubes; and

[0062] FIG. 3 IB shows the Faradic efficiency of Fe-catalyzed MSCO ₂ RR-derived carbon and evolved CO, as a function of applied cathodic potential (vs. Ag/Ag2SO4) when electrolysis temperature and time were fixed at 760°C and 30 minutes. DETAILED DESCRIPTION

[0063] The following detailed description is made with reference to the accompanying drawings, showing details and embodiments of the present disclosure for the purposes of illustration. Features that are described in the context of an embodiment may correspondingly be applicable to the same or similar features in the other embodiments, even if not explicitly described in these other embodiments. Additions and/or combinations and/or alternatives as described for a feature in the context of an embodiment may correspondingly be applicable to the same or similar feature in the other embodiments.

[0064] In the context of various embodiments, the articles “a”, “an” and “the” as used with regard to a feature or element include a reference to one or more of the features or elements.

[0065] In the context of various embodiments, the term “about” or “approximately” as applied to a numeric value encompasses the exact value and a reasonable variance as generally understood in the relevant technical field, e.g., within 10% of the specified value.

[0066] As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items.

[0067] The term “potential” may generally be used to refer to an electric potential applied to a conductive structure, such as an electrode, in relative to or with reference to another conductive structure, such as another electrode. For the sake of brevity, the term "potential" may be used to refer to any one or more of the terms "electric potential", "electrical potential ", “electrical potential energy”, "potential difference", etc., as will be understood from the context. [0068] The present disclosure relates to apparatus and methods for producing carbon nanotubes from molten salt in a carbonaceous environment via molten salt electrochemical CO2 reduction reaction (MSCO2RR). As examples, the apparatus and methods may produce carbon nanotubes from carbon dioxide while realizing improved material properties such as tensile strength, electrical conductivity, and capacitance to the carbon nanotubes. The carbon nanotubes as produced may be used in various applications such as catalyst supports, heavy metal adsorbents, supercapacitors, and lithium-ion batteries. Further, the carbon nanotubes as produced may be more widely adopted as carbon dioxide-negative additives to structural materials, coatings, fibers, etc. The chemical synthesis of the carbon nanotubes, the molten salt electrochemistry and sustainability are also described hereinbelow.

[0069] In comparison to carbon nanotubes produced via conventional ways which often include a mixture of carbon nanotubes and other carbon ‘junk’ with irregular morphologies, the carbon nanotubes produced according to embodiments of the present invention have higher selectivity, purity, quality, and yield of carbon nanotubes. The present disclosure addresses the problem of uncontrolled catalytic growth of carbon nanotubes converted from carbon dioxide via molten salt. Compared to the conventional energy-intensive mass production of carbon nanotubes by CVD, solution-phase bon nanotubes production by MSCO2RR offers a lower carbon footprint and energy consumption and allows for sustainable and eco-friendly material synthesis, reducing the reliance on traditional carbon sources.

[0070] In various embodiments, utilizing molten salt CO2 reduction reaction (MSCO2RR) for production of carbon nanotubes is described. MSCO2RR is an enabling technology to capture and transfer carbon dioxide into value-added carbon nanostructures, including carbon nanotubes, thus is green and environmentally friendly. MSCO2RR utilizes carbon dioxide, a greenhouse gas, as the carbon source for synthesizing carbon nanostructures in a solution phase, and its water-free molten electrolyte eliminates the hydrogen evolution reaction, thus ensuring a safer, Fh-free, CO ₂ -negative carbon synthesis process. Further, the carbon dioxide source may be pure carbon dioxide gas, air, or flue gas, which may reduce the need for extensive system shielding and sealing, making it highly scalable compared to CVD. Moreover, since the whole process does not consume electrolytes, there is no need for any complex or expensive catalysts, therefore enabling a low-cost and economical solution. In addition, renewable energy/energy sources may be utilized to drive the electrolysis as described above.

[0071] Various embodiments of the present disclosure enable a direct and fast conversion of a carbonaceous source, such as carbon dioxide gas, to carbon nanotubes using a liquid phase molten salt in an electrolysis setting. In some embodiments, by employing a three-electrode system, which includes a cathode, an anode, and a reference electrode, the thermodynamiccalculation-guided cathode potential for carbon nanotube growth or carbon nanotube production may be precisely controlled. As a result, the controllable growth of carbon dioxidederived carbon nanotube is realized. The obtained carbon nanotubes may possess high purity and a favorable graphitization degree (-0.24 ID/IG), and FECNT of -82.59%.

[0072] To aid understanding and not to be limiting, a detailed description of various embodiments of an apparatus for producing carbon nanotubes and a method for producing carbon nanotubes will be described below with reference to the appended figures. For the sake of brevity, the following will describe various examples with respect to the apparatus and method for producing carbon nanotubes, but it will be understood that the apparatus and method may be used in multiple scenarios and applications.

[0073] FIG. 1 is a schematic diagram of an apparatus 100 for producing carbon nanotubes. The apparatus 100 includes a cell structure including a molten salt bath 80 and an electrode system 110. The molten salt bath 80 may be maintained in a carbonaceous environment 81, or in other words, the molten salt is maintained in contact with a carbon source or carbon compound/material. In some embodiments, the molten salt bath 80 may be maintained in the carbonaceous environment 81 at an elevated temperature. For example, the elevated temperature may controlled within the range of 300°C to 1000°C, preferably between 740°C to 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 fixed and controlled at 760°C.

[0074] The electrode system 110 may be a 3 -electrode system which includes an iron-based cathode 112, an anode 114 and a reference electrode 116 disposed space apart from each other. The placement of the three electrodes 112/114/116 in the molten salt 80 may include positioning each of the electrodes 112/114/116 at equidistant. In some examples, the electrodes 112/114/116 may be vertically parallel or horizontally parallel. The cathode 112, the anode 114 and the reference electrode 116 may each be in the form or shape of a foil (such as the cathode 112 in FIG. 1), a wire (such as the reference electrode 116 in FIG. 1) or a coiled type (such as the anode 114 in FIG.l). It may be appreciated that the form or shape of each of the electrodes 112/114/116 may be interchangeable and determined based on the specific applications.

[0075] The electrode system 110 may further include a controller 118 in signal communication with the cathode 112, the anode 114 and the reference electrode 116. The controller 118 may include a power source or a voltage source. The controller 118 may be configured to apply a negative potential on 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 on the cathode 112, the anode 114 and/or the reference electrode 116 based on a targetted or preset negative potential value.

[0076] When the cathode 112, the anode 114 and the reference electrode 116 are immersed in the molten salt bath 80, the electrode system 100 may be configured to apply a negative potential at the cathode 112 for forming carbon nanotubes 82 at the cathode 112. In some embodiments, the negative potential applied on the cathode 112 is in a range of -0.58 V to -0.78 V. In some embodiments, the negative potential applied on the cathode 112 relative to the reference electrode 116 is in a range of -0.58 V to -0.78 V. [0077] In some embodiments, the three-electrode system 110 enables a stable and reliable system for precise control of the potential applied on the cathode 112 for the carbon nanotube growth, instead of a control of current or current density applied on the cathode 112. In some embodiments, the potential applied on the cathode 112 may be thermodynamic-calculation- guided. As a result, the controllable growth of carbon dioxide-derived carbon nanotubes is realized. The obtained carbon nanotubes have excellent crystallinity and graphitization degree, uniform morphology, approaching 100% yield and purity. Further, morphologies of the carbon nanotubes as obtained are comparable to conventional multiwall carbon nanotubes produced via the CVD technique.

[0078] In some embodiments, the carbon nanotubes 82 produced may include an iron core shell structure. The iron core shell structure may include an iron nucleation 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.

[0079] In some embodiments, the carbonaceous environment 81 may correspond to the molten salt bath 80 being held in an atmosphere 81 with a carbonaceous gas, such as one of or a mixture/combination of: pure carbon dioxide gas, flue gas, exhaust, air, and carbon dioxide/argon mixed gas. The cell structure may include a container 140 for holding the molten salt bath 80. The container 140 may be enclosed such that the molten salt bath 80 is confined with the carbonaceous gas collectively by the container 140, wherein a supply of carbonaceous gas is provided through at least one inlet of the cell structure to create the carbonaceous environment. In other embodiments, the container 140 has is open thus allowing a constant supply of carbonaceous gas from the surrounding atmosphere.

[0080] In alternative embodiments, the carbonaceous environment may include the carbonaceous gas being directly injected into the molten salt bath 80. In some embodiments, the carbonaceous gas may be injected via bubbling into the molten salt bath. For example, the carbonaceous gas may be bubbled into the molten salt bath via a conduit, such as a stainless- steel tube or a corundum tube.

[0081] In some embodiments, the molten salt bath 80 may include 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, ISfeCO ₃ , K2CO3, Rb2CO3, and CS2CO3. The alkaline-earth metal carbonate may include BeCO ₃ , MgCO ₃ , CaCO ₃ , SrCO ₃ , BaCO ₃ . The chloride may include LiCl, NaCl, KC1, CaCh. The fluoride may include LiF, NaF, and KF. The metal oxide may include Li2O, CaO, GeO ₂ , SnO ₂ , CuO, Fe2C>3, FesC , BaGeCh, Na2SnO3.

[0082] 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 may be heated such that the molten salt bath 80 is maintained at the elevated temperature.

[0083] FIG. 2 illustrates the mechanism of carbon dioxide (carbon dioxide) capturing and the conversion of carbon dioxide into carbon nanotubes (CNT) according to an embodiment. As an example, a constant voltage electrolysis and an electrolysis temperature of lower than 900°C is shown as an example. Under the electrolysis reaction, the reduction of CO ₃ ² ' into carbon nanotubes takes place at the cathode surface (reaction 1 below). The oxygen evolution reaction or the oxygen evolution through the discharge of released O ² ' takes place at the anode (reaction 2 below). The decomposed CO ₃ ² ' is replenished by the combination between O ² ' and gaseous CO2 (reaction 3 below) which is the CO ² capture reaction, resulting in the net electro reduction of CO2 into carbon and O2. Therefore, the net reaction is the electrosplitting of CO2 into solid solid carbon nanotubes 82 and gaseous O ² (reaction 4 below). Reaction 1 : Cathode reaction: CO ₃ ² '+ 4e' ^ C + 3O ²

Reaction 2: Anode reaction: 2O ² ' O ₂ + 4e"

Reaction 3: CO2 Capture: O ² + CO2 ^ CO ₃ ² '

Reaction 4: Net reaction: CO2 C + O ₂

[0084] The carbon nanotubes as produced according to the present disclosure does not require additional chemical reactions to functionalize the carbon nanotubes. The reason being during the MSCO2RR process, due to cathodic deoxygenation of CO ₃ ² ' and the surface modification effect of molten salt, cathodic carbon products (i.e., carbon nanotubes) may be in operando functionalized. In some embodiments, the carbon nanotubes 82 is functionalized in operando with one or a combination of a surface oxygen-containing functional group. As examples, the surface oxygen-containing functional group may include an epoxide(C-O) functional group, a hydroxyl (C-O) functional group and a carboxylate (C=O) functional group. The fabricated carbon nanotubes may be directly applied to improve properties of materials such as tensile strength, electrical conductivity, and capacitance, which reduces costs and is energy-saving and environmentally friendly.

[0085] Referring to FIG. 3, a method 300 for producing carbon nanotubes is disclosed according to various embodiments of the present disclosure. The method 300 includes in stage 310, maintaining a molten salt bath in a carbonaceous environment. The carbonaceous environment may include holding the molten salt bath in an atmosphere with a carbonaceous gas; confining the molten salt bath with a carbonaceous gas; and injecting a carbonaceous gas into the molten salt bath. The method 300 may also include in stage 320, immersing an ironbased cathode, an anode and a reference electrode of an electrode system in the molten salt bath; and in stage 330, applying a negative potential at the iron-based cathode to form carbon nanotubes at the cathode. In some embodiments, the negative potential applied on the cathode 112 may be in a range of -0.58V to -0.78V.

[0086] In some embodiments, the method 300 may further include in stage 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 include in stage 350, diffusing the iron clusters away from the surface of the cathode to form a respective iron core shell structure. In some embodiments, the iron core shell structure may include an iron nucleation tip. In some embodiments, the iron core shell structure may include iron in a body of the iron core shell structure.

[0087] In some embodiments, the method 300 may further include functionalizing in operando the carbon nanotubes with one or a combination of a surface oxygen-containing functional group. The surface oxygen-containing functional group may include an epoxide(C- O) functional group, a hydroxyl (C-O) functional group and a carboxylate (C=O) functional group. In some embodiments, the molten salt bath may be maintained at an elevated temperature within a range of 300°C to 1000°C. Preferably, the elevated temperature may be in a range between 760°C to 780°C.

[0088] In some embodiments, the method 300 further includes pre-processing the molten salt bath in a galvanostatic electrolysis system to remove moisture and impurities. The galvanostatic electrolysis system includes a graphite cathode and a inert anode, such as a platinum anode. For example, the galvanostatic electrolysis setup may include a graphite rod cathode and an inert platinum foil serving as the anode. The platinum foil may be polished with sandpapers after each electrolysis for re-use.

[0089] In some embodiments, the negative potential applied at the iron-based cathode 112 may be a negative potential relative to the anode and/or a negative potential relative to the reference electrode 116. The reference electrode 116 may aid in providing a reference potential for controlling the potential applied at the iron-based cathode 112. Therefore, with the provision of the reference electrode 116, accurate/timely control of the potential applied at the cathode 116 may be achieved.

[0090] In some embodiments, the controller 118 may control the negative potential at the cathode 112 within a range. In some embodiments, the potential applied on the cathode 112 is a negative cathode potential in relative to the reference electrode over a predetermined duration. For example, the negative cathode potential may be in a range of -6V 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 the range of -0.58 V to -0.78 V, relative to the reference electrode 116, and the predetermined duraiton may be in the 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 on the cathode 112. In some embodiments, the negative cathode potential applied on the cathode 112 may be a constant potential relative to the reference electrode 116, e.g. a fixed -0.64 V potential 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 potential in relative to the reference electrode, e.g., first step at -0.74 V, next step at -0.76 V and final step at -0.78 V in relative to the reference electrode 116.

[0091] In some embodiments, the iron-based cathode 112 may be a cathode containing iron or a pure iron cathode. In some embodiments, the iron-based cathode 112 may be made from iron with a purity of 99.9%. In some embodiments, the cathode 112 may be a poly poly crystalline iron foil cathode. In other embodiments, the iron-based cathode 112 may made from one of or a combination of: an iron alloy, 304 stainless steel, 316 stainless steel, galvanized steel or a combination thereof. [0092] In some embodiments, the anode 114 may be made from one of or a combination of a material such as: a metal anode material, a metal alloy anode material, a cermet anode material, a ceramic anode material, a non-metal anode material, and a metal/metal oxide plated anode material. Specifically, the metal anode material may include nickel, platinum, palladium, copper, iron, iridium, titanium, and gold. The metal alloy anode material may include NiioCunFe, NiuFeioCu, 304 stainless steel, and Inconel 718. The cermet anode material may be Ni-TiO ₂ , (l-x)CaTiO3-xNi). The ceramic anode material may include SnO ₂ and RuO ₂ -TiO ₂ . The non-metallic anode material may include graphite, carbon and glassy carbon. The metal/metal oxide plated anode material may include Pt coated Ti, AI2O3 coated Ni, and NiO/Ni2O3 loaded Pt.

[0093] In some embodiments, the reference electrode 116 may be made from an inert conductive material such as platinum, silver, gold and graphite. In other embodiments, the reference electrode 116 may be made from Ag/Ag2SO4 or Ag/AgCl.

[0094] Referring to FIG. 4, embodiments of an apparatus 200 for producing carbon nanotubes are disclosed. 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 carbonaceous environment, or in other words, the molten salt is maintained in contact with a carbon source or carbon compound/material. In some embodiments, the molten salt bath 80 may be maintained in the carbonaceous environment at an elevated temperature. The electrode system 210 may be a 2-electrode system which includes an iron-based cathode 212 and an anode 214 disposed space 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.

[0095] In some embodiments, one or both of the iron-based cathode 212 and the anode 214 may act as a respective reference electrode. For example, the anode 214 may be taken as a reference electrode such that the cell potential applied on the iron-based cathode 212 is relative to the reference electrode or the anode 214 to form carbon nanotubes 82. In other examples, as the iron-based cathode 212 is coupled to the anode 214, both the iron-based cathode 212 and the anode 214 act as respective reference electrodes. Therefore, the three-electrode system as described in previous embodiments is now a pseudo-three-electrode system which is in a two- electrode configuration. The two-electrode configuration is used to control the cell potential to produce carbon nanotubes this in alternative to controlling the cathode potential or the potential applied on the iron-based cathode. As an example of the cell potential range, the cell potential may be in a range of 0.001V to 100V. In some embodiments, the cell potential may be a constant potential, e.g. a fixed 1.0 V. In some embodiments, the cell potential may be a stepwise potential, e.g., first step at 0.30 V, next step at 0.40 V and final step at 1.0 V.

Examples

[0096] FIG. 5A and 5B illustrates an apparatus 400 of the present disclosure according to various examples. The apparatus may include a molten salt reactor 410 with a vertical split tube furnace 420 (working temperature < 1100°C ± 1°C) and may be flange sealed 416 to the bottom panel of a glovebox 430, which has a working atmosphere of Argon (99.9995%, Air Liquid) gas. The reactor lid 413 may include tube holes for placing and fixing electrodes 432/434/436, thermocouple 418, gas inlet, and outlet tubes 419. The electrodes 432/434/436, thermocouple 418 and gas tubes 419 may extend into the chamber of the reactor 410. The electrodes 432/434/436, thermocouple 418, gas inlet, and outlet tubes 419 may be inserted into designated tube holes on the reactor lid 413, securely sealed by tightening the nut around the Viton O-ring 412. The lid was then further sealed to the reactor body using a Viton O-ring 415 and KF100 clamp. H2O and O2 concentrations inside the glovebox were both lower than 0.01 ppm, indicating an excellent inert atmosphere to conduct experiments. A circulating cooling water (15~25°C) ring tank 417 may be welded onto the upper part of the reactor body to reduce the operating temperature on the reactor lid in the glovebox 430. Here, by maintaining a water-and oxygen-free atmosphere, the metal electrodes may be prevented from oxidizing before electrolysis. Otherwise, the cathode surface may undergo a co-or two-step- reduction, which refers to the reduction of metal oxide to metal catalysts, and the reduction of COs ² ' to carbon catalyzed by the just reduced metal catalyst may happen in sequence or simultaneously. This disordered catalytic reduction of CO, ² ' to carbon may interfere with the morphology evolution of carbon species formed, showing an indirect discontinuous metal catalytic behaviour.

[0097] FIG. 5C illustrates 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 which includes a one-ended-closed alumina tube 452 and a silver (Ag) wire 454 passing through 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 by epoxy adhesive glue 456. The reference electrode may further include a form material 458 such as Li2CO3-Ag2SO4 disposed interior of the alumina tube 452. The process of making the Ag/Ag2SO4 reference electrode is as follows. The one-end-closed alumina tube (99%, OD=8 mm, ID=5 mm, L=50 cm) may be cleaned with acetone (>99%), ethanol (99%), and deionized (DI) water respectively, and dried in an oven at 60°C overnight before putting into the glovebox. The Ag wire (99.99%, cp= 1 mm, L=55 cm) may be wiped with acetone (>99%) and ethanol (99%) respectively, and transferred back to the glovebox after cleaning. The Ag wire may be filled with Li2CO3-Ag2SO4-filled. 1g dehydrated Li2CO3 (> 99%, Sigma Aldrich) and 0.1 mmol Ag2SO4 (99.999%, Alfa Aesar) were weighed respectively, mixed, and filled into the prepared alumina tube. The alumina tube may be shaken sufficiently such that the salt reaches the tube bottom. Thereafter, the Ag wire was inserted into and went deep into the bottom of the tube. The Li2CO3-Ag2SO4-filled Ag-wire- inserted alumina tube was heated to 780°C at a heating rate of 5°C /min, maintained there for 30 min to melt the Li2CO3-Ag2SO4 electrolyte, then cooled down to the room temperature. The epoxy adhesive glue (Araldite) was used to seal the open end of the alumina tube. After the glue has set, the sealed corundum Ag/Ag2SO4 tube (Supplementary Fig. 3) was soaked in molten Li2CO3 at 780°C for 8 hrs. to activate the alumina membrane and thus to achieve the ionic conduction via the alumina tube wall/membrane. Finally, the activated corundum Ag/Ag2SO4 reference electrode was then cooled to room temperature for ready use.

[0098] In some examples of the present disclosure, Pure Li2CO3 or a salt mixture comprising Li ₂ CO ₃ (e g., Li ₂ CO3-Na2CO3-K ₂ CO3, Li ₂ CO3-CaCO3-K ₂ CO3, Li ₂ CO3-Ba2CO ₃ ) is disposed into a container such as an alumina crucible, a graphite crucible or a stainless-steel container. The container filled with salt is maintained in an atmosphere such as high purity carbon dioxide, flue gas or air, and heated to a corresponding electrolysis temperature such as 760°C, 780°C or 800°C. Three electrodes, a pure iron or alloy containing Fe cathode in a foil form, inert anode in a coil form such as a platinum electrode, a platinum-coated Titanium electrode or a SnO ₂ electrode, and a reference electrode in a wire form such as a platinum electrode or a silver electrode, are immersed into the molten salt spaced apart from each other. The electrolysis is performed by applying a negative potential on the pure iron cathode, for example, 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 duration of time, such as 60 minutes, 90 minutes or 120 minutes. Thereafter, the carbon dioxidederived solid carbon nanotubes may be collected at the copper cathode. The collected carbon dioxide-derived solid carbon nanotubes are washed by 1~7 M diluted hydrochloric acid and deionized water respectively, collected by methods such as suction filtration or centrifugation, before being dried in the oven for usage. In this example, by using high purity Li2CO3 salt without any other additives and high purity iron foil without any other elements doped within, excludes the adverse interferences from other influencing factors during the carbon nanotubes growth. As a result, carbon dioxide-derived carbon nanotubes with high yield, purity, selectivity and Faradaic efficiency are achieved.

[0099] A unitary molten salt, such as a Li2CO3 electrolyte may be used in place of a mixed salt (e.g., LiCO3-Na2CO3-K2CO3 and CaCh-NaCl-CaO), especially mixed salts contain Na+ and K+. The mixed salts with NA+ or K+ may exclude the interference from the reduced alkali earth metal and/or other metals from salt on the subsequent catalytic carbon depositions and its morphologies. FIG. 6A shows the results of thermodynamic calculations of decomposition reactions of molten Li2CO3. The decomposition of liquid Li2CO3 into solid carbon and gaseous O2 demanded the lowest Gibbs free energy and thus was the most thermodynamically favorable reaction. FIG. 6B further shows the thermodynamic calculations of the reductions of Na+ and K+ to alkali earth metals. Additionally, using a pure transition metal cathode coupled with an inert Pt anode instead of alloy electrodes, and avoiding the use of catalyst additives such as CO3O4 and NiO, helps to avoid different catalytic behaviors from different elements towards carbon morphology during electrolysis. This may facilitate controlling carbon growth or carbon nanotube growth, by determining each element's carbon catalytic behavior.

[00100] In an example of the present disclosure, referring to FIGs. 6 to 14, 180 grams of Li2CO3 (>99.0%) was filled in an alumina crucible (99%, OD=75 mm, H=90 mm). The crucible was maintained in a carbon dioxide atmosphere (99.995%), and heated to the fixed electrolysis temperature respectively including 760°C, 780°C, 800°C or 820°C. Thereafter, three electrodes, i.e., an iron foil cathode (99.9%, 2 cm* 1.5 cm*0.3 mm, 3 cm2), a coiled platinum wire anode (99.99%, <I> 1 mm, 6 cm2), and a platinum wire quasi reference electrode (99.99%, <I> 1 mm), were immersed into the molten Li2CO3 with 3 cm apart from each other. The electrolysis was conducted by applying a series of fixed negative potential (-0.58 to -0.70 V vs Pt, 0.02V step) respectively onto the cathode for 90 min. Thereafter, the carbon dioxide-derived solid carbon nanotubes were be collected at the iron cathode. The collected carbon dioxidederived solid carbon nanotubes were washed with a 2 M diluted hydrochloric acid and deionized water respectively, and collected by suction filtration (0.4 pm filter membrane) and finally dried at 60°C overnight in the oven for usage.

[00101] First, based on thermodynamic calculations, as shown in FIG. 7, the decomposition of Li2CO3 into Li2O, solid carbon and gaseous O2 requires the lowest potential and energy which is thermodynamically preferred compared to other possible competing reactions. This indicates that to achieve a high selectivity and current efficiency of carbon dioxide conversion into high yield and purity carbon nanotubes, the cell potential is to be higher than the onset potential for carbon nanotube growth but at the same time lower than potentials for other competing reactions which will decrease the current efficiency. Namely, the overall cell potential should be controlled to fall in the shaded area in FIG. 7. Furthermore, to understand the onset cathode potential (in relative to platinum reference electrode) of the carbon deposition and fine-tune the growth of carbon nanotubes by tuning the cathodic potential within the effective range, cyclic voltammetry (CV) curves were obtained by using iron as cathode, coiled platinum wire as anode and platinum wire as the quasi -reference electrode. As shown in FIG. 8, the XRD pattern of the utilized iron foil cathode was confirmed to be amorphous iron. The obtained CV curves (FIG. 9) were found with an obvious reduction peak of Cl which was the reduction peak of CO, ² ' to carbon nanotubes, providing a position of onset cathode potential applied which may result in CNT growth.

[00102] As shown in FIGs. 10A to 10G, the processed collected CNT products obtained at the fixed electrolysis temperature of 780°C and varied cathode potentials were characterized by scanning electron microscope (SEM). As the applied cathode potential increased from -0.58 V to -0.70 V (in relative to the platinum reference electrode) with a step of 0.02 V, the quality and morphologies of carbon dioxide-derived carbon nanotubes initially increases and began to decrease thereafter. The preferred electrolysis potential for CNTs growth for this example is at or around -0.64 V. This potential of -0.64V is relatively high and may induce carbon dioxide evolution and decrease the overall current efficiency. Based on fixing the cathodic potential at -0.64 V, the electrolysis temperature was also varied to modulate the carbon nanotube growth. As shown in FIG. 11, as the electrolysis temperature increased from 760°C to 820°C with a step of 20°C, the carbon nanotube quality and morphology improves from 760°C to 780°C. However, the carbon nanotubes quality either deteriorates or furthermore diminishes such that no carbon nanotubes may be found, when the temperature is greater or equal to 800°C. Taking energy consumption into consideration, 760°C may be the preferred or targeted electrolysis temperature for carbon nanotube growth. More importantly, when the electrolysis was conducted with a cathode potential of -0.64 V at 760°C, very large scale, near -100% yield and purity of the carbon dioxide-derived carbon nanotubes were produced (FIG. 12). As examples, the carbon nanotubes as formed may be characterized by TEM which shows typical hollow feature of carbon nanotubes (as shown in FIGs. 13 A to 13B). The carbon nanotubes may also be of the multiwall type ones with good crystallinity (as shown in FIGs. 13C to 13F). Furthermore, the CNTs had clear nucleation agents at their tips (FIG. 14), indicating a tipgrowth mode for iron-catalyzed carbon dioxide-derived carbon nanotubes. EDS mapping confirmed those tips contain iron elements, demonstrating the catalytic behaviors of iron element for MSCO2RR.

[00103] In another example of the present disclosure, referring to FIGs. 15A to 17, 160 grams of Li ₂ CO ₃ (>99.0%) was filled into an alumina crucible (99%, OD=65 mm, H=90 mm). The crucible was maintained in a carbon dioxide atmosphere (99.995%), and heated to the electrolysis temperature of 780°C. Thereafter, iron-containing galvanized steel (<b 1 mm, 3 cm ² ) was used as the cathode for 90 minutes with molten Li2CO ₃ as the molten salt bath. Thereafter, the carbon dioxide-derived solid CNTs were peeled off from the galvanized steel cathode. The collected carbon dioxide-derived solid CNTs were washed by 2 M diluted hydrochloric acid and deionized water respectively, collected by suction filtration (0.4 pm filter membrane) and finally dried at 60°C for 8 hours in the oven for usage.

[00104] In this example, both SEM (FIG. 15 A) and TEM (FIGs. 15B and 15C) characterization images confirmed that iron-containing galvanized steel catalyzed carbon nanotubes were types of multi-wall ones with typical hollow features. Raman spectra (FIG. 15D) results demonstrated that the obtained multi-wall carbon nanotubes (MWCNTs) have decent graphitization degree with an ID/IG ratio of 0.633. EDS mapping results (FIGs. 16A to 16C) showed the obtained multi -wall carbon nanotubes contain oxygen signals which were oxygen-containing functional groups and caused by the in operando cathodic deoxygenation of CO ₃ ² ' and the surface modification effect of molten salt.

[00105] FIG. 16D shows the XPS O scans analysis of carbon dioxide-derived MWCNTs. FIG. 17A shows the SEM images of commercial MWCNTs utilized for comparison. The XPS characterization result of O scans of the raw commercial MWCNTs are shown in FIG. 13B. A typical functionalization solution process may be used to add epoxy groups onto the raw commercial MWCNTS. FIG. 17C shows the XPS characterization result of O scans of the functionalized commercial MWCNTs. FIGs. 16D, 17B and 17C were all fitted for C-0 (i.e., epoxide and hydroxyls functional group) peaks and C=O (i.e., carboxylate functional groups) peaks. By comparing fitted XPS results of raw (FIG. 17B) and functionalized (FIG. 17C) commercial MWCNTs, carboxylates peak (C=O) intensities were fairly consistent before and after the functionalization process. This pointing to carboxylate groups being present in commercial MWCNTs when purchased, with intensities of epoxides/hydroxyls peaks (C-O) increased after the commercial MWCNTs were functionalized. By comparing carbon dioxide- derived MWCNTs via molten salt (FIG. 16D) and functionalized commercial MWCNTs (FIG. 17C), molten salt samples had comparable amount of epoxide and hydroxyl functional groups and slightly increased carboxylate functional groups. This indicating again that during the MOCO2RR electrolysis process, cathodic carbon products are in operando functionalized and added with surface functional groups such as epoxide(C-O), hydroxyl (C-O) and carboxylate (C=O) functional groups which were due to cathodic deoxygenation of CO, ² ' and the surface modification effect of molten salt.

[00106] Moreover, the in operando functionalized molten salt MWCNTs were compared with commercial ones in terms of the property of tensile strength. As shown in FIG. 18, that molten salt derived in operando functionalized MWCNTs have a slightly better tensile strength than that of raw commercial MWCNTs, indicating the methods of producing carbon nanotubes of the present disclosure is not only a green and cost-effective process, but also produce functionalized carbon nanotubes with improved properties. The above examples demonstrated the uniqueness of iron in catalyzing CNTs converted from carbon dioxide via molten salt. The synthesized MWCNTs have near -100% yield, purity and selectivity, decent graphitization degree and crystallinity. Furthermore, due to the nature of synthesis in molten melt, the MWCNTs are functionalized in operando with surface oxygen-containing functional groups added such as epoxide(C-O), hydroxyl (C-O) and carboxylate (C=O) functional groups and have improved properties such as tensile strength.

[00107] Various other examples of the carbon nanotube production as disclosed in the following section further demonstrates the advantage of using iron-based cathode for production of carbon nanotubes. MSCO2RR was conducted using a three-electrode configuration comprising a pure transition metal foil cathode, an inert platinum foil anode, an Ag/Ag2SO4 reference electrode (FIG. 5C), and a pure molten Li2CO3 electrolyte. The apparatus as shown in FIG. 5A and 5B was used to prevent impurities (such as alkali metals reduced from mixed salts, mixed transition metal catalysts, and metals reduced from pre-oxidized catalysts) from affecting the carbon’s catalytic growth and morphology. An alumina (99%) crucible with a dimension of 75 mm outer diameter (OD), 105 mm height (H), and 4 mm (wall thickness) was fixed to the reactor 410 by a support 442. The cathode and anode were assembled to a Mo lead wire (OD x length (L): 3 mm x 500 mm), respectively. The Mo wire was sealed to a two- end open alumina (99%) tube (OD x inner diameter (ID) x L: 8 mm x 5 mm x 450 mm) with alumina adhesive cement. All Sc to Cu transition metals (elements 21 to 29) with around 3 cm ² of nominal electrochemically active surface area were immersed in molten salt. The current density was in terms of the cathodic active surface area. The inert anodes were L-shaped Pt foil (99.99%, 0.2 mm thickness) and around 5 cm ² of nominal electrochemically active surface area immersed in the salt. The cathode to anode active surface area ratio was 3: 5. The error calibration between the set temperature point and the practical temperature inside the reactor chamber was performed with a Type K thermocouple (TJ36-CAXL-14G-18-CC-XSIB, Omega). The working atmosphere within the sealed reactor can be either high purity argon (99.9995%, Air Liquid) or carbon dioxide (99.995%, Air Liquid) in which both flow rates were controlled by a mass flow controller, respectively. Argon gas flow rate, carbon dioxide flow rate, and all gas pressures in this work were 50 seem, 100 seem, and 1 atm respectively. The outlet gases from the molten salt reactor were directed to gas chromatography for analysis or to a bubbler into a fume hood.

[00108] To synthesize carbon nanotubes via the apparatus, 140 g Li2CO3 (> 99%, Sigma Aldrich) was filled into the alumina crucible, dehydrated at 300°C for 24 hours under argon atmosphere. Thereafter, the Li2CO3 was melted at 780°C under carbon dioxide atmosphere and held for 2 hours to be fully melted. To further remove moisture and metallic impurities, the complete melted Li2CO3 was pre-electrolyzed for 3 hours by applying a -0.6 V (vs. Ag/Ag2SO4) potential onto a graphite rod cathode (99.995%, <I> 5 mm, 3.34 cm ² active surface area) which was coupled to the L-shaped inert Pt anode (5 cm ² active surface area). To maintain good OER activity, the Pt anode was polished with grit #2000, #3000, and #5000 silicon carbide sandpapers (Starcke) respectively for re-usage after each electrolysis.

[00109] The carbon dioxide electroreduction process was performed in the purified Li2CO3 using the three-electrode configuration of a pure transition metal cathode, pure Pt foil anode, and corundum Ag/Ag2SO4 reference electrode at the specified temperature, potential, and time. 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 flake (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 used as cathodes for electrolysis respectively. All transition metal electrodes were stored in the glovebox to avoid oxidation. Besides CV measurements which were performed under the Ar atmosphere to exclude the influence of carbon dioxide, all electrolysis was conducted under the carbon dioxide atmosphere.

[00110] Upon completion of the electrolysis, the cathode with carbon products deposited thereon was lifted to the upper part of the reactor for natural cooling and finally taken out of the reactor and glovebox for subsequent processing. The cathodic carbon products were peeled off, and washed with ~2 M diluted hydrochloric acid (Sigma Aldrich) to remove frozen salts, collected by suction filtration (0.22 pm filter membrane), and finally dried at 60°C overnight in the vacuum oven before characterizations. For carbon products deposited within short durations such as 30 seconds, 5 minutes, 6 minutes, 8minutes, and 10 minutes, the metal cathodes with carbon attached thereon were washed by a weak acid such as ~1 M diluted acetic acid (Sigma Aldrich). The weak acid removes frozen salts and more importantly avoids severe acid corrosion onto the metal cathode surface which may affect subsequent FESEM observation. [00111] Cyclic voltammetry tests, PEIS tests, and chronoamperometry electrolysis based on the three-electrode configuration were conducted with a potentiostat (SP300, Biologic) equipped with a booster (2 A, 30 V). The structures and morphologies of obtained carbon dioxide-derived carbon materials were characterized by a field-emission scanning electron microscope (FESEM, JSM-7610 FPlus), a transmission electron microscope (TEM, JEM- 21 OOF), and a scanning transmission electron microscope (STEM, FEI Titan). Elemental compositions of carbon materials were measured with energy dispersive spectroscopy (EDS) mapping equipped on TEM. The electron energy loss spectroscopy (EELS) spectrum of obtained CNTs was obtained with a Gatan Tridiem HR detector in HAADF-STEM mode. Raman analysis was conducted on LabRam HR Evolution (Horiba Scientific) with the excitation source of a 514.53 nm Ar laser. XRD patterns of carbon products and transition metal foils were collected with Bruker system by Cu K-a radiation ( = 1.5418 A). X-ray photoelectron spectroscopy analysis of as-produced CNTs was conducted by a Thermo Scientific K-Alpha XPS system (monochrome Al Ka, hv = 1486.6 eV). Calibration was performed by setting the Cis peak at 284.8 eV. The functional groups attached to the CNTs were measured with Cary 660 FTIR spectrometer (Agilent Technologies).

[00112] Results from the exampls above are presented below, which demonstrate the unique suitability of iron electrocatalyst for producing high-purity and high-yield catalysis of MSCCERR-derived CNTs. This may be due to the affinity for carbon and the stability of first- row transition metal carbides (Sc to Cu, elements 21 to 29). Upon carbon incorporation, elements 21 to 26 (Sc to Fe) formed mixture phases of transition metal and the respective carbide, with decreasing stability of the carbide as the element moved from Sc to Fe. In addition, elements 27 to 29 (Co, Ni, and Cu) remained metallic and formed metal and graphite mixture phases. 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 catalytic behaviors of Sc to Cu transition metals for carbon dioxide electroreduction under solution-phase molten Li2CO3, it was observed that only iron could catalyze solution-phase large-scale carbon nanotubes growth. This shows the suitability of iron for promoting high-yield uniform carbon nanotubes. This is in contrast to other utilized transition metal elements only catalyzed carbon spheres or carbon black.

[00113] In some examples, parameters for producing high-yield carbon nanotubes formation over other carbon growth modes was demonstrated. Cyclic voltammetry (CV) curves were performed with the iron cathode substrate (FIG. 19 A). The utilized pure iron cathode substrate was poly crystalline and ferrite phase (a-Fe) with a body-centered cubic (BCC) structure under 760°C and 1 atm Ar atmosphere. FIG. 19A showed a pair of observed reduction (Cl, located around -0.78 V) and oxidation (Al, located around -0.28 V) features. The Cl reduction peak around -0.78 V was determined to be the reduction peak of CO ₃ ² ' to carbon by screening the cathodic potential from -0.70 to -0.85 V (vs. Ag/Ag2SO4). As the potential was increased, carbon deposition (qualified via XRD, FIG. 20C) on the cathode increases from none to substantial which then decreases to a reduced amount (FIG. 21). Moreover, as the CV scan cycles were increased, more carbon was deposited on the cathode, which was reduced and oxidized during the scan, resulting in gradually enhanced peak current densities of Cl and Al peak (FIG. 19A). The cathodic potential range between -0.74 to -0.82 V centered around the Cl peak (FIG. 20 A and 20B) was further examined to determine the morphology evolution and optimal potential for CNT growth. At potentials of -0.74 to -0.78 V, only pure uniform carbon nanotubes with average diameters of ~80 nm were observed, with a transformation from 2D spiderweb to 3D island growth (FIGs. 19B to 19D). At -0.80 to -0.82 V, carbon sphere growth modes were observed mixed with carbon nanotubes, indicating decreased selectivity and yield of carbon dioxide-derived carbon nanotubes (FIG. 19E and 19F). At -0.85 V, only pure carbon spheres were observed. Thus, three potential regimes were observed for potential-dependent cathodic carbon growth: pure carbon nanotubess, mixed carbon nanotubes and carbon spheres, and pure carbon spheres.

[00114] In some examples, the potential for the transition from pure carbon nanotube growth to mixed morphologies growth was at a potential of -0.78 V, corresponding with the Cl reduction peak potential. An increased driving force, such as increased potential, may enable multiple carbon growth modes. In these examples, a potential window of 40 mV may support high-purity carbon nanotube growth via MSCO2RR. This highlights the importance of potential control for achieving high purity and yield of MSCO ₂ RR-derived carbon nanotubes.

[00115] The quality of the potential-dependent carbon products, such as carbon nanotubes, were accessed by crystallization and graphitization degree. The Raman spectroscopy features of carbon materials, namely the D band, Gband, and 2D peak, were observed at approximately 1350, 1576, and 2700 cm' ¹ , respectively (FIG. 20D). A higher ID/IG ratio indicated a greater presence of disordered or defective carbon structures, indicating a lower graphitization degree, and vice versa. A stronger 2D peak was indicative of higher-quality carbon nanotubes. XRD peaks at around -22° and 26° corresponded to the amorphous and crystalline features of carbon, respectively (FIG. 20C). The presence of the D band suggested that MSCO ₂ RR-derived carbon nanotubes contain structural distortions, defects, and incomplete graphite crystallites (i.e., amorphous fraction), which were also reflected by the -22° amorphous XRD peak. As the potential rose from -0.74 V to -0.82 V (FIGs. 14C to 14E), MSCO ₂ RR-derived carbon material showed an increasing XRD peak intensity around 26° (i.e., enhanced crystallinity) and a decreasing ID/IG ratio (i.e., increased graphitization degree). The average ID/IG ratio could reach as low as -0.24 at -0.78 V. Based on selectivity, yield, and quality (e.g., sharpness of 2D peak), -0.78 V (equivalent to an overall 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-minute reactions. FIGs. 19G to 191 showed XRD, Raman, and Cis XPS spectra of optimized CNTs. TEM characterization (FIGs. 22A to FIG. 22H) confirmed a typical hollow multiwalled carbon nanotube structure with good yield and quality. Only the elements C, O, and Fe were detected on XPS performed on the MSCO2RR-derived carbon nanotubes (FIGs. 23 A to 23C), where the element O content was mainly surface oxygen-containing functional groups on the carbon nanotubes and is characterized by FTIR spectrum (FIG. 23D). These were primarily carboxyl groups caused by molten Li2CO3 surface modification and COs ² ' deoxygenation at the cathode during electrolysis. The element Fe was also confirmed to be the metallic nucleation agent.

[00116] Temperature is another parameter which affects the morphology, yield, and quality of MSCO ₂ RR-derived CNTs due to the effect of temperature on altering mass transport and reaction kinetics. In various examples, increasing the temperature from 760°C to 800°C enhanced the cell voltage and current densities (FIGs. 24A and 24B). At 770°C (FIGs. 25A to 251), the obtained carbon products were pure carbon nanotubes with an average diameter of -100 nm, like those obtained at 760°C (FIG. 19D). However, an increasing number of carbon spheres emerged and became more prevalent between 780°C and 800°C. Only pure carbon sphere morphologies were observed as the temperature reached 800°C. Overall, higher temperatures provided a lowered Gibbs free energy of reactions, stronger driving force, and greater energy for generating more CSs. FIGs. 24C to 24E shows the temperature-dependent carbon products’ XRD pattern, Raman spectrum, and ID/IG ratio. Based on decreased amorphous XRD peak intensity (FIG. 24C), elevated temperatures may improve carbon crystallinity. At 760°C, the graphitization degree was relatively high (FIG. 24E). Moreover, temperature-dependent CO evolution was monitored by analyzing the outlet gas via online gas chromatography. Raising the temperature generally enhanced FEco (FIGs. 26A and 26B), as it lowers the reaction barrier for the competitive CO evolution reaction. Therefore, according to some examples, 760°C is identified as one of the temperature for Fe-catalyzed MSCO2RR-to- carbon nanotube growth, delivering high yield and quality carbon nanotubes with inhibited current efficiencies for CO.

[00117] Elucidating the growth mechanism of Fe-catalyzed MSCO2RR to carbon nanotubes aids in understanding and controlling carbon nanotube growth. As shown in FIG. 27A to 27F, in an example, during the initial 30 seconds of electrolysis, metallic iron particles are formed which acted as nucleation agents, and emerged from the iron foil cathode surface (FIG. 27A). After 5 minutes, the iron particles had diffused and grown into clusters around ~1 pm in size (FIG. 27B). Each of the iron particles are in a carbon-encapsulated Fe structures (Fe@C) (FIGs. 28B to 28C) that had accumulated due to the reduction of CCE ² ' to carbon catalyzed by the iron nucleation agents. After 6 minutes, the Fe@C clusters and iron nucleation agents diffused out the iron electrode surface, causing Fe-catalyzed carbon nanotubes to begin growing and branching (FIG. 27C). After 8 to 10 minutes, the diffusion of Fe@C clusters and iron nucleation agents, catalytic carbon nanotube growth and branching were occurring simultaneously (FIGs. 27D and 27E). After 15 minutes, the Fe-catalyzed carbon nanotubes continues to grow rapidly (FIG. 27F). It may be seen that the carbon nanotubes include one or more branches.

[00118] As can also be seen in FIG. 27G to 27J, the Fe-catalyzed carbon nanotube tip-growth model was observed rather than a base-growth one. The Fe@C led the carbon nanotube growth in which partial Fe catalysts may remain in carbon nanotube bodies. FIGs. 27K to 27M displays HRTEM images, lattice fringes, and the TEM diffraction ring pattern of one Fe@C catalyst particle, confirming the iron core (-0.210 nm lattice fringes) encapsulated by fully sp ² -bonded multiwall carbon shell (-0.352 nm lattice fringe), and poly crystalline BCC a-phase Fe foil cathode utilized. After electrolysis at 760°C, the a-phase Fe does not undergo phase changes. FIG. 27N shows the HAADF-STEM and EELS spectrum of one Y-shaped carbon nanotube grown in 15 minutes, illustrating the branching of carbon nanotube growth. Since only the carbon K ionization edge and the Fe L2,3 edge were detected, the carbon nanotubes were catalyzed by metallic iron instead of its oxides. Additional EELS spectra and mapping also confirmed that carbon nanotubes were catalyzed by metallic iron. Eventually, high yield, purity, and quality carbon nanotubes were obtained with longer growth times of 30 min (FIG. 19D). [00119] In other examples as shown in FIGs. 29A to 29L, the electrolysis with a cathodic potential of -0.78 V (relative to Ag/Ag2SO4 reference electrode) at 760°C and under carbon dioxide atmosphere, were performed over a range of time from 30 seconds to 15 minutes. FIGs. 30A to 30H illustrate other Fe@C core-shell structures with corresponding EDS mapping and spectrum results.

[00120] The generalized growth mechanism of Fe-catalyzed carbon dioxide-derived carbon nanotubes is schematically illustrated in FIG. 31 A, which included six steps: (i) Fe nucleus cluster formation and (ii) diffusion, (iii) carbon accumulation and (iv) encapsulation, (v) initial CNT growth and random diffusion of Fe catalysts, and (vi) branching and rapid carbon nanotubes growth. Referring to FIG. 3 IB, a comparison of the Faradaic efficiency for various carbon products produced according to various cathodic potentials are shown. By applying optimal electrolysis potential and temperature of -0.78 V and 760°C, we can produce optimal selectivity, purity, and quality CNTs with average FECNT and FEco achieved over 82% and 15%, respectively. Moreover, prolonging the electrolysis duration promoted the COs ² ' reduction to carbon (i.e., the CNT growth) and thus enhanced CNT mass yield and FECNT. For instance, after 2 and 4 hours of electrolysis, FECNT reached approximately 93% and 91%, respectively.

[00121] In embdiments of the present disclosure, a breakthrough towards producing high- yield, high-quality, homogeneous CNTs through MSCO2RR was disclosed. The 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 disclosed for different carbon growth modes, producing Fe- catalyzed MSCChRR-derived CNTs a purity approaching -100%, a favorable graphitization degree of -0.24 ID/IG, and FECNT of -82.59%.

[00122] All examples described herein, whether of apparatus, methods, materials, or products, are presented for the purpose of illustration and to aid understanding, and are not intended to be limiting or exhaustive. Modifications may be made by one of ordinary skill in the art without departing from the scope of the invention as claimed.