High purity and ultra-high purity carbon nanotube materials and methods for producing these materials

Abstract

A method for producing high purity carbon nanotube (CNT) materials, in some cases such that they contain no more than 10,000 ppm total residual catalyst and residual metal, and in some cases no more than 1,000 ppm of such contaminants. The method includes providing starting CNT material from a CNT synthesis process wherein CNTs are grown on supported active metal catalyst, performing a thermal oxidation treatment on the starting CNT material in a furnace under a controlled atmosphere in a temperature range and for at least a minimum time, to produce an oxidized CNT material wherein metals are oxidized, and performing an acid digestion treatment on the oxidized CNT material using a defined acid and for at least a minimum time, to dissolve oxidized metals and produce the high purity CNT material.

Description

[0001] High Purity and Ultra-High Purity Carbon Nanotube Materials and Methods for Producing These Materials

[0002] CROSS-REFERENCE TO RELATED APPLICATION

[0003] This application claims priority of Provisional Patent Application 63 / 704,124, filed on October 7, 2024. The entire disclosure of this priority application is incorporated by reference herein, and for all purposes.

[0004] BACKGROUND

[0005] Carbon nanotubes (CNTs) are used as additives in lithium-ion batteries to enhance the electrical conductivity and mechanical properties of both electrodes. Their high aspect ratio allows lower loadings compared to conventional conductive carbon materials while achieving similar percolation thresholds and improved electrochemical performance. However, the presence of metallic impurities, particularly Fe, Ni, Al, and Co, particularly at concentrations above 1,000 ppm, can adversely affect battery performance. These metal ions may embed in the carbon electrode during charging, reducing lithium-ion insertion and reversible capacity. Iron accelerates electrolyte decomposition, increases the thickness of the solid-electrolyte interphase (SEI) film, and raises interfacial impedance, contributing to capacity fading and potential battery failure. Therefore, ultra-high purity CNTs are helpful for optimal battery performance.

[0006] CNTs have demonstrated the ability to enhance commercial applications through their unique structure, offering superior electrical and thermal conductivity along with impressive mechanical properties. Notably, CNTs offer great potential in energy storage, serving as anodes and conductive additives for lithium-ion batteries (LiBs). CNT-based electrodes demonstrate significantly improved reversible lithium-ion capacities compared to conventional graphite-based anodes. Unlike graphite, where lithium ions interact with every second carbon hexagon, CNTs, with their tubular structures, offer improved lithium capacity, overcoming theoretical limitations associated with graphite anodes in LiBs.

[0007] Using CNTs as conductive additives in LiB cathodes enhances electrochemical properties. The high aspect ratio of CNTs (L / D) allows for lower loading levels, achieving a comparable percolation threshold. This results in improved conductivity and electrochemical properties with a smaller amount of CNTs, leading to higher energy capacity, power density, and longer charge / discharge cycles.

[0008] CNT length is a morphological factor affecting lithium-ion diffusion. Short MWCNTs offer small reversible routes for Li ions and extra intercalation sites at open ends, enhancing Li ion storage. Chemical and ball milling treatments are commonly used to increase lithium insertion and reversible capacity. Oxidation of carbon nanotubes caused by poorly controlled chemical treatments during purification reduces electrical conductivity and coulombic efficiency due to introduced large structural defects, while ball milling treatment leads to significant voltage hysteresis from surface functional groups on CNTs.

[0009] The inner and outer diameter of CNTs significantly influence lithium adsorption capacity and diffusion inside and outside the tubes. Interactions between Li ions and CNTs, including charge transfer, are enhanced by the tubes' curvature, resulting in varying lithium capacities. Exfoliation during lithium insertion depends on the inner diameter, affecting lithium capacities. Larger CNTs with unused space inside are less advantageous than smaller ones for batteries. An increase in tube diameter leads to a multi-shell structure, improving lithium capacity. Research indicates that CNTs with a diameter exceeding 5 A have greater interaction energy, making them optimal for lithium storage as an LIB anode material.

[0010] High-purity carbon nanotubes help to optimize LIB performance. Impurities, especially metal ions like iron, sodium, aluminum, and nickel, and compounds with active hydrogen in electrolytes, can compromise battery electrochemical performance. Metal impurity ions, with lower reduction potential than lithium ions, embed in the carbon electrode during charging, reducing lithium-ion insertion and reversible capacity. Iron deposits accelerate electrolyte decomposition, increase SEI film thickness, and raise interfacial impedance, causing capacity fading. High metal ion concentration decreases reversible specific capacity and may lead to battery failure. However, low metal ion concentration, < 10 ppm in the organic electrolyte, has minimal impact on battery performance due to the small radius and high migration rate of lithium ions between graphite layers.

[0011] CNTs deliver impressive thermal conductivity, enhanced heat dissipation in composites and potentially improved safety compared to electrodes with inferior carbon additives. Additionally, the mechanical strength and flexibility of CNTs helps prevent cracking during operation or in vibration environments. Their ability to entangle anode and cathode particles can address issues of active particle separation during extended cycling.

[0012] Despite the high energy capacity of LIB s, challenges in developing the next generation include improving charging rate, reducing battery weight and size, and increasing electrode cycle life. Developing controlled structure, morphology, and ultra-high-purity CNTs is helpful to enhance conventional material performance. Currently, incorporating CNTs as additives in composite electrodes offers a clear advantage in LIBs, increasing reversible capacity, improving rate capability, and enhancing cyclability.

[0013] Other commercial applications require ultra-high purity carbon nanotubes, both in terms of residual metals and tubular carbon purity. These include single wall CNTs (SWCNTs) for cancer treatment and microelectronics, CNT fibers, and CNT membranes, where the purification methods of the present invention are applicable.

[0014] The presence of certain impurities can significantly impact the performance of Li-ion batteries (LIBs). The main impurities that commonly mitigate LIB performance include:

[0015] • Metal Ions: metal ions such as iron, sodium, aluminum, and nickel are common impurities that can compromise the electrochemical performance of LIBs. These ions often have a lower reduction potential than lithium ions. During charging, metal impurity ions tend to be embedded in the carbon negative electrode, reducing the insertion position of lithium ions. This results in a reduction of the reversible capacity of the LIB.

[0016] • Substances with Active Hydrogen: Substances containing active hydrogen in electrolyte molecules can also affect LIB performance. These substances can interact with the electrochemical processes within the battery, leading to undesirable effects.

[0017] The effects of impurities on LIB performance include:

[0018] • Reduction in Reversible Capacity: Metal impurity ions, with their lower reduction potential, can displace lithium ions during charging, reducing the reversible capacity of the battery.

[0019] • Electrolyte Decomposition: Some impurities, like iron, can accelerate the decomposition of the electrolyte, leading to increased surface structural disorder in graphite electrodes. This can result in the formation of a thicker SEI film, increasing interfacial impedance and causing capacity fading during cycling.

[0020] • Formation of Ineffective Passivation Layers: High concentrations of metal impurity ions may prevent the formation of an effective passivation layer on the surface of graphite electrodes. This can lead to the destruction of the battery.

[0021] Efforts are made to ensure the high purity of materials used in the manufacturing of LIB components. Strict quality control measures are implemented to minimize the presence of these impurities, especially in the electrolyte and electrode materials, to optimize the performance, cycle life, and safety of Li-ion batteries. To mitigate these issues, it is generally required to maintain a low concentration of impurities in the LIB components. The content of each metal impurity ion in the organic electrolyte, for example, is typically recommended to be less than a certain percentage (e.g., 0.007 wt% in inorganic electrolyte) to ensure optimal performance and longevity of Li-ion batteries.

[0022] Carbon nanotubes are commercially produced using the catalytic chemical vapor deposition method (CCVD). There are two main production processes that use catalysts based on transition metals (typically Fe, Mo, Ni, Co, and combinations of these metals), either supported on a substrate (typically SiCh, AI2O3, MgO, etc.) or unsupported. The purification of carbon nanotubes is carried out to remove the residual catalyst and amorphous carbon generated during their synthesis. The most common methods used to purify CNTs include:

[0023] Acid Treatments:

[0024] Nitric Acid is commonly used to remove metals and amorphous carbon but may introduce defects in the CNT walls and surface oxidation. Acid mixtures combinations (e.g., HNO3 + H2SO4, HC1 + HNOs, H2SO4 + H2O2) are more effective for removing metals but can cause damage to CNT structures and surface oxidation at high acid concentration and temperature treatment. The acid mixture H2SO4 + H2O2 called Piranha is a strong oxidizing agent that can cause exfoliation of the tubes. Hydrofluoric Acid (HF) is specifically used to dissolve, generally at room temperature, metal particles and metal oxides, such as AI2O3 and Si O2, without affecting the CNTs. Citric Acid is used to dissolve metal oxides and other impurities without damaging CNT structures.

[0025] Oxidizing agents used in the purification of CNTs and surface functionalization, such as HNO3, H2SO4 + H2O2, ozone, HCIO4, H2CrO4, KMnO4, etc. can significantly affect their electrical conductivity. Oxidation introduces defects in the nanotube structure, such as oxygen-containing functional groups (e.g., carboxyl, hydroxyl, and carbonyl groups), which can disrupt the electron conjugation in the CNT walls and thereby reduce their electrical conductivity.

[0026] Physical Exfoliation:

[0027] Sonication in solutions disperses CNTs in solutions, with impurities separated by centrifugation.

[0028] Physical Separation Methods:

[0029] High-Speed Centrifugation separates CNTs from impurities based on density.

[0030] Membrane filtration separates CNTs based on size, removing larger or smaller particles.

[0031] SUMMARY

[0032] To address the problem of metallic impurities in CNTs, the present invention provides methods for producing high purity (HP) and ultra-high purity (UHP) carbon nanotubes suitable for lithium-ion battery applications, as well as the resulting CNTs. In at least the lithium-ion battery industry, the term ultra-high purity (UHP) carbon nanotubes refers to materials with metallic impurities below 1,000 ppm (below 0.1 wt%), while high-purity (HP) CNTs are defined as those with carbon content in the range of 99.0 to 99.9 wt% (i.e., metallic impurities between 1.0 to 0.1 wt%).

[0033] In some examples the methods combine thermal pretreatment at moderate temperatures with chemical purification to efficiently remove residual metallic impurities from CNTs without introducing structural or surface damage.

[0034] The temperature and duration of the thermal pretreatment, carried out in a furnace under an air flow or a mixture of air and inert gas (e.g., N2), are variable parameters that can be controlled to inhibit or prevent material loss due to combustion or surface oxidation of carbon atoms. In some examples a temperature range of 300-400 °C and a treatment time of 10-20 minutes has been found not to materially or substantially affect the structural integrity or electrical conductivity of the CNTs.

[0035] In some examples the subsequent chemical treatment (digestion) is carried out with a mixture of hydrochloric acid (HC1) and sulfuric acid (H2SO4). In non-limiting examples each of these are at a molar concentration between 1.0 and 3.0 M. The digestion temperature can be below or above 100 °C. In some examples the digestion is performed in a stirred tank under reflux conditions or in an autoclave, typically for 3-6 hours.

[0036] This treatment effectively dissolves the metallic elements present in the residual catalyst, enabling the production of multi-walled carbon nanotubes (MWCNTs) with purities exceeding 99.9 wt%. The resulting CNT material retains its high aspect ratio, excellent dispersibility, and electrical conductivity, making it suitable for high-performance lithium-ion battery electrodes with improved energy capacity, power density, and cycle life.

[0037] In one aspect a method for producing at least high purity (HP) carbon nanotube (CNT) material that contains no more than 10,000 ppm total residual catalyst and residual metal includes providing starting CNT material from a CNT synthesis process wherein CNTs are grown on supported active metal catalyst, wherein at least some of the starting CNT material includes at least 10,000 ppm total residual catalyst and residual metal, performing a thermal oxidation treatment on the starting CNT material in a furnace under a controlled atmosphere in a temperature range and for at least a minimum time, to produce an oxidized CNT material wherein metals are oxidized, and performing an acid digestion treatment on the oxidized CNT material using a defined acid and for at least a minimum time, to dissolve at least some of the oxidized metals and produce the HP CNT material.

[0038] In some examples the acid digestion treatment comprises one or more of: use of an HF acid solution; using a combination of HC1 and H2SO4 in a molar ratio of 3 : 1 to 1 : 1 under reflux for at least 3 hours; using a combination of HC1 and H2SO4 under reflux at different molarities and different HCI / H2SO4 ratios; and using an HNO3 solution. In some examples the acid digestion treatment uses HF, which is effective for removing alumina, silica-alumina, and silica at room temperature. In some examples the acid digestion treatment uses HN03, which is effective for removing MgO at room temperature. In some examples the acid digestion step uses a mixture of HC1 and H2SO4, which is effective under reflux conditions for removing metal oxides and metallic particles including at least one of Co, Fe, Ni, and Cr, without substantial adverse effects on the surface properties and structure of the CNTs, wherein the CNTs comprise at least one of SWCNT, DWCNT, and MWCNT.

[0039] In some examples the thermal oxidation treatment takes place at temperatures between about 200°C and about 350°C. In some examples the thermal oxidation treatment takes place for from about 20 minutes to about 8 hours. In some examples the thermal oxidation treatment takes place in an atmosphere comprising air. In some examples the controlled atmosphere in which the thermal oxidation treatment takes place comprises air and an inert gas. In some examples the inert gas comprises nitrogen.

[0040] In some examples the invention comprises an HP CNT material made by one of these methods.

[0041] In some examples the method further comprises producing an ultra-high purity (UHP) carbon nanotube (CNT) material with less than 1,000 ppm metal from the HP CNT material, by graphitizing the HP CNT material by subjecting it to high-temperature conditions at from 2500- 3000°C under vacuum or an inert gas, to restructure amorphous carbon atoms into more ordered structures comprising graphite, and separating the restructured amorphous carbon from CNTs using physical techniques comprising at least one of exfoliation and density separation. In some examples the invention comprises a UHP CNT material made by this method.

[0042] In some examples the method further comprises producing an ultra-high purity (UHP) carbon nanotube (CNT) material with less than 1,000 ppm metal from the HP CNT material, by coating metal of the HP CNT material with a layer of graphitized carbon and subjecting the graphitized HP CNT material to chlorine vapor at temperatures of at least about 950°C, such that the chlorine diffuses through the carbon layers of the CNT and reacts with the metal to form metal chloride (MxCly), the formation of MxClycauses density changes that lead to cracks in the CNT, which increases chlorine diffusion and MxClyformation, and the MxClysublimates, leaving CNT with less metal content. In some examples the invention comprises a UHP CNT material made by this method.

[0043] In some examples the method further comprises producing an ultra-high purity (UHP) carbon nanotube (CNT) material with less than 1,000 ppm metal from the HP CNT material, by preparing a suspension of the HP CNTs, placing the suspension into an electrochemical cell with a cathode, along with an electrolyte and a solvent, and applying a voltage to the cell to deposit metallic impurities onto the cathode. In some examples the invention comprises a UHP CNT material made by this method.

[0044] In another aspect a method for producing at least high purity (HP) carbon nanotube (CNT) material that contains no more than 10,000 ppm total residual catalyst and residual metal includes providing starting CNT material from a CNT synthesis process wherein CNTs are grown on supported active metal catalyst, wherein at least some of the starting CNT material includes at least 10,000 ppm total residual catalyst and residual metal, performing a thermal oxidation treatment on the starting CNT material in a furnace under a controlled atmosphere in a temperature range of from about 260°C to about 350°C and for at least a minimum time, to produce an oxidized CNT material wherein metals are oxidized, and performing an acid digestion treatment on the oxidized CNT material using a mixture of HC1 and H2SO4 for at least a minimum time, to dissolve at least some of the oxidized metals and produce the HP CNT material. In some examples the thermal oxidation treatment takes place for from about 2 hours to about 8 hours. In some examples the controlled atmosphere in which the thermal oxidation treatment takes place comprises air and an inert gas.

[0045] BRIEF DESCRIPTION OF THE DRAWINGS

[0046] Figure 1 A is a transmission electron microscopy (TEM) image of metal encapsulated in MWCNTs, and Figure IB is a TEM image of a coarse (larger) metal particle.

[0047] Figure 2 schematically illustrates several CNT purification processes.

[0048] Figure 3 includes scanning electron microscopy (SEM) images taken at 10KX, 25KX, and 50KX magnification (from left to right, respectively) of MWCNTs treated with thermal oxidation followed by HF and HC1 + H2SO4 acid digestion.

[0049] Figure 4A is a TGA analysis of a CNT sample treated with hydrofluoric acid (HF), followed by thermal treatment at 260°C, and subsequent further chemical treatment (purification) with HF, while Figure 4B is a TGA analysis where the HF purification is followed by digestion using HC1 + H2SO4.

[0050] Figure 5 is a TGA analysis of a purified FWCNT sample.

[0051] Figure 6 is a TEM image of the purified FWCNT sample of Figure 5 after thermal and chemical treatments.

[0052] Figure 7 is the Raman spectrum of the purified FWCNT sample of Figures 5 and 6, obtained with a 532 nm laser.

[0053] Figure 8A illustrates the optical absorption spectrum of SWCNTs after purification with HF, while Figure 8B is the absorption spectrum when the HF purification is followed by thermal oxidation at 250°C. Figure 9A is a TGA analysis of an as produced 85 wt% MWCNT sample, while Figure 9B is a TGA analysis of this sample after thermal and chemical purification.

[0054] Figure 10 includes SEM images taken at 5KX, 10KX, 25KX, 50 KX and 100KX magnifications (left to right, respectively) for the carbon nanotube sample containing 85 wt% MWCNT as- produced (top row), and for the same sample after thermal and chemical treatments (bottom row).

[0055] Figures 11 A is a TGA analysis of an as-produced carbon nanotube sample containing 90 wt% purity, while Figure 1 IB is a TGA analysis of the same sample after thermal and chemical purification.

[0056] Figure 12 includes Raman spectra corresponding to the 90 wt% MWCNT sample as-produced (leftmost spectrum), after thermal treatment at 270 °C for 2 hours (middle spectrum), and then after subsequent chemical treatment with an acidic mixture of 3M HC1-3M H2SO4 at 80 °C for 4 hours (rightmost spectrum).

[0057] Figure 13 includes SEM images taken at 5KX, 10KX, 25KX, 50 KX and 100KX magnifications (left to right, respectively) for the carbon nanotube sample containing 90 wt% MWCNT after thermal and chemical treatments.

[0058] DETAILED DESCRIPTION

[0059] The present invention relates to post-treatment of as-produced CNT to remove metallic impurities, so that the CNTs are better candidates for use in lithium-ion batteries.

[0060] Fig. 1A is a TEM image of metal catalyst (the dark blob) encapsulated in MWCNTs. Fig. IB is a TEM image of a coarse metal particle (the dark largely circular blob) not encapsulated in the CNTs; such larger metallic particles encapsulated with a carbon coating may also be found in the as produced CNTs. These encapsulated metal particles are contaminants that should also be removed from the CNT product. Either or both of these metallic impurities can be present in as- produced CNTs, and are largely or almost entirely removed by the methods of this invention, to result in high purity (HP) and ultra-high purity (UHP) CNTs.

[0061] One aspect of this invention leads to HP CNTs. It combines selective thermal oxidation of amorphous carbon with chemical purification methods. Even higher purity CNTs (e.g., UHP CNTs) can be produced by further processing, for example with one of three techniques for producing ultra-high purity CNTs, set forth below.

[0062] Figure 2 schematically illustrates several CNT purification methods / processes of the present invention. After the synthesis of carbon nanotubes using the catalytic chemical vapor deposition (CCVD) method in fluidized bed or rotary tube reactors, the product undergoes thermal oxidation treatment in a furnace under a controlled atmosphere, where the carbon nanotubes are exposed to a flow of air or a mixture of air and an inert gas at temperatures between about 200°C and about 350°C (both perhaps + / - 5%) for at least 2-3 hours, depending on the amount of material to be purified. Under these conditions, the amorphous carbon is removed, and the metallic particles present in the CNTs begin to oxidize on their surface. Because the CNTs are treated under moderate temperature conditions, no structural changes are expected.

[0063] Depending on the type of catalyst support, an acid digestion is performed. For example, if the catalyst contains SiCh or AI2O3 as a support, an HF acid solution can be used to efficiently dissolve the particles at room temperature, forming soluble silicon or aluminum fluoride salts. However, since HF has limited capacity to dissolve metal or metal carbide particles, a second chemical treatment is carried out using a combination of HC1 and H2SO4 in a molar ratio of 3 : 1 to 1 : 1 under reflux for at least 3 hours. Under these conditions, HC1 effectively removes metallic impurities, while the controlled proportion of H2SO4 helps maintain the structure of the CNTs without significant oxidation. This process results in HP SWCNT and HP MWCNT.

[0064] The alumina-based catalyst can be dissolved in an HC1 + H2SO4 solution under reflux conditions. The molarity and the HCI / H2SO4 ratio will depend on the alumina content, and the active metals in the product. This process results in HP MWCNT.

[0065] If the catalyst uses MgO as a support, it dissolves easily at room temperature using an HNO3 solution. Under these conditions, no oxidation occurs on the surface of the carbon nanomaterial. The metals are then dissolved using an acid mixture of HC1 and H2SO4 in the previously mentioned ratios. This process results in HP SWCNT and HP DWCNT.

[0066] In the production of SWCNTs, DWCNTs, and MWCNTs, catalysts based on SiCh, AI2O3, and MgO are used, with active metals such as Co, Ni, Fe, Mo, and combinations thereof.

[0067] The three CNT purification routes shown in Figure 2, which combine thermal oxidation with chemical treatments, will typically produce carbon nanotubes with a purity between about 1,000 and about 10,000 ppm metal. However, for CNTs to be classified as ultra-high purity (UHP) for lithium battery applications, metallic impurities must be reduced to below about 1,000 ppm. Achieving UHP CNTs requires additional treatment of the purified material. As previously mentioned, three methods can be applied: i) graphitization of CNTs at high temperatures, ii) chlorine gas treatment at T > 950°C, or iii) electrochemical treatment of pre-oxidized and chemically treated samples.

[0068] Ultra-High Purity CNTs production processes:

[0069] To produce Ultra-High Purity CNT materials (UHP, defined as residual metal < 1000 ppm), several methods can be used, including high-temperature graphitization, chlorine gas treatment at elevated temperatures, and electrochemical methods. Each method has its advantages and disadvantages, which will be discussed below.

[0070] Thermal methods are used to remove residual catalyst metals through high-temperature vaporization, typically between 2500-3000 °C, under vacuum or inert gas flow to prevent oxidation or combustion of CNTs. This high temperature promotes the restructuring of amorphous carbon atoms into more ordered structures like graphite. Once converted, the amorphous carbon can be separated from CNTs using physical techniques such as exfoliation or density separation. The result is a material with significantly reduced unwanted carbon, yielding purer and structurally refined CNTs. However, the process requires extremely high temperatures, which leads to high energy consumption and costs. If not carefully controlled, the high temperature can damage or alter the CNT structure, creating defects, particularly in single-walled and few-walled CNTs (SWCNTs, FWCNTs). This can result in decreased conductivity and negatively impact mechanical and thermal properties. Another disadvantage of this method is that the presence of metal oxides in the catalyst, such as AI2O3, SiCh, and MgO, provides a source of oxygen. During high-temperature thermal treatment, these compounds decompose, causing oxidation of the carbon nanotubes.

[0071] The mechanism for removing metal impurities using chlorine gas at high temperatures involves the following stages: 1) the active metal is coated with a layer of graphitized carbon, 2) chlorine vapor at temperatures > 950°C diffuses through the carbon layers and reacts with the metal to form metal chloride (MxCly), 3) the formation of MxClycauses density changes, leading to the first cracks in the carbon, 4) chlorine diffusion and MxClyformation increase, 5) MxClysublimates, leaving empty carbon shells. This method is effective for metal removal, but improper control of process parameters may lead to chlorine functionalization of the carbon nanotubes.

[0072] An electrochemical treatment involves preparing a suspension of CNTs and placing it into an electrochemical cell with a suitable electrolyte and solvent. A voltage higher than that used in typical lithium batteries is then applied to deposit metallic impurities onto the cathode. However, active metals are often not completely removed from the carbon nanotubes and other coarse metal particles may also not be removed, as they may be encapsulated in a graphite coating.

[0073] Example 1: High Purity MW CNTs

[0074] A catalyst containing 10 wt% Co supported on alumina was prepared, and carbon nanotubes were synthesized using ethylene as carbon source in a rotary tube reactor at 650°C with a 15- minute residence time. Details of the catalyst preparation and MWCNT synthesis conditions were previously published (see, e.g., WO 2024 / 173929 A2, and WO 2023 / 191851 A2, the entire disclosures of which are incorporated by reference herein and for all purposes). The synthesis product contained 80 wt% or 90% MWCNT. The product was subjected to thermal oxidation treatment in an air flow oven for 4 hours at temperatures between 260-300 °C.

[0075] The alumina support was dissolved in a 15 wt% HF solution for 3 hours in a polypropylene container with a Teflon -bladed mechanical stirrer at room temperature. The solid was filtered and washed with deionized water until a pH between 6 and 7 was achieved. The resulting paste was treated with an acidic solution containing HCI / H2SO4 (1: 1 molar ratio) under reflux at 80- 90°C for 4 hours. Finally, the MWCNT paste was filtered, washed to a neutral pH, and dried in an oven at 60°C for 2 hours, followed by 120°C for 2 hours.

[0076] Table 1 includes ash content results from various thermal oxidation and chemical treatments for products containing 80 wt% and 90 wt% CNTs. When the product containing 80 wt% carbon was treated with an HF solution for 3 hours, the ash content was 1.38 wt%. If this product is first treated in an air flow furnace at 260°C for 3 hours and then subjected to HF treatment, the ash content decreases to around 0.70 wt%. A subsequent treatment of this sample with a 3M HC1-3M H2SO4 acid solution under reflux at 90°C for 4 hours further reduced the ash content to approximately 0.58 wt%. The combination of thermal treatment at 260°C followed by HF and HC1 + H2SO4 acid digestion increased the material’s purity by 58%.

[0077] The same experiments were conducted with the material containing 90% MWCNT. In this case, the ash content was 1.05 wt%, 0.32 wt%, and 0.18 wt% for the samples treated with HF, 260°C -

[0078] > HF, and 260°C -> HF -> HC1 + H2SO4, respectively. In this case, the material's purity increased by 83%.

[0079] If the 90% material is pre-oxidized at 280°C and then chemically treated with the HC1 + H2SO4 solution, the ash content is 0.63 wt%. In this case, the material's purity increases by 40%.

[0080] The progressive increase in pre-oxidation temperature from 260°C to 300°C, followed by chemical treatment with HF and HC1 + H2SO4, does not substantially improve the efficiency of the product purification.

[0081] Table: 1 : Effect of Thermal and HC1 + H2SO4 Chemical Treatments on MWCNT Purification

[0082] Efficiency.

[0083] Table 2 includes energy dispersive X-ray (ED AX) analysis results for samples purified through different thermal and chemical treatments. Other metal oxides, such as Fe, Ni, Cr, Mg, Ca, and K, were detected. Significant differences in the removal of Co, Al, and other detected elements are observed across the various treatments. The lowest Co and Al residues in the ashes are found when samples are pre-oxidized at temperatures between 260-300°C and then treated with HF, followed by an HC1 + H2SO4 acid mixture.

[0084] Table 2: Residual Metal in Samples from Different Treatments Determined by ED AX.

[0085] Table 3 includes the percentage removal of Co, Al, Fe, Ni, Cr along with other elements, compared to the chemically treated sample with HF reference. Samples pre-oxidized at temperatures between 260-300°C and then chemically treated with HF followed by HC1 + H2SO4 show the highest percentage of metal removal among the different purification methods tested. Cobalt removal for these samples was about 80%, while Fe, Ni, and Cr removal exceeded 95%. The best overall metal removal results were obtained with the 260°C —> HF — > HC1 + H2SO4 purification procedure.

[0086] Table 3: Efficiency of Various Thermal and Chemical Treatments in Purifying CNTs

[0087] Figure 3 includes SEM images taken at 10KX, 25KX, and 50KX magnification (from left to right, respectively) of MWCNTs treated with thermal oxidation followed by HF and HC1 + H2SO4 acid digestion. The images clearly show completely clean multi-walled carbon nanotubes (no observable impurities) with uniform diameters of 10 ± 2 nm

[0088] Figures 4A and 4B include TGA analysis of samples treated with HF, thermal treatment at 260°C, and subsequent chemical treatment with HF (Fig. 4A) and HC1 + H2SO4 (Fig. 4B). Since the presence of Co in the product can act as a combustion catalyst, causing carbon oxidation at a lower temperature, the maximum rate of oxidation temperature shifts from 601°C to 614°C as Co active metal is removed from the product.

[0089] Example 2: High Purity FWCNTs for Conductive Applications.

[0090] In this example, a FeMo / MgO catalyst was prepared using the citrate method. This involves mixing magnesium nitrate, iron nitrate, ammonium hepta-molybdate, and citric acid used as a chelating agent with a small amount of water to form a metal-citrate viscous gel. The gel is then introduced into a preheated oven at 550°C with an air flow, where the metal salts immediately decompose into their respective metal oxides. The resulting solid is sieved to a particle size between 300 and 500 microns. The final catalyst composition is 4.36 wt% with a Fe / Mo ratio of 1.72. This method is valued for producing catalysts with well-dispersed metal oxides.

[0091] The carbon nanotube synthesis was conducted in a fluidized bed reactor using a gas mixture of 40% methane as the carbon source in hydrogen at 950°C, with a total gas flow of 20 L / min and 15 grams of catalyst for a reaction time of 10 minutes. The product contained 20 wt% FWCNT.

[0092] The obtained product is pre-oxidized at 260°C for 4 hours, followed by treatment with a 3M HNO3 solution at room temperature for 3 hours. Under these conditions, MgO shows good solubility. The product is then filtered and washed with DI water to remove excess acid and Mg(NCh)2 salts. Finally, the FWCNT paste is treated with an HCI / H2SO4 acidic solution under reflux conditions for 4 hours, then filtered, washed to a neutral pH, and dried using freeze- drying.

[0093] Table 4 includes the properties of the purified product. The carbon content, determined by the ash content method, is 99.28 wt%. The carbon nanotubes, mainly double-walled (DWCNTs), exhibit thermal stability at approximately 540°C and a maximum oxidation rate at 604°C (See the TGA analysis of Figure 5). The material's resistivity, measured using the bucky paper technique, is 287 £! / □, making it suitable for electrical conductivity applications. Table 5 also shows the elemental composition of the material, with a carbon content of 99.37%, consistent with the ash analysis, and residual Mo and Fe levels of 0.42 wt% and 0.14 wt%, respectively.

[0094] Table 4: FWCNT product properties

[0095] Figure 6 is a TEM image of the purified FWCNT sample after thermal and chemical treatments. The image clearly shows clean and uniform FWCNTs with diameters ranging from approximately 1.7 to 2.5 nm. No residual catalyst particles are visible.

[0096] Figure 7 includes the Raman spectrum of the purified FWCNT sample, obtained with a 532 nm laser. The G / D band ratio is 39, corresponding to a Q factor of approximately 0.97. The G band, located at 1562 cm is indicative of tubular carbon, while the D band, at 1327 is associated with other carbon forms and structural defects.

[0097] Example 3: High purity SWCNTs

[0098] Single walled carbon nanotubes (SWCNTs) were synthesized using a catalyst and reaction conditions disclosed in US 2022 / 0298017 Al (incorporated by reference herein) in a fluidized bed reactor with CO as the carbon source. The SWCNT product obtained was chemically treated with a 15% v / v HF acid solution, washed, and dried. It was then pre-oxidized at 250°C for 4 hours, followed by reflux treatment with a 3M HC1 + 3M H2SO4 acid solution for 4 hours.

[0099] Table 5 includes results for ash content and ED AX analysis of samples after HF treatment, and after pre-oxidation and acid treatment with HC1 + H2SO4. The results in Table 5 help establish that HF is effective for removing SiCE, which is why the ED AX analysis shows a residue of 63 ppm of Si. However, since HF is a weak acid, it is not capable of significantly reducing the metal particles (7,837 ppm of Co and 3,619 ppm of Mo). If the silica is first removed with HF, followed by a thermal treatment at 250 °C to oxidize the carbon encapsulating the metal, and then followed by acid digestion with HC1 + H2SO4 (a strong acid mixture), the metallic impurities in the product can be significantly reduced to below 1,000 ppm of Co and Mo.

[0100] Table 5: Ash Content and ED AX Analysis of SWCNT After HF Digestion and Pre-Oxidation at 250°C, Followed by Chemical Treatment with an HC1 + H2SO4 Acid Mixture

[0101] Figures 8A and 8B illustrate the optical absorption spectra of SWCNTs after purification with hydrofluoric acid (Fig. 8A) followed by thermal oxidation at 250°C (Fig. 8B). The sample retains its optical properties after oxidation. A moderate 12% decrease in signal intensity is observed for the (6,5) chirality at -984 nm, while changes for the (7,5) and (7,6) chiralities at -1036 nm and -1136 nm are minimal (< 3%).

[0102] Example 4: Influence of the thermal treatment temperature on the carbon nanotubealumina hybrid material weight loss. In one embodiment, the influence of temperature on the weight loss of a carbon nanotubealumina hybrid material was evaluated. The thermal pretreatments were conducted in a furnace under a continuous air flow of approximately 2 L / min, at temperatures ranging from 300 °C to 400 °C, for a duration of 20 minutes. The as-produced material contained varying amounts of multi-walled carbon nanotubes (MWCNTs), specifically 85 wt%, 90 wt%, and 94 wt%.

[0103] As set forth in Table 6, an increase in pretreatment temperature above 300 °C resulted in a progressive increase in weight loss. At 400 °C, the weight loss was measured as follows: (i) 6.62 wt% for the material comprising 85 wt% MWCNTs, (ii) 3.77 wt% for the material comprising 90 wt% MWCNTs, and (iii) 1.98 wt% for the material comprising 94 wt% MWCNTs. One conclusion from the data of Table 6 is that the weight loss due to carbon oxidation depends on the CNT content in the product and on the thermal pretreatment temperature. An effective thermal pretreatment temperature for the different CNT percentages in the product is 350 °C, since the weight loss is less than 1.5 wt%. When comparing the results obtained at 400 °C for 85%, 90%, and 94% CNT, it is clear that greater weight loss due to carbon oxidation occurs when there is a higher amount of residual catalyst (as in the 85 wt% CNTs).

[0104] Table 6:

[0105] Example 5; Thermal Pretreatment Temperature and Acid Digestion Conditions for producing Ultra-High Purity MWCNTs This example (summarized in Table 7) presents the results obtained from thermal pretreatment of as-produced carbon nanotubes containing different purity levels (85, 91, and 94 wt%). The pretreatments were carried out in a furnace under an airflow of 2 L / min at different temperatures (270-3 0 °C) for 20 minutes. Table 7 also reports the results obtained from chemical treatments conducted at different temperatures (80 °C and 110 °C) and with different molar concentrations of HC1 and H2SO4.

[0106] Using a sample of carbon nanotubes containing 85 wt% MWCNT, thermal treatments were performed at 300, 330, and 350 °C, followed by acid digestion under reflux conditions (110 °C) with an acid mixture of 3 M HC1 / 3 M H2SO4. The highest purity (99.94 wt% MWCNT) was obtained when the thermal treatment temperature was 350 °C. At this temperature, Table 6 shows that the weight loss due to oxidation of the graphite coating was 1.26 wt%, whereas at 300 °C the weight loss was only 0.61 wt%. This indicates that at 300 °C only partial oxidation of the graphite coating encapsulating the metal particles occurs, thereby limiting the accessibility of the acid to the encapsulated metal and reducing its removal efficiency.

[0107] It was also observed that when the treatment temperature was increased to 400 °C, where the oxidation weight loss reached 3.77 wt% for the sample containing 90 wt% MWCNT and 6.62 wt% for the sample containing 85 wt% MWCNT, the resulting purity was lower (99.87 wt%). These results indicate that the optimal thermal treatment temperature is 350 °C.

[0108] In another set of experiments, thermal treatment at 350 °C was performed on the 85 wt% MWCNT sample, followed by acid digestion at 110 °C using a less concentrated acid mixture (1.5 M HC1 1 1.5 M H2SO4) for 4 hours. The resulting purity was 99.90 wt%, thus reducing acid consumption during the purification process.

[0109] Additional experiments were performed using as-produced carbon nanotube samples with higher initial purity (90 wt% and 94 wt% MWCNT). In one experiment, the 90 wt% MWCNT sample was thermally pretreated at 270 °C for 2 hours to control graphite coating oxidation, followed by chemical treatment with 3 M HC1 and with a 3 M HC1 / 3 M H2SO4 mixture at 80 °C for 4 hours. The pretreatment with 3 M HC1 alone yielded a product with lower purity compared to the acid mixture (98.51 wt% vs. 99.90 wt%, respectively).

[0110] Similarly, a 94 wt% MWCNT sample was thermally pretreated under the same conditions (270 °C, 2 hour) and chemically treated with the acid mixture, yielding a product with 99.91 wt% purity.

[0111] Table 7: Effect of Thermal Pretreatment Temperature and Acid Digestion Conditions on the Purification of Carbon Nanotubes.

[0112] Example 6; Product Quality of the Purified Carbon Nanotubes

[0113] Figures 9A and 9B include thermogravimetric analyses corresponding to the as-produced carbon nanotube sample containing 85 wt% purity (Figure 9A), and the same sample after purification through a thermal treatment at 350 °C for 20 minutes, followed by chemical treatment with a 3M HC1 / 3M H2SO4 mixture at 110 °C for 4 hours (Figure 9B).

[0114] A symmetric signal appears at 525 °C in the as-produced MWCNT sample, which shifts to 615 °C after purification. This behavior is attributed to the presence of cobalt particles in the as- produced MWCNT sample that catalyze the carbon combustion reaction at lower temperatures. Upon removal of these catalytic particles via thermal and chemical purification, the thermal stability of the carbon nanotubes increases, and the signal becomes narrower due to a more uniform combustion reaction.

[0115] Figure 10 includes SEM images taken at 5KX, 10KX, 25KX, 50 KX and 100KX magnifications (left to right, respectively) for the carbon nanotube sample containing 85 wt% MWCNT as- produced (top row), and for the same sample after thermal and chemical treatments (bottom row). The images reveal carbon nanotubes with a bundle-type morphology, with lengths greater than 10 microns, bundle diameters between 2 and 3 microns, and individual tube diameters ranging from 9 to 12 nm. In the as-produced sample, catalyst particles and encapsulated cobalt are visible. In contrast, the purified sample shows no evidence of residual catalyst particles and only a few encapsulated Co particles.

[0116] Figures 11A and 1 IB include therm ogravimetric analyses corresponding to the as-produced carbon nanotube sample containing 90 wt% purity (Figure 11 A), and the same sample after purification through a thermal treatment at 270 °C for 2 hours, followed by chemical treatment with a 3M HC1 / 3M H2SO4 mixture at 80 °C for 4 hours (Figure 1 IB).

[0117] In this case, the signal corresponding to the as-produced carbon nanotube sample is shifted by approximately 10 °C above the maximum combustion rate temperature observed for the 85 wt% MWCNT sample (Figure 8A). Since this sample contains fewer metallic impurities, its thermal stability is higher. The purified sample exhibits a maximum combustion rate temperature of approximately 615 °C.

[0118] Figure 12 includes Raman spectra corresponding to the 90 wt% MWCNT sample as-produced (leftmost spectrum), after thermal treatment at 270 °C for 2 hours (middle spectrum), and then after subsequent chemical treatment with an acidic mixture of 3M HC1-3M H2SO4 at 80 °C for 4 hours (rightmost spectrum). The G-band signal located at about 1583 cm’1corresponds to tubular carbon, while the D-band at aboutl350 cm-1was assigned in the literature with the presence of other carbon species and structural defects. The relative intensities are presented for laser excitations at 532 nm and 638 nm. As observed, no significant structural changes occur in the carbon nanotubes after the thermal and chemical treatments under the experimental conditions employed. The G / D ratios for both lasers remain the same as those observed for the as-produced sample.

[0119] Figure 13 includes SEM images taken at 5KX, 10KX, 25KX, 50 KX and 100KX magnifications (left to right, respectively) for the carbon nanotube sample containing 90 wt% MWCNT after thermal and chemical treatments. Clean CNTs with individual tube diameters between 9 and 12 nm are observed. A few encapsulated Co particles are also visible. Table 8 includes results of the elemental chemical composition analysis performed by Inductively Coupled Plasma (ICP) on the as-produced carbon nanotube sample (90 wt% MWCNT) after thermal and chemical purification treatments.

[0120] As can be seen, the Co content is below 1000 ppm, which falls within the specifications for an ultra-high-purity carbon nanotube material. The estimated residual AI2O3 content is 429 ppm, demonstrating the high efficiency of the purification method in removing not only cobalt particles, both encapsulated and non-encapsulated by a carbon coating, but also the catalyst support.

[0121] Table 8: ICP Results for As-Produced Purified Carbon Nanotubes (90 wt% MWCNT)

[0122] A number of implementations have been described. Nevertheless, it will be understood that additional modifications may be made without departing from the scope of the inventive concepts described herein, and, accordingly, other examples are within the scope of the following claims.

Claims

What is claimed is:

1. A method for producing at least high purity (HP) carbon nanotube (CNT) material that contains no more than 10,000 ppm total residual catalyst and residual metal, comprising: providing starting CNT material from a CNT synthesis process wherein CNTs are grown on supported active metal catalyst, wherein at least some of the starting CNT material includes at least 10,000 ppm total residual catalyst and residual metal; performing a thermal oxidation treatment on the starting CNT material in a furnace under a controlled atmosphere in a temperature range and for at least a minimum time, to produce an oxidized CNT material wherein metals are oxidized; and performing an acid digestion treatment on the oxidized CNT material using a defined acid and for at least a minimum time, to dissolve at least some of the oxidized metals and produce the HP CNT material.

2. The method of claim 1 wherein the acid digestion treatment comprises one or more of: use of an HF acid solution; using a combination of HC1 and H2SO4 in a molar ratio of 3 : 1 to 1 : 1 under reflux for at least 3 hours; using a combination of HC1 and H2SO4 under reflux at different molarities and different HCI / H2SO4 ratios; and using an HNOs solution.

3. The method of claim 1 wherein the acid digestion treatment uses HF, which is effective for removing alumina, silica-alumina, and silica at room temperature.

4. The method of claim 1 wherein the acid digestion treatment uses HN03, which is effective for removing MgO at room temperature.

5. The method of claim 1 wherein the acid digestion step uses a mixture of HC1 and H2SO4, which is effective under reflux conditions for removing metal oxides and metallic particles including at least one of Co, Fe, Ni, and Cr, without substantial adverse effects on the surface properties and structure of the CNTs, wherein the CNTs comprise at least one of SWCNT, DWCNT, and MWCNT.

6. The method of claim 1 wherein the thermal oxidation treatment takes place at temperatures between about 260°C and about 350°C.

7. The method of claim 6 wherein the thermal oxidation treatment takes place for at least about 20 minutes.

8. The method of claim 6 wherein the thermal oxidation treatment takes place in an atmosphere comprising air.

9. The method of claim 8 wherein the controlled atmosphere in which the thermal oxidation treatment takes place comprises air and an inert gas.

10. The method of claim 9 wherein the inert gas comprises nitrogen.

11. An HP CNT material made by the method of claim 1.

12. The method of claim 1 further comprising producing an ultra-high purity (UHP) carbon nanotube (CNT) material with less than 1,000 ppm metal from the HP CNT material, by graphitizing the HP CNT material by subjecting it to high-temperature conditions at from 2500- 3000°C under vacuum or an inert gas, to restructure amorphous carbon atoms into more ordered structures comprising graphite, and separating the restructured amorphous carbon from CNTs using physical techniques comprising at least one of exfoliation and density separation.

13. A UHP CNT material made by the method of claim 12.

14. The method of claim 1 further comprising producing an ultra-high purity (UHP) carbon nanotube (CNT) material with less than 1,000 ppm metal from the HP CNT material, by coating metal of the HP CNT material with a layer of graphitized carbon and subjecting the graphitized HP CNT material to chlorine vapor at temperatures of at least about 950°C, such that the chlorine diffuses through the carbon layers of the CNT and reacts with the metal to form metal chloride (MxCly), the formation of MxClycauses density changes that lead to cracks in the CNT, which increases chlorine diffusion and MxClyformation, and the MxClysublimates, leaving CNT with less metal content.

15. A UHP CNT material made by the method of claim 14.

16. The method of claim 1 further comprising producing an ultra-high purity (UHP) carbon nanotube (CNT) material with less than 1,000 ppm metal from the HP CNT material, by preparing a suspension of the HP CNTs, placing the suspension into an electrochemical cell with a cathode, along with an electrolyte and a solvent, and applying a voltage to the cell to deposit metallic impurities onto the cathode.

17. A UHP CNT material made by the method of claim 16.

18. A method for producing at least high purity (HP) carbon nanotube (CNT) material that contains no more than 10,000 ppm total residual catalyst and residual metal, comprising: providing starting CNT material from a CNT synthesis process wherein CNTs are grown on supported active metal catalyst, wherein at least some of the starting CNT material includes at least 10,000 ppm total residual catalyst and residual metal; performing a thermal oxidation treatment on the starting CNT material in a furnace under a controlled atmosphere in a temperature range of from about 260°C to about 350°C and for at least a minimum time, to produce an oxidized CNT material wherein metals are oxidized; and performing an acid digestion treatment on the oxidized CNT material using a mixture of HC1 and H2SO4 for at least a minimum time, to dissolve at least some of the oxidized metals and produce the HP CNT material.

19. The method of claim 18 wherein the thermal oxidation treatment takes place for from about 20 minutes to about 8 hours.

20. The method of claim 19 wherein the controlled atmosphere in which the thermal oxidation treatment takes place comprises air and an inert gas.

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