Method for preparing carbon nano tube through CO2 in-situ synchronous absorption-electrolysis

Abstract

The invention discloses a method for preparing a carbon nano tube through CO2 in-situ synchronous absorption-electrolysis. On the basis of molten salt component optimization, the high-added-value carbon nanotubes are prepared through synchronous CO2 absorption and in-situ electrolysis, and integration of carbon capture, conversion and material preparation is achieved. According to the method, CO2 is absorbed by using high-temperature molten salt, and then a carbon source is driven to directionally grow on the surface of a metal catalyst (such as Fe and Ni-based electrodes) to form a carbon nanotube with a controllable structure by regulating and controlling current, temperature and molten salt components in an electrolysis process, so that the CO2 conversion rate reaches 85% or above, the yield of the carbon nanotube can reach 2-5g / (h.g catalyst), and the product has high purity, low defect degree and excellent conductivity. The invention provides a new path for emission reduction of industrial CO2 and low-cost preparation of the nano carbon material, has both environmental benefits and economic values, and has important significance for promoting cyclic utilization of carbon resources.

Description

Technical Field

[0001] The present invention belongs to the technical field of carbon nanotube preparation, and in particular relates to a method for preparing carbon nanotubes through in-situ synchronous absorption-electrolysis of CO2. Background Art

[0002] Carbon nanotubes (CNTs) and their growth mechanisms have fascinated the nanotechnology community for decades. Their unique structure gives them broad applications in tires, films, coatings, batteries, and other fields. Despite their promising prospects, the high cost challenge is a key factor hindering the widespread use of CNTs in mass-produced products, with prices ranging from US$200 per kilogram (large quantities of low-quality CNTs) to hundreds of dollars per gram (small quantities of single-walled CNTs). This is because the production of CNTs requires the use of traditional gas-phase synthesis methods, such as fluidized beds or other forms of chemical vapor deposition processes, which require the use of expensive energy, materials, and infrastructure. Therefore, it is very important to continuously propose new methods for producing CNTs with higher energy efficiency and high precursor utilization, which will help CNTs replace the black carbon and activated carbon currently widely used in the industry, laying the foundation for CNTs to become the next generation of mainstream materials.

[0003] In recent years, the process of preparing CNTs using molten carbonate electrolysis of CO2 has attracted increasing attention. This method uses molten carbonate as an electrolyte. After electrolytic reduction of CO2, carbon is deposited at the cathode and O2 is deposited at the anode. It can also absorb CO2 from the air to replenish the carbonate in the reaction. In this way, the carbon atoms in CO2 serve as the active material for synthesis, making the electrolysis reaction a sustainable process that continuously converts CO2 in the air into solid carbon while releasing oxygen. This technical process is relatively simple, the product morphology is easy to control, pure oxygen is produced as a by-product, the cost is relatively low, and it also realizes the resource utilization of CO2.

[0004] While molten salt electrolysis of CO₂ to produce CNTs can achieve carbon resource utilization, it has yet to be widely adopted, primarily due to the following technical bottlenecks: 1. High lithium resource cost: Li₂CO₃ accounts for ≥50% of binary / single lithium-based molten salts, and raw material costs account for over 60%. For example, prior art CN201510895049.2 uses Li₂CO₃ as the primary electrolyte, with lithium resource costs accounting for over 60%. 2. Ex-situ processes: CO₂ absorption and electrolysis are performed in separate steps, requiring secondary processing of intermediate products and increasing energy consumption by 15-20%. 3. Low mass transfer efficiency: The limited gas-liquid contact area of ​​conventional electrodes results in a CO₂ absorption rate of only 0.5-1.2 g / (h·g catalyst). For example, prior art CN201510895049.2 utilizes a Li₂CO₃-Na₂CO₃ binary molten salt system, requiring an electrolysis temperature of ≥800°C and continuous external CO₂ gas flow. Its electrode structure is flat, the gas-liquid contact efficiency is low, the CO2 mass transfer path is long (millimeter level), and the absorption efficiency is only 20-30%. Summary of the Invention

[0005] The purpose of the present invention is to provide a method for in-situ synchronous absorption-electrolysis of CO2 to produce carbon nanotubes. By synchronously absorbing CO2 and in-situ electrolyzing to produce high-value-added carbon nanotubes, the integration of "carbon capture-conversion-material preparation" is achieved. CO2 is absorbed by high-temperature molten salt, and then during the electrolysis process, the current, temperature and molten salt composition are controlled to drive the carbon source to grow in a directional manner on the surface of a metal catalyst (such as Fe, Ni-based electrodes) into structurally controllable carbon nanotubes, achieving a CO2 conversion rate of more than 85%, a carbon nanotube yield of 2-5g / (h·g catalyst), and a product with high purity (>95%), low defectivity (ID / IG<0.2) and excellent conductivity (>10 3 S / cm).

[0006] In order to achieve the above object, the technical solution adopted by the present invention is as follows:

[0007] A method for producing carbon nanotubes by in-situ simultaneous absorption-electrolysis of CO2 comprises the following steps:

[0008] (1) Weighing the following carbonates according to the proportion: Li2CO3: 20-40wt%, BaCO3: 30-50wt%, CaCO3: 30-50wt%, with a total proportion of 100wt%; thoroughly mixing them to obtain a mixed carbonate, and then drying them in a blast drying oven at 100-105°C for 24-26 hours to remove moisture from the mixed carbonate;

[0009] (2) pouring the dried mixed carbonate into an alumina crucible, heating to 730-770°C at a heating rate of 4.5-5.5°C / min, and then maintaining the temperature for 2-2.2 hours to fully dissolve the molten carbonate;

[0010] (3) Electrode pretreatment: Polish the hollow mesh electrode with 600, 1000, 1500, and 2000 mesh sandpaper until the electrode surface is bright, then use ultrasonic cleaning for 15 to 20 minutes, soak in acetone for 0.4 to 0.6 hours to remove oil, soak in dilute sulfuric acid for 8 to 9 seconds to remove the oxide film on the electrode surface, and place the pretreated electrode in a blast drying oven to dry for later use;

[0011] (4) After the molten carbonate in step (2) is fully dissolved, the pretreated electrode is inserted into the crucible to ensure full contact between the electrode and the molten salt, and the electrode is allowed to stabilize for 0.5 to 0.6 hours; at the same time, the mass flow meter switch is turned on, and carbon dioxide is passed through the hollow mesh electrode into the molten salt at a flow rate of 70 to 80 mL / min;

[0012] (5) After the electrode is stable, a DC regulated current is applied, gradually applying 3 mA / cm 2 , 5mA / cm 2 , 10mA / cm 2 The current density was adjusted to 200-210 mA / cm after the pre-electrolysis was completed. 2 Continue electrolysis for 1.8 to 2.2 hours;

[0013] (6) After the electrolysis is completed, the electrode is removed and the product is purified according to the following steps: ① Place the cathode product in a dilute hydrochloric acid solution and fully soak for 46 to 50 hours until the product is completely detached and no bubbles are generated; ② Ultrasonic cleaning of the product is performed for 15 to 20 minutes using an ultrasonic cleaner; ③ The product is filtered with a 0.45 μm water film and repeatedly washed with distilled water for 3 to 5 times; ④ The product is dried in a blast drying oven at 75 to 85 ° C for 7.5 to 8.5 hours; ⑤ Grind the product to obtain carbon nanotubes and bag them for later use.

[0014] Furthermore, in step (1), SrCO3 can be used to replace part of BaCO3, with the proportion of SrCO3 being 10-20 wt%, and the total proportion of SrCO3 and BaCO3 being 30-50 wt%.

[0015] The present invention adopts the above technical solution mainly based on the following considerations:

[0016] (1) The core role of the ternary molten salt system

[0017] 1. Ratio range: Li2CO3 (20-40wt%), BaCO3 (30-50wt%), CaCO3 (30-50wt%), totaling 100w%.

[0018] Li2CO3: The eutectic effect reduces the melting point to 650-750°C (traditional system ≥800°C) and increases the conductivity of the molten salt to 0.15-0.2S / cm.

[0019] CaCO3: Regulates pH environment, inhibits side reactions, and improves product crystallinity.

[0020] BaCO3: stable molten salt skeleton at high temperature (>700℃), Ba 2+ Catalyzes CNTs nucleation and induces tube diameter uniformity (15-25nm).

[0021] 2. Defects of the binary system:

[0022] Only Li2CO3+CaCO3: CaCO3 decomposes excessively at high temperatures (>700℃), and CO2 bubbles interfere with electrolytic stability.

[0023] Only Li2CO3+BaCO3: The pH of the molten salt is prone to fluctuate, the anode oxygen evolution side reaction is aggravated (OER efficiency>20%), and the current efficiency decreases.

[0024] Only BaCO3+CaCO3: The melting point is too high (>900℃), the energy consumption increases dramatically, and there is no Li + Improve electrical conductivity.

[0025] 3. Redundancy of four or more components:

[0026] Adding additional salts (such as Na2CO3) will introduce competitive ions and destroy Li + / Ba 2+ / Ca 2+ synergistic balance.

[0027] The complex ratio makes the viscosity of the molten salt uncontrollable, affecting the CO2 mass transfer efficiency.

[0028] 4. Optimized balance of ternary ratio:

[0029] Li2CO3 (20-40wt%): Ensure sufficient Li + Lower the melting point but avoid wasting resources;

[0030] BaCO3 (30-50 wt%): provides high temperature stability and catalyzes CNT nucleation;

[0031] CaCO3 (30-50wt%): Dynamically adjusts CO2 concentration and pH, inhibiting side reactions.

[0032] All three are indispensable, and deviation from the ratio will lead to performance degradation (such as insufficient conductivity when Li < 20%, and foaming of molten salt when Ca > 50%).

[0033] (2) Three-phase interface design of hollow mesh electrode

[0034] 1. Structural characteristics:

[0035] ① The electrodes used in this invention feature through-hole microchannels (20-200μm in diameter) that force the electrolyte to flow in a laminar manner, shortening the CO2 mass transfer path from the gas phase to the liquid phase to the micron level. The inner walls of the microchannels are treated with a hydrophobic treatment (e.g., silicon carbide coating) to reduce electrolyte retention and prevent clogging. Furthermore, the microchannel diameter can be expanded to 50-300μm to accommodate molten salt systems of varying viscosities.

[0036] ② The outer surface of the electrode is loaded with Fe / Ni catalyst nanoparticles (particle size 5-10nm) as the growth site of carbon nanotubes (CNTs). The thickness of the catalyst layer is controlled at 100-500nm, and a porous network structure (porosity >60%) is adopted to achieve both high activity and permeability.

[0037] ③CO2 is transported to the outer surface of the electrode through the microchannel and contacts with the molten salt to form HCO3 - / CO3 2 -Active species; When the molten salt flows through the outer surface of the electrode, it carries the active species to the catalyst site and reduces them to CNTs. The growth direction is regulated by the pulsed electric field (such as the growth of straight tubular CNTs in the direction perpendicular to the electric field).

[0038] 2. Synchronous reaction mechanism:

[0039] CO2 absorption: CO2 diffuses from the gas phase through microchannels into the molten salt to generate HCO3 - / CO3 2 -Active species.

[0040] In-situ electrolysis: active species are directly reduced to CNTs on the cathode surface (reaction formula: HCO3 - +H2O+e - →C(CNT)+OH - ) to avoid the loss of intermediate product migration.

[0041] Dynamic balance: CaCO3 decomposition and regeneration Maintain the pH of the molten salt (9.5-10.5) to inhibit side reactions.

[0042] (3) Process optimization (compared with traditional technology)

[0043] Traditional process: CO2 pressurized absorption → molten salt transfer → electrolysis → product separation (4 steps).

[0044] The process of the present invention: CO2 in-situ absorption and electrolysis synchronously → product collection (2 steps), energy consumption is reduced by 40%.

[0045] The present invention has the following beneficial effects:

[0046] ① This invention addresses the issues of lithium resource dependence and cost: Existing technologies (such as CN201510895049) use single or binary lithium-based molten salt systems, which have high lithium content and limited resources. This invention reduces the Li2CO3 content to 20-40% by optimizing the ratio of a ternary mixed molten salt (Li2CO3-BaCO3-CaCO3), significantly reducing the lithium content and overall cost by over 35%, thus addressing the scarcity and high cost of lithium resources.

[0047] This invention addresses the complexity of ex-situ processes: Traditional technologies require separate steps for CO2 absorption and electrolysis, resulting in a cumbersome and inefficient process. This invention utilizes a hollow mesh electrode design to create a three-phase gas-solid molten salt reaction interface, enabling simultaneous in-situ CO2 absorption and electrolysis. This reduces the number of process steps by 50%, improves process stability, and achieves higher product consistency.

[0048] ③ This invention addresses the problem of insufficient product control: Existing technologies offer limited control over the morphology and structure of carbon nanotubes (CNTs). By combining the synergistic effects of molten salt components with pulsed electric field control, this invention achieves precise, directional growth of CNTs with precise diameter (10-50nm), number of layers (3-15), and morphology (straight or curved). BRIEF DESCRIPTION OF THE DRAWINGS

[0049] Figure 1 This is a diagram of the synergistic mechanism of the ternary molten salt of the present invention.

[0050] Figure 2 Schematic diagram of the experimental device of the present invention.

[0051] Figure 3 is a SEM image of the carbon nanotubes prepared in Example 1.

[0052] Figure 4 is the XRD pattern of the carbon nanotubes prepared in Example 1. DETAILED DESCRIPTION

[0053] Example 1

[0054] A method for producing carbon nanotubes by in-situ simultaneous absorption-electrolysis of CO2 comprises the following steps:

[0055] (1) First, accurately weigh the corresponding weight of carbonate using an electronic balance, mix them thoroughly, and then place them in a blast drying oven at 105°C for 24 hours to remove moisture from the mixed carbonate;

[0056] The mixed carbonate composition is as follows: Li2CO3: 20-40wt%, BaCO3: 30-50wt%, CaCO3: 30-50wt%, and the total weight ratio is 100%.

[0057] (2) The dried mixed carbonate was poured into an alumina crucible, heated to 750°C at a heating rate of 5°C / min, and then kept at this temperature for 2 h to fully dissolve the molten carbonate.

[0058] (3) Electrode pretreatment: Use 600, 1000, 1500, and 2000 mesh sandpaper to polish the electrode until the surface is bright. Then use ultrasonic washing for 15 minutes, soak in acetone for 0.5 hours to remove oil, and soak in dilute sulfuric acid for 8 seconds to remove the oxide film on the electrode surface. Place the treated electrode in a blast drying oven to dry for later use.

[0059] (4) After the molten carbonate in step (2) is fully dissolved, the pretreated electrode is inserted into the crucible to ensure full contact between the electrode and the molten salt. The electrode is allowed to stabilize for 0.5 h. At the same time, the mass flow meter is turned on and carbon dioxide is passed through the hollow mesh electrode into the molten salt at a flow rate of 75 mL / min.

[0060] (5) After the electrode is stable, a DC regulated current is applied, gradually applying 3 mA / cm 2 , 5mA / cm 2 , 10mA / cm 2 The current density was adjusted to 200 mA / cm after the pre-electrolysis was completed. 2 The electrolysis was continued for 2 h.

[0061] (6) After the electrolysis is completed, the electrode is removed for product purification. Product purification: ① Place the cathode product in a dilute hydrochloric acid solution and soak it for 48 hours until the product is completely detached and no bubbles are generated. ② Ultrasonic wash the product with an ultrasonic cleaner for 15 minutes; ③ Filter the product with a 0.45μm water filter and wash it repeatedly with distilled water three times; ④ Dry the product in an 80℃ forced air drying oven for 8 hours; ⑤ Grind the product to obtain carbon nanotubes and bag them for later use.

[0062] (7) Product characterization

[0063] Figure 3 This is a SEM image of the carbon nanotubes prepared in Example 1. It can be seen from the image that the prepared carbon nanotubes have uniform diameters, smooth tube walls, and exhibit excellent carbon nanotube morphology.

[0064] Figure 4 This is the XRD pattern of the carbon nanotubes prepared in Example 1. It can be seen from the figure that the prepared carbon nanotubes have a sharp carbon diffraction peak at 26° and a secondary peak at around 44°, indicating that the prepared carbon nanotubes have a high degree of graphitization and good crystallinity.

[0065] Example 2

[0066] A method for producing carbon nanotubes by in-situ simultaneous absorption-electrolysis of CO2 comprises the following steps:

[0067] (1) Weighing the following carbonates according to the proportion: Li2CO3: 20-40wt%, BaCO3: 30-50wt%, CaCO3: 30-50wt%, with a total proportion of 100wt%; thoroughly mixing them to obtain a mixed carbonate, and then drying them in a blast drying oven at 100-105°C for 24-26 hours to remove moisture from the mixed carbonate;

[0068] (2) pouring the dried mixed carbonate into an alumina crucible, heating to 730-770°C at a heating rate of 4.5-5.5°C / min, and then maintaining the temperature for 2-2.2 hours to fully dissolve the molten carbonate;

[0069] (3) Electrode pretreatment: Polish the electrode with 600, 1000, 1500, and 2000 mesh sandpaper until the surface is bright, then use ultrasonic cleaning for 15 to 20 minutes, soak in acetone for 0.4 to 0.6 hours to remove oil, soak in dilute sulfuric acid for 8 to 9 seconds to remove the oxide film on the electrode surface, and place the pretreated electrode in a blast drying oven to dry for later use;

[0070] (4) After the molten carbonate in step (2) is fully dissolved, the pretreated electrode is inserted into the crucible to ensure full contact between the electrode and the molten salt, and the electrode is allowed to stabilize for 0.5 to 0.6 hours; at the same time, the mass flow meter is turned on, and carbon dioxide is passed through the hollow mesh electrode into the molten salt at a flow rate of 70 mL / min;

[0071] (5) After the electrode is stable, a DC regulated current is applied, gradually applying 3 mA / cm 2 , 5mA / cm 2 , 10mA / cm 2 The current density was adjusted to 200-210 mA / cm after the pre-electrolysis was completed. 2 Continue electrolysis for 1.8 to 2.2 hours;

[0072] (6) After the electrolysis is completed, the electrode is removed and the product is purified according to the following steps: ① Place the cathode product in a dilute hydrochloric acid solution and fully soak for 46 to 50 hours until the product is completely detached and no bubbles are generated; ② Ultrasonic cleaning of the product is performed for 15 to 20 minutes using an ultrasonic cleaner; ③ The product is filtered with a 0.45 μm water film and repeatedly washed with distilled water for 5 times; ④ The product is dried in a 75 to 85 ° C forced air drying oven for 7.5 to 8.5 hours; ⑤ Grind the product to obtain carbon nanotubes and bag them for later use.

[0073] Example 3

[0074] A method for producing carbon nanotubes by in-situ simultaneous absorption-electrolysis of CO2 comprises the following steps:

[0075] (1) Weighing the following carbonates according to the proportion: Li2CO3: 20-40wt%, BaCO3: 30-50wt%, CaCO3: 30-50wt%, with a total proportion of 100wt%; thoroughly mixing them to obtain a mixed carbonate, and then drying them in a blast drying oven at 100-105°C for 24-26 hours to remove moisture from the mixed carbonate;

[0076] (2) pouring the dried mixed carbonate into an alumina crucible, heating to 730-770°C at a heating rate of 4.5-5.5°C / min, and then maintaining the temperature for 2-2.2 hours to fully dissolve the molten carbonate;

[0077] (3) Electrode pretreatment: Polish the electrode with 600, 1000, 1500, and 2000 mesh sandpaper until the surface is bright, then use ultrasonic cleaning for 15 to 20 minutes, soak in acetone for 0.4 to 0.6 hours to remove oil, soak in dilute sulfuric acid for 8 to 9 seconds to remove the oxide film on the electrode surface, and place the pretreated electrode in a blast drying oven to dry for later use;

[0078] (4) After the molten carbonate in step (2) is fully dissolved, the pretreated electrode is inserted into the crucible to ensure full contact between the electrode and the molten salt, and the electrode is allowed to stabilize for 0.5 to 0.6 hours; at the same time, the mass flow meter is turned on, and carbon dioxide is passed through the hollow mesh electrode into the molten salt at a flow rate of 80 mL / min;

[0079] (5) After the electrode is stable, a DC regulated current is applied, gradually applying 3 mA / cm 2 , 5mA / cm 2 , 10mA / cm 2 The current density was adjusted to 200-210 mA / cm after the pre-electrolysis was completed. 2 Continue electrolysis for 1.8 to 2.2 hours;

[0080] (6) After the electrolysis is completed, the electrode is removed and the product is purified according to the following steps: ① Place the cathode product in a dilute hydrochloric acid solution and fully soak for 46 to 50 hours until the product is completely detached and no bubbles are generated; ② Ultrasonic cleaning of the product is performed for 15 to 20 minutes using an ultrasonic cleaner; ③ The product is filtered with a 0.45 μm water film and repeatedly washed with distilled water for 4 times; ④ The product is dried in a blast drying oven at 75 to 85 ° C for 7.5 to 8.5 hours; ⑤ Grind the product to obtain carbon nanotubes and bag them for later use.

Claims

1. A method for producing carbon nanotubes by in-situ simultaneous absorption and electrolysis of CO2, characterized in that: The following steps are involved: (1) Weigh the following carbonates according to the ratio: Li2CO3: 20-40 wt%, BaCO3: 30-50 wt%, CaCO3: 30-50 wt%, totaling 100 wt%; The mixture is thoroughly mixed to obtain a mixed carbonate, and then dried to remove moisture from the mixed carbonate; (2) Pour the dried mixed carbonate into an alumina crucible, heat it to 730-770°C, and keep it at this temperature for 2-2.2 hours to fully dissolve the molten carbonate; (3) Electrode pretreatment: Use sandpaper to polish the electrode until the surface is bright, then use ultrasonic cleaning for 15-20 minutes, soak in acetone for 0.4-0.6 hours to remove oil, soak in dilute sulfuric acid for 8-9 seconds to remove the oxide film on the electrode surface, and dry the pretreated electrode for later use; (4) After the molten carbonate in step (2) is fully dissolved, the pretreated electrode is inserted into the crucible to ensure full contact between the electrode and the molten salt. The electrode is allowed to stabilize for 0.5 to 0.6 h. At the same time, carbon dioxide is introduced into the electrode and the molten salt at a flow rate of 70 to 80 mL / min. (5) After the electrode is stable, a DC regulated current is applied, gradually applying 3 mA / cm 2 , 5 mA / cm 2 , 10 mA / cm 2 The current density was adjusted to 200 ~ 210 mA / cm after the pre-electrolysis was completed. 2 Continue electrolysis for 1.8-2.2 h; (6) After the electrolysis is completed, remove the electrode and purify the product according to the following steps: ① Place the cathode product in a dilute hydrochloric acid solution and fully soak for 46 to 50 hours until the product is completely detached and no bubbles are generated; ② Ultrasonic wash the product with an ultrasonic cleaner for 15 to 20 minutes; ③ Filter the product with a 0.45 μm water film and repeatedly wash it with distilled water for 3 to 5 times; ④ Dry the product; ⑤ Grind the product to obtain carbon nanotubes.

2. The method for producing carbon nanotubes by in-situ simultaneous CO2 absorption-electrolysis according to claim 1, characterized in that: The drying step (1) is to place the mixed carbonate in a blast drying oven and dry it at 100-105°C for 24-26 hours.

3. The method for producing carbon nanotubes by in-situ simultaneous CO2 absorption-electrolysis according to claim 1, characterized in that: The heating rate of step (2) is 4.5~5.5℃ / min.

4. The method for producing carbon nanotubes by in-situ simultaneous CO2 absorption-electrolysis according to claim 1, characterized in that: The grinding and polishing in step (3) are performed using sandpaper of 600, 1000, 1500 and 2000 mesh, respectively.

5. The method for producing carbon nanotubes by in-situ simultaneous CO2 absorption-electrolysis according to claim 1, characterized in that: The drying in step (6) is to dry the product in a forced air drying oven at 75-85°C for 7.5-8.5 h.

6. The method for producing carbon nanotubes by in-situ simultaneous CO2 absorption-electrolysis according to claim 1, characterized in that: In step (1), SrCO3 is used to replace part of BaCO3, the proportion of SrCO3 is 10-20 wt%, and the total proportion of SrCO3 and BaCO3 is 30-50 wt%.

Images