A method for carbon dioxide capture by capacitive deionization based on amino-functionalized carbon nanotube nanofluid

Abstract

The application discloses a kind of carbon dioxide capture methods of capacitive deionization based on aminated carbon nanotube nanofluid, comprising: preparation aminated carbon nanotube nanofluid as absorbent;The absorbent is introduced into the absorbent chamber of capacitive deionization reactor, so that the gas to be treated is contacted with the absorbent;Direct current voltage is applied to the capacitive deionization reactor to charge, so that carbon dioxide is absorbed in the absorbent and captured on the electrode surface via electrochemical action;Wherein, the aminated carbon nanotube is obtained by modifying carbon nanotube after acid oxidation treatment using silane coupling agent.The application simultaneously breaks through in three dimensions of capture efficiency, energy consumption index and system stability through the innovative design of CNT nanofluid, to provide a solution with high efficiency, economy and reliability for industrial carbon capture.

Description

Technical Field

[0001] This invention belongs to the field of capacitive deionization carbon dioxide capture technology, and particularly relates to a capacitive deionization carbon dioxide capture method based on aminated carbon nanotube nanofluids. Background Technology

[0002] Carbon dioxide (CO2) capture technology is of great significance for mitigating climate change, among which capacitive deionization (CDI) technology has attracted widespread attention due to its low energy consumption and modular design. Flow electrode capacitive deionization (FCDI), as a derivative technology of CDI, achieves CO2 capture through the synergistic effect of a flow electrode and absorbent. However, existing FCDI technologies have significant drawbacks.

[0003] First, traditional FCDI systems use deionized water or salt solutions as the absorbent, resulting in excessive mass transfer resistance at the gas-liquid interface. The low solubility and slow diffusion rate of CO2 in the liquid phase severely limit the capture efficiency. For example, the CO2 capture efficiency of the water-based FCDI system (H-FCDI) is less than 30%, far below industrial requirements. Existing research shows that in the two-film theory model of the gas-liquid boundary layer, the mass transfer resistance is concentrated in the interfacial film layer, and the lack of uniform turbulence further exacerbates the mass transfer bottleneck.

[0004] Secondly, to improve mass transfer efficiency, existing technologies attempt to introduce carbon nanotube (CNT) nanofluids as absorbent enhancers. Carbon nanoparticles can carry CO2 molecules across the gas-liquid boundary layer through physical transport, increasing the effective mass transfer area. However, CNTs are prone to agglomeration in the liquid phase, and the resulting aggregates can block mass transfer pathways, increasing system resistance. Experiments show that although CNT nanofluids (C-FCDI) improve the capture capacity by 58% compared to water-based absorbents, the agglomeration effect leads to increased energy consumption fluctuations and poor long-term operational stability. Related literature confirms that when the nanoparticle concentration exceeds a critical value, increased viscosity and weakened Brownian motion significantly weaken the mass transfer enhancement effect, even causing performance to fall below that of the basic absorbent.

[0005] Furthermore, existing capacitive capture devices suffer from high energy consumption. While supercapacitors and membrane capacitor deionization (MCDI) systems can achieve CO2 fixation, they rely on solutions with high ionic strength, resulting in energy consumption of 40-103.6 kJ / mol. The capture rates of these systems are generally below 4 × 10⁻⁶ kJ / mol. -8 The current concentration of mol / s / g is insufficient to meet the requirements for continuous operation. Although optimizing the stirring rate can temporarily improve mass transfer, excessive stirring can induce particle agglomeration, creating an irreconcilable contradiction between efficiency and energy consumption.

[0006] In summary, the core problems of current FCDI technology are: low gas-liquid mass transfer efficiency, performance degradation caused by nanoparticle aggregation, and high energy consumption bottleneck. An innovative solution is urgently needed that can enhance mass transfer, suppress aggregation, and synergistically reduce energy consumption to achieve a practical breakthrough in CO2 capture technology. Summary of the Invention

[0007] To address the aforementioned technical challenges, this invention proposes a capacitive deionization carbon dioxide capture method based on aminated carbon nanotube nanofluids. Through innovative design of CNT nanofluids, breakthroughs are achieved simultaneously in three dimensions: capture efficiency, energy consumption, and system stability, providing a highly efficient, economical, and reliable solution for industrial carbon capture.

[0008] To achieve the above objectives, the present invention provides a capacitive deionization carbon dioxide capture method based on aminated carbon nanotube nanofluids, comprising:

[0009] Prepare aminated carbon nanotube nanofluids as absorbents;

[0010] The absorbent is introduced into the absorbent chamber of the capacitor deionization reactor so that the gas to be treated comes into contact with the absorbent.

[0011] A DC voltage is applied to the capacitor deionization reactor to charge it, so that carbon dioxide is absorbed in the absorbent and captured on the electrode surface via electrochemical action.

[0012] The aminated carbon nanotubes are obtained by modifying acid-oxidized carbon nanotubes with a silane coupling agent.

[0013] Optionally, the acid oxidation process includes placing the carbon nanotubes in a mixed acid solution of concentrated sulfuric acid and concentrated nitric acid, and stirring the reaction under heating conditions.

[0014] Optionally, the modification process using a silane coupling agent includes: dispersing acid-oxidized carbon nanotubes in an ethanol solution, adding an aminosilane coupling agent, subjecting the mixture to ultrasonic treatment, and reacting it under heating conditions.

[0015] Optionally, the concentration of the aminated carbon nanotube nanofluid ranges from 0.005 wt% to 0.2 wt%.

[0016] Optionally, the process of contacting the gas to be treated with the absorbent includes: performing gas-liquid contact under stirring conditions, with a stirring rate ranging from 130 r / min to 530 r / min.

[0017] Optionally, the charging process using a DC voltage is performed in a constant voltage charging mode.

[0018] Optionally, the voltage value for constant voltage charging mode is 0.4V.

[0019] Optionally, the capacitor deionization reactor is a flowing electrode capacitor deionization reactor.

[0020] The technical effects of this invention are as follows: The aminated carbon nanotube (CNT) nanofluid-enhanced FCDI technology provided achieves breakthrough technical effects in the field of carbon dioxide capture, specifically in the following three aspects:

[0021] 1. Significantly improved capture performance:

[0022] Through the synergistic enhancement of the gas-liquid mass transfer process by CNT nanofluids, the system's carbon dioxide capture efficiency achieved a leap forward. Compared to traditional water-based absorbent systems, CNT nanofluids increased the carbon dioxide capture capacity by 133%, raising the capture efficiency from less than 30% to over 67%. This effect stems from the dual mechanism of CNTs: physical mass transfer enhancement: the Brownian motion of nanoparticles significantly accelerates the passage of carbon dioxide molecules through the gas-liquid boundary layer, expanding the effective mass transfer area; and chemical adsorption enhancement: the surface amino functional groups react with carbon dioxide to generate stable carbamates, forming a "carry-and-fix" cyclic pathway.

[0023] 2. Revolutionary reduction in energy consumption:

[0024] CNT nanofluids achieve a dramatic reduction in unit capture energy consumption by optimizing charge migration pathways and lowering system internal resistance. Experiments show that its energy consumption is reduced to 3.05 kJ / mol, a 48.6% reduction compared to traditional water-based absorbent systems and over 97% reduction compared to existing supercapacitor devices (103.6 kJ / mol). This effect stems from a triple synergy: accelerated charge migration: nanoparticles reduce absorbent resistance by 12%, promoting efficient ion transport; optimized electrochemical pathways: the capacitive capture process directly immobilizes bicarbonate / carbonate ions, avoiding high-energy-consuming phase transition processes; and streamlined operating conditions: a low-voltage (0.4V) constant-voltage mode enables precise energy control.

[0025] 3. System stability is fundamentally improved:

[0026] CNT surface amino modification completely solves the problem of carbon nanotube aggregation and ensures long-term stable operation of the system: Dispersion breakthrough: hydrophilic amino groups inhibit particle aggregation, and SEM verification shows that the structural integrity remains unchanged before and after modification; Continuous operation advantage: closed-loop circulation design supports the reuse of absorbent, and the efficiency decay is less than 25% after 10 consecutive cycles. Attached Figure Description

[0027] The accompanying drawings, which form part of this application, are used to provide a further understanding of this application. The illustrative embodiments and descriptions of this application are used to explain this application and do not constitute an undue limitation of this application. In the drawings:

[0028] Figure 1 The diagram shows the CO2 capture performance of FCDI under different absorbents in this embodiment of the invention, including pressure changes, capture rate, and energy consumption. (a) represents relative pressure, (b) represents removal rate, and (c) represents energy consumption.

[0029] Figure 2 This is a schematic diagram of the CO2 capture performance of FCDI under different @CNT nanoparticle mass ratios in embodiments of the present invention, showing the capture amount, capture rate, capture efficiency, and energy consumption, where (a) represents the CO2 capture amount and (b) represents the CO2 capture efficiency.

[0030] Figure 3 The diagram shows the CO2 capture performance of @C-FCDI under different stirring rates in this embodiment of the invention, including capture amount, capture rate, capture efficiency and energy consumption, where (a) is the CO2 capture amount and (b) is the CO2 capture efficiency.

[0031] Figure 4 This is a schematic diagram of the qualitative and quantitative analysis of surface elements of the original CNTs and @CNTs using energy dispersive spectroscopy (EDS) in an embodiment of the present invention, where (a) is the EDS image of CNTs and (b) is the EDS image of @CNTs.

[0032] Figure 5 The following is a schematic diagram of the surface morphology changes of CNTs after amino modification studied using SEM in an embodiment of the present invention, wherein (a) is the original CNTS, (b) is @CNTs, (c) is CNTs after charging, and (d) is @CNTs after charging;

[0033] Figure 6 This is a schematic diagram illustrating the sample characterization results using Fourier transform infrared spectroscopy (FTIR) in an embodiment of the present invention.

[0034] Figure 7 The XPS C1s spectra of this invention are as follows: (a) CNTs, (b) CNTs after charging, (c) CNTs after discharging, (d) @CNTs, (e) @CNTs after charging, and (f) @CNTs after discharging.

[0035] Figure 8 The XPS N1s spectra of this invention are as follows: (a) CNTs, (b) CNTs after charging, (c) CNTs after discharging, (d) @CNTs, (e) @CNTs after charging, and (f) @CNTs after discharging.

[0036] Figure 9 This is a schematic diagram of the gas-liquid mass transfer process in FCDI according to an embodiment of the present invention;

[0037] Figure 10This is a schematic diagram of Nyquist with FCDI containing different absorbents fitted according to an embodiment of the present invention;

[0038] Figure 11 This is a schematic diagram of the mechanism of CO2 capture by FCDI capacitor under the conditions of power-on and power-off in the CO2 capture device of the present invention. (a) shows the change of CO2 capture amount over time under the conditions of power-on and power-off, and (b) shows the mechanism of CO2 capture by FCDI capacitor.

[0039] Figure 12 This is a schematic diagram of the equivalent circuit model of FCDI in an embodiment of the present invention. Detailed Implementation

[0040] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. This application will now be described in detail with reference to the accompanying drawings and embodiments.

[0041] It should be noted that the steps shown in the flowchart in the accompanying drawings can be executed in a computer system such as a set of computer-executable instructions, and although a logical order is shown in the flowchart, in some cases the steps shown or described may be executed in a different order than that shown here.

[0042] This embodiment provides a capacitive deionization carbon dioxide capture method based on aminated carbon nanotube nanofluids, including:

[0043] Prepare aminated carbon nanotube nanofluids as absorbents;

[0044] The absorbent is introduced into the absorbent chamber of the capacitor deionization reactor so that the gas to be treated comes into contact with the absorbent.

[0045] A DC voltage is applied to the capacitor deionization reactor to charge it, so that carbon dioxide is absorbed in the absorbent and captured on the electrode surface via electrochemical action.

[0046] The aminated carbon nanotubes are obtained by modifying acid-oxidized carbon nanotubes with a silane coupling agent.

[0047] Furthermore, the acid oxidation process includes placing carbon nanotubes in a mixed acid solution of concentrated sulfuric acid and concentrated nitric acid, and stirring the reaction under heating conditions.

[0048] Furthermore, the modification process using silane coupling agents includes: dispersing acid-oxidized carbon nanotubes in an ethanol solution, adding an aminosilane coupling agent, subjecting the process to ultrasonic treatment, and reacting under heating conditions.

[0049] Furthermore, the concentration of the aminated carbon nanotube nanofluid ranges from 0.005 wt% to 0.2 wt%.

[0050] Furthermore, the process of contacting the gas to be treated with the absorbent liquid includes: performing gas-liquid contact under stirring conditions, with a stirring rate ranging from 130 r / min to 530 r / min.

[0051] Furthermore, the charging process using DC voltage employs a constant voltage charging mode.

[0052] Furthermore, the voltage value in constant voltage charging mode is 0.4V.

[0053] Furthermore, the capacitor deionization reactor is a flowing electrode capacitor deionization reactor.

[0054] Specifically, the implementation process of this embodiment includes:

[0055] Reactor Structure: The FCDI used in this embodiment is a three-chamber cell consisting of a polyethylene separator and an intermediate spacer (5cm × 6cm × 0.5mm) made of 100-mesh nylon fabric; two ion exchange membranes; and two graphite current collectors with etched serpentine flow channels (2.4mm wide, 2mm deep). The effective contact area between the ion exchange membranes and the flow electrode is 16.3cm². 2 These components are secured together using plexiglass endplates. The electrode slurry is connected to an external storage tank via polyethylene tubing. The intermediate chamber is also connected to the external gas-liquid contact system via polyethylene tubing. The gas-liquid contact system is a closed system of approximately 320 mL sealed with rubber rings, comprising 120 mL of absorbent and 200 mL of gas components. The system is connected to a differential pressure gauge (DP360, Sanliang, China), an air bag, an FCDI inlet pipe, and an FCDI outlet pipe via four extended pipes. The end of the pipe connecting the gas-liquid contact system to the FCDI inlet pipe extends below the water level, and the end of the pipe connecting the gas-liquid contact system to the FCDI outlet pipe is connected to a conical water distribution device made of nylon cloth. The electrode solution is a flowing electrode prepared by dispersing 2.5 wt% commercially available activated carbon in deionized (DI) water, with 60 g of electrode solution for both the cathode and anode.

[0056] Air tightness verification: After assembling the apparatus for each experiment, an air tightness verification experiment is performed before the experiment begins. The specific procedure is as follows: Before each experiment, use a syringe (500mL) to inflate the system with about 200mL of air, let it stand for 5 minutes, and observe the change in air pressure within the system to verify the air tightness of the system. If the change in air pressure is not significant, it proves that the air tightness of the system is good.

[0057] Reactor Operation: The FCDI operates in an isolated closed-loop (ICC) mode to capture CO2 from a gas mixture. Unless otherwise specified, the anolyte and cathode electrode solutions are continuously stirred throughout the experiment to maintain homogeneity. The flow rates of the intermediate absorbent and the flowing electrode are controlled at approximately 10 mL / min by a peristaltic pump. Before the experiment begins, keep valves 3 and 7 closed, open valves 1, 2, 5, 6, 8, and 9, and disconnect valve 4. Start the FCDI's inlet and outlet pumps to allow the absorbent in the gas-liquid contact system to enter the FCDI's intermediate chamber. After the first drop of absorbent reaches valve 9 via valve 2, immediately close valves 1, 2, 4, 5, 6, 8, and 9 and the pumps to fill the FCDI intermediate chamber and system piping with absorbent. Disconnect the two-way valve 9, connect the hose corresponding to valve 9 to a vacuum pump, and open valve 2 to evacuate the system. Once the vacuum pump reading stabilizes and stops decreasing, quickly close valve 2, connect valve 9, and turn off the vacuum pump. Open valve 4 to connect a 500mL gas bag pre-filled with a mixed gas (N2:CO2 = 85:15) to the system. Then open valves 1, 2, 5, 6, 7, 8, and 9 to fill the system with the standard mixed gas, and compress the gas bag to achieve an initial relative pressure of 1.7 ± 0.1 kPa. Immediately afterward, turn on the peristaltic pump, power supply, barometer, and conductivity meter to begin testing. During the charging phase, a constant voltage was applied between the two electrodes for 240 minutes, and the current, voltage, conductivity, and headspace pressure of the gas-liquid contact system were continuously monitored throughout the experiment. After charging, unless otherwise specified, a reverse voltage of 1.2V was applied for discharge for 20 minutes.

[0058] Preparation of amino-carbon nanotubes (@CNTs): 1.0 g of CNTs was placed in a beaker, and 100 mL of sulfuric acid (95%) and nitric acid (70%) in a volume ratio of 3:1 was added. The mixture was stirred at 60 °C for 12 h, cooled to room temperature, filtered, and washed until the filtrate was neutral. The solid was then vacuum dried at 100 °C for 8 h. The obtained sample was added to 60 mL of ethanol, sonicated for 30 min, and 1.5 mL of LAPTES was added. The mixture was stirred at 70 °C for 4 h, filtered, washed repeatedly with deionized water and acetone, and vacuum dried at 80 °C for 20 h to obtain @CNTs.

[0059] Nanofluid preparation: A two-step method was used to prepare the nanofluid. A certain mass of CNT / @CNT was weighed according to experimental requirements and slowly added to a deionized water-based solution. Subsequently, the solution was stirred at 200 rpm for 30 min using a magnetic stirrer, followed by ultrasonication for 1 h to obtain the nanofluid absorbent.

[0060] This embodiment uses a standard mixed gas as the inlet gas, with a gas composition of N2:CO2 = 85:15. CO2 is captured using ICC operation mode. First, a single-factor experimental design was conducted to compare the performance of three FCDI systems using water, CNT absorbent, and @CNT absorbent as the inlet gas. The capture voltage for each test was 0.4V, the nanoparticle concentration was 0.02wt%, and the absorbent stirring rate was 130 r / min; other operations were performed as described above. To optimize the particle concentration, single-factor experiments were conducted with absorbents of different mass percentages of @CNT (0, 0.005wt%, 0.02wt%, 0.05wt%, 0.2wt%). To optimize the stirring rate of the nanofluid absorbent, single-factor experiments were conducted with different stirring rates (0, 130 r / min, 350 r / min, 530 r / min). Finally, the CO2 recovery performance of the system was evaluated at a discharge voltage of 0.4V. In each cycle, fresh gas and absorbent were used instead of the inlet gas and absorbent. Meanwhile, CO2 capture experiments were conducted using the FCDI device without capacitors (0.02 wt%, 130 r / min) under the same operating conditions (all other parameters were the same). All experiments were repeated three times.

[0061] Adding nanoparticles to the absorbent can enhance the CO2 capacitive capture effect of the system. However, CNTs are prone to aggregation during the experiment, which may limit the solubilizing effect of the nanoparticles on the system. Therefore, this embodiment compares the CO2 capacitive capture effects of water absorbent, CNT absorbent, and @CNT (hydrophilic amino carbon nanotubes, which can theoretically solve the aggregation problem) absorbent as FCDI absorbents.

[0062] like Figure 1 As shown, the CO2 capture performance of FCDI under different absorbents is as follows: (a) pressure change, (b) capture rate, and (c) energy consumption (H-FCDI: FCDI with pure water as absorbent; C-FCDI: FCDI with CNT nanofluid as absorbent; @C-FCDI: FCDI with @CNT nanofluid as absorbent).

[0063] from Figure 1 It can be seen that, under the same initial gas pressure (CO2 content), C-FCDI using CNT-containing fluid as the absorbent has significantly superior CO2 capture performance compared to H-FCDI using pure water as the absorbent. Within 4 hours, all three FCDIs for capturing CO2 reached their capture limits, and the gas pressure hardly changed anymore. Figure 1(a) Ultimately, the CO2 captures of C-FCDI and @C-FCDI were 0.57 ± 0.02 mmol and 0.84 ± 0.08 mmol, respectively, representing increases of 58.33% and 133.33% compared to H-FCDI. This demonstrates the enhanced CO2 capture performance of CNTs. Figure 1 (b) Within 4 hours, the CO2 capture rate of @C-FCDI was significantly higher than that of C-FCDI and H-FCDI, indicating that amino modification can greatly enhance the CO2 capture capacity of CNTs. Ultimately, the CO2 capture efficiency in the @C-FCDI system reached 67.01%, which was 45.83% and 131.14% higher than that of C-FCDI and H-FCDI, respectively (Table 1). Furthermore, @C-FCDI also exhibited the lowest energy consumption within 4 hours. Figure 1 The energy consumption (c) of @CNT was only 3.05 ± 0.22 kJ / mol, lower than that of H-FCDI (5.93 ± 1.71 kJ / mol). This may be because @CNT reduces the internal resistance of the system and enhances the CO2 capture ability of FCDI, thus reducing the energy consumption per unit capture. However, as the capture time increases, the capture energy consumption of CNT is higher than that of the blank (water). This may be because the aggregation of CNT increases the mass transfer resistance, while @CNT particles, due to the surface modification with hydrophilic amino groups, reduce the aggregation effect and decrease the possibility of aggregation.

[0064] Within 4 hours, the CO2 capture capacity and capture rate of C-FCDI were significantly improved compared to H-FCDI using pure water as the absorbent. Furthermore, the CO2 capture capacity of @C-FCDI using @CNT as the absorbent was also much higher than that of C-FCDI, as shown in Table 1. This indicates that nanofluids can effectively enhance the CO2 capture of FCDI, and amino nanofluids can effectively promote this enhancement. For example, compared to the FCDI control using pure water, the CO2 capture rate of C-FCDI using CNTs increased by 58.5%, and the CO2 capture rate of @C-FCDI using @CNTs increased by 131.15%. This enhancement effect is consistent with previous studies, which have shown that carbon nanoparticles can enhance the gas-liquid mass transfer of CO2. Lu et al. added AC particles and CNT particles to a isothermal stirred reactor; both types of carbon nanoparticles effectively increased the gas-liquid mass transfer of CO2, with AC achieving an enhancement factor of 3.7. However, further research found that the enhancement factor of AC decreased with increasing stirring rate, while that of CNTs decreased. Rahmatmand et al. used CNTs to increase the CO2 gas-liquid mass transfer rate in a high-pressure closed system by 34%.

[0065] Table 1

[0066]

[0067] The mass percentage (concentration) of nanoparticles in a nanofluid significantly affects its solubilization effect on CO2. Adding nanoparticles to the base fluid can enhance physical convection between the gas and liquid. However, when the nanoparticle load exceeds a critical value, the viscosity of the nanofluid increases, hindering interparticle interactions and potentially reducing microconvection and mass diffusion. Therefore, this embodiment compares the effects of absorbents with different nanofluid mass percentages on the capacitive capture effect of CO2.

[0068] Figure 2 The CO2 capture performance of FCDI under different @CNT nanoparticle mass ratios: capture amount, capture rate, capture efficiency, and energy consumption. (a) represents the CO2 capture amount, and (b) represents the CO2 capture efficiency. Figure 2 It can be seen that as the mass percentage of @CNT nanoparticles in the absorbent increases from 0 to 0.02%, the CO2 capture rate increases accordingly, reaching a maximum of 0.209 ± 0.021 mmol / h when the @CNT mass percentage is 0.02 wt%, with a CO2 capture amount of 0.84 ± 0.08 mmol and a capture efficiency of 67.01 ± 6.59%. Previous studies have also found that increasing nanoparticle concentration enhances the CO2 gas-liquid transfer effect. Mehdipour et al. investigated the effect of nanoparticle concentration on the CO2 absorption enhancement factor of SiO2 and ZnO nanofluids at 0.05 and 0.1 wt%. It was reported that the enhancement factor increases with increasing nanoparticle concentration. This is because the surface adsorption of CO2 by nanoparticles helps improve the solubility of CO2 in the nanofluid. Saidi et al. studied the CO2 absorption of nanofluids using distilled water and amine nanoparticles at different nanoparticle concentrations, and the results showed that the CO2 removal efficiency was higher with increasing nanoparticle concentration.

[0069] However, as the mass percentage of @CNT nanoparticles continued to increase, the capture performance of FCDI actually decreased. When the mass percentage reached 0.2%, the capture rate was 0.121 ± 0.004 mmol / h, a decrease of 42% compared to the peak. Figure 2(a) This may be because excessive nanoparticles hinder gas transport within the system, leading to a decrease in capture performance. Previous studies have also found this phenomenon. Increasing the concentration of Fe3O4 / water nanofluid from 0 to 0.025 wt% increases CO2 absorption. However, further increases in nanoparticle concentration decrease the system's CO2 absorption capacity. This may be due to the hydrophilicity / hydrophobicity of Fe3O4 particles; increasing the nanoparticle concentration leads to particle aggregation and reduces the effective surface area for mass transfer. Nabipour et al. measured the CO2 absorption rate of a sulfonane-MDEA aqueous solution containing MWCNTs, finding a 23.2% increase at a loading of 0.02 wt% compared to a loading of 0. However, further increasing the MWCNT concentration to 0.1 wt% resulted in a decrease in CO2 absorption below that of the base solution. Excessive addition of nanoparticles causes internal aggregation. This weakens the Brownian motion of the particles and increases the viscosity of the absorbent. The enhancing effect of nanoparticles on gas-liquid mass transfer is highly dependent on the Brownian motion of the particles. Liang et al. observed a similar phenomenon when using TiO2 nanoparticles to enhance the gas-liquid mass transfer of CO2. Wang et al.'s research found that the increased solution viscosity due to nanoparticles was a major reason for the reduced enhancement effect.

[0070] Corresponding to the changes in capture performance, when the mass percentage of @CNTs increased from 0% to 0.02% and then further to 0.2%, the unit capture energy consumption decreased from 5.93±1.71kJ / mol to 3.05±0.22kJ / mol and then increased again to 5.39±0.42kJ / mol. The lowest capture energy consumption was achieved when the mass percentage was 0.02%.

[0071] The stirring rate can affect the contact area and boundary layer thickness between the gas and liquid, thus influencing the gas-liquid mass transfer efficiency. Therefore, the stirring rate of the absorbent may have a significant impact on the CO2 capacitive capture efficiency. To investigate the effect of the absorbent stirring rate on CO2 capture by an FCDI capacitor, the capture amount, removal rate, capture efficiency, and energy consumption of CO2 were compared at the same @CNT concentration using different stirring rates (0 r / min, 130 r / min, 350 r / min, 530 r / min, and 700 r / min).

[0072] Figure 3 A schematic diagram of CO2 capture performance of @C-FCDI at different stirring rates (nanoparticle mass percentage is 0.02 wt%), where (a) represents CO2 capture amount and (b) represents CO2 capture efficiency; from Figure 3It can be seen that when the stirring rate increases from 0 to 130 r / min, the FCDI capture rate increases from 0.116 ± 0.005 mmol / h to 0.209 ± 0.021 mmol / h, an increase of 80.17%. The CO2 capture amount and capture efficiency also reach their maximum at a stirring rate of 130 r / min, at 0.837 ± 0.0829 mmol and 67.01 ± 6.59%, respectively. When the stirring rate continues to increase to 530 r / min, the FCDI CO2 capture amount and capture efficiency actually decrease to 0.530 ± 0.00069 mmol and 42.40 ± 0.57%, respectively, a decrease of 36.7% compared to the maximum. This may be because excessively increasing the stirring rate can cause a certain degree of aggregation, thus reducing the capture efficiency. The energy consumption per unit of capture first decreases and then increases with the increase of the stirring rate, reaching its lowest value at 130 r / min. A certain degree of increase in stirring rate (0-130 r / min) can also promote CO2 capture by FCDI; however, a sustained increase will weaken this enhancement effect. This may also be due to the agglomeration effect caused by excessively high stirring rates.

[0073] As shown in Table 2, capacitive CO2 capture devices have evolved from supercapacitors to MCDI (Mechanical Controlled Diode). The absorbent is typically deionized water or a salt solution, which is detrimental to gas-liquid mass transfer between CO2 and the liquid, resulting in low capture rates and efficiencies, and high energy consumption. Kokoszka et al. first proposed that CO2 could be captured by capacitors based on the "Supercapacitive Swing" phenomenon; however, the energy consumption for capture was as high as 103.6 kJ / mol. Subsequently, Liu et al. designed a coin-shaped supercapacitor for CO2 capture and achieved an excellent capture rate (4 × 10⁻⁶ kJ / mol). -8 While the capture rate was 100 mol / s / gCarbon, the energy consumption for capture was still as high as 57 kJ / mol. Legrand et al. achieved CO2 capture using MCDI, but the capture rate was only 4 × 10⁻⁶. -8 The CO2 capture rate (mol / s / gCarbon) is still far lower than that of traditional capture methods such as zeolite adsorption. However, this embodiment utilizes amino-carbon nanotubes to enhance the mass transfer of water vapor within the system, thereby enhancing the CO2 capture rate of CDI. The final CO2 capture rate can reach 3.22 × 10⁻⁶ mol / s / gCarbon. -8 The energy consumption per mol / s / g Carbon is comparable to that of Liu et al.; however, this embodiment achieves a lower capture energy consumption of approximately 3 kJ / mol. According to... Figure 3 (b) The CO2 capture effect of FCDI is mainly accomplished through two processes: CO2 is captured by the absorbent liquid and HCO3 is captured by the FCDI capacitor. - and CO3 2-Therefore, the effects of CNTs and @CNTs on CO2 capture enhancement and energy reduction by FCDI are likely closely related to the promotion of physical absorption in the first stage and charge migration in the second stage. Furthermore, the scalability and continuous operation of FCDI itself give it greater development potential.

[0074] Table 2

[0075]

[0076]

[0077] To verify the successful grafting of amino functional groups onto the surface of carbon nanotubes (CNTs) and to clarify the changes in the elemental composition of amino-based carbon nanotubes (@CNTs), energy-dispersive spectroscopy (EDS) was used to perform qualitative and quantitative analysis of the surface elements of the original CNTs and @CNTs. The characterization results are as follows: Figure 4 As shown in the figure. The EDS spectra of the original CNTs detected C and trace amounts of N and O. Quantitative analysis showed that the relative content of C was as high as 97.85 wt%, O was 7.7 wt%, while the content of N was only 2.8 wt%. Figure 4 (a) C is the main component of carbon nanotubes, while the O element may mainly originate from surface oxidation impurities (such as hydroxyl and carboxyl groups) remaining during the CNTs preparation process, which is consistent with the typical elemental composition characteristics of unmodified carbon nanotubes. However, after modification with amino groups (@CNTs), the intensity of the N element peak in the EDS spectrum is significantly enhanced, as shown in Figure (a). Figure 4 (b) Quantitative results showed that the relative content of nitrogen (N) in @CNTs increased from 2.8 wt% in the original CNTs to 7.0 wt%, and the relative content of oxygen (O) increased from 7.7 wt% to 18.8 wt%. The relative content of carbon (C) decreased slightly to 74.2 wt% due to the introduction of N and O. The presence of nitrogen is direct evidence of the successful loading of amino functional groups (-NH2), while the increase in O content is related to the acid oxidation pretreatment used in the amino modification process (introducing more oxygen-containing functional groups to provide active sites for APTES grafting), further confirming the effectiveness of the modification process.

[0078] To investigate the effect of amino modification on the microstructure of carbon nanotubes (CNTs) and the stability of the nanoparticle structure during CO2 capture, scanning electron microscopy (SEM) was used to characterize pristine CNTs, @CNTs, and samples after CO2 capture. The results are as follows: Figure 5 As shown. According to Figure 5 As can be seen from (a) and (b), the morphological structures of @CNTs and the original CNTs are very similar, indicating that the modification did not change the structure of the CNTs. Furthermore, Figure 5(c) and (d) show that the two types of nanoparticles still exhibit similar regular tubular structures after capturing CO2, indicating that the adsorption process did not change the surface structure of the nanoparticles.

[0079] According to the two-film theory, a stable interface exists between the gas and liquid phases, with a gas film and a liquid film on either side. At the phase interface, the gas and liquid phases can quickly reach equilibrium. Outside the film layer, the gas and liquid flow is completely turbulent and homogeneous. Therefore, there is no concentration gradient or mass transfer resistance in the gas and liquid; the mass transfer resistance is concentrated at the gas-liquid boundary layer. Inside the reactor, the gas-liquid contact area is constant, and the gas and liquid are strongly agitated, forming a simplified two-film model. Numerous studies have been conducted on the factors and mechanisms by which nanofluids enhance mass transfer.

[0080] Current research suggests that the possible mechanisms mainly include inhibiting bubble aggregation (the presence of nanoparticles inhibits bubble aggregation, thereby increasing the gas-liquid mass transfer area), hydrodynamic effects (or boundary layer mixing effects, altering boundary layer fluid properties and reducing mass transfer resistance), transport (nanoparticles freely enter and exit the liquid boundary layer, carrying gas molecules into the liquid bulk), and inhibiting bubble coalescence (bubbles generate numerous small bubbles during collisions with nanoparticles, increasing the mass transfer area). Since the nanoparticles used in this study are small in size, and carbon nanotubes possess a unique hollow tubular structure capable of absorbing and transporting gases, transport is likely the most important mechanism. SEM images show that the morphology of the original CNTs, @CNTs, and particles before and after charge-discharge remained largely unchanged, maintaining a regular tubular structure; amino modification did not alter the surface structure of the nanoparticles. Hydrodynamic effects (including shuttle effects) are achieved through the physical motion of the nanoparticles. Since the morphological structure of the modified nanoparticles did not change significantly, the hydrodynamic effects of @CNTs and CNTs are similar. This indicates that the physical enhancement mechanism of @CNTs on the system after amino modification remained unchanged, and both are likely transport mechanisms.

[0081] To clarify the surface functional group composition of CNTs and @CNTs, and their chemical changes during CO2 capture (charging), Fourier transform infrared spectroscopy (FTIR) was used to characterize the samples. The results are as follows: Figure 6 As shown, the FTIR spectra clearly reflect the changes in surface functional groups of CNTs caused by amino modification. Compared with CNTs, @CNTS show improved performance at 2850 cm⁻¹. -1 and 2920cm -1 The nearby peaks become more pronounced, which is likely due to the presence of both symmetrical and asymmetric CH bonds. The FTIR spectra of both CNTs and @CNTs are in the range of 3018–3698 cm⁻¹. -1A broad and distinct peak appears in this range, which may be attributed to the stretching vibrations of OH and NH. CNTs show only a weak and broad absorption peak in this range, possibly attributed to the OH stretching vibrations of surface-adsorbed water. @CNTs, however, exhibit significantly enhanced peak intensity and a broader peak shape in this range. This is likely due to the introduction of the amino functional group (-NH2), which superimposes both symmetric and asymmetric NH stretching vibrations (overlapping with the OH vibration peaks), demonstrating successful amino loading onto the CNT surface. (1637 cm⁻¹) -1 The peak at that point may be attributed to bicarbonate (HCO3-). - In-plane bending vibration of the C=O bond in CNTs. After charging, @CNTs exhibit in-plane bending vibration at 1637 cm⁻¹. -1 The peaks near the point become more pronounced, reflecting the presence of HCO3 on the @CNT surface. - The generation of . Furthermore, 1550cm -1 1445cm -1 Possibly related to protonated amino groups (-NH3) + The deformation and vibration are related to 1100cm. -1 1172cm -1 The peak at that location may be related to protonated amino groups (-NH3). + The peaks in the infrared spectra of @CNTs are related to the asymmetric vibrations of CO2 and -NH2, and these peaks become more pronounced after charging, which proves the production of protonated amino groups after charging @CNTs. Protonated amino groups are important intermediate products of the reaction between CO2 and -NH2, therefore, CO2 may have chemically combined with -NH2 within the system.

[0082] To further investigate the effect of amino functionalization on the mechanism of carbon nanotubes (CNTs) in CO2 capture, X-ray photoelectron spectroscopy (XPS) was used to analyze the surface carbon chemical states of pristine CNTs, amino-functionalized carbon nanotubes (@CNTs), and their states after charging (CO2 capture) and discharging (CO2 release). The results of the fine C 1s and N 1s spectra are shown below. Figure 6 and Figure 7 As shown, this provides key molecular-level evidence for understanding the physical and chemical mechanisms by which nanofluids enhance CO2 capture.

[0083] like Figure 7 As shown, XPS C1s spectra (a) CNTs, (b) CNTs after charging, (c) CNTs after discharging, (d) @CNTs, (e) @CNTs after charging, and (f) @CNTs after discharging, for the original CNTs ( Figure 7(a) Its C1s spectrum can be decomposed into five characteristic peaks located at 284.50 eV, 285.70 eV, 286.98 eV, 288.37 eV and 290.72 eV, which are respectively attributed to CC / C=C(sp 2 The experiment showed peaks for carbon, CO (hydroxyl / ether), C=O (carbonyl), OC=O (carboxyl), and π-π transitions. This indicates that the CNTs used in the experiment themselves contain a certain amount of oxygen-containing functional groups. It is noteworthy that after charging (… Figure 7 (b) and discharge ( Figure 7 After process (c), no new characteristic peaks appeared in the C1s spectrum of CNTs, and the position and relative intensity of each peak did not change significantly compared with the original state. This phenomenon clearly shows that CNTs mainly play a physical role in their interaction with CO2 (such as promoting mass transfer through the "shuttle effect"), and the chemical state of their surface carbon does not change, and no obvious chemical reaction occurs between them and CO2.

[0084] In stark contrast to CNTs, @CNTs exhibited a crucial change in their C1s spectra after undergoing an electrochemical CO2 capture (charging) process. In addition to all the characteristic peaks present in the aforementioned CNTs, a distinct new peak was observed at ~285.36 eV. This peak position can be clearly attributed to the CN bond, providing direct evidence for the formation of carbamates (-NHCOO-). Carbamates are key intermediate products in the reversible chemical reaction between CO2 and primary amines (-NH2).

[0085] This result indicates that the amino functional groups on the surface of @CNTs undergo specific chemical interactions with dissolved CO2 during the voltage-capture process, thus surpassing the only physical enhancement mechanism of CNTs. After discharge (applying a reverse voltage to release CO2), the C1s spectrum of @CNTs ( Figure 7 In (e)), the CN peak still exists at 285.47 eV, but its intensity is weakened compared to the charging state. This indicates that the chemisorbed CO2 (in the form of carbamate) is partially reversible during electrochemical release, but not completely reversible, and some reaction products may still remain stably present on the carbon nanotube surface.

[0086] like Figure 8As shown, XPS N1s spectra (a) CNTs, (b) CNTs after charging, (c) CNTs after discharging, (d) @CNTs, (e) @CNTs after charging, and (f) @CNTs after discharging are presented. XPS N1s spectra characterize the chemical evolution of nitrogen species in nanoparticles under different states. The results show that no obvious nitrogen signal was detected in the original CNTs, while @CNTs showed amino (-NH2) and nitrogen oxide (NO) peaks at 399.88 eV and 402.01 eV, respectively, confirming successful amino grafting. After charging (CO2 capture), the N1s spectra of @CNTs showed carbamate (-NHCOO-) at 400.15 eV and protonated amine (-NH3) at 401.83 eV. + The characteristic peaks indicate that CO2 reacts chemically with the surface amine groups of @CNTs to form reversible intermediates. After discharge, the nitrogen species spectrum returned to its original state, retaining only the -NH2 and NO peaks, indicating that the chemisorption process has good electrochemical reversibility, which is beneficial for recycling and reducing energy consumption. In contrast, CNTs did not show any change in nitrogen species after charging and discharging, indicating that their performance improvement mainly depends on physical mass transfer mechanisms. This is consistent with the results of the C1s spectrum.

[0087] according to Figure 9 The morphology of the original CNTS, @CNTS, and both before and after charging remained largely unchanged, exhibiting a regular tubular structure. Amino modification did not alter the surface structure of the nanoparticles. Hydrodynamic effects (transportation, etc.) are mediated by the physical motion of the nanoparticles. Since the morphology and structure of the modified nanoparticles did not change significantly, the hydrodynamic effects of CNTs and @CNTs were similar. This indicates that the physical enhancement mechanism of the system by amino-modified @CNTs did not change. However, @CNTs exhibited superior CO2 capture performance compared to the original CNTS, suggesting that new CO2 binding sites appeared in @CNTS during transport. Figure 5 It can be observed in FTIR that the C=O functional group increases after @CNTS charging, possibly due to an increase in carbamate or bicarbonate groups. Furthermore, the protonated amino functional group also increases after charging, while the -NH3 group... + This provides important evidence for the chemical reaction between CO2 and -NH2. Therefore, we hypothesize that carbon dioxide, as the anionic component introduced into the nanofluid, interacts with some amine functional groups (as cationic components) to generate intermediates. The generated intermediates deprotonate some intermediates involved in the formation of protonated groups and carbamates on the surface of the amine-modified nanostructures. The result is the stabilization of CO2 on the nanoparticle surface via the following reaction:

[0088] RNH2 + CO2 → RNH2 + COO -;

[0089] RNH2 + COO - +RNH2→RNHCOO - +RNH3 + ;

[0090] or

[0091] RNH2 + COO - +H2O→RNH3 + +HCO3 - ;

[0092] Therefore, the mechanism by which nanoparticles in FCDI physically enhance CO2 gas-liquid mass transfer can be inferred as follows: Figure 9 As shown, the introduction of amino groups enhances the ability of nanoparticles to bind with CO2 gas, significantly improving their ability to transport CO2 molecules, thereby significantly enhancing the transport effect. According to... Figure 11 (a) Within 4 hours, the transport effect of @CNT on the system can be enhanced by up to 309%, which is higher than the 81.8% of CNT, reflecting the enhancement of the transport effect of CNT by the introduction of amino groups.

[0093] In capacitive deionization systems, the addition of particles to the absorbent not only affects the gas-liquid mass transfer efficiency but also the electrochemical performance of FCDI, thus influencing the final capacitive capture effect. Therefore, EIS testing was conducted to investigate the electrochemical performance of different FCDI systems. Based on... Figure 10 As can be seen, the Nyquist plots for FCDI in different absorbents all consist of a similar linear structure followed by an arc-shaped structure. The linear region indicates that ion capture within different systems is achieved through double-layer capacitance. The intersection of the high-frequency region of the electrolyte curve with the true impedance axis reflects the system's resistance. C-FCDI exhibits the lowest system resistance at 10.94 Ω, lower than @C-FCDI's 12.49 Ω and H-FCDI's 13.88 Ω. This suggests that the @CNT nanofluidic pure water absorbent reduces the system's internal resistance; however, compared to CNTs, the amino modification actually increases the system's internal resistance.

[0094] Figure 11 (a) CO2 capture rate over time with and without power (the numbers next to the arrows indicate the growth rate of the water absorbent compared to the CO2 capture rate). (b) Mechanism of CO2 capture by FCDI capacitor.

[0095] To analyze the impact of CNT introduction on the first and second stages of the system, the changes in CO2 capture over time were compared under the conditions of added and unadded power, as follows: Figure 11(a). By comparing the changes in the CNT-introduced system without electricity and the @CNT system, the influence of carbon-based nanoparticles on the physical absorption of CO2 in the system can be inferred. According to Figure 11 In (a), without the addition of capacitance, water and CNTs reached their adsorption limits within 1 hour, and the adsorption amounts remained unchanged thereafter. However, @CNTs maintained a continuous increasing trend in CO2 capture over 4 hours. Within 4 hours, in the uncharged system, the CO2 absorption of the CNT absorbent was 0.2 ± 0.02 mmol, an increase of 81.82% compared to the water absorbent (0.11 ± 0.01 mmol). With capacitance, the CNT absorbent showed an increase of 58.33% compared to the water absorbent. Similarly, in the uncharged system, the CO2 absorption of the @CNT absorbent increased by 309.09% compared to the water absorbent, while with capacitance, the increase was only 133.33%. This demonstrates that the main contribution of CNTs and @CNTs to promoting capacitive CO2 absorption comes from their enhanced physicochemical absorption. Furthermore, it can be observed that without capacitance, the absorption of @CNTs increased by 125% compared to CNTs, while with capacitance, the increase was 47.37%. In the first hour, the growth rates of CNT and @CNT absorbents with and without power were similar compared to those of water absorbents.

[0096] like Figure 12 As shown, this embodiment proposes an equivalent circuit for an FCDI, where the current carriers are electrons and ions, connected by a double-layer capacitor. The equivalent circuit better represents the resistance of each component, facilitating the study of current-resistance changes caused by nanofluids. Since H-FCDI, C-FCDI, and @C-FCDI are identical in all components except for the absorbent, the difference in R caused by the absorbent can be approximated as the only difference among the three FCDIs. pp ’ and R ion ’ The values ​​differ. The conductivity of the absorbent can approximately reflect the internal resistance of the absorbent. During the capture process, the conductivity of the absorbent containing CNTs in C-FCDI is consistently higher than that of the absorbent containing @CNTs in @C-FCDI, and also higher than that of the water absorbent. This indirectly reflects the Rt values ​​in C-FCDI, @C-FCDI, and H-FCDI. pp ’ +R ion ’The values ​​increase sequentially. CNT nanoparticles and @CNT nanoparticles reduce the resistance of the FCDI system. This is also reflected in the Nyquist plot, with resistances of C-FCDI, @C-FCDI, and H-FCDI being 10.94 Ω, 12.49 Ω, and 13.88 Ω, respectively. CNT nanoparticles and @CNT nanoparticles reduce the particle contact resistance and ion migration resistance of the absorbent, thereby accelerating electron transport and ion migration in FCDI, and the enhancing effect of CNTs is better than that of @CNTs. Meanwhile, according to... Figure 11 In (a), the absorption of @CNTs increased by 125% compared to CNTs without electricity, while it increased by 47.37% after electricity was applied. This indicates that @CNTs are not as effective as CNTs in promoting charge transfer. However, the final CO2 capture effect was better with @C-FCDI than with C-FCDI, which also reflects that this promoting effect on charge and ion migration did not play a dominant role in the system.

[0097] The above are merely preferred embodiments of this application, but the scope of protection of this application is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in this application should be included within the scope of protection of this application. Therefore, the scope of protection of this application should be determined by the scope of the claims.

Claims

1. A capacitive deionization carbon dioxide capture method based on aminated carbon nanotube nanofluids, characterized in that, include: Prepare aminated carbon nanotube nanofluids as absorbents; The absorbent is introduced into the absorbent chamber of the capacitor deionization reactor so that the gas to be treated comes into contact with the absorbent. A DC voltage is applied to the capacitor deionization reactor to charge it, so that carbon dioxide is absorbed in the absorbent and captured on the electrode surface via electrochemical action. The aminated carbon nanotubes are obtained by modifying acid-oxidized carbon nanotubes with a silane coupling agent.

2. The capacitive deionization carbon dioxide capture method based on aminated carbon nanotube nanofluids as described in claim 1, characterized in that, The acid oxidation process includes placing carbon nanotubes in a mixed acid solution of concentrated sulfuric acid and concentrated nitric acid, and stirring the solution under heating conditions.

3. The capacitive deionization carbon dioxide capture method based on aminated carbon nanotube nanofluids as described in claim 1, characterized in that, The process of modification using silane coupling agents includes: dispersing acid-oxidized carbon nanotubes in an ethanol solution, adding an aminosilane coupling agent, ultrasonic treatment, and reacting under heating conditions.

4. The capacitive deionization carbon dioxide capture method based on aminated carbon nanotube nanofluids as described in claim 1, characterized in that, The concentration range of the aminated carbon nanotube nanofluid is 0.005 wt% to 0.2 wt%.

5. The capacitive deionization carbon dioxide capture method based on aminated carbon nanotube nanofluids as described in claim 1, characterized in that, The process of bringing the gas to be treated into contact with the absorbent includes: gas-liquid contact under stirring conditions, with a stirring rate ranging from 130 r / min to 530 r / min.

6. The capacitive deionization carbon dioxide capture method based on aminated carbon nanotube nanofluids as described in claim 1, characterized in that, The charging process using DC voltage employs a constant voltage charging mode.

7. The capacitive deionization carbon dioxide capture method based on aminated carbon nanotube nanofluids as described in claim 1, characterized in that, The voltage value in constant voltage charging mode is 0.4V.

8. The capacitive deionization carbon dioxide capture method based on aminated carbon nanotube nanofluids as described in claim 1, characterized in that, The capacitor deionization reactor is a flowing electrode capacitor deionization reactor.

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