Method for preparing carbon nanotube-based composite microsphere energy storage material through continuous flow

Abstract

The invention discloses a method for preparing a carbon nanotube-based composite microsphere energy storage material through continuous flow. The invention discloses a preparation method for constructing a microspherical composite carbon nanotube supercapacitor material based on microfluidics, and belongs to the technical field of supercapacitors. The method comprises the following steps: acidifying carbon nanotubes to improve dispersibility; the preparation method comprises the following steps: by taking dimethyl silicone oil as an oil phase and acidified carbon nanotube dispersion liquid as a water phase, regulating and controlling a flow rate ratio through a microfluidic technology, and preparing carbon nanotube microspheres with uniform sizes; performing high-temperature pyrolysis to obtain a conductive microsphere framework; the MF-CNTs (at) ZIF-8 composite material is obtained by taking the MF-CNTs (at) ZIF-8 as a carrier, performing in-situ growth of ZIF-8 through a solution impregnation method and then performing secondary pyrolysis. According to the method, accurate and controllable preparation of the carbon nanotube microspheres is realized, and a conductive network of the carbon nanotubes and high specific surface area and pseudocapacitance activity of the ZIF-8 are synergistically exerted by combining in-situ compounding of the ZIF-8. The specific capacitance of the prepared material in a 6 M KOH electrolyte reaches 356.2 F.g <-1 >, the rate capability is excellent, and the capacity retention rate is 81.6% after 3000 cycles. The method has the advantages of controllable steps, mild conditions, low cost, easiness in continuous production and large-scale application potential.

Description

Technical Field

[0001] This invention relates to the field of advanced electrochemical energy storage materials technology, specifically to a method for preparing electrode materials for supercapacitors, and more particularly to a method for preparing composite carbon nanotube materials for high-performance supercapacitors by constructing a three-dimensional conductive framework of microspherical carbon nanotubes using microfluidic technology and in-situ composite with metal-organic framework derivatives. Background Technology

[0002] With the rapid development of renewable energy systems, smart grids, and portable / wearable electronic devices, the demand for energy storage devices that combine high power density, high energy density, and long cycle life is becoming increasingly urgent. Supercapacitors, due to their high power density, fast charge / discharge speed (seconds), long cycle life (up to hundreds of thousands of cycles), and good safety, have become a research hotspot in the energy storage field. However, compared with traditional secondary batteries, their energy density is relatively low, which limits their widespread application in applications requiring high energy storage. The key to improving the energy density of supercapacitors lies in developing electrode materials with high specific capacitance and a wide operating voltage window. Among many electrode materials, carbon nanotubes are considered ideal double-layer capacitor materials due to their extremely high specific surface area, excellent conductivity, and superior mechanical stability. However, the capacitance of pure carbon nanotube electrodes mainly depends on physical charge adsorption, and their specific capacitance still has room for improvement. To solve this problem, researchers are committed to combining carbon nanotubes with metal-organic framework materials with high Faradaic pseudocapacitance. Among them, ZIF-8, as a typical zeolite imidazole ester framework material, has an extremely high specific surface area, a regular microporous structure and abundant nitrogen elements. It can not only contribute to the double-layer capacitance through its huge specific surface area, but its nitrogen species can also induce a certain pseudocapacitance, thereby significantly improving the overall electrochemical performance of the composite material.

[0003] Metal-organic frameworks (MOFs) are a class of crystalline porous materials with periodic network structures formed by the self-assembly of metal ions / clusters and organic ligands. Among them, the zeolite imidazole ester framework material ZIF-8 possesses extremely high specific surface area, a regular microporous structure, abundant nitrogen species, and good thermal / chemical stability. When combined with carbon nanotubes, ZIF-8 not only provides a large surface area for double-layer capacitance, but its nitrogen-containing species also contribute reversible pseudocapacitance. Furthermore, its pyrolysis derivatives can form nitrogen-doped porous carbon, further optimizing the material's conductivity and wettability. However, carbon nanotube / ZIF-8 composites prepared by traditional methods (such as solution mixing and hydrothermal methods) often suffer from problems such as uneven ZIF-8 nanocrystal distribution, easy aggregation, and weak bonding between ZIF-8 and carbon nanotubes. This leads to obstructed ion / electron transport pathways and low utilization of active sites in the composite material, thus affecting the rate performance and cycle stability of the device. Therefore, achieving uniform and robust loading of ZIF-8 on the carbon nanotube surface and precisely controlling the overall microstructure of the composite material is crucial for obtaining high-performance electrode materials. Microfluidics, a cutting-edge technology capable of precisely manipulating fluid flow, mixing, and reactions within micrometer-scale channels, provides a revolutionary platform for material synthesis. It enables highly controllable spatiotemporal mixing of reactants, allowing for the reproducible preparation of micro / nanomaterials with uniform size and regular morphology. Applying microfluidics to the fabrication of electrode materials holds promise for solving the problem of uncontrollable structure development inherent in traditional methods.

[0004] Based on this, this invention proposes a novel method for constructing microspherical composite carbon nanotube supercapacitor materials using microfluidic technology. This method first utilizes microfluidic technology to prepare uniformly sized carbon nanotube microspheres, which serve as ideal three-dimensional conductive frameworks and carriers. Subsequently, ZIF-8 is grown in situ on the surface of the microspheres via solution impregnation, and then subjected to high-temperature pyrolysis to finally form a composite structure. This method aims to synergistically enhance the conductivity, specific surface area, and electrochemical activity of the material through precise microfluidic control and in-situ composite formation with ZIF-8, thereby obtaining high-performance supercapacitor electrode materials. By precisely controlling the microfluidics, the structural defects of traditional preparation methods are overcome, opening a novel and promising pathway for developing next-generation high-performance supercapacitor electrode materials. Summary of the Invention

[0005] (a) Purpose of the invention The purpose of this invention is to overcome the shortcomings of existing methods for preparing carbon nanotube-based composite electrode materials, such as uncontrollable structure, uneven dispersion of active components, and limited performance improvement. This invention provides a method for preparing microfluidically constructed microspherical composite carbon nanotube supercapacitor materials that is stable, operates under mild conditions, allows for precise control of the microstructure, and enables continuous production. The materials prepared by this method exhibit high specific capacitance, excellent rate performance, and good cycling stability.

[0006] (II) Technical Solution To achieve the above objectives, the present invention adopts the following technical solution: A method for preparing microspherical composite carbon nanotube supercapacitor materials based on microfluidic construction includes the following steps: 1. Acidification pretreatment of carbon nanotubes: Carbon nanotubes are acidified using a mixture of concentrated sulfuric acid and concentrated nitric acid to introduce oxygen-containing functional groups such as carboxyl and hydroxyl groups onto their surface, thereby improving their dispersibility in water and their ability to subsequently bind with metal ions. The preferred acid volume ratio is concentrated sulfuric acid:concentrated nitric acid = 3:1. The treatment conditions are: 15 ml of mixed acid per 100 mg of carbon nanotubes, followed by sonication for 60 min, magnetic stirring for 2 h, and finally washing until neutral.

[0007] 2. Microfluidic Preparation of Carbon Nanotube Microspheres: Dimethyl silicone oil is used as the continuous phase (oil phase), and an acidified aqueous dispersion of carbon nanotubes is used as the dispersed phase (aqueous phase). A microfluidic device (such as a flow focusing chip) is used to precisely control the flow rate ratio of the oil phase to the aqueous phase (preferably 10:1), causing the aqueous phase to be sheared into monodisperse microdroplets within the oil phase, forming a water-in-oil emulsion. These emulsion droplets are collected in a polytetrafluoroethylene reactor containing dimethyl silicone oil and heated in an oil bath at 120°C. This causes the water within the droplets to evaporate, and the carbon nanotubes self-assemble and solidify, forming uniformly sized carbon nanotube microspheres.

[0008] 3. Primary pyrolysis of carbon nanotube microspheres: The obtained carbon nanotube microspheres are subjected to high-temperature pyrolysis under nitrogen protection (preferably 700℃, held for 2 h, heating rate 1-5℃ / min). This process removes unstable oxygen-containing functional groups from the microspheres, improves the graphitization degree and conductivity of the carbon material, and forms a stable three-dimensional porous microsphere framework, providing a structural basis for subsequent loading.

[0009] 4. In-situ growth of ZIF-8 and preparation of composite materials: ZIF-8 was grown in situ using a solution method with pyrolyzed carbon nanotube microspheres as a carrier. First, a methanol solution of zinc nitrate hexahydrate (solution A) and a methanol solution of 2-methylimidazole (solution B) were prepared to control the Zn content. 2+ The molar ratio of Zn to 2-methylimidazole is 4:1. Carbon nanotube microspheres are immersed in solution A and stirred to allow Zn to... 2+ The ZIF-8 crystals adsorbed onto the surface and pores of the microspheres. Solution B was then added, and the mixture was stirred overnight at room temperature to allow for in-situ nucleation and growth of ZIF-8 crystals on and inside the microspheres. After the reaction was complete, the ZIF-8-loaded composite precursor underwent a second high-temperature pyrolysis (preferably 700°C, nitrogen atmosphere) to convert ZIF-8 into nitrogen-doped porous carbon, which then tightly bonded to the carbon nanotube microsphere framework, ultimately yielding the target composite material, denoted as MF-CNTs@ZIF-8.

[0010] As can be seen from the technical solution provided by the present invention above, the method for preparing a microspherical composite carbon nanotube supercapacitor based on microfluidic construction provided by the present invention has the following significant advantages compared with the prior art: 1. Precise and controllable structure, excellent performance: Microfluidic technology allows for the reproducible fabrication of carbon nanotube microspheres with uniform size and good sphericity, solving the problem of uneven product morphology in traditional methods. Using these microspheres as a carrier, uniform and robust loading of ZIF-8 derived nanoparticles was achieved, constructing a hierarchical porous conductive network. This unique structure facilitates rapid ion penetration and transport in the electrolyte, providing abundant electrochemical active sites, resulting in a material exhibiting a high performance of 356.2 F•g in 6 M KOH electrolyte. -1 It has high specific capacitance and excellent rate performance.

[0011] 2. Significant Synergistic Effect: This invention achieves an effective synergy between the excellent conductive network of carbon nanotubes and the high specific surface area and pseudocapacitive activity of ZIF-8 derived carbon. The carbon nanotube microsphere framework ensures rapid electron conduction, while the uniformly distributed ZIF-8 derivative contributes a large amount of double-layer capacitance and reversible pseudocapacitance, jointly enhancing the overall energy storage performance of the material.

[0012] 3. Excellent Cyclic Stability: The three-dimensional framework structure of carbon nanotube microspheres exhibits good mechanical stability. ZIF-8, through in-situ growth and pyrolysis, firmly bonds with the carbon substrate. This structure effectively buffers volume changes during charge and discharge, preventing the shedding of active materials. Tests show that this material exhibits good cycle stability at 10 A·g⁻¹. -1 After 3000 cycles at high current density, the specific capacitance retention rate can still reach 81.6%, demonstrating excellent long-term cycle stability.

[0013] 4. Advanced preparation process, easy to scale up: The microfluidic synthesis process is stable, continuous, and controllable, with mild reaction conditions (room temperature and atmospheric pressure), requiring no complex equipment. The entire process is simple, highly reproducible, uses inexpensive raw materials, is environmentally friendly, and has the potential for large-scale continuous production, which is conducive to promoting the commercial application of high-performance supercapacitor materials. Attached Figure Description

[0014] Figure 1 This is a scanning electron microscope image of the MF-CNTs@ZIF-8 composite material; Figure 2 X-ray diffraction patterns of MF-CNTs@ZIF-8 composite material, ZIF-8 and carbon nanotubes (scanning range: 5°-80°, step size: 0.01°, scan rate: 10° / min). Figure 3This is the X-ray photoelectron spectrum of the MF-CNTs@ZIF-8 composite material; Figure 4 This is a cyclic voltammetric characteristic curve of the MF-CNTs@ZIF-8 composite material (test voltage scan range: 0-0.5V). Figure 5 The constant current charge-discharge curves of MF-CNTs@ZIF-8 composite material are shown. Figure 6 This is a comparison of the specific capacitance of MF-CNTs@ZIF-8 composite materials at different calcination temperatures; Figure 7 This is a comparison chart of the specific capacitance of MF-CNTs@ZIF-8 composite materials with different ZIF-8 loadings; Figure 8 These are the electrochemical impedance spectra of MF-CNTs@ZIF-8 composite materials at different calcination temperatures; Figure 9 These are electrochemical impedance spectroscopy spectra of MF-CNTs@ZIF-8 composites with different ZIF-8 loadings; Detailed Implementation

[0015] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0016] To better understand the above technical solutions, the following will provide a detailed explanation of the technical solutions in conjunction with the accompanying drawings and specific embodiments.

[0017] like Figure 1-4 As shown, this embodiment of the invention provides a method for preparing a microspherical composite carbon nanotube supercapacitor based on microfluidic construction, comprising the following steps: Step 1: Acidification of carbon nanotubes: Prepare an acidification solution using concentrated sulfuric acid and concentrated nitric acid. Mix the aqueous dispersion of carbon nanotubes with the acidification solution, sonicate, stir magnetically, wash with ultrapure water until neutral, and then redisperse in ultrapure water for later use. The ratio of concentrated sulfuric acid to concentrated nitric acid in the acidification solution is 3:1. Use 15 ml of acidification solution for every 100 mg of carbon nanotubes, sonicate for 60 min, stir for 2 h, and then wash. Step 2: Microfluidic Preparation of Carbon Nanotube Microspheres: Dimethyl silicone oil is used as the oil phase, and an acidified aqueous dispersion of carbon nanotubes is used as the aqueous phase. These are mixed using a microfluidic device to form an oil-in-water solution containing carbon nanotube droplets. This solution is then passed into an oil bath and solidified into carbon nanotube microspheres. The flow rate ratio of the oil phase to the aqueous phase is controlled at 10:1. The receiving container for the oil-in-water solution is a polytetrafluoroethylene reactor containing dimethyl silicone oil, which is heated in an oil bath at 120°C. Step 3: The carbon nanotube microspheres are uniformly dispersed on the bottom of the ceramic boat and placed in a tube furnace for high-temperature pyrolysis under nitrogen protection, followed by natural cooling to room temperature. The specific operation of the high-temperature pyrolysis is as follows: under a pure nitrogen atmosphere, the temperature is increased to 700℃ at a rate of 1-5℃ / min, held at this temperature for 2 hours, and then naturally cooled to room temperature. Step 4: Zinc nitrate hexahydrate was added to methanol to prepare solution A, and 2-methylimidazole was added to methanol to prepare solution B. The pyrolyzed carbon nanotube microspheres were first placed in solution A and magnetically stirred, then added to solution B. After stirring overnight, the microspheres were evenly dispersed at the bottom of a ceramic boat and placed in a tube furnace for high-temperature pyrolysis under nitrogen protection. The mixture was then naturally cooled to room temperature to obtain the supercapacitor material. The molar ratio of solute in solution A to solution B was 4:1, and the pyrolysis temperature was 700℃.

[0018] This invention discloses a method for preparing a microfluidic-based microspherical composite carbon nanotube supercapacitor, and the application of this material as a supercapacitor.

[0019] The active substance described in this invention is abbreviated as MF-CNTs@ZIF-8.

[0020] This invention uses a platinum sheet electrode as the counter electrode, saturated silver chloride electrodes (Ag / AgCl) as reference electrodes, and nickel foam loaded with active material placed on a platinum sheet working electrode clamp as the working electrode.

[0021] The polymer binder added during the preparation of the working electrode described in this invention is polyvinylidene fluoride with a concentration of 10 wt%.

[0022] The working electrode of this invention is prepared by a wet coating process, in which active material, polymer binder and conductive agent (acetylene black) are compounded at a mass ratio of 8:1:1, ethanol is used as the dispersion medium, and the mixture is dispersed in a magnetic stirrer for 1 hour to form a homogeneous slurry. Subsequently, the slurry is loaded onto the surface of a pretreated nickel foam current collector (1×1 cm²) using a dip-coating method, and then dried in a vacuum drying oven at 60°C for 12 hours to achieve solvent evaporation.

[0023] All supercapacitor performance tests described in this invention were conducted in 6 M KOH electrolyte.

[0024] The composite material described in this invention requires CV activation for 3 cycles before electrochemical testing.

[0025] The catalysts described in this invention are all tested at room temperature to prevent large temperature variations from affecting the performance of the composite materials.

[0026] The present invention will be further illustrated below with reference to specific embodiments. To further understand the present invention, preferred embodiments are described in conjunction with the embodiments. However, it should be understood that these descriptions are merely for further illustrating the features and advantages of the present invention, and not for limiting the scope of the claims. Furthermore, it should be understood that after reading the disclosure of this invention, those skilled in the art can make various alterations or modifications to the present invention, and these equivalent forms also fall within the scope of protection defined by this invention.

[0027] Unless otherwise specified, the experimental methods used in the following examples are conventional methods; unless otherwise specified, the reagents and materials used in the following examples are commercially available. Example

[0028] This embodiment demonstrates a method for preparing a microspherical composite carbon nanotube supercapacitor MF-CNTs@ZIF-8 based on microfluidic construction.

[0029] (1) Acidification of carbon nanotubes: Prepare an acidification solution by mixing concentrated sulfuric acid and concentrated nitric acid. The ratio of concentrated sulfuric acid to concentrated nitric acid in the preparation of the acidification solution is 3:1. Mix the aqueous dispersion of carbon nanotubes with the acidification solution, sonicate for 60 min and stir for 2 h, wash, wash with ultrapure water until neutral, and then redisperse in ultrapure water for later use. (2) Microfluidic preparation of carbon nanotube microspheres: Dimethyl silicone oil is used as the oil phase and acidified aqueous dispersion of carbon nanotubes is used as the aqueous phase. The flow rate ratio of the oil phase and the aqueous phase is controlled at 10:1. The mixture is mixed by a microfluidic device to form an oil-in-water solution of carbon nanotube droplets. Then, it is passed into a polytetrafluoroethylene reactor containing dimethyl silicone oil heated at 120°C in an oil bath to solidify into carbon nanotube microspheres. (3) Carbon nanotube microspheres were uniformly dispersed on the bottom of a ceramic boat and placed in a tube furnace. Under a pure nitrogen atmosphere, the temperature was increased to 700℃ at a heating rate of 1-5℃ / min. After holding at this temperature for 2 hours, the temperature was naturally cooled to room temperature. (4) 0.297 g of zinc nitrate hexahydrate was added to 20 ml of methanol to prepare solution A, and 0.328 g of 2-methylimidazole was added to 20 ml of methanol to prepare solution B. The pyrolyzed carbon nanotube microspheres were first placed in solution A and magnetically stirred for 10 min, then added to solution B. After stirring overnight, the microspheres were evenly dispersed at the bottom of a ceramic boat and placed in a tube furnace. The furnace was then pyrolyzed at 700 °C under nitrogen protection and allowed to cool naturally to room temperature to obtain the supercapacitor material. The phase identification and microstructure and structure characterization of the MF-CNTs@ZIF-8 material obtained in this embodiment were performed: the phase identification of the prepared material was performed using Raman spectroscopy, powder X-ray diffraction and X-ray photoelectron spectroscopy, and the microstructure and structure of the obtained material were characterized using scanning electron microscopy.

[0030] Figure 1 This is a scanning electron microscope (SEM) image of the MF-CNTs@ZIF-8 composite material. The image shows that the material exhibits a spherical carbon nanotube structure, with ZIF-8 particles uniformly loaded between the pores of the carbon nanotubes, demonstrating successful loading of the active component. Furthermore, we can see that the material possesses numerous pores, which can provide more active sites, thereby increasing the charge storage capacity of the capacitor.

[0031] Figure 2 This is the X-ray diffraction pattern of the MF-CNTs@ZIF-8 composite material. MF-CNTs@ZIF-8 exhibits strong diffraction peaks at 2θ = 7.34 °, 10.42 °, 12.74 °, 14.72 °, 16.44 °, 18.06 °, 22.18 °, and 25.68 °, corresponding to the (011), (002), (112), (022), (013), (222), (114), and (134) crystal planes on the ZIF-8 standard card, indicating that ZIF-8 was successfully loaded onto the composite material. Furthermore, XRD characterization shows that both the MF-CNTs@ZIF-8 composite material and pure ZIF-8 exhibit strong and narrow peaks, indicating that the prepared material has good crystallinity.

[0032] Figure 3This is the X-ray photoelectron spectrum of the MF-CNTs@ZIF-8 composite material. XPS full-spectrum analysis confirmed the presence of five elements (C, N, O, S, and Zn) on the surface of the material. The carbon-carbon absorption peaks indicate that the synthesized carbon is a highly conductive carbon matrix. The high-resolution N 1s spectrum can be further decomposed into two main components. The main peak at approximately 398.6 eV is attributed to the pyridine nitrogen (Zn-N) coordinated to the zinc ion on the 2-methylimidazolium ligand in ZIF-8. The smaller peak at approximately 399.5 eV can be identified as the uncoordinated tertiary amine nitrogen (-N=) in the imidazolium ring. The shape and binding energy position of the N 1s spectrum are highly similar to those of pure ZIF-8 reported in the literature, further confirming the successful construction and maintenance of the ZIF-8 framework structure. Example

[0033] This embodiment demonstrates the electrochemical performance study of a microfluidic-constructed microspherical composite carbon nanotube supercapacitor MF-CNTs@ZIF-8.

[0034] This invention uses a platinum sheet electrode as the counter electrode, saturated silver chloride electrodes (Ag / AgCl) as reference electrodes, and nickel foam loaded with active material placed on a platinum sheet working electrode clamp as the working electrode.

[0035] The polymer binder added during the preparation of the working electrode described in this invention is polyvinylidene fluoride with a concentration of 10 wt%.

[0036] The working electrode of this invention is prepared by a wet coating process, in which active material, polymer binder and conductive agent (acetylene black) are compounded at a mass ratio of 8:1:1, ethanol is used as the dispersion medium, and the mixture is dispersed in a magnetic stirrer for 1 hour to form a homogeneous slurry. Subsequently, the slurry is loaded onto the surface of a pretreated nickel foam current collector (1×1 cm²) using a dip-coating method, and then dried in a vacuum drying oven at 60°C for 12 hours to achieve solvent evaporation.

[0037] All supercapacitor performance tests described in this invention were conducted in 6 M KOH electrolyte.

[0038] The composite material described in this invention requires CV activation for 3 cycles before electrochemical testing.

[0039] The catalysts described in this invention are all tested at room temperature to prevent large temperature variations from affecting the performance of the composite materials.

[0040] Electrochemical tests were performed on the catalyst obtained in this embodiment using the Chenhua CHI600EA17252 electrochemical workstation manufactured by Shanghai Chenhua Instrument Co., Ltd. Cyclic voltammetry (CV) was used to record the electrode potential change over time by applying a triangular wave potential scan. Quantitative analysis of the characteristic parameters of the CV curves allows for a systematic evaluation of the double-layer capacitance and pseudocapacitive contribution of the electrode material, and reveals the reversibility of its redox reactions. Galvanostatic charge-discharge (GCD) was used to perform charge-discharge cycle tests on the electrode material by applying a constant current density (e.g., 1 A / g), recording the characteristic curve of its potential change over time. Analysis of the characteristic parameters of the GCD curves allows for accurate calculation of the material's specific capacitance. Electrochemical impedance spectroscopy (EIS) was used to acquire the impedance response signal of the electrode system by applying a sinusoidal excitation signal within a specific frequency range, and then generating a characteristic impedance spectrum through Fourier transform analysis.

[0041] Figure 4 This is the cyclic voltammetric characteristic curve of the MF-CNTs@ZIF-8 composite material (test voltage scan range: 0-0.5V, scan rate: 10-50 mV / s). The CV curve in the image shows an approximately rectangular outline with broadened redox peaks, which reflects the weak pseudocapacitance introduced by ZIF-8. Based on the Randles-Sevcik equation kinetic analysis, the scan rate gradient test shows that the integral area of ​​the CV curve increases linearly with increasing scan rate, indicating that the material maintains excellent Faraday charge transfer efficiency even at high magnification.

[0042] Figure 5 The images show the charge-discharge curves of the MF-CNTs@ZIF-8 composite material. The GCD curves reveal that all curves exhibit nonlinear charge-discharge behavior, further confirming its pseudocapacitive behavior. These approximately symmetrical curves demonstrate that the electrode fabricated from the MF-CNTs@ZIF-8 composite material possesses excellent electrochemical capacitance characteristics.

[0043] Figure 6 This is a comparison of the specific capacitance of MF-CNTs@ZIF-8 composite materials obtained at different calcination temperatures. Calculations show that a suitable heat treatment temperature helps to control the pore structure, conductivity, and electrochemical activity of the material, thereby significantly improving its energy storage performance. 700℃ is considered the optimal temperature, balancing the material's conductivity and structural stability, allowing it to exhibit excellent pseudocapacitive characteristics in alkaline environments, demonstrating high application potential.

[0044] Figure 7This is a comparison chart of the specific capacitance of MF-CNTs@ZIF-8 composites with different ZIF-8 loadings. Calculations show that lower ZIF-8 loadings result in less total active material mass, limiting further improvements in capacitance and energy density. Conversely, higher ZIF-8 loadings block the electron transport paths of carbon nanotubes, leading to increased internal resistance. Therefore, the ZIF-8 loading should be kept within a reasonable range.

[0045] Figure 8 The composite material at different calcination temperatures is at 10 -2 Up to 10 5 Electrochemical impedance spectroscopy (EIS) was performed in the Hzz frequency range. Tests were conducted under open-circuit potential conditions using a three-electrode system. The EIS curves exhibited typical Nyquist characteristics, including a semi-circular shape in the high-frequency region (corresponding to charge transfer resistance Rct) and a linear portion with a 45° slope in the low-frequency region. The intersection of the curve with the real axis represents the internal resistance R0, encompassing the contact resistance, ionic resistance, and intrinsic resistance of the electrode material. Fitting analysis showed that the electrode had the lowest Rct at a calcination temperature of 700 °C, indicating a low charge transfer resistance. Although the R0 value was slightly higher than that of samples at other temperatures, the difference was not significant. In the low-frequency region, the sample calcined at 700 °C exhibited the steepest linear slope, reflecting its excellent charge transfer kinetics and high ionic conductivity.

[0046] Figure 9 The electrode made of MF-CNTs@ZIF-8 composite material was tested at a current density of 10 A·g. -1 The cycling performance graph after 3000 cycles shows a capacitance retention rate of 81.6%. The trend of the cycling curves indicates that the MF-CNTs@ZIF-8 electrode did not exhibit a significant drop in capacitance or electrode failure throughout the testing process, demonstrating its high structural toughness and chemical stability, enabling it to withstand long-term operation under high-rate conditions. Overall, this material outperforms conventional carbon materials in terms of capacitance retention, showcasing its application potential as a high-performance supercapacitor electrode material.

[0047] In summary, this invention provides an innovative, efficient, and controllable method for preparing microspherical composite carbon nanotube supercapacitor materials. This method successfully combines the precise molding advantages of microfluidic technology with the structural and functional advantages of MOF materials. The resulting MF-CNTs@ZIF-8 composite material exhibits excellent comprehensive electrochemical performance, and the preparation process is green and economical, showing broad prospects for industrial application.

[0048] The above description is merely a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A method for preparing a microfluidic-based microspherically structured composite carbon nanotube supercapacitor material, characterized by, The method comprises the following steps: Step 1: acid treatment of carbon nanotubes: mix concentrated sulfuric acid and concentrated nitric acid at a volume ratio of 3:1 to prepare an acidification solution; take the aqueous dispersion of carbon nanotubes and mix it with the acidification solution, then perform ultrasonic treatment and magnetic stirring, and then wash it with ultrapure water until it is neutral, and then disperse it in ultrapure water again to obtain an acidified carbon nanotube dispersion for later use; Step 2: preparation of carbon nanotube microspheres by microfluidic control: take dimethyl silicone oil as the oil phase and the acidified carbon nanotube dispersion obtained in step 1 as the water phase, and then pass the oil phase and the water phase into a microfluidic device, control the flow rate ratio of the oil phase to the water phase to be 10:1, and then mix to form water-in-oil emulsion droplets; collect the emulsion droplets in a polytetrafluoroethylene reaction kettle containing dimethyl silicone oil, and then place the reaction kettle in an oil bath at 120℃ for heating, so that the droplets are solidified to obtain carbon nanotube microspheres; Step 3: primary pyrolysis of carbon nanotube microspheres: uniformly disperse the carbon nanotube microspheres obtained in step 2 in a porcelain boat, place it in a tube furnace, and then perform programmed heating at a heating rate of 1-5 ℃ / min to 700℃ under the protection of a nitrogen atmosphere, and then keep the temperature at 700℃ for 2 hours, and then naturally cool to room temperature to obtain a carbon nanotube microsphere skeleton after pyrolysis and carbonization; Step 4: in-situ growth of ZIF-8 and secondary pyrolysis of the composite material: dissolve zinc nitrate hexahydrate in methanol to prepare solution A; dissolve 2-methylimidazole in methanol to prepare solution B, wherein the molar ratio of Zn²⁺ in solution A to 2-methylimidazole in solution B is 4:1; first immerse the carbon nanotube microsphere skeleton obtained in step 3 in solution A and perform magnetic stirring, then add solution B and continue to stir overnight; after the reaction is completed, uniformly disperse the microspheres loaded with precursors in a porcelain boat, place it in a tube furnace, and then perform high-temperature pyrolysis at 700℃ under the protection of a nitrogen atmosphere, and then naturally cool to obtain the supercapacitor material MF-CNTs@ZIF-8.

2. The method of claim 1, wherein: In step 1, the ratio of the amount of carbon nanotubes to the acidification solution is 15 ml of acidification solution for every 100 mg of carbon nanotubes; the ultrasonic treatment time is 60 minutes, and the magnetic stirring time is 2 hours.

3. The method of claim 1, wherein: In step 2, the microfluidic device is a flow-focusing type or a T-junction microchannel structure; the oil bath heating temperature is 120℃, and the solidification time is determined according to complete solidification of the droplets.

4. The method of claim 1, wherein: In steps 3 and 4, the heating rate of the high-temperature pyrolysis process is 1-5 ℃ / min, and the pyrolysis atmosphere is high-purity nitrogen (purity ≥ 99.99%).

5. The method of claim 1, wherein: In step 4, the volume of solution A is equal to that of solution B; the magnetic stirring speed is 300-500 rpm; and the overnight stirring time is 10-14 hours.

6. A supercapacitor electrode material characterized by: The material prepared by the method of any one of claims 1 to 5 is a composite structure of microspherical carbon nanotube skeleton uniformly loaded with ZIF-8 derived porous carbon nanoparticles, and is marked as MF-CNTs@ZIF-8.

7. Use of the supercapacitor electrode material according to claim 6, characterized in that: The MF-CNTs@ZIN-8 material is used as an active material to prepare a supercapacitor working electrode. The working electrode is prepared by mixing the active material, a polymer binder and a conductive agent in a mass ratio of 8:1:1, using ethanol as a dispersion medium, and magnetically stirring to form a homogeneous slurry; the slurry is loaded on the surface of a pretreated nickel foam current collector by dip coating, and vacuum dried at 60 DEG C for 12 hours to obtain the working electrode.

8. Use according to claim 7, characterized in that: The polymer binder is polyvinylidene fluoride, and the conductive agent is acetylene black; the size of the nickel foam current collector is 1 cm x 1 cm; and the electrochemical test of the working electrode is carried out in a 6M KOH electrolyte system.

Images