Preparation Method and Application of MWCNTs / PPy / NiCo-LDH

Abstract

In this invention, MWCNTs / PPy was synthesized by in-situ chemical oxidative polymerization of pyrrole on MWCNTs. Functional groups carboxyl and hydroxyl were introduced by pre-treating carbon nanotubes, enabling them to form hydrogen bonds with -NH on the pyrrole monomer. Keeping n(Ni<supgt;2+< / supgt>):n(Co<supgt;2+< / supgt>) unchanged at 1:2, n(py):n(APS) at 1:1, and the reaction time between Py and the oxidant at 12 h, MWCNTs and PPy formed a pearl necklace-like structure, which was explained from the reaction mechanism and electrochemical principle. Finally, the carbon nanotube / polypyrrole / nickel cobalt layered hydroxide electrode material was successfully prepared by a one-step hydrothermal method and co-precipitation method. Through comprehensive study of these three composite materials, it was found that MWCNTs / PPy could reduce the collapse and aggregation occurring during charge and discharge of the material, and improve the conductivity, rate performance and cycle stability of the layered hydroxide.

Description

Technical Field

[0001] The present invention belongs to the technical field of electrode material preparation, and specifically relates to a preparation method of MWCNTs / PPy / NiCo-LDH. Background Art

[0002] Carbon-based electrode materials are considered to be the most promising material type due to their sustainable raw material acquisition, low price, large specific surface area, and controllable physical and chemical properties, and have been commercialized in supercapacitors. On the one hand, carbon-based materials have high porosity, a unique porous structure, and good electronic conductivity, and can act as a physical carrier. On the other hand, their strong controllability, high conductivity, and strong stability are conducive to electrochemical power transmission. However, the low specific capacitance, low specific energy, and surface hydrophobicity of traditional carbon-based materials weaken their competitive advantages compared to other electrode materials. Therefore, the performance of carbon materials can be greatly improved by external doping to overcome the limitation of low specific capacitance. Among various carbon nanomaterials, carbon nanotubes are an important one-dimensional nanomaterial with high intrinsic conductivity and strong physical and chemical properties (mechanical, electrical, and thermal properties), making them an outstanding matrix for combination with other transition metal oxides, sulfides, phosphides, etc., and showing great potential in the fields of electronics, chemical sensing, semiconductor industry, power infrastructure, 3C digital, etc. Multi-walled carbon nanotubes, as a typical electric double layer material, have strong cycle stability, can effectively improve the rate performance of pseudocapacitance, provide structural support for electrode materials, establish a rich pore structure and continuous conductive network for the electrode, and alleviate the agglomeration of nanoparticles, etc.

[0003] In addition, as the best candidate for supercapacitors with high-performance pseudocapacitance, transition metal layered double hydroxides (LDHs) are composed of divalent / trivalent metal oxide octahedral main layers and charge-balancing anions located in the interlayer space. Their elemental composition has high tunability and high redox activity of the main layer metal. At the same time, LDHs is a pseudocapacitance material that can provide a large specific capacitance. Recently, researchers have synthesized some LDHs and their composites through exploration experiments and achieved good results. For example, Chen Ling et al. prepared a nickel cobalt layered double hydroxide (NiCo-LDH)@N-GH / NF composite on a nickel foam (NF) substrate equipped with three-dimensional porous nitrogen-doped graphene hydrogel (N-GH). The three-dimensional hierarchical structure has a positive effect on promoting electrochemical performance. The specific capacitance of the assembled asymmetric supercapacitor (ASC) is 1393F.g -1 (1 mA·cm -2 ), and when the power density is 260 W.kg -1 , the energy density reaches 63.33 Wh.kg -1Therefore, the morphological advantages and performance advantages of functionalized multi-walled carbon nanotubes, polypyrrole with strong conductivity, and layered metal hydroxide with a large specific surface area can be fully utilized to synergistically prepare a supercapacitor, so as to obtain better specific capacitance and more excellent energy density. Summary of the Invention

[0004] The purpose of the present invention is to provide a preparation method of MWCNTs / PPy / NiCo-LDH composite material to obtain a composite material with better electrochemical performance for application in an asymmetric supercapacitor.

[0005] I. Preparation of MWCNTs / PPy / NiCo-LDH

[0006] The preparation route of MWCNTs / PPy / NiCo-LDH composite material is as follows:

[0007]

[0008] It includes the following steps:

[0009] 1) Purification of MWCNTs: Place MWCNTs in a mixed solution of concentrated nitric acid and concentrated hydrochloric acid configured according to a volume ratio of 1:3, stir continuously, after ultrasonic treatment for 3 - 4 h, reflux in an oil bath at 80 - 90 °C for 4 h, cool to room temperature, filter by suction, and dry overnight for standby; the concentration of MWCNTs in the mixed solution of concentrated nitric acid and concentrated hydrochloric acid is 1.5 - 1.7 g / L.

[0010] 2) Preparation of MWCNTs / PPy: In the H3PO4 solvent system, using APS as an initiator, purified MWCNTs and Py as reactants, carry out a polymerization reaction at 0 °C for 8 - 24 h to obtain MWCNTs / PPy; the mass ratio of purified MWCNTs to Py is 0.7:1 - 0.9:1, and the mass ratio of APS to Py is 2:1 - 4:1.

[0011] 3) Preparation of MWCNTs / PPy / NiCo-LDH:

[0012] Preparation of MWCNTs / PPy / ZIF-67. First, take MWCNTs / PPy and Co(NO3)2 . 6H2O and dissolve them in a methanol solvent, stir and mix for 1 - 2 h, then add 2-MeIM organic ligand and stir and react at room temperature for 0.5 - 1 h, let it stand for 12 h, filter by suction and wash, dry at 70 °C for 12 h to obtain MWCNTs / PPy / ZIF-67;

[0013] Preparation of MWCNTs / PPy / NiCo-LDH: The prepared MWCNTs / PPy / ZIF-67 and Ni(NO3)2·6H2O were dissolved in deionized water and stirred evenly. Then, the mixture was reacted at 100 - 120 °C for 10 - 12 h, cooled, centrifuged, filtered by suction, washed with ethanol, and dried at 70 °C for 10 h to obtain a black-gray powder, which is MWCNTs / PPy / NiCo-LDH.

[0014] The mass ratio of MWCNTs / PPy to Co(NO3)2 . 6H2O is 0.005:1 - 0.009:1, and the mass ratio of 2-MeIM to Co(NO3)2 . 6H2O is 0.4:1 - 0.8:1. The mass ratio of MWCNTs / PPy / ZIF-67 to Ni(NO3)2·6H2O is 0.15:1 - 0.25:1.

[0015] II. Structure Characterization and Performance Evaluation of MWCNTs / PPy / NiCo-LDH

[0016] The following test data are based on the samples MPNCL-1, MPNCL-2, and MPNCL-3 prepared in the examples of the present invention.

[0017] 2.1 Structure Characterization

[0018] 1) SEM and TEM Characterization

[0019] The present invention first proposed a core-shell structure of a composite material of polypyrrole (PPy) and multi-walled carbon nanotubes (MWCNTs). Using APS as an oxidant, PPy was introduced onto the surface of MWCNTs through in-situ polymerization of pyrrole. As Figure 1 (a) and (b) show, MWCNTs have a smooth and uniform tubular structure. The diameter of the purchased MWCNTs is about 40 - 60 nm, and the length is about 10 - 20 μm. After adding the Py monomer for polymerization, the tubular surface of MWCNTs is covered with rough nodules covered by PPy. The functionalized MWCNTs and PPy are combined to form a pearl necklace-like morphology, mainly due to the different dopants added. The carbon nanotubes serve as a template, and polypyrrole grows on its surface, as Figure 1 (c) and (d) show; subsequently, Ni(NO3)2 . 6H2O was added to MWCNTs / PPy / ZIF-67, and the zeolitic imidazolate framework (ZIF-67) was topologically transformed into NiCo-LDH through a hydrothermal process. The principle is that Co released by ZIF-67 2+ is partially oxidized by O2 and nitrate to form Co 3+ , and the ionic Ni 2+ / Ni 3+and Co 2+ / Co 3+ Coprecipitation forms nickel-cobalt hydroxide. As can be seen from Figure 1 (e), it is overall honeycomb-shaped. At a magnified scale, interconnected 3D-structured stacked nanosheets can be seen growing on the surface of the nanotubes and polypyrrole, as shown in Figure 1 (f). At the same time, it can also be observed that some MWCNTs / PPy are exposed, indicating the successful synthesis of the MWCNTs / PPy / NiCo-LDH nanocomposite. The spatial orientation along the axis of the carbon nanotubes improves the spatial occupancy of the ZIF-67 hollow nanocage monomers, improves the dispersibility between particles, and enhances the mechanical stability of the hollow nanocages; the addition of polypyrrole improves the electrical conductivity of the material; the three-dimensional nanosheet array enables electrolyte ions to better contact the electrode material, promotes the transfer of electrons, and promotes the occurrence of electrochemical reactions. As can be seen more clearly from Figure 1 (g) and Figure 1 (h) are the microscopic morphology and structural characteristics of MWCNTs / PPy / NiCo-LDH. The part marked by the dotted line is MWCNTs / PPy, and the part marked by the solid line is the outer-wrapped NiCo-LDH nanosheets. Among them, the lattice fringe with a lattice spacing of 0.3 nm belongs to the carbon nanotubes, and the rest are the characteristic crystal planes of NiCo-LDH.

[0020] 2) XRD analysis

[0021] As shown in Figure 2 is the XRD pattern of the MWCNTs / PPy / NiCo-LDH composite material prepared by the present invention. It can be clearly seen from the figure that the diffraction peaks at 2θ = 21.91° and 42.58° correspond to the (002) and (100) crystal planes of the carbon nanotubes respectively, which is consistent with what most literature reports. The obvious broad diffraction peak of PPy at 2θ = 17~27° proves its amorphous structure. At the same time, the peaks of MWCNTs / PPy / NiCo-LDH at 11.44°, 34.62°, 38.8°, 52.1°, and 60.66° correspond to the (003), (100), (015), (102), and (110) diffraction planes of NiCo-LDH respectively, and the XRD of the obtained composite material is basically consistent with the XRD pattern trend of NiCo-LDH, indicating that the addition of carbon nanotubes and polypyrrole has no effect on the crystal form of the composite material.

[0022] 3) Surface element analysis

[0023] EDS tests were further carried out on MWCNTs / PPy / NiCo-LDH. The presence of C, O, N, Co, and Ni can be seen. Among them, the intensities of N and Co elements are relatively weak, mainly because the added PPy content is small and due to the topological transformation of ZIF-67. The intensities of O, C, and Ni elements are relatively strong, indicating the successful synthesis of carbon nanotubes and layered hydroxides. The H element was not detected because of its low relative atomic mass, indicating that the product is very pure and free of impurities. At the same time, in Figure 3 the mass ratios of C, N, O, Co, and Ni can be seen to be 42.8%, 5.8%, 20.9%, 5.5%, and 25.0% respectively.

[0024] 4) XPS spectrum analysis

[0025] To further analyze the surface composition, the XPS spectrum was used to characterize the structure of the prepared sample. The obtained data are shown in Figure 4.

[0026] The full XPS spectrum of MWCNTs / PPy / NiCo-LDH ( Figure 4a ) indicates the presence of Ni, Co, O, C, and N elements. For the high-resolution XPS spectrum of Ni 2p ( Figure 4b ), the two main peaks at 854.8 eV and 872.8 eV are consistent with Ni 2p 3 / 2 and Ni 2p 1 / 2 . There are two satellite peaks at 879.0 eV and 860.8 eV, indicating the presence of Ni 3+ and Ni 2+ in the MWCNTs / PPy / NiCo-LDH structure. The Co 2p spectrum is shown in Figure 4 (c). The two main peaks centered at 780.5 eV and 796.3 eV are composed of Co 2p 3 / 2 and Co2p 1 / 2 . The peaks at 785.8 eV and 801.7 eV belong to the satellite peaks of Co, indicating the presence of Co 3+ and Co 2+ . Figure 4(d) shows that the C 1s spectrum has three strong bands at binding energies of 284.3, 285.6, and 291.7 eV, which point to C-C / C=C, C-N, and C-OH respectively, proving the successful combination of carboxylated carbon nanotubes and polypyrrole with carbon nanotubes. The O 1s spectral peak is located at 530.1 eV ( Figure 4e), because the metal-oxygen bonds in the MWCNTs / PPy / NiCo-LDH sample, i.e., the coordination defects of oxygen, the peak observed at 531.6 eV originates from chemically adsorbed water, and the peak observed at 533.1 eV belongs to the hydroxide component of NiCo-LDH. The binding energies of the N 1s spectra in Fig. 4(f) are approximately at 398.8 eV, 400.0 eV, and are attributed to the electrons in -C≡N-, -NH in the pyrrole ring, and the positively charged NH in pyrrole + The presence of one peak (at about 401.2 eV) indicates that the introduced nitrate ions are embedded in the structure of NiCo-LDH. Although the content of N in the full spectrum is small, the doping of N can effectively improve the conductivity of the carbon matrix by providing p electrons to the conjugated sp 2 carbon structure. In addition, the presence of N in pyrrole will lead to a large number of defects in the carbon structure, thereby promoting mass transfer and improving the performance of the supercapacitor.

[0027] 5) Infrared spectrum and Raman spectrum analysis

[0028] The functional groups of the materials were analyzed by FT-IR. As shown in Fig. 5(a), the peak around 3434 cm -1 is attributed to the vibration mode of water molecules and the peak intensity of NiCo-LDH, indicating its good hydrophilicity and the ability to adsorb water molecules on its surface, confirming the presence of -OH. The absorption peak around 3136 cm -1 corresponds to the C-H stretching vibration peak on the aromatic ring. In addition, the characteristic peaks at 1626 cm -1 and 1108 cm -1 belong to the stretching vibrations of C=O and C-C-O on the purified multi-walled carbon nanotubes. It is worth noting that the strong peak at 1400 cm -1 is the characteristic peak of the stretching vibration of the N-O bond of nitrate on the material surface. This is because of the residue of Ni(NO3)2·6H2O and Co(NO3)2·6H2O, or it may also be the stretching vibration of the C-N bond on the pyrrole ring. The characteristic peak at 1171.5 cm -1 originates from the in-plane deformation vibration of C-H, and the peak below 800 cm -1 is due to the bending vibration of the metal-oxygen bond or the out-of-plane deformation vibration of C-H. It can be seen from the FT-IR that PPy, MWCNTs / PPy, and MWCNTs / PPy / NiCo-LDH were successfully synthesized.

[0029] The composites were characterized by Raman spectroscopy. As shown in Fig. 5(b), the typical peaks of carbon nanotubes appear at 1345 cm -1 and 1590 cm -1There, they respectively correspond to the D peak of the defect degree and the G peak of the integrity degree of the carbon nanotubes. The intensity ratio of ID / IG is 1.10. The D peak and G peak of MWCNTs / PPy / NiCo-LDH appear at 1325 cm -1 and 1594 cm -1 respectively. The intensity ratio of ID / IG is 1.25, which proves that the introduction of PPy / NiCo-LDH exposes the intrinsic defects of the microstructure of the carbon layer, changes the physical and chemical properties of the carbon-based material, leads to an increase in the defect density, and further combines the corresponding electrochemical test to analyze the charge-discharge behavior of the composite material. The increase in defects provides a capacitance contribution.

[0030] 2.2 Electrochemical performance test

[0031] Preparation of the working electrode: The electrode active material, carbon black, and binder PTFE are added in a mass ratio of 80:10:10, and an appropriate amount of ethanol is added dropwise to form a slurry. After mixing and stirring, it is evenly coated on the treated nickel foam current collector. The mass of the active substance on the prepared electrode sheet is about 2 mg·cm 2 and the thickness is about 0.1 mm. Before use, the working electrode is vacuum dried at 60 °C and pressed into a sheet at 10 MPa for 20 s. -l

[0032] The test is carried out in a three-electrode system. The working electrode is the MWCNTs / PPy / NiCo-LDH composite material, the counter electrode and the reference electrode are a platinum sheet electrode and a Hg / HgO electrode respectively, and 3 M KOH is used as the electrolyte. The cyclic voltammetry curve (CV), galvanostatic charge-discharge curve (GCD), and electrochemical impedance spectroscopy (EIS) of the material are measured by a CHI760E electrochemical workstation. The specific capacitance is calculated according to formulas (1) and (2), and finally the cycle life test is carried out on a blue electrochemical system.

[0033] (1)

[0034] Among them, is the area of the CV curve, m is the mass of the active substance, in g, Vc - Va is the voltage window difference, in V, and v is the scanning rate, in mV s -1 .

[0035] (2)

[0036] Among them, I is the applied current, in A, Δt is the discharge time, in s, m is the mass of the active substance, in g, and ΔV is the voltage difference, in V.

[0037] 1) Electrochemical performance of the three-electrode system ​

[0038] Using a three - electrode cell in 3.0 M alkaline electrolyte, the advantages of MWCNTs / PPy / NiCo - LDH composite in electrochemical performance were evaluated by CV, GCD and EIS methods, and the results are shown as follows. Figure 6(a) shows the CV curves of MWCNTs / PPy / NiCo - LDH obtained at a scan rate of 5 - 50 mV s -1 . It can be observed that there is a slight change in the peak current, the anodic peak shifts to a higher potential, while the cathodic peak shifts to the opposite potential and has distinct redox peaks, indicating the presence of a reversible Faraday reaction. The explanation is as follows:

[0039] Co(OH)2 + OH - → CoOOH + H2O + e -

[0040] CoOOH + OH - → CoO2 + H2O + e -

[0041] Ni(OH)2 + OH - → NiOOH + H2O + e -

[0042] As expected, the high redox current intensity and MPNCL area represent superior electrochemical capacitance and better ability to store charge. The GCD curve in Figure 6(b) shows a discharge plateau at about 0.28 - 0.3 V. The non - linear discharge curve clearly indicates the battery - type electrode of MPNCL, which is in good agreement with the CV results. The corresponding specific capacitance was calculated according to the equation. MPNCL has a high specific capacitance of 2114 F g -1 at 1 A g -1 , and at 10 A g -1The rate performance at this time remains at 77.5% (Table 1 shows the comparison of the specific capacitance of this composite material with that of materials in other literatures). Combining the characterization of the structure and morphology, the improvement of capacitance retention at high current density is due to the increased charge transfer rate caused by surface activation. In addition, in order to explore the source of this advantage, EIS (from 0.1 Hz to 100 kHz) was tested under open circuit conditions to examine the ion and electron transfer processes of the electrode material. Figure 6(c) shows the Nyquist plot of the prepared MPNCL. It can be seen that the intersections of the arcs and intercepts in the high-frequency regions of MPNCL-2 and MPNCL-3 are not very different (including the internal resistance of the electrode, the ionic impedance of the electrolyte, and the interfacial impedance between the electrode material and the current collector, etc.), indicating that the solution resistances (Rs) of the MPNCL-2 and MPNCL-3 electrodes are quite comparable, and their resistances are 0.627 Ω and 0.669 Ω respectively obtained by Zview software analysis. For the composite material prepared with MWCNTs / PPy reacted for 12 h as the reaction substrate, the diameter of the semicircle is significantly reduced, that is, the charge transfer resistance (Rct) is reduced (where the Rct values of MPNCL-1, MPNCL-2, and MPNCL-3 are 0.578 Ω, 0.518 Ω, and 0.560 Ω respectively). These results indicate that the reaction time has a great influence on reducing the charge transfer resistance, so the charge transfer ability of MPNCL-2 during the charge storage process is greater than that of MPNCL-1 and MPNCL-3.

[0043] Table 1 Comparison of the performance of MWCNTs / PPy / NiCo-LDH electrodes with other materials

[0044]

[0045] Figures 6(d) and (e) show the specific capacitances of the three materials, which are 1866 F g -1 、2114 F g -1 and 1738 F g -1 respectively, and it can be seen that the rate performances of the composite materials with three reaction times are 70.7%, 77.5%, and 73.4% respectively at 10 A g -1 . After 5000 cycles, Figure 6(f) shows that the cycling stability of the optimal sample is 80.1% of the initial specific capacitance.

[0046] The main reason for the above results lies in the influence of the reaction time between pyrrole monomer and carbon nanotubes. The possible mechanism is as follows: within a certain time, the conjugated chain of pyrrole increases with the increase of polymerization time, and the regular arrangement of the chain tends to be complete. By 12 h, the pyrrole monomer has completely reacted, polymerization and proton doping have been completed, and the conjugated chain length of the product reaches the optimal state. The molecular arrangement is relatively regular, the electron mobility of the large π-conjugated system is high, and the overall electrochemical performance is excellent. However, with the further extension of the reaction time, side reactions such as chain transfer and local peroxidation of the pyrrole ring increase, and the orderliness and conjugation degree of the chain decrease, which may lead to a decrease in the conductivity of the product. In summary, when the reaction time between MWCNTs and PPy is 12 h, the electrochemical performance of the composite material with NiCo-LDH is optimal.

[0047] 2) Judgment and contribution analysis of pseudocapacitance behavior in a three-electrode system

[0048] To clarify the transport kinetics, we used the following empirical formulas (3) and (4) to quantitatively analyze the relationship between the scan rate and the peak current based on the CV curves at different scan rates. In the formulas, I is the peak current density (A g -1 ), ν is the scan rate (mV s -1 ), and both a and b are adjustable parameters. Generally, it is considered that when the value of b is close to 1, the charge storage has a capacitance-like behavior, while when the value of b is less than 0.5, the charge storage ability stems from a slow Faraday reaction controlled by semi-infinite diffusion. The specific pseudocapacitance contribution was calculated according to formula (5).

[0049] (3)

[0050] (4)

[0051] (5)

[0052] As shown in Figures 7 (a) and (b), MPNCL-2 (b1 = 0.3802, b2 = 0.5209) indicates that the charge storage is mainly controlled by the diffusion process of pseudocapacitance contribution. As can be seen from Figures 7 (c) and (d), the contribution of the surface control process to the electrode charge storage ability increases with the increase of the scan rate, and the pseudocapacitance contribution rate is 22.1% at a scan rate of 30 mV s -1 .

[0053] 3) Assembly of MWCNTs / PPy / NiCo-LDH / / AC asymmetric supercapacitor

[0054] The assembly schematic of the aqueous asymmetric supercapacitor is shown in Fig. 8(a). Fig. 8(b) shows the cyclic voltammograms obtained at a scanning rate of 50 mV s -1 in a three-electrode system for the positive and negative electrodes after charge balance. The voltage window range of the MWCNTs / PPy / NiCo-LDH electrode is 0 - 0.5 V, while the potential window range of the activated carbon (AC) electrode is -1.0 - 0 V. To determine the stable test voltage window of the device, Fig. 8(c) shows the CV diagrams at different potential windows at a scanning rate of 50 mV s -1 . When the voltage window reaches 1.6 V, obvious polarization of the electrodes occurs due to the decomposition of the electrolyte. Therefore, the maximum potential window of the asymmetric device has a stable operating voltage at 1.5 V. Thus, the CV tests of the device at different scanning rates from 0 to 1.5 V were scanned, as shown in Fig. 8(d). It can be seen from the figure that with the increase of the scanning rate, no obvious shape change was observed in all CV curves. Even when the scanning rate increased to 100 mV s -1 , all CV curves still had the same shape, indicating that the device has excellent fast charge-discharge performance. Among them, the CV curve of the assembled supercapacitor shows a smooth quasi-rectangular shape, which is different from the CV curves of single electrodes.

[0055] Fig. 8(e) is the GCD curves tested at a current density of 1 - 10 A g -1 . Among them, all curves are almost symmetric, which not only indicates that the device has a fast charge storage capacity. The assembled device was continuously charged and discharged 5000 cycles at a current density of 10 A g -1 , and its capacity retention rate can reach 87.6%, indicating its excellent performance. As shown in Fig. 8(f), through calculation, it can be known that when the current density of the device is 1 A g -1 , the specific capacitance is 123 F g -1 ; when the current density is 10 A g -1 , the specific capacitance is 81.6 F g -1 , and the capacity retention rate is about 66.34%. After calculation, when the energy density is 38.44 Wh kg -1 , the power density is 754.55 W kg -1 . Even when the high power density is 7512.27 W kg -1 , the energy density can reach 25.5 Wh kg -1 . This performance is significantly higher than that reported in some literatures ( Figure 8g). As shown in the Ragone plot, the device NiAI-LDH@NF / / AC assembled by Zhang Luojiang et al. achieved a power density of 800 W kg -1 and an energy density of 30.2 Wh kg -1 . The device Ni-MOF SPANI / / AC assembled by Zhang Junye et al. achieved a power density of 824 W kg -1 and an energy density of 34.79 Wh kg -1 . The device GR-CNT@Co / / AC assembled by Zhang Weijie et al. achieved a power density of 685.3 Wkg -1 and an energy density of 36.55 Wh kg -1 .) This proves the successful synthesis of the positive composite material and the application potential of MWCNTs / PPy / NiCo-LDH / / AC in supercapacitors.

[0056] In summary, in this invention, MWCNTs / PPy was synthesized by in-situ chemical oxidative polymerization of pyrrole on MWCNTs. By pretreating carbon nanotubes to introduce functional groups carboxyl and hydroxyl, hydrogen bonds were formed with -NH on the pyrrole monomer to keep n(Ni 2 + ):n(Co 2+ ) unchanged at 1:2, n(py):n(APS) at 1:1, and when the reaction time between Py and the oxidant was 12 h, MWCNTs and PPy formed a pearl necklace-like structure, which was explained from the reaction mechanism and electrochemical principle. Finally, the carbon nanotube / polypyrrole / nickel cobalt layered hydroxide electrode material was successfully prepared by one-step hydrothermal method and coprecipitation method. By comprehensively studying these three composite materials, it was found that MWCNTs / PPy could reduce the collapse and aggregation that occurred during the charge and discharge process of the material, improve the conductivity, rate performance and cycle stability of the layered hydroxide. The specific capacitance of this material was 2114 F g -1 at 1 A g -1 , the rate performance reached 77.5% at 10 A g -1 , and it maintained good cycle stability during long cycling (the capacitance retention rate was 87.6% after 5000 cycles). When assembled into an asymmetric supercapacitor, its specific capacitance was 123 F g -1 , and when the energy density was 38.44 Wh kg -1 , the power density was 754.55 W kg -1 . This experimental scheme demonstrated the wide application of carbon-based and pseudocapacitive composite materials in the field of supercapacitors. BRIEF DESCRIPTION OF THE DRAWINGS

[0057] Figure 1Chinese: (a) and (b) are SEM images of multi-walled carbon nanotubes (MWCNTs) at magnification of 2 μm and 300 nm, respectively; (c) and (d) are SEM images of MWCNTs / PPy at magnification of 2 μm and 300 nm, respectively; (e) and (f) are SEM images of MWCNTs / PPy / NiCo-LDH at magnification of 2 μm and 300 nm, respectively; (g) is a TEM image of MWCNTs / PPy / NiCo-LDH at magnification of 50 nm, and (h) is a HRTEM image of MWCNTs / PPy / NiCo-LDH at magnification of 5 nm;

[0058] Figure 2 XRD pattern of the MWCNTs / PPy / NiCo-LDH composite material prepared by the present invention;

[0059] Figure 3 Surface elemental analysis diagram of MWCNTs / PPy / NiCo-LDH prepared by the present invention;

[0060] In Figure 4: (a) is the XPS spectrum of MWCNTs / PPy / NiCo-LDH prepared by the present invention,

[0061] (b) is the high-resolution energy spectrum of Ni 2p, (c) is the high-resolution energy spectrum of Co 2p, (d) is the high-resolution energy spectrum of C 1s, (e) is the high-resolution energy spectrum of O 1s, and (f) is the high-resolution energy spectrum of N 1s;

[0062] In Figure 5: (a) is the infrared spectrum of MWCNTs / PPy / NiCo-LDH prepared by the present invention,

[0063] (b) is the Raman spectrum of MWCNTs / PPy / NiCo-LDH prepared by the present invention;

[0064] In Figure 6: (a) are the CV curves of MPNCL-2 prepared in the examples of the present invention at different scan rates,

[0065] (b) are the galvanostatic charge-discharge curves of MPNCL-2 at different current densities,

[0066] (c) is the Nyquist fitting diagram of the EIS spectra of three composite materials, MPNCL-1, MPNCL-2, and MPNCL-3, prepared in the examples of the present invention (frequency range 0.1 - 10 KHz),

[0067] (d) is the comparative line graph of the specific capacitances of three composite materials, MPNCL-1, MPNCL-2, and MPNCL-3, at different current densities,

[0068] (e) Galvanostatic charge-discharge curves of three composite materials, MPNCL-1, MPNCL-2, and MPNCL-3, at 1 A g-1,

[0069] (f) Curve of MPNCL-2 cycled 5000 times at a current density of 10 A g-1;

[0070] In Figure 7: (a) Linear relationship between the scan rate and the anodic and cathodic peak currents of MPNCL-2 prepared in the embodiment of the present invention,

[0071] (b) Linear relationship between the scan rate and the logarithm of the anodic and cathodic peak currents of MPNCL-2, (c) Pseudocapacitance contribution rate of MPNCL-2 at a scan rate of 30 mV s -1 -1,

[0072] (d) Capacitance contribution rate diagram of MPNCL-2 at different scan rates;

[0073] In Figure 8: (a) Schematic diagram of the assembly of an asymmetric supercapacitor,

[0074] (b) CV curves of AC and MWCNTss / PPy / NiCo-LDH under a three-electrode system at a scan rate of 50 mV s -1 -1,

[0075] (c) CV curves of MWC NTss / PPy / NiCo-LDH / / AC at different voltage windows

[0076] (d) CV curves of MWCNTss / PPy / NiCo-LDH / / AC at different scan rates,

[0077] (e) GCD curves of MWCNTss / PPy / NiCo-LDH / / AC,

[0078] (f) Cycling performance diagram of MWCNTss / PPy / NiCo-LDH / / AC,

[0079] (g) Ragone diagram comparing the performance of MWCNTss / PPy / NiCo-LDH / / AC with other devices. Detailed implementation manners

[0080] The present invention will be further explained and illustrated below in conjunction with specific embodiments. Embodiment

[0081] 1) Purification of MWCNTs: MWCNTs were placed in a mixed solution of concentrated nitric acid and concentrated hydrochloric acid prepared according to a volume ratio of 1:3 and continuously stirred. After ultrasonic treatment for 3 - 4 h, they were refluxed in an oil bath at 80 - 90 °C for 4 h, cooled to room temperature, filtered by polycarbonate membrane, and dried overnight for standby;

[0082] 2) Preparation of MWCNTs / PPy: 0.36 g of purified MWCNTs was placed in a three-necked flask. 0.006 mol (0.454 mL) of Py and 50 mL of 0.1 mol / L H3PO4 were respectively poured into the three-necked flask and ultrasonicated for 10 min to form solution A. 0.006 mol of (NH4)2S2O8 (APS) was dissolved in 15 mL of 0.1 mol / L H3PO4 to form solution B. Solution B was dropped into A within 15 min using a constant pressure funnel, and reactions were carried out at 0 °C for 8 h, 12 h, and 24 h, respectively, which were recorded as sample 1, sample 2, and sample 3. After filtration, they were washed with deionized water and ethanol until colorless and dried in vacuum at 60 °C.

[0083] 3) Preparation of MWCNTs / PPy / NiCo-LDH:

[0084] ① Preparation of MWCNTs / PPy / ZIF-67. The main reason for its formation is that free Co 2+ is adsorbed onto negatively charged MWCNTs / PPy through electrostatic attraction. The specific process is as follows: 78.6 mg of MWCNTs / PPy (samples 1, 2, and 3) were respectively dispersed in 30 mL of methanol solution and ultrasonicated for 30 min to form solution A. 1.36 g of Co(NO3)2 . ·6H2O was weighed and added to the above solution and stirred for 1 h. Meanwhile, 0.833 g of 2-MeIM (2-methylimidazole) was dissolved in 30 mL of methanol to form solution B, and solution B was slowly dropped into solution A within 25 min using a constant pressure funnel and stirred for 30 min, then left standing for 12 h, filtered and washed, and dried at 70 °C for 12 h;

[0085] ② Preparation of MWCNTs / PPy / NiCo-LDH; 160 mg of MWCNTs / PPy / ZIF-67 and 356.6 mg of Ni(NO3)2·6H2O were respectively dissolved in 35 mL of deionized water, stirred for 30 min, then poured into a reaction kettle, reacted at 120 °C for 12 h, cooled, centrifuged, filtered, washed with ethanol, and dried at 70 °C for 10 h to obtain black-gray powders, which were respectively named MPNCL-1, MPNCL-2, and MPNCL-3.

[0086] Structure characterization and performance evaluation are as shown in Figure 1 ~8.

Claims

1. A preparation method of MWCNTs / PPy / NiCo-LDH, characterized in that, It includes the following steps: 1) Purification of MWCNTs: Place MWCNTs in a mixed solution of concentrated nitric acid and concentrated hydrochloric acid configured by volume ratio of 1:3 and continuously stir. After ultrasonic treatment for 3 - 4 h, reflux in an oil bath at 80 - 90 °C for 4 h, cool to room temperature, filter by suction, and dry overnight for standby; the concentration of MWCNTs in the mixed solution of concentrated nitric acid and concentrated hydrochloric acid is 1.5 - 1.7 g / L; 2) Preparation of MWCNTs / PPy: In a H3PO4 solvent system, using APS as an initiator, the purified MWCNTs and Py monomer as reactants, carry out a polymerization reaction at 0 °C for 8 - 24 h to obtain MWCNTs / PPy; the mass ratio of the purified MWCNTs to Py is 0.7:1 - 0.9:1, and the mass ratio of APS to Py is 2:1 - 4:1; 3) Preparation of MWCNTs / PPy / NiCo-LDH: Preparation of MWCNTs / PPy / ZIF-67: First, MWCNTs / PPy and Co(NO3)2 . ·6H2O were co-dissolved in methanol solvent, stirred and mixed for 1 - 2 h. Subsequently, 2-MeIM organic ligand was added and stirred at room temperature for 0.5 - 1 h, left standing for 12 h, filtered and washed, and dried at 70 °C for 12 h to obtain MWCNTs / PPy / ZIF-67; the mass ratio of MWCNTs / PPy to Co(NO3)2 . ·6H2O was 0.005:1 - 0.009:1, and the mass ratio of 2-MeIM to Co(NO3)2 . ·6H2O was 0.4:1 - 0.8:1; For the preparation of MWCNTs / PPy / NiCo-LDH, take the above-prepared MWCNTs / PPy / ZIF-67 and Ni(NO3)2·6H2O and dissolve them in deionized water and stir to mix evenly, then react at 100 - 120 °C for 10 - 12 h. After cooling, centrifuge, filter by suction, wash with ethanol, and dry at 70 °C for 10 h to obtain a black-gray powder, which is MWCNTs / PPy / NiCo-LDH; the mass ratio of MWCNTs / PPy / ZIF-67 to Ni(NO3)2·6H2O is 0.15:1 - 0.25:

1.

2. Application of MWCNTs / PPy / NiCo-LDH prepared by the method as described in claim 1 in an asymmetric supercapacitor.

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