Nickel-cobalt metal hydroxide nanosheet-coated nitrogen-doped carbon nanofiber membrane with graded core-shell structure as well as preparation method and application of nickel-cobalt metal hydroxide nanosheet-coated nitrogen-doped carbon nanofiber membrane

Abstract

The invention belongs to the field of nano materials, and relates to a graded core-shell structure nickel-cobalt metal hydroxide nanosheet coated nitrogen-doped carbon nanofiber membrane and a preparation method and application thereof.The nitrogen-doped carbon nanofiber membrane is adopted as a conductive substrate, a graded core-shell structure nickel-cobalt metal hydroxide nanosheet array grows on the surface of the nitrogen-doped carbon nanofiber membrane, and the graded core-shell structure nickel-cobalt metal hydroxide nanosheet array is prepared. The core-shell structure refers to that a nanosheet is loaded on a nanosheet. The preparation method comprises the following steps: dissolving polyacrylonitrile and 2-methylimidazole in an N, N-dimethylformamide solution, carrying out electrostatic spinning to obtain a precursor nanofiber membrane, carrying out pre-oxidation and high-temperature carbonization to obtain a nitrogen-doped carbon nanofiber membrane, and then carrying out two-step hydrothermal reaction to obtain the nitrogen-doped carbon nanofiber membrane. The nitrogen-doped carbon nanofiber membrane material coated with the nickel-cobalt metal hydroxide nanosheet with the graded core-shell structure is obtained. The composite material prepared by the invention combines the high conductivity of the nitrogen-doped carbon nanofibers, the pseudocapacitance characteristic of the nickel-cobalt metal hydroxides and the advantages of the graded core-shell structure, and has excellent supercapacitor performance.

Description

Technical Field

[0001] The invention belongs to the field of nanomaterials, and specifically relates to a graded core-shell structured nickel-cobalt metal hydroxide nanosheet coated nitrogen-doped carbon nanofiber membrane, and a preparation method and application thereof. Background Art

[0002] Supercapacitors are green and sustainable energy storage devices with the advantages of excellent specific capacitance, high power density, low cost, fast charge and discharge, and long cycle life. Electrode materials are an important component of supercapacitors. Transition metal hydroxides have a unique two-dimensional layered structure, high theoretical specific capacitance, and multi-electron Faraday redox reaction, and are one of the most popular supercapacitor electrode materials. However, in practical applications, due to the poor conductivity and severe agglomeration of transition metal hydroxides, low specific capacitance and poor cycle stability have greatly limited their widespread application in energy storage systems. Assembling transition metal hydroxides on a carbon substrate with excellent conductivity can solve the above problems.

[0003] Carbon nanofibers have the advantages of high conductivity, large aspect ratio, easy preparation, strong corrosion resistance, low price, and abundant growth space. When nitrogen-doped carbon nanofibers are compounded with transition metal hydroxides, nitrogen doping not only enhances the binding sites between carbon materials and transition metal hydroxides, but also induces electronic modulation and charge redistribution of surrounding carbon atoms, effectively promoting electrochemical processes; at the same time, it improves the conductivity of carbon nanofibers and the electron transfer rate, and improves charge storage capacity. In addition, the hierarchical core-shell structure can further increase the contact area between the electrode material and the electrolyte, provide more active sites, effectively reduce the agglomeration or accumulation of transition metal hydroxides in electrochemical reactions, and promote electron diffusion, thereby showing excellent capacitance performance and long-term stability.

[0004] In the present invention, we use electrospinning to prepare PAN nanofibers containing 2-methylimidazole, and after pre-oxidation and carbonization treatment, we use nitrogen-doped carbon nanofiber membrane as the substrate and hydrothermally grow hierarchical core-shell structured nickel-cobalt metal hydroxide nanosheets in two steps. This method is simple, fast and efficient, and the prepared composite material has good application potential in supercapacitors. Summary of the invention

[0005] The purpose of the present invention is to overcome the problem that transition metal hydroxides have poor electrical conductivity and are prone to agglomeration or accumulation in electrochemical reactions, resulting in poor specific capacitance and stability, and to provide a graded core-shell structured nickel-cobalt metal hydroxide nanosheet coated nitrogen-doped carbon nanofiber membrane and its preparation method and application. The prepared composite material uses PAN nanofibers containing 2-methylimidazole as a precursor, and after pre-oxidation and carbonization treatment, nitrogen-doped carbon nanofibers (NCNF) are obtained. The strong bonding between NCNF and nickel-cobalt metal hydroxide (NCH) nanosheets is utilized, and after hydrothermal reaction, an NCH nanosheet array with a size of 1-1.8 μm is loaded on the NCNF, and a secondary hydrothermal reaction is performed to obtain an NCH@NCH nanosheet array with a core-shell structure loaded on the NCNF. The composite material has high specific capacitance, energy density and excellent stability, and has important application value in the field of electrochemistry.

[0006] To achieve the above purpose, the technical solution adopted by the present invention is as follows:

[0007] Step (1): dissolving a certain amount of polyacrylonitrile and 2-methylimidazole in N,N-dimethylformamide (DMF) to obtain a precursor solution, and preparing precursor nanofiber PAN-M under certain conditions by electrospinning;

[0008] Step (2): placing the precursor nanofiber PAN-M obtained in step (1) in a muffle furnace, and performing a pre-oxidation treatment at a certain temperature in an air atmosphere to prepare a pre-oxidized nanofiber NPNF, and then carbonizing it in a nitrogen atmosphere to obtain a nitrogen-doped carbon nanofiber membrane NCNF;

[0009] Step (3): placing the nitrogen-doped carbon nanofiber membrane NCNF prepared in step (2) in a mixed solution of nickel and cobalt metal salts, and subjecting it to a hydrothermal reaction at a certain temperature to prepare a nickel-cobalt metal hydroxide nanosheet-coated nitrogen-doped carbon nanofiber membrane NCH / NCNF;

[0010] Step (4): placing the nickel-cobalt metal hydroxide nanosheet-coated nitrogen-doped carbon nanofiber membrane NCH / NCNF obtained in step (3) in a mixed solution of nickel-cobalt metal salt and hexamethylenetetramine, and subjecting it to a hydrothermal reaction at a certain temperature to obtain a hierarchical core-shell structured nickel-cobalt metal hydroxide nanosheet-coated nitrogen-doped carbon nanofiber membrane NCH@NCH / NCNF.

[0011] Furthermore, the mass ratio of polyacrylonitrile, 2-methylimidazole and DMF in step (1) is (1-2):(0.1-1):(5-10).

[0012] Furthermore, the electrospinning conditions in step (1) are as follows: the spinning temperature is room temperature, the spinning voltage is 15-20 kV, the distance from the nozzle to the receiving roller is 10-18 cm, the receiving speed is 100-180 r / min, and the injection speed is 0.04-0.10 mm / min.

[0013] Furthermore, the pre-oxidation treatment conditions in step (2) are as follows: the pre-oxidation temperature is 210-250° C., the heating rate is 2-10° C. / min, and the holding time is 1-3 h.

[0014] Furthermore, the carbonization treatment conditions in step (2) are as follows: carbonization temperature is 600-900°C, heating rate is 2-10°C / min, and holding time is 1-3h.

[0015] Furthermore, the hydrothermal reaction conditions in step (3) are as follows: the metal salts are Ni(NO3)2·6H2O and Co(NO3)2·6H2O, the total mass of the metal salts is 0.5 g, the ratio of NCNF to the total mass of the metal salts is 1:(50-300), the mass ratio of Ni(NO3)2·6H2O and Co(NO3)2·6H2O is (1-2):(1-2), the reaction solution is a mixed solution of DMF and methanol (1:1), the reaction temperature is 100-120°C, and the reaction time is 1-12 h.

[0016] Furthermore, in the hydrothermal reaction of step (4), the metal salts are Ni(NO3)2·6H2O and Co(NO3)2·6H2O, the total mass of the metal salts is 0.2-0.6 g, the ratio of NCH / NCNF to the total mass of the metal salts is 1:(20-60), the ratio of the total mass of the metal salts to hexamethylenetetramine is 1:(1-3), the mass ratio of Ni(NO3)2·6H2O and Co(NO3)2·6H2O is (1-2):(1-2), the reaction solution is a methanol solution, the reaction temperature is 100-120°C, and the reaction time is 1-12 h.

[0017] The present invention also provides a hierarchical core-shell structured nickel-cobalt metal hydroxide nanosheet coated with nitrogen-doped carbon nanofiber membrane prepared by the above-mentioned preparation method. The nitrogen-doped carbon nanofiber (NCNF) is obtained by high-temperature carbonization of PAN nanofiber containing 2-methylimidazole. Nitrogen doping can enhance the binding sites between the carbon material and the nickel-cobalt metal hydroxide, effectively promote the growth of nickel-cobalt metal hydroxide (NCH) nanosheets, and make the NCH nanosheets grow uniformly, vertically and orderly on the entire NCNF. The size of the NCH nanosheets is 1-1.8 μm, which plays a supporting role in the construction of the core-shell structure.

[0018] The present invention also provides a hierarchical core-shell structured nickel-cobalt metal hydroxide nanosheet coated with nitrogen-doped carbon nanofiber membrane prepared by the above preparation method, wherein a hierarchical core-shell structure is constructed, wherein the first-level skeleton is nitrogen-doped carbon nanofiber (NCNF), the second-level conductor is a large-sized (1-1.8 μm) nickel-cobalt metal hydroxide (NCH) nanosheet, and the third-level conductor is a small-sized (300-800 nm) nickel-cobalt metal hydroxide (NCH) nanosheet, and the whole constitutes a hierarchical core-shell structure (NCH@NCH / NCNF).

[0019] The present invention also provides a graded core-shell structured nickel-cobalt metal hydroxide nanosheet coated nitrogen-doped carbon nanofiber membrane prepared by the above-mentioned preparation method, wherein the core-shell structure refers to a third-level conductor (NCH@NCH) loaded on a second-level conductor, and an auxiliary agent is used to obtain the core-shell structure, and the auxiliary agent is hexamethylenetetramine. By controlling the mass of the metal salt and the hexamethylenetetramine, the size of the third-level conductor is regulated to form a core-shell structure.

[0020] The hierarchical core-shell structured nickel-cobalt metal hydroxide nanosheets coated with nitrogen-doped carbon nanofiber membranes are used as electrode materials for supercapacitors.

[0021] Compared with the prior art, the above technical solution conceived by the present invention can achieve the following beneficial effects:

[0022] (1) The preparation process of the present invention is simple and fast, with a short process, easy to operate, convenient and efficient, green and environmentally friendly, and the cost of raw materials is relatively low and easy to obtain;

[0023] (2) The composite material provided by the present invention can be used as a self-supporting electrode material without the need for a binder or a conductive agent, thereby solving the problem that the conventional electrode material needs to add an inert binder, resulting in poor conductivity and cycle stability of the electrode material;

[0024] (3) The preparation method of the present invention uses polyacrylonitrile and 2-methylimidazole as raw materials, obtains precursor nanofibers by electrospinning, and obtains nitrogen-doped carbon nanofibers after high-temperature carbonization. The fibers have a large aspect ratio, high porosity, and good permeability, which helps electrolyte ions enter the interior of the material and improves the transmission rate of the electrolyte.

[0025] (4) Using nitrogen-doped carbon nanofibers (NCNF) as the substrate, on the one hand, the nanofibers can effectively prevent the agglomeration or accumulation of nanosheets; on the other hand, nitrogen doping can induce electronic modulation and charge redistribution of the surrounding carbon atoms, effectively promoting the electrochemical process; at the same time, nitrogen doping enhances the binding sites between the carbon material and nickel cobalt metal hydroxide, effectively promoting the growth of nickel cobalt metal hydroxide (NCH) nanosheets, making the nanosheet size in the range of 1-1.8 μm, which plays a supporting role in the construction of the core-shell structure.

[0026] (5) The preparation method of the present invention constructs a hierarchical core-shell structure, wherein the first-level skeleton is nitrogen-doped carbon nanofiber (NCNF), the second-level conductor is large-sized (1-1.8 μm) nickel cobalt metal hydroxide (NCH) nanosheets, and the third-level conductor is small-sized (300-800 nm) nickel cobalt metal hydroxide (NCH) nanosheets, which cover the surfaces of the first and second-level conductors, forming a hierarchical core-shell structure (NCH@NCH / NCNF) as a whole.

[0027] (6) The core-shell structure refers to a third-level conductor (NCH@NCH) loaded on a second-level conductor. By controlling the mass of the metal salt and hexamethylenetetramine, the size of the third-level conductor is regulated to form a core-shell structure. By constructing a core-shell structure, a high-load active material can be obtained, which also provides high space utilization, large specific surface area and sufficient effective active sites for the redox reaction of NCH, ensuring effective conduction and mass transfer channels, accelerating the rapid transmission of ions or electrons, improving reaction kinetics, and thus improving electrochemical performance.

[0028] (7) The results of experiments show that the prepared hierarchical core-shell structured nickel-cobalt metal hydroxide nanosheets coated with nitrogen-doped carbon nanofiber membranes have a synergistic effect on the nitrogen-doped carbon nanofibers, nickel-cobalt metal hydroxide nanosheets and hierarchical core-shell structures. When the prepared hierarchical core-shell structured nickel-cobalt metal hydroxide nanosheets are used as electrode materials for electrochemical testing, the electrochemical performance is better than that of the prepared hierarchical core-shell structured nickel-cobalt metal hydroxide nanosheets at 1A g -1 At this current density, the specific capacitance reaches 3461.2F g -1 , at 10A g -1 At the current density, after 6000 charge and discharge cycles, the specific capacitance can retain 94.3% of the initial capacitance. It is used as the positive electrode and activated carbon as the negative electrode to assemble a supercapacitor. When the power density is 792.3W kg -1 The maximum energy density can reach 120.3Wh kg -1 , indicating that it has high specific capacitance, energy density and long cycle life. Therefore, the composite material is particularly suitable as an electrode material for supercapacitors. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 is a scanning electron microscope image of NCH@NCH / NCNF prepared in Example 1 of the present invention;

[0030] Figure 2 is the XRD pattern of NCH@NCH / NCNF prepared in Example 1 of the present invention;

[0031] Figure 3 The NCH@NCH / NCNF prepared in Example 1 of the present invention is -1 The charge and discharge curve diagram below;

[0032] Figure 4 The NCH@NCH / NCNF prepared in Example 1 of the present invention is -1 Cyclic stability curve under current density;

[0033] Figure 5 The power density and energy density curves of the supercapacitor assembled with NCH@NCH / NCNF prepared in Example 1 of the present invention as the positive electrode and activated carbon as the negative electrode;

[0034] Figure 6 Scanning electron microscopy image of NCH@NCH / NCNF-1 prepared in Example 2 of the invention;

[0035] Figure 7 Scanning electron microscopy image of NCH@NCH / NCNF-3 prepared in Example 3 of the invention;

[0036] Figure 8 The composite materials prepared in Examples 1, 2 and 3 of the present invention are -1 Comparison of charge and discharge curves under current density;

[0037] Fig. 9 is a scanning electron microscope image of NCH / NCNF prepared in Comparative Example 1 of the present invention;

[0038] Fig.10 is a scanning electron microscope image of NCH / CNF prepared in Comparative Example 2 of the present invention;

[0039] Fig.11 The composite materials prepared in Example 1, Comparative Example 1 and Comparative Example 2 of the present invention are -1 Comparison of charge and discharge curves under different current densities. DETAILED DESCRIPTION

[0040] The present invention will be further clearly and completely described below in conjunction with specific embodiments. The embodiments are only a part of the present invention and are used to illustrate the present invention, but do not limit the scope of the invention.

[0041] Example 1

[0042] The preparation method of the hierarchical core-shell structured nickel-cobalt metal hydroxide nanosheets coated with nitrogen-doped carbon nanofiber membrane in this embodiment is as follows:

[0043] Step (1): 1.4 g polyacrylonitrile and 0.5 g 2-methylimidazole were dissolved in N, N-dimethylformamide, magnetically stirred for 24 h, and allowed to stand at room temperature to obtain a precursor solution. Subsequently, an electrospinning machine was used for spinning to obtain precursor nanofibers. The spinning conditions were: spinning temperature was room temperature, spinning voltage was 18 kV, the distance from the nozzle to the receiving roller was 18 cm, the receiving speed was 150 r / min, and the injection speed was 0.05 mm / min. The precursor nanofiber was named: PAN-M.

[0044] Step (2): The PAN-M obtained in step (1) was placed in a muffle furnace for pre-oxidation treatment under the following conditions: in an air atmosphere, the temperature was raised from room temperature to 220°C at a heating rate of 5°C / min, and the temperature was kept for 1 hour. The obtained pre-oxidized nanofibers were named: NPNF. Then, high-temperature carbonization treatment was performed in a nitrogen atmosphere at a carbonization temperature of 700°C, a heating rate of 5°C / min, and the temperature was kept for 2 hours. The prepared nitrogen-doped carbon nanofibers were named: NCNF.

[0045] Step (3): Dissolve 0.5 g of Ni(NO3)2·6H2O and Co(NO3)2·6H2O in a mass ratio of 1:1 in a mixed solution of 5 ml DMF and 5 ml methanol to prepare a metal salt solution. Place 0.002 g of NCNF and the metal salt solution obtained in step (2) in the inner tank of a hydrothermal reactor, react at 120°C for 5 h, cool to room temperature, take out the nanofiber membrane, rinse and dry to obtain a nickel-cobalt metal hydroxide nanosheet-coated nitrogen-doped carbon nanofiber membrane, named: NCH / NCNF.

[0046] Step (4): 0.4 g of Ni(NO3)2·6H2O and Co(NO3)2·6H2O with a mass ratio of 1:1 and 0.25 g of hexamethylenetetramine were dissolved in 15 ml of methanol solution to prepare a mixed solution. 0.01 g of NCH / NCNF obtained in step (3) and the mixed solution were placed in the inner tank of a hydrothermal reactor, reacted at 120° C. for 3 h, cooled to room temperature, and the nanofiber membrane was taken out. After rinsing and drying, a hierarchical core-shell structured nickel-cobalt metal hydroxide nanosheet-coated carbon nanofiber membrane was obtained, named: NCH@NCH / NCNF.

[0047] Example 2

[0048] The preparation steps are the same as those in Example 1, except that the total mass of Ni(NO3)2·6H2O and Co(NO3)2·6H2O in step (4) of Example 2 is 0.2 g, and the mass of hexamethylenetetramine is 0.125 g. The material is named: NCH@NCH / NCNF-1.

[0049] Example 3

[0050] The preparation steps are the same as those in Example 1, except that the total mass of Ni(NO3)2·6H2O and Co(NO3)2·6H2O in step (4) of Example 3 is 0.6 g, and the mass of hexamethylenetetramine is 0.375 g. The material is named: NCH@NCH / NCNF-3.

[0051] Comparative Example 1

[0052] The preparation steps are the same as those in Example 1, except that only steps (1), (2) and (3) are included in Comparative Example 1. The material is named NCH / NCNF.

[0053] Comparative Example 2

[0054] The preparation steps are the same as those of Comparative Example 1, except that 2-methylimidazole is not added in step (1) of Comparative Example 2. The material is named NCH / CNF.

[0055] Figure 1 : This is a scanning electron microscope image of NCH@NCH / NCNF prepared in Example 1. It can be seen from the figure that the core-shell structured nickel cobalt metal hydroxide nanosheets (NCH@NCH) are uniformly coated on the nitrogen-doped carbon nanofiber (NCNF) membrane, the first-level skeleton is NCNF, the second-level conductor is a large-size 1-1.8μm NCH nanosheet, and the third-level conductor is a small-size (300-800nm) NCH nanosheet, which constitute a hierarchical core-shell structure as a whole. This structure can obtain a high-load active material, and also provides a high space utilization rate, a large specific surface area and sufficient active sites for the redox reaction of NCH, and provides an effective conduction and mass transfer channel, accelerates the rapid transmission of ions or electrons, improves the reaction kinetics, and thus improves the electrochemical performance.

[0056] Figure 2 This is the XRD spectrum of NCH@NCH / NCNF prepared in Example 1. It can be seen from the figure that the diffraction peaks at 2θ of 11.5°, 23.2°, 34.4°, and 59.8° correspond to the (003), (006), (101), and (110) crystal planes of hydrotalcite-like LDH, respectively, which are consistent with the standard spectra of Ni(OH)2 (JCPDS No.46-0605) and Co(OH)2 (JCPDS No.38-0715), proving the formation of NiCo-LDH nanosheets. Since the diffraction peak of NiCo-LDH nanosheets is strong, the diffraction peak of NCNF is not obvious.

[0057] Figure 3 The NCH@NCH / NCNF prepared in Example 1 is -1 The charge and discharge curves of NCH@NCH / NCNF are shown in Figure 1.-1 The charge-discharge curves under the above conditions show good symmetry and a typical potential platform, indicating that the material has good charge-discharge reversibility during the redox process. The specific capacitance of NCH@NCH / NCNF is as high as 3461.2 F g -1 .

[0058] Figure 4 The NCH@NCH / NCNF prepared in Example 1 was -1 The cycle stability curve under current density shows that after 6000 cycles, the specific capacitance can still maintain 94.3% of the initial capacitance, and the coulombic efficiency is 99.7%, indicating that the material has excellent cycle stability. This is because the hierarchical structure of the material can increase the contact area between the electrode material and the electrolyte, provide more active sites, effectively reduce the agglomeration or accumulation of transition metal hydroxides in the electrochemical reaction, and promote electron diffusion, thereby showing long-term stable performance.

[0059] Figure 5 The power density and energy density curves of the supercapacitor assembled with NCH@NCH / NCNF prepared in Example 1 as the positive electrode and activated carbon as the negative electrode are shown in the figure. It can be seen from the figure that the material has a power density of 792.3 W kg -1 When the temperature is high, it can reach 120.3Wh Kg -1 The high energy density of NCH@NCH / NCNF indicates that NCH@NCH / NCNF has high energy density and power density and has practical application value.

[0060] Figure 6 This is a scanning electron microscope image of NCH@NCH / NCNF-1 prepared in Example 2. It can be seen from the figure that the second-level conductor NCH nanosheets of NCH@NCH / NCNF-1 are all grown in an orderly manner on the nitrogen-doped carbon nanofiber membrane. Due to the small mass of the metal salt and hexamethylenetetramine, the grown third-level conductor is small in size, about 200nm, and is evenly loaded on the second-level conductor, showing a core-shell structure.

[0061] Figure 7 This is a scanning electron microscope image of NCH@NCH / NCNF-3 prepared in Example 3. It can be seen from the figure that NCH@NCH / NCNF-3 does not show an obvious core-shell structure. This is because during the secondary hydrothermal reaction, the mass of the metal salt and hexamethylenetetramine is large, and the size of the grown third-level conductor is large, which cannot be evenly loaded on the second-level conductor to form a multi-piece structure. Therefore, it can be seen from Examples 1, 2, and 3 that this patent successfully realizes the construction of a hierarchical core-shell structure nanofiber membrane by controlling the mass of the metal salt and hexamethylenetetramine.

[0062] Figure 8The composite materials prepared in Examples 1, 2 and 3 are -1 Comparison of charge and discharge curves under current density. It can be seen from the figure that NCH@NCH / NCNF has a high charge and discharge current density of 1A g -1 It has the longest discharge time and the largest specific capacitance value (3461.2F / g), which is better than samples NCH@NCH / NCNF-1 (2893.5F / g) and NCH@NCH / NCNF-3 (2410.4F / g). Compared with the multi-sheet structure NCH@NCH / NCNF-3, the core-shell structure NCH@NCH / NCNF-1 and NCH@NCH / NCNF have better capacitance performance, proving the advantages of the core-shell structure. Compared with NCH@NCH / NCNF-1, NCH@NCH / NCNF has a higher active material loading and better reaction kinetics, so it shows the highest specific capacitance.

[0063] Fig. 9 It is a scanning electron microscope image of NCH / NCNF prepared in comparative example 1. From the figure, it can be seen that NCH nanosheets grow uniformly and orderly on nitrogen-doped carbon nanofibers. Nitrogen doping can enhance the binding sites between carbon nanofibers and NCH, effectively promote the growth of NCH nanosheets, and make the nanosheet size 1-1.8μm. The larger-sized nanosheets play a supporting role in the construction of the core-shell structure.

[0064] Fig.10 This is a scanning electron microscope image of NCH / CNF prepared in Comparative Example 2. It can be seen from the figure that the PAN nanofibers without 2-methylimidazole and the carbon nanofibers obtained after high-temperature carbonization have weak binding force with metal ions, and the formed NCH nanosheets are small in size (about 400nm) and agglomeration occurs. Therefore, the surface of the carbon nanofibers is loaded with not only smaller nanosheets but also partially agglomerated nanoflowers. The smaller nanosheets and nanoflowers are not conducive to the construction of the core-shell structure.

[0065] Fig.11 The composite materials prepared in Example 1, Comparative Example 1 and Comparative Example 2 are -1 Comparison of charge and discharge curves under current density. It can be seen from the figure that NCH@NCH / NCNF with a hierarchical core-shell structure has the largest specific capacitance value (3461.2F / g), followed by NCH / NCNF (2103.8F / g) and NCH / CNF (1904.1F / g). This proves the advantages of introducing 2-methylimidazole into electrospun nanofibers and carbonizing them at high temperature to obtain nitrogen-doped carbon nanofibers in the preparation method of this patent, and obtaining a hierarchical core-shell structure with a high active material loading by controlling the mass of metal salts and hexamethylenetetramine.

[0066] The above shows and describes the basic principles and main features of the present invention and the advantages of the present invention. It should be understood by those skilled in the art that the present invention is not limited to the above embodiments. The above embodiments and descriptions are only for explaining the principles of the present invention. Without departing from the spirit and scope of the present invention, the present invention may have various changes and improvements, which fall within the scope of the present invention to be protected. The scope of protection of the present invention is defined by the attached claims and their equivalents.

Claims

1. A method for preparing a hierarchical core-shell structured nickel-cobalt metal hydroxide nanosheet-coated nitrogen-doped carbon nanofiber membrane, characterized in that: The following steps are involved: Step (1): dissolving a certain amount of polyacrylonitrile and 2-methylimidazole in N,N-dimethylformamide (DMF) to obtain a precursor solution, and preparing precursor nanofiber PAN-M by electrospinning under certain conditions; Step (2): placing the precursor nanofiber PAN-M obtained in step (1) in a muffle furnace, and performing a pre-oxidation treatment at a certain temperature in an air atmosphere to prepare a pre-oxidized nanofiber NPNF, and then carbonizing it in a nitrogen atmosphere to obtain a nitrogen-doped carbon nanofiber membrane NCNF; Step (3): placing the nitrogen-doped carbon nanofiber membrane NCNF prepared in step (2) in a mixed solution of nickel and cobalt metal salts, and subjecting it to a hydrothermal reaction at a certain temperature to prepare a nickel-cobalt metal hydroxide nanosheet-coated nitrogen-doped carbon nanofiber membrane NCH / NCNF; Step (4): placing the nickel-cobalt metal hydroxide nanosheet-coated nitrogen-doped carbon nanofiber membrane NCH / NCNF obtained in step (3) in a mixed solution of nickel-cobalt metal salt and hexamethylenetetramine, and subjecting it to a hydrothermal reaction at a certain temperature to obtain a hierarchical core-shell structured nickel-cobalt metal hydroxide nanosheet-coated nitrogen-doped carbon nanofiber membrane NCH@NCH / NCNF.

2. The method for preparing the hierarchical core-shell structured nickel-cobalt metal hydroxide nanosheet-coated nitrogen-doped carbon nanofiber membrane according to claim 1, characterized in that: The mass ratio of polyacrylonitrile, 2-methylimidazole and DMF in the step (1) is (1-2): (0.1-1): (5-10).

3. The method for preparing the hierarchical core-shell structured nickel-cobalt metal hydroxide nanosheet-coated nitrogen-doped carbon nanofiber membrane according to claim 1, characterized in that: The electrospinning conditions in step (1) are as follows: the spinning temperature is room temperature, the spinning voltage is 15-20 kV, the distance from the nozzle to the receiving roller is 10-18 cm, the receiving speed is 100-180 r / min, and the injection speed is 0.04-0.10 mm / min.

4. The method for preparing the hierarchical core-shell structured nickel-cobalt metal hydroxide nanosheet-coated nitrogen-doped carbon nanofiber membrane according to claim 1, characterized in that: The pre-oxidation treatment conditions of step (2) are as follows: the pre-oxidation temperature is 210-250 °C, the heating rate is 2-10 °C / min, and the holding time is 1-3 h.

5. The method for preparing the hierarchical core-shell structured nickel-cobalt metal hydroxide nanosheet-coated nitrogen-doped carbon nanofiber membrane according to claim 1, characterized in that: The carbonization treatment conditions of step (2) are as follows: the carbonization temperature is 600-900 °C, the heating rate is 2-10 °C / min, and the holding time is 1-3 h.

6. The method for preparing the hierarchical core-shell structured nickel-cobalt metal hydroxide nanosheet-coated nitrogen-doped carbon nanofiber membrane according to claim 1, characterized in that: The hydrothermal reaction conditions of step (3) are as follows: the metal salts are Ni(NO3)2·6H2O and Co(NO3)2·6H2O, the total mass ratio of NCNF to the metal salt is 1:(50-300), the mass ratio of Ni(NO3)2·6H2O to Co(NO3)2·6H2O is (1-2):(1-2), the reaction solution is a mixed solution of DMF and methanol in a volume ratio of 1:1, the reaction temperature is 100-120 °C, and the reaction time is 1-12 h.

7. The method for preparing the hierarchical core-shell structured nickel-cobalt metal hydroxide nanosheet-coated nitrogen-doped carbon nanofiber membrane according to claim 1, characterized in that: The hydrothermal reaction conditions of step (4) are as follows: the metal salts are Ni(NO3)2·6H2O and Co(NO3)2·6H2O, the ratio of the total mass of NCH / NCNF to the metal salt is 1:(20-60), the mass ratio of the total mass of the metal salt to hexamethylenetetramine is 1:(1-3), the mass ratio of Ni(NO3)2·6H2O to Co(NO3)2·6H2O is (1-2):(1-2), the reaction solution is a methanol solution, the reaction temperature is 100-120 °C, and the reaction time is 1-12 h.

8. The hierarchical core-shell structured nickel-cobalt metal hydroxide nanosheet-coated nitrogen-doped carbon nanofiber membrane prepared by the preparation method according to any one of claims 1 to 7, characterized in that: A hierarchical core-shell structure was constructed. The first-level skeleton was nitrogen-doped carbon nanofibers (NCNF), the second-level conductor was nickel cobalt hydroxide (NCH) nanosheets with a size of 1-1.8 μm, and the third-level conductor was nickel cobalt hydroxide (NCH) nanosheets with a size of 300-800 nm. The whole structure constituted a hierarchical core-shell structure (NCH@NCH / NCNF). The core-shell structure refers to a third-level conductor (NCH@NCH) loaded on a second-level conductor, and the core-shell structure is obtained by using an auxiliary agent, wherein the auxiliary agent is hexamethylenetetramine; by controlling the mass of the metal salt and hexamethylenetetramine, the size of the third-level conductor is regulated to form a core-shell structure.

9. The hierarchical core-shell structured nickel-cobalt metal hydroxide nanosheet-coated nitrogen-doped carbon nanofiber membrane according to claim 8, characterized in that: Nitrogen-doped carbon nanofibers (NCNF) are obtained by high-temperature carbonization of PAN nanofibers containing 2-methylimidazole. Nitrogen doping can enhance the binding sites between carbon materials and nickel-cobalt hydroxide, effectively promote the growth of nickel-cobalt hydroxide (NCH) nanosheets, and make NCH nanosheets grow uniformly and vertically in an orderly manner on the entire NCNF. The size of NCH nanosheets is 1-1.8 μm, which plays a supporting role in the construction of core-shell structure.

10. Use of the hierarchical core-shell structured nickel-cobalt metal hydroxide nanosheets coated with nitrogen-doped carbon nanofiber membrane as an electrode material in a supercapacitor as claimed in claim 8 or 9.

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