Self-Supported Nickel Cobalt Sulfide Nanobelt / Nickel Foam Electrode Material Coated with Nitrogen-Doped Carbon Layer, and Preparation Method and Application Thereof

Abstract

The present invention relates to the technical field of electrode material preparation, and specifically to a self-supporting nitrogen-doped carbon layer-coated nickel cobalt sulfide nanobelt / nickel foam electrode material, its preparation method and application. The nickel cobalt sulfide nanobelt grows in-situ on the surface of nickel foam, and the surface of the nickel cobalt sulfide nanobelt is coated with a nitrogen-doped carbon layer. In the present invention, a nickel cobalt nanobelt precursor with controllable morphology is first grown in-situ on the nickel foam skeleton. By means of the electrostatic interaction between the anionic (OH ‑ 、CO3 2‑ ) negative charge in the precursor and the amine positive charge in dopamine hydrochloride, polydopamine is coated on the surface of the nanobelt at room temperature in-situ, and then the target product is obtained through sulfidation and carbonization. The nitrogen-doped carbon layer is beneficial to accelerating the electron conduction rate of nickel cobalt sulfide, providing abundant reaction active sites, accelerating the kinetics of ion transport in the redox reaction process, improving the structural stability of the electrode material and inhibiting the volume expansion of the active substance during cycling, and improving the specific capacitance, rate and cycling stability performance of the electrode.

Description

Technical Field

[0001] The present invention belongs to the technical field of preparing electrode materials for solid-state supercapacitors, and particularly relates to a self-supporting nitrogen-doped carbon layer-coated nickel cobalt sulfide nanobelt / nickel foam electrode material, a preparation method thereof, and an application thereof. Background Art

[0002] As a new type of energy storage device, supercapacitors have attracted more and more attention due to their advantages of high power density, long cycle life, and high safety compared with lithium-ion batteries. Compared with traditional liquid electrolyte supercapacitors, solid-state supercapacitors usually use gel electrolytes, which will not cause problems such as electrolyte leakage, and can be integrated into a single chip in various geometric shapes. They are fast energy storage devices for future intelligent wearable electronic device fields. Electrode materials are crucial for the performance of supercapacitors. Optimizing and regulating the morphology, composition, and size of electrode materials are the main research directions of current electrode materials. Nickel cobalt transition bimetallic sulfides have high electrochemical activity and specific capacitance, and are potential pseudocapacitive electrode materials. However, affected by the material morphology and low intrinsic conductivity, they face problems such as slow kinetics during charge and discharge, inability to buffer volume changes, and unstable structure. Therefore, it is necessary to develop a new electrode material to solve the above problems and simultaneously obtain a large specific capacitance, rate performance, and cycle stability of the electrode material by designing and optimizing the material structure. Summary of the Invention

[0003] The purpose of the present invention is to provide a self-supporting nitrogen-doped carbon layer-coated nickel cobalt sulfide nanobelt / nickel foam electrode material, a preparation method thereof, and an application thereof. Using nickel foam as a substrate, a nickel cobalt nanobelt precursor with controllable morphology is in-situ grown through a hydrothermal reaction, polydopamine is coated on the surface of the nickel cobalt nanobelt through a room-temperature in-situ polymerization reaction, and then sulfidation and carbonization are carried out to form a nitrogen-doped carbon layer-coated nickel cobalt sulfide nanobelt / nickel foam electrode material. By adjusting the time for coating polydopamine, the thickness of the nitrogen-doped carbon layer can be controlled, and an electrode material with high specific capacitance, good rate performance, and excellent cycle stability can be prepared.

[0004] The primary object of the present invention is to propose a self-supporting nitrogen-doped carbon layer-coated nickel cobalt sulfide nanobelt / nickel foam electrode material. In this electrode material, nickel cobalt sulfide nanobelts are in-situ grown on the surface of nickel foam, and a nitrogen-doped carbon layer is coated on the surface of the nickel cobalt sulfide nanobelts. The nickel cobalt sulfide has a nanobelt structure with edges, and the width of the nickel cobalt sulfide nanobelts is 150-200 nm.

[0005] Another object of the present invention is to provide a preparation method of a self-supporting nitrogen-doped carbon layer-coated nickel cobalt sulfide nanobelt / nickel foam electrode material, which specifically includes the following steps:

[0006] (1) Pretreat the nickel foam NF;

[0007] (2) Prepare the nickel-cobalt nanobelt precursor / NF: Dissolve hydrated nickel salt, hydrated cobalt salt, urea, and ammonium fluoride in deionized water to form solution A; Immerse the pretreated nickel foam in step (1) into solution A, then transfer it to a hydrothermal reaction kettle for hydrothermal reaction. After the reaction, cool it naturally to room temperature. Take out the reacted nickel foam, wash it with deionized water, and then dry it to obtain the nickel-cobalt nanobelt precursor grown in situ on the nickel foam, denoted as nickel-cobalt nanobelt precursor / NF;

[0008] (3) Prepare the nickel-cobalt nanobelt precursor@PDA / NF: Add tris(hydroxymethyl)aminomethane to an ethanol aqueous solution, dissolve and stir evenly, adjust the pH value of the solution to 8.5, and then add dopamine hydrochloride to dissolve it to obtain solution B; Put the nickel-cobalt nanobelt precursor / NF obtained in step (2) into solution B, let it stand and react at room temperature for 24 - 96 h, replace the fresh solution B every 24 h. Take out the reacted nickel foam and wash it with deionized water, and then dry it to obtain the nickel foam with polydopamine coated in situ on the surface of the nickel-cobalt nanobelt precursor, denoted as nickel-cobalt nanobelt precursor@PDA / NF;

[0009] (4) Prepare the nickel-cobalt sulfide nanobelt@PDA / NF: Dissolve sodium sulfide nonahydrate in deionized water to form solution C. Immerse the nickel-cobalt nanobelt precursor@PDA / NF obtained in step (3) into solution C and transfer all of it to a hydrothermal reaction kettle for hydrothermal sulfidation reaction. After the reaction, cool it naturally to room temperature. Take out the reacted nickel foam, wash it with deionized water, and then dry it to obtain the nickel-cobalt sulfide nanobelt@PDA / NF;

[0010] (5) Prepare the NCS@NC / NF electrode material: Calcinate the nickel-cobalt sulfide nanobelt@PDA / NF obtained in step (4) under an argon atmosphere, the calcination temperature is 480 - 600 °C, the heating rate is 2 °C / min, and the holding time is 60 - 180 min. After the calcination, cool it naturally to room temperature and then take it out to obtain the self-supporting nitrogen-doped carbon layer coated nickel-cobalt sulfide nanobelt / nickel foam electrode material, denoted as NCS@NC / NF.

[0011] Further, in step (1) of the above preparation method, the pretreatment process of the nickel foam includes: First, rinse the nickel foam with deionized water and then immerse it in a 1 mol / L hydrochloric acid solution for ultrasonic washing for 20 - 30 min, then wash it alternately with deionized water and absolute ethanol 3 times in sequence, then immerse the nickel foam in an acetone solvent for ultrasonic washing for 20 - 30 min, and then wash it alternately with deionized water and absolute ethanol 3 times in sequence. Finally, put the nickel foam into a drying oven to dry.

[0012] Further, in step (2) of the above preparation method, the nickel salt is selected from nickel nitrate hexahydrate and nickel chloride hexahydrate, the cobalt salt is selected from cobalt nitrate hexahydrate and cobalt chloride hexahydrate, and the temperature of the hydrothermal reaction in step (2) is 120 °C and the reaction time is 8 h.

[0013] Furthermore, the concentration of hydrated nickel salt in solution A is 0.01 - 0.03 mol / L, and the concentration of hydrated cobalt salt is 0.02 - 0.06 mol / L; and the molar ratio of hydrated nickel salt, hydrated cobalt salt, urea, and ammonium fluoride in solution A is 1:2:4:4.

[0014] Further, in step (3) of the above preparation method, the concentration of tris(hydroxymethyl)aminomethane in solution B is 0.01 - 0.03 mol / L, and the concentration of dopamine hydrochloride is 0.01 - 0.06 mol / L. The ethanol aqueous solution is obtained by mixing absolute ethanol and water, and the volume ratio of absolute ethanol to water is 1:1 - 1:4.

[0015] Further, in step (4) of the above preparation method, the concentration of sodium sulfide nonahydrate in solution C is 0.2 - 0.6 mol / L, the temperature of the hydrothermal sulfidation reaction is 160 °C, and the reaction time is 6 h.

[0016] Another object of the present invention is to provide an application of the self-supporting nitrogen-doped carbon layer-coated nickel cobalt sulfide nanoribbon / nickel foam electrode material obtained by the above preparation method as an energy storage electrode material in a supercapacitor. The self-supporting nitrogen-doped carbon layer-coated nickel cobalt sulfide nanoribbon / nickel foam electrode material can be directly used as a working electrode.

[0017] In the above application, using the self-supporting nitrogen-doped carbon layer-coated nickel cobalt sulfide nanoribbon / nickel foam electrode material as the working electrode, a platinum sheet electrode as the counter electrode, and a Hg / HgO electrode as the reference electrode, with the electrolyte being 6 mol / L KOH, at 1 mA / cm 2 the areal specific capacitance of the self-supporting nitrogen-doped carbon layer-coated nickel cobalt sulfide nanoribbon / nickel foam electrode material is 3100 - 5100 mF / cm 2 ; at 10 mA / cm 2 the areal specific capacitance of the self-supporting nitrogen-doped carbon layer-coated nickel cobalt sulfide nanoribbon / nickel foam electrode material is 2200 - 4500 mF / cm 2 .

[0018] Compared with the prior art, the present invention has obvious advantages and beneficial effects. By means of the above technical solutions, the present invention can achieve considerable technological progressiveness and practicality, and has a wide range of utilization values. It has at least the following advantages:

[0019] (1) In the self-supporting nitrogen-doped carbon layer-coated nickel cobalt sulfide nanobelt / nickel foam (NCS@NC / NF) electrode material prepared by the present invention, nickel cobalt sulfide nanobelts grow in situ and uniformly on nickel foam. The nickel cobalt nanobelts are firmly bonded to the nickel foam substrate and can be directly used as a working electrode. Compared with the preparation process of powder-like electrode materials, the present invention does not require mixing of solvents and binders, nor operations such as stirring, coating, and drying. Therefore, it saves the time for electrode preparation and avoids the problems of uneven coating of electrode materials and weak bonding between active materials and nickel foam substrates during the preparation of electrodes obtained by traditional preparation methods, resulting in the shedding of active materials from the surface of nickel foam during the testing process, low result repeatability, and poor stability.

[0020] (2) The contact area between the electrode materials with nanoneedle structures and nanosheet structures and the electrolyte is small. In the NCS@NC / NF electrode material prepared by the present invention, nickel cobalt sulfide is a nanobelt structure with edges, which has a large specific surface area. In alkaline electrolytes, it can increase the contact area with OH - and provide more active sites for redox reactions, generating a large Faraday pseudocapacitance.

[0021] (3) In the NCS@NC / NF electrode prepared by the present invention, a nitrogen-doped carbon layer is coated on the surface of nickel cobalt sulfide nanobelts. The nitrogen-doped carbon layer is an excellent conductive substrate. Coating it on the surface of high specific capacity nickel cobalt sulfide is beneficial to accelerating the electron conduction rate of nickel cobalt sulfide. At the same time, the relatively high specific surface area can provide abundant reactive sites to accelerate the kinetics of ion transport in redox reactions in the electrolyte, which is conducive to improving the structural stability of the electrode material and inhibiting the volume expansion of active substances during cycling. It can better exert the synergistic effect of the high theoretical specific capacity of nickel cobalt sulfide and the excellent conductivity of carbon materials, and make up for the defects of poor conductivity of nickel cobalt sulfide and low specific capacity of carbon materials. By adjusting the coating time of polydopamine, the thickness of the nitrogen-doped carbon layer can be effectively controlled, so as to optimize the synthesis of energy storage electrode materials with the best conductivity and electrochemical active sites, which can be used in hybrid solid-state supercapacitors.

[0022] (4) Compared with the nickel cobalt sulfide / nickel foam (NC / NF) electrode without a nitrogen-doped carbon layer coating, the NCS@NC / NF electrode prepared by the present invention has relatively excellent specific capacity and rate performance, and can maintain good cycle stability, which is better than the nickel cobalt sulfide electrodes prepared by other reported methods.

[0023] (5) The preparation method of the NCS@NC / NF electrode of the present invention is simple and repeatable, and has universality. The nickel salts, cobalt salts, and urea raw materials used are cheap and easy to obtain, and the cost is low. It is also applicable to the preparation of other transition single-metal sulfide, bimetallic sulfide, and polymetallic sulfide electrodes with foam as the substrate and coated with a nitrogen-doped carbon layer. Brief Description of the Drawings

[0024] Figure 1 In (a) and (b), SEM images of the nickel-cobalt nanobelt precursor / NF prepared in step (2) of Example 1 are shown at magnification factors of 50,000 times and 700 times, respectively.

[0025] Figure 2 In (a), (b), (c), and (d), SEM images of the nickel-cobalt nanobelt precursor@PDA / NF prepared after coating with polydopamine in step (3) of Example 1, Example 2, Example 3, and Example 4 are shown, respectively.

[0026] Figure 3 In (a) and (b), SEM images of the nickel-cobalt sulfide / NF electrode prepared in the comparative example are shown at magnification factors of 50,000 times and 700 times, respectively.

[0027] Figure 4 In (a), (b), (c), and (d), SEM images of the nickel-cobalt sulfide nanobelt@PDA / NF prepared in Example 1, Example 2, Example 3, and Example 4 are shown, respectively.

[0028] Figure 5 It is the SEM image of the NCS@NC / NF electrode material prepared in Example 2 and Example 3, where (a) and (b) represent Example 2, and (c) and (d) represent Example 3.

[0029] Figure 6 It is the XRD curve of the NCS@NC / NF electrode material prepared in Example 2 and the nickel-cobalt sulfide / NF electrode material prepared in the comparative example.

[0030] Figure 7 is the XPS full spectrum of the NCS@NC / NF electrode material prepared in Example 2.

[0031] Figure 8 It is the CV curve of the nickel-cobalt sulfide / NF electrode material prepared in the comparative example at different scanning rates.

[0032] Figure 9 It is the CV curve of the NCS@NC / NF electrode material prepared in Example 2 at different scanning rates.

[0033] Figure 10 It is a comparative diagram of the galvanostatic charge-discharge (GCD) curves of the electrode materials prepared in Examples 1 to 4 and the comparative example at a current density of 1 mA / cm 2 ².

[0034] Figure 11 It is the electrode materials prepared in Example 2 and the comparative example at a frequency of 0.01 - 10 5Comparison diagram of electrochemical impedance (EIS) curves at [[Hz]] with an amplitude of 5 mV.

[0035] Figure 12 is Figure 11 Magnified comparison diagram in the medium and high frequency regions.

[0036] Figure 13 It is a comparison diagram of the specific capacitance retention rate of the NCS@NC / NF electrode material prepared in Example 2 and the nickel cobalt sulfide / NF electrode material prepared in the comparative example during 5000 cycles of charge and discharge at a current density of 5 mA / cm 2 when. Detailed implementation manners

[0037] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the present invention will be clearly and completely described below in conjunction with specific embodiments. Obviously, the described embodiments are some, but not all, of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0038] The present invention will be described in detail below with specific embodiments. For those not specified in the following embodiments, they are all carried out according to conventional conditions or conditions recommended by the manufacturer. The raw materials and reagents used, without indicating the manufacturer, are all conventional products that can be obtained through commercial purchase.

[0039] Example 1:

[0040] (1) Pretreatment of nickel foam (NF): Nickel foam with dimensions of 15×20×1 mm 3 was first rinsed with deionized water and then immersed in a hydrochloric acid solution with a concentration of 1 mol / L for ultrasonic washing for 20 min. Then, it was washed alternately with deionized water and absolute ethanol 3 times, and then the nickel foam was immersed in acetone solvent for ultrasonic washing for 20 min. Subsequently, it was washed alternately with deionized water and absolute ethanol 3 times again. Finally, the NF was placed in a drying oven and dried at 60°C for 12 h to obtain the pretreated nickel foam.

[0041] (2) Preparation of nickel cobalt nanobelt precursor / NF: 0.291 g of nickel nitrate hexahydrate, 0.582 g of cobalt nitrate hexahydrate, 0.24 g of urea, and 0.148 g of ammonium fluoride were dissolved in 50 mL of deionized water to form solution A; the NF pretreated in step (1) was immersed in solution A and all transferred to a hydrothermal reaction kettle, and hydrothermally reacted in an oven at a temperature of 120°C for 8 h. After the reaction, it was naturally cooled to room temperature. The nickel foam NF reacted in solution A was taken out, washed 3 times with deionized water, and then placed in a vacuum oven and dried at 60°C for 12 h to obtain the nickel cobalt nanobelt precursor / NF.

[0042] (3)Preparation of nickel-cobalt nanobelt precursor@PDA / NF: Dissolve 0.0726 g of tris(hydroxymethyl)aminomethane in 60 mL of an ethanol-water solution (volume ratio of ethanol to water is 1:1), stir evenly, add ammonia water to adjust the pH value of the solution to 8.5, and then add 0.2 g of dopamine hydrochloride to dissolve it to obtain solution B; Put the nickel-cobalt nanobelt precursor / NF obtained in step (2) into solution B, let it stand and react at room temperature for 24 h, take out the nickel foam after reaction in solution B, wash it 3 times with deionized water, and then put it into a vacuum oven and dry it at 60 °C for 12 h to obtain nickel foam with polydopamine in-situ coated on the surface of the nickel-cobalt nanobelt precursor, denoted as nickel-cobalt nanobelt precursor@PDA / NF.

[0043] (4)Preparation of nickel-cobalt sulfide nanobelt@PDA / NF: Dissolve 2.4 g of sodium sulfide nonahydrate in 50 mL of deionized water to form solution C. Immerse the nickel-cobalt nanobelt precursor@PDA / NF obtained in step (3) into solution C and transfer it all to a hydrothermal reaction kettle, and carry out hydrothermal sulfidation reaction in an oven at 160 °C for 6 h. After the reaction is completed, naturally cool it to room temperature. Take out the nickel foam NF after reaction in solution C, wash it 3 times with deionized water, and then put it into an oven and dry it at 60 °C for 12 h to obtain nickel-cobalt sulfide nanobelt@PDA / NF.

[0044] (5)Preparation of NCS@NC / NF: Calcinate and carbonize the nickel-cobalt sulfide nanobelt@PDA / NF obtained in step (4) under an argon atmosphere. The calcination temperature is 480 °C, the heating rate is 2 °C / min, and the holding time is 120 min. After the calcination is completed, naturally cool it to room temperature and take it out to obtain a self-supporting nitrogen-doped carbon layer-coated nickel-cobalt sulfide nanobelt / nickel foam electrode material, denoted as NCS@NC / NF.

[0045] Take the self-supporting nitrogen-doped carbon layer-coated nickel-cobalt sulfide nanobelt / nickel foam electrode (NCS@NC / NF) prepared in step (5) as the working electrode, a platinum sheet electrode as the counter electrode, and a Hg / HgO electrode as the reference electrode, and perform cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), electrochemical impedance (EIS), and cyclic stability test analysis at different scan rates in a 6 mol / L KOH electrolyte. Calculate the specific area capacitance at 1 mA / cm 2 to be 3102.2 mF / cm 2 at 10 mA / cm 2 the specific area capacitance is 2210.5 mF / cm 2 with a rate performance of 71.2% and a retention rate of the specific area capacitance of 85.8% after 5000 cycles.

[0046] Example 2:

[0047] (1) Pretreatment of nickel foam (NF): The specific operation is the same as step (1) of Example 1.

[0048] (2) Preparation of nickel-cobalt nanobelt precursor / NF: The specific operation is the same as step (2) of Example 1.

[0049] (3) Preparation of nickel-cobalt nanobelt precursor@PDA / NF: The standing reaction time at room temperature is 48 h, and fresh solution B is replaced every 24 h. The remaining operations are the same as step (3) of Example 1.

[0050] (4) Preparation of nickel-cobalt sulfide nanobelt@PDA / NF: The specific operation is the same as step (4) of Example 1.

[0051] (5) Preparation of NCS@NC / NF: The calcination temperature is 500 °C, and the remaining operations are the same as step (5) of Example 1.

[0052] The self-supported nickel-cobalt sulfide nanobelt / nickel foam electrode (NCS@NC / NF) prepared in step (5) was used as the working electrode, a platinum sheet electrode as the counter electrode, and a Hg / HgO electrode as the reference electrode. Cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), electrochemical impedance (EIS), and cycle stability tests were carried out in 6 mol / L KOH electrolyte at different scan rates. Based on the GCD curve, the specific area capacitance at 1 mA / cm 2 was 5082.6 mF / cm 2 , and the specific area capacitance at 10 mA / cm 2 was 4460.1 mF / cm 2 , the rate performance was 87.8%, and the retention rate of the specific area capacitance after 5000 cycles was 98.6%.

[0053] Example 3:

[0054] (1) Pretreatment of nickel foam (NF): The specific operation is the same as step (1) of Example 1.

[0055] (2) Preparation of nickel-cobalt nanobelt precursor / NF: The specific operation is the same as step (2) of Example 1.

[0056] (3) Preparation of nickel-cobalt nanobelt precursor@PDA / NF: The standing reaction time at room temperature is 72 h, and fresh solution B is replaced every 24 h. The remaining operations are the same as step (3) of Example 1.

[0057] (4) Preparation of nickel-cobalt sulfide nanobelt@PDA / NF: The specific operation is the same as step (4) of Example 1.

[0058] (5) Preparation of NCS@NC / NF: The calcination temperature was 550 °C, and the remaining operations were the same as those in step (5) of Example 1.

[0059] Using the self-supporting nickel cobalt sulfide nanobelt / nickel foam electrode coated with nitrogen-doped carbon layer (NCS@NC / NF) prepared in step (5) as the working electrode, a platinum sheet electrode as the counter electrode, and a Hg / HgO electrode as the reference electrode, cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), electrochemical impedance (EIS) and cyclic stability test analyses were carried out in 6 mol / L KOH electrolyte at different scan rates. Based on the GCD curve, the specific area capacitance at 1 mA / cm 2 was 5051.2 mF / cm 2 and at 10 mA / cm 2 was 4196.5 mF / cm 2 The rate performance was 83.1%, and the retention rate of the specific area capacitance after 5000 cycles was 99.1%.

[0060] Example 4:

[0061] (1) Pretreatment of nickel foam (NF): The specific operation was the same as that in step (1) of Example 1.

[0062] (2) Preparation of nickel cobalt nanobelt precursor / NF: The specific operation was the same as that in step (2) of Example 1.

[0063] (3) Preparation of nickel cobalt nanobelt precursor@PDA / NF: The standing reaction time at room temperature was 96 h, and the solution B was replaced with a new one every 24 h. The remaining operations were the same as those in step (3) of Example 1.

[0064] (4) Preparation of nickel cobalt sulfide nanobelt@PDA / NF: The specific operation was the same as that in step (4) of Example 1.

[0065] (5) Preparation of NCS@NC / NF: The specific operation was the same as that in step (5) of Example 1.

[0066] Using the self-supporting nickel cobalt sulfide nanobelt / nickel foam electrode coated with nitrogen-doped carbon layer (NCS@NC / NF) prepared in step (5) as the working electrode, a platinum sheet electrode as the counter electrode, and a Hg / HgO electrode as the reference electrode, cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), electrochemical impedance (EIS) and cyclic stability test analyses were carried out in 6 mol / L KOH electrolyte at different scan rates. Based on the GCD curve, the specific area capacitance at 1 mA / cm 2 was 4548.9 mF / cm 2 and at 10 mA / cm 2The areal specific capacitance at [specific condition] is 3602.7 mF / cm 2 , the rate performance is 79.2%, and the retention rate of the areal specific capacitance after 5000 cycles is 96.3%.

[0067] Comparative example:

[0068] (1) Pretreatment of nickel foam (NF): The specific operation is the same as step (1) of Example 1.

[0069] (2) Preparation of nickel-cobalt nanobelt precursor / NF: The specific operation is the same as step (2) of Example 1.

[0070] (3) Dissolve 2.4 g of sodium sulfide nonahydrate in 50 mL of deionized water to form solution C. Immerse the nickel-cobalt nanobelt precursor / NF obtained in step (2) into solution C and transfer all of it to a hydrothermal reaction kettle. Carry out hydrothermal sulfidation reaction at 160 °C in an oven for 6 h. After the reaction, naturally cool to room temperature. Take out the nickel foam NF reacted in solution C, wash it 3 times with deionized water, and then put it into the oven to dry at 60 °C for 12 h to obtain a nickel-cobalt sulfide / nickel foam electrode, denoted as nickel-cobalt sulfide / NF.

[0071] Use the nickel-cobalt sulfide / NF prepared in step (3) as the working electrode, a platinum sheet electrode as the counter electrode, and a Hg / HgO electrode as the reference electrode to conduct cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), electrochemical impedance (EIS), and cyclic stability test analysis at different scan rates in 6 mol / L KOH electrolyte. Calculate the areal specific capacitance at 1 mA / cm 2 based on the GCD curve, which is 2597.8 mF / cm 2 , and the areal specific capacitance at 10 mA / cm 2 is 1623.1 mF / cm 2 , the rate performance is 62.5%, and the retention rate of the areal specific capacitance after 5000 cycles is 69.4%.

[0072] The above comparative example is nickel-cobalt sulfide / NF prepared by a two-step hydrothermal reaction without a nitrogen-doped carbon layer coating.

[0073] Use a German ZEISS Sigma 360 type scanning electron microscope (SEM) to characterize the morphological changes of the nickel-cobalt nanobelt precursor / NF before and after coating with polydopamine, after hydrothermal sulfidation, and after calcination in the examples and comparative examples. Arbitrarily cut small pieces with a size of 4×4 mm 2 from the electrode, stick them on the conductive glue of the sample stage without distinguishing the front and back, and then spray gold for testing.

[0074] Figure 1 is the SEM image of the nickel-cobalt nanobelt precursor / NF prepared in step (2) of Example 1.Figure 1 The magnification of (a) in [Figure] is 50,000 times, Figure 1 and the magnification of (b) in [Figure] is 700 times. It can be seen from Figure 1 that the pretreated nickel foam can in-situ and uniformly grow nano-ribbon nickel-cobalt precursors through hydrothermal reaction. The surface of the nickel-cobalt nano-ribbons is smooth and has thin lamellae.

[0075] Figure 2 are SEM images of the nickel-cobalt nano-ribbon precursor@PDA / NF prepared after coating with polydopamine in step (3) of Example 1, Example 2, Example 3, and Example 4, and the magnification is 50,000 times for all. Among them, Figure 2 (a) in [Figure] represents Example 1, and the polydopamine coating time is 24 h, Figure 2 (b) in [Figure] represents Example 2, and the polydopamine coating time is 48 h, Figure 2 (c) in [Figure] represents Example 3, and the polydopamine coating time is 72 h, Figure 2 (d) in [Figure] represents Example 4, and the polydopamine coating time is 96 h. It can be seen from Figure 2 that from Example 1 to Example 4, as the coating time increases from 24 h to 96 h, the surface of the nickel-cobalt nano-ribbon precursor becomes rougher and rougher, indicating that the thickness of the polydopamine PDA coating gradually increases.

[0076] Figure 3 are SEM images of the nickel-cobalt sulfide / NF prepared in the comparative example. Figure 3 The magnification of (a) in [Figure] is 50,000 times, Figure 3 and the magnification of (b) in [Figure] is 700 times. It can be seen from Figure 3 (a) that after the hydrothermal sulfidation process, the surface of the nickel-cobalt precursor nano-ribbons is etched, and the ribbon morphology becomes quasi-hexagonal pyramid-like.

[0077] Figure 4 (a), (b), (c), and (d) in [Figure] are SEM images of the nickel-cobalt sulfide nano-ribbon@PDA / NF prepared after hydrothermal sulfidation in step (4) of Example 1, Example 2, Example 3, and Example 4 respectively, and the magnification is 50,000 times for all. It can be seen from Figure 4 that compared with Figure 3 the morphology of the product obtained by directly sulfiding without coating polydopamine in the comparative example, the nickel-cobalt sulfide nano-ribbon@PDA / NF formed after coating with polydopamine and then sulfiding can better retain the morphology of the nickel-cobalt sulfide nano-ribbons. In Example 1, the coating time of polydopamine is 24 h, and the etching phenomenon can be seen on the surface; in Example 2, no etching phenomenon is seen on the surface, and the surface is rough and ribbon-like; in Example 3, the surface of the nano-ribbons becomes rougher and is covered with granular polydopamine; in Example 4, a layer of nano-sheet-like structure substance covers the surface of the nano-ribbons, and there is accumulation of nano-ribbons in Example 3 and Example 4.

[0078] Figure 5 are SEM images of the self-supporting nitrogen-doped carbon layer-coated nickel cobalt sulfide nanobelt / nickel foam electrode material (NCS@NC / NF) prepared after calcination in Example 2 and Example 3. Figure 5 In (a) and (b) represent Example 2. Figure 5 In (c) and (d) represent Example 3. The magnification of (a) and (c) is 50,000 times, and the magnification of (b) and (d) is 700 times. From Figure 5 It can be seen that the nickel cobalt sulfide still retains the nanobelt morphology after calcination and grows uniformly on the nickel foam skeleton. The surface of the nanobelt is rough and has edges.

[0079] The XRD test was carried out using a Rigaku SmartLab SE type X-ray diffractometer produced in Japan. The nickel foam carrying the active material was placed on the sample stage to analyze information such as the composition and crystal structure of the material. Figure 6 are the XRD curves of the NCS@NC / NF prepared in Example 2 and the nickel cobalt sulfide / NF electrode material prepared in the comparative example. By comparison, it is found that the XRD curves of the two electrodes are very similar, and the characteristic peaks of nickel cobalt sulfides Ni3S2, NiCo2S4, and Ni appear. Among them, 2θ at 21.8°, 31.1°, 37.8°, 44.5°, 49.8°, 50.2°, and 55.3° correspond to the (100), (-110), (111), (200), (210), (-210), and (211) crystal planes of Ni3S2, respectively; 2θ at 31.5°, 38.3°, 50.4°, and 55.3° correspond to the (311), (400), (511), and (440) crystal planes of NiCo2S4, respectively, indicating that the loaded nickel cobalt sulfide product contains Ni3S2 and NiCo2S4. The remaining three sharp diffraction peaks at 2θ of 44.5°, 51.9°, and 76.4° correspond to (111), (200), and (220) of Ni, respectively, which belong to Ni in the nickel foam. Since the carbon in the nitrogen-doped carbon layer is amorphous carbon, the diffraction peak is weak and not observed in the XRD pattern of NCS@NC / NF.

[0080] The XPS test was carried out using a Thermo Scientific K-Alpha type X-ray photoelectron spectrometer produced in the United States. The nickel foam carrying the active material was placed on the sample stage to analyze the valence states of the elements in the electrode and possible bonding interactions. Figure 7It is the XPS full spectrum of the NCS@NC / NF electrode material prepared in Example 2, indicating that the NCS@NC / NF electrode contains elements such as Ni, Co, S, C, N, and O. The presence of C and N elements indicates that the polydopamine coated on the surface of nickel-cobalt sulfide nanobelts forms a nitrogen-doped carbon layer coated on the surface of the nanobelts after calcination.

[0081] The three-electrode test of the material was carried out using the Shanghai Chenhua electrochemical workstation CHI760E. The NCS@NC / NF electrode prepared in Example 2 or the nickel-cobalt sulfide / NF electrode prepared in the comparative example was used as the working electrode, the platinum wire electrode was used as the counter electrode, and the Hg / HgO electrode was used as the reference electrode. Cyclic voltammetry (CV) tests at different scan rates were carried out in 6 mol / L KOH electrolyte. Figure 8 and Figure 9 are the CV curves of the electrode materials prepared in the comparative example and Example 2 at different scan rates. From Figure 8 it can be seen that as the scan rate increases from 2 mV / s to 20 mV / s, the shape of the CV curve of the self-supporting nickel-cobalt sulfide / NF electrode material prepared in the comparative example can remain consistent. However, when it continues to increase to 50 mV / s and 100 mV / s, the CV curve is severely deformed. Figure 9 In, as the scan rate increases from 2 mV / s to 300 mV / s, the shape of the CV curve of the self-supporting NCS@NC / NF electrode material prepared in Example 2 can still remain consistent, indicating that coating a nitrogen-doped carbon layer on the surface of nickel-cobalt sulfide nanobelts can improve the rate performance of the electrode.

[0082] Figure 10 is the comparison chart of galvanostatic charge-discharge (GCD) curves of the electrode materials prepared in Examples 1 to 4 and the comparative example at a current density of 1 mA / cm 2 . At a current density of 1 mA / cm 2 , the areal specific capacitances of the electrodes prepared in Example 1, Example 2, Example 3, Example 4 and the comparative example are 3102.2 mF / cm 2 , 5082.6 mF / cm 2 , 5051.1 mF / cm 2 , 4548.9 mF / cm 2 and 2597.8 mF / cm 2 respectively, indicating that coating a nitrogen-doped carbon layer on the surface of nickel-cobalt sulfide nanobelts can significantly improve the specific capacitance of the electrode. The areal specific capacitance of the electrode obtained with a coating time of 48 h in Example 2 is the largest, indicating that the best coating effect is achieved. Continuing to extend the coating time to 72 h in Example 3 and 96 h in Example 4, after increasing the thickness of the nitrogen-doped carbon layer, partial agglomeration of the nanobelts occurs, resulting in a decrease in the areal specific capacity during discharge of the electrode material.

[0083] Figure 11 is a comparison chart of the electrochemical impedance (EIS) curves of the electrode materials prepared in Example 2 and the comparative example at a frequency of 0.01 - 10 5 Hz and an amplitude of 5 mV; Figure 12 is Figure 11 an enlarged comparison chart of the medium and high frequency regions. It can be seen from Figure 11 that compared with the nickel cobalt sulfide / NF prepared in the comparative example, the impedance curve of the NCS@NC / NF electrode prepared in Example 2 shows a larger low-frequency slope, indicating a lower diffusion resistance during the Faraday redox process and a fast charge transfer ability. In addition, it can be seen from Figure 12 that the NCS@NC / NF electrode prepared in Example 2 is semicircular in the high-frequency region and has a smaller radius, and its R s value (0.697 Ω) is significantly smaller than the R s value (1.789 Ω) of the nickel cobalt sulfide / NF electrode prepared in the comparative example, proving that coating the carbon layer on the surface of the nickel cobalt sulfide nanobelt reduces the charge transfer resistance at the contact between the electrode and the KOH electrolyte.

[0084] Figure 13 is a comparison chart of the specific capacitance retention rates of the electrode materials prepared in Example 2 and the comparative example during 5000 cycles of charge and discharge at a current density of 5 mA / cm 2 . The capacitance retention rate of the self-supporting NCS@NC / NF electrode prepared in Example 2 after 5000 cycles is 98.6%, which is significantly better than the capacitance retention rate of 69.4% of the self-supporting nickel cobalt sulfide / NF electrode prepared in the comparative example, indicating that coating the nitrogen-doped carbon layer can significantly improve the cycling stability of the nickel cobalt sulfide nanobelt / NF electrode.

[0085] The self-supporting nitrogen-doped carbon layer-coated nickel cobalt sulfide nanobelt / nickel foam electrode material disclosed by the present invention uses pretreated nickel foam as the substrate. First, nickel cobalt nanobelt precursors are in-situ grown on the surface of nickel foam through a hydrothermal reaction. Then, polydopamine is in-situ coated on the surface of the nickel cobalt nanobelt precursors at room temperature in an ethanol / aqueous solution containing tris(hydroxymethyl)aminomethane and dopamine hydrochloride. Subsequently, a sulfidation reaction occurs through hydrothermal ion exchange to obtain nickel foam with polydopamine-coated nickel cobalt sulfide nanobelts. Finally, after the calcination process, the polydopamine decomposes and carbonizes to obtain the nickel foam electrode material with a nitrogen-doped carbon layer-coated nickel cobalt sulfide nanobelt. This preparation method forms nickel cobalt precursor nanobelt morphologies by controlling the concentrations of nickel salts and cobalt salts during the hydrothermal process. Then, polydopamine is coated on the surface of the nanobelts as a carbon source and a nitrogen source through a simple in-situ polymerization reaction at room temperature. Subsequently, the target product with a stable structure is obtained through hydrothermal sulfidation and carbonization. In NCS@NC / NF, the role of nickel foam is to serve as a substrate for the in-situ and uniform growth of active materials to form nickel cobalt sulfide nanobelts. The structure of the nitrogen-doped carbon layer-coated nickel cobalt sulfide nanobelt with high conductivity and high specific surface area can increase the contact area between the active material and the electrolyte, provide more active sites for redox reactions, and generate a larger pseudocapacitance. At the same time, it can utilize the excellent conductivity of the nitrogen-doped carbon layer to accelerate the synergistic energy storage effect of ion / electron transfer rates in the electrolyte, provide more adsorption sites, and generate a larger double-layer capacitance, thereby improving the specific capacity, rate performance, and cycle stability of the electrode. The nitrogen-doped carbon layer, as an important protective layer for the stability of the nickel cobalt nanobelt structure, can control the structures of the nitrogen-doped carbon layer and the nickel cobalt sulfide nanobelt by adjusting the polydopamine coating time.

[0086] The above are only embodiments of the present invention and do not impose any form of limitation on the present invention. The present invention may also have other forms of embodiments based on the above structures and functions, which will not be listed one by one. Therefore, any person skilled in the relevant art, without departing from the scope of the technical solution of the present invention, any simple modification, equivalent change, and modification made to the above embodiments based on the technical essence of the present invention still fall within the scope of the technical solution of the present invention.

Claims

1. A method for preparing a self-supporting nitrogen-doped carbon layer-coated nickel-cobalt sulfide nanobelt / nickel foam electrode material, characterized in that: The following steps are involved: (1) Pretreatment of nickel foam NF; (2) Preparation of nickel-cobalt nanobelt precursor / NF: dissolving hydrated nickel salt, hydrated cobalt salt, urea and ammonium fluoride in deionized water to form solution A; immersing the nickel foam pretreated in step (1) in solution A, and then transferring it to a hydrothermal reactor for hydrothermal reaction. After the reaction is completed, the nickel foam is naturally cooled to room temperature. The nickel foam after the reaction is taken out, washed with deionized water, and then dried to obtain a nickel-cobalt nanobelt precursor in situ grown on the nickel foam, which is recorded as nickel-cobalt nanobelt precursor / NF; (3) Preparation of nickel-cobalt nanobelt precursor @PDA / NF: Add tris(hydroxymethyl)aminomethane to an ethanol aqueous solution and stir to dissolve it uniformly, adjust the pH value of the solution to 8.5, and then add dopamine hydrochloride thereto to dissolve it, thereby obtaining solution B; put the nickel-cobalt nanobelt precursor / NF obtained in step (2) into solution B, and allow it to react at room temperature for 24 to 96 h, replace solution B every 24 h, take out the nickel foam after the reaction, wash it with deionized water, and then dry it to obtain a nickel foam with polydopamine in situ coated on the surface of the nickel-cobalt nanobelt precursor, which is recorded as nickel-cobalt nanobelt precursor @PDA / NF; (4) Preparation of nickel cobalt sulfide nanobelts @PDA / NF: dissolving sodium sulfide nonahydrate in deionized water to form a solution C, immersing the nickel cobalt nanobelt precursor @PDA / NF obtained in step (3) in the solution C and transferring the whole of the solution into a hydrothermal reactor for a hydrothermal sulfidation reaction. After the reaction is completed, the solution is naturally cooled to room temperature. The nickel foam after the reaction is taken out, washed with deionized water, and then dried to obtain nickel cobalt sulfide nanobelts @PDA / NF; (5) Preparation of NCS@NC / NF electrode material: The nickel cobalt sulfide nanobelt@PDA / NF obtained in step (4) was calcined in an argon atmosphere at a temperature of 480-600°C, a heating rate of 2°C / min, and a holding time of 60-180 min. After the calcination, the material was naturally cooled to room temperature and taken out to obtain a self-supporting nitrogen-doped carbon layer-coated nickel cobalt sulfide nanobelt / nickel foam electrode material, denoted as NCS@NC / NF.

2. The method for preparing the self-supporting nitrogen-doped carbon layer-coated nickel-cobalt sulfide nanobelt / nickel foam electrode material according to claim 1, characterized in that: The pretreatment process of the nickel foam in step (1) includes: rinsing the nickel foam with deionized water and then immersing it in a hydrochloric acid solution with a concentration of 1 mol / L for ultrasonic washing for 20 to 30 minutes, then washing it with deionized water and anhydrous ethanol alternately for 3 times, then immersing the nickel foam in an acetone solvent for ultrasonic washing for 20 to 30 minutes, then washing it with deionized water and anhydrous ethanol alternately for 3 times, and finally drying the nickel foam in a drying oven.

3. The method for preparing the self-supporting nitrogen-doped carbon layer-coated nickel-cobalt sulfide nanobelt / nickel foam electrode material according to claim 1, characterized in that: The nickel salt in step (2) is selected from one of nickel nitrate hexahydrate and nickel chloride hexahydrate, and the cobalt salt is selected from one of cobalt nitrate hexahydrate and cobalt chloride hexahydrate. The temperature of the hydrothermal reaction is 120° C. and the reaction time is 8 h.

4. The method for preparing the self-supporting nitrogen-doped carbon layer-coated nickel-cobalt sulfide nanobelt / nickel foam electrode material according to claim 1 or 3, characterized in that: The concentration of the hydrated nickel salt in solution A is 0.01-0.03 mol / L, and the concentration of the hydrated cobalt salt is 0.02-0.06 mol / L; and the molar ratio of the hydrated nickel salt, the hydrated cobalt salt, urea, and ammonium fluoride in solution A is 1:2:4:

4.

5. The method for preparing the self-supporting nitrogen-doped carbon layer-coated nickel-cobalt sulfide nanobelt / nickel foam electrode material according to claim 1, characterized in that: In step (3), the concentration of tris(hydroxymethyl)aminomethane in solution B is 0.01 to 0.03 mol / L, the concentration of dopamine hydrochloride is 0.01 to 0.06 mol / L, and the ethanol aqueous solution is obtained by mixing anhydrous ethanol and water, and the volume ratio of anhydrous ethanol to water is 1:1 to 1:

4.

6. The method for preparing the self-supporting nitrogen-doped carbon layer-coated nickel-cobalt sulfide nanobelt / nickel foam electrode material according to claim 1, characterized in that: In step (4), the concentration of sodium sulfide nonahydrate in solution C is 0.2-0.6 mol / L, the temperature of the hydrothermal sulfidation reaction is 160° C., and the reaction time is 6 h.

7. The method for preparing the self-supporting nitrogen-doped carbon layer-coated nickel-cobalt sulfide nanobelt / nickel foam electrode material according to claim 1, characterized in that: In the prepared self-supporting nitrogen-doped carbon layer-coated nickel cobalt sulfide nanobelt / nickel foam electrode material, the nickel cobalt sulfide is an edge-containing nanobelt structure, whose surface is coated with a nitrogen-doped carbon layer. The nickel cobalt sulfide nanobelt is in situ grown on the surface of nickel foam, and the width of the nickel cobalt sulfide nanobelt is 150~200 nm.

8. Application of the self-supporting nitrogen-doped carbon layer-coated nickel cobalt sulfide nanobelt / nickel foam electrode material obtained by the preparation method of claim 1 as an energy storage electrode material in a supercapacitor.

9. The use according to claim 8, characterized in that: The self-supporting nitrogen-doped carbon layer-coated nickel-cobalt sulfide nanobelt / nickel foam electrode material is directly used as a working electrode.

10. The use according to claim 8 or 9, characterized in that: The self-supporting nitrogen-doped carbon layer coated nickel cobalt sulfide nanobelt / nickel foam electrode material was used as the working electrode, the platinum sheet electrode was used as the counter electrode, the Hg / HgO electrode was used as the reference electrode, the electrolyte was 6 mol / L KOH, and the current was 1 mA / cm 2 The area specific capacitance of the self-supporting nitrogen-doped carbon layer coated nickel cobalt sulfide nanobelt / nickel foam electrode material is 3100~5100 mF / cm 2 ; 10 mA / cm 2 The area specific capacitance of the self-supporting nitrogen-doped carbon layer coated nickel cobalt sulfide nanobelt / nickel foam electrode material is 2200~4500 mF / cm 2 .

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