Preparation method of self-supporting nitrogen-doped carbon nanotube-loaded nickel-molybdenum nitride catalyst

Abstract

The present invention discloses a method for preparing self-supporting nitrogen-doped carbon nanotube-loaded nickel molybdenum nitride. The structural unit uses carbon cloth as a precursor, nitrogen-doped carbon nanotubes (NCNTs) as a conductive network, and nickel molybdenum nitride is loaded onto the carbon cloth. Plasma-assisted chemical vapor deposition is used to deposit nickel molybdenum nitride onto the surface of the carbon cloth on which the nitrogen-doped carbon nanotubes are grown, thereby producing the self-supporting nitrogen-doped carbon nanotube-loaded nickel molybdenum nitride. The self-supporting nitrogen-doped carbon nanotube-loaded nickel molybdenum nitride prepared by the present invention is a bifunctional catalyst used in catalyzing both oxygen evolution reaction (ORR) and oxygen evolution reaction (OER) reactions. It can significantly enhance the adsorption efficiency of adsorbed gases, improve stability and conductivity, and exhibit low overpotential and cost in both ORR and OER reactions, meeting the requirements of commercial applications.

Description

Technical Field

[0001] The present invention relates to the field of oxygen reduction and oxygen evolution reaction catalyst preparation, and in particular to a method for preparing a self-supporting plasma nitrogen-doped carbon nanotube-loaded nickel-molybdenum nitride catalyst. Background Art

[0002] With the advancement of society and changes in the environment, developing new renewable energy sources and reducing the use of traditional fossil fuels are crucial to promoting sustainable development. The oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) at cathodes are important energy conversion processes, and the development of stable non-precious metal ORR / OER bifunctional electrocatalysts is particularly important. Summary of the Invention

[0003] In order to solve the problem of poor stability of non-precious metal ORR / OER bifunctional electrocatalysts, the present invention provides a method for preparing self-supporting plasma nitrogen-doped carbon nanotubes loaded with nickel molybdenum nitride. The prepared catalyst has high catalytic activity, more active sites and better stability.

[0004] The technical solution of the present invention to achieve the above-mentioned purpose is:

[0005] A method for preparing a self-supporting nitrogen-doped carbon nanotube-supported nickel-molybdenum nitride catalyst is characterized in that the method uses carbon cloth as a precursor and nitrogen-doped carbon nanotubes (NCNTs) as a conductive network, then prepares the nitrogen-doped carbon nanotube-supported nickel-molybdenum double hydroxide on the surface of the carbon cloth, and then uses plasma-assisted chemical vapor deposition to load the nickel-molybdenum nitride on the nitrogen-doped carbon nanotubes.

[0006] Furthermore, the preparation method specifically comprises the following steps:

[0007] (1) Synthesis of Cobalt-Iron Nanowires: Cobalt-Iron Nanowires were prepared from carbon cloth as a precursor, cobalt nitrate hexahydrate, iron nitrate nonahydrate, ammonium fluoride, and urea.

[0008] (2) Synthesis of nitrogen-doped carbon nanotubes: Nitrogen-doped carbon nanotubes were prepared from carbon cloth grown with cobalt-iron nanowires and dicyandiamide;

[0009] (3) Synthesis of self-supporting nitrogen-doped carbon nanotube-supported nickel-molybdenum double hydroxide: nickel-molybdenum hydroxide was prepared from nickel nitrate and sodium molybdate, and grown on the surface of carbon cloth containing nitrogen-doped carbon nanotubes to obtain self-supporting nitrogen-doped carbon nanotube-supported nickel-molybdenum double hydroxide;

[0010] (4) Synthesis of self-supporting nitrogen-doped carbon nanotube-loaded nickel-molybdenum nitride nanoparticles: NiMoN was prepared by plasma-assisted chemical vapor deposition on the surface of carbon cloth for growing nitrogen-doped carbon nanotubes to prepare self-supporting nitrogen-doped carbon nanotube-loaded nickel-molybdenum nitride nanoparticles.

[0011] Furthermore, the specific process for synthesizing the cobalt-iron nanowires in step (1) is as follows: ultrasonically treating the carbon cloth in a 10 wt.% potassium permanganate solution for 30 minutes, continuing ultrasonication in deionized water and ethanol until the solution is completely clear, and drying at 60°C for 6 hours. The carbon cloth is then placed in a mixed solution of cobalt nitrate hexahydrate, ferric nitrate nonahydrate, ammonium fluoride, urea and deionized water, stirred for 30 minutes to form a uniform mixed solution, transferred to a hydrothermal reactor, reacted at 120°C for 6 hours, the carbon cloth was taken out, washed with deionized water and ethanol, and finally dried in a vacuum oven at 60°C for 6 hours.

[0012] Furthermore, the specific process of synthesizing the nitrogen-doped carbon nanotubes in step (2) is as follows: the carbon cloth for growing cobalt-iron nanowires and dicyandiamide are placed in two different porcelain boats in a tube furnace, with the dicyandiamide located upstream of the tube furnace, and annealed at 400°C for 2h in an inert gas atmosphere, then heated to 800°C and continued annealing for 2h, with a heating rate of 5°C / min.

[0013] Furthermore, the specific process of the self-supporting nitrogen-doped carbon nanotubes loaded with nickel-molybdenum double hydroxide in step (3) is as follows: placing the carbon cloth on which the nitrogen-doped carbon nanotubes are grown in a mixed solution containing nickel nitrate, ammonium molybdate and deionized water, stirring for 30 minutes to form a uniform mixed solution, transferring it to a hydrothermal reactor, reacting it at 120°C for 6 hours, taking out the carbon cloth, washing it with deionized water and ethanol, and finally drying it in a vacuum oven at 60°C for 6 hours.

[0014] Furthermore, the specific process of the self-supporting nitrogen-doped carbon nanotubes loaded with nickel molybdenum nitride nanoparticles in step (4) is as follows: the carbon of the nitrogen-doped carbon nanotubes grown with nickel molybdenum hydroxide is arranged in a porcelain boat, dicyandiamide is added, and nickel molybdenum nitride nanoparticles are loaded on its surface by plasma-assisted chemical vapor deposition under appropriate working parameters.

[0015] Preferably, in step (1), the mass ratio of the cobalt nitrate hexahydrate, the ferric nitrate nonahydrate, the ammonium fluoride and the urea is 4:3:2:10, and the volume of the deionized water is 30 ml.

[0016] Preferably, in step (2), the mass of the dicyandiamide is 1400 to 1600 mg.

[0017] Preferably, in step (2), the inert gas atmosphere is argon.

[0018] Preferably, the mass ratio of nickel nitrate to sodium molybdate in step (3) is 8:7, and the volume of deionized water is 80-90 ml.

[0019] Preferably, the working parameters of step (4) are air pressure, power, and duration, respectively: 10-30 Pa, 100-150 W, and 50 to 80 min.

[0020] Preferably, in step (4), the mass of the dicyandiamide is 1000-3000 mg.

[0021] The nitrogen-doped self-supporting carbon nanotubes loaded with nickel molybdenum nitride nanoparticles prepared by the present invention can be used as electrode materials.

[0022] The bifunctional catalyst prepared by the present invention is used to catalyze ORR and OER reactions. Nickel molybdenum nitride nanoparticles are loaded on the surface of self-supporting nitrogen-doped carbon nanotubes, which can significantly enhance the adsorption efficiency of adsorbed gas, improve stability and conductivity, and have high catalytic activity and low cost in ORR and OER reactions, which can meet the requirements of commercial applications.

[0023] Compared with the prior art, the present invention has the following beneficial effects:

[0024] (1) Self-supporting nitrogen-doped carbon nanotubes can not only connect with each other to form an excellent conductive network, improving the weakness of insufficient conductivity of metal oxides, but also help to expose more electrochemically active substances on the surface of carbon nanotubes;

[0025] (2) Self-supporting nitrogen-doped carbon nanotubes can not only provide more electrochemically active sites, but also obtain a rougher surface, which is conducive to the deposition of nickel molybdenum nitride; self-supporting nitrogen-doped carbon nanotubes loaded with nickel molybdenum nitride have a higher specific surface area and a more uniform distribution. Nickel molybdenum nitride metal particles grow evenly on the surface of self-supporting nitrogen-doped carbon nanotubes, which can effectively solve the serious agglomeration problem in the traditional preparation process, thereby improving its electrochemical performance;

[0026] (3) Carbon nanotubes have a large specific surface area and excellent electronic properties. By creating defects through low-temperature plasma modification technology and increasing active sites, they can exert greater advantages.

[0027] (4) Nickel molybdenum nitride can be directly used as electrode material and has the advantages of high power density and high stability. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 Example 1 Microscopic morphology of self-supporting nitrogen-doped carbon nanotubes supported on nickel-molybdenum nitride catalyst under scanning electron microscope (SEM);

[0029] Figure 2 Linear sweep voltammetry (LSV) of oxygen reduction reaction (ORR) of self-supporting nitrogen-doped carbon nanotube-supported nickel molybdenum nitride catalyst and comparative example, commercial Pt / C under alkaline conditions;

[0030] Figure 3 Example Linear sweep voltammetry (LSV) of oxygen evolution reaction (OER) of self-supporting nitrogen-doped carbon nanotubes supported on nickel molybdenum nitride, comparative example and commercial ruthenium oxide under alkaline conditions. DETAILED DESCRIPTION

[0031] In order to deepen the understanding of the present invention, the present invention is further described in detail below with reference to the accompanying drawings and embodiments.

[0032] Example 1

[0033] A method for preparing a self-supporting nitrogen-doped carbon nanotube-supported nickel molybdenum nitride catalyst comprises the following steps:

[0034] (1) Synthesis of Cobalt-Fe Nanowires:

[0035] A 3cm×4cm carbon cloth was ultrasonically treated in a 10wt.% potassium permanganate solution for 10 minutes. Ultrasonication was continued in deionized water and ethanol until the solution was completely clear, and the solution was dried in a vacuum oven at 60°C for 12 hours. 200mg of cobalt nitrate hexahydrate, 150mg of ferric nitrate nonahydrate, 100mg of ammonium fluoride, and 500mg of urea were added to 40ml of deionized water and stirred for 10 minutes to form a homogeneous mixed solution. The solution was then transferred to a hydrothermal reactor, where the carbon cloth was immersed and allowed to stand at 120°C for 6 hours. The carbon cloth was then removed, washed sequentially with deionized water and ethanol, and finally dried in a vacuum oven at 40°C for 12 hours. The resulting sample is designated CoFe NWs / CC.

[0036] (2) Synthesis of nitrogen-doped carbon nanotubes:

[0037] A carbon cloth containing cobalt-iron nanowires and 1.5g of dicyandiamide (DCD) were placed in two separate porcelain boats in a tube furnace, with the DCD positioned upstream. The samples were annealed in an Ar atmosphere at 400°C for 2 hours, then raised to 800°C for another 2 hours at a heating rate of 5°C / min. The resulting samples were designated NCNTs / CC.

[0038] (3) Synthesis of Nitrogen-doped Carbon Nanotubes Supported with Nickel-Molybdenum Hydroxide:

[0039] 800mg of nickel nitrate and 700mg of sodium molybdate were added to 80ml of deionized water and stirred for 10 minutes to form a homogeneous mixed solution. The solution was then transferred to a hydrothermal reactor and immersed in the carbon cloth with nitrogen-doped carbon nanotubes. The mixture was then incubated at 160°C for 6 hours. The carbon cloth with nitrogen-doped carbon nanotubes was then removed, washed sequentially with deionized water and ethanol, and finally dried in a vacuum oven at 40°C for 12 hours. The resulting sample was designated NiMoLDH / NCNTs@CC.

[0040] (4) Synthesis of Nitrogen-doped Carbon Nanotubes Loaded with Nickel Molybdenum Nitride:

[0041] 1g of dicyandiamide and a carbon cloth containing nitrogen-doped carbon nanotubes of nickel-molybdenum hydroxide were placed on top of each other on a porcelain boat. Nickel-molybdenum nitride was deposited on the surface of the carbon cloth by plasma-assisted chemical vapor deposition (PECVD) at 10 Pa, 100 W, and 700°C in an argon-ammonia atmosphere for one hour. The resulting sample was designated NiMoN@NCNTs / CC-1.

[0042] The morphology of the NiMoN@NCNTs / CC material obtained in Example 1 was analyzed by scanning electron microscopy (SEM). Figure 1 The surface of the NCNTs shown becomes rough and opaque, and nickel molybdenum nitride nanoparticles are loaded on the surface of the NCNTs.

[0043] Example 2

[0044] A method for preparing a self-supporting nitrogen-doped carbon nanotube-supported nickel molybdenum nitride catalyst comprises the following steps:

[0045] (1) This step is the same as step (1) in Example 1.

[0046] (2) This step is the same as step (2) in Example 1.

[0047] (3) This step is the same as step (3) in Example 1.

[0048] (4) Synthesis of Nitrogen-doped Carbon Nanotubes Loaded with Nickel Molybdenum Nitride:

[0049] A carbon cloth containing 1.5g of dicyandiamide and nitrogen-doped carbon nanotubes containing nickel-molybdenum hydroxide was placed on top of the carbon cloth on a porcelain boat. Nickel-molybdenum nitride was deposited on the surface of the carbon cloth using plasma-assisted chemical vapor deposition (PECVD) at 10 Pa, 100 W, and 700°C in an argon-ammonia atmosphere for one hour. The resulting sample was designated NiMoN@NCNTs / CC-2.

[0050] Example 3

[0051] A method for preparing a self-supporting nitrogen-doped carbon nanotube-supported nickel molybdenum nitride catalyst comprises the following steps:

[0052] (1) This step is the same as step (1) in Example 1.

[0053] (2) This step is the same as step (2) in Example 1.

[0054] (3) This step is the same as step (3) in Example 1.

[0055] (4) Synthesis of Nitrogen-doped Carbon Nanotubes Loaded with Nickel Molybdenum Nitride:

[0056] 1g of dicyandiamide and a carbon cloth containing nitrogen-doped carbon nanotubes of nickel-molybdenum hydroxide were placed on top of each other on a porcelain boat. Nickel-molybdenum nitride was deposited on the surface of the carbon cloth using plasma-assisted chemical vapor deposition (PECVD) at 700°C, 100W, and 30 Pa pressure in an argon-ammonia atmosphere for one hour. The resulting sample was designated NiMoN@NCNTs / CC-3.

[0057] Example 4

[0058] A method for preparing a self-supporting nitrogen-doped carbon nanotube-supported nickel molybdenum nitride catalyst comprises the following steps:

[0059] (1) This step is the same as step (1) in Example 1.

[0060] (2) This step is the same as step (2) in Example 1.

[0061] (3) This step is the same as step (3) in Example 1.

[0062] (4) Synthesis of Nitrogen-doped Carbon Nanotubes Loaded with Nickel Molybdenum Nitride:

[0063] 1g of dicyandiamide and a carbon cloth containing nitrogen-doped carbon nanotubes of nickel-molybdenum hydroxide were placed on top of each other on a porcelain boat. Nickel-molybdenum nitride was deposited on the surface of the carbon cloth using plasma-assisted chemical vapor deposition (PECVD) at 10 Pa, 150 W power, and 700°C in an argon-ammonia atmosphere for one hour. The resulting sample was designated NiMoN@NCNTs / CC-4.

[0064] Comparative Example 1

[0065] A method for preparing self-supporting nitrogen-doped carbon nanotubes loaded with nickel molybdenum nitride comprises the following steps:

[0066] (1) Synthesis of Cobalt-Fe Nanowires:

[0067] A 3cm×4cm carbon cloth was ultrasonically treated in a 10wt.% potassium permanganate solution for 10 minutes. Ultrasonication was continued in deionized water and ethanol until the solution was completely clear, and the solution was dried in a vacuum oven at 60°C for 12 hours. 200mg of cobalt nitrate hexahydrate, 150mg of ferric nitrate nonahydrate, 100mg of ammonium fluoride, and 500mg of urea were added to 40ml of deionized water and stirred for 10 minutes to form a homogeneous mixed solution. The solution was then transferred to a hydrothermal reactor, where the carbon cloth was immersed and allowed to stand at 120°C for 6 hours. The carbon cloth was then removed, washed sequentially with deionized water and ethanol, and finally dried in a vacuum oven at 40°C for 12 hours. The resulting sample is designated CoFe NWs / CC.

[0068] (2) Synthesis of nitrogen-doped carbon nanotubes:

[0069] A carbon cloth containing cobalt-iron nanowires and 1.5g of dicyandiamide (DCD) were placed in two separate porcelain boats in a tube furnace, with the DCD positioned upstream. The samples were annealed in an Ar atmosphere at 400°C for 2 hours, then raised to 800°C for another 2 hours at a heating rate of 5°C / min. The resulting samples were designated NCNTs / CC.

[0070] (3) Synthesis of Nitrogen-doped Carbon Nanotubes Supported with Nickel-Molybdenum Hydroxide:

[0071] 800mg of nickel nitrate and 700mg of sodium molybdate were added to 80ml of deionized water and stirred for 10 minutes to form a homogeneous mixed solution. The solution was then transferred to a hydrothermal reactor and immersed in the carbon cloth with nitrogen-doped carbon nanotubes. The mixture was then incubated at 160°C for 6 hours. The carbon cloth with nitrogen-doped carbon nanotubes was then removed, washed sequentially with deionized water and ethanol, and finally dried in a vacuum oven at 40°C for 12 hours. The resulting sample was designated NiMoLDH / NCNTs@CC.

[0072] (4) Synthesis of Nitrogen-doped Carbon Nanotubes Loaded with Nickel Molybdenum Nitride:

[0073] 1g of dicyandiamide and a carbon cloth containing nitrogen-doped carbon nanotubes of nickel-molybdenum hydroxide were placed on top of each other on a porcelain boat. Nickel-molybdenum nitride was deposited on the surface via chemical vapor deposition (CVD) at 700°C in an argon-ammonia atmosphere for one hour. The resulting sample, designated T-NiMoN@NCNTs / CC, served as a comparative example, comparing this sample to Example 1 without plasma treatment. The bifunctional performance of the sample was significantly inferior to that of Example 1.

[0074] Bifunctional catalytic performance evaluation

[0075] All electrochemical tests were performed using a CHI 760E electrochemical workstation equipped with a PINE rotating disk electrode test system at room temperature.

[0076] Preparation of the working electrode: Before using the rotating disk electrode (RDE), that is, the glassy carbon electrode (GCE, d = 0.4 cm), first use Al2O3 powder to polish the electrode surface to a mirror surface on a polishing cloth, then rinse with distilled water several times, and ultrasonically vibrate for 10 seconds. After drying at room temperature, it is ready for use. Use a puncher to cut a sample with a diameter of 4 mm from the prepared sample, take 5-10 μL Nafion solution (5wt.%) and stick the carbon cloth to the GCE surface. Dry it naturally to obtain the working electrode used for the test. The comparative example was prepared and tested using the same electrode preparation method. As a control experiment, 10 mg of catalyst, 261 μL isopropanol, 652 μL deionized water, and 87 μL Nafion (5wt.%) were weighed and mixed, and the mixture was ultrasonically treated for 1 hour to obtain a homogeneous black suspension. For ORR and OER tests, the slurry prepared above was drop-coated on the surface of 1 cm × 4 cm carbon cloth, dried naturally, and prepared and tested using the same electrode preparation method to obtain the comparison electrode used in the test.

[0077] Electrochemical performance test: A standard three-electrode electrochemical test system was used during the test, wherein the counter electrode was a Pt sheet, the reference electrode was a saturated calomel electrode (SCE), and the working electrode prepared above.

[0078] The LSV curves of the NiMoN@NCNTs / CC sample, the comparative example and the commercial 20 wt.% Pt / C catalyst were tested in 1 M KOH electrolyte saturated with O2 at a rotation speed of 1600 rpm. The results are shown in Figure 2. Figure 2 The NiMoN@NCNTs / CC sample exhibited high ORR electrocatalytic activity (half-wave potential of 0.85 V vs. RHE in 1 M KOH), which was very close to that of the commercial Pt / C catalyst tested under the same conditions (half-wave potential of 0.86 V vs. RHE in 1 M KOH), indicating that the material has faster reaction kinetics in the ORR electrocatalytic process.

[0079] The OER catalytic activities of NiMoN@NCNTs / CC samples and ruthenium oxide catalysts in 1M KOH electrolyte were tested using a rotating disk electrode (RDE). Figure 3 The LSV curves of the OER catalytic performance of the NiMoN@NCNTs / CC sample of the embodiment and the comparative example are shown. The current density of the NiMoN@NCNTs / CC sample of the embodiment is 100 mA cm in 1 M KOH electrolyte. -2 The OER overpotential is 269 mV, which is much lower than that of commercial RuO2 (660 mV@100 mA cm -2), indicating that the NiMoN@NCNTs / CC sample has excellent OER electrocatalytic activity.

[0080] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the above embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing a self-supporting nitrogen-doped carbon nanotube-supported nickel molybdenum nitride catalyst, characterized in that: The method uses carbon cloth as a precursor and nitrogen-doped carbon nanotubes (NCNTs) as a conductive network, then prepares nickel-molybdenum double hydroxide supported by the nitrogen-doped carbon nanotubes on the surface of the carbon cloth, and then uses plasma-assisted chemical vapor deposition to load nickel-molybdenum nitride on the nitrogen-doped carbon nanotubes.

2. The preparation method according to claim 1, wherein The method specifically comprises the following steps: (1) Synthesis of Cobalt-Iron Nanowires: Cobalt-Iron Nanowires were prepared from carbon cloth as a precursor, cobalt nitrate hexahydrate, iron nitrate nonahydrate, ammonium fluoride, and urea. (2) Synthesis of nitrogen-doped carbon nanotubes: Nitrogen-doped carbon nanotubes were prepared from carbon cloth grown with cobalt-iron nanowires and dicyandiamide; (3) Synthesis of self-supporting nitrogen-doped carbon nanotube-supported nickel-molybdenum double hydroxide: nickel-molybdenum hydroxide was prepared from nickel nitrate and sodium molybdate, and grown on the surface of carbon cloth containing nitrogen-doped carbon nanotubes to obtain self-supporting nitrogen-doped carbon nanotube-supported nickel-molybdenum double hydroxide; (4) Synthesis of self-supporting nitrogen-doped carbon nanotube-loaded nickel-molybdenum nitride nanoparticles: NiMoN was grown on the surface of nitrogen-doped carbon cloth by plasma-assisted chemical vapor deposition to prepare self-supporting nitrogen-doped carbon nanotube-loaded nickel-molybdenum nitride nanoparticles.

3. The preparation method according to claim 2, wherein: The specific process for synthesizing the cobalt-iron nanowires in step (1) is as follows: ultrasonically treating the carbon cloth in a 10 wt.% potassium permanganate solution for 30 minutes, continuing ultrasonication in deionized water and ethanol until the solution is completely clear, and drying at 60°C for 6 hours. The carbon cloth is then placed in a mixed solution of cobalt nitrate hexahydrate, ferric nitrate nonahydrate, ammonium fluoride, urea and deionized water, stirred for 30 minutes to form a uniform mixed solution, transferred to a hydrothermal reactor, reacted at 120°C for 6 hours, the carbon cloth was taken out, washed with deionized water and ethanol, and finally dried in a vacuum oven at 60°C for 6 hours.

4. The preparation method according to claim 2, wherein The specific process of synthesizing the nitrogen-doped carbon nanotubes in step (2) is as follows: the carbon cloth for growing cobalt-iron nanowires and dicyandiamide are placed in two different porcelain boats in a tube furnace, with the dicyandiamide located upstream of the tube furnace, and annealed at 400°C for 2h in an inert gas atmosphere, then heated to 800°C and continued annealing for 2h, wherein the heating rate is 5°C / min.

5. The preparation method according to claim 2, wherein: The specific process of the self-supporting nitrogen-doped carbon nanotubes loaded with nickel-molybdenum double hydroxide described in step (3) is as follows: placing the carbon cloth on which the nitrogen-doped carbon nanotubes are grown in a mixed solution containing nickel nitrate, ammonium molybdate and deionized water, stirring for 30 minutes to form a uniform mixed solution, transferring it to a hydrothermal reactor, reacting it at 120°C for 6 hours, taking out the carbon cloth, washing it with deionized water and ethanol, and finally drying it in a vacuum oven at 60°C for 6 hours.

6. The preparation method according to claim 2, wherein: The specific process of the self-supporting nitrogen-doped carbon nanotubes loaded with nickel-molybdenum nitride nanoparticles in step (4) is as follows: the carbon of the nitrogen-doped carbon nanotubes grown with nickel-molybdenum hydroxide is arranged in a porcelain boat, dicyandiamide is added, and nickel-molybdenum nitride nanoparticles are loaded on the surface thereof by plasma-assisted chemical vapor deposition under appropriate working parameters.

7. The preparation method according to claim 3, wherein: In step (1), the mass ratio of the cobalt nitrate hexahydrate, the ferric nitrate nonahydrate, the ammonium fluoride and the urea is 4:3:2:10, and the volume of deionized water is 30 ml.

8. The preparation method according to claim 4, characterized in that: In step (2), the mass of the dicyandiamide is 1400 to 1600 mg.

9. The preparation method according to claim 5, wherein: The mass ratio of the nickel nitrate to sodium molybdate in step (3) is 8:7, and the volume of deionized water is 80-90 ml.

10. The preparation method according to claim 6, characterized in that: The working parameters of step (4) are air pressure, power and duration, respectively: 10-30 Pa, 100-150 W, 50-80 min; the mass of the dicyandiamide is 1000-3000 mg.

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