Preparation method and electrocatalytic application of self-supporting electrode of heteropoly acid derived high-valent metal and strong electronegative non-metal co-doped nickel sulfide

Abstract

The application discloses a preparation method of a self-supporting electrode of a heteropoly acid derived high-valence metal and strong electronegative nonmetal co-doped nickel sulfide and electrocatalytic application, and belongs to the technical field of electrocatalytic material synthesis. The application uses phosphomolybdic acid as a precursor, and utilizes the advantages of solubility, small size and simultaneous containing of fixed proportions of high-valence metal Mo and strong electronegative nonmetal P to prepare a P and Mo co-doped Ni3S2 catalyst with atomic level dispersion. The strategy has the advantages of simple process, wide condition and low cost, and the prepared catalyst shows excellent electrocatalytic urea oxidation performance and has high practical application value.

Description

Technical Field

[0001] This invention relates to a method for preparing a self-supporting nickel sulfide electrode co-doped with heteropolyacid-derived high-valence metals and highly electronegative nonmetals, and its electrocatalytic application, belonging to the field of electrocatalytic material synthesis technology. Background Technology

[0002] Electrocatalytic water splitting for hydrogen production is a key technology for developing clean energy. Currently, the bottleneck lies in the high theoretical voltage (1.23V) required for the oxygen evolution reaction (OER), the oxidation half-reaction. This necessitates commercial water electrolyzers requiring even higher voltages (typically greater than 1.5V), severely impacting electrolysis efficiency. Urea oxidation (UOR), as the most promising anolyte reaction to replace OER, can reduce its thermodynamic potential from 1.23V to 0.37V and simultaneously purify urea-rich wastewater. However, the UOR process involves the transfer of six electrons, generating complex intermediates and accompanied by the desorption of multiple gases, resulting in slow UOR kinetics. Therefore, highly efficient non-precious metal-based electrocatalysts are needed to accelerate this kinetically slow reaction.

[0003] Currently, Ni-based catalysts are considered ideal catalysts for UOR, and Ni 3+ A higher proportion of Ni species results in higher UOR activity. Optimizing the electronic structure of the catalyst is beneficial for increasing the content of high-valence Ni species, thereby improving its UOR performance. In recent years, doping has been considered an effective method to modify the chemical environment of Ni sites in catalysts, especially doping with high-valence W, Mo, V, etc., which can effectively modulate the electronic structure and thus improve the valence state of Ni. Xu et al. (J. Mater. Chem. A, 2021, 9, 3418-3426.) prepared Mo-doped Ni3S2 electrocatalysts. Their research showed that the incorporation of Mo modulated the electronic structure of Ni3S2, exposing more active sites and thus optimizing UOR activity. However, high-valence elements can attract more electrons, leading to a decrease in the covalent nature of the Ni-O bond, which can enhance the UOR activity of Ni. 3+The binding strength between Ni-based catalysts and *COO intermediates is crucial (Chem. Commun., 2020, 56, 11038-11041). Desorption of *COO intermediates is the rate-determining step of UOR (Angew. Chem. Int. Ed., 2019, 58, 16820-16825). Therefore, there is an urgent need to develop *COO intermediate desorption catalysts with low energy barriers and high UOR performance. Doping with strongly electronegative elements (such as S, N, and P) can modulate the downward shift of the d-band center, reducing the adsorption energy of *COO and facilitating *COO intermediate desorption (Chem. Eng. J., 2020, 394, 124926). Therefore, simultaneous doping with high-valence metals and strongly electronegative nonmetals may be an effective way to improve the UOR activity of Ni-based catalysts. Existing technologies often employ simple metal salts to dope high-valence metals. For example, patent CN112481653A discloses a method for preparing Mo-doped cobalt selenide / nanocarbon electrocatalysts. This method first dissolves cobalt salts and molybdates, then adds 2-methylimidazole solution to obtain a Mo-doped cobalt-based metal-organic framework precursor. This precursor is then further mixed with selenium powder, ground, and calcined to obtain the Mo-doped cobalt selenide / nanocarbon electrocatalyst. Alternatively, strong electronegative nonmetallic elements are doped using treatments such as nitriding, phosphating, or sulfidation in a tube furnace. For instance, patent CN113201753A discloses a method for preparing P-doped nickel selenide. In this method, the phosphating step involves placing any one of red phosphorus, NaH2PO2, NaH2PO3, or NaH2PO4 as a phosphorus source at the upper air outlet of a quartz tube, and placing nickel foam at the lower air outlet. Phosphating is performed for different times under inert gas protection and the tube furnace temperature is set to 300-500℃. All of the above doping methods have problems such as complex reaction processes, uneven mixing of raw materials, and non-uniform product morphology, making it difficult to obtain UOR catalysts with uniform high-valence metal and strong electronegative non-metal co-doping. Summary of the Invention

[0004] In view of the above-mentioned shortcomings of the prior art, the purpose of this invention is to provide a method for preparing a nickel sulfide self-supporting electrode based on co-doping of a high-valence metal and a strongly electronegative nonmetal with heteropolyacid.

[0005] Another objective of this invention is to use a nickel sulfide self-supporting electrode co-doped with heteropolyacid-derived high-valence metals and strongly electronegative nonmetals as a highly efficient UOR catalyst to replace OER for auxiliary industrial water electrolysis.

[0006] The above-mentioned objectives of the present invention will be achieved through the following technical solutions:

[0007] A method for preparing a self-supporting nickel sulfide electrode co-doped with a heteropolyacid-derived high-valence metal and a strongly electronegative nonmetal includes the following steps:

[0008] Step 1: Pretreatment of nickel foam (NF) electrodes;

[0009] Step 2: Dissolve phosphomolybdic acid and thiourea separately in 18 mL of water, then mix the two solutions to form a homogeneous solution for later use;

[0010] Step 3: Place the above solution in a 50mL stainless steel autoclave lined with polytetrafluoroethylene, put in a treated NF electrode, seal the autoclave and place it in a hydrothermal oven for reaction. After the reaction is completed and cooled to room temperature, open the autoclave and remove the NF electrode after the reaction with tweezers.

[0011] Step 4: Wash the NF electrode after reaction several times with water and ethanol, and then dry it in a vacuum drying oven at 60°C for 12 hours.

[0012] Preferably, in step 1, the NF is cut into 3*2cm pieces so that it can be placed upright in the reactor.

[0013] Preferably, the NF pretreatment step in step 1 is as follows: (1) NF is subjected to surface hydrophilic treatment in a Plasma plasma cleaner under O2 atmosphere; (2) NF is ultrasonically cleaned in 6M HCl solution for 15 min to remove the nickel oxide layer on the surface; (3) NF is ultrasonically cleaned in acetone, ethanol and water for 30 min each to remove contaminants such as grease on the NF surface; (4) The cleaned NF is dried in a vacuum oven at 50°C to avoid further oxidation.

[0014] Preferably, in step 2, the molar ratio of Mo to S in phosphomolybdic acid and thiourea is 1:2-1:6, that is, the concentration of thiourea is 0.5g / 100mL and the concentration of phosphomolybdic acid is 0.17g / 100mL-0.5g / 100mL.

[0015] Preferably, the hydrothermal reaction temperature in step 3 is 150-200℃.

[0016] Preferably, the hydrothermal reaction time in step 3 is 2-24 hours.

[0017] This invention provides a novel doping strategy for synthesizing nickel sulfide catalysts co-doped with high-valence metals and strongly electronegative nonmetals, which has the following advantages: ① Heteropolymetallic oxoacids (heteropoly acids for short) are formed by coordinating atoms (Mo, W, V, Nb, Ta, etc.) and central heteroatoms (P, As, Si, Fe, Co, etc.) connected by oxygen atoms in a certain structure, and contain both high-valence metals and strongly electronegative nonmetals. This method utilizes phosphomolybdic acid as a precursor to achieve co-doping of the high-valence metal Mo and the strongly electronegative nonmetal P, thus solving the phase separation problem that easily occurs when using two precursors or two synthesis methods in existing processes; ② Using phosphomolybdic acid as a precursor is more conducive to doping at the atomic level, thus facilitating the acquisition of biatomic doped sulfides with uniform nanoforest morphology; ③ Co-doping of P and Mo can regulate the morphology and electronic structure of Ni3S2, accelerate electron transfer, increase active sites, and the numerous gaps between the nanoforest morphology structures can serve as electron and reaction liquid transport channels, which is beneficial to improving the catalytic reaction activity; ④ The preparation method is simple, has wide operating conditions, and is low in cost. The prepared catalyst has excellent electrocatalytic urea oxidation performance and has high practical application value. Attached Figure Description

[0018] Figure 1 To prepare scanning electron microscope (SEM) images of P-Mo-Ni3S2 / NF electrode material at different magnifications using the method described in Example 1 of this invention, where a is the SEM image at 1000 magnification; b is the SEM image at 20000 magnification; and c is the SEM image at 40000 magnification.

[0019] Figure 2 To prepare transmission electron microscopy (TEM) images of P-Mo-Ni3S2 / NF electrode material at different magnifications using the method described in Example 1 of this invention, where a and b are TEM morphology images; c is a high-resolution transmission electron microscopy image of region I in Figure b; and d is a high-resolution transmission electron microscopy image of region II in Figure b.

[0020] Figure 3 To prepare linear sweep voltammetry (LSV) curves of P-Mo-Ni3S2 / NF electrode material in 1M KOH and 1M KOH + 0.5M urea solutions using the method described in Example 1 of this invention.

[0021] Figure 4 To test the UOR stability of the P-Mo-Ni3S2 / NF electrode material prepared using the method described in Example 1 of this invention in 1M KOH + 0.5 Murea solution.

[0022] Figure 5To prepare SEM and TEM images of Mo-Ni3S2 / NF electrode material using the method described in Comparative Example 1 of this invention, where a is a SEM image at 500x magnification; b is a SEM image at 20000x magnification; c is a SEM image at 40000x magnification; d and e are TEM morphology images; and f is a high-resolution transmission electron microscope image of the selected area in Figure e.

[0023] Figure 6 To prepare SEM and TEM images of Ni3S2 / NF electrode material using the method described in Comparative Example 2 of this invention, where a is a SEM image at 500x magnification; b is a SEM image at 40000x magnification; c is a TEM morphology image; d is a high-resolution transmission electron microscope image; e and f are magnified views of regions I and II in figure d, respectively.

[0024] Figure 7 The X-ray powder diffraction (XRD) patterns of the P-Mo-Ni3S2 / NF, Mo-Ni3S2 / NF, and Ni3S2 / NF electrode materials prepared using the methods of Example 1, Comparative Example 1, and Comparative Example 2 of this invention are shown.

[0025] Figure 8 LSV curves of P-Mo-Ni3S2 / NF, Mo-Ni3S2 / NF and Ni3S2 / NF electrode materials prepared using the methods described in Examples 1, 1, and 2 of this invention were obtained in 1M KOH + 0.5M urea solution.

[0026] Figure 9 To prepare SEM images of P-Mo-Ni3S2 / NF electrode material at different magnifications using the method described in Example 2 of this invention, where a is the SEM image at 5000 magnification and b is the SEM image at 20000 magnification.

[0027] Figure 10 To prepare SEM images of P-Mo-Ni3S2 / NF electrode material at different magnifications using the method described in Example 3 of this invention, where a is the SEM image at 5000 magnification and b is the SEM image at 20000 magnification.

[0028] Figure 11 LSV curves of P-Mo-Ni3S2 / NF electrode materials prepared using the methods described in Examples 1, 2, and 3 of this invention in 1M KOH + 0.5M urea solution were obtained. Detailed Implementation

[0029] The present invention will be further described in detail below with reference to specific embodiments and accompanying drawings. However, the present invention is not limited to the following embodiments. Any modifications or equivalent substitutions to the technical solutions of the present invention should be included within the protection scope of the present invention.

[0030] Example 1: (1) Weigh 0.09g of phosphomolybdic acid and dissolve it in 18mL of water. The concentration of phosphomolybdic acid is 0.25g / 100mL. Stir until dissolved. (2) Weigh 0.18g of thiourea and dissolve it in 18mL of water. The concentration of thiourea is 0.5g / 100mL. Stir until dissolved. (3) Mix the solutions in step (1) and step (2) to form a uniform liquid. Transfer it to a high-pressure reactor and place a 2*3cm NF electrode in it. Place the reactor in a 200℃ drying oven and react for 8h. Cool it to room temperature and remove the NF electrode after the reaction. (4) Wash the electrode taken out in step (3) several times with water and ethanol. Then dry it in a vacuum drying oven at 60℃ for 12h to obtain a P and Mo co-doped Ni3S2 self-supporting electrode (P-Mo-Ni3S2 / NF). Figure 1 SEM images of P-Mo-Ni3S2 / NF electrode materials prepared using the method described in Example 1 of this invention are shown. Figure 1 As shown in figure a, the P-Mo-Ni3S2 / NF electrode material exhibits a nanoforest morphology composed of nano-pine trees, with trunk lengths of 4-5 μm. Figure 1 b). Furthermore, the surface of the branches is encapsulated by thin nanosheets ( Figure 1 c). Figure 2 TEM images of P-Mo-Ni3S2 / NF electrode materials prepared using the method described in Example 1 of this invention. Figure 2 As shown in a and 2b, the main branch width of the nano-pine trees in the P-Mo-Ni3S2 / NF electrode is approximately 100-200 nm. (High-resolution TEM (HRTEM) images show...) Figure 2 c and 2d), the lattice fringes of 0.24 nm and 0.20 nm correspond to the (003) and (202) crystal planes of Ni3S2, respectively. Figure 3 The LSV curves of the P-Mo-Ni3S2 / NF electrode material prepared using the method described in Example 1 of this invention are shown in 1M KOH and 1M KOH + 0.5M urea solutions. As can be seen from the figure, the UOR performance of the P-Mo-Ni3S2 / NF electrode material is much higher than its OER performance. Figure 4 The UOR stability of the P-Mo-Ni3S2 / NF electrode material prepared using the method described in Example 1 of this invention was tested in 1M KOH + 0.5M urea solution. The figure shows that the P-Mo-Ni3S2 / NF electrode exhibits UOR stability at 500 mA cm⁻¹. -2 At current density, the potential increased by only 1.80% after 100 hours of testing, indicating that it has good UOR stability.

[0031] Comparative Example 1: (1) Weigh 0.10g of ammonium molybdate and dissolve it in 18mL of water. The concentration of ammonium molybdate is 0.28g / 100mL. Stir until dissolved. (2) Weigh 0.18g of thiourea and dissolve it in 18mL of water. The concentration of thiourea is 0.5g / 100mL. Stir until dissolved. (3) Mix the solutions in step (1) and step (2) to form a homogeneous liquid. Transfer it to a high-pressure reactor and place a 2*3cm NF electrode inside. Place the reactor in a 200℃ drying oven and react for 8h. Cool to room temperature and remove the NF electrode after reaction. (4) Wash the electrode removed in step (3) several times with water and ethanol. Then dry it in a vacuum drying oven at 60℃ for 12h to obtain a Mo-doped Ni3S2 self-supporting electrode (Mo-Ni3S2 / NF). Figure 5 SEM and TEM images of the Mo-Ni3S2 / NF electrode material were prepared using the method described in Comparative Example 1 of this invention. Figure 5 As shown in a-5d, the Mo-Ni3S2 / NF electrode material exhibits a rough, grass-like morphology. Furthermore, the lattice fringes in the HRTEM are 0.29 nm, corresponding to the Ni3S2(110) crystal plane (5e-5f).

[0032] Comparative Example 2: (1) Weigh 0.18g of thiourea and dissolve it in 36mL of water. The concentration of thiourea is 0.5g / 100mL. Stir until dissolved. (2) Transfer the solution in step (1) to a high-pressure reactor, put in a 2*3cm NF electrode, place the reactor in a 200℃ drying oven for 8h, cool to room temperature, and take out the NF electrode after reaction. (3) Wash the electrode taken out in step (2) several times with water and ethanol, and then dry it in a vacuum drying oven at 60℃ for 12h to obtain a Ni3S2 self-supporting electrode (Ni3S2 / NF). Figure 6 SEM and TEM images of the Ni3S2 / NF electrode material were prepared using the method described in Comparative Example 2 of this invention. Figure 6 As shown in a-6c, the Ni3S2 / NF electrode material exhibits a surface morphology similar to the original NF, but with a rougher surface. The lattice fringes in the HRTEM are 0.41 nm and 0.29 nm, corresponding to the (101) and (110) crystal planes (6d-6f) of Ni3S2, respectively. Furthermore, the XRD patterns of the P-Mo-Ni3S2 / NF, Mo-Ni3S2 / NF, and Ni3S2 / NF electrode materials prepared using the methods of Example 1, Comparative Example 1, and Comparative Example 2 of this invention are shown below. Figure 7As shown in the figure, the diffraction peaks correspond to NF (PDF#04-0850) and Ni3S2 (PDF#44-1418), respectively. No peaks for P and Mo were observed in the P-Mo-Ni3S2 / NF and Mo-Ni3S2 / NF electrodes, indicating that P and Mo exist in doped form. Figure 8 This study compares the UOR performance of P-Mo-Ni3S2 / NF, Mo-Ni3S2 / NF, and Ni3S2 / NF electrode materials prepared using the methods described in Examples 1, 1, and 2 of this invention. Example 1 shows the best UOR performance, i.e., the current density of the P-Mo-Ni3S2 / NF electrode can reach 1 A cm⁻¹ when the applied voltage is 1.53 V vs. RHE. -2 It is a Mo-Ni3S2 / NF electrode (0.63A cm). -2 It is 1.59 times that of the Ni3S2 / NF electrode (0.27Acm). -2 3.70 times that of ).

[0033] Example 2: (1) Weigh 0.06g of phosphomolybdic acid and dissolve it in 18mL of water. The concentration of phosphomolybdic acid is 0.17g / 100mL. Stir until dissolved. (2) Weigh 0.18g of thiourea and dissolve it in 18mL of water. The concentration of thiourea is 0.5g / 100mL. Stir until dissolved. (3) Mix the solutions in step (1) and step (2) to form a uniform liquid. Transfer it to a high-pressure reactor and place a 2*3cm NF electrode in it. Place the reactor in a 200℃ drying oven and react for 8h. Cool it to room temperature and remove the NF electrode after the reaction. (4) Wash the electrode taken out in step (3) several times with water and ethanol. Then dry it in a vacuum drying oven at 60℃ for 12h to obtain a P and Mo co-doped Ni3S2 self-supporting electrode (P-Mo-Ni3S2 / NF). Figure 9 SEM images of P-Mo-Ni3S2 / NF electrode materials prepared using the method described in Example 2 of this invention are shown. Figure 9 As shown in a and 9b, the surface of the P-Mo-Ni3S2 / NF electrode material exhibits a large micron-scale morphology, but many thin nanosheets appear on its surface.

[0034] Example 3: (1) Weigh 0.18g of phosphomolybdic acid and dissolve it in 18mL of water. The concentration of phosphomolybdic acid is 0.5g / 100mL. Stir until dissolved. (2) Weigh 0.18g of thiourea and dissolve it in 18mL of water. The concentration of thiourea is 0.5g / 100mL. Stir until dissolved. (3) Mix the solutions in step (1) and step (2) to form a homogeneous liquid. Transfer the solution to a high-pressure reactor and place a 2*3cm piece of NF in it. Place the reactor in a 200℃ forced-air drying oven and react for 8h. Cool to room temperature and remove the NF electrode after reaction. (4) Wash the electrode taken out in step (3) several times with water and ethanol. Then dry it in a vacuum drying oven at 60℃ for 12h to obtain a P and Mo co-doped Ni3S2 self-supporting electrode (P-Mo-Ni3S2 / NF). Figure 10 SEM images of P-Mo-Ni3S2 / NF electrode materials prepared using the method described in Example 3 of this invention. Figure 10 As shown in a and 10b, Example 3 exhibits a similar nanoforest morphology to Example 1, but the size of the nanosheets grown on the surface is larger. Figure 11 This study compares the UOR performance of P-Mo-Ni3S2 / NF electrode materials prepared using the methods described in Examples 1, 2, and 3 of this invention. Example 1 shows the best UOR activity.

[0035] Example 4: (1) Weigh 0.09g of phosphomolybdic acid and dissolve it in 18mL of water. The concentration of phosphomolybdic acid is 0.25g / 100mL. Stir until dissolved. (2) Weigh 0.18g of thiourea and dissolve it in 18mL of water. The concentration of thiourea is 0.5g / 100mL. Stir until dissolved. (3) Mix the solutions in step (1) and step (2) to form a homogeneous liquid. Transfer the solution to a high-pressure reactor and place a 2*3cm NF electrode in it. Place the reactor in a 150℃ drying oven and react for 8h. Cool to room temperature and remove the NF electrode after reaction. (4) Wash the electrode taken out in step (3) several times with water and ethanol. Then dry it in a vacuum drying oven at 60℃ for 12h to obtain a P and Mo co-doped Ni3S2 self-supporting electrode (P-Mo-Ni3S2 / NF).

[0036] Example 5: (1) Weigh 0.09g of phosphomolybdic acid and dissolve it in 18mL of water. The concentration of phosphomolybdic acid is 0.25g / 100mL. Stir until dissolved. (2) Weigh 0.18g of thiourea and dissolve it in 18mL of water. The concentration of thiourea is 0.5g / 100mL. Stir until dissolved. (3) Mix the solutions in step (1) and step (2) to form a homogeneous liquid. Transfer the solution to a high-pressure reactor and place a 2*3cm piece of NF in it. Place the reactor in a 200℃ forced-air drying oven and react for 2h. Cool to room temperature and remove the NF electrode after reaction. (4) Wash the electrode taken out in step (3) several times with water and ethanol. Then dry it in a vacuum drying oven at 60℃ for 12h to obtain a P and Mo co-doped Ni3S2 self-supporting electrode (P-Mo-Ni3S2 / NF).

[0037] Example 6: (1) Weigh 0.09g of phosphomolybdic acid and dissolve it in 18mL of water. The concentration of phosphomolybdic acid is 0.25g / 100mL. Stir until dissolved. (2) Weigh 0.18g of thiourea and dissolve it in 18mL of water. The concentration of thiourea is 0.5g / 100mL. Stir until dissolved. (3) Mix the solutions in step (1) and step (2) to form a homogeneous liquid. Transfer the solution to a high-pressure reactor and place a 2*3cm piece of NF in it. Place the reactor in a 200℃ forced-air drying oven and react for 24h. Cool to room temperature and remove the NF electrode after reaction. (4) Wash the electrode taken out in step (3) several times with water and ethanol. Then dry it in a vacuum drying oven at 60℃ for 12h to obtain a P and Mo co-doped Ni3S2 self-supporting electrode (P-Mo-Ni3S2 / NF).

Claims

1. A method for preparing a self-supporting nickel sulfide electrode co-doped with a heteropolyacid-derived high-valence metal and a strongly electronegative nonmetal, characterized in that, The selected heteropolyacid precursor contains both the high-valence metal Mo and the strongly electronegative nonmetal P; the substrate is selected as conductive nickel foam, and a P- and Mo co-doped nickel sulfide self-supporting electrode is formed in situ through hydrothermal sulfidation. The preparation method includes the following steps: Step 1: Pretreatment of nickel foam electrodes; Step 2: Dissolve phosphomolybdic acid and thiourea separately in water, then mix the two solutions to form a homogeneous solution for later use; Step 3: Place the above solution in a stainless steel autoclave lined with polytetrafluoroethylene, put in a piece of treated nickel foam electrode, seal the autoclave and place it in a hydrothermal box for reaction. After the reaction is completed and cooled to room temperature, open the autoclave and remove the nickel foam electrode after reaction with tweezers. Step 4: Wash the reacted nickel foam electrode several times with water and ethanol alternately, and then dry it in a vacuum drying oven at 60°C for 12 hours.

2. The preparation method according to claim 1, characterized in that, In step 2, the molar ratio of Mo to S in phosphomolybdic acid and thiourea is 1:2-1:6, the concentration of thiourea in the system is 0.5 g / 100 mL, and the concentration of phosphomolybdic acid in the system is 0.17 g / 100 mL-0.5 g / 100 mL.

3. The preparation method according to claim 1 or 2, characterized in that, The pretreatment steps of the nickel foam electrode in step 1 are as follows: (1) The surface of the nickel foam electrode is hydrophilically treated by a Plasma plasma cleaner in an O2 atmosphere; (2) The nickel foam electrode is ultrasonically cleaned in 6M HCl solution for 15 min to remove the nickel oxide layer on the surface; (3) The nickel foam electrode is ultrasonically cleaned in acetone, ethanol and water for 30 min each; (4) The cleaned nickel foam electrode is dried in a vacuum oven at 50℃ to avoid further oxidation.

4. The preparation method according to claim 1 or 2, characterized in that, In step 3, the hydrothermal reaction temperature is 150-200℃, and the reaction time is 2-24h.

5. The preparation method according to claim 3, characterized in that, In step 3, the hydrothermal reaction temperature is 150-200℃, and the reaction time is 2-24h.

6. The preparation method according to claim 1, 2, or 5, characterized in that, In step 1, the nickel foam electrode is cut into 3*2cm pieces so that it can be placed upright in the reactor.

7. The preparation method according to claim 3, characterized in that, In step 1, the nickel foam electrode is cut into 3*2cm pieces so that it can be placed upright in the reactor.

8. The preparation method according to claim 4, characterized in that, In step 1, the nickel foam electrode is cut into 3*2cm pieces so that it can be placed upright in the reactor.

9. The application of the nickel sulfide self-supporting electrode obtained by the preparation method of claim 1 in the electrocatalytic oxidation of urea.

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