Nickel phosphide nanoparticles with solid sphere structure modified electrode, preparation method and application thereof

Abstract

The application discloses a kind of nickel phosphide nanoparticles modified electrode with solid ball structure and its preparation method and application, and the modified electrode is with commercial carbon-based electrode as matrix, and the carbon fiber surface of carbon-based electrode is loaded with pure phase nickel phosphide, and the pure phase nickel phosphide is evenly distributed on the surface of carbon fiber with solid nanosphere structure.The preparation method comprises the following steps: pretreating commercial carbon-based electrode; evenly depositing metal Ni nanoparticles on the surface of commercial carbon-based material by electrochemical method; carrying out hydrothermal reaction on the commercial electrode uniformly loaded with metal Ni nanoparticles to obtain a commercial carbon-based electrode loaded with nickel phosphide; the obtained commercial carbon-based electrode loaded with nickel phosphide is phosphated for the second time; after the end of the holding stage, the tubular furnace is heated and held again, and after natural cooling, it is cleaned and dried, and the modified electrode is obtained.The modified electrode prepared by the application has good catalytic activity, high conductivity, high specific surface area and high stability, and overcomes the shortcomings of traditional carbon-based materials.

Description

Technical Field

[0001] This invention belongs to the field of electrode technology for flow batteries, and relates to a nickel phosphide nanoparticle modified electrode with a solid sphere structure, its preparation method and application. Background Technology

[0002] Redox flow batteries are the preferred energy storage technology for large-scale applications due to their advantages such as power and energy separation, long cycle life, safety and reliability, and the ability to perform rapid and deep charge / discharge cycles. Electrodes, as a key component of redox flow batteries, often directly affect important indicators such as energy conversion efficiency and cycle life. Carbon-based materials such as carbon felt, graphite felt, carbon cloth, and carbon paper are commonly used as electrodes in flow battery systems. These materials often possess a certain degree of conductivity, but their catalytic activity towards the reaction substrate is relatively poor, thus affecting the performance parameters of the flow battery.

[0003] Transition metal phosphides are binary or multi-component compounds formed by transition metals and phosphorus, exhibiting properties similar to both metals and semiconductors. They possess strength and hardness approaching that of covalent solids; simple crystal structures, high melting points, and corrosion resistance; and electromagnetic properties similar to metals. One key reason for the superior electrocatalytic performance of nickel phosphide lies in its unique crystal structure. Phosphorus atoms are arranged in an irregular, packed pattern within the basic units of nickel phosphide, resulting in a degree of distortion within these units. Consequently, nickel phosphide has an approximately spherical structure rather than a layered planar structure, exposing more active sites. Furthermore, the Ni-Ni bond lengths within the nickel phosphide structural units are remarkably similar to those in elemental nickel, giving nickel phosphide strong metallic properties and thus contributing to its excellent catalytic activity.

[0004] Using traditional carbon-based materials as a matrix and loading nickel phosphide nanoparticles onto its surface can largely overcome the drawbacks of traditional carbon-based materials, allowing the excellent catalytic properties of nickel phosphide to be utilized in flow battery systems. However, the technology of introducing nickel phosphide onto the surface of traditional carbon-based materials and ensuring its stability as an electrode during flow battery cycling remains a challenge.

[0005] Chinese patent CN 110484934 A discloses a method for preparing a three-dimensional self-supporting hydrogen evolution electrode material of nickel-phosphorus / nickel phosphide-carbon cloth. The material loads nickel-phosphorus alloy and nickel phosphide on a carbon cloth matrix, but its stability is poor and it is difficult to use in flow battery systems. Summary of the Invention

[0006] To address the aforementioned issues, this invention provides a nickel phosphide nanoparticle-modified electrode with a solid spherical structure, which possesses advantages such as good catalytic activity, high conductivity, high specific surface area, and high stability, overcoming the shortcomings of traditional carbon-based materials.

[0007] Another objective of this invention is to provide a method for preparing a nickel phosphide nanoparticle modified electrode with a solid spherical structure.

[0008] A third objective of this invention is to provide an application of a nickel phosphide nanoparticle-modified electrode with a solid spherical structure.

[0009] The technical solution adopted in this invention is a nickel phosphide nanoparticle modified electrode with a solid sphere structure, using a commercial carbon-based electrode as the substrate. Pure phase nickel phosphide is loaded on the carbon fiber surface of the carbon-based electrode, and the pure phase nickel phosphide is uniformly distributed on the carbon fiber surface in a solid nanosphere structure.

[0010] A method for preparing a nickel phosphide modified electrode with a solid spherical structure includes the following steps:

[0011] S1, pretreatment of commercial carbon-based electrodes with acid solution; removal of inorganic impurities on the surface of carbon-based materials and increase of defects on the surface of commercial electrodes make it easier and more uniform for metallic Ni nanoparticles to adhere to the surface of commercial electrodes, providing a good substrate for subsequent steps;

[0012] S2, using an electrochemical method, uniformly deposits metallic Ni nanoparticles on the surface of a commercially available carbon-based material; the electrolyte composition is 1.5–2.5 mM nickel chloride hexahydrate (NiCl2·6H2O) and 0.5–1 mM HCl / KCl; among which, after hydrolysis of NiCl2·6H2O, a large amount of free Ni is generated in the solution. 2+ The electrolyte provides a nickel source for the electrodeposition reaction, while HCl / KCl provides an acidic / neutral reaction environment, promoting the reaction. Furthermore, HCl / KCl, as an acidic / neutral solution controlling the reaction environment, does not introduce other impurities that could affect the reaction. When the amount of NiCl2·6H2O in the electrolyte is too high, the metallic Ni nanoparticles deposited on the surface of the carbon-based material will agglomerate; conversely, too low an amount will result in insufficient deposition. 2+ When reduced to Ni, the standard potential is -0.257V, which is quite negative. If the electrolyte is highly acidic, it is more likely to trigger the hydrogen evolution reaction. Therefore, HCl and KCl are added to make the electrolyte weakly acidic or neutral.

[0013] S3, prepare a hydrothermal reaction solution: 50-500 mM sodium hypophosphite, 3-5 ml surfactant, and 1-3 mM sodium hydroxide; react the hydrothermal reaction solution with a commercially available electrode uniformly loaded with metallic Ni nanoparticles at 100-200 °C to obtain a commercially available carbon-based electrode with nickel phosphide loading; the surface-attached metallic Ni nanoparticles provide stable sites for the growth of nickel phosphide during the hydrothermal reaction, effectively preventing the aggregation of synthesized materials after the hydrothermal reaction and increasing the synthesis ratio; excessive sodium hypophosphite will promote the formation of nickel phosphate, nickel hypophosphite, and other products, while insufficient sodium hypophosphite will result in incomplete reaction; the surfactant can ensure uniform distribution of reaction products, but excessive amounts can cause desorption between the products and the matrix; excessive sodium hydroxide will promote the formation of nickel hydroxide in the components, affecting the reaction process.

[0014] S4. Place the commercially available carbon-based electrode loaded with nickel phosphide obtained in S3 and 2-5 g of sodium hypophosphite in a tube furnace. After evacuation, heat the furnace to 200-300°C at a rate of 10-15°C / min under an argon or nitrogen atmosphere, and hold for 0.5-2 hours for secondary phosphating. The vacuum level should be below 5 Pa to avoid gas contamination. The purpose of secondary phosphating is to complete the phosphating treatment of Ni that did not react completely in the hydrothermal reaction, improve the utilization rate of raw materials, and increase the volume of nickel phosphide loaded on the surface of the commercial electrode. Heating the tube furnace to 200-300°C is to cause the hypophosphite to decompose and generate PH3, which then reacts with the metallic Ni on the electrode surface to complete the phosphating reaction.

[0015] S5, after the heat preservation stage, heat the tube furnace to 300-500℃ at a heating rate of 10-15℃ / min, keep it at that temperature for 1-3 hours, and then let it cool naturally before cleaning and drying to obtain the product.

[0016] Furthermore, S1 includes the following steps: placing a commercial carbon-based electrode in an acidic solution with a concentration of 0.5–3 mol / L, immersing it for 10–60 min, cleaning and drying it; placing it in a muffle furnace and holding it at 200–250°C for 4–6 h for heat treatment to obtain a pretreated commercial electrode.

[0017] Furthermore, the acidic solution is any one of sulfuric acid, nitric acid, or hydrochloric acid.

[0018] Furthermore, S2 includes the following steps: using pretreated commercial carbon-based material as the working electrode, a graphite plate as the control electrode, and Hg / Hg2SO4 or a saturated calomel electrode as the reference electrode; during the deposition process, the voltage is 0.1 to 0.6V, the deposition time is controlled between 100s and 500s, the electrolyte is continuously stirred to maintain the uniform distribution and diffusion of ions, and after deposition, the carbon felt is rinsed with deionized water 5 to 6 times and dried to obtain a commercial electrode with uniformly distributed metallic Ni nanoparticles on the carbon fiber surface.

[0019] Furthermore, S3 includes the following steps: after the hydrothermal reaction, the reaction vessel is allowed to cool naturally, the commercial electrode is taken out, washed 5-6 times with organic solution and deionized water in sequence, and then placed in a vacuum drying oven to dry the moisture, thereby obtaining a commercial electrode with nickel phosphide loading.

[0020] Furthermore, in step S4, the flow rate of argon or nitrogen gas is 10–60 sccm.

[0021] Furthermore, in step S5, after natural cooling, the electrode is washed 5-6 times with deionized water, placed in a vacuum drying oven and dried at 50-55°C for 6-8 hours, and after natural cooling, a nickel phosphide modified electrode with a solid spherical structure is obtained.

[0022] Furthermore, in S1, the commercial electrode is carbon felt, graphite felt, carbon cloth, or carbon paper.

[0023] Application of a nickel phosphide nanoparticle modified electrode with a solid sphere structure in acidic flow batteries, alkaline flow batteries and neutral flow batteries.

[0024] The beneficial effects of this invention are:

[0025] (1) In a specific embodiment of the present invention, uniformly distributed metallic Ni nanoparticles are electrodeposited on the surface of the original carbon-based material, which provides uniform attachment points for the synthesis of nickel phosphide nanoparticles in the next hydrothermal reaction process. This method of pre-depositing metallic nanoparticles on the carbon fiber surface can effectively avoid the agglomeration phenomenon that occurs in the traditional hydrothermal reaction process and ensure the uniformity of nickel phosphide distribution on the carbon fiber surface.

[0026] (2) In a specific embodiment of the present invention, the thermal decomposition properties of sodium hypophosphite are utilized to further phosphating the carbon-based material with PH3 from its decomposition products. Although the carbon-based material after hydrothermal synthesis has nickel phosphide nanoparticles on its surface, the nickel phosphide has a complex chemical composition (mainly Ni2P and Ni). 12 P5), and their properties also differ. Under high-temperature conditions, Ni 12 P5 is unstable and easily decomposes. In the secondary phosphating process, it can not only phosphate the metals remaining in the hydrothermal process, but also remove non-target nickel phosphide products, further improving the crystallinity of nickel phosphide.

[0027] (3) The nickel phosphide nanoparticle modified electrode with a solid spherical structure prepared in the specific embodiments of the present invention exhibits good catalytic activity, conductivity, and stability. The entire preparation process of the specific embodiments of the present invention has low raw material costs, low equipment prices, and simple and convenient operation procedures, which is conducive to the large-scale production of commercial electrodes for flow batteries. Attached Figure Description

[0028] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0029] Figure 1a This is a SEM image (scale bar is 3 μm) of the original carbon-based electrode surface that has not undergone modification in Example 1 of this invention.

[0030] Figure 1b This is a SEM image (scale bar is 300 nm) of the original carbon-based electrode surface that has not undergone modification in Example 1 of the present invention.

[0031] Figure 2a This is a SEM image (scale bar is 3 μm) of the modified electrode with complex solid spherical nickel phosphide nanoparticles prepared in Example 1 of this invention.

[0032] Figure 2b This is a SEM image (scale bar is 300 nm) of the modified electrode with complex solid spherical nickel phosphide nanoparticles prepared in Example 1 of the present invention.

[0033] Figure 3 This is the XRD pattern of the nickel phosphide modified electrode prepared in Example 1 of the present invention.

[0034] Figure 4 This is the Raman spectrum of the nickel phosphide modified electrode prepared in Example 1 of this invention.

[0035] Figure 5 This is the cyclic voltammetry curve of the nickel phosphide modified electrode prepared in Example 1 of this invention in vanadium oxysulfate.

[0036] Figure 6 This is the Nyquist plot of the nickel phosphide modified electrode prepared in Example 1 of this invention in vanadium oxysulfate.

[0037] Figure 7 This is a long-cycle performance diagram of a vanadium redox flow battery assembled with a nickel phosphide modified electrode prepared in Example 1 of the present invention as the positive electrode.

[0038] Figure 8 This is the cyclic voltammetry curve of the nickel phosphide modified electrode prepared in Example 4 of this invention in polysulfides.

[0039] Figure 9 This is the Nyquist plot of the nickel phosphide modified electrode prepared in Example 4 of this invention in polysulfides.

[0040] Figure 10 This is a long-cycle performance diagram of a sulfur-based flow battery assembled with a nickel phosphide modified electrode as the negative electrode, as prepared in Example 4 of this invention.

[0041] Figure 11 This is a SEM image (scale bar is 50 μm) of the nickel phosphide modified electrode prepared in Comparative Example 4 of this invention.

[0042] Figure 12 This is a SEM image (scale bar is 1 μm) of the nickel phosphide modified electrode prepared in Comparative Example 5 of this invention.

[0043] Figure 13 This is the XRD pattern of the nickel phosphide modified electrode prepared in Comparative Example 6 of this invention.

[0044] Figure 14 This is a test graph showing the long-cycle performance of a vanadium redox flow battery assembled with the nickel phosphide modified electrode prepared in Comparative Example 7 of this invention.

[0045] Figure 15 This is a test graph showing the long-cycle performance of a vanadium redox flow battery assembled with the nickel phosphide modified electrode prepared in Comparative Example 8 of this invention. Detailed Implementation

[0046] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0047] The design concept of this invention is as follows: using commercial carbon-based materials as raw materials, uniformly distributed metallic Ni nanoparticles are prepared on the surface of carbon fibers through electrochemical deposition. Subsequently, using sodium hypophosphite as a phosphorus source, the carbon-based materials are placed in a hydrothermal reactor liner for reaction. Under high temperature and high pressure, nickel phosphide is synthesized in situ on the carbon fiber surface. The carbon felt is then placed in a tube furnace, and sodium hypophosphite is used again to deepen the phosphating degree while removing impurities (tetragonal phase nickel phosphide), ensuring the consistency of the crystalline phase structure of the nickel phosphide loaded on the carbon fiber surface. This method yields a nickel phosphide-modified electrode with a solid spherical structure.

[0048] Example 1

[0049] A method for preparing a nickel phosphide modified electrode with a solid spherical structure includes the following steps:

[0050] S1. Prepare a 1M sulfuric acid solution, immerse the commercial carbon felt in the solution for 0.5 hours, then wash it 5 times with deionized water. After that, place the carbon felt in a vacuum drying oven and dry it at 50°C for 8 hours. Then, place the commercial carbon felt in a muffle furnace and heat it at 200°C for 4 hours to obtain the pretreated carbon felt.

[0051] Microstructure of untreated carbon felt as follows Figures 1a-1b As shown, its surface is smooth with vertical stripes. Its small specific surface area results in insufficient active sites on the surface when it is used as an electrode material, thus leading to poor reversibility.

[0052] S2, using a three-electrode system, uniformly distributed metallic Ni nanoparticles were electrodeposited on the surface of a pretreated carbon felt. The pretreated carbon felt served as the working electrode, the graphite plate as the control electrode, and Hg / Hg₂SO₄ as the reference electrode. The electrolyte consisted of 2.5 mM nickel chloride hexahydrate and 0.5 mM hydrochloric acid solution. The voltage was maintained at 0.4 V during deposition, and the deposition time was 250 s. After deposition, the surface was rinsed five times with deionized water and dried in a vacuum drying oven at 50 °C for 8 h to obtain a carbon felt with uniformly distributed metallic Ni nanoparticles deposited on its surface.

[0053] S3, a hydrothermal reaction solution, was prepared, consisting of 100 mM sodium hypophosphite, 5 ml ethylene glycol, and 2 mM sodium hydroxide. 55 ml of the hydrothermal reaction solution was placed together with a commercially available carbon felt uniformly loaded with metallic Ni nanoparticles in a polytetrafluoroethylene (PTFE) liner within a hydrothermal reactor. The liner was then installed inside the reactor. The reactor was placed in a forced-air drying oven at 180°C for 12 hours. After heating, the reactor cooled naturally. The commercial carbon felt was then removed, washed five times with deionized water, and dried in a vacuum drying oven at 50°C for 8 hours to obtain a commercially available carbon felt with a surface loaded with nickel phosphide.

[0054] S4. Place commercially available carbon cloth coated with nickel phosphide in a ceramic boat in a tubular furnace. Simultaneously, place 4g of sodium hypophosphite in another ceramic boat between the carbon felt and the gas inlet. After securing the collar, open the vacuum pump valve to lower the gas pressure inside the tube to below 5Pa. Increase the temperature to 200℃ at a rate of 10℃ / min and hold for 1 hour, continuously introducing argon gas at a flow rate of 20sccm during this period.

[0055] S5. After the heat preservation stage, the temperature of the tube furnace is increased to 400℃ at a heating rate of 10℃ / min and held for 2 hours. After natural cooling, it is removed and washed 5 times with deionized water. It is then dried in a vacuum drying oven at 50℃ for 8 hours to obtain a nickel phosphide modified electrode with a solid spherical structure.

[0056] The microstructure of the nickel phosphide particles with a solid spherical structure prepared in Example 1 is as follows: Figures 2a-2bAs shown, the uniformly distributed nickel phosphide nanoparticles provide numerous active sites for the reaction on the electrode surface. The XRD results of the nickel phosphide modified electrode prepared in Example 1 are as follows: Figure 3 As shown, the Ni₂P loaded on the modified electrode exhibits good crystallinity. The Raman spectrum is as follows: Figure 4 As shown, I D / I G An increase in the ratio means that more active sites are exposed.

[0057] The cyclic voltammetry curves of the nickel phosphide modified electrode prepared in Example 1 in vanadium oxysulfate, measured at room temperature, are shown below. Figure 5 As shown, the potential difference is 392 mV, which is 91 mV less than that of the original commercial carbon felt. The redox peak currents are 194 mA and 181 mA, while the redox peak currents of the original commercial electrode are 171 mA and 160 mA. Meanwhile, the Nyquist plot of the nickel phosphide modified electrode prepared in the example in vanadium oxysulfate was measured at room temperature as shown in the figure. Figure 6 As shown, the charge transfer resistance of the nickel phosphide modified electrode is 0.37Ω, which is significantly lower than that of the original commercial carbon felt (1.29Ω), greatly improving the reaction rate and enhancing the electrode catalytic activity.

[0058] To practically verify the application of nickel phosphide modified electrodes in flow batteries, two vanadium redox flow batteries were assembled using both the original commercial electrode and the nickel phosphide modified electrode as positive electrodes, respectively, and their long-cycle performance was compared. The results showed that the battery using the nickel phosphide modified electrode exhibited a higher initial energy efficiency (87%) and also showed higher energy efficiency during 400 charge-discharge cycles compared to the original commercial electrode. Figure 7 As shown.

[0059] Example 2

[0060] A method for preparing a nickel phosphide modified electrode with a solid spherical structure includes the following steps:

[0061] S1. Prepare a 0.5M hydrochloric acid solution, immerse the commercial carbon cloth in the solution for 10 minutes, then soak and wash it in deionized water for 30 minutes. After that, put the commercial carbon cloth in a vacuum oven at 50°C and dry it for 1 hour. Then, put the commercial carbon cloth in a muffle furnace and heat it at 230°C for 5 hours to obtain the pretreated commercial carbon cloth.

[0062] In step S2, pretreated carbon cloth was used as the working electrode, a graphite plate as the counter electrode, and a three-electrode system consisting of Hg / Hg₂SO₄ as the reference electrode. Metallic Ni nanoparticles were deposited on the carbon cloth surface. The deposition voltage was maintained at 0.6V, and the deposition time was 200s. The electrolyte consisted of 1.5mM hydrated nickel chloride and 0.5mM hydrochloric acid solution. After deposition, the carbon cloth was rinsed six times with deionized water and dried in a vacuum drying oven at 50°C for 1 hour to obtain commercially available carbon cloth with metallic Ni nanoparticle modification on its surface.

[0063] S3, a hydrothermal reaction solution was prepared by mixing 50 mM sodium hypophosphite, 5 ml ethylene glycol, and 1 mM sodium hydroxide. 60 ml of the hydrothermal reaction solution was placed together with a piece of commercially available carbon cloth uniformly loaded with metallic Ni nanoparticles in the liner of a hydrothermal reactor. The liner was then filled into the hydrothermal reactor. The reactor was placed in a forced-air drying oven at 200°C for 4 hours. After heating was complete and the reactor cooled naturally, the commercial carbon felt was removed, washed six times with deionized water, and dried in a vacuum drying oven at 50°C for 8 hours to obtain a commercially available carbon felt with a surface loaded with nickel phosphide.

[0064] S4. Place commercially available carbon cloth coated with nickel phosphide in a ceramic boat in a tubular furnace. Simultaneously, place 2g of sodium hypophosphite in another ceramic boat between the carbon felt and the gas inlet. After securing the collar, open the vacuum pump valve to lower the gas pressure inside the tube to below 5Pa. Increase the temperature to 200℃ at a rate of 10℃ / min and hold for 0.5h, continuously introducing argon gas at a flow rate of 10sccm during this period.

[0065] S5. After the heat preservation stage, the temperature of the tube furnace is increased to 300℃ at a heating rate of 10℃ / min and held for 1 hour. After natural cooling, the commercial carbon felt is removed, washed 6 times with deionized water, and dried in a vacuum drying oven at 50℃ for 8 hours to obtain a nickel phosphide modified electrode with a solid spherical structure.

[0066] The cyclic voltammetry curve of the nickel phosphide modified electrode prepared in Example 2 in vanadium oxysulfate, measured at room temperature, showed a potential difference of 435 mV, which was 86 mV lower than that of the original commercial carbon cloth. The redox peak currents were 189 mA and 177 mA, while the redox peak currents of the original commercial carbon cloth were 167 mA and 171 mA.

[0067] Example 3

[0068] A method for preparing a nickel phosphide modified electrode with a solid spherical structure includes the following steps:

[0069] S1. Prepare a 1M nitric acid solution, immerse the commercial graphite felt in the nitric acid solution for 1 hour, then wash it 5 times with deionized water. After that, place the carbon felt in a vacuum drying oven and dry it at 50°C for 8 hours. Then, place the commercial graphite felt in a muffle furnace and heat it at 200°C for 4 hours to obtain the pretreated graphite felt.

[0070] S2, using a three-electrode system, deposited metallic Ni nanoparticles on the surface of pretreated graphite felt. The working electrode was the pretreated graphite felt, the counter electrode was a graphite plate, and the reference electrode was Hg / Hg₂SO₄. The deposition time was 100 s, and the voltage was maintained at 0.2 V during the deposition process. The electrolyte consisted of 2.5 mM nickel chloride hexahydrate and 1 mM hydrochloric acid solution. After deposition, the surface was rinsed five times with deionized water and dried in a vacuum drying oven at 50 °C for 8 h. A commercially available graphite felt with a surface modified with metallic Ni was obtained.

[0071] S3, a hydrothermal reaction solution was prepared consisting of 150 mM sodium hypophosphite, 4 ml acetone, and 1.5 mM sodium hydroxide. 58 ml of this solution, along with a commercially available graphite felt uniformly loaded with Ni nanoparticles, was placed inside a hydrothermal reactor liner. The reactor was then placed in a forced-air drying oven. The hydrothermal reaction was carried out at 140°C for 12 hours. After the reaction was complete and the reactor cooled naturally, the commercial graphite felt was removed, washed six times with deionized water, and then dried in a vacuum drying oven at 50°C for 8 hours. This yielded a commercially available carbon felt with a surface loaded with nickel phosphide.

[0072] S4. Place commercial graphite felt in a ceramic boat in a tubular furnace. Place 3g of sodium hypophosphite in another ceramic boat between the graphite felt and the gas inlet. After securing the quartz tube collar, open the vacuum pump valve to ensure the gas pressure inside the tube is below 5Pa. Heat to 250℃ at a rate of 10℃ / min and hold for 2 hours, continuously introducing argon gas at a flow rate of 20sccm during this period.

[0073] S5, after the heat preservation stage, is heated to 400℃ at a heating rate of 10℃ / min and held at that temperature for 1 hour. After natural cooling, the commercial graphite felt is removed and washed 6 times with deionized water. It is then dried in a vacuum drying oven at 55℃ for 6 hours to obtain a nickel phosphide modified electrode with a solid spherical structure.

[0074] Cyclic voltammetry curves of the nickel phosphide modified electrode prepared in Example 3 in vanadium oxysulfate were measured at room temperature. The potential difference was 512 mV, which was 78 mV less than that of the original commercial graphite felt. The redox peak currents were 192 mA and 179 mA, while the redox peak currents of the original commercial graphite felt were 184 mA and 166 mA.

[0075] Example 4

[0076] A method for preparing a nickel phosphide modified electrode with a solid spherical structure includes the following steps:

[0077] Except that in S2, a saturated calomel electrode was used as the reference electrode, and the 0.5 mM hydrochloric acid in the electrolyte was replaced with a 0.5 mM potassium chloride solution. The deposition voltage was maintained at 0.1 V. Everything else was the same as in Example 1.

[0078] In this embodiment, the electrodeposition electrolyte was controlled to neutral conditions using KCl, and the reference electrode suitable for neutral conditions was replaced with a saturated calomel electrode. The cyclic voltammetry curve of the nickel phosphide modified electrode prepared in this embodiment in polysulfides was measured at room temperature as follows: Figure 8 As shown, the potential difference is 974 mV, which is 162 mV less than that of the commercially available original electrode. The redox peak currents are 192 mA and 145 mA, while the redox peak currents of the original commercial electrode are 140 mA and 118 mA. Simultaneously, the Nyquist plot of the nickel phosphide modified electrode prepared in the examples in polysulfides was measured at room temperature, as shown below. Figure 9 As shown, the charge transfer resistance of the nickel phosphide modified electrode is 2.23Ω, which is significantly lower than the original commercial electrode's 6.02Ω, thus accelerating the reaction process and reducing polarization.

[0079] To practically verify the application of nickel phosphide modified electrodes in flow batteries, two sulfur-based flow batteries were assembled using both the original commercial electrode and the nickel phosphide modified electrode as negative electrodes, respectively, and their long-cycle performance was compared. The results showed that the battery using the nickel phosphide modified electrode exhibited a higher initial energy efficiency (81%) and also showed higher energy efficiency during 1000 charge-discharge cycles compared to the original commercial electrode. Figure 10 As shown.

[0080] Example 5

[0081] A method for preparing a nickel phosphide modified electrode with a solid spherical structure includes the following steps:

[0082] Except for S3, sodium hydroxide is not added to the hydrothermal reaction solution.

[0083] Everything else is the same as in Example 1.

[0084] Cyclic voltammetry curves of the nickel phosphide modified electrode prepared in the example were tested at room temperature in vanadium oxysulfate. The potential difference was 432 mV, which was 51 mV less than that of the original commercial electrode, and the redox peak currents were 183 mA and 168 mA. Cyclic voltammetry curves in polysulfides under the same conditions showed a potential difference of 978 mV, which was 158 mV less than that of the original commercial electrode, and the redox peak currents were 163 mA and 124 mA.

[0085] In the hydrothermal reaction components, sodium hydroxide can adjust the pH of the reaction components and positively promote the reaction. In this embodiment, sodium hydroxide was not added during the hydrothermal reaction, which reduced the number of nickel phosphide nanoparticles synthesized on the surface of the original commercial electrode, thus affecting subsequent processing and impacting the electrode's performance. Compared to Example 1, CV test data shows that although the electrochemical performance of the sample prepared in this embodiment is improved compared to the original commercial electrode, it still lags behind the sample prepared in Example 1.

[0086] Example 6

[0087] A method for preparing a nickel phosphide modified electrode with a solid spherical structure includes the following steps:

[0088] Except for S3, where the sodium hypophosphite concentration was set to 500 mM.

[0089] Everything else is the same as in Example 3.

[0090] Cyclic voltammetry curves of the nickel phosphide modified electrode prepared in the example at room temperature in vanadium oxysulfate showed a potential difference of 425 mV, a decrease of 58 mV compared to the original commercial electrode, with redox peak currents of 187 mA and 173 mA. Cyclic voltammetry curves in polysulfides under the same conditions showed a potential difference of 993 mV, a decrease of 143 mV compared to the original commercial electrode, with redox peak currents of 159 mA and 131 mA. This example used the largest amount of sodium hypophosphite; further increasing the amount of phosphorus source in the reaction did not have a positive effect on performance improvement.

[0091] Example 7

[0092] A method for preparing a nickel phosphide modified electrode with a solid spherical structure includes the following steps:

[0093] Except for S4, where the amount of sodium hypophosphite used is 5g.

[0094] Everything else is the same as in Example 2.

[0095] Cyclic voltammetry curves of the nickel phosphide modified electrode prepared in the example were tested at room temperature in vanadium oxysulfate. The potential difference was 419 mV, which was 64 mV lower than that of the original commercial electrode. The redox peak currents were 188 mA and 169 mA. Cyclic voltammetry curves of the nickel phosphide modified electrode prepared in the example were tested under the same conditions in polysulfides. The potential difference was 995 mV, which was 141 mV lower than that of the original commercial electrode. The redox peak currents were 160 mA and 137 mA.

[0096] Example 8

[0097] A method for preparing a nickel phosphide modified electrode with a solid spherical structure includes the following steps:

[0098] Except for the argon flow rate of 60 sccm and the holding time of 1 hour in S4, everything else was the same as in Example 1.

[0099] Cyclic voltammetry curves of the nickel phosphide modified electrode prepared in the example were tested at room temperature in vanadium oxysulfate. The potential difference was 397 mV, which was 91 mV lower than that of the original commercial electrode. The redox peak currents were 184 mA and 177 mA. Cyclic voltammetry curves of the nickel phosphide modified electrode prepared in the example were tested under the same conditions in polysulfides. The potential difference was 980 mV, which was 156 mV lower than that of the original commercial electrode. The redox peak currents were 189 mA and 133 mA.

[0100] Example 9

[0101] A method for preparing a nickel phosphide modified electrode with a solid spherical structure includes the following steps:

[0102] Except for the heating time and temperature of the S3 hydrothermal reactor, which are controlled at 24 hours and 200°C, everything else is the same as in Example 2.

[0103] The hydrothermal reaction temperature and reaction time were set to their maximum values. Cyclic voltammetry curves of the nickel phosphide modified electrode prepared in the example were tested in vanadium oxysulfate at room temperature. The potential difference was 404 mV, a decrease of 79 mV compared to the original commercial electrode. The redox peak currents were 190 mA and 176 mA. Under the same conditions, the cyclic voltammetry curves of the nickel phosphide modified electrode prepared in the example were tested in polysulfides. The potential difference was 988 mV, a decrease of 148 mV compared to the original commercial electrode. The redox peak currents were 182 mA and 143 mA.

[0104] As a result, the high temperature and long reaction time altered the crystal structure of some nickel phosphide, leading to the formation of six-directional Ni. 12 P5.

[0105] Example 10

[0106] A method for preparing a nickel phosphide modified electrode with a solid spherical structure includes the following steps:

[0107] Except for replacing the gas introduced in S4 and S5 with nitrogen, everything else is the same as in Example 3.

[0108] Cyclic voltammetry curves of the nickel phosphide modified electrode prepared in the example were tested at room temperature in vanadium oxysulfate. The potential difference was 397 mV, which was 86 mV lower than that of the original commercial electrode. The redox peak currents were 182 mA and 169 mA. Cyclic voltammetry curves of the nickel phosphide modified electrode prepared in the example were tested under the same conditions in polysulfides. The potential difference was 992 mV, which was 144 mV lower than that of the original commercial electrode. The redox peak currents were 179 mA and 137 mA.

[0109] Example 11

[0110] A method for preparing a nickel phosphide modified electrode with a solid spherical structure includes the following steps:

[0111] S1. Prepare a 3M nitric acid solution, immerse the commercial graphite felt in the nitric acid solution for 1 hour, then wash it 5 times with deionized water. After that, place the carbon felt in a vacuum drying oven and dry it at 50°C for 8 hours. Then, place the commercial carbon felt in a muffle furnace and heat it at 250°C for 6 hours to obtain the pretreated graphite felt.

[0112] S2, using a three-electrode system, deposited metallic Ni nanoparticles on the surface of pretreated graphite felt. The working electrode was the pretreated graphite felt, the counter electrode was a graphite plate, and the reference electrode was Hg / Hg₂SO₄. The deposition time was 500 s, and the voltage was maintained at 0.1 V during the deposition process. The electrolyte consisted of 2 mM nickel chloride hexahydrate and 1 mM KCl solution. After deposition, the surface was rinsed five times with deionized water and dried in a vacuum drying oven at 50 °C for 8 h. A commercially available graphite felt with a surface modified with metallic Ni was obtained.

[0113] S3, a hydrothermal reaction solution was prepared using 500 mM sodium hypophosphite, 3 ml acetone, and 3 mM sodium hydroxide. 60 ml of this solution, along with a commercially available graphite felt uniformly loaded with Ni nanoparticles, was placed inside a hydrothermal reactor liner. The reactor was then placed in a forced-air drying oven. The hydrothermal reaction was carried out at 100°C for 24 hours. After the reaction was complete and the reactor cooled naturally, the commercial graphite felt was removed, washed six times with deionized water, and then dried in a vacuum drying oven at 50°C for 8 hours. This yielded a commercially available carbon felt with a surface loaded with nickel phosphide.

[0114] S4. Place commercial graphite felt in a ceramic boat in a tubular furnace. Place 5g of sodium hypophosphite in another ceramic boat between the graphite felt and the gas inlet. After securing the quartz tube collar, open the vacuum pump valve to ensure the gas pressure inside the tube is below 5Pa. Heat to 300℃ at a rate of 15℃ / min and hold for 2 hours, continuously introducing argon gas at a flow rate of 60sccm during this period.

[0115] S5, after the heat preservation stage, is heated to 500℃ at a heating rate of 15℃ / min and held at that temperature for 3 hours. After natural cooling, the commercial graphite felt is removed and washed 6 times with deionized water. It is then dried in a vacuum drying oven at 50℃ for 8 hours to obtain a nickel phosphide modified electrode with a solid spherical structure.

[0116] Cyclic voltammetry curves of the nickel phosphide modified electrode prepared in Example 11 in vanadium oxysulfate were measured at room temperature. The potential difference was 502 mV, which was 88 mV less than that of the original commercial graphite felt. The redox peak currents were 196 mA and 181 mA. Cyclic voltammetry curves of the nickel phosphide modified electrode prepared in the same example in polysulfides were measured under the same conditions. The potential difference was 982 mV, which was 154 mV less than that of the original commercial electrode. The redox peak currents were 179 mA and 137 mA.

[0117] A nickel phosphide nanoparticle-modified electrode with a solid spherical structure is disclosed, using a commercially available carbon-based electrode as the substrate. The carbon fiber surface of the carbon-based electrode is loaded with pure-phase nickel phosphide, which is uniformly distributed on the carbon fiber surface in a solid nanosphere structure. The commercially available electrode is made of carbon felt, graphite felt, carbon cloth, or carbon paper.

[0118] By increasing the specific surface area of ​​the material, nickel phosphide, with its external electronic structure similar to that of metals, readily gains and loses electrons. Therefore, this unique composite structure provides a large number of active sites for the reaction. Simultaneously, due to the metallic-like nature of nickel phosphide, this material itself possesses strong electrical conductivity. Using it as an electrocatalyst support on the surface of commercial electrodes can significantly improve the conductivity of the electrode and reduce the effects of polarization.

[0119] The nickel phosphide nanoparticle modified electrode with a solid sphere structure prepared in this invention can be applied in acidic flow batteries, alkaline flow batteries, and neutral flow batteries, and has universality.

[0120] Comparative Example 1

[0121] A method for preparing a nickel phosphide modified electrode with a solid spherical structure includes the following steps:

[0122] (1) Prepare a 1M sulfuric acid solution, immerse the commercial carbon felt in the solution for 0.5h, then wash it 5 times with deionized water, and then put the carbon felt into a vacuum drying oven and dry it at 50℃ for 8h. Then put the commercial carbon felt into a muffle furnace and heat it at 200℃ for 4h to obtain the pretreated carbon felt.

[0123] (2) Uniformly distributed metallic Ni nanoparticles were electrodeposited on the surface of a pretreated carbon felt using a three-electrode system. The pretreated carbon felt served as the working electrode, the graphite plate as the control electrode, and Hg / Hg₂SO₄ as the reference electrode. The electrolyte consisted of 2.5 M nickel chloride hexahydrate and 0.5 mM hydrochloric acid solution. The voltage was maintained at 0.4 V during deposition, and the deposition time was 250 s. After deposition, the carbon felt was rinsed five times with deionized water and dried in a vacuum drying oven at 50 °C for 8 h to obtain a carbon felt with uniformly distributed metallic Ni nanoparticles deposited on its surface.

[0124] (3) Prepare a hydrothermal reaction solution consisting of 100 mM sodium hypophosphite, 5 ml ethylene glycol, and 2 mM sodium hydroxide. Place the reaction solution and a commercially available carbon felt with uniformly loaded Ni nanoparticles on its surface together in the liner of a hydrothermal reactor. Then, fill the hydrothermal reactor with the liner. Place the hydrothermal reactor in a forced-air drying oven, setting the temperature and holding time to 180℃ and 12 h, respectively. After the heating is completed and the hydrothermal reactor cools naturally, remove the commercially available carbon felt, wash it five times with ethanol using deionized water, and dry it in a vacuum drying oven at 50℃ for 8 h to obtain a commercially available carbon felt with nickel phosphide loaded on its surface.

[0125] (4) The commercially available electrode with surface-loaded phosphating was placed in a tube furnace and heated to 500°C at a heating rate of 10°C / min in an argon atmosphere, and held at that temperature for 3 hours. After it cooled naturally, the electrode was removed, washed 6 times with deionized water, and dried in a vacuum drying oven for 8 hours to obtain the nickel phosphide modified electrode.

[0126] Step S5 was omitted, meaning the high-temperature adjustment of the crystal structure was omitted. This resulted in poor consistency in the crystal structure of nickel phosphide, and the relationship between Ni2P and Ni... 12 P5 is also present on the surface of commercial carbon-based materials; therefore, the performance improvement in electrochemical and battery performance tests is not significant compared to the original commercial carbon-based materials. The cyclic voltammetry curve of the nickel phosphide-modified electrode in vanadium oxysulfate prepared in the example, tested at room temperature, showed a potential difference of 465 mV, a decrease of 18 mV compared to the original commercial electrode. The redox peak currents were 183 mA and 162 mA. Under the same conditions, the cyclic voltammetry curve of the nickel phosphide-modified point prepared in the example in polysulfides showed a potential difference of 992 mV, a decrease of 144 mV compared to the original commercial electrode. The redox peak currents were 179 mA and 137 mA.

[0127] Comparative Example 2

[0128] (1) Prepare a 1M nitric acid solution, immerse the commercial carbon felt in the solution for 1 hour, then wash it 5 times with deionized water, and then place the carbon felt in a vacuum drying oven and dry it at 50°C for 8 hours. Then place the commercial carbon felt in a muffle furnace and heat it at 200°C for 4 hours to obtain the pretreated carbon felt.

[0129] (2) Prepare a hydrothermal reaction solution. The solution components are 200mM sodium hypophosphite, 5% acetone, 3mM sodium hydroxide, and 20mM nickel chloride hexahydrate. After stirring the solution in an ultrasonic instrument for 0.5h, put the reaction solution and commercial electrodes into the PTFE hydrothermal reactor liner. Fill the liner into the hydrothermal reactor and place it in a forced-air drying oven. Adjust the heating time and heating temperature to 24h and 200℃.

[0130] (3) After heating is complete and the hydrothermal reactor cools naturally, remove electrode 2, wash it 5 times with 95% ethanol, then wash it 5 times with deionized water, and place it in a vacuum drying oven at 80°C for 5 hours. After it cools naturally, the nickel phosphide modified electrode is obtained.

[0131] The steps of electrodeposition of metallic Ni, hydrothermal treatment, secondary phosphating, and high-temperature adjustment of crystal structure were omitted. The cyclic voltammetry curves of the nickel phosphide modified electrode prepared in the example were tested at room temperature in vanadium oxysulfate, with a potential difference of 472 mV, a decrease of 11 mV compared to the original commercial electrode. The redox peak currents were 179 mA and 165 mA. Under the same conditions, the cyclic voltammetry curves of the nickel phosphide modified electrode prepared in the example were tested in polysulfides, with a potential difference of 987 mV, a decrease of 149 mV compared to the original commercial electrode, and redox peak currents of 161 mA and 128 mA.

[0132] Comparative Example 3

[0133] (1) Prepare a 1M hydrochloric acid solution, immerse the commercial carbon felt in the solution for 2 hours, then wash it 5 times with deionized water, and then place the carbon felt in a vacuum drying oven and dry it at 50°C for 8 hours. Then place the commercial carbon felt in a muffle furnace and heat it at 200°C for 4 hours to obtain the pretreated carbon felt.

[0134] (2) Place the pretreated carbon felt in a tube furnace, place 5g of sodium hypophosphite between the commercial electrode and the gas inlet, fix the collar and open the vacuum valve to make the gas pressure in the tube lower than 5Pa. Raise the temperature to 300℃ at a rate of 10℃ / min and hold for 1.5h, during which argon gas is continuously introduced at a flow rate of 20sccm.

[0135] (3) After the heat preservation stage, the tube furnace is heated to 400℃ at a heating rate of 10℃ / min and held for 1 hour. After natural cooling, the commercial carbon felt is removed and washed 5 times with deionized water. It is then dried in a vacuum drying oven at 50℃ for 8 hours to obtain a nickel phosphide modified electrode with a solid spherical structure.

[0136] The steps of electrodeposition of metallic Ni, secondary phosphating, and high-temperature adjustment of crystal structure were omitted. Cyclic voltammetry curves of the nickel phosphide modified electrode prepared in the example were tested at room temperature in vanadium oxysulfate. The potential difference was 458 mV, 25 mV less than the original commercial electrode, and the redox peak currents were 181 mA and 174 mA. Cyclic voltammetry curves of the nickel phosphide modified electrode prepared in the example were tested under the same conditions in polysulfides. The potential difference was 985 mV, 151 mV less than the original commercial electrode, and the redox peak currents were 183 mA and 137 mA. A smaller potential difference for the electrode material is better.

[0137] Comparative Example 4

[0138] A method for preparing a nickel phosphide modified electrode with a solid spherical structure includes the following steps:

[0139] Except for S1, where the hydrothermal reactor heating time is 48 hours and the hydrothermal reactor heating temperature is 220℃, the concentrations of nickel chloride and sodium hypophosphite are 3mM and 600mM, respectively.

[0140] Everything else is the same as in Example 1.

[0141] Cyclic voltammetry curves of the nickel phosphide-modified electrode prepared in the example were tested at room temperature in vanadium oxysulfate. The potential difference was 464 mV, a decrease of 19 mV compared to the original commercial electrode. The redox peak currents were 178 mA and 160 mA. Cyclic voltammetry curves of the nickel phosphide-modified electrode prepared in the example were tested under the same conditions in polysulfides. The potential difference was 1084 mV, a decrease of 52 mV compared to the original commercial electrode. The redox peak currents were 156 mA and 136 mA. Figure 11 As shown, the severe agglomeration of nickel phosphide nanoparticles on the surface of the modified carbon felt carbon fibers prepared when the hydrothermal reaction temperature, reaction time, and reactant ratio are set outside the required range is the reason for the poor modification effect.

[0142] The temperature, reaction time, and reactant ratio of the hydrothermal reaction were set beyond the required range, resulting in negligible improvement in the performance of the prepared electrode. At the same time, the energy consumption and excessive reactants in the process also contradicted the requirement of low cost.

[0143] Comparative Example 5

[0144] Except for the S5 tube furnace, the temperature is increased to 600℃.

[0145] Everything else is the same as in Example 1.

[0146] Cyclic voltammetry curves of the nickel phosphide modified electrode prepared in the example were tested at room temperature in vanadium oxysulfate. The potential difference was 417 mV, which is 66 mV lower than that of the original commercial electrode. The redox peak currents were 180 mA and 176 mA. Cyclic voltammetry curves of the nickel phosphide modified electrode prepared in the example were tested under the same conditions in polysulfides. The potential difference was 987 mV, which is 149 mV lower than that of the original commercial electrode. The redox peak currents were 165 mA and 136 mA.

[0147] Setting the temperature for the crystal form adjustment step to 600 degrees Celsius exceeded the required temperature range, resulting in no significant improvement in the performance of the carbon felt compared to Example 1. SEM testing of the sample prepared in this comparative example revealed structural damage on the surface of the carbon fibers in the carbon felt. Figure 12 As shown, this is due to the collapse of the nickel phosphide nanolayer caused by temperature.

[0148] Comparative Example 6

[0149] Except for the S5 tube furnace, the temperature is increased to 200℃.

[0150] Everything else is the same as in Example 1.

[0151] Cyclic voltammetry curves of the nickel phosphide modified electrode prepared in the example were tested at room temperature in vanadium oxysulfate. The potential difference was 452 mV, which was 31 mV lower than that of the original commercial electrode. The redox peak currents were 185 mA and 170 mA. Cyclic voltammetry curves of the nickel phosphide modified electrode prepared in the example were tested under the same conditions in polysulfides. The potential difference was 990 mV, which was 146 mV lower than that of the original commercial electrode. The redox peak currents were 177 mA and 138 mA.

[0152] Setting the temperature for the crystal form adjustment step to 600 degrees Celsius exceeded the required temperature range, resulting in no significant improvement in the performance of the carbon felt compared to Example 1. XRD analysis of the sample prepared in this comparative example revealed that its crystal structure was hexagonal nickel phosphide (Ni5P). 12 )like Figure 13 As shown, excessively low secondary phosphating temperatures cannot achieve uniformity in crystal structure and are also a major cause of poor performance in modified electrodes.

[0153] Comparative Example 7

[0154] Except for S4, where the amount of sodium hypophosphite used in the ceramic boat is 1g.

[0155] Everything else is the same as in Example 1.

[0156] Cyclic voltammetry curves of the nickel phosphide modified electrode prepared in the example were tested at room temperature in vanadium oxysulfate. The potential difference was 414 mV, which was 69 mV lower than that of the original commercial electrode. The redox peak currents were 183 mA and 177 mA. Cyclic voltammetry curves of the nickel phosphide modified electrode prepared in the example were tested under the same conditions in polysulfides. The potential difference was 989 mV, which was 147 mV lower than that of the original commercial electrode. The redox peak currents were 169 mA and 131 mA.

[0157] To further verify the reason for the lack of significant improvement in electrode performance, the sample prepared in this comparative example was used as the positive electrode in a vanadium redox flow battery, and compared with the two batteries in Example 1. The results showed that the energy efficiency of the battery in this comparative example exhibited a trend of increasing from low to high and then continuously decreasing. Figure 14 As shown. The reason is that the relatively small amount of sodium hypophosphite did not achieve the effect of secondary phosphating, and some of the metallic nickel loaded on the surface of the commercial electrode was consumed during battery cycling.

[0158] Comparative Example 8

[0159] Except for S4, where the amount of sodium hypophosphite used in the ceramic boat is 6g.

[0160] Everything else is the same as in Example 1.

[0161] Cyclic voltammetry curves of the nickel phosphide modified electrode prepared in the example were tested at room temperature in vanadium oxysulfate. The potential difference was 409 mV, which was 74 mV lower than that of the original commercial electrode. The redox peak currents were 190 mA and 172 mA. Cyclic voltammetry curves of the nickel phosphide modified electrode prepared in the example were tested under the same conditions in polysulfides. The potential difference was 1033 mV, which was 103 mV lower than that of the original commercial electrode. The redox peak currents were 187 mA and 131 mA.

[0162] To further verify the reason for the lack of significant improvement in electrode performance, the sample prepared in this comparative example was used as the positive electrode in a vanadium redox flow battery, and compared with the two batteries in Example 1. The results showed that the energy efficiency of the battery in this comparative example exhibited a continuous decreasing trend, and the initial energy efficiency was low, such as... Figure 15 As shown. The reason is that excessive sodium hypophosphite coats the surface of the electrode during the secondary phosphating process, resulting in a decrease in the overall conductivity of the material.

[0163] In this embodiment of the invention, the surfactant in the hydrothermal reaction solution is ethylene glycol, glycerol, ethylhexyl alcohol, or stearyl alcohol, and the amount used is 3-5 ml.

[0164] In Example S3 of this invention, sodium hypophosphite is used as the phosphorus source for phosphating, which is safer than using red phosphorus or white phosphorus. PH3 plays a major role in the phosphating process among the decomposition products of sodium hypophosphite after heating. The addition of organic solvents makes PH3 more easily soluble and dispersed in the reaction solution, and sodium hydroxide adjusts the pH to a weakly alkaline state, promoting the reaction. Sodium hydroxide, as a pH adjuster, can be replaced by other reagents, such as KOH, without introducing other impurities.

[0165] In embodiment S4 of this invention, sodium hypophosphite is placed between the commercial electrode and the air inlet. During the heat preservation stage, the PH3 generated by the thermal decomposition of sodium hypophosphite is evenly swept onto the surface of the commercial electrode along with the protective gas. Under high temperature conditions, this induces the residual Ni on the electrode surface to react. If the amount of sodium hypophosphite is too large, more PH3 gas will be generated at high temperature. Over-phosphating will result in the formation of hexagonal nickel phosphide (Ni). 12 P5) This, in turn, affects the crystallinity of Ni2P. Argon or nitrogen is introduced as a protective gas to ensure that no gaseous contamination occurs during the reaction.

[0166] Under normal conditions, nickel phosphide comprises various chemical compositions, primarily tetragonal Ni₂P and hexagonal Ni₂P. 12P5. Differences in crystal structure will lead to differences in performance; Ni2P often has better thermal stability than Ni. 12 P5. In this embodiment of the invention, tetragonal (Ni2P) nickel phosphide is the target product. S5 will adjust the product's crystal structure by setting parameters after the secondary phosphating to ensure consistency. Heating the tube furnace to 400-500°C can induce Ni... 12 P5 decomposes, resulting in the supported nickel phosphide existing only in the tetragonal Ni2P crystal structure. Ni2P, compared to Ni... 12 P5 can remain stable under high temperature conditions, but high temperature can also change the microstructure of carbon-based materials. Therefore, the holding temperature and time parameters need to be explored and investigated through a large number of experiments to summarize the rules and control the reaction.

[0167] Existing methods for preparing nickel phosphide include one-step hydrothermal synthesis and pyrolysis of sodium hypophosphite. However, stabilizing its loading onto a substrate and maintaining structural stability during long-term operation is challenging. This invention first electrodeposits metallic Ni nanoparticles onto a carbon-based material surface. This not only prepares a reaction precursor but also ensures that the electrodeposited metallic Ni is stably anchored to the carbon-based material surface with sufficient bonding strength. This prevents desorption at specific sites during subsequent processing, resulting in a nickel phosphide-modified electrode with structural stability during operation. Furthermore, this invention loads pure-phase nickel phosphide (tetragonal Ni₂P) onto the surface of carbon fibers, exhibiting higher stability compared to nickel-phosphide alloy-nickel phosphide composite structures.

[0168] Hydrothermal reactions are often difficult to control, and the crystallization type of the product is random. Secondary phosphating can completely react the nickel that did not react during the hydrothermal reaction, which is an important step to improve product conversion rate, utilization rate, and crystallinity. In this embodiment of the invention, the two phosphating processes are a first hydrothermal phosphating process with sodium hypophosphite and a second pyrolytic phosphating process with sodium hypophosphite. Although both are phosphating processes, the order of the two processes is strictly required. If the pyrolytic phosphating process is performed first, the metallic Ni nanoparticles loaded on the surface of the carbon-based material may oxidize to NiO in the high-temperature environment of the tube furnace, affecting the reaction. Most nickel phosphide preparation methods involve mixing nickel and phosphorus sources and preparing them through a reaction in an aqueous system or a tube furnace system, often a direct preparation. The difficulty of this invention lies in preparing nickel phosphide and stably loading it onto a carbon-based material matrix. This makes direct preparation of nickel phosphide difficult to apply in this invention. This embodiment of the invention uses a commercial carbon-based material as the matrix and uses a three-electrode system to load metallic nickel nanoparticles onto the surface of the carbon-based material; [Ni 2+ +2e -=Ni(-0.257vs.SHE)], anchoring metallic nickel to the surface of the carbon-based material while preparing for the next hydrothermal reaction. In this next step, sodium hypophosphite decomposes into PH3 at high temperature, and PH3 plays a major phosphating role (4NaH2PO2=2PH3↑+H2O+Na4P2O7, 8Ni+4PH3=4Ni2P+6H2↑). The subsequent secondary phosphating reaction follows the same mechanism as the previous step; in this step, unreacted metallic Ni is completely reacted.

[0169] After the hydrothermal reaction, Ni2P and Ni will inevitably exist on the electrode surface. 12 P5. In this embodiment of the invention, Ni is induced by high temperature in S5. 12 After P5 decomposition, only Ni2P (homogeneous tetragonal nickel phosphide) remains on the surface, resulting in a nickel phosphide nanoparticle-modified carbon-based electrode with a solid spherical structure. This electrode exhibits advantages such as large specific surface area, good catalytic performance, good stability, and high conductivity, effectively overcoming the problems of poor catalytic activity, low conductivity, and poor stability of traditional commercial carbon-based electrodes. The electrocatalytic activity and conductivity exhibited in vanadium oxysulfate and polysulfides in this invention are far superior to the original carbon-based electrode. SEM testing confirmed that the nickel phosphide loaded on the carbon fiber surface has a solid microspherical structure. XRD testing confirmed that the loaded nickel phosphide has good crystallinity. The strict sequence of each step in this invention plays a crucial role in improving the performance of the nickel phosphide-modified electrode, providing a novel approach and method for modifying redox flow battery electrodes. The preparation method in this invention is simple and feasible, environmentally friendly, and uses low-cost raw materials, resulting in a low-cost, high-performance commercial electrode for redox flow batteries.

[0170] The above description is merely a preferred embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention are included within the scope of protection of the present invention.

Claims

1. A method for preparing a nickel phosphide nanoparticle modified electrode with a solid spherical structure, characterized in that, Includes the following steps: S1, a commercial carbon-based electrode pretreated with an acid solution; S2, uniformly deposits metallic Ni nanoparticles on the surface of commercial carbon-based materials via electrochemical means, with an electrolyte composition of 1.5~2.5mM nickel chloride hexahydrate and 0.5~1mM HCl / KCl; S3, prepare the hydrothermal reaction solution: 50~500mM sodium hypophosphite, 3~5ml surfactant and 1~3mM sodium hydroxide; A commercially available carbon-based electrode with nickel phosphide loading was obtained by hydrothermal reaction of the hydrothermal reaction solution with a commercially available electrode uniformly loaded with metallic Ni nanoparticles at 100-200 °C. S4. Place the commercially available carbon-based electrode supported on nickel phosphide obtained in S3 and 2-5g of sodium hypophosphite in a tube furnace. After evacuation, heat the furnace to 200-300℃ at a heating rate of 10-15℃ / min under an argon or nitrogen atmosphere, and hold for 0.5-2h to perform secondary phosphating. S5, after the heat preservation stage, heat the tube furnace to 300-500℃ at a heating rate of 10-15℃ / min, keep it at that temperature for 1-3 hours, and then let it cool naturally before cleaning and drying to obtain the product.

2. The method for preparing a nickel phosphide nanoparticle modified electrode with a solid spherical structure according to claim 1, characterized in that, S1 includes the following steps: placing a commercial carbon-based electrode in an acidic solution with a concentration of 0.5~3 mol / L, immersing it for 10~60 min, cleaning and drying it; placing it in a muffle furnace and holding it at 200~250 ℃ for 4~6 h for heat treatment to obtain a pretreated commercial electrode.

3. The method for preparing a nickel phosphide nanoparticle modified electrode with a solid spherical structure according to claim 2, characterized in that, The acidic solution is any one of sulfuric acid, nitric acid, or hydrochloric acid.

4. The method for preparing a nickel phosphide nanoparticle modified electrode with a solid spherical structure according to claim 1, characterized in that, The S2 includes the following steps: using pretreated commercial carbon-based material as the working electrode, a graphite plate as the control electrode, and Hg / Hg2SO4 or a saturated calomel electrode as the reference electrode; during the deposition process, the voltage is 0.1~0.6 V, the deposition time is controlled at 100s~500s, the electrolyte is continuously stirred to maintain the uniform distribution and diffusion of ions, and after deposition, the electrode is rinsed 5~6 times with deionized water and dried to obtain a commercial electrode with uniformly distributed metallic Ni nanoparticles on the carbon fiber surface.

5. The method for preparing a nickel phosphide nanoparticle modified electrode with a solid spherical structure according to claim 1, characterized in that, The S3 includes the following steps: after the hydrothermal reaction, the reaction vessel is allowed to cool naturally, the commercial electrode is taken out, washed 5-6 times with organic solution and deionized water in sequence, and then placed in a vacuum drying oven to dry the moisture, thereby obtaining a commercial electrode with nickel phosphide loading.

6. The method for preparing a nickel phosphide nanoparticle modified electrode with a solid spherical structure according to claim 1, characterized in that, In step S4, the flow rate of argon or nitrogen gas is 10~60 sccm.

7. The method for preparing a nickel phosphide nanoparticle modified electrode with a solid spherical structure according to claim 1, characterized in that, In step S5, after natural cooling, the electrode is washed 5-6 times with deionized water, placed in a vacuum drying oven and dried at 50-55 ℃ for 6-8 h, and after natural cooling, a nickel phosphide modified electrode with a solid spherical structure is obtained.

8. The method for preparing a nickel phosphide nanoparticle modified electrode with a solid spherical structure according to claim 1, characterized in that, In S1, the commercial electrode is carbon felt, graphite felt, carbon cloth, or carbon paper.

9. The application of the electrode obtained by the preparation method of the nickel phosphide nanoparticle modified electrode with a solid sphere structure as described in claim 1 in acidic flow batteries, alkaline flow batteries and neutral flow batteries.

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