Preparation method of graphite felt modified electrode for all-vanadium redox flow battery

Abstract

The present invention discloses a preparation method of a modified electrode of graphite felt for all-vanadium redox flow battery, which comprises the following steps: (1) performing pretreatment on the graphite felt to obtain pretreated graphite felt; (2) placing the pretreated graphite felt into a polytetrafluoroethylene inner liner of a hydrothermal autoclave, wherein the polytetrafluoroethylene inner liner is filled with a mixed solution of cobalt nitrate, nickel nitrate and urea; (3) placing the hydrothermal autoclave in an oven for reaction; (4) taking out the graphite felt in the hydrothermal autoclave, washing it and then placing it in an electrolytic cell for cathodic reduction electrodeposition to form a cobalt-nickel alloy on the surface of the graphite felt; (5) subjecting the electrodeposited graphite felt to high-temperature treatment in a muffle furnace and a tubular furnace in sequence to obtain a modified electrode of graphite felt. The present invention prepares a graphite felt loaded with NiCoO2 with a petal morphology and a high specific surface area, increases the specific surface area and the electrochemical specific surface area of the surface of the graphite felt, and significantly improves the reversibility of the positive oxidation-reduction reaction of the vanadium battery for the modified graphite felt.

Description

Technical Field:

[0001] The present invention belongs to the field of manufacturing all-vanadium redox flow batteries, and mainly relates to a preparation method of a modified graphite felt electrode. Background Art:

[0002] An all-vanadium redox flow battery is an electrochemical energy storage device suitable for large-scale energy storage. It has advantages such as safe use, flexible design, long lifespan, and easy maintenance. Its main components include an ion exchange membrane, electrolyte, and electrode material. Among them, the electrode material affects the voltage efficiency of the overall battery pack due to factors such as electrochemical polarization and ohmic polarization. Currently, due to its good chemical and electrochemical stability and broad potential window, graphite felt material is one of the most commonly used electrodes in all-vanadium redox flow batteries. However, it also has many problems. For example, its catalytic activity for the redox reaction of the positive electrode VO 2+ / VO2 + electrode pair is far less than that of the V 2+ / V 3+ electrode pair, and its overall electrochemical activity is also relatively low.

[0003] Currently, the main treatment methods of graphite felt can be divided into two categories: intrinsic treatment and modification by introducing catalysts. Common intrinsic treatment methods include heat treatment, microwave method, chemical oxidation method, electrochemical oxidation treatment, etc. These intrinsic treatment methods can effectively increase the specific surface area of graphite felt and the oxygen-containing functional groups on its surface, increase the mass transfer efficiency of surface vanadium ions and the number of redox active sites, and thus improve the overall efficiency of the battery. However, limited by the graphite felt material substrate, its inherent electrocatalytic effect is limited. Therefore, introducing a catalyst on the surface of graphite felt is considered an effective method to improve its electrocatalytic activity. This type of method mainly utilizes the electrocatalytic effect of the catalyst on vanadium ions to improve the battery efficiency. So far, the reported methods mainly include introducing metals, metal oxides, electroactive organic compounds, carbon-based nanomaterials, etc.

[0004] The excellent conductivity and electrocatalytic activity of nickel cobalt oxide (NiCoO2) provide the possibility for its application in the electrochemical field. In addition, studies have shown that different morphologies of NiCoO2 can be obtained by controlling the preparation technology, and it is found that it has an important impact on the electrocatalytic activity of NiCoO2. In view of this, the present invention uses a composite preparation technology to develop a NiCoO2 / graphite felt composite material with a unique morphology and applies it as the positive electrode material of an all-vanadium redox flow battery, significantly improving the comprehensive performance of the battery. Summary of the Invention:

[0005] The technical problem to be solved by the present invention is to provide a preparation method of a modified graphite felt electrode for an all-vanadium redox flow battery with strong electrochemical activity and good catalytic activity for the positive electrode reaction of the vanadium battery.

[0006] The following is a specific description of the technical solution adopted by the present invention to solve the above technical problems.

[0007] The present invention provides a preparation method for a modified graphite felt electrode for a vanadium redox flow battery, which comprises the following steps:

[0008] (1) Place the graphite felt that has been washed and dried with acetone, ethanol, and water respectively in a muffle furnace, heat it from room temperature to 400 - 550 °C, and hold for 1 - 3 h to improve its hydrophilicity, obtaining a pretreated graphite felt;

[0009] (2) Place the pretreated graphite felt into the polytetrafluoroethylene inner liner of a hydrothermal autoclave. The polytetrafluoroethylene inner liner contains a hydrothermal treatment solution, and the hydrothermal treatment solution is a mixed solution of cobalt nitrate, nickel nitrate, and urea. The solvent is a mixed solvent of ethanol and water with a volume ratio of 1 - 5:1. The concentrations of nickel nitrate and urea are 0.01 - 0.1 M and 0.01 - 1 M respectively, and the molar ratio of cobalt nitrate to nickel nitrate is 1 - 4:1;

[0010] (3) Place the hydrothermal autoclave in an oven, heat it to 60 - 120 °C, and keep it warm for 8 - 16 hours;

[0011] (4) Take out the graphite felt in the hydrothermal autoclave, wash it, and then place it in an electrolytic cell for cathodic reduction electrodeposition to form a cobalt - nickel alloy on the surface of the graphite felt. The electrolyte is an aqueous solution of cobalt sulfate, 0.01 - 0.1 M nickel sulfate, 0.05 - 0.2 M sodium chloride, 0.5 - 2 M boric acid, 0.5 - 2 M sodium formate, and 0.05 - 0.2 M sodium sulfate, and the molar ratio of cobalt sulfate to nickel sulfate is 1 - 4:1;

[0012] (5) Place the electrodeposited graphite felt in a muffle furnace at 300 - 500 °C for 1 - 4 h and in a tubular furnace at 300 - 600 °C for 1 - 4 h to obtain a modified graphite felt electrode.

[0013] Preferably, in step (1), heat it from room temperature to 500 °C and hold for 2 h.

[0014] Preferably, in step (2), the volume of the hydrothermal treatment solution accounts for 4 / 9 - 6 / 9 of the volume of the polytetrafluoroethylene inner liner, and more preferably 5 / 9.

[0015] Preferably, in step (2), the solvent is a mixed solvent of ethanol and water with a volume ratio of 4:1.

[0016] Preferably, in step (2), the molar ratio of cobalt nitrate to nickel nitrate is 2:1.

[0017] As a further preference, in step (2), the hydrothermal treatment solution is a mixed solution of 0.02 M cobalt nitrate, 0.01 M nickel nitrate, and 0.1 M urea, and the solvent is a mixed solvent of ethanol and water with a volume ratio of 4:1.

[0018] Preferably, step (3) is to place the hydrothermal reactor in an oven at 80 °C and keep it warm for 14 h.

[0019] Preferably, in step (4), the molar ratio of cobalt nitrate to nickel nitrate is 2:1.

[0020] Preferably, in step (4), the electrolyte is an aqueous solution mixture of cobalt sulfate, 0.01 - 0.02 M nickel sulfate, 0.05 - 0.1 M sodium chloride, 0.5 - 1 M boric acid, 1 - 2 M sodium formate, and 0.1 - 0.2 M sodium sulfate. The most preferred electrolyte is an aqueous solution mixture of 0.02 M cobalt sulfate, 0.01 M nickel sulfate, 0.1 M sodium chloride, 1 M boric acid, 1 M sodium formate, and 0.1 M sodium sulfate.

[0021] Preferably, in step (4), the electrodeposition is variable potential deposition. It uses a single - slot electrolytic cell. The working electrode is a graphite felt electrode clamped by a clip electrode, the counter electrode is a platinum electrode, and the saturated calomel electrode is used as the reference electrode. The reference electrode is immersed in another beaker filled with saturated potassium chloride solution and connected to the electrolytic cell by a salt bridge; the electrode distance between the working electrode and the counter electrode is 1 cm. The electrolysis is carried out in a water bath at 15 - 35 °C (more preferably 25 °C). The parameters of variable potential deposition are - 0.7 to - 0.8 V for 10 - 20 s, - 0.9 to - 1.0 V for 10 - 20 s, more preferably - 0.8 V for 15 s, - 1.0 V for 15 s, and cycled 10 - 60 times, and most preferably cycled 40 times.

[0022] Preferably, step (5) is specifically: wash the graphite felt after electrodepositing cobalt and nickel with deionized water and dry it at 60 - 100 °C for 4 - 8 h, heat the muffle furnace to 250 - 350 °C and keep it warm for 1 - 3 h (more preferably dry it at 80 °C for 6 h, heat the muffle furnace to 300 °C and keep it warm for 2 h), then place it in a tube furnace and keep it at 300 - 500 °C for 3 h in a nitrogen atmosphere. More preferably, it is kept at 300 °C for 1 h, 400 °C for 1 h, and 500 °C for 1 h. The nitrogen flow rate of the tube furnace is 40 - 80 ml / min, and the heating rate is 3 - 10 °C / min.

[0023] Principle of the present invention: On the basis of a cobalt-nickel modified graphite felt substrate treated by hydrothermal treatment, a cobalt-nickel alloy is formed on the surface of the graphite felt by variable potential deposition, and then oxidized to NiCoO2 crystals through heat treatment in a tube furnace and a muffle furnace, and the crystal-graphite felt bonding force is improved by high-temperature treatment in the muffle furnace and the tube furnace. A NiCoO2 graphite felt electrode with a petal-like morphology structure is prepared by combining two loading methods, which has an excellent specific surface area and can effectively communicate and connect each NiCoO2 crystal component. This structure effectively utilizes the advantages of the NaCl-type unit cell structure of NiCoO2 crystals, increases the reaction mass transfer efficiency of vanadium ions in the positive electrode solution of the vanadium battery, and thus improves the overall performance of the battery.

[0024] The innovation points of the present invention are as follows: (1) NiCoO2 is loaded on the graphite felt by combining the hydrothermal method and the electrodeposition method; (2) A NiCoO2-loaded graphite felt with a petal-like morphology and a high specific surface area is prepared, which increases the specific surface area and the electrochemical specific surface area of the graphite felt surface; (3) The reversibility of the redox reaction of the positive electrode of the vanadium battery is significantly improved after the modification of the graphite felt in the present invention.

[0025] The positive effects of the present invention are as follows: (1) The preparation method of this method is simple, and the raw materials used are cheap and easily available; (2) The modified graphite felt is applied to the positive electrode of the all-vanadium redox flow battery, which significantly increases the reversibility and electrochemical activity of the positive electrode redox reaction; When the graphite felt prepared by the present invention is used as the positive electrode material of the vanadium battery, it shows good performance, and the Coulomb efficiency, energy efficiency and voltage efficiency are all effectively improved; (3) The graphite felt prepared by this method has good chemical and electrochemical stability in the vanadium battery system and can be used for a long time. Description of the drawings:

[0026] Figure 1 It is the XRD pattern of the pre-treated graphite felt (Comparative Example 4) and the graphite felt of the present invention (Example 2);

[0027] Figure 2 It is the SEM comparison diagram of the pre-treated graphite felt (Comparative Example 4) and the graphite felt of the present invention (Example 2);

[0028] Figure 3 It is the nitrogen adsorption and desorption curve diagram of the pre-treated graphite felt (Comparative Example 4) and the graphite felt of the present invention (Example 2);

[0029] Figure 4 It is the cyclic voltammetry comparison diagram (scan rate 20 mV / s) of the pre-treated graphite felt (Comparative Example 4) and the graphite felt of the present invention (Example 2);

[0030] Figure 5 It is the comparison diagram of the test efficiency of the battery assembled with the pre-treated graphite felt (Comparative Example 4) and the graphite felt of the present invention (Example 2) as the positive electrode. Detailed implementation manners:

[0031] The technical solution of the present invention will be further described below with specific embodiments, but the protection scope of the present invention is not limited thereto:

[0032] Example 1:

[0033] (1) Place a 3×3 cm graphite felt that has been cleaned and dried with acetone, ethanol, and water in a muffle furnace and heat it from room temperature to 500 °C and hold for 2 h to improve its hydrophilicity (this is the pre-treated graphite felt); (2) Place the heat-treated graphite felt in a 90 ml hydrothermal autoclave, and add � ml of 0.02 M cobalt nitrate, 0.01 M nickel nitrate, 0.1 M urea, and a 4:1 ethanol aqueous solution to the polytetrafluoroethylene inner liner; (3) Place the hydrothermal autoclave in an 80 °C oven and keep it warm for 14 h; (4) Place the above-mentioned graphite felt in a single-chamber electrolytic cell containing a mixed solution of 0.02 M cobalt sulfate, 0.01 M nickel sulfate, 0.1 M sodium chloride, 1 M boric acid, 1 M sodium formate, and 0.1 M sodium sulfate. The working electrode is a 3×3 cm graphite felt electrode clamped by a clip electrode, the counter electrode is a 3×3 cm platinum electrode, and the saturated calomel electrode is used as the reference electrode. The reference electrode is immersed in another beaker containing saturated potassium chloride solution and is connected to the electrolytic cell with a salt bridge. The electrode spacing between the working electrode and the counter electrode is about 1 cm. The electrolysis is carried out in a 25 °C water bath. Use an ivium electrochemical workstation for potentiostatic deposition (-0.8 V for 15 s, -1.0 V for 15 s, cycle 40 times); (5) Wash the graphite felt after electrodepositing cobalt and nickel with deionized water and dry it at 80 °C for 6 h. Heat the muffle furnace to 300 °C and hold for two hours, then place it in a tube furnace. Under a nitrogen atmosphere, heat it at a rate of 5 °C / min, hold at 300 °C for 1 h, 400 °C for 1 h, and 500 °C for 1 h to improve the binding force of nickel cobaltate-loaded graphite felt.

[0034] The prepared sample is used for electrochemical testing and the performance testing of a vanadium redox flow battery.

[0035] Cyclic voltammetry test: Cut the graphite felt material prepared in this example into 1×1 cm using a die cutter, clamp one end with a 0.5×0.5 cm platinum sheet electrode and immerse it in a three-electrode electrolytic cell containing a solution of 0.1 M VOSO4 and 2 M H2SO4. Use a 2×2 cm platinum sheet electrode as the counter electrode and a saturated calomel electrode as the reference electrode, and connect them in the middle with a salt bridge. Set the cyclic voltammetry parameters: the starting voltage is 0.8 V, the potential range is -0.1 V to 1.6 V, and the scan rate is 20 mV·s -1 。

[0036] Battery performance test: Use a self-designed plate-frame battery (the apparent area of the electrode is 3×3 cm 2) The battery performance of various electrodes was tested. The main components of the battery from positive to negative are modified graphite felt, 1.5M VOSO4 + 3M H2SO4 electrolyte, nafion117 ion exchange membrane, 0.75M V2(SO4)3 + 3M H2SO4, and pretreated graphite felt. The VRFB test process is divided into four stages: charging stage, charging stage, static state, discharging, and static state. The test was carried out on a LANHE battery test system. The test temperature was 25 °C, the charging cut-off voltage was 1.65V, the discharging cut-off voltage was 0.8V, and the apparent current density during charge and discharge was 100 mA·h -1 , and the electrolyte flow rate was set to 20 mL·min -1 . The test was carried out on the cyclic battery for 80 cycles, and after treatment, the data showed that the battery voltage efficiency was 83.1%.

[0037] Example 2;

[0038] (1) The 3×3 cm graphite felt, which had been cleaned and dried with acetone, ethanol, and water respectively, was placed in a muffle furnace and heated from room temperature to 500 °C and maintained for 2 h to improve its hydrophilicity; (2) The heat-treated graphite felt was placed into a 90 ml hydrothermal reactor, and 50 ml of 0.02M cobalt nitrate, 0.01M nickel nitrate, 0.1M urea, and a 4:1 ethanol aqueous solution were added to the polytetrafluoroethylene inner liner; (3) The hydrothermal reactor was placed in an oven at 80 °C and kept warm for 14 h; (4) The above-mentioned graphite felt was placed in a single-tank electrolytic cell containing a mixed solution of 0.02M cobalt sulfate, 0.01M nickel sulfate, 0.1M sodium chloride, 1M boric acid, 1M sodium formate, and 0.1M sodium sulfate. The working electrode was a 3×3 cm graphite felt electrode clamped by a clip electrode, the counter electrode was a 3×3 cm platinum electrode, and the saturated calomel electrode was used as the reference electrode. The reference electrode was immersed in another beaker containing saturated potassium chloride solution and connected to the electrolytic cell with a salt bridge. The electrode spacing between the working electrode and the counter electrode was about 1 cm, and the electrolysis was carried out in a water bath at 25 °C. The ivium electrochemical workstation was used for potentiostatic deposition (-0.8V for 15 s, -1.0V for 15 s, cycled 40 times) (5) The graphite felt after cobalt-nickel electrodeposition was washed with deionized water and dried at 80 °C for 6 h. The muffle furnace was heated to 300 °C and maintained for two hours, and then placed in a tubular furnace. Under a nitrogen atmosphere, it was heated at a rate of 5 °C / min to 300 °C and maintained for 1 h, 400 °C and maintained for 1 h, and 500 °C and maintained for 1 h to improve the binding force of nickel cobaltate loaded on the graphite felt. Referring to Example 1, the cyclic charge-discharge test of the battery was carried out. The test temperature was 25 °C, the charging cut-off voltage was 1.65V, the discharging cut-off voltage was 0.8V, and the apparent current density during charge and discharge was 100 mA·h -1 , and the electrolyte flow rate was set to 20 mL·min -1 . The test was carried out on the cyclic battery for 80 cycles, and after treatment, the data showed that the battery voltage efficiency was 87.3%.

[0039] Example 3;

[0040] (1) Place a 3×3 cm graphite felt, which has been cleaned and dried with acetone, ethanol, and water respectively, in a muffle furnace and heat it from room temperature to 500 °C and hold for 2 h to improve its hydrophilicity; (2) Place the heat-treated graphite felt in a 90 ml hydrothermal autoclave. Add 60 ml of 0.02 M cobalt nitrate, 0.01 M nickel nitrate, 0.1 M urea, and a 4:1 ethanol-water solution into the polytetrafluoroethylene inner liner; (3) Place the hydrothermal autoclave in an 80 °C oven and keep it warm for 14 h; (4) Place the above-mentioned graphite felt in a single-tank electrolytic cell containing a mixed solution of 0.02 M cobalt sulfate, 0.01 M nickel sulfate, 0.1 M sodium chloride, 1 M boric acid, 1 M sodium formate, and 0.1 M sodium sulfate. The working electrode is a 3×3 cm graphite felt electrode clamped by a clip electrode, the counter electrode is a 3×3 cm platinum electrode, and the saturated calomel electrode is used as the reference electrode. The reference electrode is immersed in another beaker containing a saturated potassium chloride solution and is connected to the electrolytic cell with a salt bridge. The electrode spacing between the working electrode and the counter electrode is about 1 cm. The electrolysis is carried out in a 25 °C water bath. Use an ivium electrochemical workstation for potentiostatic deposition (-0.8 V for 15 s, -1.0 V for 15 s, cycle 40 times); (5) Wash the graphite felt after cobalt-nickel electrodeposition with deionized water and dry it at 80 °C for 6 h. Heat the muffle furnace to 300 °C and hold for two hours, then place it in a tube furnace. Under a nitrogen atmosphere, heat it at a rate of 5 °C / min, hold at 300 °C for 1 h, 400 °C for 1 h, and 500 °C for 1 h to improve the binding force of nickel cobaltate-loaded graphite felt. Refer to Example 1 for the battery cycle charge-discharge test. The test temperature is 25 °C, the charging cut-off voltage is 1.65 V, the discharging cut-off voltage is 0.8 V, and the apparent current density of charge-discharge is 100 mA·h -1 , and the electrolyte flow rate is set to 20 mL·min -1 . Test the cycle battery for 80 cycles, and the data obtained after treatment shows that the battery voltage efficiency is 86.9%.

[0041] Example 4;

[0042] (1) Place a 3×3 cm graphite felt, which has been washed and dried with acetone, ethanol, and water respectively, in a muffle furnace and heat it from room temperature to 500 °C and hold for 2 h to improve its hydrophilicity. (2) Place the heat-treated graphite felt in a 90 ml hydrothermal reactor. Add 50 ml of 0.01 M cobalt nitrate, 0.01 M nickel nitrate, 0.1 M urea, and a 4:1 ethanol-water solution into the polytetrafluoroethylene inner liner. (3) Place the hydrothermal reactor in an 80 °C oven and keep it warm for 14 h. (4) Place the above-mentioned graphite felt in a single-chamber electrolytic cell containing a mixed solution of 0.02 M cobalt sulfate, 0.01 M nickel sulfate, 0.1 M sodium chloride, 1 M boric acid, 1 M sodium formate, and 0.1 M sodium sulfate. The working electrode is a 3×3 cm graphite felt electrode clamped by a clip electrode, the counter electrode is a 3×3 cm platinum electrode, and the saturated calomel electrode is used as the reference electrode. The reference electrode is immersed in another beaker containing saturated potassium chloride solution and is connected to the electrolytic cell with a salt bridge. The electrode spacing between the working electrode and the counter electrode is about 1 cm. The electrolysis is carried out in a 25 °C water bath. Use an ivium electrochemical workstation for potentiostatic deposition (-0.8 V for 15 s, -1.0 V for 15 s, cycle 40 times). (5) Wash the graphite felt after cobalt-nickel electrodeposition with deionized water and dry it at 80 °C for 6 h. Heat the muffle furnace to 300 °C and hold for two hours, then place it in a tubular furnace. Heat it at a rate of 5 °C / min under a nitrogen atmosphere, hold at 300 °C for 1 h, 400 °C for 1 h, and 500 °C for 1 h to improve the binding force of nickel cobaltate-loaded graphite felt. Refer to Example 1 for the battery cycle charge-discharge test. The test temperature is 25 °C, the charging cut-off voltage is 1.65 V, the discharging cut-off voltage is 0.8 V, and the apparent current density of charge-discharge is 100 mA·h -1 , and the electrolyte flow rate is set to 20 mL·min -1 . Test the cyclic battery for 80 cycles, and the data voltage efficiency obtained after treatment is 86.0%.

[0043] Example 5;

[0044] (1) Place a 3×3 cm graphite felt, which has been cleaned and dried with acetone, ethanol, and water respectively, in a muffle furnace and heat it from room temperature to 500 °C and hold for 2 h to improve its hydrophilicity; (2) Place the heat-treated graphite felt in a 90 ml hydrothermal autoclave. Add 50 ml of 0.04 M cobalt nitrate, 0.01 M nickel nitrate, 0.1 M urea, and a 4:1 ethanol-water solution into the polytetrafluoroethylene inner liner; (3) Place the hydrothermal autoclave in an 80 °C oven and keep it warm for 14 h; (4) Place the above-mentioned graphite felt in a single-chamber electrolytic cell containing a mixed solution of 0.02 M cobalt sulfate, 0.01 M nickel sulfate, 0.1 M sodium chloride, 1 M boric acid, 1 M sodium formate, and 0.1 M sodium sulfate. The working electrode is a 3×3 cm graphite felt electrode clamped by a clip electrode, the counter electrode is a 3×3 cm platinum electrode, and the saturated calomel electrode is used as the reference electrode. The reference electrode is immersed in another beaker containing a saturated potassium chloride solution and is connected to the electrolytic cell with a salt bridge. The electrode distance between the working electrode and the counter electrode is about 1 cm. The electrolysis is carried out in a 25 °C water bath. Use an ivium electrochemical workstation for variable potential deposition (-0.8 V for 15 s, -1.0 V for 15 s, cycle 40 times) (5) Wash the graphite felt after cobalt-nickel electrodeposition with deionized water and dry it at 80 °C for 6 h. Heat the muffle furnace to 300 °C and hold for two hours, then place it in a tube furnace. Heat it at a rate of 5 °C / min under a nitrogen atmosphere, hold at 300 °C for 1 h, 400 °C for 1 h, and 500 °C for 1 h to improve the binding force of nickel cobaltate-loaded graphite felt. Refer to Example 1 for the battery cycle charge-discharge test. The test temperature is 25 °C, the charging cut-off voltage is 1.65 V, the discharging cut-off voltage is 0.8 V, and the apparent current density of charge-discharge is 100 mA·h -1 , and the electrolyte flow rate is set to 20 mL·min -1 . Test the cyclic battery for 80 cycles, and the data voltage efficiency obtained after treatment is 85.7%.

[0045] Example 6;

[0046] (1) Place a 3×3 cm graphite felt, which has been cleaned and dried with acetone, ethanol, and water respectively, in a muffle furnace and heat it from room temperature to 500 °C and hold for 2 h to improve its hydrophilicity; (2) Place the heat-treated graphite felt in a 90 ml hydrothermal autoclave, and add 50 ml of 0.02 M cobalt nitrate, 0.01 M nickel nitrate, 0.1 M urea, and a 4:1 ethanol-water solution into the polytetrafluoroethylene inner liner; (3) Place the hydrothermal autoclave in an 80 °C oven and keep it warm for 14 h; (4) Place the above graphite felt in a single-tank electrolytic cell containing a mixed solution of 0.01 M cobalt sulfate, 0.01 M nickel sulfate, 0.1 M sodium chloride, 1 M boric acid, 1 M sodium formate, and 0.1 M sodium sulfate. The working electrode is a 3×3 cm graphite felt electrode clamped by a clip electrode, the counter electrode is a 3×3 cm platinum electrode, and the saturated calomel electrode is used as the reference electrode. The reference electrode is immersed in another beaker containing saturated potassium chloride solution and is connected to the electrolytic cell with a salt bridge. The electrode spacing between the working electrode and the counter electrode is about 1 cm. The electrolysis is carried out in a 25 °C water bath. Use an ivium electrochemical workstation for potentiostatic deposition (-0.8 V for 15 s, -1.0 V for 15 s, cycle 40 times) (5) Wash the graphite felt after cobalt-nickel electrodeposition with deionized water and dry it at 80 °C for 6 h. Heat the muffle furnace to 300 °C and hold for two hours, then place it in a tube furnace. Heat it at a rate of 5 °C / min under a nitrogen atmosphere, hold at 300 °C for 1 h, 400 °C for 1 h, and 500 °C for 1 h to improve the binding force of nickel cobaltate-loaded graphite felt. Conduct a battery cycle charge-discharge test. The test temperature is 25 °C, the charging cut-off voltage is 1.65 V, the discharging cut-off voltage is 0.8 V, and the apparent current density of charge-discharge is 100 mA·h -1 , and the electrolyte flow rate is set to 20 mL·min -1 . Test the cyclic battery for 80 cycles, and the data voltage efficiency obtained after treatment is 86.2%.

[0047] Example 7;

[0048] (1) Place the 3×3 cm graphite felt cleaned and dried with acetone, ethanol, and water in a muffle furnace and heat it from room temperature to 500 °C and hold for 2 h to improve its hydrophilicity; (2) Place the heat-treated graphite felt in a 90 ml hydrothermal autoclave, and add 50 ml of 0.02 M cobalt nitrate, 0.01 M nickel nitrate, 0.1 M urea, and a 4:1 ethanol aqueous solution to the polytetrafluoroethylene inner liner; (3) Place the hydrothermal autoclave in an 80 °C oven and keep it warm for 14 h; (4) Place the above graphite felt in a single-tank electrolytic cell containing a mixed solution of 0.04 M cobalt sulfate, 0.01 M nickel sulfate, 0.1 M sodium chloride, 1 M boric acid, 1 M sodium formate, and 0.1 M sodium sulfate. The working electrode is a 3×3 cm graphite felt electrode clamped by a clip electrode, the counter electrode is a 3×3 cm platinum electrode, and the saturated calomel electrode is used as the reference electrode. The reference electrode is immersed in another beaker containing a saturated potassium chloride solution and connected to the electrolytic cell with a salt bridge. The electrode spacing between the working electrode and the counter electrode is about 1 cm. The electrolysis is carried out in a 25 °C water bath. Use an ivium electrochemical workstation for variable potential deposition (-0.8 V for 15 s, -1.0 V for 15 s, cycle 40 times) (5) Wash the graphite felt after cobalt-nickel electrodeposition with deionized water and dry it at 80 °C for 6 h. Heat the muffle furnace to 300 °C and hold for two hours, then place it in a tube furnace. Heat it at a rate of 5 °C / min under a nitrogen atmosphere, hold at 300 °C for 1 h, 400 °C for 1 h, and 500 °C for 1 h to improve the binding force of nickel cobaltate-loaded graphite felt. Refer to Example 1 for the battery cycle charge-discharge test. The test temperature is 25 °C, the charging cut-off voltage is 1.65 V, the discharging cut-off voltage is 0.8 V, and the apparent current density of charge-discharge is 100 mA·h -1 , and the electrolyte flow rate is set to 20 mL·min -1 . Test the cyclic battery for 80 cycles, and the data voltage efficiency obtained after treatment is 85.5%.

[0049] Example 8;

[0050] (1) Place a 3×3 cm graphite felt, which has been cleaned and dried with acetone, ethanol, and water respectively, in a muffle furnace and heat it from room temperature to 500 °C and hold for 2 h to improve its hydrophilicity; (2) Place the heat-treated graphite felt in a 90 ml hydrothermal autoclave. Add 50 ml of 0.02 M cobalt nitrate, 0.01 M nickel nitrate, 0.1 M urea, and a 4:1 ethanol-water solution into the polytetrafluoroethylene inner liner; (3) Place the hydrothermal autoclave in an 80 °C oven and keep it warm for 14 h; (4) Place the above-mentioned graphite felt in a single-tank electrolytic cell containing a mixed solution of 0.02 M cobalt sulfate, 0.01 M nickel sulfate, 0.1 M sodium chloride, 1 M boric acid, 1 M sodium formate, and 0.1 M sodium sulfate. The working electrode is a 3×3 cm graphite felt electrode clamped by a clip electrode, the counter electrode is a 3×3 cm platinum electrode, and the saturated calomel electrode is used as the reference electrode. The reference electrode is immersed in another beaker containing saturated potassium chloride solution and is connected to the electrolytic cell with a salt bridge. The electrode spacing between the working electrode and the counter electrode is about 1 cm. The electrolysis is carried out in a 25 °C water bath. Use an ivium electrochemical workstation for potentiostatic deposition (-0.8 V for 15 s, -z.0 V for 15 s, cycle 10 times) (5) Wash the graphite felt after cobalt-nickel electrodeposition with deionized water and dry it at 80 °C for 6 h. Heat the muffle furnace to 300 °C and hold for two hours, then place it in a tube furnace. Heat it at a rate of 5 °C / min under a nitrogen atmosphere, hold at 300 °C for 1 h, 400 °C for 1 h, and 500 °C for 1 h to improve the binding force of nickel cobaltate-loaded graphite felt. Refer to Example 1 for the battery cycle charge-discharge test. The test temperature is 25 °C, the charging cut-off voltage is 1.65 V, the discharging cut-off voltage is 0.8 V, and the apparent current density of charge-discharge is 100 mA·h -1 , and the electrolyte flow rate is set to 20 mL·min -1 . Test the cyclic battery for 80 cycles, and the data voltage efficiency obtained after treatment is 85.4%.

[0051] Example 9;

[0052] It should be noted that there is a "z" in "-z.0 V" in the original text which seems to be a typo. I translated it as "-z.0 V" as it is in the original. You may want to check and correct it if necessary.(1) Place the 3×3 cm graphite felt, which has been cleaned and dried with acetone, ethanol, and water respectively, in a muffle furnace and heat it from room temperature to 500 °C and hold for 2 h to improve its hydrophilicity; (2) Place the heat-treated graphite felt in a 90 ml hydrothermal autoclave, and add 50 ml of 0.02 M cobalt nitrate, 0.01 M nickel nitrate, 0.1 M urea, and a 4:1 ethanol-water solution into the polytetrafluoroethylene inner liner; (3) Place the hydrothermal autoclave in an 80 °C oven and keep it warm for 14 h; (4) Place the above graphite felt in a single-tank electrolytic cell containing a mixed solution of 0.02 M cobalt sulfate, 0.01 M nickel sulfate, 0.1 M sodium chloride, 1 M boric acid, 1 M sodium formate, and 0.1 M sodium sulfate. The working electrode is a 3×3 cm graphite felt electrode clamped by a clip electrode, the counter electrode is a 3×3 cm platinum electrode, and the saturated calomel electrode is used as the reference electrode. The reference electrode is immersed in another beaker containing saturated potassium chloride solution and connected to the electrolytic cell with a salt bridge. The electrode spacing between the working electrode and the counter electrode is about 1 cm. The electrolysis is carried out in a 25 °C water bath. Use an ivium electrochemical workstation for potentiostatic deposition (-0.8 V for 15 s, -1.0 V for 15 s, cycle 20 times) (5) Wash the graphite felt after cobalt-nickel electrodeposition with deionized water and dry it at 80 °C for 6 h. Heat the muffle furnace to 300 °C and hold for two hours, then place it in a tubular furnace. Heat it at a rate of 5 °C / min under a nitrogen atmosphere, hold at 300 °C for 1 h, 400 °C for 1 h, and 500 °C for 1 h to improve the binding force of nickel cobaltate loaded on the graphite felt. Refer to Example 1 for the battery cycle charge-discharge test. The test temperature is 25 °C, the charging cut-off voltage is 1.65 V, the discharging cut-off voltage is 0.8 V, and the apparent current density of charge-discharge is 100 mA·h -1 , and the electrolyte flow rate is set to 20 mL·min -1 . Test the cyclic battery for 80 cycles, and the obtained data voltage efficiency after treatment is 86.0%.

[0053] Example 10;

[0054] (1) Place the 3×3 cm graphite felt, which has been cleaned and dried with acetone, ethanol, and water respectively, in a muffle furnace and heat it from room temperature to 500 °C and hold for 2 h to improve its hydrophilicity; (2) Place the heat-treated graphite felt in a 90 ml hydrothermal reactor, and add 50 ml of 0.02 M cobalt nitrate, 0.01 M nickel nitrate, 0.1 M urea, and a 4:1 ethanol-water solution into the polytetrafluoroethylene inner liner; (3) Place the hydrothermal reactor in an 80 °C oven and keep it warm for 14 h; (4) Place the above graphite felt in a single-tank electrolytic cell containing a mixed solution of 0.02 M cobalt sulfate, 0.01 M nickel sulfate, 0.1 M sodium chloride, 1 M boric acid, 1 M sodium formate, and 0.1 M sodium sulfate. The working electrode is a 3×3 cm graphite felt electrode clamped by a clip electrode, the counter electrode is a 3×3 cm platinum electrode, and the saturated calomel electrode is used as the reference electrode. The reference electrode is immersed in another beaker containing saturated potassium chloride solution and connected to the electrolytic cell with a salt bridge. The electrode spacing between the working electrode and the counter electrode is about 1 cm. The electrolysis is carried out in a 25 °C water bath, and the variable potential deposition is carried out with an ivium electrochemical workstation (-0.8 V for 15 s, -1.0 V for 15 s, cycle 30 times) (5) Wash the graphite felt after cobalt-nickel electrodeposition with deionized water and dry it at 80 °C for 6 h. Heat the muffle furnace to 300 °C and hold for two hours, then place it in a tubular furnace. Heat it up at a rate of 5 °C / min under a nitrogen atmosphere, hold at 300 °C for 1 h, 400 °C for 1 h, and 500 °C for 1 h to improve the binding force of nickel cobaltate loaded on the graphite felt. Refer to Example 1 for the battery cycle charge-discharge test. The test temperature is 25 °C, the charging cut-off voltage is 1.65 V, the discharging cut-off voltage is 0.8 V, and the apparent current density of charge-discharge is 100 mA·h -1 , and the electrolyte flow rate is set to 20 mL·min -1 . Test the cyclic battery for 80 cycles, and the data voltage efficiency after treatment is 86.9%.

[0055] Example 11;

[0056] (1) Place a 3×3 cm graphite felt, which has been cleaned and dried with acetone, ethanol, and water respectively, in a muffle furnace and heat it from room temperature to 500 °C and hold for 2 h to improve its hydrophilicity. (2) Place the heat-treated graphite felt in a 90 ml hydrothermal reactor. Add 50 ml of 0.02 M cobalt nitrate, 0.01 M nickel nitrate, 0.1 M urea, and a 4:1 ethanol-water solution into the polytetrafluoroethylene inner liner. (3) Place the hydrothermal reactor in an 80 °C oven and keep it warm for 14 h. (4) Place the above-mentioned graphite felt in a single-chamber electrolytic cell containing a mixed solution of 0.02 M cobalt sulfate, 0.01 M nickel sulfate, 0.1 M sodium chloride, 1 M boric acid, 1 M sodium formate, and 0.1 M sodium sulfate. The working electrode is a 3×3 cm graphite felt electrode clamped by a clip electrode, the counter electrode is a 3×3 cm platinum electrode, and the saturated calomel electrode is used as the reference electrode. The reference electrode is immersed in another beaker containing a saturated potassium chloride solution and is connected to the electrolytic cell with a salt bridge. The electrode distance between the working electrode and the counter electrode is about 1 cm. The electrolysis is carried out in a 25 °C water bath. Use an ivium electrochemical workstation for potentiostatic deposition (-0.8 V for 15 s, -1.0 V for 15 s, cycle 50 times). (5) Wash the graphite felt after cobalt-nickel electrodeposition with deionized water and dry it at 80 °C for 6 h. Heat the muffle furnace to 300 °C and hold for two hours, then place it in a tube furnace. Heat it at a rate of 5 °C / min under a nitrogen atmosphere, hold at 300 °C for 1 h, 400 °C for 1 h, and 500 °C for 1 h to improve the binding force of nickel cobaltate-loaded graphite felt. Refer to Example 1 for the battery cycle charge-discharge test. The test temperature is 25 °C, the charging cut-off voltage is 1.65 V, the discharging cut-off voltage is 0.8 V, and the apparent current density of charge-discharge is 100 mA·h -1 , and the electrolyte flow rate is set to 20 mL·min -1 . Test the rechargeable battery for 80 cycles, and the data voltage efficiency after treatment is 85.8%.

[0057] Example 12;

[0058] (1) Place a 3×3 cm graphite felt, which has been cleaned and dried with acetone, ethanol, and water respectively, in a muffle furnace and heat it from room temperature to 500 °C and hold for 2 h to improve its hydrophilicity. (2) Place the heat-treated graphite felt in a 90 ml hydrothermal reactor. Add 50 ml of 0.02 M cobalt nitrate, 0.01 M nickel nitrate, 0.1 M urea, and a 4:1 ethanol-water solution into the polytetrafluoroethylene inner liner. (3) Place the hydrothermal reactor in an 80 °C oven and keep it warm for 14 h. (4) Place the above-mentioned graphite felt in a single-chamber electrolytic cell containing a mixed solution of 0.02 M cobalt sulfate, 0.01 M nickel sulfate, 0.1 M sodium chloride, 1 M boric acid, 1 M sodium formate, and 0.1 M sodium sulfate. The working electrode is a 3×3 cm graphite felt electrode clamped by a clip electrode, the counter electrode is a 3×3 cm platinum electrode, and the saturated calomel electrode is used as the reference electrode. The reference electrode is immersed in another beaker containing saturated potassium chloride solution and connected to the electrolytic cell with a salt bridge. The electrode spacing between the working electrode and the counter electrode is about 1 cm. The electrolysis is carried out in a 25 °C water bath. Use an ivium electrochemical workstation for potentiostatic deposition (-0.8 V for 15 s, -1.0 V for 15 s, cycle 60 times). (5) Wash the graphite felt after cobalt-nickel electrodeposition with deionized water and dry it at 80 °C for 6 h. Heat the muffle furnace to 300 °C and hold for two hours, then place it in a tube furnace. Heat it at a rate of 5 °C / min under a nitrogen atmosphere, hold at 300 °C for 1 h, 400 °C for 1 h, and 500 °C for 1 h to improve the binding force of nickel cobaltate-loaded graphite felt. Refer to Example 1 for the battery cycle charge-discharge test. The test temperature is 25 °C, the charging cut-off voltage is 1.65 V, the discharging cut-off voltage is 0.8 V, and the apparent current density of charge-discharge is 100 mA·h -1 , and the electrolyte flow rate is set to 20 mL·min -1 . Test the cycle battery for 80 cycles, and the data voltage efficiency obtained after treatment is 84.3%.

[0059] Comparative Example 1: Without electrodeposition;

[0060] (1) Place the 3×3 cm graphite felt, which has been cleaned and dried with acetone, ethanol, and water respectively, in a muffle furnace and heat it from room temperature to 500 °C and hold for 2 h to improve its hydrophilicity; (2) Place the heat-treated graphite felt in a 90 ml hydrothermal reactor. Add 50 ml of 0.02 M cobalt nitrate, 0.01 M nickel nitrate, 0.1 M urea, and a 4:1 ethanol-water solution to the polytetrafluoroethylene inner liner; (3) Place the hydrothermal reactor in an 80 °C oven and keep it warm for 14 h; (4) Wash the hydrothermally treated graphite felt with deionized water and dry it at 80 °C for 6 h. Heat the muffle furnace to 300 °C and hold for two hours, then place it in a tubular furnace. Heat it at a rate of 5 °C / min under a nitrogen atmosphere, hold at 300 °C for 1 h, 400 °C for 1 h, and 500 °C for 1 h to improve the binding force of nickel cobaltate loaded on the graphite felt. Refer to Example 1 for the battery cycle charge and discharge test. The test temperature is 25 °C, the charging cut-off voltage is 1.65 V, the discharging cut-off voltage is 0.8 V, and the apparent current density of charge and discharge is 100 mA·h -1 , and the electrolyte flow rate is set to 20 mL·min -1 . Test the cyclic battery for 80 cycles, and the data voltage efficiency obtained after treatment is 85.5%.

[0061] Comparative Example 2: Without hydrothermal treatment;

[0062] (1) Place the 3×3 cm graphite felt, which has been cleaned and dried with acetone, ethanol, and water respectively, in a muffle furnace and heat it from room temperature to 500 °C and hold for 2 h to improve its hydrophilicity; (2) Place the above-mentioned graphite felt in a single-tank electrolytic cell containing a mixed solution of 0.02 M cobalt sulfate, 0.01 M nickel sulfate, 0.1 M sodium chloride, 1 M boric acid, 1 M sodium formate, and 0.1 M sodium sulfate. The working electrode is a 3×3 cm graphite felt electrode clamped by a clip electrode, the counter electrode is a 3×3 cm platinum electrode, and the saturated calomel electrode is used as the reference electrode. The reference electrode is immersed in another beaker containing a saturated potassium chloride solution and is connected to the electrolytic cell with a salt bridge. The electrode spacing between the working electrode and the counter electrode is about 1 cm. The electrolysis is carried out in a 25 °C water bath. Use an ivium electrochemical workstation for potentiostatic deposition (-0.8 V for 15 s, -1.0 V for 15 s, cycle 40 times) (3) Wash the graphite felt after electrodepositing cobalt and nickel with deionized water and dry it at 80 °C for 6 h. Heat the muffle furnace to 300 °C and hold for two hours, then place it in a tubular furnace. Heat it at a rate of 5 °C / min under a nitrogen atmosphere, hold at 300 °C for 1 h, 400 °C for 1 h, and 500 °C for 1 h to improve the binding force of nickel cobaltate loaded on the graphite felt. Refer to Example 1 for the battery cycle charge and discharge test. The test temperature is 25 °C, the charging cut-off voltage is 1.65 V, the discharging cut-off voltage is 0.8 V, and the apparent current density of charge and discharge is 100 mA·h -1 , and the electrolyte flow rate is set to 20 mL·min -1The battery was cycled 80 times for testing, and the data voltage efficiency after treatment was 85.3%.

[0063] Comparative Example 3: Original graphite felt

[0064] The original graphite felt was used for electrochemical testing and the performance testing of a vanadium redox flow battery. Referring to Example 1, the test temperature was 25°C, the charging cut-off voltage was 1.65 V, the discharging cut-off voltage was 0.8 V, and the apparent current density during charge and discharge was 100 mA·h -1 , and the electrolyte flow rate was set at 20 mL·min -1 The battery was cycled 80 times for testing.

[0065] Comparative Example 4: Pretreated graphite felt

[0066] The 3×3 cm graphite felt after being cleaned and dried with acetone, ethanol, and water respectively was placed in a muffle furnace and heated from room temperature to 500°C and maintained for 2 h to improve its hydrophilicity, obtaining the pretreated graphite felt.

[0067] The graphite felt material prepared in this comparative example was cut into 1×1 cm using a die cutter, and one end was clamped with a 0.5×0.5 cm platinum sheet electrode and immersed in a three-electrode electrolytic cell containing a solution of 0.1 M VOSO4 and 2 M H2SO4. A 2×2 cm platinum sheet electrode was used as the counter electrode, and a saturated calomel electrode was used as the reference electrode, which were connected in the middle with a salt bridge. The cyclic voltammetry parameters were set with the starting voltage at 0.8 V, the potential range at -0.1 V to 1.6 V, and the scan rate at 20 mV·s -1 .

[0068] Referring to Example 1, the pretreated graphite felt was used for electrochemical testing and the performance testing of a vanadium redox flow battery. The test temperature was 25°C, the charging cut-off voltage was 1.65 V, the discharging cut-off voltage was 0.8 V, and the apparent current density during charge and discharge was 100 mA·h -1 , and the electrolyte flow rate was set at 20 mL·min -1 The battery was cycled 80 times for testing.

[0069] Table 1 is a summary table of the voltage efficiencies of various comparative examples and examples. It can be seen that in Example 2 of the present invention, the process conditions with the highest voltage efficiency are a hydrothermal solution of 50 ml / 90 ml, and the best cobalt-nickel ratio of the hydrothermal solution and the electrode solution is 2:1. The optimal number of electrodeposition cycles is 40 cycles, and the surface is evenly distributed exactly. Figure 1XRD patterns of the graphite felt of the present invention and the pretreated graphite felt. Diffraction peaks can be observed at 36.8°, 42.8°, 61.8°, 73.9° and 77.9° in the graphite felt of the present invention, corresponding to the (111), (200), (220), (311) and (222) crystal planes of NiCoO2 (JCPDS#10-088). This indicates that the preparation method effectively synthesizes NiCoO2 on the surface of the graphite felt. Figure 2 Changes in the surface of the graphite felt before and after modification can be seen. Figure 2 The graphite felt of the present invention in b has a rich surface. It can be observed that the petal-shaped NiCoO2 on the surface is evenly distributed on the graphite felt, and these petals are connected to each other to form a support structure. Figure 3 Low-temperature nitrogen adsorption and desorption isotherms of the pretreated graphite felt and the modified graphite felt prepared in Example 2 of the present invention. The slope of the hysteresis loop of the modified graphite felt prepared in the present invention is the highest, indicating that it has more mesopores and macropores. The BET specific surface area also confirms the same result. The BET specific surface area of the modified graphite felt prepared in the present invention is 2.27 m 2 / g, which is twice that of the pretreated graphite felt of 1.08 m 2 / g. This also corresponds to the petal-shaped surface morphology. A larger specific surface area is more conducive to the infiltration of the electrode liquid and provides more pathways for the mass transfer of vanadium ions. Figure 4 Cyclic voltammetry comparison diagrams of the two electrodes. The CV separation degree and the ratio of oxidation-reduction peak currents of the modified graphite felt prepared in Example 2 of the present invention are significantly smaller than those of the pretreated graphite felt. It is worth noting that the reduction peak of pentavalent vanadium ions has been significantly improved. This indicates that the STED-GF of nanoarray NiCoO2 has better vanadium ion electrochemical activity and redox reversibility. In Figure 5 In the battery performance test, it can be seen that the coulombic efficiency has little difference between the two batteries, but the energy efficiency and voltage efficiency are significantly improved for the graphite felt of the present invention. After 80 charge-discharge cycles, they reach approximately 78.5% and 85.2% on average, respectively, which is nearly 10 percentage points higher than 68.4% and 76.6% of the heat-treated T-GF.

[0070] Table 1

[0071]

[0072]

Claims

1. A method for preparing a graphite felt modified electrode for an all-vanadium redox flow battery, characterized in that: The preparation method comprises the following steps: (1) washing and drying the graphite felt with acetone, ethanol, and water, respectively, in a muffle furnace and heating the temperature from room temperature to 400-550° C. for 1-3 hours to improve its hydrophilicity, thereby obtaining a pretreated graphite felt; (2) placing the pretreated graphite felt into a polytetrafluoroethylene liner of a hydrothermal kettle, wherein the polytetrafluoroethylene liner is filled with a hydrothermal treatment liquid, wherein the hydrothermal treatment liquid is a mixed solution of cobalt nitrate, nickel nitrate and urea, wherein the solvent is a mixed solvent of ethanol and water in a volume ratio of 1-5:1, the concentrations of nickel nitrate and urea are 0.01-0.1M and 0.01-1M, respectively, and the molar ratio of cobalt nitrate to nickel nitrate is 1-4:1; (3) Place the hydrothermal kettle in an oven and heat to 60-120°C for 8-16 hours; (4) The graphite felt in the hydrothermal kettle is taken out and cleaned, and then placed in an electrolytic cell for cathode reduction electrodeposition to form a cobalt-nickel alloy on the surface of the graphite felt; wherein the electrolyte is an aqueous solution of cobalt sulfate, 0.01-0.1M nickel sulfate, 0.05-0.2M sodium chloride, 0.5-2M boric acid, 0.5-2M sodium formate, and 0.05-0.2M sodium sulfate, wherein the molar ratio of cobalt sulfate to nickel sulfate is 1-4:1; the electrodeposition is a variable potential deposition, and a single-tank electrolytic cell is used. The working electrode is a graphite felt electrode clamped by a clip electrode, the counter electrode is a platinum electrode, and the reference electrode is a saturated calomel electrode. The reference electrode is immersed in another beaker filled with saturated potassium chloride solution and connected to the electrolytic cell via a salt bridge. The inter-electrode distance between the working electrode and the counter electrode is 1 cm. The electrolysis is carried out in a water bath at 15-35°C. The parameters of the variable potential deposition are -0.7 to -0.8V for 10-20 seconds, -0.9 to -1.0V for 10-20 seconds, and 10-60 cycles. (5) The electrodeposited graphite felt is placed in a muffle furnace at 300-500°C for 1-4 hours and in a tube furnace at 300-600°C for 1-4 hours to obtain a graphite felt modified electrode.

2. The preparation method according to claim 1, wherein: In step (2), the volume of the hydrothermal treatment liquid accounts for 4 / 9 to 6 / 9 of the volume of the polytetrafluoroethylene liner.

3. The preparation method according to claim 1, wherein: In step (2), the volume of the hydrothermal treatment liquid accounts for 5 / 9 of the volume of the polytetrafluoroethylene liner.

4. The preparation method according to claim 1, wherein: In step (2) or (4), the molar ratio of cobalt nitrate to nickel nitrate is 2:

1.

5. The preparation method according to claim 1, wherein: In step (2), the hydrothermal treatment solution is a mixed solution of 0.02M cobalt nitrate, 0.01M nickel nitrate and 0.1M urea, and the solvent is a mixed solvent of ethanol and water in a volume ratio of 4:

1.

6. The preparation method according to claim 1, wherein: Step (3) is to place the hydrothermal kettle in an oven at 80°C and keep it warm for 14 hours.

7. The preparation method according to claim 1 or 3, wherein: In step (4), the electrolyte is a mixed aqueous solution of cobalt sulfate, 0.01-0.02M nickel sulfate, 0.05-0.1M sodium chloride, 0.5-1M boric acid, 1-2M sodium formate, and 0.1-0.2M sodium sulfate.

8. The preparation method according to claim 7, wherein: In step (4), the electrolyte is a mixed aqueous solution of 0.02M cobalt sulfate, 0.01M nickel sulfate, 0.1M sodium chloride, 1M boric acid, 1M sodium formate, and 0.1M sodium sulfate.

9. The preparation method according to claim 1, wherein: In step (4), the parameters of the variable potential deposition are -0.8 V for 15 s, -1.0 V for 15 s, and 40 cycles.

10. The preparation method according to claim 1, wherein: Step (5) is specifically as follows: the graphite felt after electrodeposition of cobalt and nickel is washed with deionized water and dried at 60-100°C for 4-8h, the muffle furnace is heated to 250-350°C and kept warm for 1-3 hours, and then placed in a tubular furnace, heated to 300-500°C in a nitrogen atmosphere and kept warm for 3h, the nitrogen flow rate of the tubular furnace is 40-80ml / min, and the heating rate is 3-10°C / min.

11. The preparation method according to claim 1, wherein: In step (5), in a tubular furnace, the temperature was raised to 300° C. and maintained for 1 h under a nitrogen atmosphere, then further raised to 400° C. and maintained for 1 h, and then further raised to 500° C. and maintained for 1 h.

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