A composite graphite felt electrode and its preparation method

Abstract

The present invention discloses a composite graphite felt electrode and a method for preparing the same, comprising at least the following steps: dissolving polyacrylonitrile in a solvent, adding a metal source, ultrasonically dispersing the solution to form a uniform spinning solution, wet spinning the composite fiber precursor, and then forming the fiber precursor into a composite white felt using a needle punching process. The composite graphite felt electrode is obtained through pre-oxidation, carbonization, graphitization, and high-temperature steam activation. All steps are compatible with existing graphite felt production lines and graphite felt high-temperature activation lines, eliminating the need for complex equipment modifications. The preparation of composite graphite felt electrodes with in-situ metal oxide modification is achieved simultaneously during the graphite felt production and activation processes. The composite graphite felt electrode prepared by the present invention utilizes the synergistic catalytic effect of metal oxides and oxygen-containing functional groups to greatly improve the electrochemical activity of the electrode.

Description

Technical Field

[0001] The present invention relates to the technical field of all-vanadium redox flow batteries, and in particular to a composite graphite felt electrode and a preparation method thereof. Background Art

[0002] All-vanadium liquid flow battery (abbreviated as vanadium battery, VRFB) is an energy storage technology based on the redox reaction of vanadium ions. 2+ / V 3+ and VO 2+ / VO2 + ) The reversible reaction in the electrolyte realizes the storage and release of energy. The all-vanadium liquid flow battery stack is mainly composed of end plates, guide plates, current collecting plates, bipolar plates, electrode frames, electrodes, diaphragms and other parts. Among them, the electrode is one of the most important components in the vanadium battery, providing reaction sites for the active substances and directly affecting the battery performance. Currently, the commonly used electrode materials are carbon materials, such as graphite felt (GF). Graphite felt not only has excellent conductivity, but also has a very large specific surface area, which greatly increases the effective reaction area of the electrode. The graphite felt sold on the market is mainly used as a thermal insulation material. If it is used directly as a battery electrode, its hydrophilicity and electrochemical activity are poor, and it must undergo subsequent activation and modification before it can be used as an electrode.

[0003] Currently, there are three main activation and modification measures commonly used: surface functionalization, deposition of metal or transition metal oxides, and bulk doping with other elements. For example, graphite felt is treated with various strong oxidants to introduce oxygen-containing functional groups on the electrode surface to improve hydrophilicity and reactive sites. However, this can easily lead to excessive oxidation and affect the mechanical strength of the electrode, and requires tedious post-activation treatments such as cleaning and drying. Depositing metal or transition metal oxides on the surface of graphite felt faces the problem of being resistant to fluid erosion and easy to fall off. The preparation process of graphite felt bulk doped with nitrogen, phosphorus, boron and other elements is complex, the conditions are harsh, and the cost is high.

[0004] Therefore, how to solve the complexity, inefficiency and instability in the activation and modification process of graphite felt electrodes is a technical problem that needs to be solved urgently. Summary of the Invention

[0005] The purpose of the present invention is to provide a composite graphite felt electrode and a preparation method thereof, which solves the problems of insufficient electrochemical activity, poor stability and complicated activation process of the graphite felt electrode in the prior art.

[0006] The technical solution adopted in the present invention is as follows:

[0007] A method for preparing a composite graphite felt electrode, the method comprising the following steps:

[0008] Step S1: dissolving 5-15 parts by weight of polyacrylonitrile in 77-94 parts by weight of a solvent, adding 1-8 parts by weight of a metal source, and ultrasonically dispersing the mixture to form a uniform spinning solution, and wet spinning the resulting composite fiber precursor with a diameter of 6-20 μm;

[0009] Step S2: preparing composite fiber precursors into composite white felt using a needle punching process;

[0010] Step S3: placing the composite white felt in a pre-oxidation furnace and heating it according to a first heating program under an air atmosphere, so that the polyacrylonitrile on the composite white felt is cyclized to form a stable trapezoidal structure, and the metal source on the composite white felt is decomposed into metal oxides, thereby obtaining a composite pre-oxidation felt;

[0011] Step S4: placing the composite preoxidized felt in a carbonization furnace, heating it according to a second heating program under an argon atmosphere, carbonizing the fibers on the composite preoxidized felt to form carbon fibers, and converting the metal oxides on the composite preoxidized felt into carbides after reacting with part of the carbon source of the carbon fibers on the composite preoxidized felt, thereby obtaining a composite carbon felt;

[0012] Step S5: placing the composite carbon felt in a graphitization furnace under an argon atmosphere and heating the temperature according to a third heating program, wherein the carbides on the composite carbon felt remain unchanged and the carbon sources of the carbon fibers on the composite carbon felt are rearranged into a graphite crystal structure, thereby obtaining a composite graphite felt;

[0013] Step S6: placing the composite graphite felt in a high-temperature steam reactor under air atmosphere, and oxidizing the carbides on the composite graphite felt into metal oxides again, thereby finally obtaining a composite graphite felt electrode in situ modified with metal oxides.

[0014] Furthermore, the solvent in step S1 is selected from at least one of dimethyl sulfoxide, N,N-dimethylformamide, N-methylpyrrolidone, and N,N-dimethylacetamide.

[0015] Furthermore, the metal source in step S1 is selected from at least one of ammonium metatungstate, ammonium dimolybdate, and ammonium heptamolybdate.

[0016] Furthermore, the needling forming process parameters in step S2 are: needling density 10-30 needles / cm², needle speed 5-10 cm / s, and needling depth 1-6 mm.

[0017] Furthermore, the composite white felt in step S2 has a thickness of 1-6 mm and a density of 0.1-0.3 g / cm 3 .

[0018] Furthermore, the reaction conditions of the high-temperature steam reactor in step S6 are: temperature 800-1000° C., steam flow rate 0.5-3 L / min, steam volume content 10-50%, and activation time 1-3 h.

[0019] Furthermore, in step S3, the first heating program is:

[0020] Starting from room temperature, the temperature was raised to 200°C at a constant speed for 1 hour;

[0021] Keep at 200℃ for 0.5 hours;

[0022] Starting from 200℃, the temperature was raised to 300℃ at a constant speed for 0.5 hours;

[0023] Keep at 300°C for 1 hour.

[0024] The first heating procedure in this embodiment is intended to avoid uneven heating of the fibers of the composite white felt during the heating process by controlling the speed of temperature change, thereby reducing the risk of fiber breakage. Specifically, the composite white felt needs to undergo a slow heating process when heated, so that the temperature rises gradually, rather than being heated quickly and concentratedly, to avoid fiber breakage due to uneven thermal expansion. At around 200°C, the fiber precursors begin to undergo a cyclization reaction, which means that a ring structure of the molecular chain is formed, increasing the strength and heat resistance of the fiber. At around 300°C, the fiber precursors will begin to undergo an oxidation reaction, which is usually due to the reaction of oxygen molecules with the fiber at high temperatures, which can improve the stability of the fiber.

[0025] Furthermore, the second heating program in step S4 is:

[0026] Starting from room temperature, the temperature was raised to 500°C at a constant rate for 2 hours;

[0027] Keep at 500℃ for 1 hour;

[0028] Starting from 500℃, the temperature was raised to 1400℃ at a constant speed within 3 hours;

[0029] Keep at 1400℃ for 3 hours.

[0030] The second heating procedure in this embodiment uses a step-by-step heating process (heating at 500°C and 1400°C). The metal source loaded into the composite pre-oxidation felt is transformed from a larger particle size to a metal oxide with a smaller particle size and higher crystallinity. This process helps improve the reactivity of the metal oxide and the subsequent carbonization reaction. At 1400°C, the metal oxide undergoes a carbothermal reduction reaction to form metal carbides. These carbides typically have a smaller particle size and higher crystallinity, possessing greater thermal stability and suitability for high-temperature environments. The composite pre-oxidation felt undergoes decomposition and carbonization reactions at 500°C and 1400°C, respectively, to produce graphite microcrystals and stable carbides. This not only improves the thermal stability of the material but also increases its mechanical strength. The step-by-step heating process ensures uniform and refined carbide formation while enhancing the overall performance of the carbon felt.

[0031] Furthermore, the third heating program in step S5 is:

[0032] Starting from room temperature, the temperature was raised to 1600°C at a constant rate for 6 hours;

[0033] Keep at 1600℃ for 2 hours;

[0034] Starting from 1600℃, the temperature was raised to 2200℃ at a constant speed for 6 hours;

[0035] Keep at 2200℃ for 3 hours.

[0036] The third heating program in this embodiment needs to ensure that the WC particles (tungsten carbide particles) are stable and do not undergo adverse changes. WC particles are generally used to enhance the hardness and wear resistance of materials. However, in high-temperature environments, WC particles may undergo physical or chemical changes. Therefore, an appropriate heating program is required to avoid the decomposition or structural changes of these particles. During the high-temperature insulation stages of 1600°C and 2200°C, the stability of the WC particles is maintained due to the change in carbon concentration during the graphitization of carbon fibers. The temperature is gradually increased during the heating process, and the temperature change is ensured to be slow, thereby avoiding the impact of temperature fluctuations on the WC particles.

[0037] Another technical solution adopted in the present invention is as follows:

[0038] A composite graphite felt electrode is prepared by the composite graphite felt electrode preparation method as described above.

[0039] The beneficial effects of the present invention include at least:

[0040] 1. The metal source is blended with PAN at the molecular level to achieve its uniform distribution on the carbon fiber, avoiding the particle agglomeration problem of traditional mechanical mixing; the metal oxide on the composite pre-oxidized felt reacts with part of the carbon source of the carbon fiber to transform into carbide, which has a certain etching effect on the carbon fiber and increases its surface area; during the graphitization process, the carbide remains stable and can serve as a heterogeneous nucleation point, reducing the energy barrier for the formation of graphite layers, thereby promoting the formation of ordered graphite structure at a lower temperature and improving conductivity.

[0041] 2. During the high-temperature steam activation stage, carbides are again in situ generated into metal oxides, which are highly chemically inert. Together with the densified structure of carbon fibers, the stability of the composite graphite felt body is achieved. The chemical bonding between carbides and carbon fibers also creates the chemical bonding between metal oxides and carbon fibers, thus improving the physical and chemical stability of the composite graphite felt electrode.

[0042] 3. The multi-level porous structure of carbon fibers is formed by controlled etching activated by high-temperature water vapor, and oxygen-containing functional groups are introduced. At the same time, metal oxides can provide additional catalytic active sites. This metal oxide / oxygen-containing functional group synergistic catalysis significantly enhances the electrode activity.

[0043] 4. All steps are compatible with the existing graphite felt production line and graphite felt high-temperature activation line, without the need for complex equipment modification. The preparation of composite graphite felt with in-situ modification of metal oxides can be achieved simultaneously during the graphite felt production and activation process. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Figure 1 This is a flow chart for preparing a composite graphite felt electrode of the present invention;

[0045] Figure 2 This is a schematic structural diagram of a composite graphite felt electrode according to the present invention;

[0046] Figure 3 This is a scanning electron microscope image of a composite graphite felt electrode of the present invention;

[0047] Figure 4 for Figure 3 A partial enlarged view of . DETAILED DESCRIPTION

[0048] The following description of at least one exemplary embodiment is merely illustrative and is in no way intended to limit the present invention, its application, or use. All other embodiments derived by persons of ordinary skill in the art based on the embodiments of the present invention without inventive effort are intended to fall within the scope of protection of the present invention.

[0049] like Figure 1As shown, polyacrylonitrile (PAN) is dissolved in a solvent, a metal source is added, and ultrasonic dispersion is performed to form a uniform spinning solution. Composite fiber precursors are prepared by wet spinning, and the composite fiber precursors are prepared by a needle punching process. The composite white felt is placed in a pre-oxidation furnace, and the temperature is increased stepwise within the range of 200-300°C in an air atmosphere. The PAN of the composite white felt is cyclized to form a stable trapezoidal structure, and the metal source on the composite white felt is decomposed into metal oxides to obtain a composite pre-oxidation felt. The composite pre-oxidation felt is placed in a carbonization furnace, and the temperature is increased stepwise within the range of 500-1400°C in an argon atmosphere. The fibers of the composite preoxidized felt are carbonized, and the metal oxides on the composite preoxidized felt react with part of the carbon source of the carbon fibers to convert into carbides, thereby obtaining a composite carbon felt; the composite carbon felt is placed in a graphitization furnace, and under an argon atmosphere, the temperature is increased stepwise within the range of 1600-2200°C, and the carbon atoms in the composite carbon felt are rearranged into a graphite crystal structure, thereby obtaining a composite graphite felt; under an air atmosphere, the composite graphite felt is placed in a high-temperature steam reactor for an activation step, and the carbides on the composite graphite felt are oxidized into metal oxides again, thereby finally obtaining a composite graphite felt electrode modified in situ with metal oxides, such as Figure 2 shown.

[0050] Example 1: Using the following technical solutions

[0051] A method for preparing a composite graphite felt electrode comprises the following steps:

[0052] Step S1: 5 parts by weight of polyacrylonitrile (PAN) was dissolved in 94 parts by weight of dimethyl sulfoxide (DMSO) solvent, 1 part by weight of ammonium metatungstate was added, and ultrasonic dispersion was performed to form a uniform spinning solution, and composite fiber precursors with a diameter of 6 μm were prepared by wet spinning.

[0053] Step S2: Using needle punching process (parameters: needle punching density 10 needles / cm 2 , needle speed 5cm / s, needle penetration depth 1mm) to make composite fiber precursor into 1mm thickness and 0.1g / cm 3 Composite white felt.

[0054] Step S3: The composite white felt is placed in a pre-oxidation furnace, and the temperature is increased stepwise in the range of 200-300°C under air atmosphere. The heating program is as follows: room temperature → uniformly increase the temperature to 200°C for 1 hour → keep at 200°C for 0.5 hour → uniformly increase the temperature to 300°C for 0.5 hour → keep at 300°C for 1 hour. The PAN of the composite white felt is cyclized to form a stable trapezoidal structure, and the ammonium metatungstate on the composite white felt is decomposed into tungsten oxide (WO3), thereby obtaining a composite pre-oxidation felt.

[0055] Step S4: placing the composite preoxidized felt in a carbonization furnace, and heating it stepwise in the range of 500-1400°C under an argon atmosphere. The stepwise heating program is as follows: room temperature → heating to 500°C at a constant speed for 2 hours → keeping at 500°C for 1 hour → heating to 1400°C at a constant speed for 3 hours → keeping at 1400°C for 3 hours. The fibers on the composite preoxidized felt are carbonized to form carbon fibers, and the WO3 on the composite preoxidized felt is converted into tungsten carbide (WC), thereby obtaining a composite carbon felt.

[0056] Step S5: placing the composite carbon felt in a graphitization furnace, and under an argon atmosphere, stepwise heating in the range of 1600-2200°C. The stepwise heating program is: room temperature → uniform heating to 1600°C for 6 hours → keeping at 1600°C for 2 hours → uniform heating to 2200°C for 6 hours → keeping at 2200°C for 3 hours. The carbide on the composite carbon felt remains unchanged, and the carbon source of the carbon fibers on the composite carbon felt is rearranged into a graphite crystal structure to obtain a composite graphite felt.

[0057] Step S6: Under air atmosphere, the composite graphite felt is placed in a high-temperature steam reactor, with the temperature controlled at 800°C, the steam flow rate at 0.5 L / min, the steam volume content at 10-50%, and the activation time at 1 hour. The WC on the composite graphite felt is oxidized to WO3, and finally a composite graphite felt electrode with in-situ modification of tungsten oxide is obtained, such as Figure 3 and Figure 4 As shown, the tungsten oxide particles are small and evenly attached to the carbon fibers.

[0058] Example 2: Using the following technical solutions

[0059] A method for preparing a composite graphite felt electrode comprises the following steps:

[0060] Step S1: 7 parts by weight of polyacrylonitrile (PAN) was dissolved in 94 parts by weight of N,N-dimethylformamide (DMF) solvent, 3 parts by weight of ammonium dimolybdate was added, and ultrasonic dispersion was performed to form a uniform spinning solution, and composite fiber precursors with a diameter of 8 μm were prepared by wet spinning.

[0061] Step S2: Using needle punching process (parameters: needle punching density 14 needles / cm 2 , needle speed 6cm / s, needle penetration depth 2mm) to make composite fiber precursor into 2mm thick and 0.15g / cm 3 Composite white felt.

[0062] Step S3: Place the composite white felt in a pre-oxidation furnace, and in an air atmosphere, stepwise heat up in the range of 200-300°C. The heating program is: room temperature → uniformly heat up to 200°C for 1 hour → keep warm at 200°C for 0.5 hour → uniformly heat up to 300°C for 0.5 hour → keep warm at 300°C for 1 hour. The PAN of the composite white felt is cyclized to form a stable trapezoidal structure, and the ammonium dimolybdate on the composite white felt is decomposed into molybdenum oxide to obtain a composite pre-oxidized felt.

[0063] Step S4: placing the composite pre-oxidized felt in a carbonization furnace, and under an argon atmosphere, stepwise heating in the range of 500-1400°C. The stepwise heating program is: room temperature → uniform heating to 500°C for 2 hours → keeping at 500°C for 1 hour → uniform heating to 1400°C for 3 hours → keeping at 1400°C for 3 hours. The fibers on the composite pre-oxidized felt are carbonized to form carbon fibers, and the molybdenum oxide on the composite pre-oxidized felt is converted into molybdenum carbide to obtain a composite carbon felt.

[0064] Step S5: placing the composite carbon felt in a graphitization furnace, and under an argon atmosphere, stepwise heating in the range of 1600-2200°C. The stepwise heating program is: room temperature → uniform heating to 1600°C for 6 hours → keeping at 1600°C for 2 hours → uniform heating to 2200°C for 6 hours → keeping at 2200°C for 3 hours. The carbide on the composite carbon felt remains unchanged, and the carbon source of the carbon fibers of the composite carbon felt is rearranged into a graphite crystal structure to obtain a composite graphite felt.

[0065] Step S6: Under air atmosphere, the composite graphite felt is placed in a high-temperature steam reactor, and the temperature is controlled to 840°C, the steam flow rate is 1L / min, the steam volume content is 10-50%, and the activation time is 1.2h. The molybdenum carbide on the composite graphite felt is oxidized to molybdenum oxide, and finally a composite graphite felt modified with molybdenum oxide in situ is obtained.

[0066] Example 3: Using the following technical solution

[0067] A method for preparing a composite graphite felt electrode comprises the following steps:

[0068] Step S1: 9 parts by weight of polyacrylonitrile (PAN) was dissolved in 87 parts by weight of N-methylpyrrolidone (NMP) solvent, 4 parts by weight of ammonium heptamolybdate was added, and ultrasonic dispersion was performed to form a uniform spinning solution, and composite fiber precursors with a diameter of 10 μm were prepared by wet spinning.

[0069] Step S2: Using needle punching process (parameters: needle punching density 18 needles / cm 2 , needle speed 7cm / s, needle penetration depth 3mm) to make composite fiber raw yarn with thickness 3mm and density 0.2g / cm 3 Composite white felt.

[0070] Step S3: Place the composite white felt in a pre-oxidation furnace, and heat it stepwise in the range of 200-300°C under an air atmosphere of 5. The heating program is: room temperature → uniformly heat to 200°C for 1 hour → keep warm at 200°C for 0.5 hour → uniformly heat to 300°C for 0.5 hour → keep warm at 300°C for 1 hour. The PAN of the composite white felt is cyclized to form a stable trapezoidal structure, and the ammonium heptamolybdate on the composite white felt is decomposed into molybdenum oxide to obtain a composite pre-oxidized felt.

[0071] Step S4: placing the composite pre-oxidized felt in a carbonization furnace, and under an argon atmosphere, stepwise heating in the range of 500-1400°C. The stepwise heating program is: room temperature → uniform heating to 500°C for 2 hours → keeping at 500°C for 1 hour → uniform heating to 1400°C for 3 hours → keeping at 1400°C for 3 hours. The fibers on the composite pre-oxidized felt are carbonized to form carbon fibers, and the molybdenum oxide on the composite pre-oxidized felt is converted into molybdenum carbide to obtain a composite carbon felt.

[0072] Step S5: placing the composite carbon felt in a graphitization furnace, and under an argon atmosphere, stepwise heating in the range of 1600-2200°C. The stepwise heating program is: room temperature → uniform heating to 1600°C for 6 hours → keeping at 1600°C for 2 hours → uniform heating to 2200°C for 6 hours → keeping at 2200°C for 3 hours. The carbide on the composite carbon felt remains unchanged, and the carbon source of the carbon fibers of the composite carbon felt is rearranged into a graphite crystal structure to obtain a composite graphite felt.

[0073] Step S6: Under air atmosphere, the composite graphite felt is placed in a high-temperature steam reactor, the temperature is controlled at 880°C, the steam flow rate is 1.5 L / min, the steam volume content is 10-50%, and the activation time is 1.6 h. The molybdenum carbide on the composite graphite felt is oxidized to molybdenum oxide, and finally a composite graphite felt modified with molybdenum oxide in situ is obtained.

[0074] Example 4: Using the following technical solutions

[0075] A method for preparing a composite graphite felt electrode comprises the following steps:

[0076] Step S1: 10 parts by weight of polyacrylonitrile (PAN) was dissolved in 85 parts by weight of N,N-dimethylacetamide (DMAC) solvent, 5 parts by weight of ammonium metatungstate was added, and ultrasonic dispersion was performed to form a uniform spinning solution, and composite fiber precursors with a diameter of 12 μm were prepared by wet spinning.

[0077] Step S2: Using needle punching process (parameters: needle punching density 20 needles / cm 2 , needle speed 8cm / s, needle penetration depth 4mm) to make composite fiber precursor into 4mm thick and 0.2g / cm 3 Composite white felt.

[0078] Step S3: The composite white felt is placed in a pre-oxidation furnace, and the temperature is increased stepwise in the range of 200-300°C under air atmosphere. The heating program is as follows: room temperature → uniformly increase the temperature to 200°C for 1 hour → keep at 200°C for 0.5 hour → uniformly increase the temperature to 300°C for 0.5 hour → keep at 300°C for 1 hour. The PAN of the composite white felt is cyclized to form a stable trapezoidal structure, and the ammonium metatungstate on the composite white felt is decomposed into tungsten oxide (WO3), thereby obtaining a composite pre-oxidation felt.

[0079] Step S4: placing the composite preoxidized felt in a carbonization furnace, and heating it stepwise in the range of 500-1400°C under an argon atmosphere. The stepwise heating program is as follows: room temperature → heating to 500°C at a constant speed for 2 hours → keeping at 500°C for 1 hour → heating to 1400°C at a constant speed for 3 hours → keeping at 1400°C for 3 hours. The fibers on the composite preoxidized felt are carbonized to form carbon fibers, and the WO3 on the composite preoxidized felt is converted into tungsten carbide (WC), thereby obtaining a composite carbon felt.

[0080] Step S5: placing the composite carbon felt in a graphitization furnace, and under an argon atmosphere, stepwise heating in the range of 1600-2200°C. The stepwise heating program is: room temperature → uniform heating to 1600°C for 6 hours → keeping at 1600°C for 2 hours → uniform heating to 2200°C for 6 hours → keeping at 2200°C for 3 hours. The carbide on the composite carbon felt remains unchanged, and the carbon source of the carbon fibers of the composite carbon felt is rearranged into a graphite crystal structure to obtain a composite graphite felt.

[0081] Step S6: Under air atmosphere, the composite graphite felt is placed in a high-temperature steam reactor, with the temperature controlled at 900°C, the steam flow rate at 2 L / min, the steam volume content at 10-50%, and the activation time at 2 h. The WC on the composite graphite felt is oxidized to WO3, and finally a composite graphite felt modified with tungsten oxide in situ is obtained.

[0082] Example 5: Using the following technical solution

[0083] A method for preparing a composite graphite felt electrode comprises the following steps:

[0084] Step S1: 11 parts by weight of polyacrylonitrile (PAN) was dissolved in 83 parts by weight of N,N-dimethylacetamide (DMAC) solvent, 6 parts by weight of ammonium metatungstate was added, and ultrasonic dispersion was performed to form a uniform spinning solution, and composite fiber precursors with a diameter of 14 μm were prepared by wet spinning.

[0085] Step S2: Using needle punching process (parameters: needle punching density 22 needles / cm 2 , needle speed 8cm / s, needle penetration depth 4mm) to make composite fiber raw yarn with thickness 4mm and density 0.25g / cm 3 Composite white felt.

[0086] Step S3: The composite white felt is placed in a pre-oxidation furnace, and the temperature is increased stepwise in the range of 200-300°C under air atmosphere. The heating program is as follows: room temperature → uniformly increase the temperature to 200°C for 1 hour → keep at 200°C for 0.5 hour → uniformly increase the temperature to 300°C for 0.5 hour → keep at 300°C for 1 hour. The PAN of the composite white felt is cyclized to form a stable trapezoidal structure, and the ammonium metatungstate on the composite white felt is decomposed into tungsten oxide (WO3), thereby obtaining a composite pre-oxidation felt.

[0087] Step S4: placing the composite preoxidized felt in a carbonization furnace, and heating it stepwise in the range of 500-1400°C under an argon atmosphere. The stepwise heating program is as follows: room temperature → heating to 500°C at a constant speed for 2 hours → keeping at 500°C for 1 hour → heating to 1400°C at a constant speed for 3 hours → keeping at 1400°C for 3 hours. The fibers on the composite preoxidized felt are carbonized to form carbon fibers, and the WO3 on the composite preoxidized felt is converted into tungsten carbide (WC), thereby obtaining a composite carbon felt.

[0088] Step S5: placing the composite carbon felt in a graphitization furnace, and under an argon atmosphere, stepwise heating in the range of 1600-2200°C. The stepwise heating program is: room temperature → uniform heating to 1600°C for 6 hours → keeping at 1600°C for 2 hours → uniform heating to 2200°C for 6 hours → keeping at 2200°C for 3 hours. The carbide on the composite carbon felt remains unchanged, and the carbon source of the carbon fibers of the composite carbon felt is rearranged into a graphite crystal structure to obtain a composite graphite felt.

[0089] Step S6: Under air atmosphere, the composite graphite felt is placed in a high-temperature steam reactor, and the temperature is controlled at 920°C, the steam flow rate is 2 L / min, the steam volume content is 10-50%, and the activation time is 2.2 h. The WC on the composite graphite felt is oxidized to WO3, and finally a composite graphite felt modified with tungsten oxide in situ is obtained.

[0090] Example 6: Using the following technical solution

[0091] A method for preparing a composite graphite felt electrode comprises the following steps:

[0092] Step S1: 13 parts by weight of polyacrylonitrile (PAN) was dissolved in 80 parts by weight of dimethyl sulfoxide (DMSO) solvent, 7 parts by weight of ammonium dimolybdate was added, and ultrasonic dispersion was performed to form a uniform spinning solution, and composite fiber precursors with a diameter of 17 μm were prepared by wet spinning.

[0093] Step S2: Using needle punching process (parameters: needle punching density 26 needles / cm 2 , needle speed 9cm / s, needle penetration depth 5mm) to make composite fiber raw yarn with thickness 5mm and density 0.25g / cm 3 Composite white felt.

[0094] Step S3: Place the composite white felt in a pre-oxidation furnace, and in an air atmosphere, stepwise heat up in the range of 200-300°C. The heating program is: room temperature → uniformly heat up to 200°C for 1 hour → keep warm at 200°C for 0.5 hour → uniformly heat up to 300°C for 0.5 hour → keep warm at 300°C for 1 hour. The PAN of the composite white felt is cyclized to form a stable trapezoidal structure, and the ammonium dimolybdate on the composite white felt is decomposed into molybdenum oxide to obtain a composite pre-oxidized felt.

[0095] Step S4: placing the composite pre-oxidized felt in a carbonization furnace, and under an argon atmosphere, stepwise heating in the range of 500-1400°C. The stepwise heating program is: room temperature → uniform heating to 500°C for 2 hours → keeping at 500°C for 1 hour → uniform heating to 1400°C for 3 hours → keeping at 1400°C for 3 hours. The fibers on the composite pre-oxidized felt are carbonized to form carbon fibers, and the molybdenum oxide on the composite pre-oxidized felt is converted into molybdenum carbide to obtain a composite carbon felt.

[0096] Step S5: placing the composite carbon felt in a graphitization furnace, and under an argon atmosphere, stepwise heating in the range of 1600-2200°C. The stepwise heating program is: room temperature → uniform heating to 1600°C for 6 hours → keeping at 1600°C for 2 hours → uniform heating to 2200°C for 6 hours → keeping at 2200°C for 3 hours. The carbide on the composite carbon felt remains unchanged, and the carbon source of the carbon fibers of the composite carbon felt is rearranged into a graphite crystal structure to obtain a composite graphite felt.

[0097] Step S6: Under air atmosphere, the composite graphite felt is placed in a high-temperature steam reactor, and the temperature is controlled at 960°C, the steam flow rate is 2.5 L / min, the steam volume content is 10-50%, and the activation time is 2.6 h. The molybdenum carbide on the composite graphite felt is oxidized to molybdenum oxide, and finally a composite graphite felt modified with molybdenum oxide in situ is obtained.

[0098] Example 7: Using the following technical solution

[0099] A method for preparing a composite graphite felt electrode comprises the following steps:

[0100] Step S1: 15 parts by weight of polyacrylonitrile (PAN) was dissolved in 77 parts by weight of N,N-dimethylformamide (DMF) solvent, 8 parts by weight of ammonium heptamolybdate was added, and ultrasonic dispersion was performed to form a uniform spinning solution, and composite fiber precursors with a diameter of 20 μm were prepared by wet spinning.

[0101] Step S2: Using needle punching process (parameters: needle punching density 30 needles / cm 2 , needle speed 10cm / s, needle penetration depth 6mm) to make composite fiber raw yarn with thickness 6mm and density 0.3g / cm 3 Composite white felt.

[0102] Step S3: Place the composite white felt in a pre-oxidation furnace, and in an air atmosphere, stepwise heat up in the range of 200-300°C. The heating program is: room temperature → uniformly heat up to 200°C for 1 hour → keep warm at 200°C for 0.5 hour → uniformly heat up to 300°C for 0.5 hour → keep warm at 300°C for 1 hour. The PAN of the composite white felt is cyclized to form a stable trapezoidal structure, and the ammonium heptamolybdate on the composite white felt is decomposed into molybdenum oxide to obtain a composite pre-oxidized felt.

[0103] Step S4: placing the composite pre-oxidized felt in a carbonization furnace, and under an argon atmosphere, stepwise heating in the range of 500-1400°C. The stepwise heating program is: room temperature → uniform heating to 500°C for 2 hours → keeping at 500°C for 1 hour → uniform heating to 1400°C for 3 hours → keeping at 1400°C for 3 hours. The fibers on the composite pre-oxidized felt are carbonized to form carbon fibers, and the molybdenum oxide on the composite pre-oxidized felt is converted into molybdenum carbide to obtain a composite carbon felt.

[0104] Step S5: placing the composite carbon felt in a graphitization furnace, and under an argon atmosphere, stepwise heating in the range of 1600-2200°C. The stepwise heating program is: room temperature → uniform heating to 1600°C for 6 hours → keeping at 1600°C for 2 hours → uniform heating to 2200°C for 6 hours → keeping at 2200°C for 3 hours. The carbide on the composite carbon felt remains unchanged, and the carbon source of the carbon fibers of the composite carbon felt is rearranged into a graphite crystal structure to obtain a composite graphite felt.

[0105] Step S6: Under air atmosphere, the composite graphite felt is placed in a high-temperature steam reactor, and the temperature is controlled at 1000°C, the steam flow rate is 3L / min, the steam volume content is 10-50%, and the activation time is 3h. The molybdenum carbide on the composite graphite felt is oxidized to molybdenum oxide, and finally a composite graphite felt modified with molybdenum oxide in situ is obtained.

[0106] Comparative Example 1: Using the following technical solutions

[0107] The preparation process is the same as that of Example 4, except that ammonium metatungstate is not added in step S1.

[0108] The composite graphite felt electrodes prepared in Examples 1 to 7 and Comparative Example 1 were assembled on a stack for testing. The coulombic efficiency, voltage efficiency, and energy efficiency were measured and recorded under the same test conditions. The test results are shown in Table 1:

[0109] Table 1 Battery performance test table of composite graphite felt electrode assembled stack

[0110]

[0111] In the above test results, the corresponding electrodes of the battery stacks of Examples 1 to 7 are all composite graphite felt electrodes modified in situ with metal oxides. Since metal oxides can provide additional catalytic active sites, the synergistic catalytic effect of the oxygen-containing functional groups of the metal oxides significantly improves the electrode activity. The corresponding electrodes of the battery stack of Comparative Example 1 are ordinary graphite felt electrodes that are not modified with metal oxides, and the absence of carbides in the graphitization process results in a higher graphitization temperature of the carbon fiber. At the same graphitization temperature, the degree of crystallization of the graphite is not as good as that of Examples 1 to 7, that is, the conductivity is slightly worse. Therefore, in terms of voltage efficiency, Comparative Example 1 is lower than that of Examples 1 to 7.

[0112] In summary, the present invention provides a composite graphite felt electrode and a preparation method thereof. In the preparation stage, a metal source is mixed with PAN at the molecular level to achieve uniform distribution on the carbon fiber, thereby avoiding the particle agglomeration problem of traditional mechanical mixing. The metal oxide on the composite pre-oxidized felt reacts with part of the carbon source of the carbon fiber to transform into carbide, which has a certain etching effect on the carbon fiber and increases its surface area. During the graphitization process, the carbide remains stable and can serve as a heterogeneous nucleation point, thereby reducing the energy barrier for the formation of graphite layers, thereby promoting the formation of an ordered graphite structure at a lower temperature and improving the conductivity. High-temperature water vaporization During the gas activation stage, the carbide is regenerated in situ to form a metal oxide, which is highly chemically inert. This, combined with the densified structure of the carbon fibers, creates the stability of the composite graphite felt itself. Furthermore, the chemical bonding between the carbide and the carbon fibers creates the chemical bonding between the metal oxide and the carbon fibers, improving the physical and chemical stability of the composite graphite felt electrode. Controlled etching activated by high-temperature water vapor forms a multi-level porous structure of the carbon fibers, introducing oxygen-containing functional groups. Simultaneously, the metal oxide provides additional catalytically active sites. This metal oxide / oxygen-containing functional group synergistic catalysis significantly enhances electrode activity. The composite graphite felt electrode and its preparation method of the present invention address the problems of insufficient electrochemical activity, poor stability, and a cumbersome activation process in graphite felt electrodes used in vanadium batteries.

[0113] The foregoing description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Those skilled in the art will readily appreciate that various modifications and variations of the present invention are possible. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present invention are intended to be within the scope of protection of the present invention.

Claims

1. A method for preparing a composite graphite felt electrode, characterized in that: The preparation method comprises the following steps: Step S1: dissolving 5-15 parts by weight of polyacrylonitrile in 77-94 parts by weight of a solvent, adding 1-8 parts by weight of a metal source, and ultrasonically dispersing the mixture to form a uniform spinning solution, and wet spinning the resulting composite fiber precursor with a diameter of 6-20 μm; Step S2: preparing composite fiber precursors into composite white felt using a needle punching process; Step S3: placing the composite white felt in a pre-oxidation furnace and heating it according to a first heating program under an air atmosphere, so that the polyacrylonitrile on the composite white felt is cyclized to form a stable trapezoidal structure, and the metal source on the composite white felt is decomposed into metal oxides, thereby obtaining a composite pre-oxidation felt; Step S4: placing the composite preoxidized felt in a carbonization furnace, heating it according to a second heating program under an argon atmosphere, carbonizing the fibers on the composite preoxidized felt to form carbon fibers, and converting the metal oxides on the composite preoxidized felt into carbides after reacting with the carbon source of the carbon fibers on the composite preoxidized felt, thereby obtaining a composite carbon felt; Step S5: placing the composite carbon felt in a graphitization furnace under an argon atmosphere and heating the temperature according to a third heating program, wherein the carbides on the composite carbon felt remain unchanged and the carbon sources of the carbon fibers on the composite carbon felt are rearranged into a graphite crystal structure, thereby obtaining a composite graphite felt; Step S6: placing the composite graphite felt in a high-temperature steam reactor under air atmosphere, and oxidizing the carbides on the composite graphite felt into metal oxides again, thereby finally obtaining a composite graphite felt electrode in situ modified with metal oxides.

2. The preparation method according to claim 1, wherein: The solvent in step S1 is selected from at least one of dimethyl sulfoxide, N,N-dimethylformamide, N-methylpyrrolidone, and N,N-dimethylacetamide.

3. The preparation method according to claim 1, wherein: The metal source in step S1 is selected from at least one of ammonium metatungstate, ammonium dimolybdate, and ammonium heptamolybdate.

4. The preparation method according to claim 1, wherein: The needling forming process parameters in step S2 are: needling density 10-30 needles / cm², needle speed 5-10 cm / s, and needling depth 1-6 mm.

5. The preparation method according to claim 1, wherein: The composite white felt in step S2 has a thickness of 1-6 mm and a density of 0.1-0.3 g / cm 3 .

6. The preparation method according to claim 1, wherein: The reaction conditions of the high-temperature steam reactor in step S6 are: temperature 800-1000° C., steam flow rate 0.5-3 L / min, steam volume content 10-50%, and activation time 1-3 h.

7. The preparation method according to claim 1, wherein: The first temperature rising procedure in step S3 is: Starting from room temperature, the temperature was raised to 200°C at a constant speed for 1 hour; Keep at 200℃ for 0.5 hours; Starting from 200℃, the temperature was raised to 300℃ at a constant speed for 0.5 hours; Keep at 300°C for 1 hour.

8. The preparation method according to claim 1, wherein: The second temperature rising procedure in step S4 is: Starting from room temperature, the temperature was raised to 500°C at a constant rate for 2 hours; Keep at 500℃ for 1 hour; Starting from 500℃, the temperature was raised to 1400℃ at a constant speed within 3 hours; Keep at 1400℃ for 3 hours.

9. The preparation method according to claim 1, wherein: The third heating procedure in step S5 is: Starting from room temperature, the temperature was raised to 1600°C at a constant rate for 6 hours; Keep at 1600℃ for 2 hours; Starting from 1600℃, the temperature was raised to 2200℃ at a constant speed for 6 hours; Keep at 2200℃ for 3 hours.

10. A composite graphite felt electrode, characterized in that: The composite graphite felt electrode is prepared by using a composite graphite felt electrode preparation method as described in any one of claims 1 to 9.

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