Composite graphite felt electrode and preparation method thereof

Abstract

The preparation method at least comprises the following steps: dissolving polyacrylonitrile in a solvent, adding a metal source, carrying out ultrasonic dispersion to form a uniform spinning solution, carrying out wet spinning to prepare a composite fiber precursor, preparing the fiber precursor into a composite white felt by using a needling molding process, and carrying out high-temperature sintering to obtain the composite graphite felt electrode. And carrying out pre-oxidation, carbonization, graphitization and high-temperature water vapor activation to obtain the high-stability composite graphite felt electrode. All the steps can be compatible with an existing graphite felt production line and an existing graphite felt high-temperature activation line, complex equipment does not need to be transformed, and preparation of the metal oxide in-situ modified composite graphite felt electrode is synchronously achieved in the graphite felt production and activation process. According to the prepared composite graphite felt electrode, the electrochemical activity of the electrode is greatly improved through the synergistic catalysis effect of the metal oxide and the oxygen-containing functional group.

Description

Technical Field

[0001] The invention relates to the technical field of all-vanadium liquid 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] At present, there are three main activation and modification measures commonly used: surface functionalization, deposition of metal or transition metal oxides, and bulk doping of 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 transition 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 not 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 by the present invention is as follows: A method for preparing a composite graphite felt electrode, the method comprising the following steps:

[0007] Step S1: Dissolve 5 - 15 parts by weight of polyacrylonitrile in 77 - 94 parts by weight of a solvent, add 1 - 8 parts by weight of a metal source, and ultrasonically disperse to form a uniform spinning solution. Then, produce a composite fiber precursor with a diameter of 6 - 20 μm through wet spinning. Step S2: Prepare a composite white felt from the composite fiber precursor using a needle punching forming process. Step S3: Place the composite white felt in a pre - oxidation furnace. Under an air atmosphere, heat it according to a first heating program. The polyacrylonitrile on the composite white felt cyclizes to form a stable ladder structure, and the metal source on the composite white felt decomposes into metal oxides to obtain a composite pre - oxidized felt. Step S4: Place the composite pre - oxidized felt in a carbonization furnace. Under an argon atmosphere, heat it according to a second heating program. The fibers on the composite pre - oxidized felt are carbonized to form carbon fibers, and the metal oxides on the composite pre - oxidized felt react with a part of the carbon source of the carbon fibers on the composite pre - oxidized felt and then transform into carbides to obtain a composite carbon felt. Step S5: Place the composite carbon felt in a graphitization furnace. Under an argon atmosphere, heat it according to a third heating program. The carbides on the composite carbon felt remain unchanged, and the carbon source of the carbon fibers on the composite carbon felt rearranges into a graphite crystal structure to obtain a composite graphite felt. Step S6: Under an air atmosphere, place the composite graphite felt in a high - temperature steam reaction furnace. The carbides on the composite graphite felt are oxidized into metal oxides again, and finally, a composite graphite felt electrode modified in - situ with metal oxides is obtained.

[0008] Further, the solvent in Step S1 is selected from at least one of dimethyl sulfoxide, N,N - dimethylformamide, N - methylpyrrolidone, and N,N - dimethylacetamide.

[0009] Further, the metal source in Step S1 is selected from at least one of ammonium metatungstate, ammonium dimolybdate, and ammonium heptamolybdate.

[0010] Further, the parameters of the needle punching forming process in Step S2 are: the needle punching density is 10 - 30 needles / cm², the needle running speed is 5 - 10 cm / s, and the needle punching depth is 1 - 6 mm.

[0011] Further, the thickness of the composite white felt in Step S2 is 1 - 6 mm and the density is 0.1 - 0.3 g / cm 3 。

[0012] Further, the reaction conditions of the high - temperature steam reaction furnace 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.

[0013] Further, the first heating program in Step S3 is: Starting from room temperature, heat up at a uniform speed to 200°C over 1 hour; Keep the temperature at 200°C for 0.5 hour; Starting from 200°C, heat up at a uniform speed to 300°C over 0.5 hour; Keep the temperature at 300°C for 1 hour.

[0014] The first heating program in this embodiment aims to avoid uneven heating of the fibers in the composite white felt during heating by controlling the rate 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 gradually rises instead of being heated rapidly and concentratedly, to avoid fiber breakage due to uneven thermal expansion. At around 200°C, the fiber filaments start to undergo a cyclization reaction, meaning the formation of a cyclic structure of the molecular chain, which increases the strength and heat resistance of the fibers. At around 300°C, the fiber filaments will start to undergo an oxidation reaction, which is usually due to the reaction of oxygen molecules with the fibers at high temperatures, and can improve the stability of the fibers.

[0015] Furthermore, the second heating program in step S4 is as follows: Starting from room temperature, heat up at a uniform speed to 500°C over 2 hours; Keep the temperature at 500°C for 1 hour; Starting from 500°C, heat up at a uniform speed to 1400°C over 3 hours; Keep the temperature at 1400°C for 3 hours.

[0016] The second heating program in this embodiment uses stepwise heating (phased heating at 500°C and 1400°C). The composite pre-oxidized felt carries a metal source, and the metal source transforms from a larger particle size to a metal oxide with a smaller particle size and higher crystallinity. This process helps to enhance the reactivity of the metal oxide and the effect of subsequent carbonization reactions. At 1400°C, the metal oxide undergoes a carbothermal reduction reaction to form metal carbides. These carbides usually have a smaller particle size and higher crystallinity, and have higher thermal stability, suitable for high-temperature environments. The composite pre-oxidized felt undergoes decomposition reactions and carbonization reactions at 500°C and 1400°C respectively, generating graphite microcrystals and stable carbides. This not only improves the thermal stability of the material but also increases its mechanical strength. Stepwise heating ensures the uniformity and refinement of carbide formation, while enhancing the overall performance of the carbon felt.

[0017] Furthermore, the third heating program in step S5 is as follows: Starting from room temperature, heat up at a uniform speed to 1600°C over 6 hours; Keep the temperature at 1600°C for 2 hours; Starting from 1600°C, heat up at a uniform speed to 2200°C over 6 hours; Keep it at 2200°C for 3 hours.

[0018] The third heating program in this embodiment needs to ensure the stable existence of WC particles (tungsten carbide particles) without adverse changes. WC particles are usually used to enhance the hardness and wear resistance of materials. In a high-temperature environment, WC particles may undergo physical or chemical changes. Therefore, an appropriate heating program is required to avoid the decomposition or structural change of these particles. During the high-temperature insulation stages at 1600°C and 2200°C, due to the change in carbon concentration during the graphitization of carbon fibers, the stability of WC particles is maintained. The temperature is gradually increased during the heating process, and it is ensured that the temperature change is slow to avoid the impact of temperature fluctuations on WC particles.

[0019] Another technical solution adopted by the present invention is as follows: A composite graphite felt electrode, which is prepared by using the preparation method of a composite graphite felt electrode as described above.

[0020] The beneficial effects of the present invention at least include: 1. The metal source is molecularly blended with PAN to achieve its uniform distribution on carbon fibers, avoiding the problem of particle agglomeration in traditional mechanical mixing; the metal oxides on the composite pre-oxidized felt react with part of the carbon source of the carbon fibers and are transformed into carbides, which have a certain etching effect on the carbon fibers and increase their surface area; during the graphitization process, the carbides remain stably present and can serve as heterogeneous nucleation sites, 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 conductivity.

[0021] 2. During the high-temperature steam activation stage, the carbides are in-situ regenerated into metal oxides again. They have high chemical inertness. Coupled with the densified structure of the carbon fibers, it creates the stability of the composite graphite felt body. Moreover, the chemical bonding between the carbides and the carbon fibers results in the chemical bonding between the metal oxides and the carbon fibers, enhancing the physical and chemical stability of the composite graphite felt electrode.

[0022] 3. Through the controllable etching of high-temperature steam activation, a hierarchical pore structure of carbon fibers is formed and oxygen-containing functional groups are introduced. At the same time, the metal oxides can provide additional catalytic active sites. This synergistic catalytic effect of metal oxides / oxygen-containing functional groups significantly improves the electrode activity.

[0023] 4. All steps are compatible with the existing graphite felt production line and graphite felt high-temperature activation line, without complex equipment modification, and the preparation of a composite graphite felt with in-situ modification of metal oxides is realized synchronously during the production and activation of graphite felt. Description of the Drawings

[0024] Figure 1 It is a flowchart for the preparation of a composite graphite felt electrode of the present invention; Figure 2 Schematic diagram of the structure of a composite graphite felt electrode of the present invention; Figure 3 Scanning electron micrograph of a composite graphite felt electrode of the present invention; Figure 4 is Figure 3 Partial enlarged view of. Detailed implementation mode

[0025] The following description of at least one exemplary embodiment is merely illustrative in nature and is in no way a limitation on the present invention or its application or use. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts fall within the scope of protection of the present invention.

[0026] As Figure 1 shown, polyacrylonitrile PAN is dissolved in a solvent, a metal source is added, and ultrasonic dispersion is carried out to form a uniform spinning solution. The composite fiber precursor is made by wet spinning, and the composite fiber precursor is made into a composite white felt by using a needle punching forming process; the composite white felt is placed in a pre-oxidation furnace, and the temperature is increased step by step in the range of 200 - 300 °C under an air atmosphere. The PAN 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 to obtain a composite pre-oxidized felt; the composite pre-oxidized felt is placed in a carbonization furnace, and the temperature is increased step by step in the range of 500 - 1400 °C under an argon atmosphere. The fibers of the composite pre-oxidized felt are carbonized, and the metal oxides on the composite pre-oxidized felt react with part of the carbon source of the carbon fiber and then turn into carbides to obtain a composite carbon felt; the composite carbon felt is placed in a graphitization furnace, and the temperature is increased step by step in the range of 1600 - 2200 °C under an argon atmosphere. The carbon atoms in the composite carbon felt are rearranged into a graphite crystal structure to obtain a composite graphite felt; under an air atmosphere, the composite graphite felt is placed in a high-temperature steam reaction furnace for an activation step. The carbides on the composite graphite felt are oxidized into metal oxides again, and finally a composite graphite felt electrode modified in-situ with metal oxides is obtained, as Figure 2 shown.

[0027] Example 1: The following technical solution is adopted A method for preparing a composite graphite felt electrode, comprising the following steps: Step S1: Dissolve 5 parts by weight of polyacrylonitrile (PAN) in 94 parts by weight of dimethyl sulfoxide (DMSO) solvent, add 1 part by weight of ammonium metatungstate, and perform ultrasonic dispersion to form a uniform spinning solution. The composite fiber precursor with a diameter of 6 μm is made by wet spinning.

[0028] Step S2: Use a needle punching forming process (parameters: needle punching density 10 needles / cm 2 , needle traveling speed 5 cm / s, needle punching depth 1 mm) to make the composite fiber precursor into a composite white felt with a thickness of 1 mm and a density of 0.1 g / cm3 Composite white felt

[0029] Step S3: Place the composite white felt in a pre-oxidation furnace. Under an air atmosphere, increase the temperature in a stepped manner within the range of 200 - 300 °C. The temperature increase program is: room temperature → uniformly increase the temperature to 200 °C in 1 h → keep the temperature at 200 °C for 0.5 h → uniformly increase the temperature to 300 °C in 0.5 h → keep the temperature at 300 °C for 1 h. The PAN on the composite white felt cyclizes to form a stable ladder structure, and ammonium metatungstate on the composite white felt decomposes into tungsten oxide (WO3) to obtain a composite pre-oxidized felt.

[0030] Step S4: Place the composite pre-oxidized felt in a carbonization furnace. Under an argon atmosphere, increase the temperature in a stepped manner within the range of 500 - 1400 °C. The stepped temperature increase program is: room temperature → uniformly increase the temperature to 500 °C in 2 h → keep the temperature at 500 °C for 1 h → uniformly increase the temperature to 1400 °C in 3 h → keep the temperature at 1400 °C for 3 h. The fibers on the composite pre-oxidized felt are carbonized to form carbon fibers, and WO3 on the composite pre-oxidized felt is transformed into tungsten carbide (WC) to obtain a composite carbon felt.

[0031] Step S5: Place the composite carbon felt in a graphitization furnace. Under an argon atmosphere, increase the temperature in a stepped manner within the range of 1600 - 2200 °C. The stepped temperature increase program is: room temperature → uniformly increase the temperature to 1600 °C in 6 h → keep the temperature at 1600 °C for 2 h → uniformly increase the temperature to 2200 °C in 6 h → keep the temperature at 2200 °C for 3 h. The carbides on the composite carbon felt remain 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.

[0032] Step S6: Under an air atmosphere, place the composite graphite felt in a high-temperature steam reaction furnace, control the temperature 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 h. The WC on the composite graphite felt is oxidized to WO3, and finally a composite graphite felt electrode modified in-situ with tungsten oxide is obtained, as Figure 3 and Figure 4 shown. The tungsten oxide particles are fine and evenly attached to the carbon fibers.

[0033] Example 2: The following technical solution is adopted A method for preparing a composite graphite felt electrode, comprising the following steps: Step S1: Dissolve 7 parts by weight of polyacrylonitrile (PAN) in 94 parts by weight of N,N-dimethylformamide (DMF) solvent, add 3 parts by weight of ammonium dimolybdate, and ultrasonically disperse to form a uniform spinning solution. Then, make composite fiber filaments with a diameter of 8 μm through wet spinning.

[0034] Step S2: Use the needle punching forming process (parameters: needle punching density 14 needles / cm 2 , needle running speed 6 cm / s, needle punching depth 2 mm) to make the composite fiber filaments into a thickness of 2 mm and a density of 0.15 g / cm3 Composite white felt

[0035] Step S3: Place the composite white felt in a pre-oxidation furnace. Under an air atmosphere, increase the temperature stepwise within the range of 200 - 300 °C. The heating program is: room temperature → uniformly heat up to 200 °C in 1 h → keep the temperature at 200 °C for 0.5 h → uniformly heat up to 300 °C in 0.5 h → keep the temperature at 300 °C for 1 h. The PAN on the composite white felt cyclizes to form a stable ladder structure, and ammonium dimolybdate on the composite white felt decomposes into molybdenum oxide, obtaining a composite pre-oxidized felt.

[0036] Step S4: Place the composite pre-oxidized felt in a carbonization furnace. Under an argon atmosphere, increase the temperature stepwise within the range of 500 - 1400 °C. The stepwise heating program is: room temperature → uniformly heat up to 500 °C in 2 h → keep the temperature at 500 °C for 1 h → uniformly heat up to 1400 °C in 3 h → keep the temperature at 1400 °C for 3 h. The fibers on the composite pre-oxidized felt are carbonized to form carbon fibers, and molybdenum oxide on the composite pre-oxidized felt is transformed into molybdenum carbide, obtaining a composite carbon felt.

[0037] Step S5: Place the composite carbon felt in a graphitization furnace. Under an argon atmosphere, increase the temperature stepwise within the range of 1600 - 2200 °C. The stepwise heating program is: room temperature → uniformly heat up to 1600 °C in 6 h → keep the temperature at 1600 °C for 2 h → uniformly heat up to 2200 °C in 6 h → keep the temperature at 2200 °C for 3 h. The carbides on the composite carbon felt remain unchanged, and the carbon source of the carbon fibers on the composite carbon felt is rearranged into a graphite crystal structure, obtaining a composite graphite felt.

[0038] Step S6: Under an air atmosphere, place the composite graphite felt in a high-temperature steam reaction furnace, control the temperature at 840 °C, the steam flow rate at 1 L / min, the steam volume content at 10 - 50%, and the activation time at 1.2 h. The molybdenum carbide on the composite graphite felt is oxidized into molybdenum oxide, and finally a composite graphite felt modified in-situ with molybdenum oxide is obtained.

[0039] Example 3: Adopt the following technical solution A method for preparing a composite graphite felt electrode, comprising the following steps: Step S1: Dissolve 9 parts by weight of polyacrylonitrile (PAN) in 87 parts by weight of N-methylpyrrolidone (NMP) solvent, add 4 parts by weight of ammonium heptamolybdate, and ultrasonically disperse to form a uniform spinning solution. Then, make composite fiber filaments with a diameter of 10 μm through wet spinning.

[0040] Step S2: Use the needle punching forming process (parameters: needle punching density 18 needles / cm 2 , needle traveling speed 7 cm / s, needle punching depth 3 mm) to make the composite fiber filaments into a composite white felt with a thickness of 3 mm and a density of 0.2 g / cm 3

[0041] ​Step S3: Place the composite white felt in a pre-oxidation furnace. Under an air atmosphere at 5, increase the temperature stepwise within the range of 200 - 300 °C. The heating program is as follows: room temperature → uniformly heat to 200 °C in 1 h → hold at 200 °C for 0.5 h → uniformly heat to 300 °C in 0.5 h → hold at 300 °C for 1 h. The PAN on the composite white felt cyclizes to form a stable ladder structure, and ammonium heptamolybdate on the composite white felt decomposes into molybdenum oxide, obtaining a composite pre-oxidized felt.

[0042] Step S4: Place the composite pre-oxidized felt in a carbonization furnace. Under an argon atmosphere, increase the temperature stepwise within the range of 500 - 1400 °C. The stepwise heating program is as follows: room temperature → uniformly heat to 500 °C in 2 h → hold at 500 °C for 1 h → uniformly heat to 1400 °C in 3 h → hold at 1400 °C for 3 h. The fibers on the composite pre-oxidized felt are carbonized to form carbon fibers, and molybdenum oxide on the composite pre-oxidized felt is transformed into molybdenum carbide, obtaining a composite carbon felt.

[0043] Step S5: Place the composite carbon felt in a graphitization furnace. Under an argon atmosphere, increase the temperature stepwise within the range of 1600 - 2200 °C. The stepwise heating program is as follows: room temperature → uniformly heat to 1600 °C in 6 h → hold at 1600 °C for 2 h → uniformly heat to 2200 °C in 6 h → hold at 2200 °C for 3 h. The carbides on the composite carbon felt remain unchanged, and the carbon source of the carbon fibers on the composite carbon felt is rearranged into a graphite crystal structure, obtaining a composite graphite felt.

[0044] Step S6: Under an air atmosphere, place the composite graphite felt in a high-temperature steam reaction furnace, control the temperature at 880 °C, the steam flow rate at 1.5 L / min, the steam volume content at 10 - 50%, and the activation time at 1.6 h. The molybdenum carbide on the composite graphite felt is oxidized into molybdenum oxide, and finally, a composite graphite felt modified in-situ with molybdenum oxide is obtained.

[0045] Example 4: Adopt the following technical solution A method for preparing a composite graphite felt electrode, comprising the following steps: Step S1: Dissolve 10 parts by weight of polyacrylonitrile (PAN) in 85 parts by weight of N,N-dimethylacetamide (DMAC) solvent, add 5 parts by weight of ammonium metatungstate, and ultrasonically disperse to form a uniform spinning solution. Then, prepare composite fiber filaments with a diameter of 12 μm through wet spinning.

[0046] Step S2: Use a needle punching forming process (parameters: needle punching density 20 needles / cm 2 , needle traveling speed 8 cm / s, needle punching depth 4 mm) to form the composite fiber filaments into a composite white felt with a thickness of 4 mm and a density of 0.2 g / cm 3 .

[0047] Step S3: Place the composite white felt in a pre-oxidation furnace. Under an air atmosphere, raise the temperature in a stepped manner within the range of 200 - 300 °C. The heating program is as follows: room temperature → uniformly heat to 200 °C in 1 h → hold at 200 °C for 0.5 h → uniformly heat to 300 °C in 0.5 h → hold at 300 °C for 1 h. The PAN on the composite white felt cyclizes to form a stable ladder structure, and ammonium metatungstate on the composite white felt decomposes into tungsten oxide (WO3) to obtain a composite pre-oxidized felt.

[0048] Step S4: Place the composite pre-oxidized felt in a carbonization furnace. Under an argon atmosphere, raise the temperature in a stepped manner within the range of 500 - 1400 °C. The stepped heating program is as follows: room temperature → uniformly heat to 500 °C in 2 h → hold at 500 °C for 1 h → uniformly heat to 1400 °C in 3 h → hold at 1400 °C for 3 h. The fibers on the composite pre-oxidized felt are carbonized to form carbon fibers, and WO3 on the composite pre-oxidized felt is transformed into tungsten carbide (WC) to obtain a composite carbon felt.

[0049] Step S5: Place the composite carbon felt in a graphitization furnace. Under an argon atmosphere, raise the temperature in a stepped manner within the range of 1600 - 2200 °C. The stepped heating program is as follows: room temperature → uniformly heat to 1600 °C in 6 h → hold at 1600 °C for 2 h → uniformly heat to 2200 °C in 6 h → hold at 2200 °C for 3 h. The carbides on the composite carbon felt remain 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.

[0050] Step S6: Under an air atmosphere, place the composite graphite felt in a high-temperature steam reaction furnace, control the temperature 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. WC on the composite graphite felt is oxidized to WO3, and finally a composite graphite felt modified in-situ with tungsten oxide is obtained.

[0051] Example 5: Adopt the following technical solution A method for preparing a composite graphite felt electrode, comprising the following steps: Step S1: Dissolve 11 parts by weight of polyacrylonitrile (PAN) in 83 parts by weight of N,N-dimethylacetamide (DMAC) solvent, add 6 parts by weight of ammonium metatungstate, and ultrasonically disperse to form a uniform spinning solution. Then, make composite fiber filaments with a diameter of 14 μm through wet spinning.

[0052] Step S2: Use a needling forming process (parameters: needling density 22 needles / cm 2 , needle moving speed 8 cm / s, needling depth 4 mm) to make the composite fiber filaments into a composite white felt with a thickness of 4 mm and a density of 0.25 g / cm 3 .

[0053] Step S3: Place the composite white felt in a pre-oxidation furnace. Under an air atmosphere, increase the temperature step by step within the range of 200 - 300 °C. The temperature increase program is: room temperature → increase the temperature uniformly to 200 °C in 1 h → hold at 200 °C for 0.5 h → increase the temperature uniformly to 300 °C in 0.5 h → hold at 300 °C for 1 h. The PAN on the composite white felt undergoes cyclization to form a stable ladder structure, and ammonium metatungstate on the composite white felt decomposes into tungsten oxide (WO3) to obtain a composite pre-oxidized felt.

[0054] Step S4: Place the composite pre-oxidized felt in a carbonization furnace. Under an argon atmosphere, increase the temperature step by step within the range of 500 - 1400 °C. The temperature increase program is: room temperature → increase the temperature uniformly to 500 °C in 2 h → hold at 500 °C for 1 h → increase the temperature uniformly to 1400 °C in 3 h → hold at 1400 °C for 3 h. The fibers on the composite pre-oxidized felt are carbonized to form carbon fibers, and WO3 on the composite pre-oxidized felt is transformed into tungsten carbide (WC) to obtain a composite carbon felt.

[0055] Step S5: Place the composite carbon felt in a graphitization furnace. Under an argon atmosphere, increase the temperature step by step within the range of 1600 - 2200 °C. The temperature increase program is: room temperature → increase the temperature uniformly to 1600 °C in 6 h → hold at 1600 °C for 2 h → increase the temperature uniformly to 2200 °C in 6 h → hold at 2200 °C for 3 h. The carbides on the composite carbon felt remain 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.

[0056] Step S6: Under an air atmosphere, place the composite graphite felt in a high-temperature steam reaction furnace, control the temperature at 920 °C, the steam flow rate at 2 L / min, the steam volume content at 10 - 50%, and the activation time at 2.2 h. The WC on the composite graphite felt is oxidized to WO3, and finally a composite graphite felt modified in-situ with tungsten oxide is obtained.

[0057] Example 6: The following technical solution is adopted A method for preparing a composite graphite felt electrode, comprising the following steps: Step S1: Dissolve 13 parts by weight of polyacrylonitrile (PAN) in 80 parts by weight of dimethyl sulfoxide (DMSO) solvent, add 7 parts by weight of ammonium dimolybdate, and ultrasonically disperse to form a uniform spinning solution. Then, make composite fiber filaments with a diameter of 17 μm through wet spinning.

[0058] Step S2: Use a needle punching forming process (parameters: needle punching density 26 needles / cm 2 , needle moving speed 9 cm / s, needle punching depth 5 mm) to make the composite fiber filaments into a composite white felt with a thickness of 5 mm and a density of 0.25 g / cm 3 .

[0059] Step S3: Place the composite white felt in a pre-oxidation furnace. Under an air atmosphere, raise the temperature stepwise within the range of 200 - 300 °C. The heating program is as follows: room temperature → uniformly heat to 200 °C in 1 h → keep the temperature at 200 °C for 0.5 h → uniformly heat to 300 °C in 0.5 h → keep the temperature at 300 °C for 1 h. The PAN on the composite white felt cyclizes to form a stable ladder structure, and ammonium dimolybdate on the composite white felt decomposes into molybdenum oxide, obtaining a composite pre-oxidized felt.

[0060] Step S4: Place the composite pre-oxidized felt in a carbonization furnace. Under an argon atmosphere, raise the temperature stepwise within the range of 500 - 1400 °C. The stepwise heating program is as follows: room temperature → uniformly heat to 500 °C in 2 h → keep the temperature at 500 °C for 1 h → uniformly heat to 1400 °C in 3 h → keep the temperature at 1400 °C for 3 h. The fibers on the composite pre-oxidized felt are carbonized to form carbon fibers, and molybdenum oxide on the composite pre-oxidized felt is transformed into molybdenum carbide, obtaining a composite carbon felt.

[0061] Step S5: Place the composite carbon felt in a graphitization furnace. Under an argon atmosphere, raise the temperature stepwise within the range of 1600 - 2200 °C. The stepwise heating program is as follows: room temperature → uniformly heat to 1600 °C in 6 h → keep the temperature at 1600 °C for 2 h → uniformly heat to 2200 °C in 6 h → keep the temperature at 2200 °C for 3 h. The carbides on the composite carbon felt remain unchanged, and the carbon source of the carbon fibers on the composite carbon felt is rearranged into a graphite crystal structure, obtaining a composite graphite felt.

[0062] Step S6: Under an air atmosphere, place the composite graphite felt in a high-temperature steam reaction furnace, control the temperature at 960 °C, the steam flow rate at 2.5 L / min, the steam volume content at 10 - 50%, and the activation time at 2.6 h. The molybdenum carbide on the composite graphite felt is oxidized into molybdenum oxide, and finally, a composite graphite felt modified in-situ with molybdenum oxide is obtained.

[0063] Example 7: Adopt the following technical solution A method for preparing a composite graphite felt electrode, comprising the following steps: Step S1: Dissolve 15 parts by weight of polyacrylonitrile (PAN) in 77 parts by weight of N,N-dimethylformamide (DMF) solvent, add 8 parts by weight of ammonium heptamolybdate, and ultrasonically disperse to form a uniform spinning solution, and prepare composite fiber filaments with a diameter of 20 μm through wet spinning.

[0064] Step S2: Use the needle punching forming process (parameters: needle punching density 30 needles / cm 2 , needle traveling speed 10 cm / s, needle punching depth 6 mm) to form the composite fiber filaments into a composite white felt with a thickness of 6 mm and a density of 0.3 g / cm 3 3.

[0065] Step S3: Place the composite white felt in a pre-oxidation furnace. Under an air atmosphere, increase the temperature step by step within the range of 200 - 300 °C. The heating program is as follows: room temperature → uniformly heat to 200 °C in 1 h → keep the temperature at 200 °C for 0.5 h → uniformly heat to 300 °C in 0.5 h → keep the temperature at 300 °C for 1 h. The PAN on the composite white felt cyclizes to form a stable ladder structure, and ammonium heptamolybdate on the composite white felt decomposes into molybdenum oxide, obtaining a composite pre-oxidized felt.

[0066] Step S4: Place the composite pre-oxidized felt in a carbonization furnace. Under an argon atmosphere, increase the temperature step by step within the range of 500 - 1400 °C. The stepwise heating program is as follows: room temperature → uniformly heat to 500 °C in 2 h → keep the temperature at 500 °C for 1 h → uniformly heat to 1400 °C in 3 h → keep the temperature at 1400 °C for 3 h. The fibers on the composite pre-oxidized felt are carbonized to form carbon fibers, and molybdenum oxide on the composite pre-oxidized felt is transformed into molybdenum carbide, obtaining a composite carbon felt.

[0067] Step S5: Place the composite carbon felt in a graphitization furnace. Under an argon atmosphere, increase the temperature step by step within the range of 1600 - 2200 °C. The stepwise heating program is as follows: room temperature → uniformly heat to 1600 °C in 6 h → keep the temperature at 1600 °C for 2 h → uniformly heat to 2200 °C in 6 h → keep the temperature at 2200 °C for 3 h. The carbides on the composite carbon felt remain unchanged, and the carbon source of the carbon fibers on the composite carbon felt is rearranged into a graphite crystal structure, obtaining a composite graphite felt.

[0068] Step S6: Under an air atmosphere, place the composite graphite felt in a high-temperature steam reaction furnace, control the temperature at 1000 °C, the steam flow rate at 3 L / min, the steam volume content at 10 - 50%, and the activation time at 3 h. The molybdenum carbide on the composite graphite felt is oxidized into molybdenum oxide, and finally a composite graphite felt modified in-situ with molybdenum oxide is obtained.

[0069] Comparative Example 1: Adopt the following technical scheme The preparation process is the same as that of Example 4, except that ammonium metatungstate is not added in Step S1.

[0070] Assemble the composite graphite felt electrodes prepared in Examples 1 - 7 and Comparative Example 1 on the stack for testing, and test and record the Coulomb efficiency, voltage efficiency, and energy efficiency under the same test conditions. The test results are shown in Table 1: Table 1 Battery performance test table of composite graphite felt electrode assembled stack

[0071] Among the above test results, the corresponding electrodes of the single cells in Examples 1 - 7 are composite graphite felt electrodes modified in-situ with metal oxides. Since metal oxides can provide additional catalytic active sites, the synergistic catalytic effect of such metal oxide oxygen-containing functional groups significantly improves the electrode activity. The corresponding electrode of the single cell in Comparative Example 1 is an ordinary graphite felt electrode not modified with metal oxides, and the absence of carbides during the graphitization process results in a relatively high graphitization temperature for carbon fibers. At the same graphitization temperature, the crystallinity of graphite is inferior to that in Examples 1 - 7, that is, the conductivity is slightly worse. Therefore, in terms of voltage efficiency, Comparative Example 1 is less than Examples 1 - 7.

[0072] In summary, for a composite graphite felt electrode and its preparation method of the present invention, during the preparation stage, a metal source is molecularly blended with PAN to achieve its uniform distribution on carbon fibers, avoiding the problem of particle agglomeration in traditional mechanical mixing; the metal oxides on the composite pre-oxidized felt react with part of the carbon source of the carbon fibers and are transformed into carbides, which have a certain etching effect on the carbon fibers and increase their surface area; during the graphitization process, the carbides remain stably present and can serve as heterogeneous nucleation sites, 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; in the high-temperature steam activation stage, the carbides are in-situ regenerated into metal oxides, which have high chemical inertness. Coupled with the densified structure of the carbon fibers, the stability of the composite graphite felt body is achieved, and the chemical bonding between the carbides and the carbon fibers results in the chemical bonding between the metal oxides and the carbon fibers, enhancing the physical and chemical stability of the composite graphite felt electrode; through the controllable etching by high-temperature steam activation, a hierarchical pore structure of carbon fibers is formed and oxygen-containing functional groups are introduced. At the same time, metal oxides can provide additional catalytic active sites, and this synergistic catalytic effect of metal oxides / oxygen-containing functional groups significantly improves the electrode activity. A composite graphite felt electrode and its preparation method of the present invention solve the problems of insufficient electrochemical activity, poor stability, and cumbersome activation process of graphite felt electrodes used in vanadium batteries.

[0073] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, various modifications and changes can be made to the present invention. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A method for preparing a composite graphite felt electrode, characterized in that, The preparation method includes the following steps: Step S1: Dissolve 5 - 15 parts by weight of polyacrylonitrile in 77 - 94 parts by weight of a solvent, add 1 - 8 parts by weight of a metal source, and ultrasonically disperse to form a uniform spinning solution. Then, make composite fiber precursor filaments with a diameter of 6 - 20 μm through wet spinning. Step S2: Prepare a composite white felt from the composite fiber precursor filaments using a needle punching forming process. Step S3: Place the composite white felt in a pre - oxidation furnace. Under an air atmosphere, heat it according to the first heating program. The polyacrylonitrile on the composite white felt cyclizes to form a stable ladder structure, and the metal source on the composite white felt decomposes into metal oxides to obtain a composite pre - oxidized felt. Step S4: Place the composite pre - oxidized felt in a carbonization furnace. Under an argon atmosphere, heat it according to the second heating program. The fibers on the composite pre - oxidized felt are carbonized to form carbon fibers, and the metal oxides on the composite pre - oxidized felt react with the carbon source of the carbon fibers on the composite pre - oxidized felt and then turn into carbides to obtain a composite carbon felt. Step S5: Place the composite carbon felt in a graphitization furnace. Under an argon atmosphere, heat it according to the third heating program. The carbides on the composite carbon felt remain unchanged, and the carbon source of the carbon fibers on the composite carbon felt rearranges into a graphite crystal structure to obtain a composite graphite felt. Step S6: Under an air atmosphere, place the composite graphite felt in a high - temperature water vapor reaction furnace. The carbides on the composite graphite felt are oxidized into metal oxides again, and finally, a composite graphite felt electrode modified in situ with metal oxides is obtained.

2. The preparation method according to claim 1, characterized in that: 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, characterized in that: 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 parameters of the needle punching forming process in Step S2 are: needle punching density 10 - 30 needles / cm², needle moving speed 5 - 10 cm / s, and needle punching depth 1 - 6 mm.

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

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

7. The preparation method according to claim 1, characterized in that: The first heating program in Step S3 is as follows: Start from room temperature and heat up to 200 °C at a constant speed for 1 hour. Keep the temperature at 200 °C for 0.5 hour. Start from 200 °C and heat up to 300 °C at a constant speed for 0.5 hour. Keep the temperature at 300 °C for 1 hour.

8. The preparation method according to claim 1, characterized in that: The second heating program in Step S4 is as follows: Start from room temperature and heat up to 500 °C at a constant speed for 2 hours. Keep the temperature at 500 °C for 1 hour. Start from 500 °C and heat up to 1400 °C at a constant speed for 3 hours. Keep the temperature at 1400 °C for 3 hours.

9. The preparation method according to claim 1, wherein: The third heating program in Step S5 is as follows: Start from room temperature and heat up to 1600 °C at a constant speed for 6 hours. Keep the temperature at 1600 °C for 2 hours. Start from 1600 °C and heat up to 2200 °C at a constant speed for 6 hours. Keep the temperature at 2200 °C for 3 hours.

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

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