Method for activating surface of graphite felt and removing amorphous carbon by utilizing photothermal effect

Abstract

The invention belongs to the technical field of all-vanadium redox flow batteries, and particularly relates to a method for activating the surface of graphite felt and removing amorphous carbon by utilizing a photothermal effect, which comprises the following steps of: 1, flattening an unactivated semi-finished graphite felt and then putting the unactivated semi-finished graphite felt into a transparent container; 2, introducing oxidizing gas meeting requirements into the transparent container, and adjusting the atmosphere in the transparent container to be the atmosphere of the oxidizing gas; and 3, arranging a light source above the transparent container to illuminate the upper surface exposed after the semi-finished graphite felt is flattened, continuously introducing the oxidizing gas into the transparent container, continuously illuminating for a period of time under the condition of keeping the atmosphere of the oxidizing gas, stopping illumination, removing amorphous carbon of the graphite felt, and activating the surface of the graphite felt. According to the method, the surface of the amorphous carbon activated graphite felt is removed, and meanwhile the technical problems that in the prior art, the process is complex, energy consumption is high, and pollution is generated by used chemical reagents are solved.

Description

Technical Field

[0001] The invention belongs to the technical field of all-vanadium liquid flow batteries, and in particular relates to a method for activating the surface of graphite felt and removing amorphous carbon by utilizing photothermal effect. Background Art

[0002] At present, the most widely used electrode material for all-vanadium liquid flow batteries is graphite felt, which is a porous felt-like fiber material woven from carbon fiber. It has a wide range of three-dimensional network structures, high specific surface area, good electrical conductivity and electrochemical stability. However, its catalytic activity is limited and it will produce a large electrochemical polarization impedance.

[0003] Graphite felt is mainly made of PAN-based pre-oxidized fiber, which is woven and processed on high-end non-woven needle-punched equipment into flat felt with specific thickness, gram per square meter and good uniformity as the raw material felt. After carbonization and high-temperature sintering, a graphite felt intermediate product with a large amount of carbon nanotubes and amorphous carbon deposited on the surface is obtained.

[0004] The intermediate product of graphite carbon fiber is pre-treated by oxidation in an activation furnace to increase the electrochemical activity of the electrode graphite felt. Activation conditions: Use water vapor activation, inject water vapor into a quartz tube containing a graphite felt electrode, and perform activation treatment at a temperature of 700°C to increase the surface active groups. In order to increase the number of active sites, Fe and Ni metal catalysts are used, but the metal elements remaining in the electrode may contaminate the electrolyte.

[0005] The amorphous carbon deposited on the fiber surface of the intermediate product of graphite carbon fiber increases the energy consumption, cost and time of electrode activation; reduces the porosity of the electrode, hinders the contact between the electrolyte and the electrode; falls off during use, affecting the performance of the electrode and the stability of the system. It has many adverse effects on the "catalytic surface"; while the large amount of carbon nanotubes deposited on the graphite surface increases the contact between the fibers and reduces the contact resistance, which improves the conductivity of the "conductive surface" and is beneficial to the system.

[0006] The traditional heat treatment operation is to heat the whole graphite felt isothermally. In fact, it is only necessary to remove the amorphous carbon between the "catalytically active surfaces" that are in direct contact with the diaphragm, so that the surface of the graphite felt can be activated while removing the amorphous carbon. However, some existing methods of removing amorphous carbon often require the use of polluting chemical reagents or complex processes, and overall heating requires high energy consumption. Therefore, a treatment method with a simpler process and reduced energy consumption and pollution is also needed. Summary of the invention

[0007] The purpose of the present invention is to provide a method for activating the surface of graphite felt and removing amorphous carbon by utilizing photothermal effect, so as to solve the technical problems in the prior art of removing amorphous carbon from the surface of activated graphite felt, such as complex process, high energy consumption or pollution caused by chemical reagents used.

[0008] The method for activating the surface of graphite felt and removing amorphous carbon by utilizing photothermal effect comprises the following steps.

[0009] Step 1: Flatten the semi-finished graphite felt that has not been activated and place it in a transparent container.

[0010] Step 2: introducing an oxidizing gas that meets the requirements into the transparent container to adjust the atmosphere in the transparent container to an oxidizing gas atmosphere.

[0011] Step three, a light source is arranged above the transparent container to illuminate the upper surface of the semi-finished graphite felt exposed after it is flattened, and the oxidizing gas is continuously introduced into the transparent container. While maintaining the oxidizing gas atmosphere, the illumination is continued for a period of time, and then the illumination is stopped to remove the amorphous carbon of the graphite felt and activate the surface of the graphite felt.

[0012] Preferably, the oxidizing gas includes one or more of air, oxygen, water vapor and carbon dioxide.

[0013] Preferably, the oxidizing gas comprises air, water vapor and carbon dioxide, wherein the volume ratio of air:carbon dioxide=1:10-1, and the oxidizing gas contains saturated water vapor.

[0014] Preferably, the light source is a xenon lamp with a focusing accessory, the distance between the xenon lamp and the surface of the graphite felt is 1 to 20 cm, and the current range of the xenon lamp during illumination is 16 to 21A.

[0015] Preferably, during the irradiation process, when the temperature of the exposed upper surface of the graphite felt rises to a set temperature, the temperature is maintained for constant temperature irradiation treatment, and the treatment time of the constant temperature irradiation is 10 to 30 minutes.

[0016] Preferably, the set temperature is 800°C.

[0017] Preferably, the felt body thickness of the semi-finished graphite felt is 3 mm.

[0018] The advantages of the present invention are:

[0019] 1. The present invention can activate the graphite felt under mild conditions by controlling the atmosphere and light conditions, thereby effectively avoiding damage to the graphite felt structure.

[0020] 2. It has little impact on the overall structure of graphite felt, and only removes amorphous carbon on the catalytic active surface. This can remove carbon deposits on the "catalytic surface" and retain the conductivity of the "conductive surface", ensuring product performance.

[0021] 3. Use one or more of air, oxygen, water, and carbon dioxide as oxidants, and do not use strong acids, strong alkalis, and other dangerous chemicals to remove amorphous carbon impurities on the surface of graphite felt. This protects the graphite felt structure and avoids possible pollution and harm caused by chemical agents.

[0022] 4. No other metal-based catalysts are introduced in the treatment process of the present invention, thereby avoiding the subsequent contamination of the vanadium electrolyte by metal ions in the electrode.

[0023] 5. The present invention requires simple equipment and low requirements for equipment, and the operation steps are simple and quick, and is easy to implement and promote. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 The present invention is a schematic flow diagram of a method for activating the surface of graphite felt and removing amorphous carbon by utilizing photothermal effect. DETAILED DESCRIPTION

[0025] The specific implementation modes of the present invention are further explained in detail below by describing the embodiments with reference to the accompanying drawings, so as to help those skilled in the art to have a more complete, accurate and in-depth understanding of the inventive concept and technical solution of the present invention.

[0026] like Figure 1 As shown, the present invention provides a method for activating the surface of graphite felt and removing amorphous carbon using photothermal effect, comprising the following steps.

[0027] Step 1: Flatten the semi-finished graphite felt that has not been activated and place it in a transparent container.

[0028] In this step, the raw materials for preparing graphite felt use commercial products, such as polyacrylonitrile-based acrylic fiber or polyacrylonitrile-based carbon fiber. The fiber raw materials are woven, needled, and other processes to form fiber felt. The thickness of the felt formed by needle punching with a needle punching machine is about 3 mm. Then the fiber felt is sent to a pre-oxidation furnace for heat treatment at a temperature of 150-260°C; the pre-oxidized fiber felt is sent to a carbonization furnace for carbonization treatment, and then sent to a graphite mold for graphitization treatment to obtain a semi-finished graphite felt that has not been activated. Then the semi-finished graphite felt is flattened and placed in a transparent container.

[0029] Step 2: introducing an oxidizing gas that meets the requirements into the transparent container to adjust the atmosphere in the transparent container to an oxidizing gas atmosphere.

[0030] The oxidizing gas includes one or more of air, oxygen, water vapor, and carbon dioxide. In this embodiment, the oxidizing gas includes air, water vapor, and carbon dioxide, wherein the volume ratio of air: carbon dioxide = 1:10-1, and the oxidizing gas contains saturated water vapor. The oxidizing gas is introduced into the transparent container to achieve the effect of adjusting the atmosphere in the transparent container to an oxidizing gas atmosphere. In this mixed atmosphere of oxidizing gas, step three is prepared, and the oxidizing gas is continuously introduced during the process of step three.

[0031] Step three, a light source is arranged above the transparent container to illuminate the upper surface of the semi-finished graphite felt exposed after it is flattened, and the oxidizing gas is continuously introduced into the transparent container. While maintaining the oxidizing gas atmosphere, the illumination is continued for a period of time, and then the illumination is stopped to remove the amorphous carbon of the graphite felt and activate the surface of the graphite felt.

[0032] In this step, the light source is a xenon lamp with a focusing accessory, the current of the xenon lamp is adjustable, and the power of the xenon lamp can be selected to be 300W; the distance between the xenon lamp and the surface of the graphite felt is adjusted to 1 to 20 cm, and the current range of the xenon lamp during illumination is 16 to 21A. When the temperature of the exposed upper surface of the graphite felt rises to the set temperature, the temperature is maintained for constant temperature illumination treatment, and the treatment time of constant temperature illumination is 10 to 30 minutes. During the illumination process, the aforementioned oxidizing gas is continuously introduced to maintain the gas atmosphere in the transparent container. After turning off the light source, the aforementioned oxidizing gas is still continuously introduced for cooling. After the graphite felt is cooled, the treated graphite felt is taken out for use.

[0033] This method controls the xenon light illumination intensity, time and atmosphere conditions (one or more of air, oxygen, water and carbon dioxide) and utilizes the photothermal effect of carbon materials to oxidize the amorphous carbon with poor crystallinity on the surface of the graphite felt fiber into CO2 or CO. Since photons can only reach a certain thickness of graphite felt, they cannot damage the structure of the other side of the graphite felt and will not affect its conductivity; therefore, this method can remove carbon deposits on the "catalytic surface" and retain the conductivity of the "conductive surface". At the same time, only the upper surface of the graphite felt is heated by light. While the upper surface of the graphite felt is heated, the heat is taken away by the continuously introduced oxidizing gas to prevent the overall temperature of the graphite felt from being too high, thereby preventing high temperature from damaging the overall structure of the graphite felt.

[0034] In the experiment, a temperature sensor for detecting the upper surface temperature of the graphite felt during the method is set in a transparent container. The samples of the semi-finished graphite felt and the treated samples are weighed before and after the treatment by the method. Based on the method provided by the present invention, taking a 10 mg sample as an example, the oxidizing gas flow rate is set to 20 mL / min, and the distance between the graphite felt surface and the xenon lamp is fixed. When the upper surface temperature of the graphite felt rises to 800°C, it is maintained for 10 to 30 minutes as the light treatment time. The results of the treatments for different times can be seen in Table 1. The maintenance times of the three experiments are 10 minutes, 20 minutes, and 30 minutes, respectively. After the experiment, the mass loss percentages of the treated graphite felt compared to the unactivated semi-finished graphite felt are: 21.5%, 29.3% and 31.7%, respectively.

[0035] Table 1. Amorphous carbon removal effect at different illumination times

[0036] Light treatment time / min 10 20 30 Mass loss / % 21.5％ 29.3％ 31.7％

[0037] This method only requires simple equipment such as a transparent container, a corresponding temperature sensor, a light source, and a ventilation device. The equipment requirements are not high, and the operation is simple and quick. This method can activate the graphite felt under mild conditions by controlling the atmosphere and light conditions, effectively avoiding damage to the graphite felt structure. Therefore, this method has little effect on the overall structure of the graphite felt, and only removes the amorphous carbon on the catalytically active surface, which can remove the carbon deposits on the "catalytic surface" and retain the conductivity of the "conductive surface".

[0038] The present invention is described above by way of example in conjunction with the accompanying drawings. It is obvious that the specific implementation of the present invention is not limited to the above-mentioned method. As long as various non-substantial improvements are made using the inventive concept and technical solution of the present invention, or the inventive concept and technical solution are directly applied to other occasions without improvement, they are all within the protection scope of the present invention.

Claims

1. A method for activating the surface of graphite felt and removing amorphous carbon by utilizing photothermal effect, characterized in that: The following steps are involved: Step 1: Flatten the semi-finished graphite felt that has not been activated and place it in a transparent container; Step 2: introducing an oxidizing gas that meets the requirements into the transparent container to adjust the atmosphere in the transparent container to an oxidizing gas atmosphere; Step three, a light source is arranged above the transparent container to illuminate the upper surface of the semi-finished graphite felt exposed after it is flattened, and the oxidizing gas is continuously introduced into the transparent container. While maintaining the oxidizing gas atmosphere, the illumination is continued for a period of time, and then the illumination is stopped to remove the amorphous carbon of the graphite felt and activate the surface of the graphite felt.

2. The method for activating the surface of graphite felt and removing amorphous carbon by utilizing photothermal effect according to claim 1, characterized in that: The oxidizing gas includes one or more of air, oxygen, water vapor and carbon dioxide.

3. A method for activating the surface of graphite felt and removing amorphous carbon by utilizing photothermal effect according to claim 1 or 2, characterized in that: The oxidizing gas includes air, water vapor and carbon dioxide, wherein the volume ratio of air to carbon dioxide is 1:10-1, and the oxidizing gas contains saturated water vapor.

4. The method for activating the surface of graphite felt and removing amorphous carbon by utilizing photothermal effect according to claim 1, characterized in that: The light source is a xenon lamp with a focusing accessory, the distance between the xenon lamp and the surface of the graphite felt is 1 to 20 cm, and the current range of the xenon lamp during illumination is 16 to 21A.

5. The method for activating the surface of graphite felt and removing amorphous carbon by utilizing photothermal effect according to claim 1, characterized in that: During the irradiation process, when the temperature of the exposed upper surface of the graphite felt rises to the set temperature, the temperature is maintained for constant temperature irradiation treatment, and the treatment time of the constant temperature irradiation is 10 to 30 minutes.

6. The method for activating the surface of graphite felt and removing amorphous carbon by utilizing photothermal effect according to claim 5, characterized in that: The set temperature is 800°C.

7. The method for activating the surface of graphite felt and removing amorphous carbon by utilizing photothermal effect according to claim 1, characterized in that: The felt body thickness of the semi-finished graphite felt is 3 mm.

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