Preparation method and application of composite flow battery carbon felt electrode containing coconut shell carbon

Abstract

The application provides a preparation method and application of a composite liquid flow battery carbon felt electrode containing coconut shell carbon. The preparation method comprises the following steps: performing plasma activation treatment on the carbon felt; impregnating the carbon felt in slurry prepared by mixing a coconut shell carbon source, KOH, cellulose nanofiber and carbon nanotube, vacuum impregnating the slurry to make the slurry penetrate into the fiber bundle, removing the excess slurry through rolling, and drying; performing primary heat treatment under the protection of inert gas to carbonize the coconut shell carbon source and simultaneously activate and form pores, washing, impregnating in a soluble bismuth salt solution, drying, and performing secondary heat treatment, thereby obtaining the composite electrode. The active layer shedding rate of the carbon felt material is less than 0.5wt%, the electrode permeability decreases slightly compared with the original carbon felt, the energy efficiency reaches 82.9~84.2% under a current density of 160mA / cm², and the process is simple and easy to scale up.

Description

Technical Field

[0001] This invention relates to the field of electrochemical energy storage electrode materials technology, specifically to a method for preparing and applying a composite flow battery carbon felt electrode containing coconut shell carbon. Background Technology

[0002] Vanadium redox flow batteries have attracted widespread attention in large-scale, long-term energy storage due to their advantages such as long cycle life, high safety, and independent design of power and capacity. Electrode materials are one of the key components determining battery performance and cost. Polyacrylonitrile-based graphite carbon felt has become the mainstream electrode material for commercial vanadium redox flow batteries due to its good conductivity, three-dimensional network structure, and acid resistance. However, the graphite carbon felt has few electrochemical active sites on its surface, poor electrolyte wettability, and slow vanadium ion redox reaction kinetics, resulting in low battery energy efficiency and limiting its application under high power density conditions. Therefore, modifying the carbon felt to improve its electrochemical activity is a key research focus in this field.

[0003] The preparation of porous carbon materials from waste biomass and their loading onto the surface of carbon felt has become a research hotspot in recent years. Coconut shells, after carbonization and activation, can yield hierarchical porous carbon with a high specific surface area, and the raw material is widely available and inexpensive. In existing technologies, a common practice for loading biomass carbon sources onto the surface of carbon felt involves mixing biomass carbon powder with binders such as polytetrafluoroethylene (PTFE) or Nafion to form a slurry, coating it onto the carbon felt surface, and then drying it to obtain a composite electrode. While this physical coating method is simple, it has inherent drawbacks: the binder is an insulating polymer, and its introduction increases the ohmic resistance of the electrode, while also blocking some of the pores of the biomass carbon, reducing the accessibility of active sites. More importantly, the contact between the carbon powder and the carbon felt fibers is maintained solely by the physical adhesion of the binder, resulting in weak interfacial bonding. Under the repeated changes in electrode volume during long-term electrolyte flushing and charge-discharge processes, the active layer is easily detached from the carbon felt surface and pulverized, leading to a rapid decline in electrode performance with increasing cycle count. Furthermore, the slurry coating process is difficult to control precisely, with a large amount of slurry filling the micron-sized macropores between the carbon felt fibers. After drying, this blocks the main channels of the electrolyte, increasing the battery's pump consumption and weakening the original advantages of the carbon felt's excellent fluid permeability. The root of these problems lies in the fact that existing technologies have consistently failed to resolve a fundamental contradiction—how to construct a continuous active layer on the surface of the carbon felt fibers that is both strong and prevents detachment while ensuring the smooth flow of the electrolyte.

[0004] To address the aforementioned issues, researchers have attempted to grow metal oxides or carbon nanoarrays in situ on the surface of carbon felt to enhance interfacial bonding. However, these methods typically involve complex multi-step processes such as hydrothermal reactions and chemical vapor deposition, which are subject to harsh conditions and poor batch-to-batch consistency. Furthermore, metal oxides pose a risk of dissolution in the strongly acidic electrolyte environment of vanadium redox flow batteries, affecting long-term stability. Another approach involves preparing biomass carbon precursors as independent thin films and then hot-pressing them onto carbon felt, utilizing carbonization shrinkage to achieve interfacial bonding. However, in this process, the large amount of gas generated during KOH activation is hindered from escaping within the confined interlayer, causing film bubbling and cracking. Moreover, large-area sintering and welding easily occur at the interface at high temperatures, making it impossible to obtain a composite electrode with a complete structure and good consistency.

[0005] Therefore, there is an urgent need for a technical solution that can construct a continuous, firm, and permeable activated carbon layer on the surface of carbon felt fibers, while also being simple in process, highly controllable, and easy to implement on a large scale. Summary of the Invention

[0006] The purpose of this invention is to overcome the shortcomings of the prior art and provide a method for preparing and applying a composite flow battery carbon felt electrode containing coconut shell carbon, thereby solving the technical problem that "in existing biomass porous carbon coated electrodes, the active carbon layer and carbon felt fibers are easily peeled off from the carbon felt surface due to physical bonding only with an adhesive, under the influence of electrolyte flushing and cyclic volume changes".

[0007] To achieve the above objectives, the present invention is implemented using the following technical solution: In a first aspect, the present invention provides a method for preparing a carbon felt electrode for a composite flow battery containing coconut shell carbon, comprising the following steps: (1) The carbon felt is subjected to plasma activation treatment to obtain activated carbon felt; (2) Disperse the coconut shell carbon source, activator and dispersing enhancer in a solvent to prepare a slurry; (3) The activated carbon felt is immersed in the slurry, and then dried by roller pressing under vacuum to obtain pretreated carbon felt; (4) The pretreated carbon felt is subjected to a heat treatment under inert gas protection to carbonize the coconut shell carbon source and simultaneously activate and create pores. After washing, it is immersed in a soluble bismuth salt solution, dried and subjected to a second heat treatment to obtain the composite flow battery carbon felt electrode.

[0008] Preferably, the coconut shell carbon source is coconut shell ultrafine powder, the activator is KOH, and the mass ratio of coconut shell ultrafine powder to KOH is 1:2~4.

[0009] Preferably, the plasma activation treatment in step (1) is carried out in an oxygen-containing atmosphere, with a radio frequency power of 100~200W, a working gas pressure of 50~100Pa, and a treatment time of 30~150 seconds.

[0010] Preferably, the dispersing enhancer is a mixture of cellulose nanofibers and carbon nanotubes, and its addition amount is 6-26% of the mass of coconut shell carbon source, wherein the mass ratio of cellulose nanofibers to carbon nanotubes is 2.5-5:1.

[0011] Preferably, the vacuum holding time for vacuum-assisted impregnation in step (3) is 10 to 30 minutes, and the absolute pressure of the vacuum environment is ≤0.01MPa.

[0012] Preferably, the rolling process in step (3) uses a rubber roller with a Shore hardness of 55 to 75 degrees and a linear pressure of 0.1 to 0.5 MPa. After rolling, the thickness of the carbon felt is compressed to 60 to 85% of the original thickness.

[0013] Preferably, the temperature of the first heat treatment in step (4) is 700~950℃ and the holding time is 1~3 hours.

[0014] Preferably, the soluble bismuth salt in step (4) is one or more of bismuth nitrate, bismuth chloride, and bismuth sulfate.

[0015] Preferably, the temperature of the secondary heat treatment is 400~550℃, and the holding time is 20~60 minutes.

[0016] Secondly, the present invention provides an application of the composite flow battery carbon felt electrode prepared by the above method in an all-vanadium redox flow battery.

[0017] Compared with the prior art, the beneficial effects achieved by the present invention are: (1) This invention uses plasma activation to introduce oxygen-containing functional groups and surface-active sites on the surface of carbon felt fibers as chemical anchoring sites. Combined with the three-dimensional carbon bridging network formed during the carbonization process of CNF / CNT, a chemical bond is formed between the coconut shell carbon coating layer and the carbon felt fibers. There is no obvious boundary at the interface between the coating layer and the carbon felt, and it does not rely on any external adhesive. Ultrasonic vibration test showed that the active layer shedding rate was less than 0.5 wt%, while the shedding rate of traditional PTFE bonded coated electrodes is usually higher than 15 wt% under the same conditions, indicating that the interfacial bonding force described in this invention is far superior to that of traditional physical coating methods.

[0018] (2) The present invention adopts a vacuum impregnation combined with a roller pressing process, so that the slurry is coated on the surface of a single carbon felt fiber in a thin layer, rather than filling the large pores between the fibers. The inherent hundreds of micron-level electrolyte flow channels of the carbon felt are effectively preserved. The electrode permeability decreases by less than 15% compared with the original carbon felt, avoiding the defects of traditional coating methods, such as electrolyte flow obstruction and increased pump consumption caused by the slurry filling the large pores. At the same time, after the coconut shell carbon is activated by KOH, it forms a multi-level structure of micropores and mesopores, which has both high specific surface area active sites and fast ion mass transfer channels. The energy efficiency reaches 82.9~84.2% at a current density of 160mA / cm², realizing the synergistic optimization of electrode permeability and catalytic activity.

[0019] (3) The coconut shell raw material of the present invention naturally contains minerals such as potassium, calcium, and magnesium, which can assist the etching reaction of KOH during the carbonization and activation process and improve the pore-forming efficiency. At the same time, the natural microporous structure retained after the carbonization of the coconut shell is conducive to ion transport. Bismuth doping is anchored on the surface of the coconut shell carbon skeleton in the form of Bi2O3 nanoparticles. As an electrocatalyst for the positive electrode reaction, it can reduce VO 2+ / VO2 + The charge transfer impedance of the redox reaction accelerates the reaction rate; the entire process does not require the use of binders and does not involve complex equipment such as hydrothermal reactions or chemical vapor deposition. It uses waste biomass coconut shells as raw materials, and the process is simple and easy to scale up for production. Attached Figure Description

[0020] Figure 1 This is a comparison chart of the capacity retention rates of Examples 1-3 and Comparative Examples 1-4 after 600 cycles. Detailed Implementation

[0021] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0022] The performance improvement of carbon felt electrodes in vanadium redox flow batteries is mainly limited by two factors: first, the carbon felt fiber surface has few electrochemical active sites; and second, it is difficult for the surface-loaded active material to form a strong bond with the carbon felt matrix.

[0023] To address the aforementioned problems, this invention proposes the following technical solution: First, oxygen-containing functional groups and surface-active sites are introduced onto the surface of carbon felt fibers through plasma activation, serving as chemical anchoring sites for subsequent carbon layer growth. Then, a slurry is prepared by mixing coconut shell carbon source with activator KOH and dispersant / reinforcing agent (CNF / CNT). This slurry is then vacuum impregnated to ensure full penetration into the carbon felt fiber bundles. Excess slurry between fiber bundles is removed by roller pressing, ensuring a thin coating on the surface of each fiber while maintaining unobstructed macropores between fibers. Subsequently, high-temperature heat treatment carbonizes the coconut shell carbon source and simultaneously activates KOH to create pores. During this process, CNF / CNT carbonization forms a three-dimensional carbon bridging network, connecting and fixing the coconut shell carbon particles to the anchoring sites on the carbon felt fiber surface, forming a continuous and robust coating layer. Finally, bismuth doping is introduced through post-impregnation, allowing Bi2O3 nanoparticles to be uniformly loaded onto the carbon framework surface, further reducing the charge transfer impedance of the positive electrode reaction. In the above scheme, plasma activation constructs chemical anchoring sites on the fiber surface required for coating growth, enabling chemical bonding between the carbon layer and the fiber rather than just physical adhesion; KOH activation creates micropores and mesopores within the coconut shell carbon layer, providing ample active sites and ion transport channels; the three-dimensional network formed by CNF / CNT carbonization connects the coating layer into a continuous integral structure, avoiding discrete distribution between particles; bismuth doping further enhances the electrocatalytic activity on the positive electrode side. Based on the above concept, the present invention proposes the following specific technical solutions: (1) Place the PAN-based graphite carbon felt in the plasma treatment chamber, evacuate it, and then introduce oxygen to the working pressure of 50~100Pa. Treat it for 30~150 seconds under the condition of radio frequency power of 100~200W to obtain activated carbon felt. (2) After removing the outer skin of the waste coconut shell, take the dense inner layer, crush it to 50-100 mesh with a crusher, use anhydrous ethanol as medium and zirconium oxide balls as grinding balls, and ball mill at 200-600 r / min for 6-12 h, and dry it to constant weight to obtain coconut shell ultrafine powder with D90≤5μm; weigh 10g of coconut shell ultrafine dry powder, 20-40g of KOH, 0.5-2g of cellulose nanofibers and 0.1-0.6g of carbon nanotubes, disperse them in deionized water, adjust the volume to the solid content of 10-30%, and sonicate for 10-60 min to obtain slurry; (3) Immerse the activated carbon felt in the slurry of step (2), vacuum it to an absolute pressure ≤0.01MPa and keep it for 10~30min, remove it after depressurization, and roll it 2~4 times with a rubber roller with a Shore hardness of 55~75 degrees and a linear pressure of 0.1~0.5MPa, each time for 2~5 seconds, so that the thickness of the carbon felt is compressed to 60~85% of the original thickness. Dry it at 50~80℃ for 6~12h to obtain the pretreated carbon felt; (4) Place the pretreated carbon felt in a tube furnace, purge with nitrogen for protection, heat to 700~950℃ at 5℃ / min and hold for 1~3h, cool to room temperature with the furnace, carbonize and boil in deionized water for 15~30min, change water and repeat 2~5 times, then ultrasonically clean with deionized water 1~3 times, and dry at 50~80℃; immerse the dried composite carbon felt in 0.01~0.05mol / L bismuth nitrate in a mixed solution of ethanol / water (volume ratio 1:1) for 2~6h, take it out and dry at 80℃, place it in a tube furnace, heat to 400~550℃ at 5~10℃ / min under nitrogen protection and hold for 20~60min to obtain bismuth-doped coconut shell porous carbon / carbon felt composite electrode.

[0024] The following specific examples further illustrate this point.

[0025] Example 1; (1) Place the PAN-based graphite carbon felt in the plasma treatment chamber, evacuate it, introduce oxygen to the working pressure of 50 Pa, and treat it for 30 seconds under the condition of 100 W radio frequency power to obtain activated carbon felt. (2) After removing the outer skin of the waste coconut shell, take the dense inner layer, crush it to 50 mesh with a crusher, use anhydrous ethanol as medium and zirconium oxide balls as grinding balls, ball mill at 200 r / min for 6 h, dry to constant weight, and obtain coconut shell ultrafine powder with D90≤5μm; weigh 10g of coconut shell ultrafine dry powder, 20g of KOH, 0.5g of cellulose nanofibers and 0.1g of carbon nanotubes, disperse them in deionized water, adjust the volume to 10% solid content, and sonicate for 10 min to obtain slurry; (3) Immerse the activated carbon felt in the slurry of step (2), vacuum it to an absolute pressure ≤ 0.01 MPa and hold for 10 min, remove it after depressurization, and roll it twice with a rubber roller with a Shore hardness of 55 degrees under a linear pressure of 0.1 MPa, each time for 5 seconds, so that the thickness of the carbon felt is compressed to 60% of the original thickness. Dry it at 50℃ for 6~12 h to obtain the pretreated carbon felt. (4) Place the pretreated carbon felt in a tube furnace, purge with nitrogen, heat to 700℃ at 5℃ / min and hold for 1h, cool to room temperature with the furnace, carbonize and boil in deionized water for 15min, change the water and repeat twice, then ultrasonically clean with deionized water once and dry at 50℃; immerse the dried composite carbon felt in 0.01mol / L bismuth nitrate in a mixed solution of ethanol / water (volume ratio 1:1) for 2h, take it out and dry at 80℃, place it in a tube furnace, heat to 400℃ at 5℃ / min and hold for 20min under nitrogen protection to obtain bismuth-doped coconut shell porous carbon / carbon felt composite electrode.

[0026] Example 2; (1) Place the PAN-based graphite carbon felt in the plasma treatment chamber, evacuate it, introduce oxygen to the working pressure of 80 Pa, and treat it for 90 seconds under the condition of 150 W radio frequency power to obtain activated carbon felt. (2) After removing the outer skin of the waste coconut shell, take the dense inner layer, crush it to 80 mesh with a crusher, use anhydrous ethanol as medium and zirconium oxide balls as grinding balls, ball mill at 400 r / min for 8 h, dry to constant weight, and obtain coconut shell ultrafine powder with D90≤5μm; weigh 10g of coconut shell ultrafine dry powder, 30g of KOH, 1g of cellulose nanofibers and 0.4g of carbon nanotubes, disperse them in deionized water, adjust the volume to 20% solid content, and sonicate for 40 min to obtain slurry; (3) Immerse the activated carbon felt in the slurry of step (2), vacuum it to an absolute pressure ≤ 0.01MPa and hold for 20 minutes, remove it after depressurization, and roll it 3 times with a rubber roller with a Shore hardness of 65 degrees under a linear pressure of 0.3MPa, each time for 3 seconds, so that the thickness of the carbon felt is compressed to 70% of the original thickness. Dry it at 70℃ for 10 hours to obtain the pretreated carbon felt. (4) Place the pretreated carbon felt in a tube furnace, purge with nitrogen, heat to 850°C at 5°C / min and hold for 2 hours, cool to room temperature with the furnace, carbonize and boil in deionized water for 20 minutes, change the water and repeat 3 times, then ultrasonically clean with deionized water twice and dry at 70°C; immerse the dried composite carbon felt in 0.03 mol / L bismuth nitrate in a mixed solution of ethanol / water (volume ratio 1:1) for 4 hours, take it out and dry at 80°C, place it in a tube furnace, heat to 500°C at 8°C / min and hold for 40 minutes under nitrogen protection to obtain bismuth-doped coconut shell porous carbon / carbon felt composite electrode.

[0027] Example 3; (1) Place the PAN-based graphite carbon felt in the plasma treatment chamber, evacuate it, introduce oxygen to the working pressure of 100 Pa, and treat it for 150 seconds under the condition of 200 W radio frequency power to obtain activated carbon felt. (2) After removing the outer skin of the waste coconut shell, take the dense inner layer, crush it to 100 mesh with a crusher, use anhydrous ethanol as medium and zirconium oxide balls as grinding balls, ball mill at 600 r / min for 12 h, dry to constant weight, and obtain coconut shell ultrafine powder with D90≤5μm; weigh 10g of coconut shell ultrafine dry powder, 40g of KOH, 2g of cellulose nanofibers and 0.6g of carbon nanotubes, disperse them in deionized water, adjust the volume to 30% solid content, and sonicate for 60 min to obtain slurry; (3) Immerse the activated carbon felt in the slurry of step (2), vacuum it to an absolute pressure ≤ 0.01 MPa and keep it for 30 min, remove it after depressurization, and roll it 4 times with a rubber roller with a Shore hardness of 75 degrees under a linear pressure of 0.5 MPa, each time for 2 seconds, so that the thickness of the carbon felt is compressed to 85% of the original thickness. Dry it at 80℃ for 12 h to obtain the pretreated carbon felt. (4) Place the pretreated carbon felt in a tube furnace, purge with nitrogen, heat to 950°C at 5°C / min and hold for 3 hours, cool to room temperature with the furnace, carbonize and boil in deionized water for 20 minutes, change the water and repeat 5 times, then ultrasonically clean with deionized water 3 times and dry at 80°C; immerse the dried composite carbon felt in 0.05 mol / L bismuth nitrate in a mixed solution of ethanol / water (volume ratio 1:1) for 6 hours, take it out and dry at 80°C, place it in a tube furnace, heat to 550°C at 10°C / min and hold for 60 minutes under nitrogen protection to obtain bismuth-doped coconut shell porous carbon / carbon felt composite electrode.

[0028] Comparative Example 1; The difference between Comparative Example 1 and Example 2 is that step (1) is omitted, and step (3) is changed to: immersing PAN-based graphite carbon felt in the slurry of step (2), evacuating to an absolute pressure ≤0.01MPa and holding for 20 minutes, removing after depressurization, and rolling it 3 times with a rubber roller with a Shore hardness of 65 degrees under a linear pressure of 0.3MPa, with each pass time of 3 seconds, so that the carbon felt thickness is compressed to 70% of the original thickness, and drying at 70°C for 10 hours to obtain pretreated carbon felt; the remaining steps are the same as in Example 2.

[0029] Comparative Example 2; The difference between Comparative Example 2 and Example 2 lies in step (2). Step (2) is changed to: after removing the outer skin of the waste coconut shell, take the dense inner layer, crush it to 80 mesh using a crusher, use anhydrous ethanol as the medium and zirconium oxide balls as the grinding balls, ball mill at 400 r / min for 8 h, and dry to constant weight to obtain coconut shell ultrafine powder with D90≤5μm; weigh 10g of coconut shell ultrafine dry powder, 1g of cellulose nanofibers and 0.4g of carbon nanotubes, disperse them in deionized water, adjust the volume to a solid content of 20%, and sonicate for 40 min to obtain a slurry; the remaining steps are the same as in Example 2.

[0030] Comparative Example 3; The difference between Comparative Example 3 and Example 2 is the difference in step (2). Step (2) is changed to: after removing the outer skin of the waste coconut shell, take the dense inner layer, crush it to 80 mesh with a crusher, use anhydrous ethanol as medium and zirconia balls as grinding balls, ball mill at 400 r / min for 8 h, dry to constant weight, and obtain coconut shell ultrafine powder with D90≤5μm; weigh 10g of coconut shell ultrafine dry powder and 30g of KOH, disperse them in deionized water, adjust the volume to a solid content of 20%, and sonicate for 40 min to obtain slurry; the remaining steps are the same as in Example 2.

[0031] Comparative Example 4; The difference between Comparative Example 4 and Example 2 is the difference in step (4). Step (4) is changed to: placing the pretreated carbon felt in a tube furnace, introducing nitrogen for protection, heating to 850°C at 5°C / min and holding for 2 hours, cooling to room temperature with the furnace, carbonizing and then boiling in deionized water for 20 minutes, changing the water and repeating 3 times, then ultrasonically cleaning with deionized water 2 times, and drying at 70°C to obtain the coconut shell porous carbon / carbon felt composite electrode; the remaining steps are the same as in Example 2.

[0032] Test and Results Analysis The composite carbon felt electrodes prepared in Examples 1-3 and Comparative Examples 1-4 were cut into electrode sheets with an effective area of ​​2 cm × 2 cm, and assembled into vanadium redox flow single cells for performance testing. The electrolyte contained 1.5 mol / L VO₂. 2+ / VO2 + A 3.0 mol / L sulfuric acid solution was used, with 50 mL of electrolyte at each electrode. A Nafion 115 proton exchange membrane was used as the separator. Charge-discharge tests were conducted at a current density of 160 mA / cm², with a voltage window of 0.8–1.6 V.

[0033] Electrode permeability was measured using a self-made permeability testing device. The testing direction was perpendicular to the thickness direction of the felt surface. A pre-pressure of 0.2 MPa was applied before testing to simulate the stack assembly state. Deionized water was used as the medium, and the volumetric flow rate of water permeating the sample per unit time was recorded under a constant pressure difference of 0.1 MPa. Permeability was calculated according to Darcy's law. For each sample, three different locations were tested repeatedly, and the average value was taken. Using the original PAN-based graphite carbon felt as a control, its permeability was measured to be 1.6 × 10⁻⁶. -12 m 2 .

[0034] The shedding rate of the electrode active material was determined by the following method: the electrode to be tested was placed in anhydrous ethanol and ultrasonically oscillated for 60 minutes at 40 kHz and 200 W, and the percentage of mass loss of the electrode before and after ultrasonication was calculated.

[0035] Electrochemical impedance spectroscopy (EIS) was performed at open circuit potential, with a frequency range of 100 kHz to 10 mHz and an amplitude of 5 mV. The charge transfer impedance (Rct) was obtained by fitting.

[0036] Energy efficiency test: A Landon CT3004A charge / discharge tester was used at a current density of 160 mA / cm². 2 A constant current charge-discharge test was performed under the specified conditions, with a voltage window of 0.8~1.6V. The stable energy efficiency value was recorded after the 50th cycle.

[0037] The test results of each embodiment and comparative example are shown in Table 1.

[0038] Table 1

[0039] As shown in Table 1, the active layer shedding rate of Examples 1-3 was less than 0.5 wt%, the energy efficiency reached 82.9-84.2%, the capacity retention rate after 600 cycles was higher than 94%, and 72-84% of the original carbon felt permeability was retained. This indicates that the coating layer constructed by plasma interface activation and CNF / CNT carbon bridging network as described in this invention forms a strong chemical bond and mechanical interlock with the carbon felt fiber. At the same time, KOH activation and pore formation endow the coconut shell carbon layer with rich micropores and mesopores at multiple levels, achieving a synergistic improvement in electrode catalytic activity and structural stability.

[0040] Comparative Example 1 lacked a plasma activation step, and the active layer shedding rate was as high as 18.73 wt%. This indicates that the surface of the carbon felt fiber without plasma activation has high chemical inertness. The coating layer and the fiber are only maintained in contact by van der Waals forces, resulting in weak bonding. The active material is continuously lost during long-term cycling. The oxygen-containing functional groups and carbon dangling bond defects introduced by plasma activation serve as chemical anchoring sites, which are key to ensuring the interfacial bonding strength of the coating layer.

[0041] Comparative Example 2, lacking KOH activation, had an energy efficiency of only 76.5% and a high Rct of 1.67 Ω·cm. 2 This indicates that the unactivated coconut shell carbon layer has a dense structure, low specific surface area, insufficient density of active sites, and severe electrochemical polarization. KOH activation to create pores is a necessary condition for constructing a multi-level structure of micropores and mesopores with high specific surface area and providing sufficient reactive active sites. Without KOH, the coconut shell carbon layer only has intrinsic pores formed by carbonization itself, lacking the large number of micropores generated by KOH chemical etching, resulting in a decrease in electrode permeability.

[0042] Comparative Example 3 lacked CNF / CNT dispersant, resulting in an active layer detachment rate as high as 23.54 wt% and an electrode permeability of only 0.52 × 10⁻⁶. -12 m 2 Compared to Example 2, the capacity decreased by about 62%, and the capacity retention rate decreased to 62.3% after 300 cycles. This indicates that without CNF / CNT, coconut shell powder is prone to agglomeration and blockage of macroporous channels between fibers during carbonization, resulting in obstructed electrolyte flow and loosening and shedding of the active layer structure. CNF / CNT not only plays a dispersing role, but the three-dimensional bridging network formed after carbonization connects the coconut shell carbon particles to each other and anchors them to the surface of the carbon felt fiber, which is the key to maintaining the integrity of the coating layer structure and electrode permeability.

[0043] Comparative Example 4, lacking bismuth doping, had an Rct of 0.82 Ω·cm² and an energy efficiency of 79.1%, indicating that Bi₂O₃ nanoparticles reduced the charge transfer impedance of the cathode reaction. The bismuth doping post-treatment was carried out after thorough washing to remove potassium salt, effectively avoiding the hydrolysis and loss of bismuth salt in the strongly alkaline KOH system, and further improving the catalytic activity and energy efficiency of the electrode.

[0044] Capacity retention test: at a current density of 160 mA / cm² 2 Under continuous charge-discharge conditions, the capacity retention rate is calculated using the formula: Capacity retention rate = C n / C 10 ×100%, where C n C is the discharge capacity of the nth cycle. 10 The discharge capacity at the 10th cycle (the capacity tends to stabilize after the 10th cycle, and this is used as a baseline) was recorded. The capacity retention rate after 600 cycles is shown in the figure. Figure 1 .

[0045] like Figure 1 As shown, the capacity retention curves of Examples 1-3 remained stable during 600 cycles, with retention rates of 94.3%, 96.1%, and 95.8% respectively after 600 cycles. The decay rates were all below 0.01% / cycle, indicating that the chemical bonding interface between the coating layer and the carbon felt fiber remained stable during long-term cycling, and no significant detachment of the active material occurred. In Comparative Example 1, the capacity retention rate without plasma activation was only 72.5% after 600 cycles, indicating that the coating layer, lacking chemical anchoring sites, continuously peeled off during cycling, and the active material gradually disappeared. Comparative Example 2, without KOH activation, had a capacity retention rate of 87.1%, with a decay rate between Examples 1 and Comparative Example 1. Its capacity loss was mainly due to increased electrochemical polarization caused by insufficient active sites in the dense carbon layer, rather than structural failure. Comparative Example 3, without CNF / CNT, showed a capacity retention of 62.3% after 600 cycles, indicating that the loosely stacked coconut shell carbon particles were prone to detachment during long-term cycling. This further demonstrates the crucial role of the CNF / CNT carbon bridging network in maintaining the structural integrity of the coating layer. Comparative Example 4, without bismuth doping, showed a capacity retention of 90.7%, slightly lower than Example 2. Its capacity degradation was mainly due to the additional polarization caused by the increased overpotential of the cathode reaction after the absence of Bi2O3 catalysis, rather than structural degradation.

[0046] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from its spirit or essential characteristics. Therefore, the embodiments should be considered in all respects as exemplary and non-limiting, and the scope of the invention is defined by the appended claims rather than the foregoing description. Thus, all variations falling within the meaning and scope of equivalents of the claims are intended to be included within the present invention. No markings in the claims should be construed as limiting the scope of the claims.

Claims

1. A method for preparing a carbon felt electrode for a composite flow battery containing coconut shell carbon, characterized in that, Includes the following steps: (1) The carbon felt is subjected to plasma activation treatment to obtain activated carbon felt; (2) Disperse the coconut shell carbon source, activator and dispersing enhancer in a solvent to prepare a slurry; (3) The activated carbon felt is immersed in the slurry, and then dried by roller pressing under vacuum to obtain pretreated carbon felt; (4) The pretreated carbon felt is subjected to a heat treatment under inert gas protection to carbonize the coconut shell carbon source and simultaneously activate and create pores. After washing, it is immersed in a soluble bismuth salt solution, dried and subjected to a second heat treatment to obtain the composite flow battery carbon felt electrode.

2. The method according to claim 1, characterized in that, The coconut shell carbon source is ultrafine coconut shell powder, and the activator is KOH. The mass ratio of ultrafine coconut shell powder to KOH is 1:2~4.

3. The method according to claim 1, characterized in that, The plasma activation treatment described in step (1) is carried out in an oxygen-containing atmosphere, with a radio frequency power of 100~200W, a working gas pressure of 50~100Pa, and a treatment time of 30~150 seconds.

4. The method according to claim 1, characterized in that, The dispersing enhancer is a mixture of cellulose nanofibers and carbon nanotubes, and its addition amount is 6-26% of the mass of coconut shell carbon source.

5. The method according to claim 1, characterized in that, The vacuum holding time for vacuum-assisted impregnation in step (3) is 10 to 30 minutes, and the absolute pressure of the vacuum environment is ≤0.01MPa.

6. The method according to claim 1, characterized in that, In step (3), the rolling process uses a rubber roller with a Shore hardness of 55 to 75 degrees and a linear pressure of 0.1 to 0.5 MPa. After rolling, the thickness of the carbon felt is compressed to 60 to 85% of the original thickness.

7. The method according to claim 1, characterized in that, The temperature of the first heat treatment in step (4) is 700~950℃, and the holding time is 1~3 hours.

8. The method according to claim 1, characterized in that, The soluble bismuth salt mentioned in step (4) is one or more of bismuth nitrate, bismuth chloride, and bismuth sulfate.

9. The method according to claim 1, characterized in that, The secondary heat treatment is performed at a temperature of 400~550℃ for 20~60 minutes.

10. The application of a composite flow battery carbon felt electrode prepared by the method according to any one of claims 1 to 9 in an all-vanadium redox flow battery.

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