Graphite felt electrode for all-vanadium redox flow battery and preparation method thereof

Abstract

The application belongs to the technical field of liquid flow batteries, and relates to a graphite felt electrode for a vanadium redox flow battery and a preparation method thereof, wherein the graphite felt electrode has a three-dimensional gradient composite structure, and comprises: a pre-oxidized activated graphite felt substrate; a reduced graphene oxide conductive layer coated on the outer surface and the internal pore surface of the graphite felt substrate; and a transition metal sulfide nanocrystal catalytic layer uniformly distributed in the outer surface and the internal pores of the graphite felt substrate, wherein the transition metal sulfide nanocrystal is generated in situ during the preparation of the electrode and is firmly combined with the reduced graphene oxide conductive layer and the surface of the graphite felt fiber. The graphite felt electrode has high specific surface area, high catalytic activity, high interface bonding strength and high loading uniformity.

Description

Technical Field

[0001] This invention belongs to the field of vanadium redox flow battery technology, and particularly relates to a graphite felt electrode for vanadium redox flow batteries and its preparation method. Background Technology

[0002] Vanadium redox flow batteries (VRFBs), as a large-scale energy storage technology, rely heavily on the electrocatalytic activity, conductivity, and stability of their electrode materials for performance. Graphite felt, due to its high conductivity, high porosity, and good chemical stability, is widely used as the electrode substrate material for VRFBs. However, the high chemical inertness and limited number of catalytically active sites on the surface of raw graphite felt make it difficult to meet the requirements for efficient electrochemical reactions. Therefore, modifying graphite felt to improve its electrochemical performance has become a key research focus in this field.

[0003] Currently, the most common modification methods include surface modification of graphite felt using reduced graphene oxide or single metal sulfides. However, the resulting modified graphite felt electrodes have low specific surface area, insufficient active sites, weak interfacial bonding with the substrate, easy shedding of active components, and poor uniformity of electrode performance, resulting in poor overall electrode performance and an inability to achieve both high catalytic activity and long-term cycling stability.

[0004] The fabrication process of this type of modified graphite felt electrode typically includes the following technical features:

[0005] The graphite felt electrode substrate undergoes only routine cleaning and drying with solvents such as acetone, ethanol, and deionized water. The modified component is typically a single component of graphene oxide or a single component of other metal sulfides other than europium sulfide (EuS). The precursor solution uses only a single dispersant or no dispersant at all, and is simply stirred and mixed. The impregnation process is mostly a conventional single impregnation at atmospheric pressure. Crystallization and reduction are mostly performed using hydrothermal methods. In the final electrode, the active and conductive components are in a simple physical attachment state on the graphite felt surface, with no strong interfacial bonding or in-situ anchoring between the components.

[0006] The aforementioned technical characteristics result in a series of inherent defects in existing modified graphite felt electrodes, making it difficult for their performance to meet the requirements of large-scale energy storage applications:

[0007] 1. Low electrode specific surface area and severely insufficient electrode active sites: Due to the use of only a single dispersant or no dispersant in the electrode precursor solution, and simple stirring and mixing, graphene oxide sheets and metal sulfide precursors are prone to stacking and agglomeration, resulting in an excessively low specific surface area of ​​the final electrode. This low specific surface area directly limits the number of active sites available for vanadium ion reactions, becoming a core bottleneck restricting the improvement of battery coulombic efficiency, voltage efficiency, and energy efficiency.

[0008] 2. Weak substrate-to-interface bonding and easy detachment of active components: Because the graphite felt electrode substrate only undergoes routine cleaning and drying, the graphite felt surface remains in its original chemically inert state. The graphite felt surface has few oxygen-containing functional groups and poor hydrophilicity, meaning the modified components can only adhere to the graphite felt surface through weak physical adsorption, resulting in weak interfacial bonding. During long-term charge-discharge cycles of vanadium redox flow batteries, the modified components easily detach from the substrate, leading to electrode structure damage and rapid decay of catalytic performance. This results in poor battery cycle stability, making it unsuitable for the long-cycle energy storage applications of vanadium redox flow batteries.

[0009] 3. The impregnation process is rough and the uniformity of electrode performance is poor: Conventional impregnation methods cannot effectively remove air from the pores inside the graphite felt. As a result, the precursor solution can easily achieve local wetting and loading on the surface of the graphite felt, but it is difficult to penetrate into the pores inside the graphite felt. This leads to large deviations in the loading of modified components in different areas of the electrode, resulting in large differences in catalytic activity and conductivity in different areas of the electrode. The battery charge-discharge curve fluctuates greatly, and the performance consistency is poor, which affects the overall operational stability and service life of the battery stack.

[0010] 4. The preparation process is complex and not environmentally friendly, making it difficult to scale up: The traditional hydrothermal method requires the use of a high-pressure reactor, which is cumbersome and generates a large amount of cleaning wastewater. The environmental pressure and production costs are high, making it difficult to meet the needs of industrial mass production.

[0011] 5. Poor overall electrode performance, unable to balance high catalytic activity and long cycle stability: The above defects are interconnected and mutually influential. Low specific surface area leads to insufficient activity, weak interfacial bonding leads to easy shedding of modified components, and uneven loading leads to performance fluctuations. As a result, the graphite felt electrodes prepared by existing technologies cannot balance high catalytic activity and long cycle stability, and cannot meet the actual application requirements of efficient, stable, and large-scale energy storage of vanadium redox flow batteries.

[0012] Therefore, there is an urgent need for a graphite felt electrode and its preparation method that can achieve high specific surface area, high catalytic activity, high interfacial bonding strength and high loading uniformity. Summary of the Invention

[0013] To address the shortcomings of existing technologies, this invention provides a graphite felt electrode for vanadium redox flow batteries and its preparation method. The graphite felt electrode has high specific surface area, high catalytic activity, high interfacial bonding strength, and high load uniformity.

[0014] To achieve the above objectives, the main technical solutions adopted by the present invention include:

[0015] In a first aspect, the present invention provides a graphite felt electrode for a vanadium redox flow battery. The graphite felt electrode has a three-dimensional gradient composite structure, comprising: a pre-oxidized and activated graphite felt substrate; a reduced graphene oxide conductive layer covering the outer surface and the surface of the internal pores of the graphite felt substrate; and a transition metal sulfide nanocrystal catalytic layer uniformly distributed on the outer surface and the internal pores of the graphite felt substrate. The transition metal sulfide nanocrystals are generated in situ during the electrode preparation process and are firmly bonded to the reduced graphene oxide conductive layer and the surface of the graphite felt fibers.

[0016] Optionally, the transition metal sulfide nanocrystals have a particle size of 15 nm to 25 nm; and / or the graphite felt electrode has a specific surface area of ​​120 m². 2 / g or more; and / or transition metal sulfides are europium sulfide.

[0017] Secondly, the present invention provides a method for preparing the graphite felt electrode for an all-vanadium redox flow battery as described above, comprising the following steps:

[0018] Step S1: Immerse the graphite felt in a hydrogen peroxide solution for pre-oxidation and activation treatment to obtain a graphite felt substrate; disperse the transition metal source, sulfur source, graphene oxide and a compound dispersant containing sodium dodecyl sulfate and hexadecyltrimethylammonium bromide in water to prepare an impregnation solution.

[0019] Step S2: Immerse the graphite felt substrate in the impregnation solution and repeat the vacuum-decompression impregnation operation at least twice.

[0020] Step S3: Under a nitrogen atmosphere, the impregnated graphite felt is subjected to programmed temperature rise calcination to transform the components in the impregnation solution into a composite material of europium sulfide and reduced graphene oxide loaded on the graphite felt, thereby obtaining a graphite felt electrode.

[0021] Optionally, in step S1, the mass concentration of the hydrogen peroxide solution is 20wt%~35wt%, the temperature of the pre-oxidation activation treatment is 70℃~90℃, and the time is 1.5h~2.5h.

[0022] Optionally, in step S1, the step of preparing the impregnation solution includes: adding 0.08g~0.12g of europium chloride, 0.8g~1.2g of thiourea, a total amount of 0.25g~0.35g of compound dispersant, and 40μL~60μL of graphene oxide dispersion with a concentration of 2 mg / mL to every 80mL of deionized water, and adjusting the pH of the mixture to 3~4; the compound dispersant is a mixture of sodium dodecyl sulfate and hexadecyltrimethylammonium bromide.

[0023] Optionally, in step S1, the step of preparing the impregnation solution further includes: after adding the graphene oxide dispersion, adding 120 μL to 200 μL of 5 wt% Nafion solution.

[0024] Optionally, in the compound dispersant, the mass ratio of sodium dodecyl sulfate to hexadecyltrimethylammonium bromide is 1:0.8~1.2.

[0025] Optionally, the transition metal source is europium salt, copper salt, nickel salt, cobalt salt or zinc salt; the sulfur source is thiourea or thiosulfate.

[0026] Optionally, in step S3, the programmed temperature calcination includes: heating to 430°C to 470°C at a rate of 3°C / min to 7°C / min under continuous nitrogen gas supply, and holding at that temperature for 1.5h to 2.5h.

[0027] Thirdly, the present invention provides a graphite felt electrode for a vanadium redox flow battery, which is prepared by the preparation method described above.

[0028] The beneficial effects of this invention are:

[0029] The graphite felt electrode provided by this invention features a pre-oxidized and activated substrate that significantly enhances surface hydrophilicity and chemical activity, providing a foundation for the robust bonding of subsequent active components. A uniformly coated reduced graphene oxide conductive layer forms a high-speed electron transport network, significantly improving the overall conductivity of the electrode. Uniformly distributed and in-situ generated transition metal sulfide nanocrystals provide abundant and stable electrocatalytic active sites. This gradient composite structure of "substrate-conductive network-catalytic site" enables the electrode to simultaneously possess high catalytic activity, excellent conductivity, superior structural stability, and high load uniformity, thus achieving both high efficiency and long cycle life in vanadium redox flow batteries and meeting the application requirements for large-scale energy storage.

[0030] The method for preparing graphite felt electrodes provided by this invention systematically solves key defects in the prior art through the synergistic combination of four core steps: "hydrogen peroxide pre-oxidation activation," "SDS / CTAB compound dispersion," "vacuum-assisted impregnation," and "nitrogen atmosphere programmed temperature calcination." Pre-oxidation activation significantly enhances the substrate's hydrophilicity and interfacial bonding, preventing the active component from detaching at its source. Compound dispersion and vacuum impregnation ensure high dispersion of the active precursor and uniform loading throughout the porous substrate, laying the structural foundation for obtaining a high specific surface area. Nitrogen calcination achieves the reduction of graphene oxide and in-situ crystallization of europium sulfide nanocrystals in one step, making the process simple and environmentally friendly. This method ultimately efficiently and reliably prepares a composite electrode with high specific surface area, high catalytic activity, excellent conductivity, and strong structural stability, enabling vanadium redox flow batteries to simultaneously achieve high energy efficiency and long cycle life. Furthermore, the process itself is easily scalable, demonstrating significant comprehensive advantages. Attached Figure Description

[0031] The present invention is described with reference to the following figures:

[0032] Figure 1 This is a schematic flowchart of a method for preparing a graphite felt electrode according to a specific embodiment.

[0033] Figure 2 A comparison diagram of the electrode specific surface area of ​​each embodiment and each comparative example;

[0034] Figure 3 The graph shows the efficiency-cycle performance results for Example 1.

[0035] Figure 4 Example 1 at 200mA cm -2 Voltage efficiency-cycle stability curve;

[0036] Figure 5 The graph shows the efficiency-cycle performance results for Comparative Example 1.

[0037] Figure 6 The efficiency-cycle performance results are shown in the graph for Comparative Example 2.

[0038] Figure 7 The graph shows the efficiency-cycle performance results for Comparative Example 3.

[0039] Figure 8 The graph shows the efficiency-cycle performance results for Example 2.

[0040] Figure 9 Example 2 at 200mA cm -2 Voltage efficiency-cycle stability curve;

[0041] Figure 10 The graph shows the efficiency-cycle performance results for Example 3.

[0042] Figure 11 Example 3 at 200mA cm -2 Voltage efficiency-cycle stability curve;

[0043] Figure 12 The graph shows the efficiency-cycle performance results for Example 4.

[0044] Figure 13 Example 4 at 200mA cm -2 Voltage efficiency-cycle stability curve;

[0045] Figure 14 The graph shows the efficiency-cycle performance results for Example 5.

[0046] Figure 15 Example 5 at 200mA cm -2 Voltage efficiency-cycle stability curve;

[0047] Figure 16 The graph shows the efficiency-cycle performance results for Example 6.

[0048] Figure 17 Example 6 at 200mA cm -2 Voltage efficiency-cycle stability curve;

[0049] Figure 18 For each comparative example at 200mA cm -2 Voltage efficiency-cycle stability curve. Detailed Implementation

[0050] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly described below with reference to the accompanying drawings. 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 skilled in the art without creative effort are within the scope of protection of the present invention.

[0051] Unless otherwise defined, all technical and scientific terms used in this invention have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains; the terminology used in the specification of this application is for the purpose of describing particular embodiments only and is not intended to limit the invention; the terms "comprising" and "having," and any variations thereof, in the specification, claims, and foregoing drawings are intended to cover non-exclusive inclusion. The terms "first," "second," etc., in the specification, claims, or foregoing drawings are used to distinguish different objects, not to describe a particular order or hierarchy.

[0052] In this invention, the reference to "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of the invention. The appearance of this phrase in various places in the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments.

[0053] In the description of this invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "installation," "connection," "linking," and "attachment" should be interpreted broadly. For example, they can refer to direct connection or indirect connection through an intermediate medium. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0054] In this invention, the term "and / or" is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, or B existing alone. Additionally, in this invention, the character " / " generally indicates that the preceding and following related objects have an "or" relationship.

[0055] In the embodiments of the present invention, the same reference numerals denote the same components, and for the sake of brevity, detailed descriptions of the same components are omitted in different embodiments. It should be understood that the thickness, length, width, and other dimensions of various components in the embodiments of the present invention shown in the accompanying drawings, as well as the overall thickness, length, width, and other dimensions of the integrated device, are merely illustrative and should not constitute any limitation on the present invention.

[0056] In this invention, "multiple" refers to two or more, including two.

[0057] This invention provides a graphite felt electrode for a vanadium redox flow battery. The graphite felt electrode has a three-dimensional gradient composite structure, comprising: a pre-oxidized and activated graphite felt substrate; a reduced graphene oxide conductive layer covering the outer surface and internal pore surfaces of the graphite felt substrate; and a transition metal sulfide nanocrystal catalytic layer uniformly distributed on the outer surface and internal pores of the graphite felt substrate. The transition metal sulfide nanocrystals are generated in situ during the electrode preparation process and are firmly bonded to the reduced graphene oxide conductive layer and the surface of the graphite felt fibers.

[0058] Thus, the pre-oxidized and activated substrate significantly enhances surface hydrophilicity and chemical activity, providing a foundation for the robust bonding of subsequent active components. The uniform coating of the substrate with the reduced graphene oxide conductive layer forms a high-speed electron transport network, significantly improving the overall conductivity of the electrode. The uniformly distributed and in-situ generated transition metal sulfide nanocrystals provide abundant and stable electrocatalytic active sites. This gradient composite structure of "substrate-conductive network-catalytic site" enables the electrode to simultaneously possess high catalytic activity, excellent conductivity, outstanding structural stability, and high loading uniformity, thereby achieving both high efficiency and long cycle life in vanadium redox flow batteries and meeting the application requirements of large-scale energy storage.

[0059] Preferably, the transition metal sulfide nanocrystals have a particle size of 15 nm to 25 nm.

[0060] Preferably, the specific surface area of ​​the graphite felt electrode is 120 m². 2 / g or more.

[0061] Preferably, the transition metal sulfide is any one of iron sulfide, copper sulfide, nickel sulfide, cobalt sulfide, and europium sulfide. More preferably, the transition metal sulfide is europium sulfide. Based on the unique electronic structure of europium sulfide, it exhibits superior intrinsic electrocatalytic activity and stability in the vanadium ion reaction of vanadium redox flow batteries, thereby further improving the overall performance of the battery.

[0062] like Figure 1 As shown, the present invention also provides a method for preparing the above-mentioned graphite felt electrode for an all-vanadium redox flow battery, comprising the following steps:

[0063] Step S1: Immerse the graphite felt in a hydrogen peroxide solution for pre-oxidation and activation treatment to obtain a graphite felt substrate; disperse the transition metal source, sulfur source, graphene oxide and a compound dispersant containing sodium dodecyl sulfate and hexadecyltrimethylammonium bromide in water to prepare an impregnation solution.

[0064] Specifically, in step S1, before pre-oxidation and activation treatment of the graphite felt, the following steps are included: cutting commercial graphite felt to the required size (e.g., 2 cm × 3 cm), and then sequentially placing it in acetone, anhydrous ethanol, and deionized water for ultrasonic cleaning, with each ultrasonic cleaning session preferably lasting 10-20 minutes. After cleaning, the graphite felt is placed in a vacuum drying oven and dried at 50-70°C for 5-8 hours to thoroughly remove impurities and solvent residues from the graphite felt surface.

[0065] Specifically, the hydrogen peroxide solution concentration is 20wt%~35wt%, the pre-oxidation activation treatment temperature is 70℃~90℃, and the time is 1.5h~2.5h. In this way, hydrogen peroxide can effectively etch the surface of the graphite felt fibers and introduce abundant oxygen-containing functional groups, thereby significantly improving its hydrophilicity and reactivity. This lays the foundation for the subsequent strong bonding of the europium sulfide active phase and the reduced graphene oxide active phase to the graphite felt substrate, while avoiding damage to the substrate structure due to excessively harsh treatment conditions.

[0066] Further, after the pre-oxidation activation treatment, the graphite felt is removed and rinsed with plenty of deionized water until neutral to remove residual hydrogen peroxide and reaction byproducts. The graphite felt is then placed in a vacuum drying oven at 50–70°C and dried for 5–8 hours to obtain a graphite felt substrate with a surface rich in oxygen-containing functional groups.

[0067] Specifically, the steps for preparing the impregnation solution include: adding 0.08g~0.12g of europium chloride (EuCl3·6H2O), 0.8g~1.2g of thiourea (CH4N2S), a total of 0.25g~0.35g of a compound dispersant, and 40μL~60μL of a graphene oxide dispersion with a concentration of 2 mg / mL, based on 80mL of deionized water, and adjusting the pH of the mixture to 3~4; the compound dispersant is a mixture of sodium dodecyl sulfate (SDS) and hexadecyltrimethylammonium bromide (CTAB). Under this formulation and pH control, europium ions and graphene oxide can be highly uniformly and stably dispersed under the action of the compound dispersant, inhibiting the aggregation of active precursors and the stacking of conductive sheets from the source, thus ensuring the subsequent calcination to obtain a high specific surface area, uniformly dispersed, and structurally stable europium sulfide / reduced graphene oxide composite catalyst layer.

[0068] Preferably, in the compound dispersant, the mass ratio of sodium dodecyl sulfate to hexadecyltrimethylammonium bromide is 1:0.8~1.2. This optimizes the synergistic effect of sodium dodecyl sulfate and hexadecyltrimethylammonium bromide in solution, thereby more effectively achieving the exfoliation of graphene oxide sheets and the stable dispersion of europium ions.

[0069] Preferably, the step of preparing the impregnation solution further includes adding 120 μL to 200 μL of a 5 wt% Nafion solution after adding the graphene oxide dispersion. By adding Nafion solution as a binder stabilizer, the interfacial bonding force between the active components and between them and the graphite felt substrate can be effectively enhanced during subsequent heat treatment, thereby improving the structural integrity and long-term cycling stability of the final composite electrode.

[0070] Further, as an example, the steps for preparing the impregnation solution include: using 80 mL of deionized water as a reference, adding concentrated hydrochloric acid dropwise to adjust the pH of the aqueous solution to a weakly acidic range of 3-4. To this acidic aqueous solution, 0.08 g-0.12 g of europium chloride and 0.8 g-1.2 g of thiourea are added sequentially, and the mixture is stirred at room temperature until both are completely dissolved. Subsequently, a total amount of 0.25 g-0.35 g of a compound dispersant, which is a mixture of sodium dodecyl sulfate and hexadecyltrimethylammonium bromide at a mass ratio of 1:0.8-1.2, is added to the solution. The mixture is stirred continuously at 55-65°C for 1 hour to ensure that the dispersant is fully dissolved and forms a uniform and stable dispersion system. Afterwards, 40-60 μL of a 2 mg / mL graphene oxide dispersion and 120-200 μL of a 5% Nafion solution are added dropwise to the system as a binder stabilizer. Finally, the entire mixture is ultrasonically stirred for 30-50 minutes to obtain the impregnation solution. By precisely controlling the pH, temperature, feeding sequence, and mixing conditions, the precursor, dispersant, and binder are systematically and reproducibly formulated, thereby ensuring the acquisition of a uniform and stable impregnation solution with highly dispersed components. This provides a reliable process basis for the final formation of a high-performance and consistent composite catalytic layer on the graphite felt.

[0071] Step S2: Immerse the graphite felt substrate in the impregnation solution and repeat the vacuum-decompression impregnation operation at least twice.

[0072] The vacuum-depressurization impregnation process involves placing the graphite felt substrate immersed in the impregnation solution in a sealed environment. First, a vacuum is applied (e.g., to -0.07 MPa ~ -0.09 MPa) to forcibly expel the air trapped within the three-dimensional porous structure of the graphite felt substrate. Then, the pressure is slowly released, using external atmospheric pressure to fully force the impregnation solution into the expelled pores. This process is repeated at least twice to ensure complete and thorough wetting and filling of all accessible pores in the graphite felt, from the surface to the interior. This lays the structural foundation for achieving uniform loading of europium sulfide and reduced graphene oxide active components throughout the graphite felt electrode.

[0073] Specifically, in step S2, during each vacuum-decompression impregnation operation, the pressure is evacuated to -0.07 MPa to -0.09 MPa and maintained for 10 to 20 minutes. Further, the vacuum-decompression impregnation operation is repeated 3 to 4 times.

[0074] Step S3: Under a nitrogen atmosphere, the impregnated graphite felt is subjected to programmed temperature rise calcination to transform the components in the impregnation solution into a composite material of europium sulfide and reduced graphene oxide loaded on the graphite felt, thereby obtaining a graphite felt electrode.

[0075] By performing programmed temperature calcination under a nitrogen inert atmosphere, the thermal reduction of graphene oxide and the in-situ generation and crystallization of europium sulfide nanocrystals were achieved in one step. This process is not only concise and environmentally friendly, facilitating large-scale production, but also ensures the simultaneous formation of high conductivity of the reduced graphene oxide, the intact crystalline phase of europium sulfide, and strong interfacial bonds between the components. This synergistically constructs a composite electrode with a stable structure, excellent conductive network, and abundant catalytically active sites.

[0076] Preferably, the programmed temperature calcination includes: heating to 430℃~470℃ at a rate of 3℃ / min~7℃ / min under continuous nitrogen gas purging, and holding at this temperature for 1.5h~2.5h. In this way, graphene oxide is fully thermally reduced to a highly conductive network, while simultaneously promoting the formation of a complete europium sulfide crystalline phase and the stable growth of nanocrystals, and forming strong interfacial bonds between the components, thereby synergistically optimizing the conductivity, catalytic activity, and structural stability of the composite electrode.

[0077] The method for preparing graphite felt electrodes provided by this invention systematically solves key defects in the prior art through the synergistic combination of four core steps: "hydrogen peroxide pre-oxidation activation," "SDS / CTAB compound dispersion," "vacuum-assisted impregnation," and "nitrogen atmosphere programmed temperature calcination." Pre-oxidation activation significantly enhances the substrate's hydrophilicity and interfacial bonding, preventing the active component from detaching at its source. Compound dispersion and vacuum impregnation ensure high dispersion of the active precursor and uniform loading throughout the porous substrate, laying the structural foundation for obtaining a high specific surface area. Nitrogen calcination achieves the reduction of graphene oxide and in-situ crystallization of europium sulfide nanocrystals in one step, making the process simple and environmentally friendly. This method ultimately efficiently and reliably prepares a composite electrode with high specific surface area, high catalytic activity, excellent conductivity, and strong structural stability, enabling vanadium redox flow batteries to simultaneously achieve high energy efficiency and long cycle life. Furthermore, the process itself is easily scalable, demonstrating significant comprehensive advantages.

[0078] Example 1:

[0079] This embodiment provides a method for preparing a graphite felt electrode for a vanadium redox flow battery, the specific steps of which are as follows:

[0080] Step S1: Cut commercial graphite felt into 2 cm × 3 cm pieces, then sequentially place them in acetone, anhydrous ethanol, and deionized water, and ultrasonically clean each for 15 minutes. After cleaning, place the graphite felt in a vacuum drying oven and dry at 60 °C for 6 hours to thoroughly remove surface impurities and solvents. Completely immerse the dried graphite felt in a 30 wt% hydrogen peroxide aqueous solution and maintain it in an 80 °C constant temperature water bath for 2 hours for pre-oxidation activation. After treatment, remove the graphite felt and rinse repeatedly with deionized water until neutral to remove residual reactants. Then, vacuum dry it again at 60 °C for 6 hours to obtain a graphite felt substrate with a surface rich in oxygen-containing functional groups.

[0081] Measure 80 mL of deionized water and add concentrated hydrochloric acid dropwise to adjust the pH of the solution to 3–4. In this weakly acidic solution, add 0.1 g of europium chloride hexahydrate (EuCl3·6H2O) and 1.0 g of thiourea sequentially, and stir at room temperature until completely dissolved. Then, add a compound dispersant consisting of 0.15 g of sodium dodecyl sulfate and 0.15 g of hexadecyltrimethylammonium bromide, and stir at 60 °C for 1 hour to ensure complete dissolution of the dispersant. Next, add 50 μL of a 2 mg / mL graphene oxide dispersion and 160 μL of a 5% (w / w) Nafion solution sequentially to the system. Finally, subject the mixed solution to ultrasonic-assisted stirring for 30 minutes to obtain a homogeneous and stable impregnation solution.

[0082] Step S2: Completely immerse the graphite felt substrate in the impregnation solution. Place the entire system in a sealed container, evacuate to -0.08 MPa and maintain for 15 minutes, then slowly release the pressure. Repeat this "vacuum-release" operation three times. Afterward, seal the system and place it in a 60 ℃ constant temperature water bath for 2 hours to impregnate.

[0083] Step S3: Remove the impregnated graphite felt and vacuum dry it at 60 °C for 6 hours. Then, transfer it to a tube furnace and heat it to 450 °C at a programmed heating rate of 5 °C / min under an inert atmosphere of continuously flowing high-purity nitrogen (flow rate 50 sccm), holding it at this temperature for 2 hours for heat treatment. Finally, allow it to cool naturally to room temperature under nitrogen protection to obtain the europium sulfide / reduced graphene oxide composite graphite felt electrode (labeled EuS / rGO / GF).

[0084] The composite graphite felt electrode prepared in Example 1 was characterized structurally and its electrochemical performance was tested.

[0085] like Figure 2As shown, the specific surface area of ​​this composite electrode, measured using the nitrogen adsorption-desorption (BET) method, is 134.3 m² / g, significantly higher than that of the conventional modified electrode mentioned in the background (approximately 90 m² / g). The composite electrode was then assembled into a vanadium redox flow battery for testing, as shown... Figure 3 As shown, at 200 mA cm -2 At the specified current density, the battery exhibits a coulombic efficiency of 98.6%, a voltage efficiency of 87.7%, and an energy efficiency of 86.4%, all at high levels, indicating excellent electrocatalytic activity and conductivity of the electrode. During 1000 charge-discharge cycles, the battery's coulombic efficiency, voltage efficiency, and energy efficiency remain stable. Specifically, as... Figure 4 As shown, the voltage efficiency decay rate after 1000 cycles is only 2.2%, which fully demonstrates that the three-dimensional gradient composite structure constructed by the integrated process of "pre-oxidation activation-compound dispersion-vacuum impregnation-nitrogen calcination" has excellent mechanical and chemical stability, which can effectively prevent the active components from falling off and deactivating during long-term cycling, and meet the application requirements of long-cycle and large-scale energy storage of vanadium redox flow batteries.

[0086] Comparative Example 1:

[0087] Compared with Example 1, this comparative example omits the hydrogen peroxide pre-oxidation activation treatment in step S1, and all other contents are the same as in Example 1, and will not be repeated here.

[0088] The composite graphite felt electrode prepared in Comparative Example 1 was characterized in structure and tested for electrochemical performance.

[0089] like Figure 2 As shown, the specific surface area of ​​the composite electrode, measured using the nitrogen adsorption-desorption (BET) method, is 110.9 m² / g. The composite electrode was then assembled into a vanadium redox flow battery for testing. Figure 5 As shown, at 200 mA cm -2 At the specified current density, the battery exhibits a coulombic efficiency of 97.0%, a voltage efficiency of 86.3%, and an energy efficiency of 83.7%. However, during a charge-discharge cycle of up to 1000 times, the battery's coulombic efficiency, voltage efficiency, and energy efficiency all show a significant decrease, particularly in the following areas: Figure 18 As shown, the voltage efficiency decays by 7.6% after 1000 cycles.

[0090] Comparative Example 2:

[0091] Compared with Example 1, this comparative example omits the addition of the compound dispersant in step S1, and all other contents are the same as in Example 1, which will not be repeated here.

[0092] The composite graphite felt electrode prepared in Comparative Example 2 was characterized in structure and tested for electrochemical performance.

[0093] like Figure 2 As shown, the specific surface area of ​​the composite electrode, measured using the nitrogen adsorption-desorption (BET) method, is 89.2 m² / g. The composite electrode was then assembled into a vanadium redox flow battery for testing. Figure 6 As shown, at 200 mA cm -2 At the specified current density, the battery exhibits a coulombic efficiency of 97.5%, a voltage efficiency of 85.8%, and an energy efficiency of 83.6%. However, during a charge-discharge cycle of up to 1000 times, the battery's coulombic efficiency, voltage efficiency, and energy efficiency all show a significant decrease, particularly in the following areas: Figure 18 As shown, the voltage efficiency decays by 8.4% after 1000 cycles.

[0094] Comparative Example 3:

[0095] Compared with Example 1, this comparative example omits the hydrogen peroxide pre-oxidation activation treatment and the addition of compound dispersant in step S1. All other contents are the same as in Example 1, and will not be repeated here.

[0096] The composite graphite felt electrode prepared in Comparative Example 3 was characterized in structure and tested for electrochemical performance.

[0097] like Figure 2 As shown, the specific surface area of ​​the composite electrode, measured using the nitrogen adsorption-desorption (BET) method, is 86.8 m² / g. The composite electrode was then assembled into a vanadium redox flow battery for testing. Figure 7 As shown, at 200 mA cm -2 At the specified current density, the battery exhibits a coulombic efficiency of 97.3%, a voltage efficiency of 85.7%, and an energy efficiency of 83.4%. However, during a charge-discharge cycle of up to 1000 times, the battery's coulombic efficiency, voltage efficiency, and energy efficiency all show a significant decrease, particularly in the following areas: Figure 18 As shown, the voltage efficiency decays by 10.7% after 1000 cycles.

[0098] Example 2:

[0099] Compared with Example 1, the impregnation solution in this example no longer contains europium chloride hexahydrate and thiourea, but instead contains 0.1 g of ferric nitrate nonahydrate and 1.0 g of sodium thiosulfate. All other contents are the same as in Example 1, and will not be repeated here.

[0100] The composite graphite felt electrode prepared in Example 2 was characterized structurally and its electrochemical performance was tested.

[0101] like Figure 2As shown, the specific surface area of ​​the composite electrode, measured using the nitrogen adsorption-desorption (BET) method, is 129.5 m² / g. This composite electrode was then tested in a vanadium redox flow battery at 200 mA cm⁻¹. -2 At current densities, such as Figure 8 As shown, the battery has a coulombic efficiency of 97.8%, a voltage efficiency of 87.1%, and an energy efficiency of 85.2%. During 1000 charge-discharge cycles, the battery's coulombic efficiency, voltage efficiency, and energy efficiency all exhibit a certain degree of stability. Specifically, as... Figure 9 As shown, the voltage efficiency decays by 4.2% after 1000 cycles.

[0102] Example 3:

[0103] Compared with Example 1, the impregnation solution in this example no longer contains europium chloride hexahydrate and thiourea, but instead contains 0.1 g of copper nitrate hexahydrate and 1.0 g of sodium thiosulfate. All other contents are the same as in Example 1, and will not be repeated here.

[0104] The composite graphite felt electrode prepared in Example 3 was characterized structurally and its electrochemical performance was tested.

[0105] like Figure 2 As shown, the specific surface area of ​​the composite electrode, measured using the nitrogen adsorption-desorption (BET) method, is 131.4 m² / g. This composite electrode was then tested in a vanadium redox flow battery at 200 mA cm⁻¹. -2 At current densities, such as Figure 10 As shown, the battery has a coulombic efficiency of 97.6%, a voltage efficiency of 87.2%, and an energy efficiency of 85.1%. During 1000 charge-discharge cycles, the battery's coulombic efficiency, voltage efficiency, and energy efficiency all exhibit a certain degree of stability. Specifically, as... Figure 11 As shown, the voltage efficiency decays by 4.5% after 1000 cycles.

[0106] Example 4:

[0107] Compared with Example 1, the impregnation solution in this example no longer contains europium chloride hexahydrate and thiourea, but instead contains 0.1 g nickel nitrate hexahydrate and 1.0 g sodium thiosulfate. All other contents are the same as in Example 1, and will not be repeated here.

[0108] The composite graphite felt electrode prepared in Example 4 was characterized structurally and its electrochemical performance was tested.

[0109] like Figure 2As shown, the specific surface area of ​​the composite electrode, measured using the nitrogen adsorption-desorption (BET) method, is 133.2 m² / g. This composite electrode was then tested in a vanadium redox flow battery at 200 mA cm⁻¹. -2 At current densities, such as Figure 12 As shown, the battery has a coulombic efficiency of 97.7%, a voltage efficiency of 87.0%, and an energy efficiency of 85.0%. During 1000 charge-discharge cycles, the battery's coulombic efficiency, voltage efficiency, and energy efficiency all exhibit a certain degree of stability. Specifically, as... Figure 13 As shown, the voltage efficiency decays by 3.3% after 1000 cycles.

[0110] Example 5:

[0111] Compared with Example 1, the impregnation solution in this example no longer contains europium chloride hexahydrate and thiourea, but instead contains 0.1 g of cobalt nitrate hexahydrate and 1.0 g of sodium thiosulfate. All other contents are the same as in Example 1, and will not be repeated here.

[0112] The composite graphite felt electrode prepared in Example 5 was characterized structurally and its electrochemical performance was tested.

[0113] like Figure 2 As shown, the specific surface area of ​​the composite electrode, measured using the nitrogen adsorption-desorption (BET) method, is 130.6 m² / g. This composite electrode was then tested in a vanadium redox flow battery at 200 mA cm⁻¹. -2 At current densities, such as Figure 14 As shown, the battery has a coulombic efficiency of 97.5%, a voltage efficiency of 86.9%, and an energy efficiency of 84.7%. During 1000 charge-discharge cycles, the battery's coulombic efficiency, voltage efficiency, and energy efficiency all exhibit a certain degree of stability. Specifically, as... Figure 15 As shown, the voltage efficiency decays by 5.9% after 1000 cycles.

[0114] Example 6:

[0115] Compared with Example 1, the impregnation solution in this example no longer contains europium chloride hexahydrate and thiourea, but instead contains 0.1 g zinc nitrate hexahydrate and 1.0 g sodium thiosulfate. All other contents are the same as in Example 1, and will not be repeated here.

[0116] The composite graphite felt electrode prepared in Example 6 was characterized structurally and its electrochemical performance was tested.

[0117] like Figure 2 As shown, the specific surface area of ​​the composite electrode, measured using the nitrogen adsorption-desorption (BET) method, is 131.9 m² / g. The composite electrode was then assembled into a vanadium redox flow battery for testing. Figure 16 As shown, at 200 mA cm -2 At the specified current density, the battery exhibits a coulombic efficiency of 97.6%, a voltage efficiency of 87.0%, and an energy efficiency of 84.9%. During a charge-discharge cycle of up to 1000 times, the battery's coulombic efficiency, voltage efficiency, and energy efficiency all demonstrate a certain degree of stability. Specifically, for example... Figure 17 As shown, the voltage efficiency decays by 4.9% after 1000 cycles.

[0118] Comparing Comparative Example 1 and Comparative Example 3, it can be seen that adding a compound dispersant can increase the active surface area of ​​the electrode reaction, thereby improving voltage efficiency and energy efficiency. Specifically, the addition of the compound dispersant increases the surface area by (110.9-86.8) / 86.8×100%=27.76%; increases the coulombic efficiency by (97.0-97.3) / 97.3×100%=-0.31%; increases the voltage efficiency by (86.3-85.7) / 85.7×100%=0.70%; increases the energy efficiency by (83.7-83.4) / 83.4×100%=0.36%; and inhibits the cycle decay rate of voltage efficiency by (10.7-7.6) / 10.7×100%=28.97%.

[0119] Comparing Comparative Example 2 with Comparative Example 3, it can be seen that the pre-oxidation activation treatment introduces oxygen-containing active groups, improves interfacial properties, and thus enhances the structural stability of the electrode. Specifically, the hydrogen peroxide pre-oxidation activation treatment increases the specific surface area by (89.2-86.8) / 86.8×100%=2.76%; increases the battery coulombic efficiency by (97.5-97.3) / 97.3×100%=0.21%; increases the battery voltage efficiency by (85.8-85.7) / 85.7×100%=0.12%; increases the battery energy efficiency by (83.6-83.4) / 83.4×100%=0.24%; and inhibits the cycle decay rate of voltage efficiency by (10.7-8.4) / 10.7×100%=21.50%.

[0120] Comparing Example 1 with Comparative Example 3, it can be seen that when pre-oxidation activation treatment and compound dispersant are performed simultaneously, the battery's three main efficiencies and specific surface area are significantly improved. Specifically, the combined effect of hydrogen peroxide pre-oxidation activation treatment and compound dispersant addition on the specific surface area is (134.3-86.8) / 86.8×100%=54.72%; the combined effect of hydrogen peroxide pre-oxidation activation treatment and compound dispersant addition on the battery's coulombic efficiency is (98.6-97.3) / 97.3×100%=1.34%; and the combined effect of hydrogen peroxide pre-oxidation activation treatment and compound dispersant addition on the battery's voltage efficiency is... The effect of hydrogen peroxide pre-oxidation activation treatment and the addition of compound dispersant on improving battery energy efficiency is (87.7-85.7) / 85.7×100%=2.33%; the combined effect of hydrogen peroxide pre-oxidation activation treatment and the addition of compound dispersant on improving battery energy efficiency is (86.4-83.4) / 83.4×100%=3.60%; the combined effect of hydrogen peroxide pre-oxidation activation treatment and the addition of compound dispersant on inhibiting the cycle decay rate of voltage efficiency is (10.7-2.2) / 10.7×100%=79.44%.

[0121] In summary, the combined surface area improvement of hydrogen peroxide pre-oxidation activation treatment and the addition of compound dispersants (54.72%) is significantly greater than the sum of the surface area improvement of hydrogen peroxide pre-oxidation activation treatment (2.76%) and the surface area improvement of compound dispersants (27.76%). The combined effect of hydrogen peroxide pre-oxidation activation treatment and the addition of compound dispersants on the coulombic efficiency improvement (1.34%) is significantly greater than the sum of the voltage efficiency improvement of hydrogen peroxide pre-oxidation activation treatment (0.21%) and the coulombic efficiency improvement of compound dispersants (-0.31%). The combined effect of hydrogen peroxide pre-oxidation activation treatment and the addition of compound dispersants on the voltage efficiency improvement (2.33%) is significantly greater than the effect of hydrogen peroxide pre-oxidation activation treatment on the overall battery efficiency improvement. The combined effect of the 0.12% improvement in voltage efficiency and the 0.70% improvement in battery voltage efficiency from the addition of the compound dispersant; the combined effect of hydrogen peroxide pre-oxidation activation treatment and the addition of the compound dispersant on battery energy efficiency is 3.60%, which is much greater than the combined effect of hydrogen peroxide pre-oxidation activation treatment on battery energy efficiency (0.24%) and the combined effect of the compound dispersant on battery energy efficiency (0.36%); the combined effect of hydrogen peroxide pre-oxidation activation treatment and the addition of the compound dispersant on inhibiting the cycle decay rate of voltage efficiency is 79.44%, which is much greater than the combined effect of hydrogen peroxide pre-oxidation activation treatment on inhibiting the cycle decay rate of voltage efficiency (21.50%) and the combined effect of the compound dispersant on inhibiting the cycle decay rate of voltage efficiency (28.97%).

[0122] As can be seen, in Example 1, the pre-oxidation activation treatment of the substrate and the addition of the compound dispersant produced a synergistic effect. Specifically, the pre-oxidation activation introduces active sites on the substrate surface, enhancing the binding ability between the substrate and the active material. The compound dispersant can inhibit particle agglomeration and increase the electrode specific surface area. Together, they optimized the electrode structure, significantly enhanced the electrode specific surface area, catalytic ability, and structural stability, thereby improving the overall electrochemical performance of the battery.

[0123] Comparing Examples 1 to 6, it can be seen that the europium sulfide-modified graphite felt electrode exhibits a significant improvement in electrochemical performance compared to graphite felt electrodes modified with other metal sulfides. The coulombic efficiency, voltage efficiency, and energy efficiency of the battery all increased by more than 0.5%. In the field of vanadium redox flow batteries, an increase of more than 0.5% in coulombic efficiency, voltage efficiency, and energy efficiency compared to previous methods is considered a significant improvement.

[0124] The above are merely preferred embodiments of this application and are not intended to limit this application. Various modifications and variations can be made to this application by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A method for preparing a graphite felt electrode for a vanadium redox flow battery, characterized in that, Includes the following steps: Step S1: Immerse the graphite felt in a hydrogen peroxide solution with a mass concentration of 20wt%~35wt% for pre-oxidation activation treatment to obtain a graphite felt substrate; disperse the transition metal source, sulfur source, graphene oxide, and a compound dispersant containing sodium dodecyl sulfate and hexadecyltrimethylammonium bromide in water to prepare an impregnation solution; wherein the mass ratio of sodium dodecyl sulfate to hexadecyltrimethylammonium bromide is 1:0.8~1.

2. Step S2: Immerse the graphite felt substrate in the impregnation solution and repeat the vacuum-decompression impregnation operation at least twice. Step S3: Under a nitrogen atmosphere, the impregnated graphite felt is subjected to programmed temperature rise calcination to transform the components in the impregnation solution into a composite material of europium sulfide and reduced graphene oxide loaded on the graphite felt, thereby obtaining a graphite felt electrode.

2. The method for preparing the graphite felt electrode for a vanadium redox flow battery according to claim 1, characterized in that, In step S1, the temperature of the pre-oxidation activation treatment is 70℃~90℃, and the time is 1.5h~2.5h.

3. The method for preparing the graphite felt electrode for an all-vanadium redox flow battery according to claim 1, characterized in that, In step S1, the step of preparing the impregnation solution includes: Based on 80 mL of deionized water, 0.08 g to 0.12 g of europium chloride, 0.8 g to 1.2 g of thiourea, 0.25 g to 0.35 g of compound dispersant, and 40 μ L to 60 μ L of graphene oxide dispersion with a concentration of 2 mg / mL were added sequentially, and the pH of the mixture was adjusted to 3 to 4. The compound dispersant was a mixture of sodium dodecyl sulfate and hexadecyltrimethylammonium bromide.

4. The method for preparing the graphite felt electrode for a vanadium redox flow battery according to claim 3, characterized in that, In step S1, the step of preparing the impregnation solution further includes: after adding the graphene oxide dispersion, adding 120 μL to 200 μL of 5 wt% Nafion solution.

5. The method for preparing the graphite felt electrode for an all-vanadium redox flow battery according to claim 1, characterized in that, The transition metal source is europium salt, copper salt, nickel salt, cobalt salt, or zinc salt; the sulfur source is thiourea or thiosulfate.

6. The method for preparing the graphite felt electrode for an all-vanadium redox flow battery according to claim 1, characterized in that, In step S3, the programmed temperature calcination includes: heating to 430℃~470℃ at a rate of 3℃ / min~7℃ / min under continuous nitrogen gas supply, and holding at this temperature for 1.5h~2.5h.

7. A graphite felt electrode for an all-vanadium redox flow battery, characterized in that, It is prepared by the preparation method according to any one of claims 1 to 6.

8. The graphite felt electrode for a vanadium redox flow battery according to claim 7, characterized in that, The graphite felt electrode has a three-dimensional gradient composite structure, including: The graphite felt substrate has undergone pre-oxidation and activation; a reduced graphene oxide conductive layer covers the outer surface and internal pore surfaces of the graphite felt substrate; and a transition metal sulfide nanocrystal catalytic layer is uniformly distributed on the outer surface and internal pores of the graphite felt substrate. The transition metal sulfide nanocrystals are generated in situ during the electrode preparation process and are firmly bonded to the reduced graphene oxide conductive layer and the surface of the graphite felt fibers.

9. The graphite felt electrode for a vanadium redox flow battery according to claim 8, characterized in that, The transition metal sulfide nanocrystals have a particle size of 15 nm to 25 nm; and / or the graphite felt electrode has a specific surface area of ​​120 m². 2 / g or more; and / or transition metal sulfides are europium sulfide.

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