Method for preparing graphite felt-graphene composite electrode, electrode and battery

Abstract

The invention relates to a method for preparing a graphite felt-graphene composite electrode through covalent interface engineering, an electrode and a battery, and the method comprises the following steps: carrying out N / S co-doping pretreatment on graphite felt GF, preparing a graphene oxide aqueous solution, and carrying out ultrasonic dispersion to obtain a uniform GO dispersion liquid; completely immersing the N / S-GF substrate into the GO dispersion liquid to adsorb graphene oxide, and drying to obtain an N / S-GF (at) GO intermediate; the N / S-GF (at) GO intermediate and hydroiodic acid HI are subjected to a reduction reaction at the high temperature of 155-165 DEG C, graphene oxide is reduced into reduced graphene oxide rGO, meanwhile, sulfydryl-SH on the surface of an N / S-GF substrate and epoxy group C-O-C are subjected to a nucleophilic ring-opening reaction, a C-S-C thioether covalent bond is formed in situ, and a reduced graphene oxide rGO nanosheet is chemically bonded to the GF substrate; and washing and drying the reacted sample to obtain the N / S-GF (at) rGO composite electrode.

Description

Technical Field

[0001] This invention relates to the field of electrode technology, and in particular to a method for preparing a graphite felt-graphene composite electrode, the electrode, and a battery through covalent interface engineering. Background Technology

[0002] Vanadium redox flow batteries are widely considered one of the most promising solutions to large-scale energy storage problems due to their excellent cycle life, independently adjustable energy and power density, high safety, and environmental friendliness. However, the commercialization of vanadium redox flow batteries is still severely constrained by their core component—electrode materials. While traditional commercial graphite felt electrodes are chemically stable and have a porous structure, their inherent hydrophobicity, low electrochemical active surface area, and susceptibility to V2O ... 2+ / V 3+ and VO 2+ / VO2 + The sluggish electron transfer kinetics of redox couples lead to severe polarization losses, low energy efficiency, and low power density, constituting a major bottleneck in the development of vanadium redox flow battery technology. To overcome this bottleneck, researchers have explored various modification strategies. On the one hand, metals or metal oxides are introduced as catalysts; however, metal-based catalysts generally face problems such as high cost, potential for hydrogen evolution side reactions, and susceptibility to metal leaching and catalyst deactivation during long-term operation in strongly acidic electrolytes, limiting their commercial application. On the other hand, composite high-specific-surface-area carbon nanomaterials are widely used to increase active sites and improve conductivity. However, these carbon nanomaterials typically rely on weak van der Waals forces for physical composite with graphite felt substrates, leading to two key problems: 1) a large interfacial charge transfer resistance hinders effective electron transport between the catalytic site and the current collector; 2) the catalyst is prone to physical stripping under long-term high-speed scouring in the electrolyte, resulting in rapid performance degradation. Therefore, existing electrode designs generally face the "trilemma" of how to synergistically solve the "catalytic activity-conductivity-stability" dilemma. There is an urgent need for a design strategy that can achieve chemical bonding, establish continuous electron transport pathways, and generate multifunctional catalytic centers in an integrated hierarchical structure.

[0003] The information disclosed in the background section is only for enhancing the understanding of the background of this invention, and therefore may contain information that does not constitute prior art known to those skilled in the art. Summary of the Invention

[0004] This invention provides a method, electrode, and battery for preparing graphite felt-graphene composite electrodes through covalent interface engineering. Reduced graphene oxide (rGO) nanosheets are firmly anchored on the graphite felt GF substrate through in-situ formed CSC sulfide covalent bonds, thus synergistically solving the problems of activity, conductivity, and stability.

[0005] A method for preparing graphite felt-graphene composite electrodes via covalent interface engineering includes:

[0006] Step 1: N / S co-doping pretreatment of graphite felt GF, wherein graphite felt GF is electrochemically oxidized in (NH4)2SO4 dispersion to achieve nitrogen / sulfur co-doping, and reactive mercapto-SH groups are introduced on the surface of graphite felt to obtain N / S-GF substrate.

[0007] Step 2: Prepare an aqueous solution of graphene oxide and disperse it ultrasonically to obtain a uniform GO dispersion;

[0008] Step 3: The N / S-GF substrate is completely immersed in the GO dispersion to adsorb graphene oxide, and then dried to obtain the N / S-GF@GO intermediate;

[0009] Step 4: The N / S-GF@GO intermediate is reduced with hydroiodic acid (HI) at a high temperature of 155-165°C for 2.5-3.5 hours to reduce graphene oxide to reduced graphene oxide (rGO). At the same time, the thiol group (-SH) on the surface of the N / S-GF substrate undergoes a nucleophilic ring-opening reaction with the epoxy group (COC) remaining on the graphene oxide and reduced graphene oxide (rGO), forming CSC sulfide covalent bonds in situ, thus chemically bonding the reduced graphene oxide (rGO) nanosheets to the GF substrate.

[0010] Step 5: Wash and dry the sample after the reaction in Step 4 to obtain the N / S-GF@rGO composite electrode.

[0011] In the method for preparing graphite felt-graphene composite electrodes by covalent interface engineering, in step 1, the concentration of the (NH4)2SO4 aqueous solution is 120–140 mg / mL, the pH value is 4.5–5.5, and the electrochemical oxidation treatment time is 90–100 seconds.

[0012] In the method for preparing graphite felt-graphene composite electrodes by covalent interface engineering, in step 2, the concentration of the GO dispersion is 0.9–1.1 mol / mL, and the ultrasonic time is 2–3 hours or more.

[0013] In the method for preparing graphite felt-graphene composite electrodes by covalent interface engineering, in step 3, the drying temperature is 55–65°C and the drying time is 11–13 hours.

[0014] In the method for preparing graphite felt-graphene composite electrodes by covalent interface engineering, in step 4, the C–S–C thioether covalent bond is formed by nucleophilic attack of the epoxy group by the thiol-SH group. The C–S–C thioether covalent bond anchors the reduced graphene oxide rGO nanosheets on the surface of the graphite felt fiber, forming a continuous electron transport channel.

[0015] In the method for preparing graphite felt-graphene composite electrodes by covalent interface engineering, in step 5, the washing is performed by alternating between anhydrous ethanol and deionized water 3–5 times, the drying temperature is 160–180°C, and the drying time is 1.5–2 hours.

[0016] A composite electrode is prepared by the method described above. The composite electrode has a three-dimensional hierarchical porous structure. Reduced graphene oxide (rGO) nanosheets are uniformly covered on the surface of N / S co-doped graphite felt fibers by C–S–C covalent bonds, and simultaneously contain nitrogen, sulfur, and oxygen heteroatoms as dopants.

[0017] The composite electrode described herein is used in a vanadium redox flow battery at a speed of 300 mA cm⁻¹. -2 Energy efficiency of no less than 87% at current density, at 500 mA cm⁻¹ -2 Energy efficiency not less than 75%, peak power density ≥1270 mW / cm² -2 Furthermore, its performance degradation is less than 5% after 2000 charge-discharge cycles.

[0018] In the aforementioned composite electrode, the composite electrode is composed of a carbon-based material.

[0019] A vanadium redox flow battery, wherein the positive and / or negative electrodes employ the aforementioned composite electrode.

[0020] Compared with existing technologies, the present invention has the following advantages: The N,S,O co-doped hierarchical electrode (N / S-GF@rGO) prepared by the present invention through an in-situ CSC sulfide covalent interface engineering strategy achieves high performance at 300 mA cm⁻¹. -2 At high current densities, the energy efficiency (EE) can reach 87.1%; at 500 mA cm⁻¹ -2 It still maintains 75.2%; its peak power density is as high as 1274.1 mW / cm³. -2 Furthermore, it exhibits outstanding long-term stability, maintaining its structural integrity and catalytic activity even after 2000 ultra-long cycles. Attached Figure Description

[0021] Various other advantages and benefits of the present invention will become apparent to those skilled in the art upon reading the detailed description of the preferred embodiments below. The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. It is obvious that the drawings described below are merely some embodiments of the invention, and those skilled in the art can obtain other drawings based on these drawings without any inventive effort. Furthermore, the same reference numerals denote the same parts throughout the drawings.

[0022] In the attached diagram:

[0023] Figure 1 This is a flow chart of the N / S-GF@rGO preparation process of the present invention;

[0024] Figure 2 This is an energy efficiency diagram of N / S-GF@rGO prepared by this invention for different HI acid reduction times;

[0025] Figure 3 Here is a SEM image of the N / S-GF@rGO-2 sample prepared according to this invention;

[0026] Figure 4 This is the XPS spectrum of N / S-GF@rGO-2 prepared in this invention;

[0027] Figure 5 This is the XPS S 2p spectrum of N / S-GF@rGO-2 prepared in this invention;

[0028] Figure 6 This is a cyclic voltammetry curve of N / S-GF@rGO-2 prepared in this invention;

[0029] Figure 7 This is an impedance curve of N / S-GF@rGO-2 prepared according to the present invention;

[0030] Figure 8 This is a charge-discharge curve of N / S-GF@rGO-2 prepared in this invention;

[0031] Figure 9 This is an energy efficiency diagram of N / S-GF@rGO-2 prepared in this invention;

[0032] Figure 10 This is a polarization curve of N / S-GF@rGO-2 prepared according to the present invention;

[0033] Figure 11 This is a graph showing the 2000-cycle performance of N / S-GF@rGO-2 prepared in this invention;

[0034] Figure 12 This is a SEM image of N / S-GF@rGO-1 prepared according to an embodiment of the present invention;

[0035] Figure 13 This is a SEM image of N / S-GF@rGO-3 prepared according to an embodiment of the present invention;

[0036] Figure 14 This is the energy spectrum of N / S-GF@rGO-4 prepared according to an embodiment of the present invention;

[0037] Figure 15This is an example of N / S-GF@rGO-4 prepared according to the present invention at 200 mA cm⁻¹ -2 Energy efficiency diagram;

[0038] Figure 16 This is a SEM image of N / S-GF@rGO-5 prepared according to an embodiment of the present invention.

[0039] The present invention will be further explained below with reference to the accompanying drawings and embodiments. Detailed Implementation

[0040] Specific embodiments of the invention will now be described in more detail with reference to the accompanying drawings. While specific embodiments of the invention are shown in the drawings, it should be understood that the invention may be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided so that this invention will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art.

[0041] It should be noted that certain terms are used in the specification and claims to refer to specific components. Those skilled in the art will understand that different terms may be used to refer to the same component. This specification and claims do not distinguish components based on differences in terminology, but rather on differences in function. The terms "comprising" or "including" used throughout the specification and claims are open-ended and should be interpreted as "comprising but not limited to." The following descriptions are preferred embodiments for carrying out the invention; however, these descriptions are for the purpose of understanding the general principles of the specification and are not intended to limit the scope of the invention. The scope of protection of this invention is determined by the appended claims.

[0042] To facilitate understanding of the embodiments of the present invention, further explanations and descriptions will be provided below with reference to the accompanying drawings and specific embodiments. The accompanying drawings do not constitute a limitation on the embodiments of the present invention.

[0043] like Figures 1 to 16 As shown, the method for preparing graphite felt-graphene composite electrodes via covalent interface engineering includes the following steps:

[0044] Step 1: N / S co-doping pretreatment of graphite felt GF, wherein graphite felt GF is electrochemically oxidized in (NH4)2SO4 dispersion to achieve nitrogen / sulfur co-doping, and reactive mercapto-SH groups are introduced on the surface of graphite felt to obtain N / S-GF substrate.

[0045] Step 2: Prepare an aqueous solution of graphene oxide and disperse it ultrasonically to obtain a uniform GO dispersion;

[0046] Step 3: The N / S-GF substrate is completely immersed in the GO dispersion to adsorb graphene oxide, and then dried to obtain the N / S-GF@GO intermediate;

[0047] Step 4: The N / S-GF@GO intermediate is reduced with hydroiodic acid (HI) at a high temperature of 155-165°C for 2.5-3.5 hours to reduce graphene oxide to reduced graphene oxide (rGO). At the same time, the thiol group (-SH) on the surface of the N / S-GF substrate undergoes a nucleophilic ring-opening reaction with the epoxy group (COC) remaining on the graphene oxide and reduced graphene oxide (rGO), forming CSC sulfide covalent bonds in situ, thus chemically bonding the reduced graphene oxide (rGO) nanosheets to the GF substrate.

[0048] Step 5: Wash and dry the sample after the reaction in Step 4 to obtain the N / S-GF@rGO composite electrode.

[0049] In a preferred embodiment of the method for preparing graphite felt-graphene composite electrodes by covalent interface engineering, in step 1, the concentration of the (NH4)2SO4 aqueous solution is 120–140 mg / mL, the pH value is 4.5–5.5, and the electrochemical oxidation treatment time is 90–100 seconds.

[0050] In a preferred embodiment of the method for preparing graphite felt-graphene composite electrodes by covalent interface engineering, in step 2, the concentration of the GO dispersion is 0.9–1.1 mol / mL, and the ultrasonication time is 2–3 hours or more.

[0051] In a preferred embodiment of the method for preparing graphite felt-graphene composite electrodes by covalent interface engineering, in step 3, the drying temperature is 55–65°C and the drying time is 11–13 hours.

[0052] In a preferred embodiment of the method for preparing graphite felt-graphene composite electrodes by covalent interface engineering, in step 4, the C–S–C thioether covalent bond is formed by nucleophilic attack of the epoxy group by the thiol-SH group, and the C–S–C thioether covalent bond anchors the reduced graphene oxide rGO nanosheets on the surface of the graphite felt fiber, forming a continuous electron transport channel.

[0053] In a preferred embodiment of the method for preparing graphite felt-graphene composite electrodes by covalent interface engineering, in step 5, the washing is performed by alternating between anhydrous ethanol and deionized water 3–5 times, the drying temperature is 160–180°C, and the drying time is 1.5–2 hours.

[0054] A composite electrode is prepared by the method described above. The composite electrode has a three-dimensional hierarchical porous structure. Reduced graphene oxide (rGO) nanosheets are uniformly covered on the surface of N / S co-doped graphite felt fibers by C–S–C covalent bonds, and simultaneously contain nitrogen, sulfur, and oxygen heteroatoms as dopants.

[0055] In a preferred embodiment of the composite electrode, the composite electrode is used in a vanadium redox flow battery at 300 mAcm. -2 Energy efficiency of no less than 87% at current density, at 500 mA cm⁻¹ -2 Energy efficiency not less than 75%, peak power density ≥1270 mW / cm² -2 Furthermore, its performance degradation is less than 5% after 2000 charge-discharge cycles.

[0056] In a preferred embodiment of the composite electrode, the composite electrode is made of a carbon-based material.

[0057] A vanadium redox flow battery, wherein the positive and / or negative electrodes employ the aforementioned composite electrode.

[0058] In one embodiment, a method for preparing a graphite felt-graphene composite electrode via covalent interface engineering comprises the following steps:

[0059] Step 1: N / S co-doping pretreatment of graphite felt (GF). Sheet-shaped raw graphite felt with length ≥ width >> height was subjected to electrochemical oxidation treatment for 90-100 s in a (NH4)2SO4 dispersion at pH 4.5-5.5 and a concentration of 120-140 mg / ml to achieve nitrogen / sulfur (N / S) co-doping and introduce reactive thiol groups (-SH) onto the graphite felt surface, resulting in an N / S-GF substrate.

[0060] Step 2: Prepare a 0.9-1.1 mol / mL graphene oxide (GO) aqueous solution in a beaker, and place it in an ultrasonicator for 2-3 hours or more to completely disperse it, so as to obtain a uniform GO dispersion.

[0061] Step 3: Immerse the N / S-GF substrate obtained in Step 1 completely into the GO solution obtained in Step 2 to allow it to fully adsorb GO, then remove it and dry it at 55-65 °C for 11-13 h to obtain the N / S-GF@GO intermediate.

[0062] Step 4: The N / S-GF@GO obtained in Step 3 is reduced with hydroiodic acid (HI acid) at a high temperature of 155-165°C for 2.5-3.5 hours. In this step, GO is efficiently reduced to rGO. At the same time, the thiol groups (-SH) on the surface of the N / S-GF substrate undergo a nucleophilic ring-opening reaction with the residual epoxy groups (COC) on GO / rGO, forming CSC thioether covalent bonds in situ, thus chemically bonding the rGO nanosheets to the GF substrate.

[0063] Step 5: Wash the sample after the reaction in Step 4 with anhydrous ethanol and deionized water alternately 3-5 times, and finally dry it at a high temperature of 160-180°C for 1.5-2 h to obtain the N / S-GF@rGO composite electrode.

[0064] Example 1

[0065] This embodiment aims to prepare N / S-GF@rGO-2 samples with optimal performance.

[0066] Step 1: N / S co-doping pretreatment of graphite felt (GF). Sheet-shaped raw graphite felt with length ≥ width >> height was electrochemically oxidized for 90 s in a (NH4)2SO4 dispersion at pH 5 and a concentration of 130 mg / ml to achieve nitrogen / sulfur (N / S) co-doping and introduce reactive thiol groups (-SH) onto the graphite felt surface, resulting in an N / S-GF substrate.

[0067] Step 2: Prepare a 1 mol / mL aqueous solution of graphene oxide (GO) in a beaker, and sonicate it in an ultrasonicator for 2 hours or more to completely disperse it and obtain a uniform GO dispersion.

[0068] Step 3: Immerse the N / S-GF substrate obtained in Step 1 completely into the GO solution obtained in Step 2 to allow it to fully adsorb GO, then remove it and dry it at 60 °C for 12 h to obtain the N / S-GF@GO intermediate.

[0069] Step 4: The N / S-GF@GO obtained in Step 3 is reduced with hydroiodic acid (HI acid) at 160 °C for 3 h. In this step, GO is efficiently reduced to rGO. At the same time, the thiol groups (-SH) on the surface of the N / S-GF substrate undergo a nucleophilic ring-opening reaction with the residual epoxy groups (COC) on GO / rGO, forming CSC thioether covalent bonds in situ, thus chemically bonding the rGO nanosheets to the GF substrate.

[0070] Step 5: Wash the sample after the reaction in Step 4 with anhydrous ethanol and deionized water three times alternately, and finally dry it at a high temperature of 160 °C for 2 h to obtain the N / S-GF@rGO composite electrode.

[0071] The N / S-GF@rGO-2 samples prepared in the above steps were characterized as follows:

[0072] Figure 3 This is a SEM image of N / S-GF@rGO-2. A thin, continuous, and highly wrinkled layer of rGO nanosheets is uniformly and densely covered on the surface of the GF carbon fibers, forming a three-dimensional hierarchical porous structure with rich layered structure. This uniform and strong adhesion not only significantly increases the specific surface area of ​​the electrode, but more importantly, it directly supports the inference of successful formation of CSC covalent bridging. Figure 4 The XPS spectrum of N / S-GF@rGO-2 shows that the contents of C, O, N and S in each sample are significantly different, confirming that the high-temperature HI acid reduction process did not cause significant damage to the N and S already doped in GF. Figure 5 This is the XPS S 2p spectrum of N / S-GF@rGO-2. The high-resolution S 2p spectrum shows a pair of distinct peaks located at 161.8 eV (S 2p). 3 / 2 ) and 162.9 eV (S 2p 1 / 2 These peaks are characteristic of thiol (-SH) groups. The presence of these active -SH groups on the surface is crucial because they are the primary reaction sites for subsequent covalent binding with GO; Figure 6 The cyclic voltammetry curve of N / S-GF@rGO-2 shows that N / S-GF@rGO-2 has better activation performance; Figure 7 The electrochemical impedance spectroscopy of N / S-GF@rGO-2 shows that N / S-GF@rGO-2 has the lowest charge transport resistance due to its abundant active sites and porosity. Figure 8 The charge-discharge curves show that N / S-GF@rGO-2 is charged and discharged at different current densities (200-500 mA cm⁻¹). -2 Excellent charge and discharge performance; Figure 9 The diagram shows the energy efficiency of N / S-GF@rGO-2, indicating that it has high energy efficiency under different current densities. Figure 10 The polarization curve of N / S-GF@rGO-2 shows that a high power density was obtained through the covalent interface structure. Figure 11 This is a long-cycle performance test chart of N / S-GF@rGO-2, demonstrating that N / S-GF@rGO-2 has an excellent service life in actual use.

[0073] In Example 1, step four, with a restoration time of 3 hours, achieves optimal performance. In one embodiment, further based on... Figure 2Insufficient reduction (2 hours) resulted in low rGO loading, poor adhesion, and minimal rGO nanosheet loading, leaving most of the fiber surface exposed and thus lacking good electrode performance. Excessive reduction (4 hours) led to excessive etching of the GF surface, disrupting structural integrity. The GF fiber surface exhibited significant roughness, erosion, and irregular pitting, and the rGO layer appeared fragmented and uneven. This excessive etching damaged the structural integrity of the GF substrate and its original conductive network, leading to a decline in its electrochemical performance. In step 2, at two points within the range of 200-500 mA / cm², using a 0.5 mol / L GO solution resulted in insufficient rGO loading on the graphite felt, weakening electron transfer and catalytic activity, thus leading to poor electrode performance. The electrode prepared using a 1.0 mol / L GO solution exhibited the highest energy efficiency (EE). Increasing the concentration to 2.0 mol / L resulted in a decrease in EE, possibly due to incomplete reduction caused by an excessively thick GO layer or hindering electrolyte migration.

[0074] Example 2

[0075] This embodiment aims to obtain a comparative N / S-GF@rGO-1 sample.

[0076] Step 1: N / S co-doping pretreatment of graphite felt (GF). Sheet-shaped raw graphite felt with length ≥ width >> height was electrochemically oxidized for 90 s in a (NH4)2SO4 dispersion at pH 4.5 and a concentration of 120 mg / ml to achieve nitrogen / sulfur (N / S) co-doping and introduce reactive thiol groups (-SH) onto the graphite felt surface, resulting in an N / S-GF substrate.

[0077] Step 2: Prepare a 0.9 mol / mL graphene oxide (GO) aqueous solution in a beaker and sonicate it in an ultrasonicator for 2 hours or more to completely disperse it and obtain a uniform GO dispersion.

[0078] Step 3: Immerse the N / S-GF substrate obtained in Step 1 completely into the GO solution obtained in Step 2 to allow it to fully adsorb GO, then remove it and dry it at 55 °C for 11 h to obtain the N / S-GF@GO intermediate.

[0079] Step 4: The N / S-GF@GO obtained in Step 3 is reduced with hydroiodic acid (HI acid) at 155 °C for 2.5 h. In this step, GO is efficiently reduced to rGO. Simultaneously, the thiol groups (-SH) on the surface of the N / S-GF substrate undergo a nucleophilic ring-opening reaction with the residual epoxy groups (COC) on GO / rGO, forming CSC thioether covalent bonds in situ, thus chemically bonding the rGO nanosheets to the GF substrate.

[0080] Step 5: Wash the sample after the reaction in Step 4 with anhydrous ethanol and deionized water three times alternately, and finally dry it at a high temperature of 160 °C for 1.5 h to obtain the N / S-GF@rGO-1 composite electrode.

[0081] The N / S-GF@rGO-1 samples obtained by the above steps were characterized as follows:

[0082] Figure 12 The image shows the SEM image of N / S-GF@rGO-1. As you can see, the surface is relatively smooth and there is no obvious loading material on it. This proves that no obvious functional groups are formed on the surface, so it is not a very ideal electrode. Figure 4 The XPS spectrum shows that although the contents of C, O, N, and S differ significantly within the N / S-GF@rGO-1 sample, the combination... Figure 12 However, due to the extremely low loading of rGO nanosheets, most of the fiber surface remains exposed, thus lacking good electrode performance; Figure 6 The cyclic voltammetry curve of N / S-GF@rGO-2 shows that the activation performance of N / S-GF@rGO-1 is indeed inferior to that of the best sample N / S-GF@rGO-2. Figure 7 The electrochemical impedance spectroscopy of N / S-GF@rGO-2 also shows that the charge transport resistance of N / S-GF@rGO-1 is not optimal due to the lack of abundant active sites. Figure 9 The energy efficiency plot of N / S-GF@rGO-2 shows that the energy efficiency of this sample is indeed not as good as that of the optimal sample under different current densities.

[0083] Example 3

[0084] This embodiment aims to obtain a comparative N / S-GF@rGO-3 sample.

[0085] Step 1: N / S co-doping pretreatment of graphite felt (GF). Sheet-shaped raw graphite felt with length ≥ width >> height was electrochemically oxidized for 100 s in a (NH4)2SO4 dispersion at pH 5.5 and a concentration of 140 mg / ml to achieve nitrogen / sulfur (N / S) co-doping and introduce reactive thiol groups (-SH) onto the graphite felt surface, resulting in an N / S-GF substrate.

[0086] Step 2: Prepare a 1.1 mol / mL graphene oxide (GO) aqueous solution in a beaker and sonicate it in an ultrasonicator for 3 hours or more to completely disperse it and obtain a uniform GO dispersion.

[0087] Step 3: Immerse the N / S-GF substrate obtained in Step 1 completely into the GO solution obtained in Step 2 to allow it to fully adsorb GO, then remove it and dry it at 65 °C for 13 h to obtain the N / S-GF@GO intermediate.

[0088] Step 4: The N / S-GF@GO obtained in Step 3 is reduced with hydroiodic acid (HI acid) at 165°C for 3.5 hours. In this step, GO is efficiently reduced to rGO. Simultaneously, the thiol groups (-SH) on the surface of the N / S-GF substrate undergo a nucleophilic ring-opening reaction with the residual epoxy groups (COC) on GO / rGO, forming CSC thioether covalent bonds in situ, thus chemically bonding the rGO nanosheets to the GF substrate.

[0089] Step 5: Wash the sample after the reaction in Step 4 with anhydrous ethanol and deionized water alternately 5 times, and finally dry it at a high temperature of 180°C for 2 h to obtain the N / S-GF@rGO-3 composite electrode.

[0090] The N / S-GF@rGO-3 samples obtained by the above steps were characterized. Figure 13 The image shows an SEM image of N / S-GF@rGO-3. It can be seen that although there are many loaded materials on the surface, the structural integrity is initially damaged due to the slightly longer reduction time. The GF fiber surface shows obvious roughness, erosion and irregular pits, so it is not a very ideal electrode. Figure 4 Although the contents of C, O, N and S in the N / S-GF@rGO-3 sample differed significantly in the XPS energy spectrum, it did not exhibit optimal performance due to the reasons mentioned above. Figure 6 The cyclic voltammetry curve of N / S-GF@rGO-2 shows that although the activation performance of N / S-GF@rGO-3 is slightly better than that of N / S-GF@rGO-1, it is still inferior to the best sample N / S-GF@rGO-2. Figure 7 The electrochemical impedance spectroscopy of N / S-GF@rGO-3 also shows that the structural damage caused by excessive etching of N / S-GF@rGO-3 resulted in the charge transport resistance not reaching the optimal level. Figure 9 The energy efficiency plot of N / S-GF@rGO-3 shows that the energy efficiency of this sample did not reach the optimal level under different current densities.

[0091] Example 4

[0092] This embodiment aims to obtain a comparative N / S-GF@rGO-4 sample.

[0093] Step 1: N / S co-doping pretreatment of graphite felt (GF). Sheet-shaped raw graphite felt with length ≥ width >> height was electrochemically oxidized for 80 s in a (NH4)2SO4 dispersion at pH 7 with a concentration of 110 mg / ml to achieve nitrogen / sulfur (N / S) co-doping and introduce reactive thiol groups (-SH) onto the graphite felt surface, resulting in an N / S-GF substrate.

[0094] Step 2: Prepare a 0.8 mol / mL graphene oxide (GO) aqueous solution in a beaker, and place it in an ultrasonicator for 1 hour or more to completely disperse it, so as to obtain a uniform GO dispersion.

[0095] Step 3: Immerse the N / S-GF substrate obtained in Step 1 completely into the GO solution obtained in Step 2 to allow it to fully adsorb GO, then remove it and dry it at 45 °C for 10 h to obtain the N / S-GF@GO intermediate.

[0096] Step 4: The N / S-GF@GO obtained in Step 3 is reduced with hydroiodic acid (HI acid) at 150°C for 2 hours. In this step, GO is efficiently reduced to rGO. Simultaneously, the thiol groups (-SH) on the surface of the N / S-GF substrate undergo a nucleophilic ring-opening reaction with the residual epoxy groups (COC) on GO / rGO, forming CSC thioether covalent bonds in situ, thus chemically bonding the rGO nanosheets to the GF substrate.

[0097] Step 5: Wash the sample after the reaction in Step 4 twice with anhydrous ethanol and deionized water alternately, and finally dry it at a high temperature of 140°C for 1 h to obtain the N / S-GF@rGO-4 composite electrode.

[0098] The N / S-GF@rGO-4 samples obtained by the above steps were characterized. Figure 14 This is the energy spectrum of N / S-GF@rGO-4. Observation reveals that due to the neutral environment during pretreatment, it essentially lacks sulfur, thus losing its significance as an N / S doped modified electrode. However, its energy spectrum at 200 mA cm⁻¹ was still tested. -2 Energy efficiency ( Figure 15 It can be seen that the energy efficiency is much lower than that of the optimal electrode N / S-GF@rGO-2 at that point. One of the main reasons for this is that the loading in the low-concentration GO aqueous solution is insufficient.

[0099] Example 5

[0100] This embodiment aims to obtain a comparative N / S-GF@rGO-4 sample.

[0101] Step 1: N / S co-doping pretreatment of graphite felt (GF). Sheet-shaped raw graphite felt with length ≥ width >> height was electrochemically oxidized for 120 s in a 150 mg / ml (NH4)2SO4 dispersion at pH 4 to achieve nitrogen / sulfur (N / S) co-doping and introduce reactive thiol groups (-SH) onto the graphite felt surface, resulting in an N / S-GF substrate.

[0102] Step 2: Prepare a 1.2 mol / mL aqueous solution of graphene oxide (GO) in a beaker, and sonicate it in an ultrasonicator for 4 hours or more to completely disperse it and obtain a uniform GO dispersion.

[0103] Step 3: Immerse the N / S-GF substrate obtained in Step 1 completely into the GO solution obtained in Step 2 to allow it to fully adsorb GO, then remove it and dry it at 55-65 °C for 14 h to obtain the N / S-GF@GO intermediate.

[0104] Step 4: The N / S-GF@GO obtained in Step 3 is reduced with hydroiodic acid (HI acid) at 175°C for 4 hours. In this step, GO is efficiently reduced to rGO. At the same time, the thiol groups (-SH) on the surface of the N / S-GF substrate undergo a nucleophilic ring-opening reaction with the residual epoxy groups (COC) on GO / rGO, forming CSC thioether covalent bonds in situ, thus chemically bonding the rGO nanosheets to the GF substrate.

[0105] Step 5: Wash the sample after the reaction in Step 4 with anhydrous ethanol and deionized water 6 times alternately, and finally dry it at a high temperature of 200°C for 3 h to obtain the N / S-GF@rGO-5 composite electrode.

[0106] The N / S-GF@rGO-5 samples prepared in the above steps were characterized. Figure 16 This is a SEM image of N / S-GF@rGO-5. It can be seen that although the increased concentration of GO aqueous solution can result in more loading on the surface, the structural integrity is initially damaged due to the overly acidic pretreatment environment and the slightly longer reduction time. The surface of the GF fiber shows obvious irregular pits caused by acid ablation, making it not a very ideal electrode. Figure 15 Its value is 200 mA cm -2 Its energy efficiency is far lower than that of the optimal electrode, therefore it cannot be considered a qualified electrode.

[0107] Furthermore, at the electron transport level, the C–S–C covalent bond acts as a “molecular wire”, establishing a low-resistance, continuous electron transport channel between the graphite felt fiber and the rGO nanosheet. This significantly reduces the interfacial contact resistance caused by van der Waals forces in traditional physical adsorption composite electrodes, greatly improving the overall conductivity and charge transfer kinetics of the electrode.

[0108] Secondly, in terms of structural stability, covalent bonding firmly anchors the rGO nanosheets to the graphite felt surface. Even under the long-term scouring of the high-flow-rate electrolyte and the stress of repeated charge and discharge in the vanadium redox flow battery, it will not peel off or fall off. This fundamentally solves the problem of easy deactivation and short lifespan of carbon-based catalyst layers, and endows the electrode with ultra-long cycle stability (no significant decay after 2000 cycles).

[0109] Finally, at the catalytic activity level, this strategy synergistically achieves multiple functions: on the one hand, the co-doping of N, S, and O heteroatoms introduces abundant defect sites and polar functional groups into the carbon framework, effectively modulating the electronic structure of carbon materials and enhancing the catalytic activity of V. 2+ / V 3+ and VO 2+ / VO2 + The intrinsic catalytic activity of the redox couple is enhanced; on the other hand, rGO nanosheets form a highly wrinkled, continuously covered three-dimensional hierarchical porous network on the surface of graphite felt fibers, significantly increasing the electrochemical active surface area and optimizing electrolyte wettability and mass transfer efficiency. Through a synergistic mechanism of "chemical bonding + heteroatom doping + structural design," the long-standing dilemma of "high activity – high conductivity – high stability" faced by vanadium redox flow battery electrode materials has been successfully solved, providing a new paradigm for the large-scale preparation of high-performance, long-life, metal-catalyst-free carbon-based composite electrodes.

[0110] Although embodiments of the present invention have been described above in conjunction with the accompanying drawings, the present invention is not limited to the specific embodiments and application fields described above. The specific embodiments described above are merely illustrative and instructive, and not restrictive. Those skilled in the art can make many other forms based on the guidance of this specification and without departing from the scope of protection of the claims of the present invention, and all of these are within the scope of protection of the present invention.

Claims

1. A method for preparing graphite felt-graphene composite electrodes via covalent interface engineering, characterized in that, Includes the following steps: Step 1: N / S co-doping pretreatment of graphite felt GF, wherein graphite felt GF is electrochemically oxidized in (NH4)2SO4 dispersion to achieve nitrogen / sulfur co-doping, and reactive mercapto-SH groups are introduced on the surface of graphite felt to obtain N / S-GF substrate. Step 2: Prepare an aqueous solution of graphene oxide and disperse it ultrasonically to obtain a uniform GO dispersion; Step 3: The N / S-GF substrate is completely immersed in the GO dispersion to adsorb graphene oxide, and then dried to obtain the N / S-GF@GO intermediate; Step 4: The N / S-GF@GO intermediate is reduced with hydroiodic acid (HI) at a high temperature of 155-165°C for 2.5-3.5 hours to reduce graphene oxide to reduced graphene oxide (rGO). At the same time, the thiol group (-SH) on the surface of the N / S-GF substrate undergoes a nucleophilic ring-opening reaction with the epoxy group (COC) remaining on the graphene oxide and reduced graphene oxide (rGO), forming CSC sulfide covalent bonds in situ, thus chemically bonding the reduced graphene oxide (rGO) nanosheets to the GF substrate. Step 5: Wash and dry the sample after the reaction in Step 4 to obtain the N / S-GF@rGO composite electrode.

2. The method for preparing a graphite felt-graphene composite electrode by covalent interface engineering according to claim 1, characterized in that, Preferably, in step 1, the concentration of the (NH4)2SO4 aqueous solution is 120–140 mg / mL, the pH value is 4.5–5.5, and the electrochemical oxidation treatment time is 90–100 seconds.

3. The method for preparing a graphite felt-graphene composite electrode via covalent interface engineering according to claim 1, characterized in that, In step 2, the concentration of the GO dispersion is 0.9–1.1 mol / mL, and the sonication time is 2–3 hours or more.

4. The method for preparing a graphite felt-graphene composite electrode by covalent interface engineering according to claim 1, characterized in that, In step 3, the drying temperature is 55–65°C and the drying time is 11–13 hours.

5. The method for preparing a graphite felt-graphene composite electrode by covalent interface engineering according to claim 1, characterized in that, In step 4, the C–S–C thioether covalent bond is formed by the nucleophilic attack of the epoxy group by the thiol –SH group. The C–S–C thioether covalent bond anchors the reduced graphene oxide rGO nanosheets on the surface of the graphite felt fiber, forming a continuous electron transport channel.

6. The method for preparing a graphite felt-graphene composite electrode by covalent interface engineering according to claim 1, characterized in that, In step 5, the washing is performed by alternating between anhydrous ethanol and deionized water 3–5 times, the drying temperature is 160–180°C, and the drying time is 1.5–2 hours.

7. A composite electrode, characterized in that, It is prepared by the method described in any one of claims 1–6. The composite electrode has a three-dimensional hierarchical porous structure. Reduced graphene oxide (rGO) nanosheets are uniformly covered on the surface of N / S co-doped graphite felt fibers through C–S–C covalent bonds, and simultaneously contain nitrogen, sulfur, and oxygen heteroatoms doped.

8. The composite electrode according to claim 7, characterized in that, The composite electrode is used in a full vanadium redox flow battery at 300 mA cm⁻¹ -2 Energy efficiency of no less than 87% at current density, at 500 mA cm⁻¹ -2 Energy efficiency not less than 75%, peak power density ≥1270 mW / cm² -2 Furthermore, its performance degradation is less than 5% after 2000 charge-discharge cycles.

9. The composite electrode according to claim 7, characterized in that, The composite electrode is made of carbon-based materials.

10. A vanadium redox flow battery, characterized in that, Its positive and / or negative electrodes employ the composite electrodes described in any one of claims 7–9.

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